Process and apparatus for the separation by cryogenic distillation of a mixture of methane, carbon dioxide and hydrogen

Abstract

In a process for the cryogenic separation of a feed mixture of at least carbon monoxide, hydrogen and methane, the feed mixture is separated in a methane wash column fed by a liquid methane stream at the top of the methane wash column to produce a gas enriched in hydrogen, a liquid stream from the bottom of the methane wash column is treated to produce a mixture of carbon monoxide and methane, the mixture of carbon monoxide and methane is separated in a separation column to produce a gas enriched in carbon monoxide and a liquid methane flow at least part of which forms a purge stream, the purge stream being varied to take account of load variations.

Claims

1. A process for controlling a cryogenic separation of a feed mixture comprising carbon monoxide, hydrogen and at least 2% methane, the process comprising the steps of: i) providing the feed mixture to a cold box, wherein the feed mixture is sourced from a syngas production facility, wherein a flow rate of the feed mixture is measured; ii) cooling the feed mixture in a heat exchanger to produce a cold feed mixture at a first temperature; iii) introducing the cold feed mixture to a methane wash column to produce a gas enriched in hydrogen and a liquid reduced in hydrogen; iv) withdrawing the liquid reduced in hydrogen from the methane wash column and introducing the liquid reduced in hydrogen to a flash column to produce an impure hydrogen gas at a top portion of the flash column and a bottoms liquid comprising carbon monoxide and methane, wherein the flash column comprises a bottom reboiler; v) introducing the bottoms liquid to a distillation column to produce a carbon monoxide-rich top gas and a methane-rich bottoms liquid; vi) removing the methane-rich bottoms liquid from a bottom section of the distillation column; vii) introducing a first portion of the methane-rich bottoms liquid to the methane wash column; viii) vaporizing a second portion of the methane-rich bottoms liquid in the heat exchanger against the feed mixture; wherein the bottom section of the distillation column comprises a liquid distributor, a reboiler section, and a storage section, wherein the liquid distributor is configured to receive falling liquid from within the distillation column and send the falling liquid to the reboiler section, wherein the storage section is configured to receive an overflow liquid from the reboiler section, wherein the liquid distributor is disposed above the storage section and the reboiler section, wherein the reboiler section and the storage section are disposed side by side within the bottom section of the distillation column, wherein a partition wall separates the storage section from the reboiler section, wherein the methane-rich bottoms liquid removed in step vi) is removed from the storage section of the bottom section; and ix) adjusting a flow rate of the methane-rich bottoms liquid removed from the bottom section in step vi) based on the flow rate of the feed mixture, wherein a flow rate of the second portion of the methane-rich bottoms liquid that is vaporized in the heat exchanger is adjusted higher if the measured flow rate of the feed mixture increases and is adjusted lower if the measured flow rate of the feed mixture decreases, wherein the flow rate of the second portion of the methane-rich bottoms liquid vaporized in the heat exchanger is adjusted in order to maintain the cold feed mixture at the first temperature.

2. The process according to claim 1, wherein a volume of liquid kept in the storage section fluctuates while the reboiler section is configured to operate with a constant liquid level.

3. The process according to claim 1, wherein a liquid level within the methane wash column is controlled by adjusting a flow rate of the liquid reduced in hydrogen withdrawn from the methane wash column in step iv), wherein the liquid level within the methane wash column has a set point that is based on the measured flow rate of the feed mixture.

4. The process according to claim 1, wherein a set point of a liquid level in the methane wash column is adjusted lower if the measured flow rate of the feed mixture increases and is adjusted higher if the measured flow rate of the feed mixture decreases.

5. The process according to claim 1, wherein, in the event of a change in the measured flow rate of the feed mixture, a sump level within the flash column is maintained constant by altering a flow rate of the bottoms liquid withdrawn from the flash column.

6. The process according to claim 1, wherein a volume of liquid in the storage section varies inversely based on the measured flow rate of the feed mixture while the reboiler section is configured to operate with a constant liquid level.

7. A process for controlling a cryogenic separation of a feed mixture comprising carbon monoxide, hydrogen and at least 2% methane, the process comprising the steps of: i) providing the feed mixture to a cold box, wherein the feed mixture is sourced from a syngas production facility, wherein a flow rate of the feed mixture is measured; ii) cooling the feed mixture in a heat exchanger to produce a cold feed mixture at a first temperature; iii) introducing the cold feed mixture to a methane wash column to produce a gas enriched in hydrogen and a liquid reduced in hydrogen; iv) withdrawing the liquid reduced in hydrogen from the methane wash column and introducing the liquid reduced in hydrogen to a flash column to produce an impure hydrogen gas at a top portion of the flash column and a bottoms liquid comprising carbon monoxide and methane, wherein the flash column comprises a bottom reboiler; v) introducing the bottoms liquid to a distillation column to produce a carbon monoxide-rich top gas and a methane-rich bottoms liquid; vi) removing the methane-rich bottoms liquid from a bottom section of the distillation column; vii) introducing a first portion of the methane-rich bottoms liquid to the methane wash column; viii) vaporizing a second portion of the methane-rich bottoms liquid in the heat exchanger against the feed mixture; wherein the bottom section of the distillation column comprises a liquid distributor, a reboiler section, and a storage section, wherein the liquid distributor is configured to receive falling liquid from within the distillation column and send the falling liquid to the reboiler section, wherein the storage section is configured to receive an overflow liquid from the reboiler section, wherein the liquid distributor is disposed above the storage section and the reboiler section, wherein the reboiler section and the storage section are disposed side by side within the bottom section of the distillation column, wherein a partition wall separates the storage section from the reboiler section, wherein the methane-rich bottoms liquid removed in step vi) is removed from the storage section of the bottom section; and ix) adjusting a flow rate of the methane-rich bottoms liquid removed from the bottom section in step vi) based on the flow rate of the feed mixture, wherein a volume of liquid kept in the storage section fluctuates while the reboiler section is configured to operate with a constant liquid level, wherein a set point of a liquid level in the methane wash column is adjusted lower if the measured flow rate of the feed mixture increases and is adjusted higher if the measured flow rate of the feed mixture decreases, wherein, in the event of a change in the measured flow rate of the feed mixture, a sump level within the flash column is maintained constant by altering a flow rate of the bottoms liquid withdrawn from the flash column, wherein a flow rate of the second portion of the methane-rich bottoms liquid that is vaporized in the heat exchanger is adjusted higher if the measured flow rate of the feed mixture increases and is adjusted lower if the measured flow rate of the feed mixture decreases, wherein the flow rate of the second portion of the methane-rich bottoms liquid vaporized in the heat exchanger is adjusted in an amount effective to maintain the cold feed mixture at the first temperature, wherein a volume of liquid in the storage section reduces when the flow rate of the second portion of the methane-rich bottoms liquid vaporized in the heat exchanger increases, wherein the volume of liquid in the storage section increases when the flow rate of the second portion of the methane-rich bottoms liquid vaporized in the heat exchanger decreases.

8. A process for controlling a cryogenic separation of a feed mixture comprising carbon monoxide, hydrogen and at least 2% methane, the process comprising the steps of: i) providing the feed mixture to a cold box, wherein the feed mixture is sourced from a syngas production facility, wherein a flow rate of the feed mixture is measured; ii) cooling the feed mixture in a heat exchanger to produce a cold feed mixture at a first temperature; iii) introducing the cold feed mixture to a methane wash column to produce a gas enriched in hydrogen and a liquid reduced in hydrogen; iv) withdrawing the liquid reduced in hydrogen from the methane wash column and introducing the liquid reduced in hydrogen to a flash column to produce an impure hydrogen gas at a top portion of the flash column and a bottoms liquid comprising carbon monoxide and methane, wherein the flash column comprises a bottom reboiler; v) introducing the bottoms liquid to a distillation column to produce a carbon monoxide-rich top gas and a methane-rich bottoms liquid, wherein the distillation column comprises a reflux capacity or condenser, wherein the distillation column also comprises a bottom section, wherein the bottom section of the distillation column comprises a liquid distributor, a reboiler section, and a storage section, wherein the liquid distributor is configured to receive falling liquid from within the distillation column and send the falling liquid to the reboiler section, wherein the storage section is configured to receive an overflow liquid from the reboiler section, wherein the liquid distributor is disposed above the storage section and the reboiler section, wherein the reboiler section and the storage section are disposed side by side within the bottom section of the distillation column, wherein a partition wall separates the storage section from the reboiler section; vi) removing the methane-rich bottoms liquid from the storage section of the distillation column; vii) introducing a first portion of the methane-rich bottoms liquid to the methane wash column; viii) vaporizing a second portion of the methane-rich bottoms liquid in the heat exchanger against the feed mixture; ix) making the following adjustments if the measured flow rate of the feed mixture is increased: a. increasing the amount of the second portion of the methane-rich bottoms liquid that is vaporized in the heat exchanger against the feed mixture in order to maintain the cold feed mixture at the first temperature; b. increasing the flow rate of the methane-rich bottoms liquid withdrawn from the storage section of the distillation column, thereby lowering the volume of liquid within the storage section of the distillation column; and x) making the following adjustments if the measured flow rate of the feed mixture is decreased: a. decreasing the amount of the second portion of the methane-rich bottoms liquid that is vaporized in the heat exchanger against the feed mixture in order to maintain the cold feed mixture at the first temperature; b. decreasing the flow rate of the methane-rich bottoms liquid withdrawn from the storage section of the distillation column, thereby raising the volume of liquid within the storage section of the distillation column.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it can admit to other equally effective embodiments.

(2) FIG. 1 provides a process of an embodiment of the invention.

(3) FIG. 2 provides a process of an embodiment of the invention.

(4) FIG. 3 provides additional details of the embodiment of FIG. 2.

DETAILED DESCRIPTION

(5) The invention will be described in greater detail with reference to the figures.

(6) FIGS. 1 and 2 show processes according to the invention and FIG. 3 shows a detail of the process of FIG. 2.

(7) The process is a cryogenic separation process taking place within a cold box 30.

(8) A feed stream 10 cooled in heat exchanger 9 and containing hydrogen, carbon monoxide and at least 2% methane is sent to the bottom of a methane wash column 1 fed by liquid methane 11 at the top of the column.

(9) A gas enriched in hydrogen 12 is removed at the top of the methane wash column 1 and warmed in the heat exchanger 9. A liquid 13 with a reduced hydrogen content is sent to a flash column 2 having a bottom reboiler 8. Gas 14 is removed from the top of the flash column and warmed in heat exchanger 9.

(10) The bottom liquid 15 from the flash column contains principally carbon monoxide and methane and is sent to the middle of a carbon monoxide/methane column 3 having a reflux capacity (or a condenser) 6 and a bottom reboiler 7. Liquid 17 from the reflux capacity 6 is sent back to column 3.

(11) Carbon monoxide rich gas 16 is removed from the top of column 3 and sent to heat exchanger 9.

(12) Methane rich liquid 18 is removed from the bottom of the column 3. The liquid from the tank 4 is pumped using pump 5 and divided into two parts (or even three parts). One part 11 is sent to the top of the methane wash column 1, the other part 20 is removed, possibly as a product. The second part may be vaporized in heat exchanger 9.

(13) The process can be controlled as follows:

(14) The flowrate of the synthesis gas feed stream 10 is measured. Variations of this stream 10 are used to lead or lag other process parameters in order to ensure the plant load change.

(15) Liquid methane stream 11 feeding the methane wash column 1 at the top is controlled in flow. The set-point of this flow controller is set via a calculation which is a function of the total synthesis gas flow 10. A lead or a lag time can be applied to the value of the set-point according the dynamics of the system.

(16) The sump level of the methane wash column 1 is controlled by the stream 13 extraction from the bottom of the methane wash column. The set point of this level controller will also be linked to the variation of the synthesis gas stream 10. This level set point will vary in the opposite direction to the plant load; this is the result of the liquid inventory variation in the distributors in the methane wash column 1.

(17) The streams used to heat reboilers 7 and 8 are controlled in flow. The set-points of these flow controllers are set via calculations which are function of the total synthesis gas flow 10. A lead or a lag time can be applied to the value of the set-point according the dynamics of the system.

(18) Sump level of the column 2 is maintained constant, by the stream 15 extraction.

(19) Reflux 17 is controlled in flow. The set-point of this flow controller is set via a calculation which is a function of the total synthesis gas flow 10. A lead or a lag time can be applied to the value of the set-point according the dynamics of the system. This set point also can be corrected by a temperature controller set in the middle of the carbon monoxide/methane column 3.

(20) Sump level of the carbon monoxide/methane column 3 is maintained constant, by the stream 18 extraction.

(21) Methane purge flow 20 is also controlled in flow. The set-point of this flow controller is set via a calculation which is a function of the total synthesis gas flow 10 so that the methane purge flow 20 increases when the synthesis gas flow 10 increases and decreases when the synthesis gas flow decreases. A lead or a lag time can be applied to the value of the set-point according the dynamics of the system.

(22) As a consequence, the level in tank 4 and the reflux capacity 6 will vary according the load of the plant. Thus if the synthesis gas flowrate increases, the level in the tank 4 will fall to allow the purge flow 20 to increase whilst leaving the liquid level in the column 3 constant. Similarly if the synthesis gas flowrate decreases, the level in the tank 4 will increase to allow the purge flow 20 to decrease whilst leaving the liquid level in the column 3 constant.

(23) Tank 4 will accumulate the methane molecules resulting from a load decrease due to the inventory change in the column liquid distributors. This accumulated methane will be used again during the load increase to reload the distributors of the methane wash column 1 with methane.

(24) Reflux capacity 6 will accumulate the liquid carbon monoxide molecules resulting from a load decrease due to the inventory change in the column liquid distributors. This accumulated liquid carbon monoxide will be used again during the load increase to reload the distributors.

(25) FIG. 2 shows processes according to the invention similar to FIG. 1, with the exception of the tank 4 which is integrated in the sump of carbon monoxide/methane column 3. In this case, it is the liquid level at the bottom of column 3 which will increase or decrease in response to the synthesis gas flowrate, so that the purge flow 20 may increase when the synthesis gas flowrate increases and vice versa.

(26) In both FIGS. 1 and 2, the column 2 may be fed at the top with pumped methane liquid from pump 5.

(27) The tank 4 may be integrated into the bottom of the carbon monoxide/methane column 3 (as shown in FIG. 3).

(28) Element 41 at the bottom of column 3 is a liquid distributor and collector which allows falling liquid to be sent from the packing above the distributor to the reboiler section 43 at one side of the sump of column 3. Tank 4 is the section 42 at the other side of the sump of column 3, separated by a partition plate 44 from where stream 21 is withdrawn to feed the pump 5.

(29) The reboiler section 43 operates at constant level and overflows into the tank section 42 where the methane inventory varies according to the plant load.

(30) While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

(31) The singular forms a, an and the include plural referents, unless the context clearly dictates otherwise.

(32) Comprising in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of comprising). Comprising as used herein may be replaced by the more limited transitional terms consisting essentially of and consisting of unless otherwise indicated herein.

(33) Providing in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary a range is expressed, it is to be understood that another embodiment is from the one.

(34) Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

(35) Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such particular value and/or to the other particular value, along with all combinations within said range.

(36) All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.