Beta-Amino Carboxylate (BAC) Solvents for Enhanced CO2 Separations

Abstract

Steric effects on amine sites beta to amide or ester functional groups are utilized to result in a family of CO.sub.2 solvents with an unprecedented hybridization of chemisorption and physisorption properties. These versatile solvents provide very high CO.sub.2 working capacities in applications typically involved with physical solvents such as Selexol, Genosorb, Fluor Solvent, Purisol, and Rectisol with the added benefit of operation near ambient conditions without the need for solvent chilling along with the potential for high pressure recovery of CO.sub.2. The non-aqueous BAC solvents can also be tailored for low partial pressure CO.sub.2 removal as required in such applications as biogas/landfill gas upgrading and CO.sub.2 removal from industrial processes such as cement and steel manufacturing where they benefit from lower energy requirements for regeneration compared to tradition aqueous amine solution, regeneration under 1 bar CO.sub.2 without steam stripping, reduced corrosion potential, reduced solvent loss, reduced environmental impact, higher volumetric CO.sub.2 uptake compared to standard commercialized processes involving physical solvents, and operation at ambient pressure without the need for feed gas compression.

Claims

1. A method of separating or removing CO.sub.2 from a CO.sub.2-containing fluid, comprising: providing a beta-amino carboxylate composition comprising at least 5 wt % of a beta-amino carboxylate or mixture of beta-amino carboxylates; combining the CO.sub.2-containing fluid and the beta-amino carboxylate or mixture of beta-amino carboxylates at a first temperature and pressure to form a reaction product of CO.sub.2 and the beta-amino carboxylate or mixture of beta-amino carboxylates; and exposing the reaction product of CO.sub.2 and the beta-amino carboxylate or mixture of beta-amino carboxylates to a second temperature and pressure to release the CO.sub.2 from the reaction product.

2. A method of separating or removing CO.sub.2 from a CO.sub.2-containing fluid, comprising: providing a beta-amino carboxylate composition comprising a beta-amino carboxylate or mixture of beta-amino carboxylates; combining the CO.sub.2-containing fluid and the beta-amino carboxylate or mixture of beta-amino carboxylates at a first temperature and pressure to form a reaction product of CO.sub.2 and the beta-amino carboxylate or mixture of beta-amino carboxylates; wherein the CO.sub.2-containing fluid comprises at least 10 mol % H.sub.2O; wherein the beta-amino carboxylate composition absorbs a higher percentage (or at least twice as high a percentage, or at least 5 times as high) of the CO.sub.2 than H.sub.2O from the CO.sub.2-containing fluid; and exposing the reaction product of CO.sub.2 and the beta-amino carboxylate or mixture of beta-amino carboxylates to a second temperature and pressure to release the CO.sub.2 from the reaction product.

3. The method of claim 1 wherein the CO.sub.2-containing fluid is a CO.sub.2-containing fluid mixture comprising CO.sub.2 and at least one other gaseous species, and wherein the step of separating or removing CO.sub.2 comprises separating CO.sub.2 from the at least one other gaseous species in the CO.sub.2-containing fluid mixture.

4. The method of claim 3 comprising pressure swing adsorption wherein the second pressure is less than the first pressure.

5. The method of claim 3 comprising temperature swing adsorption wherein the second temperature is greater than the first temperature.

6. The method of claim 1 further comprising a step of storing the released CO.sub.2 in an underground cavern.

7. The method of claim 1 wherein the beta-amino carboxylate is selected from the group consisting of: ##STR00013## ##STR00014## ##STR00015## ##STR00016## ##STR00017##

8. The method of claim 1 wherein the beta-amino carboxylate composition comprises less than 5% or less than 3% or less than 1% water by weight.

9. The method of claim 8 wherein the beta-amino carboxylate composition comprises less than 5% or less than 3% or less than 1% added water by volume.

10. The method of claim 1 wherein the beta-amino carboxylate composition comprises at least 50 wt % or at least 75 wt % or at least 90 wt % or at least 99 wt % of a beta-amino carboxylate or mixture of beta-amino carboxylates.

11. The method of claim 1 wherein the beta-amino carboxylate composition comprises at least 10 wt % of a non-aqueous solvent.

12. The method of claim 1 comprising a plurality of cycles of temperature swing absorption.

13. (canceled)

14. The method of claim 1 wherein the CO2 containing fluid comprises at least 1 mol % CO2 or at least 2 mol % CO2 or at least 5, or at least 10, or at least 20, or at least 50 mol % CO2.

15. The method of claim 14 wherein the CO2 containing fluid comprises at least 5 mol % H2O, or at least 10 mol % H2O, and a CO2/H2O in a ratio of from 0.01 to 5, and wherein, in a single cycle, more CO2 is adsorbed than H2O; and/or wherein CO2 is preferentially absorbed relative to H2O; and/or wherein at least twice as much CO2 than H2O is adsorbed.

16. The method of claim 14 wherein the CO2 containing fluid comprises 10 mol % H2O or less, or 5 mol % H2O or less, or 1 mol % H2O or less, and a CO2/H2O in a ratio greater than 0.5, and wherein, in a single cycle, more CO2 is adsorbed than H20; and/or wherein CO2 is preferentially absorbed relative to H2O; and/or wherein at least twice as much CO2 than H2O is adsorbed.

17. The method of claim 1 conducted in a temperature range of 0 to 50° C.

18. (canceled)

19. A beta-amino carboxylate composition comprising at least 5 wt % of one or more of the compounds listed in claim 7; and comprising at least 5 wt % CO.sub.2 or at least 5 wt % of the reaction product of CO.sub.2 and one or more of the compounds listed in claim 7.

20. A beta-amino carboxylate composition comprising at least 2 wt % of one or more of the compounds listed in claim 7; and comprising at least 1 wt % CO.sub.2 or at least 2 wt % of the reaction product of CO.sub.2 and one or more of the compounds listed in the Appendix or elsewhere in the specification; and further comprising at least 10 wt % (or at least 25 wt % or at least 50 wt %) of a non-aqueous solvent.

21. A beta-amino carboxylate composition comprising one or any combination of the compounds listed in claim 7; and comprising at least 5 wt % CO.sub.2 or at least 5 wt % of the reaction product of CO.sub.2 and one or any combination of the compounds listed in claim 7.

22. The composition of claim 20 wherein the beta-amino carboxylate composition comprises less than 5% or less than 3% or less than 1% water by weight.

23. The composition of claim 1 wherein the beta-amino carboxylate composition comprises at least 50 wt % or at least 75 wt % or at least 90 wt % of a beta-amino carboxylate or mixture of beta-amino carboxylates.

24. A CO.sub.2 separation system comprising the composition of claim 21.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0059] FIG. 1. Comparing CO.sub.2 absorption isotherms at 25° C. (left) and isosteric heats of CO.sub.2 absorption (right) for different BAC solvents to highlight the influence of structure on CO.sub.2 uptake in the low-pressure and high-pressure regions. Selexol is included as an industrial benchmark. The optimal operational temperature range for a solvent is determined by viscosity, vapor pressure, and CO.sub.2 solubility. The BAC solvents EMA-NH-EMA and 2AB-DECAM are more suitable for CO.sub.2 capture at high pressures with warm regeneration. The solvents 1AH-EMA, 1AA-IBMA, and 1AB-DECAM are better suited for capture at lower partial pressures of CO.sub.2, e.g. biogas upgrading.

[0060] FIG. 2. Comparing CO.sub.2 absorption isotherms for different BAC solvents with varying degrees of steric hindrance around the amine site. As the steric crowding around the amine site increases, the CO.sub.2 uptake in the low-pressure region of the isotherm decreases. This provides a powerful tool to tailor the structure of BAC solvents to control CO.sub.2 solubility at desired temperature and pressure conditions.

[0061] FIG. 3. Temperature effects on the CO.sub.2 absorption isotherms for BAC solvent IBA-DECAM. A linear adsorption isotherm for the physical solvent Selexol at 25° C. is included for comparison.

[0062] FIG. 4. (left) CO.sub.2 absorption isotherms at 40° C. and 70° C. for BAC solvents compared to those obtained for the commercial hydrophilic physical solvent Selexol. The enhanced CO.sub.2 interaction with BAC solvents provides the opportunity for CO.sub.2 capture under warm gas conditions. The higher heat of CO.sub.2 absorption in EMA-NH-EMA and 2AB-BDM-2AB provide a significant drop in CO.sub.2 loading at 1 bar with moderate warming and provides an efficient route to high working capacities.

[0063] FIG. 5. Comparing CO.sub.2 absorption for EMA-NH-EMA to a standard commercial physical solvent Selexol at 25° C. and 70° C. A large increase in CO.sub.2 uptake at 20 bar is observed in EMA-NH-EMA at 25° C., whereas the uptake in both solvents is similar at 1 bar and 70° C. regeneration condition. The CO.sub.2 absorption profile for EMA-NH-EMA provides a high CO.sub.2 working capacity without the need for solvent chilling.

[0064] FIG. 6. CO.sub.2 absorption profiles for different BAC solvents and Selexol under simulated biogas conditions of P.sub.CO2 300 mbar/P.sub.N2 700 mbar showing relative CO.sub.2 mass transfer rates, effect of regeneration temperature, relative evaporation rates and approximate solvent working capacities after 20 min. absorption time (chosen to coincide with CO.sub.2 diffusion rates in Selexol). Note that working capacities for Selexol under these conditions is effectively 0 wt % since CO.sub.2 uptake at 60° C., 1 bar is slightly higher than the CO.sub.2 uptake at 25° C., 300 mbar. As such, effective use of Selexol requires compression of the feed gas to P.sub.total 6-8 bar. (TBP=tributyl phosphate)

[0065] FIG. 7. Cycling stability test for 1AB-DECAM. One cycle includes absorption at P.sub.CO2 300 mbar, 25° C., 70% RH, followed by regeneration at P.sub.CO2 1 bar, 60° C., 0% RH, followed by N.sub.2 sweep at 1 bar, 60° C., then back to dry N.sub.2 baseline at 25° C.

[0066] FIG. 8. Water vapor absorption and relative mass changes in BAC solvent 1AH-EMA under various humidity conditions of N.sub.2 and CO.sub.2/N.sub.2 mixtures relevant to typical biogas conditions. (top left) Water vapor absorption was measured under continuous flow at 1 bar, 25° C. using blends of humidified N.sub.2 and dry N.sub.2. Wet and dry CO.sub.2 runs were measured at 25° C. in continuous flow using mixtures of P.sub.CO=300 mbar, P.sub.N2=700 mbar with humidity controlled by blending humidified N.sub.2 and dry N.sub.2. Solvent regeneration was done first at 1 bar CO.sub.2, 60° C. to get a working capacity, then followed by a 1 bar N.sub.2 sweep at 60° C. to determine baseline mass. A high working capacity can be achieved with CO.sub.2 recovery accomplished under a pure CO.sub.2 sweep at 1 bar, 60° C. without the need for steam stripping, high temperatures, vacuum, or an inert purge gas.

[0067] FIG. 9. Viscosity changes in BAC solvents between neat and CO.sub.2 loaded phases after saturation at 1 bar. Note that projected uses of the solvents are targeted for P.sub.CO2 0.2-0.4 bar which will result in even lower CO.sub.2 loaded viscosities.

[0068] FIG. 10. FTIR spectra of 1HA-EMA under dry N.sub.2, dry CO.sub.2, and 70% relative humidity CO.sub.2 showing physisorbed and chemisorbed CO.sub.2, but no carbamate/carbonate side products.

[0069] FIG. 11. (left) Comparing CO.sub.2 uptake in BAC solvent EMA-NH-EMA at temperatures 15° C. and 20° C. higher than that of Selexol. (Right) comparing theoretical CO.sub.2 working capacities of EMA-NH-EMA and Selexol (top) using a combined temperature and pressure swing process with capture at 25° C., 20 bar and regeneration at 70° C., 1.5 bar or (bottom) using a high-pressure temperature swing process with capture at 25° C., 20 bar and regeneration at 70° C., 20 bar. BAC solvent EMA-NH-EMA outperforms Selexol in theoretical working capacity in either case using the same conditions. The high-pressure solvent regeneration is particularly attractive as a means to reduce CO.sub.2 compression costs prior to pipeline injection.

[0070] FIG. 12. Comparison of the theoretical working capacities of BAC solvent 1AH-EMA and commercial physical solvent Selexol for CO.sub.2 removal from a gas stream containing 0.3 bar P.sub.CO2 at a total system pressure of 1 bar as a function of solvent regeneration temperature with regeneration done in 1 bar CO.sub.2. alternative strategies providing similar CO.sub.2 working capacities where either solvent chilling is removed, or decompression of gas in the recovery step is reduced or removed. Either alternative could lead to savings in capital costs and operation.

[0071] FIG. 13. (top) Viscosity changes in BAC solvents between CO.sub.2 lean and CO.sub.2 loaded phases after saturation at 1 bar. Note that projected uses of the solvents are targeted for P.sub.CO2 0.2-0.4 bar which will provide even lower CO.sub.2 loaded viscosities. (bottom) BAC solvent viscosities (green stars) plotted to scale with viscosities reported by Koech et al. for an alkanolguanidine CO.sub.2 capture solvent (diamonds for dry solvent and red squares for solvent with 10 wt % added water). The alkanolguanidine solvent is described in the report as a “low viscosity” solvent. The CO.sub.2-loaded viscosities for BAC solvents reported herein are significantly lower than those of reported low viscosity solvents despite a cooler temperature and higher CO.sub.2 loadings..sup.25

DETAILED DESCRIPTION OF THE INVENTION

[0072] The current invention provides CO.sub.2 solubility in non-aqueous solvents by designing the molecular structure of the solvent to include two functional groups having favorable interactions with CO.sub.2. The first functional group incorporated is the carboxylate group. The carboxylate group, in particular the ester functional group, is among the best for CO.sub.2 physical solvents..sup.1 This functional group will enhance the CO.sub.2 solubility in the higher-pressure region of the isotherm. The second functional group built into the solvent molecules is the amine functional group. Amines are known to react with CO.sub.2 to form carbamic acids. In aqueous systems with excess amine, the reaction often proceeds to the formation of an ammonium carbamate by deprotonation of the carbamic acid. Additional hydrolysis by water can result in the formation of ammonium bicarbonate/carbonates. The regeneration of CO.sub.2 is more energy intensive for bicarbonate/carbonates than it is for carbamic acids so our goal in the design of strong physical solvents for CO.sub.2 is to limit bicarbonate/carbonates formation and thus reduce the energy required for regeneration while maintaining a strong CO.sub.2 affinity.

##STR00001##

[0073] The amine solvents are tailored to maximize CO.sub.2 uptake while minimizing the energy required for solvent regeneration in three ways. The first is to use the solvents as neat solvents or nonaqueous mixtures. Removing water from the CO.sub.2 capture solvent system (that is, to use non-aqueous solvents) increases the overall efficiency of the process. Water promotes corrosion, has a high heat capacity, low boiling point, and latent heat of evaporation which combine to create a large energy penalty in the process. Water also has a very low CO.sub.2 solubility. As such, dilution of the CO.sub.2 active solvent with water will only reduce the overall volumetric CO.sub.2 solubility. Increased water content in the solvent is “dead weight” and may also increase solvent viscosity. Both of these lead to increased pumping costs. Furthermore, the recovered CO.sub.2 will have a high moisture content which will need to be remediated prior to pipeline injection. The less water in the solvent, the better. The second strategy is to locate the amine functional group beta to the carboxylate group. The electronic withdrawing effects of a functional group beta to the amine location tends to reduce the basicity and hence reactivity of the amine group. The stronger the electronic withdrawing effect, the less basic and CO.sub.2-reactive the amine. The third approach is to sterically hinder the amine site to limit the strength of the interaction of CO.sub.2 with the amino nitrogen. The more sterically crowded the amine site, the less favorable the CO.sub.2 interaction is with the amine. The “hindered amine” and water lean techniques have been investigated by other researchers as a way to increase CO.sub.2 capture efficiency, however, those studies were often hindered by one or more undesirable attributes..sup.25 For example, some studies include complex molecules which will be costly to scale up. Other studies on solvents involving branched alky amines suffer from costly complex molecules or involve highly volatile low molecular weight amines. Conversely, high molecular weight alkyl amine solvents are not favorable since high mole fractions of hydrocarbon in their structures have a negative effect on CO.sub.2 solubility due to the weak interaction of CO.sub.2 with methyl (—CH.sub.3) or methylene (—CH.sub.2—) units. Non-aqueous or water lean amine solvents often show significant increases in viscosity upon reaction with absorbed CO.sub.2.

[0074] The molecular weights of the solvents are designed to provide a balance for low vapor pressure, low viscosity, and high CO.sub.2 capacity. Physical properties along with the solvent hydrophilicity are tailored through the size and shape of the alkyl moieties. Additional alkyl hydrocarbon groups increase molecular weights and lower the vapor pressure while also reducing water affinity. These are positive effects for CO.sub.2 physical solvents. Addition of too many hydrocarbon units, however, will increase the viscosity and lower the volumetric CO.sub.2 solubility since the interaction of CO.sub.2 with hydrocarbons is much weaker than the interaction of CO.sub.2 with carboxylate groups and amines. Increased hydrocarbon mole fraction in the solvent may also increase the CH.sub.4 solubility of the solvent which is undesirable for applications involving CO.sub.2 removal from methane streams. An optimal range of hydrocarbons has been determined in which the overall molecular weight of the beta-amino carboxylate is in the range of 180-400 g/mol, with solvent densities in the range of 0.85-1.1 kg/L, viscosities at 25° C. from 2-32 cP and boiling points of at least 200° C. In some embodiments, the solvent has a molecular weight (or in the case of mixtures, a weight average molecular weight) of 350 or less or 300 Daltons or less, or in the range of 150 to 400 or 200 to 300 Daltons. BAC solvents will typically contain C1-C6 amino, amido, or ester groups. The alkyl groups can be linear, branched, or cyclic depending on the desired property of the solvent.

[0075] The syntheses of the solvents can be accomplished using established organic reactions as outlined in Scheme 2. The first step in the reaction sequence is the formation of the amide or ester derivative of acrylic acid, methacrylic acid, or crotonic acid. Many ester derivatives of acrylic acid, methacrylic acid, or crotonic acid are commercially available with the methyl, ethyl, and isobutyl esters very common and inexpensive. As such, the use of acrylic acid, methacrylic acid, or crotonic acid derivatives as a route to sterically hindered amines is a potentially more cost-effective route than preparing sterically hindered alkyl amines. Amide versions of the BAC compounds will typically require the reaction of an appropriate alkyl amine with the respective acid chloride. The next step is to react the amido or ester derivative of acrylic acid, methacrylic acid, or crotonic acid with an appropriate alkyl amine using a catalyzed Michael addition where the amine will add to the terminal alkene of the acrylate, methacrylate, or crotonate. Typical reaction times are one to a few days at temperatures ranging from 60-100° C., with lighter amines typically done in sealed reactors to minimize evaporative losses. Several catalysts which are effective for the Michael addition are known and include silica, alumina, and rare earth salts. The Michael addition can be done using a 1:1 ratio of amine to acrylate, methacrylate, or crotonate, a 2:1 ratio of amine to di-acrylate, di-methacrylate, or di-crotonate, or in a 1:2 ratio of ammonia to acrylate, methacrylate, or crotonate. The reactions are often done without the need for solvent. Many beta-amino carboxylates have been prepared and characterized in the course of this work. One skilled in the art would realize that mixed reactions are also possible for the 2:1 and 1:2 reactions in which two or more different amines could be used, or two or more different acrylate, methacrylate, or crotonate compounds could be used to give products with two or more functional groups in the final product. This might be valuable as a route to further refine physical properties of the solvents and CO2 solubilities. Physical mixtures of pure BAC solvents could also be prepared, or mixtures of BAC solvents with other organic solvents, or mixtures of BAC solvents with water, or mixtures of BAC solvents with water and other organic solvent, etc. in a nearly limitless number of ways as well. The Examples focus on the preparation and characterization as pure solvents, or in some mixtures with organic solvents including Selexol, diethyl sebacate, and tributyl phosphate, but anyone skilled in the art could easily expand the applications of these solvents through a variety of mixtures.

##STR00002## ##STR00003##

##STR00004## ##STR00005##

[0076] Structures of inventive BAC solvents include (but are not limited to) the following:

##STR00006## ##STR00007## ##STR00008## ##STR00009## ##STR00010##

Space filling modeling showed the steric effects of methyl groups adjacent to the amine site on absorbed CO.sub.2 as it interacts with the amine N site. The steric crowding limits the CO.sub.2 binding strength of the amine site. The steric effect can be tailored to optimize the CO.sub.2 solubility in BAC solvents at the appropriate temperature and pressure conditions while also reducing the energy demands for solvent regeneration The exceptional CO.sub.2 solubilities resulting from their hybrid molecular structures along with an ability to tune the strength of CO.sub.2 interaction within the BAC family of solvents makes these solvents highly versatile and very-well suited to a wide range of CO.sub.2 capture processes. The CO.sub.2 absorption isotherm data shown in FIG. 1 for different BAC solvents highlight how the CO.sub.2 uptake can be increased in the desired pressure range. Solvents 1AH-EMA, 1AA-IBMA and 1AB-DECAM are optimized for capture at lower partial pressures of CO.sub.2 in the range of 0.2-4 bar. Solvents 2AB-DECAM and EMA-NH-EMA are better suited for high pressure CO.sub.2 capture as indicated by the working capacity ratio of CO.sub.2 uptake at higher pressure to the uptake at the 1 bar regeneration pressure. The EMA-NH-EMA solvent with two ester functional groups has an enhanced CO.sub.2 capacity at high pressures typical encountered in precombustion applications. The working capacities of all BAC solvents will be significantly improved by warming the solvent during the regeneration step. The moderate to high heat of CO.sub.2 adsorption in the BAC solvents leads to a more pronounced drop in CO.sub.2 uptake as the solvent temperature is increased. This effect provides a higher working capacity with only a moderate temperature ramp of 25-50° C. above the absorption temperature.

[0077] The isotherms in FIG. 1 show the advantage of ester groups over amide groups for high CO.sub.2 capacity at elevated pressure. The BAC solvents are further tailored through the introduction of steric hinderance around the amine site. The CO.sub.2 absorption isotherms in FIG. 2 demonstrate the pronounced effects of steric hinderance on CO.sub.2 uptake in a solvent. The least hindered amine, 3AP-EA, shows the strongest CO.sub.2 uptake in the low-pressure region of the isotherms and produces a pronounced nonlinear isotherm. Addition of a methyl group two carbons from the amine nitrogen in 3AP-EMA significantly reduces the low-pressure CO.sub.2 uptake and produces a nearly linear isotherm. Placement of the methyl group on the carbon next to the amine nitrogen in the IPA-IBC sample further reduces the low-pressure CO.sub.2 uptake and yields a linear isotherm as typically seen in pure physical solvents. The steric crowding has essentially neutralized the amine nitrogen binding interaction with CO.sub.2.

[0078] The incorporation of an amine functional group into the solvent adds a variable degree of chemisorptive qualities to the CO.sub.2 interaction with the solvent. The higher heat of absorption for CO.sub.2 in the BAC solvent instills a higher sensitivity of the CO.sub.2 solubility to changes in temperature. The effects of temperature on the shape of the CO.sub.2 absorption isotherms are demonstrated in FIG. 3 for solvent IBA-DECAM. The CO.sub.2 uptake at 25-35° C. is highly nonlinear with a steep slope to the isotherm in the low-pressure region that transitions to a linear shape at higher pressures. A small ramp in temperature above 45° C. has a dramatic effect on the isotherm shape, effectively flattening the chemisorption region at low pressure and producing a near linear isotherm over the entire pressure range. Only a small temperature change was required to effectively transform the solvent from a mixed chemisorption/physisorption to a quasi-physisorption mechanism. This ability to flatten the CO.sub.2 absorption curve with a modest temperature ramp is highly beneficial for increasing the working capacity of the solvents since solvent regeneration is typically performed at 1 bar. Thus, high absorption capacity for CO.sub.2 can be achieved at an optimal absorption temperature which takes advantage of the strong amine-CO.sub.2 interactions with the release of CO.sub.2 in the regeneration step significantly enhanced with only mild to moderate heating of the solvent. Very high CO.sub.2 working capacities can be achieved in most BAC solvents using a thermal swing of only 25-50° C. above the absorption temperature.

[0079] The BAC solvents have a unique advantage in possessing an inherent boost in CO.sub.2 capacity due to an enhanced interaction with the carboxylate and hindered amine sites. These two effects combine to increase both the low-pressure and higher-pressure CO.sub.2 solubility in the solvents. Thus, the solvents can be optimized for applications involving capture of CO.sub.2 at lower partial pressures such as encountered in biogas/landfill gas,.sup.8-22 cement production.sup.23-24 and steel production or for applications involving higher partial pressure of CO.sub.2 such as H.sub.2 and NH.sub.3 production..sup.7 The benefits of these solvents will now be discussed in two contrasting applications as examples of their unique properties. The first demonstration will involve pre-combustion CO.sub.2 capture in a reforming process which generates H.sub.2 as fuel for a turbine in an Integrated Gasification Combined Cycle (IGCC) power plant where CO.sub.2 is at an elevated partial pressure of 20-25 bar at ˜50% concentration in H.sub.2. The second example will be for biogas methane upgrading which is typically performed at CO.sub.2 partial pressures near 2.5 bar at ˜35% concentration in methane.

Example 1: Pre-Combustion CO.SUB.2 .Capture

[0080] Physical solvents are ideally suited to CO.sub.2 capture under precombustion conditions due to the higher partial pressure of CO.sub.2 involved (20-25 bar)..sup.6-7 The solvents are typically chilled to further maximize the CO.sub.2 solubility even though solvent chilling introduces energy penalties associated with the higher solvent viscosity and chiller operation. Operations at warmer temperatures with similar CO.sub.2 solvent capacity would thus be beneficial. As shown in FIG. 4, BAC solvents provide much higher CO.sub.2 solubilities under warm gas condition than typically observed for physical solvents. The combined influences of carboxylate and hindered amine functional groups enhance CO.sub.2 absorption over the full range of the isotherms. The hindered beta-amine site gives a rapid rise in CO.sub.2 solubility in the 1-5 bar region, with the carboxylate groups aiding in CO.sub.2 absorption through the remainder of the pressure ramp. Absorption above ˜12 bar becomes very linear as the amine-based absorption saturates and physical absorption due to the carboxylate contribution dominates. The combined effects of the solvent functional groups result in a CO.sub.2 uptake at 20 bar which is nearly double that of the commercial standard Selexol at 40° C. A 30° C. temperature swing during regeneration effectively reduces the low-pressure CO.sub.2 solubility and provides a high working capacity of 2.5 mol/L for regeneration at 1 bar, 70° C. The working capacity can be increased further to 4.1 mol/L if the absorption temperature is dropped to 25° C. In pre-combustion CO.sub.2 capture processes, the CO.sub.2 recovery is typically done in stages using flash tanks to step down the pressure and allow recovery and recycling of co-adsorbed H.sub.2. The unique CO.sub.2 absorption properties of BAC solvents could allow the flash tanks to operate at different temperatures to maximize the purity of recovered H.sub.2 by limiting the co-release of absorbed CO.sub.2.

[0081] The higher heat of CO.sub.2 absorption in the EMA-NH-EMA compared to a typical physical solvent such as Selexol offers other potential advantages in system design for CO.sub.2 removal. With a higher heat of absorption, the CO.sub.2 uptake in the solvent is more effected by temperature swing. As such, the CO.sub.2 solubility in EMA-NH-EMA drops significantly more than in Selexol with a ramp in temperature. This temperature sensitivity provides a means for regeneration of the solvent using only a change in temperature without a need for a drop in pressure. Referring again to the isotherms shown in FIG. 5, it is clear that a significant CO.sub.2 working capacity of 3.8 mol/L can be achieved with EMA-NH-EMA by simply regenerating the solvent at 20 bar, 70° C. The potential to recover CO.sub.2 at pressures near the absorber pressure of 20-25 bar could provide substantial energy savings over the traditional pressure swing process of recovering the CO.sub.2 at 1.5 bar since compression of the recovered CO.sub.2 prior to pipeline injection is a significant energy penalty in CO.sub.2 removal processes. The work required for compression of a gas follows a logarithmic correlation with pressure. Thus, compression of CO.sub.2 to the liquification pressure of 70 bar when starting at 20 bar requires 67% less energy than required for compression starting at 1.5 bar. Additional energy savings are also possible with high pressure CO.sub.2 recovery due to the reduction in recompression demand on the CO.sub.2 capture system gas after solvent regeneration.

[0082] High Molecular Weight Solvents with Reduced Volatility

[0083] A subgroup of BAC solvents was created which contain two beta-amino carboxylate groups per molecule. These solvents were prepared from diacrylate derivatives which have a terminal double bonds and internal ester groups (bottom right in Scheme 1). The bis-(amino) nature leads to solvents with higher molecular weights. The higher molecular weights of the solvents result in projected boiling points >300° C. and very low vapor pressures, but higher room temperature viscosities. The high-boiling solvents were designed for applications where CO.sub.2 capture, and solvent regeneration could be accomplished at elevated temperatures with little solvent loss due to evaporation. One potential benefit to this application window would be in the ability to regenerate the solvent at elevated pressures using a temperature ramp as discussed above with only negligible solvent loss due to evaporation. High-boiling solvents allow higher regeneration temperatures and thus higher working capacities for regeneration at elevated pressures. As discussed above in reference to the isotherm data in FIG. 5, the high CO.sub.2 uptake in EMA-NH-EMA offers an attractive option of regenerating the solvent at elevated pressure. For higher molecular weight/higher viscosity solvents such as BAC solvent 2AB-BDM-2AB the absorption will require warmer temperatures to lower the solvent viscosity. This will lower the working capacity but savings due to reduced solvent loss, removal of solvent chilling, and high pressure recovered CO.sub.2 may offset the reduction in working capacity. The working capacity of 2AB-BDM-2AB for CO.sub.2 capture at 40° C., 20 bar followed by regeneration at 70° C., 20 bar still gives a relatively high working capacity of 2.1 mol/L. It's notable that the estimated working capacity for regeneration at 20 bar in BAC solvents can exceed the total absorbed amount of CO.sub.2 at 20 bar in the traditional physical solvents when operated at 40° C.

Example 2: CO.SUB.2 .Removal for Biogas Upgrading with BAC Solvents

[0084] Biogas methane is formed through the anaerobic digestion of municipal or agricultural waste..sup.8-22 The gas generated in the digestor averages 35% CO.sub.2 with the balance mostly methane. (Note that landfill gas can have significantly higher concentrations of N.sub.2 which would also need to be removed from the product CH.sub.4 gas in a separate stage from the CO.sub.2 capture.) In most commercial solvent processes, capture of the sub-ambient pressure CO.sub.2 in the biogas is facilitated by pressurizing the system to 6-8 bar to give a CO.sub.2 partial pressure of approximately 2.5 bar. CO.sub.2 is then removed using a circulating solvent system which typically uses water, or in some cases Selexol. Compression of the biogas is required due to the low CO.sub.2 solubility in water and Selexol. Compression of the biogas adds to the energy demands of the process and increases the CH.sub.4 solubility in the CO.sub.2 removal solvent. As such, BAC solvents with significantly higher CO.sub.2 solubility at low partial pressures of CO.sub.2 could reduce or even remove the need for compression of the feed gas and thus reduce the energy demands of the treatment process. The benefits for BAC solvents in biogas upgrading can be summarized as follows: high CO2 removal capacity without pressurization of biogas; low pressure operation reduces CH4 co-absorption in solvent; high CO2 capacity reduces size of absorber; high purity of recovered CO2; and mild solvent regeneration conditions.

[0085] To demonstrate the effectiveness of BAC solvents at removing CO.sub.2 under ambient conditions, CO.sub.2 absorption tests were run at 25° C. using a flowing 30/70 mixture of CO.sub.2/N.sub.2 at a total pressure of 1 bar. Note that Selexol is ineffective under these conditions since the CO.sub.2 solubility at the regeneration condition (P.sub.CO2 1 bar, 60° C.) is slightly higher than the CO.sub.2 solubility at the capture condition (P.sub.CO2 300 mbar, 25° C.). The working capacity of Selexol is essentially 0 wt % when the feed gas is not compressed above ambient pressure. In contrast, pure BAC solvents 1AHEMA, 1AA-iBMA and 1AB-DECAM, and a blended BAC solvent TBP/1AB-DECAM (TBP=tributyl phosphate) consisting of 20 wt % TBP and 80 wt % 1AB-DECAM all show very high CO.sub.2 uptakes under these conditions. Working capacities (WC) are indicated on the chart using conservative estimates of the absorbed amounts of CO.sub.2 after a 20-minute time window corresponding to the approximate time required for CO.sub.2 saturation in Selexol. Note that even higher working capacities are possible with extended CO.sub.2 contact times with the solvent. The working capacities are also variable depending on choice of regeneration temperature. The ultimate choices of absorption and regeneration temperatures would be guided by system parameters including CO.sub.2 mass transfer rates and solvent evaporation rates. The results presented in FIG. 6 highlight the exceptional improvements in CO.sub.2 working capacities possible with BAC solvents compared to traditional physical solvents, but the conditions of the test were not optimized to any particular system design. The BAC solvent with the highest CO.sub.2 uptake at P.sub.CO2 300 mbar is 1AH-EMA. The low viscosity of the solvent leads to fast mass transfer. Absorption tests at 20° C. and 25° C. indicate that the ultimate CO.sub.2 capacity is higher at 20° C., however the CO.sub.2 uptake in the first 20 minutes is nearly identical between the two temperatures. The optimal temperature of the absorber would thus depend on the ideal balance of mass transfer rate and CO.sub.2 solubility.

[0086] Solvent stability was evaluated for BAC solvents under simulated absorption/desorption cycling. The results of a cycling test for BAC solvent 1AB-DECAM is shown in FIG. 7. While a typical biogas upgrading process will involve a dehumidification step prior to the CO.sub.2 removal step, the 1AB-DECAM stability study was done under a more rigorous condition of 70% relative humidity to ensure sample stability in the presence of moisture. Thus, the sample was exposed to a cycle consisting of a dry N.sub.2 baseline purge, followed by exposure to moist CO.sub.2 (30% dry CO.sub.2/70% wet N.sub.2; P.sub.tot 1 bar) at 25° C. for 90 minutes, then heated to 60° C. under flowing dry CO.sub.2 at 1 bar for 60 minutes, then purged at 60° C. with dry N.sub.2 for 60 minutes, followed by cooling back to 25° C. to return to the baseline condition before repeating the cycle. The sample was cycled 14 times for a total run time of 4,000 min (67 hrs). No measurable drop in solvent performance relevant to CO.sub.2 capacity was observed over the length of the test.

[0087] Additional testing was done to determine the effect of moist CO.sub.2 on the absorption and the ability to regenerate BAC solvent 1AH-EMA since the presence of significant amounts of water in amine/CO.sub.2 mixtures can lead to the formation of carbonates which require more energy input to regenerate CO.sub.2. As noted earlier, typical biogas processing involves a water removal step prior to the CO.sub.2 removal step so moisture levels in the biogas feed are expected to be well below the levels used in the 1AH-EMA evaluation. The tests were designed to include higher moisture levels to ensure the solvent is stable. The results of the water vapor absorption and wet CO.sub.2 absorption tests with 1AH-EMA are summarized in FIG. 9. Tests using humidified N.sub.2 showed very low levels of water vapor absorption in 1AH-EMA. The solvent is relatively hydrophobic showing only ˜1.55 wt % of water vapor absorbed at 80% relative humidity at 25° C. The mass changes in the solvent were determined at 25° C. using humidified N.sub.2, dry 30/70 CO.sub.2/N.sub.2 mixture and humidified 30/70 CO.sub.2/N.sub.2 mixtures with varying amounts of humidity added at a total pressure of 1 bar. Under dry conditions, the mass change in the solvent under a 30/70 CO.sub.2/N.sub.2 mixture is 8.15 wt %. This weight change is due to CO.sub.2 absorption in the solvent. Water vapor absorption from humidified N.sub.2 streams lead to weight changes of ˜0.4 wt % at 30% RH, 0.8 wt % at 50% RH, and 1.3 wt % at 70% RH. When the solvent was exposed to humidified CO.sub.2, the total mass change was slightly higher than expected from the combined values of water vapor and CO.sub.2 determined from the independent runs. For example, in the humidified CO.sub.2 run at 30% RH, the total mass gained was 8.90 wt %, slightly larger than the sum of water vapor (0.4 wt %) and CO.sub.2 (8.15 wt %) absorption observed in the pure runs. This trend was slightly more pronounced at higher humidity levels. At this point it is unknown whether the presence of CO.sub.2 enhances water vapor absorption or if the presence of water vapor enhances the uptake of CO.sub.2. Attempts to answer this question via in situ FTIR tests were inconclusive.

[0088] The results shown in FIG. 8 confirm that the presence of water in the CO.sub.2 stream does not affect the ability to regenerate the solvent. Regeneration of the solvent under 1 bar of dry CO.sub.2 at 60° C. proceeded equally when the CO.sub.2 absorption was done under dry or wet conditions. The similarity in regeneration behavior for the wet and dry CO.sub.2 tests are strong evidence that the presence of moisture in the gas stream does not induce the formation of stable bicarbonates or carbonates. Additional evidence for this conclusion was obtained by in situ FTIR studies and will be discussed in detail below. Thus, residual moisture in the gas will not negatively impact the CO.sub.2 performance of the BAC solvent. The effect of co-adsorbed water will mainly be a system level impact involving pumping cost and the need to dry the CO.sub.2 prior to sequestration. The BAC solvents are thus flexible to humidity requirements that the system demands. Reducing the energy penalties involved for aqueous amine solvents is a common motivation for the development of non-aqueous CO.sub.2 capture solvents. Research in this area is often referred to in the literature as “water lean” amine solvents..sup.4-5 One prevalent barrier to progress in these endeavors is the large increase in solvent viscosity that typically accompanies CO.sub.2 absorption in water lean amine solvents. The increase in viscosity can be several orders of magnitude which leads to major penalties in CO.sub.2 mass transfer rates and pumping costs. To evaluate the effect of CO.sub.2 loading on viscosity, three BAC solvents were tested after saturation with CO.sub.2 at 1 bar, 25° C. The results are shown in FIG. 8. All three solvents showed only modest increases in viscosity in the CO.sub.2 loaded phase compared to the neat phase, but much lower than typically reported for other water lean amine solvents. The 1AH-EMA solvent showed the smallest CO.sub.2-loaded viscosity, even though the solvent has the highest CO.sub.2 uptake under the test conditions. It is important to note that the viscosity measurements were done on solvents saturated with CO.sub.2 at 1 bar due to equipment constraints. In some projected applications of these solvents, e.g., biogas upgrading, the partial pressure of CO.sub.2 will be significantly below 1 bar and as such the CO.sub.2 loading will be smaller as well. As such, the CO.sub.2-loaded solvent viscosities under process conditions are projected to be 30-50% lower than those shown in FIG. 8.

[0089] Water Solubility.

[0090] Water solubility in CO.sub.2 solvents is an important property to consider. High water solubility can be beneficial in some applications where co-removal of water and CO.sub.2 from the gas being upgraded is desired. This approach is often applied in natural gas upgrading where hydrophilic solvents such as Selexol, NMP, and propylene carbonate are designed to aid in dehydration of the sour gas. In other applications, such as pre-combustion CO.sub.2 capture, co-adsorption of water with the captured CO.sub.2 is not desired since water is needed for the combustion system, water reduces the CO.sub.2 capacity of the solvent, increases the pumping cost of the solvent, and must be removed from the captured CO.sub.2 prior to injection into the transmission pipeline to the sequestration site. Thus, control of water solubility is important in CO.sub.2 solvent design depending on the targeted application. Water solubility in BAC solvents can be controlled to a significant degree depending on the nature of the carboxylate and amine functional groups. In general, esters are more hydrophobic than amide functionalized BAC solvents. For example, a humidification chamber study on ester solvent EMA-NH-EMA and amide solvent 1AB-DECAM showed that EMA-NH-EMA absorbed 25 wt % of water and 1AB-DECAM absorbed 54 wt % of water after 200 hours of continuous exposure to 90% relative humidity at room temperature. Note that these evaluations were done under extreme conditions. In an actual solvent looping process, the majority of the moisture would be removed from the feed gas prior to the CO.sub.2 removal step and contact times with the solvent would be much shorter with regeneration of the solvent performed after each absorption step.

Stability/Performance in the Presence of Water.

[0091] Some carbon capture solvents containing amines undergo side reactions to form solid carbamates, bicarbonates or carbonates, particularly in the presence of water. The formation of these unwanted compounds increases the energy demand of the solvent regeneration step, reduces the capacity of the solvent by converting a portion of it irreversibly, and diminishes the amount of solvent available for reversible CO.sub.2 capture. The formation of solids can also increase solvent viscosity, clog solvent pumps, and foul system channels causing downtime. Samples of pure BAC solvents were studied by FTIR spectroscopy under both dry and wet N.sub.2 and CO.sub.2, repeatedly cycling between these conditions. The results showed that while evidence of both physisorbed and chemisorbed CO.sub.2 was seen in all BAC solvents, the amount of CO.sub.2 absorbed was not diminished in the presence of water nor were any irreversible side products observed; sample spectra always reverted to their original appearance after purging with dry N.sub.2. Results obtained for BAC solvent 1AH-EMA are shown in FIG. 10.

High Pressure CO.SUB.2 .Removal Processes

[0092] For high pressure CO.sub.2 removal processes in pre-combustion applications, the commercial solvent of choice is Selexol. The absorption process takes advantage of the highly reversible nature of CO.sub.2 absorption in the physical solvent by using a pressure swing adsorption/desorption cycle along with solvent warming. The CO.sub.2 is captured from a high partial pressure stream consisting of ˜50/50 mixture of CO.sub.2 and H.sub.2 at a total pressure near 50 bar. The absorber operates at 10° C. to enhance the volumetric CO.sub.2 solubility and reduce the size of the absorption column. The absorbed CO.sub.2 is then released near 25° C. in a series of flash tanks which step down the pressure to about 1.5 bar. Once the CO.sub.2 is recovered, it needs to be cleaned and then recompressed to 1500-2200 psi to transport it via pipeline for geologic storage, enhanced oil recovery, or CO.sub.2 utilization.

[0093] Tailored BAC solvents are designed to improve the efficiency of the process by providing high CO.sub.2 capacity without the need for solvent chilling while also providing a means for reducing compression costs of the recovered CO.sub.2 by regenerating the solvent at or near the absorber pressure using a moderate temperature ramp. The contrast in CO.sub.2 uptake at different temperatures between BAC solvent EMA-NH-EMA and Selexol are shown in FIG. 11. For a capture process operating at a P.sub.CO2 of 20 bar, the EMA-NH-EMA solvent can achieve the same volumetric CO.sub.2 uptake at 25° C. as observed for Selexol at 10° C. The results indicate that replacement of Selexol with EMA-NH-EMA could remove the need for solvent chilling and save on energy costs. Additional energy saving could be achieved when considering that dehydration of absorbed water from Selexol requires heating to 80° C. or higher. A similar temperature ramp can be applied to EMA-NH-EMA for recovery of absorbed CO.sub.2. A working capacity of >5 mol/L is theoretically possible with an absorption step at 25° C., 20 bar if a combined temperature and pressure swing is applied in the regeneration with heating to 70° C. and the pressure dropped to 1.5 bar. The working capacity of Selexol using the same conditions would be slightly above 3 mol/L. An alternative option is to take advantage of the higher CO.sub.2 heat of absorption in EMA-NH-EMA and do the regeneration at the same pressure as the absorber using only a temperature swing. A working capacity of 3.9 mol/L is theoretically possible with an absorption step at 25° C., 20 bar using only a temperature swing to 70° C. in the regeneration with the pressure maintained at 20 bar. Selexol under these conditions would only yield a working capacity slightly above 2 mol/L. To put these results into further perspective, system modeling studies for CO.sub.2 capture in an IGCC powerplant uses Selexol with an absorber temperature of 10° C. and desorption down to 1.5 bar at 25° C. Under these conditions, the theoretical working capacity of Selexol would be approximately 4.5 mol/L. However, Selexol will still require heating to desorb water since the solvent is quite hydrophilic. Thus, EMA-NH-EMA offers two alternative strategies providing similar CO.sub.2 working capacities where either solvent chilling is removed, or decompression of gas in the recovery step is reduced or removed. Either alternative could lead to savings in both capital and operation costs.

Low to Intermediate CO.SUB.2 .Pressure Processes

[0094] A number of commercially important processes generate CO.sub.2 at concentrations intermediate between those involved with post-combustion CO.sub.2 capture and syngas processes. These processes are listed in Table 1 and include biogas/landfill gas CH.sub.4 upgrading, refineries, cement production and steel manufacturing. The CO.sub.2 concentrations in these applications span a typical range of 20-40%. While different CO.sub.2 removal technologies have been proposed for some of these processes, only biogas upgrading has seen significant deployment. The advantages of BAC solvent for biogas upgrading will be applicable to other applications listed in Table 1 since these processes all involve CO.sub.2 removal under similar concentrations, however the current discussion will focus on biogas upgrading since this is the most widely employed commercial need.

[0095] Current commercial biogas upgrading processes that involve continually looping solvent systems predominantly use water or Selexol as CO.sub.2 physical solvents. Both water and Selexol have relatively low CO.sub.2 solubility and thus require compression of the biogas stream to increase CO.sub.2 uptake in the solvent. Compression of the feed gas increases the energy demand of the system and increases co-adsorption of CH.sub.4 in the solvent. Replacing Selexol with BAC solvent 1AH-EMA will allow the system to operate at ambient pressure while providing a significant enhancement in CO.sub.2 working capacity and reduction in CH.sub.4 co-adsorption. The results shown in FIG. 12 highlight the exceptional increase in CO.sub.2 working capacity achieved when 1AH-EMA is used in place of Selexol in a biogas upgrading process operating without additional compression of the biogas. Selexol is completely ineffective under these conditions since the CO.sub.2 solubility at the regeneration condition (P.sub.CO2 1 bar, 60° C.) is slightly higher than the CO.sub.2 solubility at the capture condition (P.sub.CO2 300 mbar, 25° C.).

[0096] The working capacity of Selexol is essentially 0 wt % when the feed gas is not compressed above ambient pressure. As a consequence, biogas upgrading systems that employ Selexol operate the absorber at elevated pressures in the range of 6-8 bar and regenerate the solvent at 40° C. at 1 bar. The working capacity for Selexol under these conditions can be estimated based on the CO.sub.2 Henry's constants for Selexol of 0.166 mol/(L*bar) at 20° C. and 0.107 mol/(L*bar) at 40° C. The theoretical maximum CO.sub.2 working capacity in a 30% CO.sub.2 stream at 8 bar total pressure would be 2.4 bar×0.166 mol/(L*bar)−1 bar×0.107 mol/(L*bar)=0.291 mol/L. In contrast, the theoretical CO.sub.2 working capacity for BAC solvent 1AH-EMA operating without compression of the feed gas with an absorber temperature of 25° C. and a solvent regeneration temperature of 45° C. is 0.42 mol/L. This is a 45% increase in working capacity using the same 20° C. temperature swing while removing the need for compression of the feed gas. The working capacity of 1AH-EMA can be improved significantly with a modest increase in the regeneration temperature. For example, increasing the absorption/desorption temperature swing from 20° C. to 30° C. more than doubles the working capacity from 0.42 mol/L to 1.01 mol/L. In contrast, increasing the desorption temperature for Selexol by 10° C. only shows a meager gain in working capacity from 0.29 mol/L to 0.31 mol/L. By simply increasing the temperature swing from 20° C. to 30° C., the working capacity of 1AH-EMA can be increased to 325% that of Selexol. This result is even more impressive because it does not require any compression of the feed gas.

[0097] Replacing Selexol in a biogas upgrading process with BAC solvent 1AH-EMA could theoretically increase CO.sub.2 working capacity by 300% or more while removing the need for biogas compression. Operating the system at 1 bar will also provide an addition benefit of significant reduction in co-adsorbed CH.sub.4 in the CO.sub.2 removal solvent. The absorption of CH.sub.4 in a solvent at conditions typical for biogas upgrading follows a linear correlation with pressure according to Henry's law, i.e., CH.sub.4 solubility=Pressure×(Henry's Constant for CH.sub.4). As such, the CH.sub.4 solubility in the CO.sub.2 removal solvent at 1 bar will be ⅙ the value at 6 bar or ⅛ the value at 8 bar. By operating the BAC biogas upgrading at 1 bar, the co-adsorption of CH.sub.4 will be reduced by nearly 85% from a Selexol system operating at 6 bar assuming similar CH.sub.4 henry's constants for the two solvents. In-house gravimetric measurements gave CH.sub.4 solubility in Selexol of 0.0086 mol/(L*bar) at 20° C. and CH.sub.4 solubility in 1AH-EMA of 0.016 mol/(L*bar) at 25° C. Based on these respective values, the CH.sub.4 solubility from a 30/70 mixed CO.sub.2/CH.sub.4 feed gas at a total pressure of 6 bar would be 0.036 mol/L in Selexol. The same gas stream at 1 bar total pressure would give a CH.sub.4 solubility in 1AH-EMA of 0.0112 mol/L. This equates to a 69% reduction in CH.sub.4 losses due to solvent absorption. The low CH.sub.4 uptake in 1AH-EMA will also lead to a significant enhancement in the purity of the recovered CO.sub.2. The ratio of absorbed CO.sub.2 to absorbed CH.sub.4 in 1AH-EMA at 25° C. for a gas stream consisting of 30% CO.sub.2 in CH.sub.4 at 1 bar is approximately 1.6/0.0112=143:1. Assuming complete release of co-adsorbed CH.sub.4 and 1 mol/L of CO.sub.2 during the regeneration will result in a recovered CO.sub.2 stream consisting of only 88% CO.sub.2/12% CH.sub.4 for the Selexol process versus 99% CO.sub.2/1% CH.sub.4 for the 1AH-EMA process.

[0098] The use of a non-aqueous BAC solvent such as 1AB-DECAM, 1AA-IBMA or 1AH-EMA in a CO.sub.2 removal application in processes such as biogas upgrading, landfill gas upgrading, and cement production where CO.sub.2 concentration in the feed gas is above 15% and operating near ambient conditions provides advantages over traditional aqueous amine solvents. These advantages include reduced corrosion rates and milder solvent regeneration conditions. BAC solvent such as 1AB-DECAM or 1AH-EMA can be regenerated at 1 bar under pure CO.sub.2 streams at mild temperatures of 45-60° C., whereas aqueous amine solvents typically require steam stripping at temperatures in the range of 80-160° C. The cooler desorption temperature and pure CO.sub.2 stripping sweep gas will result in significantly reduced solvent loss and reduced solvent decomposition while yielding a cleaner CO.sub.2 recovery without co-adsorbed water. The perceived benefits of water lean amine solvents have led to an increasing interest recently among research groups to develop new and improved CO.sub.2 capture solvents based on this strategy. The vast majority of these research efforts are aimed at applications involving post-combustion CO.sub.2 capture, but some of their results are indeed relevant to related applications targeted for BAC solvents such as biogas upgrading, landfill gas upgrading, and cement production. The majority of prior attempts at water lean amine solvents for CO.sub.2 capture have been hampered by significant increases in solvent viscosity in the CO.sub.2 loaded phase. The increase in viscosity can be several orders of magnitude which leads to major penalties in CO.sub.2 mass transfer rates and pumping costs. To evaluate the effect of CO.sub.2 loading on viscosity, three BAC solvents were tested after saturation with CO.sub.2 at 1 bar, 25° C. The results are shown in FIG. 13. All three solvents showed only a modest increase in viscosity in the CO.sub.2 loaded phase compared to the neat phase, but much lower than typically reported for other water lean amine solvents. To put the CO.sub.2-loaded BAC solvent viscosities in proper perspective, the results are compared against a recent literature report which touted the discovery of solvents with low CO.sub.2-loaded viscosities. The reported solvents showed ten-fold viscosity increases up to values of ˜250 cP at 40° C. after CO.sub.2 absorption of 8 wt %. In contrast, BAC solvents show significantly lower CO.sub.2-loaded viscosities even at higher CO.sub.2 loadings and lower temperature. The 1AH-EMA solvent showed the smallest CO.sub.2-loaded viscosity even though the solvent has the highest CO.sub.2 uptake under the test conditions. At 25° C. with 11 wt % CO.sub.2 absorbed; the solvent 1AH-EMA has a viscosity of only 30 cP. This CO.sub.2 loaded viscosity is an order of magnitude below the value reported for the “low viscosity” solvent in the literature. It is important to note that the viscosity measurements were done on BAC solvents saturated with CO.sub.2 at 1 bar due to equipment constraints. In the projected applications of these solvents, e.g., biogas upgrading, the partial pressure of CO.sub.2 will be significantly below 1 bar and as such the CO.sub.2 loading will be smaller as well. As such, the CO.sub.2-loaded solvent viscosities under process conditions are projected to be 30-50% lower than those shown in FIG. 14 and well within the operational window of a solvent looping CO.sub.2 removal system.

[0099] The defining chemical component of the BAC solvent molecular structures comprises the beta-amino carboxylate group shown below:

##STR00011##

[0100] The BAC carboxylate component may comprise either an ester or an amide. This core structural motif is critical for several reasons. The alkyl ester or dialkyl amide moiety controls the melting point and viscosity to ensure that the compound is liquid, stable, and has low volatility in the targeted temperature and pressure range of the application. Location of the amine site in a position beta to the carboxylate carbonyl carbon provides a unique electronic effect on amine reactivity and also provides a convenient route to synthesis using established high yield organic reactions, e.g., Michael addition, involving simple organic amines wherein the steric crowding of the amine site can be conveniently controlled. The physical properties and CO.sub.2 affinity of the solvent can be easily optimized for a particular application via the incorporation of different alkyl groups on the amide, ester, and/or amine site. The optimal size of the alkyl groups is from C1-C6 to maintain high volumetric CO.sub.2 uptake, low viscosity, and low melting point. To further illustrate the spirit of the invention, examples of BAC solvents which have been prepared and characterized are included in the appendix. The amine site functions best as a secondary amine. Two alkyl groups on the amide nitrogen are preferred to provide a melting point below room temperature.

[0101] The solvents are designed with the intention of using them as the primary CO.sub.2 capture component in a CO.sub.2 capture system. The total CO.sub.2 capacity of the solvent will be maximized when operated as a neat solvent. However, in certain applications, the rate of CO.sub.2 diffusion may be more critical than the overall CO.sub.2 capacity of the solvent. In these circumstances, it may be advantageous to blend the BAC solvent with one or more co-solvents. These co-solvents would have a lower neat viscosity than the CO.sub.2 loaded phase of the BAC solvent. The blend would then be optimized to provide the highest CO.sub.2 diffusion rate and highest CO.sub.2 absorption amount wherein one or more BAC solvents in the blend would be the main CO.sub.2 reactive component.

##STR00012##

Examples of alternative versions of BAC solvents are shown above. Two derivatives of BAC solvents with similar molecular weights and steric hinderance around the amine sites. The structure on the left is prepared as described in this disclosure using a common crotonyl derivative and butyl amine, whereas the structure on the right is prepared from a specialty chemical and ethyl amine. An alternative BAC solvent (bottom) using an ether functionalized amine in place of an alkyl amine.

[0102] The structures of the BAC solvents can be designed to minimize water affinity. In certain applications where water absorption needs to be optimized, or in applications where water vapor absorption is of little concern, functionalization of the pendant alkyl groups may be preferred. As an example, the n-butyl group on the amine site could be replaced with an ether or ester group to provide an enhanced CO.sub.2 affinity of the solvent. The BAC core structure can be derived from functionalization of an acrylate, methacrylate, or crotonate core since each of these building blocks are commodity chemicals and readily available at reasonable costs. One skilled in the art could use a specialty chemical in which the methyl group of the methacrylate or crotonate is replaced with an ethyl, propyl, butyl, pentyl or hexyl group in order to prepare a modified BAC solvent with similar molecular weight and steric influence on the amine site.