UNIT FOR SEPARATING A MIXTURE OF CARBON MONOXIDE, HYDROGEN AND METHANE BY CRYOGENIC DISTILLATION

Abstract

A unit for separating a mixture of carbon monoxide, hydrogen and methane by cryogenic distillation including a distillation column having a condenser and a reboiler, a refrigeration cycle having a compressor, a first heat exchanger, a second heat exchanger, means for sending the gas mixture to be cooled in the first heat exchanger, means for separating the gas mixture cooled in the first heat exchanger and for sending a fluid produced by the separation to the distillation column, a thermally insulated enclosure containing the first and second heat exchangers, means for sending a cycle gas from the distillation column to the second heat exchanger and then to the first heat exchanger to be heated, means for sending a cycle gas liquefied in the reboiler to be cooled in the second heat exchanger, and means for sending the cycle gas liquefied and cooled in the second heat exchanger to the condenser.

Claims

1. A unit for separating a mixture of carbon monoxide, hydrogen and methane by cryogenic distillation comprising; at least one distillation column comprising a top condenser and a bottom reboiler, a refrigeration cycle comprising a cycle compressor, a first heat exchanger, a second heat exchanger, a means for sending the gas mixture to be cooled in at least the first heat exchanger, a means for separating the gas mixture cooled in at least the first heat exchanger and for sending a fluid produced by the separation to the at least one distillation column, a thermally insulated enclosure containing the first and second heat exchangers but no scrubbing or distillation column, the thermally insulated enclosure having a length, a means for sending a cycle gas from the at least one distillation column to the second heat exchanger to be heated and then to the first heat exchanger to be heated, a means for sending a cycle gas liquefied in the bottom reboiler to be cooled in the second heat exchanger, and a means for sending the cycle gas liquefied and cooled in the second heat exchanger to the top condenser, wherein the thermally insulated enclosure (is arranged when in use with the length oriented vertically and the first heat exchanger being arranged beside the second heat exchanger.

2. The unit according to claim 1, wherein the second heat exchanger has a first end and a second end and is connected such that in operation at least one fluid to be heated is sent to the first end and exits from the second end and/or at least one fluid to be cooled is sent to the second end and exits at the first end, the first end being located at a geodesic height greater than the geodesic height of the second end.

3. The unit according to claim 1, wherein the first heat exchanger has a first end at a first geodesic height and a second end at a second geodesic height less than the first geodesic height and is connected such that in operation at least one fluid to be heated is sent to the end at the second height and exits from the end at the first height and/or at least one fluid to be cooled is sent to the end at the first height and exits at the end at the second height.

4. The unit according to claim 2, wherein a pipe is connected to the second end of the second heat exchanger and to the end of the first exchanger at the second geodesic height to transfer a fluid to be heated from the second exchanger to the first exchanger.

5. The unit according to claim 2, wherein a pipe is connected to the second end of the second heat exchanger and to the end of the first exchanger at the second geodesic height to transfer a fluid to be cooled from the first exchanger to the second exchanger.

6. The unit according to claim 2, wherein the second end of the second exchanger is substantially at the second geodesic height.

7. The unit according to claim 1, wherein the first and second exchangers are located in use in an upper part of the enclosure.

8. The unit according to claim 7, wherein other elements of the unit required to operate at a cryogenic temperature are located in a lower part of the enclosure.

9. The unit according to claim 1 comprising a third heat exchanger which is located in the thermally insulated enclosure.

10. The unit according to claim 9, wherein the third heat exchanger is located in use in a lower part of the enclosure.

11. The unit according to claim 10, wherein the third heat exchanger is located in use below the second heat exchanger.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The invention is described in greater detail with reference to the figures, in which:

[0030] FIG. 1 shows a separation unit.

[0031] FIG. 2 shows heat exchangers of the separation unit arranged in an enclosure.

[0032] FIG. 3 shows heat exchangers of the separation unit arranged in an enclosure according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0033] In FIG. 1, a mixture of carbon monoxide, hydrogen and methane SG is separated by cryogenic distillation. The gas mixture SG is first cooled from a temperature above 0 C. in a heat exchanger E1 from the hottest end to the coldest end thereof to an intermediate temperature, and then in the heat exchanger E3 from the hottest end to the coldest end thereof to a temperature below 170 C. The cooled flow SG is then sent to the bottom of a methane scrubbing column K1 top-fed with a flow of liquid methane 14. A heat exchanger E4 supplies cold to intermediate levels of the scrubbing column K1, which operates at a pressure between 15 bar (a) and 30 bar (a). The exchanger E4 is cooled by a part of the cycle gas.

[0034] A hydrogen-rich top gas 5 is drawn from the top of column K1 and is heated in the exchangers E3, E1 and the bottom liquid 3 is sent to the top of the stripping column K2. The stripping column K2 has a bottom reboiler R2 and operates at between 7 bar (a) and 15 bar (a). A methane-depleted hydrogen-enriched stream 9 is drawn from the top of the column K2 and heated in the heat exchanger E2 and then in the heat exchanger E1. The bottom liquid 7 of the column K2 is heated in the heat exchanger E3 and then split into two. One part 11 is expanded and sent to the top of column K3 and one part 9 is expanded and sent to an intermediate level of a distillation column K3. The column K3, which has a bottom reboiler R3 and a top condenser, operates at between 1.5 bar (a) and 4 bar (a).

[0035] Methane-enriched bottom liquid 14 from the column K3 is pressurized by a pump P and sent to the scrubbing column K1 as scrubbing liquid after cooling in the exchanger E3. The remainder 13 of the liquid is vaporized in the heat exchanger E1.

[0036] A carbon monoxide-rich top gas 15 from the column K3 is used as cycle gas and is first heated in the exchanger E2 and then in the exchanger E1 as gas 23. The gas 23 is compressed in a cycle compressor (not shown) to a higher pressure (for example 28 bars) and then cooled in the exchanger E1 as gas 25. A part 27 of the gas 25 is expanded in a turbine T to supply cold and then rejoins the gas 23 at an intermediate temperature of the exchanger E1. The remainder 27 of the gas leaves the exchanger E1 at 140 C. and is used to heat at least one of the reboilers R2, R3, therefore being cooled and condensed. In the figure, the two liquid parts 29, 31 of the flow 27 enter the exchanger E2 downstream of the reboilers R2, R3 where they are cooled to 180 C. The flow 29 and the flow 31 are then expanded, mixed, sent partly as a liquid flow 33 to the top condenser of the column K3 and partly as a liquid flow 35 to the exchanger E3 to be vaporized and join the flow 25 coming from the cycle compressor.

[0037] The gas 17 from the condenser of the column K3 joins the cycle gas 15 forming the flow 23.

[0038] Cycle gas 33 condenses in the top condenser of the column K3 and the liquid formed is sent to the top of the column K3. Another part 21 of the liquid from the condenser is vaporized in the exchanger E3, mixes with the gas coming from the exchanger E4 forming the gas 19 and joins the remainder of the cycle 15, 17, 23.

[0039] To summarize, the heat exchanger E2 cools the liquid flows 29, 31 from 147 C. and 151 C. to 180 C. by heat exchange with the top gas 9 which is a mixture of carbon monoxide, methane and hydrogen intended to be used as fuel and with the cycle gas 23 to be heated intended for compression in the cycle gas. None of the fluids therefore undergoes a phase change.

[0040] According to the invention, the heat exchangers E1, E3 and optionally E2 are located in the same thermally insulated enclosure B, as illustrated in FIG. 2. On the other hand, there are no columns in this enclosure.

[0041] A phase separator S is fastened to the outside of the enclosure B. The exchanger E3 and a thermosiphon TS are located in a lower part of the enclosure which is arranged with its length oriented vertically.

[0042] The upper part contains the exchanger E1 composed of two identical bodies arranged side by side and the exchanger E2 which is arranged at a geodesic level lower than the exchanger E1, at least one pipe C connecting the exchanger E2 with each body of the exchanger E1 to send thereto a gas heated in the exchanger E2 and to be heated in the exchanger E1. Similarly, there is at least one pipe (not shown) for sending a fluid cooled in the bodies of the exchanger E1 to the exchanger E2 to be further cooled therein. Obviously, the exchanger E1 may comprise a single body or more than two bodies.

[0043] It will be noted that the height of the enclosure B is greater than the sum of the heights of the exchangers E1, E2 and E3, which could lead to problems when transporting the enclosure.

[0044] In this variant, the exchangers E1, E2 are oriented conventionally with their cold end EF downwards and their hot end EC upwards. Thus, the pipe C must be longer than the height of the exchanger E2 in order to convey the gas to be heated to the exchanger E1. Thus, at least one heating agent is circulated downwards, and the reverse is true for at least one cold fluid.

[0045] This variant is well suited for cases where at least one fluid has to change phase in the exchanger E2, either by condensing at least partially for a gas or by vaporising at least partially for a liquid.

[0046] No part of the syngas SG is cooled in the exchanger E2.

[0047] According to a variant of the invention, the direction of circulation of the fluids is inverted in one of the exchangers to reduce the size of the package and thus reduce the amount of structure.

[0048] The exchanger or exchangers to be inverted include heating agents and cooling agents in gas or liquid form that do not change state (gas or liquid) through the exchanger. Typically, the heating agents are liquids to be subcooled and the cooling agents are gases to be heated.

[0049] Thus, this makes it possible to reduce the length of the pipes between the different exchangers and consequently to reduce the cost of the piping and the size of the package.

[0050] In this case, E1, E3 are arranged with the exchanger E1, optionally composed of several bodies, above the heat exchanger E3, if present. Optionally, the heat exchanger E2 can be located below the heat exchanger E3. Thus, the exchangers E1 and E2 and optionally E2 are arranged in the thermally insulated enclosure with their lengths arranged in the direction of a vertical axis of the enclosure. Thus, the cold end of the exchanger E1 faces the cold end of the exchanger E2, the columns K1 to K3 being located in at least one other thermally insulated enclosure.

[0051] Usually and as illustrated in FIG. 2, all the heat exchangers are arranged with their hot ends upwards and their cold ends downwards, so that the fluids circulate in an overall vertical direction.

[0052] According to the invention, illustrated in FIG. 3, the arrangement of these exchangers is modified in order to reduce the size of the package containing these exchangers. The best arrangement then has to be found in consideration of the constraints of the method, minimizing the amount of structure in order to reduce costs.

[0053] According to this variant, the exchanger E2 is arranged with its cold end EF upwards and its hot end EC downwards, so that the fluids circulate in an overall vertical direction, but the at least one liquid being cooled circulates upwards and the gases being heated circulate from the top downwards.

[0054] In this case, the exchanger E1 and exchanger E2 can be placed side by side so that a pipe conveying heated gas from the hot end of the exchanger E2 to the cold end of the exchanger E1 forms a U and is much shorter than it would be if the hot end were at the top for the exchanger E2. As the exchanger E2 is no longer below the exchanger E1, the enclosure B of the exchangers becomes much shorter, facilitating transportation.

[0055] Moreover, since the enclosure B comprises other elements below the exchanger E1, R2 which occupy a certain width, the arrangement of the exchanger E2 beside the exchanger E1 does not substantially change the width of the enclosure, and may not change it at all.

[0056] These other elements may be the exchanger E3 and/or a phase separator S and/or a thermosiphon TS.

[0057] In this case, it is essentially the heights of the exchangers E1 and E3 that determine the height of the enclosure B, which will be shorter than for the variant in FIG. 2.

[0058] The exchanger E3 and the thermosiphon TS are not necessarily present in the enclosure B.

[0059] In this case, it is preferable for no fluid to change phase in the exchanger E2, either by condensing at least partially for a gas being cooled or by vaporising at least partially for a liquid being heated. Preferably, the exchanger is designed so that the fluids indirectly exchanging heat therein are all gases. Optionally, at least one liquid can also be cooled therein.

[0060] No part of the syngas SG is cooled in the exchanger E2.

[0061] The enclosure in FIG. 3 typically occupies no more than 90% of the volume of the enclosure in FIG. 2 and its height is reduced by 20%.

[0062] 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.