Comparing the Absorption Performance of Packed Columns and

Jun 4, 2005 - This paper evaluates the performance of a GAM system and a packed column using the overall mass transfer coefficient (KGav) as a basis f...
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SEPARATIONS Comparing the Absorption Performance of Packed Columns and Membrane Contactors David deMontigny,† Paitoon Tontiwachwuthikul,*,† and Amit Chakma‡ Faculty of Engineering, University of Regina, Regina, Saskatchewan, Canada S4S 0A2, and Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1

Several technologies have been developed for capturing carbon dioxide (CO2), but absorption remains the most suitable method for large-scale industrial operations. In recent years the use of gas absorption membrane (GAM) systems has been explored as an alternative to traditional packed columns. This paper evaluates the performance of a GAM system and a packed column using the overall mass transfer coefficient (KGav) as a basis for comparison. The GAM system tested microporous polypropylene (PP) and poly(tetrafluoroethylene) (PTFE) hollow fiber membranes while the packed column contained Sulzer DX structured packing. Aqueous solutions of monoethanolamine (MEA) and 2-amino-2-methyl-1-propanol (AMP) were used in both absorbers. Experimental results showed that the GAM system performed better than the packed column. GAM systems deserve the attention they have been receiving from researchers as they have significant potential to replace packed columns. 1. Introduction In the last 15 years there has been a great deal of interest generated in the area of CO2 capture. This increased interest has been driven in part by environmental concerns over the release of CO2, a known greenhouse gas, into the atmosphere. Traditionally, packed columns have been used as the contacting device in absorption systems. Although packed columns have attained considerable success in industry, they suffer from various operational problems including flooding, channeling, entrainment, and foaming.1 Furthermore, packed columns tend to be large and expensive to build. One promising alternative to packed columns is the GAM system.2,3 In this arrangement a microporous membrane separates the gas and liquid phases. Contact between phases is made after the gas diffuses through the porous membrane. The concept is shown in Figure 1 for a hollow fiber membrane system. This type of arrangement allows for independent rates of liquid and gas flow, thereby eliminating the operational problems seen in packed columns. Additional advantages lie in the modular nature of membrane systems. GAM modules have a very high surface area-to-volume ratio, and scale-up is straightforward since the effective area equals the membrane surface area. On the negative side, the membrane itself adds an additional level of resistance to the mass transfer process. This resistance can be significant if the membranes are wetted by the absorption solution.4 According to Falk-Pedersen and * To whom correspondence should be addressed. Tel.: (306) 585-4160. Fax: (306) 585-4855. E-mail: paitoon@ uregina.ca. † University of Regina. ‡ University of Waterloo.

Figure 1. Mass transfer in a microporous hollow fiber GAM system.

Dannstro¨m,5 GAM systems are the best alternative to packed columns. Performance comparisons between packed columns and GAM systems have been reported in the literature by several authors. For example, Nii et al.6 used poly(dimethylsiloxane) membranes in a GAM system to absorb CO2 into aqueous solutions of sodium hydroxide (NaOH) and potassium carbonate. They achieved absorption rates comparable to results published for a packed tower containing Rashig rings. Karoor and Sirkar7 used PP membranes in a GAM system that absorbed CO2 into water. The mass transfer values obtained in their GAM system were roughly 5 times larger than published data for various randomly packed columns. Nishikawa et al.8 reported similar findings for a GAM system which absorbed CO2 into MEA solutions using polyethylene and PTFE membranes. Results were on average 5 times greater than mass transfer values published for randomly packed columns containing steel rings and ceramic Berl saddles.9 Finally, work by

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Rangwala10 studied CO2 absorption into water, NaOH, and diethanolamine solutions in a GAM system containing PP membranes. Mass transfer results varied from 3 to 9 times higher than the results published for a packed column containing Rashig rings. The research work conducted on GAM systems thus far is positive and indicates that they may be a suitable alternative to packed columns. However, previous comparisons should be considered carefully because there was no mention of both systems being analyzed under similar operating conditions. Furthermore, the comparisons with packed columns only considered absorbers containing random packings, while it is well established that structured packing offers a superior performance.11,12 These two considerations raise concern over the accuracy of performance comparisons conducted in the past. The current work attempts to eliminate some of the uncertainty by (1) conducting experiments with high-efficiency structured packing and (2) ensuring that the operating conditions in each absorber are the same. The CO2 absorption performance was compared using the KGav as a basis. The packed column contained Sulzer DX structured packing, while the GAM system tested both PP and PTFE membranes using one, two, and three modules in series. Aqueous solutions of MEA and AMP were used in both systems. The two contacting devices were operated under similar experimental conditions, placing special emphasis on the CO2 mole ratio (YA) and solution CO2 loading (R), allowing for a more reliable and accurate performance comparison. 2. Theory Absorption occurs when mass transfers from a gas phase into a liquid phase. In absorption systems with chemical reactions, the driving force for mass transfer is the difference in alkalinity between the solute and the solvent. Since CO2 is an acid gas, it can be absorbed by any alkaline solution. Mass transfer processes inside packed columns and GAM systems have been described using the film theory.6,10,13,14 The overall rate of mass transfer (NA) depends on this driving force and the resistance to mass transfer. Mathematically this is given by:

NA ) kGP(yA,G - yA,M) )

kMP (y - yA,i) ) RT A,M kL(CA,i - CA,L) (1)

where P is the system pressure, yA,G is the mole fraction of solute A in the bulk gas, yA,M is the mole fraction of solute A at the gas-membrane interface, and yA,i is the mole fraction of solute A at the membrane-liquid interface. The individual gas, membrane, and liquid mass transfer coefficients are given by kG, kM, and kL, respectively. Since the driving force for mass transfer takes place over a very small distance, it is more convenient to represent the rate of mass transfer in terms of the overall gas-phase mass transfer coefficient, KG:11

NA ) KGP(yA,G - y/A,G)

(2)

where y/A,G is the mole fraction of solute A in equilibrium with the bulk liquid. The overall gas-phase mass transfer coefficient is based on the individual gas and liquid-phase mass transfer coefficients. The film theory

assumes that mass transfer resistances are in series and are represented by the reciprocals of the individual gas and liquid-phase mass transfer coefficients, with H representing Henry’s constant3:

1 1 RT H ) + + K G kG kM kL

(3)

2.1. Overall Mass Transfer Coefficient. Transfer unit theory has been used for a number of years to design absorption columns, and it is in this theory where the KGav appears in engineering design.15 The challenge has always been to design an appropriate height for the packed column. The height of an absorption column (Z) is given by the height of a transfer unit (HTU), multiplied by the number of transfer units (NTU):

Z ) HTU × NTU

(4)

or

Z)

( ) (∫ GI × KGavP

dyA,G

yA,out

yA,in

(1 - yA,G)(yA,G - y/A,G)

)

(5)

By differentiating eq 5 and rearranging we can obtain an expression for KGav given by:

KGav )

(

GI

P(yA,G -

y/A,G)

)( ) dYA,G dZ

(6)

where KGav is the overall volumetric gas-phase mass transfer coefficient, GI is the inert gas flow rate and dYA,G/dZ is the solute concentration gradient. The y/A,G term can be evaluated using solubility data. For CO2 absorption into MEA and AMP solutions, the y/A,G term can be assumed to be zero since the chemical reaction is fast. With this in mind, eq 6 is easy to solve because all of the remaining variables can be measured in absorption experiments. The graphical determination of the dYA,G/dZ term has been explained elsewhere.11 In short, it is determined by analyzing the gas-phase CO2 concentration profile along the length of an absorber. This technique is advantageous because the KGav value can be calculated at specific CO2 mole ratios, allowing for a more accurate comparison of results between experiments. Another advantage of using the KGav as a basis for comparison is that the av term accounts for the effective surface areas in both the packed column and GAM systems. While the membrane surface area can be approximated with reasonable accuracy in GAM systems, the wetted area in a packed column is difficult to determine. Since the packed column and GAM systems have different packing densities, void fractions, and total surface areas for mass transfer, this is particularly useful. 3. Experimental Work The experiments were conducted in two phases: (1) packed column studies, and (2) GAM system studies. The packed column was tested first to establish base cases for comparison. GAM system experiments began with the PP membranes and concluded with the PTFE membranes. Both absorption systems were tested using aqueous MEA and AMP solutions. The chemical reac-

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Table 1. Characteristics of the Membranes, Membrane Modules, and Packed Column description outer diameter (mm) inside diameter (mm) membrane length (m) number of fibers outside specific area (m2/m3) module outside area (m2) void fraction (%) membrane porosity (%)

PP PTFE DX membranes membranes column 0.300 0.244 0.145 1550 2752.0 0.212 82.2 35.0

2.0 1.0 0.122 57 581.6 0.044 70.9 50.0

28.0 900.0 1.200 77.5 -

tions that occur in CO2-MEA and CO2-AMP systems have been explained elsewhere.16,17 3.1. Packed Column Study. The absorber in this study contained DX structured packing from Sulzer Chemtech Ltd. in Switzerland. This stainless steel gauze packing had a specific surface area of 900 m2/m3. The absorption column was made out of acrylic tubing with a total height of 2.40 m, an inside diameter of 28 mm, and a wall thickness of 6 mm. Based on the column design, the maximum total available packing surface area was roughly 1.20 m2. The column was operated under counter-current flow with the sour gas entering at the bottom and the lean liquid solvent entering at the top. Three liquid distributors within the column reduced liquid channeling along the wall. Packing elements were rotated 90° with respect to each other. Gas and temperature sampling points were located along the length of the column, which was wrapped with 13-mm foam insulation for adiabatic operation. 3.2. GAM System Study. The design of GAM systems predominantly depends on the process requirements.18 With this in mind, the modules were designed to be small, modular, and reusable. Since the main goal of this work was to compare the performance of the membrane unit to a traditional packed column, the design of the GAM module is similar to the design of the packed column. Modules were made from acrylic tubing with a 28-mm inside diameter and a 6-mm wall thickness. Each module was roughly 0.25 m long with gas, liquid, and temperature sampling points located along its length. Microporous PP hollow fibers from Mitsubishi Rayon Ltd. (Japan) and microporous PTFE hollow fibers from Sumitomo Electric Fine Polymer (Japan) were tested in the GAM system. The membranes were potted to removable cartridges that could be placed into a GAM module shell. Several epoxies were tested for their resistance to MEA and AMP solutions. Loctite E-60NC Hysol performed the best and was used to pot both the PP and PTFE fibers. Since PTFE is a hydrophobic material, its potting surface needed to be defluorinated.19 The ends of the PTFE fibers were etched with FluorEtch prior to potting. The characteristics of the membranes and the GAM modules are listed in Table 1 along with some packed column characteristics. Note that the membrane values are listed per membrane module. For example, if three modules were used in series, the total surface area in the PP and PTFE systems would be 0.636 m2 and 0.132 m2, respectively. This is much smaller than the total surface area of 1.20 m2 that is available in the packed column. In this study the experiments were done with the gas phase flowing through the fiber lumen and the liquid phase flowing through the shell side of the GAM module. The cost to pump the liquid through the fiber

lumen becomes excessive when the hollow fibers have a small diameter, in the range of a few hundred micrometers.20 The GAM system was operated in counter-current mode of operation with the gas entering at the top and the liquid entering at the bottom. This arrangement is opposite to the packed column, and was required to have the liquid flow contact all of the membrane surface area. If the flow orientation had been identical to that of the packed column, the liquid solution would have dripped through the shell side of the module and contacted the membranes ineffectively. GAM systems, similar to packed columns, have been shown to perform better when operated under countercurrent flow conditions.21,22 3.3. Contactor Operation and Data Collection. The operation of the packed column and GAM system is virtually identical. Prior to an experiment, an aqueous solution of either MEA or AMP was prepared to the desired concentration using deionized water. Concentrations were verified by titrating a known sample volume with 1.0 N hydrochloric acid solution using methyl orange as an indicator. Solution CO2 loading was determined using the procedure outlined by the Association of Official Analytical Chemists.23 At the start of an individual run the gas (air + CO2) was introduced to the contactor. Air and CO2 flow rates were controlled using calibrated flow meters. An infrared CO2 gas analyzer was used to verify that the desired concentration of CO2 was present in the feed gas stream. Once this had been achieved, the liquid solution was introduced to the system. Liquid flow rates were controlled with the use of a variable area flow meter, calibrated by collecting a known volume of liquid over time. Depending on the operating conditions, steady state was usually reached within 20-40 min for the packed column and 30-45 min for the GAM system. During steady-state operation the concentration of CO2 in the gas phase was measured along the length of the contactor using the infrared gas analyzer. Temperatures inside the contactor were recorded, and liquid samples were taken and analyzed for their CO2 loading. Experiments in both the packed column and GAM system were run separately in a batch mode. Figure 2 shows a schematic of the experimental setup. Experimental operating conditions are listed in Table 2 for both systems. The gas flow rates are presented on a crosssectional area basis. Since the gas flowed through the hollow fiber membranes, the values are larger for the GAM experiments. However, the total gas flow through each system was the same on a liter per minute basis. Note that the CO2 inlet gas concentrations vary more widely in the GAM system. This was required to generate the same CO2 mole ratio and solution CO2 loading values that were obtained in the packed column, allowing for a more accurate performance comparison. An added feature of the GAM design used in this study is the ability to obtain a CO2 concentration profile along the length of the GAM system. When the gas flows in the fiber lumen, the CO2 gas-phase concentration can be sampled at points located between modules. This allows for an accurate determination of the dYA,G/dZ term used in the calculation of the KGav. In cases when only one module was used the CO2 concentration profile was assumed to be linear from the inlet to the outlet. Analysis of results showed that this assumption was just as accurate as using an empirical approach.

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Figure 4. Effect of MEA solution concentration on the KGav in the GAM system using PP membranes in one module. YCO2 ) 0.16, R ) 0.29, GI ) 268.0 kmol/m2‚h.

Figure 2. Process flow diagram for the packed column and GAM system.

Figure 3. Effect of inert gas flow rate on the KGav in the GAM system using PP membranes in one module with MEA solution. MMEA ) 2.0 kmol/m3, YCO2 ) 0.16, R ) 0.08, L ) 8.45 m3/m2‚h. Table 2. Experimental Operating Conditions operating condition

packed column

GAM system

gas flow rate (kmol/m2‚h) liquid flow rate (m3/m2‚h) MEA concentration (kmol/m3) AMP concentration (kmol/m3) CO2 feed concentration (%) feed solution CO2 loading (mol/mol)

30.9 5.1-12.6 2.0 2.0 13.5-14.7 0.01-0.30

150.3-434.0 5.3-12.6 1.0-5.0 2.0 8.4-14.6 0.06-0.30

4. Results and Discussion 4.1. Effect of Operating Parameters. Previous work in our group has studied the effects of operating parameters on mass transfer performance in packed columns.11,12 Prior to comparing the performance of GAM systems and packed columns, initial work was done to determine whether the GAM system would behave in a manner similar to that of the packed column when operating conditions varied. Tests were conducted to evaluate the effect of the gas flow rate, liquid flow rate, and solution concentration. The effect of the gas flow rate on the KGav is shown in Figure 3 for the GAM

system. Changes in the gas flow rate do not affect the KGav in GAM systems, a result consistent with the findings in the packed column studies. This is because the controlling factor in the CO2 absorption process is the liquid-side mass transfer.24 Regardless of the concentration of CO2 gas at the gas-liquid interface, the reaction remains constant due to the availability of free amine. The effects of liquid flow rate and solution concentration on the KGav are shown in Figure 4 for the GAM system. Increasing the liquid flow rate improved the KGav values. At higher liquid flow rates the interaction between CO2 and amine molecules increases per unit time. This results in a higher kL, which is proportional to the KG as seen in eq 3. Results from the solution concentration study show that the KGav increases with concentration at low concentration ranges (1.0-3.0 mol/ L). This is due to an increase in the amount of free amine molecules per unit volume. At higher concentrations, in the 5.0 mol/L range, the KGav values start to decrease. This can be attributed to an increased solution viscosity, which offsets the advantage of the excess free amine molecules. These results are consistent with previous work in our group using packed columns.12 It is important to note that increased solution flow rates lead to higher circulation and regeneration costs, while increased solution concentrations are expensive and accelerate corrosion rates in steel vessels. Maximizing these two operating parameters may not necessarily improve the overall system. 4.2. Comparing the GAM System and Packed Column. Results from the GAM system were compared to those from the packed column using the KGav as a basis. All of the results reported here were measured when the CO2 mole ratio in the system was Y ) 0.09 and the solution CO2 loading was R ) 0.30 mol CO2/ mol amine. This type of comparison increases the confidence in the results since the GAM system and packed column were compared under similar operating conditions. MEA Experiments. Experiments with MEA tested the GAM system with one, two, and three modules operated in series. In all cases the GAM system performed better than the packed column; however, the degree of improvement depended on the system configuration and type of membrane being used. Figure 5 compares the results from the packed column with those from GAM systems containing PP and PTFE mem-

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Figure 5. Performance of PP and PTFE GAM systems using MEA solution in one module compared to that for the packed column. MMEA ) 2.0 kmol/m3, YCO2 ) 0.09, R ) 0.30, GI,PTFE ) 434.0 kmol/ m2‚h, GI,PP ) 270.6 kmol/m2‚h, GI,DX ) 30.9 kmol/m2‚h.

Figure 7. Performance of PP and PTFE GAM systems using MEA solution in three modules in series compared to that for the packed column. MMEA ) 2.0 kmol/m3, YCO2 ) 0.09, R ) 0.30, GI,PTFE ) 434.0 kmol/m2‚h, GI,PP ) 266.9 kmol/m2‚h, GI,DX ) 30.9 kmol/ m2‚h.

Figure 6. Performance of PP and PTFE GAM systems using MEA solution in two modules in series compared to that for the packed column. MMEA ) 2.0 kmol/m3, YCO2 ) 0.09, R ) 0.30, GI,PTFE ) 434.0 kmol/m2‚h, GI,PP ) 266.9 kmol/m2‚h, GI,DX ) 30.9 kmol/ m2‚h.

Figure 8. Effect of additional membrane modules on the KGav in GAM systems using PP and PTFE membranes with MEA solution. MMEA ) 2.0 kmol/m3, YCO2 ) 0.09, R ) 0.30, GI,PTFE ) 434.0 kmol/ m2‚h, GI,PP ) 266.9-270.6 kmol/m2‚h.

branes in one module. On average the PP and PTFE membranes performed 41% and 152% better than the packed column, respectively. Both GAM systems show a much more pronounced increase in the KGav values with liquid flow rate than the packed column. This could be due to the independent liquid and gas flow rate system in the GAM module. A similar result is seen in Figure 6 when two modules were used in series. In this case the average improvement in performance over the packed column was 126% for the PP membranes and 160% for the PTFE membranes. The improved performance of the PP membranes is the first indication that membrane wetting may be affecting the PP membranes. It has been reported in the literature that amine solutions wet PP membranes.10,25 The wetting was believed to be a result of chemical interaction between the amine and PP that changed the surface properties of the membrane. Several authors have confirmed that wetting should be avoided since it increases the resistance to mass transfer.7,26,27 Finally, Figure 7 shows the results obtained when three modules were used in series. The PP system produced mass transfer coefficients that were 81% better than the packed column, while the PTFE system achieved an average of 167% in performance.

The results presented in Figures 5-7 show that the PTFE membranes performed better than the PP membranes. In the absence of wetting it is reasonable to expect that the increase in the level of performance the PTFE membranes had over that of the PP membranes would remain consistent. This was not the case, however, as the PTFE membranes performed 83, 15, and 48% better than the PP membranes when one, two, and three modules were used, respectively. This indicates that wetting likely affected the performance of the PP membranes. Another consideration is the stability of the KGav values when additional modules are added to the system. Having one, two, or three modules in series should not affect the KGav value since it is calculated on an overall volumetric basis. This is evident in Figure 8 where the results from the PP and PTFE experiments with one, two, and three modules have been plotted on the same graph. In the case of the PTFE membranes, the KGav values fall in the same general range. This is expected since PTFE membranes are not wetted by alkanolamine solutions. The PP membranes, however, show no performance consistency with the addition of a second and third module. This can be attributed to the fact that PP membranes are wetted by alkanolamine solutions. The PP results for one module were generated

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Figure 9. Performance of PP and PTFE GAM systems using AMP solution in three modules in series compared to that for the packed column. MAMP ) 2.0 kmol/m3, YCO2 ) 0.09, R ) 0.30, GI,PTFE ) 434.0 kmol/m2‚h, GI,PP ) 266.9 kmol/m2‚h, GI,DX ) 30.9 kmol/ m2‚h.

after the inert gas flow, liquid flow rate, and solution concentration studies. It is very likely that this membrane cartridge had begun to suffer the effects of wetting by the time these experiments were conducted, resulting in the low curve seen in the graph. When the second module was added, a fresh set of PP membranes was introduced to the system, resulting in a jump in the performance. When the third module was added, there was no jump in performance. Instead, the performance dropped, due to the fact that two-thirds of the GAM system contained membranes that had been damaged by wetting. The performance with three modules was not as good as when two modules were used, but it was better than when only a single module had been used since the PP membranes in the third cartridge were new. AMP Experiments. The experiments with AMP were done using three modules in series. Initial tests with one and two modules did not provide sufficient contact time for the gas and liquid phases. Results from the AMP runs are shown in Figure 9 for both PP and PTFE membranes. Once again the GAM system has been shown to perform better than the packed column. One run with PP membranes at a low flow rate did not perform as well as the packed column. The low liquid flow rate could be a contributing factor, but a more likely cause was membrane wetting. The AMP runs were done after the MEA tests, and the PP membranes had already suffered from the effects of wetting. Had new PP membranes been available for the AMP tests, the PP curve in Figure 9 would have been higher. On average, the PP and PTFE membranes produced KGav values in the AMP-GAM system that were 18% and 430% higher than those obtained in the packed column. A direct comparison of the performance of PP and PTFE membranes is given in Figure 10 using both MEA and AMP solutions. The superior performance of the PTFE membranes is obvious for both solutions. Even though the PP membranes were susceptible to wetting, this was an unexpected result since the PP modules offered an average of 380% more surface area than the PTFE membrane modules. While the amount of surface area available for mass transfer is important, it is not the determining factor. Membrane porosity and permeability also affect system performance. In this work the porosity of the PP and PTFE membranes was 35% and

Figure 10. Comparing the performance of PP and PTFE GAM systems using MEA and AMP solutions in three modules in series. MAmine ) 2.0 kmol/m3, YCO2 ) 0.09, R ) 0.30, GI,PTFE ) 434.0 kmol/ m2‚h, GI,PP ) 266.9 kmol/m2‚h.

50%, respectively. Unfortunately, permeability data for the membranes was not available. The reduced performance of the PP membranes, when compared to that of the PTFE membranes, can be attributed to their wettability and lower porosity. Another possibility for reduced performance in the PP membrane system is liquid channeling through the module. The PP membrane cartridges were potted in bunches, whereas the PTFE membranes were potted individually. It is possible that the liquid may not have been exposed to all of the membrane surface area in the PP membrane system. Finally, it should be mentioned that MEA performed better than AMP in all situations. This is evident in Figure 10 and was expected since MEA has a faster reaction rate with CO2 when compared to AMP. Despite the lower performance when AMP solutions were used, the results show that regardless of the solvent, GAM systems perform better than packed columns. 5. Conclusions Previous comparisons between GAM systems and packed columns have been reported in the literature. However, these comparisons did not consider operating conditions and were made with packed columns containing random packings. This paper has presented a more accurate performance comparison between these two contacting devices. On the basis of the experimental work, the following conclusions can be made: (1) Changes in the gas flow rate, liquid flow rate, and solution concentration affect GAM systems in the same manner that they affect packed columns. (2) The GAM systems tested in this study produced KGav values that were up to 4 times larger than the values obtained in a packed column containing Sulzer DX structured packing. (3) The PP membranes were susceptible to wetting and were outperformed by the PTFE membranes. Other factors may have also played a role, including membrane porosity, permeability, and the effects of liquid channelling in the PP membrane system. (4) Experiments with MEA produced better results than tests with AMP; however, AMP is still a suitable solvent for use in GAM systems since results were better than those in the DX-AMP tests. The above findings present a positive case for the use of GAM systems in CO2 capture applications. Recently,

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membranes have been proposed to replace the structured packing in distillation columns.28 This could lead to smaller membrane-based regenerators. The CO2 capture process is poised to benefit from advances in membrane technology that lead to better absorbers and regenerators. Acknowledgment Financial support from the NSERC, IODE Canada, and The City of Regina is gratefully acknowledged. We also thank Sulzer Chemtech (Switzerland) and Mitsubishi Rayon (Japan) for donating the DX structured packing and polypropylene membranes. Nomenclature CA,i ) concentration of solute at the interface (kmol/m3) CA,L ) concentration of solute A in the bulk liquid (kmol/m3) GI ) inert gas flow rate (kmol/m2‚h) H ) Henry’s law constant (kPa‚m3/kmol) HTU ) height of a transfer unit (m) kG ) gas-phase mass transfer coefficient (kmol/m2‚s‚kPa) kL ) chemical liquid-phase mass transfer coefficient (m/s) kM ) membrane mass transfer coefficient (m/s) KG ) overall gas-phase mass transfer coefficient (kmol/m2‚s‚kPa) KGav ) overall volumetric gas-phase mass transfer coefficient (kmol/m3‚h‚kPa) NA ) mass transfer flux of solute A (kmol/m2‚s) NTU ) number of transfer units P ) total system pressure (kPa) R ) gas constant (m3‚kPa/kmol‚K) T ) temperature (K) yA,G ) mole fraction of solute A in the bulk gas phase yA,i ) mole fraction of solute A at the membrane-liquid interface yA,M ) mole fraction of solute A at the gas-membrane interface y/A,G ) mole fraction of solute A in equilibrium with CA,L YA ) mole ratio of solute A in the bulk gas (kmol CO2/kmol air) Z ) height of the packed column or effective membrane length (m) R ) solution CO2 loading (kmol CO2/kmol amine)

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Received for review October 20, 2004 Revised manuscript received March 9, 2005 Accepted April 25, 2005 IE040264K