Process for stripping carbamate from ion exchange resin

Abstract

In a preferred embodiment, there is provided a process for separating an amine compound or a conjugate acid thereof and a carbamate compound or a conjugate acid thereof from a mixture having the amine compound, the carbamate compound, carbon dioxide and at least one anionic contaminant salt using an anionic exchange column, the process including passing the mixture through the column to obtain a first effluent and passing through the column an extraction fluid to obtain a second effluent, where the extraction fluid most preferably includes carbonic acid.

Claims

1. A process for separating an amine compound or a conjugate acid thereof and a carbamate compound or a conjugate acid thereof from a mixture having the amine compound, the carbamate compound, carbon dioxide and at least one anionic contaminant salt using an anionic exchange column having a plurality of anion exchange sites, the amine compound being an optionally substituted piperazine or having formula (1) of R.sub.3xN(R.sup.1-Q-R.sup.1OH).sub.x and the carbamate compound being an optionally substituted piperazine carbamate or piperazine dicarbamate, or having formula (2) of (HOR.sup.1-Q-R.sup.1).sub.yNH.sub.2yC(O)O.sup., wherein at least a portion of the carbamate compound is present in the mixture from a reversible reaction between the amine compound and the carbon dioxide to produce the carbamate compound and a hydrogen atom or the conjugate acid of the amine compound, wherein x and y are independently of each other 1 or 2, R is a hydrogen atom or optionally substituted straight or branched C.sub.1-C.sub.8 alkyl, R.sup.1 is nothing or optionally substituted straight or branched C.sub.1-C.sub.4 alkylene, and Q is nothing, O or S, and wherein the optionally substituted piperazine comprises at least one ring nitrogen atom bonded to a hydrogen atom, at least one R is a hydrogen atom, and at least R.sup.1 in each said formulas (1) and (2) is optionally substituted straight or branched C.sub.1-C.sub.4 alkylene; wherein the process comprises: passing the mixture through the column to effect attachment to the at least one anionic contaminant salt to the anion exchange sites, and collecting from the column a first effluent comprising at least the amine compound or the conjugate acid thereof; passing through the column an extraction fluid at a pressure between 6 atm and 15 atm to obtain a second effluent, the extraction fluid comprising carbonic acid, to effect removal of the carbamate compound attached to the anion exchange sites, wherein the second effluent comprises at least the carbamate compound, the conjugate acid of the carbamate compound or a decomposition product thereof, said decomposition product comprising the amine compound and the carbon dioxide; and passing through the column a regeneration fluid to regenerate the column, the regeneration fluid comprising an anionic regeneration compound selected to replace the anionic contaminant salt attached to the anion exchange sites, wherein the first and second effluents are collected to obtain a separated mixture comprising at least the amine compound or the conjugate acid thereof and the carbamate compound or the conjugate acid thereof.

2. The process of claim 1, wherein the process is for use with a method for removing from a gaseous stream carbon dioxide and optionally one or more of hydrogen sulfide, sulfur dioxide, carbonyl sulfide, carbon disulfide, nitrogen dioxide and hydrogen cyanide, said mixture being an aqueous solution obtained from contacting a liquid stream comprising the amine compound with the gaseous stream to obtain a loaded fluid mixture, heating the loaded fluid mixture with a steam to remove at least a portion of the gaseous stream absorbed in the loaded fluid mixture to obtain a heated fluid mixture, and refluxing the heated fluid mixture to obtain the aqueous solution.

3. The process of claim 2, wherein the aqueous solution has a carbon dioxide loading capacity between about 0.01 and about 1.0, said loading capacity being obtained from a molar ratio between the carbon dioxide and the amine compound in the aqueous solution, and wherein the aqueous solution has a concentration of the carbamate compound between about 0.5 weight % and about 10 weight % with respect to the total weight of the aqueous solution.

4. The process of claim 2, wherein at least a portion of the at least one anionic contaminant salt is produced during one or both of said contacting the liquid stream with the gaseous stream and said heating the loaded fluid mixture with the steam, said at least one anionic contaminant salt being a heat stable salt comprising one or more of a formate salt, an acetate salt, a thiocyanate salt, a thiosulfate salt, a glycolate salt, a chloride salt, an oxalate salt, a butyrate salt, a phosphate salt, a nitrate salt, a propionate salt and a sulfate salt.

5. The process of claim 1, wherein the amine compound is one or more of monoethanolamine, diethanolamine, monoisopropanolamine, diisopropanolamine, 1-amino-2-propanol, 2-amino-2-methyl-1-propanol, 2-amino-2-methyl-1,3-propanediol, N-methylmonoethanolamine, N-ethylmonoethanolamine, N-butylmonoethanolamine, 2-(2-aminoethoxy)ethanol, N-(2-hydroxyethyl)ethylenediamine, piperazine, 1-[2-(2-Hydroxyethoxy)ethyl]piperazine, 1-methylpiperazine, 1-(2-methoxyethyl)piperazine, 1-(2-hydroxyethyl)piperazine, 1-(2-ethoxyethyl)piperazine and 1-(2-aminoethyl)piperazine.

6. The process of claim 1, wherein the carbonic acid donates a proton to the carbamate compound attached to the anion exchange sites to form the conjugate acid of the carbamate compound and optionally decomposes the conjugate acid of the carbamate compound to form the amine compound, thereby removing the carbamate compound from the anion exchange sites.

7. The process of claim 1, wherein the amine compound has formula (3) HOR.sup.2NH.sub.2, R.sup.2 being C.sub.1-C.sub.8 alkylene, and the reversible reaction is 2 HOR.sup.2NH.sub.2+CO.sub.2HOR.sup.2NH.sub.3.sup.++HOR.sup.2NHCOO.sup., and wherein, the carbonic acid donates a proton to HOR.sup.2NHCOO.sup. attached to the anion exchange sites to form HOR.sup.2NHCOOH and a bicarbonate ion, and optionally decomposes HOR.sup.2NHCOOH to HOR.sup.2NH.sub.2 and CO.sub.2.

8. The process of claim 1, wherein the regeneration fluid is an aqueous solution comprising one or more of sodium hydroxide, sodium carbonate and sodium bicarbonate at a concentration between about 10 g/L and about 150 g/L.

9. The process of claim 1, wherein the separated mixture is a separated aqueous solution having the at least one anionic contaminant salt at a concentration up to about 3 weight % with respect to the total weight of the separated aqueous solution.

10. The process of claim 1, wherein the anionic exchange column comprises a type 1 or 2 strong base anion resin, and each said anion exchange sites comprise a quaternary amino moiety.

11. A process for separating an amine compound or a conjugate acid thereof and a carbamate compound or a conjugate acid thereof from a mixture having the amine compound, the carbamate compound, carbon dioxide and at least one anionic contaminant salt using an anionic exchange column having a plurality of anion exchange sites, the amine compound being an optionally substituted piperazine or having formula (1) of R.sub.3xN(R.sup.1-Q-R.sup.1OH).sub.x and the carbamate compound being an optionally substituted piperazine carbamate or piperazine dicarbamate, or having formula (2) of (HOR.sup.1-Q-R.sup.1).sub.yNH.sub.2yC(O)O.sup., wherein x and y are independently of each other 1 or 2, R is a hydrogen atom or optionally substituted straight or branched C.sub.1-C.sub.8 alkyl, R.sup.1 is nothing or optionally substituted straight or branched C.sub.1-C.sub.4 alkylene, and Q is nothing, O or S, and wherein the optionally substituted piperazine comprises at least one ring nitrogen atom bonded to a hydrogen atom, at least one R is a hydrogen atom, and at least R.sup.1 in each said formulas (1) and (2) is optionally substituted straight or branched C.sub.1-C.sub.4 alkylene; wherein the process comprises: passing the mixture through the column to effect attachment of the at least one anionic contaminant salt to the anion exchange sites, and collecting from the column a first effluent comprising at least the amine compound or the conjugate acid thereof; passing through the column an extraction fluid at a pressure between 6 atm and 15 atm to obtain a second effluent, the extraction fluid comprising carbonic acid, to effect removal of the carbamate compound attached to the anion exchange sites, wherein the second effluent comprises at least the carbamate compound, the conjugate acid of the carbamate compound or a decomposition product thereof, said decomposition product comprising the amine compound and the carbon dioxide; and passing through the column a regeneration fluid to regenerate the column, the regeneration fluid comprising an anionic regeneration compound selected to replace the anionic contaminant salt attached to the anion exchange sites, wherein the first and second effluents are collected to obtain a separated mixture comprising at least the amine compound or the conjugate acid thereof and the carbamate compound or the conjugate acid thereof.

12. The process of claim 11, wherein the carbonic acid donates a proton to the carbamate compound attached to the anion exchange sites to form the conjugate acid of the carbamate compound, thereby removing the carbamate compound from the anion exchange sites.

13. The process of claim 11, wherein the process is for use with a method for removing from a gaseous stream carbon dioxide and optionally one or more of hydrogen sulfide, sulfur dioxide, carbonyl sulfide, carbon disulfide, nitrogen dioxide and hydrogen cyanide, said mixture being an aqueous solution obtained from contacting a liquid stream comprising the amine compound with the gaseous stream to obtain a loaded fluid mixture, heating the loaded fluid mixture with a steam to remove at least a portion of the gaseous stream absorbed in the loaded fluid mixture to obtain a heated fluid mixture, and refluxing the heated fluid mixture to obtain the aqueous solution.

14. The process of claim 13, wherein the aqueous solution has a carbon dioxide loading capacity between about 0.01 and about 1.0, said loading capacity being obtained from a molar ratio between the carbon dioxide and the amine compound in the aqueous solution, and wherein the aqueous solution has a concentration of the carbamate compound between about 0.5 weight % and about 10 weight % with respect to the total weight of the aqueous solution.

15. The process of claim 13, wherein at least a portion of the at least one anionic contaminant salt is produced during one or both of said contacting the liquid stream with the gaseous stream and said heating the loaded fluid mixture with the steam, said at least one anionic contaminant salt being a heat stable salt comprising one or more of a formate salt, an acetate salt, a thiocyanate salt, a thiosulfate salt, a glycolate salt, a chloride salt, an oxalate salt, a butyrate salt, a phosphate salt, a nitrate salt, a propionate salt and a sulfate salt.

16. The process of claim 11, wherein the amine compound is one or more of monoethanolamine, diethanolamine, monoisopropanolamine, diisopropanolamine, 1-amino-2-propanol, 2-amino-2-methyl-1-propanol, 2-amino-2-methyl-1,3-propanediol, N-methylmonoethanolamine, N-ethylmonoethanolamine, N-butylmonoethanolamine, 2-(2-aminoethoxy)ethanol, N-(2-hydroxyethyl)ethylenediamine, piperazine, 1-[2-(2-Hydroxyethoxy)ethyl]piperazine, 1-methylpiperazine, 1-(2-methoxyethyl)piperazine, 1-(2-hydroxyethyl)piperazine, 1-(2-ethoxyethyl)piperazine and 1-(2-aminoethyl)piperazine.

17. The process of claim 11, wherein at least a portion of the carbamate compound is present in the mixture from a reversible reaction between the amine compound and the carbon dioxide to produce the carbamate compound and a hydrogen atom or the conjugate acid of the amine compound, and the amine compound has formula (3) HOR.sup.2NH.sub.2, R.sup.2 being C.sub.1-C.sub.8 alkylene, wherein the reversible reaction is 2 HOR.sup.2NH.sub.2+CO.sub.2HOR.sup.2NH.sub.3.sup.++HOR.sup.2NHCOO.sup., and the carbonic acid donates a proton to HOR.sup.2NHCOO.sup. attached to the anion exchange sites to form HOR.sup.2NHCOOH and a bicarbonate ion, and optionally decomposes HOR.sup.2NHCOOH to HOR.sup.2NH.sub.2 and CO.sub.2.

18. The process of claim 11, wherein the regeneration fluid is an aqueous solution comprising one or more of sodium hydroxide, sodium carbonate and sodium bicarbonate, at a concentration between about 10 g/L and about 150 g/L.

19. The process of claim 11, wherein the separated mixture is a separated aqueous solution having the at least one anionic contaminant salt at a concentration up to about 3 weight %, with respect to the total weight of the separated aqueous solution.

20. The process of claim 11, wherein the anionic exchange column comprises a type 1 or 2 strong base anion resin, and each said anion exchange sites comprise a quaternary amino moiety.

21. The process of claim 1, wherein the amine compound is monoethanolamine.

22. The process of claim 11, wherein the amine compound is monoethanolamine.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Reference may now be had to the following detailed description taken together with the accompanying drawings in which:

(2) FIG. 1 is a flow diagram illustrating a process for extracting an alkanolamine compound and a carbamate compound in accordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(3) Reference is made to FIG. 1 which shows a schematic of a process in accordance with a preferred embodiment of the present invention, and which incorporates an anion exchange step for treating an alkanolamine solution utilized for absorbing carbon dioxide in a coal burning power plant. As will be described in greater detail below, the process is conducted in a carbon capture apparatus 100 which includes an absorber tower 1, a stripper tower 2 and an anion exchange unit 3, wherein the absorber tower 1, the stripper tower 2 and the anion exchange unit 3 are in fluid communication with one another. The process includes: a chemical absorption step to be performed in the absorber tower 1; a thermal regeneration step to be performed in the stripper tower 2; and the anion exchange step to be performed in the anion exchange unit 3. It is to be appreciated that although the process in accordance with this preferred embodiment is described for application in the coal burning power plant, the process of the present invention is not limited to such application, and may be applied in various other contexts for removing carbon dioxide.

(4) In the chemical absorption step, an exhaust gas stream 4 produced from the coal burning power plant, and which includes a flue gas containing carbon dioxide, as well as other gaseous compounds resulting from combustion of coal is channeled into the absorber tower 1 from a lower portion thereof. The gas stream 4 is contacted in the absorber tower 1 with a lean monoethanolamine stream 5 flown into the absorber tower 1 through an upper portion thereof, so as to effect countercurrent contact between the gas stream 4 and the lean monoethanolamine stream 5. The lean monoethanolamine stream 5 includes monoethanolamine of the formula HOCH.sub.2CH.sub.2NH.sub.2 alone or together with other primary or secondary alkanolamine compounds to conduct chemical absorption of carbon dioxide and the other gaseous compounds, such as but not limited to hydrogen sulfide and sulfur dioxide, during the countercurrent contact with the gas stream 4. As will be described in greater detail below, the monoethanolamine stream 5 includes a recycled monoethanolamine stream 6 from the stripper tower 2 and a treated monoethanolamine stream 7 from the anion exchange unit 3. After contact with the exhaust gas stream 4, the lean monoethanolamine stream 5 flows downwardly and out of the absorber tower 1 as a loaded monoethanolamine stream 8 having absorbed carbon dioxide at a carbon dioxide lean loading between 0.05 to 0.3 or preferably between 0.1 and 0.15 (expressed as a molar ratio between the carbon dioxide and monoethanolamine) and the other gaseous compounds. The exhaust gas stream 4 with carbon dioxide and the other gaseous compounds removed therefrom is vented from the absorber tower 1.

(5) In the thermal regeneration step, the loaded monoethanolamine stream 8 is fed into the stripper tower 2 through an upper portion thereof, and is contacted with a steam (not shown) generated by a stripper tower reboiler (not shown) to remove at least a portion of carbon dioxide and the other gaseous compounds absorbed in the loaded monoethanolamine stream 8. The loaded monoethanolamine stream 8 having contacted the steam leaves the stripper tower 2 as a stripped monoethanolamine stream 9 through a bottom portion of the stripper tower 2. A portion of the stripped monoethanolamine stream 9 is rechanneled as the recycled monoethanolamine stream 6 to form part of the lean monoethanolamine stream 5, and the remaining portion of the stream 9 is flown to the anion exchange unit 3 as a bleed monoethanolamine stream 10 for further treatment.

(6) It is to be appreciated that the thermal regeneration step does not completely remove all carbon dioxide and the other gaseous compounds from the loaded monoethanolamine stream 8, and the stripped monoethanolamine stream 9, as well as the bleed monoethanolamine stream 10, includes some portions of the carbon dioxide and the other gaseous compounds. Furthermore, the bleed monoethanolamine stream 10 also includes a number of contaminants and degradation products which may result from one or both of the chemical absorption and thermal regeneration steps. Such degradation products or corrosive heat stable salts include formate, oxalate, glycolate and/or acetate, and could causes damages to the carbon capture apparatus 100 and various components thereof.

(7) To reduce an amount of the heat stable salts in the bleed monoethanolamine stream 10, in the anion exchange chromatography step the bleed monoethanolamine stream 10 is flown through a type 1 or 2 strong base anion resin (not shown) included in the anion exchange unit 3, and which includes a plurality of anion exchange sites having a positively charged quaternary amino moiety to effect loading or attachment of the heat stable salts thereto. A first effluent eluted from the anion resin by passing the bleed monoethanolamine stream 10 therethrough is channeled towards the absorber tower 1 as the treated monoethanolamine stream 7, and is combined with the recycled monoethanolamine stream 6 to form the lean monoethanolamine stream 5 for continuous cyclic use in contacting the exhaust gas stream 4 in the absorber tower 1. Without wishing to be bound by theory, it is contemplated that some monoethanolamine in the bleed monoethanolamine stream 10 prior to or during passing through the anion resin reacts with carbon dioxide to form the carbamate anion HOCH.sub.2CH.sub.2NHCOO.sup. according to reaction (1) identified above, and the carbamate anion attaches to the anion exchange sites. The carbamate anion attached to the anion exchange sites and not recovered in the treated monoethanolamine stream 7 could represent a loss of monoethanolamine in the carbon capture apparatus 100 over time, if the attached carbamate anion, together with the attached heat stable salts, is subsequently eluted from the resin during resin regeneration as will be further described below.

(8) To avoid such loss of monoethanolamine in the apparatus 100, the anion resin is contacted with an aqueous carbonic acid stream 11 to selectively remove the carbamate anion from the anion exchange sites and to obtain a second effluent. Without wishing to be bound by theory, it has been envisioned that such selective carbamate anion removal may be described by equation (6):
RN.sup.+(HOCH.sub.2CH.sub.2NHCO.sup.)+H.sub.2CO.sub.3.fwdarw.RN.sup.+(HCO.sub.3.sup.)+HOCH.sub.2CH.sub.2NH.sub.2+CO.sub.2 (6)
The carbamate anion attached on the resin may behave as a conjugate base to accept a proton from carbonic acid, removing and decomposing the carbamate anion to form monoethanolamine and carbon dioxide. The bicarbonate anion produced from equation (6) may be attached to the anion exchange sites. The acid base reaction in equation (6) is believed to be quite rapid.

(9) It has been appreciated that carbonic acid may be exist in the aqueous stream 11 in chemical equilibrium as described by equation (7):
CO.sub.2+H.sub.2OH.sub.2CO.sub.3H.sup.++HCO.sub.3.sup.(7)
The bicarbonate anion resulting from the chemical equilibrium of equation (7) or from the acid base reaction of equation (6) may cause removal of for example the heat stable salt formate anion according to equation (8):
RN.sup.+(HCOO.sup.)+HCO.sub.3.sup..fwdarw.RN+(OH.sup.)+HCOO.sup.(8)
It has been envisioned however that carbonic acid is only weakly dissociated and consequently the bicarbonate anion concentration is low, making the carbonic acid and the bicarbonate anion a poor regenerant for the heat stable salts attached on the anion resin. Furthermore, possible formation of the conjugate acid forms of the heat stable salts may maintain low solution pH, so as to inhibit further dissociation of the carbonic acid.

(10) It has been further envisioned that as equation (6) is based on acid base neutralization reaction which stoichiometrically consumes the carbonic acid and not just the bicarbonate anion, the reaction identified by equation (6) is not significantly affected by the carbonic acid's dissociation constant or formation of the bicarbonate anion. In the presence of absorbed or dissolved carbon dioxide, any carbonic acid consumed in equation (6) may be replaced by the carbon dioxide reacting with water to produce more carbonic acid in the chemical equilibrium described by equation (7), and therefore, equation (6) in removing the carbamate anion is not limited by the carbonic acid and bicarbonate equilibrium concentration. To drive the chemical equilibrium of equation (7) towards carbonic acid, it is preferred that the aqueous carbonic acid stream 11 is contracted with the anion resin at an elevated pressure, preferably between 1 and 14 atm or more preferably between 6 and 8 atm.

(11) The anion resin is further rinsed with a water stream 12 and the water stream 12 is combined with the second effluent obtained from contacting the carbonic acid stream 11 with the anion resin to form a recovered monoethanolamine stream 13. The recovered monoethanolamine stream 13 is in turn combined with the treated monoethanolamine stream 7 for continuous cyclic use as the lean monoethanolamine stream 5 in contacting the exhaust gas stream 4 in the absorber tower 1.

(12) To regenerate the anion resin, an aqueous sodium hydroxide stream (not shown) is flown through the anion resin to obtain a third effluent containing the heat stable salts removed from the anion exchange sites, and the third effluent is discarded as a waste stream.

(13) In an experimental study, recovery of a carbamate anion from an anion exchange resin was confirmed. In the study, 20 mL of type II anion resin was loaded into a packed resin column, and the resin was converted into the hydroxide form by passing 60 mL of a sodium hydroxide solution with a concentration of 80 g/L through the column. The residual sodium hydroxide was washed from the resin by passing 60 mL of deionized water through the column. A feed solution containing 167 g/L monoethanolamine (MEA) was prepared by sparging carbon dioxide gas through the feed solution until a steady pH of 10.2 was achieved. The feed solution thus prepared was titrated with 0.1 M NaOH, and found to have a lean loading level of 0.27 mol CO.sub.2/mol MEA which is equivalent to 45.8 g MEA/L. Formic acid was then added to the feed solution to give a concentration of 11.2 g/L, which is equivalent to 11 g/L as formate.

(14) 200 mL of the feed solution was passed through the packed resin column, and an effluent was collected in a flask. The remaining liquid void in the resin column was displaced by passing air therethrough, and the displaced water was collected into the same flask giving a combined volume of 212 mL. The feed was then rinsed from the resin column with 58 mL of deionized water, and an effluent was collected separately and had a volume of 58 mL. The resin column was then regenerated and rinsed with 60 mL of 80 g/L sodium hydroxide and 50 mL of deionized water. The effluents from both the regeneration and washing steps were collected together giving a combined volume of 110 mL. Quantification of monoethanolamine and formate is summarized in Table 1:

(15) TABLE-US-00001 TABLE 1 Volume MEA Formate Sample ml g/L g g/L g Feed 200 167 33.4 11 2.2 Feed and air displacement 212 147.8 31.3 6 1.27 effluent Feed rinse effluent 58 22.3 1.29 1.63 0.09 Regenerant and rinse 110 6.13 0.67 7.69 0.85 effluent

(16) The same experiment was repeated but with inclusion of an additional carbamate rinse step incorporated between the feed rinse and regeneration steps. The carbamate rinse step involved passing 210 mL of carbonic acid at 11 atm through the packed resin column and collecting an effluent. Quantification of monoethanolamine and formate from the repeated experiment is summarized in Table 2:

(17) TABLE-US-00002 TABLE 2 Volume MEA Formate Sample ml g/L g g/L g Feed 200 167 33.4 11 2.2 Feed and air 210 150.1 31.5 5.8 1.22 displacement effluent Feed rinse effluent 65 21.7 1.41 1.46 0.095 Carbonic acid effluent 210 4.06 0.85 0.94 0.19 Regenerant and rinse 128 0 0 5 0.64 effluent
Tables 1 and 2 confirmed that with inclusion of the carbonic acid strip step, detectable amounts of monoethanolamine was found in the regenerant and rinse effluent, confirming that the carbamate anion was removed from the resin column in the carbonic acid strip step.

(18) While the invention has been described with reference to preferred embodiments, the invention is not or intended by the applicant to be so limited. A person skilled in the art would readily recognize and incorporate various modifications, additional elements and/or different combinations of the described components consistent with the scope of the invention as described herein.