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Ind. Eng. Chem. Res. 2005, 44, 1542-1546
SEPARATIONS Development of Supported Ethanolamines and Modified Ethanolamines for CO2 Capture T. Filburn,* J. J. Helble, and R. A. Weiss Department of Chemical Engineering, University of Connecticut, Storrs, Connecticut 06269-3222
Liquid amines can be immobilized within the pores of polymeric supports to provide a regenerable CO2 sorbent. This paper describes the capture of CO2 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 CO2 by a pressure swing absorption process, where low-pressure vacuum was used to desorb the CO2 and regenerate the sorbent. The effect on CO2 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 CO2 removal capacity compared to the original primary amines. These results suggest that modified amines could potentially be used for CO2 capture in space life support systems as well as for terrestrial flue gas CO2 removal applications. Introduction Long-duration, human-occupied space missions require the use of regenerable sorbents for CO2 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 CO2 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 CO2 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 Bottoms6 introduced the use of triethanolamine (TEA) for the regenerative removal of CO2 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 pro* To whom correspondence should be addressed. Current addresss: Department of Mechanical Engineering, University of Hartford, West Hartford, CT 06117. E-mail:
[email protected].
vide higher CO2 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 pKa values and consequently differing affinities for acid gases such as carbon dioxide.11 These pKa 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 CO2 removal by amines has concentrated on measuring capacities for aqueous amine solutions. The research described herein examined the CO2 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 CO2 capacity in a pressure swing absorption system. Reaction of Amines with CO2 In general, the industrial use of amine sorbents has centered on aqueous solutions of primary and secondary amines, which react directly with CO2 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.
2RNH2 + CO2 w RNHCOO- + RNH3+
(1)
RNHCOO- + H2O w RNH2 + HCO3-
(2)
10.1021/ie0495527 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/29/2005
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Figure 2. Structure of the TEPA molecule (normal). Table 1. Integral Heats of Solution for Absorption of CO21
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 CO2, which produces faster CO2 capture kinetics for the primary and secondary amines. Although carbamate formation reaction proceeds rapidly, the overall capacity for CO2 capture for both primary and secondary amines is reduced by the stoichiometry requirement of two amine molecules for each CO2 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 CO2 capture, because they do not react with CO2 to produce carbamate ions. Tertiary amines, however, can remove a stoichiometric amount of CO2 by reaction with water to produce hydroxyl ions that can then react with CO2 to produce bicarbonate ions, as shown in reactions 3 and 4.
H2O + R3N w R3NH+ + OH-
(3)
CO2(aq) + OH- w HCO3-
(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 CO2 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 CO2 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
amine
type
integral heat of solution (cal/g of CO2)
MEA DEA DGA MDEA TEA DIPA
primary secondary primary tertiary tertiary secondary
1485 1260 1476 1035 837 1296
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 CO2 from representative amines. In this paper, we describe the use of supported amine sorbents to capture CO2 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 CO2 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.,12-14 who modified tetraethylenepentamine (TEPA, Figure 2) to increase its working capacity for CO2 removal. The working capacity refers to the amount of CO2 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 CO2 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 CO2. Low levels of CO2, generally at about 7.6 mmHg in a gas stream maintained at 760 mmHg total pressure, were considered. These CO2 partial pressures and total pressures mimic those typically found in enclosedenvironment life-support systems such as submarines and the space shuttle.
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Experimental Section Commercially available ethanolamines MEA (Baker, 100% reagent grade), DEA (Aldrich, 99%), and TEA (Baker, 98.7%) were used as amine sorbents for CO2 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 for 1 h to ensure 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 m2/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 N2 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 CO2 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 CO2 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 CO2 levels, temperatures, and dew points were all monitored as shown. The small bed size (110 cm3) 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 (
