RATE ENHANCEMENT OF GAS CAPTURE PROCESSES

Abstract

The present invention relates to a method of capturing CO.sub.2 from a gas stream. The method uses a two liquid phase capture composition.

Claims

1. A method for capturing CO.sub.2 from a gas stream, the gas stream containing CO.sub.2, the method comprising: contacting the gas stream containing CO.sub.2 with a capture composition in a gas-liquid contacting apparatus to generate a loaded capture composition, the capture composition comprising: i. a first liquid phase comprising at least one capture reagent; and ii. a second liquid phase; wherein the second liquid phase is an effective solvent for CO.sub.2 and is chemically inert to CO.sub.2; wherein the first liquid phase comprises an aqueous solution of at least one salt of at least one carboxylic acid; and wherein the method further comprises releasing said CO.sub.2 from the loaded capture composition.

2. A method of claim 1, wherein the step of contacting the gas stream with the capture composition is carried out in a gas-liquid contacting apparatus selected from: a packed column (with random or structured packings, in co-flow, counter-flow, or crossflow configurations), a spray tower, a plate or tray column, a stirred tank reactor (in either continuous or batch configuration), a tubular flow reactor (under either laminar or turbulent flow condition), a bubble column reactor, a falling film reactor, or a membrane contactor.

3. A method of claim 1, wherein the cation of the at least one salt of at least one carboxylic acid is an alkali metal, an alkali earth metal or a mixture thereof.

4. A method of claim 3, wherein the at least one carboxylic acid comprises only carbon, hydrogen and oxygen.

5. A method of claim 4, wherein the at least one carboxylic acid is at least one C.sub.1-C.sub.8 aliphatic carboxylic acid.

6. A method of claim 5, wherein the at least one carboxylic acid corresponding to the at least one carboxylate salt are chosen from a list that includes acetic acid, propanoic acid, butyric acid and its branched derivative, pentanoic acid and its branched derivatives, hexanoic acid and its branched derivatives, heptanoic acid and its branched derivatives, and octanoic acid and its branched derivatives.

7. A method of claim 1, wherein the first liquid phase further comprises at least one carbonate salt.

8. A method of claim 7, wherein the at least one carbonate salt is chosen from a list that includes alkali metal carbonates, alkali earth metal carbonates or a mixture thereof.

9. A method of claim 1, wherein the first liquid phase further comprises an enzyme.

10. A method of claim 9, wherein the enzyme is a natural carbonic anhydrase or an engineered carbonic anhydrase.

11. A method of claim 1, wherein the at least one capture reagent is present in the first liquid phase at a concentration in the range 2 M to 15 M.

12. A method of claim 1, wherein the second liquid phase is an organic solvent.

13. A method of claim 12, wherein the second liquid phase is chosen from a list that includes silicones/siloxanes and ethers.

14. A method of claim 12, wherein the second liquid phase is a solvent selected from: 1,2-dimethoxypropane, 1,2-diethoxypropane, dipropylene glycol dimethyl ether, dipropylene glycol diethyl ether, dipropylene glycol methyl ethyl ether, tripropylene glycol dimethyl ether, tripropylene glycol diethyl ether, tripropylene glycol methyl ethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethylene glycol methyl ethyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol methyl ethyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol methyl ethyl ether, di-isopropyl ether, dibutyl ether, ethyl butyl ether, methyl tert-butyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, octamethyltrisiloxane, decamethyltetrasiloxane, hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, butyl acetate, pentyl acetate, pentyl propionate, hexyl propionate, hexyl butyrate, heptyl butyrate, gamma-butyrolactone, gamma-octanoic lactone, 2-pentanone, 3-heptanone, 4-octanone, hexanal, heptanal, octanal, decanal.

15. A method of claim 1, wherein the ratio of the first liquid phase to the second liquid phase may be in the range 1:3 to 9:1 by volume.

16. A method of claim 1, wherein the physical solubility of CO.sub.2 in the second liquid phase is greater than the physical solubility of CO.sub.2 in the first liquid phase.

17. A method of claim 1, wherein the gas stream comprising CO.sub.2 comprises emissions from a combustion process.

18. A method of claim 1, wherein said CO.sub.2 is released by: (i) heating the loaded capture composition; and/or (ii) subjecting the loaded capture composition to a stream of stripping gas, for example air; and/or (iii)reducing the pressure above the loaded capture composition to provide a stripped capture composition.

19. A method for capturing CO.sub.2 from a gas stream, the gas stream containing CO.sub.2, the method comprising: contacting the gas stream containing CO.sub.2 with a capture composition in a gas-liquid contacting apparatus to generate a loaded capture composition, the capture composition comprising: i. a first liquid phase, said first liquid phase comprising an aqueous solution of an alkali metal salt of a C.sub.2-C.sub.5 aliphatic carboxylic acid; wherein the solution has a concentration such that the molar ratio of salt:water in the range 1:2.5 to 1:15; and ii. a second liquid phase, said second liquid phase comprising a solvent of formula (I) or a mixture of more than one solvent of formula (I): ##STR00004## wherein R.sup.1 and R.sup.3 are each independently unsubstituted C.sub.1-C.sub.4 alkyl; R.sup.2 is independently at each occurrence selected from H and Me; and n is an integer selected from 1, 2, 3 and 4.

20. A method of claim 19, wherein the molar ratio of salt:water is in the range of 1:2.5 to 1:5.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0140] FIG. 1 CO.sub.2 absorption rate as a function of Reynolds number for example 11.

DETAILED DESCRIPTION OF THE INVENTION

[0141] The term “alkyl” refers to a linear or branched hydrocarbon chain. For example, the term “C.sub.1-.sub.6 alkyl” refers to a linear or branched hydrocarbon chain containing 1, 2, 3, 4, 5 or 6 carbon atoms, for example methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, sec-butyl, tert-butyl, n-pentyl and n-hexyl. The alkyl group may be unsubstituted.

[0142] The term “aliphatic carboxylic acid” refers to a carboxylic acid comprising a CO.sub.2H attached to an alkyl group. It may refer to a carboxylic acid comprising a CO.sub.2H attached to an unsubstituted alkyl group.

[0143] For a chemical reaction in a liquid phase involving one or more gaseous reagents, the rate of that reaction can be governed, at least in part, by the availability of the gaseous reagent(s) in said liquid phase. As per Henry’s law, the equilibrium concentration of a gas physically dissolved into a liquid phase is proportional to its partial pressure above the liquid, with the proportionality factor called the Henry’s law constant. In other words, the physical solubility of a gas in a liquid phase is its concentration in the gas phase multiplied by some number, the Henry’s law constant, that accounts for the nature of the gaseous species, the nature of the liquid phase and the temperature at which the measurement is being taken.

[0144] For carbon capture applications the concentration of CO.sub.2 in the gas stream to be treated will be dependent on the nature of the process generating said gas stream. For many potential applications of carbon capture technology, the gas stream to be treated is the product of an air-breathing combustion process and as a result may contain relatively little CO.sub.2 (Table 1). Even the waste gases from activities considered to be highly carbon intensive, such as coal-fired power generation (Table 1, Entry 4) or cement manufacture (Table 1, Entry 8), contain relatively low volumetric concentrations of CO.sub.2. Capturing the CO.sub.2 from such sources can be technically challenging, necessitating the use of large scale absorbers, adding to the capital cost of a capture plant, or mechanical process intensification, adding to the energy penalty of the capture process.

TABLE-US-00001 Typical CO.sub.2 content in the flue gas for various potential CCS applications Entry Process Typical CO.sub.2 Concentration in Stack Gas (%v/v, dry basis) CO.sub.2 Concentration in Stack Gas (mmol/L, dry basis).sup.a 1 Power generation (natural gas fired boiler) 7-10 2.9-4.1 2 Power generation (gas turbine) 3-4 1.2-1.6 3 Power generation (oil fired boiler) 11-13 4.5-5.3 4 Power generation (coal fired boiler) 12-14 4.9-5.7 5 Power generation (IGCC -after combustion) 12-14 4.9-5.7 6 Oil refinery & petrochemical plant gas fired heaters 8 3.3 7 Blast furnace gas 20-27 8.2-11.1 8 Cement kiln off-gas 14-33 5.7-13.5 9 IGCC (syngas after gasification) 8-20 3.3-8.2 .sup.acalculated using pV = nRT for p = 1 Bar, T = 293.15 K

[0145] An effective solvent for CO.sub.2, as used in this specification, means one which has a Henry’s law constant for CO.sub.2 such that the equilibrium volumetric concentration of CO.sub.2 found in the solvent under a given partial pressure of CO.sub.2 is greater than the volumetric concentration of CO.sub.2 found in the gas phase at the same partial pressure. The critical Henry’s law constant value can be calculated to be approximately 0.041 mol/L/Bar at 20° C. (Table 2). In these circumstances, a solvent with a Henry’s law constant greater than this value may be deemed to be an effective solvent for CO.sub.2, a solvent with a lower Henry’s law constant may not be deemed to be an effective solvent for CO.sub.2.

TABLE-US-00002 Equilibrium concentrations of CO.sub.2 in the liquid phase for solvents with Henry’s law constants at, above and below the critical value to be considered an effective solvent for CO.sub.2 in these circumstances at different partial pressures of CO.sub.2 Entry P.sub.CO2 (kPa) [CO.sub.2].sub.GAS (mmol/L).sup.a [CO.sub.2].sub.LIQUID (mmol/L) for Henry’s law constant of 0.02 mol/L/Bar 0.041 mol/L/Bar 0.10 mol/L/Bar 1 5 2.05 1.00 2.05 5.00 2 15 6.15 3.00 6.15 15.00 3 25 10.26 5.00 10.26 25.00 4 50 20.51 10.00 20.51 50.00 5 100 41.03 20.00 41.02 100.00 .sup.acalculated using pV = nRT for p = 1 Bar, T= 293.15K

[0146] The generality of this approach in terms of combinations of different first and second liquid phases will be illustrated with various examples as described below. All of these examples were carried out in our laboratory using our vapour-liquid equilibria (VLE) apparatus which consists of a stirred, jacketed, stainless steel vessel equipped with temperature and pressure sensors. The composition being trialled is brought to test temperature in the vessel and a given partial pressure of CO.sub.2 is added to the gas space above the composition. The CO.sub.2 is supplied from another temperature and pressure monitored reservoir and the partial pressure of CO.sub.2 in the reaction vessel maintained by a regulator. From the temperature and pressure data of the CO.sub.2 reservoir recorded as a function of time, the rate of CO.sub.2 absorption and overall amount of CO.sub.2 absorbed can be calculated. For each of the examples below, the salt(s) and water were combined in the desired ratio and the organic solvent added until a second liquid phase emerged. The composition was then run in the VLE apparatus twice; firstly with a single phase composition in which the organic solvent is present at an amount in which it is completely dissolved in the aqueous salt solution then secondly with the organic solvent present at an amount at which it forms a second liquid phase as well as being dissolved in the aqueous salt solution. In all cases, the runs in which the composition has two liquid phases were carried out with a 1:1 liquid phase ratio by volume. Absorption rates are reported as moles of CO.sub.2 absorbed per litre of composition per Bar of CO.sub.2 partial pressure per hour.

[0147] Example 1: Potassium propionate and water in a molar ratio of 1 to 3 with methyl isobutyrate. Single phase absorption experiment exhibited a maximum absorption rate of 2 mol/L/Bar/h. Two liquid phase absorption experiment exhibited a maximum absorption rate of 5 mol/L/Bar/h.

[0148] Example 2: Potassium acetate and water in a molar ratio of 1 to 3 with diethylene glycol dimethyl ether. Single phase absorption experiment exhibited a maximum absorption rate of 2.5 mol/L/Bar/h. Two liquid phase absorption experiment exhibited a maximum absorption rate of 6.5 mol/L/Bar/h.

[0149] Example 3: 7 M Potassium acetate in water with methyl isobutyrate. Single phase absorption experiment exhibited a maximum absorption rate of 2 mol/L/Bar/h. Two liquid phase absorption experiment exhibited a maximum absorption rate of 6.5 mol/L/Bar/h.

[0150] Example 4: Potassium acetate and water in a molar ratio of 1 to 3 with polymethylhydrosiloxane. Single phase absorption experiment exhibited a maximum absorption rate of 1.5 mol/L/Bar/h. Two liquid phase absorption experiment exhibited a maximum absorption rate of 4.5 mol/L/Bar/h.

[0151] Example 5: Potassium propionate and water in a molar ratio of 1 to 3.25 with 2-pentanone. Single phase absorption experiment exhibited a maximum absorption rate of 4 mol/L/Bar/h. Two liquid phase absorption experiment exhibited a maximum absorption rate of 8 mol/L/Bar/h.

[0152] Example 6: Potassium propionate and water in a molar ratio of 1 to 3.25 with butyl acetate. Single phase absorption experiment exhibited a maximum absorption rate of 4 mol/L/Bar/h. Two liquid phase absorption experiment exhibited a maximum absorption rate of 8 mol/L/Bar/h.

[0153] Example 7: Potassium propionate and water in a molar ration of 1 to 3.25 with cyclohexanone. Single phase absorption experiment exhibited a maximum absorption rate of 2.5 mol/L/Bar/h. Two liquid phase absorption experiment exhibited a maximum absorption rate of 5.5 mol/L/Bar/h.

[0154] Example 8: Potassium propionate and water in a molar ratio of 1 to 3.25 with diethylene glycol diethyl ether. Single phase absorption experiment exhibited a maximum absorption rate of 0.5 mol/L/Bar/h. Two liquid phase absorption experiment exhibited a maximum absorption rate of 7 mol/L/Bar/h.

[0155] Example 9: Potassium propionate and water in a molar ration of 1 to 3.25 with ethyl acetate. Single phase absorption experiment exhibited a maximum absorption rate of 4 mol/L/Bar/h. Two liquid phase absorption experiment exhibited a maximum absorption rate of 6.5 mol/L/Bar/h.

[0156] Example 10: 0.6 M Potassium hexanoate and 4 M potassium carbonate in water with cyclohexanone. Single phase absorption experiment exhibited a maximum absorption rate of 3.5 mol/L/Bar/h. Two liquid phase absorption experiment exhibited a maximum absorption rate of 6 mol/L/Bar/h.

[0157] Example 11: This final example provided herein is to demonstrate that the effect is general across a wide range of stirring rates, i.e. mechanical inputs. The experiments were run in the same way as in examples 1 through 10 except that for this final example, the absorption was run multiple times with the stirring rate being varied between runs. The Reynolds number was calculated for each run and a graph plotted (FIG. 1) showing the maximum absorption rate (in mol/L/Bar/h as above) as a function of Reynolds number. As can be seen, the accelerating effect of the second liquid phase is general across a wide range of stirring rates/Reynolds number. It should be noted that the axes of the graph are logarithmic so that even at the highest Reynolds number achievable in our apparatus (Re = ca. 10,000), the rate of absorption into the biphasic system was approximately twice the rate of absorption into the monophasic system.

[0158] The composition used for this final example was potassium propionate and water in a molar ratio of 1 to 3.25 with butyl acetate as the second liquid phase.