High-pressure natural gas two-stage expansion liquefaction device and process method

Abstract

The invention discloses a two-stage expansion liquefaction device for high-pressure natural gas. The device comprises two heat exchangers, five separators, two expansion compressors and two air coolers. The invention further discloses a high-pressure natural gas two-stage expansion liquefaction process method based on the device. Pressure energy of high-pressure natural gas which comes from a natural gas pipe network and is used for refrigeration is utilized, a two-stage expansion refrigeration process is used, after first-stage expansion refrigeration, one part of natural gas in the pipe network is liquefied into liquefied natural gas, and the other part of natural gas is subjected to second-stage expansion to obtain low temperature; and cold energy is provided for the main heat exchanger to liquefy the to-be-liquefied natural gas, so that the overall liquefaction rate of the device is improved, and the energy consumption and the cost are reduced.

Description

Technical Field

[0001] The present invention relates to the field of liquefied natural gas, and in particular to a high-pressure natural gas two-stage expansion liquefaction device and a process thereof. Background Art

[0002] Pipeline natural gas is usually transported at high pressure through long-distance pipelines. Before it is supplied to end users, it is usually necessary to reduce the pressure through pressure regulating equipment so that the pressure of the natural gas can match the gas-using facilities. High-pressure natural gas contains huge pressure energy, which is often wasted in the process of pressure regulation through pressure regulating equipment. If the pressure energy of high-pressure natural gas is used to provide cold energy through an expander to liquefy part of the natural gas, natural gas can be transported more efficiently and considerable benefits can be obtained with lower energy consumption.

[0003] For example, the Chinese invention patent application with publication number CN102660341A discloses a "process and device for partially liquefying natural gas using natural gas pressure energy". This process uses the expansion refrigeration process to obtain cold energy for the liquefaction of part of the natural gas in the pipeline network. The disadvantage is that this process uses a two-stage expansion compressor. After the natural gas used for refrigeration is expanded in one stage to recover cold energy, it is recompressed and returned to the main heat exchanger for cooling, and then expanded and refrigerated in the second stage. The main heat exchanger has many and complex flow channels, low refrigeration efficiency, and more high-pressure natural gas is required for refrigeration, and the liquefiable part is small. Summary of the invention

[0004] One of the technical problems to be solved by the present invention is to provide a high-pressure natural gas two-stage expansion liquefaction device, which can improve the efficiency of high-pressure natural gas expansion refrigeration and the liquefaction rate of natural gas and reduce energy consumption and cost.

[0005] In order to solve the above technical problems, the high-pressure natural gas two-stage expansion liquefaction device of the present invention mainly includes 2 heat exchangers, 5 separators, 2 expansion compressors, and 2 air coolers, wherein:

[0006] The first heat exchanger includes four heat exchange channels; the inlet of the first heat exchange channel is connected to the refrigeration natural gas pipeline, and the outlet is connected to the inlet of the first separator; the inlet of the second heat exchange channel is connected to the liquefied natural gas pipeline, and the outlet is connected to the inlet of the second separator; the inlet of the third heat exchange channel is respectively connected to the top gas phase outlets of the second separator, the second heat exchanger and the fourth separator, and the outlet is connected to the upper end inlet of the fifth separator; the inlet of the fourth heat exchange channel is connected to the gas outlet of the expansion end of the second expansion compressor, and the outlet is connected to the lower end inlet of the fifth separator;

[0007] The top gas phase outlets of the first separator and the third separator are connected to the inlets of the expansion ends of the first expansion compressor and the second expansion compressor respectively; the bottom liquid phase outlet pipelines of the second separator and the fourth separator are connected to the liquefied natural gas product storage tank after merging; a line is branched from the inlet pipeline of the liquefied natural gas product storage tank to connect to the bottom liquefied natural gas inlet of the second heat exchanger;

[0008] The gas inlet of the second heat exchanger is connected to the outlet of the expansion end of the first expansion compressor, and the gas outlet is connected to the inlet of the third separator;

[0009] The bottom liquid phase outlet of the third separator is connected to the inlet of the fourth separator; the top gas phase outlet of the fifth separator is connected to the inlet of the compression end of the second expansion compressor, and the bottom liquid phase outlet is connected to the discharge pipe;

[0010] The inlet of the first air cooler is connected to the outlet of the compression end of the first expansion compressor, and the outlet is connected to the flash gas pipeline; the inlet of the second air cooler is connected to the outlet of the compression end of the second expansion compressor, and the outlet is connected to the inlet of the compression end of the first expansion compressor.

[0011] Preferably, the liquefaction device can also be provided with a three-stage compression unit between the gas phase outlet of the fifth separator and the compression end inlet of the second expansion compressor; the three-stage compression unit includes 3 gas compressors and 3 air coolers, wherein the inlet of the first-stage gas compressor is connected to the gas phase outlet of the fifth separator; the inlets of the first-stage air cooler, the second-stage air cooler and the third-stage air cooler are respectively connected to the outlets of the first-stage gas compressor, the second-stage gas compressor and the third-stage gas compressor; the outlets of the first-stage air cooler and the second-stage air cooler are respectively connected to the inlets of the second-stage gas compressor and the third-stage gas compressor; and the outlet of the third-stage air cooler is connected to the compression end inlet of the second expansion compressor.

[0012] Preferably, the liquefaction device may also be provided with a liquefied natural gas pump, the inlet of the liquefied natural gas pump being respectively connected to the liquid phase outlets of the second separator and the fourth separator, and the outlet of the liquefied natural gas pump being respectively connected to the second heat exchanger and the liquefied natural gas product storage tank.

[0013] Preferably, flow transmitters may be respectively provided on inlet pipelines of the first heat exchange channel and the second heat exchange channel of the first heat exchanger, and a liquid level transmitter may be provided on the separator.

[0014] Preferably, throttle valves may be respectively provided on the outlet pipelines of the second heat exchange channel and the fourth heat exchange channel of the first heat exchanger and the liquid phase outlet pipeline of the third separator.

[0015] The second technical problem to be solved by the present invention is to provide a two-stage expansion liquefaction process method for high-pressure natural gas based on the above-mentioned device, which divides the high-pressure raw natural gas into two parts: natural gas to be liquefied and refrigeration natural gas. The pressure energy of the refrigeration natural gas is first used for primary expansion refrigeration to liquefy a part of the natural gas in the refrigeration natural gas, and then the pressure energy of the refrigeration natural gas is used for secondary expansion refrigeration to obtain a low temperature for the other part of the natural gas in the refrigeration natural gas, providing cold energy for the first heat exchanger to liquefy the natural gas to be liquefied. Specifically, the process method includes the following process steps:

[0016] The refrigeration natural gas in the raw natural gas enters the first heat exchange channel of the first heat exchanger for cooling, and then enters the first separator for gas-liquid separation. The gas phase is expanded and cooled by the first expansion compressor, and then heat-exchanged and cooled by the second heat exchanger. Then, it enters the third separator for separation into gas and liquid phases. The gas phase is expanded and cooled by the second expansion compressor and then returns to the fourth heat exchange channel of the first heat exchanger. The liquid phase enters the fourth separator for gas-liquid separation. The gas phase returns to the third heat exchange channel of the first heat exchanger, and the liquid phase is recovered as a liquefied natural gas product.

[0017] The liquefied natural gas in the raw natural gas enters the second heat exchange channel of the first heat exchanger for cooling, and then enters the second separator for gas-liquid separation, the gas phase returns to the third heat exchange channel of the first heat exchanger, and the liquid phase is recovered as a liquefied natural gas product;

[0018] A portion of the recovered liquefied natural gas product enters the second heat exchanger for heat exchange and then returns to the third heat exchange channel of the first heat exchanger;

[0019] After the refrigerant in the third heat exchange channel and the fourth heat exchange channel leaves the first heat exchanger, it enters the fifth separator for gas-liquid separation, and the liquid phase is discharged from the device. The gas phase is compressed and cooled in turn by the second expansion compressor and the first expansion compressor to obtain a flash gas product.

[0020] Preferably, the gas phase of the fifth separator can be subjected to three-stage compression and cooling before entering the compression end of the second expansion compressor.

[0021] The high-pressure natural gas two-stage expansion liquefaction process and device of the present invention utilizes the pressure energy of the high-pressure natural gas itself from the natural gas pipeline network for refrigeration, and uses a two-stage expansion refrigeration process. After the first-stage expansion refrigeration, a part of the natural gas in the pipeline network is liquefied into liquefied natural gas, and the other part of the natural gas is subjected to a second-stage expansion to obtain a low temperature, providing cold capacity for the main heat exchanger to liquefy the natural gas to be liquefied. The two parts of liquefied natural gas are mixed and recovered as LNG products. At the same time, the output work of the expansion compressor is used for recompression of flash gas (BOG) and refrigeration natural gas to obtain medium-pressure natural gas. Since the second-stage expansion is carried out at a relatively low temperature, the expansion refrigeration efficiency is high. Therefore, the overall liquefaction rate of the present invention is high, the energy consumption is low, and the main heat exchanger has fewer flow channels, the equipment is relatively simple, and the cost can be reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic diagram of a high-pressure natural gas two-stage expansion liquefaction device and a process flow diagram of Example 1 of the present invention.

[0023] The reference numerals in the figures are described as follows:

[0024] 1: First heat exchanger

[0025] 2: Second heat exchanger

[0026] 3: First separator

[0027] 4: Second separator

[0028] 5: The third separator

[0029] 6: The fourth separator

[0030] 7: Fifth separator

[0031] 8: First expansion compressor

[0032] 9: Second expansion compressor

[0033] 10: First throttle valve

[0034] 11: Second throttle valve

[0035] 12: Third throttle valve

[0036] 13: First stage gas compressor

[0037] 14: Second stage gas compressor

[0038] 15: Third stage gas compressor

[0039] 16: First air cooler

[0040] 17: Second air cooler

[0041] 18: First stage air cooler

[0042] 19: Second stage air cooler

[0043] 20: Third stage air cooler

[0044] 21: Liquefied Natural Gas Pump

[0045] L1~L4: The first to fourth heat exchange channels

[0046] E1, E2: expansion end

[0047] C1, C2: Compression end

[0048] FT: Flow Transmitter

[0049] LT: Level transmitter

[0050] NG: liquefied natural gas

[0051] RG: Natural gas for refrigeration DETAILED DESCRIPTION

[0052] In order to have a more specific understanding of the technical content, characteristics and effects of the present invention, the technical solution of the present invention is further described in detail in conjunction with the accompanying drawings and specific embodiments:

[0053] Example 1

[0054] The high-pressure natural gas two-stage expansion liquefaction device of this embodiment is as follows: Figure 1 As shown, it mainly includes 2 heat exchangers, 5 separators, 2 expansion compressors, 3 gas compressors, 5 air coolers and 1 liquefied natural gas pump. Among them:

[0055] The first heat exchanger 1 is a main heat exchanger and is provided with 4 heat exchange channels. The inlet of the first heat exchange channel L1 is connected to the refrigeration natural gas pipeline, and the outlet is connected to the inlet of the first separator 3 through a pipeline; the inlet of the second heat exchange channel L2 is connected to the natural gas pipeline to be liquefied, and the outlet is connected to the inlet of the second separator 4 through a pipeline, and a first throttle valve 10 is provided on the pipeline between the outlet of the second heat exchange channel L2 and the inlet of the second separator 4; the inlet of the third heat exchange channel L3 is connected to the top outlets of the second heat exchanger 2, the second separator 4, and the fourth separator 6 through pipelines, respectively, and the outlet of the third heat exchange channel L3 is connected to the upper inlet of the fifth separator 7 through a pipeline; the inlet of the fourth heat exchange channel L4 is connected to the gas outlet of the expansion end of the second expansion compressor 9 through a pipeline, and the outlet of the fourth heat exchange channel L4 is connected to the lower inlet of the fifth separator through a pipeline, and a third throttle valve 12 is provided on the pipeline between the outlet of the fourth heat exchange channel L4 and the lower inlet of the fifth separator.

[0056] The top gas phase outlet of the first separator 3 is connected to the inlet of the expansion end of the first expansion compressor 8 through a pipeline. The gas outlet of the expansion end of the first expansion compressor 8 is connected to the gas inlet of the second heat exchanger 2 through a pipeline. The gas outlet of the second heat exchanger 2 is connected to the inlet of the third separator 5 through a pipeline.

[0057] The top gas phase outlet of the third separator 5 is connected to the inlet of the expansion end of the second expansion compressor 9 through a pipeline. The bottom liquid phase outlet of the third separator 5 is connected to the inlet of the fourth separator 6 through a pipeline. A second throttle valve 11 is provided on the pipeline between the bottom liquid phase outlet of the third separator 5 and the inlet of the fourth separator 6.

[0058] The bottom liquid phase outlets of the second separator 4 and the fourth separator 6 are connected to the inlet of the liquefied natural gas pump 21 through pipelines. The outlet of the liquefied natural gas pump 21 is connected to the LNG product storage tank through a pipeline. The outlet of the LNG product storage tank is connected to the LNG inlet of the second heat exchanger 2 through a pipeline.

[0059] The top gas phase outlet of the fifth separator 7 is connected to the inlet of the first-stage gas compressor 13 through a pipeline. The inlets of the first-stage air cooler 18, the second-stage air cooler 19, and the third-stage air cooler 20 are connected to the outlets of the first-stage gas compressor 13, the second-stage gas compressor 14, and the third-stage gas compressor 15 through pipelines, respectively. The outlets of the first-stage air cooler 18 and the second-stage air cooler 19 are connected to the inlets of the second-stage gas compressor 14 and the third-stage gas compressor 15 through pipelines, respectively. The outlet of the third-stage air cooler 20 is connected to the inlet of the compression end C2 of the second expansion compressor 9 through a pipeline. The outlet of the compression end C2 of the second expansion compressor 9 is connected to the inlet of the second air cooler 17 through a pipeline. The outlet of the second air cooler 17 is connected to the inlet of the compression end C1 of the first expansion compressor 8 through a pipeline. The outlet of the compression end C1 of the first expansion compressor 8 is connected to the inlet of the first air cooler 16 through a pipeline. The outlet of the first air cooler 16 leads to the BOG product pipeline.

[0060] Each separator is provided with a liquid level transmitter LT for controlling the liquid level in the separator. The inlet pipelines of the first heat exchange channel L1 and the second heat exchange channel L2 of the first heat exchanger 1 are provided with flow transmitters FT for controlling the gas flow in the pipelines.

[0061] The process flow of the high-pressure natural gas two-stage expansion liquefaction device of this embodiment is described in detail below.

[0062] The raw gas (pressure of about 80 barg, temperature of 44°C) from the high-pressure natural gas pipeline network is divided into two parts: natural gas to be liquefied and refrigeration natural gas (natural gas without water).

[0063] The liquefied natural gas first enters the second heat exchange channel L2 of the first heat exchanger 1, is cooled to a temperature of about -143°C in the first heat exchanger 1, and then is throttled to a pressure of 0.17 barg through the first throttle valve 10, and the temperature drops to about -160°C, and enters the second separator 4 to be separated into a gas phase and a liquid phase. The gas phase portion is BOG (flash gas), which returns to the third heat exchange channel L3 of the first heat exchanger 1 as a refrigerant to provide cooling for the first heat exchanger 1; the liquid phase portion is sent to the liquefied natural gas product storage tank as a liquefied natural gas (LNG) product via a liquefied natural gas pump 21.

[0064] The refrigeration natural gas first enters the first heat exchange channel L1 of the first heat exchanger 1 and is cooled to a temperature of -44°C. It is then sent to the first separator 3 for gas-liquid separation. The gas phase enters the expansion end E1 of the first expansion compressor 8 and outputs power. The pressure drops to 11 barg and the temperature drops to about -120°C. The gas after temperature and pressure reduction enters the second heat exchanger 2 and is partially liquefied into gas and liquid phases. It then enters the third separator 5 and is separated into gas and liquid phases. The gas phase enters the expansion end E2 of the second expansion compressor 9 and outputs power. The gas pressure drops to 1 barg and the temperature drops to -152.5°C. It then returns to the fourth heat exchange channel L4 of the first heat exchanger 1 as a refrigerant to provide cooling for the first heat exchanger 1. The liquid phase is throttled to a pressure of 0.17 barg and a temperature of -159.1°C by the second throttle valve 11, and then enters the fourth separator 6 to be separated into a gas phase and a liquid phase. The gas phase returns to the third heat exchange channel L3 of the first heat exchanger 1 as a refrigerant to provide cooling for the first heat exchanger 1, and the liquid phase is sent to the liquefied natural gas product storage tank through the liquefied natural gas pump 21 as a liquefied natural gas product.

[0065] A portion of the liquefied natural gas product is used as a refrigerant to provide cooling for the second heat exchanger 2 and becomes a gas phase. This gas phase is combined with the gas phases from the second separator 4 and the fourth separator 6, and then sent to the third heat exchange channel L3 of the first heat exchanger 1 to provide cooling for the first heat exchanger 1. After being heated in the first heat exchanger 1, the temperature and pressure of this refrigerant become 40.4°C and 0.12 barg respectively, and then enters the fifth separator 7 to be separated into a gas phase and a liquid phase.

[0066] The refrigerant in the fourth heat exchange channel L4 of the first heat exchanger 1 is heated to a temperature of 40.4°C and a pressure of 0.83 barg after providing cooling for the first heat exchanger 1, and then throttled to 0.13 barg by the third throttle valve 12, and then enters the fifth separator 7 to be separated into a gas phase and a liquid phase.

[0067] The liquid phase in the fifth separator 7 is discharged from the bottom outlet pipeline of the fifth separator 7. After the gas phase at the top of the fifth separator 7 is compressed and cooled by the first to third gas compressors and the third to fifth air coolers, its temperature and pressure become 46.1°C and 16 barg respectively. The pressurized gas enters the compression end C2 of the second expansion compressor 9 (its compression power is provided by the expansion end E2 of the second expansion compressor 9), is compressed and pressurized and cooled by the second air cooler 17, and its temperature and pressure are 46.1°C and 22 barg respectively, and then is sent to the compression end C1 of the first expansion compressor 8 (its compression power is provided by the second expansion compressor 9), is compressed and cooled by the first air cooler 16, and its temperature and pressure become 46.11°C and 32 barg respectively. The multi-stage pressurized gas phase (i.e., BOG product) can be used as a raw material for a gas-fired generator set.

[0068] The above embodiments are merely feasible or preferred embodiments of the present invention and are used to illustrate the present invention rather than to limit the scope of the patent application of the present invention. Therefore, all equivalent changes and modifications made according to the scope of the patent application of the present invention should fall within the scope covered by the patent of the present invention.

Claims

1. A two-stage expansion liquefaction device for high-pressure natural gas, characterized in that: It includes 2 heat exchangers, 5 separators, 2 expansion compressors, and 2 air coolers, including: The first heat exchanger includes four heat exchange channels; the inlet of the first heat exchange channel is connected to the refrigeration natural gas pipeline, and the outlet is connected to the inlet of the first separator; the inlet of the second heat exchange channel is connected to the liquefied natural gas pipeline, and the outlet is connected to the inlet of the second separator; the inlet of the third heat exchange channel is respectively connected to the top gas phase outlets of the second separator, the second heat exchanger and the fourth separator, and the outlet is connected to the upper end inlet of the fifth separator; the inlet of the fourth heat exchange channel is connected to the gas outlet of the expansion end of the second expansion compressor, and the outlet is connected to the lower end inlet of the fifth separator; The top gas phase outlets of the first separator and the third separator are connected to the inlets of the expansion ends of the first expansion compressor and the second expansion compressor respectively; the bottom liquid phase outlet pipelines of the second separator and the fourth separator are connected to the liquefied natural gas product storage tank after merging; a line is branched from the inlet pipeline of the liquefied natural gas product storage tank to connect to the bottom liquefied natural gas inlet of the second heat exchanger; The gas inlet of the second heat exchanger is connected to the outlet of the expansion end of the first expansion compressor, and the gas outlet is connected to the inlet of the third separator; The bottom liquid phase outlet of the third separator is connected to the inlet of the fourth separator; the top gas phase outlet of the fifth separator is connected to the inlet of the compression end of the second expansion compressor, and the bottom liquid phase outlet is connected to the discharge pipe; The inlet of the first air cooler is connected to the outlet of the compression end of the first expansion compressor, and the outlet is connected to the flash gas pipeline; the inlet of the second air cooler is connected to the outlet of the compression end of the second expansion compressor, and the outlet is connected to the inlet of the compression end of the first expansion compressor.

2. The liquefaction device according to claim 1, characterized in that: The liquefaction device also includes a three-stage compression unit, which is arranged on the pipeline between the gas phase outlet of the fifth separator and the compression end inlet of the second expansion compressor; the three-stage compression unit includes three gas compressors and three air coolers, wherein the inlet of the first-stage gas compressor is connected to the gas phase outlet of the fifth separator; the inlets of the first-stage air cooler, the second-stage air cooler and the third-stage air cooler are respectively connected to the outlets of the first-stage gas compressor, the second-stage gas compressor and the third-stage gas compressor; the outlets of the first-stage air cooler and the second-stage air cooler are respectively connected to the inlets of the second-stage gas compressor and the third-stage gas compressor; and the outlet of the third-stage air cooler is connected to the compression end inlet of the second expansion compressor.

3. The liquefaction device according to claim 1, characterized in that: The liquefaction device further comprises a liquefied natural gas pump, the inlet of the liquefied natural gas pump is respectively connected to the liquid phase outlets of the second separator and the fourth separator, and the outlet of the liquefied natural gas pump is respectively connected to the second heat exchanger and the liquefied natural gas product storage tank.

4. The liquefaction device according to claim 1, characterized in that: Flow transmitters are respectively provided on the inlet pipelines of the first heat exchange channel and the second heat exchange channel of the first heat exchanger, and a liquid level transmitter is provided on the separator.

5. The liquefaction device according to claim 1, characterized in that: Throttle valves are respectively arranged on the outlet pipelines of the second heat exchange channel and the fourth heat exchange channel of the first heat exchanger and on the liquid phase outlet pipeline of the third separator.

6. A two-stage expansion liquefaction process for high-pressure natural gas based on the device according to any one of claims 1 to 5, characterized in that: The high-pressure raw natural gas is divided into two parts: natural gas to be liquefied and refrigeration natural gas. The pressure energy of the refrigeration natural gas is first used for primary expansion refrigeration to liquefy a part of the natural gas in the refrigeration natural gas. The pressure energy of the refrigeration natural gas is then used for secondary expansion refrigeration to obtain a low temperature for the other part of the natural gas in the refrigeration natural gas, providing cold capacity for the first heat exchanger to liquefy the natural gas to be liquefied.

7. The liquefaction process according to claim 6, characterized in that: The following steps are involved: The refrigeration natural gas in the raw natural gas enters the first heat exchange channel of the first heat exchanger for cooling, and then enters the first separator for gas-liquid separation. The gas phase is expanded and cooled by the first expansion compressor, and then heat-exchanged and cooled by the second heat exchanger. Then, it enters the third separator for separation into gas and liquid phases. The gas phase is expanded and cooled by the second expansion compressor and then returns to the fourth heat exchange channel of the first heat exchanger. The liquid phase enters the fourth separator for gas-liquid separation. The gas phase returns to the third heat exchange channel of the first heat exchanger, and the liquid phase is recovered as a liquefied natural gas product. The liquefied natural gas in the raw natural gas enters the second heat exchange channel of the first heat exchanger for cooling, and then enters the second separator for gas-liquid separation, the gas phase returns to the third heat exchange channel of the first heat exchanger, and the liquid phase is recovered as a liquefied natural gas product; A portion of the recovered liquefied natural gas product enters the second heat exchanger for heat exchange and then returns to the third heat exchange channel of the first heat exchanger; After the refrigerant in the third heat exchange channel and the fourth heat exchange channel leaves the first heat exchanger, it enters the fifth separator for gas-liquid separation, and the liquid phase is discharged from the device. The gas phase is compressed and cooled in turn by the second expansion compressor and the first expansion compressor to obtain a flash gas product.

8. The liquefaction process according to claim 7, characterized in that: The gas phase of the fifth separator is first compressed and cooled in three stages before entering the compression end of the second expansion compressor.

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