Method for simultaneous absorption of sulfur dioxide and nitric oxide from flue gas

• US 5,433,934 A
• Filed: 01/11/1994
• Issued: 07/18/1995
• Est. Priority Date: 08/13/1991
• Status: Expired due to Fees

• Current Assignee: The Dow Chemical Company (Dow, Inc.)
• Original Assignee: The Dow Chemical Company (Dow, Inc.)
• Inventors: Chang, Dane, Kirby, Larry H., Bedell, Stephen A.
• Primary Examiner(s): Lander, Ferris
• Assistant Examiner(s): DI MAURO, PETER T
• Application Number:
  US08/179,882
• Time in Patent Office: 553 Days
• Field of Search: 423/242.7, 423/242.6, 423/242.3, 423/243.07, 423/235, 204/130
• US Class Current: 423/235
• CPC Class Codes: B01D 53/60 Simultaneously removing sulfur oxides and nitrogen oxides
• CPC Class Codes: Y02A 50/20 Air quality improvement or preservation, e.g. vehicle emission control or emission reduction by using catalytic converters

Abstract

The simultaneous abatement of NO and SO₂ in flue gas is provided by an absorption process and apparatus utilizing an absorbent composition comprising an aqueous solution of chelates and sulfite salt for NO abatement and amine SO₂ absorbents such as piperazinones, morpholinones, piperidines, piperazines, piperazinediones, hydantoins, triazinones, pyrimidinones, oxazolidones, etc. for SO₂ abatement. SO₂ is thermally stripped from the spent absorbent and recovered. Metal chelates oxidized to an inactive state as a side-reaction are electrochemically reduced. An anionic exchange membrane in the electrochemical cell regenerates heat stable amine salt byproducts to be converted back to useable amine sorbent, and facilitates removal from the absorbent solution of other waste salts.

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 SO₂ 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, R¹ 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 R² is R¹ or a 2-hydroxyethyl group of the formula;

##STR14## wherein R³ 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 SO₂ absorbent to form an SO₂ -rich absorbent solution containing oxidized metal chelate and heat stable piperazinone salts;

(b) thermally stripping a major portion of the sulfur dioxide from said SO₂ -rich absorbent solution to obtain an SO₂ -lean absorbent solution containing only a minor portion of the SO₂ absorbed in step (a);

(c) passing said SO₂ -lean absorbent solution through an electrochemical cell to reduce said oxidized metal chelates, and to convert the heat stable piperazinone salts into free piperazinone SO₂ 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 SO₂ -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°

C. to about 95°

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 SO₂ 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 SO₂ 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 SO₂ absorbent comprises 4-(2-hydroxyethyl)-2-piperazinone.

15. The process of claim 1, wherein R² is selected from said 2-hydroxyethyl groups.

16. The process of claim 2, including the steps of supplying said SO₂ -lean absorbent solution to said contacting step and passing a side-stream of said SO₂ -lean absorbent solution through said electrochemical cell.