METHOD AND APPLIANCE FOR SEPARATING A SYNTHESIS GAS BY CRYOGENIC DISTILLATION

Abstract

The invention relates to a method for separating a synthesis gas comprising hydrogen and carbon monoxide by cryogenic distillation, according to which the synthesis gas (1, 5) is cleaned and cooled to a cryogenic temperature, the cooled synthesis gas is separated by a first means (15) in order to produce a hydrogen-depleted liquid (33), the hydrogen-depleted liquid is introduced into the upper part of a stripping column (25) and a hydrogen-enriched gas (27) is drawn off at the head of the stripping column, at least partially condensed and sent back to the upper part of the stripping column.

Claims

1. A process for the separation of a synthesis gas comprising hydrogen, carbon monoxide and methane, and possibly nitrogen, by cryogenic distillation, in which: i) the synthesis gas (1, 5) is purified and cooled down to a cryogenic temperature, ii) the cooled synthesis gas is separated by a first means (9, 15) in order to produce a liquid depleted in hydrogen (91), the separation carried out by the first means consists of a stage of scrubbing in a scrubbing column (15) with at least a part of the liquid enriched in methane withdrawn from a column for the separation of carbon monoxide and methane having an overhead condenser, the condenser being cooled by a carbon monoxide cycle, and iii) the liquid depleted in hydrogen is introduced into the upper part of a stripping column (25) which also comprises a lower part, iv) a gas enriched in hydrogen (27, 29) is withdrawn at the top of the stripping column, v) a liquid (33) is withdrawn at the bottom of the stripping column and sent to the separation column (45), a gas (43) enriched in carbon monoxide and depleted in methane is withdrawn at the top of the separation column, a liquid (77) depleted in carbon monoxide and enriched in methane is withdrawn at the bottom of the separation column and the gas enriched in carbon monoxide is heated by heat exchange with the synthesis gas from stage i) in order to form a product (29), vi) a gas (27) withdrawn in the upper part of the stripping column is at least partially condensed and returned at least in part to the upper part of the stripping column, characterized in that the gas withdrawn in the upper part of the stripping column is at least partially condensed in a heat exchanger (23) which also serves to cool at least one gas (210, 211) withdrawn from the scrubbing column, the heat exchanger also serving to heat a refrigerant.

2. The process as claimed in claim 1, in which the gas (27) withdrawn in the upper part of the stripping column condenses against at least a part (51) of the carbon monoxide cycle liquid.

3. The process as claimed in either of the preceding claims, in which the synthesis gas (1, 3) comprises nitrogen and the liquid withdrawn from the stripping column (25) or a fluid derived from this liquid is separated in a denitrogenation column (55), the liquid of which serves to at least partially condense the overhead gas from the stripping column.

4. The process as claimed in one of the preceding claims, in which the gas (27) withdrawn in the upper part of the stripping column is at least partially condensed by heat exchange with a liquid enriched in carbon monoxide withdrawn from the separation column (45), which heats up and optionally vaporizes, at least partially.

5. The process as claimed in claim 4, in which the liquid (47) enriched in carbon monoxide is withdrawn from a distillation section of the separation column (45) or from a capacity forming the top of the separation column.

6. The process as claimed in one of the preceding claims, in which the gas withdrawn from the upper part of the stripping column (27) is an overhead gas from the stripping column withdrawn at a level above any heat and mass transfer means of the stripping column.

7. The process as claimed in one of claims 1 to 6, in which the gas withdrawn from the upper part of the stripping column is withdrawn at least one theoretical plate below the top of the stripping column, a part of the liquid enriched in methane withdrawn from the separation column being sent to a level of the stripping column above the level for withdrawal of the gas.

8. The process as claimed in one of claims 1 to 6, in which the separation carried out by the first means does not comprise a stage of washing with a liquid enriched in methane.

9. The process as claimed in one of the preceding claims, which is kept cold by a cycle using the gas enriched in carbon monoxide originating from the separation column (45).

10. The process as claimed in one of the preceding claims, in which the liquid (17) depleted in hydrogen contains between 1 mol % and 3 mol % of hydrogen.

11. An apparatus for the separation of a synthesis gas comprising hydrogen, carbon monoxide, methane and optionally nitrogen by cryogenic distillation comprising a heat exchanger (7), a first separation means (9, 15) which is a methane scrubbing column, a stripping column (25) and optionally a column for the separation (45) of carbon monoxide and methane, means (3) for purifying the synthesis gas, means for sending the purified gas to be cooled in the heat exchanger to a cryogenic temperature, means for sending the cooled synthesis gas to the first means for producing a liquid depleted in hydrogen (17), means for introducing the liquid depleted in hydrogen into the upper part of the stripping column which also comprises a lower part, means for withdrawing a gas enriched in hydrogen (27) at the top of the stripping column, means for withdrawing a liquid (33) at the bottom of the stripping column, a second heat exchanger (23), means for sending there a gas (27) withdrawn in the upper part of the stripping column and means for sending at least a part of the gas at least partially condensed in the second heat exchanger to the upper part of the stripping column, means for sending the stripping column (25) bottom liquid (33) to the column for the separation of carbon monoxide and methane (45), means for withdrawing a gas (43) enriched in carbon monoxide and depleted in methane at the top of the separation column, means for withdrawing a liquid (77) depleted in carbon monoxide and enriched in methane at the bottom of the separation column and means for sending the gas enriched in carbon monoxide to heat up in the heat exchanger by heat exchange with the synthesis gas in order to form a product, characterized in that it comprises means for sending at least one gas (210, 211) withdrawn from the scrubbing column to be cooled in the second heat exchanger and means for sending at least one refrigerant to heat up in the second heat exchanger.

12. The apparatus as claimed in claim 11, in which the second heat exchanger is an indirect heat exchanger (23).

13. The apparatus as claimed in claim 11 or 12, comprising means for withdrawing the gas in the upper part of the stripping column and for sending it to condense against at least a part of the carbon monoxide cycle liquid (47, 51).

14. The apparatus as claimed in claim 11, 12 or 13, where the refrigerant is a liquid (47, 51) enriched in carbon monoxide and comprising means for withdrawing the liquid enriched in carbon monoxide is from a distillation section of the separation column or from a capacity forming the top of the separation column.

15. The apparatus as claimed in one of claims 11 to 14, in which the means for keeping the process cold comprise a cycle (47, 51, 57, 67) using the gas enriched in carbon monoxide originating from the separation column.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0054] For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

[0055] FIG. 1 schematically illustrates a process using a phase separator, a methane scrubbing column, a stripping column and a column for the separation of carbon monoxide and methane.

[0056] FIG. 2 schematically illustrates a stripping column with a methane scrubbing section, a part of the liquid pressurized and sent to the top of the stripping column and another part being sent to the top of the scrubbing column.

[0057] FIG. 3 schematically illustrates a process that does not use a methane scrubbing column.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0058] FIG. 1 shows a process using a phase separator 9, a methane scrubbing column 15, a stripping column 25 and a column for the separation of carbon monoxide and methane 45, for example containing structured packings for the columns, and capable of operating at cryogenic temperatures.

[0059] The synthesis gas 1 containing carbon monoxide, methane and carbon monoxide is purified of water and/or of carbon dioxide in the purification unit 3 before arriving at the heat exchanger 7, where it cools to a cryogenic temperature and partially condenses.

[0060] The two phases are separated in a phase separator 9, in order to form a gas 11 enriched in hydrogen and a liquid depleted in hydrogen 13. The gas 11 is sent to the bottom of the methane scrubbing column 15, which produces a gas 19 enriched in hydrogen which is heated in the exchanger. A part of this gas 19 serves to regenerate the purification unit 3.

[0061] At least one intermediate gas 21A, 21B, 21C withdrawn from the column 15 is cooled in a heat exchanger 23 by indirect heat exchange with a fluid of the process, in this instance the liquid 51.

[0062] The bottom liquid 17 from the column 15 joins the liquid 13 from the separator 9 and the mixture 91, containing between 1 mol % and 3 mol % of hydrogen, is sent to the top of a stripping column 25. An overhead gas 27 from the stripping column condenses at least partially in the heat exchanger 23. A part 31 at least of the at least partially condensed gas is returned at the top of stripping column 25 in order to provide reflux liquid. The remainder 29 can heat up in the heat exchanger 7 against the synthesis gas 5.

[0063] The refrigerant gas 27 which leaves to be cooled in the heat exchanger 23 will undergo an at least partial condensation therein. In the case of partial condensations, liquid and gas are created. It is possible that a part of the liquid created passes through another pipe 31 or else falls back via the pipe 27 from which the refrigerant gas to be cooled originates, to join the column 25. In this second case, the pipe 31 is not necessary.

[0064] A liquid 33 taken at the bottom of the stripping column 25 cools in the exchanger 7 and is sent to the separation column 45. Another part of the same liquid 35 vaporizes in a bottom reboiler 37 and is returned at the bottom of the stripping column.

[0065] The separation column comprises several sections for separation by distillation and optionally a capacity 99. It has a bottom reboiler 73 which serves to heat the bottom liquid 75, the gas formed being returned to the bottom. The bottom liquid 77 enriched in methane is divided into two. A part 83 vaporizes in the exchanger 7 in order to form fuel. The remainder 85 is pressurized by a pump 87 and is sent to the top of the scrubbing column 15.

[0066] The overhead gas from the column 43 enriched in carbon monoxide is sent to a product compressor 57, which produces a gas enriched in carbon monoxide 57. A part of the gas enriched in carbon monoxide 61 is cooled and is divided into two. A part 65 is expanded in a turbine 67 in order to provide cold. The expanded gas 89 is returned to the inlet of the compressor 57. The remainder of the gas 69 continues to cool in the exchanger 7 and serves to heat the reboilers 73 and 37 (flows 93 and 73). The gas which has served for the reboiling is thus partially condensed and feeds, as flow 97, the capacity 99 at the top of the separation column 45. The gas 41 from the capacity 99 feeds the compressor 57. The liquid 47 from the capacity 99 is sent to a phase separator 49, the liquid 51 from the separator serves as refrigerant in the heat exchanger 23 in order to cool the intermediate gases 21A, 21B, 21C as well as the overhead gas 27 from the stripping column. The liquid 51 is thus vaporized and returned to the phase separator 49, the gas 53 from which feeds the compressor 57.

[0067] A liquid withdrawn from the separation section of the separation column can replace the liquid 47 or another liquid of the process.

[0068] According to an alternative form of the process, illustrated in FIG. 2, the stripping column comprising a methane scrubbing section, a part of the liquid 85 pressurized by the pump 87 being sent to the top of the stripping column 25 and another part being sent, as for FIG. 1, to the top of the scrubbing column 15.

[0069] In this case, the gas 27 withdrawn from the stripping column 25 is taken at least one theoretical plateau below the top of the column.

[0070] The gas at least partially condensed in the exchanger 23 returns to the stripping column next to the withdrawal point and the gas rising in the upper part of the stripping column (scrubbing section 25A) becomes more enriched in hydrogen, making it possible to reduce the number of theoretical plates required.

[0071] As shown by FIG. 3, the invention also applies to the case where the process does not use a methane scrubbing column. In this instance, the first separation is carried out simply by partial condensation in the separator 9. The gas 11 is heated and the liquid depleted in hydrogen 13 is sent to the stripping column 25. The latter may or may not comprise a methane scrubbing section, as for FIG. 2, according to requirements.

[0072] In this instance, the methane scrubbing section 25A is present and thus the gas 27 withdrawn from the stripping column is taken at an intermediate level, as for FIG. 2.

[0073] Thus, all the pressurized methane is sent to the top of the stripping column 25.

[0074] In the absence of a scrubbing column 15, the heat exchanger 23 is simplified and makes possible a heat exchange between two fluids alone, the gas to be cooled 27 and the liquid to be heated 51.

[0075] If the section 25A is absent, the gas 27 is taken at the top of the column 25.

[0076] It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.