Method for liquefying natural gas

Abstract

In a natural gas liquefaction method, prepared natural gas is compressed, the heat of compression is removed, the natural gas is cooled by three circuits containing mixed refrigerants, the pressure of the cooled gas is reduced to produce a vapor-liquid mixture, and the liquefied gas is withdrawn. In each circuit, a mixed refrigerant is compressed, the heat of compression is removed, the refrigerant is sub-cooled, the pressure thereof is lowered to produce in each circuit a low-pressure mixed refrigerant, and the low-pressure mixed refrigerant is used to cool the natural gas. The second mixed refrigerant end boiling point at the pressure at which the compression thereof commences is below the first mixed refrigerant temperature after reducing the first mixed refrigerant pressure, and the third mixed refrigerant end boiling point at the commencement compression pressure is below the second mixed refrigerant temperature after reducing the second mixed refrigerant pressure.

Description

TECHNICAL FIELD

[0001] The invention pertains to technologies for liquefaction of natural gas to be subsequently transported by river or sea.BACKGROUND ART

[0002] There exist multiple well-known natural gas liquefaction methods and relevant plants, most of them based on removal of heat using an external refrigerant.

[0003] There exists a well-known natural gas liquefaction technology (EP 3299757 B1, publication date: 19 Jun. 2019), which consists in cooling pre-treated natural gas in several stages using a mixed refrigerant stream in a coil wound heat exchanger, and expanding the same to generate LNG. The mixed refrigerant is compressed, partially condensed using an air-cooled heat exchanger and fed into a separator where it is separated into a liquid phase and a vapour phase, and then the individual mixed refrigerant streams are fed into the coil wound heat exchanger. The mixed refrigerant is sub-cooled in the coil wound heat exchanger through the boiling of the low-pressure mixed refrigerant, and expanded at a Joule-Thomson valve in an isenthalpic process. The mixed refrigerant vapour is partially condensed in the coil wound heat exchanger and separated into a liquid phase and a vapour phase in a separator. The mixed refrigerant liquid is sub-cooled and the mixed refrigerant vapour is condensed in a heat exchanger through the boiling of the low-pressure mixed refrigerant.

[0004] The disadvantage of the technology consists in limited liquefied natural gas production rate caused by proportional increase in the number of compressor stages and their size limitations, as well as in poor energy efficiency as a single mixed refrigerant circuit is used.

[0005] Moreover, there exist multiple inventions, e.g. RU 2 706 892 C2, that address the start-up issue with heat exchangers in mixed-refrigerant liquefaction processes where during the initial start-up the mixed refrigerant stream condensation in the heat exchanger is incomplete resulting in slower start-up and ramp-up of the plant as well as in equipment vibration that undermine the equipment reliability.

[0006] A natural gas liquefaction method is known (DE 19716415 C1, publication date: 22 Oct. 1998), which consists in cooling natural gas, which may or may not be treated, in several stages using mixed refrigerant streams in cascading heat exchanging cycles based on three refrigerant mixtures, and expanding the same to generate LNG. In the first cycle, the natural gas is pre-cooled, the second mixed refrigerant is condensed, and the third mixed refrigerant is partially condensed through the boiling of the first refrigerant. In the second cycle, the natural gas is liquefied and the third refrigerant is condensed through the boiling of the second refrigerant. In the third cycle, the natural gas is sub-cooled through the boiling of the third refrigerant. The first refrigerant is compressed, cooled and condensed in the first cycle and expanded at a Joule-Thomson valve in an isenthalpic process. The second refrigerant is compressed, condensed in the first cycle, sub-cooled in the second refrigeration cycle and expanded at a Joule-Thomson valve in an isenthalpic process. The third refrigerant is partially condensed in the first refrigeration cycle, condensed in the second refrigeration cycle, sub-cooled in the third refrigeration cycle and expanded at a Joule-Thomson valve in an isenthalpic process.

[0007] The disadvantage of this method is that the second refrigerant is condensed in the first heat exchanger of the first refrigeration cycle, as well as that condensation of the third refrigerant in the first and second refrigeration cycles is incomplete, resulting in a two-phase stream in the heat exchanging equipment, vibration, and, therefore, lower reliability of the plant, and process control complications.

[0008] A method for natural gas liquefaction, which is the closest to the proposed one and is seen as the prototype for the same, features natural gas cooling and liquefaction in three refrigeration circuits (RU 2698565 C2, publication date: 28 Aug. 2019). The treated natural gas is compressed, with removal of the heat resulting from such compression, and cooled using three mixed refrigerant circuits, the cooled gas is depressurized resulting in a mixture of liquid and vapour, and the liquefied gas is sent further downstream, while the mixed refrigerant in each of the circuits is compressed, with removal of the heat resulting from such compression, sub-cooled, expanded, resulting in low-pressure mixed refrigerant in each of the circuits, and used to cool the natural gas, where the first mixed refrigerant in the first circuit is sub-cooled through the evaporation of the first low-pressure mixed refrigerant, the second mixed refrigerant in the second circuit is also cooled through the evaporation of the first low-pressure mixed refrigerant and sub-cooled through the evaporation of the second low-pressure mixed refrigerant, and the third mixed refrigerant in the third circuit is cooled through the evaporation of the second low-pressure mixed refrigerant and sub-cooled through the evaporation of the third low-pressure mixed refrigerant.

[0009] The plant, in which the method is implemented, that is seen as the prototype (description available under the same reference) comprises a natural gas cooling train that includes a natural gas compressor, the first air cooler and the first, second and third stages natural gas cooling heat exchange spaces of the first, second, and third multi-stream heat exchangers accordingly, the first pressure reducer and a separator, as well as the first, second, and third mixed refrigerant circuits, where the first mixed refrigerant circuit consecutively connects the first mixed refrigerant compressor, the second air or water cooler, the first collection tank, a sub-cooling heat exchange space of the first multi-stream heat exchanger, the second pressure reducer (reduction valve) and an evaporation heat exchange space of the first multi-stream heat exchanger, while the second mixed refrigerant circuit consecutively connects the second mixed refrigerant compressor, the third air-cooled heat exchanger, the second collection tank, a cooling heat exchange space of the first multi-stream heat exchanger, a sub-cooling heat exchange space of the second multi-stream heat exchanger, the third pressure reducer (reduction valve) and an evaporation heat exchange space of the second multi-stream heat exchanger, and the third mixed refrigerant circuit consecutively connects two compressors of the third mixed refrigerant, each followed by an air cooler, the fourth one and the fifth one, respectively, a cooling heat exchange space of the second multi-stream heat exchanger, the third collection tank, a sub-cooling heat exchange space of the third multi-stream heat exchanger, the fourth pressure reducer and an evaporation heat exchange space of the third multi-stream heat exchanger.

[0010] When the prototype method is implemented and the natural gas liquefaction plant is operated, the natural gas flow rate and the mixed refrigerants flow rates could become abnormally irregular leading to abnormally narrow differential between the temperatures at the front ends and the rear ends of the heat exchangers, which may result in the natural gas stream becoming colder. If the process setup is incorrect, this may lead to a reverse heat transfer due to overlapping of temperatures between the natural gas stream and the mixed refrigerant stream. This may result in partial condensation of the evaporated refrigerant by virtue of the natural gas stream. If the condensed mixed refrigerant portion ends up in the compressor, the compressor may break down and an accident may occur at the plant. If there are any separators upstream the compressors, the condensed portion is separated therein and evacuated from the plant altering the refrigerant composition, undermining energy efficiency, necessitating the processing of refrigerant condensate (e.g. its fractionation at a dedicated unit) and the adjustment of its composition to achieve the most appropriate specification.

[0011] The technical problem, which is solved with the group of inventions, consists in reducing the risk of accidents caused by the blow-by of the condensed portion of the mixed refrigerant to the compressor.SUMMARY OF THE INVENTION

[0012] The technical problem is solved with the natural gas liquefaction method, by which pre-treated natural gas is compressed, heat of compression is removed, the natural gas is cooled by three circuits containing mixed refrigerants, pressure of the cooled gas is reduced to produce a vapour-liquid mixture, and a liquefied gas is withdrawn, in each said circuit a mixed refrigerant is compressed, heat of compression is removed, the refrigerant is sub-cooled, pressure thereof is lowered to produce in each said circuit a low-pressure mixed refrigerant, and said low-pressure mixed refrigerant is used to cool down the natural gas, herewith in the first circuit the first mixed refrigerant is sub-cooled through evaporation of the first low-pressure mixed refrigerant, in the second circuit the second mixed refrigerant is also cooled through evaporation of the first low-pressure mixed refrigerant and sub-cooled through evaporation of the second low-pressure mixed refrigerant, and in the third circuit, the third mixed refrigerant is cooled through evaporation of the second low-pressure mixed refrigerant and sub-cooled through evaporation of the third low-pressure mixed refrigerant, meanwhile, according to the invention, an end boiling point of the second mixed refrigerant at the pressure at which the compression thereof commences is lower than a temperature of the first mixed refrigerant after the pressure of said first mixed refrigerant has been reduced, and an end boiling point of the third mixed refrigerant at the pressure at which the compression thereof commences is lower than a temperature of the second mixed refrigerant after the pressure of said second mixed refrigerant has been reduced.

[0013] Besides, the first and second mixed refrigerants are completely condensed to avoid formation of two-phase flows in a heat exchange space after the heat of compression is removed.

[0014] Besides, the natural gas is compressed to supercritical state to avoid phase transition.

[0015] Besides the third mixed refrigerant is compressed to supercritical state to avoid two-phase flows at inlets to heat exchangers.

[0016] Besides to reduce the energy consumption of the hole process after the heat of compression is removed from the third mixed refrigerant, the third mixed refrigerant is pre-cooled through evaporation of the first low-pressure mixed refrigerant.

[0017] Besides, the first mixed refrigerant may be a hydrocarbon mixture consisting mainly of ethane or ethylene, propane or propylene and butanes; the second mixed refrigerant may be a hydrocarbon mixture consisting mainly of methane, ethane or ethylene, propane or propylene, and the third mixed refrigerant may be a mixture consisting mainly of nitrogen, methane and ethane or ethylene.

[0018] Besides, the first, second, and third multi-stream heat exchangers are mainly of the coil-wound or plate-fin types.

[0019] The technical result of the proposed method consists in a more stable operation of the plant by way of providing that the end boiling point of the refrigerant is below the post-expansion temperature of the mixed refrigerant of the preceding circuit (lowering of pressure), which makes it impossible to cool down the natural gas in the preceding liquefaction circuit to the temperatures which correspond to a mixed refrigerant existence in the two-phase area at any flow rate or other fluctuations of the technological process parameters.LIST OF DRAWINGS

[0020] FIG. 1 shows the schematic of the plant for implementing the method without pre-cooling of the third mixed refrigerant.

[0021] FIG. 2 shows the schematic of the plant for implementing the method with pre-cooling of the third mixed refrigerant.EXAMPLES OF IMPLEMENTATION OF THE INVENTION

[0022] The natural gas liquefaction plant in FIG. 1 comprises a natural gas cooling line and circuits for the first, second, and third mixed refrigerants.

[0023] The natural gas cooling line consecutively connects natural gas compressor 1, air or water coolers 2, the first, second and third stages natural gas cooling heat exchange spaces of first, second, and third multi-stream heat exchangers 3, 4, 5, respectively, first pressure reducer 6 and a separator 7. It is advisable to use coil-wound and plate-fin types for multi-stream heat exchangers 3, 4, 5.

[0024] The first mixed refrigerant circuit consecutively connects at least one first mixed refrigerant compressor 11, at least one air or water cooler 12, air-cooled or water-cooled condenser 13, a sub-cooling heat exchange space of first multi-stream heat exchanger 3, second pressure reducer 14 and an evaporation heat exchange space of first multi-stream heat-exchanger 3.

[0025] The second mixed refrigerant circuit consecutively connects at least one second mixed refrigerant compressor 21, at least one air or water cooler 22, air-cooled or water-cooled condenser 23, a cooling heat exchange space of first multi-stream heat exchanger 3, a sub-cooling heat exchange space of second multi-stream heat exchanger 4, the third pressure reducer 24, and a evaporation heat exchange space of the second multi-stream heat exchanger 4.

[0026] The third mixed refrigerant circuit consecutively connects at least one third mixed refrigerant compressor 31, at least one air or water cooler 32, a cooling heat exchange space of second multi-stream heat exchanger 4, a sub-cooling heat exchange space of third multi-stream heat exchanger 5, fourth pressure reducer 33, and an evaporation heat exchange space of third multi-stream heat exchanger 5.

[0027] The natural gas liquefaction plant in FIG. 2 differs from the schematic in FIG. 1 by that the pre-cooling heat exchange space of first multi-stream heat exchanger 3 is introduced into the third mixed refrigerant circuit after air or water cooler 32.

[0028] Natural gas compressor 1, first mixed refrigerant compressor 11, second mixed refrigerant compressor 21, and third mixed refrigerant compressor 31 can be driven by, including but not limited to, gas turbines or electric motors, which may be connected with the compressors through multipliers (not shown in the figure).

[0029] Should the throughput capacity be insufficient, the number of natural gas compressors 1 may be increased together with parallel installation of the relevant compressors and an air or water cooler after each compressor.

[0030] Should the throughput capacity be insufficient, the number of first, second, and third mixed refrigerant compressors 11, 21, 31 may be increased together with parallel installation of the relevant compressors and an air or water cooler after each compressor.

[0031] Air-cooled or water-cooled condensers 13, 23 are heat exchangers that enable normal phase transition processes to occur without gas pockets, including, but not limited to, by using inclined single-wound heat exchange tubes.

[0032] The natural gas liquefaction method is implemented as follows.

[0033] The natural gas that was treated ahead of the liquefaction and is free of any water vapour, carbon dioxide and other impurities is supplied to natural gas compressor(s) 1 to be compressed to around 8-10 MPa and then cooled by the ambient cold in air or water cooler(s) 2 to around +15-20° C., and is sent to the heat exchange space of the first multi-stream heat exchanger 3. From the first cooling stage, the gas at around minus 30 to minus 40° C. is supplied to the heat exchange space of second multi-stream heat exchanger 4. From the second cooling stage, the gas at around minus 60 to minus 70° C. is supplied to the heat exchange space of second multi-stream heat exchanger 5 where it is cooled to around minus 155 to minus 159° C. The gas is then sent to first pressure reducer 6, in this case, a throttle valve, where it is depressurized to 0.1 MPa resulting in a liquid and vapour stream, and then sent to the separator 7 to separate the liquid phase and the vapour phase. The liquid portion is liquefied natural gas.

[0034] The first mixed refrigerant is a mixture of mostly ethane or ethylene, propane or propylene, and butane, but these are not the only substances that are used. The first mixed refrigerant vapour from the evaporation heat exchange space of first multi-stream heat exchanger 3 is supplied to first mixed refrigerant compressor(s) 11 to be compressed to around 2.5 MPa and then cooled in air or water cooler(s) 12 and condensed in condenser 13 at +15-20° C. The first mixed refrigerant liquid is sent to the heat exchange space of first multi-stream heat exchanger 3 where it is sub-cooled to around minus 30 to minus 40° C. Once sub-cooled, the first mixed refrigerant is depressurized with the help of the second pressure reducer 14, in this case a throttle valve, resulting in a temperature decrease to around minus 32 to minus 40° C., followed by evaporation of the first low-pressure mixed refrigerant in the evaporation heat exchange space of first multi-stream heat exchanger 3, resulting in cooling of the natural gas, sub-cooling of the first mixed refrigerant and cooling of the second mixed refrigerant. The first mixed refrigerant vapour from multi-stream heat exchanger 3 is sent to compressor 11, compressed, cooled and condensed, and then used again along the circuit to cool the gas and the mixed refrigerants.

[0035] The second mixed refrigerant is a hydrocarbon mixture of mostly methane, ethane or ethylene, and propane or propylene, but these are not the only substances that are used. The second mixed refrigerant vapour is in the evaporation heat exchange space of second multi-stream heat exchanger 4 at around 0.35 to 0.45 MPa, meaning that its end boiling point is around minus 45 to minus 50° C., which is lower than the first mixed refrigerant temperature following expansion at second pressure reducer 14, by at least 5 ° C., precluding any condensation of the evaporated refrigerant in case of the natural gas flow rate becoming abnormally irregular and the natural gas becoming colder. The second mixed refrigerant is then sent to second mixed refrigerant compressor(s) 22 to be further pressurized to around 3.5 to 4.5 MPa, cooled in air or water cooler(s) 21, and condensed in condenser 23 at around +15-20° C. The second mixed refrigerant liquid is cooled through evaporation of the first low-pressure mixed refrigerant in multi-stream heat exchanger 3 to around minus 30 to minus 40° C. and sub-cooled in multi-stream heat exchanger 4 to around minus 62 to minus 70° C. Once sub-cooled, the second mixed refrigerant is depressurized with the help of third pressure reducer 24, in this case a throttle valve, resulting in a temperature decrease to around minus 62 to minus 73° C., followed by evaporation of the second low-pressure mixed refrigerant in the evaporation heat exchange space of second multi-stream heat exchanger 4, resulting in cooling of the natural gas, sub-cooling of the second mixed refrigerant, and cooling of the third mixed refrigerant, respectively. The second mixed refrigerant vapour from said heat exchanger 4 is sent to compressor 21, compressed, cooled and condensed, and then used again along the circuit to cool the gas and the mixed refrigerants.

[0036] The third mixed refrigerant is a mixture of mostly nitrogen, methane, and ethane or ethylene, but these are not the only substances that are used. The third mixed refrigerant vapour is in the evaporation heat exchange space of third multi-stream heat exchanger 5 at around 0.35 to 0.45 MPa, meaning that its end boiling point is around minus 75 to minus 85° C., which is lower than the first mixed refrigerant temperature following expansion at third pressure reducer 24, by at least 5 degrees, precluding any condensation of the refrigerant evaporation in case of the natural gas flow rate becoming abnormally irregular and the natural gas becoming colder. The third mixed refrigerant is then sent to third mixed refrigerant compressor(s) 31 to be further pressurized to around 8 to 9 MPa, cooled in air or water cooler(s) 32. The third mixed refrigerant vapor is consecutively sent to second multi-stream heat exchanger 4 to be cooled to around minus 60 to minus 70, and to third multi-stream heat exchanger 5 to be cooled to around minus 155 to minus 159° C. Once cooled, the third mixed refrigerant is depressurized at fourth pressure reducer 34, in this case a throttle valve. Once depressurized, the third mixed refrigerant is supplied to the evaporation heat exchange space of the third multi-stream heat exchanger 5 where the third low-pressure mixed refrigerant evaporates, resulting in cooling of the natural gas and sub-cooling of the third mixed refrigerant. The third mixed refrigerant vapour from multi-stream heat exchanger 5 is sent to compressor 31, compressed, cooled and condensed, and then used again along the circuit to cool the gas and the mixed refrigerants.

[0037] In case of high ambient temperatures, the arrangement in FIG. 2 is advisable wherein, after cooling in air or water cooler(s) 32, the third mixed refrigerant vapour is first sent to the pre-cooling heat exchange space of first multi-stream heat exchanger 3 to be pre-cooled to around minus 30 to minus 40° C., and then to second multi-stream heat exchanger 4 to be cooled to around minus 60 to minus 70° C., and to third multi-stream heat exchanger 5 to be cooled to around minus 155 to minus 159. The third mixed refrigerant then follows the circuit as shown in FIG. 1.

[0038] In the proposed method, it is advisable to condense at least two of the mixed refrigerants in the air-cooled or water-cooled condenser 13 and 23, where the second mixed refrigerant is sub-cooled through boiling of the first mixed refrigerant resulting in better energy efficiency since the cool down is deeper and the size of the heat exchangers is smaller because there is no two phase stream on the tube side, as well as in a lower portion of vapour in the second mixed refrigerant stream after expansion in an isenthalpic or isentropic process. The proposed method also implies that the cooling of the natural gas and the third mixed refrigerant occurs at preferable pressures above the critical levels to preclude any phase transitions in the heat exchangers, therefore enabling lower steel intensity and higher reliability of the plant.

Claims

1: A natural gas liquefaction method, by which a pre-treated natural gas is compressed, heat of compression is removed, the natural gas is cooled by three circuits containing mixed refrigerants, pressure of a cooled gas is reduced to produce a vapor-liquid mixture, and a liquefied gas is withdrawn, in each said circuit a mixed refrigerant is compressed, heat of compression is removed, the mixed refrigerant is sub-cooled, pressure thereof is lowered to produce in each circuit a low-pressure mixed refrigerant, and such low-pressure mixed refrigerant is used to cool down the natural gas, meanwhile in the first circuit the first mixed refrigerant is sub-cooled through evaporation of the first low-pressure mixed refrigerant, in the second circuit the second mixed refrigerant is also cooled through evaporation of the first low-pressure mixed refrigerant and sub-cooled through evaporation of the second low-pressure mixed refrigerant, in the third circuit the third mixed refrigerant is cooled through evaporation of the second low-pressure mixed refrigerant and sub-cooled through evaporation of the third low-pressure mixed refrigerant, wherein an end boiling point of the second mixed refrigerant at the pressure at which the compression thereof commences is lower than a temperature of the first mixed refrigerant after the pressure of said first mixed refrigerant has been reduced, and an end boiling point of the third mixed refrigerant at the pressure at which the compression thereof commences is lower than a temperature of the second mixed refrigerant after the pressure of said second mixed refrigerant has been reduced.2: The method according to claim 1, wherein the first and second mixed refrigerants are completely condensed after the heat of compression is removed.3: The method according to claim 1, wherein the natural gas is compressed to supercritical state.4: The method according to claim 1, wherein the third mixed refrigerant is compressed to supercritical state.5: The method according to claim 1, wherein after the heat of compression is removed from the third mixed refrigerant, the third mixed refrigerant is pre-cooled through evaporation of the first low-pressure mixed refrigerant.6: The method according to claim 1, wherein the first mixed refrigerant is a hydrocarbon mixture comprising ethane or ethylene, propane or propylene and butane; the second mixed refrigerant is a hydrocarbon mixture comprising methane, ethane or ethylene, propane or propylene; the third mixed refrigerant is a mixture comprising nitrogen, methane and ethane or ethylene.

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