System for the capture and release of acid gases

Abstract

In one aspect, the invention provides a method for the capture of at least one acid gas in a composition, the release of said gas from said composition, and the subsequent regeneration of said composition for re-use, said method comprising performing, in order, the steps of: (a) capturing the at least one acid gas by contacting said at least one gas with a capture composition comprising at least one salt of a carboxylic acid and at least one water-miscible non-aqueous solvent; (b) releasing said at least one acid gas by adding at least one protic solvent or agent to said composition; and (c) regenerating the capture composition by partial or complete removal of said added protic solvent or agent from said composition. Optionally, said capture composition comprising at least one salt of a carboxylic acid and at least one water-miscible non-aqueous solvent additionally comprises water or another protic solvent. In another aspect, the invention envisages a composition which additionally comprises at least one protic solvent or agent and release of the at least one acid gas is achieved solely by subjecting the composition to the application of heat or stripping with a stream of air. The method is typically applied to the capture and subsequent release of carbon dioxide, and offers a convenient and simple process which uses inexpensive consumables and offers significant advantages over the methods of the prior art.

Claims

1. A method for the capture of CO.sub.2 in a composition, the release of said CO.sub.2 from said composition, and the subsequent regeneration of said composition for re-use, said method comprising performing, in order, the steps of: (a) capturing CO.sub.2 by contacting said CO.sub.2 with a capture composition comprising at least one alkali metal salt of an non-amino-acid alkyl monocarboxylic acid, at least one non-aqueous solvent and at least one protic solvent selected from water and an alcoholic solvent; wherein said at least one alkali metal salt of an non-amino-acid alkyl monocarboxylic acid is initially present in said composition at a level of between 1M and 14M; and wherein the components of the composition are selected such that the pKa of the at least one non-amino-acid alkyl monocarboxylic acid that is present in the capture composition in the form of the at least one metal salt is increased relative to the pKa of said at least one non-amino-acid alkyl monocarboxylic acid in water; and the pKa of said CO.sub.2 in the capture composition is increased relative to the pKa of said CO.sub.2 in water; and wherein the resultant relative magnitudes of the pKas of said CO.sub.2 and non-amino-acid alkyl monocarboxylic acid in the capture composition cause the capture composition to capture said CO.sub.2; (b) releasing said CO.sub.2 by subjecting said composition to the application of heat and/or stripping with a stream of air or releasing said CO.sub.2 by adding at least one release agent to said composition; and (c) regenerating the capture composition by cooling or regenerating the capture composition by partial or complete removal of the added release agent from said composition.

2. A method as claimed in claim 1 wherein said at least one non-aqueous solvent is a glycol ether.

3. A method as claimed in claim 2, wherein said glycol ether is selected from diethylene glycol dimethyl ether, diethylene glycol monobutyl ether, diethylene glycol monoethyl ether, diethylene glycol monomethyl ether, diethylene glycol monopropyl ether, dipropylene glycol dimethyl ether, ethylene glycol dimethyl ether, ethylene glycol monobutyl ether, ethylene glycol monoethyl ether, ethylene glycol monomethyl ether, ethylene glycol monopropyl ether, propylene glycol monobutyl ether and triethylene glycol dimethyl ether, propylene glycol dimethyl ether and diethylene glycol diethyl ether.

4. A method as claimed in claim 1 wherein said non-amino-acid alkyl monocarboxylic acid is selected from straight chained, branched or cyclic carboxylic acids which may be substituted or unsubstituted and/or wherein said at least one salt of an non-amino-acid alkyl monocarboxylic acid is selected from salts of C.sub.1-20 non-amino-acid alkyl monocarboxylic acid.

5. A method as claimed in claim 4, wherein said non-amino-acid alkyl monocarboxylic acid is acetic acid, propionic acid, butyric acid, isobutyric acid, isovaleric acid or pivalic acid.

6. A method as claimed in claim 1, wherein the release agent is a further portion of the at least one protic solvent.

7. A method as claimed in claim 1 wherein said alkali metal is selected from lithium, sodium or potassium.

8. A method as claimed in claim 1 wherein said protic solvent is water.

9. A method as claimed in claim 1 wherein said protic solvent is present at levels of 5-20% v/v.

10. A method as claimed in claim 1 wherein said at least one non-aqueous solvent comprises at least one at least partially water-miscible polar solvent or at least one polar aprotic solvent.

11. A method as claimed in claim 1 wherein said at least one non-aqueous solvent is selected from dimethylsulphoxide (DMSO), N-methylpyrrolidinone (NMP), dimethylformamide (DMF), tetrahydrofuran (THF), 2-methyltetrahydrofuran, acetonitrile, sulfolane, 1,1,3,3-tetramethylurea (TMU), N,N-dimethyl-N,N-trimethyleneurea (1,3-dimethyl-3,4,5,6-tetrahydro-2-pyrimidinone (DMPU)), 1,3-dimethyl-2-imidazolidinone (DMI), dioxane, 1,3-dioxolane, lactate esters or polyethers.

12. A method as claimed in claim 1 wherein said at least one non-aqueous solvent is selected from dimethylsulphoxide (DMSO), tetrahydrofuran (THF), 2-methyltetrahydrofuran, sulfolane, dioxane, 1,3-dioxolane, lactate esters or polyethers.

13. A method as claimed in claim 1 wherein said composition comprises a solution, a slurry, a dispersion or a suspension.

14. A method as claimed in claim 1 wherein said CO.sub.2 is contacted with the composition at pressures in the range of from 1 to 50 bar.

15. A method as claimed in claim 1 wherein said release of said CO.sub.2 is achieved by heating up to 80 C. and optionally is achieved by heating to temperatures in the region of 30-80 C.

16. A method as claimed in claim 1 comprising performing, in order, the steps of: (a) capturing the CO.sub.2 by contacting said CO.sub.2 with a capture composition comprising at least one alkali metal salt of an non-amino-acid alkyl monocarboxylic acid, at least one non-aqueous solvent and at least one protic solvent selected from water and an alcoholic solvent; wherein said at least one alkali metal salt of an non-amino-acid alkyl monocarboxylic acid is initially present in said composition at a level of between 1M and 14M; and wherein the components of the composition are selected such that the pKa of the at least one non-amino-acid alkyl monocarboxylic acid that is present in the capture composition in the form of the at least one alkali metal salt is increased relative to the pKa of said at least one non-amino-acid alkyl monocarboxylic acid in water; and the pKa of CO.sub.2 in the capture composition is increased relative to the pKa of CO.sub.2 in water; and wherein the resultant relative magnitudes of the pKas of said CO.sub.2 and non-amino-acid alkyl monocarboxylic acid in the capture composition cause the capture composition to capture said CO.sub.2; (b) releasing said CO.sub.2 by subjecting said composition to the application of heat and/or stripping with a stream of air; and (c) regenerating the capture composition by cooling.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:

(2) FIG. 1 shows a schematic representation of a conventional post-combustion capture process;

(3) FIG. 2 provides a graphical representation of the rate of CO.sub.2 uptake for 1M potassium butyrate and 1M tetrabutylammonium butyrate in the method according to the invention;

(4) FIG. 3 is a graphical illustration of the carbon capture capacity of saturated tetrabutylammonium acetate solutions as a function of the water content in the method of the invention;

(5) FIG. 4 shows a graphical representation of the vapour liquid equilibria of a potassium pivalate system in sulfolane with varying water content;

(6) FIG. 5 provides a graphical illustration of the vapour liquid equilibria of a potassium pivalate system in ethylene glycol monobutyl ether with varying water content;

(7) FIG. 6 provides a schematic representation of a low temperature capture process according to the invention;

(8) FIG. 7 depicts the use of a pervaporation membrane for solvent regeneration according to the invention;

(9) FIG. 8 illustrates the use of a conventional membrane for solvent regeneration according to the invention;

(10) FIG. 9 provides a schematic representation of a water separation process using hot carrier gas according to the invention;

(11) FIG. 10 is a schematic representation of a further low temperature capture process according to the invention;

(12) FIG. 11 is a graphical representation of the results of a distillation process using 3 M potassium pivalate in diglyme with 16% w/w water;

(13) FIG. 12 provides a graphical illustration of the vapour liquid equilibria of diglyme-water at 60 C.; and

(14) FIG. 13 shows a graphical representation of changes in water content observed after addition of sorbent materials to organic-water mixtures.

DESCRIPTION OF THE INVENTION

(15) The present inventors have provided a new system of acid gas capture compositions that provides significant advantages over the methods of the prior art and finds potential application in areas such as power stations, cement manufacture, steel manufacture, glass making, brewing, syngas processes, natural gas and biogas purification and other chemical processes, such as ammonia production, as well as any other acid gas producing industrial processes. In a particular application, the defined method may be applied to the capture of carbon dioxide from the atmosphere.

(16) As well as such applications, the compositions provided by the present invention are also appropriate for use in smaller scale specialist applications such as, for example, in submarines, spacecraft and other enclosed environments.

(17) A particular embodiment of the invention envisages the application of the disclosed method to the capture and subsequent release of carbon dioxide. The incorporation of carbon dioxide into a substrate is known as carboxylation; the removal of the same group is decarboxylation. This carboxylation/decarboxylation process is key to effective CO.sub.2 capture and absorbent regeneration.

(18) The present invention also envisages the use of the disclosed formulations in the capture and subsequent release of other acid gases (e.g. H.sub.2S, SO.sub.2) or mixtures thereof, in applications such as natural gas sweetening and desulphurisation.

(19) In specific embodiments of the invention, systems are provided wherein CO.sub.2 gas is captured by the use of formulations which comprise potassium or tetrabutylammonium salts of acetic, propionic or butyric acid in solvent systems comprising water in combination with, for example, dimethylsulphoxide or N-methylpyrrolidinone. The use of such systems facilitates the highly efficient capture of the gas at ambient temperature and pressure. Subsequent release of the captured gas may conveniently be achieved by the addition of further water to the gas-loaded compositions.

(20) Thus, the inventors have conducted a range of trials utilising equipment adapted for the measurement of vapour-liquid equilibria (VLE). Specifically a system was provided which comprised a stainless steel vessel (400 mL) equipped with a pressure transducer, two temperature probes (one for monitoring vapour temperature, one for solvent temperature), a safety release valve, and an assembly for purging/refilling the chamber with a known atmosphere.

(21) From the data collected for a given formulation, it was possible to directly determine the rate of CO.sub.2 absorption, the proportion of CO.sub.2 recoverable depending on solvent composition, and the CO.sub.2 absorption/desorption profile at different partial pressures of CO.sub.2.

(22) By the use of a spray nozzle and syringe assembly, it was also possible to determine the rate of CO.sub.2 uptake by a given formulation with mass transfer effects minimised so as to provide an insight into the mechanism of CO.sub.2 absorption by solvents.

(23) In order to demonstrate the highly effective nature of the systems of the invention, various aliphatic carboxylate salts were studied, together with alternative organic acid salts, specifically salts of aromatic compounds, in particular phenolate salts and salts of an aromatic carboxylic acid, salicylic acid.

(24) The salts were dissolved in mixtures of dimethylsulphoxide (DMSO) and water; suitable ratios were found to be between 1000:1 and 1:1 and particularly favourable results were demonstrated by the aliphatic carboxylate salts at a DMSO:water ratio of 3:1. In water alone, it was demonstrated that the carboxylate salts do not capture CO.sub.2, as the pK.sub.a of a carboxylic acid is too low (pK.sub.a=5); a phenolate does, however, capture CO.sub.2 in these circumstances as, in water, this salt has a much higher pK.sub.a (10). In a mixture of DMSO and water (3:1), however, both capture agents would have significantly higher pK.sub.a and so would be expected to show improved CO.sub.2 capture efficiency.

(25) Each solution under test was exposed to CO.sub.2 at ambient pressure and the absorption process was allowed to reach equilibrium. The results which were obtained are presented in Table 2.

(26) Thus, it was observed that, in DMSO and water (3:1), salts of carboxylic acids (e.g. 1M Na-acetate, K-propionate and K-butyrate, entries 1, 2 and 3) captured significant amounts of CO.sub.2 (up to 0.63 mol.sub.CO2/L) clearly demonstrating the enhanced basicity of the carboxylate under these conditions; by way of comparison, the pK.sub.a of acetic acid is 12.4 in pure DMSO. The phenolate salt also showed high CO.sub.2 capture (entry 4), as would be expected due to increased pK.sub.a (the pK.sub.a of phenol in pure DMSO is 18).

(27) TABLE-US-00002 TABLE 2 VLE Performance of Carboxylate and Phenolate Salts in Mixed Aqueous-Organic Solvents Full Fast Rate Capacity Recovered Entry Salt.sup.a Solvent (1/s) (mol.sub.CO2/L) CO.sub.2 (%).sup.b 1 Na-acetate DMSO:H.sub.2O 3:1 0.017 0.29 65 2 K-propionate DMSO:H.sub.2O 3:1 0.018 0.50 93 3 K-butyrate DMSO:H.sub.2O 3:1 0.010 0.63 78 4 K-phenolate DMSO:H.sub.2O 3:1 0.009 0.89 5 K-propionate.sup.c NMP:H.sub.2O 3:1 0.005 0.92 88 6 K-butyrate DMSO:H.sub.2O 3:1 0.008 0.56 2.sup.d 7 K-butyrate DMSO:H.sub.2O 3:1 0.008 0.56 19.sup.e 8 K-butyrate DMSO:H.sub.2O 3:1 0.008 0.56 93 9 K-butyrate NMP:H.sub.2O 5.3:1 0.006 0.71 66.sup.f 10 TBA.sup.g butyrate NMP (1M H.sub.2O) 0.074 0.56 88.sup.h 11 DBU-isovalerate NMP:H.sub.2O 9:1 0.010 1.15 74 12 DBU-isovalerate Sulfolane:H.sub.2O 9:1 0.030 1.05 73 .sup.a1M solution; .sup.bupon increasing H.sub.2O content to 1:1 (5 mL added); .sup.c2M solution; .sup.dEtOH content to 3:1 (5 mL); .sup.eMeOH content to 3:1 (5 mL); .sup.fMeOH content to 1:1 (10 mL); .sup.gTBA = tetrabutylammonium; .sup.hUpon increasing H.sub.2O content to 2:1 (5 mL added)

(28) Following capture of the CO.sub.2, water was added to the composition at ambient temperature in order to increase the ratio of water in the solvent mixtures to 1:1 so as to induce CO.sub.2 release by lowering the pK.sub.a of the acids formed on carboxylation. The amount of CO.sub.2 recovered was then determined.

(29) The procedure was shown to be successful for a range of aliphatic carboxylic acid salts (e.g. potassium propionate, sodium acetate and potassium butyrate), all of which efficiently captured CO.sub.2 and released up to 93% of the absorbed CO.sub.2 simply upon water addition at ambient temperature.

(30) It was significant, however, that no CO.sub.2 was liberated from the composition comprising the phenolate using this process, as the pK.sub.a was still too high to allow the gas to be liberated at room temperature, even in 100% water (entry 4). This provides further evidence that it is the pK.sub.a value that is of key significance in the decarboxylation process.

(31) The use of higher concentrations of carboxylic acid salts (2M potassium propionate, entry 5) showed almost a doubling of capacity, with a negligible decrease in recovered CO.sub.2 liberated on addition of water (93 to 88%).

(32) Although water appears to be the optimum solvent for addition to facilitate CO.sub.2 liberation, other protic solvents, most particularly methanol, were also found to be successful in achieving this objective (entries 7 and 9). Other alcohols (e.g. ethylene glycol, glycerol, ethanol, trifluoroethanol, dihydroxymethane (hydrated formaldehyde) have also been shown to be useful in this regard, together with various other high polarity solvents. Further hydroxyl containing compounds which induce release of CO.sub.2 include sugars such as glucose and fructose, oligosaccharides such as cotton, starch and alginic acid, and amino acids such as serine and related oligomers.

(33) Thus, by means of the method of the present invention, it is possible to facilitate the use of capture agents for acid gases, specifically, the inexpensive and non-toxic carboxylic acids, which would usually be considered ineffective for this purpose because of their low pK.sub.a in water. Indeed, it is possible to achieve quite readily up to 93% decarboxylation at room temperature simply by increasing the water content of the solvent, and higher CO.sub.2 loadings can also be achieved by increasing the concentration of the capture agent.

(34) It is also apparent that a variety of water-miscible non-aqueous solvents, particularly polar aprotic solvents, can be used in this process. Thus, for example, changing from DMSO to N-methyl pyrrolidinone (NMP) showed comparable effects (entries 5, 9 and 10), and further options are discussed below.

(35) As previously noted, the process is successfully carried out using carboxylates with a range of cations, including sodium, potassium, lithium, ammonium (including choline salts and betaine), and phosphonium salts.

(36) The inventors have, however, also established that the particular cationic group and solvent composition may affect the rate of CO.sub.2 capture. Thus, for example, it was observed that the tetrabutylammonium cation showed an enhanced rate of CO.sub.2 uptake when compared to the corresponding potassium salt, as illustrated in FIG. 2.

(37) In one possible embodiment of the invention blends of different counter-ions, such as tetraalkylammonium- and alkali metal carboxylates may be used in a carbon capture process to ensure optimal performance, whilst solvent costs stay at a minimum.

(38) It has also been established that the method of the invention may be successfully applied over a range of CO.sub.2 pressures, as shown in Table 3. Thus, a lower initial pressure (0.6 bara) causes the CO.sub.2 uptake capacity and rate to decrease slightly, whilst the release becomes more effective (entry 2). Higher initial pressure (1.5 bara), however, causes the CO.sub.2 uptake capacity and rate to increase slightly, while the release becomes somewhat less effective (entry 3).

(39) TABLE-US-00003 TABLE 3 VLE Performance at Different CO.sub.2 Initial Pressures P.sub.CO2 initial Fast Rate Fast Capacity Full Capacity Recovered Entry (bara).sup.a (1/s) (mol.sub.CO2/L) (mol.sub.CO2/L) CO.sub.2 (%).sup.b 1 1 0.043 0.77 0.83 88 2 0.6 0.038 0.62 0.68 90 3 1.5 0.080 0.83 0.88 80 .sup.a1.36M TBAA solution in NMP containing 1 eq. water (sat. @40 C.), 10 mL of solvent was injected to VLE under CO.sub.2 atmosphere; .sup.b3 mL water was added to induce release

(40) The partial pressure of CO.sub.2 in the gas stream which is to be purified influences the performance of the capture solvent. In order to ensure that the presently disclosed method is generally applicable to waste streams from a wide range of sources, several different gas compositions with varying CO.sub.2 partial pressures were tested and the results are presented in Table 4.

(41) TABLE-US-00004 TABLE 4 Representative Examples of Effect of Changing CO.sub.2 Partial Pressure on Absorption Performance pCO.sub.2 Capacity Initial Rate Entry (mbar) Capture System (mol.sub.CO2/L).sup.a (mmol.sub.CO2/L/s).sup.b 1 50 DBUP.sup.c in Octanol 0.14 0.11 2 150 K-Piv.sup.d in Sulfolane 0.49 0.10 3 300 K-Piv.sup.d in Diglyme 1.12 0.41 4 500 DBUP.sup.c in Octanol 0.37 0.49 .sup.aRefers to the moles of CO.sub.2 captured per litre of solvent during the measurement; .sup.brefers to the rate of CO.sub.2 absorption in the initial 500 s interval of the measurement; .sup.c1,8-Diazabicyclo[5.4.0]undec-7-ene propionate; .sup.dPotassium pivalate; .sup.eDiethylene glycol dimethyl ether

(42) These effects are, however, seen to be marginal over the studied pressure range, and this demonstrates that the approach should be applicable over a wide variation of CO.sub.2 concentrations, such as those in a real carbon capture processes (CO.sub.2 levels of 3-15% of total flue gas), and for atmospheric CO.sub.2 capture (ca. 400 ppm CO.sub.2 present in air at 2013 levels).

(43) Various commercially available solvents have been evaluated to exemplify the general applicability of the method, and the results are presented in Table 5. In general, all of the solvents tested showed a good level of performance, demonstrating the flexibility of the approach. It is important to emphasise that, in this instance, the term solvent relates to organic molecules which solvate the capture agents (e.g. the carboxylate), rather than being the capture agents themselves (which is common terminology in post-combustion Carbon Capture and Storage technology (CCS)).

(44) TABLE-US-00005 TABLE 5 VLE Performance of Tetrabutylammonium Acetate Dissolved in Various Water-Miscible Solvents Recov- Concen- Fast Full Water ered tration Rate Capacity Added CO.sub.2 Entry Solvent.sup.a (mol.sub.TBAA/L) (1/s) (mol.sub.CO2/L) (mL) (%) 1 TMU 1.45 0.058 0.85 3.0 86 2 DMPU 1.50 0.035 0.81 3.0 90 3 DMI 1.64 0.055 0.86 3.0 83 4 THF.sup.b 1.47 0.046 0.57 1.5 81 5 MeCN 1.50 0.066 0.62 2.0 85 6 NMP 1.50 0.051 0.88 5.0.sup.c 95 7 Sulfolane 1.41 0.026 0.69 3.0 91 .sup.aTBAA + 1 eq. water (2-4%), saturated @40 C., 10 mL of solvent was injected to VLE under 1 bara CO.sub.2 atmosphere; .sup.bnot corrected for solvent partial pressure, capacity is likely higher; .sup.cearly experiment, 3 mL water suffices to release the bulk of CO.sub.2

(45) From these data, it is apparent that suitable solvents for this purpose include, but are not limited to, dimethylsulphoxide (DMSO), N-methylpyrrolidinone (NMP), 1,1,3,3-tetramethylurea (TMU), N,N-dimethyl-N,N-trimethyleneurea (1,3-dimethyl-3,4,5,6-tetrahydro-2-pyrimidinone (DMPU)), 1,3-dimethyl-2-imidazolidinone (DMI), tetrahydrofuran (THF), acetonitrile and sulfolane.

(46) The rate of CO.sub.2 uptake varied only slightly with different solvents, and remained in the range of 0.035-0.066 1/s, whilst the capacity of CO.sub.2 uptake varied with different solvents, but remained in the range of 0.57-0.88 mol.sub.CO2/L. These values are not optimised, however, and it is believed that higher capacities can be achieved by using, for example, more concentrated solutions of the capture agents.

(47) The amount of water necessary to induce an acceptable CO.sub.2 release was found to be similar in the various solvents, although acetonitrile and THF (entries 4 and 5) required addition of significantly less water. These results suggest that the decisive factor between the solvent candidates will be commercial (price, availability) or their ability to separate from water (for example, using membranes) in the key solvent regeneration step which is essential for the cyclic absorption process.

(48) It has also been observed that the initial water content of a solvent can have an effect on its activity, thereby allowing solvent composition to be tuned to optimise solvent performance for specific formulations and applications. The results of these trials are presented in Table 6.

(49) TABLE-US-00006 TABLE 6 Effect of Water Content on VLE Performance of Tetrabutylammonium Acetate Solutions Water Content Concentration Fast Rate Full Capacity Recovered Entry (V %).sup.a (mol.sub.TBAA/L) (1/s) (mol.sub.CO2/L) CO.sub.2 (%).sup.b 1 0 0.94 0.090 0.36 77 2 2 (1 eq) 0.93 0.075 0.64 88 3 10 1.09 0.046 0.56 89 4 20 1.28 0.016 0.45 84 5 30 1.57 0.012 0.38 86 6 40 1.64 0.015 0.28 75 7 50 2.02 0.010 0.26 71 .sup.aTBAA solution in NMP-water mixture saturated @20 C., 10 mL of solvent was injected to VLE under 1 bara CO.sub.2 atmosphere; .sup.b3 mL water was added to induce release

(50) It is observed that the rate of CO.sub.2 absorption is reduced by increasing water content (consistent with the reduced pKa of the capture agent). Trace amounts of water are seen to be particularly effective (entry 1), but reasonable activity is still shown with larger amounts of water present. The performance of the absorber drops steadily with more water present, but solutions containing 10-20% v/v water are still effective, which increases the operational window of this chemistry, as illustrated in FIG. 3.

(51) The method of the invention is also shown to be successfully applied with a broad range of carboxylate salts, as shown in Table 7.

(52) The results particularly show that the use of certain organic cations (e.g. using DBU as base) is successful in the process, provided that the carboxylate is formed substantially. Thus, for example, the salt formed from the reaction of butyric acid and a strong organic base such as DBU results in a viscous oil, which is soluble and/or miscible with NMP, and gives high uptake capacity and acceptable release properties. The use of weaker TEA (triethylamine) as a base was probably ineffective due to its weaker basic character and lower concentrations of carboxylate (entry 2), whilst guanidine acetate showed poor performance due to solubility limitations (entry 3). However, various multiprotic carboxylic acid salts (e.g. citrate and oxalate) showed reasonable activity, as did carboxylic acid salts containing -hydroxy groups although the possible optimisation of the solventand hence pK.sub.aleaves scope for improvement in terms of reactivity and solubility. The results also show that ammonium salts such as choline, TMA (tetramethylammonium) and TBA (tetrabutylammonium) cations are effective (entries 8, 10 and 11).

(53) TABLE-US-00007 TABLE 7 VLE Performance of Various Organic Base-Carboxylic Acid Salts in N-Methylpyrrolidinone Recov- Concen- Fast Full Water ered tration Rate Capacity Added CO.sub.2 Entry Salt.sup.a (mol/L) (1/s) (mol.sub.CO2/L) (mL) (%) 1 DBU.sup.b butyrate 2.18 0.016 0.94 4 79 2 TEA.sup.c butyrate 1.00 nd 0.10 2 0 3 Guanidine 0.50 0.029 0.34 2 83 acetate 4 TBA citrate 0.33 0.083 0.44 10.sup.d 47 5 TBA citrate 0.50 0.054 0.63 10.sup.d 38 6 TBA oxalate 1.50 0.042 0.41.sup.e 0.6 48 7 TBA lactate 1.48 0.038 0.24 2.3 70 8 TMA.sup.f 2.48 0.014 0.92 3.4 72 propionate 9 BTMA.sup.g 0.70 0.065 0.44 3.0 79 propionate 10 Choline 1.56 0.047 0.51 3.3 69 propionate 11 Choline 2.50 0.017 0.66 3.9 69 propionate .sup.aSolution in NMP containing 1 eq. water to carboxylate groups (sat. @r.t.), 10 mL of solvent was injected to VLE under CO.sub.2 atmosphere, release was triggered by 3 mL water; .sup.bDBU = 1,8-Diazabicycloundec-7-ene, the salt and NMP are miscible, the concentration displayed was chosen after preliminary screening; .sup.cTEA = Triethylamine, not saturated, concentration is arbitrarily chosen; .sup.d10 mL MeOH added; .sup.e5 mL of solvent was injected to VLE under CO.sub.2 atmosphere; .sup.fTMA = Tetramethylammonium; .sup.gBTMA = Benzyltrimethylammonium

(54) The relative ease of access and efficiency of DBU-carboxylate salts enabled a further solvent screen using DBU propionate. A wide variety of organic solvents was found to be compatible with the general concept as summarised below in Table 8.

(55) TABLE-US-00008 TABLE 8 Absorption Performance of DBU Propionate in Various Organic Solvents Capacity Initial Rate Entry Carrier Solvent (mol.sub.CO2/L).sup.a (mmol.sub.CO2/L/s).sup.b 1 1,3-Dioxolane 0.66 0.96 2 Dioxane 0.94 1.43 3 2-Ethyl-1-hexanol 0.72 0.93 4 3,5,5-Trimethyl-1-hexanol 0.71 0.72 5 4-Methyl-2-pentanol 0.70 1.05 6 1-Butanol 0.57 0.85 7 1-Pentanol 0.58 0.84 8 1-Hexanol 0.65 1.00 9 1-Heptanol 0.73 0.92 10 1-Octanol 0.68 0.88 11 Dimethylsulfoxide (DMSO) 0.96 1.41 12 1,1,3,3-Tetramethylurea (TMU) 0.93 1.36 13 Dimethylformamide (DMF) 0.92 1.42 14 1,3-Dimethyl-2-imidazolidinone 0.98 1.39 (DMI) 15 1,3-Dimethyl,3,4,5,6-tetrahydro- 1.03 1.11 2-pyrimidinone (DPMU) Capture composition: 1-4M 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) propionate, 5-10% w/w water and the corresponding carrier solvent; absorption conditions: 1 bar CO.sub.2 at 40 C.; .sup.arefers to the moles of CO.sub.2 captured per litre of solvent during the measurement; .sup.brefers to the rate of CO.sub.2 absorption in the initial 500 s interval of the measurement

(56) Furthermore, absorption performance was tested under more realistic conditions comprising of low CO.sub.2 partial pressure (150 mbar) at an absorption temperature of 40 C. In this case, potassium pivalate was chosen as a representative substrate, and the results are presented in Table 9.

(57) TABLE-US-00009 TABLE 9 Absorption Performance of Potassium Pivalate in Various Organic Solvents Initial Rate Capacity (mmol.sub.CO2/ Entry Carrier Solvent (mol.sub.CO2/L).sup.a L/s).sup.b 1 Sulfolane 0.46 0.05 2 Cyclohexanone 0.51 0.20 3 Diethylene glycol dimethyl ether 0.85 0.19 4 Diethylene glycol diethyl ether 0.87 0.15 5 Diethylene glycol monomethyl ether 0.10 0.10 6 Diethylene glycol monoethyl ether 0.52 0.11 7 Diethylene glycol monopropyl ether 0.38 0.10 8 Diethylene glycol monobutyl ether 0.52 0.09 9 Ethylene glycol monoethyl ether 0.45 0.15 10 Ethylene glycol monopropyl ether 0.42 0.14 11 Ethylene glycol monobutyl ether 0.58 0.11 12 Propylene glycol monobutyl ether 0.32 0.12 13 Triethylene glycol dimethyl ether 0.55 0.15 14 Dipropylene glycol dimethyl ether 0.79 0.15 15.sup.c Dipropylene glycol dimethyl ether 0.56 0.24 Capture composition: 1-4M Potassium pivalate, 10-25% w/w water and the corresponding carrier solvent; absorption conditions: 150 mbar CO.sub.2 at 40 C.; .sup.arefers to the moles of CO.sub.2 captured per litre of solvent during the measurement; .sup.brefers to the rate of CO.sub.2 absorption in the initial 500 s interval of the measurement; .sup.cpotassium isobutyrate (rather than potassium pivalate)

(58) Polymeric carboxylic acids can also be successfully used for this process. For example, alginic acid is a naturally occurring oligosaccharide extracted from seaweed that contains a carboxylic acid functional group on each monomer unit. An appropriate amount of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) was added to a suspension of alginic acid in N-methylpyrrolidone (NMP) and water to form the corresponding DBU carboxylate salt. The resulting slurry showed substantial capacity for absorbing CO.sub.2 (0.79 M) at room temperature. Increasing the water content of this carboxylated system to approximately 20% v/v resulted in 54% of the total CO.sub.2 content being released.

(59) In a further embodiment of the invention, blends of two or more carboxylate salts were employed and demonstrated a high level of performance, as illustrated by the data in Table 10.

(60) TABLE-US-00010 TABLE 10 Absorption Performance of Carboxylate Blends Capacity Initial Rate Entry Composition (mol.sub.CO2/L).sup.a (mmol.sub.CO2/L/s).sup.b 1 1M KPiv.sup.c, 2M KOAc.sup.d 0.62 0.39 2 1.25M KPiv, 1.75M KOAc 0.66 0.33 3 1.5M KPiv, 1.5M KOAc 0.69 0.31 4 1.75M KPiv, 1.25M KOAc 0.70 0.18 5 2M KPiv, 1M KOAc 0.78 0.21 6 2.25M KPiv, 0.75M KOAc 0.77 0.22 7 0.5M KPiv, 2.5M KProp.sup.e 0.64 0.27 8 0.75M KPiv, 2.25M KProp 0.58 0.30 9 1M KPiv, 2M KProp 0.74 0.29 10 1.25M KPiv, 1.75M KProp 0.72 0.19 11 1.5M KPiv, 1.5M KProp 0.75 0.21 12 1.75M KPiv, 1.25M KProp 0.78 0.20 13 1.5M KPiv, 1.5M KOAc 0.53 0.25 Capture composition: stated salt concentration, 10-25% w/w water in diethylene glycol dimethyl ether; absorption conditions: 150 mbar CO.sub.2 at 40 C.; .sup.arefers to the moles of CO.sub.2 captured per litre of solvent during the measurement; .sup.brefers to the rate of CO.sub.2 absorption in the initial 500 s interval of the measurement; .sup.cpotassium pivalate; .sup.dpotassium acetate; .sup.epotassium propionate; .sup.fsolvent is diethylene glycol diethyl ether

(61) In addition, the inventors investigated the effects achieved by the addition of accelerants to the system. Specifically, several different amine derivatives were tested as potential accelerants (alongside the potassium pivalate) in a representative solvent in order to establish their influence on CO.sub.2 uptake rates. Representative examples of primary, secondary, and tertiary amines were screened to broaden the scope of the study and as shown in Table 11, an increase in absorption rate was observed, although there is the possibility that cyclic capacity may be affected due to the presence of the amine.

(62) TABLE-US-00011 TABLE 11 Effect of Amine Accelerants on the Performance of Potassium Pivalate in Ethylene Glycol Monobutyl Ether Capacity Initial Rate Entry Amine Accelerant (mol.sub.CO2/L).sup.a (mmol.sub.CO2/L/s).sup.b 1 3-(Diethylamino)-1,2-propanediol (DAPD) 0.91.sup.c 0.11 2 Tetramethylethylenediamine (TMEDA) 0.54 0.15 3 2-Diethylaminoethanol (DEAE) 0.48 0.12 4 Tetraethylmethanediamine (TEMDA) 1.17 0.29 5 Tetramethylmethanediamine (TMMDA) 0.71 0.30 6 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) 0.70.sup.d 0.47 7 Tetramethyl-1,3-diaminopropane (TMPDA) 1.15.sup.c 0.16 8 Triethylamine (TEA) 0.41 0.15 9 Monoethanolamine (MEA) 0.80.sup.e 0.40 10 Morpholine 0.82.sup.f 0.40 11 Piperazine 0.63.sup.g 0.40 Capture composition: 1-4M Potassium pivalate, 16-25% w/w water, 16% amine relative to the salt in ethylene glycol monobutyl ether; absorption conditions: 150 mbar CO.sub.2 at 40 C.; .sup.arefers to the moles of CO.sub.2 captured per litre of solvent during the measurement; .sup.brefers to the rate of CO.sub.2 absorption in the initial 500 s interval of the measurement; .sup.ccyclic capacity 0.66M; .sup.dcyclic capacity 0.19M; .sup.ecyclic capacity 0.27M; .sup.fcyclic capacity 0.50M; .sup.gno release

(63) As previously discussed, regeneration of the capture composition, such that it may be used for further capture and release operations with further acid gases, is an important consideration, particularly in the context of providing a commercially attractive process. Consequently, it is desirable to not only provide for the capture and liberation of CO.sub.2 or other acid gases, but also to provide a system which facilitates a return to the original ratio of solvents in the active capture solvent to allow for the subsequent re-use of the composition in further procedures. The regeneration process typically requires separation of the gas-releasing additivethe protic solvent or agentfrom the original composition.

(64) It is necessary to determine the minimum amount of release agent (usually water) necessary for CO.sub.2 stripping. The energy demand associated with the removal of said agent in the subsequent solvent reset stage has a profound effect on the overall energetics of the process. Therefore, it is necessary to maximise the volume of CO.sub.2 that can be recovered from a carboxylated solvent per unit of release agent added. A revised stripping experiment was designed (1 bar CO.sub.2 atmosphere, 60 C. solvent temperature) to simulate realistic conditions and test release performance with different amounts of release agent.

(65) Two representative examples are presented in FIGS. 4 and 5, with sulfolane and ethylene glycol monobutyl ether being employed as solvents. The results clearly show the different CO.sub.2 loadings possible with different water content on the basis of VLE experiments and it is seen that the water content could be varied from values of 10% and 36% to control the potential CO.sub.2 loading.

(66) Various alternative procedures are available in order to achieve control of release agent content, and the choice usually depends on the amount of added release agenttypically waterthat needs to be removed from the composition, and the nature of the water-miscible, non-aqueous solvent.

(67) Removal of the gas-releasing additiveparticularly watermay, for instance, be achieved by the use of a suitable membrane, examples of which would be readily apparent to those skilled in the art. In a possible embodiment of the invention, membranes may be used to separate organic solvents from water. It has been demonstrated that several types of permeable membranes can be used to selectively permeate either an organic solvent or water from a mixture from a mixture of the two materials. Hydrophilic membranes may be used to selectively permeate water, and hydrophobic membranes can be used to selectively permeate organic solvents. This has been shown for a variety of different solvents over several temperatures, as summarised in Table 12.

(68) TABLE-US-00012 TABLE 12 Composition of Permeates for Different Solvents Through Either Hydrophilic or Hydrophobic Membranes Permeate Composition.sup.a (% w/w) Separation Solvent Pressure Temperature Organic Factor Entry (% w/w) Membrane (Bar) ( C.) Solvent H.sub.2O (S.sub.F).sup.b 1 Diglyme-water PTFE 1 45 67.3 32.7 2.06 (50-50) 2 Diglyme-water PTFE 1 60 68.4 31.6 2.16 (50-50) 3 Diglyme-water Silica 1 30 67 33 0.39 (84-16) 4 Diglyme-water Silica 3 60 75 25 0.57 (84-16) 5 Diglyme-water Silica 2 45 54 46 0.58 (67-33) 6 Diglyme-water Silica 3 30 49 51 0.96 (50-50) 7 Diglyme-water Silica 1 30 10 90 0.26 (30-70) 8 Diglyme-water Silica 3 30 24 76 0.74 (30-70) 9 Sulfolane-water PTFE 1 30 68 32 1.10 (66-34) 10 Sulfolane-water PTFE 1 45 70 30 1.18 (66-34) 11 Sulfolane-water Silica 2 30 66 34 0.97 (66-34) 12 TMU-water Silica 2 30 69 31 0.94 (70-30) 13 TMU-water Silica 1.2 45 69 31 0.94 (70-30) 14 TMU-water Silica 2 45 69 31 0.97 (70-30) 15 TMU-water Silica 1.2 60 69 31 0.97 (70-30) 16 BuOH-water Silica 2 30 64 36 0.77 (70-30) 17 BuOH-water Silica 2 45 64 36 0.76 (70-30) 18 BuOH-water Silica 2 60 62 38 0.69 (70-30) .sup.aRefers to the composition of the solvent that permeated through the membrane; .sup.brefers to the permeation flux through the membrane

(69) In FIG. 6, there is illustrated an example of a system and process which utilises the specific features of the present invention. The absorption part of the process is essentially the same as for a conventional process. The gas-rich solvent is then combined with a water rich stream, thereby inducing a pKa change in the solvent, which pressurises the liquid flow. The flow is heated with waste heat to introduce the heat required to overcome the desorption enthalpy, and the warm rich solvent is then flashed in several vessels in series, where the first vessel will release most of the CO.sub.2 at high pressure; this reduces the need for CO.sub.2 compressors and/or large size equipment.

(70) After releasing at high pressure, the solvent can be further stripped at lower pressures; this results in lower flows of low pressure CO.sub.2 compared to the processes of the prior art, thus allowing for the use of smaller, low pressure compressors and, consequently, requiring less compression energy on the whole apparatus. In a separate unit operation, the water fraction in the solvent is lowered to reset the pKa value of the capturing composition to the optimum level for acid gas absorption, and this solvent is fed to the absorber. The separated water-rich phase is fed back to the gas-rich solvent stream.

(71) Substantial energy savings can be achieved by creating a high pressure CO.sub.2 stream in the release stage, thereby reducing compressor duty. Stripping efficiency is expected to be lower at elevated CO.sub.2 partial pressures as it creates a considerable strain on release. In a possible embodiment of the invention, a cascade of release tanks is envisaged to strip a portion of CO.sub.2 at higher pressure and the rest in subsequent lower pressure steps to maintain high cyclic capacity while lowering the energy costs associated with compression. To determine the feasibility of stripping at higher pressures, a process was conducted using a 1 M solution of tetrabutylammonium acetate in NMP. The carboxylated solution was placed in a pressure reactor and additional CO.sub.2 was introduced to increase the pressure inside to 23 bar. Upon water addition, the pressure rose to 30 bar indicating CO.sub.2 release. Additionally, a series of release experiments was conducted using DBU propionate solutions in NMP, demonstrating that 29% of the total CO.sub.2 content can be stripped against CO.sub.2 partial pressure as high as 3.8 bar at 60 C.

(72) Several water separation unit operations are suitable for use in the context of the present invention. Specific unit operations are illustrated in FIGS. 7 to 9, although these are by no means limiting of the scope of the invention.

(73) FIGS. 6 to 8 are illustrations of such unit operations but the invention is not limited to these unit operations. Indeed, any water separation process can be selected from those which are known to those skilled in the art. Specifically, FIGS. 7 and 8 show a pervaporation membrane process and a conventional membrane process, respectively.

(74) Thus, lean solvent obtained after release of the acid gas is fed to a membrane unit, slightly heated by waste heat to overcome the energy required for separation (e.g. evaporation enthalpy of the water and/or heat of dissolution) where the water is separated by diffusing through the membrane by a pressure difference over the membrane, provided by a high liquid pressure inside the membrane and/or a low pressure on the permeate side, which may be provided by e.g. a vacuum pump. The low pressure on the permeate side can also be achieved by a vertical liquid column and liquid pump, as only a slightly negative pressure is required on the permeate side; such an arrangement would result in a more energy efficient process and less chance of losing the permeate product to e.g. the vacuum line.

(75) Investigations with the pervaporation system have shown that two liquid fractions can be collected. After the solvent permeates the membrane, the higher boiling point fraction condenses immediately and can be collected near the membrane, whilst the vapour fraction containing predominantly water can be condensed in a separate phase. The conventional membrane system, on the other hand, generates one permeate stream.

(76) Hence, pervaporation can be used for the partial or complete removal of a release agent. An experiment with a TiO.sub.2 pervaporation membrane demonstrated that a solvent feed of NMP-water could be enriched in water on the permeate side as shown in Table 13.

(77) TABLE-US-00013 TABLE 13 Water Separation from Capture Solvents Using Pervaporation Evaporation Settings Condensate Solvent Water Pressure Temperature Solvent Water Solvent (% v/v) (% v/v) (mbar) ( C.) (% v/v) (% v/v) NMP.sup.a 80 20 250 76 21 79 NMP.sup.a 50 50 250 76 16 84 .sup.aN-Methylpyrrolidinone

(78) A further process for the separation of water may comprise a distillation process, an example of which is illustrated in FIG. 9. In this process the lean solvent is heated using waste heat to overcome the heat of evaporation. The warm solvent is contacted with warm carrier gas (e.g. air), preheated using waste heat, in a liquid gas contactor (e.g. a packed column) where the water vapour is preferentially evaporated into the gas stream. The dry solvent can be recovered from the bottom of the stripper, whilst the wet carrier gas is subsequently cooled and the condensate is recovered, and the cold carrier gas is heated up again by waste heat and fed to the liquid gas contactor. Experiments and calculations have shown that such a process can operate with a very small temperature difference between cold and hot carrier gas, and this scenario is especially suited for utilising waste heat.

(79) In laboratory scale studies, the removal of water at 80 C. in an air stream has been achieved and this can significantly reduce energy requirements, and this indicates that the water content of the solution can be adjusted back within the desired range by such an approach, as shown in Table 14.

(80) TABLE-US-00014 TABLE 14 Water Removal by Distillation at 80 C. in Air Stream Density Refractive Fraction.sup.a Volume (mL) (g/mL) Index Water Content (%) 1 10 0.994 1.3351 98 2 19 0.996 1.3371 97 3 18 0.997 1.3419 93 4 10 1.01 1.3760 71 Residue ~150 nd 1.4679 5-7 .sup.a210 mL NMP-water mixture (2:1) was heated up to 80 C. and air was bubbled through it. Different fractions of the distillate were collected and their water content determined by measuring refractive indexes

(81) In an embodiment of the invention, CO.sub.2 stripping is carried out by using a stream of air passing through the solvent without the addition of a release agent. In this way, the energy penalty associated with the removal of said release agent can be avoided. A procedure was conducted to demonstrate the feasibility of this approach, wherein a solution of potassium pivalate in diethylene glycol dimethyl ether was carboxylated in the usual manner, followed by air stream stripping at 40 C., resulting in a considerable decrease in CO.sub.2 loading (c=0.6M).

(82) As previously reported, the use of alternative protic solvents or agents, most particularly lower boiling point, small chain alcohols, has been successfully demonstrated for CO.sub.2 release and may minimise energy consumption. Experiments have confirmed that methanol is a suitable, albeit slightly less effective, alternative to water in the release phase.

(83) A further alternative method for removal of the gas-releasing additive is to flash the lean solvent under a slight negative pressure. The lean solvent is heated with waste heat and pumped through a valve. After the valve, the solvent is subjected to a slight negative pressure to provide a driving force for the water to evaporate. The slight negative pressure can be provided by a vertical liquid column and liquid pump, or by a conventional vacuum pump.

(84) Alternatively, a gas-releasing additive such as water could be removed by distillation, at atmospheric or reduced pressure.

(85) Different forms of distillation are well established on both laboratory and industrial scales. The feasibility of applying distillation to manipulate the amount of release agent in the solvent was investigated with several different compositions. Representative examples were selected to serve as proof of principle. A solution of 3 M potassium pivalate in diethylene glycol dimethyl ether (diglyme) and 16% w/w water was distilled at 300 mbara to give a distillation temperature of about 70 C. The vapour contained 555% w/w water based on 15 independent experiments, the remaining fraction consisting of diglyme. The pivalic acid and potassium pivalate content of the condensate was negligible. The same solvent was also distilled at about 80 C. and 500 mbara but no significant difference in gas phase composition was observed (53% w/w water in vapour), thus showing that distillation temperature and pressure have little effect on the equilibrium.

(86) In the experimental procedure, solvent compositions were fractionally distilled to remove water in a 100 mL round bottom flask using an electrical heater jacket. The temperature of the vapour and liquid was recorded with T-type thermocouples and values were logged with Labview. The pressure in the distillation apparatus was controlled by an Edwards vacuum pump and the pressure was monitored with an Omega manometer. The vapour was condensed using a water cooled glass condenser and the composition of the condensate was analysed by NMR and verified by Karl Fischer titration. The results obtained are illustrated in FIG. 11.

(87) The vapour obtained from the solvent as described above may require further purification as its water content is low for use in the release step. The vapour liquid equilibrium for diglyme and water was experimentally determined at 60 C. and is shown in FIG. 12. Mass balance calculations based on experimental data can be used to predict the vapour and condensate compositions in subsequent equilibrium stages. One additional stage would result in a water content of 77-83% w/w in the condensate, which is high enough for reuse as a release agent. A third stage would result in 88-89% w/w pure water. Instead of distillation, the solvent could also be flashed under similar conditions in 2 to 3 stages to achieve the same purity.

(88) As illustrated by the data presented in Table 15, structurally similar carrier solvents demonstrate slight variation in the water content of the vapour phase.

(89) TABLE-US-00015 TABLE 15 Vapour Composition Above Selected Capture Solvents at 60 C. Solvent) Vapour Composition (% w/w) Entry Component 1 Component 2 Component 1 Component 2 Component 3 1 Proglyde.sup.a 39 61 2 Proglyde.sup.a Diglyme.sup.b 24 24 52 3 Triglyme.sup.c Diglyme.sup.b 10 35 55 4 EGBE.sup.d Diglyme.sup.b 10 33 57 Solvent composition: 3M Potassium pivalate, 16% w/w water in Component 1:Component 2 50:50; vapour phase was sampled at an equilibrium temperature of 60 C.; vapour composition was determined by NMR; .sup.apropylene glycol dimethyl ether; .sup.bethylene glycol dimethyl ether; .sup.ctriethylene glycol dimethyl ether; .sup.dethylene glycol monobutyl ether

(90) In an embodiment of the invention a mechanical vapour recompression (MVR) evaporator is used to minimise the energy requirement for flashing or distillation by compressing the vapour to slightly elevated pressures and temperatures to condense it and thereby provide the heat required for evaporation. Alternatively, steam can be injected to compress the vapour to elevated pressures and temperatures. In this manner, only a small heat input is required to evaporate large quantities of the release agent, resulting in an energy efficient method of separation.

(91) According to this procedure, solvent was fed at a rate of 10-20 L/h to an evaporation unit utilising mechanical vapour recompression. Between 0.5 and 2 L/h of release agent was evaporated at temperatures ranging from 60 to 97 C. and pressures ranging from 100 to 800 mbara. The results are presented in Table 16.

(92) TABLE-US-00016 TABLE 16 Water Separation from Capture solvents Using Mechanical Vapour Recompression (MVR) Evaporation Feed Composition Setting Outlet Composition Condensate (% w/w) Pressure T (% w/w) (% w/w) Solvent Salt.sup.a Solvent Water (mbara) ( C.) Salt.sup.a Solvent Water Solvent Water EGBE.sup.b 35 37 28 478 98 41 39 20 22 78 EGBE.sup.b 35 37 28 600 97 38 39 23 13 87 Diglyme.sup.c 35 39 26 350 75 40 39 21 37 63 Diglyme.sup.c 35 39 26 350 84 43 38 19 43 57 .sup.aPotassium pivalate; .sup.bethylene glycol monobutyl ether; .sup.cdiethylene glycol dimethyl ether

(93) The condensate composition calculated from the mass balance was in good agreement with that determined experimentally by NMR. Average water contents of 55% and 90% w/w were found for diglyme and EGBE respectively. It was demonstrated that the diglyme condensate can be further purified by an additional evaporation step or by fractional condensation (vide supra). Alternatively the condensate can be further purified by membrane filtration.

(94) Still further approaches to the removal of the gas-releasing additive include the adsorption of water into hydrophilic materials, such as cellulose, cotton, starch, polyethylene glycols or polyacrylic acids or, when using other protic solvents, adsorption of these solvents into more hydrophobic materials, for example organic polymers such as polyesters, polystyrene, polyvinylchloride, polyethylene, polypropylene, polylactic acid, ion exchange resins, perfluorinated materials and siloxanes.

(95) A variety of hydrophilic and hydrophobic materials was found to be able to alter the composition of aqueous-organic mixtures by selective sorption of either water or the organic component. Hydrophobic materials selectively adsorbed organic solvents, and hydrophilic materials selectively adsorbed water, as demonstrated with representative examples and illustrated in FIG. 13, which shows results at a solvent to sorbent ratio of 5:1 and an initial water content of 30% w/w for various hydrophilic materials (to the left of the chart) and hydrophobic material (to the right of the chart).

(96) A variation of this general approach is to use stimuli-responsive polymers, such as thermally responsive polymers, which are also known as smart or environmentally sensitive polymers, which undergo physical structural changes in response to changes in their environment. One such example is poly(N-isopropylacrylamide) which, when heated above its lower critical solubility temperature, switches from a hydrophilic to a hydrophobic state, thereby expelling the water contained therein and losing about 90% of its volume in the process. Such a polymer, being present in situ during the capture process, could allow enhancement of water levels on warming, and hence induce decarboxylation and, on cooling, would rehydrate (and hence effectively remove water from the active solvent) to reactivate the solvent for CO.sub.2 capture.

(97) Alternatively, other processes well known in the art may be employed, including, for example, thermal regeneration, solvent stripping, the use of vacuum or pressure, or mechanical regeneration (e.g. removal of solvent by squeezing an absorbent material composition between rollers or under pressure). It is noted that such processes may also be applied to the regeneration of hydrophilic or hydrophobic materials which have been utilised for the adsorption of protic solvents or agents.

(98) A further alternative process may involve electrolysis of the water rich solution after CO.sub.2 release. Although such a procedure would require the use of electricity, and thereby reduce the likely efficiency of the overall process, it would also produce oxygen and hydrogen gas as by-products and these materials themselves are useful and valuable commodities which would offset the cost of additional electricity generation.

(99) A still further alternative process utilises solvent mixtures that allow a phase separation at some stage in the process, thereby facilitating separation of the non-aqueous water-miscible solvent from the release solvent (typically water). In this case, solvents which have partial miscibility with the release solvent (typically water) are particularly suitable.

(100) The inventors have also demonstrated the overall greater efficiency of the carboxylate-based formulations which are used according to the present invention when compared with the typically amine-based, compositions disclosed by the prior art. The heat of formation of carbamates in amine based absorbers accounts for a large portion of the energy demand of CO.sub.2 release, whilst solutions of carboxylic acid salts are more easily stripped of their CO.sub.2 content, which would lead to an expectation that lower desorption temperatures might be achieved. The efficiency of stripping will vary depending on the effective pKa of the acid in the process, and this can be fine-tuned by optimisation of the solvent composition.

(101) TABLE-US-00017 TABLE 17 Absorption Performance and Low Temperature Thermal Regeneration of TBAA Solutions in NMP-Water Mixed Solvent Water Fast Recovered Content Concentration Rate Full Capacity CO.sub.2 Entry (V %).sup.a (mol.sub.TBAA/L) (1/s) (mol.sub.CO2/L) @80 C. (%).sup.b 1 0 0.94 0.090 0.36 74 2 2 (1 eq) 0.93 0.075 0.64 69 3 10 1.09 0.046 0.56 80 4 20 1.28 0.016 0.45 72 5 30 1.57 0.012 0.38 74 6 40 1.64 0.015 0.28 67 7 50 2.02 0.010 0.26 62 .sup.aTBAA solution in NMP-water mixture saturated @20 C., 10 mL of solvent was injected to VLE under 1 bara CO.sub.2 atmosphere; .sup.bThe carbon rich solution was heated up to 80 C. without changing the NMP/water ratio of the mixture

(102) In order to illustrate this point, carboxylate-based formulations, comprising tetra-N-butyl ammonium acetate (TBAA) solutions in NMP, were prepared made up with varying initial water contents and were then tested under conventional absorption-thermal regeneration cycles. It was found that effective stripping of carbon dioxide was achieved at temperatures as low as 80 C., as can be seen from the data presented in Table 17.

(103) The general applicability of the method of the invention was exemplified by repeating the above tests using potassium acetate as the carboxylate salt, and the results obtained are shown in Table 18.

(104) From these data, the uptake rate is demonstrated by the fast capacity of the solvent, which refers to the overall CO.sub.2 captured faster than 0.0009 l/s. Increasing water content has a profound effect of the solubility of the acetate salt; on the other hand, it is detrimental to the speed at which CO.sub.2 is absorbed (entries 2-5). In the case of potassium acetate, the water content cannot be effectively lowered below 20% without phase separation. The more hydrophobic (or greasier) potassium isobutyrate allows for a reduction in water content (10%), which leads to superior CO.sub.2 capture performance (entry 8). Two experiments with different initial CO.sub.2 pressure were conducted to demonstrate that the system was able to capture low pressure CO.sub.2, and that desorption was effective against considerable CO.sub.2 backpressure (entries 6 and 7).

(105) TABLE-US-00018 TABLE 18 Absorption Performance and Low Temperature Thermal Regeneration of Potassium Acetate Solutions in NMP-Water Mixed Solvent Water Concen- Fast Full Recovered Content tration Capacity Capacity CO.sub.2 Entry (V %).sup.a (mol.sub.salt/L) (mol.sub.CO2/L) (mol.sub.CO2/L) @80 C. (%).sup.b 1 10.sup.c nd nd nd nd 2 20 3.12 0.67 1.38 63 3 30 4.22 0.37 1.41 63 4 40 5.50 0.08 1.36 65 5 50 6.22 0.09 1.29 68 6 20.sup.d 3.12 0.13 0.67 64 7 20.sup.e 3.12 1.08 1.74 55 8 10.sup.f 1.73 0.96 1.32 48 .sup.aKOAc solution in NMP-water mixture saturated @20 C., 10 mL of solvent was injected to VLE under 1 bara CO.sub.2 atmosphere; .sup.bThe carbon rich solution was heated up to 80 C. without changing the NMP/water ratio of the mixture; .sup.cPhase separation was observed upon dissolving the salt; .sup.d10 mL of solvent was injected to VLE under 0.5 bara CO.sub.2 atmosphere; .sup.e10 mL of solvent was injected to VLE under 1.5 bara CO.sub.2 atmosphere; .sup.fK-iso-butyrate was used under 1 bara CO.sub.2 atmosphere

(106) Throughout the description and claims of this specification, the words comprise and contain and variations of them mean including but not limited to, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

(107) Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

(108) The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

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