A mixed refrigerant liquefied natural gas plant with refrigerated brine pre-cooling and method thereof

Abstract

The application discloses a kind of frozen brine pre-cooling mixed refrigerant liquefied natural gas device and method thereof, device includes buffer tank, pre-cooling heat exchanger, refrigeration water unit, mixed refrigerant separator, heavy hydrocarbon separator, main heat exchanger, liquid phase refrigerant separator and gas phase refrigerant separator.Mixed refrigerant in buffer tank can be pre-cooled in pre-cooling heat exchanger and then enter mixed refrigerant separator, and purified natural gas can enter pre-cooling heat exchanger for pre-cooling;Natural gas pre-cooled by pre-cooling heat exchanger can enter main heat exchanger for heat exchange, and then enter heavy hydrocarbon separator;Heavy hydrocarbon separator can be communicated with heavy hydrocarbon recovery device and liquefied natural gas recovery device;The discharge of mixed refrigerant separator can be returned to buffer tank after heat exchange in main heat exchanger.The application can effectively avoid the change of mixed refrigerant component with the change of ambient temperature, make the process more stable, reduce the energy consumption of refrigerant compressor, and save cost.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of natural gas liquefaction, and particularly relates to a mixed refrigerant liquefied natural gas device with frozen brine pre-cooling and a method thereof. BACKGROUND

[0002] At present, common natural gas liquefaction processes in China are mainly cascade liquefaction process, liquefaction process with expander and mixed refrigerant liquefaction process. The mixed refrigerant natural gas process has been widely applied in large LNG liquefaction plants due to its low energy consumption and simple process.

[0003] The mixed refrigerant in the mixed refrigerant natural gas liquefaction process is mainly composed of various gases such as hydrocarbons and nitrogen. The matching degree of the heat exchange temperature between the mixed refrigerant and the natural gas determines the total efficiency of the refrigeration cycle of the liquefaction system, so the combination of the mixed refrigerant is the main factor determining the efficiency of the refrigeration cycle of the liquefaction system. The refrigeration effect of the refrigerant and the complexity of the ratio are mutually restricted, so the reasonable ratio of the mixed refrigerant is one of the difficulties in the mixed refrigerant liquefaction process.

[0004] In terms of the general properties of the mixture, the proportion of the heavy component in the mixed refrigerant can greatly affect the refrigeration effect. In the mixed refrigerant with a high proportion of heavy components, the dew point temperature of the mixture is higher, and the temperature range of the gas-liquid two-phase region is wider. Especially under low pressure conditions, the heavy component can effectively expand the refrigeration and heat exchange interval of the mixed refrigerant, and has a very obvious effect on improving the heat exchange efficiency of the system. The increase of the heavy component can significantly enhance the refrigeration capacity of the mixed refrigerant, thereby reducing the circulation amount of the mixed refrigerant and the power consumption of the compressor. However, if the content of the heavy component is too high, the mixed refrigerant will be in a gas-liquid two-phase state before entering the compressor, which will cause liquid hammer of the compressor, which is a great damage to the compressor. Therefore, it is particularly important to reasonably match the proportion of each component in the mixed refrigerant. SUMMARY

[0005] The present application aims to overcome the defects in the prior art and provide a mixed refrigerant liquefied natural gas device with frozen brine pre-cooling and a method thereof.

[0006] The specific technical solutions adopted by the present application are as follows:

[0007] In a first aspect, the application provides a mixed refrigerant liquefied natural gas device pre-cooled by frozen brine, which comprises a buffer tank, a pre-cooling heat exchanger, a frozen water unit, a mixed refrigerant separator, a heavy hydrocarbon separator, a main heat exchanger, a liquid-phase refrigerant separator and a gas-phase refrigerant separator; the buffer tank is used for containing mixed refrigerant, and its outlet is connected to the pre-cooling heat exchanger through a pipeline in sequence after a compressor and an air cooler; the mixed refrigerant can be pre-cooled in the pre-cooling heat exchanger and then enter the mixed refrigerant separator; and the purified natural gas can enter the pre-cooling heat exchanger through a first natural gas pipeline for pre-cooling; the pre-cooling heat exchanger is provided with a frozen water circulation pipeline for pre-cooling the mixed refrigerant and the natural gas, and the frozen water circulation pipeline is connected to an external frozen water unit;

[0008] The natural gas pre-cooled by the pre-cooling heat exchanger can enter the main heat exchanger through a second natural gas pipeline for heat exchange, and then enter the heavy hydrocarbon separator through a first heat exchange pipeline; the heavy hydrocarbon separator comprises a first liquid-phase outlet and a first gas-phase outlet; the first liquid-phase outlet is connected to a heavy hydrocarbon recovery device through a second heavy hydrocarbon separation pipeline; the first gas-phase outlet enters the main heat exchanger through a first heavy hydrocarbon separation pipeline for heat exchange, and then is connected to a liquefied natural gas recovery device through a second heat exchange pipeline, a first throttling valve and a first throttling pipeline;

[0009] The mixed refrigerant separator comprises a second liquid-phase outlet and a second gas-phase outlet; the second liquid-phase outlet is connected to the main heat exchanger through a second refrigerant separation pipeline, and then is connected to a liquid-phase refrigerant separator through a third heat exchange pipeline, a second throttling valve and a second throttling pipeline; the third liquid-phase outlet and the third gas-phase outlet of the liquid-phase refrigerant separator are respectively connected to a reflux refrigerant pipeline in the main heat exchanger through pipelines; the second gas-phase outlet is connected to the main heat exchanger through a first refrigerant separation pipeline, and then is connected to a gas-phase refrigerant separator through a fourth heat exchange pipeline, a third throttling valve and a third throttling pipeline; the fourth liquid-phase outlet and the fourth gas-phase outlet of the gas-phase refrigerant separator are respectively connected to the reflux refrigerant pipeline in the main heat exchanger through pipelines; the reflux refrigerant pipeline is connected to the buffer tank through a fifth heat exchange pipeline, and can make the mixed refrigerant after heat exchange with the natural gas in the main heat exchanger return to the buffer tank.

[0010] Preferably, the pre-cooling heat exchanger and the main heat exchanger are both plate-fin heat exchangers; and the compressor is a screw compressor.

[0011] Preferably, the mixed refrigerant separator, the heavy hydrocarbon separator, the liquid-phase refrigerant separator and the gas-phase refrigerant separator are all gas-liquid separators.

[0012] Preferably, the main heat exchanger, the liquid-phase refrigerant separator and the gas-phase refrigerant separator are all arranged in a cold box for heat preservation.

[0013] Preferably, a throttling valve is arranged on the fifth heat exchange pipeline, and the pipeline is also connected to pipelines for supplementing nitrogen and mixed refrigerant.

[0014] As a preference, the first gas phase outlet of the heavy hydrocarbon separator is further connected to the liquid phase refrigerant separator through a methane make-up line for returning methane gas.

[0015] In a second aspect, the present application provides a method for liquefying natural gas using the mixed refrigerant liquefied natural gas plant according to any one of the first aspect, which is specifically as follows:

[0016] The mixed refrigerant in the buffer tank is compressed and cooled by the compressor and air cooler, and then enters the pre-cooling heat exchanger. Meanwhile, the purified natural gas enters the pre-cooling heat exchanger through the first natural gas line. In the pre-cooling heat exchanger, the natural gas and the mixed refrigerant are pre-cooled by the chilled water circulating line.

[0017] The natural gas pre-cooled by the pre-cooling heat exchanger enters the main heat exchanger through the second natural gas line for further heat exchange and cooling, and then enters the heavy hydrocarbon separator through the first heat exchange line for heavy hydrocarbon removal treatment. The liquid phase heavy hydrocarbon separated by the heavy hydrocarbon separator flows out from the first liquid phase outlet and enters the heavy hydrocarbon recovery device through the second heavy hydrocarbon separation line. The gas phase separated by the heavy hydrocarbon separator flows out from the first gas phase outlet and enters the main heat exchanger through the first heavy hydrocarbon separation line for further heat exchange and cooling. The super-cooled product gas flows out from the main heat exchanger, enters the liquefied natural gas recovery device through the second heat exchange line, the first throttling valve and the first throttling line.

[0018] The mixed refrigerant pre-cooled by the pre-cooling heat exchanger enters the mixed refrigerant separator for gas-liquid separation. The gas phase refrigerant separated by the mixed refrigerant separator flows out from the second gas phase outlet and enters the main heat exchanger through the first refrigerant separation line for further heat exchange and cooling, and then enters the gas phase refrigerant separator through the fourth heat exchange line, the third throttling valve and the third throttling line for gas-liquid separation. The gas and liquid separated by the gas phase refrigerant separator enter the reflux refrigerant line in the main heat exchanger and jointly form the first stage mixed refrigerant, which further provides cold energy to the main heat exchanger through re-evaporation. The liquid phase refrigerant separated by the mixed refrigerant separator flows out from the second liquid phase outlet and enters the main heat exchanger through the second refrigerant separation line for further heat exchange and cooling, and then enters the liquid phase refrigerant separator through the third heat exchange line, the second throttling valve and the second throttling line for gas-liquid separation. The gas and liquid separated by the liquid phase refrigerant separator enter the reflux refrigerant line in the main heat exchanger and jointly form the second stage mixed refrigerant, which further provides cold energy to the main heat exchanger through re-evaporation. The first stage mixed refrigerant and the second stage mixed refrigerant flow out from the main heat exchanger and are returned to the buffer tank through the fifth heat exchange line, so as to realize the refrigeration cycle of the mixed refrigerant.

[0019] As a preference, the components of the mixed refrigerant in the buffer tank include methane, nitrogen, ethylene, propane and isobutane.

[0020] Preferably, the medium in the chilled water circulation pipeline is chilled brine.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] This invention replaces isopentane with isobutane in the heavy components required by common mixed refrigerants. It also adds chilled water circulation piping, a pre-cooling heat exchanger, and feed gas. While optimizing the mixed refrigerant ratio and increasing process stability, it further reduces the compressor load, lowers the overall system energy consumption, and saves on equipment investment costs for natural gas separation. This invention replaces isopentane with isobutane in commonly used mixed refrigerants because the lubricating oil used in screw compressors has similar properties to isopentane, making separation difficult. Lubricating oil entering the plate-fin heat exchanger in the cold box can cause freezing and blockage of the heat exchanger channels, damaging the heat exchanger. Replacing isopentane with isobutane avoids this problem. Attached Figure Description

[0023] Figure 1 A schematic diagram of a mixed refrigerant liquefied natural gas (LNG) unit;

[0024] Figure 2 This is a schematic diagram of another structure for a liquefied natural gas (LNG) unit with mixed refrigerants.

[0025] Figure 3 for Figure 1 Pipeline identification diagram;

[0026] In the diagram: Buffer tank 1, Compressor 2, Air cooler 3, Pre-cooling heat exchanger 4, Chilled water unit 5, Mixed refrigerant separator 6, Heavy hydrocarbon separator 7, Main heat exchanger 8, Cold box 9, Liquid phase refrigerant separator 10, Gas phase refrigerant separator 11, First refrigerant line 801, Second refrigerant line 802, Third refrigerant line 803, Fourth refrigerant line 804, First refrigerant separation line 805, Second refrigerant separation line 806, First natural gas line 807, Second natural gas line 808, First refrigeration unit Water pipe 809, second chilled water pipe 810, methane replenishment pipe 501, first heat exchange pipe 502, first heavy hydrocarbon separation pipe 503, second heat exchange pipe 504, first throttling pipe 505, second heavy hydrocarbon separation pipe 506, fourth heat exchange pipe 509, third throttling pipe 510, second throttling pipe 511, third heat exchange pipe 512, refrigerant return pipe 513, fifth heat exchange pipe 514, first throttling valve HV1, second throttling valve HV2, third throttling valve HV3. Detailed Implementation

[0027] The present invention will be further described and illustrated below with reference to the accompanying drawings and specific embodiments. The technical features of each embodiment of the present invention can be combined accordingly, provided that there is no mutual conflict.

[0028] As Figure 1 and 3 The mixed refrigerant liquefied natural gas device provided by the application is a refrigerated brine pre-cooled mixed refrigerant liquefied natural gas device, which mainly comprises a buffer tank 1, a pre-cooling heat exchanger 4, a refrigerated water unit 5, a mixed refrigerant separator 6, a heavy hydrocarbon separator 7, a main heat exchanger 8, a liquid-phase refrigerant separator 10 and a gas-phase refrigerant separator 11. The structure and connection mode of each component will be described in detail below.

[0029] The buffer tank 1 is used for containing mixed refrigerant, and the outlet thereof is connected with a compressor 2 through a first refrigerant pipeline 801. The compressor 2 is used for pressurizing the mixed refrigerant. The outlet of the compressor 2 is connected with an air cooler 3 through a second refrigerant pipeline 802. The air cooler 3 is used for preliminarily cooling the mixed refrigerant. The outlet of the air cooler 3 is connected with the pre-cooling heat exchanger 4 through a third refrigerant pipeline 803. The material conveyed in the third refrigerant pipeline 803 can be preliminarily cooled and reduced in temperature in the pre-cooling heat exchanger 4, and then enters the mixed refrigerant separator 6 through a fourth refrigerant pipeline 804. The pre-cooling heat exchanger 4 is externally connected with a first natural gas pipeline 807. The first natural gas pipeline 807 can send the raw material gas (natural gas) which is qualified after being purified by acid removal, dehydration and hydrocarbon removal into the pre-cooling heat exchanger 4 to perform a pre-cooling reaction. The pre-cooling heat exchanger 4 is provided with a refrigerated water circulation pipeline which is connected with an external refrigerated water unit 5. The refrigerated water circulation pipeline can perform pre-cooling heat exchange on the mixed refrigerant and the natural gas in the pre-cooling heat exchanger 4. The refrigerated water circulation pipeline comprises a first refrigerated water pipeline 809 and a second refrigerated water pipeline 810. The inlet end of the first refrigerated water pipeline 809 is connected with the refrigerated water unit 5, and the outlet end thereof is connected with the pre-cooling heat exchanger 4, which is used for sending the cooled refrigerated water into the pre-cooling heat exchanger 4. The inlet end of the second refrigerated water pipeline 810 is connected with the pre-cooling heat exchanger 4, and the outlet end thereof is connected with the refrigerated water unit 5, which is used for sending the refrigerated water which is heated in the pre-cooling heat exchanger 4 into the refrigerated water unit 5 to be cooled.

[0030] The pre-cooling heat exchanger 4 is provided with a second natural gas pipeline 808, which is used to send the natural gas pre-cooled by the pre-cooling heat exchanger 4 into the main heat exchanger 8 for heat exchange, and then into the heavy hydrocarbon separator 7 through the first heat exchange pipeline 502. The heavy hydrocarbon separator 7 is mainly used to realize the separation of the heavy hydrocarbon liquid phase, and includes a first liquid phase outlet and a first gas phase outlet. The first liquid phase outlet of the heavy hydrocarbon separator 7 is connected with the heavy hydrocarbon recovery device through a second heavy hydrocarbon separation pipeline 506, and is used to recover the separated heavy hydrocarbon. The first gas phase outlet of the heavy hydrocarbon separator 7 enters the main heat exchanger 8 for heat exchange through a first heavy hydrocarbon separation pipeline 503, and then is connected with the liquefied natural gas recovery device through a second heat exchange pipeline 504, so as to recover the liquefied natural gas further separated. In actual application, the second heat exchange pipeline 504 can be provided with a first throttle valve HV1, and the product obtained after heat exchange in the main heat exchanger 8 is sent into the liquefied natural gas recovery device through the first throttle pipeline 505 after pressure reduction by the throttle valve.

[0031] The mixed refrigerant separator 6 is used to separate the gas phase and liquid phase of the mixed refrigerant, and includes a second liquid phase outlet and a second gas phase outlet. The second liquid phase outlet of the mixed refrigerant separator 6 is connected to the main heat exchanger 8 through a second refrigerant separation pipeline 806, and the material in the second refrigerant separation pipeline 806 is communicated to the liquid phase refrigerant separator 10 through the third heat exchange pipeline 512, the second throttling valve HV2 and the second throttling pipeline 511 after heat exchange in the main heat exchanger 8. The liquid phase refrigerant separator 10 is used to further separate the gas and liquid phases of the liquid phase refrigerant. A third gas phase outlet is arranged at the top of the liquid phase refrigerant separator 10, and the third gas phase outlet is connected to a separation gas pipeline and is connected to the return flow refrigerant pipeline 513 in the main heat exchanger 8 through the pipeline. A third liquid phase outlet is arranged at the bottom of the liquid phase refrigerant separator 10, and the third liquid phase outlet is connected to a separation liquid pipeline and is connected to the return flow refrigerant pipeline 513 in the main heat exchanger 8 through the pipeline. The separation gas pipeline and the separation liquid pipeline connected to the liquid phase refrigerant separator 10 respectively extend into the main heat exchanger 8 and are communicated and combined into one pipeline in the main heat exchanger 8. The second gas phase outlet of the mixed refrigerant separator 6 is connected to the main heat exchanger 8 through a first refrigerant separation pipeline 805, and the material in the first refrigerant separation pipeline 805 is communicated to the gas phase refrigerant separator 11 through the fourth heat exchange pipeline 509, the third throttling valve HV3 and the third throttling pipeline 510 after heat exchange in the main heat exchanger 8. The gas phase refrigerant separator 11 is used to further separate the gas and liquid phases of the gas phase refrigerant. A fourth gas phase outlet is arranged at the top of the gas phase refrigerant separator 11, and the fourth gas phase outlet is connected to a separation gas pipeline and is connected to the return flow refrigerant pipeline 513 in the main heat exchanger 8 through the pipeline. A fourth liquid phase outlet is arranged at the bottom of the gas phase refrigerant separator 11, and the fourth liquid phase outlet is connected to a separation liquid pipeline and is connected to the return flow refrigerant pipeline 513 in the main heat exchanger 8 through the pipeline. The separation gas pipeline and the separation liquid pipeline connected to the gas phase refrigerant separator 11 respectively extend into the main heat exchanger 8 and are communicated and combined into one pipeline in the main heat exchanger 8. The return flow refrigerant pipeline 513 is connected to the buffer tank 1 through the fifth heat exchange pipeline 514, and the mixed refrigerant after heat exchange with the natural gas in the main heat exchanger 8 can be returned to the buffer tank 1.

[0032] In actual application, the pre-cooling heat exchanger 4 and the main heat exchanger 8 can be plate-fin heat exchangers, the compressor 2 can be a screw compressor, the mixed refrigerant separator 6, the heavy hydrocarbon separator 7, the liquid phase refrigerant separator 10 and the gas phase refrigerant separator 11 can be gas-liquid separators, and the throttling valve can be a J / T valve. In order to further ensure the cooling effect, the main heat exchanger 8, the liquid phase refrigerant separator 10 and the gas phase refrigerant separator 11 can be arranged in the cold box 9 to achieve the heat preservation effect of the devices. Figure 2As shown, in order to further ensure the separation effect of the mixed refrigerant in the main heat exchanger 8, so that the mixed refrigerant component entering the buffer tank 1 through the fifth heat exchange pipeline 514 remains stable, a methane supplement pipeline 501 for supplementing methane gas can be connected to the liquid-phase refrigerant separator 10 at the first gas-phase outlet of the heavy hydrocarbon separator 7, so as to supplement the methane gas into the liquid-phase refrigerant separator 10.

[0033] In actual application, the fifth heat exchange pipeline 514 is provided with a throttle valve and is connected to pipelines for supplementing nitrogen gas and mixed refrigerant respectively. When the device is just started, there is no mixed refrigerant capable of circulating operation in the buffer tank 1, so the mixed refrigerant can be added into the buffer tank 1 through the pipeline for supplementing mixed refrigerant, and then, when the device is normally operated and has no leakage, the mixed refrigerant can be returned to the buffer tank 1 through the fifth heat exchange pipeline 514 after providing cold energy, so as to realize the recycling of the mixed refrigerant, at this time, it is not necessary to add mixed refrigerant into the buffer tank 1 through the pipeline for supplementing mixed refrigerant. However, if there is leakage or other abnormal conditions of the device, which causes deviation of the components or content of the mixed refrigerant, the corresponding mixed refrigerant components can be supplemented into the buffer tank 1 through the pipelines for supplementing nitrogen gas and mixed refrigerant while the mixed refrigerant is returned through the fifth heat exchange pipeline 514.

[0034] The natural gas liquefaction method using the mixed refrigerant natural gas liquefaction device is as follows:

[0035] The mixed refrigerant in the buffer tank 1 is compressed and cooled by the compressor 2 and the air cooler 3 and then enters the precooling heat exchanger 4, and the raw material gas (natural gas) that has passed the purification treatment of deacidification, dehydration and dehydrocarbon enters the precooling heat exchanger 4 through the first natural gas pipeline 807. In the precooling heat exchanger 4, the natural gas and the mixed refrigerant are pre-cooled through the chilled water circulation pipeline.

[0036] The natural gas pre-cooled by the precooling heat exchanger 4 enters the main heat exchanger 8 through the second natural gas pipeline 808 for further heat exchange and cooling, and then enters the heavy hydrocarbon separator 7 through the first heat exchange pipeline 502 for heavy hydrocarbon removal treatment. The liquid-phase heavy hydrocarbon separated by the heavy hydrocarbon separator 7 flows out from the first liquid-phase outlet and enters the heavy hydrocarbon recovery device through the second heavy hydrocarbon separation pipeline 506. The gas phase separated by the heavy hydrocarbon separator 7 flows out from the first gas-phase outlet and enters the main heat exchanger 8 through the first heavy hydrocarbon separation pipeline 503 for further heat exchange and cooling. The over-cooled product gas flows out from the main heat exchanger 8, enters the liquefied natural gas recovery device through the second heat exchange pipeline 504, the first throttle valve HV1 and the first throttle pipeline 505.

[0037] The pre-cooled mixed refrigerant enters the mixed refrigerant separator 6 for gas-liquid separation after being pre-cooled by the pre-cooling heat exchanger 4. The gaseous refrigerant separated by the mixed refrigerant separator 6 flows out from the second gas outlet and enters the main heat exchanger 8 through the first refrigerant separation pipeline 805 for further heat exchange and cooling. Subsequently, it enters the gas phase refrigerant separator 11 through the fourth heat exchange pipeline 509, the third throttle valve HV3 and the third throttle pipeline 510 for gas-liquid separation. The gas and liquid separated by the gas phase refrigerant separator 11 enter the return refrigerant pipeline 513 in the main heat exchanger 8 and together form the first-stage mixed refrigerant. The first-stage mixed refrigerant further provides low-temperature cooling capacity to the main heat exchanger 8 through reheating and evaporation. The liquid refrigerant separated by the refrigerant separator 6 flows out from the second liquid outlet and enters the main heat exchanger 8 through the second refrigerant separation pipeline 806 for further heat exchange and cooling. Then, it enters the liquid refrigerant separator 10 through the third heat exchange pipeline 512, the second throttling valve HV2, and the second throttling pipeline 511 for gas-liquid separation. The gas and liquid separated by the liquid refrigerant separator 10 enter the return refrigerant pipeline 513 in the main heat exchanger 8 and together form the second-stage mixed refrigerant. The second-stage mixed refrigerant further provides low-temperature cooling to the main heat exchanger 8 through reheat evaporation. After flowing out of the main heat exchanger 8, both the first-stage and second-stage mixed refrigerants return to the buffer tank 1 through the fifth heat exchange pipeline 514 to achieve the refrigeration cycle of the mixed refrigerant.

[0038] In practical applications, the mixed refrigerant components in buffer tank 1 include methane, nitrogen, ethylene, propane, and isobutane. This invention employs chilled brine precooling because, compared to commonly used process units without precooling, chilled brine precooling significantly improves the compression efficiency of the refrigerant compressor, and is easier to debug and start up, resulting in stable operation.

[0039] Example

[0040] This embodiment utilizes a mixed refrigerant liquefied natural gas (LNG) unit to liquefy natural gas. The natural gas is a qualified feed gas that has undergone acid removal, dehydration, and hydrocarbon removal purification treatment, with the following parameters: temperature ~35℃, pressure 4.0~5.36MPa. The liquefaction process is as follows:

[0041] The mixed refrigerant in buffer tank 1 is at approximately 0°C, with the following composition and proportions: methane 21.2%, nitrogen 15.8%, ethylene 37.5%, propane 13%, and isobutane at a volume percentage of 12.5%. After compression and cooling by compressor 2 and air cooler 3, the mixed refrigerant reaches a temperature of approximately 50°C and then enters precooling heat exchanger 4. Simultaneously, feed gas (natural gas) enters precooling heat exchanger 4 via the first natural gas pipeline 807. The feed gas temperature is approximately 35°C, and the pressure is 4.0~5.36 MPa. Inside precooling heat exchanger 4, the natural gas and mixed refrigerant are precooled through a chilled water circulation pipeline. The parameters of the chilled water circulation pipeline are as follows.

[0042] The natural gas, pre-cooled by the pre-cooling heat exchanger 4 to ~3°C, enters the main heat exchanger 8 via the second natural gas pipeline 808 for further cooling to -50~-60°C. It then enters the heavy hydrocarbon separator 7 via the first heat exchange pipeline 502 for heavy hydrocarbon removal. The liquid heavy hydrocarbons separated by the heavy hydrocarbon separator 7 flow out from the first liquid phase outlet and enter the heavy hydrocarbon recovery unit via the second heavy hydrocarbon separation pipeline 506. The gaseous phase separated by the heavy hydrocarbon separator 7 flows out from the first gas phase outlet and enters the main heat exchanger 8 via the first heavy hydrocarbon separation pipeline 503 for further cooling to approximately -162°C. The subcooled product gas flows out from the main heat exchanger 8, passes through the second heat exchange pipeline 504, is depressurized by the throttle valve HV1, and then enters the liquefied natural gas storage unit via pipeline 505.

[0043] After being pre-cooled by the pre-cooling heat exchanger 4, the temperature of the mixed refrigerant drops to ~3℃ and enters the mixed refrigerant separator 6 for gas-liquid separation. The gaseous refrigerant separated by the mixed refrigerant separator 6 flows out from the second gas phase outlet and enters the main heat exchanger 8 through the first refrigerant separation pipeline 805 for further heat exchange and cooling to about -162℃. Then, it enters the third throttling valve HV3 through the fourth heat exchange pipeline 509 for throttling and pressure reduction, and then enters the gas phase refrigerant separator 11 through the third throttling pipeline 510 for gas-liquid separation. The gas and liquid separated by the gas phase refrigerant separator 11 enter the return refrigerant pipeline 513 in the main heat exchanger 8 and together form the first-stage mixed refrigerant. The first-stage mixed refrigerant further provides low-temperature cooling capacity to the main heat exchanger 8 through reheating and evaporation. The liquid refrigerant separated by the refrigerant separator 6 flows out from the second liquid outlet and enters the main heat exchanger 8 through the second refrigerant separation pipeline 806 for further heat exchange and cooling to about -50°C. Then, it enters the second throttling valve HV2 through the third heat exchange pipeline 512 for throttling and pressure reduction, and then enters the liquid refrigerant separator 10 through the second throttling pipeline 511 for gas-liquid separation. The gas and liquid separated by the liquid refrigerant separator 10 enter the return refrigerant pipeline 513 in the main heat exchanger 8 respectively and together form the second-stage mixed refrigerant. The second-stage mixed refrigerant further provides cooling capacity to the high-temperature section of the main heat exchanger 8 through reheating and evaporation. After the first-stage and second-stage mixed refrigerants flow out of the main heat exchanger 8, they both return to the buffer tank 1 through the fifth heat exchange pipeline 514 to achieve the refrigeration cycle of the mixed refrigerant.

[0044] This invention is based on the conventional mixed refrigerant natural gas liquefaction process. Its purpose is to improve the problem of difficult mixed refrigerant proportioning. It replaces isopentane, a heavy component required for mixed refrigerant proportioning, with isobutane. At the same time, it adds a chilled water precooling cycle to precool the feed gas and mixed refrigerant. This not only effectively avoids changes in the mixed refrigerant composition with changes in ambient temperature, making the process more stable, but also reduces the energy consumption of the refrigerant compressor and saves costs.

[0045] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the invention. Therefore, all technical solutions obtained through equivalent substitution or transformation fall within the protection scope of the present invention.

Claims

1. A mixed refrigerant liquefied natural gas plant pre-cooled with chilled brine, characterized by, The application relates to a natural gas liquefying device, which comprises a buffer tank (1), a precooling heat exchanger (4), a refrigerated brine unit (5), a mixed refrigerant separator (6), a heavy hydrocarbon separator (7), a main heat exchanger (8), a liquid-phase refrigerant separator (10) and a gas-phase refrigerant separator (11); the buffer tank (1) is used for containing mixed refrigerant, and the outlet of the buffer tank (1) is connected with the precooling heat exchanger (4) through a pipeline in sequence after passing through a compressor (2) and an air cooler (3); the mixed refrigerant can be precooled in the precooling heat exchanger (4) and then enter the mixed refrigerant separator (6); and the purified natural gas can enter the precooling heat exchanger (4) through a first natural gas pipeline (807) to be precooled; the precooling heat exchanger (4) is provided with a refrigerated brine circulating pipeline for precooling the mixed refrigerant and the natural gas, and the refrigerated brine circulating pipeline is connected with an external refrigerated brine unit (5); The natural gas pre-cooled through the precooling heat exchanger (4) can enter the main heat exchanger (8) through a second natural gas pipeline (808) to be heat-exchanged, and then enter the heavy hydrocarbon separator (7) through a first heat exchange pipeline (502); the heavy hydrocarbon separator (7) comprises a first liquid-phase outlet and a first gas-phase outlet; the first liquid-phase outlet is connected with a heavy hydrocarbon recovery device through a second heavy hydrocarbon separation pipeline (506); the first gas-phase outlet enters the main heat exchanger (8) through a first heavy hydrocarbon separation pipeline (503) to be heat-exchanged, and then is connected with a liquefied natural gas recovery device through a second heat exchange pipeline (504), a first throttling valve (HV1) and a first throttling pipeline (505); The mixed refrigerant separator (6) comprises a second liquid-phase outlet and a second gas-phase outlet; the second liquid-phase outlet is connected with the main heat exchanger (8) through a second refrigerant separation pipeline (806), and then is connected with the liquid-phase refrigerant separator (10) through a third heat exchange pipeline (512), a second throttling valve (HV2) and a second throttling pipeline (511); the third liquid-phase outlet and the third gas-phase outlet of the liquid-phase refrigerant separator (10) are respectively connected with a backflow refrigerant pipeline (513) in the main heat exchanger (8) through pipelines; the second gas-phase outlet is connected with the main heat exchanger (8) through a first refrigerant separation pipeline (805), and then is connected with the gas-phase refrigerant separator (11) through a fourth heat exchange pipeline (509), a third throttling valve (HV3) and a third throttling pipeline (510); the fourth liquid-phase outlet and the fourth gas-phase outlet of the gas-phase refrigerant separator (11) are respectively connected with the backflow refrigerant pipeline (513) in the main heat exchanger (8) through pipelines; the backflow refrigerant pipeline (513) is connected with the buffer tank (1) through a fifth heat exchange pipeline (514), and can make the mixed refrigerant heat-exchanged with the natural gas in the main heat exchanger (8) backflow to the buffer tank (1).

2. The mixed refrigerant LNG plant of claim 1, wherein, The precooling heat exchanger (4) and the main heat exchanger (8) are both plate-fin heat exchangers; and the compressor (2) is a screw compressor.

3. The mixed refrigerant LNG plant of claim 1 wherein, The mixed refrigerant separator (6), the heavy hydrocarbon separator (7), the liquid-phase refrigerant separator (10) and the gas-phase refrigerant separator (11) are all gas-liquid separators.

4. The mixed refrigerant LNG plant of claim 1 wherein, The main heat exchanger (8), the liquid-phase refrigerant separator (10) and the gas-phase refrigerant separator (11) are all arranged in a cold box (9) for heat preservation.

5. The mixed refrigerant LNG plant of claim 1 wherein, The fifth heat exchange pipeline (514) is provided with a cut-off valve and is connected with pipelines for supplementing nitrogen, ethylene, propane and isobutane respectively.

6. The mixed refrigerant LNG plant of claim 1 wherein, The first gas phase outlet of the heavy hydrocarbon separator (7) is further connected with the liquid phase refrigerant separator (10) through a methane supplement pipeline (501) for refluxing methane gas.

7. A method for liquefying natural gas using the mixed refrigerant liquefying natural gas apparatus according to any one of claims 1 to 6, characterized by, The specific implementation is as follows: The mixed refrigerant in the buffer tank (1) is compressed and cooled by the compressor (2) and the air cooler (3) and then enters the pre-cooling heat exchanger (4), and at the same time, the purified natural gas enters the pre-cooling heat exchanger (4) through the first natural gas pipeline (807), and in the pre-cooling heat exchanger (4), the natural gas and the mixed refrigerant are pre-cooled by the circulating pipeline of the chilled brine; The natural gas pre-cooled by the pre-cooling heat exchanger (4) enters the main heat exchanger (8) through the second natural gas pipeline (808) for further heat exchange and cooling, and then enters the heavy hydrocarbon separator (7) through the first heat exchange pipeline (502) for heavy hydrocarbon removal treatment; the liquid phase heavy hydrocarbon separated by the heavy hydrocarbon separator (7) flows out from the first liquid phase outlet and enters the heavy hydrocarbon recovery device through the second heavy hydrocarbon separation pipeline (506); the gas phase separated by the heavy hydrocarbon separator (7) flows out from the first gas phase outlet and enters the main heat exchanger (8) through the first heavy hydrocarbon separation pipeline (503) for further heat exchange and cooling; the supercooled product gas flows out from the main heat exchanger (8), enters the liquefied natural gas recovery device through the second heat exchange pipeline (504), the first throttling valve (HV1) and the first throttling pipeline (505); The mixed refrigerant pre-cooled by the pre-cooling heat exchanger (4) enters a mixed refrigerant separator (6) to be separated into gas and liquid phases. The gas phase refrigerant separated by the mixed refrigerant separator (6) flows out from the second gas phase outlet and enters the main heat exchanger (8) through the first refrigerant separation pipeline (805) to be further cooled. Subsequently, the refrigerant enters a gas phase refrigerant separator (11) through the fourth heat exchange pipeline (509), the third throttling valve (HV3) and the third throttling pipeline (510) to be separated into gas and liquid phases. The gas and liquid separated by the gas phase refrigerant separator (11) enter the reflux refrigerant pipeline (513) in the main heat exchanger (8) to form a first stage mixed refrigerant. The first stage mixed refrigerant is further evaporated to provide cold energy to the main heat exchanger (8). The liquid phase refrigerant separated by the mixed refrigerant separator (6) flows out from the second liquid phase outlet and enters the main heat exchanger (8) through the second refrigerant separation pipeline (806) to be further cooled. Subsequently, the refrigerant enters a liquid phase refrigerant separator (10) through the third heat exchange pipeline (512), the second throttling valve (HV2) and the second throttling pipeline (511) to be separated into gas and liquid phases. The gas and liquid separated by the liquid phase refrigerant separator (10) enter the reflux refrigerant pipeline (513) in the main heat exchanger (8) to form a second stage mixed refrigerant. The second stage mixed refrigerant is further evaporated to provide cold energy to the main heat exchanger (8). The first stage mixed refrigerant and the second stage mixed refrigerant flow out from the main heat exchanger (8) and are returned to the buffer tank (1) through the fifth heat exchange pipeline (514) to realize the refrigeration cycle of the mixed refrigerant.

8. The natural gas liquefaction process of claim 7, wherein, The mixed refrigerant components in the buffer tank (1) include methane, nitrogen, ethylene, propane and isobutane.

9. The natural gas liquefaction process of claim 7, wherein, The medium in the refrigerated brine circulation pipeline is refrigerated brine. The medium in the refrigerated brine circulation pipeline is refrigerated brine.

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