Regenerable solvent mixtures for acid-gas separation

Abstract

A solvent system comprising a diluent and a nitrogenous base for the removal of CO.sub.2 from mixed gas streams is provided. Also provided is a process for removing CO.sub.2 from mixed gas streams using the disclosed solvent system. The solvent system may be utilized within a gas processing system.

Claims

1. A process for the removal of acid gas from a gas stream, comprising contacting an acid gas-containing gas stream with a solvent system comprising a solution formed of: a nitrogenous base having a nitrogen with a hydrogen atom leaving group; and a non-aqueous diluent, wherein the nitrogenous base has a structure such that it reacts with an acidic gas so as to form a carbamate salt or a heteroatom analogue of a carbamate salt without any substantial formation of a carbonate ester or heteroatom analogue of a carbonate ester, wherein the solvent system is substantially insoluble in water, and wherein the diluent is selected from the group consisting of 2,2,3,3,4,4,5,5-octafluoropentanol ( OFP); 2,2,3,3 -tetrafluoropropanol (TFP); 2,2,3,3,3 -pentafluoropropanol (PFP); 2,2,3,3,4,4-hexafluorobutanol (HFB); nonafluoro-1-hexanol; 4,4,5,5,6,6,7,7,7-nonafluoroheptanol ; 1,1,3,3-hexafluoro-2-phenyl-2-propanol; 4-methoxyphenol (4-MeOPh); 4-ethoxyphenol (4-EtOPh); 2-ethoxyphenol; 4-propoxyphenol; benzimidazole; 1-trifluoroacetylimidazole; 2-trifluoromethylpyrazole; 3,5 -bistrifluoromethylpyrazole; 3-trifluoromethylpyrazole; and mixtures thereof.

2. The process of claim 1, further comprising outputting an acid gas-rich solvent and an acid gas-lean gas stream.

3. The process of claim 1, further comprising regenerating the acid gas-rich solvent by applying heat to form a regenerated solvent comprising a lower content of acid gas than present in the acid gas-rich solvent.

4. The process of claim 1, wherein the heat applied is derived from a source selected from the group consisting of low-pressure steam, hot flue gas, or a combination thereof.

5. The process of claim 1, further comprising regenerating the acid gas-rich solvent by applying heat sufficient to release CO.sub.2 from the carbamate salt or the heteroatom analogue of the carbamate salt to form a regenerated solvent comprising a lower content of acid gas than present in the acid gas-rich solvent.

6. The process of claim 1, wherein the acid gas-containing gas stream is fossil fuel combustion flue gas.

7. The process of claim 1, wherein the contacting is conducted within an absorber selected from the group consisting of a spray-tower absorber, packed-bed absorber, tray-tower absorber, venture absorber, and ejector absorber.

8. The process of claim 7, wherein the absorber is configured to route the solvent system to a unit wherein water can be decanted, centrifuged, or otherwise removed from the solution.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a scheme showing a reaction pathway employed for capturing CO.sub.2 using solvent mixtures comprising an acid component and a nitrogenous base;

(2) FIG. 2 is a diagram of a reboiler-based system embodied by the present invention for the capture and regeneration of acidic gases from a mixed gas stream;

(3) FIG. 3 is a diagram of a reboiler-free system embodied by the present invention for the capture of acidic gases from a mixed gas stream;

(4) FIG. 4 is a diagram of a reboiler-assisted system embodied by the present invention for the capture of acidic gases from a mixed gas stream;

(5) FIG. 5 is a diagram of a waste heat reboiler system embodied by the present invention for the capture of acidic gases from a mixed gas stream;

(6) FIG. 6 is a diagram of a waste heat utilization system embodied by the present invention for the capture of acidic gases from a mixed gas stream;

(7) FIG. 7 is a fluorine NMR spectrum of 2-fluorophenethylamine and 2,2,3,3,4,4,5,5-octafluoropentanol before (top) and after (bottom) reaction with CO.sub.2, showing six unique resonances in the product;

(8) FIG. 8 is fluorine NMR spectra of 2-FPEA before (top) and after (bottom) reaction with CO.sub.2;

(9) FIG. 9 is a .sup.1H NMR spectrum showing transformation of 2-fluorophenethylamine into 2-fluorophenethylamine carbamate upon reaction with carbon dioxide;

(10) FIG. 10 shows reaction pathways with carbon dioxide for the solvent 3-fluoro-N-methylbenzylamine in 4,4,5,5,6,6,7,7,7-nonafluorheptanol, where all relevant hydrogen and fluorine nuclei are labeled for discussion herein;

(11) FIG. 11 is fluorine NMR spectra following the transformation of 3-FNMBA/NFHp solvent (top) with the addition of CO.sub.2 (bottom);

(12) FIG. 12 is .sup.1H NMR spectra of the transformation occurring with purging of 3-FNMBA/NFHp with CO.sub.2, showing fluorinated alcohol .sup.1H resonances unaffected by CO.sub.2 absorption;

(13) FIG. 13 is fluorine NMR spectra of 3-FNMBA before (top) and after (bottom) reaction with CO.sub.2; and

(14) FIG. 14 is .sup.1H NMR spectra showing transformation of 3-fluoro-N-methylbenzylamine into 3-fluoro-N-methylbenzylamine carbamate upon reaction with carbon dioxide.

DETAILED DESCRIPTION OF THE INVENTION

(15) The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the inventions are shown. Indeed, these inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements. As used in this specification and the claims, the singular forms a, an, and the include plural referents unless the context clearly dictates otherwise.

(16) In one aspect of the present invention is provided a liquid solvent system. The solvent system may be used for the separation of acidic gases from gas mixtures. The term acid gas is intended to refer to any gas component that can result in formation of an acid when mixed with water. Non-limiting examples of acid gases encompassed by the present invention include CO.sub.2, SO.sub.2, COS, CS.sub.2 and NOx. For simplicity, the invention is described below in relation specifically to CO.sub.2. It is understood, however, that the present invention encompasses methods and systems for removal of any acid gas component from a gas stream.

(17) In certain embodiments, the solvent system is regenerable in that the acidic gases can be released from the solvent, and the solvent can be reused to separate additional acidic gases from further gas mixtures. In particular embodiments, the solvent system is regenerable at temperatures lower than those typically required for solvents used for such purposes.

(18) In some embodiments, the solvent system of the present invention comprises a mixture of a nitrogenous base component with a non-aqueous diluent. The non-aqueous diluent can be, but is not necessarily, a relatively acidic component. The term relatively acidic component as used herein is interchangeable with the term acidic component and is understood to mean a material having an acidity that is greater than the acidity of water, preferably substantially greater than the acidity of water. For example, in some embodiments, the diluent can have a pKa of less than about 15, less than about 14, less than about 13, less than about 12, less than about 11, or less than about 10. In some embodiments, the diluent has a pKa of the alcohol component is about 6 to about 15, about 7 to about 15, about 8 to about 15, about 9 to about 15, about 6 to about 14, about 7 to about 14, about 8 to about 13, about 9 to about 13, about 6 to about 12, about 7 to about 12, about 8 to about 12, about 9 to about 12, about 6 to about 11, about 7 to about 11, about 8 to about 11, about 9 to about 11, about 6 to about 10, about 7 to about 10, or about 8 to about 10.

(19) Exemplary classes of diluents (e.g., relatively acidic diluents) that may be used according to the invention include, but are not limited to the following: fluorinated alcohols; optionally substituted phenols; and nitrogen heterocycles. Fluorinated alcohols useful according to the invention may comprise any compound having the formula ROH, where R is an alkyl group (e.g., C.sub.1-C.sub.10 alkyl, C.sub.1-C.sub.8 alkyl, C.sub.1-C.sub.6 alkyl, C.sub.2-C.sub.10 alkyl, C.sub.2-C.sub.8 alkyl, C.sub.2-C.sub.6 alkyl, C.sub.3-C.sub.10 alkyl, C.sub.3-C.sub.8 alkyl, or C.sub.3-C.sub.6 alkyl) and wherein one or more hydrogen atoms of the alkyl group is substituted with fluorine. In some embodiments, the number of hydrogen atoms replaced with fluorine can be two, three, four, five, six, seven, eight, nine, or even more as may be deemed useful. In further embodiments, one or more of the hydrogen atoms of the alkyl group may optionally be replaced with one or more other substituents, including, but not limited to, C.sub.1-C.sub.6 alkyl, C.sub.1-C.sub.6 alkoxy, and halo substituents. Optionally substituted phenols useful in the invention are understood to mean phenols wherein one or more of the hydrogen atoms on the phenyl ring may be replaced with a substituent. Non-limiting, exemplary replacement groups for one or more of the hydrogen atoms on the phenyl ring include C.sub.1-C.sub.6 alkyl, C.sub.1-C.sub.6 alkoxy, and halo. Nitrogen heterocycles are understood to mean any cyclic compound including at least one nitrogen atom in the ring structure (including but not limited to imidazoles, pyrazoles, and triazoles) and being optionally substituted such that one or more of the hydrogen atoms on the ring structure may be replaced with a substituent. Non-limiting, exemplary replacement groups for one or more of the hydrogen atoms on the ring include C.sub.1-C.sub.6 alkyl, C.sub.1-C.sub.6 alkoxy, and halo.

(20) In some specific embodiments, the diluent (e.g., relatively acidic diluents) may be selected from the group consisting of: 2,2,3,3,4,4,5,5-octafluoropentanol (OFP); 2,2,3,3-tetrafluoropropanol (TFP); 2,2,3,3,3-pentafluoropropanol (PFP); 2,2,3,3,4,4-hexafluorobutanol (HFB); 2,2,2-trifluoroethanol (TFE); nonafluoro-1-hexanol; 4,4,5,5,6,6,7,7,7-nonafluoroheptanol; 1,1,3,3-hexafluoro-2-phenyl-2-propanol 4-methoxyphenol (4-MeOPh); 4-ethoxyphenol (4-EtOPh); 2-ethoxyphenol; 4-propoxyphenol; imidazole; benzimidazole; N-methyl imidazole; 1-trifluoroacetylimidazole; 1,2,3-triazole; 1,2,4-triazole; 2-trifluoromethylpyrazole; 3,5-bistrifluoromethylpyrazole; 3-trifluoromethylpyrazole; and mixtures thereof.

(21) In other embodiments, the non-aqueous diluent is not a relatively acidic component, and does not have a pKa that falls within the ranges noted above. For example, the non-aqueous diluent may, in certain embodiments, have a pKa greater than about 15.

(22) In certain embodiments, the non-aqueous diluent used in the solvent system may be generally selected from the group consisting of alcohols, ketones, aliphatic hydrocarbons, aromatic hydrocarbons, nitrogen heterocycles, oxygen heterocycles, aliphatic ethers, cyclic ethers, esters, and amides and mixtures thereof. In more specific embodiments, the diluent may be selected from the group consisting of fluorinated alcohols, fluorinated ketones, fluorinated aliphatic hydrocarbons, fluorinated aromatic hydrocarbons, fluorinated nitrogen heterocycles, fluorinated oxygen heterocycles, fluorinated aliphatic ethers, fluorinated cyclic ethers, fluorinated esters, and fluorinated amides and mixtures thereof. In specific embodiments the diluent may be selected from the group consisting of toluene, p-xylene, 1-methylnaphthalene, 2,4,6-dimethylaminophenol, benzylalcohol, 2,6-dimethylcyclohexanone, 3,5-lutidine, cyclohexanone, aniline, pyridine, 2-fluoroacetylphenone, 1-fluorodecane, 2,4-difluorobenzophenone, 2-fluoro-3-trifluoromethylaniline, 2-fluoroaniline, 4-fluoroaniline, 3-trifluoromethylacetophenone, 2-trifluoromethylacetophenone, bis(2,2,2-trifluoroethyl)methylpho sphonate, 4-fluoro-3-(trifluoromethyl)benzaldehyde and mixtures thereof. Further, diluents within this list can be combined with diluents noted to be relatively acidic diluents as listed above.

(23) The nitrogenous base according to the present invention can be characterized as any nitrogenous base having a proton that can be donated from a nitrogen, which reacts with an acid gas via a carbamate pathway and avoids reaction with the acid gas to form carbonate esters. The nitrogenous base component may, in certain embodiments, be almost any nitrogenous base that meets this requirement including, but not limited to, primary amines, secondary amines, diamines, triamines, tetraamines, pentamines, cyclic amines, cyclic diamines, amine oligomers, polyamines, alcoholamines, guanidines, amidines, and the like. In some embodiments, the nitrogenous base can have a pKa of about 8 to about 15, about 8 to about 14, about 8 to about 13, about 8 to about 12, about 8 to about 11, or about 8 to about 10. In certain embodiments, the nitrogenous base component has a pKa less than about 11.

(24) A primary amine is understood to be a compound of the formula NH.sub.2R, where R can be a carbon-containing group, including but not limited to C.sub.1-C.sub.20 alkyl. A secondary amine is understood to be a compound of the formula NHR.sub.1R.sub.2, wherein R.sub.1 and R.sub.2 are independently carbon-containing groups, including but not limited to C.sub.1-C.sub.20 alkyl, wherein R, R.sub.1, and R.sub.2 are independently carbon-containing groups, including but not limited to C.sub.1-C.sub.20 alkyl. One or more of the hydrogens on R, R.sub.1, and R.sub.2 may optionally be replaced with one or more substituents. For example, one or more of the hydrogens on R, R.sub.1, or R.sub.2 may be replaced with optionally substituted C.sub.1-C.sub.6 alkyl, optionally substituted C.sub.1-C.sub.6 alkoxy, optionally substituted C.sub.2-C.sub.10 alkenyl; optionally substituted C.sub.2-C.sub.10 alkynyl; optionally substituted alkaryl; optionally substituted arylalkyl; optionally substituted aryloxy; optionally substituted heteroaryl; optionally substituted heterocycle; halo (e.g., Cl, F, Br, and I); hydroxyl; halogenated alkyl (e.g., CF.sub.3, 2-Br-ethyl, CH.sub.2F, CH.sub.2CF.sub.3, and CF.sub.2CF.sub.3); optionally substituted amino; optionally substituted alkylamino; optionally substituted arylamino; optionally substituted acyl; CN; NO.sub.2; N.sub.3; CH.sub.2OH; CONH.sub.2; C.sub.1-C.sub.3 alkylthio; sulfate; sulfonic acid; sulfonate esters (e.g., methanesulfonyl); phosphonic acid; phosphate; phosphonate; mono-, di-, or triphosphate esters; trityl or monomethoxytrityl; CF.sub.3S; CF.sub.3SO.sub.2; or silyl (e.g., trimethylsilyl, dimethyl-t-butylsilyl, and diphenylmethylsilyl). Cyclic amines are amines wherein the nitrogen atom forms part of the ring structure, and may include, but are not limited to, aziridines, azetidines, pyrrolidines, piperidines, piperazines, pyridines, pyrimidines, amidines, pyrazoles, and imidazoles. Cyclic amines may comprise one or more rings and may optionally be substituted with one or more substituents as listed above. In some embodiments, the nitrogenous base has a guanidine structure, which is optionally substituted with one or more substituents as noted above. In some embodiments, the nitrogenous base has an amidine structure, which is optionally substituted with one or more substituents as noted above. In some embodiments, the nitrogenous base may be a diamine. In some embodiments, the nitrogenous base may be a primary or secondary alcoholamine. Alcoholamines are also known as amino alcohols and contain both an alcohol and amine group. The amine group of the alcoholamine may be any type of amine as disclosed herein. In some embodiments, the alcoholamine is a primary, secondary, or tertiary alcohol amine.

(25) In certain embodiments, the primary or secondary amine may be selected from amines functionalized with fluorine-containing-alkyl-aromatic groups. In specific embodiments, the amine may be selected from the group consisting of 2-fluorophenethylamine, 3-fluorophenethylamine, 4-fluorophenethylamine, 2-fluoro-N-methylbenzylamine, 3-fluoro-N-methylbenzylamine, and 4-fluoro-N-methylbenzylamine, 3,5-di-fluorobenzylamine, D-4-fluoro-alpha-methylbenzylamine, and L-4-fluoro-alpha-methylbenzylamine.

(26) In certain embodiments, the nitrogenous base may be selected from the group consisting of 1,4-diazabicyclo-undec-7-ene (DBU); 1,4-diazabicyclo-2,2,2-octane; piperazine (PZ); triethylamine (TEA); 1,1,3,3-tetramethylguanidine (TMG); 1,8-diazabicycloundec-7-ene; monoethanolamine (MEA); diethylamine (DEA); ethylenediamine (EDA); 1,3-diamino propane; 1,4-diaminobutane; hexamethylenediamine; 1,7-diaminoheptane; diethanolamine; diisopropylamine (DIPA); 4-aminopyridine; pentylamine; hexylamine; heptylamine; octylamine; nonylamine; decylamine; tert-octylamine; dioctylamine; dihexylamine; 2-ethyl-1-hexylamine; 2-fluorophenethylamine; 3-fluorophenethylamine; 3,5-difluorobenzylamine; 3-fluoro-N-methylbenzylamine; 4-fluoro-N-methylbenzylamine; imidazole; benzimidazole; N-methyl imidazole; 1-trifluoroacetylimidazole; 1,2,3-triazole; 1,2,4-triazole; and mixtures thereof. In certain embodiments, the nitrogenous base may be a guanidine or amidine.

(27) In some embodiments, the solvent system may include a mixture comprising a nitrogenous base and a diluent, which components may be present in roughly equal proportions by molarity (i.e. are present in equimolar amounts). In certain embodiments, the diluent is present in excess. In certain embodiments, the nitrogenous base is present in excess. For example, the molar ratio of nitrogenous base to diluent can be about 1:1 to about 100:1, for example, about 1.1:1 to about 20:1, 1.1:1 to about 15:1, 1.1:1 to about 10:1, 1.1:1 to about 5:1, 1.1:1 to about 3:1, about 2:1 to about 20:1, about 2:1 to about 15:1, 2:1 to about 10:1, 2:1 to about 5:1, about 3:1 to about 20:1, about 3:1 to about 15:1, about 3:1 to about 10:1, about 4:1 to about 20:1, about 4:1 to about 15:1, about 4:1 to about 10:1, about 5:1 to about 20:1, about 5:1 to about 15:1, or about 5:1 to about 10:1. Although not wishing to be bound by theory, it is believed that the use of an additional component can be useful to reduce or prevent precipitation of solids in the solvent system. In some embodiments, the solvent system may further comprise one or more additional components. The additional components may be added, for example, to increase the solubility of the captured CO.sub.2 product in the solvent system, and thus avoid the formation of precipitates. In other embodiments, however, solids formation may be desirable, and such formation may be enhanced by altering the concentration of one or more solvent components.

(28) In some embodiments, the solvent system of the present invention is particularly useful for capturing CO.sub.2 from a gas stream. The gas stream may be a mixed gas stream, having one or more other components in addition to CO.sub.2. When a solution comprising a solvent system of the present invention is purged with a gas mixture containing CO.sub.2, the components of the solvent system undergo a chemical reaction with CO.sub.2, binding the CO.sub.2 in the solution. In some embodiments, the solvent systems of the present invention have high CO.sub.2 loadings. For example, the solvent systems may be useful for capturing or removing greater than about 0.05 moles CO.sub.2 per mole of nitrogenous base, greater than about 0.1 moles CO.sub.2 per mole of nitrogenous base, greater than about 0.2 moles CO.sub.2 per mole of nitrogenous base, greater than about 0.3 moles CO.sub.2 per mole of nitrogenous base, greater than about 0.4 moles CO.sub.2 per mole of nitrogenous base, greater than about 0.5 moles CO.sub.2 per mole of nitrogenous base, greater than about 0.6 moles CO.sub.2 per mole of nitrogenous base, greater than about 0.7 moles CO.sub.2 per mole of nitrogenous base, greater than about 0.8 moles CO.sub.2 per mole of nitrogenous base, greater than about 0.9 moles CO.sub.2 per mole of nitrogenous base, or greater than about 1 mole CO.sub.2 per mole of nitrogenous base.

(29) FIG. 1 illustrates the reaction pathway for capturing CO.sub.2 using solvent mixtures comprising a nitrogenous base and a non-aqueous diluent according to the present invention. The reversible capture of CO.sub.2 according to this process involves a reaction with two equivalents of a nitrogenous base diluted in a non-aqueous diluent. The reaction involves formation of a carbamate and avoids the formation of a carbonate ester. As illustrated in FIG. 1, only the nitrogenous base (i.e., not the diluent) is shown to react with CO.sub.2 from the gas stream, with capture of CO.sub.2 substantially (including solely) as a carbamate salt. The resulting solution can be either ionic or non-ionic.

(30) Accordingly, the invention provides a solvent system comprising a solution formed of: a nitrogenous base having a nitrogen with a hydrogen atom leaving group; and a diluent, wherein the nitrogenous base has a structure such that it reacts with an acidic gas so as to form a carbamate salt or a heteroatom analogue of a carbamate salt without any substantial formation of a carbonate ester, preferably with no formation of a carbonate ester. Substantial is defined herein as meaning that the product of the reaction is at least about 80% carbamate salt, at least about 98% carbamate salt, at least about 99% carbamate salt, at least about 99.5% carbamate salt, at least about 99.9% carbamate salt and most desirably, 100% carbamate salt. Thus, the product of the reaction can be characterized as having less than about 20% carbonate ester, less than about 2% carbonate ester, less than about 1% carbonate ester, less than about 0.5% carbonate ester, less than about 0.1% carbonate ester, and most desirably, 0% carbonate ester.

(31) In certain embodiments, the diluent is selected such that it has low miscibility with water. For example, in some embodiments, the diluent has a solubility of less than or equal to about 10 g/100 mL in water at 25 C. (i.e., 10 g of solvent per 100 mL of water). In other embodiments, the diluent has a solubility in water of less than or equal to about 0.01 g/100 mL, less than or equal to about 0.1 g/100 mL, less than or equal to about 0.5 g/100 mL, less than or equal to about 1 g/100 mL, less than or equal to about 1.5 g/100 mL, less than or equal to about 2 g/100 mL, less than or equal to about 2.5 g/100 mL, less than or equal to about 3 g/100 mL, less than or equal to about 4 g/100 mL, less than or equal to about 5 g/100 mL, less than or equal to about 6 g/100 mL, less than or equal to about 7 g/100 mL, less than or equal to about 8 g/100 mL, or less than or equal to about 9 g/100 mL in water at 25 C. In some embodiments, the diluent is completely immiscible with water. Using diluents with low water solubility may result in solvent systems that display one or more of the following attributes: they may require less energy for regeneration; may have high CO.sub.2 loading capacities; may be able to tolerate water in the gas stream; and/or may be able to be separated from water without a large energy penalty.

(32) In additional embodiments, the nitrogenous based component of the solvent system is similarly selected such that it has low miscibility with water. In preferred embodiments, the nitrogenous base has higher miscibility with the diluent than with water. In some embodiments, the nitrogenous base has high solubility in the diluent. Examples of such nitrogenous bases include, but are not limited to, aliphatic amines with one or more hydrocarbon chains composed of three or more carbons, and aliphatic amines with one or more hydrocarbon chains composed of three or more carbons with one or more fluorine atoms substituted for hydrogen in the hydrocarbon chain. It is noted that although diluents and/or nitrogenous bases having low miscibility with water are preferred, the present invention also encompasses solvent systems wherein the diluents, nitrogenous base, and/or combination thereof are at least partially miscible with water.

(33) In some embodiments, the solvent system is tolerant to the presence of water. In certain embodiments, the solvent system tolerates water up to or equal to about 30% water by volume. For example, in some embodiments, the solvent system tolerates up to or equal to about 25% water by volume, up to or equal to about 20%, up to or equal to about 15%, up to or equal to about 10%, up to or equal to about 5%, up to or equal to about 2%, or up to or equal to about 1% water by volume. In some embodiments, tolerance to the presence of water means that there is little to no degradation of the solvent performance up to the indicated volume of water. In some embodiments, the solvent system maintains at or near its initial capacity for CO.sub.2 loading up to the indicated volume of water.

(34) In preferred embodiments, the CO.sub.2 captured using the solvent system of the present invention may be released to regenerate the solvent system for reuse. It is preferred that the solvent system is regenerable (or reaction with the CO.sub.2 is reversible) under mild conditions (e.g., at a low temperature). In some embodiments, the release of CO.sub.2 and corresponding regeneration of the solvent system is effectuated by heating the solution. When the solution containing bound CO.sub.2 is heated, the chemical reaction is reversed and the CO.sub.2 is released, producing a concentrated CO.sub.2 stream.

(35) In some embodiments, the present application relates to a solvent system and process for the removal of CO.sub.2 from a gas stream. The present invention applies to any gas stream containing CO.sub.2. For example, in particular embodiments, the invention relates to a process for the removal of CO.sub.2 from fossil fuel combustion flue gas, a natural gas mixture, or a mixture of respiration gases from closed environments containing CO.sub.2. The process involves passing the mixed gas stream through a solvent system comprising a diluent and a nitrogenous base component. In some embodiments, the present invention further relates to the regeneration of the solvent system, which releases the CO.sub.2. In some embodiments, regeneration of the solvent system involves heating the solvent system at a temperature sufficient to release the CO.sub.2. In some embodiments, the process involves heating the solvent system at a temperature at or below about 200 C., for example, at or below about 185 C., at or below about 150 C., or at or below about 125 C. In preferred embodiments, the process involves heating the solvent system at a temperature at or below about 100 C., for example, at a temperature at or below about 95 C., at or below about 90 C., at or below about 85 C., at or below about 80 C., at or below about 75 C., or at or below about 70 C. In some embodiments, the CO.sub.2 may be released at ambient temperature. In certain embodiments, the CO.sub.2 is captured in a non-aqueous phase under conditions in which water accumulates as a separate, lower density phase. This phase can be sent to the regenerator with the rich, non-aqueous phase to be regenerated at a lower temperature than the corresponding rich aqueous phase alone. This can be followed by phase separation from the lean, regenerated solvent before being sent back to the absorber.

(36) In certain embodiments, at or about 100% of the CO.sub.2 is removed from the CO.sub.2-rich solvent system. However, in other embodiments, less than 100% of the CO.sub.2 is removed from the CO.sub.2-rich solvent system. In preferred embodiments, about 50 to 100% of the captured CO.sub.2 is removed from the CO.sub.2-rich solvent system, preferably about 75% to 100%, about 80% to 100%, about 90% to 100%, about 95% to about 100%, or about 98% to 100%. For example, in some embodiments, at least about 98%, 95%, 90%, 85%, 80%, 75%, 70%, 60%, or 50% of the captured CO.sub.2 is removed from the CO.sub.2-rich solvent system.

(37) In some embodiments, the removal of CO.sub.2 from gas mixtures containing H.sub.2O in addition to CO.sub.2 can lead to the accumulation of H.sub.2O in the solvent system, either as a single phase or biphase solution, depending upon the reaction conditions. As noted above, the presence of H.sub.2O in the solvent mixture may be disadvantageous because of an undesirable side reaction, and more energy will be required for solvent regeneration due to the necessity of removing water from the solvent. Thus, the accumulation of H.sub.2O in the solvent system may increase the regeneration energy demand, decreasing the efficiency of the regeneration system.

(38) In some embodiments, the process of the present invention provides a method by which the detrimental effects of H.sub.2O accumulation in the solvent system may be avoided. For example, the detrimental effect of H.sub.2O accumulation on the solvent system regeneration energy demand may be minimized, by providing a process by which the CO.sub.2 is captured within the solvent system at a temperature greater than the H.sub.2O saturation temperature of the gas mixture. Additionally, the detrimental effect of H.sub.2O accumulation on the solvent system regeneration energy demand may be minimized by providing a process by which the H.sub.2O accumulates as a separate, aqueous phase within the solvent system. This process involves the use of a solvent system that exhibits little or no solubility in water. In such a system, water that collects is present as a separate phase. The separate, aqueous phase may be decanted or centrifuged off by mechanical, rather than thermal, processes, minimizing the energy required to maintain an efficient CO.sub.2 removal system. For example, as the hydrocarbon chain of aliphatic alcohols is increased in length, the solubility of the alcohol in water decreases. This is also true for fluorinated alcohols. For example, 2,2,3,3,4,4,5,5-octafluoropentanol (OFP) is essentially immiscible with water. Thus, a solvent system comprising an appropriate nitrogen base and OFP forms a biphasic liquid solution when combined with water. In such a solvent, water can be separated from the solvent system without distillation or the use of a membrane by decanting or centrifugation of the aqueous layer from the fluorinated phase. In some embodiments, after removal of the H.sub.2O, the CO.sub.2-rich solvent system can be regenerated at a low temperature with the addition of low boiling diluents to satisfy the partial pressure requirements. The solvent system could thus avoid the added energy penalty associated with the distillation of water. By providing a non-aqueous CO.sub.2 absorbing solvent system with low water solubility, the solvent system has lower energy demands and milder regeneration conditions than those of aqueous or high-water affinity CO.sub.2 solvent systems.

(39) In some embodiments, a system for the removal of CO.sub.2 from a gas stream is provided. A schematic of an exemplary system of the present invention is depicted in FIGS. 2 through 6. The CO.sub.2 removal system 10 includes an absorber 12 configured with an inlet to receive a gas stream. The gas stream may come directly from, e.g., a combustion chamber of a boiler system in a power generation plant. The gas stream may or may not be passed through other cleaning systems prior to entering the CO.sub.2 removal system. The absorber may be any chamber wherein a solvent system for the removal of CO.sub.2 is contained, having an inlet and outlet for a gas stream, and wherein the gas stream may be brought into contact with the solvent system. Within the absorber, the CO.sub.2 may be transferred from gaseous phase to liquid phase according to the principles discussed herein. The absorber may be of any type; for example, the absorber may comprise a spray-tower absorber, packed-bed absorber (including countercurrent-flow tower or cross-flow tower), tray-tower absorber (having various tray types, including bubble-cap trays, sieve trays, impingement trays, and/or float valve trays), venture absorber, or ejector absorber. The temperature and pressure within the absorber may be controlled. For example, in one embodiment, the temperature of the absorber may be maintained at or near 50-60 C. and the absorber may be maintained at or near atmospheric pressure. Thus, the absorber may be equipped with a heating/cooling system and/or pressure/vacuum system.

(40) Within the absorber, the gas stream is brought into fluid contact with and passed through a solvent system comprising a diluent and a nitrogenous base component. The solvent system reacts with the CO.sub.2 present in the gas stream, capturing it from the remaining components of the gas, and the resulting CO.sub.2-free gas stream is released from the absorber through an outlet. The solvent system continues to react with entering CO.sub.2 as the mixed gas stream is passed through, until it becomes rich with CO.sub.2. The absorber is optionally connected to one or more components. For example, the absorber is preferably configured with a means for routing solvent to a unit wherein water may be decanted, centrifuged, or otherwise removed from the system.

(41) At any stage in the process of CO.sub.2 capture, the solvent system may be regenerated. The system therefore includes an optional regeneration system 14 to release the captured CO.sub.2 via a separate CO.sub.2 gas stream and thus regenerate the solvent system. The regeneration system is configured to receive a feed of rich solvent from absorber and to return regenerated solvent to the absorber once CO.sub.2 has been separated from the rich solvent. The regeneration system may simply comprise a chamber with a heating unit to heat the solvent system at a temperature sufficient to release the gas, along with a release valve to allow the CO.sub.2 to be removed from the regeneration system. It may also be a distillation column and have essentially the same design as described above for the absorption column. The regenerator may be optionally connected to one or more components. For example, the regenerator is preferably configured with a means for routing solvent to a unit wherein water may be decanted, centrifuged, or otherwise removed from the system.

(42) The released CO.sub.2 can be output to storage or for other predetermined uses. The regenerated solvent is again ready to absorb CO.sub.2 from a gas stream, and may be directed back into the absorber.

(43) Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

EXPERIMENTAL

(44) The following example is provided for the purpose of complete disclosure and is not to be viewed as limiting of the invention.

Example 1: Reaction between CO2 and 2-fluorophenethylamine in Solution with 4-methoxy phenol as Diluent

(45) Reaction between CO.sub.2 and 2-fluorophenethylamine (2-FPEA) in solution with a methyl substituted phenol, specifically for this example 4-methoxy phenol (4-MeOPh), was initially observed using a standard semi-batch reactor system in which the amine solution was contained in the reaction vessel and a gas stream containing CO.sub.2 was bubbled through the amine solution. The CO.sub.2 concentration of the feed gas and reactor outlet gas streams was monitored via a NDIR CO.sub.2 analyzer to determine extent of CO.sub.2-amine reaction. Upon introduction of the CO.sub.2 containing gas stream to the amine solution resulted in a reduction in the CO.sub.2 content of the reactor outlet gas stream indicating that CO.sub.2 was reacting with the amine solution. Reaction with CO.sub.2 was accompanied by a small temperature rise in amine solution temperature from 25.5 to 27 C. due to the exothermic nature of the reaction. The reaction was continued until the CO.sub.2 concentration in the reactor outlet stream achieved >95% of the feed concentration. Integration of the quantity of CO.sub.2 reacted with the amine solution indicated that a CO.sub.2:amine molar ratio of 0.37:1 was achieved.

(46) Reversibility of the reaction between CO.sub.2 and the 2-FPEA/4-MeOPh solvent system was observed by ramping the temperature of the 2-FPEA/4-MeOPh/CO.sub.2 system, described above, to 80 C. under a N.sub.2 purge. The release of CO.sub.2 from the CO.sub.2-containing solution was monitored via a NDIR CO.sub.2 analyzer. Integration of the quantity of CO.sub.2 released indicated that CO.sub.2 was completely released from both solvent systems.

Example 2: Reaction between CO2 and 2-fluorophenethylamine in Solution with 2,2,3,3-tetrafluoropropanol (TFP) and 2,2,3,3,4,4,5,5-octafluoropentanol (OFP) as Diluents

(47) Reaction between CO.sub.2 and 2-fluorophenethylamine (2-FPEA) in solution with fluorinated alcohols, specifically for this example 2,2,3,3-tetrafluoropropanol (TFP) and 2,2,3,3,4,4,5,5-octafluoropentanol (OFP), was initially observed using a standard semi-batch reactor system in which the amine solution was contained in the reaction vessel and a gas stream containing CO.sub.2 was bubbled through the amine solution. The CO.sub.2 concentration of the feed gas and reactor outlet gas streams was monitored via a NDIR CO.sub.2 analyzer to determine extent of CO.sub.2-amine reaction. For both amine solutions, 2-FPEA/TFP and 2-FPEA/OFP, the uptake of CO.sub.2 was very rapid and near complete removal of CO.sub.2 from the flowing gas stream was observed. Reaction with CO.sub.2 was accompanied by a rise in amine solution temperature from 26.5 to 29.5 C. (2-FPEA/TFP) and 23.6 to 26.6 C. (2-FPEA/OFP) due to the exothermic nature of the reaction. The reaction was continued until the CO.sub.2 concentration in the reactor outlet stream achieved >95% of the feed concentration. Integration of the quantity of CO.sub.2 reacted with the amine solution indicated that a CO.sub.2:amine molar ratio of 0.43:1 for the 2-FPEA/TFP and 0.47:1 for 2-FPEA/OFP was achieved.

(48) .sup.19F NMR was performed for the 2-FPEA/OFP/CO.sub.2 mixture to characterize the product of the amine-CO.sub.2 reactions. Deuterated chloroform was used as the NMR solvent. .sup.19F NMR of the 2-FPEA/OFP mixture is shown in the top spectrum of FIG. 7. Five unique .sup.19F resonances correspond to the fluorine nuclei labeled F1 associated with 2-FPEA and F2, F3, F4, and F5 associated with OFP are observable. After purging the 2-FPEA/OFP mixture with CO.sub.2, a total of six unique .sup.19F resonances are observed that correspond to the fluorine nuclei labeled F1, F1, F2, F3, F4, and F5 as shown in the bottom spectrum of FIG. 7. The new F1 resonance is associated with the CO.sub.2-amine reaction product2-fluorophenethylamine carbamate. No shift in the fluorine nuclei associated with OFP was observed, indicating that the fluorinated alcohol was not involved in the CO.sub.2-amine reaction pathway and serves as a diluent.

Example 3: Reaction between CO2 and 2-fluorophenethylamine in Solution with Chloroform as Diluent

(49) Observation of the reaction between CO.sub.2 and 2-fluorophenethylamine (2-FPEA) in solution with chlorinated hydrocarbons, specifically for this example deuterated chloroform, was observed by .sup.1H and .sup.19F NMR. An NMR experiment was conducted in deuterated chloroform solvent containing only 2-fluorophenethylamine. .sup.19F NMR of 2-FPEA/deuterated chloroform mixture is shown in the top spectrum of FIG. 8 and shows a single .sup.19F resonance corresponding to the fluorine nuclei labeled F1 associated with 2-FPEA. After purging the 2-FPEA/deuterated chloroform mixture with CO.sub.2, a total of two unique .sup.19F resonances are observed that correspond to the fluorine nuclei labeled F1 and F1 as shown in the bottom spectrum of FIG. 8. The new F1 resonance is associated with the CO.sub.2-amine reaction product2-fluorophenethylamine carbamate. The corresponding .sup.1H NMR spectra, shown in FIG. 9 also indicate that the 2-fluorophenethylamine carbamate is formed. Notably, the resonances for the amide NH (H8) and protonated NH.sub.3.sup.+(H1) are clearly observable. No evidence for the involvement of the involvement of the chlorinated hydrocarbon in the CO.sub.2-amine reaction pathway was observed indicating that it acts as a diluent.

Example 4: Reaction between CO2 and 3-fluoro-N-methylbenzylamine with 2,2,3,3,4,4-hexafluorobutanol (HFB) and 4,4,5,5,6,6,7,7,7-nonafluoroheptanol (NFHp) as Diluent

(50) Reaction between CO.sub.2 and 3-fluoro-N-methylbenzylamine (3-FNMBA) in solution with fluorinated alcohols, specifically for this example 2,2,3,3,4,4-hexafluorobutanol (HFB) and 4,4,5,5,6,6,7,7,7-nonafluoroheptanol (NFHp), was initially observed using a standard semi-batch reactor system in which the amine solution was contained in the reaction vessel and a gas stream containing CO.sub.2 was bubbled through the amine solution. The CO.sub.2 concentration of the feed gas and reactor outlet gas streams was monitored via a NDIR CO.sub.2 analyzer to determine extent of CO.sub.2-amine reaction. For both amine solutions, 3-FNMBA/HFB and 3-FNMBA/NFHp, the uptake of CO.sub.2 was very rapid and near complete removal of CO.sub.2 from the flowing gas stream was observed. Reaction with CO.sub.2 was accompanied by a rise in amine solution temperature from 25.2 to 28.7 C. (3-FNMBA/HFB) and 21.9 to 26.4 C. (3-FNMBA/NFHp) due to the exothermic nature of the reaction. The reaction was continued until the CO.sub.2 concentration in the reactor outlet stream achieved >95% of the feed concentration. Integration of the quantity of CO.sub.2 reacted with the amine solution indicated that a CO.sub.2:amine molar ratio of 0.36:1 for the 3-FNMBA/HFB and 0.47:1 for 3-FNMBA/NFHp was achieved.

(51) .sup.1H and .sup.19F NMR was performed on the 3-FNMBA/NFHp/CO.sub.2 mixture to characterize the product of the amine-CO.sub.2 reactions. Deuterated chloroform was used as the NMR solvent. FIG. 10 shows the two reaction pathways with CO.sub.2 which could be theorized for the solvent. All relevant hydrogen and fluorine nuclei are labeled. .sup.19F NMR of the 3-FNMBA/NFHp mixture is shown in the top spectrum of FIG. 11. After purging the 3-FNMBA/NFHp mixture with CO.sub.2, a total of six unique .sup.19F resonances are observed that correspond to the four associated with the diluent fluorinated heptanol (F2, F3, F4, F5), one associated with the amine starting material (F1), and one associated with the carbamate salt (F1) as shown in the bottom spectrum of FIG. 11. The new F1 resonance is associated with the CO.sub.2-amine reaction product3-FNMBA carbamate. Examination of the .sup.1H NMR (FIG. 12) of the 3-FNMBA/NFHp mixture (top) compared to the .sup.1H NMR of the 3-FNMBA/NFHp/CO.sub.2 mixture (bottom) also shows no change in the resonances associated with the methylene CH bonds of the fluorinated alcohol (H1, H2, H3). Instead, only .sup.1H resonances associated with carbamate formation are observed (H6, H7, H8). No shift in the proton or fluorine nuclei associated with NFHp was observed indicating that the fluorinated alcohol was not involved in the CO.sub.2-amine reaction pathway and serves as a diluent.

(52) Reversibility of the reaction between CO.sub.2 and the 3-FNMBA/HFB and 3-FNMBA/NFHp solvent systems was observed by ramping the temperature of the 3-FNMBA/HFB/CO.sub.2 and 3-FNMBA/NFHp/CO.sub.2 systems, described above, to 80 C. under a N.sub.2 purge. The release of CO.sub.2 from the CO.sub.2-containing solution was monitored via a NDIR CO.sub.2 analyzer. Integration of the quantity of CO.sub.2 released from the 3-FNMBA/HFB/CO.sub.2 and 3-FNMBA/NFHp/CO.sub.2 systems indicated that CO.sub.2 was completely released from both solvent systems.

Example 5: Reaction between CO2 and 3-fluoro-N-methylbenzylamine in Solution with Chloroform as Diluent

(53) Observation of the reaction between CO.sub.2 and 3-fluoro-N-methylbenzylamine (3-FNMBA) in solution with chlorinated hydrocarbons, specifically for this example deuterated chloroform, was observed by .sup.1H and .sup.19F NMR. An NMR experiment was conducted in deuterated chloroform solvent containing only 3-FNMBA. .sup.19F NMR of 3-FNMBA/deuterated chloroform mixture is shown in the top spectrum of FIG. 13 and shows a single .sup.19F resonance corresponding to the fluorine nuclei labeled F1 associated with 3-FNMBA. After purging the 3-FNMBA/deuterated chloroform mixture with CO.sub.2, a total of two unique .sup.19F resonances are observed that correspond to the fluorine nuclei labeled F1 and F1 as shown in the bottom spectrum of FIG. 13. The new F1 resonance is associated with the CO.sub.2-amine reaction product3-fluoro-N-methylbenzylamine carbamate. The corresponding .sup.1H NMR spectra, shown in FIG. 14 also indicate that the 3-fluoro-N-methylbenzylamine carbamate is formed. Notably, the resonances for the amide NH (H2) and protonated NH.sub.2.sup.+(H2) are clearly observable. No evidence for the involvement of the involvement of the chlorinated hydrocarbon in the CO.sub.2-amine reaction pathway was observed indicating that it acts as a diluent.