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On the use of membrane reactors in biomass utilization

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On the use of membrane reactors in biomass utilization

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Content i

ON THE USE OF MEMBRANE REACTORS IN BIOMASS UTILIZATION
By Mingyang Tan
UNIVERSITY OF SOUTHERN CALIFORNIA

ii

Contents
List of Tables ................................................................................................................................................... iii
List of Figures ................................................................................................................................................. iii
ABSTRACT ....................................................................................................................................................... iv
1. INTRODUCTION ..................................................................................................................................... 1
2. EXPERIMENTAL SYSTEM AND PROCEDURES ......................................................................... 10
2.1. Experimental System ................................................................................................................. 10
2.2. MR Experiments .......................................................................................................................... 12
2.3. Packed-Bed Reactor Experiments ......................................................................................... 13
3. RESULTS AND DISCUSSION ............................................................................................................. 14
3.1. Single Gas Permeation Before and After Experiments ................................................. 14
3.2. Kinetics of the Reactions .......................................................................................................... 16
3.3. CMSM and Catalyst Performance with Presence of Toluene and Naphthalene ... 18
3.4. Comparison of CO Conversion between MR and Packed-Bed Reactions ............... 20
3.5. CO Conversion under Different Experimental Conditions .......................................... 21
3.6. H 2 Recovery under Different Experimental Conditions ............................................... 22
3.7. H 2 Purity under Different Experimental Conditions ..................................................... 23
4. CONCLUSIONS ...................................................................................................................................... 26
5. REFERENCES ........................................................................................................................................ 28
6. ACKNOWLEDGEMENTS .................................................................................................................... 36
7. NOMENCLATURE ................................................................................................................................ 37
8. APPENDIX .............................................................................................................................................. 40

iii

List of Tables

Table 1 Different biomass feedstocks used for hydrogen production
11
.................................... 3
Table 2 Single-gas permeation data for the CMSM before and after the MR
experiments ................................................................................................................................................... 15
Table 3 WGS reaction rate expression parameters ......................................................................... 41

List of Figures

Figure 1Experimental set-up used in the membrane reactor experiments .......................... 10
Figure 2 Experimental vs. calculated CO conversion using the power law rate
expression
16, 46
under various packed-bed reactor experimental conditions ...................... 17
Figure 3 CO conversion under 3 bars of reactor pressure and at 300
o
C for different
sweep ratio ..................................................................................................................................................... 21
Figure 4 CO conversion under 5 bar of reactor pressure and at 300
o
C for different
sweep ratio. .................................................................................................................................................... 22
Figure 5 Hydrogen recovery under various experimental conditions .................................... 23
Figure 6 Permeate-side H 2 purity (dry basis) under various experimental conditions ... 24

iv

ABSTRACT
IGCC power plants show promise for environmentally-benign power generation.
In these plants coal and/or biomass are first gasified into syngas, which is then
processed in a water gas-shift (WGS) reactor to further enhance its hydrogen
content for power generation. However, impurities in the syngas, primarily H 2S, NH 3,
various organic vapors and tar-like species are detrimental to catalyst life and must
be removed before the gas enters the WGS reactor. This, today, means cooling the
syngas for clean-up and then reheating it to the WGS reaction temperature. For use
in various industrial applications, and potentially for CO 2 capture/sequestration,
hydrogen purification is required. This, today, is accomplished by conventional
absorption/desorption processes, which results in significant process complexity
and energy penalty for the overall plant. Ideally, one would like to establish a
“one-box” process in which the syngas is fed directly into the WGS reactor, which
then effectively converts the CO into hydrogen in the presence of all the
aforementioned impurities, and delivers a contaminant-free hydrogen product. In
this study, the development of such a process is described. It includes a catalytic
membrane reactor (MR) making use of a hydrogen-selective, carbon molecular sieve
membrane, and a sulfur-tolerant Co/Mo/Al 2O 3 catalyst. The membrane reactor’s
behavior has been investigated for different experimental conditions and compared
with the modeling results. The model is used to further discuss the scale-up of the
proposed process.
Keywords: Hydrogen Production, IGCC Power Plant, Process Intensification,
Membrane Reactor (MR), Carbon Molecular Sieve (CMS) Membrane, Water-Gas Shift
Reaction (WGS)
1

1. INTRODUCTION
Air pollution is today a serious problem throughout the world, especially in the
industrialized and developing regions, with power plant, factory and motor vehicle
emissions being major contributors. The use of hydrogen, as an alternative energy
source to substitute for fossil fuels, is generally acknowledged as a potentially
effective way for reducing such air emissions
1
.One of the benefits of adopting
hydrogen as a future energy source, in addition to reducing carbon dioxide
emissions, is that it can be produced from readily available resources like coal and
renewable biomass; this then diminishes the need to use the world’s dwindling
crude-oil resources.
Although hydrogen is an abundant element on earth, because of its high
reactivity it is very rarely found in its indigenous form as gas. As noted above, a
number of raw materials including fossil fuels, coal and renewable biomass can be
used to produce hydrogen. Coal is an attractive and relatively inexpensive raw
material to produce hydrogen as well as a key energy source for power generation in
a number of the largest economies of the world including the USA and China where
it is found as an abundant resource
2
.To produce hydrogen, coal must be first gasified
via the aid of an oxidant like air or pure oxygen at high temperatures
2
.This then
produces a gas mixture known as coal-gasifier off-gas or syngas that contains as key
species H 2, CO, CO 2, H 2O, CH 4, and other gaseous by-products such as various organic
vapors, tars, H 2S, NH 3, etc.
2
The exact syngas composition depends on the operating
conditions of the gasifier (e.g., pressure, temperature, coal and oxidant flow rates,
gasifier configuration, etc.), and the type of coal and oxidant used
2
. To produce
2

hydrogen, the syngas must be first cleaned of its undesired components that include,
in addition to the gaseous by-products noted above, various particulates like coke,
and inorganic matter known as ash
2
(for example, H 2S can be removed using a
granular solid sorbents
3-5
, via catalytic conversion
6
, or through a
solvent-based
7
absorption process). The clean syngas is then processed in a reactor
which converts the CO via its catalytic reaction with steam, known as the water gas
shift (WGS) reaction into additional hydrogen and carbon dioxide; the WGS reactor
effluent must be further processed to produce a pure hydrogen product
8
.When coal
(and/or biomass – see further discussion below) gasification and hydrogen
generation are combined with electricity production (e.g., via the use of the
hydrogen in a turbine or a fuel cell) this is known as the integrated gas combined
(power generation) cycle or IGCC
9
. IGCC power plants are attracting renewed
interest today because they are ideally suited for carbon dioxide capture for storage
and sequestration (CCS)
9
.
Another abundant energy source,and also a promising raw material for
hydrogen production, as well environmentally-benign power generation via IGCC, is
biomass
10
. The term biomass, as an energy source, encompasses a broad range of
materials, including lingo-cellulosic products such as wood and wood waste,
agricultural products and by-products, food processing and municipal waste, algae
and various other aquatic plants, etc
11
.– see Table 1 for a list of biomass types and
the various techniques applied for their beneficial use as energy sources.
Moderately-dried biomass can be directly used as a fuel (e.g., via co-firing in coal
power plants) for electricity generation. Lingo-cellulosic materials can be used to
produce liquid fuels (e.g., ethanol) via hydrolysis followed by fermentation of the
resulting sugars
11
.A broader range of liquid fuels can be produced via biomass
3

pyrolysis
11
,though the process is still not commercial.
For environmentally-benign power generation from biomass, IGCC offers the
best option (biomass has an added environmental advantage over coal, as it is a
renewable energy source and thus offers a much greater potential for CO 2emission
reductions). Here, as with coal-based IGCC, biomass is first gasified in the presence
of oxygen and steam to produce a gasifier off-gas containing similar main
components as with coal-derived syngas (albeit at different concentrations)
together with additional impurities, including H 2S, NH 3, and various high molecular
weight compounds known as tars
12,13
.

Table 1 Different biomass feed stocks used for hydrogen production
11

Biomass feedstock Major conversion technology
Kraft lignin Steam gasification
Almond shell Steam gasification
Peanut shell Pyrolysis
Tea waste Pyrolysis
Rice straw/Danish wheat straw Pyrolysis
Pine sawdust Steam reforming
Crumb rubber Supercritical conversion
4

Maple sawdust slurry Supercritical conversion
MSW Supercritical conversion
Composted municipal refuse Supercritical conversion
Starch biomass slurry Supercritical conversion
Micro-algae Gasification
Paper and pulp waste Microbial conversion

For IGCC power generation this syngas must be further reacted with steam in a
WGS reactor to enrich its hydrogen content. The WGS reaction is exothermic and is
favored by low temperatures, with equilibrium conversion decreasing as
temperature increases. Therefore, typically two reactors are needed to overcome
both equilibrium and kinetic limitations, and to increase the H 2 conversion
14
. One of
these, known as the high-temperature shift (HTS) reactor, operates at high
temperatures, using Fe/Cr-based catalysts, and the other, known as the
low-temperature shift (LTS) reactor, operates at lower temperatures using a
Cu/Zn-based catalyst
15
. As is the case with coal-derived syngas, the WGS reactor
product must be further processed in order to produce pure hydrogen and a CO 2
stream for further storage and sequestration. The whole process is complicated and
highly energy-intensive. In its place, our team has recently proposed -- see further
discussion to follow -- a novel process
16
, termed the “one-box” process that
substitutes the conventional dual-bed WGS reactor with a catalytic membrane
5

reactor (MR). By using a hydrogen-permselective carbon molecular sieve (CMS)
membrane and a sulfided Co/Mo catalyst, which are both resistant to the impurities
found in biomass-derived and coal-derived syngas, this novel process avoids the
need to use a separate syngas pre-treatment step as well as a hydrogen purification
step, thus significantly simplifying process design.
WGS membrane reactors are not new, however. Uemiyaet al.
17
were the first to
study an atmospheric pressure Pd-membrane WGS reactor treating pure CO and
using Ar as a sweep. At 400° C (with a stoichiometric H 2O/CO ratio) they achieved a
maximum CO conversion of 96%. Since the WGS reaction requires higher
temperatures, most studies today (with a few notable exceptions
18
) utilize
high-temperature membranes including dense Pd
19-24
, microporous silica
25-28
,
microporous zeolite
29
,and carbon molecular sieve membranes.
16,30-32

Pd or Pd-alloy membrane reactors have attracted most of the attention, starting
with earliest studied by Uemiyaet al.
17
noted above. Bi et al.
23
studied a MR
operating at 375 ° C and a pressure of 1.2 bar with a simulated syngas feed (H 2, 7%;
CO, 25%; CO 2, 15%; N 2, 53%) and N 2 sweep gas rate of 28.3cm
3
/min using a
conventional Co/Cr catalyst and a porous glass supported Pd membrane. They
obtained a maximum CO conversion of 98%.Augustine et al.
33
used a
WGS-membrane reactor to treat a simulated syngas mixture(H 2, 22.0%; H 2O,45.4%;
CO, 22.7%;CO 2, 9.9%)with dense Pd membrane supported on porous stainless steel.
They obtained a maximum CO conversion of 98% and H 2 recovery of 88% with a
H 2O/CO ratio of 2.6 at the reactor temperature of 450° C, feed side pressure of 14.4
bar for GHSV= 2900 h
-1
. Ma and coworkers
34
also studied the WGS reaction at
temperatures ranging 420—440 ° C and pressures ranging from 7 to 20 bar using a
6

larger size composite Pd membrane, a simulated syngas mixture (H 2, 40%; CO,
42.2%; CO 2 17.8%), and H 2O/CO ratios ranging from 2.5 to 3.5. They obtained a
maximum CO conversion of 98.1% and a H 2 recovery of 85.1% at 440
o
C, and a GHSV
= 1130 h
-1
.
As the above studies indicate, Pd membranes when used for the WGS
reaction(for a more recent concise review see
35
)can deliver high CO conversions,
and a high-purity hydrogen product; their main drawback for the WGS application
(other than the limited availability of the metal, which may ultimately, however,
hinder their widespread use for this large-scale application) is that they are
sensitive to the syngas impurities, particularly H 2S which adversely affects their
characteristics, even at single-digit ppm levels. Exposure of Pd to H 2S has been
shown to reduce its permeability, and to result into the formation of a surface scale
of Pd sulfide
36
, as well as in dramatic pitting of the membrane surface
37
. An
exhaustive clean-up step for the syngas, therefore, becomes necessary
38, 39
.
WGS membrane reactors making use of silica membranes have been studied
and shown excellent performance as well
40-42
.The drawback with silica membranes
for such a reaction is well known, in that silica undergoes condensation in the
presence of steam. Efforts to improve hydrothermal stability include
functionalization of the membranes using surfactants to form a hydrophobic silica
surface
40
, and incorporation into the silica structure of various metals
41, 42
and
carbon
43
during the preparation step, but all these efforts have, so far, found limited
success (microporous silica membranes, on the other hand, show good potential for
application to other reactions where the presence of steam is not required
44, 45
.
7

CMS membranes (CMSM), as a result of the challenges other types of
membranes face with the WGS reaction, have attracted recent attention
16, 30, 31, 38, 46
.
These membranes are made via the pyrolysis of polymeric precursors in various
atmospheres, important conditions influencing their properties being: (1) the type
of precursor utilized
47
; (2) the pyrolysis conditions such as the atmosphere and
heating protoclss
48
; and (3) post-pyrolysis modifications such as activation,
oxidation and stabilization
49
. Initial efforts by our group in using CMS membranes
for the WGS reaction
30, 31
were important to prove that these membranes show good
performance and good stability in the presence of high-temperature steam. However,
these studies were performed with feeds that did not contain the impurities
typically encountered in coal- and biomass-derived syngas. In two most recent
studies
16, 46
by our group, CMSM were utilized to treat simulated coal-derived
46
and
biomass-derived
16
containing realistic concentrations of H 2S and NH 3. In these
studies use was also made of the so-called sour-shift commercial catalysts
50, 51
, and
during the lab-scale investigations lasting more than a month both membranes and
catalysts exhibited good and stable performance in the temperature range of
250-350
o
C(recently Dong and coworkers
29
used silylated zeolite membranes to
carry out the HTS reaction in the presence of H 2S; though the reported steam
stability of the silica surfaces contrasts prior studies by other groups, see above, the
results are nevertheless promising and will, hopefully, provide impetus for the
further study and development of these membranes).
In addition to H 2Sand NH 3, the presence of organic vapors and of tar-like
species is a concern for the operation of WGS membrane reactors, particularly those
processing biomass-derived syngas. Appropriate modifications in gasifier design
and operating conditions along with using catalysts and additives, as well as novel
8

technologies such as supercritical water gasification
16
help to minimize tar
formation, and also to convert organic vapors in the syngas into H 2 and CO via steam
reforming. However, their presence in the syngas cannot be completely eliminated,
and they must, therefore, be taken into account during the design of downstream
systems for further syngas clean-up and processing. High concentrations of organic
vapors were previously shown, for example, to be highly detrimental to the
performance of glassy polymeric membranes used in the separation of
syngas-relevant mixtures. For example, White et al.
53
performed the separation of a
(CO 2/CH 4) binary mixture saturated with toluene using polyimide membranes. The
presence of the organic vapor reduced the selectivity by about 50%. Taniharaet
al.
53
also reported similar losses in selectivity (~85%) when separating an equimolar
(H 2/CH 4) mixture in the presence of (1600-7600) ppm of toluene vapor.
Accumulation of the tar-like species on the membrane surface and
condensation of organic vapors within the membrane pore structure, thus blocking
transport and leading to severe reductions in performance remained, prior to recent
studies by our group
16, 54
a key concern for the use of CMS membranes in
biomass-derived syngas environments. An earlier study by Vu et al.
55
had hinted,
however, that CMS membranes may be significantly more robust to the presence of
organic vapors than their polymeric counterparts. They studied the influence of the
presence of toluene vapor (70 ppm) on the selectivity of a CMS membrane treating a
(10% CO 2/90% CH 4) mixture at a pressure of 3448 KPa and at 35° C. In tests lasting
as long as 60 h, the membrane showed stable performance, which Vu et al.
55

attributed to the inability of the toluene vapor to enter the membrane nanopores.
During our lab-scale testing
16
the CMS membranes were exposed to simulated
syngas mixtures containing realistic amounts of naphthalene (as a model tar-like
9

species) and toluene (as a model organic vapor).The membranes performed well
16

with very little impact on throughput and selectivity observed. However,
temperature is a key consideration with stable performance observed as long as the
operating temperature stayed above 250
o
C, with losses in performance observed
for lower temperatures, becoming particularly severe below 200
o
C. These results
were also validated in field tests
54
with a slip-stream of real coal-derived and/or
biomass-derived syngas at DOE’s NCCC facility with a pilot-scale CMSM module
containing 86 membrane tubes. Again, the membranes performed stably as long as
the testing temperature remained above 250
o
C, with progressively increasing losses
in membrane performance observed upon lowering the test temperatures.
The current plan by our team is to field-test the “one-box” process using the
86-tube, pilot-scale module. Prior to (and in preparation for) such testing, however,
our team recently completed a lab-scale investigation using a CMSM with properties
similar to the membranes used to prepare the pilot-scale module. In these studies
the membrane and catalyst were challenged with a simulated biomass-derived
syngas that contained in addition to H 2S and NH 3 realistic concentrations of a model
organic vapor (toluene) and a model tar-like species (naphthalene). The results of
these studies are detailed in this paper, and provide impetus and further motivation
for undertaking the field-testing of the technology with real biomass-derived syngas
on the way to the eventual technology commercialization.

10

2. EXPERIMENTAL SYSTEM AND PROCEDURES
2.1. Experimental System

Figure 1Experimental set-up used in the membrane reactor experiments
Figure 1 is a schematic of the membrane reactor experimental set-up we used
to perform the WGS reaction experiments and the gas permeation tests which are
reported here. For the experiments, the CMS membrane (25.4cm long) is inserted in
the middle of the tubular stainless steel reactor and sealed there with the aid of
graphite o-rings and compression fittings. The commercial Süd-Chemie
Co/Mo/Al 2O 3 C-25 WGS catalyst particles (~ 10 g) are mixed with quartz particles
(~ 80 g), and then loaded in the annular volume in between the membrane and the
reactor housing. Prior to loading, the catalyst and quartz particles are crushed
separately into smaller particles, and their sizes are sorted with the aid of
11

mesh-screens in the range of 600-800 𝝁 m. (We dilute the catalyst with inert quartz
particles in order to completely fill the annular reactor volume, and to be able to
operate the reactor bed under isothermal conditions). The experimental system
consists of the feed, the reactor, and the analysis section. The feed section consists of
gas cylinders, mass flow controllers, syringe pumps to deliver the water and the
organic compounds (toluene and naphthalene), and the steam and organic vapor
generating units. The reactor section includes the MR, a furnace for heating-up the
reactor, gauges or measuring the pressure, two condensers and two moisture traps
to remove the water and the organic vapors from the reject and permeate side
streams of the reactor, two traps to remove H 2S, two traps for removing NH 3 and
another two traps to remove trace organic vapors from the same streams. The
analysis section consists of an on-line gas chromatograph to analyze the
concentration of the exit gas streams, two bubble flow-meters for measuring the
total flow-rates, and Draeger tubes for measuring the H 2S and NH 3 concentration
(via slip streams). For the liquid collected in the moisture trap, we measure the
organic phase composition with a GC-MS after water is removed, see further details
below.
The reactor and a section of the feed and sweep lines (in order to preheat the
feed gas mixture and the sweep steam to the reactor temperature) are maintained at
isothermal conditions in a six-zone furnace. The temperature in each zone is
controlled by using six temperature controllers and thermocouples installed in six
different locations in the bed. A sliding thermocouple is also used to monitor the
temperature along the length of the bed. Mass flow meters are used to control the
flow rates of the feed and sweep gas streams. The pressure is controlled by adjusting
the needle valves at the exit of the reactor and sweep sides. Pressure gauges are
12

installed in the feed, reject and permeate sides in order to monitor the pressure.
Two syringe pumps, one for the feed and the other for the permeate side, are used to
supply a controlled flow of water into the two evaporators (steam-generating) units.
Another syringe pump and evaporator are combined to deliver the toluene and
naphthalene vapors into the reaction system (the naphthalene is dissolved in the
toluene at predetermined concentration before being loaded into the syringe). These
evaporator units are stainless steel (SS) vessels packed with quartz beads in order to
accelerate water/organic vapor evaporation and to dampen out any fluctuations in
their flow. Heating tapes and temperature controllers have been used to heat and
control the temperature of the vessels. The steam generators along with all the
stream lines including feed, sweep, permeate, and reject lines are insulated and
“heat-traced” using heating tapes. Their temperature is also controlled with
temperature controllers.
2.2. MR Experiments
During the MR experiments, the gas streams exiting the reject and permeate
sides flow through condensers and then through the moisture-traps in order to
capture the water the organic vapors and the adsorbent beds to remove the H 2S and
NH 3. The flow rates of the water-free and organic-free streams are then measured by
a bubble flow-meter and its composition is measured with an online gas
chromatograph. For the reactor experiments reported here we use typical
composition of an air-blown biomass gasifier off-gas
(H 2/CO/CO 2/N 2/CH 4/NH 3/H 2S)= (0.67:1.00:1.00:2.67:0.2:0.00067:0.0006)
16
along
with 0.8 vol% of naphthalene and 6.4 vol% toluene added to the syngas feed. For the
reactor experiments we use a near stoichiometric H 2O/CO ratio in the feed of 1.1.
13

2.3. Packed-Bed Reactor Experiments
The same procedure is followed in performing packed-bed reactor experiments
(to compare its performance with that of the MR), and for measuring the catalytic
reaction kinetics, with the only difference being that the inlet and exit valves for the
sweep gas are closed. The same system has also been used for single-gas permeation
studies in order to characterize the membrane properties. For such experiments, the
sweep gas (permeate side) inlet is closed, gas flows into the feed-side and the flow
rates of permeate and reject streams are measured.
Another key goal of the project was to further validate, by WGS-MR experiments,
an isothermal MR model, previously utilized by our group for describing such
reactors
16,46
so that simulations can be performed to identify optimized reactor
operating conditions (in terms of residence time, reactor pressure and temperature,
and steam purge rate) for the various end-use applications. Further details about the
model can be found in the Appendix and elsewhere
16, 46
.

14

3. RESULTS AND DISCUSSION
3.1. Single Gas Permeation Before and After Experiments
As noted above, the experimental system in Figure 1 has also been used for
single-gas permeation studies in order to characterize the membrane properties. For
such experiments, the sweep gas (permeate side) inlet is closed, gas flows into the
feed-side and the flow rates of permeate and reject streams are measured. For
measuring the water permeance, an Ar gas stream containing a predetermined
concentration of water is fed into the system. The permeate stream then passes
directly through an adsorbent bed where the water is captured. The amount of
water that permeates is then calculated by measuring the weight of the adsorbent
before and after water permeation.
As noted above, these membranes were previously tested in the laboratory in
the presence of a model organic vapor (toluene) and a model tar (naphthalene)
under non-reactive conditions
16, 46
as well as in field-scale studies where they were
exposed to real coal-derived and/or biomass-derived syngas
54
and they were shown
to perform well. The main goal in this study, therefore, was to test their ability to
perform satisfactorily as well under reactive conditions in the presence of the
aforementioned impurities. Prior to the initiation of the MR experiments the
membrane was characterized through single-gas permeation experiments. The
permeances of the various species for(calculated with respect to the inside
membrane area) for the fresh membrane are shown in Table 2. (Our previous
studies with CMS membranes indicate
16, 46
that the mixed-gas permeance of the
various gases generally remain fairly close to the values measured during the
single-gas experiments, so no such experiments were performed here). For the H 2S
15

and NH 3 syngas contaminants, the MR experiments indicate that they do not
permeate through the membrane, and their permeance was therefore set equal to
zero for the simulations shown below (In extensive studies in which both the surface
of the membrane module and the plumping were specifically coated to avoid
potential wall adsorption, the H 2S permeance was always found to lie in between the
permeance of N 2 and CH 4). A key conclusion from these experiments, lasting for
more than 8 weeks, is that the catalyst and membrane exhibited fairly robust
behavior. We observed, for example, no notable changes in catalyst activity. The
single-gas permeances (other than for water, for which the permeance was
measured as described above) of the various species were also measured after all
the reactor experiments were completed (see Table 2). The permeance of the less
permeable species (CO, N 2, CH 4) changed very little, and the permeance of H 2
decreased by ~ 7%, which is very much in line with our field-scale observations
with these membranes
54
.
Table 2 Single-gas permeation data for the CMSM before and after the MR experiments
Gas
Permeance(m
3
/m
2
*hr*bar)
Before After
H 2 2.21 2.04
CO 0.036 0.037
H 2O Not measured 0.73
CO 2 0.073 0.079
CH 4 0.007 0.007
N 2 0.016 0.017
16

3.2. Kinetics of the Reactions
Another goal of the study was to investigate the impact the presence of an
organic vapor and model tar-like species may have on catalyst performance. The
reaction kinetics of this particular catalyst (Süd-Chemie Co/Mo/Al 2O 3 C-25 WGS
catalyst) was extensively investigated previously by our group and are detailed in
our previous papers
16, 46
. Specifically, an empirical rate law first proposed by Weller
and coworkers
56-59
and indicated below, was used to fit an extensive reactor data set
that included studying syngas compositions corresponding to biomass and coal
gasifiers, under a broad range of pressure, temperatures, and reactor residence
times.
𝑟 𝐶𝑂
= 𝐴 𝑒 −
𝐸 𝑅𝑇
𝑝 𝐶 𝑂 𝑎 𝑝 𝐻 2
𝑂 𝑏 𝑝 𝐶𝑂
2
𝑐 𝑝 𝐻 2
𝑑 (1 − 𝛽 ) (1)
𝛽 =
1
𝐾 𝑒𝑞
(𝑃 𝐶𝑂
2
.𝑃 𝐻 2
)
(𝑃 𝐶𝑂
. 𝑃 𝐻 2
𝑂
)
(2)
In the above expression r COis the reaction rate with respect to CO, T is the
temperature. P j is the partial pressure for component j, and K eq is the overall reaction
equilibrium constant. The values of parameters A, E, a, b, c, d have been reported
elsewhere
16, 46
and are also shown in the Appendix.
As reported in our previous papers
16, 46
, this empirical rate law performed well
in fitting both our packed-bed reactor as well as our membranes reactor data.
However, the simulated syngas used in the present investigation contains a
substantial amount of two model impurities (toluene and naphthalene) and their
impact on the reaction kinetics of the catalyst was not known prior to this study
17

(such impurities, for example, could have undergone catalytic cracking to produce
coke that deactivates the catalyst or could have interfered, in some other way, with
its WGS activity). For further investigating this issue, additional reactor data were
generated and compared with the predictions of the empirical rate law (all these
experiments were carried out at 300
o
C and a CO/H 2O=1.1, which are the conditions
utilized in the MR experiments reported here). The agreement between data and the
predictions from the empirical rate model are shown in Figure 2, which compares
the experimentally measured conversions with calculated conversions from the
model. The experimental data agrees well with the model indicating that under the
conditions studied in this paper the organic vapor and tar-like model impurities
have no impact on catalytic activity. Furthermore, as noted previously, the catalyst
activity remained stable through the 8 weeks of experiments.

Figure 2 Experimental vs. calculated CO conversion using the power law rate
expression
16, 46
under various packed-bed reactor experimental conditions
18

Since the empirical reaction rate model previously developed was shown to
perform adequately was also utilized for all the reactor simulations reported in this
paper, as discussed below.
3.3. CMSM and Catalyst Performance with Presence of Toluene and
Naphthalene
One of the major potential advantages of the proposed “one-box” process is that
it is a multi-functional system that combines the WGS reaction, the separation of the
hydrogen product, and the removal of the various syngas impurities (H 2S, NH 3,
organic vapors and tars, etc.) into a single step. In particular, the ability of the CMSM
to separate in situ the various syngas impurities offers great benefit in that it
eliminates the need for using a warm-gas clean-up unit (WGCU) for removing these
impurities from the syngas prior to its being processed in the WGS reactor. In our
previous studies we reported on the ability of the CMSM to remove H 2S and NH 3
16, 39,
54
. However, due to experimental limitations we were not able to check their
separation characteristics towards the organic vapors and the tar-like species. In
this study, we have carried out a series of experiments in order to specifically check
for the ability of such membranes to remove these types of impurities. Since these
membranes are permeable to water (and also steam is used as the sweep gas) and
toluene and naphthalene are somewhat soluble in water (see below) it is not
straightforward to check the separation characteristics under reactive conditions.
(In principle, through the use of condensers on the permeate side one could
condense both water and the organic vapor and tar-like species. However, because
the CMSM does not allow such impurities to go through – see further discussion
below -- no separate organic phase can be detected). Instead, an experiment was
19

carried out in which the CMS membrane was exposed to a flowing gas mixture of 6.4
vol% toluene in hydrogen at 300
o
C and at 5 bar, and the gas phase exiting the
permeate side (virtually pure hydrogen) was sampled via GC/MS for the presence of
toluene. In these experiments we were unable to detect any toluene in the permeate
stream (analytical instrument detection limit <1 ppm). Thus, we conclude, based on
these experiments, that toluene does not permeate through this tight-pore CMS
membrane. Since naphthalene is a much bigger molecule than toluene, hence one
may also conclude (barring a set of unknown circumstances) that naphthalene (and
the tar-like species in real syngas) will be unlikely to penetrate through the CMS
membrane either.
During the membrane reactor experiments, we collected the liquids from the
condenser on the reject side and studied their volume and composition. The key
reason for doing that, is so that we are able to investigate whether toluene and
naphthalene react to a substantial extent in the WGS MR, and also whether they
leak-through to the permeate side to during the process. These experiments,
typically, involved carrying out the MR experiments for a certain period of time
(while collecting all the condensable liquids produced), and then switching the feed
flow into pure He gas, depressurizing the reactor and under the same temperature
flushing the reactor system with He for an additional 8 hr. Since the solution of
naphthalene in toluene (a 1:8 molar ratio) has a lower density than water, the liquid
phase collected consists of two phases, with the organic phase residing on the top.
We then carefully separated the organic phase from the water phase and weighed
the organic phase. The total amount of organic phase collected corresponds to more
than 95% of the amount of organic impurities (toluene + naphthalene) fed to the
reactor during the period the liquid phase was collected. The composition of the
20

organic phase was tested using a Bruker GC450 - MS300 (making use of a 30-m
DB-5 non-polarized column. The analysis procedure starts at 50 °C and keeping the
oven at that temperature for 2 min. Then the oven temperature increased to 250 °C,
at 50°C /min. After reaching 250 °C, the column was kept at that temperature for 4
min).The GC/MS was calibrated using (toluene/naphthalene) mixtures in methanol
with a molar ratio ranging from 1:1 to 16:1). The ratio of (toluene/naphthalene) in
the organic phase collected from the reactor was (7.7:1). Given that toluene has a
small but finite solubility in water (the toluene’s solubility in water is 490 mg/L, and
the naphthalene’s solubility is 30 mg/L) and it is also volatile, it is not surprising
that both the total amount of liquids but also the amount of toluene collected is
somewhat smaller than the corresponding feed amounts (albeit less than 5%).
Nevertheless, one can conclude from these results than neither of these compounds
gets substantially converted in the membrane reactor during the WGS reaction
experiments (which is in line with the experimental findings that they do not impact
the reaction kinetics) nor do they leak-though the membrane to the permeate side
(which is a finding consistent with the permeation studies with the toluene in
hydrogen mixtures noted above).
3.4. Comparison of CO Conversion between MR and Packed-Bed Reactions
Figure 3 shows the CO conversion as a function Wcat/Fco (the weight of catalyst
which for these experiments is 10 g, over the molar feed flow rate of CO) during the
membrane reactor experiments for a feed pressure of 3 bar and two different sweep
ratios (SR=0.1 and 0.3). Shown on the same Figure are the simulations based on the
MR model using the independently measured reaction rate expression and the
membranes permeances (the average values measured for the fresh membrane and
21

the membrane after 8 weeks of experiments, see Table 2). For comparison purposes,
packed-bed experimental data and simulations are also shown on the same Figure. A
key observation from Figure 3 is that the model does an adequate job in describing
both the MR as well as the packed-bed reactor experiments. The membrane
reactor’s conversion is higher than the packed-bed reactor’s conversion, and both
conversions increase with Wcat/Fco as expected. In addition, increasing the sweep
ratio also improves the reactor conversion.

Figure 3 CO conversion under 3 bars of reactor pressure and at 300
o
C for different
sweep ratio
3.5. CO Conversion under Different Experimental Conditions
Figure 4 shows the CO conversion as a function Wcat/Fco during the membrane
reactor experiments for a different feed pressure of 5 bar and two different sweep
ratios (SR=0.1 and 0.3). Shown on the same Figure, in addition, are the simulations
22

based on the MR model. Once more, as Figure 4 indicates, the model does an
adequate job in describing the MR. The membrane reactor’s conversion again
increases with Wcat/Fco and the the sweep ratio. When comparing the results
between Figures 3 and 4, it is clear that running the reactor at higher pressures
benefits performance, an important result in terms of the eventual
commercialization of the technology, since typical gasifiers are run at much higher
pressures (20 bar and above).

Figure 4 CO conversion under 5 bar of reactor pressure and at 300
o
C for different sweep
ratio.
3.6. H
2
Recovery under Different Experimental Conditions
Figure 5 shows the hydrogen recovery, defined as the fraction of total hydrogen
that ends up as part of the permeate stream
16, 39
as a function of Wcat/Fco and the
reactor pressure and sweep ratio. Shown on the same Figure are also the recoveries
calculated using the data-validated MR model. As can be seen in Figure 5, the model
23

does an adequate job, again, in describing the hydrogen recoveries for all conditions
studied. As expected higher sweep ratios and reactor pressure increase H 2 recovery,
and for a sweep ratio=0.3 and a pressure of 5 bar, a H 2 recovery higher than 70% is
achieved. (The recoveries shown in Figure 5 are rather low due to limitations with
the size of our laboratory system, which accommodates only one small size CMS
membrane – under optimized conditions, see discussion below, recoveries in excess
of 90% can be attained under realistic IGCC conditions).

Figure 5 Hydrogen recovery under various experimental conditions
3.7. H
2
Purity under Different Experimental Conditions
Finally Figure 6 shows the hydrogen purities (dry-basis) on the permeate side
as a function of Wcat/Fco and the reactor pressure and sweep ratio. Shown on the
same Figure are also the hydrogen purities calculated using the data-validated MR
model, which does a decent job in predicting the experimental values (as a result of
24

the un-optimized lab-scale reactor operation these purities are rather -- under
optimized conditions
54
purities in excess of 90% with a CO content of a few hundred
ppm can be attained under realistic IGCC conditions). The results in Figures 5 and 6
manifest the challenge one faces in optimizing such reactors, whereby conditions
maximizing recovery lead to diminished hydrogen purity. Thus, to optimize the
reactor operating conditions, one needs a fine balance between Wcat/Fco, the sweep
ratio and the reactor pressure in order to reach high CO conversion with acceptable
hydrogen recovery and purity.

Figure 6 Permeate-side H
2
purity (dry basis) under various experimental conditions
Since the model performs reasonably well in describing the experimental
results, it can be used to further study the effect of various parameters on WGS-MR
performance, in terms of reactor conversion, hydrogen recovery, and purity. The
target here is to choose appropriate conditions which maximize both the CO
conversion and H 2 recovery, and minimize the CO content of the hydrogen product.
25

Results of such process design and scale-up simulations have been presented
elsewhere
16, 39, 46, 54
. For example, simulations, based upon membrane properties
measured during field testsdemonstrate
54
that the “one-box” process, operating on a
typical oxygen-blown gasifier off-gas, can deliver more than 90% hydrogen recovery
at more than 90% purity (dry-basis), and thus shows good promise for commercial
application.

26

4. CONCLUSIONS
A novel MR system termed as the “one-box” process, in which syngas clean-up,
reaction and product separation are combined in the same unit was successfully
utilized for producing hydrogen from a feed with a simulated biomass-derived
syngas composition containing common impurities such as H 2S and NH 3, a model
organic vapor (toluene) and a model tar-like species (naphthalene). A CMS
membrane was used for the in-situ hydrogen separation. The membrane was
characterized in terms of its gas permeances which were used for the model
predictions. The CMS membrane stability was also investigated in the presence of
these impurities, and the membrane proved to be stable under the experimental
WGS reaction conditions.
The kinetics of the WGS reaction over a commercial Co/Mo/Al 2O 3 sour-shift
catalyst was also investigated in the presence of the organic vapor and model
tar-like species as part of our study, and no impact of these impurities was observed.
The performance of the MR (the “one-box” process) using such membranes and
catalysts was investigated experimentally for a range of pressures and sweep ratios;
the MR showed higher conversions compared with those of the traditional
packed-bed reactor. Parallel modeling investigations indicated good agreement with
the experimental data.
The “one-box” process shows several advantages over the traditional
packed-bed system, including improvements in CO conversion and H 2 purity, while
allowing one to perform the reaction in the presence of hydrogen sulfide and
ammonia and being able to deliver a contaminant-free hydrogen product. Use of the
process in hydrogen production from biomass-derived syngas should, therefore,
27

result in considerable energy savings.

28

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36

6. ACKNOWLEDGEMENTS
I express my special thanks to all members of my Thesis Committee, Professor
Theodore T . Tsotsis, Professor Muhammad Sahimi and Professor Katherine Shing. I
express my deepest appreciation to my advisor, Professor Tsotsis for his guidance,
professional advice and constructive feedback over the past two years. He teaches
me how to be a good chemical engineer and researcher. Also I would like to thank
Jiang Yu, who is my co-worker and helps a lot over the last two years.
My deepest gratitude to my parents, Yi-xin Li and Xi-sheng Tan, and my
girlfriend Ling-ting Jiang for giving me unlimited love and support me throughout
my Master study. Without their support, I cannot achieve the goal of completing the
study and thesis.
Finally, the support of the US Department of Energy (Award Number
DE-FC26-07NT43057) is gratefully acknowledged.

37

7. NOMENCLATURE
A
F
cross-sectional area for the feed-side (m
2
)
d p particle diameter in the feed side (m)
f
friction factor
F j molar flux for component j (mol/m
2
. h)
F
H
2
molar flux for hydrogen (mol/m
2
. h)
g c gravity conversion factor
G
F
superficial mass flow velocity in the feed side (g/m
2
∙h)
ID inner diameter of the membrane (m)
k reaction rate constant (mol/g.h.bar
2
)
K eq equilibrium constant
L length of membrane (m)
n pressure exponent
n j
F
molar flow rate for component j in the feed side (mol/h)
n
H
2
F

molar flow rate for hydrogen in the feed side (mol/h)
n j
P
molar flow rate for component j in the permeate side (mol/h)
n
H
2
P

molar flow rate for hydrogen in the permeate side(mol/h)
Re
F
N

Reynolds number for the feed-side
OD outer diameter of the membrane (m)
P j partial pressure for component j (bar)
38

P
H
2
partial pressure for hydrogen(bar)
P
F
feed-side pressure (bar)
P j
F
partial pressure for component j in the feed-side (bar)
P
H
2
F

partial pressure for hydrogen in the feed side (bar)
P
P
permeate side pressure (bar)
P j
P
partial pressure for component j in the permeate side (bar)
P
H
2
P

partial pressure for hydrogen in the permeate side (bar)
r overall reaction rate expression (mol /g. h)
Re
H
2
hydrogen recovery, defined by Eq. (16)
SR
steam sweep gas ratio

n
j0
P

n
j0
F
  

T temperature
u
F
superficial flow velocity on the feed side (m/h)
U j membrane permeance for component j (mol/m
2
.h.bar)
𝑈 𝐻 2
H 2permeance(mol/m
2
.h.bar
n
)
V reactor volume variable (m
3
)
W C weight of the catalyst (g)
X
CO
carbon monoxide conversion, defined by Eq. (15)
m

membrane area per feed side reactor volume (m
2
/m
3
)

equilibrium coefficient
39

c

fraction of solid volume occupied by catalyst
v

bed porosity in the feed side
F

viscosity of the fluid (g/m∙h)
ρ
F
average density of the fluid (g/m
3
)
ρ
c
catalyst density (g/m
3
)
ν
j
stoichiometric coefficient for component j

40

8. APPENDIX
In several previous papers published by our group
16, 46
, we developed an
isothermal co-current membrane reactor model to describe the behavior of our CMS
membrane reactor and also the kinetics for the WGSR catalyst. Based on our
experimental findings, we assume that toluene and naphthalene will not react and
can be treated as inert gas. Following, is a brief description of the model.
Empirical equation for hydrogen transport through the Pd membrane:
𝐹 𝐻 2
=
𝑈 𝐻 2
((𝑃 𝐻 2
𝐹 )
𝑛 − (𝑃 𝐻 2
𝑃 )
𝑛 ) (1)
Membrane mass transport for species other than H 2:
𝐹 𝑗 = 𝑈 𝑗 (𝑃 𝐽 𝐹 − 𝑃 𝐽 𝑃 ) (2)
H 2 mass balances in the feed and permeate sides:
𝑑 𝑛 𝐻 2
𝐹 𝑑𝑉
= −𝛼 𝑚 𝑈 𝐻 2
((𝑃 𝐻 2
𝐹 )
𝑛 − (𝑃 𝐻 2
𝑃 )
𝑛 ) + (1 − 𝜀 𝑣 )𝛽 𝑐 𝜌 𝑐 𝑟 𝐹 (3)
𝑑 𝑛 𝐻 2
𝑝 𝑑𝑉
= 𝛼 𝑚 𝑈 𝐻 2
((𝑃 𝐻 2
𝐹 )
𝑛 − (𝑃 𝐻 2
𝑃 )
𝑛 ) (4)
Mass balances for components other than H 2 in the feed and permeate sides:
𝑑 𝑛 𝑗 𝐹 𝑑𝑉
= −𝛼 𝑚 𝑈 𝑗 (𝑃 𝑗 𝐹 − 𝑃 𝑗 𝑃 ) + 𝜈 𝑗 (1 − 𝜀 𝑣 )𝛽 𝑐 𝜌 𝑐 𝑟 𝐹 (5)
𝑑 𝑛 𝑗 𝑃 𝑑𝑉
= 𝛼 𝑚 𝑈 𝑗 (𝑃 𝑗 𝐹 − 𝑃 𝑗 𝑃 ) (6)
41

The pressure drop in the packed-bed is calculated using the Ergun equation:
−
𝑑 𝑃 𝐹 𝑑𝑉
= 10
−6
𝑓 (𝐺 𝐹 )
2
𝐴 𝐹 𝑔 𝐶 𝑑 𝑃 𝜌 𝐹 (7)
𝑓 = ⌊
1−𝜀 𝜐 𝜀 𝜐 3
⌋ ⌊1.75 +
150 (1−𝜀 𝜐 )
𝑁 𝑅𝑒
𝐹 ⌋(8)
𝑁 𝑅𝑒
𝐹 =
𝑑 𝑃 𝐺 𝐹 𝜇 𝐹 (9)
Boundary conditions:
At V = 0: 𝑛 𝑗 𝐹 = 𝑛 𝑗 0
𝐹 ,𝑛 𝑗 𝑃 = 𝑛 𝑗 0
𝑃 , 𝑃 𝐹 = 𝑃 0
𝐹 , 𝑃 𝑃 = 𝑃 0
𝑃 (10)
WGS reaction rate expression:
𝑟 𝐶𝑂
= 𝐴 𝑒 −
𝐸 𝑅𝑇
𝑝 𝐶𝑂
𝑎 𝑝 𝐻 2
𝑂 𝑏 𝑝 𝐶𝑂
2
𝑐 𝑝 𝐻 2
𝑑 (1 − 𝛽 ) (11)
𝛽 =
1
𝐾 𝑒𝑞
(𝑃 𝐶𝑂
2
.𝑃 𝐻 2
)
(𝑃 𝐶𝑂
. 𝑃 𝐻 2
𝑂
)
(12)
The rate expression parameters are listed in Table 1 below.

Table 3 WGS reaction rate expression parameters
16

E 28.145 (kJ/mol)
k 0 4.9 mol/(atm
(a+b+c+d)
*h*g)
a 0.61(±0.0605)
42

b 0.37(±0.0752)
c -0.38(±0.0103)
d -0.54(±00079)

Abstract (if available)

Abstract IGCC power plants show promise for environmentally-benign power generation. In these plants coal and/or biomass are first gasified into syngas, which is then processed in a water gas-shift (WGS) reactor to further enhance its hydrogen content for power generation. However, impurities in the syngas, primarily H₂S, NH₃, various organic vapors and tar-like species are detrimental to catalyst life and must be removed before the gas enters the WGS reactor. This, today, means cooling the syngas for clean-up and then reheating it to the WGS reaction temperature. For use in various industrial applications, and potentially for CO₂ capture/sequestration, hydrogen purification is required. This, today, is accomplished by conventional absorption/desorption processes, which results in significant process complexity and energy penalty for the overall plant. Ideally, one would like to establish a ""one-box"" process in which the syngas is fed directly into the WGS reactor, which then effectively converts the CO into hydrogen in the presence of all the aforementioned impurities, and delivers a contaminant-free hydrogen product. In this study, the development of such a process is described. It includes a catalytic membrane reactor (MR) making use of a hydrogen-selective, carbon molecular sieve membrane, and a sulfur-tolerant Co/Mo/Al₂O₃ catalyst. The membrane reactor’s behavior has been investigated for different experimental conditions and compared with the modeling results. The model is used to further discuss the scale-up of the proposed process.

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University of Southern California Dissertations and Theses

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Asset Metadata

Creator Tan, Mingyang (author)

Core Title On the use of membrane reactors in biomass utilization

School Viterbi School of Engineering

Degree Master of Science

Degree Program Chemical Engineering

Publication Date 07/31/2013

Defense Date 06/24/2013

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag carbon molecular sieve (CMS) membrane,hydrogen production,IGCC power plant,membrane reactor (MR),OAI-PMH Harvest,process intensification,water-gas shift reaction (WGS)

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Tsotsis, Theodore T. (committee chair), Sahimi, Muhammad (committee member), Shing, Katherine (committee member)

Creator Email mingyant@usc.edu,tanmy1988@gmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c3-308900

Unique identifier UC11293721

Identifier etd-TanMingyan-1902.pdf (filename),usctheses-c3-308900 (legacy record id)

Legacy Identifier etd-TanMingyan-1902.pdf

Dmrecord 308900

Document Type Thesis

Format application/pdf (imt)

Rights Tan, Mingyang

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA