Liquefaction process for a carbon dioxide-rich flow

Abstract

Title of the invention; Liquefaction process for a flow rich in carbon dioxide. In a process for liquefying a flow (1) containing CO2, using a single cooling cycle with CO2 as the refrigerant, the flow to be liquefied is sent to a heat exchanger (51) where it liquefies. The refrigerant (5) is introduced into the exchanger at a temperature T1, at a pressure P1 and subcooled to an intermediate temperature. A portion (7) of the refrigerant subcooled to pressure P1 exits the exchanger at a temperature T5, is expanded to an intermediate pressure P5 and introduced into the exchanger where it vaporizes by heat exchange with the flow to be liquefied and the refrigerant at pressure P1. Another portion (9) of the refrigerant is subcooled in the exchanger to a temperature T10, where T10 > T5 > T1, expanded to a pressure P10 where P10

Description

Title of the invention: Method for liquefying a flow rich in carbon dioxide

[0001] The present invention relates to a method for liquefying a flow rich in carbon dioxide. A flow rich in carbon dioxide contains at least 50 mol% of carbon dioxide, or even at least 90 mol% of carbon dioxide.

[0002] In the context of global warming, industry is encouraged to reduce its CO2 emissions. One solution is to store CO2 underground. Underground CO2 storage can be located far from CO2 emitters, which can justify sending CO2 in liquid form in ships rather than by pipeline in gaseous (or supercritical) form. Today, two processes are being evaluated for CO2 liquefaction, the first is a simple CO2 cycle process, as described in FR2975478, which uses pure CO2 as a refrigerant and a second more complex and more efficient in certain cases where there are two cycles of pure components, the first cycle with ammonia, and the second cycle with CO2.

[0003] Both methods have some drawbacks: • CO2 cycle: high pressure to allow condensation of CO2 against the cooling fluid at ambient temperature • Ammonia and CO2 cascade cycles: the high toxicity and explosiveness of ammonia present certain operational risks.

[0004] It is an object of the present invention to provide a method having at least one of the following advantages: greater efficiency, less danger and less risk of causing global warming.

[0005] Most natural gas liquefaction processes use mixed refrigerant cycles. A mixed refrigerant consists of at least two components.

[0006] Compared with a cascade process where each refrigerant has a single component and its own cycle including a compressor, mixed refrigerant processes use fewer refrigerant compressors and reduce the compressor outlet pressure.

[0007] In the field of liquefied natural gas, mixed refrigerants are composed of nitrogen and hydrocarbons (methane, ethane or ethylene, propane, propylene, isobutane or n-butane, etc.). For the liquefaction of CO2, these components should be avoided to minimize industrial risks and safety zones. In addition, to reduce harmful effects on the environment, gases with a low global warming potential (GWP), i.e. presenting little risk of causing global warming. Moreover, refrigerants must have a boiling point between 40°C and - 50°C at a pressure range between 5 and 50bar.

[0008] In order to liquefy CO2, the following refrigerants meeting the above criteria: • Fluoromethane (R41) with a normal boiling point at -78°C and a GWP of 92 • Propane (R290) with a normal boiling point of -44°C and a GWP below 25 • 2,3,3,3-tetrafluoropropene (R1234-yf) with a normal boiling point of -29°C and a GWP of 4 • 1,3,3,3-tetrafluoropropene (R1234-ze) with a normal boiling point of -19°C and a GWP of 6 • Carbon dioxide (R744) with a normal melting point of -78°C and a GWP of 1

[0009] can be used.

[0010] According to an object of the invention, there is provided a process for liquefying a flow containing at least 50 mol% of CO2, or even at least 90 mol% of CO2 in which the liquefied gas leaves the liquefaction process at a temperature between -45 and -55°C, using a single cooling cycle in which the refrigerant is CO2 or CO2 as well as at least one other refrigerant chosen from the list: propane, 2,3,3,3-tetrafluoropropene, 1,3,3,3-tetrafluoropropene and fluoromethane • and in which • The flow to be liquefied is sent to a heat exchanger where it liquefies. • The refrigerant is introduced into the heat exchanger at a temperature Tl, at a pressure PI between 40 and 90 bars and a density greater than 500 kg / m3 and subcooled to an intermediate temperature between -20 and 10°C • A portion of the subcooled refrigerant at pressure PI leaves the exchanger at a temperature T5, is expanded to an intermediate pressure P5 and introduced into the heat exchanger where it vaporizes by indirect heat exchange with the flow to be liquefied and the refrigerant at pressure PI • Another part of the refrigerant is subcooled in the heat exchanger to a temperature T10 between -45 to -55°C, where T10>T5>Tl, expanded to a pressure P10 where P10 <P5<Pl et introduit dans l’échangeur de chaleur où elle se vaporise par échange de chaleur indirect avec le débit à liquéfier et le réfrigérant à la pression PI • The refrigerant vaporized at pressure P10 heats up in the heat exchanger and is sent to at least one first stage of compression where it is compressed to pressure P5, • The refrigerant vaporized at pressure P5 heats up in the heat exchanger and is sent to at least one second compression stage where it is compressed to pressure PI, after being mixed with the refrigerant compressed in the at least one first compression stage • The refrigerant compressed to PI pressure is cooled to increase its density to at least 500kg / m3 by indirect heat exchange with a refrigerant flow.

[0011] According to other optional aspects: • at least another portion of the subcooled refrigerant at pressure PI leaves the exchanger at a temperature between T1 and T10, is expanded to an intermediate pressure lower than PI and higher than P10 and introduced into the heat exchanger where it vaporizes by indirect heat exchange with the flow to be liquefied and the refrigerant at pressure PL • at least one other part of the refrigerant subcooled at pressure PI leaves the exchanger at a temperature between T1 and T10 and is expanded to an intermediate pressure lower than P5. • at least one other part of the refrigerant subcooled at pressure PI leaves the exchanger at a temperature between T1 and T10 and is expanded to an intermediate pressure greater than P5. • the at least one first compression stage and the at least one second compression stage are part of a centrifugal compressor. • the heat exchanger is a brazed aluminum plate and fin exchanger. • at least part of the refrigerant is expanded in a valve. • at least part of the refrigerant is expanded in a turbine. • the cycle is a closed cycle

[0012] The invention will be described in more detail with reference to the figures where:

[0013] [Fig-1] illustrates a process for liquefying a CO2-rich gas where two parts of the cycle refrigerant are each expanded to a different pressure, vaporized at that pressure and sent to the cycle compressor to be compressed

[0014] [Fig.2] illustrates a process for liquefying a CO2-rich gas where three portions of the cycle refrigerant are each expanded to a different pressure, vaporized at that pressure, and sent to the cycle compressor for compression.

[0015] [Fig.3] illustrates a process for liquefying a CO2-rich gas where four parts of the cycle refrigerant are each expanded to a different pressure, vaporized at that pressure, and sent to the cycle compressor for compression.

[0016] In [Fig.l] a gas flow 1 containing at least 50 mol% CO2, or even at least 90 mol% CO2 liquefies in a heat exchanger E51 which may be a brazed aluminum plate and fin exchanger. The process uses a single refrigeration cycle to liquefy the CO2, this cycle preferably being closed.

[0017] A high pressure refrigerant 5 PI (between 40 and 90 bar) comprising at least CO2 and optionally at least one other refrigerant which can be chosen from the list: R1234-yf, R1234-ze, R290, R41 at an ambient temperature T1 is sent to a heat exchanger E1. A portion 9 of this refrigerant is cooled in the exchanger E51 to the cold end at a temperature T10 where -55°C <T10<-45°C. Le réfrigérant 9 est détendu dans une vanne ou une turbine jusqu’à une pression P10 où 4.2 barg < P10 < 4.6 barg.

[0018] The refrigerant 9 is then vaporized in a low-pressure passage of the heat exchanger E51 and superheated. Then it is sent to at least a first stage of a compressor CIO.

[0019] Between the hot inlet of the refrigerant at pressure PI and ambient temperature T1 and the cold outlet at T10, another part of the refrigerant 7 leaves the exchanger at a temperature T5 where T10 <T5<Tl et est détendue par une vanne ou une turbine jusqu’à une pression intermédiaire P5.

[0020] The refrigerant 7 is then vaporized in a medium pressure passage of the heat exchanger E51. Then it is sent to at least a second stage of a compressor CIO, after being mixed with the flow of refrigerant 9 compressed in the compressor CIO up to P5.

[0021] The mixture is compressed in the compressor CIO to the pressure PI and then is cooled to increase its density to at least 500 kg / m3 at ambient temperature Tl by indirect heat exchange with a refrigerant flow (for example water or air) in a heat exchanger E10. The refrigerant after this densification can be in liquid or supercritical form.

[0022] The method according to the invention may comprise an expansion of a portion of the refrigerant cooled to an intermediate temperature up to between two and four pressure levels. At each level, the refrigerant is vaporized and returned to a compressor stage at a temperature at least 10°C above its dew point.

[0023] Each compression stage compresses the flow of refrigerant arriving directly from the exchanger to the pressure of the inlet of the stage and in addition the refrigerant compressed in the previous stage(s).

[0024] Instead of sending the expanded refrigerant flow in the valve directly into the heat exchanger, it is possible to use phase separation upstream of the exchanger and downstream of the valve (pots V54, V56) and to combine the gaseous and liquid phases coming from the pot upstream of the exchanger.

[0025] In [Fig. 2], compared to [Fig. 1], an additional portion 6 of the refrigerant 5 leaves the exchanger at a temperature higher than T5 and lower than T1, is expanded to a pressure higher than P5 and lower than PI and vaporizes in the heat exchanger before being sent to the compressor CIO at the pressure to which it was expanded.

[0026] Instead of sending the expanded refrigerant flow in the valve directly into the heat exchanger, it is possible to use phase separation upstream of the exchanger and downstream of the valve (pots V54, V56, V57) and to combine the gaseous and liquid phases coming from the pot upstream of the exchanger.

[0027] In [Fig.3], compared to [Fig.2], an additional portion 7 of the refrigerant 5 leaves the exchanger at a temperature higher than T10, lower than T5 and lower than T1, is expanded to a pressure higher than P10 and lower than P5 and vaporizes in the heat exchanger before being sent to the compressor CIO at the pressure to which it was expanded.

[0028] Instead of sending the expanded refrigerant flow in the valve directly into the heat exchanger, it is possible to use phase separation upstream of the exchanger and downstream of the valve (pots V54, V55, V56, V57) and to combine the gaseous and liquid phases coming from the pot upstream of the exchanger.

Claims

Claims

1. Process for liquefying a flow (1) containing at least 50 mol% of CO2, or even at least 90 mol% of CO2 in which the liquefied gas (3) leaves the liquefaction process at a temperature between -45 and -55°C, using a single cooling cycle in which the refrigerant is CO2 or CO2 as well as at least one other refrigerant chosen from the list: propane, 2,3,3,3-tetrafluoropropene, 1,3,3,3-tetrafluoropropene and fluoro-methane and in which • The flow to be liquefied is sent to a heat exchanger (51) where it liquefies. • The refrigerant (5) is introduced into the heat exchanger at a temperature Tl, at a pressure PI between 40 and 90 bars and a density greater than 500 kg / m3 and subcooled to an intermediate temperature between -20 and 10°C • A portion (7) of the subcooled refrigerant at pressure PI leaves the exchanger at a temperature T5, is expanded to an intermediate pressure P5 and introduced into the heat exchanger where it vaporizes by indirect heat exchange with the flow to be liquefied and the refrigerant at pressure PI • Another part (9) of the refrigerant is subcooled in the heat exchanger to a temperature T10 between -45 to -55°C, where T10>T5>Tl, expanded to a pressure P10 where P10 <P5<Pl et introduite dans l’échangeur de chaleur où elle se vaporise par échange de chaleur indirect avec le débit à liquéfier et le réfrigérant à la pression PI • The refrigerant vaporized at pressure P10 heats up in the heat exchanger and is sent to at least one first compression stage (CIO) where it is compressed to pressure P5, • The refrigerant vaporized at pressure P5 heats up in the heat exchanger and is sent to at least a second compression stage where it is compressed to pressure PI, after having been mixed with the compressed refrigerant in at least one first compression stage • The refrigerant compressed to pressure PI is cooled (E10) to increase its density to at least 500kg / m3 by indirect heat exchange with a refrigerant flow.

2. Method according to claim 1 in which at least another portion (7, 8) of the subcooled refrigerant at pressure PI leaves the exchanger (E51) at a temperature between T1 and T10, is expanded to an intermediate pressure lower than PI and higher than P10 and introduced into the heat exchanger where it vaporizes by indirect heat exchange with the flow to be liquefied and the refrigerant at pressure PL

3. Method according to claim 2 in which the at least one other part (6) of the refrigerant subcooled at pressure P1 leaves the exchanger at a temperature between T1 and T10 and is expanded to an intermediate pressure lower than P5.

4. Method according to claim 2 or 3 in which the at least one other part (8) of the refrigerant subcooled at pressure P1 leaves the exchanger at a temperature between T1 and T10 and is expanded to an intermediate pressure greater than P5.

5. Method according to one of the preceding claims in which the at least one first compression stage and the at least one second compression stage are part of a centrifugal compressor (CIO).

6. Method according to one of the preceding claims in which the heat exchanger (E51) is a brazed aluminum plate and fin exchanger.

7. Method according to one of the preceding claims in which at least a portion (6,7,8,9) of the refrigerant is expanded in a valve.

8. Method according to one of the preceding claims in which at least a portion (6,7,8,9) of the refrigerant is expanded in a turbine.

9. A method according to any preceding claim wherein the refrigerant is CO2.

10. Method according to one of the preceding claims 1 to 8 in which the refrigerant is CO2 as well as at least one other refrigerant chosen from the list: propane, 2,3,3,3-tetrafluoropropene, 1,3,3,3-tetrafluoropropene and fluoromethane

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