BR112013005146B1 - SOLVENT SYSTEM COMPRISING A NON-REVERSIBLE IONIC LIQUID, AND ITS USE FOR REMOVAL OF ACID GAS FROM A GAS STREAM - Google Patents

Abstract

Regenerable ionic liquid solvent system for acid-gas separation and process for removing acidic gases from a gas stream. The present invention relates to a solvent system comprising an ionic liquid formed from a relatively acidic component and a nitrogenous base for removing acidic gases from mixed gas streams. Also provided is a process for removing acidic gases from mixed gas streams using the disclosed solvent system. The solvent system can be used within a gas processing system.

Description

FIELD OF INVENTION

[001] The present invention relates to solvent systems for removing specific components from gas streams, as well as devices and methods using such systems. More specifically, the invention may provide for removal of acid gases, such as CO2, SO2, COS, CS2 and NOX. The invention may further provide for continuous operation of sports and methods using the system. Further, the processes of the invention may utilize multiple absorption/desorption means, including gas absorption/desorption or enhanced phase absorption/desorption.

BACKGROUND OF THE INVENTION

[002] Several strategies are being pursued to minimize the production and/or release of undesirable emissions from combustion processes. One such strategy is the development of technologies for the specific removal of acid gases from gas mixtures, such as the exhausts of carbon combustion processes. The separation of acid gases, such as CO2, from gas mixtures has been carried out industrially for over a hundred years, although any known processes have been used on a large scale such as that required by large industrial plants. Of the various processes used for CO2 separation, current technology focuses primarily on the use of various solvents, such as alkali carbonates in the BENFIELD® Process (UOP, LLC), alcoholamines in the ECONAMINE FG PLUS® process (Fluor Corporation), and alcohols, diols, and ethers in the RECTISOL® process (Lurgi, GMBH) and the SELEXOL® solvent (The Dow Chemical Company). In a typical solvent-based process, the gas mixture to be treated is passed through a liquid solvent that interacts with acidic compounds in the gas stream (e.g., CO2 and SO2) and separates them from non-acidic components. The liquid becomes rich in the acid-gas components, which are then removed under a different set of operating conditions so that the solvent can be recycled for further acid-gas removal.

[003] Methods for removing acid-gas components from rich solvents involve pressure and temperature changes. Depending on the temperature of the gas mixture and the partial pressure of the acid-gas in the mixture, certain solvents are preferred for specific applications. When a solvent operates by chemical absorption, an exothermic chemical reaction with the acid-gas occurs. Reversal of this reaction requires at least as much energy to be added back to the rich solvent as was produced by the forward reaction, not to mention the energy required to bring the rich solvent to the temperature where reversal is appreciable and to maintain conditions for completion of the reverse reaction to an appreciable degree. The energy required to obtain purified acid-gas from rich solvent contributes to the cost of the purified product. In particular, the cost of purified acid-gas has become a significant obstacle to the application of solvent technologies to fossil fuel-fired power plants for the removal of acid gases from flue gas.

[004] Nonaqueous solvents have been used to remove CO2 from natural gas streams and require less energy for regeneration. Single-component alcohol physisorption solvents such as RECTISOL® and SELEXOL® are commercially available for CO2 separation, but they perform poorly in the humid, near-ambient pressure conditions associated with flue gas. Alcoholamines and amines have been combined with alcohols, diols, and cyclic carbonates by several investigators to form “hybrid solvents” whose reaction mechanisms and kinetics have been studied in the literature. See Alvarez-Fuster et al., Chem. Eng. Sci. 1981, 36, 1513; Ali et al., Separation and Purification Technology 2000, 18, 163; Usubharatana et al., Energy Procedia 2009, 1, 95; and Park et al., Sep. Sci. Technol. 2005, 40, 1885. Furthermore, a process known as the “phase transitional absorption method” has been disclosed in relation to methods for deacidification of gas mixtures, which generally consist of the absorption of acid gases in an “absorption phase” of lower density than water consisting of a nitrogenous base and an alcohol, followed by transfer of the absorbed acid gas to an aqueous “carrier phase.” The aqueous carrier phase can be regenerated in a regenerator. The process claims to save energy by absorbing an acid gas at a faster rate than in an absorption phase alone, and by avoiding the energy required to pump a rich absorption phase to a separate regenerator by using gravity to transfer the acid gas between the phases in a single column for absorption and regeneration.

[005] Ionic liquids are another non-aqueous solvent currently under development. These solutions consist entirely of ion pairs that are in a liquid state at near room temperature. They have few regeneration requirements, but have not surpassed aqueous amine solvents in performance due to factors including CO2 loading capacity, viscosity, cost, and, importantly, degradation by water. Use of a non-aqueous liquid solvent to separate CO2 from gas mixtures containing water vapor can lead to the accumulation of H2O in the liquid solution as a single-phase or two-phase solution, depending on the process conditions (e.g., pressure, temperature, H2O concentration) and the affinity of the non-aqueous solvent for H2O. Accumulation of H2O is detrimental to the CO2 separation and purification process, since more energy will be required for solvent regeneration due to the need to continually remove water from the solvent.

[006] Another group of nonaqueous liquids that could be developed to address many of the problems affecting CO2 solvents are the room temperature exchangeable ionic liquids. These equimolar mixtures of amidine or guanidine nitrogen bases and alcohols are room temperature nonionic liquids that react with CO2 to form room temperature ionic liquids. Typically, the conductivity of equimolar mixtures increases by one to two orders of magnitude when CO2 is added. Importantly, these solvents have higher CO2 loadings than some aqueous amines, and are regenerable under mild conditions. Although these solvents are a promising alternative technology, those that have been previously disclosed are poorly suited for flue gas applications due to their chemistries with respect to water, which is typically a major component of flue gas. CO2 is captured through the formation of amidine and guanidine alkyl carbonate salts derived from the conjugate bases of the deprotonated alcohol components. However, if the conjugate base of the alcohol is a weaker acid, an acid–base equilibrium is established between the alcohol conjugate base and water, which favors deprotonation of water and reformation of the protonated alcohol. The conjugate base of water, the hydroxide ion, reacts favorably with CO2 to form a bicarbonate anion, which requires more energy to reverse than alkyl carbonate anions.

[007] Thus, it would be beneficial to formulate a new solvent system capable of effectively removing acidic gases from gas streams (particularly gas streams containing water) and that could be regenerated at lower temperatures and energy loads than solvents currently used for such purposes.

SUMMARY OF THE INVENTION

[008] In one aspect, the present invention relates to a solvent system for removing acidic gases, such as CO2, from a gas stream. In some embodiments, a solvent system according to the invention comprises a nitrogenous base and an acidic component. Specifically, the acidic component may have a pKa of less than about 15.

[009] In some embodiments, the invention provides a solvent system for removing acidic gaseous components (e.g., CO2, SO2, and NO2) from a gas stream, wherein the solvent system can be described as a non-reversible ionic liquid comprising a nitrogenous base and an acidic component having a pKa of less than about 15, which reacts with said acidic gaseous components to form a carbonate ester or heteroatom analog of a carbonate ester, and wherein said acidic gaseous components reversibly bind to the ionic liquid solvent to form an ionic liquid product.

[0010] In some embodiments, the invention provides a solvent system comprising an ionic liquid comprised of: the conjugate base of an acidic component, the acidic component having a pKa of less than about 15; and the conjugate acid of a nitrogenous base selected from the group consisting of amidines, guanidines, and combinations thereof; the conjugate base of the acidic component having a structure such that it can react with an acidic gas to form a carbonate ester or a heteroatom analog of a carbonate ester. The components of the ionic liquid may vary.

[0011] For example, in certain embodiments, the acidic component is selected from the group consisting of fluorinated alcohols, optionally substituted phenols, nitrogen heterocycles, and mixtures thereof. In specific embodiments, the acidic component includes, but is not limited to, 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; 3-fluorophenol; 3-trifluoromethylphenol; and mixtures thereof.

[0012] In certain embodiments, the amidine, guanidine, or combinations thereof have a pKa of about 12 to about 15. Certain amidines or guanidines that may be particularly useful in accordance with the present invention include amidines and guanidines where one or more hydrogen atoms are replaced with fluorine atoms. In specific embodiments, the amidines and guanidines are selected from the group including, but not limited to, 1,1,3,3-tetramethylguanidine (“TMG”), N-tert-butyl-1,1,3,3-tetramethylguanidine, diphenylguanidine, Ditolylguanidine, and 1,8-diazabicyclo(5.4.0)undec-7-ene.

[0013] In certain embodiments, the solvent system can be characterized in terms of its conductivity. For example, the solvent system can be described as having a conductivity of greater than about 100 microsiemens/cm2, greater than about 200 microsiemens/cm2, greater than about 250 microsiemens/cm2, greater than about 300 microsiemens/cm2, greater than about 350 microsiemens/cm2, or greater than about 400 microsiemens/cm2. The solvent system can, in certain embodiments, be characterized as immiscible in water. For example, in some embodiments, the solvent system has a solubility in water of less than about 10 g of solvent per 100 mL of water.

[0014] In another aspect of the invention there is provided a method for removing acidic gas components using the solvent systems described herein. For example, in some embodiments, the present invention may relate to a process for removing acidic gas components from a gas stream by contacting the gas stream with a solvent system comprising an ionic liquid formed from: the conjugate base of an acidic component, wherein the acidic component has a pKa of less than about 15; and the conjugate acid of a nitrogenous base selected from the group consisting of amidines, guanidines, and combinations thereof; wherein the conjugate base of the acidic component has a structure such that it can react with the acidic gas components to form a carbonate ester or a heteroatom analog of a carbonate ester. In specific embodiments, the gas stream may contain water. Preferably, the absorbed acid gas does not react with water to an appreciable degree to form a water-derived compound (e.g., a bicarbonate anion in the embodiment where CO2 is removed).

[0015] In specific embodiments, contact of the gas stream with a solvent system according to the invention may cause formation of a solvent having a density greater than water. Specifically, such a higher density solvent may form a biphasic mixture in the presence of water (e.g., where the lower phase is an organic phase). In other embodiments, water present in the system does not particularly accumulate in the absorber column. In additional embodiments, the processes of the invention may include capture of the acid gas. In addition, an acid-gas rich solvent formed upon contact with the acid gas may be sent to a regenerator for removal of the acid-gas components.

[0016] In certain embodiments, the acid gas is captured in a non-aqueous phase under conditions where water accumulates as a separate, lower density phase. This phase may 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 may be followed by phase separation of the lean, regenerated solvent before being sent back to the absorber.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is a schematic showing a reaction course employed for CO2 capture using solvent mixtures comprising an ionic liquid formed from an acidic component and a nitrogenous base;

[0018] Figure 2 is a diagram of a reboiler-based system embodied by the present invention for capturing and regenerating acid gases from a mixed gas stream;

[0019] Figure 3 is a diagram of a reboiler-free system embodied by the present invention for capturing acid gases from a mixed gas stream;

[0020] Figure 4 is a diagram of a reboiler-assisted system embodied by the present invention for capturing acid gases from a mixed gas stream;

[0021] Figure 5 is a diagram of a secondary heat reboiler system embodied by the present invention for capturing acid gases from a mixed gas stream;

[0022] Figure 6 is a diagram of a secondary heat utilization system embodied by the present invention for capturing acid gases from a mixed gas stream;

[0023] Figure 7 is a CO2 loading curve for an equimolar solution of 1,1,3,3-tetramethylguanidine with 2,2,2-trifluoroethanol;

[0024] Figure 8 is a fluorine NMR spectrum showing 1,1,3,3-tetramethylguanidine with 2,2,2-trifluoroethanol before (top) and after (bottom) reaction with CO2, showing that a new fluorine resonance appears for the CO2-containing carbonate ester product;

[0025] Figure 9 is a graph of conductivity measurements in an equimolar solution of 1,1,3,3-tetramethylguanidine and 2,2,3,3-tetrafluoropropanol during CO2 absorption, where carbon dioxide is introduced into the mixture approximately one minute after the start of the evaluation;

[0026] Figure 10 is a graph of conductivity measurements in an equimolar solution of 1,1,3,3-tetramethylguanidine and 2,2,3,3,4,4-hexafluorobutanol with CO2 absorption, where carbon dioxide is introduced into the mixture approximately one minute after the start of the evaluation;

[0027] Figure 11 is a graph of conductivity measurements in an equimolar solution of N-tert-butyl-1,1,3,3-tetramethylguanidine and 2,2,3,3,4,4,5,5-octafluoropentanol with CO2 absorption, where carbon dioxide is introduced into the mixture at approximately one minute after the start of the evaluation; and

[0028] Figure 12 is a CO2 loading curve of a solvent composed of equimolar 1,8-diaza-bicyclo-undec-7-ene and 2,2,3,3-tetrafluoropropanol.

DETAILED DESCRIPTION OF THE INVENTION

[0029] The present invention will be described more fully hereinafter with reference to the accompanying drawings, wherein some, but not all, embodiments of the invention are shown. Indeed, the present invention may be embodied in many different forms and should not be construed as limited to the embodiments shown herein; rather, such embodiments are provided so that the present invention will satisfy applicable legal requirements. Like numerals refer to like elements. As used in the specification and claims, the singular forms “a,” “an,” and “the,” “the” include plural referents unless the context clearly dictates otherwise.

[0030] In one aspect of the present invention there is provided a liquid solvent system. The solvent system can be used for the separation of acidic gases from gas mixtures. The term “acidic gas” is intended to refer to any gas component that can result in the formation of an acid when mixed with water. Non-limiting examples of acidic gases encompassed by the present invention include CO2, SO2, CS2, and COS. For simplicity, the invention is described below with specific respect to CO2. It is understood, however, that the present invention encompasses methods and systems for removing any acidic gas component from a gas stream.

[0031] In certain embodiments, the solvent system is regenerable in that acid gases can be released from the solvent and the solvent can be reused to separate additional acid gases from additional gas mixtures. In particular embodiments, the solvent system is regenerable at temperatures lower than those typically required for solvents used for such purposes.

[0032] In some embodiments, the solvent system of the present invention comprises a mixture of a nitrogen-based component with 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 acidic component 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 relatively acidic component can have a pKa of about 9 to about 15, about 10 to about 15, about 11 to about 15, about 12 to about 15, about 13 to about 15, about 9 to about 14, about 9 to about 13, about 9 to about 12, or about 9 to about 11, about 10 to about 12, about 10 to about 13, about 10 to about 14, about 11 to about 13, or about 11 to about 14.

[0033] Exemplary classes of relatively acidic components that may be used in accordance with the invention include, but are not limited to, the following: fluorinated alcohols; optionally substituted phenols; and nitrogen heterocycles. Particularly preferred are relatively acidic components selected from fluorinated alcohols and optionally substituted phenols. Fluorinated alcohols useful in accordance with the invention may comprise any compound having the formula R-OH, where R is an alkyl group (e.g., C1-C10 alkyl, C1-C8 alkyl, C1-C6 alkyl, C2-C10 alkyl, C2-C8 alkyl, C2-C6 alkyl, C3-C10 alkyl, C3-C8 alkyl or C3-C6 alkyl) and where one or more hydrogen atoms of the alkyl group are replaced 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 deemed useful. In additional embodiments, one or more of the hydrogen atoms of the alkyl group can be optionally substituted with one or more other substituents, including, but not limited to, C1-C6 alkyl, C1-C6 alkoxy, and halo substituents. Optionally substituted phenols useful in the invention are understood to mean phenols where one or more of the hydrogen atoms on the phenyl ring can be replaced with a substituent. Exemplary, non-limiting substitution groups for one or more of the hydrogen atoms on the phenyl ring include C1-C6 alkyl, C1-C6 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 in the ring structure can be replaced with a substituent. In certain embodiments, at least one nitrogen atom in the ring structure has an acidic hydrogen atom with a pKa of less than about 15 (e.g., between about 8 and about 15). Exemplary, non-limiting substitution groups for one or more of the hydrogen atoms in the ring include C1-C6 alkyl, C1-C6 alkoxy, and halo.

[0034] In some specific embodiments, the relatively acidic component can 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”); 4-propoxyphenol; imidazole; benzimidazole; N-methyl imidazole; 1- trifluoroacetylimidazole; 1,2,3-triazole; 1,2,4-triazole; 2-trifluoromethylpyrazole; 3,5-bistrifluoromethylpyrazole; 3-trifluoromethylpyrazole, 2-fluorophenol, 3-fluorophenol, 4-fluorophenol, 2-trifluoromethylphenol, 3-trifluoromethylphenol, 4-trifluoromethylphenol and mixtures thereof.

[0035] The nitrogenous base component is typically selected from relatively strong bases, such as amidines and guanidines. In certain embodiments, the nitrogenous base is a guanidine, which is understood to be a compound of the structure RNC(NR1R2)2, where R, R1, and R2 are independently H or carbon-containing groups, including, but not limited to, C1-C20 alkyl. One or more of the hydrogen atoms in R, R1, and/or R2 may be optionally substituted with one or more substituents. For example, one or more of the hydrogens in R, R1, R2, and R3 may be substituted with optionally substituted C1-C6 alkyl, optionally substituted C1-C6 alkoxy, optionally substituted C2-C10 alkenyl; optionally substituted C2-C10 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., CF3, 2-Br-ethyl, CH2F, CH2CF3, and CF2CF3); optionally substituted amino; optionally substituted alkylamino; optionally substituted arylamino; optionally substituted acyl; CN; NO2; N3; CH2OH; CONH2; C1-C3 alkylthio; sulfate; sulfonic acid; sulfonate esters (e.g., methanesulfonyl); phosphonic acid; phosphate; phosphonate; mono-, di-, or triphosphate esters; trityl or monomethoxytrityl; CF3S; CF3SO2; or silyl (e.g., trimethylsilyl, dimethyl-t-butylsilyl, and diphenylmethylsilyl).

[0036] In certain embodiments, the nitrogenous base is an amidine, including, but not limited to, a carboxamidine/carboximidamide, which is understood to be a compound of the structure RC(=NH)NR1R2, where R, R1, and R2 are independently H or carbon-containing groups, including, but not limited to, C1-C20 alkyl. One or more of the hydrogen atoms in R, R1, and/or R2 may be optionally substituted with one or more substituents. For example, one or more of the hydrogens in R, R1, R2, and R3 may be substituted with optionally substituted C1-C6 alkyl, optionally substituted C1-C6 alkoxy, optionally substituted C2-C10 alkenyl; optionally substituted C2-C10 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., CF3, 2-Br-ethyl, CH2F, CH2CF3, and CF2CF3); optionally substituted amino; optionally substituted alkylamino; optionally substituted arylamino; optionally substituted acyl; CN; NO2; N3; CH2OH; CONH2; C1-C3 alkylthio; sulfate; sulfonic acid; sulfonate esters (e.g., methanesulfonyl); phosphonic acid; phosphate; phosphonate; mono-, di-, or triphosphate esters; trityl or monomethoxytrityl; CF3S; CF3SO2; or silyl (e.g., trimethylsilyl, dimethyl-t-butylsilyl, and diphenylmethylsilyl).

[0037] In more specific embodiments, the nitrogenous base can be selected from the group consisting of 1,1,3,3-tetramethylguanidine (“TMG”), N-tert-butyl-1,1,3,3-tetramethylguanidine, diphenylguanidine, Ditolylguanidine, or 1,8-diazabicyclo(5.4.0)undec-7-ene. Other exemplary guanidines that may be useful in certain embodiments according to the present invention include, but are not limited to, 1,1,3-trimethyl-3-(2,2,3,3-tetrafluoropropyl)guanidine; 1,1,3-trimethyl-3-(2,2,3,3,3-pentafluoropropyl)guanidine; 1,3-dimethyl-1,3-bis(2,2,2-trifluoroethyl)-guanidine; 1,3-bis(2,2,3,3-tetrafluoropropyl)guanidine; 1,3-bis(4-fluorophenyl)guanidine; 1,3-bis(3-fluorophenyl)guanidine; and 1,3-bis(2-fluorophenyl)guanidine. Other amidines that may be useful in certain embodiments according to the invention are 2-(2,2,2-trifluoroethyl)-1,4,5,6-tetrahydropyrimidine; 2-(2,2,3,3-tetrafluoropropyl)-1,4,5,6-tetrahydropyrimidine; 3,3,4,4-tetrafluoro-N,N-dimethylbutanimidamide; and 3,3,3-trifluoro-N,N-dimethylpropanimidamide. Still other nitrogenous bases that may be used in accordance with the present invention include, for example, those disclosed in U.S. Patent Application Publication No. 2008/0058549 to Jessop et al., the disclosure of which is incorporated herein by reference. In certain embodiments, the nitrogenous base may have a pKa of about 12 to about 15, about 12 to about 14, or about 13 to about 15. For example, in some embodiments, the nitrogenous base may have a pKa of about 12, about 13, about 14, or about 15.

[0038] In some embodiments, the solvent system may include a mixture comprising a nitrogenous base and a relatively acidic alcohol, which components may be present in proportions roughly equal in molarity (i.e., are present in equimolar amounts). In some embodiments, the solution may be diluted, such as with water or using an excess of a relatively acidic component. For example, the molar ratio of nitrogenous base to relatively acidic component may be 1.1 to about 10, 1.1 to about 15, 1.1 to about 10, 1.1 to about 5, 1.1 to about 3, about 2 to about 20, about 2 to about 15, 2 to about 10, 2 to about 5, about 3 to about 20, about 3 to about 15, about 3 to about 10, about 4 to about 20, about 4 to about 15, about 4 to about 10, about 5 to about 20, about 5 to about 15, or about 5 to about 10. While not wishing to be limited by theory, it is believed that the use of the diluent may be useful in reducing or preventing 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 CO2 product in the solvent system, and thus prevent the formation of precipitates. In other embodiments, however, formation of solids may be desirable, and such formation may be increased by altering the concentration of one or more solvent components.

[0039] In some embodiments, the solvent system of the present invention is particularly useful for capturing CO2 from a gas stream. The gas stream may be a mixed gas stream, having one or more other components in addition to CO2. When a solution comprising a solvent system of the present invention is purged with a gas mixture containing CO2, the components of the solvent system undergo a chemical reaction with CO2, binding the CO2 in the solution. In some embodiments, the solvent systems of the present invention have high CO2 loadings. For example, the solvent systems may be useful for capturing or removing more than about 0.05 mole of CO2 per mole of nitrogenous base, more than about 0.1 mole of CO2 per mole of nitrogenous base, more than about 0.2 mole of CO2 per mole of nitrogenous base, more than about 0.3 mole of CO2 per mole of nitrogenous base, more than about 0.4 mole of CO2 per mole of nitrogenous base, more than about 0.5 mole of CO2 per mole of nitrogenous base, more than about 0.6 mole of CO2 per mole of nitrogenous base, more than about 0.7 mole of CO2 per mole of nitrogenous base, more than about 0.8 mole of CO2 per mole of nitrogenous base, more than about 0.9 mole of CO2 per mole of nitrogenous base, or more than about 1 mole of CO2 per mole of nitrogenous base. mole of CO2 per mole of nitrogenous base.

[0040] Figure 1 illustrates reaction pathways for CO2 capture using ionic liquids comprising a relatively acidic alcohol and a nitrogenous base. Reversible CO2 capture according to the present process involves reaction with an ionic liquid formed from the conjugate base of the alcohol and the conjugate acid of the nitrogenous base. In Figure 1, the nitrogenous base is shown reacting with an acidic nucleophile (alcohol) to deprotonate the alcohol and form the ionic liquid. The solvent system can be classified as an ionic liquid by any means known in the art. For example, the solvent system can be determined to have a conductivity greater than about 100 microsiemens/cm2. For example, in preferred embodiments, the solvent system exhibits a conductivity greater than about 400 microsiemens/cm2. The ionic liquid is capable of capturing CO2 as a carbonate ester. It is appreciated that by varying the acidic gas being removed from the system, the structure of the captured product will need to vary as well. In this manner, in certain embodiments, the acid gas will be captured as a heteroatom analogue of a carbonate ester.

[0041] The solvent system of the present invention is designed such that the relatively acidic component (e.g., an alcohol in the above embodiment) is more acidic than water, and thus its conjugate base is comparatively less susceptible to protonation by water. In this manner, the present invention ameliorates the chemical degradation impact by water that exists for some non-aqueous solvents. The reaction of the disclosed solvent system with CO2 is fully reversible under certain conditions. For example, the reaction is reversible under elevated CO2 pressure and elevated temperature (e.g., when heated to about 50°C and above).

[0042] In certain embodiments, the relatively acidic component is selected such that it exhibits low mixing ability with water. For example, in some embodiments, the relatively acidic component exhibits 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 relatively acidic component exhibits 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, less than or equal to about 9 g/100 mL, or less than or equal to about 10 g/100 mL. g/100 mL in water at 25 °C. In some embodiments, the relatively acidic component is completely immiscible in water. Use of relatively acidic components with low water solubility can result in solvent systems that exhibit one or more of the following attributes: they may require less energy for regeneration; they may have high CO2 loading capacities; they may be able to tolerate water in the gas stream; and/or they may be able to be separated from water without a large energy penalty.

[0043] In additional embodiments, the nitrogenous base component of the solvent system is similarly selected such that it exhibits low mixing capacity with water. In preferred embodiments, the nitrogenous base exhibits greater mixing capacity with the relatively acidic component than with water. In some embodiments, the nitrogenous base exhibits high solubility in the relatively acidic component. Examples of such nitrogenous bases include, but are not limited to, guanidines or amidines, such as those having one or more substituted or unsubstituted hydrocarbon chains, one or more substituted or unsubstituted aromatic moieties (e.g., fluorine-substituted aromatic moieties), and/or one or more substituted or unsubstituted alkylaromatic moieties (e.g., fluorine-substituted alkylaromatic moieties).

[0044] 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 or no degradation of the solvent's performance up to the indicated volume of water. In some embodiments, the solvent system remains at or near its initial capacity for CO2 loading up to the indicated volume of water.

[0045] In preferred embodiments, the CO2 sequestered using the solvent system of the present invention can be released to regenerate the solvent system for reuse. It is preferred that the solvent system be regenerable using mild conditions. In some embodiments, the release of CO2 and corresponding regeneration of the solvent system is accomplished by heating the solution. When the solution containing bound CO2 is heated, the chemical reaction is reversed and the CO2 is released, producing a stream of concentrated CO2.

[0046] In some embodiments, the present application relates to a solvent system and process for removing CO2 from a gas stream. The present invention applies to any gas stream containing CO2. For example, in particular embodiments, the invention relates to a process for removing CO2 from fossil fuel combustion gas, a natural gas mixture, or a mixture of indoor respiratory gases containing CO2. The process involves passing the mixed gas stream through a solvent system comprising a relatively acidic component and a nitrogenous base component. In some embodiments, the present invention further relates to regeneration of the solvent system, which releases the CO2. In some embodiments, regeneration of the solvent system involves heating the solvent system to a temperature sufficient to release the CO2. In some embodiments, the process involves heating the solvent system to a temperature at or below about 200 °C, e.g., 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 to a temperature at or below about 100 °C, e.g., 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. For example, in some embodiments, where the relatively acidic component is a fluorinated alcohol, the CO2 is captured as a fluoroalkylcarbonate, which can be decomposed to release CO2 by heating the solvent system to a temperature between about 40 °C and about 85 °C. In some embodiments, CO2 may be released at room temperature.

[0047] In certain embodiments, at or about 100% of the CO2 is removed from the CO2-rich solvent system. However, in other embodiments, less than 100% of the CO2 is removed from the CO2-rich solvent system. In preferred embodiments, about 50 to 100% of the captured CO2 is removed from the CO2-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 CO2 is removed from the CO2-rich solvent system.

[0048] In some embodiments, removal of CO2 from gas mixtures containing H2O in addition to CO2 may lead to accumulation of H2O in the solvent system, either as a single-phase or biphase solution, depending on the reaction conditions. As mentioned above, the presence of H2O 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 need to remove water from the solvent. Thus, accumulation of H2O in the solvent system may increase the energy demand for regeneration, decreasing the efficiency of the regeneration system.

[0049] In some embodiments, the process of the invention provides a method by which the detrimental effects of H2O accumulation in the solvent system can be avoided. For example, the detrimental effect of H2O accumulation on the regeneration energy demand of the solvent system can be minimized by providing a process by which CO2 is sequestered within the solvent system at a temperature above the H2O saturation temperature of the gas mixture. Further, the detrimental effect of H2O accumulation on the regeneration energy demand of the solvent system can be minimized by providing a process by which H2O 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 separated, aqueous phase can be decanted or centrifuged by mechanical, rather than thermal, processes, minimizing the energy required to maintain an efficient CO2 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 in water. Thus, a solvent system comprising a suitable nitrogen base and OFP forms a two-phase 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 centrifuging the aqueous layer of the fluorinated phase. In some embodiments, after removal of the H2O, the CO2-rich solvent system can be regenerated at low temperature with the addition of low boiling diluents to meet partial pressure requirements. The solvent system could then avoid the additional energy penalty associated with distilling water. By providing a non-aqueous CO2 absorption solvent system with low water solubility, the solvent system exhibits lower energy demands and milder regeneration conditions than those of aqueous or high water affinity CO2 solvent systems.

[0050] In some embodiments, a system for removing CO2 from a gas stream is provided. A schematic of an exemplary system of the present invention is shown in Figures 2-6. The CO2 removal system 10 includes an absorber 12 configured with an inlet for receiving a gas stream. The gas stream may come directly from, for example, a combustion chamber of a boiling system in a power generation plant. The gas stream may or may not be passed through other cleaning systems before entering the CO2 removal system. The absorber may be any chamber where a solvent system for removing CO2 is contained, having an inlet and outlet for a gas stream, and where the gas stream may be brought into contact with the solvent system. Within the absorber, CO2 may be transferred from the gas phase to the liquid phase in accordance with the principles discussed herein. The absorber may be of any type; for example, the absorber may comprise a spray tower absorber; jacketed absorber (including counter-current flow tower or cross-flow tower), tray-tower absorber (having various types of trays, including bubble-cap trays, sieve trays, impingement trays and/or float valve trays), hazard 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. In this manner, the absorber may be equipped with a heating/cooling system and/or pressure/vacuum system.

[0051] Within the absorber, the gas stream is brought into fluid contact with and passed through a solvent system comprising a relatively acidic component and a nitrogenous base component. The solvent system reacts with the CO2 present in the gas stream, sequestering it from the remaining gas components, and the resulting CO2-free gas stream is released from the absorber through an outlet. The solvent system continues to react with incoming CO2 as the mixed gas stream is passed through it, until it becomes “rich” in CO2. The absorber is optionally connected to one or more components. For example, the absorber is preferably configured with a means for routing the solvent to a unit where water can be decanted, centrifuged, or otherwise removed from the system.

[0052] At any stage in the CO2 capture process, the solvent system may be regenerated. The system then includes an optional regeneration system 14 to release the captured CO2 via a separate CO2 gas stream and then regenerate the solvent system. The regeneration system is configured to receive a feed of “rich” solvent from the absorber and return regenerated solvent to the absorber once the CO2 has been separated from the “rich” solvent. The regeneration system may comprise only a chamber with a heating unit to heat the solvent system to a temperature sufficient to release the gas, along with a release valve to allow the CO2 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 optionally be connected to one or more components. For example, the regenerator is preferably configured with a means to route solvent to a unit where water may be decanted, centrifuged or otherwise removed from the system.

[0053] The released CO2 may be released for storage or other predetermined uses. The regenerated solvent is again ready to absorb CO2 from a gas stream and may be directed back to the absorber.

[0054] Many modifications and other embodiments of the invention disclosed herein will be understood by one skilled in the art to which the present invention belongs having the benefit of the teachings set forth in the above specification and the associated drawings. Accordingly, it should be understood that the invention is not 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

[0055] The following examples are provided for the purpose of complete disclosure and are not to be construed as limiting the invention.

Example 1: Absorption of CO2 by 1,1,3,3-tetramethylguanidine/2,2,2-trifluoroethanol resulting in formation of a carbonate ester

[0056] An equimolar solution of 1,1,3,3-tetramethylguanidine (“TMG”) and 2,2,2-trifluoroethanol (“TFE”) was prepared, resulting in an exothermic event with moderate heat generation. The solution was placed in a collision apparatus and purged with a binary gas mixture containing approximately 13% CO2 (nitrogen balance). The CO2 concentration of the off-gas was monitored using an NDIR CO2 analyzer. The CO2 loading curve is shown in Figure 7. Upon heating the solution to 80 °C in a flowing nitrogen stream, the absorption was reversed, resulting in the release of approximately 0.56 mol CO2/mol amine.

[0057] Figure 8 provides a nuclear magnetic resonance (NMR) spectrum of the resultant reaction between the TMG/TFE solvent system and CO2 gas. In an NMR tube at room temperature, 1,1,3,3-tetramethylguanidine (0.6 mmol) was combined with 2,2,2-trifluoroethanol (0.6 mmol) in deuterated chloroform (CDCl3, 0.6 grams). A 19F NMR spectrum of the starting solution was recorded. A single resonance appeared in the NMR with a chemical shift of approximately -77 ppm corresponding to the chemically equivalent fluorine environment, as shown in the upper portion of Figure 8.

[0058] The NMR tube was then purged for 30 minutes with a gas mixture of 13.3% CO2 and equilibrium nitrogen. A second 19F NMR spectrum was recorded at this time, shown in the lower portion of Figure 8. This spectrum clearly indicates the appearance of a new peak (indicative of a new fluorine-containing product), shifted by approximately 3 ppm.

[0059] This data provides evidence that a carbonate ester was formed under the experimental conditions. When fluorine nuclei are present in alcoholic reactants (as they are), fluorine NMR is a convenient way to identify involvement of the alcohol in CO2 capture as a carbonate ester. Formation of a new product involving the alcohol will result in new 19F resonances in the NMR spectrum. Fluorine spectra shown are proton decoupled.

Example 2: Absorption of CO2 by an ionic liquid consisting of an equimolar mixture of 1,1,3,3-tetramethylguanidine and 1,1,3,3-tetrafluoropropanol

[0060] 1,1,3,3-Tetramethylguanidine was combined with an equimolar amount of 2,2,3,3-tetrafluoropropanol, giving an exothermic reaction that occurs resulting in the formation of an ionic liquid at room temperature, with an initial conductivity equal to approximately 440 μS/cm2, as shown in Figure 9. This figure further shows that, when the ionic liquid was purged with a binary gas mixture composed of approximately 13.3% CO2 (equilibrium nitrogen), the conductivity of the solution increased to a maximum of approximately 1175 μS/cm2. This data (an increase in conductivity) corresponds to the absorption of CO2 by the solution as evidenced by the observed decrease in the concentration of the gas passing through the solution as monitored with a CO2 analyzer using NDIR spectroscopy. The solution absorbed approximately 0.35 mol of CO2/mol of amine. When heated to approximately 80 °C under nitrogen purge, the solvent was fully regenerable, releasing all of the captured CO2.

Example 3: Absorption of CO2 by an ionic liquid consisting of an equimolar mixture of 1,1,3,3-tetramethylguanidine and 2,2,3,3,4,4-hexafluorobutanol

[0061] An equimolar mixture of 1,1,3,3-tetramethylguanidine with 2,2,3,3,4,4-hexafluorobutanol formed an ionic liquid at room temperature with an initial conductivity of approximately 400 μS/cm2 as shown in Figure 10. As further shown in this figure, when the ionic liquid was purged with a binary gas mixture composed of approximately 13.3% CO2 (equilibrium nitrogen) the conductivity of the solution increased to a maximum of approximately 800 μS/cm-1. This data (an increase in conductivity) shows that the solution absorbs CO2 as evidenced by the observed decrease in the concentration of the gas passing through the solution as monitored with a CO2 analyzer using NDIR spectroscopy. This indicates that the lean solvent is an ionic liquid whose conductivity increases with absorption of carbon dioxide. The solution absorbed a total of approximately 0.25 mol CO2/mol amine. When heated to approximately 80 °C, the solvent was fully regenerated.

Example 4: Absorption of carbon dioxide by an ionic liquid composed of N-tert-butyl-1,1,3,3-tetramethylguanidine and 2,2,3,3,4,4,5,5-octafluoropentanol

[0062] An equimolar mixture of N-tert-butyl-1,1,3,3-tetramethylguanidine was combined with 2,2,3,3,4,4-hexafluorobutanol, giving an ionic liquid at room temperature with an initial conductivity of approximately 150 μS/cm2 as shown in Figure 10. The ionic liquid was purged with a binary gas mixture composed of approximately 13.3% CO2 (equilibrium nitrogen). As demonstrated in Figure 11, the conductivity of the solution increased to a maximum of approximately 200 μS/cm2. This data demonstrates (an increase in conductivity) that the solution absorbs CO2, as evidenced by the observed decrease in the concentration of gas passing through the solution as monitored with a CO2 analyzer using infrared spectroscopy. This indicates that the lean solvent is an ionic liquid whose conductivity increases as carbon dioxide absorption increases. The solvent absorbed a total of approximately 0.67 mol CO2/mol amine and was fully regenerable upon heating to 80 °C under nitrogen purge.

Example 5: Absorption of carbon dioxide by an ionic liquid composed of 1,8-diaza-bicyclo-undec-7-ene (“DBU”) and 2,2,3,3-tetrafluoropropanol.

[0063] An ionic liquid was prepared by mixing 1,8-diaza-bicycloundec-7-ene (“DBU”) and 2,2,3,3-tetrafluoropropanol. The initial conductivity of the system was 135 μS/cm2, indicating that the as-formed solution was a non-reversible ionic liquid. The solution was placed in a collision apparatus and purged with a binary gas mixture containing approximately 13% CO2 (equilibrium nitrogen). The CO2 concentration of the off-gas from the reactor was monitored using an NDIR CO2 analyzer. The CO2 loading curve is shown in Figure 12. As shown, the solution absorbed approximately 0.56 mol CO2/per mol amine and was reversible upon heating the solution to 80 °C in a flowing nitrogen stream.

Claims (3)

1. A solvent system comprising a non-reversible ionic liquid formed from: the conjugate base of an acidic component, the acidic component having a pKa of less than 15 and 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; 3-fluorophenol; 3-trifluoromethylphenol; and mixtures thereof; and the conjugate acid of a nitrogenous base selected from the group consisting of 1,1,3,3-tetramethylguanidine (“TMG”); N-tert-butyl-1,1,3,3-tetramethylguanidine; diphenylguanidine; ditalylguanidine; 1,8-diazabicyclo(5.4.0)undec-7-ene, and mixtures thereof, wherein said nitrogenous base has a pKa of 12 to 15, wherein said solvent system has a solubility in water of less than 10 g of solvent per 100 mL of water, and wherein said solvent system has a conductivity greater than 100 microsiemens/cm2. 2. Solvent system, according to claim 1, characterized by the fact that it presents a conductivity greater than 400 microsiemens/cm2. 3. Use of a solvent system as defined in claim 1 or 2, characterized in that it is for removing acid gas from a gas stream.