Process for natural gas liquefaction

Abstract

The present invention relates to a technology for liquefying natural gas subsequently transported by river or sea. The technical result of the proposed method is a more stable plant operation due to the fact that the final boiling point of the refrigerant is lower than the post-expansion temperature of the mixed refrigerant in the preceding circuit, thereby making it impossible to cool the natural gas in the preceding liquefaction circuit to a temperature corresponding to the presence of the mixed refrigerant in the two-phase region, regardless of any fluctuations in flow rate or other technological process parameters. The prepared natural gas is compressed, the heat of compression is removed, and the natural gas is cooled through three circuits containing the mixed refrigerant. The pressure of the cooled gas is reduced to produce a vapor-liquid mixture, and the liquefied gas is withdrawn. In each circuit, the mixed refrigerant is compressed, the heat of compression is removed, the refrigerant is subcooled, and its pressure is reduced to produce a low-pressure mixed refrigerant in each circuit, which is used to cool the natural gas.

Description

[Technical Field]

[0001] The present invention relates to a technique for the liquefaction of natural gas that is subsequently transported by river or sea. [Background technology]

[0002] There are several known natural gas liquefaction processes and associated plants, most of which are based on the use of an external refrigerant to remove heat.

[0003] A known natural gas liquefaction technology (EP 3299757, published June 19, 2019) involves cooling pre-processed natural gas in several stages using a mixed refrigerant stream in a coil-wound heat exchanger and expanding it to produce LNG. The mixed refrigerant is compressed, partially condensed using an air-cooled heat exchanger, and pumped into a separator where it is separated into liquid and vapor phases. The individual mixed refrigerant streams are then pumped into the coil-wound heat exchanger. The mixed refrigerant is subcooled in the coil-wound heat exchanger by boiling the low-pressure mixed refrigerant and expanded in a Joule-Thomson valve in an isenthalpic process. The mixed refrigerant vapor is partially condensed in the coil-wound heat exchanger and separated into liquid and vapor phases in the separator. The mixed refrigerant liquid is subcooled, and the mixed refrigerant vapor is condensed in the heat exchanger by boiling the low-pressure mixed refrigerant.

[0004] The drawbacks of this technology are the limited liquefied natural gas production caused by the proportional increase in the number of compressor stages, its size limitations, and poor energy efficiency when a single mixed refrigerant circuit is used.

[0005] Additionally, there are several inventions, such as Russian Patent Invention No. 2706892, that address start-up issues related to heat exchangers in mixed refrigerant liquefaction processes, where the mixed refrigerant stream in the heat exchanger during initial start-up is incompletely condensed, resulting in slow plant start-up and ramp-up, as well as equipment vibrations that compromise equipment reliability.

[0006] A method for liquefying natural gas is known (German Patent No. 19716415, published October 22, 1998), which consists in cooling natural gas, which may or may not be processed, in several stages using a mixed refrigerant stream in a cascade heat exchange cycle based on a three-refrigerant mixture and expanding it to produce 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 by boiling off the first refrigerant. In the second cycle, the natural gas is liquefied, and the third refrigerant is condensed by boiling off the second refrigerant. In the third cycle, the natural gas is subcooled by boiling off the third refrigerant. The first refrigerant is compressed, cooled, and condensed in the first cycle and expanded in a Joulet-Thomson valve in an isenthalpic process. The second refrigerant is compressed and condensed in the first cycle, subcooled in the second refrigeration cycle, and expanded in a Joulet-Thomson valve in an isenthalpic process. The third refrigerant is partially condensed in the first refrigeration cycle, condensed in the second refrigeration cycle, subcooled in the third refrigeration cycle, and expanded in a Joulet-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, and the condensation of the third refrigerant in the first and second refrigeration cycles is incomplete, resulting in a two-phase stream in the heat exchanger and vibrations, thus reducing the reliability of the plant and complicating process control.

[0008] The closest proposed method of natural gas liquefaction, which can be considered as a prototype, is characterized by the cooling and liquefaction of natural gas in three refrigeration circuits (Russian Patent No. 2698565, published on August 28, 2019). The processed natural gas is compressed, with the removal of heat due to such compression, and cooled using three mixed refrigerant circuits; the cooled gas is depressurized, resulting in a mixture of liquid and vapor; and the liquefied gas is sent further downstream, while the mixed refrigerant in each of the circuits is compressed, with the removal of heat due to such compression, subcooled, and expanded, resulting in a low-pressure mixed refrigerant in each of the circuits, used to cool the natural gas; where a first mixed refrigerant in a first circuit is subcooled by evaporation of the first low-pressure mixed refrigerant; a second mixed refrigerant in a second circuit is also cooled by evaporation of the first low-pressure mixed refrigerant and subcooled by evaporation of the second low-pressure mixed refrigerant; and a third mixed refrigerant in a third circuit is cooled by evaporation of the second low-pressure mixed refrigerant and subcooled by evaporation of the third low-pressure mixed refrigerant.

[0009] The plant in which this method is implemented and which is considered as a prototype (the description of which is available under the same criteria) comprises a natural gas cooling train, which includes a natural gas compressor, a first air cooler, first, second and third stages of natural gas cooling heat exchanger spaces corresponding to first, second and third multi-stream heat exchangers, a first pressure reducer, a separator, and first, second and third mixed refrigerant circuits, wherein the first mixed refrigerant circuit connects in series the first mixed refrigerant compressor, the second air or water cooler, the first collection tank, the subcooling heat exchange space of the first multi-stream heat exchanger, the second pressure reducer (pressure reducer), and the evaporative heat exchange space of the first multi-stream heat exchanger, while The second mixed refrigerant circuit successively connects the second mixed refrigerant compressor, the third air-cooled heat exchanger, the second collecting tank, the cooling heat exchange space of the first multi-stream heat exchanger, the subcooling heat exchange space of the second multi-stream heat exchanger, the third pressure reducer (pressure reducing valve), and the evaporative heat exchange space of the second multi-stream heat exchanger; the third mixed refrigerant circuit successively connects the two compressors of the third mixed refrigerant, the cooling heat exchange space of the second multi-stream heat exchanger, the third collecting tank, the subcooling heat exchange space of the third multi-stream heat exchanger, the fourth pressure reducer, and the evaporative heat exchange space of the third multi-stream heat exchanger, each followed by an air cooler, i.e., a fourth and a fifth one, respectively.

[0010] When this prototype method was implemented and a natural gas liquefaction plant was operated, the natural gas flow rate and mixed refrigerant flow rate could become abnormally irregular, leading to an abnormally narrow temperature difference between the front and back ends of the heat exchanger, resulting in the natural gas stream becoming colder. If the process configuration is not correct, this can lead to reverse heat transfer due to temperature overlap between the natural gas stream and the mixed refrigerant stream. As a result, partial condensation of the evaporated refrigerant can occur due to the natural gas stream. If the condensed mixed refrigerant portion terminates in the compressor, the compressor may break down, causing an accident in the plant. If a separator is present upstream of the compressor, the condensed portion will be separated within it and will be discharged from the plant with a different refrigerant composition, reducing energy efficiency and requiring treatment of the refrigerant condensate (e.g., its separation in a dedicated unit) and adjustment of its composition to achieve optimal specifications.

[0011] The technical problem solved by the group of present inventions is to reduce the risk of an accident caused by blow-by of the condensed part of the mixed refrigerant into the compressor. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] European Patent No. 3299757 [Patent Document 2] Russian Patent Invention No. 2706892 [Patent Document 3] German Patent No. 19716415 [Patent Document 4] Russian Patent Invention No. 2698565 Summary of the Invention [Means for solving the problem]

[0013] The technical problem is a method for liquefying natural gas, in which pre-treated 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 an evaporation-liquid mixture, the liquefied gas is withdrawn, in each of said circuits the mixed refrigerant is compressed, the heat of compression is removed, the refrigerant is sub-cooled and its pressure is reduced to produce a low-pressure mixed refrigerant in each of said circuits, said low-pressure mixed refrigerant is used to cool the natural gas, and in the first circuit the first mixed refrigerant is sub-cooled by evaporation of the first low-pressure mixed refrigerant, and in the second circuit the second mixed refrigerant is also In addition, in a third circuit, a first low-pressure mixed refrigerant is cooled by evaporation and subcooled by evaporation of a second low-pressure mixed refrigerant, and in a third circuit, a third mixed refrigerant is cooled by evaporation of the second low-pressure mixed refrigerant and subcooled by evaporation of the third low-pressure mixed refrigerant, while according to the present invention, the final boiling point of the second mixed refrigerant at the pressure at which compression starts is lower than the temperature of the first mixed refrigerant after the pressure of the first mixed refrigerant is reduced, and the final boiling point of the third mixed refrigerant at the pressure at which compression starts is lower than the temperature of the second mixed refrigerant after the pressure of the second mixed refrigerant is reduced.

[0014] In addition, the first and second mixed refrigerants are fully condensed to avoid the formation of two-phase flow in the heat exchange space after the heat of compression is removed.

[0015] Additionally, the natural gas is compressed to a supercritical state to avoid phase transitions.

[0016] Additionally, the third mixed refrigerant is compressed to a supercritical state to avoid two-phase flow at the inlet of the heat exchanger.

[0017] Additionally, to reduce the energy consumption of the Hall process, the third mixed refrigerant is pre-cooled by evaporation of the first low-pressure mixed refrigerant after the heat of compression is removed from the third mixed refrigerant.

[0018] Additionally, the first mixed refrigerant may be a hydrocarbon mixture consisting primarily of ethane or ethylene, propane or propylene, and butane; the second mixed refrigerant may be a hydrocarbon mixture consisting primarily of methane, ethane, or ethylene, propane or propylene; and the third mixed refrigerant may be a mixture consisting of nitrogen, methane, and ethane or ethylene.

[0019] In addition, the first, second, and third multi-stream heat exchangers are mainly of the coil-wound or plate-fin type.

[0020] The technical result of the proposed method is that the final boiling point of the refrigerant falls below the temperature after expansion of the mixed refrigerant in the preceding circuit (pressure drop), thereby making it impossible to cool the natural gas in the preceding liquefaction circuit to the temperature corresponding to the presence of the mixed refrigerant in the two-phase region at any flow rate or other fluctuations in technological process parameters, thereby making the plant operation more stable. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 is a schematic diagram of a plant for carrying out the process without pre-cooling of the third mixed refrigerant. [Figure 2] FIG. 1 is a schematic diagram of a plant for carrying out the process with pre-cooling of the third mixed refrigerant. DETAILED DESCRIPTION OF THE INVENTION

[0022] The natural gas liquefaction plant of FIG. 1 includes a natural gas refrigeration line and circuits for first, second, and third mixed refrigerants.

[0023] The natural gas cooling lines successively connect the natural gas compressor 1, the air or water cooler 2, the natural gas cooling heat exchange spaces of the first, second and third stages of the first, second and third multi-stream heat exchangers 3, 4 and 5, respectively, the first pressure reducer 6 and the separator 7. The multi-stream heat exchangers 3, 4 and 5 are preferably of the coil-wound and plate-fin types.

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

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

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

[0027] The natural gas liquefaction plant of FIG. 2 differs from the schematic diagram of FIG. 1 in that the pre-cooling heat exchange space of the first multi-stream heat exchanger 3 is introduced into the third mixed refrigerant circuit after the 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 may be driven by, but are not limited to, a gas turbine or an electric motor, which may be connected to the compressors by a multiplier (not shown).

[0029] If the processing capacity is insufficient, the number of natural gas compressors 1 may be increased together by installing the associated compressors in parallel and an air or water cooler after each compressor.

[0030] If the processing capacity is insufficient, the number of first, second and third mixed refrigerant compressors 11, 21, 31 may be increased together by installing the associated compressors in parallel and an air or water cooler after each compressor.

[0031] The air-cooled or water-cooled condensers 13, 23 are heat exchangers that allow the normal phase transition process to occur without gas pockets, such as by using, but not limited to, inclined single-wound heat exchanger tubes.

[0032] The natural gas liquefaction process is carried out as follows.

[0033] Natural gas, which has been treated prior to liquefaction and is free of any water vapor, carbon dioxide, and other impurities, is fed to a natural gas compressor 1 and compressed to approximately 8-10 MPa. It is then cooled to approximately +15-20°C by ambient cold air in an air or water cooler 2 and sent to the heat exchange space of a first multi-stream heat exchanger 3. From the first cooling stage, gas at approximately -30--40°C is fed to the heat exchange space of a second multi-stream heat exchanger 4. From the second cooling stage, gas at approximately -60--70°C is fed to the heat exchange space of a second multi-stream heat exchanger 5, where it is cooled to approximately -155--159°C. The gas is then sent to a first pressure reducer 6, in this case a throttle valve, where it is reduced in pressure to 0.1 MPa, resulting in liquid and vapor streams, which are then sent to a separator 7 for separation into liquid and vapor phases. The liquid portion is liquefied natural gas.

[0034] The first mixed refrigerant is mostly a mixture of ethane or ethylene, propane or propylene, and butane, although these are not the only materials used. The first mixed refrigerant vapor from the evaporative heat exchange space of the first multi-stream heat exchanger 3 is fed to the first mixed refrigerant compressor 11, compressed to about 2.5 MPa, then cooled in an air or water cooler 12, and condensed in a condenser 13 at +15 to +20°C. The first mixed refrigerant liquid is sent to the heat exchange space of the first multi-stream heat exchanger 3, where it is subcooled to about -30 to -40°C. Once subcooled, the first mixed refrigerant is decompressed by second pressure reducer 14, in this case a throttle valve, resulting in a drop in temperature to about -32 to about -40°C, and then the first low-pressure mixed refrigerant evaporates in the evaporative heat exchange space of first multi-stream heat exchanger 3, resulting in cooling of the natural gas, subcooling of the first mixed refrigerant, and cooling of the second mixed refrigerant. The first mixed refrigerant vapor from multi-stream heat exchanger 3 is sent to compressor 11, compressed, cooled, condensed, and then used again along the circuit to cool the gas and mixed refrigerant.

[0035] The second mixed refrigerant is a hydrocarbon mixture of mostly methane, ethane, or ethylene, and propane or propylene, although these are not the only substances used. The second mixed refrigerant vapor is in the evaporative heat exchange space of the second multi-stream heat exchanger 4 at approximately 0.35-0.45 MPa. This means that its final boiling point is approximately -45 to -50°C, at least 5°C lower than the first mixed refrigerant temperature after expansion in the second pressure reducer 14, eliminating any condensation of evaporated refrigerant if the natural gas flow becomes abnormally irregular and the natural gas becomes colder. The second mixed refrigerant is then sent to the second mixed refrigerant compressor 22, where it is further compressed to approximately 3.5 to 4.5 MPa, cooled in an air or water cooler 21, and condensed in a condenser 23 at approximately +15 to +20°C. The second mixed refrigerant liquid is cooled to approximately -30 to -40°C by evaporation of the first low-pressure mixed refrigerant in multi-stream heat exchanger 3 and subcooled to approximately -62 to -70°C in multi-stream heat exchanger 4. Upon subcooling, the second mixed refrigerant is reduced in pressure by third pressure reducer 24, in this case a throttling valve, resulting in a temperature drop to approximately -62 to -73°C, after which the second low-pressure mixed refrigerant evaporates in the evaporative heat exchange space of second multi-stream heat exchanger 4, thereby cooling the natural gas, subcooling the second mixed refrigerant, and cooling the third mixed refrigerant, respectively. The second mixed refrigerant vapor from heat exchanger 4 is sent to compressor 21, compressed, cooled, condensed, and then used again along the circuit to cool the gas and mixed refrigerant.

[0036] The third mixed refrigerant is mostly a mixture of nitrogen, methane, and ethane or ethylene, although these are not the only substances used. The third mixed refrigerant vapor resides in the evaporative heat exchange space of the third multi-stream heat exchanger 5 at approximately 0.35 to 0.45 MPa. This means that its final boiling point is approximately -75 to -85°C, at least 5 degrees lower than the first mixed refrigerant temperature after expansion in the third pressure reducer 24, eliminating any condensation of the refrigerant vapor if the natural gas flow becomes abnormally irregular and the natural gas becomes colder. The third mixed refrigerant is then sent to the third mixed refrigerant compressor 31, where it is further compressed to approximately 8 to 9 MPa, and cooled in the air or water cooler 32. The third mixed refrigerant vapor is then sent successively to the second multi-stream heat exchanger 4, where it is cooled to approximately -60 to -70°C, and to the third multi-stream heat exchanger 5, where it is cooled to approximately -155 to -159°C. Once cooled, the third mixed refrigerant is reduced in pressure in fourth pressure reducer 34, in this case a throttling valve. Once reduced in pressure, the third mixed refrigerant is fed to the evaporative heat exchange space of third multi-stream heat exchanger 5, where the third low-pressure mixed refrigerant vaporizes, resulting in cooling of the natural gas and subcooling of the third mixed refrigerant. The third mixed refrigerant vapor from multi-stream heat exchanger 5 is sent to compressor 31, compressed, cooled, condensed, and then used again along the circuit to cool the gas and mixed refrigerant.

[0037] 2 is preferred when the ambient temperature is high, and after cooling in air or water cooler 32, the third mixed refrigerant vapor is first sent to the pre-cooling heat exchange space of first multi-stream heat exchanger 3 to be pre-cooled to about −30 to −40° C., then sent to second multi-stream heat exchanger 4 to be cooled to about −60 to −70° C., and then sent to third multi-stream heat exchanger 5 to be cooled to about −155 to −159° C. The third mixed refrigerant then follows the circuit shown in FIG.

[0038] In the proposed method, it is desirable to condense at least two of the mixed refrigerants in the air- or water-cooled condensers 13 and 23, where the second mixed refrigerant is subcooled by boiling the first mixed refrigerant, resulting in better energy efficiency because of the greater cooling effect and smaller heat exchanger size since, after expansion in an isenthalpic or isentropic process, there are no two phase streams on the tube side and in the lower vapor portion of the second mixed refrigerant stream. The proposed method also suggests that the cooling of the natural gas and the third mixed refrigerant occurs at a preferred pressure above the critical level, eliminating any phase transitions in the heat exchangers and thus allowing for reduced steel strength and improved plant reliability.

Claims

1. 1. A natural gas liquefaction method, wherein pre-processed natural gas is compressed, heat of compression is removed, the natural gas is cooled through three circuits containing mixed refrigerants, the pressure of the cooled gas is reduced to produce an evaporated-liquid mixture, and the liquefied gas is withdrawn, and in each of the circuits the mixed refrigerant is compressed, heat of compression is removed, the mixed refrigerant is sub-cooled and its pressure is reduced to produce a low-pressure mixed refrigerant in each circuit, and the low-pressure mixed refrigerant is used to cool the natural gas, while in a first circuit the first mixed refrigerant is sub-cooled by evaporation of the first low-pressure mixed refrigerant, and in a second circuit a second mixed refrigerant is also sub-cooled by evaporation of the first low-pressure mixed refrigerant. a first circuit in which a first mixed refrigerant is cooled by the evaporation of a first low-pressure mixed refrigerant and subcooled by the evaporation of a second low-pressure mixed refrigerant, and a third circuit in which a third mixed refrigerant is cooled by the evaporation of the second low-pressure mixed refrigerant and subcooled by the evaporation of a third low-pressure mixed refrigerant, characterized in that the final boiling point of the second mixed refrigerant at the pressure at which compression starts is lower than the temperature of the first mixed refrigerant after the pressure of the first mixed refrigerant is reduced, and the final boiling point of the third mixed refrigerant at the pressure at which compression starts is lower than the temperature of the second mixed refrigerant after the pressure of the second mixed refrigerant is reduced.

2. 10. The method of claim 1, wherein the first and second mixed refrigerants are fully condensed after the heat of compression is removed.

3. 10. The method of claim 1, wherein the natural gas is compressed to a supercritical state.

4. 10. The method of claim 1, wherein the third mixed refrigerant is compressed to a supercritical state.

5. 2. The method of claim 1, wherein after the heat of compression is removed from the third mixed refrigerant, the third mixed refrigerant is pre-cooled by evaporation of the first low pressure mixed refrigerant.

6. 2. The method of claim 1, wherein the first mixed refrigerant is a hydrocarbon mixture consisting of ethane or ethylene, propane or propylene, and butane; the second mixed refrigerant is a hydrocarbon mixture consisting of methane, ethane, or ethylene, propane or propylene; and the third mixed refrigerant is a mixture consisting of nitrogen, methane, and ethane or ethylene.

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