Methods 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 The pressure of the cooled gas is reduced to produce a vapor-liquid mixture, and the liquefied gas is withdrawn.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 technology for liquefying natural gas that is subsequently transported by a river or sea.

Background Art

[0002] There are a plurality of well-known natural gas liquefaction methods and related plants, most of which are based on removing heat using an external refrigerant.

[0003] There is a well-known natural gas liquefaction technology (European Patent No. 3299757 published on June 19, 2019), which cools pre-treated natural gas in several stages using a mixed refrigerant stream in a coil-wound heat exchanger, expands this to generate LNG. The mixed refrigerant is compressed, partially condensed using an air-cooled heat exchanger, fed into a separator where it is separated into a liquid phase and a vapor phase, and then the individual mixed refrigerant streams are fed into the coil-wound heat exchanger. The mixed refrigerant is subcooled in the coil-wound heat exchanger by the boiling of 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 a liquid phase and a vapor phase in the separator. The mixed refrigerant liquid is subcooled, and the mixed refrigerant vapor is condensed in the heat exchanger by the boiling of the low-pressure mixed refrigerant.

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

[0005] Furthermore, there are several inventions, such as Russian Patent No. 2706892, that address startup problems related to heat exchangers in the liquefaction process of mixed refrigerants. In this process, during the initial startup, the condensation of the mixed refrigerant stream in the heat exchanger is incomplete, resulting in slow plant startup and rise time, as well as equipment vibrations that impair the reliability of the equipment.

[0006] A method for liquefying natural gas is publicly known (German Patent No. 19716415, issued October 22, 1998), which involves cooling and expanding a mixed refrigerant stream in several stages using a cascade heat exchange cycle based on a mixture of three refrigerants 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 the boiling of the first refrigerant. In the second cycle, the natural gas is liquefied, and the third refrigerant is condensed by the boiling of the second refrigerant. In the third cycle, the natural gas is supercooled by the boiling of the third refrigerant. The first refrigerant is compressed, cooled, and condensed in the first cycle and expanded in a Jul-Thomson valve in an isenthalpy process. The second refrigerant is compressed and condensed in the first cycle, supercooled in the second refrigeration cycle, and expanded in the Jule-Thomson valve in the isenthalpy process. The third refrigerant is partially condensed in the first refrigeration cycle, condensed in the second refrigeration cycle, supercooled in the third refrigeration cycle, and expanded in the Jule-Thomson valve in the isenthalpy process.

[0007] The drawbacks of this method are that the second refrigerant condenses 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 within the heat exchanger, causing vibrations, which in turn reduces plant reliability and complicates process control.

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

[0009] A plant in which this method is implemented and which can be considered as a prototype (the description is available under the same criteria) is equipped with a natural gas cooling train, the natural gas cooling train comprising 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 multistream heat exchangers, a first pressure reducer, a separator, and first, second, and third mixed refrigerant circuits, in which case the first mixed refrigerant circuit continuously connects the first mixed refrigerant compressor, a second air or water cooler, a first collection tank, the subcooled heat exchange space of the first multistream heat exchanger, the second pressure reducer (pressure reducing valve), and the evaporative heat exchange space of the first multistream heat exchanger, on the other hand, The second mixed refrigerant circuit continuously connects the second mixed refrigerant compressor, the third air-cooled heat exchanger, the second collection tank, the cooling heat exchange space of the first multistream heat exchanger, the subcooled heat exchange space of the second multistream heat exchanger, the third pressure reducer (pressure reducing valve), and the evaporative heat exchange space of the second multistream heat exchanger. The third mixed refrigerant circuit continuously connects the two compressors of the third mixed refrigerant, the cooling heat exchange space of the second multistream heat exchanger, the third collection tank, the subcooled heat exchange space of the third multistream heat exchanger, the fourth pressure reducer, and the evaporative heat exchange space of the third multistream heat exchanger, with air coolers, namely the fourth and fifth, following each of them.

[0010] When this prototype method is implemented and the natural gas liquefaction plant is operated, the natural gas flow rate and the mixed refrigerant flow rate may become abnormally irregular, potentially resulting in an abnormally narrow temperature difference between the front and rear ends of the heat exchanger, which could cause the natural gas stream to become more cooled. If the process configuration is not precise, this can lead to reverse heat conduction due to the temperature overlap between the natural gas stream and the mixed refrigerant stream. As a result, partial condensation of the evaporated refrigerant may occur due to the natural gas stream. If the condensed mixed refrigerant portion terminates in the compressor, the compressor may fail, potentially causing an accident in the plant. If a separator is present upstream of the compressor, the condensed portion will be separated within it, altering the refrigerant composition before being discharged from the plant, reducing energy efficiency and requiring treatment of the refrigerant condensate (e.g., its fractionation in a dedicated unit) and adjustment of its composition to achieve the most suitable specifications.

[0011] The technical problem to be solved by the present invention group is to reduce the risk of accidents caused by the blow-out of condensed portions of a refrigerant mixture into the compressor. [Prior art documents] [Patent Documents]

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

[0013] The technical problem is a natural gas liquefaction method in which pre-treated natural gas is compressed, the heat of compression is removed, the natural gas is cooled by three circuits containing a refrigerant mixture, the pressure of the cooled gas is reduced to produce an evaporation-liquid mixture, the liquefied gas is drawn out, the refrigerant mixture is compressed in each circuit, the heat of compression is removed, the refrigerant is supercooled, its pressure is reduced to produce a low-pressure refrigerant mixture in each circuit, the low-pressure refrigerant mixture is used to cool the natural gas, and in the first circuit, the first refrigerant mixture is supercooled by the evaporation of the first low-pressure refrigerant mixture, and in the second circuit, the second refrigerant mixture is also supercooled. Furthermore, the first low-pressure mixed refrigerant is cooled by evaporation and supercooled by evaporation of the second low-pressure mixed refrigerant, and in the third circuit, the third mixed refrigerant is cooled by evaporation and supercooled by evaporation of the second low-pressure mixed refrigerant, while in the present invention, the final boiling point of the second mixed refrigerant at the pressure at which its compression begins is lower than the temperature of the first mixed refrigerant after the pressure of the first mixed refrigerant has been reduced, and the final boiling point of the third mixed refrigerant at the pressure at which its compression begins is lower than the temperature of the second mixed refrigerant after the pressure of the second mixed refrigerant has been reduced, which is resolved by a natural gas liquefaction method.

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

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

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

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

[0018] In addition, the first mixed refrigerant may be a hydrocarbon mixture mainly composed of ethane or ethylene, propane or propylene, and butane, the second mixed refrigerant may be a hydrocarbon mixture mainly composed of methane, ethane, or ethylene, propane or propylene, and the third mixed refrigerant may be a mixture composed 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 type or the plate-fin type.

[0020] The technical result of the proposed method is that the final boiling point of the refrigerant is lower than the temperature after the 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 in any flow rate or other variation of the technical process parameters, which leads to making the operation of the plant more stable.

Brief Description of the Drawings

[0021] [Figure 1] It is a schematic diagram of a plant for implementing the method without pre-cooling of the third mixed refrigerant. [Figure 2] It is a schematic diagram of a plant for implementing the method with pre-cooling of the third mixed refrigerant.

Modes for Carrying Out the Invention

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

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

[0024] The first mixed refrigerant circuit continuously 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, the 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, the 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 in Figure 2 is different from the schematic diagram in Figure 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] The natural gas compressor 1, the first mixed refrigerant compressor 11, the second mixed refrigerant compressor 21, and the third mixed refrigerant compressor 31 may be connected to the compressor by a multiplier (not shown in the figure), and may be driven by a gas turbine or an electric motor, etc.

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

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

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

[0032] The natural gas liquefaction method is carried out as follows:

[0033] Natural gas, treated before liquefaction and free of any water vapor, carbon dioxide, and other impurities, is supplied to a natural gas compressor 1 where it is compressed to approximately 8-10 MPa, then cooled to approximately +15-20°C by ambient coolant 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 supplied 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 supplied 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 to 0.1 MPa, resulting in liquid and vapor streams, which are then sent to a separator 7 to separate the 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, but these are not the only substances used. The first mixed refrigerant vapor from the evaporative heat exchange space of the first multistream heat exchanger 3 is supplied to the first mixed refrigerant compressor 11, where it is 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 multistream heat exchanger 3, where it is supercooled to about -30 to -40°C. When supercooled, the first refrigerant mixture is depressurized by the second pressure reducer 14, in this case a throttle valve, resulting in a temperature drop to approximately -32 to approximately -40°C. Subsequently, the first low-pressure refrigerant mixture evaporates within the evaporative heat exchange space of the first multi-stream heat exchanger 3, resulting in the cooling of the natural gas, the supercooling of the first refrigerant mixture, and the cooling of the second refrigerant mixture. The first refrigerant mixture vapor from the multi-stream heat exchanger 3 is sent to the compressor 11, where it is compressed, cooled, and condensed, and then reused along the circuit to cool the gas and refrigerant mixture.

[0035] The second mixed refrigerant is mostly a hydrocarbon mixture of methane, ethane or ethylene, and propane or propylene, but these are not the only substances used. The second mixed refrigerant vapor is in the evaporative heat exchange space of the second multistream heat exchanger 4 at approximately 0.35–0.45 MPa, which means its final boiling point is at least 5°C lower than the first mixed refrigerant temperature after expansion in the second pressure reducer 14, at approximately -45 to -50°C, thus eliminating any condensation of the evaporated refrigerant in the event that the natural gas flow rate becomes unusually irregular and the natural gas is cooled more. The second mixed refrigerant is then sent to the second mixed refrigerant compressor 22, where it is further pressurized to approximately 3.5–4.5 MPa, cooled in an air or water cooler 21, and condensed in a condenser 23 at approximately +15–20°C. The second mixed refrigerant liquid is cooled to approximately -30 to -40°C by the evaporation of the first low-pressure mixed refrigerant in the multi-stream heat exchanger 3, and then supercooled to approximately -62 to -70°C in the multi-stream heat exchanger 4. Upon supercooling, the second mixed refrigerant is depressurized by the third pressure reducer 24, in this case a throttle valve, resulting in a temperature drop to approximately -62 to -73°C. Subsequently, the second low-pressure mixed refrigerant evaporates in the evaporative heat exchange space of the second multi-stream heat exchanger 4, resulting in the cooling of the natural gas, the supercooling of the second mixed refrigerant, and the cooling of the third mixed refrigerant, respectively. The second mixed refrigerant vapor from the heat exchanger 4 is sent to the compressor 21, where it is compressed, cooled, condensed, and then reused 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, but these are not the only substances used. The third mixed refrigerant vapor is in the evaporative heat exchange space of the third multistream heat exchanger 5 at approximately 0.35–0.45 MPa, which means its final boiling point is at least 5 degrees lower than the expanded first mixed refrigerant temperature in the third pressure reducer 24, at approximately -75 to -85°C, thus eliminating any condensation of refrigerant evaporation in the event that the natural gas flow rate becomes unusually irregular and the natural gas is cooled more. The third mixed refrigerant is then sent to the third mixed refrigerant compressor 31 where it is further pressurized to approximately 8–9 MPa and cooled in an air or water cooler 32. The third mixed refrigerant vapor is continuously sent to the second multistream heat exchanger 4 where it is cooled to approximately -60 to -70°C, and then to the third multistream heat exchanger 5 where it is cooled to approximately -155 to -159°C. Once cooled, the third refrigerant mixture is depressurized in the fourth pressure reducer 34, in this case a throttle valve. Upon depressurization, the third refrigerant mixture is supplied to the evaporative heat exchange space of the third multistream heat exchanger 5, where the third low-pressure refrigerant mixture evaporates, resulting in the cooling of the natural gas and the subcooling of the third refrigerant mixture. The vapor of the third refrigerant mixture from the multistream heat exchanger 5 is sent to the compressor 31, where it is compressed, cooled, and condensed, and then reused along the circuit to cool the gas and the refrigerant mixture.

[0037] When the ambient temperature is high, the configuration shown in Figure 2 is preferable. After cooling in the air or water cooler 32, the third mixed refrigerant vapor is first sent to the pre-cooled heat exchange space of the first multi-stream heat exchanger 3 to be pre-cooled to approximately -30 to -40°C, then sent to the second multi-stream heat exchanger 4 to be cooled to approximately -60 to -70°C, and then sent to the third multi-stream heat exchanger 5 to be cooled to approximately -155 to -159°C. The third mixed refrigerant then follows the circuit as shown in Figure 1.

[0038] In the proposed method, it is desirable to condense at least two of the mixed refrigerants in air-cooled or water-cooled condensers 13 and 23, in which case the second mixed refrigerant is supercooled by the boiling of the first mixed refrigerant, resulting in better energy efficiency because the cooling effect is greater, and also because, after expansion in an isenthalpic or isentropic process, there are no two phase streams on the tube side and in the lower part of the vapor in the second mixed refrigerant stream, thus allowing for a smaller heat exchanger size. 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 exchanger and thus enabling a reduction in steel strength and an improvement in plant reliability.

Claims

1. A natural gas liquefaction method comprising: compressing natural gas pretreated by the method, removing the heat of compression; cooling the natural gas through three circuits containing a refrigerant mixture; reducing the pressure of the cooled gas to produce an evaporation-liquid mixture; and drawing out the liquefied gas, compressing the refrigerant mixture in each circuit, removing the heat of compression; supercooling the refrigerant mixture, reducing its pressure to produce a low-pressure refrigerant mixture in each circuit; and using the low-pressure refrigerant mixture to cool the natural gas, wherein in the first circuit, the first refrigerant mixture is supercooled by the evaporation of the first low-pressure refrigerant mixture; and in the second circuit, the second refrigerant mixture is also supercooled by the evaporation of the first A natural gas liquefaction method comprising: a first low-pressure mixed refrigerant being cooled by the evaporation of a second low-pressure mixed refrigerant and supercooled by the evaporation of a second low-pressure mixed refrigerant; and in a third circuit, a third mixed refrigerant being cooled by the evaporation of the second low-pressure mixed refrigerant and supercooled by the evaporation of the third low-pressure mixed refrigerant, characterized in that the final boiling point of the second mixed refrigerant at the pressure at which its compression begins is lower than the temperature of the first mixed refrigerant after its pressure has been reduced, and the final boiling point of the third mixed refrigerant at the pressure at which its compression begins is lower than the temperature of the second mixed refrigerant after its pressure has been reduced.

2. The method according to claim 1, characterized in that the first and second mixed refrigerants are completely condensed after the heat of compression is removed.

3. The method according to claim 1, characterized in that the natural gas is compressed to a supercritical state.

4. The method according to claim 1, characterized in that the third mixed refrigerant is compressed to a supercritical state.

5. The method according to claim 1, characterized in that, after the heat of compression is removed from the third mixed refrigerant, the third mixed refrigerant is pre-cooled by the evaporation of the first low-pressure mixed refrigerant.

6. The method according to claim 1, characterized in that 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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