Low energy regenerable SO2 scrubbing process

• US 20050034600A1
• Filed: 08/13/2003
• Published: 02/17/2005
• Est. Priority Date: 08/13/2003
• Status: Active Grant

• Current Assignee: Shell Oil Company (Shell Plc)
• Original Assignee: Cansolv Technologies Inc. (Shell Plc)
• Inventors: Sarlis, John Nicolas, Ravary, Patrick M., Parisi, Paul Joseph, Hakka, Leo E.
• Granted Patent: US 7,214,358 B2
• Time in Patent Office: Days
• US Class Current: 95/235
• CPC Class Codes: B01D 53/1481 Removing sulfur dioxide or sulfur trioxide
• CPC Class Codes: B01D 53/1493 Selection of liquid materials for use as absorbents
• CPC Class Codes: Y02A 50/20 Air quality improvement or preservation, e.g. vehicle emission control or emission reduction by using catalytic converters

Abstract

Sulphur dioxide is removed from gas streams by contacting the gas stream with an absorbing medium containing an amine capable of forming an amine salt, heat stable salt and sulfite. The level of heat stable salt is selected such that during the regeneration process of the sulphur dioxide rich amine, the pH of the absorbing medium is at a selected level or below when the level of sulfite in the absorbing medium has been reduced to a specified value. The amine that absorbs the sulphur dioxide has a pKa less than that of sulfite. If the absorbent includes a diamine, then the spent absorbing medium is regenerated under conditions such that at least one amine group remains in salt form.

Claims

1. A cyclic process for the removal of sulphur dioxide from a sulphur dioxide containing gas stream using an absorbent medium and for the regeneration of the absorbent medium comprising

a) contacting the gas stream with a lean aqueous absorbing medium containing a sulphur dioxide absorbent to absorb sulphur dioxide from the gas to form a sulphur dioxide lean treated gas stream and spent absorbing medium;

b) steam stripping gaseous sulphur dioxide from the spent absorbing medium at a temperature to form a regenerated aqueous absorbing medium;

c) recovering the gaseous sulphur dioxide;

d) monitoring the regenerated aqueous absorbing medium and adjusting the level of heat stable salt to maintain the pH of the regenerated aqueous absorbing medium at a selected pH level; and

e) recycling the regenerated aqueous absorbing medium to the contacting step.

2. The process as claimed in claim 1 wherein the selected level is 6 or less.

3. The process as claimed in claim 1 wherein the selected level is 5 or less.

4. The process as claimed in claim 1 wherein the selected pH level is determined based on a targeted concentration of total dissolved SO₂ in the regenerated aqueous absorbing medium.

5. The process as claimed in claim 4 wherein the targeted concentration of total dissolved SO₂ in the regenerated aqueous absorbing medium is less than 1.0 wt % based on the total weight of the regenerated aqueous absorbing medium.

6. The process as claimed in claim 4 wherein the targeted concentration of total dissolved SO₂ in the regenerated aqueous absorbing medium is less than 0.5 wt % based on the total weight of the regenerated aqueous absorbing medium.

7. The process as claimed in claim 1 wherein the absorbent is a diamine that is selected so that, in free base form, the diamine has a molecular weight of less than about 300 and the diamine in half salt form having a pKa value for the free nitrogen atom of about 3.0 to about 5.5, the aqueous absorbing medium containing at least 1 mole of water for each mole of sulphur dioxide to be absorbed and having a heat sable salt concentration of greater than 1 equivalents of acid per mole of diamine prior to the lean aqueous absorbing medium contacting the sulphur dioxide containing gas.

8. The process as claimed in claim 1 wherein the absorbent is selected to have a pKa from 3.0 to 5.5 and the selected pH level is 5.5,

9. The process as claimed in claim 1 wherein the absorbent is selected to have a pKa from 3.2 to 5.0 and the selected pH level is 5,

10. The process as claimed in claim 1 wherein the absorbent is selected to have a pKa from 3.5 to 4.5 and the selected pH level is 5.

11. A cyclic process for the removal of sulphur dioxide from a sulphur dioxide containing gas stream using an absorbent medium and for the regeneration of the absorbent medium comprising

a) contacting the gas stream with a lean aqueous absorbing medium containing a water-soluble half salt of a diamine to absorb sulphur dioxide from the gas to form a sulphur dioxide lean treated gas stream and spent absorbing medium, the diamine in free base form having a molecular weight of less than about 300 and the diamine in half salt form having a pKa value for the free nitrogen atom of about 3.0 to about 5.5, the aqueous absorbing medium containing at least 1 mole of water for each mole of sulphur dioxide to be absorbed and having a heat sable salt concentration of greater than 1 equivalents of acid per mole of diamine prior to the lean aqueous absorbing medium contacting the sulphur dioxide containing gas;

b) steam stripping gaseous sulphur dioxide from the spent absorbing medium at a temperature to form a regenerated aqueous absorbing medium;

c) recovering the gaseous sulphur dioxide; and

d) recycling the regenerated aqueous absorbing medium to the contacting step.

12. The process as claimed in claim 11 wherein the gas stream is contacted with the aqueous absorbing medium at a temperature of from about 10°

to about 60°

C. to absorb sulphur dioxide from the gas stream in amounts of at least about 100 g of sulphur dioxide per kg of absorbing medium, and gaseous sulphur dioxide is stripped from the spent absorbing medium at a temperature of about 50°

to about 110°

C. and at least about 30°

C. greater than the contact temperature to form the regenerated aqueous absorbing medium.

13. The process as claimed in claim 11 wherein the heat sable salt concentration of the lean aqueous absorbing medium prior to contacting the sulphur dioxide containing gas is selected to be from 1.03 to 1.7 equivalents of acid per mole of diamine.

14. The process as claimed in claim 11 wherein the heat sable salt concentration of the lean aqueous absorbing medium prior to contacting the sulphur dioxide containing gas is selected to be from 1.05 to 1.45 equivalents of acid per mole of diamine.

15. The process as claimed in claim 11 wherein the heat sable salt concentration of the lean aqueous absorbing medium prior to contacting the sulphur dioxide containing gas is selected to be from 1.1 to 1.3 equivalents of acid per mole of diamine.

16. The process as claimed in claim 11 wherein the heat sable salt concentration is selected to reduce the concentration of sulfur dioxide in the treated gas stream to a selected level.

17. The process as claimed in claim 11 wherein the heat sable salt concentration is selected to reduce steam consumption in the steam-stripping step.

18. The process as claimed in claim 11 wherein the heat sable salt concentration is selected to reduce steam consumption in the steam stripping step to an amount in the range 25 kg steam/kg SO₂ recovered to 2 kg steam/kg SO₂ recovered while producing a sulphur dioxide lean treated gas stream having less than 10 ppm SO₂.

19. The process as claimed in claim 11 wherein the heat sable salt concentration is selected to reduce steam consumption in the steam stripping step to an amount in the range 10 kg steam/kg SO₂ recovered to 2 kg steam/kg SO₂ recovered while producing a sulphur dioxide lean treated gas stream having less than 50 ppm SO₂.

20. The process as claimed in claim 11 wherein the heat sable salt concentration is selected to reduce steam consumption in the steam stripping step to an amount in the range 10 kg steam/kg SO₂ recovered to 2 kg steam/kg SO₂ recovered while producing a sulphur dioxide lean treated gas stream having less than 20 ppm SO₂.

21. The process as claimed in claim 11 wherein the pH of the regenerated aqueous absorbing medium is monitored and the level of heats stable salts in the aqueous absorbing medium is adjusted to maintain the pH of the regenerated aqueous absorbing medium at a selected level.

22. The process as claimed in claim 11 wherein the selected level is 6 or less.

23. The process as claimed in claim 11 wherein the selected level is 5 or less.

24. A method of reducing the concentration of sulfur dioxide in a treated gas stream comprising:

a) contacting a gas stream with an aqueous absorbing medium containing an amine capable of forming an amine salt, heat stable salt and sulfite, the amine having a pKa less than that of sulfite;

b) stripping gaseous sulphur dioxide from the spent absorbing medium at a temperature to form a regenerated aqueous absorbing medium;

c) recovering the gaseous sulphur dioxide;

d) recycling the regenerated aqueous absorbing medium to contacting step (a); and

,

e) adjusting the aqueous absorbing medium to have a heat stable salt concentration sufficient to at least essentially neutralize the sulfite in the aqueous absorbing medium.

25. The method as claimed in claim 24 wherein the amine is part of a diamine and the method comprises adjusting the heat sable salt concentration of the aqueous absorbing medium to be greater than 1 equivalents of acid per mole of diamine.

26. The method as claimed in claim 24 wherein the aqueous absorbing medium contains a water-soluble half salt of a diamine to absorb sulphur dioxide from the gas stream to form a sulphur dioxide lean treated gas stream and a spent aqueous absorbing medium, the diamine in free base form having a molecular weight of less than about 300 and the diamine in half salt form having a pKa value for the free nitrogen atom of about 3.0 to about 5.5, the aqueous absorbing medium containing at least 1 mole of water for each mole of sulphur dioxide to be absorbed and comprising less than about 80 wt. % of water and the method comprises adjusting the heat sable salt concentration of the aqueous absorbing medium to be from 1.05 equivalents of acid per mole of diamine to 1.45 equivalents of acid per mole of diamine.

27. The method as claimed in claim 24 wherein the method further comprises adjusting the composition of the aqueous absorbing medium such that the pH of the regenerated aqueous absorbing medium is 6 or less.

28. The method as claimed in claim 24 wherein the method further comprises adjusting the composition of the aqueous absorbing medium such that the pH of the regenerated aqueous absorbing medium is 5.0 or less.

29. The method as claimed in claim 24 wherein the composition of the aqueous absorbing medium is adjusted by varying the level of heat stable salt in therein.