An unconventional natural gas liquefaction system and its control method

Abstract

The present invention discloses an unconventional natural gas liquefaction system, which includes a precooling system, a refrigeration system and a cryogenic liquefaction system, and is mainly composed of a multi-stream heat exchanger, a distillation column, a wound tube heat exchanger, a compressor and several heat exchangers. In the present invention, on the one hand, the refrigerant in the precooling system is led to the heat exchanger between the outlet of the top of the distillation column and the condenser to precool the components at the top of the distillation column. On the other hand, the refrigerant in the refrigeration system is led to the condenser of the distillation column to cool the components at the top of the distillation column. By respectively adjusting the flow rates of the refrigerant entering the top heat exchanger and the condenser of the distillation column in the liquefaction system, the condensation load of the condenser is adjusted so that it changes with the change of the methane concentration in the raw gas, ensuring the purity and recovery rate of the liquefied product.

Description

Technical Field

[0001] The present invention relates to a natural gas liquefaction system, and more particularly to an unconventional natural gas liquefaction system and a control method thereof. Background Art

[0002] Unconventional natural gas is a common backup application resource. Since unconventional natural gas (such as coalbed methane, etc.) is often located in remote mountainous areas, far from the natural gas pipeline network, and its gas quality is different from that of conventional natural gas, it is not suitable to directly enter the existing pipeline network for transportation. Liquefaction can effectively solve the problems of "scattered, small, and partial" of unconventional natural gas (such as coalbed methane, etc.), which is convenient for transportation, has less investment, and good returns. Therefore, the current common way is to recover and utilize it through liquefaction.

[0003] The invention patent with the name of "Unconventional Natural Gas Liquefaction System Based on Spiral Wound Heat Exchanger (Publication No. CN103542692B)" provides an unconventional natural gas liquefaction system based on a spiral wound heat exchanger. This system realizes heat transfer with a large temperature difference by using a spiral wound heat exchanger, and it is applicable not only to conventional natural gas but also to unconventional natural gas rich in hydrogen and nitrogen. However, the technical solution adopted is to first liquefy the incoming gas and then perform cryogenic distillation. The methane contents of the three embodiments of this technical solution are 60.11%, 55.32%, and 78.08% respectively, and the liquefaction of raw material gases with three different methane concentrations is carried out. Therefore, the device is only applicable to the liquefaction of raw material gases with a concentration of at least 50% and is not applicable to the liquefaction of low-concentration unconventional natural gas with a concentration below 50%.

[0004] In the paper "Analysis of the Operating Conditions of the Liquefaction Distillation Column for Low-Concentration Coalbed Methane" published by Xiao Lu, the influence of the methane concentration of the raw material gas on the load of the distillation column is analyzed in detail. "As the methane concentration of the raw material gas increases, the condenser load (absolute value) continuously decreases, while the reboiler load continuously increases". On the contrary, when the methane concentration in the raw material gas is smaller, the condenser load (absolute value) is larger. Under the condition that the flow rate, pressure, and temperature of the raw material gas are stable, as the methane concentration of the raw material gas decreases, the liquid flow rate that needs to be condensed by the condenser increases, resulting in an increase in the condenser load (absolute value). Therefore, the technical solution provided by the invention patent with the name of "Unconventional Natural Gas Liquefaction System Based on Spiral Wound Heat Exchanger (Publication No. CN103542692B)" is not applicable to the liquefaction of low-concentration unconventional natural gas.

[0005] In actual operation, the condensation load of the condenser in the distillation column of the liquefaction system should change with the change of the methane concentration in the raw material gas to ensure the purity and recovery rate of the liquefied product.

[0006] The operation practice of domestic liquefied natural gas plants shows that due to the influence of the quality fluctuation of unconventional natural gas, the refrigerant parameters during the actual operation of the device, such as refrigerant composition, compressor inlet temperature, and pressure ratio, will all change to a certain extent. And this change directly affects the power consumption of the refrigerant compressor unit, thereby affecting its operation economy and the energy consumption level of the device. How to reduce power consumption, optimize the process, and stabilize the liquefaction rate of natural gas by optimizing the pressure ratio, inlet parameters, and refrigerant circulation volume of the refrigerant compressor is an urgent problem to be solved at present. Summary of the Invention

[0007] Object of the Invention: Aiming at the above problems, the object of the present invention is to provide an unconventional natural gas liquefaction system, so that the condensation load of the condenser changes with the change of the methane concentration in the raw gas, ensuring the purity and recovery rate of the liquefied product. And its control method is provided.

[0008] Technical Solution: An unconventional natural gas liquefaction system includes a precooling system, a refrigeration system, and a low-temperature liquefaction system;

[0009] The multi-stage compressor of the precooling system is successively connected to a first three-way valve through a third heat exchanger and an eighth expansion valve and then branched into two pipelines. The first pipeline is connected to a multi-stream heat exchanger, and the second pipeline is connected to the multi-stream heat exchanger outlet after passing through a first heat exchanger and then jointly connected to the multi-stage compressor;

[0010] The second heat exchanger of the refrigeration system is connected to a first gas-liquid separation tank through a multi-stream heat exchanger. Its gas-phase outlet is successively connected to the shell side of a wound-tube heat exchanger through a sixth expansion valve. Its shell-side outlet is successively connected to a refrigerant pump and a second compressor and then connected to the second heat exchanger. Its liquid-phase outlet is branched into two pipelines. The first pipeline is connected to a fourth expansion valve, and the second pipeline is successively connected to the tube side of a ninth expansion valve and a condenser and then converges with the fourth expansion valve outlet and jointly connected to the inlet of the wound-tube heat exchanger and then connected to the shell side of the wound-tube heat exchanger through a seventh expansion valve;

[0011] In the low-temperature liquefaction system, the external raw gas is connected to a rectification tower through a multi-stream heat exchanger. The top outlet of the rectification tower is connected to the shell side of the condenser through a first heat exchanger. Its upper shell-side outlet is connected to a second gas-liquid separation tank through a wound-tube heat exchanger. The liquid-phase outlet of the second gas-liquid separation tank is connected to an external ethane or propane storage tank. Its gas-phase outlet is successively connected to a seventh gas-liquid separation tank through a wound-tube heat exchanger and a fifth expansion valve. The gas-phase outlet of the seventh gas-liquid separation tank is connected to the outside, and its liquid-phase outlet is connected to an LNG storage tank. The pipeline at the bottom outlet of the rectification tower is branched into two pipelines. One pipeline is connected to the bottom inlet of the rectification tower through a sixth heat exchanger, and the other pipeline is connected to an external heavy hydrocarbon treatment system. The shell side of the condenser is connected to the top inlet of the rectification tower.

[0012] Furthermore, the multi-stream heat exchanger is provided with a first heat exchange tube, a second heat exchange tube, and a third heat exchange tube. The first heat exchange tube connects the first inlet and the fourth outlet of the multi-stream heat exchanger. The first three-way valve is connected to the first heat exchange tube of the multi-stream heat exchanger through the first inlet and converges at the fourth outlet; the second heat exchange tube connects the fifth inlet and the fifth outlet of the multi-stream heat exchanger. The second heat exchanger is connected to the second heat exchange tube of the multi-stream heat exchanger through the fifth inlet, and the fifth outlet is connected to the first gas-liquid separator; the third heat exchange tube connects the sixth inlet and the sixth outlet of the multi-stream heat exchanger. The external raw material gas is connected to the third heat exchange tube of the multi-stream heat exchanger through the sixth inlet, and the sixth outlet is connected to the distillation column. A third compressor is provided between the external raw material gas and the sixth inlet.

[0013] Preferably, the pre-cooling system further includes a fourth gas-liquid separator, a fifth gas-liquid separator, a sixth gas-liquid separator, a first expansion valve, a second expansion valve, and a third expansion valve. The first heat exchange tube is of a multi-section structure, and between the first inlet and the fourth outlet, it also successively passes through a first outlet, a second inlet, a second outlet, a third inlet, a third outlet, and a fourth inlet. The first outlet of the multi-stream heat exchanger is connected to the inlet of the fourth gas-liquid separator. The gas-phase outlet of the fourth gas-liquid separator is connected to the first inlet of the multi-stage compressor. The liquid-phase outlet of the fourth gas-liquid separator is connected to the second inlet of the multi-stream heat exchanger through the first expansion valve. The second outlet of the multi-stream heat exchanger is connected to the inlet of the fifth gas-liquid separator. The gas-phase outlet of the fifth gas-liquid separator is connected to the second inlet of the multi-stage compressor. The liquid-phase outlet of the fifth gas-liquid separator is connected to the third inlet of the multi-stream heat exchanger through the second expansion valve. The third outlet of the multi-stream heat exchanger is connected to the inlet of the sixth gas-liquid separator. The gas-phase outlet of the sixth gas-liquid separator is connected to the third inlet of the multi-stage compressor. The liquid-phase outlet of the sixth gas-liquid separator is connected to the fourth inlet of the multi-stream heat exchanger through the third expansion valve.

[0014] Preferably, the refrigeration system further includes a fourth compressor, an eighth gas-liquid separator, and a second refrigerant pump. The left-end outlet of the second heat exchanger is connected to the inlet of the eighth gas-liquid separator. The gas-phase outlet of the eighth gas-liquid separator is connected to the inlet of the fourth compressor. The liquid-phase outlet of the eighth gas-liquid separator is connected to the second refrigerant pump. The outlets of the fourth compressor and the second refrigerant pump are joined together in a pipeline and then commonly connected to the fifth inlet of the multi-stream heat exchanger.

[0015] Further, the spiral wound tube heat exchanger is provided with a first heat exchange tube, a second heat exchange tube, a third heat exchange tube, and a fourth heat exchange tube. The first heat exchange tube connects the first inlet and the first outlet of the spiral wound tube heat exchanger. The first outlet is connected to a seventh expansion valve, and the seventh expansion valve is connected to the shell side of the spiral wound tube heat exchanger through the sixth inlet of the spiral wound tube heat exchanger. The second heat exchange tube connects the second inlet and the second outlet of the spiral wound tube heat exchanger. The second outlet is connected to a sixth expansion valve, and the sixth expansion valve is connected to the shell side of the spiral wound tube heat exchanger through the fifth inlet of the spiral wound tube heat exchanger. The third heat exchange tube connects the third inlet and the third outlet of the spiral wound tube heat exchanger, and the third outlet is connected to the second gas-liquid separation tank. The fourth heat exchange tube connects the fourth inlet and the fourth outlet of the spiral wound tube heat exchanger, and the fourth outlet is connected to a fifth expansion valve.

[0016] Further, the tube side inlet and the tube side outlet of the condenser are connected through a pipeline. The tube side inlet is connected to a ninth expansion valve, and the tube side outlet is connected to the spiral wound tube heat exchanger. One inlet and two outlets are also arranged at intervals on the shell side of the condenser, namely the shell side inlet, the upper shell side outlet, and the lower shell side outlet. The first heat exchanger is connected to the shell side inlet, the spiral wound tube heat exchanger is connected to the upper shell side outlet, and the top inlet of the distillation column is connected to the lower shell side outlet.

[0017] Preferably, the system further includes a flow ratio controller, a first flow meter, and a second flow meter. A first flow meter is arranged in the pipeline connecting the gas phase outlet of the first gas-liquid separation tank and the spiral wound tube heat exchanger. A second flow meter is arranged in the pipeline connecting the fourth expansion valve and the spiral wound tube heat exchanger. The signals of the first flow meter and the second flow meter are connected to the flow ratio controller through wires. The output signal of the flow ratio controller is connected to the fourth expansion valve through a wire. The seventh expansion valve is signal-connected to the flow ratio controller.

[0018] Preferably, the system further includes a first ball valve, a check valve, and a temperature controller. Two pipelines are connected in parallel at the liquid phase outlet of the first gas-liquid separation tank. The branch pipeline connected to the condenser is further sequentially connected with a check valve and a first ball valve. A temperature controller is arranged in the pipeline connecting the condenser and the spiral wound tube heat exchanger. The signal of the temperature controller is connected to the first ball valve through a wire.

[0019] A control method for the above unconventional natural gas liquefaction system includes two feedback control loops;

[0020] The first control loop sets the ratio of the liquid mixed refrigerant and the gas mixed refrigerant as the key control variable, and effectively controls the LNG outlet temperature at the fourth outlet of the spiral wound tube heat exchanger by controlling the seventh expansion valve;

[0021] The second control loop sets the temperature of the spiral wound tube heat exchanger as the key control variable, and effectively controls the temperature of the light components in the distillation column by controlling the first ball valve, thereby indirectly controlling the liquefaction rate of LNG.

[0022] Further, the control method of the first control loop includes the following steps:

[0023] Step 1: The first flowmeter and the second flowmeter measure the flow rate of the fluid in the pipeline where they are located, and transmit the signals to the flow ratio controller through wires.

[0024] Step 2: The flow ratio controller calculates and displays the ratio, compares the calculated ratio value with the preset ratio value, and then transmits the control signal to the seventh expansion valve through wires, and controls the ratio of the gaseous refrigerant to the liquid refrigerant entering the spiral wound heat exchanger by adjusting the valve opening of the seventh expansion valve.

[0025] The control method of the second control loop includes the following steps:

[0026] Step 1: The temperature controller measures the temperature of the fluid in the pipeline where it is located.

[0027] Step 2: The temperature controller compares the measured temperature value with the preset temperature value, and then transmits the control signal to the first ball valve through wires, and controls the flow rate of the refrigerant entering the condenser by controlling the valve opening of the first ball valve.

[0028] Propane is selected as the refrigerant in the precooling system, and a mixed refrigerant is selected as the refrigerant in the refrigeration system.

[0029] Beneficial effects: Compared with the prior art, the advantages of the present invention are:

[0030] 1. In an unconventional natural gas liquefaction system of the present invention, on the one hand, the refrigerant in the precooling system is led to the heat exchanger between the outlet of the top of the rectification column and the condenser, and a part of the cold energy of the precooling system is used to condense and reflux the gas at the top of the rectification column. On the other hand, by leading the refrigerant in the refrigeration system to the condenser of the rectification column, the components at the top of the rectification column are cooled. By leading the cold energy of the precooling system and the refrigeration system to the top of the rectification column, the condensation load of the condenser in the rectification column in the liquefaction system is adjusted to change with the change of the methane concentration in the raw gas, so as to be applicable to the liquefaction of low-concentration unconventional natural gas below 50%, and ensure the purity and recovery rate of the liquefied product.

[0031] 2. In the working method of an unconventional natural gas liquefaction system of the present invention, on the one hand, the flow ratio controller is used to adjust the flow ratio of the gas-liquid components of the refrigerant to match the change of the heat load on the unconventional natural gas side, so as to ensure that the cold and hot fluids in the heat exchanger operate at a lower heat transfer temperature difference, reduce the irreversible loss in the heat transfer process, and improve the operability and automation level of the device. Description of the Drawings

[0032] Figure 1It is a schematic diagram of the system connection in the first embodiment of the present invention;

[0033] Figure 2 It is a schematic diagram of the system connection in the second embodiment of the present invention. Detailed implementation manners

[0034] The present invention will be further clarified below with reference to the accompanying drawings and specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention.

[0035] Embodiment 1:

[0036] An unconventional natural gas liquefaction system, as Figure 1 shown, includes a precooling system, a refrigeration system, and a cryogenic liquefaction system.

[0037] The connection mode of the precooling system is as follows:

[0038] The outlet 8b of the multi-stage compressor 8 is connected to the upper end inlet 34a of the third heat exchanger 34. The lower end outlet 34b of the third heat exchanger 34 is connected to the inlet of the first three-way valve 33 via the eighth expansion valve 37. The right end inlet 34c of the third heat exchanger 34 is connected to the external cooling water inlet. The left end outlet 34d of the third heat exchanger 34 is connected to the external cooling water outlet. The first outlet of the first three-way valve 33 is connected to the first inlet ca of the multi-stream heat exchanger 1. After the fourth outlet ch of the multi-stream heat exchanger 1 converges with the pipeline at the right end outlet 5b of the first heat exchanger 5 in the cryogenic liquefaction system, they are jointly connected to the fourth inlet 8a of the multi-stage compressor 8. The second outlet of the first three-way valve 33 is connected to the left end inlet 5a of the first heat exchanger 5.

[0039] In the multi-stream heat exchanger 1, the first inlet ca and the fourth outlet ch are connected by a pipeline.

[0040] The connection mode of the refrigeration system is as follows:

[0041] The fifth inlet ba of the multi-stream heat exchanger 1 is connected to the fifth outlet bb of the multi-stream heat exchanger 1. The fifth outlet bb of the multi-stream heat exchanger 1 is connected to the first gas-liquid separator 2. The gas-phase outlet of the first gas-liquid separator 2 is connected to the second inlet 9h of the spiral wound heat exchanger 9. The second outlet 9j of the spiral wound heat exchanger 9 is connected to the fifth inlet 9c of the spiral wound heat exchanger 9 via the sixth expansion valve 25. Two pipelines are connected in parallel at the liquid-phase outlet of the first gas-liquid separator 2. The first pipeline is connected to the tube-side inlet 3d of the condenser 3 after passing through the check valve 31, the first ball valve 23, and the ninth expansion valve 38 in sequence. The tube-side outlet 3e of the condenser 3 converges with the pipeline at the outlet of the fourth expansion valve 22 and then jointly connects to the first inlet 9q of the spiral wound heat exchanger 9. The second pipeline converges with the pipeline of the tube-side outlet 3e of the condenser 3 after passing through the fourth expansion valve 22 and then jointly connects to the first inlet 9q of the spiral wound heat exchanger 9. The first outlet 9i of the spiral wound heat exchanger 9 is connected to the sixth inlet 9b of the spiral wound heat exchanger 9 via the seventh expansion valve 26.

[0042] The fifth outlet 9a of the spiral wound heat exchanger 9 is connected to the right-end inlet 13b of the second heat exchanger 13 via the first refrigerant pump 12 and the second compressor 11 in sequence. The left-end outlet 13a of the second heat exchanger 13 is connected to the fifth inlet ba of the multi-stream heat exchanger 1. The lower-end inlet 13d of the second heat exchanger 13 is connected to the external cooling water outlet, and the upper-end outlet of the second heat exchanger 13 is connected to the external cooling water inlet.

[0043] The first inlet 9q of the spiral wound heat exchanger 9 is connected to the first outlet 9i of the spiral wound heat exchanger 9 through a pipeline.

[0044] The second inlet 9h of the spiral wound heat exchanger 9 is connected to the second outlet 9j of the spiral wound heat exchanger 9 through a pipeline.

[0045] Among them, a second flowmeter 29 is further provided in the pipeline connecting the fourth expansion valve 22 and the first inlet 9q of the spiral wound heat exchanger 9. A first flowmeter 28 is further provided in the pipeline connecting the gas-phase outlet of the first gas-liquid separator 2 and the second inlet 9h of the spiral wound heat exchanger 9. The signals of the first flowmeter 28 and the second flowmeter 29 are respectively connected to the flow ratio controller 27 through wires, and the output signal of the flow ratio controller 27 is connected to the seventh expansion valve 26 through a wire.

[0046] The connection mode of the low-temperature liquefaction system is as follows:

[0047] The raw gas inlet is connected to the sixth inlet aa of the multi-stream heat exchanger 1 via the third compressor 35. The sixth outlet ab of the multi-stream heat exchanger 1 is connected to the first inlet 4a of the distillation column 4.

[0048] The sixth inlet aa of the multi-stream heat exchanger 1 is connected to the sixth outlet ab of the multi-stream heat exchanger 1 through a pipeline.

[0049] The top outlet 4b of the rectifying column 4 is connected to the lower inlet 5d of the first heat exchanger 5. The pipeline at the bottom outlet 4d of the rectifying column 4 is branched into two pipelines. One is connected to the lower inlet 36b of the sixth heat exchanger 36. The upper outlet 36a of the sixth heat exchanger 36 is connected to the bottom inlet 4e of the rectifying column 4. The other is connected to the external heavy hydrocarbon treatment system. The right end inlet 36c of the sixth heat exchanger 36 is connected to the external heat source outlet, and the left end outlet 36d of the sixth heat exchanger 36 is connected to the external heat source inlet. The second inlet 4c of the rectifying column 4 is connected to the shell side lower outlet 3c of the condenser 3. The upper outlet 5c of the first heat exchanger 5 is connected to the shell side inlet 3a of the condenser 3. The shell side upper outlet 3b of the condenser 3 is connected to the third inlet 9g of the spiral wound heat exchanger 9. The third outlet 9f of the spiral wound heat exchanger 9 is connected to the inlet of the second gas-liquid separation tank 6. The gas phase outlet of the second gas-liquid separation tank 6 is connected to the fourth inlet 9e of the spiral wound heat exchanger 9. The liquid phase outlet of the second gas-liquid separation tank 6 is connected to the external ethane or propane storage tank (C2 or C3).

[0050] The fourth outlet 9d of the spiral wound heat exchanger 9 is connected to the inlet of the seventh gas-liquid separation tank 30 through the fifth expansion valve 24. The gas phase outlet of the seventh gas-liquid separation tank 30 is connected to the outside, and the liquid phase outlet of the seventh gas-liquid separation tank 30 is connected to the LNG storage tank 7.

[0051] The fourth inlet 9e and the fourth outlet 9d of the spiral wound heat exchanger 9 are connected by a pipeline.

[0052] The third inlet 9g and the third outlet 9f of the spiral wound heat exchanger 9 are connected by a pipeline.

[0053] A temperature controller 32 is also provided in the pipeline connecting the shell side upper outlet 3b of the condenser 3 and the third inlet 9g of the spiral wound heat exchanger 9. The signal of the temperature controller 32 is connected to the first ball valve 23 through a wire.

[0054] The working process of an unconventional natural gas liquefaction system.

[0055] Working process of the precooling system: Circulating refrigeration is achieved by using methane as the refrigerant. After being compressed by the multi-stage compressor 8, methane enters the third heat exchanger 34 for water cooling. After the temperature of the refrigerant A is reduced, it enters the first three-way valve 33 through the eighth expansion valve 37 for flow splitting. A part of the methane enters the multi-stream heat exchanger 1 to precool the raw natural gas and is led out through the fourth outlet ch of the multi-stream heat exchanger 1. Another part of the methane enters the first heat exchanger 5 to cool the natural gas led out from the top of the distillation column 4, sharing the cooling load of the condenser 3. After the methane led out from the fourth outlet ch of the multi-stream heat exchanger 1 and the right-end outlet 5b of the fifth heat exchanger 5 are combined, all enter the multi-stage compressor 8 for compression and enter the next cycle.

[0056] Working process of the refrigeration system: Circulating refrigeration is achieved by using a mixed refrigerant. After being compressed by the second compressor 11, the mixed refrigerant enters the second heat exchanger 13 for water cooling. After the temperature of the refrigerant B is reduced, it enters the multi-stream heat exchanger 1 for heat exchange. After partial vaporization of the mixed refrigerant, it enters the first gas-liquid separation tank 2 for gas-liquid separation. The gaseous refrigerant enters the spiral wound heat exchanger 9 for heat exchange, and the liquid refrigerant is split.

[0057] For the liquid-phase outlet of the first gas-liquid separation tank 2, a part of the liquid refrigerant enters the condenser 3 through the ninth expansion valve 38 to cool the overhead gas of the distillation column 4 after temperature reduction, and another part enters the spiral wound heat exchanger 9 through the fourth expansion valve 22 after temperature reduction and is combined with the refrigerant flowing back in the condenser 3 and then enters the spiral wound heat exchanger 9 through the first inlet 9q of the spiral wound heat exchanger 9 for heat exchange. After being led out through the first outlet 9i of the spiral wound heat exchanger 9, it is cooled by the seventh expansion valve 27 and then enters the shell side through the sixth inlet 9b of the spiral wound heat exchanger 9.

[0058] The refrigerant at the gas-phase outlet of the first gas-liquid separation tank 2 enters the spiral wound heat exchanger 9 through the second inlet 9h of the spiral wound heat exchanger 9. Then, the refrigerant B is led out through the second outlet 9j of the spiral wound heat exchanger 9 and cooled by the sixth expansion valve 25, and then enters the shell side through the fifth inlet 9c of the spiral wound heat exchanger 9.

[0059] The refrigerant entering the shell side is mixed in the shell side of the spiral wound heat exchanger 9 and then led out through the fifth outlet 9a of the spiral wound heat exchanger 9, enters the second compressor 11 through the first refrigerant pump 12, and enters the next cycle.

[0060] Working process of the low-temperature liquefaction system: The raw natural gas is first precooled in the multi-stream heat exchanger 1 and then enters the distillation column 4 for rectification. The heavy hydrocarbons of C4 and above (C4+) are drawn from the bottom of the column, and methane, ethane, propane, a small amount of heavy hydrocarbons, and non-condensable gases are drawn from the top of the distillation column 4. They are heat-exchanged with methane in the first heat exchanger 5, and after cooling down, they enter the shell side of the condenser 3 for heat exchange. Methane, ethane, and propane are drawn from the upper outlet 3b on the shell side of the condenser 3 and enter the spiral wound heat exchanger 9 for heat exchange. The heavy hydrocarbons are drawn from the lower outlet 3c on the shell side of the condenser 3 and return to the distillation column through the second inlet 4c of the distillation column 4. After heat exchange in the spiral wound heat exchanger 9, methane, ethane, and propane enter the second gas-liquid separator 6. Ethane and propane are liquefied into the liquid phase and drawn from the liquid phase outlet of the second gas-liquid separator 6. Methane and non-condensable gases are drawn from the gas phase outlet of the second gas-liquid separator 6 and then return to the spiral wound heat exchanger 9 for further cooling. Then they are separated in the seventh gas-liquid separation tank 30. Non-condensable gases such as nitrogen are drawn from the gas phase outlet of the seventh gas-liquid separation tank 30, and LNG enters the LNG storage tank from the liquid phase outlet of the seventh gas-liquid separation tank 30.

[0061] A control method for an unconventional natural gas liquefaction system includes two feedback control loops:

[0062] The first control loop sets the ratio of the liquid mixed refrigerant to the gas mixed refrigerant as the key control variable, and effectively controls the LNG outlet temperature by controlling the seventh expansion valve 26.

[0063] The specific implementation method is as follows: The first flowmeter 28 and the second flowmeter 29 measure the flow rate of the fluid in the pipeline where they are located, and transmit the signals to the flow ratio controller 27 through wires. The flow ratio controller 27 calculates and displays their ratio, compares the calculated ratio value with the pre-set ratio value, and then transmits the control signal to the fourth expansion valve 22 through wires. By adjusting the valve opening of the seventh expansion valve 26, the ratio of the gaseous refrigerant to the liquid refrigerant entering the spiral wound heat exchanger 9 is controlled.

[0064] The second control loop sets the temperature of the fluid at the third inlet 9g of the spiral wound heat exchanger 9 as the key control variable, and effectively controls the temperature of the light components in the distillation column 4 by controlling the first ball valve 23, thereby indirectly controlling the liquefaction rate of LNG.

[0065] The specific implementation method is as follows: The temperature controller 32 measures the temperature of the fluid in the pipeline where it is located, compares the measured temperature value with the pre-set temperature value at the same time, and then transmits the control signal to the first ball valve 23 through wires. By controlling the valve opening of the first ball valve 23, the flow rate of the refrigerant entering the condenser 3 is controlled.

[0066] Example 2:

[0067] An unconventional natural gas liquefaction system, as Figure 2 shown, includes a precooling system, a refrigeration system, and a cryogenic liquefaction system.

[0068] The connection mode of the precooling system is as follows:

[0069] The outlet 8b of the multi-stage compressor 8 is connected to the upper end inlet 34a of the third heat exchanger 34. The lower end outlet 34b of the third heat exchanger 34 is connected to the inlet of the first three-way valve 33 via the eighth expansion valve 37. The right end inlet 34c of the third heat exchanger 34 is connected to the external cooling water inlet, and the left end outlet 34d of the third heat exchanger 34 is connected to the external cooling water outlet. The first outlet of the first three-way valve 33 is connected to the first inlet ca of the multi-stream heat exchanger 1. The first outlet cb of the multi-stream heat exchanger 1 is connected to the inlet of the fourth gas-liquid separation tank 16. The gas phase outlet of the fourth gas-liquid separation tank 16 is connected to the first inlet 8c of the multi-stage compressor 8. The liquid phase outlet of the fourth gas-liquid separation tank 16 is connected to the second inlet cc of the multi-stream heat exchanger 1 via the first expansion valve 19. The second outlet cd of the multi-stream heat exchanger 1 is connected to the inlet of the fifth gas-liquid separation tank 17. The gas phase outlet of the fifth gas-liquid separation tank 17 is connected to the second inlet 8d of the multi-stage compressor 8. The liquid phase outlet of the fifth gas-liquid separation tank 17 is connected to the third inlet ce of the multi-stream heat exchanger 1 via the second expansion valve 20. The third outlet cf of the multi-stream heat exchanger 1 is connected to the inlet of the sixth gas-liquid separation tank 18. The gas phase outlet of the sixth gas-liquid separation tank 18 is connected to the third inlet 8e of the multi-stage compressor 8. The liquid phase outlet of the sixth gas-liquid separation tank 18 is connected to the fourth inlet cg of the multi-stream heat exchanger 1 via the third expansion valve 21. After the fourth outlet ch of the multi-stream heat exchanger 1 converges with the pipeline at the right end outlet 5b of the first heat exchanger 5 in the cryogenic liquefaction system, they are jointly connected to the fourth inlet 8a of the multi-stage compressor 8. The second outlet of the first three-way valve 33 is connected to the left end inlet 5a of the first heat exchanger 5 in the cryogenic liquefaction system.

[0070] In the multi-stream heat exchanger 1, the first inlet ca is connected to the first outlet cb through a pipeline, the second inlet cc is connected to the second outlet cd through a pipeline, the third inlet ce is connected to the third outlet cf through a pipeline, and the fourth inlet cg is connected to the fourth outlet ch through a pipeline.

[0071] The connection mode of the refrigeration system and the connection mode of the cryogenic liquefaction system are the same as those in Embodiment 1.

[0072] Working process of the precooling system: The cycle precooling is achieved by propane refrigerants at 4 different pressure levels. After being compressed by the multi-stage compressor 8, the propane refrigerants enter the third heat exchanger 34 for water cooling, and then expand through the eighth expansion valve 37. After the temperature is reduced, the propane refrigerants are branched through the first three-way valve 33. Approximately 75% to 90% of the refrigerants enter the multi-stream heat exchanger 1 to precool the raw natural gas. After that, they are led out from the first outlet cb of the multi-stream heat exchanger 1 of the multi-stream heat exchanger 1 and enter the fourth gas-liquid separation tank 16 for gas-liquid separation. The ultra-high pressure gaseous refrigerants obtained by separation enter the multi-stage compressor 8 through the first inlet 8c of the multi-stage compressor 8 for further compression. The liquid-phase refrigerants obtained by separation enter the multi-stream heat exchanger 1 for heat exchange after the temperature is reduced through the first expansion valve 19. After partial refrigerant methane is vaporized, gas-liquid separation is carried out in the fifth gas-liquid separation tank 17. The high-pressure gaseous refrigerants obtained by separation enter the multi-stage compressor 8 through the second inlet 8d of the multi-stage compressor 8 for further compression. The liquid-phase refrigerants obtained by separation enter the multi-stream heat exchanger 1 for heat exchange after the temperature is reduced through the second expansion valve 20. After partial methane refrigerant is vaporized, gas-liquid separation is carried out in the sixth gas-liquid separation tank 18. The medium-pressure gaseous refrigerants obtained by separation enter the multi-stage compressor 8 through the third inlet 8e of the multi-stage compressor 8 for compression. The liquid-phase refrigerants obtained by separation enter the multi-stream heat exchanger 1 for heat exchange after the temperature is reduced through the third expansion valve 21. After being vaporized, the low-pressure gaseous refrigerants are led out from the fourth outlet ch of the multi-stream heat exchanger 1 and then return to the multi-stage compressor 8 for compression.

[0073] The precooling cycle can precool the natural gas and refrigerant B to -40°C to -35°C by using propane at 4 pressure levels.

[0074] The advantages of choosing propane as the refrigerant for the precooling system include: 1. The system is easy to operate; 2. Propane generally exists in the raw gas and can be fractionated on-site; 3. The physical properties of propane are suitable for cooling within the required temperature range.

[0075] The remaining part of the refrigerants enters the first heat exchanger 5 to cool the methane led out from the top of the rectification column 4 and share the cooling load of the condenser 3. After the refrigerants led out from the fourth outlet of the multi-stream heat exchanger 1 and the right-end outlet 5b of the fifth heat exchanger 5 are converged, they all enter the multi-stage compressor 8 for compression and enter the next cycle.

[0076] The working process of the refrigeration system and the working process of the low-temperature liquefaction system are generally the same as those in Embodiment 1. The main difference is that the left-end outlet 13a of the second heat exchanger 13 is connected to the inlet of the eighth gas-liquid separation tank 14. The gas-phase outlet of the eighth gas-liquid separation tank 14 is connected to the inlet of the fourth compressor 10. The liquid-phase outlet of the eighth gas-liquid separation tank 14 is connected to the second refrigerant pump 15. After the outlet of the fourth compressor 10 and the pipeline at the outlet of the second refrigerant pump 15 are converged, they are jointly connected to the fifth inlet ba of the multi-stream heat exchanger 1.

[0077] After the refrigerant is compressed, gas-liquid separation is carried out. The liquid phase is compressed by the second refrigerant pump, and the gas phase is compressed by the compressor, so that the power consumption is smaller.

Claims

1. An unconventional natural gas liquefaction system, characterized in that: It includes a precooling system, a refrigeration system, and a cryogenic liquefaction system; The multi-stage compressor (8) of the precooling system is successively connected to the first three-way valve (33) via the third heat exchanger (34) and the eighth expansion valve (37), and then branched into two pipelines. The first pipeline is connected to the multi-stream heat exchanger (1), and the second pipeline is connected to the multi-stream heat exchanger (1) after converging with the outlet of the first heat exchanger (5) at the outlet of the multi-stream heat exchanger (1), and then jointly connected to the multi-stage compressor (8); The second heat exchanger (13) of the refrigeration system is connected to the first gas-liquid separation tank (2) via the multi-stream heat exchanger (1). Its gas-phase outlet is successively connected to the shell side of the spiral wound heat exchanger (9) via the spiral wound heat exchanger (9) and the sixth expansion valve (25). Its shell-side outlet is successively connected to the second heat exchanger (13) via a refrigerant pump (12) and the second compressor (11). Its liquid-phase outlet is branched into two pipelines. The first pipeline is connected to the fourth expansion valve (22), and the second pipeline is successively connected to the tube side of the ninth expansion valve (38) and the condenser (3) and then converges with the outlet of the fourth expansion valve (22), and jointly connected to the inlet of the spiral wound heat exchanger (9), and then connected to the shell side of the spiral wound heat exchanger (9) via the seventh expansion valve (26); In the cryogenic liquefaction system, the external raw material gas is connected to the distillation column (4) via the multi-stream heat exchanger (1). The top outlet (4b) of the distillation column (4) is connected to the shell side of the condenser (3) via the first heat exchanger (5). Its upper shell-side outlet is connected to the second gas-liquid separation tank (6) via the spiral wound heat exchanger (9). The liquid-phase outlet of the second gas-liquid separation tank (6) is connected to the external ethane or propane storage tank. Its gas-phase outlet is successively connected to the seventh gas-liquid separation tank (30) via the spiral wound heat exchanger (9) and the fifth expansion valve (24). The gas-phase outlet of the seventh gas-liquid separation tank (30) is connected to the outside, and its liquid-phase outlet is connected to the LNG storage tank (7). The pipeline at the bottom outlet (4d) of the distillation column (4) is branched into two pipelines. One pipeline is connected to the bottom inlet (4e) of the distillation column (4) via the sixth heat exchanger (36), and the other pipeline is connected to the external heavy hydrocarbon treatment system. The shell side of the condenser (3) is connected to the top inlet (4c) of the distillation column (4); The spiral wound heat exchanger (9) is provided with heat exchange tube one, heat exchange tube two, heat exchange tube three, and heat exchange tube four. Heat exchange tube one connects the first inlet (9q) and the first outlet (9i) of the spiral wound heat exchanger (9). The first outlet (9i) is connected to the seventh expansion valve (26). The seventh expansion valve (26) is connected to its shell side through the sixth inlet (9b) of the spiral wound heat exchanger (9). Heat exchange tube two connects the second inlet (9h) and the second outlet (9j) of the spiral wound heat exchanger (9). The second outlet (9j) is connected to the sixth expansion valve (25). The sixth expansion valve (25) is connected to its shell side through the fifth inlet (9c) of the spiral wound heat exchanger (9). Heat exchange tube three connects the third inlet (9g) and the third outlet (9f) of the spiral wound heat exchanger (9). The third outlet (9f) is connected to the second gas-liquid separation tank (6). Heat exchange tube four connects the fourth inlet (9e) and the fourth outlet (9d) of the spiral wound heat exchanger (9). The fourth outlet (9d) is connected to the fifth expansion valve (24).

2. The unconventional natural gas liquefaction system according to claim 1, wherein: The multi-stream heat exchanger (1) is provided with a first heat exchange tube, a second heat exchange tube, and a third heat exchange tube. The first heat exchange tube connects the first inlet (ca) and the fourth outlet (ch) of the multi-stream heat exchanger (1). The first three-way valve (33) is connected to the first heat exchange tube of the multi-stream heat exchanger (1) through the first inlet (ca) and converges at the fourth outlet (ch); the second heat exchange tube connects the fifth inlet (ba) and the fifth outlet (bb) of the multi-stream heat exchanger (1). The second heat exchanger (13) is connected to the second heat exchange tube of the multi-stream heat exchanger (1) through the fifth inlet (ba), and the fifth outlet (bb) is connected to the first gas-liquid separation tank (2); the third heat exchange tube connects the sixth inlet (aa) and the sixth outlet (ab) of the multi-stream heat exchanger (1). The external raw material gas is connected to the third heat exchange tube of the multi-stream heat exchanger (1) through the sixth inlet (aa), and the sixth outlet (ab) is connected to the distillation column (4). A third compressor (35) is provided between the external raw material gas and the sixth inlet (aa).

3. An unconventional natural gas liquefaction system according to claim 2, characterized in that: The pre-cooling system further includes a fourth gas-liquid separation tank (16), a fifth gas-liquid separation tank (17), a sixth gas-liquid separation tank (18), a first expansion valve (19), a second expansion valve (20), and a third expansion valve (21). The first heat exchange tube is of a multi-section structure, and between the first inlet (ca) and the fourth outlet (ch), it also successively passes through a first outlet (cb), a second inlet (cc), a second outlet (cd), a third inlet (ce), a third outlet (cf), and a fourth inlet (cg). The first outlet (cb) of the multi-stream heat exchanger (1) is connected to the inlet of the fourth gas-liquid separation tank (16). The gas-phase outlet of the fourth gas-liquid separation tank (16) is connected to the first inlet (8c) of the multi-stage compressor (8). The liquid-phase outlet of the fourth gas-liquid separation tank (16) is connected to the second inlet (cc) of the multi-stream heat exchanger (1) through the first expansion valve (19). The second outlet (cd) of the multi-stream heat exchanger (1) is connected to the inlet of the fifth gas-liquid separation tank (17). The gas-phase outlet of the fifth gas-liquid separation tank (17) is connected to the second inlet (8d) of the multi-stage compressor (8). The liquid-phase outlet of the fifth gas-liquid separation tank (17) is connected to the third inlet (ce) of the multi-stream heat exchanger (1) through the second expansion valve (20). The third outlet (cf) of the multi-stream heat exchanger (1) is connected to the inlet of the sixth gas-liquid separation tank (18). The gas-phase outlet of the sixth gas-liquid separation tank (18) is connected to the third inlet (8e) of the multi-stage compressor (8). The liquid-phase outlet of the sixth gas-liquid separation tank (18) is connected to the fourth inlet (cg) of the multi-stream heat exchanger (1) through the third expansion valve (21).

4. An unconventional natural gas liquefaction system according to claim 2, characterized in that: The refrigeration system further includes a fourth compressor (10), an eighth gas-liquid separation tank (14), and a second refrigerant pump (15). The left end outlet (13a) of the second heat exchanger (13) is connected to the inlet of the eighth gas-liquid separation tank (14). The gas phase outlet of the eighth gas-liquid separation tank (14) is connected to the inlet of the fourth compressor (10). The liquid phase outlet of the eighth gas-liquid separation tank (14) is connected to the second refrigerant pump (15). The outlets of the fourth compressor (10) and the second refrigerant pump (15) converge and are jointly connected to the fifth inlet (ba) of the multi-stream heat exchanger (1).

5. An unconventional natural gas liquefaction system according to claim 1, characterized in that: The tube side inlet (3d) and the tube side outlet (3e) of the condenser (3) are connected by a pipeline. The tube side inlet (3d) is connected to the ninth expansion valve (38). The tube side outlet (3e) is connected to the spiral wound heat exchanger (9). The shell side of the condenser (3) is also provided with an inlet and two outlets at intervals, namely the shell side inlet (3a), the upper shell side outlet (3b), and the lower shell side outlet (3c). The first heat exchanger (5) is connected to the shell side inlet (3a). The spiral wound heat exchanger (9) is connected to the upper shell side outlet (3b). The top inlet (4c) of the rectification column (4) is connected to the lower shell side outlet (3c).

6. An unconventional natural gas liquefaction system according to claim 1, characterized in that: It further includes a flow ratio controller (27), a first flowmeter (28), and a second flowmeter (29). A first flowmeter (28) is provided in the pipeline connecting the gas phase outlet of the first gas-liquid separation tank (2) to the spiral wound heat exchanger (9). A second flowmeter (29) is provided in the pipeline connecting the fourth expansion valve (22) to the spiral wound heat exchanger (9). The signals of the first flowmeter (28) and the second flowmeter (29) are connected to the flow ratio controller (27) through wires. The output signal of the flow ratio controller (27) is connected to the seventh expansion valve (26) through a wire.

7. An unconventional natural gas liquefaction system according to claim 6, characterized in that: It further includes a first ball valve (23), a check valve (31), and a temperature controller (32). In the two parallel pipelines at the liquid phase outlet of the first gas-liquid separation tank (2), the branch connected to the condenser (3) is further connected with a check valve (31) and a first ball valve (23) in sequence. A temperature controller (32) is provided in the pipeline connecting the condenser (3) and the spiral wound heat exchanger (9). The signal of the temperature controller (32) is connected to the first ball valve (23) through a wire.

8. A control method for an unconventional natural gas liquefaction system as claimed in claim 7, characterized in that: It includes two feedback control loops; The first control loop sets the ratio of the liquid mixed refrigerant and the gas mixed refrigerant as the key control variable, and effectively controls the LNG outlet temperature at the fourth outlet (9d) of the spiral wound heat exchanger (9) by controlling the seventh expansion valve (26); The second control loop sets the temperature of the spiral wound heat exchanger (9) as the key control variable, and effectively controls the temperature of the light components in the rectification column (4) by controlling the first ball valve (23), thereby indirectly controlling the liquefaction rate of LNG.

9. The control method of an unconventional natural gas liquefaction system according to claim 8, characterized in that The control method of the first control loop includes the following steps: Step 1: The first flowmeter (28) and the second flowmeter (29) measure the flow rate of the fluid in the pipeline where they are located, and transmit the signals to the flow ratio controller (27) through wires; Step 2: The flow ratio controller (27) calculates and displays the ratio, compares the calculated ratio value with a preset ratio value, and then transmits a control signal to the seventh expansion valve (26) through a wire, and controls the ratio of the gaseous refrigerant to the liquid refrigerant entering the spiral wound heat exchanger (9) by adjusting the valve opening of the seventh expansion valve (26); The control method of the second control loop includes the following steps: Step 1: The temperature controller (32) measures the temperature of the fluid in the pipeline where it is located; Step 2: The temperature controller (32) compares the measured temperature value with a preset temperature value, and then transmits a control signal to the first ball valve (23) through a wire, and controls the refrigerant flow rate entering the condenser (3) by controlling the valve opening of the first ball valve (23).

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