Removal of carbon dioxide from a gas stream by using aqueous ionic liquid

Abstract

The present invention relates to the separation of gases, and more specifically to an inventive process for the removal of carbon dioxide gas using carefully selected ionic liquid absorbents together with water in a carefully selected ratio.

Claims

1. A process for removing carbon dioxide from a gaseous stream comprising contacting the gaseous stream with a carbon dioxide absorbent comprising a mixture of an ionic liquid and water in a molar ratio of from 55:45 to 45:55 and recovering a gaseous stream having reduced carbon dioxide content, wherein the ionic liquid has the formula
[Cat+][X] wherein [Cat+] represents a cationic species selected from tetrasubstituted phosphonium cations, tetrasubstituted ammonium cations, trisubstituted sulfonium cations, guanidinium cations and quinolinium cations; and [X] represents an anionic species which is a conjugate base of a carboxylic acid; and wherein [Cat+] is selected from: (i)[P(Ra)(Rb)(Rc)(Rd)].sup.+, [N(Ra)(Rb)(Rc)(Rd)].sup.+, and [S(Rb)(Rc)(Rd)].sup.+, wherein Ra, Rb, Rc, and Rd are each independently selected from a C1 to C20 straight chain or branched alkyl group, a C3 to C8 cycloalkyl group, or a C6 to C10 aryl group, or wherein any two of Ra, Rb, Rc, and Rd together form a methylene chain of the formula (CH.sub.2).sub.q, where q is an integer of from 4 to 7, and wherein said alkyl, cycloalkyl or aryl groups, or said methylene chain are unsubstituted; (ii) quinolinium cations of the formula ##STR00003## wherein Ra, Rb, Rc, Rd, Re, Rf, Rg, Rh, and Ri are each independently selected from hydrogen, a C1 to C20 straight chain or branched alkyl group, a C3 to C8 cycloalkyl group, or a C6 to C10 aryl group, or any two of Rb, Rc, Rd, Re, Rf, Rh and Ri attached to adjacent carbon atoms may form a saturated methylene chain (CH.sub.2).sub.q wherein q is from 3 to 6, and wherein said alkyl, cycloalkyl or aryl groups, or said methylene chain are unsubstituted; or (iii) guanidinium cations of the formula ##STR00004## wherein Ra, Rb, Rc, Rd, Re, and Rf are each independently selected from a C1 to C20 straight chain or branched alkyl group, a C3 to C8 cycloalkyl group, or a C6 to C10 aryl group, or any two of Rb, Rc, Rd, Re, Rf, Rh, and Ri attached to adjacent carbon atoms may form a saturated methylene chain (CH.sub.2).sub.q wherein q is from 3 to 6, and wherein said alkyl, cycloalkyl, or aryl groups, or said methylene chain are unsubstituted.

2. A process according to claim 1, wherein [Cat+] is selected from [P(Ra)(Rb)(Rc)(Rd)]+ and [N(Ra)(Rb)(Rc)(Rd)]+ and wherein Ra, Rb, Rc, and Rd are as defined in claim 1.

3. A process according to claim 1, wherein [Cat+] comprises [P(Ra)(Rb)(Rc)(Rd)]+ and wherein Ra, Rb, Rc, and Rd are as defined in claim 1.

4. A process according to claim 1, wherein Ra, Rb, Rc, and Rd are each independently selected, where present, from a C1 to C16 straight chain or branched alkyl group, or wherein any two of Ra, Rb, Rc, and Rd together form a methylene chain of the formula (CH2)q, where q is an integer of from 4 or 5.

5. A process according to claim 1, wherein [X] comprises an anion having the formula [RxCO2], wherein Rx is selected from hydrogen, a C1 to C10 straight chain or branched alkyl group, a C3 to C8 cycloalkyl group, or a C6 to C10 aryl group, wherein said alkyl, cycloalkyl or aryl groups are unsubstituted or may be substituted by one or more groups selected from F, Cl, OH, CN, NO2, SH, O, and CO2H.

6. A process according to claim 5, wherein [X] is selected from formate, acetate, trifluoroacetate, hydroxyacetate, propanoate, pentafluoropropanoate, lactate, butanoate, isobutanoate, pivalate, pyruvate, thiolactate, oxalate, tartrate, malonate, succinate, adipate, and benzoate.

7. A process according to claim 6, wherein [X] is selected from formate, acetate, trifluoroacetate, hydroxyacetate, propanoate, pentafluoropropanoate, lactate, butanoate, isobutanoate, and pivalate.

8. A process according to claim 7, wherein [X] is selected from formate or acetate.

9. A process according to claim 1, wherein the ionic liquid has a melting point of 100 C. or less.

10. A process according to claim 9, wherein the ionic liquid has a melting point of 50 C. or less.

11. A process according to claim 10, wherein the ionic liquid has a melting point of 25 C. or less.

12. A process according to claim 1, wherein the molar ratio of ionic liquid to water is about 50:50.

13. A process according to claim 1, wherein the gaseous stream is contacted with the carbon dioxide absorbent at a temperature of from 10 to 80 C.

14. A process according to claim 13, wherein the gaseous stream is contacted with the carbon dioxide absorbent at a temperature of from 10 to 50 C.

15. A process according to claim 14, wherein the gaseous stream is contacted with the carbon dioxide absorbent at a temperature of from 20 to 30 C.

16. A process according to claim 1, wherein the gaseous stream is contacted with the carbon dioxide absorbent at a pressure of from 100 to 2000 kPa.

17. A process according to claim 16, wherein the gaseous stream is contacted with the carbon dioxide absorbent at a pressure of from 200 to 1000 kPa.

18. A process according to claim 1, wherein carbon dioxide is subsequently released from the carbon dioxide absorbent.

19. A process according to claim 18, wherein the carbon dioxide is subsequently released by subjecting the carbon dioxide absorbent to reduced pressure, or by sparging the carbon dioxide absorbent with a gas at elevated temperature.

20. A process according to claim 1, wherein the gaseous stream is a hydrocarbon-containing gaseous stream.

21. A process according to claim 20, wherein the gaseous stream is a methane-containing gaseous stream.

22. A process according to claim 21, wherein the gaseous stream is a natural gas stream.

23. A process according to claim 21, wherein the gaseous stream is a biogas-derived stream.

24. A process according to claim 1, wherein the gaseous stream is a flue gas stream.

25. A process according to claim 1, wherein the gaseous stream is a breathing gas stream for a life support system.

26. A process according to claim 17, wherein the gaseous stream is contacted with the carbon dioxide absorbent at a pressure of about 500 kPa.

27. A process for removing carbon dioxide from a gaseous stream comprising contacting the gaseous stream with a carbon dioxide absorbent comprising a mixture of an ionic liquid and water in a molar ratio of from 55:45 to 45:55 and recovering a gaseous stream having reduced carbon dioxide content, wherein the ionic liquid has the formula
[Cat+][X] wherein [Cat+] represents a cationic species selected from tetrasubstituted phosphonium cations, tetrasubstituted ammonium cations, trisubstituted sulfonium cations, guanidinium cations, and quinolinium cations; and [X] represents an anionic species which is a conjugate base of a carboxylic acid; and wherein: (i) [Cat.sup.+] is selected from [P(Ra)(Rb)(Rc)(Rd)].sup.+, [N(Ra)(Rb)(Rc)(Rd)].sup.+, and [S(Rb)(Rc)(Rd)].sup.+, wherein Ra, Rb, Rc, and Rd are each independently selected from a C1 to C20 straight chain or branched alkyl group, a C3 to C8 cycloalkyl group, or a C6 to C10 aryl group, or wherein any two of Ra, Rb, Rc, and Rd together form a methylene chain of the formula (CH.sub.2).sub.q, where q is an integer of from 4 to 7, and wherein said alkyl, cycloalkyl or aryl groups, or said methylene chain are unsubstituted; (ii) [Cat.sup.+] is selected from quinolinium cations of the formula ##STR00005## wherein Ra, Rb, Rc, Rd, Re, Rf, Rg, Rh, and Ri are each independently selected from hydrogen, a C1 to C20 straight chain or branched alkyl group, a C3 to C8 cycloalkyl group, or a C6 to C10 aryl group, or any two of Rb, Rc, Rd, Re, Rf, Rh and Ri attached to adjacent carbon atoms may form a saturated methylene chain (CH.sub.2).sub.q wherein q is from 3 to 6, and wherein said alkyl, cycloalkyl or aryl groups, or said methylene chain are unsubstituted; or (iii) [Cat.sup.+] is selected from guanidinium cations of the formula ##STR00006## wherein Ra, Rb, Rc, Rd, Re, and Rf are each independently selected from a C1 to C20 straight chain or branched alkyl group, a C3 to C8 cycloalkyl group, or a C6 to C10 aryl group, or any two of Rb, Rc, Rd, Re, Rf, Rh and Ri attached to adjacent carbon atoms may form a saturated methylene chain (CH.sub.2).sub.q wherein q is from 3 to 6, and wherein said alkyl, cycloalkyl or aryl groups, or said methylene chain are unsubstituted; and subsequently releasing the carbon dioxide from the carbon dioxide absorbent by at least one of (i) subjecting the carbon dioxide absorbent to reduced pressure or (ii) sparging the carbon dioxide absorbent with a gas at elevated temperature.

28. A process according to claim 1, wherein the step of contacting the gaseous stream with a carbon dioxide absorbent in the process further comprises removing water from the gaseous stream, and wherein the process further comprises subsequently releasing water from the absorbent by drying at elevated temperature or by sparging with a dry gas at elevated temperature.

29. A process according to claim 27, wherein the step of contacting the gaseous stream with a carbon dioxide absorbent in the process further comprises removing water from the gaseous stream, and wherein the process further comprises subsequently releasing water from the absorbent by drying at elevated temperature or by sparging with a dry gas at elevated temperature.

30. A process according to claim 27, wherein the carbon dioxide is subsequently released by sparging the carbon dioxide absorbent with a gas at elevated temperature.

31. A process according to claim 27, wherein the carbon dioxide is released from the carbon dioxide absorbent by subjecting the carbon dioxide absorbent to reduced pressure, wherein subsequent to the step of contacting the gaseous stream with the carbon dioxide absorbent, and prior to the step of subjecting the carbon dioxide absorbent to reduced pressure, the absorbent is not exposed to pressures higher than the pressure used during the contacting step.

Description

(1) The present invention will now be described by way of Examples, and with reference to the attached figures, wherein:

(2) FIG. 1 is a histogram showing the carbon dioxide solubility in the ionic liquid/water mixtures described in Example 1 at 500 kPa and at 25 C. FIG. 1 also shows the carbon dioxide solubilities of the comparative absorbent systems discussed in Examples 3, 4 and 5;

(3) FIG. 2 shows the variation of carbon dioxide solubility with water content for a variety of ionic liquids;

(4) FIG. 3 is the FTIR spectrum of nitrogen gas saturated with water;

(5) FIG. 4 is the FTIR spectrum of the nitrogen gas of FIG. 3 after being bubbled through an absorbent ionic liquid composition;

(6) FIG. 5 shows the CO.sub.2 uptake of a mixture of tributylmethylphosphonium propanoate and water in an ionic liquid:water ratio of 40:60 (see Example 9); and

(7) FIG. 6A shows the IR spectrum of air spiked with CO.sub.2. FIG. 6B shows the corresponding IR spectrum when the air spiked with CO.sub.2 has been bubbled through tributylmethylphosphonium propanoate and water in an ionic liquid;water ratio of 40:60 (see Example 10).

EXAMPLES

Example 1

(8) This example describes the general experimental method used to determine the solubility of carbon dioxide in the ionic liquid water mixtures.

(9) In a typical experiment, the volume of a pressure vessel [Parr pressure system] was first determined by evacuating it under reduced pressure and subsequently pumping a known amount of gas at a certain temperature and pressure into the vessel. Measurement of the amount of gas was read as the volume of gas at standard conditions from the mass flow controller [BROOKS Smart Massflow]. The ideal gas law was used to calculate the actual volume of the pressure vessel.

(10) A known volume of a tetraalkylphosphonium ionic liquid having the formula ([P.sub.nnnm][R.sup.xCO.sub.2], where n and m are integers which indicate the number of carbon atoms in the alkyl chain, and R.sup.x is hydrogen or a C.sub.1 to C.sub.10 alkyl) and water (ca. 5.0 mL) was placed in the pressure vessel, degassed for 5 min under reduced pressure. The carbon dioxide was then pumped into the stirred pressure vessel (1000 rpm) through the mass flow controller up to 500 kPa and at 25.0 C. The system was allowed to equilibrate for 15 min or until no more gas was added according to the mass flow controller.

(11) Calculation of the total amount of gas introduced in the pressure vessel was conducted from the reading in the mass flow controller. The actual amount of gas in the gas phase was calculated by the ideal gas law, where the volume of the gas phase was equal to the volume of the pressure vessel minus the volume of the liquid phase. The amount of gas dissolved in the liquid phase was calculated by subtracting the actual amount of gas in the gas phase from the total amount of gas introduced in the pressure vessel.

(12) Results expressed as a molar concentration (mol.Math.L.sup.1) and as SCF/100 gal are shown in Table 1, where mole fraction water indicates the mole fraction of water in the mixture of ionic liquid and water The amount of water in the liquid mixtures was quantified by Karl-Fischer titration, and/or .sup.1H NMR.

(13) TABLE-US-00001 TABLE 1 Mole Solubility Solubility Ionic Liquid fraction water (mol .Math. L.sup.1) (SCF/100 gal) P.sub.2,2,2,8 formate 0.50 1.900 600.34 P.sub.4,4,4,4 formate 0.51 2.070 654.05 P.sub.4,4,4,6 formate 0.50 1.66 538.47 P.sub.4,4,4,8 formate 0.53 1.877 605.47 P.sub.4,4,4,10 formate 0.53 2.149 692.46 P.sub.4,4,4,12 formate 0.52 1.464 473.19 P.sub.6,6,6,14 formate 0.48 1.100 347.56 P.sub.2,2,2,8 acetate 0.53 2.480 783.6 P.sub.4,4,4,6 acetate 0.49 2.137 689.26 P.sub.4,4,4,8 acetate 0.51 2.067 666.34 P.sub.4,4,4,10 acetate 0.49 2.753 886.26 P.sub.4,4,4,12 acetate 0.48 2.307 743.29

Example 2

Solubility of Carbon Dioxide in Recycled Tetraalkylphosphonium Carboxylate/Water Mixtures

(14) The mixtures of ionic liquids and water containing absorbed carbon dioxide from Example 1 were recycled by stirring in a glass round bottom flask attached to a reflux condenser while N.sub.2 gas was bubbled through the solution for 15 min at 60 to 70 C. The water content of the resulting solutions was determined by Karl-Fischer titration and, if needed, additional water was added up to the desired composition. Solubility of carbon dioxide in the resulting liquid mixtures of [P.sub.4444][HCO.sub.2], and water was measured as described in Example 1. After a first recycle, the solubility of carbon dioxide was found to be 2.01 mol.Math.L.sup.1. After a second recycle, the solubility of carbon dioxide was found to be 2.07 mol.Math.L.sup.1. Thus, it can be observed that the ability of the absorbent to absorb carbon dioxide is not reduced after recycling.

Comparative Example 3

Solubility of Carbon Dioxide in 1-butyl-3-methylimidazolium bis[(trifluoromethyl)-sulfonyl]imide

(15) The solubility of carbon dioxide in dry 1-butyl-3-methylimidazolium bis[(trifluoromethyl)-sulfonyl]imide ([bmim][NTf.sub.2]) was measured at 500 kPa and at 25.0 C. as described in Example 1. This ionic liquid was chosen as a comparative example of a strictly physical CO.sub.2 absorber. The solubility of carbon dioxide in the ionic liquid was found to be 0.594 mol.Math.L.sup.1. By way of a further comparison, the solubility of carbon dioxide in Genosorb, a commercially available absorbent solvent based on polyethylene glycol dimethyl ethers. The solubility of carbon dioxide in Genosorb was found to be 0.64 mol.Math.L.sup.1.

Comparative Example 4

Solubility of Carbon Dioxide in Dry [P.SUB.66614.][CH.SUB.3.CO.SUB.2.]

(16) The solubility of carbon dioxide in dry [P.sub.66614][CH.sub.3CO.sub.2] (<0.25 wt % water content) was measured at 500 kPa and at 25.0 C. as described in Example 1. The solubility of carbon dioxide in the ionic liquid was found to be 0.52 mol.Math.L.sup.1.

Comparative Example 5

Solubility of Carbon Dioxide in Monoethanolamine/Water Mixtures

(17) The solubility of carbon dioxide in a mixture of monoethanolamine (MEA) and water (30:70 MEA/H.sub.2O weight ratio) was measured at 500 kPa and at 25.0 C. as described in Example 1. This liquid solution was chosen as a comparative example of a carbon dioxide chemical absorber used commercially in industry, especially in natural gas processing operations. The solubility of carbon dioxide in the monoethanolamine/water mixture was found to be 3.57 mol.Math.L.sup.1.

Example 6

Solubility of Methane in Tetraalkylphosphonium Carboxylate/Water Mixtures

(18) The solubility of methane in the [P.sub.4444[HCO.sub.2]/water mixture of Example 1 was measured at 500 kPa and 25.0 C. as described in Example 1. The amount of water in the liquid mixture was quantified by Karl-Fischer titration, and/or .sup.1H NMR. The solubility of methane in the ionic liquid/water mixture was found to be 0.07 mol.Math.L.sup.1, clearly demonstrating the selectivity of the process of the invention.

Example 7

Separation of Methane from a CO.SUB.2./CH.SUB.4 .Gas Mixture Using Tetraalkylphosphonium Carboxylate/Water Mixtures

(19) A known amount of [P.sub.4444][HCO.sub.2]/water mixture (ca. 10 mL) was placed in a pressure vessel and degassed by stirring for 5 min under reduced pressure. A gas mixture containing 7.22 mol % CO.sub.2 in CH.sub.4 (model natural gas) was then pumped into the reactor up to a pressure of ca. 3000 kPa. The mixture was vigorously stirred at 25.0 C. until pressure equilibration, which took about 10 min. The gas was then sampled out and analysed on the chromatographic gas analyser. The ratio of CO.sub.2 was found to have been reduced to 2.32 mol %.

Example 8

Variation of Carbon Dioxide Absorption with Water Content

(20) The solubility of carbon dioxides in absorbents comprising the ionic liquids [P.sub.4444][HCO.sub.2], [P.sub.4444][CH.sub.3CO.sub.2], and [P.sub.66614][HCO.sub.2] with varying amounts of water was measured. The results are shown in FIG. 2.

Example 9

Chemical and Physical Absorption

(21) To observe the chemical and physical absorption behaviour of ionic liquid/water mixtures, the solubility of CO.sub.2 in a mixture of tributylmethylphosphonium propanoate and water at a ratio of 40:60 (ionic liquid:water) was plotted against CO.sub.2 partial pressure. As shown in FIG. 5, typical chemical absorption behaviour is observed at low CO.sub.2 pressure, with the CO.sub.2 uptake increasing asymptotically as the 1:1 molar ratio is approached. Once the saturation pressure is reached (i.e. a 1:1 molar ratio of ionic liquid and CO.sub.2), the system switches to the linear response expected of a physical CO.sub.2 absorber. The linear increase in absorption is observed continuously to the highest CO.sub.2 partial pressure observed.

Example 10

Separation of Carbon Dioxide from Air

(22) The removal of carbon dioxide from air was examined by analysis of IR absorption spectra.

(23) FIG. 6A shows the IR absorption spectrum of air containing an elevated level of CO.sub.2 (ca. 40 mol %). The same air/CO.sub.2 mixture was bubbled through a mixture of tributylmethyl-phosphonium propanoate and water in an ionic liquid:water ratio of 40:60) over a period of 20 minutes. As shown in FIG. 6B, the IR absorption peaks due to the carbon dioxide stretching frequencies are not observed, indicating that substantially all CO.sub.2 has been removed from the CO.sub.2/air mixture.

Example 11

Removal of Water

(24) The removal of water by the ionic liquid compositions of the invention was examined by bubbling nitrogen gas saturated with water through [P.sub.6668][HCO.sub.2] at 15 C. The Fourier transform infrared spectra (FTIR) of the gas were measured both before and after contact with the ionic liquid (see FIGS. 3 and 4 respectively). It will be observed that the peaks in the region 3400 to 4000 cm.sup.1 attributable to water are significantly reduced.