Waste gas treatment process including removal of mercury

• US 20060286017A1
• Filed: 06/20/2005
• Published: 12/21/2006
• Est. Priority Date: 06/20/2005
• Status: Active Grant

• Current Assignee: Cansolv Technologies Inc. (Shell Plc)
• Original Assignee: Cansolv Technologies Inc. (Shell Plc)
• Inventors: Hakka, Leo Ernest
• Granted Patent: US 7,384,616 B2
• Time in Patent Office: Days
• US Class Current: 423/220
• CPC Class Codes: B01D 2251/90 Chelants
• CPC Class Codes: B01D 53/1475 Removing carbon dioxide
• CPC Class Codes: B01D 53/60 Simultaneously removing sulfur oxides and nitrogen oxides
• CPC Class Codes: B01D 53/64 Heavy metals or compounds thereof, e.g. mercury
• CPC Class Codes: B01D 53/96 Regeneration, reactivation or recycling of reactants
• CPC Class Codes: Y02A 50/20 Air quality improvement or preservation, e.g. vehicle emission control or emission reduction by using catalytic converters
• CPC Class Codes: Y02C 20/40 of CO2

Abstract

A process for removing contaminants from a waste gas stream comprises treating the waste gas stream to remove at least one of SO₂ and NO_x and to obtain a lean stream having a reduced level of at least one of SO₂ and NO_x and, contacting the lean gas stream with a mercury absorbent solution comprising permanganate to remove mercury vapour and to obtain a mercury lean stream and a mercury rich absorbent solution. The mercury rich absorbent solution may be subsequently treated on a batch basis to remove precipitated manganese dioxide and obtain a solution containing mercury ions.

Claims

1. A process for removing contaminants from a waste gas stream comprising:

a) treating the waste gas stream to remove at least one of SO₂ and NO_x and to obtain a lean stream having a reduced level of at least one of SO₂ and NO_x; and

,

b) contacting the lean gas stream with a mercury absorbent solution comprising permanganate to remove mercury vapour and to obtain a mercury lean stream and a mercury rich absorbent solution.

2. The process as claimed in claim 1 further comprising selecting an aqueous alkali metal or alkaline earth metal permanganate solution as the mercury absorbent solution.

3. The process as claimed in claim 1 further comprising selecting an aqueous alkali metal as the mercury absorbent solution.

4. The process as claimed in claim 3 further comprising selecting as the mercury absorbent solution a solution comprising at least one of a potassium salt of permanganate, a sodium salt of permanganate, a magnesium salt of permanganate and a calcium salt of permanganate.

5. The process as claimed in claim 3 wherein the mercury absorbent solution is acidic.

6. The process as claimed in claim 1 wherein the mercury absorbent solution comprises an aqueous alkali metal permanganate solution having a pH less than about 1.

7. The process as claimed in claim 6 wherein the aqueous alkali metal permanganate solution comprises about 0.1 to 5% by weight permanganate salt.

8. The process as claimed in claim 1 further comprising treating the waste gas stream to remove SO₂ and NO_x and to obtain a lean stream having a reduced level of SO₂ and NO_x.

9. The process as claimed in claim 1 further comprising treating the waste gas stream to remove SO₂ and to subsequently remove NO_x and to obtain a lean stream having a reduced level of SO₂ and NO_x.

10. The process as claimed in claim 1 further comprising treating the waste gas stream to remove SO₂, CO₂ and NO_x and to obtain a lean stream having a reduced level of SO₂, CO₂ and NO_x.

11. The process as claimed in claim 1 further comprising:

a) subjecting the waste gas stream to a SO₂ removal step using a regenerated SO₂ absorbent stream and recovering a SO₂ lean stream and a SO₂ rich absorbent stream;

b) treating the SO₂ rich absorbent stream to obtain the regenerated SO₂ absorbent stream and a first gaseous stream;

c) subjecting the SO₂ lean stream to a NO_x removal step using a regenerated NO_x absorbent stream and recovering the lean stream and a NO_x rich absorbent stream; and

,

d) treating the NO_x rich absorbent stream to obtain the regenerated NO_x absorbent stream and a second gaseous stream.

12. The process as claimed in claim 1 further comprising:

a) subjecting the waste gas stream to a SO₂ removal step using a regenerated SO₂ absorbent stream and recovering a SO₂ lean stream and a SO₂ rich absorbent stream;

b) treating the SO₂ rich absorbent stream to obtain the regenerated SO₂ absorbent stream and a first gaseous stream;

c) subjecting the SO₂ lean stream to a CO₂ removal step using a regenerated CO₂ absorbent and recovering a CO₂ lean stream and a CO₂ rich absorbent stream;

d) treating the CO₂ absorbent to obtain the regenerated CO₂ absorbent stream and a third gaseous stream;

e) subjecting the CO₂ lean stream to a NO_x removal step using a regenerated NO_x absorbent stream and recovering the lean stream and a NO_x rich absorbent stream; and

,

f) treating the NO_x rich absorbent stream to obtain the regenerated NO_x absorbent stream and a second gaseous stream.

13. The process as claimed in claim 1 wherein the lean stream comprises less than 100 ppm SO₂ and NO_x.

14. The process as claimed in claim 1 wherein the lean stream comprises less than 50 ppm SO₂ and NO_x.

15. The process as claimed in claim 1 wherein at least some of the permanganate reacts upon expose to mercury vapour to produce manganese dioxide and the process further comprises treating the mercury rich absorbent stream to remove precipitated manganese dioxide.

16. The process as claimed in claim 15 wherein the mercury rich absorbent stream is treated to remove precipitated manganese dioxide on a batch basis.

17. The process as claimed in claim 1 wherein the mercury rich absorption solution is acidic and contains Hg⁺⁺ ions and Mn⁺⁺ ions and the process further comprises treating the mercury rich absorption solution to precipitate manganese dioxide.

18. The process as claimed in claim 17 further comprising treating the mercury rich absorption solution with permanganate to precipitate manganese dioxide.

19. The process as claimed in claim 17 further comprising subsequently removing the dissolved Hg⁺⁺ by one or more of selective ion exchange and precipitation.

20. The process as claimed in claim 19 wherein the Hg⁺⁺ is precipitated by the addition of a precipitation agent comprising one or more of sulfide and trimercapto-s-triazine trisodium salt.

21. The process as claimed in claim 1 further comprising monitoring the concentration of permanganate in the mercury absorbent solution and adding alkali metal permanganate to maintain the concentration of permanganate in the mercury absorbent solution in the range 0.01 to 5% wt.

22. The process as claimed in claim 1 wherein the mercury rich absorbent solution is neutralized to a pH>

4 and sufficient permanganate is added to oxidize manganese in the +2 oxidation state to the +4 oxidation state whereby essentially all the manganese salts precipitate as MnO₂.

23. The process as claimed in claim 22 further comprising treating the mercury rich absorbent solution having precipitate therein to separate the precipitate and subsequently treating the mercury rich absorbent solution to remove dissolved Hg⁺⁺.

24. The process as claimed in claim 1 wherein the mercury rich absorption solution contains Hg⁺⁺ ions and precipitated manganese dioxide and the process further comprises separating the precipitates manganese dioxide and obtaining a liquid stream.

25. The process as claimed in claim 24 further comprising subsequently removing the dissolved Hg⁺⁺ from the liquid stream by one or more of selective ion exchange and precipitation.

26. The process as claimed in claim 25 wherein the Hg⁺⁺ is precipitated by the addition of a precipitation agent comprising one or more of sulfide and trimercapto-s-triazine trisodium salt.