Process and apparatus for separating CO2 by partial condensation

Abstract

Title: Process and apparatus for CO2 separation by partial condensation In a CO2 separation process, a gas stream (1) is compressed in a compressor (C), partially condensed by a refrigeration cycle (CC, R, V4) and a part (19) of the liquid formed is expanded, vaporized against the gas stream or a gas formed by the partial condensation and returned to the compressor to be compressed. Fig. 1

Description

Title of the invention: Method and apparatus for separating CO2 by partial condensation

[0001] The present invention relates to a method and apparatus for separating CO2 by partial condensation and optionally by distillation.

[0002] In the case of liquid CO2 production, the conventional configurations for generating cold from a CO2 capture unit by cryogenic means are as follows: 1. Expansion to a pressure close to the triple point of at least a portion of the liquid CO2 generated by partial condensation of the stream to be purified, vaporization of the liquid CO2 obtained, compression of the vaporized CO2 to a pressure allowing its condensation in the face of an external refrigerant (ammonia for example) which vaporizes, liquefaction and production of CO2. 2. The liquid CO2 resulting from the partial condensation of the stream to be purified is directly exported as a product and a CO2 refrigeration loop is used in parallel: expansion to a pressure close to the triple point of at least part of the liquid / dense CO2 contained in the loop, vaporization of the liquid CO2 obtained, compression of the CO2, vaporization to a pressure allowing its condensation against an external refrigerant or against air or water and finally liquefaction or densification of the CO2.

[0003] In these two configurations, in addition to the compression step of the stream to be purified, a CO2 compressor is required. To this machine, a refrigeration unit can be added for the liquefaction of the CO2 using an external refrigerant when liquid CO2 is required. Minimizing the number of machines is therefore key to reducing the costs of the unit as well as its footprint.

[0004] When no external refrigerant is used, this compressor will have a high output pressure in order to be able to liquefy the CO2 at room temperature.

[0005] Since according to the invention an external refrigerant is used, the compressor of the gas stream to be treated will have a moderate outlet pressure, for example between 15 and 60 bars but a refrigerant generation unit is necessary; this unit is preferably composed of a cycle involving compression, condensation, expansion and vaporization stages.

[0006] It is known from "Gas quality impacts, assessment and control in oxyfuel technology for CCS" ANLEC R&D Project 6-0710-00661 of 2014, page 11 to compress a gas to be separated containing CO2, to dry the gas, to cool it and to separate it by partial condensation and distillation. The top gas of the column is mixed with a part of the vaporized tank liquid and the mixture is sent to the compressor. The CO2 is produced entirely in liquid form.

[0007] According to an object of the invention, there is provided a method for separating and liquefying CO2 from a gas stream to be treated comprising the following steps: i. Compression of the gas stream to be treated from a first pressure to a second pressure in a compressor, ii. Cooling the gas stream to the second pressure and partially condensing the cooled gas stream to a first temperature by indirect heat exchange with an external vaporizing liquid refrigerant, separating the partially condensed gas stream into a first liquid enriched in carbon dioxide and a first gas depleted in carbon dioxide, iii. Optionally cooling at least a portion of the first carbon dioxide-depleted gas to a second temperature at which it partially condenses, producing a second carbon dioxide-enriched liquid and a second carbon dioxide-depleted gas, iv. Expansion of a. A portion of the CO2-enriched liquid, generated by the partial condensation of step ii) or at least one of the two partial condensations of steps ii) and iii), up to a third pressure, higher than the first pressure and lower than the second pressure and producing another portion of the CO2-enriched liquid as product or b. A CO2-enriched liquid, produced by separation in a mass and heat exchange column fed with at least one CO2-enriched liquid generated by the partial condensation of step ii) or at least one of the two partial condensations of steps ii) and iii), up to a third intermediate pressure between the first pressure and the second pressure and production of a portion of the column bottom liquid as product, v. Vaporizing the expanded CO2-rich liquid to the third pressure, in order to cool and partially condense the gas stream and / or the first CO2-depleted gas from the gas stream of step iii), thereby generating a CO2-rich gas flow at a third pressure and vi. Sending at least a portion of the CO2-rich gas flow at the third pressure to the compressor of the gas stream to be treated from step i) to be compressed together with the gas stream from a pressure.

[0008] According to other optional features: • in a process described above variant b) of step iv) where the liquid generated by at least one of the two partial condensations is expanded and sent to the top of the column. in a process described above variant b) of step iv) a part of the liquid in the column bottom vaporizes by heat exchange with at least a part of the gas depleted in carbon dioxide. the vaporized tank liquid constitutes the only gas flow sent to the column, in a process described above variant b) of step iv) the CO2-rich liquid is withdrawn from the column at an intermediate level thereof or forms part of the tank liquid of the column. the coolings of steps ii) and iii) take place in a heat exchanger, the cooling of step iii) cools the at least part of the gas to the coldest end of the heat exchanger. the external liquid refrigerant vaporizes according to step ii), is compressed by a cycle compressor, is cooled in the heat exchanger, is expanded and vaporizes again in the heat exchanger, forming a closed refrigeration cycle. The liquid refrigerant is propane or ammonia. gas from the distillation column is sent to the compressor of the gas stream to be treated from step i) to be compressed together with the gas stream from a second intermediate pressure between the first intermediate pressure and the second pressure. no part of the CO2-rich liquid expanded to the third pressure which has been vaporized is compressed in the compressor without having been mixed with the gas stream to be treated. no part of the liquid produced by a CO2-rich liquid distillation column expanded to the third pressure which has been vaporized is compressed in the compressor without having been mixed with the gas stream to be treated. The gas stream at the second pressure is cooled, dried and cooled again before being partially condensed. the external liquid refrigerant is also used to cool water intended to cool the gas stream at the second pressure upstream of a gas stream drying step. the gas stream contains at least one other component lighter than CO2 which may be hydrogen, nitrogen, oxygen, methane, carbon monoxide. The gas stream contains water which is removed by a drying step downstream of the compressor. • the second pressure is between 15 and 60 bars. • the gas stream is cooled from the first temperature to the inlet temperature of a phase separator by heat exchange with the CO2-rich liquid expanded to the third pressure, before or after vaporization of the CO2-rich liquid.

[0009] According to an object of the invention, there is provided an apparatus for separating and liquefying CO2 from a gas stream to be treated comprising a compressor for compressing the gas stream to be treated from a first pressure to a second pressure, means for cooling the gas stream compressed in the compressor to the second pressure by partially condensing it by indirect heat exchange with an external vaporizing liquid refrigerant, a first phase separator for separating the partially condensed gas stream into a first liquid enriched in carbon dioxide and a first gas depleted in carbon dioxide, optionally means for cooling at least a portion of the first gas depleted in carbon dioxide to a second temperature at which it partially condenses producing a second liquid enriched in carbon dioxide and a second gas depleted in carbon dioxide, optionally a second phase separator, a.Means for expanding a portion of the CO2-enriched liquid, coming from the first separator or from at least one of the first and second separators, to a third pressure, higher than the first pressure and lower than the second pressure and means for outputting another portion of the CO2-enriched liquid as product or . b. A mass and heat exchange column, means for expanding liquid from the first separator or from at least one of the first and second separators, means for sending the expanded liquid to the top of the column, means for expanding a CO2-enriched liquid, produced by separation in the column, to a third intermediate pressure between the first pressure and the second pressure and means for withdrawing another part of the bottom liquid from the column as product and

[0010] means for vaporizing CO2-rich liquid expanded to the third pressure, by heat exchange with the compressed gas stream and / or the first CO2-depleted gas, thereby generating a CO2-rich gas flow at a third pressure and means for sending at least a portion of the CO2-rich gas flow at the third pressure to the compressor of the gas stream to be treated from step i) to be compressed together with the gas stream from a pressure.

[0011] The invention makes it possible to avoid the use of a CO2 compressor, allowing reductions in the investment costs of the unit as well as its footprint.

[0012] It also avoids vaporizing all the CO2 produced. Thus, part of the CO2 is produced directly in liquid form.

[0013] The invention may comprise the steps of: • Cool and partially condense the stream to be purified directly against the external refrigerant to an intermediate temperature. • Cooling and partially condensing the gaseous stream resulting from the partial condensation of the stream to be purified against vaporizing liquid CO2 resulting from a partial condensation, for example from one of two partial condensation stages or separated by distillation after at least one partial condensation stage. • Recycle vaporized CO2 into the stream compressor to be purified instead of sending it to a dedicated CO2 compressor. • Use at least part of the CO2 from partial condensation as liquid production without having to vaporize it.

[0014] This configuration therefore makes it possible to avoid the use of a CO2 compressor, while minimizing the quantity of gas recycled to the compressor of the stream to be purified because the cold at intermediate temperatures is generated by an external liquid refrigerant, preferably by an external refrigeration cycle.

[0015] In addition, the external refrigerant refrigeration unit can be used in conjunction with other heat exchanges, such as the generation of refrigeration water to cool the stream to be purified before drying. This contributes to the pooling of equipment, further limiting the number of rotating machines.

[0016] The invention will be described in more detail with reference to the figures.

[0017] [Fig. 1] illustrates a process for distilling a gas mixture comprising CO2 and at least one lighter component by partial condensation according to the invention.

[0018] [Fig.2] illustrates a process for distilling a gas mixture comprising CO2 and at least one lighter component by partial condensation and distillation according to the invention.

[0019] [Fig.3] illustrates another method of distilling a gas mixture comprising CO2 and at least one lighter component by partial condensation and distillation according to the invention.

[0020] [Fig. 1] illustrates a process in which refrigeration for the process is provided by vaporizing a liquid stream 19 formed by partial condensation and vaporizing an external refrigerant 27.

[0021] A gas stream 1 of a gas mixture comprises CO2 and at least one component lighter than CO2 such as oxygen, argon, nitrogen, carbon monoxide, methane, hydrogen, ethane or another hydrocarbon...

[0022] This gas stream 1 is compressed in a compressor C from a first pressure up to a second pressure, for example between 15 and 60 bars. The stream 5 at the second pressure partially condenses in a heat exchanger E, leaves the exchanger E and is separated in a phase separator SI, forming a first liquid 9 enriched in CO2 and depleted in the lighter component and a gas depleted in CO2 and enriched in the at least one light component 7. The valve VI if present can make it possible to balance the pressure differences.

[0023] In a first variant of the invention, there is a single partial condensation step, the liquid 9 constitutes the product of the process and the flow 7 is reheated in the exchanger E. A part of the liquid 9 becomes the liquid 19 expanded in the valve V3 which is sent to the intermediate level of the compressor C.

[0024] A second partial condensation step may optionally follow in which the gas 7 partially condenses in the heat exchanger E and is sent to a second phase separator S2. In this separator a second liquid 13 enriched in CO2 and depleted in the lighter component and a gas depleted in CO2 and enriched in the at least one light component 11 are formed.

[0025] A CO2-rich liquid consisting of at least a portion of liquid 9 and / or at least a portion of liquid 13 is the liquid product of the process.

[0026] A CO2-rich liquid 19 consisting of at least a portion of the liquid 9 expanded to a third pressure and / or at least a portion of the liquid 13, expanded to a third pressure in a valve V3, vaporizes at the third pressure in the heat exchanger E and is sent to be compressed in the compressor C from the third pressure which is between the first and second pressures.

[0027] Refrigerants for the process are provided by an external liquid refrigerant 27 which vaporizes to cool the gas stream at the second pressure to a first temperature by indirect heat exchange in the heat exchanger E. At this first temperature, the gas stream has begun to condense.

[0028] In the case of a single phase separator process, the remainder of the frigories required to partially condense the gas stream by bringing the gas stream up to the cold end temperature of the exchanger E are provided by vaporization of the expanded liquid 19.

[0029] In the case of a process with several phase separators, the external liquid refrigerant 27 vaporizes to cool and partially condense the gas stream at the second pressure to a first temperature by indirect heat exchange in the heat exchanger E, the first temperature being an intermediate temperature of the heat exchanger E. At this first temperature, the gas stream continues to partially condense by heat exchange with the CO2 which vaporizes, before being sent to the first phase separator SL. Then the cold to cool the gas of the first phase separator and possibly the gas of the se- following phase separations is provided by the CO2-rich liquid 19.

[0030] The frigories for cooling the gas stream to the first temperature can be provided by a closed refrigeration circuit, comprising a cycle compressor CC, a cooler R where the compressed refrigerant condenses and an expansion valve V4 for expanding the condensed refrigerant. The condensed refrigerant cools in the heat exchanger E before being expanded in the valve V4 and then sent to an intermediate level of the exchanger E. The liquid refrigerant 27 is vaporized in the heat exchanger E to cool the hot end of the heat exchanger E. The vaporized liquid is then returned to the cycle compressor CC. The cycle fluid is preferably propane or ammonia.

[0031] The gas 11 from the separator S2 is heated in the heat exchanger E.

[0032] [Fig.2] illustrates a process in which refrigeration for the process is provided by vaporizing a flow drawn from the distillation column.

[0033] A flow 1 of a gas mixture comprises CO2 and at least one component lighter than CO2 such as oxygen, argon, nitrogen, carbon monoxide, methane, hydrogen, ethane or another hydrocarbon...

[0034] This flow 1 is compressed in a compressor C from a first pressure to a second pressure, for example between 15 and 60 bars, forming a compressed flow 5. The flow 5 partially condenses in a heat exchanger E, leaves the exchanger and is separated in a phase separator SI, forming a liquid 9 enriched in CO2 and depleted in the lighter component and a gas depleted in CO2 and enriched in the at least one light component 7. The liquid 9 passes into a valve VI and then into a valve V2 and then sent to the top of a distillation column K containing trays or packings to promote the exchange of mass and heat. The distillation pressure is necessarily above 5.2 bars. Due to the expansion in the valves VI, V2, the pressure of the flow 5 will be higher than that of the column K.

[0035] A second partial condensation step follows the first, in which the gas 7 partially condenses in the heat exchanger E and is sent from the cold end of the exchanger E to a second phase separator S2. In this separator S2 a liquid 13 enriched in CO2 and depleted in the lighter component and a gas depleted in CO2 and enriched in the at least one light component 11 are formed. The liquid 13 is sent to the top of a distillation column K, here mixing with the expanded flow in the valve VI to form a liquid 15 which is expanded in a valve V2 upstream of the column K. The gas 11 heats up in the heat exchanger E.

[0036] A top gas 17 from column K depleted in CO2 and enriched in at least one light component is reheated in exchanger E to cool the mixture and is sent to compress in compressor C if present.

[0037] A liquid 19 is drawn from the column at least one theoretical plate below the head of column K and at least one theoretical plate above the tank of column K. It is expanded in valve V3 and vaporizes in exchanger E before being sent to compress in compressor C if present.

[0038] Here the liquid 19 is illustrated as being withdrawn in the upper part of the column K. However, according to the calculations, a better performance is obtained by withdrawing the liquid 19 in the lower part of the column, that is to say at an intermediate level between the first and the N / 2nd theoretical plate of the column, the column comprising N theoretical plates, the first being in the bottom of the column.

[0039] If the column includes an odd number of theoretical plates, the value of N / 2 is rounded down to an integer.

[0040] Preferably, the stream 19 drawn off at an intermediate level is drawn off between the N / 4th and the N / 2nd theoretical plate of the column.

[0041] If the column includes an odd number of theoretical plates or a number that is not a multiple of 4, the value of N / 2 is rounded down to a whole number and the value of N / 4 is rounded up to a whole number.

[0042] A liquid 21 is withdrawn from the bottom of the column being rich in CO2, therefore containing at least 80 mol% of CO2. A part 23 of this liquid serves as liquid product and the remainder 25 vaporizes in the heat exchanger E and is sent in gaseous form to the bottom of the column K to provide reboiling, without having been cooled in the heat exchanger E.

[0043] Refrigerants for the process are provided by an external liquid refrigerant 27 which vaporizes to cool the gas stream at the second pressure to a first temperature by indirect heat exchange. The remainder of the refrigerants required to bring the gas stream to an intermediate temperature of the exchanger lower than the first temperature, and the gas from the first separator S1 to the temperature of the cold end of the exchanger E are provided by the heating of the gas 17, the vaporization and heating of the expanded liquid 19, the vaporization and heating of the liquid 25. Partial condensation begins at the dew point which is higher than the vaporization temperature of the refrigerant with refrigerants coming from the liquid refrigerant 27 but ends without any cold supply from the refrigerant 27.

[0044] The intermediate temperature of the exchanger E is the inlet temperature of the separator SL. The frigories for cooling the gas stream to the first temperature can be provided by a closed refrigeration circuit, comprising a cycle compressor CC and a cooler R which condenses the refrigerant as well as a valve V4. The refrigerant compressed by the compressor CC, and cooled by the cooler R, for example with water, cools in the hot end of heat exchanger E, is expanded in a valve V4, vaporizes in the heat exchanger and is sent to the CC compressor, the cycle being closed. The cycle fluid is preferably propane or ammonia.

[0045] It will be understood that this variant can be simplified by eliminating the partial condensation in S2, so that the column is supplied only by the expanded liquid 9 and the gas 7 is reheated in the exchanger E.

[0046] [Fig.3] differs from [Fig.2] in that the liquid 19 vaporized to provide refrigeration is a part of the bottom liquid 21 of the column. Here too the second partial condensation in S2 is optional.

[0047] It will be understood that the cycle of the figures could be replaced by sending liquid to the heat exchanger which vaporizes there, providing cold.

[0048] In the case where there is a cycle, the only compressors in the apparatus are the gas stream compressor and the cycle compressor.

Claims

Claims

1. A method for separating and liquefying CO2 from a gas stream to be treated (1) comprising the following steps: i. Compression of the gas stream to be treated from a first pressure to a second pressure in a compressor (C), ii. Cooling the gas stream to the second pressure and partially condensing the cooled gas stream to a first temperature by indirect heat exchange with an external vaporizing liquid refrigerant, separating the partially condensed gas stream into a first liquid (9) enriched in carbon dioxide and a first gas (7) depleted in carbon dioxide, iii. Optionally cooling at least a portion of the first carbon dioxide-depleted gas to a second temperature at which it partially condenses producing a second carbon dioxide-enriched liquid (13) and a second carbon dioxide-depleted gas (11), iv. Relaxation (VI, V2, V3) of a. A portion of the CO2-enriched liquid (9, 13, 15), generated by the partial condensation of step ii) or at least one of the two partial condensations of steps ii) and iii), up to a third pressure, higher than the first pressure and lower than the second pressure and production of another portion of the CO2-enriched liquid as product or b. A liquid (19) enriched in CO2, produced by separation in a mass and heat exchange column (K) fed with at least one liquid (9, 13, 15) enriched in CO2 generated by the partial condensation of step ii) or at least one of the two partial condensations of steps ii) and iii), up to a third intermediate pressure between the first pressure and the second pressure and production of a part of the bottom liquid (21) of the column as product (23) v. Vaporizing the expanded CO2-rich liquid to the third pressure, in order to cool and partially condense the gas stream (5) and / or the first CO2-depleted gas (7) from the gas stream of step iii), thereby generating a CO2-rich gas flow at a third pressure and vi. Sending at least a portion of the CO2-rich gas flow at the third pressure to the compressor of the gas stream to be treated from step i) to be compressed together with the gas stream from a pressure.

2. Method according to claim 1 variant b) of step iv) where the liquid (9, 13, 15) generated by at least one of the two partial condensations (SI, S2) is expanded and sent to the top of the column (K).

3. Method according to claim 1 variant b) of step iv) or of claim 2 in which a part (25) of the bottom liquid (21) of the column vaporizes by heat exchange with the at least one part (7) of the gas depleted in carbon dioxide.

4. A method according to claim 3 wherein the vaporized tank liquid constitutes the only gas flow sent to the column (K).

5. Method according to claim 1 variant b) of step iv) or of claim 2 or 3 in which the CO2-rich liquid (19) is withdrawn from the column at an intermediate level thereof or forms part of the bottom liquid (21) of the column.

6. Method according to one of the preceding claims in which the cooling of steps ii) and iii) takes place in a heat exchanger (E).

7. A method according to claim 6 wherein the cooling of step iii) cools the at least part of the gas (7) to the coldest end of the heat exchanger (E).

8. Method according to one of the preceding claims in which the external liquid refrigerant vaporizes according to step ii), is compressed by a cycle compressor (CC), is cooled in the heat exchanger (E), is expanded (V4) and vaporizes again in the heat exchanger, forming a closed refrigeration cycle.

9. A method according to any preceding claim wherein the liquid refrigerant is propane or ammonia.

10. A method according to any preceding claim wherein gas (17) of the distillation column (K) is sent to the compressor (C) of the gas stream to be treated (1) of step i) to be compressed together with the gas stream from a second intermediate pressure between the first intermediate pressure and the second pressure.

11. A method according to any preceding claim wherein a. no part of the CO2-rich liquid (19) expanded to the third pressure which has been vaporized or b. no part of the liquid produced by a CO2-rich liquid distillation column expanded to the third pressure which has been vaporized is compressed in the compressor without having been mixed with the gas stream to be treated (1).

12. A method according to any preceding claim wherein the gas stream at the second pressure (5) is cooled, dried and cooled again before being partially condensed (SI).

13. Method according to one of the preceding claims in which the external liquid refrigerant also serves to cool water intended to cool the gas stream at the second pressure upstream of a step of drying the gas stream (1).

14. Method according to one of the preceding claims in which the gaseous stream (1) contains at least one other component lighter than CO2 which may be hydrogen, helium, nitrogen, oxygen, methane, carbon monoxide.

15. Method according to one of the preceding claims in which the gaseous stream (1) contains water which is removed by a drying step downstream of the compressor (C).

16. Method according to one of the preceding claims in which the second pressure is between 15 and 60 bars.

17. Method according to one of the preceding claims in which the gas stream (1) is cooled from the first temperature to the inlet temperature of a phase separator (SI) by heat exchange with the CO2-rich liquid (19) expanded to the third pressure, before or after the vaporization of the CO2-rich liquid.

18. Apparatus for separating and liquefying CO2 from a gas stream to be treated comprising a compressor (C) for compressing the stream gaseous stream to be treated (1) from a first pressure to a second pressure, means (E) for cooling the compressed gaseous stream in the compressor to the second pressure by partially condensing it by indirect heat exchange with an external vaporizing liquid refrigerant, a first phase separator (S1) for separating the partially condensed gaseous stream into a first liquid (9) enriched in carbon dioxide and a first gas (7) depleted in carbon dioxide, optionally means (E) for cooling at least a portion of the first gas depleted in carbon dioxide to a second temperature at which it partially condenses producing a second liquid (13) enriched in carbon dioxide and a second gas (11) depleted in carbon dioxide, optionally a second phase separator (S2), a. Means (VI, V2) for expanding a portion of the CO2-enriched liquid, coming from the first separator (SI) or from at least one of the first and second separators (SI, S2), to a third pressure, higher than the first pressure and lower than the second pressure and means for outputting another portion of the CO2-enriched liquid as product or b. A mass and heat exchange column (K), means for expanding liquid (15) coming from the first separator or from at least one of the first and second separators, means for sending the expanded liquid to the top of the column, means (V3) for expanding a liquid (19) enriched in CO2, produced by separation in the column, up to a third intermediate pressure between the first pressure and the second pressure and means for withdrawing another part producing a part of the bottom liquid from the column as product and Means (E) for vaporizing CO2-rich liquid expanded to the third pressure, by heat exchange with the compressed gas stream (5) and / or the first CO2-depleted gas (7), thus generating a CO2-rich gas flow at a third pressure and means for sending at least part of the CO2-rich gas flow at the third pressure to the compressor of the gas stream to be treated step i) to be compressed together with the gas stream from a pressure.