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Preparation of polyetherimide nanoparticles by electrospray drying, and their use in the preparation of mixed-matrix carbon molecular-sieve (CMS) membranes
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Preparation of polyetherimide nanoparticles by electrospray drying, and their use in the preparation of mixed-matrix carbon molecular-sieve (CMS) membranes
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Content
PREPARATION OF POLYETHERIMIDE NANOPARTICLES BY
ELECTROSPRAY DRYING, AND THEIR USE IN THE PREPARATION OF
MIXED-MATRIX CARBON MOLECULAR-SIEVE (CMS) MEMBRANES
by
Faezeh Bagheri-Tar
A Dissertation Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(CHEMICAL ENGINEERING)
December 2007
Copyright 2007 Faezeh Bagheri-Tar
ii
Dedication
Dedicated to the memory of
My sister Behjat, my role model
My mother Tavooss, my first teacher
My father Abbas, my foremost supporter
iii
Acknowledgments
I would like to thank both of my advisors. Professor Muhammad Sahimi, a
kind and smart scientist, with excellent sense of humor, who has not only provided
me with guidance in my research, but has also given me his time generously to
encourage, advise and support me. Professor Theodore T. Tsotsis, a very intelligent
and hard working scientist, who has given me considerable advice whenever I
could not make further progress in solving difficult problems in my research. I
have learnt a lot from him. I would also like to thank Professor Tsotsis' wife, Betty,
who has always been very kind to me.
I am grateful to my Ph.D. committee members, Professor Thieo E.
Hogen Esch, Dr. Katherine S. Shing, and Dr. C. Ted Lee for their valuable
comments and suggestions. In particular, Professor Hogen Esch has had
considerable impact on my work by giving me advice very generously during my
study at USC. I also want to thank Dr. T. J. Williams who accepted at the last
minute to be the replacement as the outside committee member.
I thank Dr. Li Yang, a postdoctoral fellow in our group, who contributed to the
making of the flat membrane set-up.
I would like to acknowledge the great support of Mrs. Margie Berti, Associate
Dean of the Viterbi School of Engineering, who always helped me in my studies
iv
by providing me with financial support in the form of a graduate fellowship. I also
thank Dr. Richard Tithecott, Associate Dean of the Graduate School, who also
provided me with financial support in the form of a thesis fellowship.
It is my great pleasure to acknowledge my brother Ali and his wife Farahnaz
who have always been supportive, as well as my brother Ja'far, my sister Maryam
and her hausband Dr. Naser, for their moral support, my nephew, Ehsan Bagheri,
and my niece, Elham Bagheri, who cared about me with their endless support.
I want to thank Tina Silva, our lab technician, who has been a great help and
support for all of us, as well as our staff members, Karen Woo and Brendan Char,
who provided much needed support. I would like to thank many friends in our
research group, T. Kim, Aadesh Haraleland, Hyun Hwang, and Mitra Abdollahi,
for their help in some of the mixed-gas experiments, as well as my friends Megha
Dadwhal and Raudel Sanchez who always helped me.
I would also like to express my appreciation for the help that Dr. Paul Liu of
Process & Media Technology, provided me with.
I am grateful for the financial support provided by the California Energy
Commission, and the Powell, WiSE, the Rob Queen, and Hal Clark fellowships.
v
Table of Contents
Dedication
ii
Acknowledgments
iii
List of Tables
vii
List of Figures
ix
Abstract
xii
Preface
xiv
Chapter 1: Introduction 1
1.1 Energy Problem Targeted and its Impact 2
1.2 Membranes Classifications 4
1.3 Membranes and Gas Separation 10
1.4 Background on Natural Gas Processing 13
1.5 Mixed-Matrix & Molecular-Sieve Membranes vs. Polymeric
Membranes
18
1.6 Polyetherimide and Carbon Molecular Sieves 25
1.7 Carbon Molecular-Sieve Nanoparticles 27
1.8 Nanoparticle Preparation Methods 28
1.9 Electrohydrodynamic Atomization 38
1.9.1 Droplet Production and Charging 40
1.9.2 Neutralization of the Aerosol 46
1.9.3 Electrospray Applications 48
References for Chapter 1
53
Chapter 2: Preparation of PEI Nanoparticles by an Electrospray
Technique
66
2.1 Introduction 66
2.2 Experimental Set-up, Materials and Procedures 75
2.3 Results and Discussion 78
2.3.1 The Effect of Distance between the Various Electrodes 78
2.3.2 Capillary Selection 80
2.3.3 EHDA Spray Modes 83
2.3.4 Effect of V oltage 84
2.3.5 Effect of the PEI Concentration 87
2.3.6 Effect of Liquid Flow Rate 91
2.3.7 CMS Particles and their Characterization 93
2.4 Conclusions 95
References for Chapter 2 97
vi
Chapter 3: Preparation of Mixed-Matrix Carbon Molecular-Sieve
Membranes Using Nano-Sized Particles
100
3.1 Introduction 100
3.2 Materials and Procedure 114
3.2.1 CMS Nanoparticles Obtained Using Conventional
Particle Size Reduction Techniques
114
3.2.2 Diffusion of CO
2
and CH
4
in the CMS Nanoparticles 118
3.3 Experimental Set-up and Procedure 121
3.3.1 Mixed Matrix Polymeric Membrane Films 121
3.3.1.1 Preparation of Homogeneous Dense Films 122
3.3.1.2 Experimental Set-up 124
3.3.1.3 Gas Permeation Properties of Nanocomposite
MMP Membrane Films
128
3.3.1.4 Single-Gas Permeation Measurements of MMP
Membrane Films
129
3.3.2 Mixed-Matrix CMS Tubular Membranes 132
3.3.2.1 Preparation of MM-CMS Tubular Membranes 132
3.3.2.2 Experimental Set-up 133
3.3.2.3 Single-Gas Tests for the MM-CMS Tubular
Membranes
136
3.3.2.4 Mixture Gas Tests for the MM-CMS Membrane 139
3.4 Supported Mixed-Matrix Membranes Using the Second Batch
of CMS Nanoparticles
141
3.4.1 Effect of the CMS and Total Solids Loading 141
3.4.2 Mixed-Matrix and Pure CMS Tubular Membranes 142
3.4.3 Effect of the Total PEI Fraction and CMS Content on
the MM-CMS Membranes
144
3.5 Conclusions 152
References for Chapter 3
155
Bibliography 161
vii
List of Tables
Table 1.1 Typical compositions of natural gas wells and the required
composition specifications for sales (Lee et al., 1994, 1995).
15
Table 2.1 BET Surface Area, Pore Size, and Diffusion Coefficient (D/r
2
)
of the CMS Particles Prepared by the Wet Grinding and Dry
Grinding of Pyrolyzed Pellets, as Well as by Electrospray
Pyrolysis.
95
Table 3.1 Permeation properties of mixed matrix films using Matrimid®
5218 or Ultem® 1000 as matrices at various loadings of
carbon molecular sieve inserts (adopted from Vu et al., 2003).
112
Table 3.2 Pure gas permeation data for MMP membranes at various
loadings of the CMS nanoparticles.
129
Table 3.3 Pure gas permeation measurements of MMP membranes.
130
Table 3.4 Permeance and ideal separation factor of a four-times coated
MM-CMS membrane for different gases.
137
Table 3.5 Comparison of performance of the MM-CMS membrane with
a pure CMS membrane.
138
Table 3.6 Mixed-gas permeation results for the four-times coated MM-
CMSM.
140
Table 3.7 Data of quaternary gas mixture permeation, using the M&PT
substrate.
140
Table 3.8 Experimental conditions for the MM-CMS & CMS
membranes.
142
Table 3.9 Comparison of the performance of the supported polymeric
membrane with a different PEI % using the M&PT substrates.
144
Table 3.10 Permeance and ideal selectivities for membranes M
9
and M
11
145
Table 3.11 Permeance and ideal selectivities for membrane M
10
146
Table 3.12 Permeance and ideal selectivities for membranes M
1
-M
6
.
146
Table 3.13 Performance of membrane M
6
147
viii
Table 3.14 Performance of membrane M
5
148
Table 3.15 Performance of membrane M
4
148
Table 3.16 Performance of membrane M
3
149
Table 3.17 Performance of membrane M
2
149
Table 3.18 Performance of membrane M
1
made with 6wt.% of the PEI
solutions (1
st
coat) and 2wt. % of the PEI solutions (2
nd+
coat).
149
Table 3.19 Performance of membrane M
7
150
Table 3.20 Performance of membrane M
8
150
Table 3.21 Mixed-gas results of membrane M
new
151
Table 3.22 Same as in Table 3.21, but at T=170°C. 151
ix
List of Figures
Figure 1.1 Classification of membranes based on their selectivity
mechanisms.
5
Figure 1.2 Solution–diffusion transport mechanism (Koros, 1995). 6
Figure 1.3 The contender technologies for large volume gas separations
(Koros, 2002).
7
Figure 1.4 Milestones in the development of membrane gas separations
(Baker, 2002).
14
Figure 1.5 Basic molecular structure of polyimide (Yamamoto et al.,
1997).
26
Figure 1.6 Polyetherimide ULTEM
1000 (adopted from GE). 26
Figure 1.7 Basic set-up used for electrohydrodynamic atomization
(EHDA).
39
Figure 1.8 Electrospray modes (after Cloupeau and Prunet-Foch, 1990). 42
Figure 1.9 Cone shape, droplet size and electric current depend on the
forces acting on liquid and dissolved ions: gravity, electric
field, surface tension (Hartman, 1998). Right: Cone-jet
picture adopted from Pantano et al. (1996).
45
Figure 1.10 Electrospray chamber and experimental set-up of bipolar
coagulation between liquid droplets (Camelot et al., 1998).
47
Figure 1.11 Electrospray chamber and experimental setup of electrospray
pyrolysis (Lenggoro and Okuyama, 1999).
47
Figure 1.12 Evaporation and desorption process according to Iribarne and
Thomson (1979).
50
Figure 1.13 Left: Multiple electrode electrospray apparatus of Taylor
cones established on capillary electrodes placed opposite a
slotted flat plate electrode; Right: Photograph of a linear
array with, R/S =0.31, P=1.39 (Rulison and Flagan, 1993).
52
Figure 2.1 Basic set-up used for electrospraying (electrohydrodynamic
atomization, EHDA, adapted from Ijsebaert et al., (2001)).
69
Figure 2.2 Electrospray modes (after Cloupeau and Prunet-Foch, 1990). 70
x
Figure 2.3 Schematic of the experimental EHDA system (adapted from
Ijsebaert et al., 2001).
76
Figure 2.4 (a) Particles obtained with the stainless steel 20-μm-id
capillary with a solution flow rate of 0.5 ml/h, solution
concentration of 0.14 wt.% PEI, capillary voltage of 13.8 kV ,
and ring voltage of 7 kV . (b) Higher-magnification image,
showing agglomerated particles in addition to fine primary
particles.
82
Figure 2.5 Scanning electron microscopy (SEM) images of particles
produced with a constant ring voltage of 7 kV and a range of
capillary voltages of 11.8-15.8 kV; flow rate of 0.3 ml/h, PEI
concentration of 0.14 wt. %.
86
Figure 2.6 Particle size analysis distribution (PASD) of particles
produced with a constant ring voltage of 7 kV and a range of
capillary voltages of 10.8-13.8 kV; flow rate of 0.3 ml/h, PEI
concentration of 0.14 wt.%. (Dm is the average particle
diameter.)
87
Figure 2.7 Average particle diameter of particles produced with a flow
rate of 0.3 ml/h, using PEI solutions of varying concentration
(0.02-0.5 wt.%), with a ring voltage of 7 kV and a capillary
voltage of 13.8 kV .
88
Figure 2.8 SEM images of the particles produced with a flow rate of 0.3
ml/h, using PEI solutions of varying concentration (0.02-0.5
wt.%), with a ring voltage of 7 kV and a capillary voltage of
13.8 kV.
89
Figure 2.9 (a) SEM image (right) and particle size distribution (left) of
particles produced using a flow rate of 0.05 ml/h of a 0.05 wt
% PEI solution (capillary voltage of 13.3 kV and ring voltage
of 7 kV). (b) SEM image (right) and particle size distribution
(left) of particles produced using a flow rate of 0.05 ml/h of a
0.14 wt.% PEI solution (capillary voltage of 13.3 kV and ring
voltage of 7 kV).
90
Figure 2.10 Average particle diameter of particles produced with different
flow rates (0.05-2.0 ml/h) of a 0.14 wt.% PEI solution (ring
voltage of 7 kV and a capillary voltage of 13.8 kV).
91
Figure 2.11 SEM images of particles produced with different flow rates
(0.05-2.0 ml/h) of a 0.14 wt.% PEI solution (ring voltage of 7
kV and capillary voltage of 13.8 kV).
92
xi
Figure 3.1 Particle size distribution of CMS feed (dry-ground), particle
sizing done by Union Process).
116
Figure 3.2 Particle size distribution of wet-ground CMS particle
(NETZSCH, Inc.).
116
Figure 3.3 Particle size distribution of CMS feed (dry-ground, particle
sizing done by NETZSCH, Inc.).
117
Figure 3.4 Particle size distribution of wet-ground CMS particle after
120 minutes grinding (NETZSCH, Inc.), Median: 0.503μm
and Mean: 0.533μm.
117
Figure 3.5 ln (1-m
t
/m
e
) against t for the uptake of CO
2
at 298 K for the
1
st
batch of CMS batch (mean particle size: 584nm).
120
Figure 3.6 ln (1-m
t
/m
e
) against t for the uptake of CO
2
at 298 K for the
2
nd
batch of CMS batch (mean particle size: 533nm).
121
Figure 3.7 Permeation apparatus for testing flat membrane films. 125
Figure 3.8 Cross-sectional view of the final mask containing the
membrane film where the annulus region (adhesive) of the
top aluminum mask forms the seal with the lower portion of a
membrane cell.
126
Figure 3.9 CO
2
permeability and ideal separation factor of MMP films. 131
Figure 3.10 Schematic of membrane module and γ-alumina substrate. 134
Figure 3.11 The effect of number of layers on the permeance and ideal
separation factor of the membrane with a carbon loading of
10 wt. %; T: 293K, P Δ : 30psi.
136
xii
Abstract
The goal of the first part of the Thesis is to prepare carbon molecular-sieve
(CMS) particles to be utilized in the preparation of mixed-matrix (MM)
membranes. Thus an experimental investigation was carried out using electrospray
of polyetherimide (Ultem-1000 PEI) solutions in dichloroethane to produce fairly
monodisperse, fine PEI particles. The effect of three key experimental parameters
was investigated, namely, the applied voltage, the liquid flow rate, and the polymer
concentration. The liquid flow rate was found to have the most important effect in
determining the particle size. An optimal range of flow rates often exists. Particles
obtained within the optimal range of the flow rate have a narrower size distribution,
and a dense and spherical morphology, compared with those produced with other
liquid flow rates. The CMS particles, prepared ex-situ by the pyrolysis of the
electrospray PEI particles, are compared in their properties with the CMS particles
that were generated by conventional grinding of pyrolyzed PEI pellets. They were
found to, generally, have similar structural properties.
In the second part of the Thesis, the technical feasibility of utilizing the MM-
CMS membranes in the separation of H
2
and CO
2
from gas mixtures of relevance
to power generation was studied. The MM membranes were fabricated by
incorporating the CMS nanoparticles into a polymer matrix. The CMS
xiii
nanoparticles were prepared by wet-grinding of pyrolyzed pellets of PEI. Flat-sheet
MM polymer-base (MMP) membranes, and supported MM-CMS membranes were
both prepared. For the MMP membranes, both the CO
2
and CH
4
permeabilities
increased with increasing the CMS content, while the CO
2
/CH
4
selectivity
remained either unchanged or was slightly reduced. This may be due to the poor
adhesion between the polymer and the CMS nanoparticles. In the case of the
MM-CMS membranes, the incorporation of the CMS nanoparticles greatly
enhanced the permeability and selectivity. An ideal CO
2
/CH
4
separation factor of
120 was obtained with a CO
2
permeance around 9.0 × 10
-8
mol/m
2
.s.Pa for
membranes with a CMS particle loadings of 10 wt.%. The corresponding H
2
separation factor was 130 and with a H
2
permeance of 9.7 × 10
-8
mol/m
2
.s.Pa.
Also investigated was the influence of the total solid fraction on the permeation
properties of the MM-CMS and CMS membranes.
xiv
Preface
The amount of energy consumed in separation processes in chemical and
petrochemical plants has been significantly increasing in recent years. This has had
an adverse impact on the environment and the operating costs of the processes,
which in turn has motivated extensive studies for developing methods to replace
the energy-demanding conventional separation processes with more energy-
efficient technologies. One class of such technologies consists of membrane-based
separations. Such processes are used in chemical and petrochemical plants in
combination with conventional processes in order to reduce their energy
consumption in various separation applications.
Despite impressive performance advances in the 1980s with polymer-based gas
separation membranes, this traditional approach has recently yielded diminishing
returns. Zeolites and carbon molecular-sieves (CMS) exhibit gas transport
properties far superior to polymeric materials, but are very difficult to
economically process on large scale. The so-called "mixed-matrix" (MM)
membranes, comprised of molecular-sieving (MS) domains dispersed in an
appropriate continuous matrix, are described in this Thesis. The MM materials
comprised of molecular-sieves domains embedded in processable polymer
matrices have the potential to provide membranes with higher permselecvitiy and
xv
equivalent productivity compared to existing membrane materials. The proposed
novel MM membranes indicate potential for application in the separation of H
2
and
CO
2
from their mixtures. Such separations are of importance in a number of
important processes of significance to environmentally-benign power generation,
including the utilization of renewable fuels and hydrogen-based fuel cells. This
Thesis evaluates the technical feasibility of utilizing MM-CMS membranes, made
by incorporating CMS nanoparticles into a polymeric matrix, in separations of H
2
and CO
2
from gas mixtures of relevance to power generation.
The Thesis is organized into 3 chapters. Chapter 1 introduces the background
information, and provides a literature review on the CMS and MM membranes. It
also provides a brief overview of the materials and experimental techniques
employed to produce micron- and submicron-size particles via spray techniques, as
well as a bries literature review on electrohydrodynamic atomization (EHDA) and
its applications.
Chapter 2 provides a broader literature review on EHDA, details the results on
electrospray drying of polyetherimide (PEI) solutions, and describes the
experimental apparatus and techniques for characterization of the nano-sized
particles.
xvi
Finally, Chapter 3 provides a detailed literature review on the CMS and MM
membranes and describes the results of the characterization of the MM membranes
using the CMS sub-micron particles embedded in a PEI matrix.
1
Chapter 1
Introduction
Many of the processes associated with electric power and energy generation
involve the separation of hydrogen and carbon dioxide from their mixtures with
other gases. Such processes include various advanced power generation cycles
involving chemical recuperation, integrated gas combined cycles, the utilization of
renewable fuels for power generation, and CO
2
sequestration. Currently, hydrogen
and CO
2
are separated using either adsorption/desorption (temperature- or pressure-
swing adsorption) or absorption (e.g., amine scrubbing for CO
2
separation)
methods. Both approaches are very capital intensive, and very much "economy of
scale" dependent. As a result, membrane-based separations are making significant
inroads in this area. Typically, polymeric membranes are utilized, the advantage of
which is that the polymers are generally cheap and easily processable into a variety
of shapes (sheets, tubes, hollow-fibers, etc.). Their disadvantage is that the
segmental flexibility of polymers limits their separatory ability, particularly towards
fixed gases, such as hydrogen and CO
2
. To improve on the separation properties of
polymeric membranes, several groups, including the USC group, have prepared
membranes made of molecular sieving materials, like zeolites and carbon
2
molecular-sieves (CMS). Such materials are, however, expensive, brittle, and
typically difficult to process into efficient membrane systems.
The use of nanocomposite (mixed-matrix) membranes, which consist of a
polymeric matrix, and a certain fraction of a molecular sieve material, has been
proposed, instead. Such membranes are thought to combine the advantages of
polymeric membranes (cheap and easily processable) with those of the molecular
sieving membranes (superior separation characteristics). The efforts, so far, have
not been quite satisfactory, and have fallen significantly short of what is
theoretically attainable. The main reason for the lack of success has been the
apparent incompatibility between the base polymeric matrix and the molecular
sieving material. In this Thesis a new approach is proposed for the preparation of
nanocomposite membranes, which we believe overcomes some of the limitations of
the existing methods, and will prepare membranes which exhibit superior
performance in the various power/energy generation related applications, as
outlined in the next section.
1.1 Energy Problem Targeted and its Impact
Many of the processes associated with electric power and energy generation
involve the separation of H
2
and CO
2
from their mixtures with other gases. The
following is only a partial list of such processes:
3
(i) Various Advanced Power Generation Cycles Involving Chemical
Recuperation (CRGT). These cycles typically involve the utilization of waste heat
to drive an endothermic reaction. The most common reactions are steam or dry
reforming of methane (CH
4
), which produce a hydrogen-rich gaseous product,
containing unreacted CH
4
, carbon monoxide (CO), and carbon dioxide (CO
2
) (Ren
et al., 2001). The success of such cycles is strongly dependent on the ability to
effectively separate the valuable hydrogen product from the rest of the mixture
components.
(ii) Various Integrated Gas Combined Cycles (IGCC). These cycles involve, as
the first step, the gasification of the (typically) solid fuel (coal, biomass, etc.) in
order to convert it to synthesis gas (a mixture of CO and H
2
), followed by a water
gas-shift (WGS) step to further convert the CO to CO
2
and H
2
(Bracht et al., 1995).
Subsequent to the WGS step, the hydrogen must be separated, in order to be
utilized as a clean fuel for electricity production in a turbine or a fuel cell.
(iii) Renewable Fuels for Power Generation. A number of gaseous renewable
fuels, such as landfill gas and biogas, typically consist of equal amounts of CO
2
and
methane. These fuels represent an important, but significantly underutilized,
source of energy. One difficulty with their utilization is that the large fraction of
CO
2
that they contain makes them difficult and/or uneconomic to burn. To improve
on the economics of utilization of the renewable fuels an effective means must be
4
utilized to upgrade their quality, i.e., to separate and remove the substantial
amounts of CO
2
found in these fuels (Prosser et al., 2000).
(iv) CO
2
Sequestration. CO
2
is thought to be the key culprit responsible for the
greenhouse effect, and global warming (Crane et al., 1998). The power/energy
generation industry is among the key producers of CO
2
. In the long-term the
industry will be expected to address the issue of CO
2
emissions. The first step
towards eliminating/reducing such emissions will involve the effective separation
of CO
2
from the flue gas.
1.2 Membrane Classifications
Membrane separation processes have become one of the emerging
technologies, which have undergone a rapid growth during the past few decades. A
membrane is defined as a selective barrier between two fluid phases. There are six
major membrane processes that are widely used in industrial application. They are
microfiltration, ultrafiltration, reverse osmosis, electrodialysis, gas separation, and
pervaporation. Gas separation using membranes is known as a developing process,
with the most gas separation membranes being of the solution-diffusion
mechanism type.
5
There are a number of important factors that need to be considered before
selecting a membrane for a given application, such as the overall cost, productivity,
separation efficiency, and mechanical integrity of the membrane at the given
operating conditions (Koros and Pinnau, 1994). Among these requirements,
selectivity and productivity are the most important ones.
A membrane can be selective by various mechanisms, which may be broadly
classified into three groups, as described below (Fig. 1.1). The membranes used in
most commercial applications are polymeric, and follow a solution-diffusion
transport mechanism (Spillman, 1989; Puri, 1996; Meindersma and Kuczynskyi,
1996).
Figure 1.1 Classification of membranes based on their selectivity mechanisms.
1-‘Simple’ sorption–diffusion
2-‘Complex’ sorption–diffusion
3-Ion-conducting
membranes
Polymeric solution-diffusion
Molecular sieving
Selective surface flow
Facilitated transport for various gas types
Palladium and related alloys for H
2
Solid oxides types
Proton exchange types
Mixed ionic electronic conductors
(MIEC) Solid oxides
6
The membrane materials used in most separations are glassy polymers, which
derive their selectivity primarily from their ability to separate gases based on subtle
differences in penetrant size. Transport occurs when there is the creation, next to
the penetrant molecule, of a transient gap of sufficient size to accommodate the
penetrant, thereby permitting a diffusion step (Koros and Hellums, 1989) to occur
(Fig. 1.2 & Fig. 1.3 A).
Figure 1.2 Solution–diffusion transport mechanism (Koros, 1995).
Polymeric membranes typically lose their performance stability at high
temperatures, high pressures, and in the presence of highly sorbing components;
their segmental flexibility which, on the one hand, makes them economical to
prepare, limits, on the other hand, their discriminating ability for similarly sized
penetrants (Singh and Koros, 1996).
7
Figure 1.3 The contender technologies for large-volume gas separations
(Koros, 2002).
8
Molecular sieving (MS) materials, an alternative to polymers, rely primarily
on differences in molecular size to achieve separation. The ultra-microporous
membranes made from MS materials have sufficiently small pores to exclude some
molecules, while allowing others to pass through (Fig. 1.3B), and show attractive
permeation performance (Morooka and Kusakabe, 1999; Tsapatis and Gavalas,
1999) on a laboratory scale. Because of their rigidity, MS membranes do not
typically lose their stability when exposed to adverse conditions (high temperature,
pressure, and highly sorbing components) which can cause polymeric membranes
to plasticize. However, since these membranes are fragile, difficult to process, and
expensive to fabricate, they are not commercially significant today, except in niche
applications.
Both polymeric and MS membranes, favor typically, the smallest components
in a mixture. On the other hand, “surface selective flow” membranes work by the
selective adsorption of the more strongly adsorbing components onto the pore
surface, followed by surface diffusion of the adsorbed molecules across the pore
(Fig. 1.3C). These membranes are used to permeate the larger-sized penetrants and
to retain the smaller components (Rao and Sircar, 1993, 1997).
In facilitated transport membranes, the penetrant dissolves in the membrane
(or reacts with a complexing agent or a carrier agent) and diffuses down its own
concentration gradient (or diffuses down a concentration gradient of a carrier gas
9
complex); see Fig. 1.3D. Although these membranes are highly selective and can
potentially achieve high permeabilities at a low concentration driving force (Way
and Noble, 1992; Cussler, 1994), they are typically not stable (i.e., the membranes
dry out or the carrier species is lost). They are configured either as an immobilized
liquid film, a solvent swollen polymer or a solid polymer film containing reactive
functional groups.
Palladium-based membranes are highly selective to hydrogen (Ma, 1999;
Wood, 1968) and exhibit good transport properties, as well as temperature
resistance (Ma, 1999). The permeation of hydrogen through such membranes
involves the dissociative adsorption of hydrogen onto the surface (Fig. 1.3D).
However, there are still considerable difficulties in preparing these membranes for
economic operation on a large scale.
Mixed ionic electronic conductors are materials capable of conducting both
oxygen ions and electrons. They are typically mixed oxide solutions called
perovskites, and have the generic formula ABO
3
, where A is a large cation with a
12-fold coordination, and B is a smaller cation with a 6-fold coordination with
oxygen ions. In some instances the solid oxide material can conduct only oxygen
ions and not electrons (Fig. 1.3F). These membranes have exceptionally high
selectivity towards oxygen and high fluxes, compared to polymeric membranes,
and typically operate at high temperature (700
o
C). Problems associated with such
10
membranes relate to difficulties with proper sealing, as well as high sensitivity of
the membranes to the temperature gradients that can result in membrane cracking
(Bessarabov, 1999).
Proton exchange membranes, which can be polymeric or inorganic (Fig.
1.3E), are nominally similar to the solid oxide membranes described earlier, in the
sense that they also only conduct protons and not electrons and, thus, can
potentially be used in fuel cells and batteries. The most popular of these is
Nafion®, a sulfonated polymer.
A special class of molecular sieve membranes is made of carbon. Ismail and
David (2001) provided a review on the latest developments in the field of carbon
membranes for gas separation. In another paper, Saufi and Ismail (2004) reviewed
the fabrication aspects of carbon membranes, which can be divided into six steps:
precursor selection, polymeric membrane preparation, pretreatment of the
precursor, pyrolysis process, post-treatment of the pyrolyzed membranes, and
module construction.
1.3 Membranes and Gas Separation
A membrane can be described as a device capable of imposing certain
restrictions on the permeation flux of various substances. The development of
11
membrane processes for the separation of gas mixtures has exhibited a remarkable
progress during the last two decades. In gas separation applications, membranes are
used to purify gas streams having two or more components. Gas separations
involving such gas pairs as O
2
/N
2
, CO
2
/CH
4
, H
2
/CH
4
, H
2
/CO, and H
2
O/CH
4
, have
been actively explored with membranes (Koros and Fleming, 1993; Bessarabov,
1999; Sedigh et al., 1998, 1999, 2000a, and references therein). Membrane-based
gas separation offers many advantages such as, low capital investment, ease of
installation, low space and weight requirements, environmentally friendly nature,
easy adaptability due to their modular design, and lower operating and maintenance
costs when compared with other competing separation technologies (Spillman and
Cooley, 1989; Spillman and Sherwin, 1990). These benefits have made industrial
gas separations one of the fastest growing uses for membranes (Membrane
Technology, 1998).
Although membranes for liquid separations (microfiltration, ultrafiltration,
reverse osmosis, etc.) remain the largest sector of membrane usage (accounting for
about 54% of membrane and module sales, or over $1 billion worldwide in 1995),
as separation membranes accounted for sales of $123 million worldwide in 1995,
and are projected to continue a healthy growth of 8% annually over the next decade
(Crull, 1996, 1997). A large majority (60%) of the gas separation membrane sales
remains in air separations, while 25% is in the hydrogen separations (Crull, 2000).
12
Strong interest has existed in the synthesis of gas separation membranes based
on materials that provide better selectivity, thermal stability and chemical stability
than those that already exist (i.e., polymeric membranes) (Stern, 1994; Fuertes
and Centeno, 1998a). Nonpolymeric and inorganic membranes have attracted
considerable attention in recent years in studies aimed at improving the
performance of membrane materials for gas separation. Among non-polymeric
membranes, those made from MS materials, such as silica, zeolites and carbon have
the potential to push the upper boundary of the permeability vs. selectivity tradeoff
relationship (Fuertes and Centeno, 1998a). The CMS materials have shown
attractive characteristics among the MS materials, such as excellent shape
selectivity for planar molecules, high hydrophobicity, heat and high corrosion
resistance (Kyotani, 2000). In addition, it is more feasible to form CMS membranes
(Hayashi et al., 1997; Jones and Koros, 1995). The economic and environmental
driving forces for the more cost-effective alternatives, and the lack of developments
in competitive polymeric membrane materials with the necessary durability and
performance requirements, have greatly improved the commercial viability of CMS
membranes (Spillman and Sherwin, 1990).
13
1.4 Background on Natural Gas Processing
The first large industrial application of gas separation by membranes goes back
to 1980, when Permea (now a division of Air Products) launched its hydrogen-
separating Prism membrane (MacLean et al., 1986; Henis, 1994). Since then, as
noted above, membrane-based gas separation has grown into a large business, and
substantial growth in the near future is likely. More than 90% of this business
involves the separation of noncondensable gases: nitrogen from air; carbon dioxide
from methane; and hydrogen from nitrogen, argon, or methane. However, a much
larger potential market for membrane gas separation lies ahead in separating
mixtures containing condensable gases, such as the C
3
+
hydrocarbons from
methane or hydrogen, propylene from propane, and n-butane from isobutane. A
milestone chart summarizing the development of membrane gas separation
technology is displayed in Fig. 1.4 (Baker, 2002).
By the mid-1980’s, Cynara (now part of Natco), Separex (now part of UOP),
and GMS (now part of Kvaerner) were using cellulose acetate membranes to
remove carbon dioxide from natural gas (Spillman, 1989).
At about the same time, Generon (now part of MG) introduced a membrane
system to separate nitrogen from air. These first air separation systems were based
on poly(4-methyl-1-pentene) (TPX) membranes with an oxygen/nitrogen
14
selectivity of about 4. They were only competitive in a few niche areas, requiring
95% nitrogen, but by 1990, Generon, Praxair, and Medal (Sanders et al., 1988;
Ekiner et al., 1992) had all produced custom polymers with oxygen/nitrogen
selectivities of 6-8. Membranes made from these polymers could produce better
than 99% nitrogen and offered a cost-competitive alternative to delivered liquid
nitrogen for many small users.
Figure 1.4 Milestones in the development of membrane gas separations
(Baker, 2002).
15
Although natural gas is viewed as a clean fuel in comparison to fossil fuels, in
terms of their combustion byproducts, the natural gas found in reservoirs is not
necessarily clean and free of impurities. With increasing shift towards cleaner fuel
sources in electricity generation, industry, and transportation, natural gas has
emerged as an important energy resource of the future. Natural gas purification has
become an important area where gas separation membranes can make a significant
impact. Indeed, impurities, such as water, hydrogen sulfide, carbon dioxide, and
other compounds must be removed to very low concentrations before being
transported through the pipeline. Table 1.1 lists typical wellhead compositions of
natural gas found in the field, and the required specifications necessary for
transport through the pipeline and eventual commercial use.
Table 1.1 Typical compositions of natural gas wells and the required
composition specifications for sales (Lee et al., 1994, 1995).
Component Typical Feed Sales Composition
CH
4
70-80% 90%
CO
2
5-20% < 2%
C
2
H
6
3-4% 3-4%
C
3
-C
5
~3% ~3%
N
2
~1-4% <4%
H
2
S < 100 ppm < 4 ppm
H
2
O Saturated < 100 ppm
C
6
+
0.5-1% 0.5-1%
16
Numerous studies have indicated economically-favorable cost analyses of
membrane-based natural gas processing in comparison to traditional separation
processes (e.g., amine/glycol system) (Bhide and Stern, 1993; Spillman, 1989,
1995; Babcock et al., 1988; Fournie and Agostini, 1987; Schell et al., 1983; Mazur
and Chan, 1982), or have shown successful implementation of gas separation
membranes (Lee et al., 1994, 1995; Cook and Losin, 1995; Falk-Pedersen and
Dannstrom, 1997; Watanabe, 1999). Capital and operating costs for membrane
systems were either comparable or less than those associated with amine/glycol
systems, especially when CO
2
concentrations were high (> 20%) and at lower flow
rates (Tabe-Mohammadi, 1999; Lee et al., 1995). Meyer (1996) indicated savings
of over 50% for a single-stage membrane unit, in comparison to diethanolamine
(DEA) and methyl diethanolamine (MDEA) systems. Relying on a pressure (or
more rigorously, fugacity) driving force, membrane systems are most advantageous
when the fast permeating component (i.e., CO
2
) is highest in concentration (or
partial pressure) in the feed. Therefore, bulk removal of CO
2
in high-pressure
natural gas streams with high CO
2
content is very efficient with membranes. On the
other hand, in the case of low CO
2
concentration (that still requires additional
processing to meet product specifications), membranes may not be as effective, and
it may be necessary to use another process (e.g., amine-based or cryogenic system)
to create a hybrid system (Tabe-Mohammadi, 1999).
17
Moreover, traditional separation processes like chemical absorption (using,
e.g., amine solutions), physical solvent processes (e.g., Selexol
TM
and Fluor
TM
) for
CO
2
and H
2
S removal, and glycol (normally triethylene glycol) dehydration
processes for water removal (Tannehill et al., 1994; Kohl and Nielsen, 1997;
Goddin, 1982), involve circulating liquids with rotating equipments (pumps,
compressors, etc.). Such processes have costly problems with corrosion, solvent
regeneration and disposal, and environmental concerns (emissions, pollution, and
potential environmental damage from spills). Another important advantage of
membranes over other separation processes is their minimal recompression costs
before sending the natural gas into the pipeline. Membranes can separate the fast
permeating impurities (CO
2
, H
2
S, and H
2
O) into the permeate (or downstream)
side, while natural gas is enriched on the retentate (or upstream) side with
negligible pressure loss. Since membrane-based gas separation avoids all of the
problems of the more conventional technologies, its installation could result in
substantial capital, operating cost, safety, and environmental benefits. In addition,
membrane modules require generally a small space. This advantage is particularly
important for offshore drilling applications, where space is very limited, and having
processing units with minimum usage of space is economically critical.
18
1.5 Mixed-Matrix & Molecular-Sieve Membranes vs. Polymeric
Membranes
Asymmetric gas separation membranes utilized in gas separations consist of a
cheap macroporous polymeric support coated with a thin dense layer of a higher
performance polymer. Polymeric materials for membrane preparation are highly
advantageous, in that they are relatively cheap, and easily processable into a variety
of membrane shapes (hollow-fiber, flat sheet or disk, tube, etc.). On the other hand,
the segmental mobility of the polymer limits the separatory characteristics of such
membranes (Zimmerman et al., 1997; Mahajan and Koros, 2000). As mentioned
earlier, in recent years a variety of groups have prepared microporous membranes
made from molecular sieving materials, such as zeolites, and CMS (Sedigh et al.,
1998, 1999, 2000a, and references therein). These membranes possess excellent
separation characteristics, but are also expensive to prepare, are brittle, and difficult
to process in commercially desirable shapes (hollow-fiber, spiral-wound, etc.). The
existing deficiencies in both polymeric and molecular-sieve type membranes
indicate the need for a different type of membrane, which combines the advantages
of both systems, while avoiding the pitfalls of both (Zimmerman et al., 1997;
Mahajan and Koros, 2000). These composite membranes consist typically of a
polymeric base, in which one imbeds a certain fraction of a molecular sieve, which
shows desirable properties for a given gas separation.
19
Over the last couple of decades a great many works have been completed, and
numerous papers have been published on the topic of polymeric membranes. There
has also been significant interest over the last decade in the subject of molecular
sieve membranes. Relatively speaking, over the same period only scant attention
has been paid to the topic of nanocomposite membranes (Zimmerman et al., 1997;
Mahajan and Koros, 2000). The earliest study that we are aware of is by Paul and
Kemp (1973), who incorporated 5A zeolite particles in silicone rubber membranes,
but did not observe any substantial changes in steady-state permeation, though the
presence of the zeolite seemed to increase the time-lag for CO
2
and CH
4
in the
diffusion cell, probably due to their adsorption on the zeolite particles. Jia et al.
(1991) incorporated silicalite (an all-silica hydrophobic zeolite) particles into a
silicone rubber membrane. Adding the silicalite to the membrane improved its
separation characteristics towards for a variety of gas mixtures. The addition of
silicalite, for example, seemed to favorably influence the separation of the (O
2
/N
2
),
(H
2
/N
2
), and (CO
2
/N
2
) gas pairs, but to negatively impact the separation of the
(CH
4
/N
2
) pair. The positive/negative effects were enhanced with increasing the
fraction of zeolite in the polymeric matrix. Jia et al. (1991) defined a parameter,
called the facilitation ratio of zeolite, in order to characterize the function of zeolite
in the membrane. Using this parameter as a guide, they were able to confirm that
the zeolite played an important role in the molecular transport, and that the altered
20
selectivities and permeances were the result of the molecular sieving effects of the
zeolite.
Kulprathipanja and his colleagues at UOP hold the first patents on the topic of
mixed-matrix (MM) membranes (Kulprathipanja, 1988, 1992). In these patents
they reported on the use of cellulose acetate as a base material in the preparation of
the composite membranes. To prepare the membranes, they added silicalite
particles to the cellulose acetate matrix. The resulting membranes seemed to have
increased separation properties towards the (O
2
/N
2
), and the (CO
2
/H
2
) gas pairs.
MM membranes have also been prepared by a variety of glassy polymers. The
use of these polymers is due to their better performance towards fixed-gas
separations than the rubbery polymers. The addition of zeolite particles, as a result,
shows higher promise for preparing commercially viable membranes. Suer and
Yilmaz (1994), for example, used a polyethersulfone glassy polymer and
hydrophilic zeolite 13X and 4A particles, in order to prepare a variety of composite
membranes. The membranes showed moderate improvements in their separation
characteristics towards the (O
2
/N
2
), and the (CO
2
/H
2
) gas pairs.
The difficulty with the nanocomposite membranes based on glassy polymers
and zeolites is that, they interact weakly with each other. This results in a series of
non-selective porous cavities within the polymer matrix surrounding the zeolite
particles, resulting in membrane properties that are often inferior to those of the
21
starting base glassy material. Mahajan and Koros (2000), for example, reported the
preparation of a MM membrane from 4A zeolite, and a commercial polyimide
(Matrimid). The resulting MM membrane had separation properties that were
inferior to those of the original polymeric membrane. The transport data were
consistent with the SEM photomicrographs of the same membrane, which clearly
showed that voids exist at the interface between the two materials, allowing the gas
to simply bypass the sieve, and resulting in higher permeability and inferior
selectivity. Various authors have used a variety of remedies to alleviate these
problems. Duvall et al. (1993), for example, reported the use of silane coupling
agents to promote adhesion between the zeolite particles and a PEI matrix. Though
photomicrographs indicated good adhesion, the transport investigations were not
very promising. Mahajan and Koros (2002a) reported similar disappointing results
with the use of silane coupling agents during preparation of a MM membrane from
4A zeolite, and a commercial polyimide (Matrimid). Though the procedure seemed
to reduce the size of the defects at the interface between the zeolite and the
polymer, they remained substantial enough to adversely impact membrane
performance. Utilizing polyimides with a lower glass transition temperature (T
g
)
and, carrying out the solvent removal step during the membrane formation process
at temperatures 30
o
C higher than the polymer T
g
(~ 250
o
C), seemed to improve the
adhesion between the polymer and the zeolite sieve, and to produce membranes
22
with better separation characteristics. However, such high processing temperatures
are not appropriate for the large-scale preparation of these materials.
The key conclusion one can draw from the survey of works in the literature is
that though MM membranes show great potential for the preparation of inexpensive
membranes with superior performance, the limited efforts have, so far, fallen short
of the promise, mostly due to incompatibility of the polymeric matrix, and the
molecular-sieve zeolite particles. Such particles are not prepared specifically and
optimally for the purpose of membrane preparation, and show either poor adhesion
properties with the underlying matrix, or porous characteristics that are not
appropriate for membrane applications. There are also other key challenges facing
the field of zeolite-based composite membranes for fixed-gas separations. For
example, there are only a limited number of zeolite systems that show significant
intrinsic permselectivity towards important fixed gas pairs (e.g., CH
4
/H
2
, O
2
/N
2
).
A-type zeolites are a well-known example. However, it is very difficult to produce
zeolite-A crystals with submicron sizes. This places severe restrictions on how thin
composite membranes one is able to prepare. Glassy polymers (e.g., polyimides)
have very low permeability, and this means that one needs to produce very thin
(submicron) composite membrane films; this is not possible, however, with the
existing zeolite particles. A-type zeolites are, in addition, highly hydrophilic
23
materials, and their use with gas (other than bone-dry) mixtures presents
challenges.
One goal of this Thesis is to assess the technical feasibility of preparing novel
nanocomposite membranes made of a polymeric base matrix and CMS particles
made by a novel technique (controlled electrospray pyrolysis), that are optimally
suited for nanocomposite membrane preparation. These membranes will be utilized
in separation of hydrogen and CO
2
from binary, ternary, and quaternary gas
mixtures of relevance to power generation.
We are only aware of a few other published efforts devoted to the preparation
of MM membranes with the CMS particles (Duvall et al., 1993, Vu et al., 2003 a,
b). The membranes produced by Duvall et al. (1993) indicated substandard
performance; however, the commercially available CMS particles utilized had a
mostly dead-end (not interconnected) porous structure.
Recently, Vu et al. (2003 a) reported formation of successful MM membrane
films with high loadings (15 to 38 vol.%) of the CMS particles (using polymeric
precursors (6FDA-BPDA/DAM and Matrimid® 5218) dispersed within polymer
(Ultem® 1000 and Matrimid® 5218) matrices achieved from flat-sheet solution
casting. Conducting both pure gas and high-pressure, mixed-gas permeation
experiments on these MM films, they observed very impressive CO
2
/CH
4
enhancements of selectivity (as much as 45% increase in CO
2
/CH
4
selectivity of the
24
MM membrane, in the range of 50 to 65), over the intrinsic CO
2
/CH
4
selectivities
of the original polymer matrices, and CO
2
productivity enhancements.
Mathematically, gas transport through a MM medium presents a complex
problem. Several theoretical models have been used to predict the permeation
properties of the MM membranes as functions of the permeabilities of the
continuous and dispersed phases. Petropoulos (1985) presented a comparative
summary of various models. Vu et al. (2003 b) have examined the Maxwell and
Bruggeman models (both have been used previously for the MM work; Sahimi,
2003) as a guide for their expected membrane improvements, enhancement with the
MM concept, and for comparison with the experimental permeation data of their
MM films as well. Based on their studies, the Bruggeman model consistently
predicts higher gas permeabilities for the MM films and higher permselectivities
than the Maxwell model, especially at high CMS loadings. They demonstrated that
laboratory-synthesized CMS particles can be successfully incorporated into two
glassy polymers, Matrimid® 5218 and Ultem® 1000, to form MM membrane films
with enhanced permeation properties over those of the original polymer matrices.
Vu et al. (2003 a) also prepared MM membrane films, using polymer matrices
6FDA-IPDA and 6FDA-6FpDA, two very attractive experimental polyimides with
transport properties lying very close to the Robeson (1991) upper bound. However,
in contrast to Ultem® and Matrimid® MM films, they reported poor polymer-sieve
25
contact and minor or no enhancement in permselectivities, possibly due to the
presence of the bulky, polar trifluoromethyl groups in the chemical structures of
these polyimides, which may have repulsive interaction with the CMS surface.
1.6 Polyetherimide and Carbon Molecular Seives
Polyimides are among the most stable classes of polymers. They can be used
at temperatures up to 300ºC, and will usually decompose before reaching their
melting point. Because they do not go through a melting phase transition or lose
their shape, polyimides are good precursors for glassy carbon. Many researchers
have used polyimides as their precursor for making carbon membranes. Jones and
Koros (1994) reported that the best carbon membranes, in terms of both the
separation and mechanical properties, were produced from the pyrolysis of
aromatic polyimides. Moreover, polyimides are rigid, high-melting point, high
glass transition temperature (Tg), thermally-stable polymers formed by the
condensation reactions of dianhydrides with diamines; see Fig. 1.5 (Vu et al.,
2002).
Polyimides are synthesized at lab scale with different types of dianhydrides
and diamines, in order to tailor their separation properties when being used as
membrane material, as shown in Fig. 1.5. Hayashi et al. (1995), Yamamoto et al.
26
(1997), and Fuertes and Centeno (1998 a) discussed further details about the
preparation of polyimides in their papers.
Figure 1.5 Basic molecular structure of polyimide (Yamamoto et al., 1997).
One economical form of polyimides used to form carbon membranes (the
polyimide of interest in this Thesis) is polyetherimide (PEI), which is manufactured
by General Electric under the trade name of Ultem
1000 (Fig. 1.6). This material
Figure 1.6 Polyetherimide ULTEM
1000 (adopted from GE)
27
has been used to prepare polymeric membranes due to its superior strength and
chemical resistance (Fuertes and Centeno, 1998 b; Sedigh et al., 1999). As can be
seen, the presence of bulky C(CH
3
)
2
groups combined with a highly stiff structure
of the polymer makes the Ultem 1000 polymer a very promising candidate for
polymeric membranes. Since during carbonization at moderate temperatures the
backbone of the starting polymer remains, most likely, unchanged, one expects the
CMS membranes utilizing PEIs as polymeric precursors to maintain their superior
performance as well.
1.7 Carbon Molecular-Seive Nanoparticles
Although several techniques, such as chemical vapor deposition (Marangoni,
et al., 2002; Venegoni et al. (2001); Serp et al., 2001) arc-discharge (Sano et al.,
2002), ultrasonic treatment (Wang et al., 2003), and explosion (Wu et al., 2002)
have been developed to fabricate nanometer-sized carbon in the gas or the liquid
phase, they may not be suitable for the preparation of the CMS nanoparticles.
Therefore, the CMS particles with well-defined micropores are usually prepared by
controlled pyrolysis of solid organic precursors, such as pitch-like materials or rigid
and cross-linked polymers (Inagaki and Radovic, 2002).
28
To keep the desired diffusion and separation properties, the CMS nanoparticles
can only be prepared by two methods:
1. Spray pyrolysis of polymeric solution in an inert atmosphere,
2. Grinding (namely, wet-grinding) of pyrolyzed polymeric precursors.
In what follows several nanoparticle preparation methods are described briefly.
1.8 Nanoparticle Preparation Methods
Ultrafine particles or nanoparticles (with a size between a few nm and 100
nm) are of interest, because their chemical and physical behavior is unprecedented
and remarkably different from those in the bulk form. They have great potential for
use in applications in the electronic, chemical or mechanical industries, as well as
technologies associated with them, such as superconductors, catalysts, drug
carriers, sensors, magnetic materials, pigments, and as structural and electronic
materials. Spherical particles with a narrow size distribution (monodisperse) are
preferred for applications and technologies, requiring the compacting and sintering
of particles. To be industrially relevant, the process for producing such particles
needs to be low-cost, and to involve both continuous operation and a high
production rate.
29
A key avenue for producing nanoparticles involves the formation of aerosols.
There are at least two different routes for the preparation of ultra-fine particles by
aerosol processing. The first involves gas-to-particle conversion (a build-up
method), and the second liquid-to-solid particle conversion (a break-down method).
In the gas-to-particle conversion method, particles are generated by cooling a
supersaturated vapor (Okuyama et al., 1991). The primary advantages of the gas-
to-particle conversion method are the small particle size (a few nm to μm), narrow
size distribution, and high purity of the particles produced. One disadvantage,
however, is the formation of hard agglomerates in the gas phase which leads to
difficulties in preparing high-quality bulk materials. Many types of reactors, such
as flame, furnace, plasma, or laser, have been used to facilitate the gas-to-particle
conversion route.
Liquid-to-particle conversion (i.e., spray-drying) is a representative “break-
down” method for aerosol processing. The spray method is often classified as a
liquid-phase technique because solutions or sols are used. Relative to the gas-to-
particle conversion route, the spray-drying method is a simple and low-cost
process; the process, itself, is not well understood (Lenggoro et al., 2000a). To
prepare fine particles by the spray-drying method, colloidal suspension/sols are
used as precursors (Iskandar et al., 2001). The suspension, which contains liquid
and solid particles, is sprayed and the liquid phase (the solvent) evaporates from the
30
droplets. The average size and size distribution of the final particles can be roughly
determined from the size of the atomized droplets, and the initial concentration of
the starting solution. In some cases, in order to prevent oxidation of the materials, a
mixture of N
2
and H
2
are used as the carrier gas. The basis of the spray pyrolysis
process assumes that one droplet forms one product particle. To date,
submicrometer-to-micrometer-sized particles are typically formed in a spray-drying
process, a one step process to convert a liquid into a fine powder by atomization of
the feed solution and subsequent drying. Upon contact with the hot drying air,
particles dry very rapidly with drying times of < 1 s for spray droplets < 100 µm
(Masters, 1991). Most studies have used a variety of typical atomizers, such as twin
fluid, or an ultrasonic nebulizer to generate the droplets. These atomizers are
capable of producing droplets with an average size in the range of several microns.
For a typical initial droplet with a diameter of 5 μm to dry into a particle with a
diameter of 100 nm, the initial volume fraction of polymer dissolved in the volatile
solvent must be less than 0.0008%. In practice, such low solute concentrations may
lead to a low rate of particle generation, and may ultimately affect the purity of the
particles. In other words, the preparation of ultrafine material particles with
diameters of less than 100 nm via a conventional spray pyrolysis method remains a
problem. To overcome this problem the drying may take place in a high-
temperature flame or combustion environmnet (flame spray pyrolysis, FSP); the
31
abrupt evolution of considerable heat and gas aids in breaking or fragmenting the
large droplets into smaller pieces; this process is coupled with the evaporation-
derived particle formation process. Particles are formed from concurrent gas-to-
particle and liquid-to-particle conversions.
Flame processes are, by far, the most widely used ones for the manufacture of
commercial quantities of nanoparticles. Particle synthesis by spray pyrolysis
involves the atomization of a precursor solution into discrete droplets. These
droplets are subsequently transported through a heating zone (e.g., electrical
furnace) where the solvent is evaporated and the dissolved species react to form the
particulates. The most important products today are carbon blacks made by Cabot,
Columbia, Degussa-Huls, etc., fumed silica (Cabot, Degussa-Huls), pigmentary
titania (DuPont, Ishihara, Millenium, Kerr-McGee) and optical fibers (Corning,
Heraeus, Lucent, Sumitomo). The success and widespread application of this
technology is based on its apparent simplicity of a one-step process and “no-
moving parts” machinery. At the same time, this is a complex process, as all
particle characteristics are determined within a few milliseconds, and can be
influenced by many process variables. More specifically, reactant mixing, additives
or electric fields are used to control primary particle size and the extent of
agglomeration when gaseous precursors are used (Pratsinis, 1998). To prepare fine
particles by spray pyrolysis, a starting solution is prepared by dissolving, usually,
32
the metal salt of the product in solvent (e.g., ethanol or toluene). The droplets that
are atomized from a starting solution are introduced into the furnace. Evaporation
of the solvent, diffusion of the solute, drying, precipitation, reaction between the
precursor and the surrounding gas, pyrolysis, or sintering may occur inside the
furnace in order to form the final product.
FSP overcomes the limitations of gaseous precursors that are required by the
industrially established gas-fed flame reactors for the synthesis of nanostructured
commodities (Pratsinis, 1998). FSP is used on a large scale today to produce carbon
black, fumed silica and titania pigments. In general, flame synthesis and especially
FSP are continuous, well-controllable and versatile processes for the production of
a wide variety of different nanoparticles (Pratsinis, 1998; Kammler et al., 2001).
The FSP processes are quite attractive as they can employ a wide array of
precursors, so a broad spectrum of new nanosized powders can be synthesized.
Electrospray is another empirical technique which is also capable of
generating fine droplets as well as nanoparticles. Low-pressure electrospray
pyrolysis has, therefore, also been used to fragment particles to give nanoparticles.
Electrospray is always an alternative to other spraying techniques. Consequently,
there is research done in the field of liquid atomization, which is traditionally
dominated by conventional sprays. Particles in the nanometer size range are often
produced. The advantage of the electrospray is due to the low agglomeration of the
33
particles, and their narrow size distribution. Park and Burlitch (1996) produced
non-agglomerated titania particles in the size range of 20 nm by spraying an
ethanolic solution of titanium alkoxide. Ijsebaert et al. (2001) spray-dried drug
solutions for inhalatation purposes. By using the conejet mode forming a
monodisperse droplet size distribution, it was possible to form drug particles in a
very narrow size range, which is shown to improve inhaling efficiency, e.g., for
antiasthma drug delivery. Gomez et al. (1999) reported the production of protein
nanoparticle with the average aerodynamic diameter ranging from about 88 to
110nm; the resulting particle distributions were quasi-monodisperse with relative
standard deviation estimated at approximately 10%. The preparation of zinc sulfide
(ZnS) particles below 50 nm in diameter by an electrospray pyrolysis method has
also been reported (Lenggoro et al., 2000b).
In recent years, the use of supercritical fluids as media for the formation of
micro-particles has also shown good promise (Eckert and Knutson, 1996).
Supercritical fluids offer significant advantages over other conventional routes of
microparticulate formation, namely, mild operating temperatures, high purity of
products, and production of solvent-free, dry particles (Knutson et al., 1996).
Young et al. (2000) sprayed supercritical solutions directly into an aqueous
surfactant solution to stabilize small particles by minimizing the flocculation and
the agglomeration resulting from particle collisions. They mentioned that by using
34
the RESS (Rapid Expansion of a Supercritical Solution) technique, cyclosporine
particles 500-700 nm in diameter were stabilized for drug concentrations as high as
6.20 and 37.5 mg/ml in 1.0% and 5.0% (w/w) Tween-80 solutions, respectively.
Emulsion polymerization and mini-emulsions are other processes that have
been used to prepare polymer (or encapsolated inorganic) micro- and nano-
particles with the aid of emulsifiers. A system where small droplets with high
stability in a continuous phase are created by using high shear (Blythe et al., 2000;
Landfester, 2000) is classically called a “mini-emulsion”. For a typical oil-in-water
mini-emulsion, an oil, one or several hydrophobic agents, an emulsifier, and water
are homogenized by high shear to obtain homogeneous and monodisperse droplets
in the size range of 30 to 500 nm (Landfester, 2001). The same procedure of mini-
emulsification can be performed to obtain polymer particles. Hydrophilic
monomers are mini-emulsified in an organic phase, and hydrophobic monomers are
mini-emulsified in water. Hardening of the droplets can then be achieved by
subsequent polymerization in each droplet. As one example, polymeric
nanoparticles can be formed via polyaddition of a diepoxide and a diamine
(Landfester et al., 2000).
On the other hand, attrition mills, such as fluid-energy mills (jet-mills), are
able to produce particles < 5 µm in diameter. As the mill contains no moving parts,
the milling force is provided by the kinetic energy of the high-velocity air or inert
35
gas. The particles are accelerated in a high velocity gas stream, and are reduced by
inter-particle collisions or impact against other solid surfaces (Lubas, 1999). The
mill can incorporate an internal air-classifier using centrifugal forces for separation
(cyclone separation). Particles of a given size leave the mill with the gas stream,
while larger particles remain on a wide circumferential path within the grinding
chamber for further size reduction. Particles generated by this method are charged,
which tends to increase the problems with cohesiveness. The preparation of small
batches and the low risk of contamination make jet-milling especially suitable for
pharmaceutical applications. Little information is available, however, about the
effects of pulverization on protein stability. Lyophilized recombinant human
growth hormone (rhGH), for example, suffers aggregation on milling (Maa and
Prestrelski, 2000), which could not be prevented by lowering the milling
temperature.
Nanotechnology applications in the pharmaceutical, materials, and chemical
industries have renewed interest in the use of wet-grinding in stirred media mills
for the production of colloids and nanoparticles. However, challenges arise in the
production of sub-micron particles that are, in part, due to colloidal surface forces
influencing slurry stability and rheology. As often observed in the literature, a
lower bound of ~1 μm is usually reached, despite the high-energy input and
aggressive milling conditions. Furthermore, the product agglomerate size can even
36
increase with increases in energy input, a seemingly counterintuitive result that
may be attributed to aggregation of fine particles during the comminution process.
As mentioned earlier, one can categorize the possible CMS producing
processes as follows:
(a) Spray pyrolysis of the polymer or polymeric solutions that can be classified as
gas (melt) atomization of the polymer followed by pyrolysis, flame spray pyrolysis
of the polymer solution, and spray pyrolysis of the polymer solutions (namely,
using atomizer, supercritical fluids, or electrospray).
(b) CMS nanoparticles can be prepared by (wet) grinding of pyrolyzed polymeric
precursors.
PEI is a glassy polymer (Tg =209°C) which decomposes before reaching its
melting point. Because it does not go through a melting phase transition to lose its
shape, gas atomization may not be a good process for it and, in addition, from an
economical point of view it will need a lot of investment for the high temperature
reactor and special atomizers required. On the other hand, the PEI pyrolysis needs
to be done in an inert atmosphere to avoid oxidation, and so flame spray pyrolysis
is not a good choice at all, as it will burn all the carbons to CO and CO
2
.
Among the other remaining spray pyrolysis methods, electrospray is capable
of producing monodispers submicron droplets and/or particles that are highly
charged, so as to repel each other and avoid aggregation. It can be done in mild
37
atmospheric conditions (it does not need elevated pressure or temperatures), and its
neutralized particles can be easily directed to the pyrolysis reactor. Since there are
several other nanoparticles prepared via electrospray pyrolysis, if the desired
particle size can be obtained via this method, the only remaining challenge will be
the scale-up and pyrolysis of the nanoparticles afterwards.
Although a supercritical process seems to be a good candidate, too, producing
monodisperse submicron PEI particles via supercritical methods needs special and
costly nozzles. Moreover, suitable solvents that can be used with the PEI and CO
2
in supercritical conditions are very strong solvents, like dichloromethane or
dichloroethane. Furthermore, there will be other costs associated with the process,
since it is required to have special piping and pumps, and other accessories that are
involved. The supercritical method has a higher production rate but, so far, we are
unaware of any pyrolyzed perticles produced with the supercritical method.
At present, there is not such a commercial polymerization process that leads to
the PEI nanoparticle. Ultem 1000 is a patented polyetherimide from GE Plastics,
but is not produced in a nanoparticulate form. Furthermore, we are not aware of any
other patented GE polyetherimides that are produced in a nanoparticulate form.
Other than electrospray, another method that could potentially produce submicron
PEI particles is wet bead-grinding of the pyrolyzed PEI, whihc is a productive and
economical technique. Since for the CMS particles it is desired that they have a
38
narrow pore size distribution, concerne exists whether wet-gridnding may ruin the
backbone of the starting polymer, or at least a large number of superfine particles
may block the pores of the final CMS particles. Other potential disadvantages
include polydispersity and particle aggregation.
The electrospray method seems to be the best method for achieving our goal
of producing monodisperse and nonaggregated PEI nanoparticles. In this Thesis
we will also study the alternative technique of directly grinding of pyrolyzed PEI
pellets, as a more productive method, to obtain the CMS micro- and nanoparticles,
in order to prepare the MM-CMS and MM- polymeric membranes.
1.9 Electrohydrodynamic Atomization
Electrohydrodynamic spraying has been studied for many years since Zeleny’s
first systematic investigations (Zeleny, 1917). However, recently, it has been
getting more attention due to a widening of its applications to diverse scientific and
industrial processes.
In “pure” electrohydrodynamic sprays the liquid is disintegrated only by the
applied electric potential. The typical electrospray set-up consists of the capillary-
plate configuration. The liquid is flowing through the metal capillary exit, where
the high electric field accelerates the liquid, resulting in jets that break-up into
39
droplets (Fig 1.7). Depending on the liquid properties, the applied potential and the
set-up geometry, different electrospray modes appear (Cloupeau and Prunet-Foch,
1990).
Figure 1.7 Basic set-up used for electrohydrodynamic atomization (EHDA).
Electrohyrodynamic atomization (EHDA) is a process that uses electrostatic
forces to break-up a liquid into fine charged droplets through the Coulombic
interaction of charges in the liquid and the applied electric field. To implement
EHDA, one feeds a sufficiently electroconductive liquid through a metal capillary
charged at an appropriately high electrical potential (typically tens of kilovolts)
relative to a ground electrode positioned a few cm away.
The motivation for the technical application of charged droplets and electrified
sprays is in the possibility of controling droplet transport, evaporation, and life-time
40
by applying external electric fields. The advantage for deposition purposes is
evident, as the droplets can be directed to the desired location of impact by tailoring
the external electric field. Therefore, deposition is one of the main applications of
EHDA, due to the intrinsic high deposition efficiency (Siefert, 1984).
1.9.1 Droplet Production and Charging
In order to atomize bulk liquids into droplets, energy has to be brought into the
system. Mechanical energy can be added to the liquid by applying high pressure,
vibration or kinetic acceleration. Further, electric energy can be added to the system
by applying a high voltage, which disrupts the liquid body into droplets. For the
production of charged droplets, mostly the “pure” electrospray method of applying
a high voltage to a capillary is used. But hybrid electrosprays are also applied
extensively, e.g., for the production of atomized charged fuel sprays (Romat and
Badri, 2001), where an electric field is applied to a conventional pressure nozzle.
Atomizers applying mechanical energy to the liquid body mostly rely on a high
relative velocity of liquid and surrounding gas medium. The type of atomizer using
electric force to overcome the surface tension of the liquid is called electrostatic or
electrohydrodynamic (EHD).
41
Lord Rayleigh (1879) found that an electrically charged droplet becomes
unstable when the outward electrostatic forces balance the surface tension. The
maximum charge q on a droplet with size d can be described according to Rayleigh
(1879) by:
5 . 0 3
) ( 8 d q
R
σ ε π
°
= (1.1)
where ε
◦
is the dielectric constant, σ the surface tension and d the droplet diameter.
Rayleigh observed that the instability of a droplet resulted in the emission of a
liquid jet. The EHDA is based on this effect. Typically the set-up of EHDA
consists of an electrically conducting capillary, to which a high potential is applied,
and a grounded counter-plate. The liquid is fed through the capillary, and is
atomized by the electric field at the capillary exit.
Cloupeau and Prunet-Foch (1990) described the different spraying modes that
can appear depending on the set-up geometry (e.g., the distance between the
capillary and the plate or the capillary radius), volume feed rate, liquid properties
(surface tension, electrical conductivity) and the applied potential (Fig. 1.8).
First, there is the dripping mode. In the absence of an electric field the liquid
flows drop by drop. Increasing the potential from zero increases the droplet
dripping frequency while decreasing the droplet size. This behavior has two causes:
The liquid is attracted to the grounded plate due to the action of the external electric
42
field, and the surface tension is reduced further due to the accumulation of charges
on the surface of the pending droplet.
Figure 1.8 Electrospray modes (after Cloupeau and Prunet-Foch, 1990).
Second comes the microdripping mode and, later, the cone-jet mode. The
liquid droplet at the capillary exit is deformed by the electric field and takes the
shape of a cone. A droplet is formed directly at the droplet apex in the
microdripping mode. In the cone-jet mode the cone is extended by a jet, which is
breaking up into droplets. Clopeau and Prunet-Foch (1989) showed that the
break-up of the jet is comparable to the uncharged jet-breakup investigated by Lord
Rayleigh.
The first mentioning of the cone-jet dates back to 1600, as Gilbert (1958)
reports in his book “De Magnete” it is since that time that a liquid drop that is
43
subject to strong electric forces adopts a roughly conical shape. But it still took
three hundred years until Zeleny (1917) was the first to investigate electrosprays
systematically in the capillary-plate configuration. Vonnegut and Neubauer (1952)
investigated electrosprays with D.C. and A.C. currents, and were also able to see
different spraying modes, among them the dripping and con-jet modes. They
produced a monodisperse cone-jet spray with droplet sizes around 1 µm. Like
Zeleny (1917), they found that it is hard to establish cone-jets with undistilled
water, due to its high electrical conductivity. But use of alcohol, lubricating oil and
distilled water proved successful. There are a number of works dealing with the
stability limits of conejets, but it is hard to give general rules for the required values
of such parameters as conductivity or surface tension of the liquid, as they do not
simulateously influence the spray formation process.
For the lower limit of the conductivity, the estimates range from 10
-8
to
10
-11
S/m (Cloupeau and Prunet-Foch, 1989). The estimates for the upper limit also
vary significantly: According to Mutoh et al. (1979), the upper conductivity limit is
10
-5
S/m, but Smith (1986) established a conejet at 10
-1
S/m. The situation is
similar with the surface tension. Cone-jets can be established with glycerine
(γ = 0.063 N/m) and even with water (γ =0.073 N/m). Fernandez dela Mora and
Loscertales (1994) studied the electric current that is produced by this process and
44
emitted through an electrified jet as a function of the volumetric feed rate, the
liquid properties, the set-up geometry and the electric potential.
When applying an electric field to a pending droplet, it is deformed into a
cone. The cone liquid surface is accelerated towards the apex due to a tangential
electric stress on the free ions in the liquid, and a jet is formed which breakes-up
into droplets. Cone and jet shape, electric current and droplet size depend,
therefore, on the viscosity and conductivity of the liquid (See Fig. 1.9). The cone-
jet mode shows the best performance for particle production. The monodispersity
of the droplet produced in the cone depends on the jet break-up mechanism, which
depends on the ratio of the electric stress over the surface tension stress. At low
stress ratios, the jet breaks-up due to varicose perturbations, whereas high stress
ratios the jet shows a whipping motion. Theoretical description of the cone-jet was
started by Taylor (1964) who was the first to explain the cone shape of the pending
droplet at the capillary exit. He calculated the cone angle to be 49.3°. In his honor
the cone-jet is often called the Taylor-cone. Joffre et al. (1982) proposed a
numerical model for shape calculation of a stable droplet at the capillary exit under
the influence of an electric field. Smith (1986) investigated the stability of the cone
with respect to the onset potential, capillary radius, liquid conductivity, and the
viscosity. Shtern and Barrero (1994) included the electric Marangoni effect as one
45
of the driving forces of the swirling motions in the Taylor-cone. Finally, Hartman
et al. (1999) presented a model that is able to calculate the shape of cone and jet.
Figure 1.9 Left: Cone shape, droplet size and electric current depend on the
forces acting on liquid and dissolved ions: gravity, electric field, surface tension
(Hartman, 1998). Right: Cone-jet mode on a nanospray tip (New Objective Inc.)
Gañán-Calvo et al. (1994) developed a Langragian type numerical model for a
single droplet tracking between the droplet break-up location and the grounded
counter-plate. The results were compared to the experimental droplet size and
velocity values obtained with a Phase Doppler Anemometer (PDA). The axial
droplet velocity was shown to be very close to the capillary exit (~10 m/s) and
decreasing towards the grounded plate, following the decreasing electric field
gradient. Due to the nature of the break-up, the spray is very narrow close to the
capillary exit, but spread towards the plate due to the mutual repulsion of the
46
charged droplets. Furthermore, size segregation could be seen both in the
measurements and calculations. The smaller droplets were pushed towards the
spray edges due to their higher mobility.
1.9.2 Neutralization of the Aerosol
Neutralization methods were discussed by only a few authors in the past,
which is probably due to the fact that most of the work on EHDA has to do with the
spraying itself, which the charge of the droplets often was an advantage in their
applications. Neutralization (at least partial) of the droplets is necessary in many
cases to reduce their electrical mobility (Cloupeau, 1986) in order to produce finer
droplets by evaporation, without the Rayleigh disintegration (Yurkstas and
Meisenzehl, 1964), and more generally in order to obtain free aerosols (Noakes
et al., 1989). Droplets may be neutralized to a greater or lesser extent by placing
them near the head, a source capable of delivering the reverse polarity charges
which will be picked-up by the droplets. For the initially positive droplets the ions
of the opposite sign may be produced by negative corona discharge, negative EHD
spraying of volatile liquids, thermoelectric emission, or flames (Cloupeau, 1994).
Camelot et al. (1998) used a bipolar coagulation process of two electrosprays
with opposite polarities, placed facing each other (Fig. 1.10). In each of the
47
electrosprays, a solution containing one chemical reactant was sprayed. When the
two sprays were mixed, neutral droplets were formed in which the chemical
reaction between the two reactants would take place.
Figure 1.10 Electrospray chamber and experimental set-up of bipolar coagulation
between liquid droplets (Camelot et al., 1998).
Recently, several other groups have used α radioactive sources (see Fig. 1.11)
(Lengorro and Okuyama, 1997) to neutralize charged droplets and particles
produced by the EDHA, in order to avoid wall deposition, and Rayleigh
disintegration during the drying/pyrolysis process which broadens the initial narrow
SD (Lenggoro et al., 2000 a, b).
Figure 1.11 Electrospray chamber and experimental set-up of electrospray
pyrolysis (Lenggoro and Okuyama, 1997).
48
1.9.3 Electrospray Applications
Compared to other film deposition techniques, electrostatic spray deposition
(ESD) bears the advantage of high deposition efficiency (up to 80%), as the
droplets are transported by electrical forces (Siefert, 1984), and do not need a
carrier gas as in conventional sprays. Tailored electric fields enable the deposition
of charged droplets in the desired locations, as shown by Kim and Ryu (1994).
They used electrified masks to deposit silica nanoparticles in geometric
micrometer-sized patterns on GaAs substrates. Chen et al. (1999) used ESD for
deposition of ZnO, ZrO
2
, and Al
2
O
3
from precursor liquid sols. Besides functional
films, ceramic powders can also be synthesized, such as nanocrystalline SnO
2
(Vercoulen et al, 1993). Consequently, the ESD finds applications in processes as
diverse as pesticide spraying on crops (Law, 2001) and painting of automobiles
(Domnick et al., 2003). The challenges facing the automotive finishing industry
are increasing the paint transfer efficiency and reducing volatile emissions without
sacrificing the surface quality or line speed (Im et al., 2001). Chen et al. (1996)
listed important parameters for the ESD process that influence the product quality:
spray production, aerosol transport, solvent evaporation and droplet disruption,
preferential landing of droplets on the substrate, discharge and spreading of
droplets on the surface, decomposition and reaction of the solute (precursor).
49
Numerous works have been published on the production of ceramic particles
and films via electrospray pyrolysis deposition (Vercoulen et al., 1993; Kim and
Ryu, 1994; Chen et al., 1996, 1997; Su and Choy, 2000; Lapham et al., 2001;
Balachandran et al., 2001; Nguyen and Djurado, 2001). The electrospray pyrolysis
deposition set-up consists, generally, of a charged capillary nozzle and a grounded
heated substrate. A precursor is dissolved in a liquid and sprayed against the
substrate. Depending on the process parameters different film morphologies can
appear. Wilhelm et al., 2003 investigated electrospray transport, evaporation and
deposition on a heated substrate theoretically by Lagrangian tracking of single
droplets. They calculated the droplet mass and heat transfer under forced
convection and compared it to limiting cases of electrospraytransport only or
droplet evaporation only.
Electrospray ionization has also emerged as a powerful tool for mass
spectrometry analysis of large and complex molecules. In this technique, the
molecules are dissolved in a liquid that is sprayed via a charged capillary electrode
(Fenn et al., 1989). The solvent of the charged solution droplets evaporates and the
charge density on the droplet surface increases. Eventually, the Rayleigh limit of
the charge density is reached. The surface becomes unstable and starts emitting
charged satellite droplets, relaxing the negative pressure produced by the high
charge density on the droplet surface. If the droplet radius is small enough, ions in
50
the droplet desorb from the droplet into the ambient gas. Attached to the ions are
solvent or solute species that are not ions themselves. This mechanism was
proposed by Iribarne and Thomson (1976). The schematic of the process can be
seen in Fig. 1.12. Another source of the ions is the electrospraying of liquid metal
(liquid metal ion sources – LMIS). The very high conductivity of liquid metals
(K ~106 S/m) leads to the formation of exceedingly sharp tips in the conejet mode
which may emit predominantly single ions (Prewett and Mair, 1991). But, due to
the large surface tension, LMIS is only applicable in vacuum (Gamero-Castaño
et al., 1997).
Figure 1.12 Evaporation and desorption process according to Iribarne and
Thomson (1976).
Encapsulation is another application of the EHDA. Loscertales et al. (2002)
developed an encapsulation method by using the cone-jet electrospray. Two
51
concentric cylinders deliver the components for encapsulation (the inner cylinder
the actual product, the outer the polymer, or the polymeric solution).
Electrosprays are also capable of dispersing the “logistical fuels” (jet fuel,
diesel) which are difficult to burn cleanly, in a way that combustion proceeds in
much the same manner as for a premixed flame with a dispersion energy input of
only a few milliwatts (Kelly, 1984).
Another application of electrospray can be found in thruster technology. The
colloid thruster based on cone-jet electrospraying that was developed by, for
example, Gamero-Castaño and Hruby (2001) is able to deliver a force of 0.3 nN at
an acceleration voltage of 1300V. The ability to deliver thrust in the range of μN is
needed for controlling small satellites and the execution of space missions, in
which very accurate positioning of spacecrafts is needed.
The disadvantage of the electrospray process is the low production rate per
nozzle, which is on the order of g/h or less. For comparison purposes, powder
production rates of air-assisted spray flames are easily in the kg/h range (Müller
et al., 2003). The liquid throughput in electrospray atomization can be increased by
operating a number of Taylor cones in parallel (Rulison and Flagan 1993). Rulison
and Flagan (1994) produced Lanthana-doped Yttria nanoparticles with electrospray
pyrolysis (with high concentration solutions of up to 400g/L). They found
conditions that led to high-quality powders, composed of dense, spheroidal,
52
submicrometer, and nanocrystalline oxide particles. To investigate the feasibility of
increasing the liquid throughput rate in electrospray atomizers, in their previous
work they constructed a linear array of capillary electrodes opposite a slotted flat
plate counterelectrode; see Fig. 1.13 (Rulison and Flagan, 1993). They also
mentioned that, although the capillaries were all at the same potential and feed flow
rate, they did not experience the same electric field due to the end-effects. To
reduce the end-effects, capillaries without liquid flow were added at each end of the
array. Electrospray and its application in making nanoparticles will be discuused in
the following Chapter in greater detail.
Figure 1.13 Left: Multiple electrode electrospray apparatus of Taylor cones
established on capillary electrodes placed opposite a slotted flat plate electrode;
Right: Photograph of a linear array with, R/S=0.31, P=1.39 (Rulison and Flagan,
1993).
53
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66
Chapter 2: Preparation of Polyetherimide Nanoparticles by an
Electrospray Technique
2.1 Introduction
As it has been discussed in the previous Chapter, a number of processes
associated with electric power and energy generation involve the separation of
mixtures containing hydrogen, carbon dioxide, and methane. H
2
and CO
2
separation is today achieved using either adsorption/desorption (temperature or
pressure-swing methods), or by absorption. Both approaches are, however, energy
and capital-intensive. Membrane-based separations, as a result, are making
significant inroads in this area. Strong interest exists in membranes with better
selectivity, and thermal and chemical stability than the existing polymeric
membranes. Mixed-matrix (MM) membranes, which are prepared by incorporating
porous molecular-sieve particles into a polymeric matrix, have, as a result,
been attracting attention for the separation of gas mixtures. The reason for this
interest is that such membranes promise to retain the processability characteristics
of polymeric membranes, while exhibiting the desirable separation properties of the
molecular sieves.
Though great strides have been made (Mahajan and Koros 2000, 2002a, b),
the efforts, so far, have not been completely satisfactory. One reason for the lack of
67
success has been the apparent incompatibility between the base polymeric matrix
(typically glassy polymers) and the molecular-sieve materials (typically zeolites)
utilized. To overcome the incompatibility of the polymeric matrix and the
molecular-sieve component, our approach is to utilize carbon molecular sieve
(CMS) particles rather than inorganic sieves (e.g., zeolites). In addition, the CMS
particles are prepared by the pyrolysis of the same polymeric material which is
used as the base-matrix in the preparation of the MM membranes.
Another challenge in the preparation of mixed-matrix membranes is that the
sieve particles must be of adequately small dimensions, and have a narrow particle
size distribution (PASD). The reason is that the base polymer has generally a very
low permeability, and membranes prepared by such a material must be asymmetric
with a fairly thin (at most a few μm thick) top permselective layer. It is the goal of
this research, therefore, to prepare CMS fine particles to be utilized in the
preparation of MM membranes. Nanoparticles of materials, such as CMS, are also
of fundamental interest, relating to whether their chemical and physical properties
are remarkably different from those of the same material in the bulk form. They
also show potential, in addition to their stated use during membrane preparation,
for use as catalyst supports and adsorbents, in the preparation of sensors, and as
pigments, and structural, electronic and magnetic materials.
68
The technique we utilize in our own work for the preparation of fine CMS
particles is the pyrolysis of polymeric precursor particles prepared by electrospray
(electrohyrodynamic) atomization (EHDA). This process uses electrostatic forces
to break-up a liquid stream into fine charged droplets, through the Coulombic
interaction of charges in the liquid and the applied electric field. To carry out
EHDA, one feeds an electroconductive liquid through a metal capillary charged at
an appropriately high electric potential (typically tens of kV) relative to a ground
electrode positioned a few cm away (Fig. 2.1). The liquid, as it exits the metal
capillary, is accelerated by the high electric field, resulting in jets that break-up into
droplets. The EHDA technique has been shown to be capable of generating fine
droplets, as well as nanoparticles. For example, the preparation of ZnS particles of
less than 50 nm in diameter by an electrospray pyrolysis method was reported by
Lenggoro et al. (2000 b), and nanocrystalline SnO
2
was prepared by Vercoulen
et al. (1993). Park and Burlitch (1996) produced TiO
2
particles 20 nm in diameter
by spraying an ethanolic solution of titanium alkoxide. Besides being used in the
preparation of ceramic powders, EHDA has also been used to prepare functional
films, such as, for example, ZnO, ZrO
2
, and Al
2
O
3
layers from precursor liquid sols
(Chen et al., 1999).
69
Figure 2.1 Basic set-up used for electrospraying (electrohydrodynamic
atomization, EHDA, adapted from Ijsebaert et al. (2001)).
Depending on the properties of the liquid utilized, the applied potential, and
the geometry of the electrospray set-up, different modes of operation (Fig. 2.2)
often appear (Cloupeau and Prunet-Foch, 1990). Often, during the electrospray
process, the liquid is observed to form a conical meniscus at the capillary outlet,
through the apex of which a fine liquid ligament is ejected that breaks-up into a
spray of droplets further downstream (Tang and Gomez, 1994; Fernandez de la
Mora et al., 1990; Chen et al., 1995). This EHDA operation is usually referred to as
the “cone-jet” mode (Cloupeau and Prunet-Foch, 1989). The droplets that are
generated are charged, and the Coulombic repulsion among them, typically,
prevents their coalescence. One of the earliest attempts to understand the
70
phenomena that take place during the con-jet mode is by Taylor (1964), and in his
honor the cone-jet mode is often also referred to as the Taylor-cone.
Figure 2.2 Electrospray modes (after Cloupeau and Prunet-Foch, 1990).
EHDA is a flexible process that can take place in various atmospheres (air,
inert gas, vacuum, etc.), using a range of fluids, and generates droplets through a
variety of operational modes, such as single-jet, multi-jet, emission collapse, etc.
(Grace and Marijnissen, 1994). It has been studied extensively, as a result, for
applications that are traditionally dominated by conventional spray techniques.
Advantages of the electrospray technique include the production of finer size
particles (often in the nm range), lower particle agglomeration, and narrower
71
PASD. Electrospray drying finds industrial applications for converting liquids into
powders, particularly in the pharmaceutical industry for the production of heat-
sensitive materials (Masters, 1991). Ijsebaert et al. (2001), for example, reported
using EHDA for the preparation of fine particles of an asthma drug with a
monodisperse PASD, and with improved inhaling efficiency. Gomez et al. (1999)
have reported using EHDA for the production of protein nanoparticles (88 to 110
nm in diameter). Using high-frequency AC electrospray Yeo et al. (2005) reported
the synthesis of poly-DL-lactic acid (PLA) nanoparticles, drug encapsulation in
mono-dispersed micron-sized biodegradable polymer shells, and the synthesis of 1
µm biodegradable fibers with adjustable pore sizes. The only disadvantage of the
electrospray technique, as previously noted in Chapter 1, is its low production rate
per nozzle (of the order of g/h or less). The throughput can, however, be increased
by operating a number of Taylor cones in parallel (Rulison and Flagan, 1993), or in
a multi-jet configuration with one Taylor cone.
Variables that play a role in EHDA include the applied voltage to the capillary,
the liquid flow rate, the geometry of the system used, the dielectric strength of the
ambient medium, and the liquid physical properties, such as electric conductivity,
surface tension, viscosity, and the dielectric constant (Grace and Marijnissen, 1994;
Tang and Gomez, 1996; Cloupeau and Prunet-Foch, 1990). The cone-jet mode, in
particular, can produce droplets with a wide range of sizes, from molecular
72
dimensions to hundreds of microns, depending on the values of the experimental
variables selected.
EHDA has also been extensively utilized in recent years for the formation of
polymeric films. Only a few recent studies (Berkland et al., 2004; Uematsu et al.,
2004; Morota et al., 2004; Saf et al., 2004) have, however, systematically studied
the morphology of the resulting polymer films as a function of the various EHDA
variables. Berkland et al. (2004), for example, used a modified electrospray
technique termed the flow-limited field-injection electrostatic spraying (FFESS) to
study the formation of biological films and particles of a polyester (poly(D,L-
lactide-co-glicolide) or PLG), and investigated the effect of the type of solvent
used, the applied voltage, the flow rate, and the precursor concentration. They
compared their technique with conventional electrospraying (CES) and concluded
that FFESS is superior to CES in the preparation of films and particles of uniform
diameter and morphology. Morota et al. (2004) prepared poly(ethylene oxide)
(PEO) thin films and investigated the effect of applied voltage, and of the various
solution properties (viscosity, surface tension, conductivity, and polymer molecular
weight), on their properties. Swarbrick et al. (2006) studied PEO layers
electrosprayed on silicon samples using AFM. They reported different film
structures depending on the voltage applied, and solution composition. Rietveld
et al. (2006) prepared polyvinylidene fluoride films and studied the effect of
73
solvent, spray concentration, temperature, solution conductivity, and polymer size
on their properties. The role of chain entanglement on the production of
polystyrene particles produced by electrospray has been investigated by Festag
et al. (1997, 1998), with an emphasis on whether the phenomenon can be
incorporated into the mechanistic electrospray models, which have focused
primarily on low mass solutes. They concluded that the size of polymer particles
produced suggests an important role that chain entanglements plays in limiting the
particle subdivision process.
As noted above, relatively few studies have been devoted to the formation of
polymer particles by EHDA (Festag et al., 1997, 1998), and relatively little is
known about what determines polymer particle morphology, and PASD. In
particular, we are not aware of any study dealing with the production by EHDA of
nanoparticles of imide-type glassy polymers that are typically utilized in the
preparation of MM membranes. Polyetherimide (Ultem-1000 PEI), purchased
from GE Plastics, is the glassy polymer used in our studies of MM membranes.
This material has been used to prepare polymeric membranes due to its superior
strength and chemical resistance, as well as in the preparation of CMS membranes
(Fuertes and Centeno, 1998a; Sedigh et al., 1999). The only study we are aware of
on the production of PEI microparticles is by GE Plastics researchers (Vallance and
Kruglov, 2002). They have prepared micronized polyetherimide powders from the
74
Ultem
-1000P resin powder, a GE Plastics commercial product. The commercially
available Ultem
-1000P resin powder has a typical average particle size of 500 µm.
To reduce the particle size, Vallance and Kruglov (2002) used both jet-milling and
controlled-flow cavitation. The controlled-flow cavitation approach yielded the
smallest size particles (21 and 31 µm average size in two different separate trials,
with a standard deviation of 1.5µm). Particles of this size are, however, too large to
be directly utilized in the preparation of MM membranes.
Though the approach we follow in our studies for the preparation of CMS
nanoparticles involves the in-situ pyrolysis of EHDA-generated PEI particles, one
must note that other techniques have also been utilized to prepare small carbon
particles. They include chemical vapor deposition, arc- and glow-discharge, and
rapid gas phase reaction (explosion). These techniques are not appropriate for
preparing CMS nanoparticles for membrane applications, however, since such
particles must have well-defined micropores with molecular dimensions; they are
usually prepared by the controlled pyrolysis of glassy polymers. None of the
existing techniques, other than EHDA, is capable of producing monodisperse,
nonagglomerated nanoparticles of such glassy polymers, which can then be
pyrolyzed in controlled atmospheres to produce pure CMS particles appropriate for
membrane preparation. It is the aim of this Thesis, therefore, to study the
75
preparation of PEI fine particles, and to study the conditions that control their size,
PASD, and particle morphology.
2.2 Experimental Set-up, Materials and Procedures
Fig. 2.3 shows a schematic of the experimental set-up used to prepare particles
by EHDA. It consists of a Pyrex glass reactor, in which the aerosol generator
system is housed. This system contains a metal shielding ring (with an i.d. of 3.7
cm, and an o.d. of 4.7 cm), and an electrode connected to the ground functioning as
the particle discharger and collector. The polymeric solution is fed with the aid of a
syringe pump (Orion, Sage model 362) to a capillary constructed from a conducting
metal. The capillary is connected to a variable D.C. voltage source (Bertan
Associates Inc., High V oltage power supply, model (205)-30p). The shielding ring
is maintained at the same positive polarity as the capillary and helps to create a
more stable liquid cone, and a concentrated stream of droplets (Meesters et al.,
1992). The field between the ring and ground electrode creates a corona discharge,
which supplies neutralizing ions of opposite polarity to the droplets. The high
voltage electrodes are discharged at the conclusion of each experiment. For the
production of CMS particles, the set-up is connected to a quartz pyrolysis tube
76
heated by a multi-zone furnace, with the CMS particles collected on glass fiber
filters (the CMS particles reported here, however, were generated ex situ)
Figure 2.3 Schematic of the experimental EHDA system
(adapted from Ijsebaert et al., 2001).
Polymeric particles were produced using PEI (GE-ULTEM
-1000, purchased
from General Electric Plastics) solutions in dichloroethane (Mallinckrodt, USA) of
different concentrations. The solutions were prepared by dissolving the PEI pellets,
as received, in dichloroethane and filtering the solution through a PTFE 0.2-µm
syringe filter (bought from Fisher Scientific or Cole-Parmer). The PEI solution was
injected into the EHDA chamber at a constant flow rate using a syringe pump with
the aid of 5 ml, gas-tight disposable syringes; the selected flow rate was verified
77
prior to every experiment by measuring the time needed for an air bubble to move a
measured distance in the carrier Teflon tube. In the experiments reported here,
unless otherwise noted, a capillary with an i.d. of 0.050 mm (50 μm), and an o.d. of
0.3175 cm (referred to as the 1/8 in capillary) is utilized. An Ar flow was
maintained through the EHDA chamber at room temperature to assist in the drying
of the particles, and the removal of the dichloroethane solvent.
To characterize the morphology and size of the particles, samples were
collected on microscope slides (placed on the collection cup), and routinely
analyzed via an optical microscope. A number of these samples were further
characterized via scanning electron microscopy (SEM), and transmission electron
microscopy (TEM) studies, performed at the Center for Electron Microscopy and
Microanalysis of the University of Southern California. The electron microscopy
images were analyzed using the software ImageJ (available from the National
Institute of Health, Bethesda, Maryland, USA), in order to obtain the average
particle size and PASD.
78
2.3 Results and Discussion
2.3.1 The Effect of the Distance between the Various Electrodes
The distance between the capillary (C) and the ring (R), and between the ring
and the ground electrode (either a needle, or a metal tube, or a metal foil) have a
profound influence on the dynamics of the EHDA process, and the size and
morphology of the particles that are generated. Generally speaking, for a fixed
distance between the ground electrode and the ring, and for a fixed voltage on the
ring, there is only a range of voltages and distances for which a stable cone-jet
mode is established, and fine particles with a narrow PASD are generated. For
example, in a series of experiments using dilute solutions (in order to allow for
uninterrupted operation for extended periods of time), a ring voltage of 7 kV , and a
needle as the ground electrode set at a fixed distance of 8 cm from the ring, we
have found that a stable cone-jet mode operation is obtained in a C-R distance
range of 1.0-1.3 cm, and a C range of voltages of 12.5-14 kV . After establishing the
cone-jet mode, at a given fixed distance and voltage, increasing the C-R distance
(e.g., by slowly moving the capillary away from the ring), without changing the
voltage would eventually lead to instability, and the onset of the dripping mode. To
re-establish the cone-jet mode, we had to increase the C voltage, but this would
79
then often lead to powder deposition on the surface of the ring. After establishing
the cone-jet mode at a given fixed distance and voltage, decreasing the C-R
distance (e.g., by slowly moving the capillary closer to the ring) would again
destabilize the system; but at the smaller distances (< 0.9 cm) we could not even re-
establish the cone-jet mode by increasing the voltage. In fact, often, when
increasing the voltage, sparking would commence between the C and R electrodes,
and electrospraying would cease.
To determine the effect of the distance between the ring and the ground
electrode, experiments were conducted beginning with configurations in which a
stable cone-jet mode was established. Subsequently the C voltage and C-R distance
were set and kept constant, while the distance between the ring and the ground
electrode and/or the voltage of the ring were varied. Again, a range of distances and
voltages were found for optimal operation. For the case of using a needle (N), as a
ground electrode, for example, we found a R-N distance range of 7-10 cm, and a
R-N potential difference range of 6-8 kV to be optimal.
During the electrospray process the role of the ground electrode is to provide
the electron density required to completely discharge the electrospray droplets and
particles, which can then be swept away by an inert gas flow for further processing
(e.g., pyrolysis). We found, however, that the grounded needle often did not
succeed in completely discharging the particles, which would then attach to the
80
needle itself or the side walls of the reactor near the needle. Coverage of the needle
by particles would then destabilize the EHDA operation. To avoid this from
happening, other types of grounded electrodes were also utilized, such as stainless
steel tubing or alumina foils placed horizontally at the bottom of the electrospray
chamber under the ring electrode. Though particles did collect on these electrodes
(these are the particles whose characteristics are discussed here) the larger surface
area of these electrodes allowed for more extended periods of operation. As we
move forward towards implementing the in-situ pyrolysis of the PEI particles, an
additional means for discharging the particles is contemplated (e.g., the use of a
radioactive source).
2.3.2 Capillary Selection
We have tested a variety of capillaries in our effort to select the type which
would produce fine, uniformly-sized particles, with an extended period of
operation. We started with commercial 1/16 in (o.d.) stainless steel (SS) tubes with
three different inside diameters of 300, 200, and 100 μm. These capillaries with the
large openings afford relatively trouble-free operation, without capillary clogging,
particularly for higher PEI concentrations (~0.5 wt. % and higher). The capillary
with the smaller i.d. (100 μm) gave the best performance with respect to particle
81
size, electrospray stability at the cone-jet mode, and a wider window of operational
voltages. In general the capillaries with the larger i.d. produce fairly large particles.
To create capillaries of smaller diameter we selected two different SS capillaries
(purchased from McMaster Co.), one with o.d. of 1/16 in (i.d. of 0.0225 in), and
another with o.d. of 1/4 in (i.d of 0.12 in). One end of these capillaries was capped
with a SS plate (1 mm thick), and holes of various sizes were then drilled using a
laser. Two types of capillaries were created using the 1/16 in tubes: Capillaries that
had a single 50 μm hole, and those that had two 20 μm holes. Three different
capillaries were created using the 1/4 in tubes, one containing a single 50 μm hole,
another a single 20 μm hole, and a third containing four 30 μm holes.
The 1/16 in type of capillaries proved difficult to work with, clogging
immediately upon the start of the experiment; once clogged the capillaries proved
impossible to regenerate. The 1/4 in capillaries fared only slightly better than the
1/16 in capillaries in that we were able to run stably long enough for us to collect
particles for characterization. Fig. 2.4 shows, for example, the particles collected
with the single 20 μm capillary. There are very fine single particles (150-300 nm)
together with large agglomerates of the same particles, signifying incomplete
discharging, as discussed previously. However, even the 1/4 in capillaries
eventually clogged irreversibly.
82
Figure 2.4 (a) Particles obtained with the stainless steel 20-μm-id capillary with a
solution flow rate of 0.5 ml/h, solution concentration of 0.14 wt.% PEI, capillary
voltage of 13.8 kV , and ring voltage of 7 kV . (b) Higher-magnification image,
showing agglomerated particles in addition to fine primary particles.
Subsequently, we acquired a set of SS capillaries from the Lennox Laser
Company (Glen Arm, MD). These capillaries with o.d. of 1/8 in, and i.d. of
0.105±0.002 in, are closed and rounded at the tip (with radius of 0.062±0.003 in)
and have a laser-drilled hole at the apex of the round tip; these holes come in a
variety of diameters. We tested Lennox capillaries with hole diameters of 20, 35
and 50 µm. We had difficulties with the 20 µm capillary, which clogged, but the 35
and 50 µm capillaries performed equally well in terms of the particles produced,
whereas the 50 µm capillary was less susceptible to clogging, and was selected for
all the further investigations reported in this paper. We also studied the performance
of two different types of nonconductive capillaries, namely fused silica tubes, from
Western Analytical Products, and also glass capillaries (sold as PicoTip Emitters,
from Scientific Instrument Services, Ringoes NJ). We used metal unions to connect
83
these nonconductive tubes to high voltage, but we were not able to attain stable
electrospray, because the liquid was not charged effectively.
2.3.3 EHDA Spray Modes
As did previous workers [Cloupeau and Prunet-Foch, 1990], we were also able
to observe four different electrospray modes under different operating conditions,
namely, single-cone spray, multiple-cone spray, and the dripping and micro
dripping electrospray modes (the various EHDA modes are shown schematically in
Fig. 2.2). In the single-cone spay mode we observed one steady spray-cone with its
cone axis sometimes slightly rotating around the nozzle axis; in the multiple-cone
spray mode, two or more unstable thin cones, or a corona shape at the edge of
capillary (Fig. 2.2) were observed, typically revolving around the rim of the
capillary.
In a typical EHDA experiment to study the onset of the various spray modes
(using pure solvent, with flow rates higher than 0.3 ml/h, to avoid potential
clogging problems) the voltage on the capillary (V
c
) was set at 14 kV , and the
voltage applied on the ring (V
r
) was increased from 0 to 12.8 kV; with increasing
V
r
the spray mode transitioned from the multiple-cone spray, to single-cone spray,
to micro-dripping and dripping modes. In experiments where the V
r
was kept
84
constant at 7 kV , and the V
c
was increased from 7 to 17 kV , the spray mode
transitioned from the dripping mode to the single-spray cone, to multi-spray cone
mode. In between the two distinct modes, there was often, a transition zone in
which it was possible to observe both modes simultaneously.
A similar pattern of spray modes was obtained when more concentrated PEI
solutions were sprayed, the main difference being an enlarged window of voltages
in which one was able to operate stably in the single-spray cone regime. For 0.14%
PEI solution, for example, with a flow rate of 0.3 ml/h, with a V
r
=7 kV , the single-
spray cone V
c
regime extended from 10 to 14 kV .
2.3.4 Effect of Voltage
As previously noted, the applied voltages at the various electrodes have
significant effect on EHDA operation. We found, for example, in a set of
experiments in which we kept the ring voltage at 7 kV , that increasing the capillary
voltage (within the cone-jet regime) often leads to smaller cone-jet angles and/or
multi-jets. Though these phenomena lead to higher charges on the droplet surfaces
and the onset of Rayleigh effects (which causes droplet break-up), they are also
accompanied by undesirable polydisperse particle formation and the onset of
unstable jets as well. For a solution of a given polymer concentration, we found
85
that a stable and monodisperse electrospray can be established only within certain
range of the liquid flow rates and the applied capillary voltages differences. For
example, with a flow rate of 0.3 ml/h of 0.14 wt% PEI solutions, the required C-R
voltage difference (to be able to operate in the cone-jet mode) is from 3 to 9 kV; if
the additional requirement is to produce small particles with a unimodal PASD,
smaller C-R voltage differences (7-9 kV) would be required.
Fig. 2.5 shows the SEM images of some of the PEI particles obtained with the
flow rate of 0.3 ml/h and 0.14 wt.% of PEI solutions while varying the C-R voltage
difference between the capillary and ring from 4 to 9 kV (the ring voltage is kept
constant at 7 kV). For capillary voltages below ~ 12 kV hollow particles are
obtained. In the voltage range between 13-15 kV the particles are dense, fairly
monodisperse and spherical. Outside this region hollow spheres are again obtained.
Fig. 2.6 shows the corresponding particle size distributions for the same range of
conditions.
86
Figure 2.5 Scanning electron microscopy (SEM) images of particles produced with
a constant ring voltage of 7 kV and a range of capillary voltages of 11.8-15.8 kV;
flow rate of 0.3 ml/h, PEI concentration of 0.14 wt. %.
87
Vc=10.8kV, Dm=630nm
0
5
10
15
20
25
30
35
40
275 425 575 725 875 1025 1174 1181 1284
Diameter (nm)
Count%
Vc=11.8kV, Dm=570nm
0
5
10
15
20
25
30
35
40
175 325 475 625 725 1008 1036 1072 1128
Diameter (nm)
Count%
Vc=12.8kV, Dm=415nm
0
5
10
15
20
25
30
35
150 250 350 450 550 650 730 744
Diameter (nm)
Co u n t%
Vc=13.8kV,Dm=383nm
0
5
10
15
20
25
30
35
40
45
150 250 350 450 550 650
Diameter (nm)
Cou n t%
Figure 2.6 Particle size analysis distribution (PASD) of particles produced with a
constant ring voltage of 7 kV and a range of capillary voltages of 10.8-13.8 kV;
flow rate of 0.3 ml/h, PEI concentration of 0.14 wt.%. (Dm is the average
particle diameter.)
2.3.5 Effect of the PEI Concentration
Changing the PEI concentration in the solution affects simultaneously a
number of the liquid’s physical properties, including viscosity, density, surface
tension and, more importantly, conductivity, and is expected, as a result, to have a
great influence on EHDA performance. In addition, too high of a concentration
causes premature drying of the droplets at the orifice tip, while too low of
88
concentration often results in incomplete solvent evaporation, which may result in
particles sticking together or on the chamber walls.
We conducted a series of experiments in which the flow rate was set at 0.3
ml/h, the ring voltage at 7 kV, the capillary voltage at 13.8, and the PEI
concentration was varied between 0.02-0.5 wt.%. What we have observed is that
increasing the PEI concentration increases the particle size, as seen in Fig. 2.7.
0
500
1000
1500
2000
2500
0 0.1 0.2 0.3 0.4 0.5 0.6
PEI wt.%
Diameter (nm)
Figure 2.7 Average particle diameter of particles produced with a flow rate of 0.3
ml/h, using PEI solutions of varying concentration (0.02-0.5 wt.%), with a ring
voltage of 7 kV and a capillary voltage of 13.8 kV.
In addition, higher concentrations lead to more polydisperse and shell-type
(donut-shaped) particles, as can be seen in Fig. 2.8. Other considerations that come
into the picture, in addition to the size of particles and their structure, in
determining the right concentration for one to use is the potential of the solution for
89
drying and clogging at the capillary tip, the ability to attain a stable cone-jet spray
within a wider window of flow rates, and an adequate particle production rate. As a
result, for given C and R voltage and a liquid flow-rate, a certain concentration
exists that is optimal both in terms of the size and the structure of the resulting
nanoparticles, but also in terms of assuring stable EHDA performance.
Figure 2.8 SEM images of the particles produced with a flow rate of 0.3 ml/h,
using PEI solutions of varying concentration (0.02-0.5 wt.%), with a ring
voltage of 7 kV and a capillary voltage of 13.8 kV.
Fig. 2.9 shows a comparison between the PASD and morphology of the
particles generated at a lower flow rate, by keeping the flow rate (0.05 ml/h), as
well as the C and R applied potentials (13.3 kV and 7 kV respectively) constant,
90
and varying the concentration. Notice that for both concentrations the particles are
solid and spherical with very little agglomeration; however, for the lower
concentration the PASD is shifted towards the lower sizes.
Figure 2.9 (a) SEM image (right) and particle size distribution (left) of particles
produced using a flow rate of 0.05 ml/h of a 0.05 wt.% PEI solution (capillary
voltage of 13.3 kV and ring voltage of 7 kV). (b) SEM image (right) and particle
size distribution (left) of particles produced using a flow rate of 0.05 ml/h of a
0.14 wt.% PEI solution (capillary voltage of 13.3 kV and ring voltage of 7 kV).
91
2.3.6 Effect of Liquid Flow Rate
The liquid flow rate has also a determining effect on particle size and
morphology. Again, an optimal range of values often exists. To study the influence
of the liquid flow rate, we conducted, for example, a series of experiments at a
fixed concentration of 0.14 wt.% PEI, a C voltage of 13.8, and R voltage of 7 kV
with flow rates ranging from 0.05 to 2 ml/h. The average particle size increases as
the liquid flow rate increases, as can be seen in Fig. 2.10.
0
200
400
600
800
1000
1200
0 0.5 1 1.5 2 2.5
Flow Rate (ml/hr)
Diameter (nm)
Figure 2.10 Average particle diameter of particles produced with different flow
rates (0.05-2.0 ml/h) of a 0.14 wt. % PEI solution (ring voltage of 7 kV and a
capillary voltage of 13.8 kV).
Furthermore, increasing the flow rate while keeping the electrode potentials
and the PEI concentration constant, results in a higher number of agglomerated
92
particles and changes the morphology of the particles themselves from dense and
spherical to hollow-shell (donut-shaped) particles, (see Fig. 2.11).
Figure 2.11 SEM images of particles produced with different flow rates
(0.05-2.0 ml/h) of a 0.14 wt.% PEI solution (ring voltage of 7 kV and
capillary voltage of 13.8 kV).
93
2.3.7 CMS Particles and their Characterization
The CMS particles were prepared by the ex-situ pyrolysis of PEI particles and
compared with the CMS particles that were prepared by the pyrolysis of the as-
received PEI pellets and further reduction of their size by conventional grinding
techniques. The latter CMS particles have been successfully utilized in the
preparation of MM membranes. The goal of this side-by-side comparison,
therefore, is to compare the structural characteristics of both types of particles. The
same pyrolysis procedure was followed in both cases, namely raising the
temperature to 623 K at a rate of 1 K/min in a flowing atmosphere of Ar, holding
the samples at this temperature for 30 min, then raising the temperature again to
873 K, holding the samples there for an additional 4 h, and subsequently cooling
down to the room temperature at a rate of 2 K/min.
The pore size distribution and surface area of the two different types of
particles were measured by nitrogen adsorption techniques. The CMS particles
prepared by the EHDA and the conventional techniques are both microporous, with
the average pore and the surface areas being shown in Table 2.1. Notice that the
median micropore diameters are very similar for all three samples. On the other
hand, the surface area of the particles prepared by a wet-grinding technique is
significantly higher that that of the electrospray particles and those generated by the
94
dry-grinding technique. This may simply indicate the smaller diameters of the CMS
particles prepared by the wet-grinding technique, as the EHDA particles did
agglomerate during the ex-situ pyrolysis procedure.
CO
2
adsorption experiments were also carried out using a thermogravimetric
(TGA) system in order to measure the permeation characteristics of CO
2
. The
purpose of these studies was to investigate whether these CMS particles had an
open pore structure which was accessible to this gas. We found that the CO
2
uptake
by the three different types of CMS particles is described to a good approximation
by a Fickian diffusion model. The D/r
2
values (where D is the diffusivity and r the
average radius of the particles) at 298 K for the three CMS samples are also
reported in Table 2.1. They appear to be fairly close to each other (given the
uncertainty in the analysis of this type of experimental data), which is consistent
with the very similar median pore diameters measured with the three different types
of particles. The somewhat larger value of D/r
2
at 298 K for the EHDA CMS may
potentially indicate (given that the median pore diameters are fairly close to each
other) that the electrospray particles have a somewhat higher open porosity than the
particles prepared by the wet- and dry- grinding techniques.
95
Table 2.1 BET surface area, pore size, and diffusion coefficient (D/r
2
) of the CMS
particles prepared by the wet grinding and dry grinding of pyrolyzed pellets, as
well as by electrospray pyrolysis
CMS Particles
BET Surface
Area (m²/g)
Median Pore
Diameter (nm)
D/r
2
(s
-1
)
0.5≤ M
t
/M
∞
≤0.8
Wet Grinding
(pyrolyzed pellets)
560 0.62 1.56×10
-3
Dry Grinding
(pyrolyzed pellets)
417 0.61 1.15×10
-3
Electrospray Pyrolysis 440 0.59 2.80×10
-3
2.4 Conclusions
We have studied here the preparation of PEI fine particles through the use of
the EHDA technique, with the eventual goal of preparing CMS nanoparticles to be
utilized in the preparation of mixed-matrix (MM) membranes. The focus in this
investigation was on understanding the effect that the various parameters of the
EHDA process have on determining the size, morphology and structural
characteristics of the particles that are generated. Such parameters include the
applied voltages at the various electrodes (capillary, ring, and ground), the distance
between these electrodes, and the type of the ground electrode that is utilized
(needle, flat plate, or S.S. tube).
Monodisperse, dense, spherical PEI particles with nm dimensions were
prepared for a broad range of conditions. It was observed, that for a given
concentration of PEI, a stable and monodisperse electrospray can be established
96
(defined as the cone-jet mode) within a certain range of liquid flow rates and
applied voltage differences. Within the cone-jet mode region, the particle size and
its morphology can be controlled by adjusting the flow rate and the solute (PEI)
concentration. The particle size increases monotonically with the liquid flow rate,
and decreases with an increase in the applied voltage. Decreasing the PEI
concentration also decreases the particle size. Increasing the flow rate, while
keeping the applied voltages and PEI concentration constant, changes the
morphology of the particles, leading to particle agglomeration, and hollow-shell
type particles. The type of capillary also plays an important role, with conductive
capillaries with a round tip shape providing the best results. Smaller i.d. capillaries
lead to smaller primary droplets and particles; however, clogging is typically a
major problem with smaller capillaries, when working with glassy polymers such
as PEI.
The CMS particles, prepared by controlled pyrolysis of the EHDA-generated
PEI particles have median pore diameters, and transport properties that are very
similar to those prepared by more conventional techniques. These preliminary
observations bode well for the eventual use of the EHDA-generated CMS particles
for the preparation of MM membranes. Our focus in our ongoing studies is on
reducing further the size of the produced nanoparticles, and on enhancing the
production rates of these particles through the aid of multi-capillary configurations.
97
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100
Chapter 3: Preparation of Mixed-Matrix Carbon Molecular-
Sieve Membranes Using Nano-Sized Particles
3.1 Introduction
Separation of H
2
and CO
2
mixtures is a problem of industrial importance.
These gases are currently separated using either adsorption/desorption methods (by
temperature- or pressure-swing adsorption), or by absorption methods (e.g., by
amine scrubbing used for CO
2
separation). The problem with both approaches is
that they are capital intensive, and very much "economy-of-scale" dependent. It is
for this reason that membrane-based separation processes are increasingly viewed
as viable alternatives, and their use is making significant inroads in this area.
Traditionally, polymeric membranes have been utilized, with their main
advantages being the fact that they are generally cheap, and easily processable into
a variety of shapes (sheets, tubes, hollow-fiber, etc.). Such membranes often
possess good separation efficiency for molecules that are similar in size and shape,
the separation of which poses considerable challenges for the conventional
techniques. The good separation properties of polymeric membranes are due to the
mechanism of transport of gas in them, which involves solvation of the molecules
into the polymeric film, followed by solid-phase molecular diffusion. Differences
101
in the permeation characteristics among a mixture's molecules are attributed either
to the differences in the solubility, or to those in the transport properties, or both. If
the right polymer and preparation technique are selected, a number of polymeric
membranes are obtained that exhibit high separation factors for a variety of
gaseous mixtures. It is for such reasons that separation and purification of a variety
of gaseous mixtures using nanoporopus or dense polymeric membranes are
currently done at commercial scales (Stern, 1994). One of the problems with
polymeric membranes in the past was their thermal resistance, but in recent years
several glassy polymeric membranes with good thermal resistance have been
synthesized. Such glassy polymeric membranes include polyimides,
polycarbonates, polysulfones and cellulosic polymers (e.g., cellulose acetate). Over
the past two decades several papers have discussed these and other polymeric
membranes for gas separation (Koros and Chern, 1987; Koros, et al., 1988;
Zolandz and Fleming, 1992; Kesting and Fritzsche, 1993; Paul and Yampolskii,
1994).
The past decade has also witnessed much improvement in synthesizing more
attractive membranes by designing glassy polymers with two desirable
characteristics, namely, (1) high free volume which provides for high diffusion
rates and permeabilities, and (2) rigid, constricted regions that give rise to sieving
of penetrants based on the size or shape differences. These features can be
102
developed by introducing polymer backbone structures that inhibit interchain
packing and torsional motion, as well as other structures that minimize mobile
linkages to create rigid, constricted regions between the polymer backbone chains
(Zimmerman, 1998). Despite their increased thermal resistance, the safe operating
temperatures for most glassy membranes are still generally below those that are
appropriate for high-temperature petrochemical applications. As a result, the
application of such membranes is restricted to the range of low to moderate
temperatures. Another disadvantage of polymeric membranes is that their
segmental flexibility limits their separatory ability, particularly toward such gases
as H
2
and CO
2
.
To overcome the deficiencies of the polymeric membranes, several groups,
including the USC group, have prepared inorganic membranes made of molecular
sieving materials, the framework of which consists of rigid constrictions with
angstrom-scale dimensions that are capable of discriminating and separating
molecules based on their dimensions. Two major categories of such materials
include carbon molecular sieves (CMS) that are formed by pyrolysis of polymeric
precursors and other methods, and zeolites that are commercially produced, mostly
by hydrothermal synthesis techniques.
In particular, the CMS are nanoporous materials with pore sizes and
interconnectivities that make fast transport of gas molecules possible (Karger and
103
Ruthven, 1992). They possess high internal surface area and adsorptive capacity,
and have been used successfully in adsorption processes in which kinetic
separation is achieved through the differences in the diffusivities of gases (Rao,
and Sircar 1993). The CMS materials can be prepared from a variety of natural and
synthetic sources (Manoj and Himmat 1994), and display exceptional permeation
properties for a variety of gas separations. They exhibit good separation and
mechanical properties when prepared through pyrolysis of polymeric precursors,
such as polyimides and polypyrrolones. Extensive work by several research groups
has revealed a range of CMS morphologies and permeation properties that depend
upon the polymer precursor [such as polyimide, polypyrrolone, polyfurfuryl
alcohol, and polyetherimide (PEI)], the precursor's geometry, and the pyrolysis
conditions (Chen and Yang, 1994; Jones and Koros, 1994; Kita, et al., 1997; Suda
and Haraya, 1997; Fuertes and Centeno, 1998b; Sedigh et al., 1998, 1999, 2000b).
The pyrolysis temperature has a strong influence on the membrane structure,
separation performance (permeability and selectivity), and the transport
mechanism for gas separation. The optimal temperature depends very much on the
type of the precursor used, and generally an increase in the pyrolysis temperature
will result in a decrease in the gas permeability and in an increase in the selectivity
(Geiszler and Koros, 1996; Suda and Haraya, 1997; Vu and Koros, 2002). The
increase in pyrolysis temperature will also lead to a carbon membrane with higher
104
compactness, a more turbostratic structure, higher crystallinity and density, and
smaller average interplanar spacing between the graphite layers of the carbon
(Tanihara et al., 1999).
The CMS membranes also offer many advantages over zeolites, the most
important of which is the ability for forming homogeneous, defect-free
membranes. For these reasons, over the past two decades an extensive body of
research work on the CMS membranes has been developing. While there have
been numerous studies on the CMS membranes for air separation (Soffer, et al.,
1987; Hatori et al., 1992; Armor, 1994; Acharya and Foley, 1999; Singh-Ghosal
and Koros, 2000), the CO
2
/CH
4
separation is also gaining increasing attention in
recent years, as such membranes have demonstrated extraordinary transport
properties in comparison to polymeric membranes. Kita et al. (1997) reported high
permeabilities and CO
2
/CH
4
selectivities (> 40) for unsupported pyrolyzed
polypyrrolone films, under a N
2
atmosphere. Centeno and Fuertes (1999) prepared
flat composite CMS membranes via vacuum pyrolysis at 700°C of a thin phenolic
resin on a disk support. The phenolic resin carbonized at 700°C showed an O
2
/N
2
selectivity around 10, and separation factors of 160 and 45 for CO
2
/CH
4
and
CO
2
/N
2
, respectively, in gas mixture separation experiments. They reported higher
permselectivity (by a factor of 2) for the mixture than for the pure gas experiments.
105
A number of groups (Rao and Sircar, 1993; Hayashi et al., 1995; and Foley
et al., 1997) have also reported the preparation of CMS membranes by the
carbonization of polymeric films, previously deposited on porous inorganic and
metal substrates. Rao and Sircar (1993), for example, prepared CMS membranes
by carbonization at 1000°C (under a N
2
atmosphere) of a thin, uniform layer of
poly (vinylidene chloride)acrylate terpolymer latex film, deposited on a
macroporous graphite or alumina support from an aqueous suspension. They have
measured pure-gas permeabilities of He and H
2
, as well as mixed-gas
permeabilities of H
2
-hydrocarbon mixtures. For a ternary mixture of CO
2
, CH
4
,
and H
2
a CO
2
/CH
4
separation factor of 18 was observed (Rao et al., 1994). Their
membranes were tested in a pilot plant-scale unit (Naheiri et al., 1997). Hayashi
et al. (1995) prepared the CMS membranes by the carbonization of thin polyimide
films, deposited on the outer surface of α-alumina tubular supports (with a mean
pore diameter 1400 Å) at temperatures between 500-900°C. The resulting
membranes had a CO
2
/CH
4
separation factor of about 100 (for a carbonization
temperature of 800°C) at 30°C. They showed that, for the CO
2
/N
2
and CO
2
/CH
4
binary mixtures, the higher the carbonization temperature is, the greater the
selectivity and the lower the permeance of the resulting membranes.
Sedigh et al. (1998) formed supported CMS membranes by pyrolyzing
poly(furfuryl) alcohol films on flat and tubular ceramic supports at 600°C under an
106
argon atmosphere. They obtained CO
2
/CH
4
selectivities in the 34-37 range, and
CO
2
permeances that were about 130 GPU (1 GPU=1×10
-6
cm
3
(STP)/cm
2
⋅s⋅cm Hg)
with single-gas and feed mixtures under a 30 psi pressure gradient at 20°C. Later,
Sedigh et al. (1999) examined the PEI as the precursor, and reported
CO
2
/ CH
4
selectivities for the resulting CMS membranes as high as 150, with
equimolar gas mixtures under transmembrane pressure difference of 20 psi at
20°C. They also reported that the CO
2
/CH
4
selectivity was higher in the binary
CO
2
/CH
4
mixture than in the pure gas experiments, and attributed the enhancement
to pore blocking by the adsorbed CO
2
molecules. Jones and Koros (1994) prepared
CMS membranes by carbonization of commercial asymmetric hollow fiber
polyimide membranes, under vacum at two temperatures of 500 and 550°C. They
carried out limited studies that indicated that the CO
2
/CH
4
selectivities were
around 140, with a CO
2
permeance of 50 GPU for an equimolar feed mixture at
150 psi and 25°C.
Even though the CMS and zeolites (Breck, 1974) offer very attractive
transport properties, with permeabilities and selectivities that are significantly
higher than those of the polymeric materials, processing challenges and high costs
hinder their industrial application. For example, many difficulties with the
preparation of zeolite membrane have been reported (Saracco, 1999), while
fabrication of the CMS membranes involves high-temperature processes (Steel,
107
2000). One way of addressing these issues is by incorporating the sieving materials
into a processable polymeric matrix, which results in mixed-matrix (MM)
membranes that offer an attractive alternative to the molecular-sieving membranes.
Such membranes combine some of the higher selectivity benefits of molecular-
sieving media, and avoid the costly processing of purely homogeneous molecular-
sieving membranes.
In the early work on MM membranes, elastomeric or rubbery polymers were
used as the continuous matrix phase. Many researchers (Duval et al., 1994;
Vankelecom et al., 1995; Boom, 1998) identified difficulties with obtaining good
polymer-sieve contact with rigid, glassy polymers, such as polyimides. Glassy
polymeric MM membranes often possess poor polymer-sieve adhesion, resulting
in macroscopic voids and no selectivity enhancement. To promote adhesion
between the rigid polymer chains and molecular sieves, extensive and intensive
studies were carried out by applying thermal treatment, silane coupling agents,
integral chain linkers, and polymer coating on the molecular sieve surface (Rojey
et al., 1990; Vankelecom et al., 1995; Zimmerman et al., 1997; Yong et al., 2001;
Vu et al., 2003a; Kulkarni, 2003). For example, Rojey et al. (1990) obtained
selectivity enhancement for H
2
/CH
4
gas mixtures using flat-sheet MM membranes
consisting of zeolite 4A and the Ultem® matrix.
108
Vankelecom et al. (1995) examined the incorporation of various zeolites into
several polyimides for the pervaporative separation of xylene isomers, and
reported that poor polymer-sieve contact is the main difference between the
polyimide-zeolite MM films and the previous PDMS-zeolite films. Duval et al.
(1994) studied the effect of various silane coupling agents on silicalite for a range
of glassy polymers. Although improved polymer-sieve contact was obtained, no
improvement in the selectivity from gas permeation measurements resulted. They
also attempted film formation above the glass-transition temperature of the
polymer and high temperature post-treatment of the MM films with no success.
Huang et al. (2006) fabricated polymer-zeolite MM membranes by incorporating
nanosized or microsized zeolite 4A into polyethersulfone, and reported that the
MM membranes exhibited decreased gas permeabilities, but permselectivities that
were greatly enhanced for He/N
2
, H
2
/N
2
, He/ CO
2
, and H
2
/ CO
2
binary mixtures.
Lee et al. (2007) prepared carbon membranes from blending poly(2,6-
dimethyl-1,4-phenylene oxide) (PPO) with polyvinylpyrrolidone (PVP), and
investigated the influences of the pyrolysis temperature and the blend ratio of PVP
on the gas permeation performances of the membranes. Cong et al. (2007) reported
synthesis of brominated poly(2,6-diphenyl-1,4-phenylene oxide) (BPPOdp) as a
new membrane material that forms flexible membranes, with CO
2
permeability
and CO
2
/N
2
selectivity higher than that of poly(2,6-dimethyl-1,4-phenylene oxide)
109
(PPOdm) membranes. By inserting 10 and 30 nm silica nanoparticles, the resulting
MM membranes had a greatly enhanced CO
2
permeability, while maintaining the
CO
2
/N
2
and CO
2
/CH
4
selectivities of the pure BPPOdp membranes. Moreover, the
CO
2
permeability increased as the silica content of the membrane also increased.
In a recent work, Anson et al. (2004) observed that the incorporation of activated
carbon into acrylonitrile–butadiene–styrene (ABS) terpolymer (0.624 volume
fraction) significantly improved CO
2
/CH
4
selectivity from 24.1 to 50.5.
A few groups studied the MM membranes that contained carbon nanotubes
(CNTs). Kim et al. (2007) prepared MM membranes by inserting a single-walled
CNT in a polysulfone matrix. Cong et al. (2007) fabricated CNT–impregnated
BPPOdp membranes, and reported that the resulting MM membrane had increased
tensile-strength and CO
2
permeability, and CO
2
/N
2
selectivity higher than that of
the corresponding pure polymer membranes. They also claimed that multi-walled
CNTs were more effective at increasing the gas permeability than the single-walled
ones, although their diameter and length had little effect on CO
2
/N
2
selectivity of
the membrane. Several groups (Kusuki et al., 1997; Soffer et al., 1997; Vu et al.,
2002; Saufi and Ismail, 2002; Li et al., 2006; Jiang et al., 2006, 2007) used hollow
carbon fiber MM membranes for gas separations. Jiang et al. (2007), for example,
fabricated a dual-layer hollow fiber composed of a polysulfone-beta zeolite (PSF-
beta) MM outer layer and a Matrimid inner layer, with O
2
/N
2
and CO
2
/CH
4
110
selectivities of 9.3 and 150, respectively. The performance of the MM membrane
was much better than that of the hollow carbon fiber derived from single-layer
Matrimid hollow fiber. Husain and Koros (2007) used organic-inorganic MM
asymmetric hollow fiber (zeolite incorporated in an Ultem® 1000 PEI matrix).
They reported sieve-in-a-cage defect in Ultem® sized modification of the zeolite,
while grignard treated ones showed significant ideal selectivity enhancement for
CO
2
/CH
4
and 25% higher for mixed-gas CO
2
/CH
4
pairs over the neat polymer
results. Ghalei and Semsarzadeh (2007) used poly(urethane)/poly(vinyl acetate)
(PU/PV Ac) blend membranes with varying blend ratios, in order to improve the
PU membrane morphology and its CO
2
selectivity. Adding 20 wt.% PV Ac
decreased the permeability of O
2
, N
2
and CH
4
, but increased the permeability of
CO
2
.
The objective of this study is to investigate the use of CMS particles for
fabricating MM membranes. The preparation and permselectivity of a MM
membrane material, composed of highly-selective CMS particles, dispersed in a
continuous high-performance glassy polymer matrix is reported. The resulting MM
membrane combines the advantages of each material (polymer and the CMS
particles), benefiting from the high selectivities of the CMS particles, while
maintaining the desirable mechanical properties and economical processing
capabilities of the polymers. The hypothesis is tested is that the MM membrane
111
should exhibit permeation properties that are in between the intrinsic properties of
the pure CMS and pure polymer. Because the CMS particles that constitute the
dispersed phase have significantly higher combined permeability and selectivity
properties than those of the polymer matrix, the goal is to prepare a MM
membrane that possesses enhanced properties beyond those of the pure polymer
alone.
The approach that we develop may enable one to fabricate MM membranes
with properties that are significantly improved over the upper bounds possible by
the pure polymer. In particular, successful engineering of the interfacial
morphology (between the CMS particles and the polymer matrix) should eliminate
the gaps or defects between the CMS particles and the glassy polymer matrix,
which is currently the most difficult hurdle to overcome in the MM membrane
formation. Unlike the past work with the MM membranes using flexible rubbery
polymers, the rigid nature of the polymer chains in glassy polymers is believed to
inhibit chain mobility near the sieve surface, hence leading to poor contact and
adhesion. Recent work to improve polymer-sieve adhesion with glassy polymers
reports possible remedies, such as zeolite surface modifications, high temperature
formation, use of plasticizing or anti-plasticizing agents, and chemical-reactive
techniques (Vankelecom et al., 1996; Mahajan, 2000; Yong, 2001), but completely
successful solutions are not yet available.
112
To our knowledge, the only other study of the preparation of the MM
membranes through the addition of the CMS particles was previously reported by
Koros and co-workers. They used rather large CMS particles (with their average
size being in the microns range), which are difficult to adopt for the preparation of
asymmetric hollow fiber membranes. Their results are summarized in Table 3.1
below. In terms of both the permeability and separation factors, their results are
very similar to the data reported in this thesis.
Table 3.1 Permeation properties of mixed matrix films using Matrimid® 5218 or
Ultem® 1000 as matrices at various loadings of carbon molecular sieve inserts
(adoppted from Vu et al., 2003 a).
Mixed Matrix Films Permeability (Barrer) Permselectivity
CO
2
CH
4
CO
2
/CH
4
Continuous Phase:
Matrimid® 5218
10.0 0.28 35.3
Disperse Phase:
CMS 800-2
44.0 0.22 200
17 vol.% CMS
10.3 0.23 44.4
19 vol.% CMS 10.6 0.23 46.7
33 vol.% CMS 11.5 0.24 47.5
36 vol.% CMS
12.6 0.24 51.7
Continuous Phase:
Ultem® 1000
1.45 0.037 38.8
Disperse Phase:
CMS 800-2
44.0 0.22 200
16 vol.% CMS 2.51 0.058 43.0
20 vol.% CMS 2.90 0.060 48.1
35 vol.% CMS
4.48 0.083 53.7
(CMS 800-2 obtained via pyrolysis of Matrimid® 5218 at 800
◦
C)
Pure gas permeation measurements with 50 psia upstream for CO
2
and CH
4
.
Temperature: 35
◦
C, (Barrer=1×10
-10
cm
3
(STP)⋅cm/cm
2
⋅s⋅cm Hg).
113
The polymer that is used in this work is the PEI. It has previously been used to
prepare the CMS membranes, due to its superior strength and chemical resistance
(Fuertes and Centeno, 1998; Sedigh et al., 1999). For example, as mentioned
earlier, Sedigh et al. (1999) dip-coated a PEI film inside a ceramic tube and then
carbonized it at 600ºC under flowing argon for 4 h. They measured the transport
characteristics of their PEI carbon membrane using single gases (CH
4
, H
2
, CO
2
),
and binary (CO
2
/CH
4
, H
2
/CH
4
, CO
2
/H
2
), as well as ternary mixtures
(CO
2
/H
2
/CH
4
). They reported CO
2
/CH
4
separation factors as high as 145 for an
equimolar binary, and 155 for the ternary mixture. The reason for such excellent
performance is the presence of a bulky C(CH
3
)
2
group, combined with a highly
stiff structure of the polymer, which make the PEI a very promising materials for
fabricating membranes. Since during carbonization at moderate temperatures the
backbone of the starting polymer remains, most likely, intact, one expects the CMS
membranes utilizing the PEI as the polymeric precursors to maintain their superior
performance as well.
114
3.2 Materials and Procedure
3.2.1 CMS Nanoparticles Obtained Using Conventional Particle
Size Reduction Techniques
The CMS powders used in this study were formed by high-temperature
pyrolysis of polyimide precursors followed by dry and wet grinding. The first
stage was to synthesize the CMS material by pyrolysis of the polymer precursor. In
this study, Ultem® 1000, a commercially available polyetherimide (GE Plastics,
Mount Vernon, IN) was used as the precursor polymer. The polymer pellets were
pyrolyzed in the presence of flowing argon (99.997% pure) in a cylindrical
furnace, 0.0508 m diameter and 0.6096 m long, externally heated and controlled
by a programmable Omega CN3000 controller. The crucible containing the pellets
was placed in between two thermocouples about 0.05 m apart, with the tip of the
thermocouples close to the outside surface of the crucible to make sure that no
temperature gradient exists along the axis of the furnace. An argon flow rate of
1-1.33×10
-6
m
3
/s was used to remove the gases evolved during carbonization and
to maintain the inert atmosphere. The pyrolysis protocol involved raising the
temperature to 623 K at a rate of 1 K/min, holding the samples at this
temperature for 30 min, then raising the temperature to 873 K holding the samples
115
there for an additional 4 hr, and finally cooling to room temperature at a rate of
2 K/min.
For use in mixed-matrix (MM) films and tubular membranes, the CMS were
further formed into very fine particles (submicron to micron). A ball mill (Norton-
Akron, Ohio) was used to crush the CMS into finer particle sizes. First, the CMS
were dried at 250°C in a vacuum oven for at least 12 h. Then, they were loaded
into a 2.25 -in. diameter, 3 -in. high Teflon container. Zirconia ball bearings (1 cm
in diameter) were placed inside the container before sealing the container with an
O-ring top and cap.
The CMS fine powder (the particle sizes range from about 1 to 100 μm, with
an average particle size of 15μm; see Fig. 3.1) was sent to the grinding company
(NETZSCH, Inc.) to be wet-ground further into very fine particles (submicron to
micron).
Fig. 3.2 shows the particle size distribution of the CMS particles that were
produced by the wet-grinding technique using the MiniZeta w/0.65 mm YTZ
grinding media (NETZSCH, Inc.) in dichloroethane. After 160 min of grinding, the
particle sizes range from about 350 nm to 1.5 μm, with the average particle size
being 584 nm.
116
Figure 3.1 Particle size distribution of CMS feed (dry ground,
particle sizing done by Union Process).
0.000
2.000
4.000
6.000
8.000
10.000
12.000
14.000
16.000
18.000
0.01 0.10 1.00 10.00 100.00 1000.00 10000.00
Microns
% in Channel
Feed Pow der
5 minutes grinding
30 minutes grinding
60 minutes grinding
120 minutes grinding
150 minutes grinding
160 minutes grinding
Figure 3.2 Particle size distributions of wet-ground CMS particles (NETZSCH, Inc.)
The second batch of dry ground CMS particles (the particle sizes range from
about 0.2 to 20 μm, Fig. 3.3) was also sent to NETZSCH Inc. to obtain finer
particles using the MiniZeta w/0.30mm YTZ in dichloroethane as the dispersing
117
media. After 120 minutes the particle sizes range from about 90 nm to 1.5 μm,
with the average particle size being 533 nm (Fig. 3.4). The particle size was more
suitable for our purpose after 105 minutes of grinding (500 nm, and range of 100
nm to 1300 nm), so grinding was stopped after 120 minutes.
Figure 3.3 Particle size distribution of the CMS feed
(dry ground, particle sizing done by NETZSCH, Inc.).
Figure 3.4 Particle size distribution of the wet-ground CMS after 120 minutes
grinding (NETZSCH, Inc.), median: 0.503 μm and mean: 0.533 μm.
118
The second batch of the CMS particles was used to prepare the entire new
round of the CMS and MM-CMS membranes (all membranes were prepared using
Media and Process Technology (M & PT) Inc., substrates, unless specified
otherwise).
3.2.2 Diffusion of CO
2
and CH
4
in the CMS Nanoparticles
Experiments were carried out using a thermogravimetric (TGA) system in
order to measure the permeation characteristics of CO
2
and CH
4
in the CMS
nanoparticles prepared by the wet-grinding technique, and utilized in the
preparation of the membranes. The purpose of these studies was to investigate
whether these nanoparticles had an open pore structure and were permeable to
these two gases. Before each measurement, about 120 mg sample of the CMS
nanoparticles were first degassed overnight under vacuum at 393 K, and then
exposed to an argon flow rate of 30 ml/min
at 298K. The gas was subsequently
switched to either CH
4
or CO
2
, also flowing at 30 ml/min, and its weight change
(resulting form the gas uptake) with time was monitored. For isothermal diffusion
in a spherical adsorbent particle (radius r), the change in the mass of the sample
due to sorption is given by
2 2 2
/
1
2 2
1 6
1
r Dt n
n
e
t
e
n m
m
π
π
−
∞
=
∑
− =
(3.1)
119
where m
t
is the gas uptake at time t, m
e
the gas uptake at equilibrium, and D is the
diffusion coefficient. This expression assumes that the sorbate pressure remains
constant following the initial step change. In the short-time region, Eq. (3.1)
approaches the limiting form
π
Dt
r m
m
t
6
1− =
∞
(3.2)
At longer times the higher-order terms in Eq. (3.1) become negligible so that
the expression simplifies to
2 2
/
2
6
1
r Dt t
e
m
m
π
π
−
∞
− =
(3.3)
A graph of ln (1-m
t
/m
∞
) for CO
2
against time t for the CMS particles (first
batch), as well as a theoretical curve calculated using Eq. (3.3), is shown in
Fig. 3.5. It is clear from the figure that the CO
2
uptake by the CMS is described to
a good approximation by the Fickian model. The diffusion coefficient D/r
2
at 298
K is 4.1×10
-4
s
-1
. Other investigators also report similar values for the CMS [see for
example, Kappoor (1989)]. The uptake of CH
4
into the CMS material was very
slow, making equilibrium uptake and time difficult to determine using the TGA
technique, but also signifying that the CMS nanoparticles are potentially capable
of separating CO
2
from CH
4
. Measurements of the BET surface area and pore size
of the CMS particles indicate a surface area of 560.2 m²/g and a median pore
diameter of 0.62 nm. For the second batch of the CMS particles prepared by dry
120
and wet grinding (mean particle size = 533 nm), the BET surface area is higher
(585.43 m²/g), and also is the median pore diameter (0.624 nm). A graph of
ln (1-m
t
/m
∞
) for CO
2
uptake against time t for the second batch of the CMS
particles, as well as a theoretical curve calculated using Eq. (3.6), are shown in
Fig. 3.6. The calculated D/r
2
at 298 K is 1.22 × 10
-3
s
-1
, which is about three times
larger than the one measured for the first batch of the CMS particles prepared by
the wet-grinding technique. Given that the median pore diameters are fairly close
to each other, these results would tend to indicate that the second CMS particles
have a higher open porosity than the first batch, although both were prepared by
the same wet-grinding technique.
0 200 400 600 800
Time (s)
-4
-3
-2
-1
0
ln(1-m
t
/m
e
)
Figure 3.5 ln (1-m
t
/m
e
) against t for the uptake of CO
2
at 298 K for the 1
st
batch of CMS (mean particle size: 584nm).
121
-2
-1.5
-1
-0.5
0
0
20 40 60 80 100
Time(s)
ln(1- M t / M ∞)
0.5< M
t
/M <0.8
Figure 3.6 ln (1-m
t
/m
e
) against t for the uptake of CO
2
at 298 K for the 2
nd
batch of CMS (mean particle size: 533nm).
Attempts to use the TGA to measure the uptake of CH
4
and CO
2
by the
original PEI proved rather unsuccessful, indicating that diffusion in the original
polymeric precursor is rather slow, as expected.
3. 3 Experimental Set-up and Procedure
In this section, we describe experimental aparatuse, and the procedure that was
used for the measurements.
3.3.1 Mixed Matrix Polymeric Membrane Films
The first step of the experiments involves the preparation of the MM
polymeric membrane films.
122
3.3.1.1 Preparation of Homogeneous Dense Films
Homogeneous, dense polymer films were prepared using a solution-casting
technique. The polymer was first dried at 100°C for at least 12 h in a vacuum oven
to remove the moisture. Immediately after removal from the vacuum oven, the
polymer was added to a dried, pre-rinsed 150-ml flask containing the desired
amount of 1,2-dichloroethane (C
2
H
4
Cl
2
) to prepare a dilute solution of
approximately 11 to 15 polymer wt.%. The polymer solutions were prepared by 24
h of stirring (with a magnetic stirrer), with a slight heat applied in the early stages
of dissolution. Then, the polymer solutions were poured into a dried, pre-rinsed
40-ml vial (Teflon®-lined septa are used for the caps), and mixed by rolling the
vial containing the solution on parallel rollers that provide mechanical agitation
through rotation of the rollers by a continuous duty motor (Norton; Akron, Ohio
Laboratory Products). Typically, the polymer solutions were rolled for at least 12 h
prior to casting. After the mixing, the vials were allowed to stand for a period of
time (typically, 30 minutes to 1 h) to degas the bubbles that may have formed
during mixing.
The formation of MM membrane films followed the same procedure as that
for the formation of homogeneous, dense polymer films. The MM polymer-based
(MMP) subscale prototype, nanocomposite membranes were prepared by solution-
123
casting of a slurry of fine CMS particles, prepared by dispersing the particles in a
polymeric solution. Generally, the slurry contained about 13-18 wt.% of solids (the
CMS particles and the PEI) in the solvent. The slurry was prepared by the
following procedure: Approximately 10 wt.% of the total amount of the PEI to be
utilized was first added to a 40 ml vial containing the CMS particles (about 4
wt.%) in 1, 2-dichlorethane, so as to “prime” the CMS particles. The PEI and CMS
particles slurry were then mixed well by rolling the vial on rotating parallel rollers
for 12 h. After that, the remaining amount of the PEI was added to the slurry, and
the resulting slurry was again mixed well by rotating the vial on parallel rollers for
an additional 24 h. The final CMS loading was around 15-35 wt.% with respect to
the total weight of the CMS and PEI.
The polymer solution (or polymer-sieve slurry for the MMP membranes) was
then poured onto a flat, level, and clean horizontal glass surface. Prior to film
deposition, the glass surface was cleaned with acetone, and the dust particles were
removed from the surface by using compressed inert gas. The film was formed
using a stainless steel film applicator (Gardco Adjustable Micrometer “Microm”
Film Applicator from Paul N. Gardner Co., Pompano Beach, FL) to draw-spread
the polymer-sieve slurry to a uniform thickness. Prior to casting, if particulates
were observed in solution, the polymer solution was filtered with a PTFE 0.2-µm
syringe filter (Fisher or Cole-Parmer) and a syringe. Casting of all the films was
124
performed at room temperature. To control the evaporation rate and prevent dust or
particulate contamination, the wet film was covered by a large, inverted glass dish
to prevent the formation of temperature gradients. The solvent from the mixed
matrix film was first allowed to evaporate overnight. The membrane film was
further dried, after the initial evaporation, at a temperature of 373 K for at least 12
h in a vacuum oven to remove the residual solvent and moisture. After evaporation
the resulting MMP films had a thickness in the range of 25-55 μm.
3.3.1.2 Experimental Set-up
The flat membrane film permeation system included a chamber containing a
membrane cell, an absolute pressure transducer (MKS Instruments, Andover, 626
A, MA, Baratron®, 0.10 torr range) for the downstream, a pressure gauge for the
upstream, stainless steel tubing, bellow seal valves (Swagelok), and Cajon VCR®
(Swagelok) metal face seal connections (see Fig. 3.7). A two-stage mechanical
vacuum pump (Vacuum, Alcatel) was used to apply vacuum on the upstream and
downstream. All measurements in this study with flat membranes were at room
temperature.
To measure the transport characteristics of the MMP membrane films, the
constant volume method (Pye, 1976 a, b) was utilized. This method involves
125
maintaining a constant pressure of a gas penetrant (the permeability of which is to
be measured) on the upstream face of the membrane, and measuring the gas flow
across the membrane film of known thickness and area. The flow (or permeation
rate) is measured from the pressure rise due to the permeating gas into a known
constant downstream volume, using the ideal gas law in order to calculate the flow.
V- - 2
Retentate
Membrane
Feed
H
2
CH4
V- - 2
P
Needle Valve
Vacuum
pump
Vacuum Gauge
V- - 2
V- - 2
V- - 2
Permeate
Volume (V)
V- - 2
Retentate
Membrane
Feed
H
2
CH4
V- - 2
P
Needle Valve
Vacuum
pump
Vacuum Gauge
V- - 2
V- - 2
V- - 2
Permeate
Volume (V)
Figure 3.7 Permeation apparatus for testing flat membrane films.
Inside the membrane cell in Fig. 3.7, the flat membrane film was masked
[with a procedure similar to Vu et al. (2003 a), and references therein] with
adhesive aluminum foil having a circular, pre-cut, exposed area for permeation
through the membrane. The aluminum masks were obtained from Avery Dennison
Specialty Tape Division (Pasadena, CA) and contained a pressure-sensitive
Fasson® (Model 800, 802) acrylic-based adhesive with single aluminum foil face-
stock on one side and paper liner on the other side. The outer rim of the
126
“sandwiched” mask was cut to reduce the diameter and a third circular mask was
used to mask the sandwiched mask containing the membrane film to the actual
permeation cell. Thus, the adhesive in the annulus (or outer rim) of the third mask
provided the seal to the lower half of the permeation cell. The masking procedure
is depicted in Fig. 3.8.
Figure 3.8 Cross-sectional view of the final mask containing the membrane
film where the annulus region (adhesive) of the top aluminum mask forms
the seal with the lower portion of a membrane cell.
The permeation cell consisted of two custom-machined cylindrical stainless
steel halves. Before the “sandwiched” mask was placed in the cell, 10 to 20
thickness measurements were taken of the membrane film using a dial gauge
micrometer (Chicago Brand) with a resolution of 0.00005 inch (or 0.05 mil). An
average thickness was used for the permeability calculations.
Generally, a 4.5 cm diameter, circular piece of medium-porosity filter paper
(Fisherbrand) was first placed over the sintered metal support to provide extra
127
cushioning of the membrane film against the rough surface of the sintered metal
support. The “sandwiched” mask was placed last at the center of the lower half of
the cell, and the third mask was carefully applied to secure the masked film to the
cell. Without the top half, the lower half of the permeation cell was attached to the
downstream section of the permeation system. Vacuum was then applied on the
film for approximately 1 to 2 h. 5-min epoxy (Devcon, Danvers, MA) or Stycast®
2651 (Emerson & Cuming, Billerica, MA) was applied to the interface between the
membrane and the aluminum mask to prevent non-selective gas flow between the
aluminum mask adhesive and the membrane. After curing the epoxy by applying
more than 12 h continued vacuum on the film and the downstream, the top half of
the permeation cell was placed firmly by pressurizing the lower half, and the entire
cell was attached to the permeation system.
Both the upstream and downstream sections of the permeation cell and the
entire system were evacuated for about 24 h to 48 h in order to be degassed.
Permeation tests of the membrane were performed by introducing in the top cell
the desired gas at the desired pressure. The permeation rate was measured at
steadystate. Following the permeation testing of a given gas, both the upstream
and downstream sections were again evacuated for about 12 to 24 h before
permeation testing of the next gas. For the pure gas permeation tests, compressed
gas cylinders of H
2
, CO
2
, and CH
4
were supplied by the Southern California Gas
128
Company. These gases were either ultra high pure (UHP) or instrument grade
(i.e., >99.00%).
3.3.1.3 Gas Permeation Properties of Nanocomposite MMP
Membrane Films
To measure the transport characteristics of the MMP membrane films, the
constant volume method (Pye, 1976 a, b) was utilized, as previously described.
This method involves maintaining a gas penetrant at a constant pressure (the
permeability of which is to be measured) on the upstream face of the membrane,
and measuring the gas flow across the membrane film of a known thickness and
area. The flow (or permeation rate) is measured from the pressure rise due to the
permeation of the gas into a known constant downstream volume, using the ideal
gas law in order to calculate the flow.
The gas permeability,
i
P [in barrer=1×10
-10
cm
3
(STP).cm/cm
2
.s.cm Hg], is then
calculated from the following equation:
dt
dP
P
AT
Vd
P
i
)
7 . 14
76
(
760
10 273
0
10
×
×
= (3.4)
where V is the volume of the downstream chamber (cm
3
), A is the effective area of
the film (cm
2
), d is the thickness of the membrane (cm), P
0
is the pressure of the
129
feed gas in the upstream chamber (psia), T is the absolute temperature during the
measurement (K), and
dt
dP
is the rate of pressure change in the downstream
chamber (mm Hg/s).
3.3.1.4 Single-Gas Permeation Measurements of the MMP
Membrane Films
We evaluated the separation characteristics of the MMP membrane films using
pure gas permeability measurements with the constant-volume method. Table 3.2,
for example, presents data of two MMP membrane films taken at 293 K with an
upstream pressure of 40 psi.
Table 3.2 Pure gas permeation data for the MMP membranes
at various loadings of the CMS nanoparticles.
Mixed-Matrix Films Permeability (Barrer) Permselectivity
CO
2
CH
4
CO
2
/CH
4
Pure PEI 1.24 0.031 40.5
20 wt.% CMS 1.78 0.045 39.4
30 wt.% CMS 2.60 0.067 38.4
Both films have a thickness of 35 μm, but contain different amounts of the
CMS. Both the CO
2
and CH
4
permeability increased with increasing carbon
content, while the CO
2
/CH
4
selectivity was only slightly reduced or unchanged.
130
Table 3.3 shows measurements (at 293 K and an upstream pressure of 40 psia)
with a MMP membrane with a different thickness (~55 μm). Though the
permeability values are generally similar to those found with the smaller thickness
membranes, the separation factors are significantly higher. A slight decrease in the
separation factor for CO
2
/CH
4
is observed, in line with the data in Table 3.2.
However, one observes larger drops in the H
2
/CH
4
separation factor.
Table 3.3 Pure gas permeation measurements of MMP membranes.
Mixed-Matrix Films Permeability (Barrer) Permselectivity
CO
2
CH
4
H
2
H
2
/CH
4
CO
2
/CH
4
Pure PEI 1.23 0.021 4.08 194.5 58.0
18 wt% CMS 1.50 0.027 3.98 146.7 55.2
Fig. 3.9 shows the CO
2
permeabilitiy and the CO
2
/CH
4
ideal separation factor
for the two different MMP membranes (MM-PM) described above. The same
permeation properties of the MM films (Vu et al., 2003 a), prepared using
Ultem®1000 as a matrix with various loadings of the CMS inserts (those obtained
via pyrolysis of Matrimid® 5218 at 800
◦
C), are also shown in the same figure.
Though our results and those of Vu et al. (2003 a) indicate an increase in
permeability with increasing the CMS content, the results of Vu et al. (2003 a) also
indicate an increase in the ideal separation factor, which is not true for the
131
membranes prepared by us. On the other hand, the separation factor of our own
membrane (with thickness of 55 μm) is higher than that of Vu et al. (2003 a).
Figure 3.9 CO
2
permeability and ideal separation factor of MMP films
132
3.3.2 Mixed-Matrix CMS Tubular Membranes
We now describe the results obtained with the MM-CMS tubular membranes.
3.3.2.1 Preparation of the MM-CMS Tubular Membranes
Supported MM-CMS membranes were also prepared by a dip-coating method
using the CMS nanoparticle slurries. Tubular porous alumina support substrates
(7 mm i.d., 10 mm o.d., and 4.5 cm long), manufactured by U.S. Filter, containing
a thin inside layer of γ–alumina, were utilized. The CMS slurry was prepared as
described previously.
To prepare the MM-CMS membranes, the outer surface of the substrate was
wrapped with Teflon tape. The substrate was then dip-coated into a solution
containing 6 wt.% of total solids (PEI+CMS) for 3 min, and was then pulled out of
the solution at a constant pulling rate of 2 cm/min. After coating, the membrane
was dried in air for 24 h, and was carbonized in the presence of flowing argon
(99.997% pure) in a cylindrical furnace, 0.0508 m diameter and 0.6096 m long,
externally heated and controlled by a programmable Omega CN3000 controller.
The membrane was placed between two thermocouples about 0.05 m apart with
the tip of the thermocouples being close to the outside surface of the membrane, in
133
order to make sure that no temperature gradient exists along the axis of the
membrane. An Ar flow rate of 1-1.33 × 10
-6
m
3
/s was used to remove the gases
evolved during carbonization and to maintain the inert atmosphere.
The carbonization procedure used was the same as the pyrolysis procedures
for the PEI pellets described above. The coating/carbonization procedure was
repeated as many times as needed in order to modify the selective layer and
achieve as high a separation factor as possible, while maintaining a desirable
permeance level. The permeation of CH
4
and CO
2
through the membrane was
measured, after each coating/carbonization step, in order to evaluate the
performance of the membrane.
3.3.2.2 Experimental Set-up
The experimental apparatus utilized for the permeation measurement of the
MM-CMS membranes has been described in detail elsewhere (Champagnie,
1992). Argon, methane, hydrogen, and carbon dioxide were all supplied from
high-pressure gas cylinders equipped with regulators. The flow rates were
controlled with Tylan FC-260 mass flow controllers. All the gases used were
99.95% pure or better. They were, in addition, purified by passing them through
Drierite, to capture any water vapor impurity that might be present. They were then
134
fed into the testing module containing the MM-CMS membranes. The testing
module was made of 316 SS and was supplied with inlet and outlet ports for its
tube and permeate sides. Fig. 3.10 shows schematics of the membrane module and
the γ-alumina substrate.
Figure 3.10 Schematic of membrane module and γ-alumina substrate.
The membrane was sealed in the module using Vition O-ring and compression
fittings. The permeate- and tube-side effluents (for the mixed-gas studies) were
measured online using a Varian 3400 gas chromatograph. The tube side pressure
Shell side inlet Pp
1
Membrane
Tube side outlet
P
t2
Shell side outlet Pp
2
Tube side inlet
P
t1
Tc Tc Tc
135
was controlled by a needle valve placed in the outlet. The permeate side pressure
was maintained at atmospheric pressure. Pressure in the system was measured with
an accuracy of 0.1 psia by Omega DP2000 pressure transducers. The tube-side
pressure was controlled by a needle valve placed in the outlet. The permeate side
was maintained at atmospheric pressure. Measurement of the outlet gas flow rates
was accomplished using a soap-bubble flow-meter. Since for the MM-CMS
membranes the thickness of the permselective layer is not precisely known, the
permeance (
j
P = d P
e
/ ) in engineering units of (P
j
) is reported in lieu of the
permeability which is calculated by
) )( 2 (
0
0
j m
m
j
p RL p T
T QP
P
Δ
=
π
(3.5)
where Q is the volumetric flow of the gas across the membrane (cm
3
/min), P
m
is
the measurement pressure (psia), T
m
is the measurement temperature (K), L is the
membrane length (cm), R is the membrane radius (cm), P
0
= 14.969 psia, and
T
0
= 273.15 K,
j
p Δ is the log-mean pressure difference between the tube and
permeate side defined by
2 2
1 1
2 2 1 1
log
) ( ) (
p t
p t
p t p t
j
P P
P P
P P P P
P
−
−
− − −
= Δ (3.6)
where P
p1
and P
p2
are the test pressures at the permeate side inlet and outlet, and
P
t1
and P
t2
the pressures at the tube side inlet and outlet, all for species j in psia,
respectively.
136
3.3.2.3 Single-Gas Tests for the MM-CMS Tubular Membranes
The single-gas permeation tests for the MM-CMS membrane indicated that
the membrane permeances and separation factors are a function of the number of
coating/carbonization steps. Fig. 3.11, for example, shows the permeance and the
corresponding ideal separation factors for CO
2
and CH
4
.
0
20
40
60
80
100
120
140
1 2 3 4
No. of Layers
Permeance
(10
8
mole/cm
2
.s.Pa)
0
20
40
60
80
100
120
CO
2
/CH
4
Sepn. Factor
CO2 CH4 CO2/CH4
Figure 3.11 The effect of number of layers on the permeance and ideal separation
factor of the membrane with a carbon loading of 10 wt.%; T=293K, P Δ =30psi.
After the first coating/carbonization step, the permeances of both gases
substantially decreased. The permeance of CO
2
showed only slight decrease after
the second coating/carbonization step. However, the ideal separation factor
increased with each additional layer up to 4 layers. The increase in the separation
137
factor may be attributed to some pore structure narrowing, but more likely is due
to the additional coatings tending to repair the existing pinholes and cracks. When
the number of coating layers rose above 4, the separation factor decreased, which
might be due to cracks developing with in the membrane for high carbon loadings.
Pure-gas permeation data with CO
2
, CH
4
, H
2
and N
2
with a MM-CMS
membrane (at 293 K and P Δ =30psi) that has been coated with four layers of the
PEI, containing 10 wt.% CMS, are shown in Table 3.4. A very high CO
2
/CH
4
separation factor of 120 was obtained with a CO
2
permeance about 9.0×10
-8
mol/m
2
.s.Pa. The corresponding H
2
/CH
4
separation factor was 130 with a H
2
permeance of 9.7×10
-8
mol/m
2
.s.Pa. It is observed that the permeation rate of these
gases through the membrane correlated well with the kinetic diameter, instead of
the molecular weight of the gases, and decreases as the molecular sizes of gases
increase, indicative of the molecular sieving nature of the membranes. This means
that the microporosity of the carbon film is very narrow and, as a consequence, it
can discern between the gas molecules, depending on their molecular size.
Table 3.4 Permeance and ideal separation factor of a four-times
coated MM-CMS membrane for different gases.
Permeance (10
8
mol/m
2
.s.Pa) Permselectivity
H
2
CO
2
N
2
CH
4
H
2
/CH
4
CO
2
/CH
4
H
2
/N
2
CO
2
/N
2
9.7 9.0 0.14 0.074 130.4 120.3 69.2 63.8
138
Comparison between the performance of the MM-CMS membrane (at a CMS
loading of 10 wt.%, T=293K, and P Δ =30psi) with that of a pure CMS membrane
(one that was made from pure PEI without any CMS particle) is shown in
Table 3.5. It can be seen that the incorporation of the CMS particles greatly
enhances the membrane ideal separation factor. The mechanism by which the
dispersed CMS particles enhance the membrane permeation characteristics is still
unclear, and is currently under investigation.
Table 3.5 Comparison of performance of the
MM-CMS membrane with a pure CMS membrane.
CMS membrane without carbon loading
Layer 1
st
2
nd
3
rd
4
th
CO
2
59.0 10.4 1.29 1.4
Permeance
(10
8
mol/m
2
.s.Pa)
CH
4
17.8 0.68 0.027 0.02
Sepn. Factor CO
2
/CH
4
3.32 15.2 47.8 67.2
MM-CMS membrane with 10wt.% carbon loading
Layer 1
st
2
nd
3
rd
4
th
CO
2
123.5 10.7 8.94 8.91
Permeance
(10
8
mol/m
2
.s.Pa)
CH
4
6.86 0.17 0.09 0.07
Sepn. Factor CO
2
/CH
4
18.0 64.0 95.1 119.6
139
3.3.2.4 Mixture Gas Tests for the MM-CMS Membrane
Table 3.6 shows mixed-gas (CO
2
+CH
4
) permeation results for the four-times coated
MM-CMS membrane with a CMS loading of 10 wt.% (at T=293K and P Δ =30psi), the
single gas permeation data of which were reported above. Both the CO
2
permeability
and CO
2
/CH
4
separation factor in the binary mixture were lower than the separation
factor measured from the single-gas permeation experiments (Table 3.5). The
experiments indicate that the separation factor of CO
2
/CH
4
decreases with increasing
the CH
4
content in the feed composition. The same trends were observed with ternary-
gas mixture (H
2
/CO
2
/CH
4
) experiments. For example, a membrane with an ideal
CO
2
/CH
4
separation factor of 17.4, and a H
2
/CH
4
separation factor of 18.2 was tested
with a ternary mixture. However, the CO
2
/CH
4
separation factor in the ternary mixture
was found to be 7.33, while the H
2
/CH
4
separation factor was 2.35. We are not certain
what causes the differences in the separation factors between the single-gas and mixed-
gas experiments. It appears that the CH
4
permeance in the single-gas and binary-gas
experiments are very similar but, the CO
2
permenaces are quite lower in the mixed-gas
experiments. One explanation would be poisoning and plugging of the micropores of
the MM-CMS membranes through which CO
2
transports (the mixed-gas experiments
were carried later than the single gas-experiments). Whatever adsorbs in the
micropores appears to leave the mesopores through which CH
4
transports is
unaffaected.
140
Table 3.6 Mixed-gas permeation results for the
four-times coated MM-CMS membrane.
Gas Mixture (30psi) Permeance (10
8
mol/m
2
.s.Pa) Permselectivity
CO
2
CH
4
CO
2
/CH
4
50%CO
2
+ 50%CH
4
1.19 0.097 12.27
75%CO
2
+ 25%CH
4
2.16 0.077 27.9
90%CO
2
+ 10%CH
4
3.78 0.060 63.0
Pure CMS membranes prepared using Media & Process Technology (M&PT),
Inc. substrates (tubular porous γ-alumina support substrates with 3.5 mm i.d., and 7
mm o.d. being cut about 8.0 cm long) and tested with quaternary mixtures
containing CO, show good correlations between the single-gas and mixed-gas
separation factors, as shown in Table 3.7.
Table 3.7 Data of quaternary gas mixture permeation, using the M&PT substrate.
Single Gas, 30pisg Quaternary Gas, 45psig
Permeance
10
8
mol/m
2
.s.Pa
Sepn. Factor
Based on H
2
Permeance
10
8
mol/m
2
.s.Pa
Sepn. Factor
Based on H
2
H
2
38.8 1.0 48.4 1.0
CO 1.29 29.7 1.08 43.4
CO
2
3.88 10.0 3.67 13.4
CH
4
0.53 73.1 0.50 97.7
141
3.4 Supported Mixed Matrix Membranes Using the Second Batch
of CMS Nanoparticles
In this section we report on the preparation of MM-CMS membranes prepared
using the M&PT substrates. Compared to the US Filter substrates, the M&PT
substrates have a smaller inside diameter, and a somewhat less uniform top
mesoporous alumina layer. We conducted a set of experiments in order to study the
effect of the total solid fractions, as well as the CMS loadings, on the permeation
properties of the MM-CMS and CMS membranes made using M& PT supports.
3.4.1 Effect of the CMS and Total Solids Loading
To study the effect of the CMS and/or of total solids fractions on the
performance of the MM-CMS membranes, we carried out two different sets of
experiments (see Table 3.8):
1. We kept the total solids fraction in the slurry solution constant (at 6 wt.% for
the first coating and 2 wt.% for the subsequent coatings), while changing the ratio
of (MS/total solid from 0-15 % for the first coating and 0-12% for the second and
subsequent coatings (cases M
1
-M
3
in Table 3.8, part a).
142
Table 3.8 Experimental conditions for the MM-CMS & CMS membranes
a 1
st
coat 2
nd+
coat
%
No.
Type
TS CMS/TS PEI CMS/TS
M
1
CMS 6 0.0 6 0
M
2
MM-CMS 6 ~4 <6
M
3
MM-CMS 6 ~10 >5
M
4
MM-CMS <6 ~7 >5
M
5
MM-CMS <6 ~4 >5
M
7
MM-CMS 5 ~12 <5
0-12
M
6
CMS 5.4 0.0 5.4 0
b 1
st
coat 2
nd
coat 3
rd
coat and after
PEI%
M
8
CMS 5 2 2
M
9
CMS 4 2 2
M
10
CMS 4 4 2
M
11
CMS 3 2 2
TS: Total Solid
2. We also worked with lower fractions of total solids (less than 6), as also
indicated in Table 3.8 (cases M
4
-M
11
).
3.4.2 Mixed-Matrix and Pure CMS Tubular Membranes
The formation of MM and pure-CMS membranes followed the same
procedure as previously reported for the formation of supported MM-CMS
membranes using the US filter substartes. In this part of our study, tubular porous
alumina substrates (3.5 mm i.d., and 7 mm o.d., cut to about 8.0 cm long glazed at
both ends), manufactured by the M&PT and containing a thin inside layer of
143
γ–alumina, were utilized. The permeances of Ar, N
2
, and He, and the ideal
selectivities using the He/Ar and He/N
2
gas pairs were used to evaluate the
performance of each substrate. The closer these selectivities are to the ideal
Knudsen values the more ideal the substrates are thought for further deposition.
To prepare the MM-CMS and CMS membranes, the outer surface of the
substrate was again wrapped with Teflon tape. It was then dip-coated by a solution
containing the desired concentrations of the solids (PEI+CMS, based on Table 3.8)
for 3 min, and was then pulled out of the solution at a constant pulling rate of 2
cm/min. After coating, the membrane was dried in air for 24 h, and carbonized in
the presence of flowing argon (99.997% pure) in a cylindrical furnace (with the
same procedure as before). The coating-carbonization procedure was repeated as
many times as needed in order to modify the selective layer and achieve as high a
separation factor as possible, while maintaining a desirable permeance level. The
permeation of CH
4
, CO
2
, and H
2
through the membrane was then measured, after
each coating-carbonization step, in order to evaluate its performance. Unless
specified otherwise, all the measurements were at room temperature and a pressure
difference of about 30 psig.
144
3.4.3 Effect of the Total PEI Fraction and CMS Content on the
MM-CMS Membranes
To investigate the influence of the fraction of the PEI in the slurry used to
prepare the first layer, we prepared several polymeric membranes using the PEI
solutions with different concentrations ranging, from 3 to 6 wt.%. The permeation
characteristics of these membranes were tested, and are shown in Table 3.9.
Table 3.9 Comparison of the performance of the supported polymeric
membranes with a different PEI% using the M&PT substrates.
without carbon loading (all 1
st
coat)
PEI% 3% 4% 5% 5.4% 6%
H
2
8 4.16 5.25 4.27 4.52
CH
4
0.86 0.28 1.69 0.97 1.72
Permeance
10
8
mol/m
2
.s.Pa
CO
2
2.13 1.07 1.74 1.23 1.53
H
2
/CH
4
9.27 15 3.12 4.41 2.63
Permselectivity
CO
2
/CH
4
2.47 3.84 1.03 1.26 0.89
As seen in Table 3.9, the permeance for all three gases decreases by increasing
the fraction of the PEI up to 4%, while the ideal separation factors increases.
Beyond this concentration of the PEI, performance significantly deteriorates. We
attribute this to the increasing viscosity of these solutions, which makes it difficult
to deposit thin polymeric layers on the inside surface of the smaller diameter
M&PT membranes without defects and voids.
145
Unfortuantely, despite the fact that coating with the 3% and 4% PEI solutions
results in better quality polymeric membranes, we were not able upon further
pyrolysis to prepare good quality CMS membranes. Table 3.10 shows the
permeation characteristics of two different membranes, namely, M
9
and M
11
(see
Table 3.8 for further details about the preparation procedure of these membranes).
Note that the ideal selectivity of both membranes (despite the deposition of five
layers for membrane M
9
, for example) is close to the Knudsen values, indicative of
the presence of many defects and micro-voids.
Table 3.10 Permeance and ideal selectivities for membranes M
9
and M
11
without carbon loading M
9
M
11
Layer 4
th
5
th
2
nd
H
2
358.74 123.42 965.20
CH
4
144.23 49.95 584.16
Permeance
(10
8
mol/m
2
.s.Pa)
CO
2
121.96 46.74 428.64
H
2
/CH
4
2.49 2.47 1.65
Permselectivity
CO
2
/CH
4
0.85 0.94 0.73
Table 3.11 shows the permeation characteristics of a third membrane, M
10
(see
Table 3.8 for further details about the preparation procedure). Again, the results are
disappointing.
Experiments are currently under way to reproduce these observations in order
to reach a definite conclusion as to whether the poor performance of the resulting
146
membranes, does indeed relate to the lower concentrations of the PEI solutions
utilized.
Table 3.11 Permeance and ideal selectivities for membrane M
10
CMS membrane without carbon loading, M
10
Layer 1
st
2
nd
3
rd
4
th
H
2
509.15
216.86 100.80
28.94
CH
4
248.75
100.77 46.25
13.1
Permeance
10
8
mol/m
2
.s.Pa
CO
2
173.93
85.94 38.45
10.1
H
2
/CH
4
2.05 2.15 2.18 2.21
Permselectivity
CO
2
/CH
4
0.67 0.85 0.83 0.77
We also tested membranes M
1
-M
6
(see Table 3.8) after the first layer was
deposited and prior to the pyrolysis, and the results are shown in Table 3.12. As
can be seen, the M
4
and M
5
membranes show the best ideal separation factors.
Table 3.12 Permeance and ideal selectivities for membranes M
1
-M
6
.
(all after desposition of the first layer and before pyrolysis)
condition M
1
M
3
M
4
M
5
M
6
H
2
4.52 3.25 2.5 4.16 4.27
CH
4
1.72 0.74 0.24 0.72 0.97
Permeance
10
8
mol/m
2
.s.Pa
CO
2
1.53 0.74 0.72 1.12 1.22
H
2
/CH
4
2.63 4.38 10.43 5.77 4.41
Permselectivity
CO
2
/CH
4
0.89 1 2.98 1.55 1.26
147
Table 3.13 shows the performance of the M
6
membrane, which is a pure CMS
membrane. Though it shows ideal separation factors higher than Knudsen, this
membrane is not highly permselective. Table 3.14 shows the permeation
characteristics of membrane M
5
, which is a MM-CMS membrane. Up to the third
coating this membrane shows excellent separation characteristics. After deposition
of a 4
th
layer, however, the membrane performance seems to deteriorate.
Table 3.15 shows the permeation performance of membrane M
4
, which is a
MM-CMS one. In contrats to membrane M
5
the performance of this membrane is
not very good indicating mostly Knudsen-like type of transport. This is despite the
fact that membrane M
4
had a better performance before pyrolysis than M
5
. This, of
course, is indicative of the fact that the preparation technique involves a number of
factors that we still cannot control and, therefore, the success rate in making good
membranes is still rather low.
Table 3.13 Performance of membrane M
6
5.4wt.% of PEI for the 1
st
coat
Layer 1
st
2
nd
3
rd
4
th
H
2
34.48 7.39 5.11 4.02
CH
4
74.10 2.59 0.95 0.53
Permeance
10
8
mol/m
2
.s.Pa
CO
2
101.05 12.81 1.34 2.71
H
2
/CH
4
0.47 2.85 5.4 7.65
Permselectivity
CO
2
/CH
4
1.36 4.94 1.41 5.16
148
Table 3.14 Performance of membrane M
5
With ~4 wt. % carbon loading (PEI >5 wt.%)
Layer 1
st
* 2
nd
3
rd
4
th
H
2
71.47 5.26 1.25
CH
4
33.07 0.054 0.036
Permeance
10
8
mol/m
2
.s.Pa
CO
2
48.55 5.53 4
H
2
/CH
4
2.16 98.11 35.05
Permselectivity
CO
2
/CH
4
1.47 103.3 39.33
*1
st
layer has been measured at 125°C
Table 3.15 Performance of membrane M
4
With ~7 wt% carbon loading (PEI >5 wt.%)
Layer 1
st
2
nd
3
rd
4
th
H
2
673.67 216.94 99.78 66.12
CH
4
331.69 69.74 37.41 30.1
Permeance
10
8
mol/m
2
.s.Pa
CO
2
339.58 129 35.41 23.28
H
2
/CH
4
2.03 3.11 2.67 2.2
Permselectivity
CO
2
/CH
4
1.02 1.85 0.95 0.77
Tables 3.16-3.18 show the permeation performance for membranes M
1
-M
3
. All
these membranes were made by keeping the total solid wt.% fraction constant at
6%, and the CMS fractional loading from 0-15%. Unfortunately, the MM-CMS
membranes (M
2
and M
3
) exhibit behavior which is ifferior to that of the pure CMS
membrane, indicating again the difficulty in the preparation of these membranes.
Once more it must be emphasized that this work is preliminary, and work in this
area is continuing.
149
Table 3.16 Performance of membrane M
3
With ~10 wt.% carbon loading (TS=6, PEI=~5.4 wt. %)
Layer 1
st
2
nd
3
rd
4
th
H
2
73.59 11.32 1.94 Cont.
CH
4
30.36 4.35 0.71
Permeance
10
8
mol/m
2
.s.Pa
CO
2
36.55 6.51 1.23
H
2
/CH
4
2.42 2.6 2.72
Permselectivity
CO
2
/CH
4
1.20 1.5 1.73
Table 3.17 Performance of membrane M
2
With ~4 wt. % carbon loading (TS=6)
Layer 1
st
2
nd
3
rd
H
2
52.71 27.28 Cont.
CH
4
75.042 18.62
Permeance
10
8
mol/m
2
.s.Pa
CO
2
61 16.7
H
2
/CH
4
0.70 1.47
Permselectivity
CO
2
/CH
4
0.81 0.9
Table 3.18 Performance of membrane M
1
made with 6wt. % of the PEI
solutions (1
st
coat) and 2wt. % of the PEI solutions (2
nd+
coat).
without carbon loading (TS=6)
Layer 1
st
2
nd
3
rd
4
th
H
2
369.13 30.89
7 1.39
CH
4
232.13 8.031
0.31 0.087
Permeance
10
8
mol/m
2
.s.Pa
CO
2
219.07 9.20
6.33 0.73
H
2
/CH
4
1.59 3.85 22.53 15.94
Permselectivity
CO
2
/CH
4
0.94 1.14 20.4 8.32
150
Tables 3.19 and 3.20 show the characteristics of the membranes M
7
(a MM-
CMS membrane) and membrane M
8
(a CMS- membrane), for which the conditions
of preparation are shown in Table 3.8. They were made by keeping the total solid
constant at 5 wt.%, CMS fractional loadings of 0 and 12%. Unfortunately, neither
membrane shows good permeation characteristics. Work is continuing with
membranes made under the same conditions, in an attempt to understand the
reasons that lead to poor membrane performance.
Table 3.19 Performance of membrane M
7
With ~12 wt% of carbon loading (TS=5)
Layer 2
nd
3
rd
4
th
5
th
H
2
918.90
678.63 436.77
244.07
CH
4
358.97
233.63 151.32
90.35
Permeance
10
8
mol/m
2
.s.Pa
CO
2
312.97
224.27 142.86
80.49
H
2
/CH
4
2.56 2.95 2.89 2.70
Permselectivity
CO
2
/CH
4
0.87 0.96 0.944 0.89
Table 3.20 Performance of membrane M
8
without carbon loading (TS=5)
Layer 1
st
2
nd
3
rd
H
2
41.93 5.81 7.69
CH
4
16.64 0.84 2.8
Permeance
10
8
mol/m
2
.s.Pa
CO
2
88.60 3.27 3.02
H
2
/CH
4
6.95 2.52 2.75
Permselectivity
CO
2
/CH
4
3.92 5.32 1.08
151
Though the membranes described above are those that show poor permeation
characteristics (other than membrane M
5
), we are on occasion able to prepare
membranes with good separation charcteristics. An example of one of these
MM-CMS membranes (M
new
), which has single-gas separation factors as high as
340 and 120 at room temperature for H
2
/CH
4
and CO
2
/CH
4
, respectively, after six
coatings and carbonization, is shown in Tables 3.21 and 3.22.
Table 3.21 Mixed-gas resultsof membrane M
new
120°C
ΔP (psig)
Permeance
10
8
mole/m
2
.s.Pa
Permselectivity
based on H
2
H
2
32.8
9.02
1.0
CH
4
32.1
0.103
87.6
CO
2
33.3
1.88
4.8
Mixed H
2
:CH
4
:CO
2
=2.0:2.0:2.0, ΔP=30.7 psig
H
2
7.39
1.0
CH
4
0.13
55.7
CO
2
1.57
4.7
Table 3.22 Same as in Table 3.21, but at T=170°C.
170°C
ΔP (psi)
Permeance
10
8
mole/m
2
.s.Pa
Permselectivity
based on H
2
H
2
30.4 10.67 1.0
CH
4
30.6 0.29 36
CO
2
30.6 2.02 5.3
Mixed H
2
:CH
4
:CO
2
=2.0:2.0:2.0, ΔP=31.2 psig
H
2
9.90 1.0
CH
4
0.24 41.8
CO
2
1.83 5.4
152
These tables show mixed-gas results with an equimolar gas mixture
(H
2
+CO
2
+CH
4
) measured at 120°C and 170°C ( P Δ =30.7 and 31.2 psi),
respectively. As can be seen M
new
performs very well and is stable at the two
temperatures.
3.5 Conclusions
We have studied in this Thesis the preparation of MM-CMS membranes
through the use of the CMS nanoparticles. These submicron particles were
prepared by the pyrolysis of the PEI pellets and further reduction of their size by
conventional dry- and wet-grinding techniques. Two different batches of CMS
particles were prepared and used. The CMS particles in both batches have
transport properties that are very similar to those reported in the literature.
The focus of this study was on evaluating the technical feasibility of utilizing
the MM-CMS membranes in the separation of H
2
and CO
2
from gas mixtures.
Both flat (unsupported) and tubular supported membranes, pure and mixed-matrix
CMS were studied. Prior to the preparation of the supported membranes, the
starting MMP membranes were also evaluated.
For the flat membranes CO
2
and CH
4
permeabilities increased with increasing
the CMS content, while the CO
2
/CH
4
selectivity remained either unchanged or
153
slightly reduced. This may be due to the poor adhesion between the polymer and
the CMS particles.
For the MM-CMS membranes with the US filter support, the incorporation of
the CMS nanoparticles greatly enhanced their permeability and selectivity. An
ideal CO
2
/CH
4
separation factor of 120 was obtained with a CO
2
permeance
around 9.0 × 10
-8
mol/m
2
.s.Pa for membranes with a CMS particle loadings of
10 wt.%. The corresponding H
2
/CH
4
separation factor was 130 with a H
2
permeance of 9.7 × 10
-8
mol/m
2
.s.Pa.
The effect of the total solids fraction and the CMS content on the performance
of the CMS and MM-CMS membranes, made of the M&PT substrates, was also
studied in detail. It is more difficult to make good membranes with such substartes
as they have smaller diameters. It was observed that for a given concenteration of
the PEI, higher permeances and permselectivities can be achieved only within a
certain range of the CMS content.
Ideal CO
2
/CH
4
and H
2
/CH
4
separation factor of 103 and 98 were obtained
with one of the membranes with a CMS particle loading of about 4 wt. %, based
on single-gas tests using the M&PT substrates.
Another membrane showed single-gas separation factors of 120 and 340 for
CO
2
/CH
4
and H
2
/CH
4
, respectively, using the M&PT substrate. This membrane
154
also exhibited very good performance and stability at 170°C and 120°C based on
the equimolar ternary gas mixture of H
2
, CO
2
, and CH
4
.
The proposed novel nanocomposite MM membranes show great potential for
application in the separation of mixtures of H
2
and CO
2
which is of prime
importance in a number of important processes of significance to environmentally-
benign power generation, including the utilization of renewable fuels and
hydrogen-based fuel cells.
Our focus in our ongoing studies is on simultaneously increasing the
permeances and permselectivities by changing the total solid fraction, CMS
content, and several other experimental conditions. We are currently working on
reducing the number of coatings, and carbonization steps needed by optimizing the
aforementioned variables.
155
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Abstract (if available)
Abstract
The goal of the first part of the Thesis is to prepare carbon molecular-sieve (CMS) particles to be utilized in the preparation of mixed-matrix (MM) membranes. Thus an experimental investigation was carried out using electrospray of polyetherimide (Ultem-1000 PEI) solutions in dichloroethane to produce fairly monodisperse, fine PEI particles. The effect of three key experimental parameters was investigated, namely, the applied voltage, the liquid flow rate, and the polymer concentration. The liquid flow rate was found to have the most important effect in determining the particle size. An optimal range of flow rates often exists. Particles obtained within the optimal range of the flow rate have a narrower size distribution, and a dense and spherical morphology, compared with those produced with other liquid flow rates. The CMS particles, prepared ex-situ by the pyrolysis of the electrospray PEI particles, are compared in their properties with the CMS particles that were generated by conventional grinding of pyrolyzed PEI pellets. They were found to, generally, have similar structural properties.
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Creator
Bagheri-Tar, Faezeh
(author)
Core Title
Preparation of polyetherimide nanoparticles by electrospray drying, and their use in the preparation of mixed-matrix carbon molecular-sieve (CMS) membranes
School
Viterbi School of Engineering
Degree
Doctor of Philosophy
Degree Program
Chemical Engineering
Publication Date
12/17/2009
Defense Date
11/01/2007
Publisher
University of Southern California
(original),
University of Southern California. Libraries
(digital)
Tag
carbon molecular-sieve (CMS),electrospray,mixed-matrix membrane,nanoparticles,OAI-PMH Harvest,plyetherimide (PEI),separation of gases
Language
English
Advisor
Tsotsis, Theodore T. (
committee chair
), Sahimi, Muhammad (
committee member
), Williams, Travis J. (
committee member
)
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bagherit@usc.edu
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https://doi.org/10.25549/usctheses-m983
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texts
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(contributing entity),
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cisadmin@lib.usc.edu
Tags
carbon molecular-sieve (CMS)
electrospray
mixed-matrix membrane
nanoparticles
plyetherimide (PEI)
separation of gases