C1-C8 carboxylic acid salt solution for the absorption of CO.SUB.2

Abstract

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. The method comprises the step of capturing an acid gas by contacting said acid gas with a capture composition comprising at least one salt of a carboxylic acid dissolved in a solvent system consisting substantially of water.

Claims

1. A method for the capture of at least one acid gas in a composition, said method comprising: a. capturing an acid gas by contacting said acid gas with a capture composition comprising a metal salt of a C.sub.5-C.sub.8-aliphatic carboxylic acid and an additive dissolved in a solvent system to form a loaded capture composition, wherein said solvent system comprises at least 95% water, wherein the solvent system is a mixture of liquids in which the carboxylic acid salt is dissolved, wherein said capture composition comprises no more than 1% of amino acids or salts of amino acids, wherein said acid gas comprises carbon dioxide; wherein the additive is selected from a carbonate salt and a metal salt of a C.sub.1-C.sub.4-carboxylic acid.

2. A method of claim 1, further comprising performing, in order, the steps of: b. releasing said at least one acid gas by heating the loaded capture composition and/or by subjecting the loaded capture composition to a stream of stripping gas and/or by applying a lower pressure than employed for absorption; c. regenerating the capture composition by cooling and/or increasing the pressure.

3. A method as claimed in claim 2 wherein said release of said at least one acid gas occurs at a temperature in the range 60° to 100° C.

4. A method as claimed in claim 2 wherein said release of said at least one acid gas occurs at a pressure of 1 Bara or less.

5. A method as claimed in claim 1, wherein the mixture capture composition is a mixture of at least one metal salt of a C.sub.1-C.sub.4 aliphatic carboxylic acid, that may be straight chained or branched, and at least one metal salt of a C.sub.5-C.sub.6 aliphatic carboxylic acid, that may be straight chained or branched.

6. A method as claimed in claim 1, wherein the base additive is a carbonate.

7. A method as claimed in claim 6, wherein the carbonate is lithium, potassium, sodium, magnesium or calcium carbonate, or a combination thereof.

8. A method as claimed in claim 1 wherein said metal salt of a C.sub.5-C.sub.8 aliphatic carboxylic acid and/or C.sub.1-C.sub.4 carboxylic acid is a potassium salt.

9. A method as claimed in claim 1, wherein the acid gas further comprises hydrogen sulphide or sulphur dioxide.

10. The method of claim 1, wherein the single metal salt of a C.sub.5-C.sub.8-aliphatic carboxylic acid is present at a concentration of greater than 5 M.

11. The method of claim 1, wherein the single metal salt of a C.sub.5-C.sub.8-aliphatic carboxylic acid is present at a concentration of greater than 6 M.

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 illustrates the relationship between the maximum capture capacity of concentrated carboxylic acid salt solutions under 1 Bar partial pressure of CO.sub.2 and the water:salt molar ratios found in said solutions.

(3) FIG. 2 shows the improvement in CO.sub.2 capture rate and capacity garnered through blending potassium hexanoate and potassium acetate as compared to each component alone.

(4) FIG. 3 illustrates the relationship between the number of carbon atoms in various carboxylic acid salts and the maximum observed rate of carbon dioxide absorption under 1 bar partial pressure of CO.sub.2.

(5) FIG. 4 shows the improvement in rate of carbon dioxide absorption into a 2M solution of potassium carbonate via the addition of 2M potassium hexanoate.

DESCRIPTION OF THE INVENTION

(6) The term ‘consisting substantially of water’ means that the solvent system in which the carboxylic acid salt and any additional components are dissolved is substantially free of organic solvents. Thus, it may be that the solvent system in which the carboxylic acid salt and any additional components are dissolved comprises at least 50% water, e.g. at least 75% water, 90% water, 95% water or 98% water, such as at least 99% water. Thus, where the capture composition comprises only a carboxylic acid salt, the carboxylic acid salt is dissolved in a solvent system that comprises at least 50% water, e.g. at least 75%, at least 90% water, at least 95% water, 98% water, or at least 99% water. Where the capture composition comprises a base additive, the carboxylic acid salt and the base additive are dissolved in a solvent system that comprises at least 50% water, e.g. at least 75% water, at least 90% water, at least 95% water, at least 98% water, or at least 99% water.

(7) The term ‘solvent system’ refers to the mixture of liquids in which the carboxylic acid salt and any base additive or other components are dissolved. The term ‘capture composition’ refers to the total composition, i.e. both the solvent system and the carboxylic acid salt and any base additive or other components. Because the concentration of the carboxylic acid salt can be very high, the ‘capture composition’ may contain only a small amount of the ‘solvent system’ and therefore only a small amount of water. Nevertheless, the liquid in which the carboxylic acid is dissolved (i.e. the solvent system) will consist substantially of water.

(8) 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 applications in areas such as power stations, cement manufacture, iron and steel manufacture, glass making, brewing, syngas processes, natural gas and biogas purification and other chemical process such as ammonia production, hydrogen production, power-to-gas (e.g. Sabatier reaction) processes, as well as any other acid gas producing industrial, commercial or domestic processes. In a particular application, the defined method may be applied to the capture of carbon dioxide directly from the atmosphere.

(9) 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.

(10) 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 carbon dioxide is known as decarboxylation. This carboxylation/decarboxylation process is key to effective carbon dioxide capture and capture formulation regeneration.

(11) The present invention also envisages the use of the disclosed formulations in the capture and subsequent release of other acid gases, for example hydrogen sulfide or sulfur dioxide, in applications such as natural gas sweetening, biogas upgrading and desulfurisation. Removal of NO.sub.x species from, for example, the waste gases from a combustion process may be an additional application.

(12) In specific embodiments of the invention, systems are provided wherein carbon dioxide is captured by use of formulations which comprise potassium acetate, potassium propionate, potassium iso-butyrate and potassium hexanoate in water either solely or in combination with each other or in combination with, for example, potassium carbonate or potassium phenolate. The use of such systems facilitates the highly efficient capture of the gas at ambient temperature and pressure.

(13) Thus, the inventors have conducted a series of trials utilising equipment adapted for the measurement of vapour-liquid equilibria (VLE). Specifically a system was provided which comprised a jacketed stainless steel vessel with the jacket connected to a temperature controlled circulating bath and the vessel equipped with two temperature probes (one for monitoring vapour temperature, one for solvent temperature), a 0-7 Bara pressure transducer, a safety release valve and emergency rupture disc, a vacuum port for removing the internal atmosphere, a carbon dioxide inlet, an air inlet and gas entraining mechanical stirring. Carbon dioxide is supplied from a temperature and pressure monitored high pressure burette via a regulator to control the internal pressure of the VLE vessel and thus the partial pressure of carbon dioxide.

(14) TABLE-US-00003 TABLE 3 Maximum absorption rate and capacity for carboxylate salts Maximum Absorption Maximum CO.sub.2 Rate [Salt] Capacity (mol.sub.CO2/ Entry Salt (mol/L) (mol/L) L/Bar/h) 1 Potassium acetate 8 1.24 2.5 2 Potassium acetate 9 1.84 1.6 3 Potassium propionate 7 1.77 2.9 4 Potassium iso-butyrate 7 1.97 2.5 5 Potassium valerate 4 0.37 6.2 6 Potassium hexanoate 2.5 0.30 6.6 7 Potassium octanoate 2.4 0.45 3.5 150 mL of aqueous solution in the VLE vessel under 1 Bar of CO.sub.2 at initial temperature of 20° C.

(15) From the data collected (150 mL of aqueous solution in the VLE vessel under 1 Bar of CO.sub.2 at initial temperature of 20° C. Table 3), for a given formulation, it is possible to determine the absolute rate of carbon dioxide absorption, the rate of carbon dioxide absorption as a function of carbon dioxide partial pressure, the overall carbon dioxide capacity as a function of temperature and/or carbon dioxide partial pressure. Combining data from a series of experiments for a particular formulation allows predictions to be made regarding the performance of said formulation in a real process.

(16) In order to demonstrate the highly effective nature of the systems, experimental campaigns were undertaken with a broad range of single carboxylate salts, mixed carboxylate salts and other capture agents accelerated by carboxylate salts all in aqueous solution.

(17) From the data collected with the VLE, it is possible to draw a graph of maximum solution capacity vs. water:salt molar ratio (FIG. 1). As can be seen, there is a significant increase in the capacity of these formulations once the water:salt molar ratio drops below approximately 6. This supports the claim that the contribution of the salt to the overall solvent character of the formulation is an important factor when determining the apparent pKa of carboxylate salts in such formulations.

(18) The efficacy of stripping from these formulations is boosted by the mechanism of acid gas absorption. Under these conditions, the thermodynamics of the reaction between hydrated carbon dioxide and the carboxylates salts is more finely balanced than for more conventional capture agents, allowing regeneration to be less energy intensive. Experiments undertaken in the VLE with 7M potassium acetate in water at a variety of temperatures prove this to be the case. With a ΔT of just 20° C. between the absorber and the desorber, in principle a cyclic carbon dioxide capacity of nearly 0.6 mol.sub.CO2/L is possible (150 mL of 7M KOAc solution in VLE under 1 Bar CO2 Table 4).

(19) TABLE-US-00004 TABLE 4 Maximum CO.sub.2 capacity of 7M KOAc solutions at different absorption temperatures Temperature Max Capacity Entry (° C.) (mol/L) 1 10 1.01 2 20 0.69 3 30 0.43 150 mL of 7M KOAc solution in VLE under 1 Bar CO.sub.2

(20) Varying the pressure of carbon dioxide during absorption has an influence on both the absolute rate of carbon dioxide absorption as well as the maximum capacity of the solution. To ensure that the herein disclosed method is generally applicable, experiments have been undertaken at partial pressures of carbon dioxide both above and below 1 Bar (150 mL of solution in the VLE at 20° C. under pressure of CO2 Table 5).

(21) TABLE-US-00005 TABLE 5 Effect of CO.sub.2 pressure on absolute absorption rate and capacity CO.sub.2 Absolute Maximum Pressure Rate Capacity Entry Solution (Bar) (mM.sub.CO2/s) (mol.sub.CO2/L) 1 7M Potassium acetate 1 0.6 0.69 2 7M Potassium acetate 2 1.3 1.05 3 7M Potassium acetate 3 2.2 1.28 4 7M Potassium acetate 4 3.0 1.48 5 7M Potassium iso-butyrate 0.3 0.15 1.61 6 7M Potassium iso-butyrate 1.0 0.7 1.97 7 7M Potassium iso-butyrate 2.0 1.5 2.35 150 mL of solution in the VLE at 20° C. under pressure of CO.sub.2

(22) The variation in CO.sub.2 capacity of these formulations as a function of CO.sub.2 partial pressure demonstrates that the formulations may be used as physical solvents as well as chemical solvents. The advantage such a system configuration would present would be to have two CO.sub.2 absorption mechanisms, both a physical and a chemical one, working in tandem. This would reduce the overall energy intensity of the CO.sub.2 separation process either by increasing the CO.sub.2 capacity of the working fluid at a given absorption pressure or by reducing the working absorption pressure necessary to achieve a given CO.sub.2 capacity.

(23) Combining two or more salts in the same solutions can have unexpected effects. For example, a solution containing 1.5M potassium hexanoate and 4M potassium iso-butyrate has a maximum capacity of 1.3 mol.sub.CO2/L, significantly greater than either individual component or the sum of their individual capacities. Similarly, a blend of 2M potassium hexanoate with 4M potassium acetate shows greater capacity than either component or the sum of its parts, at a greater absorption rate than would be expected from either component. This is demonstrated in FIG. 2. As can be extrapolated from FIG. 2, 2M potassium hexanoate would be expected to have a capacity of about 0.2 to 0.25M CO.sub.2 and 4M potassium acetate a capacity of about 0.1 to 0.15M; while FIG. 2 indicates that a blend of 2M potassium hexanoate with 4M potassium acetate has a capacity of at least 1M CO.sub.2.

(24) There is a relationship between the total number of carbon atoms in the carboxylate salt and the maximum observed rate. As can be seen, this peaks at around five or six carbon atoms in the carboxylate salt (FIG. 3).

(25) This can be exploited by blending faster carboxylate salts with a second component that is basic enough to effect deprotonation of the carboxylic acid as it forms and holding the captured carbon dioxide as bicarbonate in solution. Specific examples include carbonate and phenolate salts of, for example, lithium, sodium, potassium, calcium or magnesium. Adding 2 mol/L potassium hexanoate to a 2M solution of potassium carbonate has little impact on the overall capacity of the solution but increases the rate of carbon dioxide absorption by more than a factor of two (FIG. 4).

(26) Similarly, a solution consisting of 2.5M potassium hexanoate and 1M potassium phenolate exhibits a faster rate of absorption (+25%) and greater capacity (+100%) as compared to a 2.5M solution of potassium hexanoate alone.

(27) This accelerating effect extends to other capture agents that might otherwise be considered “slow”. Adding 1M potassium hexanoate to a 5M solution of 2-amino-2-methyl-1-propanol (AMP) increases the rate of carbon dioxide absorption by approximately 30%. This effect remains after a full absorption and release cycle for a second absorption.

(28) Carboxylate solutions themselves can have their rate of carbon dioxide absorption accelerated by the usual accelerants known to those skilled in the art. For example, including 250 mg/L of carbonic anhydrase in a 7M solution of potassium acetate increases the rate of carbon dioxide absorption by approximately 15%.