PROCESS AND DEVICE FOR THE CRYOGENIC SEPARATION OF SYNTHESIS GAS

Abstract

The invention relates to a process and device for the cryogenic separation of a methane-containing feed gas predominantly consisting of hydrogen and carbon monoxide, that is partially condensed in this case by cooling, in order to obtain a hydrogen-containing first liquid phase predominantly consisting of carbon monoxide and methane, from which first liquid phase, in an H.sub.2 separation column that is heated via a circulation heater, a second liquid phase is generated by separating off hydrogen, from which second liquid phase, in a CO/CH.sub.4 separation column, a carbon monoxide-rich gas phase is obtained having a purity that permits release thereof as carbon monoxide product. It is characteristic in this case that a low-methane material stream is withdrawn from the H.sub.2 separation column and is then applied to the CO/CH.sub.4 separation column as reflux.

Claims

1. Process for the cryogenic separation of a methane-containing feed gas (1) predominantly consisting of hydrogen and carbon monoxide, that is partially condensed in this case by cooling, in order to obtain a hydrogen-containing first liquid phase (5) predominantly consisting of carbon monoxide and methane, from which first liquid phase, in an H.sub.2 separation column (T1) that is heated via a circulation heater (8), a second liquid phase (11) is generated by separating off hydrogen (9), from which second liquid phase, in a CO/CH.sub.4 separation column (T2), a carbon monoxide-rich gas phase (28) is obtained having a purity that permits release thereof as carbon monoxide product (29), characterized in that a low-methane material stream (26, 34) is withdrawn from the H.sub.2 separation column (T1) and is then applied to the CO/CH.sub.4 separation column (T2) as reflux.

2. Process according to claim 1, characterized in that the low-methane material stream (26) is withdrawn in the gaseous state from the H.sub.2 separation column (T1) and liquefied by cooling before introduction thereof into CO/CH.sub.4 separation column (T2).

3. Process according to claim 1, characterized in that the low-methane material stream (26) is withdrawn from the H.sub.2 separation column (T1) below the sixth practical separation step.

4. Process according to claim 3, characterized in that the low-methane material stream (26) is withdrawn from the sump space and/or between the first and third practical separation steps.

5. Process according to claim 1, characterized in that the low-methane material stream (34) is withdrawn in the liquid state from the H.sub.2 separation column.

6. Process according to claim 1, characterized in that the carbon monoxide-rich gas phase (28) obtained in the CO/CH.sub.4 separation column is warmed and released without pressure elevation as carbon monoxide product (29).

7. Process according to claim 1, characterized in that peak cold is provided via a nitrogen circuit.

8. Device for the cryogenic separation of a methane-containing feed gas (1) predominantly consisting of hydrogen and carbon monoxide, having at least one heat exchanger (E1, E2) for cooling and partial condensation of the feed gas (1), a separator (D1) in which a first liquid phase (5) can be separated off from the partially condensed feed gas (2), an H.sub.2 separation column (T1) that is heatable via a circulation heater (8) and in which a second liquid phase (11) can be generated from the first liquid phase (5) by separating off hydrogen (9), and also a CO/CH.sub.4 separation column (T2), in which a carbon monoxide-rich gas phase (28) can be separated off from the second liquid phase (11) at a purity that permits release thereof as carbon monoxide product (29), characterized in that the H.sub.2 separation column (T1) is connected to the CO/CH.sub.4 separation column (T2) in such a manner that a low-methane material stream (26, 34) can be withdrawn from the H.sub.2 separation column (T1) via a take-off site and can be applied to the CO/CH.sub.4 separation column (T2) as reflux.

9. Device according to claim 8, characterized in that a cooling appliance (E2) for liquefying a low-methane material stream (26) that is withdrawn from the H.sub.2 separation column in the gaseous state is arranged between the H.sub.2 separation column (T1) and the CO/CH.sub.4 separation column (T2).

10. Device according to claim 8, characterized in that the take-off site for the low-methane material stream (26) is arranged beneath the sixth practical separation step of the H.sub.2 separation column.

11. Device according to claim 10, characterized in that the take-off site for the low-methane material stream (26) is arranged between the sump space and the third practical separation step of the H.sub.2 separation column (T1).

12. Device according to claim 8, characterized in that the H.sub.2 separation column (T1) is constructed in the lower part thereof as a dividing-wall column, from which the low-methane material stream (34) can be withdrawn in the liquid state.

13. Device according to claim 8, characterized in that it comprises a cooling circuit that is operable with nitrogen as refrigerant.

14. Device according to claim 8, characterized in that the H.sub.2 separation column (T1) has sieve trays and/or slotted bubble-cap trays and/or structured packings and/or dumped-bed packings as practical separation stages.

Description

[0026] Hereinafter, the invention is to be described in more detail with reference to two exemplary embodiments which are schematically illustrated in FIGS. 1 and 2.

[0027] FIG. 1 shows an embodiment of the process according to the invention, in which a material stream provided as reflux for the CO/CH.sub.4 separation column is withdrawn from the H.sub.2 separation column in the gaseous state.

[0028] FIG. 2 shows a different embodiment of the process according to the invention, in which a material stream provided as reflux for the second CO/CH.sub.4 separation column is withdrawn in the liquid state from the H.sub.2 separation column.

[0029] In both figures, the same plant components and process streams are labelled with the same reference signs.

[0030] In FIG. 1, a methane-containing feed gas 1 that is to be separated and predominantly consists of hydrogen and carbon monoxide, that is present at a pressure between 30 and 60 bar(a) is cooled in the first heat exchanger E1 and the second heat exchanger E2 against process streams that are to be warmed, wherein a two-phase mixture of material 2 is formed by the condensation of components, which mixture of material is separated in the separator D1 into a hydrogen-containing liquid phase substantially consisting of carbon monoxide and methane and a hydrogen-rich gas phase. The gas phase is withdrawn via line 3 from the separator D1 and, after warming in the heat exchangers E2 and E1, is released as crude hydrogen 4 at the plant limits. The liquid phase 5, in contrast, is fed to the H.sub.2 separation column T1. For this purpose, it is split into two substreams, of which the first 6 is expanded as reflux into the head of the H.sub.2 separation column T1, while the second substream 7, after expansion and partial vaporization in the heat exchanger E2, is applied to the central part of the H.sub.2 separation column T1 as interstage heating.

[0031] The H.sub.2 separation column T1 is operated at a pressure that is between one third and one half of the pressure of the feed gas 1, and serves for removing the hydrogen dissolved in the liquid phase 5. It is heated by a circulation heater 8 that is integrated in the heat exchanger E2.

[0032] The hydrogen-rich overhead fraction 9 from the H.sub.2 separation column T1, after it is warmed in the heat exchangers E2 and E1, is released as flash gas 10 at the plant limits, whereas the substantially hydrogen-free sump fraction 11 consisting of carbon monoxide and methane is expanded into the CO/CH.sub.4 separation column T2 that is operated at a pressure between 8.5 and 9 bar(a). For this purpose, the sump fraction 11 is split into two substreams, of which one 12 serves as intermediate reflux, and the second 13, after vaporization in the heat exchanger E2, serves as interstage heating. The CO/CH.sub.4 separation column T2 is heated via a circulation heater 14 integrated into the heat exchanger E3.

[0033] The peak cold required for the process is obtained via a nitrogen circuit driven by the two-stage circuit compressor V. Nitrogen 15 leaves the second compressor stage C2 at a pressure that is typically between 16 and 21 bar(a), is subsequently cooled in the heat exchanger E1 and condensed in the heat exchanger E3 against sump product 14 of the CO/CH.sub.4 separation column T2 that is to be warmed. The condensed nitrogen 16 is expanded to an intermediate pressure between 7 and 9 bar(a), wherein a two-phase mixture of material 17 is formed which is separated in the separator D2 into a gas phase 18 and a liquid phase 19. A material stream 21 formed from the gas phase 18 and a part 20 of the liquid phase 19 is completely vaporized at the intermediate pressure level in the heat exchanger E2 and further warmed in the heat exchanger E1 before it is fed on the suction side to the second compressor stage C2. The remaining liquid phase 22 is further expanded to a low-pressure level between 3 and 5 bar(a), vaporized in the heat exchanger E2 and, after it is warmed in the heat exchanger E1, is recirculated via the suction side of the first compressor stage C1 into the circuit compressor V. The liquid phase 19 is divided into the two substreams 20 and 22 in such a manner that the temperature required at the separator D1 is achieved.

[0034] If required, external nitrogen can be fed to the closed nitrogen circuit via the low-pressure passage 22, wherein gaseous nitrogen 23 is introduced on the warm side of the heat exchanger E1 and liquid nitrogen 24 on the cold side of the heat exchanger

[0035] E2. Surplus nitrogen 25 is removed on the pressure side of the circuit compressor V.

[0036] To generate a reflux for the CO/CH.sub.4 separation column T2, a low-methane gas phase 26 is withdrawn from the H.sub.2 separation column T1 below the sixth practical separation stage, cooled and condensed in the heat exchanger E2 and then conducted via line 27 to the head of the CO/CH.sub.4 separation column T2. The overhead product 28 of the CO/CH.sub.4 separation column T2 has the purity required for a carbon monoxide product and is available at a pressure that is high enough in order to be able to release it, after it is warmed in the heat exchangers E2 and E1, as carbon monoxide product 29 without further compression. In the sump of the CO/CH.sub.4 separation column T2, a methane-rich carbon monoxide-containing liquid phase 30 collects that, after vaporization and warming in the heat exchangers E2 and E1 is released as fuel gas 31.

[0037] The exemplary embodiment shown in FIG. 2 permits the carbon monoxide product 29 to be generated at a higher purity than is possible with the configuration shown in FIG. 1. For this purpose, a column T3 is used for stripping off hydrogen from the liquid phase 5, which column T3, in the lower region thereof, is subdivided by a dividing wall into two segments S1 and S2. At the top end of the segment S1 there is situated the feed site for the substream 7 of the liquid phase 5 that serves as interstage heating, whereas at the top end of the segment S2, a condenser E4 is arranged in which a part 32 of the sump fraction 11 consisting of carbon monoxide and methane is used as refrigerant. The warmed and vaporized refrigerant 33 is then fed together with the substream 13 to the CO/CH.sub.4 separation column T2 as interstage heating. In order to avoid methane contaminants of the liquid phase in segment S2, the liquid phase flowing away from the upper region of the column T3 is fed alone to the segment S1. Below the condenser E4, therefore, from the segment S2 a low-methane carbon monoxide fraction 34 can be withdrawn in the liquid state that serves as reflux at the head of the CO/CH.sub.4 separation column T2.