Small skid-mounted liquefied natural gas device integrated with methane refrigeration system and operation method thereof

Abstract

This invention discloses a small skid-mounted liquefied natural gas (LNG) unit and its operation method integrating a methane refrigeration system, belonging to the field of natural gas liquefaction technology. The unit includes a methane-rich gas liquefaction system, an expansion refrigeration system, and a methane open-loop refrigeration cycle system. Addressing the shortcomings of existing small skid-mounted LNG technologies, such as large footprint, high investment, and complex operation, this invention innovatively adopts an integrated architecture of "single-stage expansion + methane open-loop refrigeration cycle." The expansion refrigeration system employs a single-stage process of pre-cooling followed by expansion; the methane open-loop cycle uses methane from the feed gas as the refrigerant, eliminating the need for external refrigerant. Through designs such as expansion refrigeration refrigeration backflow branch and methane refrigeration branch, precise cascade utilization of cooling capacity is achieved. This method has few operating variables and is simple to operate. Overall, this invention achieves a highly compact system, low investment and maintenance costs, high energy utilization efficiency, and strong reliability, perfectly meeting the core requirements of small skid-mounted scenarios.

Description

Technical Field

[0001] This invention relates to the field of natural gas liquefaction technology, and in particular to a small skid-mounted liquefied natural gas unit with an integrated methane refrigeration system and its operation method. Background Technology

[0002] With the rapid improvement of distributed energy networks, the increasing demand for development in remote oil and gas fields, and the normalization of emergency gas supply scenarios, small skid-mounted liquefied natural gas (LNG) systems are becoming a hot topic in industry research and application due to their advantages of high mobility, short deployment cycle, and wide adaptability. Currently, the mainstream technologies in the LNG liquefaction field include MR mixed refrigerant process, nitrogen expansion process, and traditional two-stage methane expansion process, but these technologies all have significant shortcomings when adapted to small skid-mounted scenarios. MR mixed refrigerant process: As the mainstream technology for large-scale LNG plants, it achieves high-efficiency cooling through phase change cascade heat exchange of multi-component refrigerants, with outstanding energy efficiency advantages. However, this process relies on precise refrigerant ratio control, involving more than ten operational variables such as pressure, temperature, and composition, requiring extremely high precision from the control system. In small skid-mounted scenarios, limited equipment integration leads to a sharp increase in maintenance difficulty. Furthermore, under the core requirements of "low cost and ease of operation," component reconfiguration significantly increases operational complexity, making it difficult to adapt to the needs of this scenario.

[0003] Nitrogen expansion process: Using nitrogen as the single refrigerant, the process is relatively simple and the refrigerant is readily available. However, nitrogen has low refrigeration efficiency, and to meet the needs of cryogenics, an additional nitrogen chiller and multi-stage expander unit must be configured, resulting in higher equipment investment than traditional processes and insufficient economic efficiency in small skid-mounted scenarios; moreover, the cyclic compression of cryogenic nitrogen further increases the equipment maintenance load, and nitrogen needs to be replenished additionally, further increasing maintenance costs.

[0004] Traditional two-stage methane expansion process: Some projects adopt a two-stage configuration of "room temperature expansion + low temperature expansion" in an attempt to improve the efficiency of cold energy utilization through staged expansion. However, this process requires two or more expanders, which occupy a large area and have a high initial investment. In addition, "room temperature expansion" increases the cost of gas compression, resulting in low system energy utilization efficiency and high unit energy consumption. Furthermore, the large number of expanders also increases maintenance costs.

[0005] In summary, existing mainstream LNG liquefaction technologies are ill-suited to the core requirements of small-scale skid-mounted LNG liquefaction scenarios—namely, their complexity in operation and high maintenance costs, large equipment investment and low energy efficiency, and reliance on additional refrigerants. The industry urgently needs a small-scale skid-mounted LNG liquefaction technology that requires fewer operating parameters, no additional refrigerant replenishment, small footprint, low investment, and high energy efficiency to address the compatibility shortcomings of existing technologies. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a system and method that are adapted to the compactness of small skids, reduce maintenance costs and operational complexity, improve system energy utilization efficiency, and optimize expander flow rate adaptation and processing performance.

[0007] This invention is achieved using the following technical solution: a small skid-mounted liquefied natural gas (LNG) unit integrating a methane refrigeration system, comprising a methane-rich gas liquefaction system, an expansion refrigeration system, and a methane open-loop refrigeration cycle system; the methane-rich gas liquefaction system includes a pre-cooling heat exchanger and a main heat exchanger, as well as a primary gas-liquid separator and a secondary gas-liquid separator connected in sequence; the expansion refrigeration system includes a methane closed-loop compressor, a methane closed-loop compressor cooler, a turbine booster expander, a turbine booster expander cooler connected in sequence, and a system connected to the turbine booster expander... An expansion refrigeration forward flow pipeline connects the expander to the main heat exchanger; the methane open-loop refrigeration cycle system includes a methane open-loop first-stage compressor, a methane open-loop first-stage compressor cooler, a methane open-loop second-stage compressor, and a methane open-loop second-stage compressor cooler connected in sequence; external methane-rich gas is connected to the pre-cooling heat exchanger via a pipeline, and after exchanging heat with the return refrigerant from the expansion refrigeration system, it merges with the methane refrigerant from the methane open-loop refrigeration cycle system after exchanging heat with the pre-cooling heat exchanger, and then enters the first-stage gas-liquid separator. The gas phase outlet of the first-stage gas-liquid separator is connected to the main heat exchanger via a pipeline, and after reheating, it is connected to the inlet of the methane open-type two-stage compressor. The liquid phase outlet of the first-stage gas-liquid separator is divided into two paths: one path is throttled and led out, and the other path is connected to the main heat exchanger via a pipeline and throttled after subcooling. The two liquid phases mix and enter the second-stage gas-liquid separator. The LNG liquid phase outlet of the second-stage gas-liquid separator is throttled and output, and its flash vapor gas phase outlet is connected to the main heat exchanger via a pipeline, and after reheating, it is connected to the inlet of the methane open-type two-stage compressor. The inlet of the primary compressor; after the expansion refrigeration return medium from the precooling heat exchanger and the main heat exchanger merges, it enters the methane closed compressor, and after being compressed, cooled, pressurized, and then cooled in sequence, it enters the expansion refrigeration forward flow pipeline of the main heat exchanger for precooling, and then enters the turbine booster expander for expansion; after expansion, the medium is divided into two paths, one path returns to the precooling heat exchanger to provide cooling capacity, and the other path, after being throttled, merges with a portion of the return medium that is branched off and throttled from the precooling heat exchanger, and enters the main heat exchanger to provide cooling capacity.

[0008] Furthermore, the precooling heat exchanger is a plate-fin heat exchanger, the main heat exchanger is a plate-fin heat exchanger, the primary gas-liquid separator is a primary gas-liquid separator, and the secondary gas-liquid separator is a secondary gas-liquid separator.

[0009] Furthermore, the precooling heat exchanger is provided with a methane-rich gas feed channel, a methane refrigeration forward flow channel, and an expansion refrigeration reflux channel; the main heat exchanger is provided with an expansion refrigeration forward flow channel, a first-stage flash vapor reflux channel, a second-stage flash vapor reflux channel, and a methane-rich liquid subcooling channel.

[0010] Furthermore, the methane closed-loop compressor is a methane closed-loop compressor, and the turbine booster expander is a turbine booster expander.

[0011] Furthermore, the methane open-type primary compressor is a methane open-type primary compressor, and the methane open-type secondary compressor is a methane open-type secondary compressor.

[0012] Furthermore, in the precooling heat exchanger, a portion of the methane refrigerant is drawn out and throttled after being split into multiple heat exchangers, and then enters the main heat exchanger for subcooling. After subcooling, it merges with the medium before entering the first-stage gas-liquid separator.

[0013] Furthermore, regulating valves are provided on the pipeline from which the return medium is drawn out from the precooling heat exchanger, and on the pipeline from which the medium is diverted from the expansion end outlet of the turbine booster expander.

[0014] Furthermore, the turbocharger expander is a single-stage expander.

[0015] An operation method for a small skid-mounted liquefied natural gas unit with an integrated methane refrigeration system includes the following steps: Methane-rich gas liquefaction steps: External methane-rich gas is pre-cooled and throttled in a pre-cooling heat exchanger, and then throttled together with methane refrigerant that has undergone heat exchange in the pre-cooling heat exchanger. The resulting mixed medium enters a primary gas-liquid separator for gas-liquid separation. The separated gas phase enters the main heat exchanger for reheating and is then sent to a methane open-loop refrigeration system. Part of the separated liquid phase is directly throttled, and the other part enters the main heat exchanger for subcooling and throttled. The two liquid phases are mixed and then enter a secondary gas-liquid separator for further separation. LNG product is obtained from the secondary gas-liquid separator, and flash vapor is separated. Expansion refrigeration step: The expansion refrigeration return medium from the precooling heat exchanger and the main heat exchanger is combined, compressed and cooled, then pressurized and cooled again, and then enters the main heat exchanger for precooling, and then undergoes expansion refrigeration; part of the expanded medium returns to the precooling heat exchanger to provide cooling capacity, and the other part is throttled and then combined with the part of return medium that is branched off and throttled from the precooling heat exchanger, and enters the main heat exchanger to provide cooling capacity; The methane open-loop refrigeration cycle steps are as follows: the flash vapor from the secondary gas-liquid separator is reheated by the main heat exchanger and then subjected to primary compression and cooling; the gas phase from the primary gas-liquid separator is reheated by the main heat exchanger and then combined with the medium after primary compression and cooling for secondary compression and cooling; the cooled medium is returned to the pre-cooling heat exchanger for heat exchange and participates in the methane-rich gas liquefaction step as the methane refrigerant.

[0016] The small skid-mounted liquefied natural gas unit and its operating method with an integrated methane refrigeration system described in this invention have the following advantages: 1. Adapts to the compact requirements of small skid-mounted systems, significantly reducing equipment footprint and initial investment. The single-stage expansion process replaces the traditional two-stage expansion process, reducing the configuration of one expander and its supporting buffer tank and valve group. This reduces the footprint of the equipment module, lowers the initial equipment investment, and meets the requirements for compact deployment in small skids.

[0017] 2. Reduce operation and maintenance costs and operational complexity, and improve system economy and stability. Employing an open-loop methane refrigeration cycle, using methane-rich feed gas as the refrigerant, eliminates the need for additional MR mixed refrigerant or nitrogen, reducing annual maintenance costs by 50-70% compared to traditional closed-loop cycles. The number of operating variables is reduced from over 10 in MR processes to 3-5, significantly lowering operational complexity and the risk of human error. The single-stage expansion design reduces equipment failure points (from 8 in traditional two-stage processes to 5), further reducing maintenance load and downtime losses.

[0018] 3. Improve system energy utilization efficiency and optimize expander flow rate adaptation and processing performance. By employing a single-stage process of "pre-cooling before expansion," the enthalpy drop loss during "room temperature expansion" in traditional two-stage expansion is avoided (reduced from over 15% to below 5%), significantly improving system energy utilization efficiency. Integrating the "room temperature expansion" and "low temperature expansion" from traditional two-stage expansion processes into a single expander increases the throughput of a single expander by approximately 2 times, reducing the processing difficulty of small expanders (eliminating the need to process multiple small-scale expanders and reducing processing precision requirements). Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. The drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0020] Figure 1 This is a schematic diagram of the structure of the present invention; In the diagram, 1-precooling heat exchanger, 2-main heat exchanger, 3-first-stage gas-liquid separator, 4-second-stage gas-liquid separator, 5-open-stage methane compressor, 6-open-stage methane compressor, 7-closed-stage methane compressor, 8-turbocharger expander, 9-open-stage methane compressor cooler, 10-open-stage methane compressor cooler, 11-closed-stage methane compressor cooler, 12-turbocharger expander cooler, 13-regulating valve, 14-liquid level regulating valve, 15-first regulating valve, 16-second regulating valve, 17-third regulating valve, 18-fourth regulating valve, 19-fifth regulating valve, 20-sixth regulating valve; 201-Methane-rich gas feed channel, 202-Methane refrigeration forward flow channel, 203-Expansion refrigeration reflux channel, 204-Methane refrigeration branch channel, 205-Expansion refrigeration reflux branch channel, 301-Expansion refrigeration reflux channel, 302-Expansion refrigeration forward flow channel, 303-Methane refrigeration branch subcooling channel, 304-First-stage flash vapor reflux channel, 305-Second-stage flash vapor reflux channel, 306-Methane-rich liquid subcooling channel; 1101 - Raw material methane-rich gas feed line; 1102 - Pre-cooled methane-rich gas throttling line; 1103 - Methane-rich gas and methane refrigerant convergence line into the first-stage separator; 1104 - First-stage separator liquid phase splitting line; 1105 - First-stage separator liquid phase throttling line; 1106 - First-stage liquid phase subcooling line into the main heat exchanger; 1107 - Subcooled liquid phase throttling line; 1108 - Second-stage separator LNG product output line. Lines 1109-LNG final discharge pipeline, 1201-first stage flash steam reflux line into main heat exchanger, 1202-first stage flash steam reheated and then fed into second stage compressor, 1203-second stage flash steam reflux line into main heat exchanger, 1204-second stage flash steam reheated and then fed into first stage compressor, 1205-methane open first stage compressor outlet pipeline, 1206-first stage compressor cooled and then fed into second stage compressor, 120 7 - Methane open-type two-stage compressor outlet line; 1208 - Second-stage compressor cooling line into pre-cooling heat exchanger; 1209 - Methane refrigeration forward flow heat exchange throttling line; 1211 - Methane refrigeration branch line outlet line; 1212 - Methane branch line into main heat exchanger subcooling line; 1213 - Methane branch line subcooling and subsequent merging line; 1301 - Pre-cooling heat exchanger expansion reflux outlet line; 1302 - Main heat exchanger expansion reflux outlet line. 1303 - Methane closed-circuit compressor outlet line; 1304 - Closed-circuit compressor cooling line into expander booster end line; 1305 - Expander booster end outlet line; 1306 - Booster cooling line into main heat exchanger forward flow line; 1308 - Expander expansion end outlet main line; 1309 - Expansion refrigerant branch throttling line; 1310 - Expansion reflux branch throttling line; 1311 - Expansion reflux convergence line into main heat exchanger. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0022] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention. Example

[0023] like Figure 1 As shown, this embodiment provides a small skid-mounted liquefied natural gas (LNG) unit with an integrated methane refrigeration system and its operation method. It mainly consists of a precooling heat exchanger 1, a main heat exchanger 2, a primary gas-liquid separator 3, a secondary gas-liquid separator 4, a methane open-type primary compressor 5, a methane open-type secondary compressor 6, a methane closed-type compressor 7, a turbine booster expander 8, a methane open-type primary compressor cooler 9, a methane open-type secondary compressor cooler 10, a methane closed-type compressor cooler 11, a turbine booster expander cooler 12, and regulating valves 13, 14, 15, 16, 17, 18, 19, and 20.

[0024] The precooling heat exchanger 1 integrates a methane-rich gas feed channel 201, a methane refrigeration forward flow channel 202, an expansion refrigeration reflux channel 203, a methane refrigeration branch channel 204, and an expansion refrigeration reflux branch channel 205. The main heat exchanger 2 integrates an expansion refrigeration reflux channel 301, an expansion refrigeration forward flow channel 302, a methane refrigeration branch subcooling channel 303, a first-stage flash vapor reflux channel 304, a second-stage flash vapor reflux channel 305, and a methane-rich liquid subcooling channel 306.

[0025] The connection relationship of the methane-rich gas liquefaction system is as follows: the external raw material methane-rich gas feed line 1101 is connected to the inlet of the methane-rich gas feed channel 201 of the precooling heat exchanger 1. The outlet of the methane-rich gas feed channel 201 is connected to one end of the precooled methane-rich gas throttling line 1102, which is equipped with a regulating valve 13. The outlet of the methane refrigeration forward flow channel 202 of the precooling heat exchanger 1 is connected to one end of the methane refrigeration forward flow heat exchange throttling line 1209, which is equipped with a third regulating valve 17. The other end of the precooled methane-rich gas throttling line 1102 and the methane refrigeration forward flow heat exchange throttling line 1209 merge to form a methane-rich gas and methane refrigerant converging into the first-stage separator line 1103. The outlet of the methane refrigeration branch channel 204 of the precooling heat exchanger 1 is connected to one end of the methane refrigeration branch outlet pipeline 1211. A fourth regulating valve 18 is installed on the methane refrigeration branch outlet pipeline 1211. The other end of the methane refrigeration branch outlet pipeline 1211 is connected to the methane branch inlet subcooling pipeline 1212 of the main heat exchanger. The methane branch inlet subcooling pipeline 1212 is connected to the inlet of the methane refrigeration branch subcooling channel 303 of the main heat exchanger 2. The outlet of the methane refrigeration branch subcooling channel 303 is connected to one end of the methane branch subcooling merge pipeline 1213. The other end of the methane branch subcooling merge pipeline 1213 merges with the methane-rich gas and methane refrigerant into the first-stage separator pipeline 1103, and then both merge into the inlet of the first-stage gas-liquid separator 3. The gas phase outlet of the first-stage gas-liquid separator 3 is connected to one end of the first-stage flash vapor inlet return line 1201 to the main heat exchanger. The other end of the first-stage flash vapor inlet return line 1201 is connected to the inlet of the first-stage flash vapor inlet return channel 304 of the main heat exchanger 2. The outlet of the first-stage flash vapor inlet return channel 304 is connected to the first-stage flash vapor reheating line 1202 leading to the second-stage compressor. The liquid phase outlet of the first-stage gas-liquid separator 3 is divided into two paths: one path is connected to the first-stage separator liquid phase diversion line 1104 (equipped with a liquid level regulating valve 14), which becomes the first-stage separator liquid phase throttling line 1105 after throttling; the other path is connected to the first-stage liquid phase inlet subcooling line 1106, entering the methane-rich liquid subcooling channel 306 of the main heat exchanger 2. The outlet of the methane-rich liquid subcooling channel 306 is connected to one end of the subcooled liquid phase throttling line 1107, which is equipped with a first regulating valve 15. The liquid phase throttling line 1105 of the first-stage separator merges with the other end of the subcooled liquid phase throttling line 1107 and is then connected to the inlet of the second-stage gas-liquid separator 4. The LNG liquid phase outlet of the second-stage gas-liquid separator 4 is connected to one end of the LNG product output line 1108 of the second-stage separator. The LNG product output line 1108 of the second-stage separator is equipped with a second regulating valve 16, and after throttling, the LNG is output to the storage tank through the final LNG discharge line 1109.The flash vapor outlet of the secondary gas-liquid separator 4 is connected to one end of the secondary flash vapor inlet return line 1203 to the main heat exchanger, and the other end of the secondary flash vapor inlet return line 1203 to the main heat exchanger is connected to the inlet of the secondary flash vapor return channel 305 of the main heat exchanger 2. The outlet of the secondary flash vapor return channel 305 is connected to the secondary flash vapor reheating line 1204 to the primary compressor.

[0026] The expansion refrigeration system is connected as follows: the outlet of the expansion refrigeration return channel 203 of the precooling heat exchanger 1 is connected to one end of the expansion refrigeration return outlet pipeline 1301 of the precooling heat exchanger. The outlet of the expansion refrigeration return channel 301 of the main heat exchanger 2 is connected to one end of the expansion refrigeration return outlet pipeline 1302 of the main heat exchanger. The other ends of the expansion refrigeration return outlet pipeline 1301 and the main heat exchanger expansion refrigeration return outlet pipeline 1302 merge and are connected to the inlet of the methane closed-loop compressor 7. The outlet of the methane closed-loop compressor 7 is connected to one end of the methane closed-loop compressor outlet pipeline 1303, and the other end of the methane closed-loop compressor outlet pipeline 1303 is connected to the inlet of the methane closed-loop compressor cooler 11. The outlet of the methane closed-loop compressor cooler 11 is connected to one end of the closed-loop compressor cooling and then entering the expander booster end pipeline 1304, and the other end of the closed-loop compressor cooling and then entering the expander booster end pipeline 1304 is connected to the booster end inlet of the turbine booster expander 8. The turbocharger expander 8's pressurization outlet is connected to one end of the expander pressurization outlet pipeline 1305, and the other end of the expander pressurization outlet pipeline 1305 is connected to the inlet of the turbocharger expander cooler 12. The outlet of the turbocharger expander cooler 12 is connected to one end of the pressurization and cooling inlet to the main heat exchanger forward flow pipeline 1306, and the other end of the pressurization and cooling inlet to the main heat exchanger forward flow pipeline 1306 is connected to the inlet of the expansion refrigeration forward flow channel 302 of the main heat exchanger 2. The outlet of the expansion refrigeration forward flow channel 302 is connected to the expansion end inlet of the turbocharger expander 8. The expansion end outlet of the turbocharger expander 8 is divided into two paths: one path is connected to the expander expansion end outlet main pipeline 1308, which is connected to the inlet of the expansion refrigeration return flow channel 203 of the precooling heat exchanger 1; the other path is connected to the expansion refrigerant diversion and throttling pipeline 1309, which is equipped with a sixth regulating valve 20. The outlet of the expansion refrigerant refrigeration refrigeration branch channel 205 of the precooling heat exchanger 1 is connected to one end of the expansion refrigerant refrigeration branch throttling line 1310, which is equipped with a fifth regulating valve 19. The expansion refrigerant branch throttling line 1309 merges with the other end of the expansion refrigerant refrigeration branch throttling line 1310 to form the expansion refrigerant merging line into the main heat exchanger 1311, which is connected to the inlet of the expansion refrigerant refrigeration refrigeration branch channel 301 of the main heat exchanger 2.

[0027] The connection relationship of the methane open-loop refrigeration cycle system is as follows: The reheated second-stage flash vapor enters the first-stage compressor via pipeline 1204, which is connected to the inlet of the first-stage methane open-loop compressor 5. The outlet of the first-stage methane open-loop compressor 5 is connected to one end of the outlet pipeline 1205, and the other end of the outlet pipeline 1205 is connected to the inlet of the cooler 9. The outlet of the cooler 9 is connected to one end of the cooling first-stage compressor entering the second-stage compressor via pipeline 1206. Simultaneously, the other end of the reheated first-stage flash vapor enters the second-stage compressor via pipeline 1202, which is also connected to the cooling first-stage compressor entering the second-stage compressor via pipeline 1206. The cooling first-stage compressor entering the second-stage compressor via pipeline 1206 is connected to the inlet of the second-stage methane open-loop compressor 6. The outlet of the second-stage methane open-loop compressor 6 is connected to one end of the outlet pipeline 1207, and the other end of the outlet pipeline 1207 is connected to the inlet of the cooler 10. The outlet of the methane open-type two-stage compressor cooler 10 is connected to one end of the pipeline 1208 that enters the precooling heat exchanger after the two-stage compression cooling, and the other end of the pipeline 1208 that enters the precooling heat exchanger after the two-stage compression cooling is connected to the inlet of the methane refrigeration positive flow channel 202 of the precooling heat exchanger 1.

[0028] The operation method of the device described in this embodiment is for the associated gas recovery scenario in a remote oil and gas field in western my country (processing capacity of 50,000 Nm³ / d, pressure of 5.0~6.0 MPa, and temperature of 25°C for methane-rich gas), and includes the following steps: 1. Operation of the methane-rich gas liquefaction system External raw material methane-rich gas enters the methane-rich gas feed channel 201 of the precooling heat exchanger 1 via the raw material methane-rich gas feed line 1101. It exchanges heat with the low-temperature refrigerant (-110~-100℃) in the expansion refrigeration return channel 203, and is precooled to -107~-97℃. After precooling, the methane-rich gas is throttled through the precooling heat exchanger 1102 and then throttled out through the regulating valve 13. Simultaneously, the methane refrigerant (temperature -107~-97℃, pressure 5.0~7.0MPa), having completed heat exchange in the precooling heat exchanger 1, flows out from the methane refrigeration forward channel 202, and after methane refrigeration forward heat exchange, is throttled through the methane refrigeration forward channel 1209 and then throttled out through the third regulating valve 17. The gas-liquid mixture after these two throttling processes merges with the methane refrigerant at the point where it enters the first-stage separator line 1103. Additionally, in the middle section of the precooling heat exchanger 1, a portion of the methane refrigerant is drawn out after heat exchange via the methane refrigeration branch channel 204. It then passes through the methane refrigeration branch outlet pipeline 1211 and is throttled by the fourth regulating valve 18. From there, it enters the main heat exchanger subcooling pipeline 1212 and then the methane refrigeration branch subcooling channel 303 of the main heat exchanger 2. There, it is further subcooled to -125 to -115°C, forming the methane branch subcooling confluence pipeline 1213. This confluence pipeline 1213 merges with the methane-rich gas and methane refrigerant in the first-stage separator pipeline 1103, forming an even lower-temperature mixed medium. This mixture then enters the first-stage gas-liquid separator 3 for gas-liquid separation.

[0029] The gas phase separated by the first-stage gas-liquid separator 3 (mainly unliquefied flash vapor) enters the first-stage flash vapor return channel 304 of the main heat exchanger 2 via the first-stage flash vapor return line 1201. After recovering the cold energy and reheating to approximately 20~25℃, it enters the second-stage compressor line 1202 and is then transported to the methane open-loop refrigeration system. The liquid phase separated by the first-stage gas-liquid separator 3 is divided into two paths: one path passes through the first-stage separator liquid phase diversion line 1104 and is throttled by the liquid level regulating valve 14, forming the first-stage separator liquid phase throttling line 1105; the other path passes through the first-stage liquid phase into the main heat exchanger subcooling line 1106 and enters the methane-rich liquid subcooling channel 306 of the main heat exchanger 2, where it is deeply cooled to -160~-155℃. After passing through the cooled liquid phase throttling line 1107 and being throttled by the first regulating valve 15, it is then transported to the methane-rich liquid subcooling channel 306 of the main heat exchanger 2. The two liquid phases are mixed before the inlet of the secondary gas-liquid separator 4 after throttling, and then enter the secondary gas-liquid separator 4 for final separation. The LNG product (approximately 2000 Nm³ / h) obtained from the bottom of the secondary gas-liquid separator 4 is throttled to approximately 0.15 MPa via the secondary separator LNG product output pipeline 1108 and the second regulating valve 16, and then transported to the LNG storage tank via the final LNG discharge pipeline 1109. The flash vapor separated from the top of the secondary gas-liquid separator 4 enters the secondary flash vapor return channel 305 of the main heat exchanger 2 via the secondary flash vapor return pipeline 1203. After recovering the cold energy and reheating to approximately 20~25°C, it is then reheated by the secondary flash vapor and sent to the primary compressor pipeline 1204 to the methane open refrigeration cycle system.

[0030] 2. Operation of the expansion refrigeration system The reflux medium (temperature 20~25℃, pressure 1.0~1.8MPa) from the expansion reflux channel 203 of the precooling heat exchanger 1 merges with the reflux medium (temperature -20~25℃, pressure 1.0~1.9MPa) from the expansion reflux channel 301 of the main heat exchanger 2 via the expansion reflux outlet pipeline 1301 of the precooling heat exchanger, and enters the methane closed-circuit compressor 7 for compression to 4.5~5.0MPa. The compressed working fluid enters the methane closed-circuit compressor cooler 11 via the methane closed-circuit compressor outlet pipeline 1303, is cooled to about 45℃, and then enters the turbocharger booster end pipeline 1304 of the expander to further boost pressure to 5.5~6.5MPa at the turbocharger booster expander 8. The pressurized working fluid enters the turbocharger expander cooler 12 via the expander pressurization outlet pipeline 1305, where it is cooled again to 45°C. After pressurization and cooling, it enters the expansion and refrigeration forward flow channel 302 of the main heat exchanger 2 via the main heat exchanger forward flow pipeline 1306, where it is pre-cooled to -45~-35°C. Subsequently, the pre-cooled high-pressure working fluid enters the expansion end of the turbocharger expander 8 for expansion and refrigeration, producing a low-temperature medium (pressure 1.2~2.0MPa, temperature -107~-100°C).

[0031] The expanded cryogenic medium is divided into two paths: the main path returns to the expansion refrigeration return channel 203 of the precooling heat exchanger 1 via the main pipeline 1308 at the expansion end outlet of the expander, providing the main cooling capacity for the feed gas; the other path involves a portion of the working medium being led out after heat exchange via the expansion refrigeration return branch channel 205 in the middle section of the precooling heat exchanger 1, passing through the expansion refrigeration branch throttling pipeline 1310 and being throttled by the fifth regulating valve 19. The throttled working medium from this path merges with the other working medium throttled via the expansion refrigerant branch throttling pipeline 1309 and the sixth regulating valve 20, forming an expansion refrigeration convergence line 1311 that enters the expansion refrigeration return channel 301 of the main heat exchanger 2, providing cooling capacity for the subcooled section. The return media flowing out from channels 203 and 301 eventually merge, completing a closed loop. This design avoids the enthalpy drop loss of "room temperature expansion" in traditional two-stage expansion by using a single-stage process of "pre-cooling and then expansion". It also achieves precise and stepped distribution of cooling capacity between the pre-cooling section and the main heat exchange section (subcooling section) by using branch channels 205 and 301 and regulating valves 19 and 20, thereby optimizing the overall thermodynamic efficiency of the system.

[0032] 3. Operation of the methane open-loop refrigeration system Flash vapor (approximately 0.12 MPa, -160°C) from the secondary gas-liquid separator 4 enters the secondary flash vapor return channel 305 of the main heat exchanger 2 via the secondary flash vapor return line 1203. After being reheated to 20-25°C, it enters the primary compressor line 1204 and then the open-stage methane compressor 5, where it is compressed to 4.0-4.5 MPa. The compressed gas then enters the open-stage methane compressor cooler 9 via the open-stage methane compressor outlet line 1205, where it is cooled to 45°C. After being cooled by the primary compressor, it enters the secondary compressor line 1206 for delivery. Simultaneously, flash vapor from the primary gas-liquid separator 3 enters the primary flash vapor return channel 304 of the main heat exchanger 2 via the primary flash vapor return line 1201. After being reheated to 20-25°C, it enters the secondary compressor line 1202 after being reheated by the primary flash vapor and also merges into the secondary compressor line 1206 after primary compression cooling. The two gas streams merge and enter the open-type two-stage methane compressor 6, where they are compressed to 6.5-7.0 MPa. The compressed gas enters the open-type two-stage methane compressor cooler 10 via the methane open-type two-stage compressor outlet line 1207, where it is cooled to 45°C. After secondary compression cooling, it enters the pre-cooling heat exchanger line 1208 and then enters the methane refrigeration forward channel 202 of the pre-cooling heat exchanger 1, completing the refrigeration cycle.

[0033] The device adopts an integrated architecture of "single-stage expansion + open-loop methane refrigeration cycle," integrating the cooling capacity of the traditional two-stage expansion process with a single turbocharger expander. The equipment is compact, occupies a small area, and has low initial investment, perfectly suited for deployment requirements in small skid-mounted scenarios. The open-loop methane refrigeration cycle directly uses the methane component in the feed gas as the refrigerant, eliminating the need for additional preparation and replenishment of external refrigerant, significantly reducing the system's operational complexity and annual maintenance costs. Simultaneously, through optimized process and precise cooling capacity allocation design, the system's energy utilization efficiency is significantly improved. This device and method effectively solve the technical shortcomings of existing small skid-mounted LNG liquefaction technologies, such as large footprint, high investment, complex operation, and reliance on external refrigerants.

[0034] The above embodiments describe the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Modifications and variations made by those skilled in the art without departing from the spirit and scope of the invention should be within the protection scope of the appended claims.

Claims

1. A small skid-mounted liquefied natural gas unit with an integrated methane refrigeration system, characterized in that, The system includes a methane-rich gas liquefaction system, an expansion refrigeration system, and a methane open-loop refrigeration cycle system. The methane-rich gas liquefaction system includes a precooling heat exchanger (1) and a main heat exchanger (2), as well as a first-stage gas-liquid separator and a second-stage gas-liquid separator connected in sequence. The expansion refrigeration system includes a methane closed-loop compressor, a methane closed-loop compressor cooler, a turbine booster expander, a turbine booster expander cooler connected in sequence, and an expansion refrigeration forward flow pipeline connected between the turbine booster expander and the main heat exchanger (2). The methane open-loop refrigeration system... The circulation system includes a methane open-loop primary compressor, a methane open-loop primary compressor cooler, a methane open-loop secondary compressor, and a methane open-loop secondary compressor cooler connected in sequence. External methane-rich gas is connected to the pre-cooling heat exchanger (1) via a pipeline, and after exchanging heat with the return refrigerant from the expansion refrigeration system, it merges with the methane refrigerant from the methane open-loop refrigeration cycle system after exchanging heat with the pre-cooling heat exchanger (1), and then enters the primary gas-liquid separator. The gas phase outlet of the primary gas-liquid separator is connected to the main... The heat exchanger (2) is reheated and then connected to the inlet of the open-type two-stage methane compressor; the liquid phase outlet of the first-stage gas-liquid separator is divided into two paths, one path is throttled and then led out, and the other path is connected to the main heat exchanger (2) via a pipeline and throttled after subcooling. The two liquid phases are mixed and then enter the second-stage gas-liquid separator; the LNG liquid phase outlet of the second-stage gas-liquid separator is throttled and then output, and its flash vapor gas phase outlet is connected to the main heat exchanger (2) via a pipeline, and after reheating, it is connected to the inlet of the open-type first-stage methane compressor; the liquid phase from the precooling exchanger... After the expansion refrigeration return medium of the heat exchanger (1) and the main heat exchanger (2) merges, it enters the methane closed compressor. After being compressed, cooled, pressurized and cooled again in sequence, it enters the expansion refrigeration forward flow pipeline of the main heat exchanger (2) for pre-cooling, and then enters the turbine booster expander for expansion. After expansion, the medium is divided into two paths. One path returns to the pre-cooling heat exchanger (1) to provide cooling capacity, and the other path is throttled and merges with the part of the return medium that is branched out and throttled from the pre-cooling heat exchanger (1) and enters the main heat exchanger (2) to provide cooling capacity.

2. A small skid-mounted liquefied natural gas unit with an integrated methane refrigeration system according to claim 1, characterized in that, The precooling heat exchanger (1) is a plate-fin heat exchanger, the main heat exchanger (2) is a plate-fin heat exchanger, the first-stage gas-liquid separator is a first-stage gas-liquid separator (3), and the second-stage gas-liquid separator is a second-stage gas-liquid separator (4).

3. A small skid-mounted liquefied natural gas unit with an integrated methane refrigeration system according to claim 2, characterized in that, The precooling heat exchanger (1) is provided with a methane-rich gas feed channel (201), a methane refrigeration forward flow channel (202), and an expansion refrigeration refrigeration return flow channel (203); the main heat exchanger (2) is provided with an expansion refrigeration forward flow channel (302), a first-stage flash vapor refrigeration channel (304), a second-stage flash vapor refrigeration channel (305), and a methane-rich liquid subcooling channel (306).

4. A small skid-mounted liquefied natural gas unit with an integrated methane refrigeration system according to claim 2, characterized in that, The methane closed-circuit compressor is a methane closed-circuit compressor (7), and the turbine booster expander is a turbine booster expander (8).

5. A small skid-mounted liquefied natural gas unit with an integrated methane refrigeration system according to claim 2, characterized in that, The methane open-type primary compressor is a methane open-type primary compressor (5), and the methane open-type secondary compressor is a methane open-type secondary compressor (6).

6. A small skid-mounted liquefied natural gas unit with an integrated methane refrigeration system according to claim 1, characterized in that, In the precooling heat exchanger, a portion of the methane refrigerant is drawn out and throttled after being split into multiple heat exchangers, and then enters the main heat exchanger for subcooling. After subcooling, it merges with the medium before entering the first-stage gas-liquid separator.

7. A small skid-mounted liquefied natural gas unit with an integrated methane refrigeration system according to claim 1, characterized in that, Regulating valves are provided on the pipeline from which the return medium is drawn out from the precooling heat exchanger, and on the pipeline from which the medium is diverted from the expansion end outlet of the turbine booster expander.

8. A small skid-mounted liquefied natural gas unit with an integrated methane refrigeration system according to claim 1, characterized in that, The turbocharger expander (8) is a single-stage expander.

9. A method for operating a small skid-mounted liquefied natural gas unit with an integrated methane refrigeration system as described in any one of claims 1-8, characterized in that, Includes the following steps: Methane-rich gas liquefaction steps: External methane-rich gas is pre-cooled and throttled in a pre-cooling heat exchanger, and then throttled together with methane refrigerant that has exchanged heat in the pre-cooling heat exchanger. The resulting mixed medium enters a primary gas-liquid separator for gas-liquid separation. The separated gas phase enters the main heat exchanger for reheating and is then sent to the methane open-loop refrigeration system. Part of the separated liquid phase is directly throttled, and the other part enters the main heat exchanger for subcooling and throttled. The two liquid phases are mixed and then enter a secondary gas-liquid separator for further separation. LNG product is obtained from the secondary gas-liquid separator, and flash vapor is separated out; Expansion refrigeration step: The expansion refrigeration return medium from the precooling heat exchanger and the main heat exchanger is combined, compressed and cooled, then pressurized and cooled again, and then enters the main heat exchanger for precooling, and then undergoes expansion refrigeration; part of the expanded medium returns to the precooling heat exchanger to provide cooling capacity, and the other part is throttled and then combined with the part of return medium that is branched off and throttled from the precooling heat exchanger, and enters the main heat exchanger to provide cooling capacity; The steps of the methane open-loop refrigeration cycle are as follows: the flash vapor from the secondary gas-liquid separator is reheated by the main heat exchanger and then subjected to primary compression and cooling; The gas phase from the first-stage gas-liquid separator is reheated by the main heat exchanger and then combined with the medium after the first-stage compression and cooling for a second-stage compression and cooling. The cooled medium is returned to the pre-cooling heat exchanger for heat exchange and participates in the methane-rich gas liquefaction step as the methane refrigerant.

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