CN113701446A - Natural gas liquefaction system with supersonic two-phase expansion refrigeration cycle - Google Patents

Abstract

The invention provides a natural gas liquefaction system for a supersonic two-phase expansion refrigeration cycle, comprising a plurality of counter-current heat exchangers and a plurality of supersonic two-phase expanders, and the natural gas flows through the plurality of counter-current heat exchangers in sequence and forms a first A circulation pipeline, each of the supersonic two-phase expanders is arranged between the adjacent counter-flow heat exchangers, and forms a plurality of refrigeration units; wherein, the refrigerant passes through the plurality of refrigeration units in sequence to make The gaseous natural gas introduced into the first circulation pipeline is liquefied. Through the above method, the supersonic two-phase expander is used as the expansion and cooling device, which has the advantages of high expansion refrigeration efficiency, low pressure drop, low energy consumption, simple and compact structure, safe and reliable without moving parts, and low processing difficulty, and greatly reduces the number of equipment. , simplify the system process and improve the system efficiency.

Description

Natural gas liquefaction system of supersonic two-phase expansion refrigeration cycle

technical field

The invention relates to the technical field of liquefaction systems, in particular to a natural gas liquefaction system of a supersonic two-phase expansion refrigeration cycle.

Background technique

As a clean and environmentally friendly energy, natural gas has been widely used in the world in recent years. The world is rich in natural gas resources, but some large natural gas wells are mostly located in desert areas, far from densely populated and industrially developed areas, and are often blocked by large areas of ocean and complex topography, long-distance pipelines, and even transoceanic gaseous states. The transportation of natural gas is often restricted by cost and technical problems. Therefore, the long-distance transportation of natural gas is mostly carried out by means of liquefied natural gas (LNG). It is of great significance to discuss the natural gas liquefaction process.

At present, there are many natural gas liquefaction processes, among which the common ones are the cascade refrigeration cycle liquefaction process, the mixed refrigerant refrigeration cycle liquefaction process and the expander refrigeration cycle liquefaction process, but all have corresponding problems. The cascade refrigeration cycle natural gas liquefaction process is complex, with many units, huge systems, and difficult maintenance and management investment; the mixed refrigerant refrigeration cycle natural gas liquefaction process has multiple gas-liquid separators and throttle valves, the equipment is more complex, and the control and management are difficult. In the natural gas liquefaction process of the expander refrigeration cycle, the expander has the hidden danger of unsafe and unstable operation of mechanical moving parts, and the expander consumes a lot of power and the system efficiency is low.

SUMMARY OF THE INVENTION

The embodiments of the present invention provide a natural gas liquefaction system with a supersonic two-phase expansion refrigeration cycle, which is used to solve the technical problems of complex processes and low system efficiency in the natural gas liquefaction system in the prior art.

The embodiment of the present invention provides a natural gas liquefaction system for a supersonic two-phase expansion refrigeration cycle, comprising: a plurality of countercurrent heat exchangers, and natural gas flows through the plurality of countercurrent heat exchangers in sequence to form a first circulation pipeline;

A plurality of supersonic two-phase expanders, each supersonic two-phase expander is arranged between the adjacent counter-flow heat exchangers, and forms a plurality of refrigeration units; wherein,

The refrigerant passes through a plurality of the refrigeration units in sequence to liquefy the gaseous natural gas introduced into the first circulation pipeline.

According to a natural gas liquefaction system of a supersonic two-phase expansion refrigeration cycle according to an embodiment of the present invention, each of the refrigeration units includes one of the countercurrent heat exchangers and one of the supersonic two-phase expanders;

At least one group of gaseous heat exchange tubes and low temperature heat exchange tubes are arranged in the countercurrent heat exchanger, the gaseous heat exchange tubes are used for transporting gaseous working medium towards the supersonic two-phase expander, and the low temperature heat exchange tubes are used for receiving The liquid working substance formed through the supersonic two-phase expander.

According to a natural gas liquefaction system of a supersonic two-phase expansion refrigeration cycle according to an embodiment of the present invention, the gaseous working medium flows through a plurality of the refrigeration units to form a gas circulation pipeline, and the liquid working medium flows out of the refrigeration units to form a low temperature return line;

The gas circulation pipeline and the low temperature return pipeline are connected end to end.

According to a natural gas liquefaction system of a supersonic two-phase expansion refrigeration cycle according to an embodiment of the present invention, the supersonic two-phase expander includes an air inlet, an air outlet and a liquid outlet;

The air inlet and the air outlet are communicated with the gas circulation pipeline, and the liquid outlet is communicated with the low temperature return pipeline.

According to a natural gas liquefaction system of a supersonic two-phase expansion refrigeration cycle according to an embodiment of the present invention, the supersonic two-phase expander includes a cyclone mechanism, a nozzle, a cyclone separation pipe, a liquid discharge mechanism and a diffuser connected in sequence ;

The air inlet is communicated with the swirl mechanism, and the swirling mechanism generates centrifugal force to form a low temperature effect on the gaseous working medium entering through the air inlet in the nozzle, and in the swirl separation pipe The generated liquid working medium flows to the low temperature return line through the liquid discharging mechanism, and the gaseous working medium flows to the gas circulation line through the diffuser.

According to the natural gas liquefaction system of the supersonic two-phase expansion refrigeration cycle according to an embodiment of the present invention, a compressor and a cooler are arranged on the low-temperature return line flowing toward the gas circulation line;

The inlet side of the compressor is communicated with the low temperature return line, the outlet side of the compressor is communicated with the inlet side of the cooler, and the outlet side of the cooler is communicated with the gas circulation line.

According to the natural gas liquefaction system of the supersonic two-phase expansion refrigeration cycle according to an embodiment of the present invention, along the direction of the first circulation pipe, the refrigerant in the low temperature return pipe in the plurality of countercurrent heat exchangers The temperature of the working medium is decreased to decrease the temperature of the natural gas in the first circulation pipeline.

According to the natural gas liquefaction system of the supersonic two-phase expansion refrigeration cycle according to an embodiment of the present invention, a throttle valve is further provided at the connection between the gas circulation pipeline and the low-temperature return pipeline after passing through the plurality of refrigeration units;

The inlet side of the throttle valve is communicated with the gas outlet of the supersonic two-phase expander, and the outlet side of the throttle valve is communicated with the low temperature return line.

According to the natural gas liquefaction system of the supersonic two-phase expansion refrigeration cycle according to an embodiment of the present invention, the number of the refrigeration units is three.

According to a natural gas liquefaction system of a supersonic two-phase expansion refrigeration cycle according to an embodiment of the present invention, the refrigerant includes nitrogen and one or more of butane, propane, ethylene, and methane combined with the nitrogen.

The natural gas liquefaction system of the supersonic two-phase expansion refrigeration cycle provided by the embodiment of the present invention includes a plurality of countercurrent heat exchangers and a plurality of supersonic two-phase expanders, and the countercurrent heat exchangers and the supersonic two-phase expanders cooperate to form multiple There is a refrigeration unit, whereby the gaseous natural gas circulating in the first circulation pipeline will exchange heat with the refrigeration unit for refrigeration, thereby producing a liquefaction effect to realize the liquefaction of the natural gas. The application uses a supersonic two-phase expander as the expansion and cooling device, which has the advantages of high expansion refrigeration efficiency, low pressure drop, low energy consumption, simple and compact structure, safe and reliable without moving parts, and low processing difficulty, greatly reducing the number of equipment, simplifying System flow, improve system efficiency.

Description of drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the accompanying drawings that need to be used in the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description These are some embodiments of the present invention. For those of ordinary skill in the art, other drawings can also be obtained according to these drawings without creative efforts.

1 is a schematic structural diagram of the composition of a natural gas liquefaction system of a supersonic two-phase expansion refrigeration cycle according to an embodiment of the present invention;

Fig. 2 is the structural representation of the supersonic two-phase expander in Fig. 1;

3 is a schematic diagram of the composition of another embodiment of the natural gas liquefaction system of the supersonic two-phase expansion refrigeration cycle shown in FIG. 1;

Reference number:

10. Countercurrent heat exchanger; 110. First circulation pipeline; 120. Gaseous heat exchange tube;

130. Low temperature heat exchange tube; 20. Supersonic two-phase expander; 210. Refrigeration unit;

220, air inlet; 230, air outlet; 240, liquid outlet;

250, swirl mechanism; 260, nozzle; 270, swirl separation pipe;

280, Drainage mechanism; 290, Diffuser; 310, Gas circulation pipeline;

320, low temperature return pipeline; 40, compressor; 50, cooler;

60. Throttle valve.

Detailed ways

In order to make the purposes, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments These are some embodiments of the present invention, but not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

It should be noted that all directional indications (such as up, down, left, right, front, back...) in the embodiments of the present invention are only used to explain the relationship between various components under a certain posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication also changes accordingly.

1, the present invention provides a natural gas liquefaction system of a supersonic two-phase expansion refrigeration cycle, comprising a plurality of countercurrent heat exchangers 10 and a plurality of supersonic two-phase expanders 20, and the natural gas flows through a plurality of countercurrent heat exchangers in turn. the device 10 and form the first circulation pipeline 110 . Each supersonic two-phase expander 20 is disposed between adjacent counter-flow heat exchangers 10 and forms a plurality of refrigeration units 210 . Wherein, the refrigerant passes through the plurality of refrigeration units 210 in sequence to liquefy the gaseous natural gas introduced into the first circulation pipeline 110 .

In an embodiment of the present invention, each refrigeration unit 210 includes a counter-flow heat exchanger 10 and a supersonic two-phase expander 20, and at least one set of gaseous heat exchange tubes 120 and low-temperature heat exchange tubes are arranged in the counter-flow heat exchanger 10 130 , the gaseous heat exchange tube 120 is used for transporting the gaseous working medium towards the supersonic two-phase expander 20 , and the low temperature heat exchange tube 130 is used for receiving the liquid working medium formed by the supersonic two-phase expander 20 . The gaseous heat exchange tube 120 is used to transport the refrigeration working medium, that is, the gaseous refrigeration working medium is transported to the supersonic two-phase expander 20, and the liquid working medium is generated by the supersonic two-phase expander 20 and is stored in the low temperature heat exchange tube 130. The low temperature liquid in the low temperature heat exchange tube 130 exchanges heat with the natural gas in the first circulation pipeline 110 to realize the cooling and liquefaction of the gaseous natural gas. Because the cold energy is transferred to the gaseous natural gas, the phase state of the low temperature liquid changes. into a gaseous outflow.

Further, the gaseous working medium flows through the plurality of refrigeration units 210 to form a gas circulation pipeline 310 , and the liquid working medium flows out of the refrigeration units 210 to form a low temperature return pipeline 320 . The refrigerating working medium completes the cycle to realize the cyclic use of the refrigerating working medium.

The supersonic two-phase expander 20 includes an air inlet 220 , an air outlet 230 and a liquid outlet 240 . The air inlet 220 and the air outlet 230 are communicated with the gas circulation pipeline 310 , and the liquid outlet 240 is communicated with the low temperature return line 320 . That is to say, when the refrigerant passes through the supersonic two-phase expander 20, the two flow directions are divided, and the gaseous working medium flows to the downstream countercurrent heat exchanger 10 and communicates with the gas circulation pipeline 310 to participate in the circulation. The formed liquid working medium flows to the low temperature return line 320, and when it flows to the low temperature heat exchange pipe 130 in the countercurrent heat exchanger 10, it exchanges heat with the natural gas flowing through the first circulation line 110 of the countercurrent heat exchanger 10, so as to Cool natural gas.

2, specifically, the supersonic two-phase expander 20 includes a swirl mechanism 250, a nozzle 260, a swirl separation pipe 270, a liquid discharge mechanism 280 and a diffuser 290 connected in sequence; The flow mechanism 250 is connected, and the swirl mechanism 250 generates centrifugal force to form the low temperature effect of the gaseous working medium entering through the air inlet 220 in the nozzle 260, and flows the generated liquid working medium to the cyclone separation pipe 270 through the liquid discharge mechanism 280. The low temperature return line 320 and the diffuser 290 flow the gaseous working medium to the gas circulation line 310 . The liquid discharge mechanism 280 may be a pipeline formed by the outer periphery of the diffuser 290 and the cyclone separation pipe 270, or a separate liquid discharge pipe, which is not limited herein.

A compressor 40 and a cooler 50 are provided on the low temperature return line 320 flowing to the gas circulation line 310 . The inlet side of the compressor 40 is communicated with the low temperature return line 320 , the outlet side of the compressor 40 is communicated with the inlet side of the cooler 50 , and the outlet side of the cooler 50 is communicated with the gas circulation line 310 . The compressor 40 is used to pressurize and heat up the gaseous working medium, and the cooler 50 is used to cool down the gaseous working medium delivered by the compressor 40 to participate in the cycle.

A throttle valve 60 is also provided at the connection between the gas circulation pipeline 310 and the low temperature return pipeline 320 after passing through the plurality of refrigeration units 210; , the outlet side of the throttle valve 60 is communicated with the low temperature return line 320 . The setting of the throttle valve 60 is used for throttling and cooling.

Along the direction of the first circulation pipe 110 , the temperature of the refrigerant in the low-temperature return pipes 320 in the plurality of countercurrent heat exchangers 10 decreases gradually, so as to reduce the temperature of the natural gas in the first circulation pipe 110 . In an embodiment of the present invention, the number of refrigeration units 210 is three, and three counter-flow heat exchangers 10 and three supersonic two-phase expanders 20 are correspondingly arranged to cool the natural gas in the first circulation pipeline 110 . Cool down.

It should be noted that each refrigeration unit 210 forms a first-stage refrigeration cycle, and three refrigeration units 210 form a three-stage refrigeration cycle, and the setting of the throttle valve 60 is a fourth-stage refrigeration cycle.

Specifically as follows, the refrigerant includes nitrogen combined with one or more of butane, propane, ethylene, and methane, so that the system can reach the liquefaction temperature region of nitrogen. Among them, the liquefaction temperature of butane, propane, ethylene, methane and nitrogen decreases. In an embodiment of the present invention, taking butane, propane, ethylene, methane and nitrogen all participating in the refrigeration cycle as an example for illustration, the lowest temperature finally reached corresponds to the liquefaction temperature region of nitrogen, that is, lower than the temperature region of natural gas liquefaction. It can be understood that the number of refrigerants corresponds to the number of refrigeration units 210 . That is, on the basis of selecting nitrogen, each addition of butane, propane, ethylene, and methane corresponds to a refrigeration unit 210 .

Butane, propane, ethylene, methane, and nitrogen are multi-component mixed working fluids, and when the multi-component mixed working fluid participates in the primary refrigeration cycle, the corresponding participation is the first refrigeration unit 210 . The multi-component mixed working medium enters the first-stage supersonic two-phase expander 20, generates centrifugal force through the swirl mechanism 250, and produces a low temperature effect by isoentropic expansion, cooling and depressurization in the nozzle 260. After the temperature is lowered, high-boiling point gases, such as butane, Condensation and nucleation will occur, droplets will be formed and further growth will occur. The liquid butane will continue to condense and liquefy in the cyclone separation tube 270 due to the tangential velocity and centrifugal effect generated by the rotation, and is discharged through the liquid discharge mechanism 280 to achieve gas-liquid separation, and the remaining The gas phase, namely propane, ethylene, methane and nitrogen, is discharged after being decelerated and heated up by the diffuser 290, so most of the pressure energy can be recovered, which greatly reduces the pressure loss at the inlet and outlet. The liquid butane flows to the low temperature return line 320, and after heat exchange through the first-stage countercurrent heat exchanger 10, adiabatic compression through the compressor 40 is used to increase the pressure value and the gas pressure when entering the first-stage supersonic two-phase expander 20 is Similarly, after passing through the cooler 50, it enters the primary countercurrent heat exchanger 10 for heat exchange, and then re-enters the primary supersonic two-phase expander 20 to complete the primary cycle.

When the multi-component mixed working medium participates in the secondary refrigeration cycle, the corresponding participation is the second refrigeration unit 210 located downstream of the first refrigeration unit 210, and the remaining gas phase after the primary refrigeration cycle includes propane, ethylene, methane and nitrogen. , the remaining mixed gas phase flows to the secondary supersonic two-phase expander 20, generates centrifugal force through the swirl mechanism 250, and produces a low temperature effect by isoentropic expansion, cooling and depressurization in the nozzle 260, and the high-boiling point gas propane will condense after the temperature is lowered. Nucleation, generation of droplets and further growth, liquid propane continues to condense and liquefy in the cyclone separation tube 270 due to the tangential velocity and centrifugal action generated by the rotation, and is discharged through the liquid discharge mechanism 280 to achieve gas-liquid separation, and the remaining gas phase is ethylene. , methane and nitrogen are discharged after the diffuser 290 decelerates, heats up and increases pressure, so most of the pressure energy can be recovered, which greatly reduces the pressure loss at the inlet and outlet. The liquid propane flows to the low-temperature return line 320, enters the secondary countercurrent heat exchanger 10 for heat exchange, and then merges with the low-temperature liquid-phase propane discharged from the primary supersonic two-phase expander 20, participates in the primary circulation, and completes the second stage. level cycle.

When the multi-component mixed working medium participates in the tertiary refrigeration cycle, the corresponding participation is the third refrigeration unit 210 located downstream of the second refrigeration unit 210. After the secondary refrigeration cycle, the remaining gas phase includes ethylene, methane and nitrogen, and the remaining The mixed gas phase flows to the three-stage supersonic two-phase expander 20, generates centrifugal force through the swirl mechanism 250, and produces a low temperature effect by isoentropic expansion, cooling, and depressurization in the nozzle 260. The droplets are generated and grow further. The liquid-phase ethylene continues to condense and liquefy in the cyclone separation tube 270 due to the tangential velocity and centrifugal effect generated by the rotation, and is discharged through the liquid discharge mechanism 280 to realize gas-liquid separation, and the remaining gas phase is methane and nitrogen. After the diffuser 290 decelerates the temperature and increases the pressure, it is discharged, so most of the pressure energy can be recovered, which greatly reduces the pressure loss of the inlet and outlet. The liquid ethylene flows to the low-temperature return line 320, enters the secondary countercurrent heat exchanger 10 for heat exchange, and then merges with the low-temperature liquid-phase butane discharged from the secondary supersonic two-phase expander 20 to participate in the secondary circulation and primary In the cycle, the three-level cycle is completed.

After the three-stage refrigeration cycle, the remaining gas phase includes methane and nitrogen, and the remaining gas phase discharged by the three-stage supersonic two-phase expander 20 diffuser 290 is throttled and cooled by the throttle valve 60, and the gas phase methane is liquefied, and part of the nitrogen is liquefied. , and jointly flow to the low-temperature return line 320, merge with the liquid phase ethylene produced by the tertiary refrigeration cycle, and participate in the tertiary refrigeration cycle, the secondary refrigeration cycle and the primary refrigeration cycle to complete the four-stage refrigeration cycle.

The first circulation pipeline 110 sequentially passes through the countercurrent heat exchanger 10 in the primary refrigeration cycle, the secondary refrigeration cycle and the tertiary refrigeration cycle for heat exchange and continuous cooling, so that the gaseous natural gas in the first circulation pipeline 110 is liquefied.

Please refer to FIG. 3 , it should be noted that multiple compressors 40 and coolers 50 may also be provided, for example, the supersonic two-phase expander 20 in the secondary circulation refrigeration flows through the secondary countercurrent heat exchange in the low temperature return line 320 After the heat exchanger 10 and the primary heat exchanger, a compressor 40 and a cooler 50 are used to compress the liquid butane to heat up, cool down, and then combine with the gaseous butane flowing out of the countercurrent heat exchanger 10 in the primary cycle refrigeration, and further gasification cooling. Similarly, a compressor 40 and a cooler 50 are arranged in the third-stage cycle and the fourth-stage cycle, thereby facilitating the rapid gasification of liquid propane, liquid ethylene and liquid methane to participate in the refrigeration cycle, thereby improving the efficiency of the multi-component mixed working fluid. Cycle cooling efficiency.

Further, the energy conservation analysis in the supersonic two-phase expander 20 is as follows: from the steady flow energy equation:

The potential energy change is not considered and no external work is done, that is, the whole process only has the conversion between enthalpy and kinetic energy, and the energy equation can be simplified as:

The process of the gaseous working medium passing through the gas outlet 230 of the supersonic two-phase expander 20 to the liquid outlet 240 is obtained by energy conservation:

where m ₁ is the mass flow of the gas entering the supersonic two-phase expander 20 , m _2g is the mass flow of the gas flowing to the diffuser 290 , and m _2l is the mass flow of the liquid flowing through the drain mechanism 280 . u _2g is the flow rate of the gas flowing to the diffuser 290 , and u _2l is the flow rate of the liquid flowing through the drain mechanism 280 . u ₁ is the flow rate of the gaseous working medium before entering the supersonic two-phase expander 20 . h _2g is the specific enthalpy of the gas flowing to the diffuser 290 , and h _2l is the specific enthalpy of the liquid flowing through the drain mechanism 280 .

The change process of the gaseous working medium from the diffuser 290 to the gas outlet 230 is obtained by the conservation of energy:

From the isentropic equation, s _2g =s ₃ is obtained. Among them, h _2g is the specific enthalpy of the gas flowing to the diffuser 290 , and h ₃ is the specific enthalpy of the gas flowing out of the gas outlet 230 . u _2g is the flow rate of the gas flowing to the diffuser 290 , and u ₃ is the flow rate of the gas flowing out of the gas outlet 230 . s _2g is the specific entropy of the gas after the two-phase separation, and s ₃ is the specific entropy of the gas flowing out of the gas outlet 230 .

In summary, the supersonic two-phase expander 20 has the characteristics of high expansion refrigeration efficiency, low pressure drop, low energy consumption, simple and compact structure, safe and reliable without moving parts, low processing difficulty, and can realize expansion in the two-phase region. The refrigeration process, combined with the countercurrent heat exchanger 10, can efficiently liquefy gaseous natural gas, and solve the technical problem of low liquefaction system efficiency in the existing natural gas liquefaction system.

It should be noted that the terms "comprising" and "having" and any modifications thereof in the embodiments of the present application are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device comprising a series of steps or units is not limited to the listed steps or units, but optionally also includes unlisted steps or units, or optionally also includes For other components or units inherent to these processes, methods, products or devices.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, but not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that it can still be The technical solutions described in the foregoing embodiments are modified, or some technical features thereof are equivalently replaced; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims (10)

1. a natural gas liquefaction system of supersonic two-phase expansion refrigeration cycle, is characterized in that, comprises: a plurality of counter-current heat exchangers, through which natural gas flows in sequence to form a first circulation pipeline; A plurality of supersonic two-phase expanders, each supersonic two-phase expander is arranged between the adjacent counter-flow heat exchangers, and forms a plurality of refrigeration units; wherein, The refrigerant passes through a plurality of the refrigeration units in sequence to liquefy the gaseous natural gas introduced into the first circulation pipeline. 2. The natural gas liquefaction system of the supersonic two-phase expansion refrigeration cycle according to claim 1, wherein each of the refrigeration units comprises a countercurrent heat exchanger and a supersonic two-phase expander; At least one group of gaseous heat exchange tubes and low temperature heat exchange tubes are arranged in the countercurrent heat exchanger, the gaseous heat exchange tubes are used for transporting gaseous working medium towards the supersonic two-phase expander, and the low temperature heat exchange tubes are used for receiving The liquid working substance formed through the supersonic two-phase expander. 3. The natural gas liquefaction system of the supersonic two-phase expansion refrigeration cycle according to claim 2, wherein the gaseous working medium flows through a plurality of the refrigeration units to form a gas circulation pipeline, and the liquid working medium flows out The refrigeration unit forms a low temperature return line; The gas circulation pipeline and the low temperature return pipeline are connected end to end. 4. the natural gas liquefaction system of supersonic two-phase expansion refrigeration cycle according to claim 3, is characterized in that, described supersonic two-phase expander comprises air inlet, air outlet and liquid outlet; The air inlet and the air outlet are communicated with the gas circulation pipeline, and the liquid outlet is communicated with the low temperature return pipeline. 5. The natural gas liquefaction system of the supersonic two-phase expansion refrigeration cycle according to claim 4, wherein the supersonic two-phase expander comprises a swirl mechanism, a nozzle, a swirl separation pipe, a row Hydraulic mechanism and diffuser; The air inlet is communicated with the swirl mechanism, and the swirling mechanism generates centrifugal force to form a low temperature effect on the gaseous working medium entering through the air inlet in the nozzle, and in the swirl separation pipe The generated liquid working medium flows to the low temperature return line through the liquid discharging mechanism, and the gaseous working medium flows to the gas circulation line through the diffuser. 6. The natural gas liquefaction system of supersonic two-phase expansion refrigeration cycle according to claim 3, characterized in that, a compressor and a cooler are provided on the low temperature return line flowing toward the gas circulation line; The inlet side of the compressor is communicated with the low temperature return line, the outlet side of the compressor is communicated with the inlet side of the cooler, and the outlet side of the cooler is communicated with the gas circulation line. 7 . The natural gas liquefaction system of the supersonic two-phase expansion refrigeration cycle according to claim 3 , wherein, along the direction of the first circulation pipe, the low temperature return pipes in the plurality of countercurrent heat exchangers The temperature of the refrigerant in the pipeline decreases gradually, so as to reduce the temperature of the natural gas in the first circulation pipeline. 8. The natural gas liquefaction system of the supersonic two-phase expansion refrigeration cycle according to claim 3, characterized in that, after the gas circulation pipeline passes through a plurality of the refrigeration units, the connection between the low temperature return pipeline and the low temperature return pipeline is also with a throttle valve; The inlet side of the throttle valve is communicated with the gas outlet of the supersonic two-phase expander, and the outlet side of the throttle valve is communicated with the low temperature return line. 9 . The natural gas liquefaction system of the supersonic two-phase expansion refrigeration cycle according to claim 8 , wherein the number of the refrigeration units is three. 10 . 10. The natural gas liquefaction system of the supersonic two-phase expansion refrigeration cycle according to claim 9, characterized in that, the refrigerating medium comprises nitrogen and one of butane, propane, ethylene, and methane combined with the nitrogen. one or more.

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