A system and process for preparing LNG by throttling high-pressure natural gas at wellhead

Abstract

The present invention provides a system and process for preparing LNG by throttling high-pressure natural gas at a wellhead, the process comprising desanding the high-pressure natural gas from the wellhead, filtering and separating the desanded natural gas to remove free water, some heavy hydrocarbons and impurities carried in the natural gas; drying the filtered and separated natural gas for the first time; throttling and reducing the pressure and temperature of the natural gas after the first drying; precooling the high-pressure saturated mixed refrigerant with the throttled low-temperature natural gas to obtain the reheated natural gas; drying the reheated natural gas for the second time after deacidification treatment; the natural gas after the second drying is precooled by the mixed refrigerant in a liquefied cold box and then heavy hydrocarbons are separated, and the natural gas after the heavy hydrocarbons are separated is cooled in the liquefied cold box and then discharged from the liquefied cold box to obtain LNG. The present invention can realize the recycling of natural gas from remote natural gas wells and fully release the production capacity of remote natural gas wells, thereby improving economic and social benefits.

Description

Technical Field

[0001] The invention relates to a system and a process for preparing LNG by throttling high-pressure natural gas at a wellhead, belonging to the technical field of oil and gas field exploitation. Background Art

[0002] With the continuous development of domestic gas fields, the number of scattered wells and remote wells is increasing. The output of some wells does not meet the requirements for laying pipelines, and some wells are in remote locations and cannot be incorporated into the gathering and transportation pipeline network. As a result, such gas wells are shut down all year round, their production capacity cannot be released, resources cannot be reasonably utilized, and the initial drilling investment cannot be recovered in time, resulting in a huge waste of energy.

[0003] Small LNG unit is a relatively mature gas recovery unit for remote gas wells in China. This unit can effectively recover natural gas from remote wells by taking advantage of the small size and high storage and transportation efficiency of LNG. At the same time, the skid-mounted design can facilitate transportation, quick installation, and flexible relocation, while improving the utilization performance of the unit.

[0004] Since the wellhead natural gas pressure from the formation is generally high, especially for the large gas fields in the northwest, the wellhead gas pressure is mostly above 15MPa. The current conventional practice is to throttle and reduce the pressure of the high-pressure natural gas to about 5MPa before it enters the LNG production device, resulting in a huge waste of pressure energy.

[0005] Therefore, providing a new system and process for preparing LNG by throttling high-pressure natural gas at wellhead has become a technical problem that urgently needs to be solved in this field. Summary of the invention

[0006] In order to solve the above-mentioned shortcomings and deficiencies, an object of the present invention is to provide a system for preparing LNG by throttling wellhead high-pressure natural gas.

[0007] Another object of the present invention is to provide a process for preparing LNG by throttling wellhead high-pressure natural gas.

[0008] In order to achieve the above objectives, on the one hand, the present invention provides a system for preparing LNG by throttling high-pressure natural gas at a wellhead, wherein the system for preparing LNG by throttling high-pressure natural gas at a wellhead comprises:

[0009] Desander, filter separator, primary molecular sieve dehydration skid, deacidification skid, secondary molecular sieve dehydration skid, precooling heat exchanger, liquefaction cold box and heavy hydrocarbon separator;

[0010] The high-pressure natural gas outlet of the wellhead gas tree is connected to the gas inlet of the desander through a pipeline, the gas outlet of the desander is connected to the gas inlet of the primary molecular sieve dehydration skid through a pipeline via a filter separator, the gas outlet of the primary molecular sieve dehydration skid is connected to the gas inlet of the precooling heat exchanger through a pipeline via a manual throttling angle valve, so as to precool the high-pressure saturated mixed refrigerant in the precooling heat exchanger to obtain reheated natural gas, and the precooled high-pressure saturated mixed refrigerant outlet of the precooling heat exchanger is connected to the refrigerant channel inlet of the liquefied cold box through a pipeline;

[0011] The reheated natural gas outlet of the precooling heat exchanger is connected to the natural gas precooling channel inlet of the liquefied cold box through a pipeline via a deacidification skid and a secondary molecular sieve dehydration skid in sequence; the natural gas precooling channel outlet of the liquefied cold box is connected to the gas inlet of the heavy hydrocarbon separator through a pipeline; the gas outlet of the heavy hydrocarbon separator is connected to the natural gas deep cold channel inlet of the liquefied cold box through a pipeline; and the natural gas deep cold channel outlet is connected to an LNG storage tank or a tank truck through a pipeline.

[0012] Preferably, the system further comprises a throttling pressure regulating valve, and the gas outlet of the primary molecular sieve dehydration skid is connected to the gas inlet of the precooling heat exchanger via a pipeline in sequence via a manual throttling angle valve and a throttling pressure regulating valve;

[0013] More preferably, the system further comprises a pressure transmitter, and the gas outlet of the primary molecular sieve dehydration skid is connected to the gas inlet of the precooling heat exchanger via a pipeline in sequence via a manual throttling angle valve, a throttling pressure regulating valve and a pressure transmitter.

[0014] The pressure transmitter is used to detect the pressure behind the throttling pressure regulating valve to determine the opening of the throttling pressure regulating valve, thereby ensuring that the pressure behind the throttling pressure regulating valve is within a relatively stable range.

[0015] Preferably, the system further comprises a demercuration and debenzenation skid, and the gas outlet of the secondary molecular sieve dehydration skid is connected to the gas inlet of the liquefaction cold box via a pipeline via the demercuration and debenzenation skid.

[0016] In the system described above, preferably, the heavy hydrocarbon outlet of the heavy hydrocarbon separator is connected to the heavy hydrocarbon channel inlet of the liquefied cold box through a pipeline via a heavy hydrocarbon JT valve to provide cooling for the liquefied cold box, and the heavy hydrocarbon channel outlet is connected to the fuel gas system through a pipeline to use the heavy hydrocarbons reheated to room temperature as fuel for the fuel gas system.

[0017] In the above-mentioned system, preferably, the natural gas cryogenic channel outlet of the liquefied cold box is connected to the LNG storage tank or tank truck through a pipeline via a liquefied natural gas JT valve.

[0018] Preferably, the system further comprises several stages of mixed refrigerant compressors and several stages of mixed refrigerant coolers; the mixed refrigerant is alternately compressed and cooled by the several stages of mixed refrigerant compressors and several stages of mixed refrigerant coolers, and then enters the precooling heat exchanger, and exchanges heat with the throttled natural gas in the precooling heat exchanger.

[0019] Preferably, the system also includes a low-temperature mixed refrigerant separator, and the refrigerant channel outlet of the liquefied cold box is connected to the inlet of the low-temperature mixed refrigerant separator through a pipeline via a mixed refrigerant JT valve to separate the throttled two-phase mixed refrigerant into two phases. The two phases are fully mixed at the inlet of the reflux channel of the liquefied cold box and then enter the liquefied cold box to provide cooling capacity for the liquefied cold box; the two-phase mixture is reheated into a gas phase and then exits the reflux channel of the liquefied cold box to enter several stages of mixed refrigerant compressors and several stages of mixed refrigerant coolers to be alternately compressed and cooled again before being reused.

[0020] In the above-mentioned system, the pore size of the filter element used in the filter separator is larger than 40 μm, so as to filter solid particles and macromolecules with a size larger than 40 μm.

[0021] In the above-mentioned system, the precooling heat exchanger and the liquefaction cold box are both aluminum plate-fin heat exchangers.

[0022] In the above-described system, the mixed refrigerant compressor is a screw compressor.

[0023] Due to the fluctuation of natural gas pressure at the wellhead, the throttling pressure regulating valve used in the above system should have good adaptability to the fluctuation of valve inlet pressure within 3MPa.

[0024] The equipment used in the above-mentioned system, such as desander, filter separator, primary molecular sieve dehydration skid, deacidification skid, secondary molecular sieve dehydration skid, precooling heat exchanger, liquefaction cold box, low-temperature mixed refrigerant separator and heavy hydrocarbon separator, etc. are all conventional equipment and can be purchased commercially.

[0025] The systems for preparing LNG by throttling high-pressure natural gas at the wellhead provided by the present invention are all designed and constructed in a skid-mounted form, which can save investment, significantly reduce the specifications and dimensions of major equipment and thus reduce the floor space, shorten the construction period, facilitate relocation and secondary utilization at any time, and contribute to the modularization, skid-mounting and integration of marginal well natural gas liquefaction equipment.

[0026] On the other hand, the present invention also provides a process for preparing LNG by throttling wellhead high-pressure natural gas, wherein the process for preparing LNG by throttling wellhead high-pressure natural gas comprises:

[0027] (1) Desanding the high-pressure natural gas from the wellhead, and then filtering and separating the desanded natural gas to remove free water, some heavy hydrocarbons and impurities carried in the natural gas;

[0028] (2) Drying the filtered and separated natural gas for the first time;

[0029] (3) throttling, reducing pressure and cooling the natural gas after the first drying;

[0030] (4) using the throttled low-temperature natural gas to precool the high-pressure saturated mixed refrigerant and obtain reheated natural gas;

[0031] (5) drying the reheated natural gas for a second time after deacidification;

[0032] (6) The natural gas after the second drying is pre-cooled by the mixed refrigerant in the liquefied cold box and then the heavy hydrocarbons are separated. The natural gas after the heavy hydrocarbons are separated is cooled in the liquefied cold box and then leaves the liquefied cold box to obtain LNG.

[0033] In the above-mentioned process, since the wellhead pressure of natural gas from the formation is generally more than 15MPa, and it will contain water and sand to varying degrees, the natural gas out of the wellhead must be pretreated before liquefaction. The process includes: sand removal, filtration and separation of free water and impurities, primary drying, throttling, pressure regulation, deacidification, and secondary drying. At this point, the contents of H2S, CO2, water and impurities in the purified natural gas are all less than the indicators required for natural gas liquefaction, and enter the natural gas mixed refrigeration liquefaction device to liquefy into product LNG.

[0034] In the above process, preferably, in step (2), the natural gas after filtration and separation is dried for the first time, so that the dew point of the natural gas after the first drying at 5 MPa is lower than -50°C.

[0035] In the above process, preferably, in step (3), throttling, reducing pressure and cooling the natural gas after the first drying are performed, including:

[0036] The natural gas after the first drying is initially throttled to 5-7MPa, and then further throttled and pressure adjusted to 4.5-5.5MPa. At this time, the natural gas temperature drops to below -36.3℃.

[0037] In the above process, preferably, in step (4), the temperature of the reheated natural gas is 30-35°C.

[0038] In the process described above, preferably, in step (5), the CO2 content in the natural gas after deacidification treatment is less than 50 ppm, and the H2S content is less than 4 ppm.

[0039] In the above process, preferably, in step (5), the water dew point of the natural gas after the second drying is lower than -70°C.

[0040] In the above process, preferably, in step (6), the temperature of the pre-cooled natural gas is -65°C to -68°C.

[0041] In the above process, preferably, in step (6), the temperature of the natural gas after cooling is below -158°C.

[0042] In the above process, preferably, in step (6), the LNG is throttled to 0.05-0.2 MPa by a liquefied natural gas JT valve before entering the LNG storage tank or tank truck.

[0043] In the process described above, the deacidification treatment and drying treatment can be achieved by conventional deacidification treatment and drying processes in the art. For example, in a specific embodiment of the present invention, an amine liquid method is used to remove acidic gases in a deacidification skid, and a molecular sieve adsorption method is used to adsorb saturated water in a primary molecular sieve dehydration skid and a secondary molecular sieve dehydration skid to achieve drying treatment.

[0044] Preferably, the process also includes cooling and throttling the high-pressure saturated mixed refrigerant after pre-cooling and then separating the two phases. The two phases obtained after separation are fully mixed at the inlet of the reflux channel of the liquefied cold box and then enter the liquefied cold box to provide cooling capacity for the liquefied cold box; the two-phase mixture is reheated into a gas phase and then exits the reflux channel of the liquefied cold box to enter several stages of mixed refrigerant compressors and several stages of mixed refrigerant coolers to be alternately compressed and cooled again before being reused.

[0045] In the above-mentioned process, preferably, the temperature of the high-pressure saturated mixed refrigerant after cooling is below -161°C.

[0046] Preferably, the process further comprises, before step (6), removing mercury and benzene from the natural gas after the second drying.

[0047] Preferably, the process further comprises recycling the heavy hydrocarbons obtained from the separation of heavy hydrocarbons in step (6) to the liquefaction cold box to provide cooling capacity for the liquefaction cold box; and then using the heavy hydrocarbons reheated to room temperature as fuel for the fuel gas system.

[0048] The system and process for preparing LNG by throttling high-pressure natural gas at the wellhead provided by the present invention can throttle the high-pressure natural gas at the wellhead that is remote and cannot be connected to the gathering and transportation system on the spot and utilize its cold capacity, then reheat it for deacidification and secondary drying, and then liquefy it into LNG that can be transported by road. The system and process provided by the present invention fully utilize the pressure energy of the formation by utilizing the cold capacity after throttling the high-pressure natural gas at the wellhead. Compared with conventional processes, it can reduce the energy consumption of the mixed refrigerant compressor of the liquefaction device by more than 19%, reduce equipment investment, realize the natural gas recovery and utilization of remote natural gas wells and fully release the production capacity of remote natural gas wells, improve economic and social benefits, and on the other hand, improve the safety performance of the system.

[0049] The system and process provided by the present invention can realize the direct liquefaction of natural gas at the wellhead on site, eliminating the upstream investment in gas field gathering and purification treatment, thereby significantly reducing investment.

[0050] The system and process provided by the present invention are not limited by the number of wellheads in the well field, and are applicable to both single-well well fields and multi-well well fields. The total processing scale of the system and process can reach 5000Nm 3 / d-30×10 4 Nm 3 / d. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0052] Figure 1 This is a schematic diagram of the system structure for preparing LNG by throttling wellhead high-pressure natural gas provided in Example 1 of the present invention.

[0053] Description of main figures:

[0054] 1. Wellhead gas tree;

[0055] S-01, desander;

[0056] V-01, filter separator;

[0057] J-01, manual throttle angle valve;

[0058] PV-01, throttling pressure regulating valve;

[0059] PI-01, pressure transmitter;

[0060] B-01, primary molecular sieve dehydration skid;

[0061] B-02, deacidification sled;

[0062] B-03, secondary molecular sieve dehydration skid;

[0063] B-04, mercury and benzene removal skid;

[0064] C-01, one-stage mixed refrigerant compressor;

[0065] C-02, two-stage mixed refrigerant compressor;

[0066] E-01, primary cooler;

[0067] E-02, secondary cooler;

[0068] E-03, precooling heat exchanger;

[0069] E-04, liquefied cold box;

[0070] JT-01, heavy hydrocarbon JT valve;

[0071] JT-02, mixed refrigerant JT valve;

[0072] JT-03, liquefied natural gas JT valve;

[0073] V-02, heavy hydrocarbon separator;

[0074] V-03, low temperature mixed refrigerant separator. DETAILED DESCRIPTION

[0075] In order to have a clearer understanding of the technical features, purposes and beneficial effects of the present invention, the technical scheme of the present invention is now described in detail in conjunction with the following specific embodiments, but it should not be understood as limiting the applicable scope of the present invention.

[0076] Example 1

[0077] This embodiment provides a system for preparing LNG by throttling high-pressure natural gas at a wellhead, and its structural schematic diagram is shown in FIG. Figure 1 As shown, from Figure 1 As can be seen, the system comprises:

[0078] Desander S-01, filter separator V-01, primary molecular sieve dehydration skid B-01, deacidification skid B-02, secondary molecular sieve dehydration skid B-03, demercuration and debenzene skid B-04, precooling heat exchanger E-03, liquefaction cold box E-04 and heavy hydrocarbon separator V-02;

[0079] The high-pressure natural gas outlet of the wellhead gas production tree 1 is connected to the gas inlet of the desander S-01 through a pipeline, and the gas outlet of the desander S-01 is connected to the gas inlet of the primary molecular sieve dehydration skid B-01 through a pipeline via the filter separator V-01, and the gas outlet of the primary molecular sieve dehydration skid B-01 is connected to the gas inlet of the precooling heat exchanger E-03 through a pipeline in sequence via a manual throttling angle valve J-01, a throttling pressure regulating valve PV-01, and a pressure transmitter PI-01, so as to precool the high-pressure saturated mixed refrigerant in the precooling heat exchanger E-03 to obtain the reheated natural gas, and the precooled high-pressure saturated mixed refrigerant outlet of the precooling heat exchanger E-03 is connected to the refrigerant channel inlet of the liquefied cold box E-04 through a pipeline;

[0080] The pressure transmitter PI-01 is used to detect the pressure after the throttling pressure regulating valve PV-01 to determine the opening of the throttling pressure regulating valve PV-01, thereby ensuring that the pressure after the throttling pressure regulating valve PV-01 is within a relatively stable range;

[0081] The reheated natural gas outlet of the precooling heat exchanger E-03 is connected to the natural gas precooling channel inlet of the liquefied cold box E-04 through a pipeline in sequence via the deacidification skid B-02, the secondary molecular sieve dehydration skid B-03, and the demercuration and debenzenization skid B-04. The natural gas precooling channel outlet of the liquefied cold box E-04 is connected to the gas inlet of the heavy hydrocarbon separator V-02 through a pipeline. The gas outlet of the heavy hydrocarbon separator V-02 is connected to the natural gas deep cold channel inlet of the liquefied cold box E-04 through a pipeline. The natural gas deep cold channel outlet is connected to the LNG storage tank or tank truck through a pipeline via the liquefied natural gas JT valve JT-03; the heavy hydrocarbon outlet of the heavy hydrocarbon separator V-02 is connected to the heavy hydrocarbon channel inlet of the liquefied cold box E-04 through a pipeline via the heavy hydrocarbon JT valve JT-01 to provide cooling for the liquefied cold box, and the heavy hydrocarbon channel outlet is connected to the fuel gas system through a pipeline so that the heavy hydrocarbons reheated to room temperature are used as fuel for the fuel gas system;

[0082] In this embodiment, the system further includes a primary mixed refrigerant compressor C-01, a primary cooler E-01, a secondary mixed refrigerant compressor C-02, and a secondary cooler E-02. The mixed refrigerant is alternately compressed and cooled by the primary mixed refrigerant compressor C-01, the primary cooler E-01, the secondary mixed refrigerant compressor C-02, and the secondary cooler E-02 to obtain a high-pressure saturated mixed refrigerant. The high-pressure saturated mixed refrigerant enters the precooling heat exchanger E-03 and exchanges heat with the throttled natural gas in the precooling heat exchanger E-03.

[0083] In this embodiment, the system also includes a low-temperature mixed refrigerant separator V-03, and the refrigerant channel outlet of the liquefied cold box E-04 is connected to the inlet of the low-temperature mixed refrigerant separator V-03 through a pipeline via the mixed refrigerant JT valve JT-02 to separate the throttled two-phase mixed refrigerant into two phases. The two phases are fully mixed at the reflux channel inlet of the liquefied cold box E-04 and then enter the liquefied cold box E-04 to provide cooling capacity for the liquefied cold box E-04; the two-phase mixture is reheated into a gas phase and then exits the reflux channel of the liquefied cold box E-04, and is alternately compressed and cooled by the first-stage mixed refrigerant compressor C-01, the first-stage cooler E-01, the second-stage mixed refrigerant compressor C-02, and the second-stage cooler E-02, and then reused to the pre-cooling heat exchanger E-03.

[0084] Example 2

[0085] This embodiment provides a process for preparing LNG by throttling high-pressure natural gas at a wellhead, which is implemented by using the system for preparing LNG by throttling high-pressure natural gas at a wellhead provided in the above embodiment 1. The process includes the following specific steps:

[0086] The high-pressure natural gas from the wellhead passes through the desander S-01 to remove impurities such as fine sand in the formation that comes out of the wellhead with the natural gas;

[0087] The desanded natural gas enters the filter separator V-01, where the free water, some heavy hydrocarbons and impurities carried in the natural gas are filtered and separated;

[0088] The filtered and separated natural gas enters the primary molecular sieve dehydration skid B-01 for the first drying, so that the dew point of the natural gas after the first drying is lower than -50℃ at 5MPa;

[0089] After that, the natural gas after the first drying is initially throttled to about 5-7MPa through the manual throttle valve J-01, and then further throttled and pressure-regulated to 4.5MPa through the throttling pressure regulating valve PV-01. At this time, the natural gas temperature drops to below -36.3℃;

[0090] The low-temperature natural gas after throttling and pressure regulation enters the precooling heat exchanger E-03, which precools the high-pressure saturated mixed refrigerant after two-stage compression by the primary mixed refrigerant compressor C-01 and the secondary mixed refrigerant compressor C-02 and two-stage cooling by the primary cooler E-01 and the secondary cooler E-02; the low-temperature natural gas after throttling and pressure regulation is reheated to 30°C in the precooling heat exchanger E-03, and the reheated natural gas enters the deacidification skid B-02 to remove CO2 and H2S in the raw natural gas, so that the CO2 content is less than 50ppm and the H2S content is less than 4ppm; the deacidified natural gas enters the secondary molecular sieve dehydration skid B-03 for secondary drying of the natural gas, so that the water dew point of the dried natural gas is less than -70°C;

[0091] After the dried and qualified natural gas meets the index of entering the natural gas liquefaction system, it enters the natural gas pre-cooling channel of the liquefied cold box E-04, where the natural gas is pre-cooled to about -65℃ and then enters the heavy hydrocarbon separator V-02 to separate heavy hydrocarbons. The gas phase exiting the heavy hydrocarbon separator enters the natural gas deep cooling channel and is cooled to below -158℃ before exiting the cold box. The LNG throttled to 0.05MPa(G) by the liquefied natural gas JT valve JT-03 enters the LNG storage tank or tank truck;

[0092] The heavy hydrocarbons at the bottom outlet of the heavy hydrocarbon separator V-02 are cooled again after being throttled by the heavy hydrocarbon JT valve JT-01, and then enter the heavy hydrocarbon channel of the liquefied cold box E-04 to provide cooling capacity for the liquefied cold box. The heavy hydrocarbons themselves are reheated to room temperature and then enter the fuel gas system to be used as fuel;

[0093] The high-pressure mixed refrigerant precooled in the precooling heat exchanger E-03 enters the refrigerant channel of the liquefied cold box E-04, and the throttled reflux refrigerant is further cooled to a supercooled state, such as below -161°C. After throttling and reducing the pressure, the two-phase mixed refrigerant is throttled by the mixed refrigerant JT valve JT-02. The throttled two-phase mixed refrigerant is separated into two phases by the low-temperature mixed refrigerant separator V-03. The two phases are fully mixed at the inlet of the reflux channel of the liquefied cold box E-04 and then enter the reflux channel of the liquefied cold box E-04, further providing cooling capacity for the cold box to cool the natural gas to below -158°C. At the same time, the two-phase mixture is reheated to the gas phase, and then the gas phase exits the reflux channel of the liquefied cold box E-04, and is alternately compressed and cooled by the first-stage mixed refrigerant compressor C-01, the first-stage cooler E-01, the second-stage mixed refrigerant compressor C-02, and the second-stage cooler E-02, and then recycled to the precooling heat exchanger E-03.

[0094] The above is only a specific embodiment of the present invention, and cannot be used to limit the scope of the invention. Therefore, the replacement of equivalent components, or equivalent changes and modifications made according to the protection scope of the patent of the present invention, should still fall within the scope of this patent. In addition, the technical features of the present invention can be freely combined with each other, with each other and with each other, and with each other.

Claims

1. A system for preparing LNG by throttling high-pressure natural gas at a wellhead, characterized in that: The system for preparing LNG by throttling high-pressure natural gas at the wellhead comprises: Desander, filter separator, primary molecular sieve dehydration skid, deacidification skid, secondary molecular sieve dehydration skid, precooling heat exchanger, liquefaction cold box and heavy hydrocarbon separator; The high-pressure natural gas outlet of the wellhead gas tree is connected to the gas inlet of the desander through a pipeline, the gas outlet of the desander is connected to the gas inlet of the primary molecular sieve dehydration skid through a pipeline via a filter separator, the gas outlet of the primary molecular sieve dehydration skid is connected to the gas inlet of the precooling heat exchanger through a pipeline via a manual throttling angle valve, so as to precool the high-pressure saturated mixed refrigerant in the precooling heat exchanger to obtain reheated natural gas, and the precooled high-pressure saturated mixed refrigerant outlet of the precooling heat exchanger is connected to the refrigerant channel inlet of the liquefied cold box through a pipeline; The reheated natural gas outlet of the precooling heat exchanger is connected to the natural gas precooling channel inlet of the liquefied cold box through a pipeline in sequence via a deacidification skid and a secondary molecular sieve dehydration skid. The natural gas precooling channel outlet of the liquefied cold box is connected to the gas inlet of the heavy hydrocarbon separator through a pipeline. The natural gas deep cold channel outlet of the liquefied cold box is connected to an LNG storage tank or a tank truck through a pipeline via a liquefied natural gas JT valve. The gas outlet of the heavy hydrocarbon separator is connected to the natural gas deep cold channel inlet of the liquefied cold box through a pipeline. The natural gas deep cold channel outlet is connected to an LNG storage tank or a tank truck through a pipeline. The heavy hydrocarbon outlet of the heavy hydrocarbon separator is connected to the heavy hydrocarbon channel inlet of the liquefied cold box through a pipeline via a heavy hydrocarbon JT valve to provide cooling for the liquefied cold box. The heavy hydrocarbon channel outlet is connected to a fuel gas system through a pipeline so that the heavy hydrocarbons reheated to room temperature can be used as fuel for the fuel gas system.

2. The system according to claim 1, characterized in that The system also includes a demercuration and debenzenation skid, and the gas outlet of the secondary molecular sieve dehydration skid is connected to the gas inlet of the liquefaction cold box through a pipeline via the demercuration and debenzenation skid.

3. The system according to claim 1, characterized in that The system also includes a throttling pressure regulating valve, and the gas outlet of the primary molecular sieve dehydration skid is connected to the gas inlet of the precooling heat exchanger through a pipeline via a manual throttling angle valve and a throttling pressure regulating valve in sequence.

4. The system according to claim 3, characterized in that The system also includes a pressure transmitter, and the gas outlet of the primary molecular sieve dehydration skid is connected to the gas inlet of the precooling heat exchanger through a pipeline via a manual throttling angle valve, a throttling pressure regulating valve, and a pressure transmitter in sequence.

5. The system according to claim 1, characterized in that The system also includes several stages of mixed refrigerant compressors and several stages of mixed refrigerant coolers; the mixed refrigerant enters the precooling heat exchanger after being alternately compressed and cooled by the several stages of mixed refrigerant compressors and several stages of mixed refrigerant coolers, and exchanges heat with the throttled natural gas in the precooling heat exchanger.

6. The system according to any one of claims 1 to 5, characterized in that: The system also includes a low-temperature mixed refrigerant separator. The refrigerant channel outlet of the liquefied cold box is connected to the inlet of the low-temperature mixed refrigerant separator through a pipeline via a mixed refrigerant JT valve to separate the throttled two-phase mixed refrigerant into two phases. The two phases are fully mixed at the inlet of the reflux channel of the liquefied cold box and then enter the liquefied cold box to provide cooling capacity for the liquefied cold box; the two-phase mixture is reheated into a gas phase and then exits the reflux channel of the liquefied cold box to enter several stages of mixed refrigerant compressors and several stages of mixed refrigerant coolers to be alternately compressed and cooled again for reuse.

7. A process for preparing LNG by throttling high-pressure natural gas at a wellhead, characterized in that: The process for preparing LNG by throttling wellhead high-pressure natural gas comprises: (1) Desanding the high-pressure natural gas from the wellhead, and then filtering and separating the desanded natural gas to remove free water, some heavy hydrocarbons and impurities carried in the natural gas; (2) Drying the filtered and separated natural gas for the first time; (3) throttling, reducing pressure and cooling the natural gas after the first drying; (4) using the throttled low-temperature natural gas to precool the high-pressure saturated mixed refrigerant and obtain reheated natural gas; (5) drying the reheated natural gas for a second time after deacidification; (6) The natural gas after the second drying is pre-cooled by the mixed refrigerant in the liquefied cold box and then the heavy hydrocarbons are separated. The natural gas after the heavy hydrocarbons are separated is cooled in the liquefied cold box and then leaves the liquefied cold box to obtain LNG.

8. The process according to claim 7, characterized in that In step (2), the filtered and separated natural gas is dried for the first time, so that the dew point of the natural gas after the first drying is lower than -50°C at 5 MPa.

9. The process according to claim 7, characterized in that In step (3), the natural gas after the first drying is throttled, depressurized and cooled, including: The natural gas after the first drying is initially throttled to 5-7MPa, and then further throttled and pressure adjusted to 4.5-5.5MPa. At this time, the natural gas temperature drops to below -36.3℃.

10. The process according to claim 7, characterized in that In step (4), the temperature of the reheated natural gas is 30-35°C.

11. The process according to claim 7, characterized in that In step (5), the CO2 content in the natural gas after the deacidification treatment is less than 50 ppm, and the H2S content is less than 4 ppm.

12. The process according to claim 7 or 11, characterized in that In step (5), the water dew point of the natural gas after the second drying is lower than -70°C.

13. The process according to claim 7, characterized in that In step (6), the temperature of the pre-cooled natural gas is -65°C to -68°C.

14. The process according to claim 7 or 13, characterized in that In step (6), the temperature of the natural gas after cooling is below -158°C.

15. The process according to claim 7 or 13, characterized in that In step (6), the LNG is throttled to 0.05-0.2 MPa by the liquefied natural gas JT valve and then enters the LNG storage tank or tank truck.

16. The process according to claim 7 or 10, characterized in that The process also includes cooling and throttling the high-pressure saturated mixed refrigerant after pre-cooling and then separating the two phases. The two phases obtained after separation are fully mixed at the inlet of the reflux channel of the liquefied cold box and then enter the liquefied cold box to provide cooling capacity for the liquefied cold box; the two-phase mixture is reheated into a gas phase and then exits the reflux channel of the liquefied cold box to enter several stages of mixed refrigerant compressors and several stages of mixed refrigerant coolers to be alternately compressed and cooled again for reuse.

17. The process according to claim 16, characterized in that The temperature of the high-pressure saturated mixed refrigerant after cooling is below -161°C.

18. The process according to claim 7, characterized in that The process further comprises, before step (6), removing mercury and benzene from the natural gas after the second drying.

19. The process according to claim 7, characterized in that The process also includes recycling the heavy hydrocarbons obtained by separating the heavy hydrocarbons in step (6) to the liquefaction cold box to provide cooling capacity for the liquefaction cold box; and then using the heavy hydrocarbons that have been reheated to room temperature as fuel for the fuel gas system.

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