Arctic cascade modified process for natural gas liquefaction and plant for its implementation.

Abstract

The present invention relates to a technology for liquefying natural gas, which is subsequently transported by rivers and oceans and ultimately regasified. Pretreated natural gas is compressed by a compressor 1, the heat of compression is removed in an air or water cooler 2, and multi-stage pre-cooling is subsequently performed by evaporating a heavy refrigerant in an evaporator 3. The light refrigerant vapor and its pre-evaporation cold are recovered in a shell-and-tube heat exchanger 7, where the pressure of the gas to be liquefied is reduced by a pressure reducer 10, unliquefied gas is separated, and liquefied natural gas is discharged. The heavy refrigerant obtained by evaporation is compressed by a compressor 4, condensed in an air or water cooler 5, and reused in the multi-stage pre-cooling of natural gas. A light refrigerant is compressed by at least two compressors 11, the heat of compression is removed in an air or water cooler 12, and then passes through continuous cooling by a low-pressure light refrigerant in a double-flow superheater 12 and an evaporating heavy refrigerant in an evaporator 3. The high-pressure light refrigerant is then split into two streams. The first stream is expanded through an isentropic process in an expander 14. The second stream passes through continuous cooling in a shell-and-tube heat exchanger 7, then expanded through an isenthalpic process in a throttle valve 15, and evaporated by subcooling natural gas in an evaporator 8. The streams are then mixed at equal pressure and used as a single stream of low-pressure light refrigerant to cool the second stream of light refrigerant before expansion and evaporation in the isenthalpic process with natural gas in the shell-and-tube heat exchanger 7, and to cool a single stream of high-pressure light refrigerant in a double-flow superheater 13. The present invention provides stronger cooling of the natural gas, reduces the amount of boil-off gas and the work required for its recompression and recycling, and provides a simpler hardware design for the heat exchanger.

Description

[Technical Field]

[0001] The present invention relates to technology for the liquefaction of natural gas, which is subsequently transported by rivers and oceans and ultimately regasified. [Background technology]

[0002] A number of well-known methods for liquefying natural gas are known, many of which are based on the removal of heat using an external refrigerant.

[0003] A method for liquefying natural gas is known, as disclosed in EP-A-0 358 100, filed 30 / 08 / 1989, characterized in that boil-off gas generated in an LNG storage system is compressed to a predetermined pressure, followed by successive cooling and subcooling in heat exchangers in a nitrogen refrigerant circuit, in which the superheated nitrogen is compressed and the heat resulting from such compression is removed, after which the nitrogen is separated into two portions and fed separately to a heat exchanger in parallel with the gas to be liquefied to be cooled through heat exchange with a low-pressure nitrogen stream, one of the cooled streams being expanded in a Joule-Thomson valve in an isenthalpic process, and the other cooled stream being expanded in an expander in an isentropic process in which energy is recovered so that it can be used to drive a final compression stage, and then used to cool the first and second nitrogen streams and the natural gas stream to be liquefied. The low-pressure nitrogen stream superheated using waste heat recovery is mixed with the nitrogen downstream of the first compression stage and sent back for further compression, while the nitrogen stream expanded in an isenthalpic process is also used to cool the first and second nitrogen streams and the gas to be liquefied, and as a result is superheated and then fed to the first or third compression stage depending on the temperature to which the gas needs to be cooled, while the liquefied gas generated after decompression of the subcooled liquid is discharged from the heat exchanger and delivered to a storage system using a pump.

[0004] A drawback of the known method and the plants in which it is implemented is that its applicability is limited to boil-off gases with a high content of low-boiling components, especially nitrogen, and with low initial temperatures of the gas, around -94 to -130 °C, and its efficiency is limited when liquefying natural gas with an initial temperature close to ambient temperature. Furthermore, separating the nitrogen stream and subsequently removing the heat generated during compression and expansion to different pressure levels in isenthalpic and isentropic processes requires the use of multi-pass heat exchangers with multiple streams at both the hot and cold ends, which means that plate-fin heat exchangers must be used, making the technology incompatible with high production rates, including medium- and large-scale LNG production, and that process control is complex.

[0005] The Arctic Cascade process and its implementation plant, described in Russian Patent No. 2645185 by PAO NOVATEK and utilized as part of the fourth liquefaction line at the Yamal LNG facility in Sabetta, are considered the closest to the proposed system and are considered the prototype. The process involves pre-cooling pre-treated natural gas, separating ethane, sub-cooling the gas to be liquefied using cryogenic nitrogen as a refrigerant, depressurizing the gas to be liquefied, separating unliquefied gas, and discharging the liquefied natural gas. Prior to pre-cooling, natural gas is compressed, and ethane is separated in a multi-stage pre-cooling of the gas to be liquefied, with simultaneous ethane evaporation using cryogenic ethane as a refrigerant. The resulting boil-off ethane is compressed, condensed, and used as a refrigerant to cool the gas to be liquefied and the nitrogen, which is then compressed, cooled, expanded, and fed to the natural gas sub-cooling stage.

[0006] The liquefaction plant includes a natural gas liquefaction line, an ethane circuit, and a nitrogen circuit; the natural gas liquefaction line is a series connection of a natural gas compressor, a cooler, an ethane evaporator, a subcooling end heat exchanger, and a separator; the ethane circuit is a series connection of at least one ethane compressor, a cooler, and the ethane evaporator, the output of which is connected to the input of at least one compressor; and the nitrogen circuit is a series connection of a cooler, the ethane evaporator with a nitrogen-nitrogen heat exchanger therebetween, a turboexpander, the subcooling end heat exchanger, the nitrogen-nitrogen heat exchanger, and a turbine compressor connected to the nitrogen compressor input.

[0007] The method and plant are characterized by nitrogen cooling in a nitrogen-nitrogen heat exchanger, which makes the process configuration technically more complex and increases the number of parts of the equipment; furthermore, the temperature reduction in the nitrogen circuit depends only on the expansion in the turboexpander, which means that the natural gas is not cooled sufficiently before expansion, less LNG is produced and more boil-off occurs, resulting in a higher workload for the boil-off gas compressor and reduced energy efficiency. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] European Patent Application Publication No. 0358100 [Patent Document 2] Russian Patent Invention No. 2645185 Summary of the Invention [Problem to be solved by the invention]

[0009] The technical challenges solved by the proposed natural gas liquefaction technology include process simplification, expansion of applications, simplification of equipment, reduction of energy consumed to produce LNG, and increase of production capacity. [Means for solving the problem]

[0010] The technical problem is solved by a natural gas liquefaction method, the gist of which is that according to the proposed natural gas liquefaction method, pre-treated natural gas is compressed, the heat of compression is removed, the natural gas is multi-stage pre-cooled by evaporating a heavy refrigerant, and sub-cooled by recovering cold from the light refrigerant vapor and pre-evaporation, the natural gas to be liquefied is depressurized, the non-liquefied gas is separated, and the liquefied gas is removed, while the heavy refrigerant obtained by evaporation is compressed, condensed and reused in the multi-stage pre-cooling of the natural gas, and the light refrigerant is compressed, cooled and used to sub-cool the natural gas, wherein the light refrigerant according to the present invention is The refrigerant is compressed before cooling and the heat of compression is removed, followed by continuous cooling of the natural gas with a low-pressure light refrigerant and an evaporating heavy refrigerant, followed by separation of the high-pressure light refrigerant into two streams, isentropic expansion of the first stream and continuous cooling of the second stream, isenthalpic expansion, and evaporation through subcooling of the natural gas, followed by mixing the streams together at equal pressure and using the single stream of low-pressure light refrigerant to cool the second stream of natural gas and light refrigerant before isenthalpic expansion and evaporation, and to cool the single stream of high-pressure light refrigerant.

[0011] Furthermore, it is preferred that the gas be pre-cooled in multiple stages at different levels of boiling pressure of the heavy refrigerant, provided by reducing the pressure of the heavy refrigerant towards boiling before each stage, with at least the final stage being used to cool the high pressure light refrigerant.

[0012] Additionally, it is preferred that the energy released from the isentropic expansion of the first stream be used to compress the light refrigerant.

[0013] Additionally, nitrogen may be used as the light refrigerant and ethane or ethylene may be used as the heavy refrigerant.

[0014] Furthermore, the pressure of the gas to be liquefied after subcooling by recovering refrigeration from the vapor of a light refrigerant can be further reduced, depending on its composition, to a pressure that precludes the formation of two-phase flow for a given gas composition.

[0015] The technical problem is also solved by a plant for liquefying natural gas, comprising a natural gas pre-cooling line, a natural gas sub-cooling circuit, a heavy refrigerant circuit, and a light refrigerant circuit, wherein the natural gas pre-cooling line comprises a series of at least one natural gas compressor, at least one air or water cooler, and a heavy refrigerant evaporator inter-tube space, the sub-cooling circuit comprises a shell-and-tube heat exchanger and a first pressure reducer, the heavy refrigerant circuit comprises a series of at least one heavy refrigerant compressor, at least one air or water cooler, and a heavy refrigerant evaporator inter-tube space, the output of the inter-tube space being connected to the heavy refrigerant compressor, and the light refrigerant circuit comprises at least two light refrigerant compressors, at least one air or water cooler arranged downstream of each light refrigerant compressor, a double-flow light refrigerant superheater, a heavy refrigerant evaporator inter-tube space, a first shell-and-tube heat exchanger, and a series of expanders, and according to the invention the natural gas sub-cooling line comprises a series of at least one shell-and-tube heat exchanger and a second shell-and-tube heat exchanger. the output of the second tube space is connected to the first heat exchange space of the light refrigerant evaporator, and the output of the first heat exchange space is connected to the first pressure reducer; in the light refrigerant circuit, the last air cooler or water cooler is connected in series to the first heat exchange space of the double-flow superheater and the second tube space of the heavy refrigerant evaporator; the light refrigerant output of the second tube space of the heavy refrigerant evaporator is connected to two light refrigerant lines, and the first line includes the expansion device; The second line includes a series of a first tube space of the shell-tube heat exchanger, a second pressure reducer, and a second heat exchange space of the light refrigerant evaporator, the gas output of the second heat exchange space together with the output of the expander is connected to the inter-tube space of the shell-tube heat exchanger, the output of the inter-tube space of the shell-tube heat exchanger is connected to the second heat exchange space of the double-flow superheater, and the second heat exchange space of the double-flow superheater is connected to the first light refrigerant compressor in the flow direction.

[0016] Furthermore, the subcooling line may further include a third pressure reducer, the input of which is connected to the output of the first tube space of the shell-and-tube heat exchanger, and the output of which is connected to the input of the first heat exchange space of the light refrigerant evaporator.

[0017] Preferably, the expander is kinematically coupled to at least one stage of the light refrigerant compressor. [Effects of the Invention]

[0018] The technical result achieved by using the proposed method and plant is a stronger cooling of natural gas in a heat exchanger with a simpler hardware design, while reducing the amount of boil-off gas and the work required for its recompression and recycling.

[0019] Compared to the "Arctic Cascade" (prototype) process, the nitrogen cooling circuit of the proposed "Arctic Cascade Modified" ("Arctic Cascade M") process envisages boiling of the light refrigerant, enabling stronger cooling of the natural gas, reducing the amount of boil-off gas, and eliminating the need to cool the light refrigerant in nitrogen-nitrogen heat exchangers at each stage of multi-stage pre-cooling. Notably, this allows for a reduction in the number and size of process units, resulting in a simpler hardware design and smaller footprint for the process, as nitrogen cooling occurs only in the final pre-cooling stage, and reduces heat losses through hydraulic resistance and the recovery of cold from the evaporated light refrigerant.

[0020] Compared to the technical solution of EP 0 358 100 A, the proposed method and plant envisage further cooling of natural gas and high-pressure light refrigerant by evaporating the heavy refrigerant, making it possible to reduce energy consumption and use the invention to liquefy gases with high input temperatures. Furthermore, the light refrigerant stream is separated in the circuit after cooling with the superheated vapor and heavy refrigerant, making it possible to use a shell-and-tube double-flow heat exchanger. Another difference of the proposed technical solution is that the pressure of the second stream after evaporation is equal to the pressure of the first stream after isentropic expansion, making it possible to mix them and use them as a single stream in the shell-and-tube heat exchanger. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 shows a proposed plant layout. DETAILED DESCRIPTION OF THE INVENTION

[0022] The natural gas liquefaction plant includes a natural gas pre-cooling line, a natural gas sub-cooling circuit, a heavy refrigerant circuit, and a light refrigerant circuit.

[0023] The natural gas pre-cooling line includes the tube space of a natural gas compressor 1, an air or water cooler 2, and a heavy refrigerant (in this case, the heavy refrigerant is ethane) evaporator 3 connected in series.

[0024] The natural gas subcooling circuit includes, connected in series, the second tube side of the shell-and-tube heat exchanger 7, a third pressure reducer 9, which is a throttle valve, a first heat exchange space (including the tube space) of the light refrigerant (in this case, the light refrigerant is nitrogen) evaporator 8, and a second pressure reducer 10, which is also a throttle valve.

[0025] The heavy refrigerant circuit includes a heavy refrigerant compressor 4, at least one air or water cooler, and a tube-to-tube space of a heavy refrigerant evaporator 3 connected in series. A throttle valve 6 is installed at the input of each evaporator 3. The gas output of the tube-to-tube space of the evaporator 3 is connected to the compressor 4. The heavy refrigerant circuit may include two or more compressors connected in series, depending on the compressor power rating and refrigerant compression requirements.

[0026] The light refrigerant circuit includes at least two light refrigerant compressors 11, at least one air or water cooler 12 after each compressor 11, a first heat exchange space of a double-flow superheater 13, and a second tube space of a light refrigerant evaporator 3. The output of the second tube space is connected to two light refrigerant lines. The first line includes an expander 14. The second light refrigerant line includes a first tube space of a shell-and-tube heat exchanger 7, a throttle valve 15, and a second heat exchange space (including the inter-tube space) of the light refrigerant evaporator 8. The gas output of the second heat exchange space is connected to the cold side input of the inter-tube space of the shell-and-tube heat exchanger 7 together with the output of the first light refrigerant expander 14. The output of the low-temperature side of the intertube space of the shell-and-tube heat exchanger 7 is connected to the second heat exchange space (including the intertube space) of the double-flow superheater 13, and the output of the second heat exchange space is connected to the input of the first light refrigerant compressor 11 in the flow direction.

[0027] The natural gas compressor 1, the heavy refrigerant compressor 4, and the light refrigerant compressor 11 may be driven by, but are not limited to, a gas turbine or an electric motor, which may be connected to the compressors through a multiplier (not shown).

[0028] One of the light refrigerant compressors 11 can also be driven by a portion of the power generated in the expander 14 by establishing that the expander 14 is kinematically coupled to the shaft of the compressor 11.

[0029] The natural gas liquefaction process is carried out as follows.

[0030] Natural gas that has been treated before liquefaction and is free from water vapor, carbon dioxide, and other impurities is fed to a natural gas compressor 1 and compressed to approximately 10 MPa. It is then cooled to approximately +15°C by ambient cold air in an air or water cooler 2 and sent to a heavy refrigerant evaporator 3 for multi-stage pre-cooling. After a series of cooling processes in the evaporator 3, the gas to be liquefied at a temperature of approximately -84°C is sent to a second heat exchange space, where it is cooled to a temperature level of approximately -144.5°C, and further sent for expansion in a third pressure reducer 9, where the pressure is reduced to approximately 2 MPa, thereby eliminating the occurrence of two-phase flow for this gas composition. The gas is then subcooled to a temperature of -152.3°C in an evaporator 8 and sent to a first pressure reducer 10, where the pressure is reduced to 0.1 MPa, producing a gas-liquid mixture, the liquid portion of which becomes liquefied natural gas.

[0031] Although not limited thereto, ethane is used as the heavy refrigerant. Ethane gas at different pressures from the evaporator 3 is fed to a heavy refrigerant multi-stage compressor 4, where the pressure is increased to approximately 3 MPa, and condensed in an air or water cooler 5 at a temperature of +10°C. The liquid ethane is then sent to the evaporator 3, where the natural gas is cooled to a temperature of approximately -84°C by heavy refrigerants at different pressure levels, and the heavy refrigerant also cools the light refrigerant stream in addition to the natural gas, at least in the last evaporator 3. The different pressure levels in the evaporators 3 are achieved by reducing the pressure in a throttle valve 6. The ethane gas from the evaporator 3 is then directed to the heavy refrigerant compressor 4, compressed, condensed, and used again in the multi-stage pre-cooling of the natural gas along the circuit.

[0032] Although not limited to this application, nitrogen is used as the light refrigerant. The low-pressure light refrigerant gas from the double-flow superheater 13 is fed to at least one light refrigerant compressor 11 and at least one air or water cooler 12, where the pressure is increased to 10.3 MPa and the temperature is reduced to +15°C. The resulting high-pressure nitrogen gas is then directed to the double-flow superheater 13, where it is cooled to approximately -65°C by refrigeration recovery from the low-pressure nitrogen, and then split into two streams. The first stream is then directed down a first line to an expander 14, where isentropic expansion with energy removal occurs, reducing the pressure to 2.1 MPa and the temperature to -147.5°C. The second stream is directed down a second line to shell-and-tube heat exchanger 7, where it is cooled to approximately -139°C by recovering refrigeration from the combined light refrigerant stream, and expanded in a second pressure reducer 15 in an isenthalpic process to 2.1 MPa to produce a vapor-liquid stream. The low-pressure light refrigerant vapor-liquid stream is directed to evaporator 8, where it is evaporated by subcooling the natural gas stream, and then mixed with the first low-pressure light refrigerant stream, and directed to shell-and-tube heat exchanger 7 in a combined stream, where it is heated to cool the high-pressure light refrigerant stream and the natural gas stream. The combined nitrogen stream from heat exchanger 7 is then directed to double-flow superheater 13, where it is superheated by cooling the high-pressure nitrogen after air or water cooler 12.

[0033] The energy released during the isentropic expansion in the expander 14 can be used to provide energy to drive at least one compressor 11 .

[0034] The process can operate in a nominal mode at ambient temperatures below +5° C. At temperatures above +5° C., the productivity of the production line begins to decrease. Since the process is developed for Arctic and Antarctic regions, in warm seasons, the heat exchange for condensing heavy refrigerants (especially ethane) can use water from Arctic or Antarctic oceans, bays, and other reservoirs, which have low temperatures even in summer.

Claims

1. The pre-treated natural gas is compressed, The heat of compression is removed, the natural gas is multi-stage pre-cooled by evaporating a heavy refrigerant and sub-cooled by recovering cold from the light refrigerant vapor and pre-evaporation; The natural gas to be liquefied is depressurized; The gas that did not liquefy is separated, The liquefied gas is removed, on the other hand, The heavy refrigerant obtained by evaporation is compressed, condensed and reused in the multi-stage pre-cooling of the natural gas; the light refrigerant is compressed, cooled, and used to subcool the natural gas; 1. A method for liquefying natural gas, comprising: The light refrigerant is compressed before being cooled; The heat of compression is removed, Subsequently, continuous cooling of the light refrigerant by the low pressure light refrigerant and the evaporating heavy refrigerant is performed; Subsequently, separation of the high pressure light refrigerant into two streams is carried out; an isentropic expansion of the first stream; Sequential cooling of the second stream, isenthalpic expansion, and evaporation through subcooling of the natural gas; But it was done, The streams are then mixed together at equal pressures and used as the single stream of low-pressure light refrigerant to cool the natural gas and the second stream of light refrigerant before isenthalpic expansion and evaporation, and to cool the single stream of high-pressure light refrigerant. A method characterized by:

2. the multi-stage pre-cooling of the natural gas is performed at different levels of boiling pressure of the heavy refrigerant, provided in each stage by reducing the pressure of the heavy refrigerant towards boiling before each stage; At least the final stage is used to cool the high-pressure light refrigerant.

2. The method of claim 1.

3. Energy released from the isentropic expansion of the first stream is used to compress the light refrigerant.

2. The method of claim 1.

4. Nitrogen is used as the light refrigerant; Ethane or ethylene is used as the heavy refrigerant 2. The method of claim 1.

5. The pressure of the natural gas to be liquefied, after subcooling by recovering cold from the vapor of the light refrigerant, is further reduced to a pressure that precludes the formation of two-phase flow.

2. The method of claim 1.

6. 1. A plant for liquefying natural gas, comprising a natural gas pre-cooling line, a natural gas sub-cooling circuit, a heavy refrigerant circuit, and a light refrigerant circuit, the natural gas pre-cooling line includes a series of at least one natural gas compressor, at least one air or water cooler, and a heavy refrigerant evaporator inter-tube space; the subcooling circuit includes a shell-and-tube heat exchanger and a first pressure reducer; the heavy refrigerant circuit includes at least one heavy refrigerant compressor, at least one air or water cooler, and an inter-tube space of the heavy refrigerant evaporator; an output of the inter-tube space connected to the heavy refrigerant compressor; the light refrigerant circuit includes a series of at least two light refrigerant compressors, at least one air cooler or water cooler disposed downstream of each light refrigerant compressor, a double-flow light refrigerant superheater, a tube space of the heavy refrigerant evaporator, a first tube space of the shell-and-tube heat exchanger, and an expander; In the plant, the natural gas subcooling line includes a second tube space of the shell-and-tube heat exchanger; The output of the second tube space is connected to a first heat exchange space of a light refrigerant evaporator; an output of the first heat exchange space is connected to the first pressure reducer; In the light refrigerant circuit, a final air cooler or water cooler is connected in series with the first heat exchange space of the double-flow superheater and the second tube space of the heavy refrigerant evaporator; a light refrigerant output of the second tube space of the heavy refrigerant evaporator connected to two light refrigerant lines; a first line including the expander; a second line including a series of the first tube space of the shell-and-tube heat exchanger, a second pressure reducer, and a second heat exchange space of the light refrigerant evaporator; a gas output of the second heat exchange space, together with an output of the expander, connected to an inter-tube space of the shell-and-tube heat exchanger; an output of the intertube space of the shell-and-tube heat exchanger is connected to a second heat exchange space of the double-flow superheater; The second heat exchange space of the double-flow superheater is connected to the first light refrigerant compressor in the flow direction. A plant characterized by:

7. the subcooling line includes a third pressure reducer; an input of the third pressure reducer is connected to an output of the first tube space of the shell-and-tube heat exchanger; The output of the third pressure reducer is connected to the input of the first heat exchange space of the light refrigerant evaporator.

7. Plant according to claim 6, characterized in that

8. the expander is kinematically coupled to at least one stage of the light refrigerant compressor; 7. Plant according to claim 6, characterized in that

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