Carbon Dioxide-Selective Membranes for High-Pressure Synthesis

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Carbon Dioxide-Selective Membranes for High-Pressure Synthesis Gas Purification He Bai and W. S. Winston Ho* William G. Lowrie Department of Chemical and Biomolecular Engineering and Department of Materials Science and Engineering, The Ohio State University, 2041 College Road, Columbus, Ohio 43210-1178, United States ABSTRACT: The CO2-selective facilitated transport membranes based on cross-linked poly(vinyl alcohol) (PVA) and sulfonated polybenzimidazole (SPBI) matrixes were synthesized and characterized for high-pressure synthesis gas purification. The crosslinked PVA-based membranes showed CO2/H2 selectivity higher than 30 and CO2 permeability as high as close to 1000 barrers for a membrane thickness of 1530 μm at a feed pressure of 220 psia, which are desirable for the intended gas purification. The SPBIbased membranes generally showed lower separation performance because of their lower hydrophilicity. The cross-linked PVAbased membranes showed the best separation performance at 106 °C. However, the SPBI-1 copolymer-based membranes exhibited the best separation performance at 100 °C because of also their lower hydrophilicity.

1. INTRODUCTION Hydrogen is a very important raw material in many industrial processes, such as chemical and refining hydrogenation as well as fuel cell applications.1 Most hydrogen is produced by steam reforming of hydrocarbon fuels, such as CH4, followed by the water gas shift (WGS) reaction.2 Thus, the resulting synthesis gas mainly contains H2 and CO2, and the removal of CO2 from H2 to produce high purity H2 has become an important industrial process. In our previous publications, the advantages of the membrane process compared with the other conventional CO2/H2 separation processes, for example, absorption and pressure swing adsorption technologies, have been described.3,4 In addition, the advantages of using CO2-selective facilitated transport membranes compared to the other H2- or CO2-selective solution-diffusion membranes have also been described in detail.4 Ho and co-workers have extensively studied the CO2-selective facilitated transport membranes based on cross-linked poly(vinyl alcohol) (PVA) and sulfonated polybenzimidazole (SPBI) matrixes for low-pressure synthesis gas purification for fuel cell applications.313 A review article has been published as a summary of these studies.4 Ho’s research group first proposed, synthesized, and experimentally demonstrated novel CO2-selective membranes capable of possessing high CO2 permeability and CO2/H2 selectivity at relatively high temperatures up to 180 °C. At a low feed gas pressure of 2 atm, a CO2 permeability of over 6000 barrers and a CO2/H2 selectivity of over 260 were achieved at the optimum operating temperatures of 100120 °C. The CO2 permeability was about 3500 barrers or higher for temperatures ranging from 100 to 150 °C, and still as high as about 1900 barrers as the temperature increased to 180 °C. Such performance is very desirable for synthesis gas purification and almost nonexistent in the literature. On the other hand, the CO2/H2 selectivity was about 80 or higher for temperatures ranging from 100 to 150 °C. The selectivity reduced slightly as the temperature increased to 170 °C and still as high about 30 at 180 °C, which are also very desirable for synthesis gas purification. Thus, the CO2-selective r 2011 American Chemical Society

facilitated transport membranes showed wide viable operating temperatures ranging up to as high as 180 °C.313 Ho’s research group also elucidated the unusual phenomenon of both permeability and selectivity increases with temperature below 100 °C.5 Less than 10 parts per million (ppm) of CO in the H2 product was achieved with a WGS membrane reactor using the membrane to drive the WGS reaction to the product side via CO2 removal. The data were in good agreement with the model prediction.313 This was the first WGS membrane reactor using a CO2-selective membrane. In addition, less than 10 parts per billion (ppb) of H2S in the H2 product was also achieved since the membrane was also highly H2S-selective. However, the requirements by the refining and petrochemical industries for the applications of CO2-selective membranes for CO2/H2 separation are more challenging since feed pressures of 220 psia or higher are preferred. At high pressures, the facilitated transport membranes generally have the carrier saturation problem, which results in lower CO2/H2 selectivity and CO2 permeability.6 Xing and Ho reported the cross-linked polyvinylalcohol-polysiloxane/fumed silica mixed matrix membranes containing amines for high pressure CO2/H2 separation.14 In this paper, the CO2-selective facilitated transport membranes based on the aforementioned cross-linked PVA and SPBI matrixes were investigated for high-pressure (220 psia or higher) synthesis gas purification. PVA was chosen as the matrix for the facilitated transport membranes because of its high hydrophilicity, good compatibility with carriers, and good film forming ability. However, PVA without cross-linking could be easily dissolved in water at a temperature of 70 °C or higher.15 For certain applications where a temperature of more than 100 °C is required, cross-linking of PVA is very crucial to improve its thermal stability. In this work, Received: April 9, 2011 Accepted: September 12, 2011 Revised: August 29, 2011 Published: September 14, 2011 12152

dx.doi.org/10.1021/ie2007592 | Ind. Eng. Chem. Res. 2011, 50, 12152–12161

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Figure 1. Chemical structures of the mobile and fixed carriers for CO2 facilitated transport.

PVA was cross-linked by the previously reported chemistry.6,16 Both formaldehyde and glutaraldehyde could be used as the cross-linking reagent. When formaldehyde was used as the crosslinking reagent, the cross-linking degree of 60 mol % and the cross-linking time of 16 h at 80 °C were required.16 However, when glutaraldehyde was used as the cross-linking reagent, the cross-linking degree of 15 mol % and the cross-linking time of 2 h at 80 °C were already sufficient.16 Moreover, the membranes cross-linked with glutaraldehyde have been found to show better separation performance.16 Thus, glutaraldehyde was the principal cross-linking reagent used in this work. In this work, SPBI copolymers were also synthesized, characterized, and successfully used as the polymer matrix for facilitated transport membranes. As described in our previous publications,3,4 the SPBI copolymer-based facilitated transport membranes generally showed lower CO2/H2 separation performance because of the lower hydrophilicity of SPBI copolymer matrix compared to the cross-linked PVA matrix. However, SPBI copolymers showed higher polymer decomposition temperature, higher glass transition temperature, and lower potential of water swelling compared to cross-linked PVA, which provided them the advantages of being used in harsh operating conditions and in asymmetric membrane preparation by the phase inversion technique (credited to the high copolymer Tg and the water insoluble property of SPBI). In this work, the membranes synthesized contained both 2-aminoisobutyric acid-potassium salt (AIBA-K) and potassium carbonate-potassium bicarbonate (K2CO3KHCO3, converted from KOH) as the mobile carriers, and poly(allylamine) (PAA) or 2-bromobutane functionalized PAA (PAA-C4H9) as the fixed carrier for CO2 transport. The chemical structures of the carriers are displayed in Figure 1. The reaction mechanisms of CO2 with both fixed and mobile carriers have been described in detail in our previous publications.3,4 Generally, the mobile carriers contribute to the CO2 flux more than the fixed carriers.5 However, since the mobile carriers are small molecules, they are not bound as strongly as the fixed carriers in the membranes. The mobile carrier AIBA-K and fixed carrier PAA-C4H9 belong to the sterically hindered amine whereas the fixed carrier PAA belongs to the nonsterically hindered amine. The reaction mechanisms of CO2 with both hindered and unhindered amine carriers have been described in detail in our previous publications.3,4 The

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reaction mechanism of CO2 with KHCO3K2CO3 is also described in our previous publication.4 As reported previously,3,4 a sterically hindered amine can double CO2 absorption capacity and increase CO2 facilitated transport efficiency compared to a nonsterically hindered amine. But, we must also consider another factor: when PAA is modified into PAA-C4H9, its molecular weight almost doubles, and thus the concentration of amino groups in PAA-C4H9 is only about half of that in PAA with using a same weight percent in the membrane. In view of both factors, it is hard to conclude which fixed-carrier is more preferable without an extensive study. However, our limited initial work showed that with the same amount of fixed carrier loading in the membrane, the fixed carrier PAA-C4H9 showed some improved CO2/H2 separation performance compared to the fixed carrier PAA. More systematic studies are being carried out in our research group, and conclusions may be drawn in our future publications. In this work, the sterically hindered PAA-C4H9 was the primary fixed carrier used for the facilitated transport of CO2. For the cross-linked PVA matrix, water was used as the solvent for all of the blending components, and the blend solution was coated on the microporous polysulfone support. However, for the SPBI copolymer matrix, dimethyl sulfoxide (DMSO) had to be used as the solvent for blending components. Thus, the inorganic ceramic support was used as the support layer since polysulfone could be dissolved in DMSO. Microporous Teflon supports are highly porous and soft, which makes them subject to compaction at high pressures. Thus, they could not be used as the support layer for high-pressure synthesis gas purification. There are two parameters to characterize the separation performance of a membrane. One is the selectivity (or the separation factor), which is defined as αij ¼

yi =yj xi =xj

ð1Þ

Another parameter is the permeability Pi, which is defined as Pi ¼

Ni Δpi =l

ð2Þ

The common unit of Pi is Barrer, which is 1010 cm3 (STP) cm/(cm2 s cmHg). (Pi/l) is referred to as the permeance, and its common unit is the gas permeation unit (GPU), which is 106 cm3 (STP)/(cm2 s cmHg).6,17

2. EXPERIMENTAL SECTION 2.1. Materials. Poly(vinyl alcohol) (PVA, 99+%, hydrolyzed powder, MW = 8900098000), glutaraldehyde (50 wt % aqueous solution), 2-aminoisobutyric acid (AIBA, 98%), 2-bromobutane (98%), potassium hydroxide (KOH, 85+%), ethylenediamine (EDA, 99+%), polyphosphoric acid (PPA, 115 wt % H3PO4 equivalent), and dimethyl sulfoxide (DMSO, 99.5+%), all from Aldrich (Milwaukee, WI), were used as received without further purification. Poly(allylamine hydrochloride) (PAAHCl, MW = 60000) was purchased from Polysciences Inc. (Warrington, PA). 3,30 -Diaminobenzidine (DABD, 99%, Aldrich), 4,40 -oxybis(benzoic acid) (OBBA, 99%, Aldrich), and 5-sulfoisophthalic acid monosodium salt (SIPA-Na, 98%, Acros Organics, Morris Plaines, NJ) were dried in a vacuum oven at 120 °C overnight before their use for reaction. The microporous polysulfone support used in our experiments, GE A1 (with a thickness 12153

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Industrial & Engineering Chemistry Research of about 50 μm excluding a nonwoven fabric support and an average pore size of about 9 nm), was kindly provided by GE Infrastructure (Vista, CA). The inorganic ceramic support used (α-alumina, a pore size of 5080 nm) was kindly provided by Dr. Verweij’s research group in the Materials Science and Engineering Department of The Ohio State University (Columbus, OH). A feed gas with a certified composition consisting of 20% CO2 and 80% H2 (all on dry basis) was purchased from Praxair Inc. (Danbury, CT) for the gas permeation tests. 2-Bromobutane functionalized PAA (PAA-C4H9) was synthesized by the following method.18 Free PAA was obtained by mixing PAA-HCl with a stoichiometric amount of KOH in methanol for 48 h. The resulting free PAA was dissolved in methanol, and KCl precipitated from the solution. After removing the salts, the stoichiometric amounts of 2-bromobutane and KOH were added to the PAA/methanol solution. The reaction proceeded at 50 °C for 48 h. The resulting PAA-C4H9 product was still soluble in methanol, and the byproduct KBr precipitated from the solution. 2.2. Synthesis of Cross-Linked PVA-Based Facilitated Transport Membranes. PVA was first dissolved in water in the concentration of around 15 wt % at 80 °C under vigorous stirring. A calculated amount of cross-linking reagent (aqueous glutaraldehyde solution) and a certain amount of potassium hydroxide (dissolved in water) were added into the PVA aqueous solution to achieve a certain degree of cross-linking. The cross-linking degree was defined as the relative ratio of hydroxyl groups that have been reacted in PVA. The relative ratio (by weight) of PVA (before cross-linking) to KOH was fixed to 2.59 based on our previous work.6,10 The PVA/cross-linking reagent/KOH aqueous solution was heated at about 80 °C for a certain time (22.5 h for glutaraldehyde cross-linking) under vigorous stirring. After this initial cross-linking process, the solution became dark brown and viscous, but all components remained soluble in the solution. The solution had a total solid concentration of around 16 wt % (the solids included both the cross-linked PVA and KOH). Separately, the AIBA-K solution was prepared by adding a stoichiometric amount of KOH into an aqueous AIBA solution with stirring to obtain a homogeneous solution with the concentration of 20.5 wt %. By using the method described in Section 2.1, free PAA and PAA-C4H9 solutions in methanol with the concentration of around 10 wt % could be obtained. Then, methanol was evaporated with minor heating in a N2 environment and replaced with water as the solvent. The calculated amounts of the aforementioned cross-linked PVA (including KOH) solution, AIBA-K solution, and PAA/PAA-C4H9 solution were blended to reach a desired composition. The blend solution with a total solid concentration of 1617 wt % was stirred at room temperature for more than 3 h to obtain the final homogeneous blend solution. After centrifuging the solution in an Eppendorf Centrifuge (Model: 5804, Brinkmann Instruments, Inc., Westbury, NY) at 8000 rpm for 3 min, the membrane with a controlled thickness was prepared by casting the solution, using a GARDCO adjustable micrometer film applicator (Paul N. Gardner Company, Pompano Beach, FL), onto the microporous polysulfone support (GE A1) that had been spread on a glass plate. The as-cast film was dried in a hood overnight, followed by drying in an oven at 120 °C for 6 h. The heat-treatment ensured the complete removal of water and the complete cross-linking reaction of PVA with the cross-linking agent. A Mitutoyo Electronic Indicator (Model: 543252B, Mitutoyo America Corporation, Aurora, IL) was used to measure the

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Figure 2. Schematic of gas permeation apparatus (originally drawn by Dr. Jian Zou6). Adapted with permission from ref 6. Copyright 2006 Elsevier.

membrane thickness with an accuracy of (0.5 μm. For each thickness determination, several measurements at various membrane positions were made and recorded, and an average thickness value was reported for that membrane. The thickness of a membrane to be mentioned hereafter refers to the thickness of the active layer (top dense layer excluding the microporous support). 2.3. Synthesis of SPBI Copolymer-Based Facilitated Transport Membranes. The procedure for the synthesis of the SPBI-1 copolymer with the composition of DABD/SIPA (70 mol %)/ OBBA (30 mol %) has been described in our previous publications.3,19 In addition, the blending process of the SPBI-1 copolymer with the other carriers has also been described in our previous publication.3 In this work, the membranes with the composition of 20 wt % SPBI-1-EDA/60 wt % PAA/20 wt % AIBA-K were successfully synthesized and used for gas permeation measurements. The final blend solution with the concentration of around 10 wt % was coated on the inorganic ceramic support during membrane preparation. The inorganic ceramic support was not flat and had a curved surface. Thus, the prepared membranes by the solutioncasting method were relatively thick (around 50 μm). 2.4. Polymer Matrix and Membrane Characterization. The polymer matrixes (both cross-linked PVA and copolymer SPBI-1) and the resulting facilitated transport membranes were characterized with Fourier transform infrared (FTIR) spectra, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and gas permeation measurements. FTIR spectra were recorded on a Nexus 470 FTIR Spectrometer (Thermo Nicolet, Madison, WI), using Smart MIRacle Single Reflection Horizontal ATR (attenuated total reflectance). TGA was performed to estimate the thermal stability of the membranes with a Pyris 1 TGA thermogravimetric analyzer (PerkinElmer, Shelton, CT). DSC was performed to determine the glass transition temperatures of the membranes with a Diamond DSC differential scanning calorimeter (PerkinElmer, Shelton, CT). The procedures of carrying out these tests have been described in our previous work.3 2.4.1. Scanning Electron Microscopy. SEM images of membranes were taken using an S-3000 scanning electron microscope (Hitachi High Technologies America, Inc., Schaumburg, Illinois). The SEM samples were freeze-fractured in liquid nitrogen, dried in a vacuum oven, and then coated with Au/Pd. 2.4.2. Gas Permeation Measurement. The gas permeation apparatus has been described in our previous publications.3,6 The 12154


Industrial & Engineering Chemistry Research gas permeation tests were conducted by using a permeation cell inside an oven (Bemco Inc., Simi Valley, CA) for the accurate control of temperature. The schematic diagram of the permeation apparatus is shown in Figure 2. A circular stainless steel cell with an active membrane area of 5.7 cm2 was used for measuring transport properties of the membranes. In this cell, the feed and the sweep gas flows were countercurrent. Argon was used as the sweep gas for the ease of gas chromatography (GC) analysis. Gas flow rates were controlled by Brooks flow-meters (Brooks Instrument, Hatfield, PA). Proper amounts of water were pumped into the two vessels inside the oven using two Varian Prostar 210 pumps (Varian Inc., Palo Alto, CA) to control the water contents of the feed gas and the sweep gas, respectively, before they entered the permeation cell. The pressure of the retentate was controlled by a back-pressure regulator and measured with a pressure gauge. The pressure on the permeate side was set close to atmospheric pressure via a near-ambient pressure regulator and measured with a pressure gauge. After leaving the oven, both the retentate and the permeate streams were cooled to ambient temperature in their respective water knockout vessels, which removed the condensed water. The compositions for both the retentate and the permeate gases were then analyzed using an Agilent 6890N gas chromatograph with two thermal conductivity detectors (TCDs) (Agilent Technologies, Palo Alto, CA). Helium and argon were used as the carrier gases for the front and back TCD detectors, respectively. The GC columns used were SUPELCO Carboxen 1004 micropacked columns (Sigma-Aldrich, St. Louis, MO). Each of the membrane permeation measurements was taken after the membrane had been exposed to the feed and permeate streams under a specific condition (temperature, feed pressure, and water rates) for at least 6 h, which allowed for steady-state permeation.

3. RESULTS AND DISCUSSION 3.1. Polymer Matrix and Membrane Characterization. The FTIR spectra of the membranes of pure PVA and cross-linked PVA have been shown in our previous publication,16 which could confirm the formation of acetal linkages during cross-linking.20 The FTIR spectrum of the SPBI-1 copolymer has also been discussed in our previous publications.3,19 The absorption peak assignments could prove that the SPBI-1 copolymer with a designed structure was successfully synthesized.3,19 The TGA curve of the cross-linked PVA has been shown in our previous paper,21 and the TGA curve of the SPBI-1 copolymer has also been illustrated in our previous publications.3,19 The TGA curves clearly indicate that the cross-linked PVA and SPBI-1 copolymer were thermally stable up to around 230 and 460 °C, respectively. Thus, they are all suitable for high temperature gas separations. The DSC curve of the cross-linked PVA-based membrane has appeared in our previous paper,16 and the DSC curve of the SPBI-1 copolymer-based membrane has been reported in our previous publication.3 For both the membrane with 50 wt % cross-linked PVA/50 wt % carriers and the membrane with 50 wt % SPBI-1/50 wt % carriers, only one Tg could be observed. This suggested that a desirable homogeneous blending was achieved in these systems. In other words, both cross-linked PVA and SPBI-1 copolymer had good compatibility with the amine carriers. However, the SPBI-1-based membrane showed a much higher Tg (228 °C) than that of the cross-linked PVA-based membrane (84 °C). Thus, the SPBI-1-based membrane has the advantage of being used for asymmetric thin membrane preparation

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Figure 3. SEM image of the cross-section of the cross-linked PVAbased facilitated transport membrane synthesized on a microporous polysulfone support (GE A1).

by the phase inversion technique. In other words, the resulting asymmetric membrane could maintain its sustainable porous structure at the bottom of the membrane, even at high temperatures and pressures. Figure 3 shows the SEM image of the cross-section of a crosslinked PVA-based membrane on the GE A1 microporous polysulfone support. It can be seen that the membrane mainly consisted of two portions: a dense active layer at the top to provide separation and the microporous polysulfone support at the bottom to provide mechanical strength. This composite structure maximizes the mechanical strength while minimizes the mass transfer resistance of the membrane. This figure also shows that the active layer of this membrane was around 15 μm, which was very close to the membrane thickness determined using the Mitutoyo Electronic Indicator (15 μm). It is worthwhile to mention that the SEM only showed the membrane thickness at one local position, not an average value throughout the whole membrane area. However, the Mitutoyo Electronic Indicator gave the average value of the membrane thickness throughout the whole membrane area. 3.2. Separation Performance of Cross-Linked PVA-Based Membranes. The cross-linked PVA-based facilitated transport membranes were tested using the gas permeation apparatus as described in Section 2.4.2. Unless specified otherwise, the feed and the sweep gas flow rates were 60 and 30 cm3/min, respectively. The feed gas flow rate of 60 cm3/min was high enough that the retentate gas composition did not change much compared to the feed gas composition. Thus, the results were accurate and reliable. The sweep gas flow rate of 30 cm3/min was high enough to dilute the permeate gas mixture and provide a high driving force. If not specified, the liquid water injection rates were 0.01 (the lowest setting of the water pump) and 0.18 cm3/min on the feed side and permeate side, respectively. At the above gas flow rates and liquid water injection rates as well as at the temperature of 106 °C and the feed pressure of 220 psia, the relative humidity (RH) values on the feed side and permeate side of the membrane were 100% and 73.7%, respectively. However, since the water could transfer from the feed side to the permeate side, the exact permeate-side relative humidity should be higher than 73.7%. The pressure on the permeate side was maintained at around 15 psia. The feed gas with the 12155



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Table 1. Compositions of Synthesized Crosslinked PVA-Based Facilitated Transport Membranes membrane abbreviation

membrane composition

Syngas-PVA-1

40 wt % cross-linked PVA (15 mol % cross-linking with glutaraldehyde)/14.6 wt % KOH/20.7 wt % AIBA-K/24.7 wt % PAA-C4H9

Syngas-PVA-2


Syngas-PVA-3


composition of 20% CO2 and 80% H2 (all on dry basis) was used to simulate the composition of the synthesis gas from the steammethane reforming and the subsequent WGS reaction. The permeation cell was a small one with an effective membrane separation area of 5.7 cm2. 3.2.1. Effects of Feed Pressure on Separation Performance. The effects of feed pressure on CO2/H2 selectivity, CO2 permeability, and CO2 permeance were investigated using the membranes Syngas-PVA-1 and Syngas-PVA-2 on the GE A1 microporous polysulfone support (membrane compositions are listed in Table 1). The feed pressure ranged from 220 to 420 psia, and the temperature was maintained at 106 °C. The thickness of the membranes was about 15 μm. As shown in Figure 4, increasing feed pressure caused the decrease of CO2/H2 selectivity, CO2 permeability, and CO2 permeance, which was due to the carrier saturation phenomenon.6,22 When the CO2 partial pressure is high, carrier saturation will occur. In this situation, the CO2 flux of the membrane reaches constant. The increase of CO2 partial pressure does not further increase the CO2 flux, and hence CO2 permeability and permeance drop according to eq 2. However, H2 permeates through the membrane by solution-diffusion mechanism, and its permeability does not change significantly with pressure. As a result, CO2/H2 selectivity reduces with the increase of feed pressure. The membrane Syngas-PVA-2 showed better performance than the membrane Syngas-PVA-1 since the former contained a higher total carrier concentration in the membrane. The effects of membrane composition will be further discussed in Section 3.2.4. 3.2.2. Effects of Temperature on Separation Performance. Our previous work showed that at temperatures ranging from 50 to 100 °C, both the CO2/H2 selectivity and the CO2 permeability increased as the temperature increased, which was due to the higher reaction and diffusion rates for the CO2-carrier reaction and reaction products, respectively, at higher temperatures. More discussion on this effect can be found in the literature.5 On the other hand, at temperatures higher than 110 °C, both the CO2/H2 selectivity and the CO2 permeability reduced as the temperature increased, which was due to the reduction of water retention in the membrane.6 Thus, in this work, the membrane performance at temperatures between 100 and 120 °C was studied. The effects of temperature on CO2/H2 selectivity, CO2 permeability, and CO2 permeance were investigated using the membrane Syngas-PVA-1 on the GE A1 microporous polysulfone support (membrane composition is listed in Table 1). The temperature ranged from 106 to 120 °C, and the feed pressure was maintained at 220 psia. The thickness of this membrane was 15 μm. As shown in Figure 5, both CO2 permeability and CO2 permeance reduced with the increase of temperature, which was due to the reduction of water retention in the membrane.6 However, the CO2/H2 selectivity maintained at a relatively stable value in this temperature range. The reason is that even though

Figure 4. Effects of feed pressure on (a) CO2/H2 selectivity and (b) CO2 permeability and CO2 permeance for the membranes Syngas-PVA-1 (]) and Syngas-PVA-2 (4). T = 106 °C, sweep side pressure = 15 psia, feed gas flow rate = 60 cm3/min, sweep gas flow rate = 30 cm3/min, feed-side liquid water injection rate = 0.01 cm3/min, sweep-side liquid water injection rate = 0.18 cm3/min, both membrane thickness = 15 μm.

CO2 permeability reduced (not very significantly at this temperature range) with the increase of temperature, H2 permeability also reduced because of the H2 solubility reduction with the increase of temperature according to Henry’s law. As a result, CO2/H2 selectivity did not change significantly in this temperature range. To achieve both the best CO2/H2 selectivity and CO2 permeability, the optimum operating temperature for the membrane application was at 106 °C. The membrane permeation measurements at a specific condition (temperature, feed pressure, and water rates) were taken after the membrane had been exposed to the feed and permeate streams for at least 6 h, which allowed for steady-state permeation. After reaching the steady state, the permeation data were recorded every hour, and totally 45 data points were obtained, which were used for selectivity and permeability calculations. Finally, the average of these 45 data points was calculated and reported. After the testing condition was changed, the same 12156



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Figure 5. Effects of temperature on (a) CO2/H2 selectivity and (b) CO2 permeability and CO2 permeance for the membrane SyngasPVA-1. Feed side pressure = 220 psia, sweep side pressure = 15 psia, feed gas flow rate = 60 cm3/min, sweep gas flow rate = 30 cm3/min, feed-side liquid water injection rate = 0.01 cm3/min, sweepside liquid water injection rate = 0.18 cm3/min, membrane thickness = 15 μm.

procedure described above was repeated before obtaining the new, reliable data. In Figure 5, at each testing condition, all the recorded 45 data points are reported, and their error bars of 1.5% based on the average value are also shown. Figure 5 clearly shows that all the experimental data points were within the 1.5% error bar, which supported that the permeation system had reached the steady state and the data obtained were reliable. Similarly, the data reported in all the other figures were also reliable steady-state permeation results, which had a testing error of less than 1.5%. 3.2.3. Effects of Membrane Thickness on Separation Performance. The effects of membrane thickness on CO2/H2 selectivity, CO2 permeability, and CO2 permeance were investigated using the membranes Syngas-PVA-1 and Syngas-PVA-2 on the GE A1 microporous polysulfone supports (membrane compositions are listed in Table 1). The feed pressure and temperature were maintained at 220 psia and 106 °C, respectively. The thicknesses of both membranes were 10 and 15 μm. With the decrease of the membrane thickness, CO2 permeance (flux)

Figure 6. Effects of membrane thickness on (a) CO2/H2 selectivity, (b) CO2 permeability, and (c) CO2 permeance for the membranes SyngasPVA-1 (]) and Syngas-PVA-2 (Δ). T = 106 °C, feed side pressure = 220 psia, sweep side pressure = 15 psia, feed gas flow rate = 60 cm3/min, sweep gas flow rate = 30 cm3/min, feed-side liquid water injection rate = 0.01 cm3/min, sweep-side liquid water injection rate = 0.18 cm3/min.

increased, but both CO2 permeability and CO2/H2 selectivity reduced for both membranes as shown in Figure 6. The CO2 permeation rate in the facilitated transport membrane is determined by the reaction rate between CO2 and carrier 12157



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Table 2. Separation Performance of Membranes with Various Compositionsa

membrane

thickness (μm)

CO2/H2 selectivity

CO2 permeability (Barrer)

CO2 permeance (GPU)

Syngas-PVA-1 Syngas-PVA-2 Syngas-PVA-3

15 15 15

43.5 42.9 37.9

623 797 915

41.5 53.2 61.0

a T = 106 °C, feed side pressure = 220 psia, sweep side pressure = 15 psia, feed gas flow rate = 60 cm3/min, sweep gas flow rate = 30 cm3/min, feed side liquid water injection rate = 0.01 cm3/min (feed side RH = 100%), sweep side liquid water injection rate = 0.18 cm3/min (sweep side RH = 73.7%).

molecules (both the forward and reverse reactions) and the diffusion rate of CO2-carrier reaction products. When the membrane is thin, the diffusion rate may be high enough to be comparable with the reaction rate; thus, none of them can be neglected. Only the diffusion rate is related to membrane thickness, and the reaction rate is independent of membrane thickness. Thus, when the membrane thickness reduced, the CO2 permeance (flux) did not proportionally increase since the CO2 permeation is not only diffusion controlled but also reaction controlled. For example, for the membrane Syngas-PVA-2, when the membrane thickness reduced from 15 to 10 μm, the CO2 permeance (flux) only increased from 53 to 58 GPU, not as pronounced as 1.5 times. By definition, permeability is the product of permeance and membrane thickness. When the membrane thickness reduced and CO2 permeance only had a slight increase, CO2 permeability reduced. However, H2 permeates in the membrane by the solutiondiffusion mechanism, and its permeability does not change with the membrane thickness. As a result, CO2/H2 selectivity reduced with the decrease of the membrane thickness. The membrane Syngas-PVA-2 showed better performance than the membrane Syngas-PVA-1 since the former had a higher total carrier concentration in the membrane. The effects of membrane composition will be further discussed in Section 3.2.4. The above analyses of thickness effects are for thin facilitated transport membranes. Only when the membrane is thin, the diffusion rate will be high enough to be comparable with the reaction rate, and the reaction part thus needs to be considered to evaluate the CO2 transport rate. That is the reason why the membrane with a thickness of 15 μm showed higher permeability than the membrane with a thickness of 10 μm. However, for very thick membranes, the diffusion rate is much lower, which is not comparable with the reaction rate any more. Thus, the CO2 transport rate will be solely determined by the diffusion rate (the reaction rate is relatively fast and needs not to be considered any more). In this case, the transport process may be similar to the solution-diffusion process, and the membrane permeability may not be affected by membrane thickness significantly. 3.2.4. Effects of Membrane Composition on Separation Performance. Table 1 lists the compositions of the membranes, and Table 2 lists their separation performance. All these membranes had the same thickness of 15 μm. The feed pressure and temperature were maintained at 220 psia and 106 °C, respectively. The membrane Syngas-PVA-2 had a higher total carrier concentration than the membrane Syngas-PVA-1. As a result, the membrane Syngas-PVA-2 showed a similar CO2/H2 selectivity, but much higher CO2 permeability and permeance. The membrane Syngas-PVA-3 had the same total carrier amount, but a higher mobile carrier concentration compared

to the membrane Syngas-PVA-2. As a result, the membrane Syngas-PVA-3 showed higher CO2 permeability and permeance. The reason is that the mobile carrier has higher mobility, and it can contribute more to the facilitated transport of CO2.5 However, mobile carriers are small molecules and are not bound to polymer chains as strongly as fixed carriers because of lack of polymer chain entanglement; thus, the resulting membrane with a higher content of mobile carrier might be less dense. As a result, even though the membrane Syngas-PVA-3 showed higher CO2 permeability than the membrane Syngas-PVA-2, the former also showed significantly higher H2 permeability than the latter, which resulted in the lower CO2/H2 selectivity of the membrane Syngas-PVA-3. In membrane preparation, the increase of the total carrier amount and the increase of the mobile carrier amount both decreased the mechanical properties of the membranes. Thus, the carrier concentration must be controlled below a certain limit. In this work, the new membranes synthesized with all these membrane compositions showed very desirable mechanical properties. In summary, all these cross-linked PVA-based facilitated transport membranes showed very desirable CO2/H2 selectivity (>30). For the membranes with a thickness of 15 μm, the achieved permeability was as high as close to 1000 barrers, for example, the membrane Syngas-PVA-315 μ showed a permeability as high as 915 barrers. These membranes have great potential to be used for high-pressure synthesis gas purification. 3.2.5. More Membrane Performance Data. Table 3 lists more data achieved in our work for various synthesized membranes (PVA-Rep-1 to PVA-Rep-9). These membranes had the compositions either similar to Syngas-PVA-1 or similar to Syngas-PVA-2. These membranes possessed various PVA cross-linking degrees ranging from 15 mol % to 40 mol % with glutaraldehyde and had the thicknesses between 20 and 30 μm. As shown in Table 3, very desirable CO2/H2 selectivity, CO2 permeability, and CO2 permeance results were achieved for all these membranes. The testing conditions of these membranes are listed in the following: T = 106 °C, feed side pressure = 220 psia, sweep side pressure = 15 psia, feed gas flow rate = 166 cm3/min, sweep gas flow rate = 30 cm3/min, feed side liquid water injection rate = 0.01 cm3/min (feed side RH = 94.2%), sweep side liquid water injection rate = 0.120.15 cm3/min (sweep side RH = 69.072.1%). In this work, we did not carry out a systematic study on crosslinking degrees of the membranes. However, we did observe that a higher PVA cross-linking degree caused a lower membrane CO2 permeability. For example, membranes PVA-Rep-8 and PVA-Rep-9 with 40 mol % cross-linking were thicker compared to membranes PVA-Rep-1, PVA-Rep-2, and PVA-Rep-3 with 15 mol % cross-linking, and thus the former membranes should have given higher CO2 permeability based on the effect of membrane thickness studied in Section 3.2.3. However, the former membranes actually showed lower CO2 permeability, which was due to the higher PVA cross-linking degree. This observation is in accordance with our previous publication14 that the CO2-carrier reaction products, which are larger molecules than H2, are more difficult to transfer through the polymer network with a higher degree of cross-linking. This is also consistent with the CO2 permeability results obtained for PVA-Rep-7 and PVA-Rep-8; PVA-Rep-7 with a lower degree of cross-linking (30 mol %) showed a higher CO2 permeability than PVA-Rep-8 with a higher degree of cross-linking (40 mol %). But, the permeation of H2, which is a small molecule, is much less affected by the cross-linked network than that of the CO2-carrier reaction products. Thus, the 12158



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Table 3. More Separation Performance Data of Various Crosslinked PVA-Based Facilitated Transport Membranesa

Table 4. Separation Performance of SPBI-1-EDA-Based Facilitated Transport Membranea

PVA

CO2

cross-

membrane

linking

thickness

degree

(μm)

selectivity

(Barrer)

(GPU)

PVA-Rep-1 15 mol %

20

38.0

903

45.2

PVA-Rep-2 15 mol %

20

41.4

902

45.1

PVA-Rep-3 15 mol %

20

42.2

938

46.9

PVA-Rep-4 30 mol %

25

37.6

759

30.4

PVA-Rep-5 30 mol %

30

49.4

889

29.6

PVA-Rep-6 30 mol %

25

36.7

869

34.8

PVA-Rep-7 30 mol % PVA-Rep-8 40 mol %

30 30

62.4 45.5

929 784

31.0 26.1

PVA-Rep-9 40 mol %

30

39.8

800

26.7

membrane

CO2

CO2/H2

membrane

(μm)

selectivity

(Barrer)

(GPU)

Syngas-SPBI-1

50

20.1

892

17.8

CO2/H2 permeability permeance

T = 106 °C, feed side pressure = 220 psia, sweep side pressure = 15 psia, feed gas flow rate = 166 cm3/min, sweep gas flow rate = 30 cm3/min, feed side liquid water injection rate = 0.01 cm3/min (feed side RH = 94.2%), sweep side liquid water injection rate = 0.120.15 cm3/min (sweep side RH = 69.072.1%). a

CO2/H2 selectivity for PVA-Rep-7 with a lower degree of crosslinking (30 mol %) was higher than that for PVA-Rep-8 with a higher degree of cross-linking (40 mol %). 3.3. Separation Performance of SPBI-1-EDA-Based Facilitated Transport Membranes. In addition to the cross-linked PVA matrix, the SPBI-1 copolymer matrix was also studied in this work. The applications of SPBI-based facilitated transport membranes for low-pressure fuel cell synthesis gas purification have been reported from our previous work.3,4 Since the SPBI-1 copolymer is not water-soluble, DMSO was used as the solvent for all of the components in solution and membrane preparation. AIBA-K and PAA were used as the mobile and fixed carrier, respectively. The K2CO3KHCO3 mobile carrier was not used in this case because of its low solubility in DMSO. The microporous polysulfone support cannot be used since polysulfone can be dissolved in DMSO. Therefore, the microporous inorganic ceramic support was used for the SPBI-1-EDA-based facilitated transport membrane preparation. The surface of the ceramic support had a curvature. Thus, the prepared membranes by the solution-casting method were relatively thick (around 50 μm). Table 4 shows the performance of one example membrane Syngas-SPBI-1 with the composition of 20 wt % SPBI-1-EDA/60 wt % PAA/20 wt % AIBA-K. The SPBI copolymer is much more hydrophobic than the cross-linked PVA. Thus, the SPBI-1-EDA-based membrane showed the best performance at 100 °C instead of 106 °C for the cross-linked PVA-based membrane.3 The feed pressure was maintained at 220 psia. Similar to the testing for the cross-linked PVA-based membranes, the feed gas flow rate of 60 cm3/min, the sweep gas flow rate of 30 cm3/min, the feed side water injection rate of 0.01 cm3/min (100% RH in the feed side), and the sweep side water injection rate of 0.18 cm3/min (91.2% RH in the sweep side) were used in the testing. The effective membrane separation area was 5.7 cm2. By comparison, it could be seen that similar to the trend in low-pressure synthesis gas purification, the SPBI-1-EDA-based facilitated transport membrane showed significantly lower CO2/H2 separation performance compared to the cross-linked PVA-based membrane even for the high-pressure condition. The reason is

CO2

thickness

CO2

permeability permeance

T = 100 °C, feed side pressure = 220 psia, sweep side pressure = 15 psia, feed gas flow rate = 60 cm3/min, sweep gas flow rate = 30 cm3/min, feed side liquid water injection rate = 0.01 cm3/min (feed side RH = 100%), sweep side liquid water injection rate = 0.18 cm3/min (sweep side RH = 91.2%). a

that the cross-linked PVA matrix is much more hydrophilic than the SPBI-1 copolymer. Thus, the cross-linked PVA-based membranes could retain more water. As described earlier, water is very important for both the reaction between CO2 and carriers and the diffusion of CO2-carrier reaction products. However, as described earlier in this paper and in our previous publications,3,4 the SPBI-1-based membranes have the potential to be used for thin membrane preparation by the phase inversion technique because of the high copolymer Tg. Thus, the resulting asymmetric membrane could maintain its sustainable bottom porous structure even at high temperatures and pressures. In addition, water could be used as nonsolvent in phase inversion, which meets the common practice in the industrial process. The goal of this research work was the development of high performance membranes for high-pressure synthesis gas purification. Because of lower CO2/H2 separation performance, the SPBI-based membrane was not extensively studied. Hence, only limited data are presented in this section. 3.4. Preposition on Combination of CO2Selective Membranes with WGS Reaction. Low-temperature WGS reaction operates well at around 150 °C using commercial Cu/ZnO/ Al2O3 catalyst because of the exothermic nature of this reaction. As reported in our previous publications,8,9 in low-pressure synthesis gas purification, two approaches were used to combine CO2-selective facilitated transport membranes with WGS reactor to achieve CO reduction (99%), and the second step was the low-temperature WGS reaction. Since CO2 was almost completely removed in the first step, the WGS reaction equilibrium could be significantly shifted forward to reduce the exit CO concentration to less than 10 ppm in the second step. The CO2-selective facilitated transport membranes performed best at temperatures of 100120 °C, and the WGS reaction performed very well at around 150 °C. Consequently, the advantage of using this two-step process is that CO2 removal 12159


Industrial & Engineering Chemistry Research by the membrane can be done in the first step at 110120 °C, and the WGS reaction can be carried out in the second step at 150 °C. Thus, both of them can perform very well at their preferable temperatures, which can maximize the system performance.9 This work was an earlier investigation of CO2-selective facilitated transport membranes in high-pressure synthesis gas purification. For future studies in this purification, one may use similar approaches mentioned above.8,9 Considering that the membrane performance at high pressures was not as exceptional as that at low pressures because of carrier saturation and that the membrane performance at 150 °C will reduce further compared to the data at 100120 °C reported in this work, the best implementation at high feed gas pressures may be the aforementioned twostep process.9 In this process, CO2 removal by the membrane can be done in the first step at 106 °C, and the WGS reaction can be carried out in the second step at 150 °C. Thus, both of them can perform very well at their preferable temperatures, which can maximize the system performance. In addition, as the membrane operating temperature (106 °C) is not far from the operating temperature of WGS reactor (150 °C), the additional heating/ cooling energy consumption can be minimized compared to that of most solution-diffusion polymeric membranes, which only show desirable selectivity at around room temperature or lower.

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’ ACKNOWLEDGMENT We would like to thank Debbie de la Cruz and GE Infrastructure for giving us the GE A1 microporous polysulfone support and Dr. Henk Verweij’s research group in the Materials Science and Engineering Department of The Ohio State University for providing us the inorganic ceramic support. We would also like to thank the Shell Oil Company, the National Science Foundation, and the Office of Naval Research for the financial support of this work. Part of this material is based upon work supported by the National Science Foundation under Grants CBET-1033131 and IIP-1127812. ’ NOMENCLATURE l = membrane thickness (cm) N = steady-state permeation flux (cm3 (STP)/(cm2 s)) P = permeability (Barrer) p = pressure (atm) Δp = partial pressure difference between the feed and permeate sides (atm) x = retentate molar fraction y = permeate molar fraction Greek Letter

α = selectivity Subscripts

4. CONCLUSIONS The CO2-selective facilitated transport membranes based on the cross-linked PVA and SPBI-1 matrixes were successfully synthesized and characterized. The cross-linked PVA-based membranes showed the best separation performance at 106 °C. Higher temperatures resulted in lower separation performance because of the reduction of water retention in the membrane, and lower temperatures resulted in poorer separation performance because of the lower reaction and diffusion rates for the CO2carrier reaction and reaction products, respectively. However, the SPBI-1 copolymer-based membranes exhibited the best separation performance at 100 °C because of their lower hydrophilicity. The SPBI-based membranes generally showed lower separation performance because of also their lower hydrophilicity. For the cross-linked PVA-based membranes, with the increase of feed pressure, CO2/H2 selectivity, CO2 permeability, and CO2 permeance of the membranes all reduced because of carrier saturation. With the decrease of membrane thickness, CO2 permeance (flux) increased, but both CO2/H2 selectivity and CO2 permeability reduced because of the facilitated transport mechanism that is controlled by both diffusion and reaction. The increase of either the total carrier amount or the mobile carrier ratio can increase the separation performance of the resulting membrane. As a result, at the temperature of 106 °C and a high feed pressure of 220 psia, the crosslinked PVA-based membranes with thicknesses of 1530 μm synthesized in this work showed very desirable CO2/H2 selectivity (higher than 30) and CO2 permeability (close to 1000 barrers). ’ AUTHOR INFORMATION Corresponding Author

*Phone: (614) 292-9970. Fax: (614) 292-3769. E-mail: [email protected].

i, j = species

’ REFERENCES (1) Ghenciu, A. F. Review of Fuel Processing Catalysts for Hydrogen Production in PEM Fuel Cell System. Curr. Opin. Solid State Mater. Sci. 2002, 6 (5), 389. (2) Kroschwitz, J. I.; Howe-Grant, M. Encyclopedia of Chemical Technology, 4th ed.; John Wiley & Sons: New York, 1995. (3) Bai, H.; Ho, W. S. W. New Carbon Dioxide-Selective Membranes Based on Sulfonated Polybenzimidazole (SPBI) Copolymer Matrix for Fuel Cell Applications. Ind. Eng. Chem. Res. 2009, 48, 2344. (4) Bai, H.; Ho, W. S. W. Recent Developments in Fuel-Processing and Proton-Exchange Membranes for Fuel Cells. Polym. Int. 2011, 60, 26. (5) Tee, Y. H.; Zou, J.; Ho, W. S. W. CO2-Selective Membranes Containing Dimethylglycine Mobile Carriers and Polyethylenimine Fixed Carrier. J. Chin. Inst. Chem. Eng. 2006, 37, 37. (6) Zou, J.; Ho, W. S. W. CO2-Selective Polymeric Membranes Containing Amines in Crosslinked Poly(Vinyl Alcohol). J. Membr. Sci. 2006, 286, 310. (7) Huang, J.; El-Azzami, L.; Ho, W. S. W. Modeling of CO2Selective Water Gas Shift Membrane Reactor for Fuel Cell. J. Membr. Sci. 2005, 261, 67. (8) Zou, J.; Huang, J.; Ho, W. S. W. CO2-Selective Water Gas Shift Membrane Reactor for Fuel Cell Hydrogen Processing. Ind. Eng. Chem. Res. 2007, 46, 2272. (9) Zou, J.; Ho, W. S. W. Hydrogen Purification for Fuel Cells by Carbon Dioxide Removal Membrane Followed by Water Gas Shift Reaction. J. Chem. Eng. Jpn. 2007, 40 (11), 1011. (10) Huang, J.; Zou, J.; Ho, W. S. W. Carbon Dioxide Capture Using a CO2-Selective Facilitated Transport Membrane. Ind. Eng. Chem. Res. 2008, 47, 1261. (11) Huang, J.; Ho, W. S. W. Effects of System Parameters on The Performance of CO2-Selective WGS Membrane Reactor for Fuel Cells. J. Chin. Inst. Chem. Eng. 2008, 39, 129. (12) Mandal, B.; Ho, W. S. W. Synthesis Gas Purification by Polymeric Membranes Containing Fixed and Mobile Carriers. Int. J. Chem. Sci. 2007, 5 (4), 1938. 12160



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(13) Huang, J.; Zou, J.; Ho, W. S. W. In Hydrogen and Syngas Production and Purification Technologies; Liu, K., Song, C., Subramani, V., Eds.; John Wiley & Sons: New York, 2010; Chapter 9, pp 385413. (14) Xing, R.; Ho, W. S. W. Crosslinked Polyvinylalcohol-Polysiloxane/Fumed Silica Mixed Matrix Membranes Containing Amines for CO2/H2 Separation. J. Membr. Sci. 2011, 367, 91. (15) Pritchard, J. G. Poly(Vinyl Alcohol), Basic Properties and Uses; Gordon and Breach Science Publishers: London, U.K., 1970. (16) Zou, J. Carbon Dioxide-Selective Membranes and Their Applications in Hydrogen Processing. Ph.D. Dissertation, The Ohio State University, Columbus, Ohio, 2007. (17) Ho, W. S. W.; Sirkar, K. K. Membrane Handbook; Chapman & Hall: New York, 1992. (18) Yamaguchi, T.; Boetje, L. M.; Koval, C. A.; Noble, R. D.; Bowman, C. N. Transport Properties of Carbon Dioxide through Amine Functionalized Carrier Membranes. Ind. Eng. Chem. Res. 1995, 34, 4071. (19) Bai, H.; Ho, W. S. W. New Sulfonated Polybenzimidazole (SPBI) Copolymer-Based Proton-Exchange Membranes for Fuel Cells. J. Taiwan Inst. Chem. Eng. 2009, 40, 260. (20) Jegal, J.; Lee, K. Nanofiltration Membranes Based on Poly(Vinyl Alcohol) and Ionic Polymers. J. Appl. Polym. Sci. 1999, 72, 1755. (21) Huang, J. CO2 (H2S) Membrane Separations and WGS Membrane Reactor Modeling for Fuel Cells. Ph.D. Dissertation, The Ohio State University, Columbus, Ohio, 2006. (22) Ho, W. S. W.; Dalrymple, D. C. Facilitated Transport of Olefins in Ag+-Containing Polymer Membranes. J. Membr. Sci. 1994, 91, 13.

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