Removal of acid gases and sulfur compounds from hydrocarbon gas streams in a caustic tower
The present invention provides a process for treating a hydrocarbon feed gas stream containing acid gases (CO 2 , H 2 S and mercaptans) by contacting the feed gas stream in a multi-section caustic tower, the bottom sections employing a recirculating caustic solution to remove the CO 2 and H 2 S down to low single digits parts per million concentration and the upper sections employing a stronger caustic solution on a once-through basis to produce a mercaptans depleted gas stream.
1. A process for removing acid components from a hydrocarbon containing gas stream, said process comprising:
(a) feeding a gaseous feed stream comprising hydrocarbons, carbon dioxide, hydrogen sulfide and mercaptans to a caustic recirculation zone in a multi-stage caustic tower, wherein said multi-stage caustic tower has a least one said caustic recirculation zone and at least one once-through caustic zone located downstream of the caustic recirculation zone(s);
(b) contacting said gaseous feed stream in said at least one caustic recirculation zone with a caustic stream that exits said tower and is recirculated to said tower to remove substantially all of said carbon dioxide and hydrogen sulfide from said gaseous stream and provide a partially treated gaseous stream comprising hydrocarbons and mercaptans;
(c) passing said partially treated gaseous stream to said at least one once-through caustic zone;
(d) contacting said partially treated gaseous stream in said at least once-through caustic zone with a caustic solution on a once-through basis to remove substantially all of the mercaptans from said partially treated gaseous stream and produce a product stream substantially free of carbon dioxide, hydrogen sulfide and mercaptans.
2. A process as defined in claim 1 wherein said gaseous feed stream comprises an effluent from a steam cracker.
3. A process as defined in claim 1 wherein said multi-stage caustic tower comprises two caustic recirculation zones.
4. A process as defined in claim 3 wherein the gaseous feed stream is contacted with a first recirculated caustic stream in a first caustic recirculation zone to remove a portion of the carbon dioxide and hydrogen sulfide from said gaseous feed stream to produce a first partially treated gas stream and wherein said first partially treated gas stream is fed to a second recirculated caustic zone for contact with a second recirculated caustic stream to substantially remove remaining carbon dioxide and hydrogen sulfide and produce a second partially treated gas stream.
5. A process as defined in claim 1 wherein said caustic stream in said caustic recirculation zone has a caustic strength less than the strength of the caustic in said once-through caustic zone.
6. A process as defined in claim 1 wherein said caustic employed in said tower is selected from the group consisting of ammonia, sodium hydroxide, potassium hydroxide and mixtures thereof.
7. A process as defined in claim 4 wherein said first recirculated caustic stream comprises sodium hydroxide.
8. A process as defined in claim 7 wherein said second recirculated caustic stream comprises sodium hydroxide.
9. A process as defined in claim 4 wherein said caustic strength of the second recirculated caustic ranges from about 2 to about 20 weight percent.
10. A process as defined in claim 9 wherein said caustic strength of said second recirculated caustic ranges from about 4 to about 15 weight percent.
11. A process as defined in claim 1 wherein said tower comprises one once through caustic zone.
12. A process as defined in claim 11 wherein the once through caustic in said once through caustic zone has a caustic strength ranging from about 15 to about 50 weight percent.
13. A process as defined in claim 12 wherein the once through caustic in said once through caustic zone has a caustic strength ranging from about 20 to about 30 weight percent.
14. A process as defined in claim 1 wherein spent caustic from said once through caustic zone is treated to remove mercaptans and mercaptan salts.
15. A process as defined in claim 14 wherein said spent caustic treatment comprises oxidizing said spent caustic over a catalyst comprising a metal phthalocyanine or derivative thereof to form a dialkyl disulfide oil and then separating the dialkyl disulfide oil from said caustic solution.
16. A process as defined in claim 1 further comprising water washing the product stream from said once-through caustic zone to remove entrained caustic and produce a water washed product stream.
17. A process as defined in claim 1 wherein said product stream comprises less than about 5 vppm CO 2 , less than about 1 vppm H 2 S and less than about 5 vppm mercaptides.
18. A process as defined in claim 17 wherein said product stream comprises less than about 1 vppm CO 2 , less than about 0.1 vppm H 2 S and less than about 2 vppm mercaptides.