Method for liquefying natural gas and device for implementing same

Abstract

The present invention relates to a natural gas liquefaction technique which subsequently transports and ultimately regasifies natural gas. Natural gas prepared for liquefaction is compressed, cooled in an air cooler or a water cooler, then subjected to multi-stage precooling by evaporating a heavy refrigerant in an evaporator, and then supercooled by recovering the cooling capacity of light refrigerant vapor through a shell-and-tube heat exchanger. In the evaporator, the pressure of the gas to be liquefied is reduced and the liquefied natural gas is extracted. And the heavy refrigerant obtained in the evaporation process is compressed and condensed and is reused for cooling the natural gas. The light refrigerant is compressed and the heat of compression is removed in either an air cooler or a water cooler, then cooled in sequence in a double flow superheater by a low pressure light refrigerant and in an evaporator by a heavy refrigerant being evaporated. The light refrigerant is divided into two streams. The first flow is expanded by an expander. The second stream is sequentially cooled in the heat exchanger, expanded in the throttle valve, and evaporated in the evaporator. The streams are then mixed under isobaric pressure. The present invention provides deeper cooling of natural gas, thereby reducing the amount of boil-off gas and the workload required for re-compression and recovery.

Description

Technical Field

[0001] The present invention involves the technology of liquefying natural gas, followed by its transportation by river and sea and its eventual regasification. Background Art

[0002] There are several well-known methods for liquefying natural gas, most of which are based on removing heat using an external refrigerant.

[0003] EP0358100A2 (August 30, 1989) discloses a natural gas liquefaction method characterized by compressing boil-off gas generated in a liquefied natural gas storage system to a predetermined pressure and then continuously cooling and sub-cooling it in a heat exchanger within a nitrogen refrigerant circuit, wherein the superheated nitrogen is compressed and removes the heat generated by the compression. The nitrogen is then divided into two parts, each of which is sent to a heat exchanger connected in parallel with the gas to be liquefied for cooling. Through heat exchange with a low-pressure nitrogen stream, one of the after-cooling streams is expanded in an isenthalpic process at a Joule-Thomson valve, and the other after-cooling stream is expanded in an isentropic process in an expander, wherein energy is recovered and can be used to drive the final compression stage, and then used to cool the first and second nitrogen streams and the natural gas stream to be liquefied. Waste heat recovery technology is used to superheat the low-pressure nitrogen flow, mix it with the nitrogen downstream of the first compression stage, and send it back to the subsequent compression. At the same time, the nitrogen flow expanded in the isenthalpic process is also used to cool the first and second nitrogen flows and the gas to be liquefied, making them superheated, and then supplied to the first or third compression stage according to the temperature required. The liquefied gas produced after the supercooled liquid is decompressed is extracted from the heat exchanger and transported to the storage system by a pump.

[0004] Disadvantages of the known method and its implementation apparatus are that its applicability is limited to boil-off gases with a high content of low-boiling-point components (particularly nitrogen) and with relatively low initial gas temperatures of approximately -94 to -130°C. Furthermore, its efficiency is limited when liquefying natural gas whose initial temperature is close to ambient temperature. Furthermore, the separation of the nitrogen stream after removing the heat generated during compression and its expansion to different pressure levels during isenthalpic and isentropic processes require the use of multi-pass heat exchangers with multiple streams at both the hot and cold ends, necessitating the use of plate-fin heat exchangers. Furthermore, this technology is not suitable for high-production rates, including those for medium- and large-scale liquefied natural gas production, and process control is also relatively complex.

[0005] The Arctic Cascade method and its implementation apparatus, which is closest to the proposed scheme and considered a prototype, are embodied in PAO NOVATEK's patent RU2645185C1, which is part of the fourth liquefaction train at the Yamal LNG facility in Sabetta. The method involves precooling the processed natural gas, separating ethane, subcooling the gas to be liquefied using cold nitrogen as a refrigerant, depressurizing the gas to be liquefied, separating unliquefied gas, and evacuating the liquefied natural gas. Prior to precooling, the natural gas is compressed, and ethane is separated in a multi-stage precooling process of the gas to be liquefied. Simultaneously, the ethane is evaporated using cold ethane as a refrigerant. The evaporated ethane produced by this evaporation is compressed, condensed, and used as a refrigerant to cool the gas to be liquefied and nitrogen, which is compressed, cooled, expanded, and supplied to the natural gas subcooling stage.

[0006] The liquefaction device includes a natural gas liquefaction pipeline, an ethane circuit, and a nitrogen circuit. The natural gas liquefaction pipeline is formed by continuously connecting a natural gas compressor, a cooler, an ethane evaporator, a subcooling end heat exchanger, and a separator. The ethane circuit is continuously connected by at least one ethane compressor, a cooler, and the outlet of the ethane evaporator connected to the inlet of at least one compressor. The nitrogen circuit is continuously connected by at least one nitrogen compressor, a cooler, the ethane evaporator and a nitrogen-nitrogen heat exchanger therebetween, a turbine expander, the subcooling end heat exchanger, the nitrogen-nitrogen heat exchanger, and a turbine compressor connected to the inlet of the nitrogen compressor.

[0007] The method and apparatus are characterized by the use of nitrogen cooling in a nitrogen-nitrogen heat exchanger, which makes the process layout technically more complex and increases the number of apparatuses; in addition, the temperature reduction in the nitrogen loop relies entirely on expansion in the turbine expander, which means that the natural gas is not sufficiently cooled before expansion, resulting in less liquefied natural gas and more evaporation, resulting in a higher boil-off gas compressor load and impaired energy efficiency.

[0008] The technical problem to be solved by the present invention is to simplify the natural gas liquefaction process, expand the scope of application, reduce the strength of the device, reduce the energy consumption of LNG production, and improve production capacity. Summary of the Invention

[0009] This technical problem is solved by a natural gas liquefaction method, the essence of which is that, through the proposed natural gas liquefaction method, pre-treated natural gas is compressed and the compression heat is removed, multi-stage pre-cooling is performed by evaporating heavy refrigerant, sub-cooling is performed by recovering cold from the light refrigerant vapor and preliminary evaporation, the gas to be liquefied is depressurized, the non-liquefied gas is separated and the liquefied gas is removed, and at the same time the heavy refrigerant obtained by evaporation is compressed, condensed and reused for the multi-stage pre-cooling of natural gas, and the light refrigerant is compressed, cooled and used for sub-cooling of natural gas. According to the present invention, the light refrigerant is compressed before cooling to remove the compression heat, and then the natural gas is continuously cooled by low-pressure light refrigerant and evaporated heavy refrigerant, and then the high-pressure light refrigerant is separated into two streams, the first stream is isentropically expanded, and the second stream is continuously cooled, isenthalpic expanded and evaporated by sub-cooling the natural gas, and then these streams are mixed together at isobaric pressure and used as a single stream of low-pressure light refrigerant, used to cool the second stream of natural gas and light refrigerant before isenthalpic expansion and evaporation, and used to cool the single stream of high-pressure light refrigerant.

[0010] Furthermore, it is proposed to carry out multi-stage precooling of gases at boiling pressures of different grades of heavy refrigerants in the various stages by reducing the pressure of the heavy refrigerant to boiling before each stage and at least the last stage for cooling the high-pressure light refrigerant.

[0011] Furthermore, it is proposed to use the energy released from the isentropic expansion of the first stream to compress the light refrigerant.

[0012] Furthermore, nitrogen may be used as the light refrigerant, and ethane or ethylene may be used as the heavy refrigerant.

[0013] Furthermore, after subcooling by recovering the cold vapor of the light refrigerant, the pressure of the gas to be liquefied can be further reduced, depending on its composition, to a pressure at which no two-phase flow will form for a given gas composition.

[0014] The technical problem is also solved by a device for liquefying natural gas, which includes a natural gas precooling pipeline, a natural gas subcooling circuit, a heavy refrigerant circuit and a light refrigerant circuit; the natural gas precooling pipeline includes at least one natural gas compressor, at least one air cooler or water cooler and the inter-tube space of the heavy refrigerant evaporator in sequence; 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 cooler or water cooler and the inter-tube space of the heavy refrigerant evaporator in sequence, and the outlet of the heavy refrigerant evaporator is connected to the heavy refrigerant compressor; the light refrigerant circuit includes at least two light refrigerant compressors, at least one air cooler or water cooler located downstream of each light refrigerant compressor, and a double-flow light refrigerant superheater, the tube space of the heavy refrigerant evaporator, the first tube space of the shell and tube heat exchanger, and an expander in sequence. According to the present invention, The natural gas subcooling pipeline includes the second tube space of the shell and tube heat exchanger, the outlet of the shell and tube heat exchanger is connected to the first heat exchange space of the light refrigerant evaporator, and the outlet of the light refrigerant evaporator is connected to the first pressure reducer; in the light refrigerant circuit, the last air cooler or water cooler is connected in sequence to the first heat exchange space of the double-flow superheater and the second tube space of the heavy refrigerant evaporator, and the light refrigerant outlet of the heavy refrigerant evaporator is connected to two light refrigerant pipelines, wherein the first pipeline includes the expander, and the second pipeline includes in sequence the first tube space of the shell and tube heat exchanger, the second pressure reducer, and the second heat exchange space of the light refrigerant evaporator, the gas outlet of the light refrigerant evaporator is connected to the tube space of the shell and tube heat exchanger together with the outlet of the expander, the outlet of the shell and tube heat exchanger is connected to the second heat exchange space of the double-flow superheater, and the outlet of the double-flow superheater is connected to the first light refrigerant compressor along the flow direction.

[0015] In addition, the subcooling line may further include a third pressure reducer having an inlet connected to an outlet of the first inter-tube space of the shell and tube heat exchanger and an outlet connected to an inlet of the first heat exchange space of the light refrigerant evaporator.

[0016] It is proposed that the expander is kinematically coupled to at least one stage of the light refrigerant compressor.

[0017] The technical effect achieved by the proposed method and apparatus is deep cooling of natural gas, thereby reducing the amount of boil-off gas and the workload required for recompression and recovery, and simplifying the hardware design of the heat exchanger.

[0018] Compared to the "Arctic Cascade" (prototype) process, the nitrogen cooling circuit of the proposed "Arctic Cascade Modified" ("Arctic Cascade M") process envisions boiling of the light refrigerant, allowing for deeper cooling of the natural gas, reducing the amount of boil-off gas, and eliminating the need for cooling the light refrigerant at each stage of the multi-stage pre-cooling process in the nitrogen-nitrogen heat exchanger. This can reduce the number and size of process units, thereby simplifying the process hardware design and reducing the footprint, as nitrogen cooling only occurs in the final stage of pre-cooling, and reduces hydraulic resistance and heat losses by recovering cold from the evaporated light refrigerant.

[0019] Compared to the proposed solution EP0358100A2, the proposed method and apparatus reduces energy consumption by evaporating the heavy refrigerant to additionally cool the natural gas and high-pressure light refrigerant. Furthermore, the present invention can be used to liquefy gases with relatively high inlet temperatures. Furthermore, the light refrigerant stream is separated within the circuit after being cooled by the superheated vapor and heavy refrigerant, enabling the use of a shell-and-tube dual-flow heat exchanger. Another difference of the proposed solution is that the pressure of the second stream after evaporation is equal to the pressure of the first stream after isentropic expansion, allowing them to be mixed and used as a single stream in the shell-and-tube heat exchanger. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 The layout of the proposed device is shown. DETAILED DESCRIPTION

[0021] The natural gas liquefaction device includes a natural gas pre-cooling pipeline, a natural gas super-cooling pipeline, a heavy refrigerant circuit, and a light refrigerant circuit.

[0022] The natural gas pre-cooling pipeline includes a pipe space of a natural gas compressor 1, an air cooler or a water cooler 2, and a heavy refrigerant (heavy refrigerant in this example is ethane) evaporator 3 connected in sequence.

[0023] The natural gas subcooling pipeline includes the second tube side 7 of the shell and tube heat exchanger, the third pressure reducer 9, i.e., the throttle valve, the first heat exchange space (including the space between the tubes) of the light refrigerant (in this example, the light refrigerant is nitrogen) evaporator 8, and the second pressure reducer 10, i.e., the throttle valve, which are connected in sequence.

[0024] The heavy refrigerant circuit includes a heavy refrigerant compressor 4, at least one air cooler or water cooler, and the inter-tube space of the heavy refrigerant evaporator 3, which are connected in series. A throttle valve 6 is installed at the inlet of each evaporator 3. The gas outlet of the inter-tube space of the evaporator 3 is connected to the compressor 4. The heavy refrigerant circuit can include two or more compressors connected in series, depending on the rated power of the compressor and the refrigerant pressure requirement.

[0025] The light refrigerant circuit includes, in sequence, at least two light refrigerant compressors 11, at least one air cooler or water cooler 12 following each compressor 11, a first heat exchange space of a dual-flow superheater 13, and a second heat exchange space of a light refrigerant evaporator 3. The outlet of the light refrigerant evaporator 3 is connected to two light refrigerant lines. The first line includes an expander 14. The second light refrigerant line includes, in sequence, the first heat exchange space of a shell-and-tube heat exchanger 7, a throttle valve 15, and the second heat exchange space (including the inter-tube space) of a light refrigerant evaporator 8. The gas outlet of the light refrigerant evaporator 8, along with the outlet of the first light refrigerant expander 14, is connected to the inlet of the cold-side inter-tube space of the shell-and-tube heat exchanger 7. The outlet of the cold-side inter-tube space of the shell-and-tube heat exchanger 7 is connected to the cold-side second heat exchange space (including the inter-tube space) of the dual-flow superheater 13. The outlet of the dual-flow superheater 13 is connected to the inlet of the first flow light refrigerant compressor 11.

[0026] The natural gas compressor 1, the heavy refrigerant compressor 4, and the light refrigerant compressor 11 can be driven by, but not limited to, a gas turbine or an electric motor, which can be connected to the compressor via a multiplier (not shown).

[0027] The light refrigerant compressor 11 can also be driven by part of the power generated by the expander 14 by ensuring that one of the light refrigerant compressors 11 is kinematically coupled to the shaft of the compressor 11 .

[0028] The natural gas liquefaction method is specifically as follows.

[0029] After pre-liquefaction treatment, the natural gas, free of water vapor, carbon dioxide, and other impurities, is sent to natural gas compressor 1 for compression to approximately 10 MPa. It is then cooled to approximately +15°C using ambient cooling in an air or water cooler 2 and sent to heavy refrigerant evaporator 3 for preliminary multi-stage cooling. After progressive cooling in evaporator 3, the liquefied gas, at approximately -84°C, is sent to the second heat exchange space, where it is cooled to approximately -144.5°C. It is then sent to the third pressure reducer 9 for expansion, where the pressure is reduced to approximately 2 MPa, eliminating the two-phase flow of this gas component. The gas is then subcooled to -152.3°C in evaporator 8 and sent to the first pressure reducer 10, where its pressure is reduced to 0.1 MPa, producing a gas-liquid mixture, the liquid portion of which is liquefied natural gas.

[0030] Ethane is used as a heavy refrigerant, but its applications are not limited to this. Gaseous ethane at various pressures from evaporator 3 is fed to heavy refrigerant multi-stage compressor 4, where its pressure is raised to approximately 3 MPa and condensed to +10°C in air or water coolers 5. Liquid ethane is then fed to evaporators 3, where the natural gas is cooled to approximately -84°C by heavy refrigerants at various pressure levels. In at least the last evaporator 3, the heavy refrigerant not only cools the natural gas but also the light refrigerant stream. The varying pressure levels in evaporator 3 are achieved by reducing the pressure of throttle valve 6. The gaseous ethane from evaporator 3 is then fed to heavy refrigerant compressor 4, where it is compressed, condensed, and then reused in the multi-stage precooling of the natural gas along the circuit.

[0031] Nitrogen is used as a light refrigerant, but its application is not limited to this. The gaseous, low-pressure light refrigerant from the dual-flow superheater 13 is fed to at least one light refrigerant compressor 11 and at least one air or water cooler 12, where its pressure is raised to 10.3 MPa and its temperature is reduced to +15°C. The resulting high-pressure gaseous nitrogen is then fed to the dual-flow superheater 13, where it is cooled to approximately -65°C by recovering refrigeration from the low-pressure nitrogen and further divided into two streams. The first stream is then fed along a first line to the expander 14, where it undergoes isentropic expansion and energy removal, reducing its pressure to 2.1 MPa and its temperature to -147.5°C. The second stream flows along a second line into the 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 to 2.1 MPa in an isenthalpic process in the second reducer 15, forming a gas-liquid stream. The low-pressure light refrigerant gas-liquid stream is introduced into evaporator 8, where it is subcooled by the natural gas stream and evaporated. After mixing with the first stream of low-pressure light refrigerant, the combined stream is introduced into shell-and-tube heat exchanger 7, where it is heated, thereby cooling the high-pressure light refrigerant and natural gas streams. The combined stream of nitrogen from heat exchanger 7 is then introduced into dual-flow superheater 13, where the high-pressure nitrogen, after passing through air or water cooler 12, is cooled and superheated.

[0032] The energy released by the expander 14 during the isentropic expansion process can be used to provide energy for driving the at least one compressor 11 .

[0033] The process operates in nominal mode at ambient temperatures of +5°C or below. Above +5°C, the line's production efficiency begins to decline. Because the process was developed for Arctic and Antarctic latitudes, the heat exchangers used to condense heavy refrigerants (especially ethane) during the hot season may use water from Arctic or Antarctic oceans, bays, and other reservoirs, which have lower temperatures even in summer.

Claims

1. A natural gas liquefaction method comprising compressing pretreated natural gas to remove heat of compression, performing multi-stage precooling of the natural gas by evaporating a heavy refrigerant, performing subcooling by recovering cold from a light refrigerant vapor and performing preliminary evaporation, depressurizing the natural gas to be liquefied, separating unliquefied gas and removing liquefied gas, compressing and condensing the heavy refrigerant obtained by evaporation and reused for multi-stage precooling of the natural gas, and compressing and cooling the light refrigerant and using it for subcooling of the natural gas, characterized in that: The light refrigerant is compressed to remove the heat of compression before cooling, and the low-pressure light refrigerant is then used to continuously cool and evaporate the heavy refrigerant. The high-pressure light refrigerant is then separated into two streams, a first stream being isentropically expanded and a second stream being continuously cooled, isenthalpically expanded, and evaporated by subcooling the natural gas. These streams are then mixed together at isobaric pressure and used as a single stream of low-pressure light refrigerant for cooling the second stream of natural gas and light refrigerant prior to isenthalpic expansion and evaporation, and for cooling the single stream of high-pressure light refrigerant.

2. The method according to claim 1, characterized in that Multi-stage precooling of natural gas is carried out in stages at different levels of heavy refrigerant boiling pressure by reducing the pressure of the heavy refrigerant to boiling before each stage, and at least the last stage is used to cool the high-pressure light refrigerant.

3. The method according to claim 1, characterized in that The energy released from the isentropic expansion of the first stream is used to compress the light refrigerant.

4. The method according to claim 1, wherein Nitrogen is used as the light refrigerant, and ethane or ethylene is used as the heavy refrigerant.

5. The method according to claim 1, wherein After subcooling by recovering the cold vapor of the light refrigerant, the pressure of the natural gas to be liquefied is additionally reduced to a pressure at which no two-phase flow occurs.

6. An apparatus for liquefying natural gas, comprising a natural gas precooling line, a natural gas subcooling circuit, a heavy refrigerant circuit, and a light refrigerant circuit; the natural gas precooling line sequentially comprises at least one natural gas compressor, at least one air cooler or water cooler, and the inter-tube space of a heavy refrigerant evaporator; the subcooling circuit comprises a shell and tube heat exchanger and a first pressure reducer; the heavy refrigerant circuit sequentially comprises at least one heavy refrigerant compressor, at least one air cooler or water cooler, and the inter-tube space of a heavy refrigerant evaporator, the outlet of the heavy refrigerant evaporator being connected to the heavy refrigerant compressor; the light refrigerant circuit sequentially comprises at least two light refrigerant compressors, at least one air cooler or water cooler located downstream of each light refrigerant compressor, a dual-flow light refrigerant superheater, the tube space of the heavy refrigerant evaporator, the first tube space of the shell and tube heat exchanger, and an expander, characterized in that The natural gas subcooling pipeline includes the second tube space of the shell and tube heat exchanger, the outlet of the shell and tube heat exchanger is connected to the first heat exchange space of the light refrigerant evaporator, and the outlet of the light refrigerant evaporator is connected to the first pressure reducer; in the light refrigerant circuit, the last air cooler or water cooler is sequentially connected to the first heat exchange space of the double-flow superheater and the second tube space of the heavy refrigerant evaporator, and the light refrigerant outlet of the heavy refrigerant evaporator is connected to two light refrigerant pipelines, wherein the first pipeline includes the expander, and the second pipeline includes the first tube space of the shell and tube heat exchanger, the second pressure reducer, and the second heat exchange space of the light refrigerant evaporator in sequence, the gas outlet of the light refrigerant evaporator is connected to the tube space of the shell and tube heat exchanger together with the outlet of the expander, the outlet of the shell and tube heat exchanger is connected to the second heat exchange space of the double-flow superheater, and the outlet of the double-flow superheater is connected to the first light refrigerant compressor along the flow direction.

7. The device according to claim 6, characterized in that The subcooling line includes a third pressure reducer, an inlet of the third pressure reducer is connected to an outlet of the first tube space of the shell and tube heat exchanger, and an outlet of the shell and tube heat exchanger is connected to an inlet of the first heat exchange space of the light refrigerant evaporator.

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

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