Claims
- 1. A process for abating nitric oxide and sulfur dioxide from a flue gas, comprising the steps of:
- (a) contacting flue gas with an aqueous absorbent solution, in an absorption stream comprising a reduced metal chelate and sulfites to absorb nitric oxide from said flue gas, and a piperazinone SO.sub.2 absorbent of the formula: ##STR13## wherein R is hydrogen, an alkyl group having one to six carbon atoms or aryl or aralkyl group having from six to twelve carbon atoms, R.sup.1 is hydrogen, an alkyl or hydroxyalkyl group having from one to six carbon atoms, or an aryl or aralkyl group having from six to twelve carbon atoms, and R.sup.2 is R.sup.1 or a 2-hydroxyethyl group of the formula: ##STR14## wherein R.sup.3 is hydrogen or an alkyl group having one or two carbon atoms to reversibly absorb sulfur dioxide from said flue gas, at a pH between 3 and 8 effective to maintain compatibility between the chelate, sulfites and SO.sub.2 absorbent to form an SO.sub.2 -rich absorbent solution containing oxidized metal chelate and heat stable piperazinone salts;
- (b) thermally stripping a major portion of the sulfur dioxide from said SO.sub.2 -rich absorbent solution to obtain an SO.sub.2 -lean absorbent solution containing only a minor portion of the SO.sub.2 absorbed in step (a);
- (c) passing said SO.sub.2 -lean absorbent solution through an electrochemical cell to reduce said oxidized metal chelates, and to convert the heat stable piperazinone salts into free piperazinone SO.sub.2 absorbent, to regenerate said absorbent solution; and
- (d) recycling said regenerated absorbent solution to said contacting step.
- 2. The process of claim 1, wherein said electrochemical cell comprises a two compartment cell having an anode in an anode compartment separated by an anionic membrane from a cathode in a cathode compartment, said SO.sub.2 -lean absorbent solution is directed to said cathode compartment and wherein an electrical potential is imposed across said anode and said cathode to reduce said oxidized metal chelates in said cathode compartment, and to separate heat stable salt anions from the cathode compartment through the anionic membrane into the anode compartment, to obtain the regenerated absorbent solution.
- 3. The process of claim 1, wherein absorbent temperature is in a range of from about 5.degree. C. to about 95.degree. C. in contact with said flue gas.
- 4. The process of claim 1, wherein said metal chelate comprises metal complexed with a chelating agent selected from the group consisting of ethylenediaminetetracetic acid, N-hydroxyethylethylenediaminetriacetic acid and nitrilotriacetic acid.
- 5. The process of claim 1, wherein said metal chelate is selected from the group consisting of iron, nickel and copper chelates.
- 6. The process of claim 1, wherein said absorbent solution in step (a) comprises from about 1000 to about 10,000 parts per million of said metal chelate.
- 7. The process of claim 1, wherein said sulfites are selected from the group consisting of sulfites and bisulfites of sodium, potassium, lithium, magnesium and ammonium.
- 8. The process of claim 1, wherein said sulfites comprise at least about 0.05 g moles per liter of said absorbent solution in step (a).
- 9. The process of claim 1, wherein said SO.sub.2 absorbent comprises at least about 0.1 g moles per liter of said absorbent solution in said contacting step.
- 10. The process of claim 1, wherein steps (a) and (b) operating pressure is in a range of from ambient to about 10 atmospheres.
- 11. The process of claim 1, wherein step (b) includes recovering essentially pure sulfur dioxide.
- 12. The process of claim 2, further comprising circulating an aqueous electrolyte solution through said anode compartment.
- 13. The process of claim 1, wherein said SO.sub.2 absorbent comprises a piperazinone selected from the group consisting of
- 4-(2-hydroxyethyl)-2-piperazinone,
- 4-(2-hydroxyethyl)-1-methyl-2-piperazinone,
- 4-(2-hydroxyethyl)-3-methyl-2-piperazinone,
- 4-(2-hydroxyethyl)-5-methyl-2-piperazinone,
- 3-ethyl-4-(2-hydroxyethyl)-2-piperazinone,
- 6-ethyl-4-(2-hydroxyethyl)-2-piperazinone,
- 4-(2-hydroxyethyl)-5,6-dimethyl-2-piperazinone,
- 1-ethyl-4-(2-hydroxyethyl)-2-piperazinone,
- 4-(2-hydroxyethyl)-3-phenyl-2-piperazinone,
- 1,4-bis-(2-hydroxyethyl)-2-piperazinone,
- 4-(2-hydroxyethyl)-2-piperazinone,
- 4-(2-hydroxypropyl)-2-piperazinone,
- 4-(2-hydroxybutyl)-2-piperazinone and
- 4-(2-hydroxy-propyl)-6-methyl-2-piperazinone.
- 14. The process of claim 1, wherein said SO.sub.2 absorbent comprises 4-(2-hydroxyethyl)-2-piperazinone.
- 15. The process of claim 1, wherein R.sup.2 is selected from said 2-hydroxyethyl groups.
- 16. The process of claim 2, including the steps of supplying said SO.sub.2 -lean absorbent solution to said contacting step and passing a side-stream of said SO.sub.2 -lean absorbent solution through said electrochemical cell.
CROSS REFERENCE TO RELATED APPLICATION
This is a continuation of application Ser. No. 07/744,157, filed Aug. 13, 1991, now abandoned.
US Referenced Citations (25)
Foreign Referenced Citations (4)
Number |
Date |
Country |
368423 |
May 1990 |
EPX |
376485 |
Jul 1990 |
EPX |
51-14881 |
Feb 1976 |
JPX |
1464050 |
Feb 1977 |
GBX |
Continuations (1)
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Number |
Date |
Country |
Parent |
744157 |
Aug 1991 |
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