CN202494271U - Device for liquefying natural gas through refrigeration of single mixed working medium - Google Patents

Abstract

The utility model relates to a device for liquefying natural gas by using a single mixed working medium to refrigerate. The device includes a two-stage mixed working medium compressor driven by a motor, two coolers, a liquid pump, four gas-liquid separators, Two throttling devices, a set of plate-fin heat exchangers and an LNG storage tank, the process adopted by the utility model compresses and separates the mixed refrigerant step by step, reducing the power consumption of gas compression; and adopts multiple Stage heat exchange makes the heat transfer curves of the cold fluid and the hot fluid in the whole heat exchange process more matched, effectively reducing the flow rate of the mixed refrigerant; in addition, the process adopted by the utility model is well adapted to the variable load operation of the device It can effectively solve the problem of liquid accumulation at the bottom of the cold box.

Description

A device for refrigerating liquefied natural gas using a single mixed working medium

technical field

The utility model relates to the liquefaction production of gas rich in hydrocarbons, in particular to a device for refrigerating liquefied natural gas with a single mixed working medium.

Background technique

Due to its environmental protection, natural gas has become the best substance to replace other fuels, and its application fields have gradually expanded to power generation, automobile gas, industrial gas, urban residential gas, chemical gas, etc.

With the growth of natural gas consumption, as one of the most efficient forms of supply and use of natural gas, the trade volume of liquefied natural gas has become one of the fastest growing areas of the energy market. The continuous development of the liquefied natural gas industry puts forward higher requirements on energy consumption, investment and efficiency of natural gas liquefaction methods and devices.

At present, relatively mature natural gas liquefaction processes mainly include: cascade refrigeration process, expansion refrigeration process and mixed refrigerant refrigeration process. Among them, the single mixed working fluid refrigeration process is more favored by medium-sized LNG installations.

In the existing natural gas liquefaction method with single mixed working medium refrigeration, the refrigerant compression system is two-stage compression, and the natural gas liquefaction adopts one-stage heat exchange.

Existing process technology: as shown in Figure 1, the device used includes a motor-driven two-stage mixed working medium compressor, two coolers, two gas-liquid separators, two liquid pumps, and a plate Fin heat exchanger and an LNG storage tank; the mixed working medium composed of C1~C5 and N2 enters the inlet of the compressor after a reasonable ratio, and is compressed to 0.6~1MPa in one stage, and then enters the primary cooler to cool to 30 ~40°C, then enter the first-stage gas-liquid separation tank for gas-liquid separation, the gas separated from the top of the first-stage gas-liquid separation tank continues to enter the second-stage inlet of the compressor, and is compressed to 1.6-2.5MPa by the second-stage, and the first-stage separation The liquid separated at the bottom is pressurized by the liquid pump and mixed with the gas from the outlet of the second-stage compressor to enter the secondary cooler to cool to 30-40°C. The cooled mixed working medium then enters the secondary gas-liquid separation tank for gas-liquid separation , the separated liquid is pressurized by the secondary liquid pump, mixed with the gas obtained from the top of the separator, and then enters the plate-fin heat exchanger, pre-cooled to a certain temperature, throttled and then returned to the plate-fin heat exchanger, as The whole heat exchange process provides cooling capacity, and the natural gas enters the LNG storage tank after passing through the plate-fin heat exchanger.

In the above process, in order to ensure that liquid and gas enter the same plate-fin heat exchanger channel to participate in heat exchange, the liquid at the bottom of the final gas-liquid separator must be pressurized to overcome the liquid outlet at the bottom of the separator to the plate-fin heat exchanger. The liquid column pressure brought by the height difference of the top refrigerant inlet must be realized by adding the final liquid pump. The heat exchange process between the refrigerant and natural gas in the plate-fin heat exchanger is a first-stage heat exchange. The optimization of the heat exchange temperature difference between the streams is limited to a certain extent, and the energy consumption of the device is high. Moreover, this process is prone to produce the bottom of the cold box. Effusion does not have good adaptability to the variable load operation of the device.

Utility model content

The utility model uses a single mixed working medium to refrigerate the natural gas.

The utility model provides a device for refrigerating liquefied natural gas using a single mixed working medium, which includes a mixed refrigerant compression system and a cold box system, wherein the mixed refrigerant compression system uses a two-stage mixed working medium compressor for compression, and the compression The system includes a two-stage mixed working medium compressor, two coolers (namely a first cooler and a second cooler) respectively connected to the first section and the second section of the two-stage mixed working medium compressor, The first gas-liquid separator and the second gas-liquid separator connected to the first cooler and the second cooler respectively, and the first gas-liquid separator among the two gas-liquid separators A liquid pump connected to the device, wherein the cold box system includes a group of plate-fin heat exchangers connected to the liquid phase end of the second gas-liquid separator in the two gas-liquid separators, and the Two throttling devices connected to the heat exchange passages of the plate-fin heat exchanger group, a refrigerant separator connected to the two heat exchange passages of the plate-fin heat exchanger group and one of the above throttling devices, and A natural gas heavy hydrocarbon separator connected to an independent heat exchange channel of the plate-fin heat exchanger group, wherein the gas phase end of the first gas-liquid separator among the two gas-liquid separators is mixed with the second-stage The second compression section of the working fluid compressor is connected, and the liquid phase end of the first gas-liquid separator is connected to the second of the two coolers after being merged with the outlet pipeline of the second compression section through a liquid pump Cooler, the gas phase end and liquid phase end of the second gas-liquid separator are respectively connected to the two heat exchange channels of the set of plate-fin heat exchangers; the above-mentioned first throttling device is connected to the refrigerant separation The gas phase end and the liquid phase end of the device are connected to a heat exchange channel of the plate-fin heat exchanger group, and the other end of the other throttling device is connected to a heat exchange channel and then connected to the middle of the refrigerant separator , the natural gas pipeline is connected to the heavy hydrocarbon separator through the independent heat exchange channel of the heat exchanger group, and the top gas phase end of the heavy hydrocarbon separator is connected to the LNG storage tank after passing through a heat exchange channel of the heat exchanger group.

The utility model adopts a single mixed working medium refrigeration device to liquefy natural gas, which is divided into a natural gas cycle and a mixed working medium refrigeration cycle. In the mixed working medium circuit, the mixed working medium is accompanied by step-by-step gas-liquid separation during its step-by-step compression process. The liquid phase stream separated by the first-stage compression does not participate in the subsequent compression process, effectively reducing the subsequent gas Compression power consumption; the gas phase and liquid phase mixed working medium streams obtained by compression enter different channels of the heat exchanger group to throttling heat exchange, compared with the traditional process, the final liquid pump is omitted (that is, only one liquid pump is used) ), and the use of secondary heat transfer makes the heat transfer curves of the hot stream and the cold stream better match in the whole process; Effusion.

The device for liquefying natural gas by using a single mixed working medium refrigeration described in the utility model includes a two-stage mixed working medium compressor, a cooler, a gas-liquid separator, a throttling device, a group of plate-fin heat exchanger groups and a Taiwan LNG storage tank. Its mixed refrigerant compression system includes a two-stage mixed working medium compressor, two coolers, two gas-liquid separators and a liquid pump. The cold box system includes a set of plate-fin heat exchangers, Two gas-liquid separators (one heavy hydrocarbon separator and one refrigerant separator) and two throttling devices; the mixed working fluid and natural gas complete the entire heat exchange process in the cold box system.

In the mixed refrigerant compression system, the outlet of the first compression section of the compressor is connected to the first-stage cooler, and the first-stage cooler is then connected to the first-stage gas-liquid separator, and the gas phase of the first-stage gas-liquid separator The end of the gas-liquid separator is connected to the second compression section, and the bottom liquid phase end of the first-stage gas-liquid separator is connected to a liquid pump. The output pipe of the liquid pump is connected to the second-stage cooler after the outlet pipe of the second compression section is merged. , the second-stage cooler is connected with the second-stage gas-liquid separator, and the top gas-phase end of the second-stage gas-liquid separator is connected with the first heat exchange channel (gas-phase channel) of the heat exchanger group; the second-stage gas-liquid The bottom liquid phase end of the separator is connected with the second heat exchange channel of the heat exchanger group;

In the cold box system, the liquid phase end of the second-stage gas-liquid separator derived from the mixed refrigerant compression system is connected to one end of the first throttling device through the second heat exchange channel in the heat exchanger group, and the first section The other end of the flow device is connected to the refrigerant separator, and the top gas phase end and the bottom liquid phase end of the refrigerant separator are connected to the third heat exchange channel and then connected to the first compression section; the gas phase obtained at the top of the second-stage gas-liquid separator The end is pre-cooled through the first heat exchange channel (gas phase channel) of the heat exchanger group, and then connected to one end of the second throttling device, and the other end of the second throttling device is connected to the fourth heat exchange channel of the heat exchanger group Connect the refrigerant separator; the natural gas pipeline is connected to the heavy hydrocarbon separator through the fifth heat exchange channel of the heat exchanger group, and the top gas phase end of the heavy hydrocarbon separator passes through the remaining stages of heat exchangers in the heat exchanger group (such as the first Six heat exchange channels) and then connected to the LNG storage tank.

The technical scheme of the utility model is summarized as follows:

A device that uses a single mixed working medium to refrigerate liquefied natural gas, including a mixed refrigerant compression system and a cold box system,

Wherein the compression system includes a two-stage mixed working medium compressor, a first cooler and a second cooler respectively connected to the first section and the second section of the two-stage mixed working medium compressor, respectively connected to The first gas-liquid separator and the second gas-liquid separator connected to the first cooler and the second cooler and a liquid connected to the first of the two gas-liquid separators Pump,

The cold box system includes: a group of plate-fin heat exchangers, which contains at least six heat exchange channels, that is, at least the first, second, third, fourth, fifth and sixth heat exchange channels, so The input ends of the second heat exchange channel and the first heat exchange channel are respectively connected to the liquid phase end and the gas phase end of the second gas-liquid separator in the mixed refrigerant compression system through two pipes, and connected to the third heat exchange channel The output end of the hot channel is connected to the first compression section via a pipeline;

A first throttling device connected to the output end of the second heat exchange channel of the plate-fin heat exchanger group;

A second throttling device connected to the output end of the first heat exchange channel and the input end of the fourth heat exchange channel of the plate-fin heat exchanger group;

and

A refrigerant separator connected to the input end of the third heat exchange passage, the output end of the fourth heat exchange passage and the first throttling device of the plate-fin heat exchanger group;

A natural gas heavy hydrocarbon separator connected to an independent heat exchange channel of the plate-fin heat exchanger group, that is, the fifth heat exchange channel,

Among the two gas-liquid separators, the gas phase end of the first gas-liquid separator is connected to the second compression section of the two-stage mixed working medium compressor, and the liquid phase end of the first gas-liquid separator is connected to the first gas-liquid separator through a liquid pump. The outlet pipes of the second compression section are connected to the second cooler of the two coolers after being merged, and the gas-phase end and liquid-phase end of the second gas-liquid separator exchange heat with the set of plate-fin heat exchangers respectively. The two heat exchange channels of the group, that is, the input ends of the first heat exchange channel and the second heat exchange channel are connected; the above-mentioned first throttling device is connected to the refrigerant separator, and the top gas phase end and the bottom of the refrigerant separator The liquid phase ends are connected to the input end of the third heat exchange channel after confluence, the output end of the third heat exchange channel is connected to the first stage of the two-stage mixed working medium compressor, and the above-mentioned second throttling device is connected to the fourth heat exchange channel The input end is connected to the refrigerant separator, and the pipeline for transporting purified natural gas is connected to the heavy hydrocarbon separator through the above-mentioned independent heat exchange channel of the heat exchanger group, that is, the fifth heat exchange channel, and the top gas phase end of the heavy hydrocarbon separator is A heat exchange channel of the heat group, that is, the sixth heat exchange channel, is connected to the liquefied natural gas storage tank afterward.

In addition, the gas phase end at the top of the heavy hydrocarbon separator passes through the sixth heat exchange passage of the heat exchanger group in turn, and then further passes through another seventh heat exchange passage of the heat exchanger group to connect to the LNG storage tank.

The "first stage of compression" or "one stage of compression" and "first stage of compression" mentioned here can be used interchangeably, and so on.

The operation procedure is described below.

The device of the two-stage mixed working medium compression system described in the utility model, in the mixed refrigerant compression system, the outlet gas of the first stage of the compressor enters the first stage cooler to be cooled and then passes through the first stage gas-liquid separation The separated gas phase continues to enter the second compression section, and the separated liquid phase is pressurized by the liquid pump and merges with the hot gas compressed in the second section, and then enters after being cooled by the second stage cooler. Gas-liquid separation is carried out in the second-stage gas-liquid separator, and the separated gas phase enters the first heat exchange channel (gas phase channel) of the downstream heat exchanger; the liquid obtained at the bottom of the second-stage gas-liquid separator enters the downstream heat exchanger respectively The second liquid phase heat exchange channel. In the cold box system, the liquid refrigerant from the bottom of the second-stage gas-liquid separator of the refrigerant compression system enters the second liquid-phase heat exchange channel of the heat exchanger group to be pre-cooled and then passes through the first throttling device. The throttling stream enters the refrigerant separator and is divided into two phases of gas and liquid. After the two phases of gas and liquid are combined, they return to the third heat exchange channel of the heat exchanger group to provide cooling capacity. The flow out of the third heat exchange channel The mixed refrigerant is sent back to the first compression section. The gas-phase refrigerant from the top of the second-stage gas-liquid separator is pre-cooled through the first heat exchange channel of the heat exchanger group, and then flows through the second throttling device for throttling, and the throttling stream is reversed. Enter the fourth heat exchange channel of the heat exchanger group, reheat to a certain temperature, and lead to the refrigerant separator. The natural gas first passes through the fifth heat exchange channel of the heat exchanger group, is cooled to a certain temperature, and then enters the heavy hydrocarbon separator for separation. The heavy hydrocarbon components are obtained at the bottom, and the gas phase part obtained at the top continues to enter the remaining stages of the heat exchanger group. The heat exchanger (such as the sixth heat exchange channel) conducts heat exchange, cools to a supercooled state, and the obtained LNG enters the LNG storage tank for storage.

The process flow of the device for liquefied natural gas using single mixed working medium refrigeration is as follows:

Natural gas cycle:

The purified raw natural gas first enters the fifth heat exchange channel of the plate-fin heat exchanger group for pre-cooling, and after being cooled to -30°C ~ -80°C, it enters the heavy hydrocarbon separator for gas-liquid separation. The gas phase stream separated at the top continues to enter the heat exchangers of the remaining stages of the heat exchanger group (such as the sixth heat exchange channel), where it is cooled to -130°C ~ -166°C, and the obtained liquefied natural gas is sent to LNG Store in tanks.

Mixed refrigerant cycle:

Mixed working fluid consisting of C1～C5 and N2, that is, four, five or six selected from C1, C2, C3, C4 and C5 alkanes and _N2 , they are in any volume ratio or in approximately equivalent The volume ratio enters the inlet of the compressor, is compressed to 0.6-1.8MPaA in the first stage, enters the first-stage cooler to cool to 30°C-40°C, and then enters the first-stage gas-liquid separator for gas-liquid separation. The gas separated from the top of the first-stage gas-liquid separator continues to enter the inlet of the second stage of the compressor, and is compressed to 1.2-5.4MPaA by the second stage, and the liquid separated from the liquid phase end at the bottom of the first-stage gas-liquid separator is pressurized by a liquid pump After merging with the hot gas from the second-stage compressed outlet pipeline, it enters the second-stage cooler to be cooled to 30°C-40°C, and the cooled mixed working medium then enters the second-stage gas-liquid separator for gas-liquid separation , the top gas of the second-stage gas-liquid separator then enters the first heat exchange channel of the main heat exchanger group to participate in heat exchange, and the liquid separated from the bottom of the second-stage gas-liquid separator enters the second heat exchange channel of the main heat exchanger group The hot channel participates in heat exchange;

The liquid drawn from the bottom of the second-stage gas-liquid separator of the mixed working medium compression system first enters the second heat exchange channel of the heat exchanger group, where it is pre-cooled to about -30 ° C ~ -80 ° C, and passes through the first section After the flow valve is throttled to 0.2-0.8MPaA, it enters the refrigerant separator, and the gas-phase stream of the mixed working medium separated from the top of the second-stage gas-liquid separator passes through the gas-phase channel of the heat exchanger group (the first heat exchange channel) Cool to -135°C ~ -169°C, then throttling to 0.2 ~ 0.8MPaA through the second throttle valve, and then reversely enter the fourth heat exchange channel of the heat exchanger group to provide cooling capacity for the heat exchanger, reheat to - After 30℃～-80℃, the heat exchanger group is drawn out and enters the middle part of the refrigerant separator to be mixed with the above-mentioned flow stream that has been throttled by the first-stage throttle valve and also enters the refrigerant separator. The two are in the refrigerant separator. It is separated into two phases of gas and liquid, and after the two phases of gas and liquid are merged, they enter the third-stage heat exchange channel in reverse to provide cooling capacity for the heat exchanger group, and then return to the first compression section.

Here, the pressure unit MPaA is megapascal, absolute pressure. In the present invention, generally speaking, the connection between one device and another is realized through pipelines.

Advantage of the utility model:

1. It has good adaptability to the variable load operation of the device. The final gas phase enters the refrigerant separator after being throttled back and reheated, which can effectively avoid liquid accumulation at the bottom of the cold box, thereby ensuring low-load conditions , the energy consumption of the product is close to the energy consumption under normal working conditions.

2. The utility model adopts a two-stage mixed refrigerant compressor, which compresses and separates the mixed refrigerant step by step, reducing the power consumption of gas compression.

3. The liquid stream at the bottom of the first-stage gas-liquid separator does not participate in the subsequent compression process, which reduces the influence of the fluctuation of the mixed refrigerant ratio on the operating conditions of the compressor unit to a certain extent, making the whole device easier to operate.

4. The use of two-stage heat exchange makes the heat transfer curves of the cold fluid and the hot fluid better match in the entire heat exchange process, effectively reducing the flow rate of the mixed refrigerant.

Description of drawings

Fig. 1 is a kind of structural diagram of prior art;

Fig. 2 is a device configuration diagram of the mixed working medium refrigeration system of the present invention.

Among them: 1 two-stage mixed working medium compressor, 6 heavy hydrocarbon separator, 7 refrigerant separator, 21, 22 cooler, 31, 32 gas-liquid separator, 4, 4' liquid pump, 8 plate-fin exchanger Heater, 9LNG storage tank, 51, 52 throttle valve

Detailed ways

Further description below in conjunction with accompanying drawings.

The device shown in Figure 2 comprises a two-stage mixed working medium compressor 1, a first cooler 21, a second cooler 22, a first gas-liquid separator 31, a second gas-liquid separator 32, two gas-liquid separators device (heavy hydrocarbon separator 6, refrigerant separator 7), liquid pump 4, first throttling device 51, second throttling device 52, a group of plate-fin heat exchanger groups 8 (namely, the main heat exchanger group ) and an LNG storage tank 9. Its mixed refrigerant compression system includes a two-stage mixed working medium compressor 1, two coolers 21, 22, two gas-liquid separators 31, 32, and a liquid pump 4. The cold box system includes a set of Plate-fin heat exchanger group 8 (two-stage heat exchange), a heavy hydrocarbon separator 6, a refrigerant separator 7 and two throttling devices 51, 52; mixed working fluid and natural gas in the cold box system Complete the entire heat transfer process. In the mixed refrigerant compression system, the outlet of the first stage of the compressor 1 is connected to the first-stage cooler 21, and the first-stage cooler 21 is connected to the first-stage gas-liquid separator 31, and the first-stage gas-liquid separator The gas phase end of 31 is connected to the second compression section, the liquid phase end at the bottom of the first-stage gas-liquid separator 31 is connected to the liquid pump 4, and the output end of the liquid pump 4 is connected to the second compression section after the outlet pipeline merges. The first stage cooler 22, the second stage cooler 22 is connected with the second stage gas-liquid separator 32 again, the gas phase end of the top of the second stage gas liquid separator 32 and the first heat exchange channel (gas phase channel) of the heat exchanger group 8 connection; the liquid phase end at the bottom of the second stage gas-liquid separator 32 is connected with the second liquid phase heat exchange channel of the heat exchanger group 8;

In the cold box system, the liquid phase end of the second-stage gas-liquid separator 32 from the mixed refrigerant compression system is connected to one end of the first throttling device 51 through the second heat exchange channel in the heat exchanger group 8, and the second The other end of a throttling device 51 is connected to the middle of the refrigerant separator 7, and the gas phase end at the top of the refrigerant separator 7 and the liquid phase end at the bottom are connected to the third heat exchange channel of the heat exchanger group 8 and then connected to the first Compression section; the gas phase end obtained from the top of the secondary gas-liquid separator 32 is precooled through the first heat exchange channel of the heat exchanger group 8, and then connected to one end of the second throttling device 52, and the other end of the second throttling device 52 One end is connected to the fourth heat exchange channel of the heat exchanger group 8 and then connected to the middle part of the refrigerant separator 7; the natural gas pipeline is connected to the heavy hydrocarbon separator 6 through the fifth heat exchange channel of the heat exchanger group, and the heavy hydrocarbons are separated The gas phase end at the top of the tank 6 is connected to the LNG storage tank 9 after successively passing through the remaining stages of heat exchangers of the heat exchanger group 8 (for example, the sixth heat exchange passage, and optionally the seventh heat exchange passage).

The technological process of using the device of Fig. 2 is as follows:

Natural gas cycle:

As shown in Figure 2, the purified raw natural gas first enters the fifth heat exchange channel in the plate-fin main heat exchanger group 8 to be pre-cooled, and then enters heavy hydrocarbons after being cooled to -30°C ~ -80°C Gas-liquid separation is carried out in the separator 6, and the gas-phase stream separated from the top of the heavy hydrocarbon separator 6 continues to enter the remaining heat exchangers of the main heat exchanger group 8, and is cooled to -130°C~-166°C in it. °C, the obtained liquefied natural gas is sent to the LNG storage tank 9 for storage, and the bottom of the heavy hydrocarbon separator 6 obtains liquefied petroleum gas (LPG).

Mixed refrigerant cycle:

Mixed working fluid consisting of C1～C5 and N2, that is, four, five or six selected from C1, C2, C3, C4 and C5 alkanes and _N2 , they are in any volume ratio or in approximately equivalent The volume ratio enters the inlet of compressor 1, undergoes a stage of compression to 0.6-1.8MPaA, enters the primary cooler 21 to cool to 30°C-40°C, and then enters the primary gas-liquid separator 31 for gas-liquid separation. The gas separated from the top of the liquid separator 31 continues to enter the second-stage inlet of the compressor, and is compressed to 1.2-5.4MPaA through the second stage. After compression, it merges with the hot gas from the outlet of the second-stage compression, and then enters the second-stage cooler 22 to cool to 30°C to 40°C, and the cooled mixed working medium then enters the second-stage gas-liquid separator 32 for gas-liquid separation The gas at the top of the second-stage gas-liquid separator 32 then enters the first heat exchange channel of the main heat exchanger group 8 to participate in heat exchange, and the liquid separated from the bottom of the second-stage gas-liquid separator 32 enters the main heat exchanger group 8 The second heat exchange channel participates in heat exchange;

The liquid drawn from the bottom of the second-stage gas-liquid separator 32 of the mixed working fluid compression system first enters the second heat exchange channel of the heat exchanger group, where it is pre-cooled to about -30°C to -80°C, and is throttled. The valve 51 is throttled to 0.2-0.8MPaA and enters the refrigerant separator 7 to be separated into gas-liquid two-phase. The hot passage) provides cooling capacity for the heat exchanger group 8, and then returns to the first compression section. The gas-phase stream of the mixed working medium separated from the top of the second-stage gas-liquid separator 32 is cooled to -135°C ~ -169°C through the gas-phase channel of the heat exchanger group 8 (ie, the first heat exchange channel), and then passed through the second The second throttling valve 52 throttles to 0.2~0.8MPaA, then reversely enters the fourth heat exchange channel of the heat exchanger group 8 to provide cooling capacity for the heat exchanger group, and after reheating to -30°C~-80°C, leads out The heater group enters the middle part of the refrigerant separator 7, and merges with the liquid phase at the bottom of the above-mentioned secondary gas-liquid separator 32 after cooling and throttling, and as mentioned above, the gas phase at the top of the refrigerant separator 7 After merging with the bottom liquid phase, it reversely enters the previous heat exchanger (that is, the third heat exchange channel) to provide cooling capacity for the heat exchanger group 8, and then returns to the first compression section.

Claims (2)

1. A device for refrigerating liquefied natural gas using a single mixed working medium, characterized in that the device includes a mixed refrigerant compression system and a cold box system, Wherein the compression system includes a two-stage mixed working medium compressor, a first cooler and a second cooler respectively connected to the first section and the second section of the two-stage mixed working medium compressor, respectively connected to The first gas-liquid separator and the second gas-liquid separator connected to the first cooler and the second cooler and a liquid connected to the first of the two gas-liquid separators Pump, The cold box system includes: A group of plate-fin heat exchangers comprising at least six heat exchange channels: first, second, third, fourth, fifth and sixth heat exchange channels, the second heat exchange channel and the first The input end of the heat exchange channel is respectively connected to the liquid phase end and the gas phase end of the second gas-liquid separator in the mixed refrigerant compression system via two pipelines, and the output end of the third heat exchange channel is connected to the the first compression section; A first throttling device connected to the output end of the second heat exchange channel of the plate-fin heat exchanger group; A second throttling device connected to the output end of the first heat exchange channel and the input end of the fourth heat exchange channel of the plate-fin heat exchanger group; and A refrigerant separator connected to the input end of the third heat exchange passage, the output end of the fourth heat exchange passage and the first throttling device of the plate-fin heat exchanger group; A natural gas heavy hydrocarbon separator connected to an independent heat exchange channel of the plate-fin heat exchanger group, that is, the fifth heat exchange channel, Among the two gas-liquid separators, the gas phase end of the first gas-liquid separator is connected to the second compression section of the two-stage mixed working medium compressor, and the liquid phase end of the first gas-liquid separator is connected to the first gas-liquid separator through a liquid pump. The outlet pipes of the second compression section are connected to the second cooler of the two coolers after being merged, and the gas-phase end and liquid-phase end of the second gas-liquid separator exchange heat with the set of plate-fin heat exchangers respectively. The two heat exchange channels of the group, that is, the input ends of the first heat exchange channel and the second heat exchange channel are connected; the above-mentioned first throttling device is connected to the refrigerant separator, and the top gas phase end and the bottom of the refrigerant separator The liquid phase ends are connected to the input end of the third heat exchange channel after confluence, the output end of the third heat exchange channel is connected to the first stage of the two-stage mixed working medium compressor, and the above-mentioned second throttling device is connected to the input end of the fourth heat exchange channel The end is connected to the refrigerant separator. The pipeline for transporting purified natural gas is connected to the heavy hydrocarbon separator through the above-mentioned independent heat exchange channel of the heat exchanger group, that is, the fifth heat exchange channel. The top gas phase end of the heavy hydrocarbon separator passes through the heat exchanger in turn A heat exchange channel of the heat group, that is, the sixth heat exchange channel, is connected to the liquefied natural gas storage tank afterward. 2. The device for refrigerating liquefied natural gas using a single mixed working medium according to claim 1, characterized in that: The gas phase end at the top of the heavy hydrocarbon separator sequentially passes through the sixth heat exchange channel of the heat exchanger group and then further passes through another seventh heat exchange channel of the heat exchanger group to be connected to the LNG storage tank.

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