Oxygen removal from contaminated gases
The invention relates to a method for achieving low oxygen levels in a natural gas stream without the use of a catalytic system. In one embodiment, the method comprises: membrane treatment for the removal of the bulk of CO 2 and oxygen in the natural gas feed and the addition of a PSA system using a carbon molecular sieve adsorbent for the adsorption of residual oxygen.
1. A process for the improved separation of oxygen from a natural gas feed stream which comprises:
a. passing said natural gas feed stream to a first separation unit so as to partially separate oxygen from said feed stream and produce a first product stream enriched with methane and a waste stream having a higher molar concentration of oxygen than said feed stream; and
b. passing said first product stream to a second separation unit containing an oxygen-selective adsorbent comprising a carbon molecular sieve and operated with a pressure swing so as to preferentially adsorb residual said oxygen from said first product stream to produce a low pressure gas stream enriched with oxygen and produce a higher pressure second product stream enriched with methane and having an oxygen content of less than 2,000 ppm.
2. The process of claim 1 , wherein the natural gas feed stream comes from landfill gas, coal mine methane, coal bed methane, natural gas wells, or biogas.
3. The process of claim 1 , wherein said natural gas is from landfill gas.
4. The process of claim 1 , wherein said first separation unit is a membrane unit.
5. The process of claim 4 , wherein said first product stream is a non-permeate stream.
6. The process of claim 1 , wherein said first separation unit is a pressure swing adsorption unit.
7. The process of claim 1 , wherein said low pressure gas stream of said second separation unit is recycled back to said natural gas feed stream.
8. The process of claim 1 , wherein the oxygen concentration of said second product stream is about 1000 ppm or less.
9. The process of claim 1 , wherein the oxygen concentration of said second product stream is about 100 ppm or less.
10. The process of claim 1 , further comprising: prior to passing said first product to said second separation unit;
a. passing said first product stream to a pressure swing adsorption nitrogen removal unit so as to remove nitrogen from said first product stream to produce a low pressure tail gas enriched with nitrogen and produce a higher pressure nitrogen-depleted product stream; and
b. passing said nitrogen-depleted product stream to said second separation unit.
11. The process of claim 10 , wherein the nitrogen removal unit contains a molecular sieve selective for adsorbing nitrogen.
12. The process of claim 11 , wherein nitrogen concentration of said second product stream is about 7% or less.
13. The process of claim 11 , wherein the nitrogen concentration of said second product stream is about 4% or less.
14. The process of claim 1 comprising passing said first product stream to a pressure swing adsorption unit containing a combination of said carbon molecular sieve adsorbent and a nitrogen selective adsorbent.
15. The process of claim 14 , wherein said nitrogen selective adsorbent is ETS-4 or CTS-1.
16. The process of claim 1 , wherein said second product stream is directed to a pressure swing adsorption unit containing a nitrogen selective adsorbent.
17. The process of claim 11 , wherein said molecular sieve is ETS-4 or CTS-1.
18. The process of claim 1 wherein said first separation unit also removes carbon dioxide.
19. A process for the improved separation of oxygen from a natural gas feed stream, which comprises:
a. passing said natural gas feed stream to a membrane separation unit so as to partially separate oxygen from said feed stream and produce a first product stream enriched with methane and a permeate waste stream having a higher molar concentration of oxygen than said feed stream;
b. passing said first product stream to a pressure swing adsorption unit containing a carbon molecular sieve so as to preferentially adsorb residual oxygen from said first product stream to produce a low pressure gas stream enriched with oxygen and to produce a second product stream enriched with methane and having an oxygen content of 1,000 ppm or less; and
c. recycling said low pressure gas stream back to said feed stream for said membrane separation unit.
20. The process of claim 19 , wherein said first separation unit also removes carbon dioxide.
21. The process of claim 19 , further comprising: prior to passing said first product to said second separation unit;
a. passing said first product stream to a pressure swing adsorption nitrogen removal unit so as to remove nitrogen from said first product stream to produce a low pressure tail gas enriched with nitrogen and produce a higher pressure nitrogen-depleted product stream; and
b. passing said nitrogen-depleted product stream to said second separation unit.