A cascade refrigeration natural gas liquefaction system and process

Abstract

The present invention discloses a cascade refrigeration natural gas liquefaction system and process, belonging to the technical field of refrigeration and cryogenic engineering. The process includes an ethane refrigeration cycle and a mixed refrigerant refrigeration cycle. The refrigerants used in the present invention are propane, ethane, and mixed refrigerant, and the components of the mixed refrigerant are composed of nitrogen, methane, and ethane. Except for nitrogen which uses public works, all the other refrigerants are components in natural gas, and the refrigerants do not need to be purchased externally. The comprehensive unit liquefaction energy consumption of the present invention is lower than that of the propane precooled mixed refrigerant process and the conventional cascade process. Although it is slightly higher compared with the winter condition of the dual mixed refrigerant process, the energy consumption of the dual mixed refrigerant process is very high in summer. Overall, the total energy consumption of the process of the present invention is still relatively low.

Description

Technical Field

[0001] The present invention relates to the technical field of refrigeration and cryogenic engineering, and in particular to a cascade refrigeration natural gas liquefaction system and process. Background Art

[0002] Liquefied Natural Gas (LNG) is currently the fastest growing energy source in the world. Vigorously developing LNG will play an important role in optimizing the energy structure, effectively solving the dual problems of energy supply security and ecological environmental protection, and achieving sustainable economic and social development.

[0003] LNG has many advantages as a vehicle fuel. First, it saves about 25-35% of fuel costs, and the economic benefits of replacing oil with gas are considerable. Secondly, natural gas is a high-octane fuel, and octane number is an important indicator for evaluating fuel performance. When a car uses a high-octane fuel, the engine is less likely to experience knock combustion, which is very beneficial to extending the life of the engine. Finally, LNG is cheaper than gasoline, diesel, and LPG, is safe and environmentally friendly, has high storage efficiency, long vehicle mileage, long engine life, and the cold released during engine operation can be used for air conditioning. This not only saves energy but also makes comprehensive use of energy.

[0004] For small and medium-sized natural gas liquefaction plants, they are mainly built near the natural gas main network to serve the surrounding markets. Basically, a single mixed refrigerant refrigeration (SMR) process is selected. Although the energy consumption of this process is higher than that of the propane pre-cooling mixed refrigerant process (C3MR) or the cascade process (Cascade), the process equipment is simple and the investment is small. From the perspective of the comprehensive investment return cycle, it is the optimal process.

[0005] However, for large-scale LNG plants with complex bases, the use of SMR technology is not the best solution. Because when the scale of liquefaction increases, the power of the refrigeration compressor must increase. From the current manufacturing capacity of motors or gas turbines, when the power is too large, one unit cannot meet the requirements at all, so multiple compressors need to be connected in parallel. In this way, when the scale of a single production line is large, if the single mixed refrigerant process is still used, multiple compressors need to be connected in parallel, and its fixed investment cost will increase greatly, and the energy consumption is also high. Therefore, at this time, the use of propane pre-cooled mixed refrigerant (C3-MR) process or cascade process (Cascade), or other cascade refrigeration processes has obvious advantages. Compared with the single mixed refrigerant process, the fixed investment of the cascade refrigeration process is close to or even lower, and the energy consumption can be greatly reduced. Therefore, at present, large-scale base load-type natural gas liquefaction plants basically use cascade refrigeration processes, especially C3-MR and Cascade processes, and a small number of double mixed refrigerant processes and multi-stage mixed refrigerant processes are also used. Summary of the invention

[0006] The purpose of the present invention is to solve the problems existing in the prior art and to provide a large-scale natural gas liquefaction system and process with cascade refrigeration.

[0007] The specific technical solutions adopted by the present invention are as follows:

[0008] In a first aspect, the present invention provides a cascade refrigeration natural gas liquefaction system, the natural gas liquefaction system comprising an ethane refrigeration cycle system and a mixed refrigerant refrigeration cycle system;

[0009] The ethane refrigeration cycle system comprises a multi-stage ethane compressor, an ethane air cooler, an ethane buffer tank, a multi-stage throttling refrigeration device, a multi-stage natural gas precooler, and a multi-stage mixed refrigerant precooler; the multi-stage ethane compressor, the ethane air cooler, the ethane buffer tank, and the multi-stage throttling refrigeration device are sequentially connected through an ethane delivery pipeline to form an ethane circulation loop; ethane is compressed by the multi-stage ethane compressor and cooled by the ethane air cooler, and then buffered and stored in the ethane buffer tank, and then enters the multi-stage throttling refrigeration device to be throttled and depressurized step by step for refrigeration, and then refluxes to the multi-stage ethane compressor after providing cooling capacity to the multi-stage natural gas precooler and the multi-stage mixed refrigerant precooler; the input end of the multi-stage natural gas precooler is connected to the natural gas inlet I, and the input natural gas to be liquefied is precooled step by step in the multi-stage natural gas precooler using the ethane cooling capacity, and then transported to the mixed refrigerant refrigeration cycle system through the natural gas delivery pipeline;

[0010] The mixed refrigerant refrigeration cycle system comprises a mixed refrigerant compressor, a mixed refrigerant air cooler, a mixed refrigerant gas-liquid separation tank, and a main heat exchanger cold box; the mixed refrigerant output port of the mixed refrigerant compressor is connected to the mixed refrigerant air cooler, the multi-stage mixed refrigerant precooler, and the mixed refrigerant gas-liquid separation tank in sequence through a mixed refrigerant delivery pipeline; after the mixed refrigerant is compressed by the mixed refrigerant compressor and air-cooled by the mixed refrigerant air cooler, the mixed refrigerant is precooled step by step in the multi-stage mixed refrigerant precooler using ethane cooling capacity and enters the mixed refrigerant gas-liquid separation tank for gas-liquid separation The gas phase outlet and liquid phase outlet of the mixed refrigerant gas-liquid separation tank are respectively connected to the inlet ends of the two throttling refrigeration units in the main heat exchanger cold box, and the two throttling refrigeration units provide the main heat exchanger cold box with the cooling capacity required for natural gas supercooling through throttling refrigeration of the mixed refrigerant; the outlet ends of the two throttling refrigeration units are connected to the mixed refrigerant reflux inlet of the mixed refrigerant compressor; a heat exchange pipeline for supercooling natural gas is provided in the main heat exchanger cold box, the input end of the heat exchange pipeline is connected to the natural gas transmission pipeline, and the output end is connected to the liquefied natural gas outlet O.

[0011] As a preferred embodiment of the above-mentioned first aspect, the multi-stage ethane compressor, multi-stage throttling refrigeration device, multi-stage natural gas precooler and multi-stage mixed refrigerant precooler contain the same number of stages and the stages correspond one to one, and each stage of the throttling refrigeration device is composed of a gas-liquid separation tank and a throttling valve installed on the inlet pipe of the gas-liquid separation tank, wherein the liquid phase outlet of the gas-liquid separation tank is divided into three routes, which are respectively transported to the refrigerant inlets of the natural gas precooler and the mixed refrigerant precooler of the corresponding levels, and the refrigerant outlets of the natural gas precooler and the mixed refrigerant precooler and the gas phase outlet of the gas-liquid separation tank are connected to the inlet of the ethane compressor of the corresponding level.

[0012] As a preferred embodiment of the first aspect, the number of stages included in the multi-stage ethane compressor, the multi-stage throttling refrigeration device, the multi-stage natural gas precooler and the multi-stage mixed refrigerant precooler is 3 to 5.

[0013] As a preferred embodiment of the above-mentioned first aspect, each of the throttling refrigeration units includes an inlet heat exchange tube, a throttling valve, a two-phase flow distribution tank and an outlet heat exchange tube, and the inlet heat exchange tube, the two-phase flow distribution tank and the outlet heat exchange tube are all located in the main heat exchanger cold box; the inlet of the inlet heat exchange tube is used to input the mixed refrigerant output from the mixed refrigerant gas-liquid separation tank, and the outlet of the inlet heat exchange tube is connected to the throttling valve and the inlet of the two-phase flow distribution tank in sequence, the gas phase outlet and the liquid phase outlet of the two-phase flow distribution tank are respectively connected to the inlet of the outflow heat exchange tube, and the outlet of the outflow heat exchange tube is connected to the mixed refrigerant reflux inlet of the mixed refrigerant compressor; the mixed refrigerant output from the mixed refrigerant gas-liquid separation tank absorbs cold in the inlet heat exchange tube and is further throttled and cooled through the throttling valve, and then enters the two-phase flow distribution tank for gas-liquid distribution and then input into the outflow heat exchange tube to provide cold for natural gas and mixed refrigerant.

[0014] As a preferred embodiment of the first aspect, a propane precooler or a water cooler is provided between the ethane air cooler and the ethane buffer tank.

[0015] As a preferred embodiment of the first aspect, a propane precooler or a water cooler is provided between the mixed refrigerant air cooler and the multi-stage mixed refrigerant precooler.

[0016] As a preferred embodiment of the first aspect, a propane precooler or a water cooler is provided between the natural gas inlet I and the multi-stage natural gas precooler.

[0017] In a second aspect, the present invention provides a cascade refrigeration natural gas liquefaction process, which is implemented based on the cascade refrigeration natural gas liquefaction system described in any one of the solutions of the first aspect above, and the natural gas liquefaction process comprises:

[0018] S1. Perform multi-stage compression on ethane gas by a multi-stage ethane compressor, and then perform air cooling of the compressed ethane into liquid ethane by an ethane air cooler and store it in an ethane buffer tank; the liquid ethane in the ethane buffer tank is stably transported to a multi-stage throttling refrigeration device to be throttled and depressurized step by step, and each stage of the throttling refrigeration device throttles and depressurizes the input liquid ethane and further performs gas-liquid separation, and the separated gaseous ethane is refluxed and transported to the multi-stage ethane compressor for re-compression, and part of the separated liquid ethane is transported to the next stage of the throttling refrigeration device, and part of it is transported to a multi-stage natural gas precooler and a multi-stage mixed refrigerant precooler to provide cooling capacity through heat exchange, and the ethane gas after heat exchange is refluxed and transported to the multi-stage ethane compressor for re-compression;

[0019] S2, compressing the mixed refrigerant by a mixed refrigerant compressor, and then air-cooling the compressed mixed refrigerant into a liquid mixed refrigerant by a compressor air cooler, and then passing the liquid mixed refrigerant through a multi-stage mixed refrigerant precooler, the liquid mixed refrigerant exchanges heat with the liquid ethane transported in the multi-stage throttling refrigeration device, thereby utilizing the cold energy provided by the liquid ethane to gradually reduce the temperature to complete precooling, and the liquid mixed refrigerant after precooling enters the mixed refrigerant gas-liquid separation tank for gas-liquid separation; the gaseous mixed refrigerant and the liquid mixed refrigerant obtained by the gas-liquid separation enter the main heat exchanger cold box respectively to further absorb cold energy for cooling, and then each passes through the throttling refrigeration unit for throttling and depressurization, and then returns to the main heat exchanger cold box to provide cold energy and heat up the natural gas and the mixed refrigerant, and the heated mixed refrigerant flows back to the mixed refrigerant compressor for re-compression;

[0020] S3. The purified natural gas to be liquefied is fed into the multi-stage natural gas precooler through the natural gas inlet I, and is heat exchanged with the liquid ethane transported in the multi-stage throttling refrigeration device, so that the precooling is completed by stepwise cooling using the cooling capacity provided by the liquid ethane; the natural gas after precooling further enters the main heat exchanger cold box, and is heat exchanged with the mixed refrigerant returned to the main heat exchanger cold box after throttling and pressure reduction, so as to complete liquefaction and supercooling, and the liquefied and supercooled natural gas is output through the liquefied natural gas outlet O.

[0021] As a preferred embodiment of the second aspect, the temperature of the pre-cooled liquid mixed refrigerant and the pre-cooled natural gas is -60 to -80°C.

[0022] As a preferred embodiment of the above second aspect, the mixed refrigerant consists of nitrogen, methane and ethane.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] 1. Conventional cascade processes use ethylene in the secondary refrigeration stage, and ethylene needs to be purchased externally. The present invention uses a natural gas liquefaction process with a cascade refrigeration cycle, and the refrigerants used in the process include propane, ethane and a mixed refrigerant, wherein the mixed refrigerant is composed of nitrogen, methane and ethane. Except for nitrogen, which needs to be supplied externally (such as from a public utility), the remaining refrigerants are all derived from the natural gas itself and do not need to be purchased additionally.

[0025] 2. At present, the most commonly used process for liquefied natural gas is C3-MR, which is the mixed refrigerant process with propane precooling. The biggest advantage of using this process is that the device has low energy consumption and is easy to operate. However, the biggest problem with this process is that the power of the propane compressor and the mixed refrigerant compressor are quite different. The mixed refrigerant power is limited by the maximum power of the gas turbine or motor, which limits the load of the single production line. At present, in order to solve this problem, the American APCI company has adopted the AP-Split MR® process, that is, a part of the power of the mixed refrigerant compressor is distributed to the gas turbine or motor that drives the propane compressor. In this way, the power of the gas turbine or motor that drives the mixed refrigerant compressor can be reduced, so as to maximize the production capacity of a single production line. In this way, the control system is relatively complex. One gas turbine or motor needs to drive two compressor systems, and the start and stop control of the compressor is very complex.

[0026] The present invention uses ethane precooled mixed refrigerant, which can cool natural gas and mixed refrigerant to -80°C. In contrast, propane precooling can only reach -40°C. Therefore, the present invention can achieve a lower precooling temperature, and by adjusting the temperature after precooling, the power of the ethane compressor and the mixed refrigerant compressor can be similar, thereby maximizing the production capacity of a single production line. At the same time, this also avoids the complex solution of one gas turbine or motor controlling two compressors.

[0027] 3. At present, the C3MR process and the cascade process usually use three-stage or four-stage propane precooling to precool the raw gas and the refrigerant to about -35°C. However, the present invention can add a propane refrigeration or seawater precooling step before ethane precooling, and the propane refrigeration can further adopt a one-stage or two-stage refrigeration method according to demand to further improve the efficiency of the entire device. The advantage of using the process of the present invention is that the process only uses one or two-stage propane precooling, or uses seawater auxiliary precooling to cool the raw gas and the refrigerant to about 12°C. This process is very suitable for areas with large temperature differences between winter and summer, especially in cold and polar regions. When the ambient temperature is low, the propane compressor refrigeration cycle or seawater precooling can be stopped, and the raw gas and the refrigerant can be cooled to 12°C or even lower by the ambient temperature, which can greatly reduce the operating energy consumption. The present invention is particularly suitable for cold polar regions, where the ambient temperature is only higher than 0°C for one or two months a year, and the propane precooling compressor or seawater precooling only needs to be turned on for a short time. The conventional three- to four-stage propane precooling process requires cooling the raw gas and refrigerant to -35°C. Even in the winter in the cold zone, it is difficult to cool to this temperature through the ambient air temperature. Therefore, the propane compressor must be running all the time, which wastes a lot of energy.

[0028] 4. The comprehensive unit liquefaction energy consumption of the present invention is lower than that of the propane precooling mixed refrigerant process and the conventional cascade process. Although it is slightly higher than the winter working condition of the double mixed refrigerant, the energy consumption of the double mixed refrigerant process is very high in summer. In general, the present invention still has an energy consumption advantage. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a structural schematic diagram of a cascade natural gas liquefaction system;

[0030] Figure 2 for Figure 1 An enlarged schematic diagram of the ethane refrigeration cycle system;

[0031] Figure 3 for Figure 2 An enlarged schematic diagram of a mixed refrigerant refrigeration cycle system;

[0032] Figure 4 This is a schematic diagram of the structure of another cascade natural gas liquefaction system.

[0033] Figure 5 for Figure 4 An enlarged schematic diagram of the ethane refrigeration cycle system;

[0034] Figure 6 for Figure 4 An enlarged schematic diagram of a mixed refrigerant refrigeration cycle system;

[0035] The reference numerals in the figure are as follows: ethane refrigeration cycle system A, mixed refrigerant refrigeration cycle system B; ethane refrigeration cycle system A includes: multi-stage ethane compressor C-201, ethane air cooler AC-201, ethane buffer tank V-201, first ethane gas-liquid separation tank V-202, second ethane gas-liquid separation tank V-203, third ethane gas-liquid separation tank V-204, fourth ethane gas-liquid separation tank V-205, natural gas-ethane primary precooler E-301, natural gas Gas-ethane secondary precooler E-302, natural gas-ethane tertiary precooler E-303, natural gas-ethane quaternary precooler E-304, mixed refrigerant-ethane primary precooler E-101, mixed refrigerant-ethane secondary precooler E-102, mixed refrigerant-ethane tertiary precooler E-103, mixed refrigerant-ethane quaternary precooler E-104, the first valve Val-201, the second valve Val-202, the third valve Val-203, the The fourth valve Val-204, the fifth valve Val-205, the sixth valve Val-206, the seventh valve Val-207, the eighth valve Val-208, the ninth valve Val-209, the tenth valve Val-210, the eleventh valve Val-211, and the twelfth valve Val-212; the mixed refrigerant refrigeration cycle system B includes: a mixed refrigerant compressor C-101, a mixed refrigerant air cooler AC-101, a mixed refrigerant gas-liquid separation Tank V-101, first two-phase flow uniform distribution tank V-401, second two-phase flow uniform distribution tank V-402, main heat exchanger cold box CB-401, first throttle valve Val-401, second throttle valve Val-402; the connecting pipelines in the two systems include: ethane transmission pipeline P-1, natural gas transmission pipeline P-2, mixed refrigerant transmission pipeline P-3, first inlet heat exchange pipe P-4, second inlet heat exchange pipe P-5, outlet heat exchange pipe P-6, heat exchange pipeline P-7. DETAILED DESCRIPTION

[0036] In order to make the above-mentioned purpose, features and advantages of the present invention more obvious and easy to understand, the specific implementation mode of the present invention is described in detail below in conjunction with the accompanying drawings. In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below. The technical features in each embodiment of the present invention can be combined accordingly without conflicting with each other.

[0037] In the description of the present invention, it is to be understood that when an element is considered to be "connected" to another element, it may be directly connected to the other element or indirectly connected, that is, there are intermediate elements. On the contrary, when an element is said to be "directly" connected to another element, there are no intermediate elements.

[0038] In the description of the present invention, it should be understood that the terms "first" and "second" are only used to distinguish the description purpose, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features.

[0039] The present invention provides a natural gas liquefaction system with cascade refrigeration, in which the purified natural gas and the compressed and cooled mixed refrigerant are respectively pre-cooled by ethane in multiple stages, and then the pre-cooled natural gas and the mixed refrigerant are cooled, liquefied and supercooled by the throttled, depressurized and reversed mixed refrigerant, the liquefied and supercooled natural gas is sent to a downstream LNG storage tank for storage, and the liquefied and supercooled mixed refrigerant is throttled and returned to the main heat exchanger to provide cooling capacity for the system.

[0040] See also Figure 1 As shown, in a preferred embodiment of the present invention, a specific implementation form of the above-mentioned cascade refrigeration natural gas liquefaction system is shown. The cascade refrigeration natural gas liquefaction system includes an ethane refrigeration cycle system A and a mixed refrigerant refrigeration cycle system B.

[0041] See also Figure 2 As shown, the ethane refrigeration cycle system A in the embodiment of the present invention includes a multi-stage ethane compressor C-201, an ethane air cooler AC-201, an ethane buffer tank V-201, a multi-stage throttling refrigeration device V-2, a multi-stage natural gas precooler E-3, and a multi-stage mixed refrigerant precooler E-1; the multi-stage ethane compressor C-201, the ethane air cooler AC-201, the ethane buffer tank V-201 and the multi-stage throttling refrigeration device V-2 are connected in sequence through an ethane delivery pipeline P-1 to form an ethane circulation loop. After being compressed by the multi-stage ethane compressor C-201 and cooled by the ethane air cooler AC-201, the ethane is buffered and stored in the ethane buffer tank V-201, and then enters the multi-stage throttling refrigeration device V-2 to be throttled and depressurized step by step, and then provides cooling capacity to the multi-stage natural gas precooler E-3 and the multi-stage mixed refrigerant precooler E-1, and then flows back to the multi-stage ethane compressor C-201. The input end of the multi-stage natural gas precooler E-3 is connected to the natural gas inlet I. The input natural gas to be liquefied is precooled step by step in the multi-stage natural gas precooler E-3 using ethane cooling capacity, and then transported to the mixed refrigerant refrigeration cycle system B through the natural gas transmission pipeline P-2.

[0042] It should be noted that the multi-stage ethane compressor C-201 is a compressor with a multi-stage compression function, which is formed by a plurality of compression units connected in cascade. Each compression unit can receive the ethane output by the previous compression unit for further compression. At the same time, each compression unit can also have its own air inlet. If there is ethane input from the air inlet of the unit itself, it can also be mixed with the ethane output by the previous compression unit and compressed together. Thus, ethane can be compressed to the required pressure step by step, so as to facilitate subsequent cooling and liquefaction. The multi-stage throttling refrigeration device V-2 is formed by a plurality of throttling refrigeration devices connected in cascade, which can throttle and reduce the pressure of the liquid ethane after cooling and liquefaction step by step, thereby generating cold to further cool the liquid ethane. A part of the liquid ethane after cooling by each throttling refrigeration device enters the next throttling refrigeration device, and the other part can provide the cold required for pre-cooling and cooling of natural gas and mixed refrigerant to the outside, thereby evaporating to form gaseous ethane. The gaseous ethane generated by throttling and the gaseous ethane formed by pre-cooling and evaporation can be returned to the multi-stage ethane compressor C-201 for compression. The multi-stage natural gas precooler E-3 is formed by a plurality of precoolers in cascade, and each stage of the precooler can use the liquid ethane provided by the throttling refrigeration device as a refrigerant to provide cooling capacity, thereby precooling the natural gas after heat exchange with the natural gas, so that the natural gas and are precooled to a certain low temperature before entering the mixed refrigerant refrigeration cycle system B. The multi-stage mixed refrigerant precooler E-1 is also formed by a plurality of precoolers in cascade, and each stage of the precooler can use the liquid ethane provided by the throttling refrigeration device as a refrigerant to provide cooling capacity, thereby precooling the mixed refrigerant after heat exchange with the mixed refrigerant, so that the mixed refrigerant is precooled to a certain low temperature before entering the mixed refrigerant refrigeration cycle system B. The specific temperature at which the mixed refrigerant and the natural gas are precooled before entering the mixed refrigerant refrigeration cycle system B can be adjusted according to actual needs. In an embodiment of the present invention, the temperature of the liquid mixed refrigerant after precooling and the natural gas after precooling is preferably -60~-80℃.

[0043] The mixed refrigerant in the present invention can be selected according to actual needs, and preferably a mixed refrigerant composed of nitrogen, methane and ethane is used.

[0044] In addition, the number of stages included in the above-mentioned multi-stage ethane compressor C-201, multi-stage throttling refrigeration device V-2, multi-stage natural gas precooler E-3 and multi-stage mixed refrigerant precooler E-1 can be adjusted according to actual needs, and the number of stages can generally be set to 3 to 5. The number of stages included in the multi-stage ethane compressor C-201, multi-stage throttling refrigeration device V-2, multi-stage natural gas precooler E-3 and multi-stage mixed refrigerant precooler E-1 can be the same or different. In an embodiment of the present invention, the multi-stage ethane compressor C-201, the multi-stage throttling refrigeration device V-2, the multi-stage natural gas precooler E-3 and the multi-stage mixed refrigerant precooler E-1 each contain the same number of stages and the stages correspond one to one. Each stage of the throttling refrigeration device is composed of a gas-liquid separation tank and a throttling valve installed on the inlet pipe of the gas-liquid separation tank, wherein the liquid phase outlet of the gas-liquid separation tank is divided into three routes, which are respectively transported to the refrigerant inlets of the natural gas precooler and the mixed refrigerant precooler of the corresponding levels, and the refrigerant outlet of the natural gas precooler, the refrigerant outlet of the mixed refrigerant precooler and the gas phase outlet of the gas-liquid separation tank are connected to the inlet of the ethane compressor of the corresponding level.

[0045] In the embodiment of the present invention, the number of stages included in the above-mentioned multi-stage ethane compressor C-201, multi-stage throttling refrigeration device V-2, multi-stage natural gas precooler E-3 and multi-stage mixed refrigerant precooler E-1 are all 4. The multi-stage ethane compressor C-201 includes 4 compressors, which are a four-stage compressor, a three-stage compressor, a two-stage compressor and a one-stage compressor in the order of compression. Each stage compressor has an inlet for inputting gas ethane and also receives ethane compressed by the previous stage compressor. The multi-stage throttling refrigeration device V-2 includes 4 gas-liquid separation tanks, namely the first ethane gas-liquid separation tank V-202, the second ethane gas-liquid separation tank V-203, the third ethane gas-liquid separation tank V-204, and the fourth ethane gas-liquid separation tank V-205. Twelve valves are also installed on the inlet and outlet pipelines of the four gas-liquid separation tanks, namely the first valve Val-201, the second valve Val-202, the third valve Val-203, the fourth valve Val-204, the fifth valve Val-205, the fifth valve Val-206, the fifth valve Val-207, the fifth valve Val-208, the fifth valve Val-209, the fifth valve Val-210, the fifth valve Val-211, the fifth valve Val-212, the fifth valve Val-213, the fifth valve Val-214, the fifth valve Val-215, the fifth valve Val-216, the fifth valve Val-217, the fifth valve Val-218, the fifth valve Val-219, the fifth valve Val-220, the fifth valve Val-230, the fifth valve Val-241, the fifth valve Val-252, the fifth valve Val-263, the fifth valve Val-274, the fifth valve Val-2 Val-205, the sixth valve Val-206, the seventh valve Val-207, the eighth valve Val-208, the ninth valve Val-209, the tenth valve Val-210, the eleventh valve Val-211, and the twelfth valve Val-212, among which the first valve Val-201, the third valve Val-203, the sixth valve Val-206, and the ninth valve Val-209 are all throttle valves, and the remaining valves are used to control the on-off of the pipelines in which they are located. The multi-stage natural gas precooler E-3 includes a natural gas-ethane primary precooler E-301, a natural gas-ethane secondary precooler E-302, a natural gas-ethane tertiary precooler E-303, and a natural gas-ethane quaternary precooler E-304, while the multi-stage mixed refrigerant precooler E-1 includes a mixed refrigerant-ethane primary precooler E-101, a mixed refrigerant-ethane secondary precooler E-102, a mixed refrigerant-ethane tertiary precooler E-103, and a mixed refrigerant-ethane quaternary precooler E-104.

[0046] The first valve Val-201 is installed on the connecting pipeline between the ethane buffer tank V-201 and the first ethane gas-liquid separation tank V-202, the second valve Val-202 is installed on the connecting pipeline between the first ethane gas-liquid separation tank V-202 and the mixed refrigerant-ethane primary precooler E-101, the third valve Val-203 is installed on the connecting pipeline between the first ethane gas-liquid separation tank V-202 and the second ethane gas-liquid separation tank V-203, and the fourth valve V Val-204 is installed on the connecting pipeline between the first ethane gas-liquid separation tank V-202 and the natural gas-ethane primary precooler E-301, the fifth valve Val-205 is installed on the connecting pipeline between the second ethane gas-liquid separation tank V-203 and the mixed refrigerant-ethane secondary precooler E-102, the sixth valve Val-206 is installed on the connecting pipeline between the second ethane gas-liquid separation tank V-203 and the third ethane gas-liquid separation tank V-204, the seventh valve V Val-207 is installed on the connecting pipeline between the second ethane gas-liquid separation tank V-203 and the natural gas-ethane secondary precooler E-302. The eighth valve Val-208 is installed on the connecting pipeline between the third ethane gas-liquid separation tank V-204 and the mixed refrigerant-ethane tertiary precooler E-103. The ninth valve Val-209 is installed on the connecting pipeline between the third ethane gas-liquid separation tank V-204 and the fourth ethane gas-liquid separation tank V-205. The tenth valve Val-210 is installed on the connecting pipeline between the third ethane gas-liquid separation tank V-204 and the natural gas-ethane three-stage precooler E-303, the eleventh valve Val-211 is installed on the connecting pipeline between the fourth ethane gas-liquid separation tank V-205 and the mixed refrigerant-ethane four-stage precooler E-104, and the twelfth valve Val-212 is installed on the connecting pipeline between the fourth ethane gas-liquid separation tank V-205 and the natural gas-ethane four-stage precooler E-304.

[0047] It should be noted that the ethane working medium pressure in the first ethane gas-liquid separation tank V-202, the second ethane gas-liquid separation tank V-203, the third ethane gas-liquid separation tank V-204, and the fourth ethane gas-liquid separation tank V-205 is gradually reduced, and the four ethane gas-liquid separation tanks correspond to the ethane high-pressure gas-liquid separation tank, the ethane high-pressure gas-liquid separation tank, the ethane low-pressure gas-liquid separation tank, and the ethane low-low-pressure gas-liquid separation tank, respectively. However, the high-pressure, high-pressure, low-pressure, and low-low-pressure above only indicate the relative height of the pressure, and are not limitations on the absolute value of the pressure.

[0048] In the above-mentioned ethane refrigeration cycle system A with 4 stages, the ethane circulation precooling process is as follows:

[0049] After being compressed in multiple stages by the multi-stage ethane compressor C-201 and cooled into liquid by the ethane air cooler AC-201, the ethane is sent to the ethane buffer tank V-201. Then the liquid ethane enters the first ethane gas-liquid separation tank V-202 for gas-liquid separation after throttling by the first valve Val-201, and the gas produced by the gas-liquid separation enters the inlet of the four-stage compressor of the multi-stage ethane compressor C-201. A part of the liquid ethane produced by the gas-liquid separation in the first ethane gas-liquid separation tank V-202 enters the natural gas-ethane primary precooler E-301 as a precoolant to precool the natural gas through the fourth valve Val-204, and a part of it enters the mixed refrigerant-ethane primary precooler E-101 as a precoolant to precool the mixed refrigerant through the second valve Val-202. The evaporated ethane after precooling in the natural gas-ethane primary precooler E-301 and the mixed refrigerant-ethane primary precooler E-101 also enters the inlet of the four-stage compressor of the multi-stage ethane compressor C-201.

[0050] Another part of the liquid ethane produced by the gas-liquid separation in the first ethane gas-liquid separation tank V-202 continues to enter the ethane high-pressure gas-liquid separator V-203 for gas-liquid separation after throttling through the third valve Val-203. The gaseous ethane produced by the gas-liquid separation enters the third compressor inlet of the multi-stage ethane compressor C-201. One part of the liquid ethane produced by the gas-liquid separation in the ethane high-pressure gas-liquid separator V-203 enters the natural gas-ethane secondary precooler E-302 as a precoolant to precool the natural gas through the seventh valve Val-207, and one part enters the mixed refrigerant-ethane secondary precooler E-102 as a precoolant to precool the mixed refrigerant through the fifth valve Val-205. The evaporated ethane after precooling in the natural gas-ethane secondary precooler E-302 and the mixed refrigerant-ethane secondary precooler E-102 enters the third compressor inlet of the multi-stage ethane compressor C-201 together.

[0051] Another part of the liquid produced by the gas-liquid separation in the second ethane gas-liquid separation tank V-203 continues to enter the ethane low-pressure gas-liquid separator V-204 for gas-liquid separation after throttling through the sixth valve Val-206. The gaseous ethane produced by the gas-liquid separation enters the secondary compressor inlet of the multi-stage ethane compressor C-201. A part of the liquid ethane produced by the gas-liquid separation in the ethane low-pressure gas-liquid separator V-204 enters the natural gas-ethane three-stage precooler E-303 through the tenth valve Val-210 as a precooling agent to precool the natural gas, and a part enters the mixed refrigerant-ethane three-stage precooler E-103 through the eighth valve Val-208 as a precooling agent to precool the mixed refrigerant. The ethane evaporated after precooling in the natural gas-ethane three-stage precooler E-303 and the mixed refrigerant-ethane three-stage precooler E-103 enters the secondary compressor inlet of the multi-stage ethane compressor C-201 together.

[0052] Another part of the liquid produced by the gas-liquid separation in the third ethane gas-liquid separation tank V-204 continues to enter the ethane low-pressure gas-liquid separator V-205 for gas-liquid separation after throttling through the ninth valve Val-209. The gaseous ethane produced by the gas-liquid separation enters the first-stage compressor inlet of the multi-stage ethane compressor C-201. A part of the liquid ethane produced by the gas-liquid separation in the ethane low-pressure gas-liquid separator V-205 enters the natural gas-ethane first-stage precooler E-304 as a precoolant to precool the natural gas through the twelfth valve Val-212, and a part enters the mixed refrigerant-ethane first-stage precooler E-104 as a precoolant to precool the mixed refrigerant through the eleventh valve Val-211. The ethane evaporated after precooling in the natural gas-ethane first-stage precooler E-304 and the mixed refrigerant-ethane first-stage precooler E-104 enters the first-stage compressor inlet of the multi-stage ethane compressor C-201 together.

[0053] The number of stages included in each multi-stage device in the above-mentioned ethane refrigeration cycle system A is 4, corresponding to the pre-cooling method of four-stage refrigeration. But in fact, the number of stages included in each multi-stage device can also be 3 or 5, corresponding to three-stage or five-stage refrigeration. In the three-stage refrigeration mode, three pre-cooling heat exchangers are selected, and the investment cost of the ethane compressor is reduced, but the power of the ethane compressor is increased, and the energy consumption of the device is increased; in the five-stage refrigeration mode, five pre-cooling heat exchangers are selected, and the equipment investment of the ethane compressor is increased, but the power of the ethane compressor is reduced, and the energy consumption of the device is reduced. Therefore, the specific number of stages can be determined according to actual needs.

[0054] See also Figure 3As shown, the mixed refrigerant refrigeration cycle system B in the embodiment of the present invention includes a mixed refrigerant compressor C-101, a mixed refrigerant air cooler AC-101, a mixed refrigerant gas-liquid separation tank V-101, and a main heat exchanger cold box CB-401. The mixed refrigerant outlet of the mixed refrigerant compressor C-101 is connected to the mixed refrigerant air cooler AC-101, the multi-stage mixed refrigerant precooler E-1, and the mixed refrigerant gas-liquid separation tank V-101 in sequence through the mixed refrigerant delivery pipeline P-3. After the mixed refrigerant is compressed by the mixed refrigerant compressor C-101 and air-cooled by the mixed refrigerant air cooler AC-101, the mixed refrigerant is precooled step by step in the multi-stage mixed refrigerant precooler E-1 using ethane cooling capacity and enters the mixed refrigerant gas-liquid separation tank V-101 for gas-liquid separation. The gas phase outlet and liquid phase outlet of the mixed refrigerant gas-liquid separation tank V-101 are respectively connected to the inlet ends of two throttling refrigeration units in the main heat exchanger cold box CB-401. The two throttling refrigeration units provide the main heat exchanger cold box CB-401 with the cooling capacity required for natural gas supercooling through throttling refrigeration of the mixed refrigerant; the outlet ends of the two throttling refrigeration units are connected to the mixed refrigerant reflux inlet of the mixed refrigerant compressor C-101; a heat exchange pipeline P-7 for supercooling natural gas is provided in the main heat exchanger cold box CB-401, the input end of the heat exchange pipeline is connected to the natural gas transmission pipeline P-2, and the output end is connected to the liquefied natural gas outlet O.

[0055] The two throttling refrigeration units arranged in the main heat exchanger cold box CB-401 are mainly used to throttle and reduce the pressure of the mixed refrigerant, thereby further reducing the temperature of the mixed refrigerant. Theoretically, any equipment that can achieve throttling refrigeration of the mixed refrigerant can be used as the throttling refrigeration unit. The two throttling refrigeration units work independently, and the gas phase outlet and liquid phase outlet of the mixed refrigerant gas-liquid separation tank V-101 are respectively connected to different throttling refrigeration units. The outlet ends of the two throttling refrigeration units can be connected to the mixed refrigerant refrigeration inlet of the mixed refrigerant compressor C-101 after merging, or they can be connected to the mixed refrigerant refrigeration inlet of the mixed refrigerant compressor C-101 respectively.

[0056] In the embodiment of the present invention, see Figure 3As shown, each throttling refrigeration unit includes an inlet heat exchange tube, a throttling valve, a two-phase flow distribution tank and an outlet heat exchange tube, and the inlet heat exchange tube, the two-phase flow distribution tank and the outlet heat exchange tube are all located in the main heat exchanger cold box CB-401. The inlet of the inlet heat exchange tube is used to input the mixed refrigerant output from the mixed refrigerant gas-liquid separation tank V-101, the outlet of the inlet heat exchange tube is connected to the throttling valve and the inlet of the two-phase flow distribution tank in sequence, the gas phase outlet and the liquid phase outlet of the two-phase flow distribution tank are respectively connected to the inlet of the outlet heat exchange tube, and the outlet of the outlet heat exchange tube is connected to the mixed refrigerant reflux inlet of the mixed refrigerant compressor C-101. After absorbing the cold in the inlet heat exchange tube, the mixed refrigerant output from the mixed refrigerant gas-liquid separation tank V-101 is further throttled and cooled by the throttling valve, and then enters the two-phase flow distribution tank for gas-liquid distribution and then enters the outlet heat exchange tube to provide cold for natural gas and mixed refrigerant. In the embodiment of the present invention, two throttling refrigeration units share the same outlet heat exchange tube, but the inlet heat exchange tubes in the two throttling refrigeration units are independent, so the throttling refrigeration unit connected to the liquid phase outlet of the mixed refrigerant gas-liquid separation tank V-101 includes the first inlet heat exchange tube P-4, the first throttling valve Val-401, the first two-phase flow uniform distribution tank V-401 and the outlet heat exchange tube P-6, and the throttling refrigeration unit connected to the gas phase outlet of the mixed refrigerant gas-liquid separation tank V-101 includes the second inlet heat exchange tube P-5, the second throttling valve Val-402, the second two-phase flow uniform distribution tank V-402 and the outlet heat exchange tube P-6. The mixed refrigerants input by the first inlet heat exchange tube P-4 and the second inlet heat exchange tube P-5 are finally converged into the outlet heat exchange tube P-6. After the natural gas transported by the natural gas transmission pipeline P-2 enters the heat exchange pipe P-7 in the main heat exchanger cold box CB-401, it can further absorb the coldness of the mixed refrigerant in the outflow heat exchange pipe P-6, thereby supercooling the natural gas to the required output temperature.

[0057] In the embodiment of the present invention, it is preferred that the mixed refrigerant compressor C-101 adopts a two-stage compressor, and thus in the mixed refrigerant refrigeration cycle system B, the overall processing flow is as follows: the mixed refrigerant compressor C-101 adopts two-stage compression, and the mixed refrigerant is output from the two-stage compression outlet to the mixed refrigerant air cooler AC-101 for air cooling after the two-stage compression, and then enters the mixed refrigerant-ethane primary precooler E-101, the mixed refrigerant-ethane secondary precooler E-102, the mixed refrigerant-ethane tertiary precooler E-103, and the mixed refrigerant-ethane quaternary precooler E-104 in sequence, so that the mixed refrigerant absorbs the coldness of the liquid ethane and gradually cools down. After the ethane quaternary precooling, the mixed refrigerant is cooled to a preset precooling temperature (which can be set to -60~ -80℃). The pre-cooled mixed refrigerant is then sent to the gas-liquid separation tank V-101. The gas phase refrigerant coming out from the top of the gas-liquid separation tank V-101 enters the second inlet heat exchange pipe P-5 in the main heat exchanger cold box CB-401 to absorb cold air. It is further throttled by the second throttle valve Val-402 for refrigeration and then distributed by the second two-phase flow distribution tank V-402 for gas-liquid uniform distribution, and then sent to the bottom of the outlet heat exchange pipe P-6. The liquid refrigerant comes out from the bottom of the separation tank V-101 and enters the first inlet heat exchange pipe P-4 of the main heat exchanger cold box CB-401 to absorb cold energy. It is further throttled by the first throttle valve Val-401 and refrigerated by the first two-phase flow distribution tank V-401 for gas-liquid uniform distribution. It is then sent to the middle of the outlet heat exchange pipe P-6, mixed with the gas-phase refrigerant flowing upward from the bottom, and releases cold energy to the natural gas in the heat exchange pipe P-7. After being reheated, it exits the main heat exchanger cold box CB-401 and enters the first compression inlet of the mixed refrigerant compressor C-101 to continue the refrigeration cycle.

[0058] It should be noted that in the above-mentioned ethane refrigeration cycle system A and the mixed refrigerant refrigeration cycle system B, the control temperature of natural gas in each link can be reasonably regulated according to actual needs. In the natural gas liquefaction system, the level of cooling temperature directly affects the size of the flash evaporation amount, which is generally manifested as: the higher the cooling temperature, the greater the flash evaporation amount, and the lower the cooling temperature, the smaller the flash evaporation amount. Therefore, the final specific cooling temperature of natural gas can be determined according to the actual demand of the system for the flash steam amount. In an embodiment of the present invention, the temperature of natural gas in each link can be controlled according to the following temperature range: natural gas after purification and ambient air cooling enters the multi-stage natural gas precooler E-3 to gradually cool down the natural gas temperature. After ethane precooling, the natural gas is cooled to -60~-80℃. The precooled natural gas then enters the main heat exchanger cold box CB-401, and is cooled to -150~-162℃ in this heat exchanger before being output from the liquefied natural gas outlet O.

[0059] In addition, in the present invention, in the ethane refrigeration cycle system A and the mixed refrigerant refrigeration cycle system B, air coolers are used for the first pre-cooling of natural gas, ethane and mixed refrigerant. However, when the ambient temperature is high, it is difficult to cool down the natural gas and the mixed refrigerant by air cooling, or even if they can be cooled down, the energy consumption of the subsequent process will be very high. Therefore, when the present invention is used in a scene with a high ambient temperature, an additional one-stage or two-stage propane pre-cooling system or seawater pre-cooling system can be added before or after the air cooler to improve the system efficiency. In another embodiment of the present invention, three water coolers can be added to cooperate with the original air cooler, such as Figure 4 , Figure 5 and Figure 6 As shown, the specific method is: add a first water cooler E-100 between the mixed refrigerant air cooler AC-101 and the multi-stage mixed refrigerant precooler E-1, add a second water cooler E-200 between the ethane air cooler AC-201 and the ethane buffer tank V-201, and add a third water cooler E-300 between the natural gas inlet I and the multi-stage natural gas precooler E-3. After adding three water coolers, it can be ensured that the mixed refrigerant, ethane and raw natural gas are cooled to about 13°C through a combination of air cooling and water cooling, and then subsequent cooling is carried out. The three added water coolers preferably use seawater as cooling water, that is, a seawater precooler is used to reduce the energy consumption cost of water cooling. Of course, the three added water coolers can also be replaced by propane precoolers, which can be selected according to actual needs.

[0060] In an embodiment of the present invention, based on the above-mentioned cascade refrigeration natural gas liquefaction system, a cascade refrigeration natural gas liquefaction process can also be provided, and the steps thereof include steps S1 to S3.

[0061] S1. The ethane gas is compressed in multiple stages by a multi-stage ethane compressor C-201, and then the compressed ethane is air-cooled into liquid ethane by an ethane air cooler AC-201 and stored in an ethane buffer tank V-201; the liquid ethane in the ethane buffer tank V-201 is stably transported to a multi-stage throttling refrigeration device V-2 to be throttled and depressurized step by step, and each stage of the throttling refrigeration device throttles and depressurizes the input liquid ethane and further performs gas-liquid separation, and the separated gaseous ethane is refluxed and transported to the multi-stage ethane compressor C-201 for re-compression, and part of the separated liquid ethane is transported to the next stage of the throttling refrigeration device, and part of it is transported to the multi-stage natural gas precooler E-3 and the multi-stage mixed refrigerant precooler E-1 to provide cooling capacity through heat exchange, and the ethane gas after heat exchange is refluxed and transported to the multi-stage ethane compressor C-201 for re-compression.

[0062] S2. The mixed refrigerant is compressed by the mixed refrigerant compressor C-101, and then the compressed mixed refrigerant is air-cooled into liquid mixed refrigerant by the compressor air cooler AC-101. The liquid mixed refrigerant is then passed through the multi-stage mixed refrigerant precooler E-1, and the liquid mixed refrigerant is heat exchanged with the liquid ethane transported in the multi-stage throttling refrigeration device V-2, so that the precooling is completed by step-by-step cooling using the cooling capacity provided by the liquid ethane. After the precooling, the liquid mixed refrigerant enters the mixed refrigerant gas-liquid separation tank V-101 for gas-liquid separation; the gaseous mixed refrigerant and liquid mixed refrigerant obtained by the gas-liquid separation respectively enter the main heat exchanger cold box CB-401 to further absorb cooling capacity and cool down, and then each is throttled and depressurized by the throttling refrigeration unit and then returns to the main heat exchanger cold box CB-401 to provide cooling capacity and temperature increase for the cooling of natural gas and the mixed refrigerant. The heated mixed refrigerant flows back to the mixed refrigerant compressor C-101 for re-compression.

[0063] S3. The purified natural gas to be liquefied is input into the multi-stage natural gas precooler E-3 through the natural gas inlet I, and is heat exchanged with the liquid ethane transported in the multi-stage throttling refrigeration device V-2, so that the precooling is completed by step-by-step cooling using the cooling capacity provided by the liquid ethane; the natural gas after precooling further enters the main heat exchanger cold box CB-401, and is heat exchanged with the mixed refrigerant that returns to the main heat exchanger cold box CB-401 after throttling and reducing the pressure, so as to complete liquefaction and supercooling, and the liquefied and supercooled natural gas is output through the liquefied natural gas outlet O.

[0064] In the above natural gas liquefaction process, it is preferred to control the temperature of the pre-cooled liquid mixed refrigerant and the pre-cooled natural gas to -60 to -80°C.

[0065] The specific implementation and technical effect of the above-mentioned cascade natural gas liquefaction system and process are demonstrated below through a specific embodiment.

[0066] Example

[0067] The cascade natural gas liquefaction system of this embodiment is as follows Figure 1 As shown, the specific structures of the ethane refrigeration cycle system A and the mixed refrigerant refrigeration cycle system B are not described in detail. The following mainly describes the fluid circulation process in the two systems and the corresponding process parameters.

[0068] In this embodiment, the pressure of the purified natural gas raw gas is 55 kg / cm 2, passing through the natural gas-ethane primary precooler E-301, the natural gas-ethane secondary precooler E-302, the natural gas-ethane tertiary precooler E-303, and the natural gas-ethane quaternary precooler E-304, and being cooled to -13°C, -33°C, -54°C, and -70°C in turn, and then entering the main heat exchanger cold box CB-401 to continue cooling to -162°C, and then output from the liquefied natural gas outlet O to the cold box, and finally sent to the LNG storage tank for storage as liquefied supercooled natural gas.

[0069] The mixed refrigerant in this embodiment is composed of nitrogen, methane and ethane. The mixed refrigerant is pressurized to 55 kg / cm 2 After that, it enters the mixed refrigerant air cooler AC-101 and is cooled to about 13℃, then passes through the mixed refrigerant-ethane primary precooler E-101, the mixed refrigerant-ethane secondary precooler E-102, the mixed refrigerant-ethane tertiary precooler E-103, and the mixed refrigerant-ethane quaternary precooler E-104, and is cooled to -13℃, -33℃, -54℃, and -70℃ in turn, and then enters the mixed refrigerant gas-liquid separation tank V-101. The liquid refrigerant comes out from the bottom of the mixed refrigerant gas-liquid separation tank V-101 and enters the main heat exchanger cold box CB-401 to continue cooling, and is drawn out through After passing through the first throttle valve Val-401, it enters the first two-phase flow uniform distribution tank V-401, and then is injected into the middle of the outlet heat exchange tube P-6; the gas phase refrigerant coming out from the top of the mixed refrigerant gas-liquid separation tank V-101 enters the main heat exchanger cold box CB-401 and continues to cool. After cooling to -162℃, it is extracted and passed through the second throttle valve val-402 to enter the second two-phase flow uniform distribution tank V-402, and then is injected into the bottom of the outlet heat exchange tube P-6, and absorbs the heat of the natural gas in the heat exchange pipe P-7 in the heat exchanger and is reheated to -73℃ before exiting the cold box and returning to the mixed refrigerant compressor C-101.

[0070] After being pressurized by a multi-stage ethane compressor C-201 with a total number of 4 stages, ethane enters the ethane air cooler AC-201, is condensed into liquid and stored in the ethane buffer tank V-201, and then the first ethane gas-liquid separation tank V-202, the second ethane gas-liquid separation tank V-203, the third ethane gas-liquid separation tank V-204, the fourth ethane gas-liquid separation tank V-205, the first valve Val-201, the third valve Val-203, the sixth valve Val-206, and the ninth valve Val-209 are used as the main body to form a four-stage throttling refrigeration device. The ethane gas is throttled and depressurized step by step in the middle, and provides cooling capacity to the natural gas-ethane primary precooler E-301, the natural gas-ethane secondary precooler E-302, the natural gas-ethane tertiary precooler E-303, the natural gas-ethane quaternary precooler E-304 and the mixed refrigerant-ethane primary precooler E-101, the mixed refrigerant-ethane secondary precooler E-102, the mixed refrigerant-ethane tertiary precooler E-103, and the mixed refrigerant-ethane quaternary precooler E-104, and then vaporizes and flows back to the multi-stage ethane compressor C-201 for re-compression step by step. The specific processing process of the ethane flow path is as described above and will not be repeated here.

[0071] It should be noted that the specific form of the main heat exchanger cold box CB-401 is not limited, and the four types of heat exchange tubes, namely the first inlet heat exchange tube P-4, the second inlet heat exchange tube P-5, the outlet heat exchange tube P-6, and the heat exchange pipe P-7, need to be designed according to the type of heat exchanger. In an embodiment of the present invention, the main heat exchanger cold box CB-401 can be implemented by a plate-fin heat exchanger with a heat exchange channel inside. In this case, the heat exchange tube form can be directly the heat exchange channel inside the heat exchanger. In addition, if the main heat exchanger cold box CB-401 adopts a coil heat exchanger, the heat exchange tube form can be a built-in heat exchange tube.

[0072] Based on the cascade natural gas liquefaction system and process provided in this embodiment, a comparative analysis of energy consumption and benefits of the present invention and other prior art processes is performed below.

[0073] Energy efficiency analysis of the process of the present invention: Taking the purified natural gas flow rate of 3.3 million tons / year as an example, the shaft power of the multi-stage ethane compressor in the present invention is about 49,800KW, and the shaft power of the mixed refrigerant compressor is about 49,600KW. The shaft power of the ethane compressor and the mixed refrigerant compressor are basically the same, so it can avoid being limited by the maximum power of the gas turbine and the motor, maximize the single-line production capacity, and the present invention also avoids the use of complex control. This system and process of the present invention are suitable for high-latitude cold areas, especially the Arctic region where the ambient temperature is cold all year round, and the conditions for using this process can be achieved through air cooling. The unit liquefaction energy consumption of the process of the present invention for natural gas is as low as about 0.18KW / Nm 3 .

[0074] Energy consumption and benefit analysis of the first comparative process: If the traditional propane pre-cooling mixed refrigerant process is used, taking the purified natural gas flow of 3.3 million tons / year as an example, the shaft power of the propane compressor is 30,300KW, and the shaft power of the mixed refrigerant compressor is about 75,300KW. The power of the two compressors is quite different, especially the shaft power of the mixed refrigerant compressor is too large. The unit liquefaction energy consumption of this process is about 0.21KW / Nm 3 .

[0075] Energy efficiency analysis of the second comparative process: If the dual mixed refrigerant process is adopted, still taking the purified natural gas flow of 3.3 million tons / year as an example, in summer conditions, the mixed refrigerant power of the pre-cooling section is about 79,000KW, and the mixed refrigerant power of the deep cooling section is about 53,000KW. In winter conditions, the mixed refrigerant power of the pre-cooling section is about 35,000KW, and the mixed refrigerant power of the deep cooling section is about 53,000KW. It can be seen that in summer, the mixed refrigerant power of the pre-cooling section is larger, while in winter it is smaller. Therefore, in theory, the dual mixed refrigerant process is also a LNG process that is more suitable for areas with lower ambient temperatures. However, the problem with this process is that the working conditions of the two mixed refrigerants are complex and difficult to operate on site. The mixed refrigerant in the pre-cooling section needs to adjust the refrigerant ratio frequently according to the ambient temperature. Otherwise, it can only be adjusted by reflux through the compressor, which will cause too high energy consumption. The unit liquefaction energy consumption of this process in summer is about 0.256KW / Nm 3 In winter, the unit liquefaction energy consumption is as low as about 0.175KW / Nm 3 .

[0076] Energy consumption and benefit analysis of the third comparative process: If the conventional cascade process is used, taking the purified natural gas flow of 3.3 million tons / year as an example, the shaft power of the propane compressor is 53,000KW, the shaft power of the ethylene compressor is 41,300KW, and the shaft power of the methane compressor is 14,200KW. This process requires three compressor units, the propane compressor has a larger power, and the total power is larger, and the energy consumption is higher. The unit liquefaction energy consumption of this process is about 0.234KW / Nm 3 .

[0077] It can be seen that the comprehensive unit liquefaction energy consumption of the present invention is lower than that of the mixed refrigerant process with propane precooling and the conventional cascade process. Although it is slightly higher than the winter working condition of the double mixed refrigerant, the energy consumption of the double mixed refrigerant process is very high in summer. In summary, the comprehensive energy consumption of the process of the present invention has obvious advantages over other processes.

[0078] The above-described embodiments are only some preferred implementations of the present invention, but are not intended to limit the present invention. A person skilled in the relevant technical field may make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, any technical solution obtained by equivalent replacement or equivalent transformation falls within the protection scope of the present invention.

Claims

1. A cascade refrigeration natural gas liquefaction system, characterized in that: The natural gas liquefaction system comprises an ethane refrigeration cycle system (A) and a mixed refrigerant refrigeration cycle system (B); The ethane refrigeration cycle system (A) comprises a multi-stage ethane compressor (C-201), an ethane air cooler (AC-201), an ethane buffer tank (V-201), a multi-stage throttling refrigeration device (V-2), a multi-stage natural gas precooler (E-3), and a multi-stage mixed refrigerant precooler (E-1); the multi-stage ethane compressor (C-201), the ethane air cooler (AC-201), the ethane buffer tank (V-201) and the multi-stage throttling refrigeration device (V-2) are connected in sequence through an ethane transmission pipeline (P-1) to form an ethane circulation loop, and ethane is compressed by the multi-stage ethane compressor (C-201) and the ethane air cooler. After being cooled, (AC-201) is buffered and stored in an ethane buffer tank (V-201), and then enters a multi-stage throttling refrigeration device (V-2) to be throttled and depressurized step by step for refrigeration, and provides cooling capacity to a multi-stage natural gas precooler (E-3) and a multi-stage mixed refrigerant precooler (E-1), and then flows back to a multi-stage ethane compressor (C-201); the input end of the multi-stage natural gas precooler (E-3) is connected to a natural gas inlet (I), and the input natural gas to be liquefied is precooled step by step in the multi-stage natural gas precooler (E-3) using ethane cooling capacity, and then is transported to a mixed refrigerant refrigeration cycle system (B) through a natural gas transmission pipeline (P-2); The mixed refrigerant refrigeration cycle system (B) comprises a mixed refrigerant compressor (C-101), a mixed refrigerant air cooler (AC-101), a mixed refrigerant gas-liquid separation tank (V-101), and a main heat exchanger cold box (CB-401); the mixed refrigerant output port of the mixed refrigerant compressor (C-101) is connected to the mixed refrigerant air cooler (AC-101), the multi-stage mixed refrigerant precooler (E-1), and the mixed refrigerant gas-liquid separation tank (V-101) in sequence through a mixed refrigerant transmission pipeline (P-3); after the mixed refrigerant is compressed by the mixed refrigerant compressor (C-101) and air-cooled by the mixed refrigerant air cooler (AC-101), the mixed refrigerant is precooled step by step in the multi-stage mixed refrigerant precooler (E-1) using ethane cooling capacity and The gas enters the mixed refrigerant gas-liquid separation tank (V-101) for gas-liquid separation; the gas phase outlet and the liquid phase outlet of the mixed refrigerant gas-liquid separation tank (V-101) are respectively connected to the inlet ends of two throttling refrigeration units in the main heat exchanger cold box (CB-401), and the two throttling refrigeration units provide the main heat exchanger cold box (CB-401) with the cooling capacity required for natural gas supercooling through throttling refrigeration of the mixed refrigerant; the outlet ends of the two throttling refrigeration units are connected to the mixed refrigerant reflux inlet of the mixed refrigerant compressor (C-101); a heat exchange pipeline for supercooling natural gas is provided in the main heat exchanger cold box (CB-401), the input end of the heat exchange pipeline is connected to the natural gas transmission pipeline (P-2), and the output end is connected to the liquefied natural gas outlet (O).

2. The cascade refrigeration natural gas liquefaction system according to claim 1, characterized in that: The multi-stage ethane compressor (C-201), the multi-stage throttling refrigeration device (V-2), the multi-stage natural gas precooler (E-3) and the multi-stage mixed refrigerant precooler (E-1) have the same number of stages and the stages correspond one to one. Each stage of the throttling refrigeration device is composed of a gas-liquid separation tank and a throttling valve installed on the inlet pipe of the gas-liquid separation tank, wherein the liquid phase outlet of the gas-liquid separation tank is divided into three routes, which are respectively transported to the refrigerant inlets of the natural gas precooler and the mixed refrigerant precooler of the corresponding levels, and the refrigerant outlets of the natural gas precooler and the mixed refrigerant precooler and the gas phase outlet of the gas-liquid separation tank are connected to the inlet of the ethane compressor of the corresponding level.

3. The cascade refrigeration natural gas liquefaction system according to claim 1, characterized in that: The multi-stage ethane compressor (C-201), the multi-stage throttling refrigeration device (V-2), the multi-stage natural gas precooler (E-3) and the multi-stage mixed refrigerant precooler (E-1) include 3 to 5 stages.

4. The natural gas liquefaction system of cascade refrigeration according to claim 1, characterized in that: Each of the throttling refrigeration units comprises an inlet heat exchange tube, a throttling valve, a two-phase flow distribution tank and an outlet heat exchange tube, wherein the inlet heat exchange tube, the two-phase flow distribution tank and the outlet heat exchange tube are all located in the main heat exchanger cold box (CB-401); the inlet of the inlet heat exchange tube is used to input the mixed refrigerant output from the mixed refrigerant gas-liquid separation tank (V-101), the outlet of the inlet heat exchange tube is connected to the throttling valve and the inlet of the two-phase flow distribution tank in sequence, the gas phase outlet and the liquid phase outlet of the two-phase flow distribution tank are respectively connected to the inlet of the outlet heat exchange tube, and the outlet of the outlet heat exchange tube is connected to the mixed refrigerant reflux inlet of the mixed refrigerant compressor (C-101); the mixed refrigerant output from the mixed refrigerant gas-liquid separation tank (V-101) absorbs cold energy in the inlet heat exchange tube, is further throttled and cooled by the throttling valve, and then enters the two-phase flow distribution tank for gas-liquid distribution and is then input into the outlet heat exchange tube to provide cold energy for natural gas and the mixed refrigerant.

5. The cascade refrigeration natural gas liquefaction system according to claim 1, characterized in that: A propane precooler or a water cooler is provided between the ethane air cooler (AC-201) and the ethane buffer tank (V-201).

6. The natural gas liquefaction system of cascade refrigeration according to claim 1, characterized in that: A propane precooler or a water cooler is provided between the mixed refrigerant air cooler (AC-101) and the multi-stage mixed refrigerant precooler (E-1).

7. The natural gas liquefaction system of cascade refrigeration according to claim 1, characterized in that: A propane precooler or a water cooler is provided between the natural gas inlet (I) and the multi-stage natural gas precooler (E-3).

8. A natural gas liquefaction process using cascade refrigeration, characterized in that: The process is implemented based on the natural gas liquefaction system of the cascade refrigeration as claimed in any one of claims 1 to 7, and the natural gas liquefaction process comprises: S1. The ethane gas is compressed in multiple stages by a multi-stage ethane compressor (C-201), and then the compressed ethane is air-cooled into liquid ethane by an ethane air cooler (AC-201) and stored in an ethane buffer tank (V-201); the liquid ethane in the ethane buffer tank (V-201) is stably transported to a multi-stage throttling refrigeration device (V-2) to be throttled and depressurized step by step, and each stage of the throttling refrigeration device throttles and depressurizes the input liquid ethane and further performs gas-liquid separation, and the separated gaseous ethane is refluxed and transported to the multi-stage ethane compressor (C-201) for re-compression, and part of the separated liquid ethane is transported to the next stage of the throttling refrigeration device, and part of it is transported to a multi-stage natural gas precooler (E-3) and a multi-stage mixed refrigerant precooler (E-1) to provide cooling capacity through heat exchange, and the ethane gas after heat exchange is refluxed and transported to the multi-stage ethane compressor (C-201) for re-compression; S2. The mixed refrigerant is compressed by a mixed refrigerant compressor (C-101), and the compressed mixed refrigerant is air-cooled into a liquid mixed refrigerant by a compressor air cooler (AC-101). The liquid mixed refrigerant is then passed through a multi-stage mixed refrigerant precooler (E-1), and the liquid mixed refrigerant is heat-exchanged with liquid ethane transported from a multi-stage throttling refrigeration device (V-2), so that the precooling is completed by step-by-step cooling using the cooling capacity provided by the liquid ethane. After the precooling, the liquid mixed refrigerant enters a mixed refrigerant gas-liquid separation tank (V-101) for gas-liquid separation. The gaseous mixed refrigerant and liquid mixed refrigerant obtained by the gas-liquid separation enter the main heat exchanger cold box (CB-401) respectively to further absorb cooling capacity and cool down. After throttling and reducing pressure through a throttling refrigeration unit, they are returned to the main heat exchanger cold box (CB-401) to provide cooling capacity and heat up the natural gas and the mixed refrigerant. The heated mixed refrigerant flows back to the mixed refrigerant compressor (C-101) for re-compression. S3. The purified natural gas to be liquefied is input into the multi-stage natural gas precooler (E-3) through the natural gas inlet (I), and heat exchanged with the liquid ethane transported in the multi-stage throttling refrigeration device (V-2), so as to complete precooling by using the cooling capacity provided by the liquid ethane to reduce the temperature step by step; the natural gas after precooling further enters the main heat exchanger cold box (CB-401), and heat exchanged with the mixed refrigerant returned to the main heat exchanger cold box (CB-401) after throttling and reducing the pressure, so as to complete liquefaction and subcooling, and the liquefied and subcooled natural gas is output through the liquefied natural gas outlet (O).

9. The natural gas liquefaction process using cascade refrigeration as claimed in claim 8, characterized in that: The temperature of the pre-cooled liquid mixed refrigerant and the pre-cooled natural gas is -60 to -80°C.

10. The natural gas liquefaction process using cascade refrigeration as claimed in claim 8, characterized in that: The mixed refrigerant consists of nitrogen, methane and ethane.

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