Department of Chemical Engineering, University of Connecticut, Storrs, Connecticut

Transcription

1 1542 Ind. Eng. Chem. Res. 2005, 44, SEPARATIONS Development of Supported Ethanolamines and Modified Ethanolamines for CO 2 Capture T. Filburn,* J. J. Helble, and R. A. Weiss Department of Chemical Engineering, University of Connecticut, Storrs, Connecticut Liquid amines can be immobilized within the pores of polymeric supports to provide a regenerable CO 2 sorbent. This paper describes the capture of CO 2 by a range of ethanolamines (primary, secondary, and tertiary) immobilized within the pores of high-surface-area poly(methyl methacrylate) beads. These supported amines were used to remove low concentrations (7.6 mmhg) of CO 2 by a pressure swing absorption process, where low-pressure vacuum was used to desorb the CO 2 and regenerate the sorbent. The effect on CO 2 capture of modification of primary amines to secondary amines by reaction with acrylonitrile was also evaluated. The modified amines provided nearly a factor of 2 increase in CO 2 removal capacity compared to the original primary amines. These results suggest that modified amines could potentially be used for CO 2 capture in space life support systems as well as for terrestrial flue gas CO 2 removal applications. Introduction Long-duration, human-occupied space missions require the use of regenerable sorbents for CO 2 capture. Regenerable sorbents provide a reduction in overall system weight and volume compared to single-use sorbents and, therefore, decrease the storage volume and launch weight of the space vehicle. Liquid amines are particularly suited to this purpose, as their efficacy in removing carbon dioxide and their regenerability have been demonstrated in a host of industrial applications. 1,2 Liquid-amine-based scrubbing systems, however, generally require large towers for contacting the liquid amine absorbent with the process gas stream, making them impractical for confined-space applications. This problem has been circumvented by immobilizing liquid amines within the pores of a solid support, thus permitting their use without requiring a separate phase separation step. 3,4 CO 2 removal and sorbent regeneration are subsequently accomplished though pressure swing absorption. 5 These supported amines therefore provide an attractive means for using liquid amines as CO 2 removal agents in the microgravity environment of space. In designing a supported-amine-based system, the selection of the optimum liquid amine to be immobilized within a support remains a challenging problem. Liquid alkanolamine sorbents have been used for the removal of carbon dioxide from gas streams for many years, since Bottoms 6 introduced the use of triethanolamine (TEA) for the regenerative removal of CO 2 from natural gas streams. Since that time, numerous refinements have been made in the use of these absorbents, 7-9 including the utilization of alternate amine formulations to provide higher CO 2 removal capacities and lower regeneration energy costs. The first of these changes switched from the relatively low capacity and low reactivity of TEA (a tertiary amine) to monoethanolamine (MEA), a primary amine. A more recent innovation has been the use of secondary amines (e.g., diethanolamine, DEA) or hindered primary amines, which have lowered the regeneration energy necessary to reuse the amine solutions. 10 It is well-known that primary, secondary, and tertiary amines have different pk a values and consequently differing affinities for acid gases such as carbon dioxide. 11 These pk a values will also be affected by the physical state of the amines. In aqueous solutions, the basicities of the amine increase from the least basic primary amine to tertiary and finally to secondary. In the gas phase, the basicity increases from primary to secondary to tertiary amine. 11 Most prior research on CO 2 removal by amines has concentrated on measuring capacities for aqueous amine solutions. The research described herein examined the CO 2 removal capacities of different amines immobilized on a solid support, and the specific goal of the research was to ascertain how the type of amine (primary, secondary, or tertiary) affected steady-state CO 2 capacity in a pressure swing absorption system. Reaction of Amines with CO 2 In general, the industrial use of amine sorbents has centered on aqueous solutions of primary and secondary amines, which react directly with CO 2 to form carbamate ions, RNHCOO -. Reaction 1 shows the formation of the carbamate ion for a primary amine. A similar reaction occurs for secondary amines. * To whom correspondence should be addressed. Current addresss: Department of Mechanical Engineering, University of Hartford, West Hartford, CT filburn@hartford.edu. 2RNH 2 + CO 2 w RNHCOO - + RNH 3 + RNHCOO - + H 2 O w RNH 2 + HCO 3 - (1) (2) /ie CCC: $ American Chemical Society Published on Web 01/29/2005

2 Ind. Eng. Chem. Res., Vol. 44, No. 5, Figure 2. Structure of the TEPA molecule (normal). Table 1. Integral Heats of Solution for Absorption of CO 2 1 amine type integral heat of solution (cal/g of CO 2) MEA primary 1485 DEA secondary 1260 DGA primary 1476 MDEA tertiary 1035 TEA tertiary 837 DIPA secondary 1296 Figure 1. Chemical structures of ethanolamines (primary, MEA; secondary, DEA; and tertiary, TEA). Water can hydrolyze the carbamate and regenerate one amine molecule (reaction 2), but because of the stability of the carbamate ion, this reaction does not occur readily. The ability to form carbamate ions allows for direct reaction of the amine with CO 2, which produces faster CO 2 capture kinetics for the primary and secondary amines. Although carbamate formation reaction proceeds rapidly, the overall capacity for CO 2 capture for both primary and secondary amines is reduced by the stoichiometry requirement of two amine molecules for each CO 2 molecule reacted. The chemical structures of example alkanolamines for each of the three amine types are shown in Figure 1. Tertiary amines are generally not used for CO 2 capture, because they do not react with CO 2 to produce carbamate ions. Tertiary amines, however, can remove a stoichiometric amount of CO 2 by reaction with water to produce hydroxyl ions that can then react with CO 2 to produce bicarbonate ions, as shown in reactions 3 and 4. H 2 O + R 3 N w R 3 NH + + OH - (3) CO 2 (aq) + OH - w HCO 3 - (4) The formation of the bicarbonate ion (reaction 4) is relatively slow compared to the carbamate ion formation reaction (reaction 1), however, so that the kinetics of CO 2 removal by tertiary amine are generally slower than for primary and secondary amines. Energy costs play a significant role in the feasibility of any commercial CO 2 removal system. For aminebased systems, the most significant energy demand is for the amine regeneration step. Primary amines typically have higher heats of absorption than secondary and tertiary amines. This higher heat of absorption produces a commensurate energy penalty for primary amines during the regeneration step. Secondary amines, therefore, provide a useful compromise between the low reaction rates of tertiary amines and high heat of absorption for primary amines. As a result, since the 1980s, secondary amines have seen increasing use in industrial applications for acid gas removal. 1 At present, however, most aqueous solutions of secondary amines limit the amine concentration to 20% because of the use of carbon steel in absorption vessels; a relatively low amine concentration is required to reduce the rate of corrosion within the process system. 1 Table 1 lists average heats of solution for capturing CO 2 from representative amines. In this paper, we describe the use of supported amine sorbents to capture CO 2 and the subsequent regeneration of the supported amine using pressure swing absorption at low ( 1 mmhg) vacuum pressure. Specifically, we examined the most common commercial ethanolamines representing the three amine types, monoethanolamine (MEA, primary), diethanolamine (DEA, secondary), and triethanolamine (TEA, tertiary); see Figure 1. In contrast to these single amine molecules, multiamine molecules can contain more than one type of amine functionality, which suggests the possibility of developing multifunctional amine sorbents that optimize their CO 2 capture behavior, i.e., provide tradeoffs between kinetic and heat of absorption limitations. In this paper, the development of new highercapacity solid amine sorbents by modification of the amine functional group of the immobilized sorbent is also described. MEA was modified by reaction with acrylonitrile to convert some of the primary amine groups into secondary amines. The justification for using reaction-modified amines is based on work described by Giavarini et al. and Rinaldi et al., who modified tetraethylenepentamine (TEPA, Figure 2) to increase its working capacity for CO 2 removal. The working capacity refers to the amount of CO 2 that can be absorbed and successfully removed during regeneration of the sorbent. Giavarini et al. and Rinaldi et al. modified TEPA by reacting it with various ratios of phenol, formaldehyde, and combinations of the two. These modified TEPA molecules showed increased working capacity for gas-phase CO 2 removal in aqueous systems. In the present study, the amine sorbents were used in an adsorbed state, immobilized on a nonionic polymeric support. The main objective was to discern the amine type most effective in removing carbon dioxide from a gas stream containing low levels of CO 2. Low levels of CO 2, generally at about 7.6 mmhg in a gas stream maintained at 760 mmhg total pressure, were considered. These CO 2 partial pressures and total pressures mimic those typically found in enclosedenvironment life-support systems such as submarines and the space shuttle.

3 1544 Ind. Eng. Chem. Res., Vol. 44, No. 5, 2005 Experimental Section Commercially available ethanolamines MEA (Baker, 100% reagent grade), DEA (Aldrich, 99%), and TEA (Baker, 98.7%) were used as amine sorbents for CO 2 capture. The ethanolamines were used as received. MEA was also modified by reaction with acrylonitrile (Aldrich, >99%). The acrylonitrile/amine reaction formed a Michael adduct much like the TEPA amines modified by Rinaldi et al. 14 via phenol and formaldehyde. The acrylonitrile tends to react predominantly with primary amines converting them to secondary amines. 15 Two different ratios of acrylonitrile to MEA were examined to provide a mixture of primary and secondary amines: (1) a Michael adduct formed with 1 mol of acrylonitrile (AN) to 4 mol of MEA and (2) a Michael adduct formed when 2 mol of AN were reacted with 3 mol of MEA. To produce the adducts, 188 ml of liquid MEA was added to a standard 500-mL round-bottom glass flask with a mechanical stirrer. An ice/water bath surrounding the flask was used to moderate the reaction exotherm.. An addition funnel was used to slowly ( 3-5 min) add 50 ml of acrylonitrile (for the 1:4 molar ratio synthesis) into the stirred amine. The rate of acrylonitrile addition was limited to prevent the solution temperature from exceeding 45 C. The solution temperature rose slightly to approximately 30 C and then slowly returned to the ice/water bath temperature. Two batches of amine/acrylonitrile reactants were produced. In the first, 0.25 mol of acrylonitrile was added per 1 mol of amine (MA14). In the second, 0.67 mol of acrylonitrile was added per 1 mol of MEA (MA23). After the addition had been completed, the solution was slowly heated to 50 C, and stirring was then continued for1htoensure complete reaction of the acrylonitrile and amine. The amine sorbents were impregnated into a nonpolar commercial poly(methyl methacrylate) (PMMA) support. This high-specific-surface-area (BET, 470 m 2 /g, as measured by the manufacturer) support bead also provided a large pore volume (1.2 ml/g) with a mean pore diameter of 17 nm (based on N 2 adsorption). These beads had a range of diameter from 0.35 to 0.84 mm. The solid polymer beads containing immobilized liquid amines were prepared using a solvent evaporation process. The PMMA beads were initially wetted by dispersing them in methanol to facilitate impregnation of the amine into the pores. An amine solution (equal volumes of amine and methanol) was then added to the beads, and the amine solution was rotated within a rotary evaporator flask at room temperature for 5 min to produce a homogeneous slurry. The volatile methanol was then removed by heating the rotary evaporator flask in a 90 C water bath. Care was taken within the first few minutes of solvent evaporation to prevent bumping of the slurry, which would carry solid support material into the condensation tube. The impregnation procedure produced sorbents consisting of a PMMA polymeric support with 30-85% of its theoretical pore volume filled with liquid amine. The ethanolamine loadings achieved were 0.34 g of MEA, 0.47 g of DEA, and 0.53 g of TEA per gram of dry PMMA bead. Because of the differences in molecular weight, those mass loadings produced nearly equal molar loadings of 2 mol of amine per liter of support. The use of the volume concentration is useful, because the pressure swing absorption experiments were conducted with a constant volume (0.1 L) of sorbent. The CO 2 absorption measurements were made in a semicontinuous two-bed adsorption system. The fixedbed reactor contained an open-cell reticulated aluminum foam that allowed the conduction of heat from the absorption chamber into the desorption bed. 16 A schematic diagram of the experimental system is shown in Figure 3. As indicated, nitrogen and carbon dioxide supplies were mixed to produce a fixed inlet CO 2 concentration (generally 1 kpa). Not shown are the humidifiers, which permitted variation of the inlet dew point from -40 C to a fully saturated ambient-temperature condition. Inlet and outlet CO 2 levels, temperatures, and dew points were all monitored as shown. The small bed size (110 cm 3 ) and the efficient thermal conduction paths provided by the aluminum foam made both the cyclic and equilibrium capacity measurements operate isothermally, limiting temperature variations between the absorbing and desorbing bed to less than 2 C, and also limited run-to-run absorbing or desorbing temperature variations to much smaller values (<1 C). Once the amines had been fabricated, tests were conducted to examine the CO 2 removal capacity for all sorbents. No capacity for CO 2 removal was expected from the PMMA support; this material is nonionic and has no affinity for CO 2 capture. Sorption measurements were conducted only with the supported amine samples. The carbon dioxide removal capacity of the solid amines was measured using a fixed absorption time of 25 min, followed by a low-pressure vacuum desorption step for another 25 min. During absorption, the gas flow was such that a gas residence time of 1.75 s and an inlet dew-point temperature of 7 C were maintained. The absorption/desorption cycle was repeated until steadystate conditions were achieved. That typically required four cycles. The steady-state values are what are reported in this article. Results and Discussion Initial tests of CO 2 absorption capacity were conducted with each chamber exposed to a continuous stream containing CO 2 at a partial pressure of 7.6 mmhg (in N 2 ) with an inlet dew point maintained at 7 ( 1 C. This stream passed through one bed of the reactor while the second bed was exposed to vacuum for regeneration. After 25 min of operating in this mode, the functions of the two beds, i.e., sorption and desorption, were alternated. Figure 4 represents the steadystate CO 2 loading on the amine bed after four cycles of absorption and desorption. This graph presents a comparison of the breakthrough capacities for the three ethanolamines (MEA, DEA, TEA) loaded onto supports and operated at 20 C. Figure 4 demonstrates the higher capacity of the secondary ethanolamine relative to the primary and tertiary amines for removing CO 2 in a PSA system. 1 For these experiments, the pressure reached 1 mmhg within the desorption bed at the end of the 25-min desorption cycle. In Figure 4, the slight change in slope between the primary and secondary amines in the min time period results from a small variation in amine loading within the support. These small slope changes between MEA and DEA are not important; it is only the larger variations and overall capacity differences that are significant. The large variation at times greater than 10 min comes from the marked change in capacity between the primary and secondary amines. The large difference between the primary and secondary amines

4 Ind. Eng. Chem. Res., Vol. 44, No. 5, Figure 3. Bench-scale system used in all CO 2 capture experiments. Figure 4. Comparison of suppported, pure amine working capacities. and the tertiary amine demonstrates the large difference in affinity for low concentrations of CO 2 between amines that form carbamates and amines that do not. The tertiary amines do not form carbamates and, as demonstrated in Figure 4, showed little affinity for CO 2 at the levels considered in these tests. A further demonstration of the efficacy of secondary amines for CO 2 capture was obtained from the capacity results of the modified MEA samples, i.e., where some of the primary amine was converted to secondary amine by reaction with acrylonitrile. Absorption tests were conducted at the same loadings and test conditions for each as discussed previously. Comparisons of the results for pure MEA and the modified MEAs also allowed us to ascertain the influence of the hydroxyl group on CO 2 capacity. The hydroxyl ion is known to be an important chemical base in CO 2 capture. In fact, aqueous amine solutions will also generate hydroxyl ions, as an intermediate in their CO 2 capture mechanism. Although the ratio of amine/hydroxyl in MEA, DEA, and TEA inherently varied, so that the effect of the hydroxyl could not be unambiguously resolved, MEA and all of the modified Figure 5. Modified MEA working capacity, normalized. MEAs had the same ratio of amine/hydroxyl. In the latter case, only the ratio of secondary to primary amine changed. Moreover, comparisons between the fully reacted MEA, which contained predominantly secondary amines, and DEA (solely secondary amines) isolated the effect of the hydroxyl group concentration. This allowed us to confirm that it is the amine type (secondary amine) and not the quantity of hydroxyl groups attached to the amine molecule that governs the capture of CO 2 in this cyclic PSA process. No change in CO 2 capacity appears to be related to the quantity of hydroxyl groups on the ethanolamines, as the data in Figure 5 show similar capacities between MEA reacted with acrylonitrile and DEA. Note that these molecules have a 2:1 ratio in the number of hydroxyl groups present in the tests. Normalization of the capacity data by the amount of amine available on the support surface permits isolation of capacity effects associated with the amine structure (see Figure 5). All of the data are for samples in which the total amount of amine on the support surface was held constant. The only difference is in the mass loading, which is due to the addition of the acrylonitrile, which

5 1546 Ind. Eng. Chem. Res., Vol. 44, No. 5, 2005 increased the molecular weight of the amine molecule. The modification of the primary MEA amine with acrylonitrile increased the cyclic capacity for CO 2 capture. The pure MEA curve in Figure 5 plateaus at an amine capacity of about 0.14 mol of CO 2 per mole of amine available within the support. Modification of the MEA with a 1:4 ratio of AN/amine increased the cyclic CO 2 removal capacity of the supported amine to 0.16 mol of CO 2 per mole of amine. Increasing the AN/amine ratio to 2:3 further increased the cyclic CO 2 removal capacity to nearly 0.19 mol of CO 2 per mole of amine. Conclusions Although liquid amines have long been used for removing acid gases from pressurized gas streams, they have generally been used as bulk liquids in aqueous absorption systems. In this report, a novel method for contacting the gas phase and absorbent by using amines supported within the pores of a polymeric support has been demonstrated. The results indicate that secondary amines are most beneficial for removing low concentrations of CO 2 from a gas stream with a supported absorbent. This information comes from a comparison of MEA, DEA, and TEA immobilized within the solid support at the same molar loading. Working capacity test results showed a large increase in capacity and utilization of the secondary amine. In addition, MEA was modified by reaction with acrylonitrile to convert this primary amine into a secondary amine. The acrylonitrile molecule formed a Michael adduct with the primary amine of the MEA, converting it into a secondary amine. The reaction mechanism was tailored to react predominantly with primary amines. This new molecule (MEA-AN, now a secondary amine) provided a large increase in cyclic CO 2 removal capacity compared to the unaltered primary amine molecule. These experiments demonstrate the importance of amine type in removing low levels of CO 2 from a PSA process while using a polymeric support. These immobilized amines clearly have the potential to provide CO 2 removal for life-support applications The further ability of the support to resist corrosion and foaming while increasing the concentration of amine compared to pumped aqueous systems might provide utility in other applications. By eliminating the need for additives and increasing the amine concentration within the support, these solid amine beads might be useful in terrestrial applications, such as natural gas sweetening or flue gas CO 2 removal. Acknowledgment The authors thank Hamilton Sundstrand Space Systems International for their support of this research effort. Literature Cited (1) Kohl, A.; Nielsen, R. Gas Purification, 5th ed.; Gulf Publishing Co.: Houston, TX, (2) Newman, S. A., Ed. Acid and Sour Gas Treating Processes; Gulf Publishing Co.: Houston, TX, (3) Filburn, T.; Genovese, J.; McNamara, L.; Thomas, G. Rapid Cycling Amine Development Test. Presented at the 48th International Astronautical Congress, Turin, Italy, Oct 6-10, 1997; Paper IAA-97-IAA (4) Filburn, T.; Lantazakis, M.; Taddey, E.; Graf, J. An Orbiter Upgrade Demonstration Test Article for a Fail-Safe Regenerative CO 2 Removal System. Presented at the 28th International Conference on Environmental Systems, Danvers, Massachusetts, Jul 13-16, 1998; Paper SAE (5) Filburn, T.; Satyapal, S.; Birbara, P.; Graf, J. Performance and Properties of a Solid Amine Sorbent for CO 2 Removal in Space Life Support. Presented at 1998 American Institute of Chemical Engineers, Annual Meeting, November 15-20, Miami, FL. (6) Bottoms, R. R. U.S. Process for Separating Acid Gases. Patent 1,783,901, (7) Dankwerts, P. The Reaction of CO 2 with Ethanolamines. Chem. Eng. Sci. 1979, 14, (8) Shulik, L.; Sartori, G.; Ho, W.; Thaler, W.; Milliman, G. Primary Hindered Amino Acids for Promoted Acid Gas Scrubbing Process. U.S. Patent 4,919,904, (9) Hagewiesche, D.; Ashour, S.; Al-Ghawas, H.; Sandall, O. Absorption of Carbon Dioxide into Aqueous Blends of Monoethanolamine and N-Methyldiethanolamine. Chem. Eng. Sci. 1995, 50 (7), (10) Steve Bedell, Dow Chemical Co. Freeport, Texas, Personal communication. (11) Carey, F. Organic Chemistry; McGraw-Hill: New York, (12) Faichetti, A.; Giavarini, C.; Moresi, M.; Sebastiani, E. Absorption of CO 2 in aqueous solutions: Reaction kinetics of modified tetraethylenepentamine. Ing. Chim. Ital. 1981, 17 (1-2), 1-7. (13) Rinaldi, G. Acid Gas Absorption by Means of Aqueous Solutions of Regenerable Phenol-Modified Polyalkylenepolyamine. Ind. Eng. Chem. Res. 1997, 36, (14) Filipsis, P.; Giavarini, C.; Maggi, C.; Rinaldi G.; Silia, R. Modified Polyamines for CO 2 Absorption. Product Preparation and Characterization. Ind. Eng. Chem. Res. 2000, 39, (15) Fletcher, R.; Tepper E. Regenerable Device for Scrubbing Breathable Air of CO 2 and Moisture Without Special Heat Exchanger Equipment. U.S. Patent 4,046,529, (16) Whitmore, F.; Mosher, H.; Adams, R.; Taylor, R.; Chapin, E.; Weisel, C.; Yanko, W. Basically Substituted Aliphatic Nitriles and Their Catalytic Reduction to Amines. J. Am. Chem. Soc. 1944, 66, (17) Astarita, G.; Savage, D.; Bisio, A. Gas Treating with Chemical Solvents; John Wiley and Sons: New York, (18) Bird, R.; Stewart, W.; Lightfoot, E. Transport Phenomena; John Wiley and Sons: New York, Received for review May 24, 2004 Revised manuscript received October 29, 2004 Accepted November 20, 2004 IE