Process and apparatus for separating gases

1. An apparatus, comprising:

(i) a feed stream separation stage and a retentate separation stage which are membrane separation stages,

wherein the feed stream separation stage is configured to separate a feed stream comprising at least two components into a first permeate stream and a first retentate stream,

wherein the retentate separation stage is configured to separate the first retentate stream into a second permeate stream and a second retentate stream, and

wherein the second permeate stream being combined with a raw gas stream to provide the feed stream;

(ii) at least one device selected from the group consisting of a compressor arranged upstream of the feed stream separation stage, a vacuum device arranged in a permeate stream of the feed stream separation stage, and a vacuum device arranged in a retentate stream of the retentate separation stage;

(iii) a heater arranged between the feed stream separation stage and the retentate separation stage; and

(iv) optionally a purification treatment upstream of the feed stream separation stage;

wherein the membranes of the retentate separation stage have a higher total capacity, measured for nitrogen Grade 4.8 under standard conditions, than the membranes of the feed stream separation stage, and

wherein the retentate separation stage comprises a back pressure regulating valve in the second retentate stream, configured to adjust the retentate pressure to 1 to 100 bar.

2. The apparatus of claim 1 , wherein a ratio of the total capacity of the membranes of the retentate separation stage to the total capacity of the membranes of the feed stream separation stage is in a range of from 1.05 to 10.

3. The apparatus of claim 1 , wherein the membranes of the feed stream separation stage have a pure gas selectivity for carbon dioxide over methane of at least 30.

4. The apparatus of claim 1 , wherein the material of a separation-active layer of the membranes is at least one selected from the group consisting of a polyamide, a polyetherimide, a polyaramide, a polybenzoxazole, a polybenzothiazole, a polybenzimidazole, a polysulfone, a cellulose acetate, a cellulose acetate derivative, a polyphenylene oxide, a polysiloxane, a polymer with intrinsic microporosity, a mixed matrix membrane, a facilitated transport membrane, a polyethylene oxide, a polypropylenexide, and a polyimide.

5. The apparatus of claim 4 , wherein said material is a polyimide consisting of the monomer units A and B:

where x is from 0 to 0.5 and y is from 0.5 to 1, and R is the same or different radical selected from the group consisting of radicals L1, L2, L3 and L4:

6. The apparatus of claim 1 , wherein the compressor is present, and is a multistage compressor, and optionally the second permeate stream is introduced into the compressor between two compression stages.

7. The apparatus of claim 1 , wherein the membrane separation stages consist of hollow fiber gas separation membrane modules.

8. A process for separating gases, carried out in an apparatus comprising:

(i) a feed stream separation stage and a retentate separation stage which are membrane separation stages, the membranes of the retentate separation stage having a higher total capacity, measured for nitrogen Grade 4.8 under standard conditions, than the membranes of the feed stream separation stage;

(ii) at least one device selected from the group consisting of a compressor arranged upstream of the feed stream separation stage, a vacuum device arranged in a permeate stream of the feed stream separation stage, and a vacuum device arranged in a retentate stream of the retentate separation stage;

(iii) a heater arranged between the feed stream separation stage and the retentate separation stage; and

(iv) optionally a purification treatment upstream of the feed stream separation stage;

the process comprising:

separating a feed stream comprising at least two components into a first permeate stream and a first retentate stream in the feed stream separation stage,

separating the first retentate stream into a second permeate stream and a second retentate stream in the retentate separation stage;

removing the first permeate stream as a first product, further processing the first permeate stream, or, if the second retentate stream is removed or further processed, discarding the first permeate stream;

removing the second retentate stream as a second product, further processing the second retentate stream, or, if the first permeate stream is removed or further processed, discarding the second retentate stream;

combining the second permeate stream with the feed stream; and

heating the first retentate stream to a temperature higher than the temperature of the feed stream, before introducing the first retentate stream to the retentate separation stage.

9. The process of claim 8 , wherein a ratio of the total capacity of the membranes of the retentate separation stage to the total capacity of the membranes of the feed stream separation stage is in a range of from 1.05 to 10.

10. The process of claim 8 , wherein the membranes of the feed stream separation stage have a pure gas selectivity for carbon dioxide over methane of at least 30.

11. The process of claim 8 , wherein the material of a separation-active layer of the membranes is at least one selected from the group consisting of a polyamide, a polyetherimide, a polyaramide, a polybenzoxazole, a polybenzothiazole, a polybenzimidazole, a polysulfone, a cellulose acetate, a cellulose acetate derivative, a polyphenylene oxide, a polysiloxane, a polymer with intrinsic microporosity, a mixed matrix membrane, a facilitated transport membrane, a polyethylene oxide, a polypropylenexide, and a polyimide.

12. The process of claim 11 , wherein said material is a polyimide consisting of the monomer units A and B:

where x is from 0 to 0.5 and y is from 0.5 to 1, and R is the same or different radical selected from the group consisting of radicals L1, L2, L3 and L4:

13. The process of claim 8 , wherein the compressor is present, and is a multistage compressor, and optionally the second permeate stream is introduced into the compressor between two compression stages.

14. The process of claim 8 , wherein the membrane separation stages consist of hollow fiber gas separation membrane modules.

15. The process of claim 8 , further comprising adjusting the retentate pressure of the retentate separation stage with a back pressure regulating valve in the second retentate stream to 1 to 100 bar.

16. The process of claim 8 , wherein the driving force used for the separation task is a partial pressure difference of the permeate gas between the retentate side and the permeate side in the respective membrane separation stages which is generated by a compressor in the feed stream, by a vacuum device in the second permeate stream, by a permeate-side flushing-gas stream, or a combination thereof.

17. The process of claim 8 , wherein the first retentate stream is heated to a temperature 5 to 50° C. higher than the temperature of the feed stream before the first retentate stream is introduced to the retentate separation stage.

18. The process of claim 8 , wherein the temperature of the feed stream is in the range of from 15 to 45° C. before the feed stream is introduced to the feed stream separation stage.

19. The process of claim 8 , wherein the gas volume recycled with the second permeate stream totals less than 40% by volume of the raw gas stream.

20. The process of claim 8 , wherein the raw gas stream is selected from the group consisting of a biogas, a natural gas, air, a gas mixture comprising carbon dioxide and methane, a gas mixture comprising hydrogen and methane, a gas mixture comprising carbon monoxide and methane, a gas mixture comprising helium and methane, a gas mixture comprising helium and nitrogen, a gas mixture comprising hydrogen and carbon monoxide, a gas mixture comprising a permanent gas having a boiling point of less than 110 K at 1 atm and a non permanent gas having a boiling point above or equal to 110 K at 1 atm, a gas mixture comprising carbon dioxide and a hydrocarbon, and a gas mixture comprising nitrogen and a hydrocarbon.