A cascaded refrigeration cycle LNG liquefaction system and method

Abstract

Embodiments of the present invention provide a cascaded refrigeration cycle LNG liquefaction system and method, which relate to the field of refrigeration cycles. The cascaded refrigeration cycle LNG liquefaction system includes a first refrigeration component, a second refrigeration component, and a third refrigeration component. The first refrigeration component, the second refrigeration component, and the third refrigeration component each include a compressor, a heat exchanger, an evaporator, and a separation tank; the refrigerant discharged from the compressor undergoes heat exchange and separation; the separated gas phase returns to the compressor after recovering sensible heat through the heat exchanger; by adding a new heat exchanger and adjusting the connection modes of multiple compressors, heat exchangers, evaporators, and separation tanks in the cascaded refrigeration cycle LNG liquefaction system, on the one hand, the cold energy of the gas-phase refrigerant is recovered, and on the other hand, it is ensured that the temperature of the gas-phase refrigerant returning to the compression unit is normal temperature, and a normal-temperature compressor can be selected for the compressor, reducing the cost investment of the compressor.

Description

Technical Field

[0001] The present invention relates to the field of refrigeration cycles, and in particular, to a cascaded refrigeration cycle LNG liquefaction system and method. Background Art

[0002] The cascaded refrigeration cycle is a relatively classic refrigeration cycle, also known as "step-by-step", "cascade" or "tandem". This cycle is composed of several refrigeration cycles operating at different low temperatures in cascade. The cascaded refrigeration cycle was first applied to liquefied natural gas products in 1939, using NH 3 、C 2 H 4 as the first and second stage refrigerants. However, the classic cascaded refrigeration cycle is a three-stage refrigeration cycle process using C 3 H 8 、C 2 H 4 and CH 4 as refrigerants, adopting three-stage throttling and low-temperature compressors to provide the required cooling capacity for purifying natural gas liquefaction.

[0003] However, in the existing cascaded refrigeration cycle, after the gas-liquid two-phase is separated in the separation tank, the gas phase directly returns to the low-temperature compressor, so the cost investment of the compressor is relatively large.

[0004] In view of this, the present application is specifically proposed. Summary of the Invention

[0005] The objectives of the present invention include, for example, providing a cascaded refrigeration cycle LNG liquefaction system and method.

[0006] The embodiments of the present invention can be implemented as follows:

[0007] In a first aspect, the present invention provides a cascaded refrigeration cycle LNG liquefaction system, including a first refrigeration component, a second refrigeration component, and a third refrigeration component;

[0008] The first refrigeration component includes a normal-temperature first refrigerant compressor, a first refrigerant heat exchanger, and a first refrigerant evaporator. The second refrigeration component includes a normal-temperature second refrigerant compressor, a second refrigerant heat exchanger, and a second refrigerant evaporator. The third refrigeration component includes a normal-temperature third refrigerant compressor, a third refrigerant heat exchanger, and a third refrigerant evaporator;

[0009] The refrigerants discharged from the normal-temperature first refrigerant compressor, the normal-temperature second refrigerant compressor, and the normal-temperature third refrigerant compressor are respectively evaporated and separated through the first refrigerant evaporator, the second refrigerant evaporator, and the third refrigerant evaporator;

[0010] The gaseous phase of the first refrigerant separated by the first refrigerant evaporator returns to the normal-temperature first refrigerant compressor after recovering sensible heat through the first refrigerant heat exchanger.

[0011] The gaseous phase of the second refrigerant separated by the second refrigerant evaporator returns to the normal-temperature second refrigerant compressor after recovering sensible heat through the second refrigerant heat exchanger.

[0012] The gaseous phase of the third refrigerant separated by the third refrigerant evaporator returns to the normal-temperature first refrigerant compressor after recovering sensible heat through the third refrigerant heat exchanger.

[0013] In an alternative embodiment, the first refrigeration assembly further includes a first refrigerant separation tank, the second refrigeration assembly further includes a second refrigerant separation tank, and the third refrigeration assembly further includes a third refrigerant separation tank; the refrigerant discharged from the normal-temperature first refrigerant compressor, the normal-temperature second refrigerant compressor, and the normal-temperature third refrigerant compressor first undergoes evaporation through the first refrigerant evaporator, the second refrigerant evaporator, and the third refrigerant evaporator respectively, and then enters the first refrigerant separation tank, the second refrigerant separation tank, and the third refrigerant separation tank for separation; alternatively, the refrigerant first undergoes separation through the first refrigerant separation tank, the second refrigerant separation tank, and the third refrigerant separation tank respectively, and then enters the first refrigerant evaporator, the second refrigerant evaporator, and the third refrigerant evaporator for evaporation.

[0014] In an alternative embodiment, the gaseous phase of the second refrigerant enters the first refrigerant heat exchanger for further heat exchange after passing through the second refrigerant heat exchanger; the gaseous phase of the third refrigerant passes through at least one of the second refrigerant heat exchanger and the first refrigerant heat exchanger for further heat exchange after passing through the third refrigerant heat exchanger.

[0015] In an alternative embodiment, before the first refrigerant discharged from the normal-temperature first refrigerant compressor enters the first refrigerant evaporator, it further includes passing the first refrigerant through the first refrigerant heat exchanger for heat exchange.

[0016] Before the first refrigerant discharged from the normal-temperature second refrigerant compressor enters the second refrigerant evaporator, it further includes passing the second refrigerant through the first refrigerant heat exchanger and the first refrigerant evaporator for heat exchange.

[0017] Before the third refrigerant discharged from the normal-temperature third refrigerant compressor enters the third refrigerant evaporator, it further includes passing the third refrigerant through the first refrigerant heat exchanger, the first refrigerant evaporator, the second refrigerant heat exchanger, and the second refrigerant evaporator for heat exchange.

[0018] In an alternative embodiment, the first refrigerant heat exchanger and the first refrigerant evaporator are integrally provided, the second refrigerant heat exchanger and the second refrigerant evaporator are integrally provided, and the third refrigerant heat exchanger and the third refrigerant evaporator are integrally provided.

[0019] In an alternative embodiment, the first refrigerant heat exchanger and the first refrigerant evaporator are separately provided and have N-stage heat exchange and N-stage evaporation, the second refrigerant heat exchanger and the second refrigerant evaporator are separately provided and have N-stage heat exchange and N-stage evaporation, and the third refrigerant heat exchanger and the third refrigerant evaporator are separately provided and have N-stage heat exchange and N-stage evaporation.

[0020] In an alternative embodiment, N in the N-stage heat exchange and N-stage evaporation is greater than or equal to 2, and the number of the first refrigerant separation tank, the second refrigerant separation tank, and the third refrigerant separation tank is N-1.

[0021] In an alternative embodiment, a first liquid-phase throttle valve is provided on the first refrigerant inlet pipeline of each stage of the first refrigerant evaporator, a second liquid-phase throttle valve is provided on the second refrigerant inlet pipeline of each stage of the second refrigerant evaporator, and a third liquid-phase throttle valve is provided on the third refrigerant inlet pipeline of each stage of the third refrigerant evaporator.

[0022] In a second aspect, the present invention provides a cascaded refrigeration cycle LNG liquefaction method, which is carried out by using the cascaded refrigeration cycle LNG liquefaction system according to any one of the foregoing embodiments, and includes the following steps:

[0023] Evaporating and separating the first refrigerant discharged from the normal-temperature first refrigerant compressor through the first refrigerant evaporator into a gas-liquid two-phase state. Among them, the gas phase enters the inlet of the first refrigerant compressor after recovering the sensible heat of the gas phase through the first refrigerant heat exchanger, and the liquid phase enters the first refrigerant heat exchanger after recovering the latent heat of the liquid phase through the first refrigerant evaporator to recover the sensible heat of the gas phase and enters the inlet of the first refrigerant compressor;

[0024] Evaporating and separating the second refrigerant discharged from the normal-temperature second refrigerant compressor through the second refrigerant evaporator into a gas-liquid two-phase state. Among them, the gas phase sequentially enters the inlet of the second refrigerant compressor after recovering the sensible heat of the gas phase through the second refrigerant heat exchanger, and the liquid phase enters the second refrigerant heat exchanger and the first refrigerant heat exchanger after recovering the latent heat of the liquid phase through the second refrigerant evaporator to recover the sensible heat of the gas phase and enters the inlet of the second refrigerant compressor;

[0025] The third refrigerant discharged from the normal-temperature third refrigerant compressor is evaporated and separated into a gas-liquid two-phase through a third refrigerant evaporator. Among them, the gas phase sequentially enters the inlet of the third refrigerant compressor after recovering the sensible heat of the gas phase through the third refrigerant heat exchanger, and the liquid phase enters the third refrigerant heat exchanger, the second refrigerant heat exchanger, and the first refrigerant heat exchanger in sequence after recovering the latent heat of the liquid phase through the third refrigerant evaporator to recover the sensible heat of the gas phase and enters the inlet of the third refrigerant compressor.

[0026] In an alternative embodiment, the first refrigerant, the second refrigerant, and the third refrigerant are different from each other and are each selected from any one of nitrogen, methane, ethane, ethylene, propane, propylene, butane, butene, pentane, and pentene.

[0027] In an alternative embodiment, when the first refrigerant is propane, the second refrigerant is ethylene, and the third refrigerant is methane, the pressure of the first refrigerant discharged from the normal-temperature first refrigerant compressor ≥ 0.47 MpaA, and the pressure of the first refrigerant entering the first refrigerant evaporator is 0.1 - 0.47 MPaA, and the temperature is -43 - 0 °C;

[0028] The pressure of the second refrigerant discharged from the normal-temperature second refrigerant compressor ≥ 1.38 MPaA, and the pressure of the second refrigerant entering the second refrigerant evaporator is 0.1 - 1.38 MPaA, and the temperature is -104 - -42 °C;

[0029] The pressure of the third refrigerant discharged from the normal-temperature third refrigerant compressor ≥ 2.44 MPaA, and the pressure of the third refrigerant entering the third refrigerant evaporator is 0.1 - 2.44 MPaA, and the temperature is -161.8 - -102 °C.

[0030] The beneficial effects of the embodiments of the present invention include, for example:

[0031] The embodiments of the present invention provide a cascaded refrigeration cycle LNG liquefaction system, which forms a new cascaded refrigeration cycle LNG liquefaction process by adding a first refrigerant heat exchanger, a second refrigerant heat exchanger, and a third refrigerant heat exchanger, and adjusting the connection modes of multiple compressors, heat exchangers, evaporators, and separation tanks in the cascaded refrigeration cycle LNG liquefaction system 100. On the one hand, the present invention recovers the cold energy of the gaseous first refrigerant, gaseous second refrigerant, and gaseous third refrigerant, and on the other hand, ensures that the temperature of the gaseous refrigerant of the return compression unit is at normal temperature, and a normal-temperature compressor can be selected for the compressor, reducing the cost investment of the compressor. The unit energy consumption index of the inventive cascaded refrigeration cycle LNG liquefaction system 100 is 0.25 kW / Nm 3 Purified natural gas, compared with the conventional cascaded refrigeration cycle system (0.30 kW / Nm 3The purified natural gas is reduced by approximately 15% compared to the MRC mixed refrigerant refrigeration cycle system (0.33 kW / Nm 3 The purified natural gas is reduced by approximately 25%. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention, and therefore should not be regarded as a limitation of the scope. For those of ordinary skill in the art, without creative efforts, other related drawings can also be obtained based on these drawings.

[0033] Figure 1 Process flow diagram of the cascade refrigeration cycle LNG liquefaction system provided in Embodiment 1 of the present application;

[0034] Figure 2 Flow channel diagram in the process flow diagram of the cascade refrigeration cycle LNG liquefaction system provided in Embodiment 1 of the present application;

[0035] Figure 3 Process flow diagram of the cascade refrigeration cycle LNG liquefaction system provided in Embodiment 2 of the present application;

[0036] Figure 4 Process flow diagram of the cascade refrigeration cycle LNG liquefaction system provided in Embodiment 3 of the present application;

[0037] Figure 5 Process flow diagram of the cascade refrigeration cycle LNG liquefaction system provided in Embodiment 4 of the present application;

[0038] Figure 6 Schematic diagram of the integrated process flow of the cascade refrigeration cycle LNG liquefaction system provided in Embodiment 4 of the present application;

[0039] Figure 7 Process flow diagram of the conventional cascade refrigeration cycle LNG liquefaction system provided in Comparative Example 1 of the present application.

[0040] Reference numerals: 100 - cascade refrigeration cycle LNG liquefaction system;

[0041] 110 - First refrigeration assembly; 111 - Normal temperature first refrigerant compressor; 112 - First buffer tank; 113 - First refrigerant heat exchanger; 1131 - First - stage first refrigerant heat exchanger; 1132 - Second - stage first refrigerant heat exchanger; 1133 - Third - stage first refrigerant heat exchanger; 114 - First refrigerant evaporator; 1141 - First - stage first refrigerant evaporator; 1142 - Second - stage first refrigerant evaporator; 1143 - Third - stage first refrigerant evaporator; 115 - First refrigerant separation tank; 1151 - First - stage first refrigerant separation tank; 1152 - Second - stage first refrigerant separation tank; 116 - First refrigerant throttle valve; 117 - First liquid - phase throttle valve; 118 - First refrigerant heat - exchange and evaporation integrator; 119 - First refrigerant cooler;

[0042] 120 - Second refrigeration assembly; 121 - Normal temperature second refrigerant compressor; 122 - Second buffer tank; 123 - Second refrigerant heat exchanger; 1231 - First - stage second refrigerant heat exchanger; 1232 - Second - stage second refrigerant heat exchanger; 1233 - Third - stage second refrigerant heat exchanger; 124 - Second refrigerant evaporator; 1241 - First - stage second refrigerant evaporator; 1242 - Second - stage second refrigerant evaporator; 1243 - Third - stage second refrigerant evaporator; 125 - Second refrigerant separation tank; 1251 - First - stage second refrigerant separation tank; 1252 - Second - stage second refrigerant separation tank; 126 - Second refrigerant throttle valve; 127 - Second liquid - phase throttle valve; 128 - Second refrigerant heat - exchange and evaporation integrator; 129 - Second refrigerant cooler;

[0043] 130 - Third refrigeration assembly; 131 - Normal temperature third refrigerant compressor; 132 - Third buffer tank; 133 - Third refrigerant heat exchanger; 1331 - First - stage third refrigerant heat exchanger; 1332 - Second - stage third refrigerant heat exchanger; 134 - Third refrigerant evaporator; 1341 - First - stage third refrigerant evaporator; 1342 - Second - stage third refrigerant evaporator; 1343 - Third - stage third refrigerant evaporator; 135 - Third refrigerant separation tank; 1351 - First - stage third refrigerant separation tank; 1352 - Second - stage third refrigerant separation tank; 136 - Third refrigerant throttle valve; 137 - Third liquid - phase throttle valve; 1371 - First - stage third liquid - phase throttle valve; 1372 - Second - stage third liquid - phase throttle valve; 138 - Third refrigerant heat - exchange and evaporation integrator; 139 - Third refrigerant cooler;

[0044] 201 - First high - pressure flow path; 202 - Second high - pressure flow path; 203 - Third high - pressure flow path; 204 - Raw material gas flow path; 205 - First gas - phase sensible heat recovery flow path; 206 - First liquid - to - gas - phase sensible heat recovery flow path; 207 - First - stage first liquid - phase latent heat recovery flow path; 208 - Second - stage first liquid - phase latent heat recovery flow path; 209 - Second gas - phase sensible heat recovery flow path; 210 - Second liquid - to - gas - phase sensible heat recovery flow path; 211 - First - stage second liquid - phase latent heat recovery flow path; 212 - Second - stage second liquid - phase latent heat recovery flow path; 213 - Third gas - phase sensible heat recovery flow path; 214 - Third liquid - to - gas - phase sensible heat recovery flow path; 215 - First - stage third liquid - phase latent heat recovery flow path; 216 - Second - stage third liquid - phase latent heat recovery flow path;

[0045] 300 - Conventional cascaded refrigeration cycle LNG liquefaction system; 301 - Low - temperature propylene compressor unit; 302 - Low - temperature ethylene compressor unit; 303 - Low - temperature methane compressor unit; 304 - Propylene first - stage separation tank; 305 - Propylene second - stage separation tank; 306 - Ethylene first - stage separation tank; 307 - Ethylene second - stage separation tank; 308 - Methane separation tank; 309 - Propylene first - stage evaporator; 310 - Propylene second - stage evaporator; 311 - Propylene third - stage evaporator; 312 - Ethylene first - stage evaporator; 313 - Ethylene second - stage evaporator; 314 - Ethylene third - stage evaporator; 315 - Methane heat exchanger; 316 - High - pressure propylene cooler; 317 - High - pressure ethylene cooler; 318 - High - pressure methane cooler; 319 - High - pressure propylene buffer tank; 320 - High - pressure ethylene buffer tank; 321 - High - pressure methane buffer tank;

[0046] A - Refrigerant compressor unit; B - On - vehicle LNG liquefaction tank. Detailed implementation manners

[0047] To make the objectives, 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. Apparently, the described embodiments are some, but not all, of the embodiments of the present invention. Usually, the components of the embodiments of the present invention described and illustrated in the accompanying drawings here can be arranged and designed in various different configurations.

[0048] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed present invention, but merely represents selected embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0049] It should be noted that: Similar reference numerals and letters denote similar items in the following drawings. Therefore, once an item is defined in one drawing, it does not need to be further defined and explained in subsequent drawings.

[0050] In the description of the present invention, it should be noted that if terms such as "upper", "lower", "inner", "outer", etc. are used to indicate the orientation or positional relationship, it is based on the orientation or positional relationship shown in the drawings, or the orientation or positional relationship in which the invention product is usually placed during use. This is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, it should not be construed as a limitation to the present invention.

[0051] In addition, if terms such as "first", "second", etc. are only used for distinguishing descriptions, they cannot be understood as indicating or implying relative importance.

[0052] It should be noted that, without conflict, the features in the embodiments of the present invention can be combined with each other.

[0053] Please refer to Figures 1-5 , this embodiment provides a cascaded refrigeration cycle LNG liquefaction system 100, including a first refrigeration component 110, a second refrigeration component 120, and a third refrigeration component 130;

[0054] The first refrigeration component 110 includes a normal-temperature first refrigerant compressor 111, a first buffer tank 112, a first refrigerant heat exchanger 113, a first refrigerant evaporator 114, a first refrigerant separation tank 115, and a first refrigerant cooler 119;

[0055] The second refrigeration component 120 includes a normal-temperature second refrigerant compressor 121, a second buffer tank 122, a second refrigerant heat exchanger 123, a second refrigerant evaporator 124, a second refrigerant separation tank 125, and a second refrigerant cooler 129;

[0056] The third refrigeration component 130 includes a normal-temperature third refrigerant compressor 131, a third buffer tank 132, a third refrigerant heat exchanger 133, a third refrigerant evaporator 134, a third refrigerant separation tank 135, and a third refrigerant cooler 139.

[0057] The refrigerants discharged from the normal-temperature first refrigerant compressor 111, the normal-temperature second refrigerant compressor 121, and the normal-temperature third refrigerant compressor 131 are evaporated and separated through the first refrigerant evaporator, the second refrigerant evaporator, and the third refrigerant evaporator;

[0058] The gaseous phase of the first refrigerant separated by the first refrigerant evaporator returns to the normal-temperature first refrigerant compressor 111 after recovering sensible heat through the first refrigerant heat exchanger 113; the gaseous phase of the second refrigerant separated by the second refrigerant evaporator returns to the normal-temperature second refrigerant compressor 121 after recovering sensible heat through the second refrigerant heat exchanger 123; the gaseous phase of the third refrigerant separated by the third refrigerant evaporator returns to the normal-temperature first refrigerant compressor 111 after successively recovering sensible heat through the third refrigerant heat exchanger 133.

[0059] In this application, the first refrigeration assembly 110 further includes a first refrigerant separation tank 115, the first refrigeration assembly 110 further includes a second refrigerant separation tank 125, and the first refrigeration assembly 110 further includes a third refrigerant separation tank 135;

[0060] There are two ways to evaporate and separate the refrigerant discharged from the normal-temperature first refrigerant compressor 111, the normal-temperature second refrigerant compressor 121, and the normal-temperature third refrigerant compressor 131. It can first be evaporated and then separated, or it can first be separated and then evaporated. Specifically, the refrigerant can first pass through the first refrigerant evaporator 114, the second refrigerant evaporator 124, and the third refrigerant evaporator 134 respectively for evaporation, and then enter the first refrigerant separation tank 115, the second refrigerant separation tank 125, and the third refrigerant separation tank 135 for separation; or the refrigerant first passes through the first refrigerant separation tank 115, the second refrigerant separation tank 125, and the third refrigerant separation tank 135 respectively for separation, and then enters the first refrigerant evaporator for evaporation, the second refrigerant evaporator 124, and the third refrigerant evaporator 134 for evaporation. In this application, by adding the first refrigerant heat exchanger 113, the second refrigerant heat exchanger 123, and the third refrigerant heat exchanger 133, on the one hand, the cold energy of the gaseous-phase first refrigerant, gaseous-phase second refrigerant, and gaseous-phase third refrigerant is recovered, and on the other hand, it is ensured that the temperature of the gaseous refrigerant returning to the compression unit is normal temperature, and a normal-temperature compressor can be selected for the compressor, reducing the cost investment of the compressor.

[0061] Further, to improve the heat exchange efficiency, in the present application, after passing through the second refrigerant heat exchanger 123, the second refrigerant gas phase can further enter the first refrigerant heat exchanger 113 for continued heat exchange, and then be introduced into the normal temperature second refrigerant compressor 121 after heat exchange in the first refrigerant heat exchanger 113; after passing through the third refrigerant heat exchanger 133, the third refrigerant gas phase can further pass through at least one of the second refrigerant heat exchanger 123 and the first refrigerant heat exchanger 113 for continued heat exchange, and then be introduced into the normal temperature third refrigerant compressor 131 after heat exchange is completed. It can be seen from the heat exchange route that the heat exchange route of the present application can be adjusted or selected according to the actual situation to achieve a better heat exchange effect, while ensuring that the temperature of the gaseous refrigerant after heat exchange is normal temperature, and a normal temperature compressor can be selected for the compressor, reducing the cost investment of the compressor.

[0062] In addition, to avoid a large temperature difference when the refrigerant discharged from the compressor directly enters the evaporator or the separation tank, in the present application, before the first refrigerant discharged from the normal temperature first refrigerant compressor 111 enters the first refrigerant evaporator, it further includes passing the first refrigerant through the first refrigerant heat exchanger 113 for heat exchange; before the second refrigerant discharged from the normal temperature second refrigerant compressor 121 enters the second refrigerant evaporator, it further includes passing the second refrigerant through the first refrigerant heat exchanger 113 and the first refrigerant evaporator 114 for heat exchange; before the third refrigerant discharged from the normal temperature third refrigerant compressor 131 enters the third refrigerant evaporator, it further includes passing the third refrigerant through the first refrigerant heat exchanger 113, the first refrigerant evaporator 114, the second refrigerant heat exchanger 123, and the second refrigerant evaporator 124 for heat exchange. Through the settings of the first refrigerant heat exchanger 113, the second refrigerant heat exchanger 123, and the third refrigerant heat exchanger 133 in the present application, the temperature difference between the compressor and the evaporator or the separation tank can be reduced, making the temperature gradient more, the product temperature lower, and the device more energy-efficient.

[0063] In this application, multiple flow channels are provided in each heat exchanger and evaporator to facilitate the entry and exit of different materials. In this application, a passage for the same substance is named a flow channel. Specifically, the first refrigerant heat exchanger 113 in this application is provided with a first high-pressure flow channel 201 for the entry and exit of high-pressure first refrigerant, and the first refrigerant heat exchanger 113 and the first refrigerant evaporator 114 are both provided with a second high-pressure flow channel 202 for the sequential entry and exit of high-pressure second refrigerant. The first refrigerant heat exchanger 113, the first refrigerant evaporator 114, the second refrigerant heat exchanger 123, and the second refrigerant evaporator 124 are all provided with a third high-pressure flow channel 203 for the sequential entry and exit of high-pressure third refrigerant; the first refrigerant heat exchanger 113, the first refrigerant evaporator 114, the second refrigerant heat exchanger 123, the second refrigerant evaporator 124, the third refrigerant heat exchanger 133, and the third refrigerant evaporator 134 are all provided with a raw material gas flow channel 204 for the sequential entry and exit of raw material gas.

[0064] The first refrigerant heat exchanger 113 is provided with a first gas-phase sensible heat recovery flow channel 205 and a first liquid-to-gas-phase sensible heat recovery flow channel 206. The first refrigerant evaporator 114 is provided with a first-stage first liquid-phase latent heat recovery flow channel 207 and a second-stage first liquid-phase latent heat recovery flow channel 208; the first refrigerant heat exchanger 113 and the second refrigerant heat exchanger 123 are both provided with a second gas-phase sensible heat recovery flow channel 209 and a second liquid-to-gas-phase sensible heat recovery flow channel 210. The second refrigerant evaporator 124 is provided with a first-stage second liquid-phase latent heat recovery flow channel 211 and a second-stage second liquid-phase latent heat recovery flow channel 212; the first refrigerant heat exchanger 113, the second refrigerant heat exchanger 123, and the third refrigerant heat exchanger 133 are all provided with a third gas-phase sensible heat recovery flow channel 213 and a third liquid-to-gas-phase sensible heat recovery flow channel 214. The second refrigerant evaporator 124 is provided with a first-stage third liquid-phase latent heat recovery flow channel 215 and a second-stage third liquid-phase latent heat recovery flow channel 216.

[0065] Please refer to Figure 2 , the outlet of the normal-temperature first refrigerant compressor 111 is communicated with the first high-pressure flow channel 201. The first high-pressure flow channel 201 is communicated with the first-stage first liquid-phase latent heat recovery flow channel 207. A first refrigerant throttle valve 116 is provided on the pipeline between the first high-pressure flow channel 201 and the first-stage first liquid-phase latent heat recovery flow channel 207. The first-stage first liquid-phase latent heat recovery flow channel 207 is communicated with the first refrigerant separation tank 115. The gas-phase outlet of the first refrigerant separation tank 115 is communicated with the first gas-phase sensible heat recovery flow channel 205. The liquid-phase outlet of the first refrigerant separation tank 115 is communicated with the second-stage first liquid-phase latent heat recovery flow channel 208. The second-stage first liquid-phase latent heat recovery flow channel 208 is communicated with the first liquid-to-gas-phase sensible heat recovery flow channel 206. Both the first gas-phase sensible heat recovery flow channel 205 and the first liquid-to-gas-phase sensible heat recovery flow channel 206 are communicated with the inlet of the normal-temperature first refrigerant compressor 111;

[0066] The outlet of the normal-temperature second refrigerant compressor 121 is communicated with the second high-pressure flow channel 202. The second high-pressure flow channel 202 is communicated with the first-stage second liquid-phase latent heat recovery flow channel 211. A second refrigerant throttle valve 126 is arranged on the pipeline between the second high-pressure flow channel 202 and the first-stage second liquid-phase latent heat recovery flow channel 211. The first-stage second liquid-phase latent heat recovery flow channel 211 is communicated with the second refrigerant separation tank 125. The gas-phase outlet of the second refrigerant separation tank 125 is communicated with the second gas-phase sensible heat recovery flow channel 209. The liquid-phase outlet of the second refrigerant separation tank 125 is communicated with the second-stage second liquid-phase latent heat recovery flow channel 212. The second-stage second liquid-phase latent heat recovery flow channel 212 is communicated with the second liquid-to-gas-phase sensible heat recovery flow channel 210. Both the second gas-phase sensible heat recovery flow channel 209 and the second liquid-to-gas-phase sensible heat recovery flow channel 210 are communicated with the inlet of the normal-temperature second refrigerant compressor 121;

[0067] The outlet of the normal-temperature third refrigerant compressor 131 is communicated with the third high-pressure flow channel 203. The third high-pressure flow channel 203 is communicated with the first-stage third liquid-phase latent heat recovery flow channel 215. A third refrigerant throttle valve 136 is arranged on the pipeline between the third high-pressure flow channel 203 and the first-stage third liquid-phase latent heat recovery flow channel 215. The first-stage third liquid-phase latent heat recovery flow channel 215 is communicated with the third refrigerant separation tank 135. The gas-phase outlet of the third refrigerant separation tank 135 is communicated with the third gas-phase sensible heat recovery flow channel 213. The liquid-phase outlet of the third refrigerant separation tank 135 is communicated with the second-stage third liquid-phase latent heat recovery flow channel 216. The second-stage third liquid-phase latent heat recovery flow channel 216 is communicated with the third liquid-to-gas-phase sensible heat recovery flow channel 214. Both the third gas-phase sensible heat recovery flow channel 213 and the third liquid-to-gas-phase sensible heat recovery flow channel 214 are communicated with the inlet of the normal-temperature third refrigerant compressor 131.

[0068] It should be understood that Figure 2 only a typical but non-limiting example is shown in this application. For example, in other embodiments, “the first-stage second liquid-phase latent heat recovery flow channel 211 is communicated with the second refrigerant separation tank 125” can also be replaced with “the first-stage second liquid-phase latent heat recovery flow channel 211 is communicated with the first refrigerant evaporator 114”. Therefore, there can be various replacement options for the connection manner in this application, and some of the structures can be omitted.

[0069] In this application, the first refrigerant, the second refrigerant, and the third refrigerant are respectively introduced into the normal-temperature first refrigerant compressor 111, the normal-temperature second refrigerant compressor 121, and the normal-temperature third refrigerant compressor 131 for pressurization operations, and then throttled through throttle valves. Among them, the designs of the first liquid-phase throttle valve 117, the second liquid-phase throttle valve 127, and the third liquid-phase throttle valve 137 facilitate the pressure reduction of the liquid phase separated from gas-liquid separation. Gradually, heat exchange is carried out with the purified natural gas in subsequent heat exchangers (the first refrigerant heat exchanger 113, the second refrigerant heat exchanger 123, and the third refrigerant heat exchanger 133) and evaporators (the first refrigerant evaporator 114, the second refrigerant evaporator 124, and the third refrigerant evaporator 134), thereby providing cooling capacity for the liquefaction of the purified natural gas.

[0070] The first buffer tank 112 is arranged between the normal-temperature first refrigerant compressor 111 and the first high-pressure flow channel 201, the second buffer tank 122 is arranged between the second high-pressure flow channel 202 and the first-stage second liquid-phase latent heat recovery flow channel 211, and the third buffer tank 132 is arranged between the third high-pressure flow channel 203 and the first-stage third liquid-phase latent heat recovery flow channel 215. In this application, through the first buffer tank 112, the second buffer tank 122, and the third buffer tank 132, the refrigerant increased by the compressor can be buffered to a certain extent, which is beneficial to protecting the stability of the subsequent flow channels.

[0071] In this application, the number and setting method of the heat exchangers and evaporators can be adjusted according to the actual situation. For example, different heat exchangers and evaporators can be set for different scales of natural gas liquefaction processing capacity or application scenarios.

[0072] For example, when applied to a small-scale device, the first refrigerant heat exchanger 113 and the first refrigerant evaporator 114 are integrated and set, the second refrigerant heat exchanger 123 and the second refrigerant evaporator 124 are integrated and set, and the third refrigerant heat exchanger 133 and the third refrigerant evaporator 134 are integrated and set. At this time, the device has a high integration degree and a smaller floor area.

[0073] For example, when applied to vehicle-mounted LNG, the normal-temperature first refrigerant compressor 111, the normal-temperature second refrigerant compressor 121, and the normal-temperature third refrigerant compressor 131 are highly integrated into a refrigerant compression unit; the first refrigerant heat exchanger 113, the first refrigerant evaporator 114, the second refrigerant heat exchanger 123, the second refrigerant evaporator 124, the third refrigerant heat exchanger 133, and the third refrigerant evaporator 134 are highly integrated into a vehicle-mounted LNG liquefaction tank to realize the application of vehicle-mounted LNG. At this time, the device has a high integration degree and a smaller floor area.

[0074] For example, when applied to large-scale devices, the first refrigerant heat exchanger 113 and the first refrigerant evaporator 114 are separately arranged and have N-stage heat exchange and N-stage evaporation. The second refrigerant heat exchanger 123 and the second refrigerant evaporator 124 are separately arranged and have N-stage heat exchange and N-stage evaporation. The third refrigerant heat exchanger 133 and the third refrigerant evaporator 134 are separately arranged and have N-stage heat exchange and N-stage evaporation. In the N-stage heat exchange and N-stage evaporation, N is greater than or equal to 2, and the numbers of the first refrigerant separation tank 115, the second refrigerant separation tank 125, and the third refrigerant separation tank 135 are N - 1.

[0075] A first liquid-phase throttle valve 117 is provided on the first refrigerant inlet pipeline of the first refrigerant evaporator 114 at each stage. A second liquid-phase throttle valve 127 is provided on the second refrigerant inlet pipeline of the second refrigerant evaporator 124 at each stage. A third liquid-phase throttle valve 137 is provided on the third refrigerant inlet pipeline of the third refrigerant evaporator 134 at each stage.

[0076] In addition, the present application also provides a cascaded refrigeration cycle LNG liquefaction method, which is carried out using the cascaded refrigeration cycle LNG liquefaction system 100 according to any one of the foregoing embodiments, and includes the following steps:

[0077] The first refrigerant discharged from the normal-temperature first refrigerant compressor 111 is passed through the first refrigerant heat exchanger 113 for heat exchange, and then introduced into the first refrigerant evaporator to recover the latent heat. Subsequently, it is separated into gas-liquid two phases by the first refrigerant separation tank 115. Among them, the gas phase enters the inlet of the first refrigerant compressor after recovering the sensible heat of the gas phase through the first refrigerant heat exchanger 113, and the liquid phase enters the first refrigerant heat exchanger 113 after recovering the latent heat of the liquid phase through the first refrigerant evaporator 114 to recover the sensible heat of the gas phase and enters the inlet of the first refrigerant compressor;

[0078] The second refrigerant discharged from the normal-temperature second refrigerant compressor 121 is first precooled by the first refrigerant heat exchanger and the first refrigerant evaporator 114, then the latent heat is recovered through the second refrigerant evaporator 124, and subsequently, it is separated into gas-liquid two phases by the second refrigerant separation tank 125. Among them, the gas phase enters the inlet of the second refrigerant compressor after sequentially recovering the sensible heat of the gas phase through the second refrigerant heat exchanger 123 and the first refrigerant heat exchanger 113, and the liquid phase enters the inlet of the second refrigerant compressor after sequentially recovering the latent heat of the liquid phase through the second refrigerant evaporator 124 and the second refrigerant heat exchanger 123 and the first refrigerant heat exchanger 113 to recover the sensible heat of the gas phase;

[0079] The third refrigerant discharged from the normal-temperature third refrigerant compressor 131 is first precooled by the first refrigerant heat exchanger 113, the first refrigerant evaporator 114, the second refrigerant heat exchanger 123, and the second refrigerant evaporator 124, then the latent heat is recovered by the third refrigerant evaporator 134, and then the gas-liquid two-phase is separated by the third refrigerant separation tank 135. Among them, the gas phase sequentially passes through the third refrigerant heat exchanger 133, the second refrigerant heat exchanger 123, and the first refrigerant heat exchanger 113 to recover the sensible heat of the gas phase and then enters the inlet of the normal-temperature third refrigerant compressor 131. The liquid phase recovers the latent heat of the liquid phase through the third refrigerant evaporator 134 and then sequentially enters the third refrigerant heat exchanger 133, the second refrigerant heat exchanger 123, and the first refrigerant heat exchanger 113 to recover the sensible heat of the gas phase and enters the inlet of the normal-temperature third refrigerant compressor 131.

[0080] The first refrigerant, the second refrigerant, and the third refrigerant are different from each other and are each selected from any one of nitrogen, methane, ethane, ethylene, propane, propylene, butane, butene, pentane, and pentene.

[0081] When the first refrigerant is propane, the second refrigerant is ethylene, and the third refrigerant is methane, the pressure of the first refrigerant discharged from the normal-temperature first refrigerant compressor 111 ≥ 0.47 MpaA, and the pressure of the first refrigerant entering the first refrigerant evaporator 114 is 0.1 - 0.47 MPaA, and the temperature is -43 - 0 °C;

[0082] The pressure of the second refrigerant discharged from the normal-temperature second refrigerant compressor 121 ≥ 1.38 MPaA, and the pressure of the second refrigerant entering the second refrigerant evaporator 124 is 0.1 - 1.38 MPaA, and the temperature is -104 - -42 °C;

[0083] The pressure of the third refrigerant discharged from the normal-temperature third refrigerant compressor 131 ≥ 2.44 MPaA, and the pressure of the third refrigerant entering the third refrigerant evaporator 134 is 0.1 - 2.44 MPaA, and the temperature is -161.8 - -102 °C.

[0084] Next, the technical solution of the present application will be described in combination with specific embodiments.

[0085] Example 1

[0086] Take Figure 1 as an example. This process is a two-stage throttling cascade refrigeration cycle LNG liquefaction process using normal-temperature compressors. Among them, both the first refrigerant heat exchanger 113 and the first refrigerant evaporator 114 are two-stage.

[0087] The high-pressure liquid-phase propane (10 - 50°C, pressure ≥ saturation pressure) from the normal-temperature first refrigerant compressor 111 enters the first buffer tank 112 for buffering after being cooled by the first refrigerant cooler 119, and then flows into the first-stage first refrigerant heat exchanger 1131. The high-pressure propane is precooled to about 10°C and then passes through the first refrigerant throttle valve 116, with the pressure reduced to about 0.298 MPaA. The medium-pressure propane (0.298 MPaA, -15°C) enters the first-stage first refrigerant evaporator 1141. After recovering part of the latent heat of the liquid-phase propane, it enters the first refrigerant separation tank 115 to be separated into gas-liquid two phases. The gaseous propane enters the second-stage inlet of the normal-temperature first refrigerant compressor 111 after recovering the sensible heat of the gaseous propane (37°C, 0.295 MPaA) through the first-stage first refrigerant heat exchanger 1131. The liquid-phase propane passes through the first liquid throttle valve 117 and the pressure is reduced to about 0.124 MPaA. The low-pressure propane (0.124 MPaA, -37.36°C) enters the second-stage first refrigerant evaporator 1142. After recovering all the latent heat of the liquid-phase propane, it enters the first-stage inlet of the normal-temperature first refrigerant compressor 111 after recovering the sensible heat of the gaseous propane (37°C, 0.12 MPaA) through the second-stage first refrigerant heat exchanger 1132 and the first-stage first refrigerant heat exchanger 1131.

[0088] The high-pressure ethylene (1.85 MPaA, 10 - 40°C) from the normal-temperature second refrigerant compressor 121 enters the first-stage first refrigerant heat exchanger 1131, the first-stage first refrigerant evaporator 1141, the second-stage first refrigerant heat exchanger 1132, and the second-stage first refrigerant evaporator 1142 after being cooled by the second refrigerant cooler 129. After being precooled to -35°C, it enters the second buffer tank 122 for buffering, and then passes through the second refrigerant throttle valve 126, with the pressure reduced to about 0.325 MPaA. The medium-pressure ethylene (0.325 MPaA, -81°C) enters the first-stage second refrigerant evaporator 1241. After recovering part of the latent heat of the liquid-phase ethylene, it enters the second refrigerant separation tank 125 to be separated into gas-liquid two phases. The gaseous ethylene enters the second-stage inlet of the normal-temperature second refrigerant compressor 121 after recovering the sensible heat of the gaseous ethylene (37°C, 0.316 MPaA) through the first-stage second refrigerant heat exchanger 1231, the second-stage first refrigerant heat exchanger 1132, and the first-stage first refrigerant heat exchanger 1131. The liquid-phase ethylene passes through the second liquid throttle valve 127 and the pressure is reduced to about 0.127 MPaA. The low-pressure ethylene (0.127 MPaA, -99°C) enters the second-stage second refrigerant evaporator 1242. After recovering all the latent heat of the liquid-phase ethylene, it enters the first-stage inlet of the normal-temperature second refrigerant compressor 121 after recovering the sensible heat of the gaseous ethylene (37°C, 0.12 MPaA) through the second-stage second refrigerant heat exchanger 1232, the first-stage second refrigerant heat exchanger 1231, the second-stage first refrigerant heat exchanger 1132, and the first-stage first refrigerant heat exchanger 1131.

[0089] High-pressure methane (3.2 MPaA, 10 - 40 °C) from the normal-temperature tertiary refrigerant compressor 131 is cooled by the tertiary refrigerant cooler 139 and then enters the primary first refrigerant heat exchanger 1131, primary first refrigerant evaporator 1141, secondary first refrigerant heat exchanger 1132, secondary first refrigerant evaporator 1142, primary second refrigerant heat exchanger 1231, primary second refrigerant evaporator 1241, secondary second refrigerant heat exchanger 1232, and secondary second refrigerant evaporator 1242. After being precooled to -98 °C, it enters the third buffer tank 132 for buffering, and then passes through the tertiary refrigerant throttle valve 136 to reduce the pressure to about 1.091 MPaA. Medium-pressure methane (1.091 MPaA, -122 °C) enters the primary tertiary refrigerant evaporator 1341. After recovering part of the latent heat of the liquid-phase methane, it is separated into gas-liquid two phases by the primary tertiary refrigerant separation tank 1351. The gaseous methane recovers sensible heat of the gaseous methane (37 °C, 0.108 MPaA) through the tertiary refrigerant heat exchanger 133, secondary second refrigerant heat exchanger 1232, primary second refrigerant heat exchanger 1231, secondary first refrigerant heat exchanger 1132, and primary first refrigerant heat exchanger 1131 and then enters the tertiary inlet of the normal-temperature tertiary refrigerant compressor 131. The liquid-phase methane is depressurized to about 0.388 MPaA by the primary tertiary liquid throttle valve 1371. Low-pressure methane (0.388 MPaA, -142 °C) enters the secondary tertiary refrigerant evaporator 1342. After recovering part of the latent heat of the liquid-phase methane, it is separated into gas-liquid two phases by the secondary tertiary refrigerant separation tank 1352. The gaseous methane recovers sensible heat of the gaseous methane (37 °C, 0.108 MPaA) through the tertiary refrigerant heat exchanger 133, secondary second refrigerant heat exchanger 1232, primary second refrigerant heat exchanger 1231, secondary first refrigerant heat exchanger 1132, and primary first refrigerant heat exchanger 1131 and then enters the secondary inlet of the normal-temperature tertiary refrigerant compressor 131. The liquid-phase methane is depressurized to about 0.128 MPaA by the secondary tertiary liquid throttle valve 1372. Ultra-low-pressure methane (0.128 MPaA, -158.7 °C) enters the tertiary tertiary refrigerant evaporator 1343. After recovering all the latent heat of the liquid-phase methane, it recovers sensible heat of the gaseous methane (37 °C, 0.12 MPaA) through the tertiary refrigerant heat exchanger 133, secondary second refrigerant heat exchanger 1232, primary second refrigerant heat exchanger 1231, secondary first refrigerant heat exchanger 1132, and primary first refrigerant heat exchanger 1131 and then enters the primary inlet of the normal-temperature tertiary refrigerant compressor 131.

[0090] The natural gas purified at normal temperature (40°C) is gradually cooled to -158°C after passing through the first-stage first refrigerant heat exchanger 1131, the first-stage first refrigerant evaporator 1141, the second-stage first refrigerant heat exchanger 1132, the second-stage first refrigerant evaporator 1142, the first-stage second refrigerant heat exchanger 1231, the first-stage second refrigerant evaporator 1241, the second-stage second refrigerant heat exchanger 1232, the second-stage second refrigerant evaporator 1242, the third refrigerant heat exchanger 133, the first-stage third refrigerant evaporator 1341, the second-stage third refrigerant evaporator 1342, and the third-stage third refrigerant evaporator 1343, and then is sent to the LNG storage tank or the LNG tanker after being depressurized to 0.11 - 0.15 MPaA (the product pressure is adjustable) through the product J-T valve.

[0091] Example 2

[0092] Taking Figure 3 as an example, this process is a three-stage throttling cascade refrigeration cycle LNG liquefaction process. It uses a normal temperature compressor and is improved on the basis of Example 1, with more temperature gradients, lower product temperature, and more energy-efficient equipment.

[0093] The high-pressure liquid-phase propane (10 - 50°C, pressure ≥ saturation pressure) from the normal-temperature first refrigerant compressor 111 is buffered in the first buffer tank 112 after being cooled by the first refrigerant cooler 119, and then enters the primary first refrigerant heat exchanger 1131. The high-pressure propane is precooled to about 10°C and is depressurized to about 0.6 MPaA through the first refrigerant throttle valve 116. The medium-pressure propane (0.6 MPaA, 7.8°C) enters the primary first refrigerant evaporator 1141. After recovering part of the latent heat of the liquid-phase propane, it is separated into gas-liquid two phases in the primary first refrigerant separation tank 1151. The gaseous propane enters the three-stage inlet of the normal-temperature first refrigerant compressor 111 after recovering the sensible heat of the gaseous propane (35°C, 0.6 MPaA) through the primary first refrigerant heat exchanger 1131. The liquid-phase propane is depressurized to about 0.24 MPaA through the first liquid throttle valve 117 (primary stage). The low-pressure propane (0.24 MPaA, -20°C) enters the secondary first refrigerant evaporator 1142. After recovering part of the latent heat of the liquid-phase propane, it is separated into gas-liquid two phases in the secondary first refrigerant separation tank 1152. The gaseous propane enters the second-stage inlet of the normal-temperature first refrigerant compressor 111 after recovering the sensible heat of the gaseous propane (35°C, 0.24 MPaA) through the secondary first refrigerant heat exchanger 1132 and the primary first refrigerant heat exchanger 1131 (10). The liquid-phase propane is depressurized to about 0.1 MPaA through the first liquid throttle valve 117 (secondary stage). The very low-pressure propane (0.1 MPaA, -42°C) enters the first-stage inlet of the normal-temperature first refrigerant compressor 111 after recovering all the latent heat and sensible heat of the propane (0.1 MPaA, 35°C) through the tertiary first refrigerant evaporator 1143, the tertiary first refrigerant heat exchanger 1133, the secondary first refrigerant heat exchanger 1132, and the primary first refrigerant heat exchanger 1131.

[0094] The high-pressure ethylene (1.5 MPaA, 40 °C) from the normal-temperature secondary refrigerant compressor 121 is cooled by the secondary refrigerant cooler 129 and then passes through the primary first refrigerant heat exchanger 1131, the primary first refrigerant evaporator 1141, the secondary first refrigerant heat exchanger 1132, the secondary first refrigerant evaporator 1142, the tertiary first refrigerant heat exchanger 1133, and the tertiary first refrigerant evaporator 1143, and is precooled to -40 °C and then enters the second buffer tank 122 for buffering. After passing through the secondary refrigerant throttle valve 126, the pressure is reduced to about 0.6 MPaA. The medium-pressure ethylene (0.6 MPaA, -66 °C) enters the primary secondary refrigerant evaporator 1241. After recovering a part of the latent heat of the liquid-phase ethylene, it is separated into gas-liquid two phases by the primary secondary refrigerant separation tank 1251. The gaseous ethylene recovers the sensible heat of the gaseous ethylene (35 °C, 0.6 MPaA) through the primary secondary refrigerant heat exchanger 1231, the tertiary first refrigerant heat exchanger 1133, the secondary first refrigerant heat exchanger 1132, and the primary first refrigerant heat exchanger 1131 and then enters the tertiary inlet of the normal-temperature secondary refrigerant compressor 121. The liquid-phase ethylene is depressurized to about 0.25 MPaA through the second liquid-phase throttle valve 127. The low-pressure ethylene (0.25 MPaA, -87 °C) enters the secondary secondary refrigerant evaporator 1242. After recovering a part of the latent heat of the liquid-phase ethylene, it is separated into gas-liquid two phases by the secondary secondary refrigerant separation tank 1252 (7). The gaseous ethylene recovers the sensible heat of the gaseous ethylene (35 °C, 0.24 MPaA) through the secondary secondary refrigerant heat exchanger 1232, the primary secondary refrigerant heat exchanger 1231, the tertiary first refrigerant heat exchanger 1133 (14), the secondary first refrigerant heat exchanger 1132 (12), and the primary first refrigerant heat exchanger 1131 and then enters the secondary inlet of the normal-temperature secondary refrigerant compressor 121. The liquid-phase ethylene is depressurized to about 0.1 MPaA through the ethylene tertiary J-T valve. The very low-pressure ethylene (0.1 MPaA, -104 °C) recovers all the latent heat and sensible heat of the ethylene (0.1 MPaA, 35 °C) through the tertiary secondary refrigerant evaporator 1243 (21), the tertiary secondary refrigerant heat exchanger 1233 (20), the secondary secondary refrigerant heat exchanger 1232 (18), the primary secondary refrigerant heat exchanger 1231 (16), the tertiary first refrigerant heat exchanger 1133 (14), the secondary first refrigerant heat exchanger 1132 (12), and the primary first refrigerant heat exchanger 1131 (10) and then enters the primary inlet of the normal-temperature secondary refrigerant compressor 121.

[0095] The high-pressure methane (2.55 MPaA, 40 °C) from the normal-temperature third refrigerant compressor 131 is cooled by the second refrigerant cooler 129 and then passes through the first-stage first refrigerant heat exchanger 1131, the first-stage first refrigerant evaporator 1141, the second-stage first refrigerant heat exchanger 1132, the second-stage first refrigerant evaporator 1142, the third-stage first refrigerant heat exchanger 1133, the third-stage first refrigerant evaporator 1143, the first-stage second refrigerant heat exchanger 1231, the first-stage second refrigerant evaporator 1241 (17), the second-stage second refrigerant heat exchanger 1232, the second-stage second refrigerant evaporator 1242 (19), the third-stage second refrigerant heat exchanger 1233, and the third-stage second refrigerant evaporator 1243, and is precooled to -102 °C and then enters the third buffer tank 132 for buffering. After passing through the third refrigerant throttle valve 136, the pressure is reduced to about 0.86 MPaA. The medium-pressure methane (0.86 MPaA, -127 °C) enters the first-stage third refrigerant evaporator 1341. After recovering part of the latent heat of the liquid-phase methane, it is separated into gas-liquid two phases by the first-stage third refrigerant separation tank 1351. The gaseous methane passes through the third refrigerant heat exchanger 133, the third-stage second refrigerant heat exchanger 1233, the second-stage second refrigerant heat exchanger 1232, the first-stage second refrigerant heat exchanger 1231, the third-stage first refrigerant heat exchanger 1133, the second-stage first refrigerant heat exchanger 1132 (12), and the first-stage first refrigerant heat exchanger 1131 (10) to recover the sensible heat of the gaseous methane (35 °C, 0.85 MPaA) and then enters the third-stage inlet of the normal-temperature third refrigerant compressor 131. The liquid-phase methane is depressurized to about 0.3 MPaA by the first-stage third liquid-phase throttle valve 1371. The low-pressure methane (0.3 MPaA, -147 °C) enters the second-stage third refrigerant evaporator 1342. After recovering part of the latent heat of the liquid-phase methane, it is separated into gas-liquid two phases by the second-stage third refrigerant separation tank 1352. The gaseous methane passes through the third refrigerant heat exchanger 133 (22), the third-stage second refrigerant heat exchanger 1233 (20), the second-stage second refrigerant heat exchanger 1232 (18), the first-stage second refrigerant heat exchanger 1231 (16), the third-stage first refrigerant heat exchanger 1133 (14), the second-stage first refrigerant heat exchanger 1132 (12), and the first-stage first refrigerant heat exchanger 1131 (10) to recover the sensible heat of the gaseous methane (35 °C, 0.28 MPaA) and then enters the second-stage inlet of the normal-temperature third refrigerant compressor 131. The liquid-phase methane is depressurized to about 0.1 MPaA by the second-stage third liquid-phase throttle valve 1372.Low-pressure methane (0.1 MPaA, -162°C) enters the third-stage third refrigerant evaporator 1343 (25), and after recovering all the latent heat of the liquid methane, it passes through the third refrigerant heat exchanger 133 (22), the third-stage second refrigerant heat exchanger 1233 (20), the second-stage second refrigerant heat exchanger 1232 (18), the first-stage second refrigerant heat exchanger 1231 (16), the third-stage first refrigerant heat exchanger 1133 (14), the second-stage first refrigerant heat exchanger 1132 (12), and the first-stage first refrigerant heat exchanger 1131 (10) to recover the sensible heat of the gaseous methane (35°C, 0.1 MPaA), and then enters the first-stage inlet of the normal temperature third refrigerant compressor 131.

[0096] Normal temperature purified natural gas (40°C) passes through the first refrigerant heat exchanger 1131 (10), the first refrigerant evaporator 1141 (11), the second refrigerant heat exchanger 1132 (12), the second refrigerant evaporator 1142 (13), the third first refrigerant heat exchanger 1133 (14), the third first refrigerant evaporator 1143 (15), the first second refrigerant heat exchanger 1231 (16), the first second refrigerant evaporator 1241 (17), the second second refrigerant heat exchanger 1232 (18), the second The second refrigerant evaporator 1242 (19), the third-stage second refrigerant heat exchanger 1233 (20), the third-stage second refrigerant evaporator 1243 (21), the third refrigerant heat exchanger 133 (22), the first-stage third refrigerant evaporator 1341 (23), the second-stage third refrigerant evaporator 1342 (24), and the third-stage third refrigerant evaporator 1343 (25) are cooled step by step to below -158°C, and are reduced in pressure to 0.11-0.15 MPaA (product pressure is adjustable) by the product JT valve and then sent to the LNG storage tank or LNG tank truck.

[0097] Example 3

[0098] by Figure 4 For example, the process is a three-stage throttling cascade refrigeration cycle LNG liquefaction process, which adopts a normal temperature compressor and is improved on the basis of Example 1. The first refrigerant heat exchanger 113 and the first refrigerant evaporator 114 in Example 1 are integrated into a first refrigerant heat exchange and evaporation integrator 118, the second refrigerant heat exchanger 123 and the second refrigerant evaporator 124 are integrated into a second refrigerant heat exchange and evaporation integrator 128, and the third refrigerant heat exchanger 133 and the third refrigerant evaporator 134 are integrated into a third refrigerant heat exchange and evaporation integrator 138. The device has a high degree of integration and occupies a smaller area, and is suitable for small-scale devices and vehicle-mounted LNG.

[0099] The high-pressure liquid-phase propane (10 - 50°C, pressure ≥ saturation pressure) from the normal-temperature first refrigerant compressor 111 is buffered in the first buffer tank 112 after being cooled by the first refrigerant cooler 119, and then enters the first refrigerant heat exchange and evaporation integrator 118. The high-pressure propane is precooled to about 11°C and is depressurized to about 0.6 MPaA through the first refrigerant throttle valve 116. The medium-pressure propane (0.6 MPaA, 7.8°C) enters the first refrigerant heat exchange and evaporation integrator 118. After recovering part of the latent heat of the liquid-phase propane, it is separated into gas-liquid two phases by the first-stage first refrigerant separation tank 1151. The gaseous propane enters the third-stage inlet of the normal-temperature first refrigerant compressor 111 after recovering the sensible heat of the gaseous propane (35°C, 0.6 MPaA) through the first refrigerant heat exchange and evaporation integrator 118. The liquid-phase propane is depressurized to about 0.24 MPaA through the first liquid throttle valve 117. The low-pressure propane (0.24 MPaA, -20°C) enters the first refrigerant heat exchange and evaporation integrator 118. After recovering part of the latent heat of the liquid-phase propane, it is separated into gas-liquid two phases by the second-stage first refrigerant separation tank 1152. The gaseous propane enters the second-stage inlet of the normal-temperature first refrigerant compressor 111 after recovering the sensible heat of the gaseous propane (35°C, 0.24 MPaA) through the first refrigerant heat exchange and evaporation integrator 118. The liquid-phase propane is depressurized to about 0.1 MPaA through the first liquid throttle valve 117. The very low-pressure propane (0.1 MPaA, -42°C) enters the first refrigerant heat exchange and evaporation integrator 118. After recovering all the latent heat and sensible heat of the propane (0.1 MPaA, 35°C), it enters the first-stage inlet of the normal-temperature first refrigerant compressor 111.

[0100] The high-pressure ethylene (1.5 MPaA, 40 °C) from the normal-temperature secondary refrigerant compressor 121 enters the first buffer tank 112 for buffering after being cooled by the first refrigerant cooler 119, and then enters the first refrigerant heat exchange and evaporation integrator 118. The high-pressure ethylene is precooled to about -40 °C and enters the second buffer tank 122 for buffering. After passing through the secondary refrigerant throttle valve 126, the pressure is reduced to about 0.6 MPaA. The medium-pressure ethylene (0.6 MPaA, -66 °C) enters the second refrigerant heat exchange and evaporation integrator 128. After recovering part of the latent heat of the liquid-phase ethylene, it is separated into gas-liquid two phases by the first-stage secondary refrigerant separation tank 1251(6). The gaseous ethylene enters the third-stage inlet of the normal-temperature secondary refrigerant compressor 121 after recovering the sensible heat of the gaseous ethylene through the second refrigerant heat exchange and evaporation integrator 128 and the first refrigerant heat exchange and evaporation integrator 118 (35 °C, 0.6 MPaA). The liquid-phase ethylene is depressurized to about 0.25 MPaA through the second liquid-phase throttle valve 127. The low-pressure ethylene (0.25 MPaA, -87 °C) enters the second refrigerant heat exchange and evaporation integrator 128. After recovering part of the latent heat of the liquid-phase ethylene, it is separated into gas-liquid two phases by the second-stage secondary refrigerant separation tank 1252(7). The gaseous ethylene enters the second-stage inlet of the normal-temperature secondary refrigerant compressor 121 after recovering the sensible heat of the gaseous ethylene through the second refrigerant heat exchange and evaporation integrator 128 and the first refrigerant heat exchange and evaporation integrator 118 (35 °C, 0.24 MPaA). The liquid-phase ethylene is depressurized to about 0.1 MPaA through the ethylene third-stage J-T valve. The very low-pressure ethylene (0.1 MPaA, -104 °C) passes through the second refrigerant heat exchange and evaporation integrator 128 and the first refrigerant heat exchange and evaporation integrator 118, and enters the first-stage inlet of the normal-temperature secondary refrigerant compressor 121 after recovering all the latent heat and sensible heat of the ethylene (0.1 MPaA, 35 °C).

[0101] The high-pressure methane (2.55MPaA, 40°C) from the third refrigerant compressor 131 at normal temperature is cooled by the third refrigerant cooler 139 and then passes into the first refrigerant heat exchange evaporation integrator 118 and the second refrigerant heat exchange evaporation integrator 128. The high-pressure methane is pre-cooled to about -102°C and enters the third buffer tank 132 for buffering. It is then reduced in pressure to about 0.86MPaA by the third refrigerant throttle valve 136. Medium-pressure methane (0.86 MPaA, -127°C) enters the third refrigerant heat exchanger 133 (12), recovers part of the latent heat of the liquid methane, and is separated into gas and liquid phases through the first-level third refrigerant separation tank 1351 (8). The gaseous methane is recovered through the third refrigerant heat exchange evaporation integrated device 138 (12), the second refrigerant heat exchange evaporation integrated device 128, and the first refrigerant heat exchange evaporation integrated device 118. After the sensible heat of the gaseous methane is recovered (35°C, 0.85 MPaA), it enters the third-stage inlet of the third refrigerant compressor 131 at normal temperature. The liquid methane is reduced in pressure to about 0.3 MPaA through the first-level third liquid phase throttle valve 1371. Low-pressure methane (0.3MPaA, -147°C) enters the third refrigerant heat exchange evaporator integrated unit 138 (12), recovers part of the latent heat of the liquid methane, and is separated into gas and liquid phases through the secondary third refrigerant separation tank 1352 (9). The gaseous methane is recovered through the third refrigerant heat exchange evaporator integrated unit 138 (12), the second refrigerant heat exchange evaporator integrated unit 128, and the first refrigerant heat exchange evaporator integrated unit 118. After the sensible heat of the gaseous methane is recovered (35°C, 0.28MPaA), the gaseous methane enters the secondary inlet of the third refrigerant compressor 131 at normal temperature, and the liquid methane is reduced in pressure to about 0.1MPaA through the secondary third liquid phase throttle valve 1372. Low-pressure methane (0.1 MPaA, -162°C) passes through the third refrigerant heat exchanger 133 (12), the second refrigerant heat exchange and evaporation integrated device 128, and the first refrigerant heat exchange and evaporation integrated device 118, and after recovering all the latent heat and sensible heat of ethylene (0.1 MPaA, 35°C), it enters the primary inlet of the normal temperature third refrigerant compressor 131. Normal temperature purified natural gas (40°C) passes through the first refrigerant heat exchange and evaporation integrated device 118, the second refrigerant heat exchange and evaporation integrated device 128, and the third refrigerant heat exchanger 133 (12), and is cooled step by step to below -158°C, and is reduced in pressure to 0.11-0.15 MPaA (product pressure is adjustable) by the product JT valve and then delivered to the LNG storage tank or LNG tank truck.

[0102] Example 4

[0103] by Figure 5 As an example, improvements are made on the basis of Example 1 and Example 2. Compared with Example 1 and Example 2, this method ensures that the refrigerant entering the evaporator is in full liquid phase, reducing the difficulty of operation. The process is a two-stage throttling + three-stage throttling combined cascade refrigeration cycle LNG liquefaction process, and the throttling process route is different from that of Example 1 and Example 2, and a normal temperature compressor is used.

[0104] The high-pressure liquid-phase propane (10 - 50°C, pressure ≥ saturation pressure) from the normal-temperature first refrigerant compressor 111 is cooled by the first refrigerant cooler 119 and then enters the first buffer tank 112 for buffering. Subsequently, it is fed into the primary first refrigerant heat exchanger 1131, where the high-pressure propane is precooled to around 10°C. After passing through the first refrigerant throttle valve 116, the pressure is reduced to around 0.298 MPaA. The medium-pressure propane (0.298 MPaA, -15°C) enters the first refrigerant separation tank 115, where it is separated into gas and liquid phases. Part of the liquid-phase propane enters the primary first refrigerant evaporator 1141. After recovering the latent heat of the liquid phase, it is combined with the medium-pressure gas-phase propane and enters the second-stage inlet of the normal-temperature first refrigerant compressor 111 after recovering the sensible heat of the gas-phase propane (37°C, 0.295 MPaA) through the primary first refrigerant heat exchanger 1131. Part of the medium-pressure liquid-phase propane is depressurized to around 0.124 MPaA through the first liquid throttle valve 117. The low-pressure propane (0.124 MPaA, -37.36°C) enters the secondary first refrigerant evaporator 1142. After recovering all the latent heat of the liquid-phase propane, it enters the first-stage inlet of the normal-temperature first refrigerant compressor 111 after recovering the sensible heat of the gas-phase propane (37°C, 0.12 MPaA) through the secondary first refrigerant heat exchanger 1132 and the primary first refrigerant heat exchanger 1131.

[0105] The high-pressure ethylene (3.3 MPaA, 10 - 40 °C) from the normal-temperature secondary refrigerant compressor 121 is cooled by the secondary refrigerant cooler 129 and then passes through the primary first refrigerant heat exchanger 1131, the primary first refrigerant evaporator 1141, the secondary first refrigerant heat exchanger 1132, and the secondary first refrigerant evaporator 1142. After being precooled to about -35 °C, it enters the second buffer tank 122 for buffering, and then passes through the secondary refrigerant throttle valve 126 to reduce the pressure to about 0.67 MPaA. The medium-pressure ethylene (0.67 MPaA, -63 °C) enters the primary second refrigerant separation tank 1251, where it is separated into gas and liquid phases. The liquid-phase ethylene is directly depressurized to about 0.3 MPaA through the ethylene secondary J-T valve. The gas-phase ethylene returns to the tertiary inlet of the normal-temperature secondary refrigerant compressor 121 after recovering sensible heat through the primary second refrigerant heat exchanger 1231, the secondary first refrigerant heat exchanger 1132, and the primary first refrigerant heat exchanger 1131 (0.66 MPaA, 30 °C). The low-pressure ethylene (0.3 MPaA, -84 °C) enters the secondary second refrigerant separation tank 1252, where it is separated into gas and liquid phases. Part of the liquid-phase ethylene enters the primary second refrigerant evaporator 1241. After recovering the latent heat of the liquid-phase ethylene, it is combined with the low-pressure gas-phase ethylene and enters the secondary inlet of the normal-temperature secondary refrigerant compressor 121 after recovering the sensible heat of the gas-phase ethylene through the primary second refrigerant heat exchanger 1231, the secondary first refrigerant heat exchanger 1132, and the primary first refrigerant heat exchanger 1131 (37 °C, 0.28 MPaA). Part of the low-pressure liquid-phase ethylene is depressurized to about 0.13 MPaA through the second liquid throttle valve 127. The very low-pressure ethylene (0.13 MPaA, -99 °C) enters the secondary second refrigerant evaporator 1242. After recovering all the latent heat of the liquid-phase ethylene, it enters the primary inlet of the normal-temperature secondary refrigerant compressor 121 after recovering the sensible heat of the gas-phase ethylene through the secondary second refrigerant heat exchanger 1232, the primary second refrigerant heat exchanger 1231, the secondary first refrigerant heat exchanger 1132, and the primary first refrigerant heat exchanger 1131 (37 °C, 0.12 MPaA).

[0106] High-pressure methane (5.3 MPaA, 10 - 40 °C) from the normal-temperature third refrigerant compressor 131 is cooled by the third refrigerant cooler 139 and then passes through the first-stage first refrigerant heat exchanger 1131, the first-stage first refrigerant evaporator 1141, the second-stage first refrigerant heat exchanger 1132, the second-stage first refrigerant evaporator 1142, the first-stage second refrigerant heat exchanger 1231, the first-stage second refrigerant evaporator 1241, the second-stage second refrigerant heat exchanger 1232, and the second-stage second refrigerant evaporator 1242. After being precooled to -96 °C, it enters the third buffer tank 132 for buffering, and then passes through the third refrigerant throttle valve 136 to reduce the pressure to about 1.1 MPaA. Medium-pressure methane (1.1 MPaA, -122 °C) enters the first-stage third refrigerant separation tank 1351. After separating into gas-liquid two phases, part of the liquid-phase methane passes through the first-stage third refrigerant evaporator 1341 to recover the latent heat of the liquid-phase methane, and then is aggregated with the medium-pressure gas-phase methane and passes through the first-stage third refrigerant heat exchanger 1331, the second-stage second refrigerant heat exchanger 1232, the first-stage second refrigerant heat exchanger 1231, the second-stage first refrigerant heat exchanger 1132, and the first-stage first refrigerant heat exchanger 1131 to recover the sensible heat of the gas-phase methane (37 °C, 1.08 MPaA) and then enters the third-stage inlet of the normal-temperature third refrigerant compressor 131. Part of the medium-pressure liquid-phase methane is depressurized to about 0.37 MPaA by the first-stage third liquid throttle valve 1371. Part of the low-pressure methane (0.37 MPaA, -143 °C) enters the second-stage third refrigerant evaporator 1342 to recover the latent heat of the liquid-phase methane, and then is aggregated with the low-pressure gas-phase methane and passes through the second-stage third refrigerant heat exchanger 1332, the first-stage third refrigerant evaporator 1341, the first-stage third refrigerant heat exchanger 1331, the second-stage second refrigerant heat exchanger 1232, the first-stage second refrigerant heat exchanger 1231, the second-stage first refrigerant heat exchanger 1132, and the first-stage first refrigerant heat exchanger 1131 to recover the sensible heat of the gas-phase methane (37 °C, 0.35 MPaA) and then enters the second-stage inlet of the normal-temperature third refrigerant compressor 131. Part of the low-pressure liquid-phase methane is depressurized to about 0.13 MPaA by the second-stage third liquid throttle valve 1372. Ultra-low-pressure methane (0.13 MPaA, -159 °C) enters the third-stage third refrigerant evaporator 1343 to recover all the latent heat of the liquid-phase methane, and then passes through the second-stage third refrigerant heat exchanger 1332, the first-stage third refrigerant evaporator 1341, the first-stage third refrigerant heat exchanger 1331, the second-stage second refrigerant heat exchanger 1232, the first-stage second refrigerant heat exchanger 1231, the second-stage first refrigerant heat exchanger 1132, and the first-stage first refrigerant heat exchanger 1131 to recover the sensible heat of the gas-phase methane (37 °C, 0.12 MPaA) and then enters the first-stage inlet of the normal-temperature third refrigerant compressor 131.

[0107] The natural gas purified at normal temperature (40°C) is gradually cooled to -158°C through the first-stage first refrigerant heat exchanger 1131, the first-stage first refrigerant evaporator 1141, the second-stage first refrigerant heat exchanger 1132, the second-stage first refrigerant evaporator 1142, the first-stage second refrigerant heat exchanger 1231, the first-stage second refrigerant evaporator 1241, the second-stage second refrigerant heat exchanger 1232, the second-stage second refrigerant evaporator 1242, the first-stage third refrigerant heat exchanger 1331, the first-stage third refrigerant evaporator 1341, the second-stage third refrigerant heat exchanger 1332, the second-stage third refrigerant evaporator 1342, and the third-stage third refrigerant evaporator 1343, and then is depressurized to 0.11 - 0.15 MPaA (the product pressure is adjustable) through the product J-T valve and sent to the LNG storage tank or the LNG tank truck.

[0108] According to the above application case, the cascade refrigeration cycle LNG liquefaction system 100 of this embodiment can be applied to the on-vehicle natural gas liquefaction device. The normal temperature first refrigerant compressor 111, the normal temperature second refrigerant compressor 121, and the normal temperature third refrigerant compressor 131 are respectively highly integrated into the refrigerant compressor unit A; the first-stage first refrigerant heat exchanger 1131, the first-stage first refrigerant evaporator 1141, the second-stage first refrigerant heat exchanger 1132, the second-stage first refrigerant evaporator 1142, the first-stage second refrigerant heat exchanger 1231, the first-stage second refrigerant evaporator 1241, the second-stage second refrigerant heat exchanger 1232, the second-stage second refrigerant evaporator 1242, the first-stage third refrigerant heat exchanger 1331, the first-stage third refrigerant evaporator 1341, the second-stage third refrigerant heat exchanger 1332, the second-stage third refrigerant evaporator 1342, and the third-stage third refrigerant evaporator 1343 are highly integrated into the on-vehicle LNG liquefaction box B, as detailed in Figure 6 .

[0109] Comparative Example 1

[0110] Taking Figure 7 as an example, the process adopted by the conventional cascade refrigeration cycle LNG liquefaction system 300 uses C 3 H 6 , C 2 H 4 and CH 4 as refrigerants and adopts three-stage throttling and a low-temperature compressor.

[0111] The high-pressure liquid-phase propylene (0.92 MPaA, 16 °C) from the low-temperature propylene compressor unit 301 enters the high-pressure propylene buffer tank 319 for buffering after being cooled by the high-pressure propylene cooler 316, and then is depressurized to about 0.52 MPaA through the first-stage J-T valve for propylene. The medium-pressure propylene (0.52 MPaA, -4 °C) enters the first-stage propylene evaporator 309. After recovering part of the latent heat of the liquid-phase propylene, it is separated into gas-liquid two phases by the first-stage propylene separation tank 304. The gaseous propylene directly enters the inlet of the third stage of the low-temperature propylene compressor unit 301, and the liquid-phase propylene is depressurized to about 0.3 MPaA through the second-stage J-T valve for propylene. The low-pressure propylene (0.3 MPaA, -28 °C) enters the second-stage propylene evaporator 310. After recovering part of the latent heat of the liquid-phase propylene, it is separated into gas-liquid two phases by the second-stage propylene separation tank 305. The gaseous propylene directly enters the inlet of the second stage of the low-temperature propylene compressor unit 301, and the liquid-phase propylene is depressurized to about 0.13 MPaA through the third-stage J-T valve for propylene. The very-low-pressure propylene (0.13 MPaA, -42 °C) enters the inlet of the first stage of the low-temperature propylene compressor unit 301 after recovering all the latent heat of the propylene (0.13 MPaA, -42 °C) through the third-stage propylene evaporator 311.

[0112] The high-pressure ethylene (1.8 MPaA, -36 °C) from the low-temperature ethylene compressor unit 302 enters the high-pressure ethylene buffer tank 320 for buffering after being cooled by the high-pressure ethylene cooler 317 and then passing through the first-stage propylene evaporator 309, the second-stage propylene evaporator 310, and the third-stage propylene evaporator 311 and being precooled to -36 °C. It is depressurized to about 0.4 MPaA through the first-stage J-T valve for ethylene. The medium-pressure ethylene (0.4 MPaA, -77 °C) enters the first-stage ethylene evaporator 312. After recovering part of the latent heat of the liquid-phase ethylene, it is separated into gas-liquid two phases by the first-stage ethylene separation tank 306. The gaseous ethylene directly enters the inlet of the second stage of the low-temperature ethylene compressor unit 302, and the liquid-phase ethylene is depressurized to about 0.25 MPaA through the second-stage J-T valve for ethylene. The low-pressure ethylene (0.25 MPaA, -87 °C) enters the second-stage ethylene evaporator 313. After recovering part of the latent heat of the liquid-phase ethylene, it is separated into gas-liquid two phases by the second-stage ethylene separation tank 307. The gaseous ethylene directly enters the inlet of the second stage of the low-temperature ethylene compressor unit 302, and the liquid-phase ethylene is depressurized to about 0.13 MPaA through the third-stage J-T valve for ethylene. The very-low-pressure ethylene (0.13 MPaA, -104 °C) enters the inlet of the first stage of the low-temperature ethylene compressor unit 302 after recovering all the latent heat of the ethylene (0.13 MPaA, -104 °C) through the third-stage ethylene evaporator 314.

[0113] The high-pressure methane (4.3 MPaA, 40 °C) from the low-temperature methane compressor unit 303 is cooled by the high-pressure methane cooler 318 and then enters the propylene primary evaporator 309, propylene secondary evaporator 310, propylene tertiary evaporator 311, ethylene primary evaporator 312, ethylene secondary evaporator 313, and ethylene tertiary evaporator 314. After being precooled to -102 °C, it enters the high-pressure methane buffer tank 321 for buffering, and then passes through the methane J-T valve to reduce the pressure to about 0.24 MPaA. The low-pressure methane (0.24 MPaA, -150 °C) is separated into gas-liquid two phases in the methane separation tank 308. The gaseous methane and liquid methane recover the cold energy of the gaseous and liquid methane through the methane heat exchanger 315 ((0.24 MPaA, -105 °C)) and then directly enter the inlet of the low-temperature methane compressor unit 303.

[0114] The normal-temperature purified natural gas (40 °C) is gradually cooled to below -150 °C through the propylene primary evaporator 309, propylene secondary evaporator 310, propylene tertiary evaporator 311, ethylene primary evaporator 312, ethylene secondary evaporator 313, ethylene tertiary evaporator 314, and methane heat exchanger 315. After being depressurized to 0.11 - 0.15 MPaA (the product pressure is adjustable) through the product J-T valve, it is sent to the LNG storage tank or LNG tank truck.

[0115] In addition, it should be noted that since recovering cold energy at -30 °C may not necessarily save energy, in this application, methane and ethylene can be selectively returned to the first heat exchanger at the inlet according to specific circumstances, making the operation more flexible and adaptable to various climate conditions. The last heat exchanger of the present invention is considered to be one open and one standby to solve the problem of increased liquid phase and reduced evaporation refrigerant caused by propane and ethane impurities in methane. The equipment of the present invention is smaller in size and occupies less land, and can be used in fields such as natural gas liquefaction plants and on-vehicle natural gas liquefaction, with a wider application range.

[0116] Furthermore, the refrigerant composition of the present invention is not limited to propane, ethylene, and methane. According to the requirements of the product temperature, any combination of nitrogen, methane, ethane, ethylene, propane, propylene, butane, butene, pentane, and pentene can be used. The process flow remains unchanged. After the refrigerant passes through one or more stages of throttling, the product is cooled or condensed to the temperature value required by the process. Each time the refrigerant passes through throttling, it corresponds to a gaseous refrigerant heat exchanger and a liquid refrigerant evaporator to recover the corresponding sensible heat of the gas phase and latent heat of the liquid phase. It is also possible to have multiple stages of throttling corresponding to one gaseous refrigerant heat exchanger, but one stage of throttling must correspond to one liquid refrigerant evaporator.

[0117] The cascaded refrigeration cycle LNG liquefaction system 100 provided by the present invention is applicable to purification natural gas liquefaction devices under different inlet pressure conditions (≥2.0 MPa.G, pressurization is required when it is lower than 2.0 MPa.G) and different processing scales. For natural gas liquefaction devices with a relatively large processing scale (≥200,000 m³ / day), the system with secondary throttling provided in Embodiment 1 can be adopted, or the system with tertiary throttling provided in Embodiment 2 can be adopted. For natural gas liquefaction devices with a relatively small processing scale (≤200,000 m³ / day), the integrated system provided in Embodiment 3 can be adopted. Moreover, the cascaded refrigeration cycle LNG liquefaction system 100 provided in this application can be applied to on-vehicle natural gas liquefaction devices through integration.

[0118] In summary, the embodiment of the present invention provides a cascaded refrigeration cycle LNG liquefaction system 100, which forms a new cascaded refrigeration cycle LNG liquefaction process by adding a first refrigerant heat exchanger 113, a second refrigerant heat exchanger 123, and a third refrigerant heat exchanger 133, and adjusting the connection modes of multiple compressors, heat exchangers, evaporators, and separation tanks in the cascaded refrigeration cycle LNG liquefaction system 100. On the one hand, the present invention recovers the cold energy of the gaseous first refrigerant, gaseous second refrigerant, and gaseous third refrigerant. On the other hand, it ensures that the temperature of the gaseous refrigerant of the return compression unit is normal temperature, and normal temperature compressors can be selected for the compressors, reducing the cost investment of the compressors. The unit energy consumption index of the inventive cascaded refrigeration cycle LNG liquefaction system 100 is 0.25 kW / Nm 3 Purified natural gas, which is about 15% less than that of a conventional cascaded refrigeration cycle system (0.30 kW / Nm 3 Purified natural gas), and about 25% less than that of an MRC mixed refrigerant refrigeration cycle system (0.33 kW / Nm 3 Purified natural gas).

[0119] The above is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered by the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claimed rights.

Claims

1. A cascaded refrigeration cycle LNG liquefaction system, characterized in that, it includes a first refrigeration component, a second refrigeration component, and a third refrigeration component, the first refrigeration component includes a normal temperature first refrigerant compressor, a first refrigerant heat exchanger, and a first refrigerant evaporator, the second refrigeration component includes a normal temperature second refrigerant compressor, a second refrigerant heat exchanger, and a second refrigerant evaporator, and the third refrigeration component includes a normal temperature third refrigerant compressor, a third refrigerant heat exchanger, and a third refrigerant evaporator; the refrigerants discharged from the normal temperature first refrigerant compressor, the normal temperature second refrigerant compressor, and the normal temperature third refrigerant compressor respectively recover the latent heat of the liquid phase through the first refrigerant evaporator, the second refrigerant evaporator, and the third refrigerant evaporator; the first refrigerant gas after recovering the latent heat of the liquid phase through the first refrigerant evaporator recovers sensible heat through the first refrigerant heat exchanger and then returns to the normal temperature first refrigerant compressor; the second refrigerant gas after recovering the latent heat of the liquid phase through the second refrigerant evaporator recovers sensible heat through the second refrigerant heat exchanger and then returns to the normal temperature second refrigerant compressor; the third refrigerant gas after recovering the latent heat of the liquid phase through the third refrigerant evaporator recovers sensible heat through the third refrigerant heat exchanger and then returns to the normal temperature third refrigerant compressor; the first refrigeration component further includes a first refrigerant separation tank, the second refrigeration component further includes a second refrigerant separation tank, and the third refrigeration component further includes a third refrigerant separation tank; the refrigerants discharged from the normal temperature first refrigerant compressor, the normal temperature second refrigerant compressor, and the normal temperature third refrigerant compressor first evaporate through the first refrigerant evaporator, the second refrigerant evaporator, and the third refrigerant evaporator respectively, and then enter the first refrigerant separation tank, the second refrigerant separation tank, and the third refrigerant separation tank for separation; or, the refrigerants first enter the first refrigerant separation tank, the second refrigerant separation tank, and the third refrigerant separation tank for separation respectively, and then enter the first refrigerant evaporator for evaporation, the second refrigerant evaporator, and the third refrigerant evaporator for evaporation; the second refrigerant gas enters the first refrigerant heat exchanger for further heat exchange after passing through the second refrigerant heat exchanger; the third refrigerant gas passes through at least one of the second refrigerant heat exchanger and the first refrigerant heat exchanger for further heat exchange after passing through the third refrigerant heat exchanger; before the first refrigerant discharged from the normal temperature first refrigerant compressor enters the first refrigerant evaporator, it further includes passing the first refrigerant through the first refrigerant heat exchanger for heat exchange; before the second refrigerant discharged from the normal temperature second refrigerant compressor enters the second refrigerant evaporator, it further includes passing the second refrigerant through the first refrigerant heat exchanger and the first refrigerant evaporator for heat exchange; Before the third refrigerant discharged by the normal-temperature third refrigerant compressor enters the third refrigerant evaporator, it also includes passing the third refrigerant through the first refrigerant heat exchanger, the first refrigerant evaporator, the second refrigerant heat exchanger, and the second refrigerant evaporator for heat exchange; The first refrigerant heat exchanger and the first refrigerant evaporator are separately arranged and have N-stage heat exchange and N-stage evaporation. The second refrigerant heat exchanger and the second refrigerant evaporator are separately arranged and have N-stage heat exchange and N-stage evaporation. The third refrigerant heat exchanger and the third refrigerant evaporator are separately arranged and have N-stage heat exchange and N-stage evaporation; In the N-stage heat exchange and N-stage evaporation, N is greater than or equal to 2, and the number of the first refrigerant separation tank, the second refrigerant separation tank, and the third refrigerant separation tank is N - 1; A first liquid-phase throttle valve is arranged on the first refrigerant inlet pipeline of each stage of the first refrigerant evaporator, a second liquid-phase throttle valve is arranged on the second refrigerant inlet pipeline of each stage of the second refrigerant evaporator, and a third liquid-phase throttle valve is arranged on the third refrigerant inlet pipeline of each stage of the third refrigerant evaporator.

2. A cascaded refrigeration cycle LNG liquefaction method, characterized in that, it is carried out using the cascaded refrigeration cycle LNG liquefaction system as described in claim 1, and it includes the following steps: The first refrigerant discharged from the normal-temperature first refrigerant compressor is evaporated and separated into gas-liquid two phases through the first refrigerant evaporator. Among them, the gas phase enters the inlet of the first refrigerant compressor after recovering the sensible heat of the gas phase through the first refrigerant heat exchanger, and the liquid phase enters the first refrigerant heat exchanger to recover the sensible heat of the gas phase and enters the inlet of the first refrigerant compressor after recovering the latent heat of the liquid phase through the first refrigerant evaporator; The second refrigerant discharged from the normal-temperature second refrigerant compressor is evaporated and separated into gas-liquid two phases through the second refrigerant evaporator. Among them, the gas phase enters the inlet of the second refrigerant compressor after sequentially recovering the sensible heat of the gas phase through the second refrigerant heat exchanger, and the liquid phase enters the second refrigerant heat exchanger and the first refrigerant heat exchanger to recover the sensible heat of the gas phase and enters the inlet of the second refrigerant compressor after recovering the latent heat of the liquid phase through the second refrigerant evaporator; The third refrigerant discharged from the normal-temperature third refrigerant compressor is evaporated and separated into gas-liquid two phases through the third refrigerant evaporator. Among them, the gas phase enters the inlet of the third refrigerant compressor after sequentially recovering the sensible heat of the gas phase through the third refrigerant heat exchanger, and the liquid phase enters the third refrigerant heat exchanger, the second refrigerant heat exchanger, and the first refrigerant heat exchanger to recover the sensible heat of the gas phase and enters the inlet of the third refrigerant compressor after recovering the latent heat of the liquid phase through the third refrigerant evaporator; When the first refrigerant is propane, the second refrigerant is ethylene, and the third refrigerant is methane, the pressure of the first refrigerant discharged by the normal-temperature first refrigerant compressor ≥ 0.47 MpaA, and the pressure of the first refrigerant entering the first refrigerant evaporator is 0.1 - 0.47 MPaA, with a temperature of -43 - 0 °C; The pressure of the second refrigerant discharged by the normal-temperature second refrigerant compressor ≥ 1.38 MPaA, and the pressure of the second refrigerant entering the second refrigerant evaporator is 0.1 - 1.38 MPaA, with a temperature of -104 - -42 °C; The pressure of the third refrigerant discharged by the normal-temperature third refrigerant compressor ≥ 2.44 MPaA, and the pressure of the third refrigerant entering the third refrigerant evaporator is 0.1 - 2.44 MPaA, with a temperature of -161.8 - -102 °C.

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