Three-stage membrane gas separation process

1. A process for separating carbon dioxide from methane in a gas stream comprising the steps of:

(a) providing a first membrane having a first feed side and a first permeate side and being selective for carbon dioxide over methane;

(b) passing the gas stream as a first feed stream at a first pressure of at least about 400 psia across the first feed side;

(c) withdrawing from the first feed side a first residue stream depleted in carbon dioxide compared with the gas stream;

(d) withdrawing from the first permeate side a first permeate stream enriched in carbon dioxide compared with the gas stream;

(e) compressing the first permeate stream;

(f) providing a second membrane having a second feed side and a second permeate side and being selective for carbon dioxide over methane;

(g) passing the compressed first permeate stream as a second feed stream across the second feed side;

(h) withdrawing from the second feed side a second residue stream depleted in carbon dioxide compared with the second feed stream;

(i) withdrawing from the second permeate side a second permeate stream enriched in carbon dioxide compared with the second feed stream;

(j) recirculating the second residue stream to step (b);

(k) providing a third membrane having a third feed side and a third permeate side and being selective for carbon dioxide over methane;

(l) passing the second permeate stream as a third feed stream without compression across the third feed side;

(m) withdrawing from the third feed side a third residue stream depleted in carbon dioxide compared with the third feed stream;

(n) withdrawing from the third permeate side a third permeate stream enriched in carbon dioxide compared with the third feed stream;

(o) recirculating the third residue stream to step (e).

2. The process of claim 1 , wherein the gas stream comprises natural gas.

3. The process of claim 1 , wherein the gas stream contains at least about 4% carbon dioxide.

4. The process of claim 1 , wherein the first pressure is greater than about 500 psia and less than about 1,200 psia.

5. The process of claim 1 , wherein the gas stream contains less than 20% carbon dioxide.

6. The process of claim 1 , wherein the first permeate stream is withdrawn from the first permeate side at a second pressure of at least 20 psia and compressed in step (e) to at least about 400 psia.

7. The process of claim 1 , wherein the first residue stream contains less than 4% carbon dioxide.

8. The process of claim 1 , wherein the third permeate stream contains between 70% and 95% carbon dioxide.

9. The process of claim 1 , wherein the second permeate stream is withdrawn from the second permeate side and passed across the third feed side at a third pressure of at least about 60 psia.

10. The process of claim one wherein the first, second and third membranes have a selectivity for carbon dioxide over methane in the range 5-30.

11. The process of claim 1 , wherein the first, second and third membranes have a methane permeance of at least about 5 gpu.

12. The process of claim 1 , wherein the first, second and third membranes each have a selective layer comprising the same polymer.

13. The process of claim 1 , wherein the first membrane comprises a polymer having a repeat unit comprising a fluorinated cyclic structure.

14. The process of claim 1 , wherein at least about 90% of the methane contained in the gas stream is recovered in the first residue stream.

15. The process of claim 1 , wherein the gas stream further comprises nitrogen.

16. The process of claim 1 , where the gas stream further contains an aggregate percentage of carbon dioxide and nitrogen that is at least 4%.

17. A process for separating nitrogen from methane in a gas stream comprising the steps of:

(a) providing a first membrane having a first feed side and a first permeate side and being selective for nitrogen over methane;

(b) passing the gas stream as a first feed stream at a first pressure of at least about 400 psia across the first feed side;

(c) withdrawing from the first feed side a first residue stream depleted in nitrogen compared with the gas stream;

(d) withdrawing from the first permeate side a first permeate stream enriched in nitrogen compared with the gas stream;

(e) compressing the first permeate stream;

(f) providing a second membrane having a second feed side and a second permeate side and being selective for nitrogen over methane;

(g) passing the compressed first permeate stream as a second feed stream across the second feed side;

(h) withdrawing from the second feed side a second residue stream depleted in nitrogen compared with the second feed stream;

(i) withdrawing from the second permeate side a second permeate stream enriched in nitrogen compared with the second feed stream;

(j) recirculating the second residue stream to step (b);

(k) providing a third membrane having a third feed side and a third permeate side and being selective for nitrogen over methane;

(l) passing the second permeate stream as a third feed stream without compression across the third feed side;

(m) withdrawing from the third feed side a third residue stream depleted in nitrogen compared with the third feed stream;

(n) withdrawing from the third permeate side a third permeate stream enriched in nitrogen compared with the third feed stream;

(o) recirculating the third residue stream to step (e).

18. The process of claim 17 , wherein the gas stream comprises natural gas.

19. The process of claim 17 , wherein the gas stream contains at least about 4% nitrogen.

20. The process of claim 17 , wherein the first pressure is greater than about 500 psia and less than about 1,200 psia.

21. The process of claim 17 , wherein the gas stream contains less than 20% nitrogen.

22. The process of claim 17 , wherein the first permeate stream is withdrawn from the first permeate side at a second pressure of at least 20 psia and compressed in step (e) to at least about 400 psia.

23. The process of claim 17 , wherein the first residue contains no more than about 4% nitrogen.

24. The process of claim 17 , wherein the second permeate stream is withdrawn from the second permeate side and passed across the third feed side at a third pressure of at least about 50 psia.

25. The process of claim 17 , wherein the first, second and third membranes have a nitrogen permeance of at least about 50 gpu.

26. The process of claim 17 , wherein the first, second and third membranes each have a selective layer comprising the same polymer.

27. The process of claim 17 , wherein the first membrane comprises a polymer having a repeat unit comprising a fluorinated cyclic structure.

28. The process of claim 17 , wherein at least about 75% of the methane contained in the gas stream is recovered in the first residue stream.

29. The process of claim 1 , further comprising maintaining a fourth pressure on the third permeate side that is no lower than atmospheric pressure.

30. The process of claim 1 , wherein a second membrane pressure ratio is maintained across the second membrane, a third membrane pressure ratio is maintained across the third membrane and the third membrane pressure ratio is lower than the second membrane pressure ratio.

31. The process of claim 29 , wherein:

a second membrane pressure ratio of between 3 and 20 is maintained across the second membrane,

a third membrane pressure ratio between 3 and 15 is maintained across the third membrane and the second and third membrane pressure ratios are maintained by selecting a second permeate pressure to be maintained on the second permeate side without compression between the second permeate side and the third feed side.

32. The process of claim 17 , further comprising maintaining a fourth pressure on the third permeate side that is no lower than atmospheric pressure.

33. The process of claim 32 , wherein:

a second membrane pressure ratio of between 3 and 12 is maintained across the second membrane,

a third membrane pressure ratio between 3 and 12 is maintained across the third membrane and the second and third membrane pressure ratios are maintained by selecting a second permeate pressure to be maintained on the second permeate side without compression between the second permeate side and the third feed side.