METHOD FOR FROSTING CARBON DIOXIDE CONTAINED IN LIQUID METHANE

Abstract

Method for extracting carbon dioxide in liquid methane, including expanding a liquid methane the carbon dioxide content being greater than 280 ppmv, the expansion carried out from a pressure greater than 6 bar to a pressure of 1 bar, the temperature of the liquid methane expanded being about 161.5 C., passing the methane gas phase and the carbon dioxide solid phase into a first liquid-solid-gas separator, with extraction of the solid carbon dioxide by filtration, and separation of the methane gas, to obtain a first liquid methane phase, partially decarbonized, transferring this first liquid methane phase to a second solid-liquid separator, the first liquid methane phase at 161 C., the second separator being an exchanger at less than 170 C., the carbon dioxide depositing in the second exchanger, to form a second liquid methane phase, the concentration of carbon dioxide in this second liquid methane phase at 170 C. being less than 200 ppmv.

Claims

1. A method for extracting carbon dioxide contained in liquid methane, the method comprising: a step of expansion of a liquid methane the carbon dioxide content of which is greater than 280 parts per million by volume, and in particular about 3000 parts per million by volume, the expansion being carried out from a pressure greater than 6 bar to a pressure of 1 bar, the temperature of the liquid methane thus expanded being about 161.5 C., the methane vaporising and the carbon dioxide crystallising during this expansion, to form a three-phase liquid-vapour-solid mixture of methane and carbon dioxide, the resulting liquid methane being supersaturated with carbon dioxide, and a step of passing the methane gas phase and the solid phase of carbon dioxide into a first liquid-solid-gas separator, with extraction of the solid carbon dioxide by filtration, and separation of the gaseous methane, to obtain a first liquid methane phase, partially decarbonized, a step of transferring this first liquid methane phase to a second solid-liquid separator, the first liquid methane phase being at a temperature of 161 C., the second separator being an exchanger the temperature of which is less than 170 C., the carbon dioxide depositing in the second exchanger, to form a second liquid methane phase, the concentration of carbon dioxide in this second liquid methane phase at 170 C. being less than 200 ppmv, advantageously below 100 ppmv.

2. The method according to claim 1, wherein the carbon dioxide content of the first liquid methane phase, from the first separator, is about 300 ppmv.

3. The method according to claim 1, wherein the temperature of the second separator is 176 C., the carbon dioxide content of the second liquid methane phase, at the outlet of the second separator, being less than 50 ppmv.

4. The method according to claim 1, comprising a step of extracting the solid carbon dioxide deposited in the second separator, this extraction being carried out in the gas phase at a pressure of about 500 mbar.

5. The method according to claim 1, comprising a step of extracting the solid carbon dioxide deposited in the second separator, this extraction being carried out in liquid phase at a pressure of about 6 bar.

6. The method according to claim 1, comprising a step of measuring the pressure loss on the liquid methane between the inlet and outlet of the first separator.

7. The method according to claim 6, wherein when the pressure loss on the liquid methane between the inlet and outlet of the first separator is greater than a predetermined threshold, the extraction of the carbon dioxide deposited in the first separator is interrupted.

8. The method according to claim 7, wherein the first separator comprises two enclosures, each enclosure being provided with a micron filter for recovery of solid carbon dioxide.

9. The method according to claim 8, wherein when the extraction of the carbon dioxide deposited in the first separator is interrupted, the method comprises a step of heating the filters.

10. The method according to claim 8, comprising a measurement of the temperature of the fluid circulating in the filters, downstream of the filters, the extraction of the carbon dioxide being completed when this temperature exceeds a predetermined threshold value, advantageously of about 10 C.

11. The method according to claim 8, wherein when the extraction of carbon dioxide is interrupted, the methane flow is diverted from a first enclosure to the second enclosure of the first separator.

12. The method according to claim 1, comprising a step of measuring the pressure loss on the liquid methane between the inlet and outlet of the second separator.

13. The method according to claim 12, wherein the second separator is a finned-tube exchanger.

14. The method according to claim 13, wherein the maximum speed of the liquid methane in the channels formed by the inter-fin spaces of the second exchanger is about 0.2 m/s.

15. The method according to claim 12, wherein when the pressure loss on the liquid methane between the inlet and outlet of the second separator is greater than a predetermined threshold, the extraction by frosting of the carbon dioxide deposited in the second separator is interrupted.

16. The method according to claim 12, wherein the second separator comprises two enclosures, the method comprises a step of heating an enclosure when the frosting of the carbon dioxide deposited in this enclosure of the second separator is interrupted.

17. The method according to claim 16, comprising a measurement of the temperature of the fluid circulating in the enclosures, downstream of the enclosures, the defrosting of the carbon dioxide being completed when this temperature exceeds a predetermined threshold value.

18. The method according to claim 17, wherein when the frosting of the carbon dioxide is completed in a first enclosure of the second separator, the methane flow is diverted to the second enclosure of the second separator, the second separator thus operating alternately, one enclosure of the second separator being in the frosting phase when the other enclosure of the second separator is in the defrosting phase.

19. A device for extracting the carbon dioxide contained in liquid methane, for implementing the method according to claim 1, the device comprising: a tank of liquid methane the carbon dioxide content of which is about 3000 parts per million by volume, at a pressure greater than 6 bar; an expansion valve downstream of the tank, to reduce the pressure of the liquid methane to a value of 1 bar, the temperature of the liquid methane thus expanded being about 161.5 C., to form a three-phase liquid-vapour-solid mixture of methane and carbon dioxide, the liquid methane obtained being supersaturated with carbon dioxide; a first liquid-solid-gas separator, wherein between the three-phase liquid-vapour-solid mixture, with extraction of the solid carbon dioxide by filtration, and separation of the gaseous methane, to obtain a first phase of liquid methane, partially decarbonized; a second liquid-solid separator, wherein between the first liquid methane phase at a temperature of 161 C., the second separator being an exchanger the temperature of which is less than 170 C., carbon dioxide depositing in the second separator, to form a second liquid methane phase, the concentration of carbon dioxide in this second phase of liquid methane at 170 C. being less than 100 ppmv.

20. The device according to claim 19, wherein the first separator comprises two enclosures, each enclosure being provided with a micron filter for recovery of solid carbon dioxide.

21. The device according to claim 20, wherein the two enclosures of the first separator are identical.

22. The device according to claim 20, wherein in that the micron filter has a solid matrix, with a porosity of about 10 micrometres.

23. The device according to claim 19, wherein the second separator comprises two enclosures, each enclosure being provided with a finned-tube exchanger.

Description

[0097] The complete device preferably comprises a central unit, capable of implementing the method as previously described.

[0098] Other objects and advantages of the invention will appear in light of the description of an embodiment, made hereinafter in reference to the appended figures.

[0099] FIG. 1 is a schematic representation of a first separation device of a carbon dioxide extraction device from the liquid phase of methane;

[0100] FIG. 2 is a schematic representation of a second separation device of a carbon dioxide extraction device from the liquid phase of methane.

[0101] The device 1 for filtering and frosting the CO.sub.2 contained in liquid methane or LNG comprises a first separation device 20 shown schematically in FIG. 1, and a second separation device 30, shown in FIG. 2.

[0102] The methane leaving the first separation device 20 has a CO.sub.2 content of about 300 ppm, and a pump 141 will circulate this methane in a line 14, to the second separation device 30.

[0103] Refer first to FIG. 1.

[0104] The device 1 comprises a liquid methane tank 10 at a pressure greater than 6 bar and the CO.sub.2 concentration of which is about 3000 ppm.

[0105] The device 1 comprises an expansion valve 12 for reducing the pressure of the liquid methane to 1 bar, installed on a connecting pipe 11 between the tank 10 and two enclosures 21, 22 of the first separation device 20.

[0106] Each enclosure 21, 22 comprises a micron filter 23, 24.

[0107] Downstream of the expansion valve 12, the pipe 11 forms two branches 111, 112.

[0108] The enclosures 21, 22 are respectively supplied with liquid methane by the branches 111, 112 and solenoid valves 113, 114 control the supply to each of these enclosures 21, 22.

[0109] The solid CO.sub.2 is recovered by the filters 23, 24.

[0110] The methane gas phase is discharged by a line 13 connected to the aspiration of a biomethane compressor, via a branch 211 for the enclosure 21 and a branch 221 for the enclosure 22.

[0111] A valve 216 and a temperature sensor 214 are placed on the branch 211.

[0112] Similarly, a valve 226 and a temperature sensor 224 are placed on the branch 221.

[0113] An anti-drip device 212 prevents the driving of droplets to the compressor, for the enclosure 21.

[0114] Similarly, an anti-drip device 222 prevents the driving of droplets to the compressor, for the enclosure 22.

[0115] The liquid methane is aspirated by a pump 141 installed on the line 14 that connects the first separator 20 to the second separator 30.

[0116] This pump 141 aspirates the liquid from a branch 251 for the enclosure 21, or from a branch 252 for the enclosure 22.

[0117] The liquid phase methane is extracted via a dip tube associated with each branch 251 and 252.

[0118] The line 14 is provided with a flowmeter 142, downstream of the pump 141.

[0119] The branch 251 is provided with a valve 253.

[0120] Similarly, the branch 252 is provided with a valve 254.

[0121] A line 15 supplies the enclosures 21, 22 with hot gaseous methane, at about 50 C. An expansion valve 16 is placed on this line 15, upstream of two supply branches of the enclosures 21, 22.

[0122] Each of the two supply branches of the hot gaseous methane enclosures is provided with a valve 151, 152.

[0123] The circulation of the methane flow is controlled by a PLC.

[0124] The PLC will open the series of valves 113, 216, 253 when methane is circulating in enclosure 21.

[0125] The PLC will open the series of valves 114, 226, 254 when methane is circulating in enclosure 22.

[0126] When one of these series of valves 113, 216, 253 is opened, the other series of valves 114, 226, 254 is closed.

[0127] The enclosure 21 is provided with a differential pressure gauge 213 measuring the pressure loss between the common inlet of the three-phase mixture and the liquid outlet on the branch 251, when the methane flow passes through the enclosure 21.

[0128] Similarly, the enclosure 22 is provided with a differential pressure gauge 223 measuring the pressure loss between the common inlet of the three-phase mixture and the liquid outlet on branch 252, when the methane flow passes through the enclosure 22.

[0129] When the separation is carried out in enclosure 21 and the high pressure loss threshold indicated by differential pressure gauge 213 is reached, then the PLC generates the following actions: switching of the methane flow from enclosure 21 to enclosure 22, by opening the series of valves 114, 226, 254 and closing valves 113, 216, the valve 253 remaining open so that pump 141 empties enclosure 21 of its liquid.

[0130] The valve 253 is closed again when the flowmeter 142 indicates a lower and constant flow value, the excess flow corresponding to the emptying of the volume of liquid extracted from the enclosure 21 which is known by construction.

[0131] Once the enclosure 21 has been emptied of its liquid, the PLC opens valves 216 and 151 and the expansion valve 16 installed on line 15, in order to circulate hot gaseous methane at about 50 C. to sublimate the CO.sub.2 trapped in the filter 23.

[0132] The methane flow comes from the high pressure of the methane compressor, expanded by the expansion valve 16 and the rinsing flow of the filter 23 is mixed with the gaseous flow of branch 221 and these two flows are aspirated by the methane compressor, via the line 13.

[0133] When the outlet temperature of the enclosure 21 measured by the temperature sensor 214 reaches a temperature above 10 C., then the sublimation of the CO.sub.2 trapped in the filter 23 is completed, and the PLC closes the expansion valve 16 and the valves 216 and 151.

[0134] The separator is ready for the three-phase separation of the next cycle.

[0135] The description of the purge of the filter 23 by sublimation of the CO.sub.2 is identical for purge of the filter 24, with of course the opening and closing of valves corresponding to the valves of the enclosure 22.

[0136] The methane exiting from the first separation device 20 has a CO.sub.2 content of about 300 ppm.

[0137] The pump 141 will circulate this methane in the second separation device 30, shown schematically in FIG. 2.

[0138] The second separation device 30 comprises two enclosures 31, 32. Each enclosure 31, 32 houses a finned-tube exchanger 33, 34.

[0139] The line 14 supplies the enclosure 31 via a branch 311, a valve 313 being arranged on this branch 311, upstream of the enclosure 31.

[0140] The line 14 supplies the enclosure 32 via a branch 312, a valve 314 being arranged on this branch 312, upstream of the enclosure 32.

[0141] The exchanger 33 of the enclosure 31 is supplied with coolant at 170 C. by a coolant line 330. A valve 335 is arranged on the line 330. A heat transfer circuit 331 is connected to the line 330 downstream of the valve 330. The heat-transfer circuit 331 is provided with a valve 337.

[0142] Similarly, the exchanger 34 of the enclosure 32 is supplied with coolant at 170 C. by a coolant line 320. A valve 326 is arranged on the line 320. A heat transfer circuit 321 is connected to the line 320 downstream of the valve 326. The heat transfer circuit 321 is equipped with a valve 328.

[0143] When the valve 335 of the coolant line 330 is open and the valve 337 of the heat transfer circuit 331 is closed, the exchanger 33 is supplied with coolant at 170 C. and the coolant exits the exchanger 33 via a line 333 to a cryogenic system (not shown), after cooling of the liquid methane in the exchanger 33.

[0144] A temperature sensor 334 is mounted on the line 333.

[0145] Similarly, when the valve 326 of the coolant line 320 is open and the valve 328 of the heat transfer circuit 321 is closed, the exchanger 34 is supplied with coolant at 170 C. and the coolant exits the exchanger 34 via a line 322 to a cryogenic system (not shown), after cooling of the liquid methane in the exchanger 34.

[0146] A temperature sensor 324 is mounted on the line 322.

[0147] The enclosures 31, 32 are connected to a liquid methane tank 50, by a line 350 on which a pump 351 and a flowmeter 352 are mounted. A valve 317 is downstream of the enclosure 31 and upstream of the tank 50, a valve 316 being downstream of the enclosure 32 and upstream of the tank 50.

[0148] The enclosures 31, 32 are connected to a pressurisation tank 43, by a line 44 whereon a valve 412 is mounted.

[0149] The line 44 is in communication with the enclosure 31 by a branch 413 whereon a valve 415 is mounted.

[0150] The line 44 is in communication with the enclosure 32 by a branch 414 whereon a valve 416 is mounted.

[0151] The enclosures 31, 32, are connected to a purge line 40 on which a vacuum pump 60, a dump valve 431 and a vacuum gauge 432 are mounted.

[0152] The enclosures 31, 32 are also connected to a liquid CO.sub.2 recovery tank 42 by a line 420.

[0153] A valve 417 is mounted on the branch 419 of the line 420.

[0154] A valve 418 is mounted on the branch 416 of line 420.

[0155] Downstream of branches 416, 419, the line 420 is provided with a valve 421 and a temperature sensor 423.

[0156] The enclosures 31, 32 are each provided with a multifunctional pressure sensor 318, measuring the absolute pressure in each of the enclosures 31 and 32 and measuring the differential pressure between the pressure measured on the inlet branch 311 and the outlet branch 354 for the enclosure 31 and the inlet branch 312 and the outlet branch 353 for the enclosure 32.

[0157] If the frosting of the CO.sub.2 is carried out in the enclosure 31, valves 313, 317 are opened and valve 314 is closed. The liquid methane circulating at low speed is cooled on the fins of the finned-tube exchanger 33 and the CO.sub.2 frosts on the fins as the temperature on the exchanger 33 drops. The purified methane is aspirated by the pump 351 and stored in the tank 50.

[0158] Similarly, if the separation takes place in the enclosure 32, valves 314, 316 are opened and valve 313 is closed. The liquid methane circulating at low speed is cooled on the fins of the finned-tube exchanger 34 and the CO.sub.2 frosts on the fins as the temperature on the exchanger 34 drops. Similarly, the purified methane is aspirated by the pump 351 and stored in the tank 50.

[0159] When the pressure difference measured by the sensor 318 between the inlet and outlet of the enclosure 31 or of the enclosure 32 is greater than a threshold of about 50 millibar, then the defrosting phase is launched. In the defrosting phase of the CO.sub.2 exchanger 34 in vapour, the PLC performs the following actions: the valve 314 is closed, the pump 351 on the line 350 creates the additional vacuum that allows the enclosure 32 to be emptied of its purified liquid methane in the storage tank 50, the valve 316 is closed again when the flowmeter 352 indicates a lower and constant flow value, the excess flow corresponding to the emptying of the volume of liquid extracted from the enclosure 32 which is known by construction.

[0160] Then the valve 416 as well as the dump valve 431 are opened, the vacuum pump 60 of the vacuum line 40 is switched on and the residual atmosphere with residual vaporisation of liquid methane is carried out until the vacuum gauge 432 indicates a residual pressure of less than 1 mbar.

[0161] The coolant circuit 320 is closed by the valve 326 and the coolant circuit 321 is opened by the valve 328, the CO.sub.2 pressure rises in the enclosure 32 and is maintained at about 500 mbar by the vacuum pump 60 until the temperature sensor 324 installed on the outlet line 322 of the coolant indicates a temperature above 10 C.

[0162] On the other hand, if liquid-phase defrosting is chosen, the dump valve 431 is closed, the vacuum pump 60 is stopped, defrosting is carried out in the same way by circulation of the heat transfer fluid in the exchanger and when the pressure inside the enclosure 32 measured by the pressure sensor 316 reaches 5.2 bar, triple point pressure, the CO.sub.2 liquefies.

[0163] Similarly, the circulation of the coolant is stopped by closing the valve 328 when the temperature indicated by the measurement of sensor 324 of the circuit 322 is higher than 10 C.

[0164] The line 44 will connect, by opening valves 412, 416, the pressurisation tank 43 to about 8 bar and the enclosure 32.

[0165] The pressure in the enclosure 32 will increase from 5.2 bar to approximately 6 bar, pressure measured by the sensor 318, the PLC then opens the valves 418, 421 so that the liquid CO.sub.2 is transferred to the CO.sub.2 tank 42.

[0166] When the temperature sensor 423 indicates that the temperature rises from about 50 C. to a temperature above 45 C., this indicates the end of the CO.sub.2 flow, the valves 421, 418 are closed as well as the valve 412, which isolates tanks 43, 42 again.

[0167] To purge the CO.sub.2 vapour from the enclosure 32, the dump valve 431 is opened and it maintains a maximum pressure of 1.2 bar downstream to avoid a pressure surge at the vacuum pump 60 aspiration.

[0168] When the pressure reaches 1 mbar measured by the vacuum gauge 432, the vacuum pump 60 is stopped, the valves 431 and 414 are closed.

[0169] The enclosure 32 is ready to begin a new CO.sub.2 frosting cycle in the liquid methane.

[0170] A first advantage of the method thus described is that it is not necessary to use solutions of all kinds to extract the CO.sub.2. As a result, the device 1 is simplified.

[0171] A second advantage is that it is no longer necessary to recycle substrates of all kinds, which are obtained in conventional washing facilities.