METHOD AND APPARATUS FOR THE CRYOGENIC SEPARATION OF A SYNTHESIS GAS CONTAINING A NITROGEN SEPARATION STEP

Abstract

A method for separating a gas mixture comprising carbon monoxide, nitrogen and hydrogen involves sending a hydrogen-depleted fluid to a denitrification column (K2) having a top condenser (C1) and a bottom reboiler (R2) in order to produce a nitrogen-enriched gas at the top of the column and a nitrogen-depleted liquid at the bottom of the column, cooling the condenser of the denitrification column by means of a nitrogen cycle using a nitrogen compressor (V1, V2, V3), vaporising, in the heat exchanger of the condenser, the liquid nitrogen (53) from the nitrogen cycle, and returning the nitrogen (55) vaporised in the heat exchanger to the nitrogen compressor.

Claims

1.-13. (canceled)

14. A process for separating a gas mixture comprising carbon monoxide, nitrogen, hydrogen, and methane, the method comprising: i) cooling the gas mixture in a heat exchanger, ii) separating the gas mixture cooled in the heat exchanger by at least one scrubbing and/or distillation and/or partial condensation step, to form a hydrogen-depleted fluid containing carbon monoxide and nitrogen, iii) introducing the hydrogen-depleted fluid into a denitrification column comprising a column top, a top condenser, a column bottom, and a bottom reboiler, thereby producing a nitrogen-enriched gas at the column top and a nitrogen-depleted liquid at the column bottom, iv) cooling the top condenser by means of a nitrogen cycle using a nitrogen compressor comprising at least a first stage and a second stage, the first stage comprising a first entry pressure and the second stage comprising a second entry pressure, the first entry pressure being lower than that of the second entry pressure, v) expanding the nitrogen-depleted liquid and sending the expanded nitrogen-depleted liquid to the top condenser wherein it is at least partially vaporized by heat exchange in the condenser with the nitrogen-enriched gas, thereby condensing the expanded nitrogen-depleted liquid, vi) vaporizing a liquid nitrogen from the nitrogen cycle in the condenser and returning the vaporized nitrogen to the heat exchanger at an inlet of the second stage of the nitrogen compressor, and a) sending the bottom liquid from the denitrification column to a methane and carbon monoxide separation column comprising a top condenser which is a bath vaporizer placed in a bath of liquid, or b) the separation in step ii) comprises a distillation step in a methane and carbon monoxide separation column for separating a methane-depleted flow from a methane-enriched flow, and at least a portion of the methane-depleted flow constitutes the hydrogen-depleted fluid supplying the denitrification column, the methane and carbon monoxide separation column comprising a top condenser which is a bath vaporizer placed in a bath of liquid, the bath of liquid of a) or b) being supplied with liquid nitrogen from the nitrogen cycle.

15. The process of claim 14, wherein liquid nitrogen from the top condenser of the methane and carbon monoxide separation column is sent to vaporize in the top condenser of the denitrification column.

16. The process of claim 14, wherein the gas mixture cooled in the heat exchanger is separated by at least one partial condensation step so as to form a hydrogen-depleted gas, the hydrogen-depleted gas is sent to an intermediate level of a stripping column comprising a bottom reboiler, and bottom liquid of the stripping column is sent to a denitrification column in case a) or to the methane and carbon monoxide separation column in case b).

17. The process of claim 14, wherein the reboiler of the stripping column and/or the reboiler of the methane and carbon monoxide separation column is reheated with at least a portion of the gas mixture.

18. The process of claim 14, wherein the operating pressure of the denitrification column is at least 7 bar abs and/or the operating pressure of the methane and carbon monoxide separation column is at least 5 bar abs.

19. The process of claim 14, wherein the top condenser of the CO/CH.sub.4 column is cooled solely by cycle nitrogen.

20. The process of claim 14, wherein the reboiler of the denitrification column is reheated by means of the cycle nitrogen.

21. The process of claim 20, wherein the nitrogen used for reheating the reboiler of the denitrification column is at the maximum pressure of the nitrogen cycle.

22. The process of claim 14, wherein the nitrogen sent to the bath of the condenser of the column CO/CH.sub.4 is condensed at the maximum pressure of the nitrogen cycle.

23. The process of claim 14, wherein the operating pressure of the methane and carbon monoxide separation column is at least 5 bar.

24. An apparatus for separating a gas mixture comprising carbon monoxide, nitrogen, hydrogen and methane, comprising a heat exchanger for cooling the gas mixture, means for separating the gas mixture cooled in the heat exchanger by at least one scrubbing and/or distillation and/or partial condensation step, to form a hydrogen-depleted fluid containing carbon monoxide and nitrogen, a denitrification column comprising a top condenser and a bottom reboiler, a pipe for sending the hydrogen-depleted fluid to the denitrification column, to produce a nitrogen-enriched gas at the column top and a nitrogen-depleted liquid at the column bottom, a nitrogen cycle using a nitrogen compressor comprising at least a first stage and a second stage, the entry pressure of the first stage being lower than that of the second stage, means for sending liquid of the nitrogen cycle to the condenser of the denitrification column, means for expanding bottom liquid from the denitrification column, means for sending the expanded liquid to the top condenser of the denitrification column for at least partial vaporization by heat exchanger in a heat exchanger of the condenser with the nitrogen-enriched gas, which is thereby condensed, means for sending nitrogen vaporized in the heat exchanger of the condenser to the inlet of the second stage of the nitrogen compressor, a methane and carbon monoxide separation column comprising a top condenser which is a bath vaporizer placed in a bath of liquid, a) means for sending the bottom liquid from the denitrification column to the methane and carbon monoxide separation column, or b) the methane and carbon monoxide separation column, means forming part of the means for separating the gas mixture cooled in the heat exchanger by at least one distillation step, the apparatus further comprising means for sending liquid nitrogen from the nitrogen cycle to the top condenser of the methane and carbon monoxide separation column.

25. The apparatus of claim 24, comprising means for sending liquid nitrogen from the top condenser of the methane and carbon monoxide separation column to the top condenser of the denitrification column.

26. The apparatus of claim 24, further comprising at least one phase separator for separating the gas mixture cooled in the heat exchanger by a partial condensation step, to form a hydrogen-depleted gas, a stripping column, and means for sending the hydrogen-depleted gas to an intermediate level of the stripping column.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0060] 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:

[0061] FIG. 1 illustrates a schematic representation a separation process in accordance with one embodiment of the present invention.

[0062] FIG. 2 illustrates another schematic representation a separation process in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0063] In FIG. 1, a gas mixture 1, resulting for example from coal gasification, contains carbon monoxide, hydrogen, methane, water, and nitrogen. The gas 1 is purified in adsorbent beds 3A, 3B and cooled in a cooler 4, Then it is sent to a first heat exchanger E1 to be cooled. Partial flows of the synthesis gas are used to reheat the reheaters R1, R2, which are drawn twice at different places in the drawing for reasons of clarity. Following expansion in a valve, separation takes place in a phase separator S1, forming a gas 5 and a liquid 7. The gas 5 is cooled in a heat exchanger E2, expanded, and sent to a phase separator S4, The gas 9 from this phase separator S4, which is rich in hydrogen, is reheated in the heat exchangers E2, E1, and a portion of the gas is used to regenerate the adsorbent beds 3A, 3B, A portion 11 of the liquid from the phase separator S4 is expanded and sent to the top of a stripping column K1 operating at 17.6 bar. The column K1 has no top condenser, but has a bottom reboiler R1. The remainder 13 of the liquid from the phase separator S4 is expanded and sent to a phase separator S3. The top gas 17 from the column K1 is reheated in the exchangers E1, E2.

[0064] The liquid 7 from the phase separator S1 mixes with other fluids (top gas from separator S3, derived from the liquid 13 from the separator S4) to form the flow 8, which is sent to a phase separator S2 and then to an intermediate level of the stripping column K1.

[0065] The gas from the phase separator S3 and the liquid from the phase separator 53, after vaporization in the exchanger E2, are mixed with the fluid 7 to supply the column K1.

[0066] Bottom liquid 19 from the column K1 is taken at 154 C., expanded at 8.3 bar, and sent to the phase separator S5, and the gas and the liquid from the phase separator are sent to an intermediate level of the CO/N.sub.2 column K2 operating at 8.3 bar. The column K2 has a top condenser C1, consisting of a plate-type heat exchanger, and a bottom reboiler R2.

[0067] The top gas 27 from the column K2 is partially condensed in the condenser C1, and the resulting liquid L, 29, is returned to the top of the column K2 and in part, and the remaining gas V, enriched in nitrogen, is reheated in the exchangers E2, E1 as gas 31.

[0068] A liquid 53 from the top condenser C2 of the column K3 is vaporized by heat exchange with the gas 27 in the condenser C1, forming the gas 55, which is sent to the entry of the compressor V3.

[0069] The bottom liquid 33 enriched in carbon monoxide and depleted in nitrogen is divided into two, 21, 35, and expanded. An expanded portion 21 at 6.5 bar is sent to a phase separator, the liquid from which is used in part to cool the condenser C1, Accordingly, the top condenser C1 of the CO/N.sub.2 column K2 is cooled by vaporization of at least a portion of the bottom liquid 33 from the CO/N.sub.2 column K2 after expansion and vaporization of the liquid nitrogen 53 at medium pressure. Vaporizing the bottom liquid 33 after expansion enables a considerable reduction in the nitrogen cycle flow to be vaporized in the condenser C1 thereby reducing the nitrogen cycle flow and hence the power of the nitrogen cycle compressor V1, V2, V3.

[0070] The remainder of the liquid from the separator S8 and the fraction 35 supply the CO/CH.sub.4 column K3 after passage through a phase separator S6, from which the gas and the liquid are sent to different intermediate levels of the column K3.

[0071] The column K3 has a top condenser C2, consisting of a plate-type heat exchanger disposed in a bath of liquid for vaporization, and a bottom reboiler R3, The carbon monoxide-enriched top gas is condensed in the condenser C2, and the methane-rich bottom liquid 39 is expanded and reheated in the exchanger E1. Column K3 functions at 6.6 bar.

[0072] The plate-type exchanger is surrounded by an annular barrier forming an overflow wall P. Accordingly, the liquid surrounding the exchanger is able to pass over the barrier P to be withdrawn as liquids 43, 53.

[0073] The top condenser C2 of the column K3 is cooled by compressed and expanded nitrogen 59 from the nitrogen cycle compressor V1, V2, V3 after cooling in the exchangers E1, E2. The vaporized nitrogen is returned upstream of the last stage V3 of the nitrogen cycle compressor. The nitrogen at the exit pressure of the stage V3 is also used to reboil the reboiler R2 of the column K2.

[0074] The reboilers R1 and R3 of the columns K1 and K3 are reheated by partial flows of the feed 1 downstream of the exchanger E1 and upstream of the phase separator S1, This reboiling of the CO/CH.sub.4 column K3 by cooling of the synthesis gas has the advantage of enabling an increase in the pressure of the column CO/CH.sub.4 without an increase in the exit pressure of the nitrogen cycle compressor. The partial flows sent to the reboilers R1, R3 are at the same temperature and at the same pressure.

[0075] Liquid nitrogen 53 from the bottom of the condenser C2 of the column K3 is sent for vaporization in the condenser C1 of the column K3 and is subsequently returned downstream of the stage V2 and upstream of the stage V3. Accordingly, the nitrogen vaporized in the condensers C1, C3 of the CO/N.sub.2 column K2 and CO/CH.sub.4 column K3 returns at an inter-stage of the nitrogen compressor V1, V2; the N.sub.2 flow 57 vaporized in the condensers C1, C2 returns to the N.sub.2 compressor at a higher pressure than the N.sub.2 flow required for the cooling of the synthesis gas. In this case, this is made possible by operating the CO/N.sub.2 column K2 at a higher pressure (8.5 bar) relative to the prior art (2.6 bar).

[0076] A carbon monoxide-rich gas 41 leaves the column K3 at 6.6 bar at 170.4 C. and is reheated in the heat exchangers E1, E2. Preferably no carbon monoxide compressor is used. It constitutes a product of the process and has not been compressed.

[0077] A supply of liquid nitrogen 69 enables compensation of the leaks from the nitrogen cycle. Sent to a phase separator S7, the liquid formed is vaporized in the exchanger E2, and mixes with the gas from the separator S7, and is sent to the entry of the compressor V1.

[0078] A portion 47 of the liquid nitrogen in the condenser C2 is expanded and sent to the separator SI, and the gas 49 formed enters at the entry of the compressor V1.

[0079] Another portion 45 of the same liquid is expanded at a lower pressure and is sent to the exit of the compressor V1 and the entry of the compressor V2.

[0080] The operating pressure of the denitrification column K2 is at least 7 bar abs or even 8 bar abs; the operating pressure of the methane and carbon monoxide separation column K3 is at least 5 bar abs or even 6 bar abs.

[0081] In FIG. 2, the order of the nitrification and methane and carbon monoxide separation columns is reversed.

[0082] Accordingly, the liquid 19 from the bottom of the stripping column is sent not to the denitrification column but instead to an intermediate point of the CO/CH.sub.4 separation column K3, after separation by a phase separator S5.

[0083] The CO/CH.sub.4 column K3 has a bottom reboiler R3 which is heated by the feed, and a top condenser C2, which is used to condense the top gas 51, which is returned to the column K3 in condensed form. The condenser is cooled with condensed nitrogen 61, 63 produced by condensing the cycle nitrogen 59 from the compressor V3 in the exchangers E1, E2 and in the reboiler R2. The liquid is partially vaporized, producing a gas 55 which is returned to the entry of the compressor V3, and a liquid which passes over the barrier P. A portion 31 of the liquid is vaporized in the exchanger E2 and returns to the entry of the compressor V3. The other portion, 53, is used to cool the top condenser C1 of the column K2, as before.

[0084] The bottom methane 39 of the column K3 is reheated in the exchanger E1, to leave the apparatus as a product. The top gas 26, enriched in carbon monoxide and containing nitrogen, leaves toward the middle of the denitrification column K2.

[0085] The column K2 has a top condenser C1, consisting of a plate-type heat exchanger, and a bottom reboiler R2, which is heated by cycle nitrogen. The top gas 27 from the column K2 is partially condensed in the condenser C1, and the liquid L formed, 29, is returned to the top of the column K2 and in part, and the remaining gas V, enriched in nitrogen, is reheated in the exchangers E2, E1 as gas 31.

[0086] A liquid 53 from the top condenser C2 of the column K3 is vaporized by heat exchange with the gas 27 in the condenser C1, forming the gas 55 which is sent to the entry of the compressor V3.

[0087] The carbon monoxide-enriched and nitrogen-depleted bottom liquid 21 is expanded. This liquid, at 6.5 bar, is sent to a phase separator, the liquid from which is used in part to cool the condenser C1. Accordingly, the top condenser C1 of the CO/N.sub.2 column K2 is cooled by vaporization of at least part of the bottom liquid 33 from the CO/N.sub.2 column K2, after expansion and vaporization of the liquid nitrogen 53 at medium pressure. The vaporization of the bottom liquid 33 after expansion enables a considerable reduction in the nitrogen cycle flow to be vaporized in the condenser C1, thereby reducing the nitrogen cycle flow and hence the power of the nitrogen cycle compressor V1, V2, V3.

[0088] The gas 31 is a carbon monoxide-rich product of the process.

[0089] The operating pressure of the denitrification column K2 is at least 7 bar abs or even 8 bar abs; the operating pressure of the methane and carbon monoxide separation column K3 is at least 5 bar abs or even 6 bar abs.

[0090] 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.