System for separating light hydrocarbon by using cold energy of liquefied natural gas

Abstract

The utility model discloses a system for separating light hydrocarbon by using cold energy of liquefied natural gas. The system comprises a condensing unit, a flash absorption tower, a feeding pump, a demethanizing tower and a deethanizing tower, the condensing unit is connected with the flash absorption tower and provides the heated LNG rich liquid for the flash absorption tower; the feeding pump is respectively connected with a tower bottom outlet of the flash absorption tower and a feeding hole of the demethanizer; a tower bottom outlet of the demethanizer is connected with a feeding hole of the deethanizer; lPG products generated by the deethanization column are discharged from the bottom of the column to an LPG storage and transportation facility outside the boundary area, and gas-phase ethane generated by the deethanization column is discharged from the top of the column. The working pressure of the demethanizer is set to be 0.5-2.5 MPag, the working pressure of the deethanizer is set to be 0.1-2.0 MPag, the demethanizer and the deethanizer both operate under low pressure, the separation efficiency is high, the number of equipment tower plates is small, the pressure grade is low, the tower top condensation load and the tower bottom reboiling load are small, and the energy consumption is reduced; the system can efficiently extract the light hydrocarbon in the LNG rich liquid, and is good in raw material adaptability, high in operation stability, low in investment, low in energy consumption and high in light hydrocarbon yield.

Description

Technical Field

[0001] The present application belongs to the technical field of light hydrocarbon recovery, and specifically relates to a system for separating light hydrocarbons by utilizing cold energy of liquefied natural gas. Background Art

[0002] Liquefied natural gas (LNG) has been widely developed around the world in recent years due to its advantages of cleanliness, efficiency and flexible supply. With the adjustment of China's energy consumption structure, the annual consumption of natural gas has increased rapidly. As an important supplement to overseas natural gas resources, the import volume of LNG has generally shown an upward trend year by year. In 2021, China's LNG imports reached 79.27 million tons, surpassing Japan to rank first in the world. The main component of LNG resources is methane, and it also contains some ethane and above light hydrocarbons, and contains a large amount of cold energy. Making full use of LNG resources has important economic and social significance.

[0003] Liquefied natural gas is divided into lean liquid and rich liquid according to the light hydrocarbon content. Rich liquid has a high light hydrocarbon content, so light hydrocarbons can be separated at the LNG receiving station for calorific value adjustment, or high value-added products such as ethane and liquefied petroleum gas (LPG) can be obtained as high-quality chemical raw materials. Although the calorific value of LNG decreases after removing light hydrocarbons, it can still meet the calorific value requirements of pipeline natural gas.

[0004] In the light hydrocarbon separation device in the prior art, the demethanizer needs to operate at a relatively high operating pressure (mostly 2.6-4.5 MPag) to condense the methane-lean gas after the light hydrocarbons are removed. The separation efficiency is low, the actual number of tower plates is large, the reboiler load is large, and the energy consumption is high. Utility Model Content

[0005] In order to solve the technical problems in the prior art that the demethanizer needs to operate at a higher pressure to condense the methane-lean gas after the light hydrocarbons are removed, the separation efficiency is low, the reboiler load is large, and the energy consumption is high, the present application provides a system for separating light hydrocarbons using the cold energy of liquefied natural gas.

[0006] The present application provides a system for separating light hydrocarbons by utilizing cold energy of liquefied natural gas, comprising a condensing unit, a flash absorption tower, a feed pump, a demethanizer and a deethanizer, wherein:

[0007] The condensing unit is connected to the flash absorption tower and is used to heat the LNG rich liquid directly delivered from the LNG receiving station without being pressurized, and provide the heated LNG rich liquid to the flash absorption tower;

[0008] The feed pump is connected to the bottom outlet of the flash absorption tower and the feed inlet of the demethanizer respectively, and is used to supply the bottom product of the flash absorption tower to the demethanizer;

[0009] The bottom outlet of the demethanizer is connected to the feed inlet of the deethanizer to supply the bottom product of the demethanizer to the deethanizer, and the working pressure of the demethanizer is 0.5-2.5 MPag;

[0010] The LPG product produced by the deethanizer is discharged from the bottom of the tower to the LPG storage and transportation facilities outside the boundary area, and the gaseous ethane produced by the deethanizer is discharged from the top of the tower. The working pressure of the deethanizer is 0.1-2.0MPag.

[0011] In some optional embodiments, the condensing unit includes a first condenser and a second condenser; the first condenser is connected to the LNG receiving station and is used to perform primary heating of the LNG rich liquid; the second condenser is respectively connected to the first condenser and the flash absorption tower, and is used to perform secondary heating of the LNG rich liquid and then provide it to the flash absorption tower.

[0012] In some optional embodiments, the top of the flash absorption tower is connected to the first condenser, so as to provide part of the generated lean gas to the first condenser without pressurization;

[0013] It also includes: a first condensate tank, a condensate pump, a second condensate tank and a lean liquid pump, wherein:

[0014] The first condensate tank is connected to the first condenser, and the LNG rich liquid in the first condenser condenses the lean gas to obtain LNG lean liquid, and the obtained LNG lean liquid enters the first condensate tank for storage;

[0015] The condensate pump is connected to the first condensate tank and the second condensate tank respectively to pressurize the LNG lean liquid and then transport it to the second condensate tank for storage;

[0016] The second condensate tank is connected to the flash absorption tower and the lean liquid pump respectively, and is used to transport the LNG lean liquid to the flash absorption tower as absorption liquid;

[0017] The lean liquid pump is connected to the demethanizer and is used for pressurizing the LNG lean liquid and transporting it to the demethanizer as top reflux.

[0018] In some optional embodiments, a branch pipe is connected to the pipeline connecting the top of the flash absorption tower and the first condenser, and the branch pipe is connected to an external flare to provide the remaining lean gas to the flare.

[0019] In some optional embodiments, the second condenser is connected to the top of the demethanizer to receive the gaseous methane generated at the top of the demethanizer;

[0020] Also included are the inlet surge tank and booster;

[0021] The top of the flash absorption tower is connected to the inlet buffer tank, so as to provide the generated lean gas to the inlet buffer tank;

[0022] The inlet buffer tank is connected to the supercharger; the supercharger is connected to the second condenser, and is used to supercharge the lean gas and provide it to the second condenser;

[0023] The second condensate tank is connected to the second condenser, and the LNG rich liquid in the second condenser condenses the lean gas and the gaseous methane to obtain LNG lean liquid, and the obtained LNG lean liquid enters the second condensate tank for storage.

[0024] In some optional embodiments, a third condenser and a third condensate tank are further included; the third condenser is connected to the top of the deethanizer and the third condensate tank respectively, and is used to condense the gaseous ethane produced at the top of the deethanizer into liquid ethane, and provide the liquid ethane to the third condensate tank; the third condensate tank is connected to the deethanizer, and is used to transport the liquid ethane to the deethanizer as reflux.

[0025] In some optional embodiments, the lean liquid pump is connected to the third condenser for conveying the LNG lean liquid to the third condenser as a cold source.

[0026] In some optional embodiments, the lean liquid pump is connected to the LNG receiving station for transporting the remaining LNG lean liquid back to the LNG receiving station.

[0027] In some optional embodiments, the first reboiler and the second reboiler are further included, wherein the first reboiler is connected to the demethanizer for heating the demethanizer; and the second reboiler is connected to the deethanizer for heating the deethanizer.

[0028] In some optional embodiments, the first reboiler and the second reboiler use steam, circulating hot water, electric current and environmental heat source as heat sources.

[0029] Compared with the prior art, the system for separating light hydrocarbons by utilizing the cold energy of liquefied natural gas provided by the utility model has the following beneficial effects:

[0030] The working pressure of the demethanizer is 0.5-2.5MPag, and the working pressure of the deethanizer is 0.1-2.0MPag. Both the demethanizer and the deethanizer operate at relatively low pressures, with high separation efficiency, small number of equipment plates, low pressure level, small top condensation load and bottom reboiler load, thereby reducing energy consumption. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1A schematic structural diagram of a system for separating light hydrocarbons using cold energy of liquefied natural gas in one or more embodiments of the utility model is shown.

[0032] Explanation of the reference numerals: 1-first condenser; 2-second condenser; 3-flash absorption tower; 4-feed pump; 5-demethanizer; 6-deethanizer; 7-first condensate tank; 8-condensate pump; 9-second condensate tank; 10-lean liquid pump; 11-inlet buffer tank; 12-booster; 13-third condenser; 14-third condensate tank; 15-first reboiler; 16-second reboiler. DETAILED DESCRIPTION

[0033] In order to make the technical personnel in the technical field to which the present application belongs to understand the present application more clearly, the technical scheme in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in the field without creative work are within the scope of protection of the present application.

[0034] Please refer to Figure 1 As shown, the embodiment of the present application provides a system for separating light hydrocarbons using cold energy of liquefied natural gas, including a condensing unit, a flash absorption tower 3, a feed pump 4, a demethanizer 5 and a deethanizer 6, wherein:

[0035] The LNG storage tank in the LNG receiving station is directly connected to the condensing unit, and is used to transport LNG rich liquid to the condensing unit; the LNG storage tank is located in the LNG receiving station, and the LNG rich liquid directly enters the boundary area and reaches the condensing unit without pressurization. The pressure of the LNG rich liquid is about 1.0 MPag, where MPag is the unit of apparent pressure;

[0036] The condensing unit is connected to the flash absorption tower 3, and is used to heat the LNG rich liquid and provide the heated LNG rich liquid to the flash absorption tower 3;

[0037] The feed pump 4 is connected to the bottom outlet of the flash absorption tower 3 and the feed inlet of the demethanizer 5 respectively, and is used to provide the bottom product of the flash absorption tower 3 to the demethanizer 5;

[0038] The bottom outlet of the demethanizer 5 is connected to the feed port of the deethanizer 6, so as to provide the bottom product of the demethanizer 5 to the deethanizer 6. The working pressure of the demethanizer 5 is 0.5-2.5 MPag. The bottom product of the demethanizer 5 is C2+ light hydrocarbons, which are sent to the deethanizer 6 as feed.

[0039] The LPG product produced by the deethanizer 6 is discharged from the bottom of the tower to the LPG storage and transportation facilities outside the boundary area, and the gaseous ethane produced by the deethanizer 6 is discharged from the top of the tower. The working pressure of the deethanizer 6 is 0.1-2.0MPag.

[0040] In the system for separating light hydrocarbons by utilizing cold energy of liquefied natural gas provided in the embodiment of the present application, the working pressure of the demethanizer 5 is 0.5-2.5 MPag, and the working pressure of the deethanizer 6 is 0.1-2.0 MPag. Both the demethanizer 5 and the deethanizer 6 operate at relatively low pressures, with high separation efficiency, a small number of equipment plates, a low pressure level, small loads for condensation at the top of the tower and reboiling at the bottom of the tower, and reduced energy consumption.

[0041] In some optional embodiments, the condensing unit includes a first condenser 1 and a second condenser 2; the first condenser 1 is connected to the LNG receiving station and is used to perform primary heating on the LNG rich liquid; the second condenser 2 is connected to the first condenser 1 and the flash absorption tower 3, respectively, and is used to perform secondary heating on the LNG rich liquid and then provide it to the flash absorption tower 3. By performing stepwise heating on the LNG rich liquid by the first condenser 1 and the second condenser 2, the LNG rich liquid can be better heated to the required temperature, in preparation for the subsequent absorption of lean gas in the flash absorption tower 3.

[0042] In some optional embodiments, the top of the flash absorption tower 3 is connected to the first condenser 1, so as to provide part of the generated lean gas to the first condenser 1 without pressurization;

[0043] The system for separating light hydrocarbons by utilizing cold energy of liquefied natural gas further comprises: a first condensate tank 7, a condensate pump 8, a second condensate tank 9 and a lean liquid pump 10, wherein:

[0044] The first condensate tank 7 is connected to the first condenser 1, and the LNG rich liquid in the first condenser 1 condenses the lean gas to obtain LNG lean liquid, and the obtained LNG lean liquid enters the first condensate tank 7 for storage;

[0045] The condensate pump 8 is connected to the first condensate tank 7 and the second condensate tank 9 respectively, so as to pressurize the LNG lean liquid and transport it to the second condensate tank 9 for storage;

[0046] The second condensate tank 9 is connected to the flash absorption tower 3 and the lean liquid pump 10 respectively, and is used to transport the LNG lean liquid to the flash absorption tower 3 as absorption liquid;

[0047] The lean liquid pump 10 is connected to the demethanizer 5 and is used to pressurize the LNG lean liquid and transport it to the demethanizer 5 as a top reflux.

[0048] In this embodiment, the methane content in the lean gas at the top of the flash absorption tower 3 is greater than 99% mol. In order to utilize the LNG cooling capacity in a cascade manner, part of the lean gas (accounting for about 20% of the LNG rich liquid entering the first condenser 1) does not need to be pressurized, but directly enters the first condenser 1 to be condensed by the LNG feed into an LNG lean liquid at about -130°C, enters the first condensate tank 7 for storage, and then enters the second condensate tank 9 after being pressurized by the condensate pump 8.

[0049] In some optional embodiments, a branch pipe is connected to the pipeline connecting the top of the flash absorption tower 3 and the first condenser 1, and the branch pipe is connected to an external flare to provide the remaining lean gas to the flare. The main component of the lean gas is methane, and the excess lean gas provided to the flare can be used as fuel gas to achieve full utilization of LNG rich liquid.

[0050] In some optional embodiments, the second condenser 2 is connected to the top of the demethanizer 5, and is used to receive the gaseous methane generated at the top of the demethanizer 5;

[0051] The system for separating light hydrocarbons using cold energy of liquefied natural gas also includes an inlet buffer tank 11 and a booster 12;

[0052] The top of the flash absorption tower 3 is connected to the inlet buffer tank 11, so as to provide the generated lean gas to the inlet buffer tank 11;

[0053] The inlet buffer tank 11 is connected to the supercharger 12; the supercharger 12 is connected to the second condenser 2, and is used to supercharge the lean gas and provide it to the second condenser 2;

[0054] The second condensate tank 9 is connected to the second condenser 2 , and the LNG rich liquid in the second condenser 2 condenses the lean gas and the gaseous methane to obtain LNG lean liquid, and the obtained LNG lean liquid enters the second condensate tank 9 for storage.

[0055] In this embodiment, most of the lean gas generated at the top of the flash absorption tower 3 is sent to the first condenser 1, and the remaining lean gas enters the inlet buffer tank 11 and is pressurized by the booster 12 to the top pressure of the demethanizer 5. The top of the demethanizer 5 produces gaseous methane at -116°C, which is mixed with the lean gas at the outlet of the booster 12 and enters the second condenser 2, where it is condensed into a lean liquid at about -126°C by the LNG feed and enters the second condensate tank 9 for storage. This step-by-step condensation process significantly reduces the power consumption of the booster 12.

[0056] In some optional embodiments, the system for separating light hydrocarbons using cold energy of liquefied natural gas also includes a third condenser 13 and a third condensate tank 14; the third condenser 13 is connected to the top of the deethanizer 6 and the third condensate tank 14, respectively, for condensing the gaseous ethane produced at the top of the deethanizer 6 into liquid ethane, and providing the liquid ethane to the third condensate tank 14; the third condensate tank 14 is connected to the deethanizer 6, for transporting the liquid ethane to the deethanizer 6 as reflux.

[0057] In some optional embodiments, the lean liquid pump 10 is connected to the third condenser 13 to transport the LNG lean liquid to the third condenser 13 as a cold source. Part of the LNG lean liquid in the secondary condensate tank enters the top of the flash absorption tower 3 as an absorption liquid, and the rest is pressurized by the lean liquid pump 10, and one stream is sent to the demethanizer 5 as a tower top reflux, and one stream is sent out of the boundary area after providing cold for the top of the deethanizer 6.

[0058] In some optional embodiments, the lean liquid pump 10 is connected to the LNG receiving station to transport the remaining LNG lean liquid back to the LNG receiving station. The LNG lean liquid pressurized by the lean liquid pump 10 is separated into one stream and sent to the demethanizer 5 and the other stream to the deethanizer 6, and the remaining LNG lean liquid is returned to the inlet of the gasifier of the LNG receiving station for recycling.

[0059] In some optional embodiments, the system for separating light hydrocarbons by utilizing cold energy of liquefied natural gas further comprises a first reboiler 15 and a second reboiler 16, wherein the first reboiler 15 is connected to the demethanizer 5 for heating the demethanizer 5, and the second reboiler 16 is connected to the deethanizer 6 for heating the deethanizer 6. The demethanizer 5 is heated by the first reboiler 15, and the deethanizer 6 is heated by the second reboiler 16, so as to provide heat support for the fractionation of the demethanizer 5 and the deethanizer 6, and gaseous methane and gaseous ethane can be fractionated respectively by heating to the required temperature by utilizing the difference in boiling points.

[0060] In some optional embodiments, the first reboiler 15 and the second reboiler 16 use steam, circulating hot water, electric current and environmental heat source (such as seawater) as heat sources. The optional range of heat sources is very wide, and since the operating pressure of the demethanizer 5 and the deethanizer 6 is relatively low, the energy consumption is significantly reduced.

[0061] The working principle of the system for separating light hydrocarbons by utilizing the cold energy of liquefied natural gas proposed in the utility model is:

[0062] The LNG raw material rich liquid pumped out of the storage tank of the LNG receiving station enters the boundary area directly without pressurization, with a pressure of about 1.0MPag. After being heated in turn by the two first condensers 1 and the second condensers 2 connected in series, it enters the flash absorption tower 3. The methane content in the lean gas at the top of the flash absorption tower 3 is greater than 99% mol. In order to utilize the LNG cooling capacity in a cascade, part of the lean gas (accounting for about 20% of the LNG feed) does not need to be pressurized, but directly enters the first condenser 1 and is condensed into a lean liquid of about -130°C by the LNG feed, enters the first condensate tank 7 for storage, and then enters the second condensate tank 9 after being pressurized by the condensate pump 8; the remaining lean gas enters the inlet buffer tank 11 and is pressurized by the lean gas booster 12 to the top pressure of the demethanizer 5. This cascade condensation process significantly reduces the power consumption of the booster 12;

[0063] The bottom product of flash absorber 3 is pressurized by feed pump 4 and enters demethanizer 5, which operates at 0.5-2.5MPag. Methane at -116℃ is produced at the top of the tower, mixed with the lean gas at the outlet of lean gas booster 12 and enters the second condenser 2, where it is condensed into lean liquid at about -126℃ by LNG feed and stored in the secondary condensate tank. The bottom of demethanizer 5 is C2+ light hydrocarbons, which are sent to deethanizer 6 as feed;

[0064] Part of the methane condensate in the secondary condensate tank enters the top of the flash absorption tower 3 as absorption liquid, and the rest is pressurized by the LNG lean liquid pump 10, and one stream is sent to the demethanizer 5 as tower top reflux, and the rest is sent out of the boundary area after providing cooling capacity for the top of the deethanizer 6 and returned to the inlet of the vaporizer of the LNG receiving station;

[0065] The deethanizer 6 operates at 0.1-2.0 MPag. The gaseous ethane at the top of the tower is condensed into a liquid phase of about -71°C in the top condenser of the deethanizer 6 with the lean liquid before returning to the receiving station, and enters the third condensate tank 14 at the top of the tower. Part of the condensate returns to the top of the deethanizer 6 as reflux, and the rest is pressurized to the boundary pressure by the ethane pump and sent out of the boundary for storage or external transmission.

[0066] The system for separating light hydrocarbons by utilizing the cold energy of liquefied natural gas proposed in the utility model has the following advantages:

[0067] (1) The recovery rate of light hydrocarbons such as ethane is high, and the purity and yield can reach more than 98%;

[0068] (2) The comprehensive utilization of cold energy is fully realized. The cold energy of LNG itself can be used to recover ethane and other light hydrocarbons. No additional cold energy is required. The cold energy utilization rate is high. The cascade utilization of cold energy saves nearly 1 / 4 of the energy consumption of the booster 12 and improves the heat exchange of the condenser ( )efficiency;

[0069] (3) Through the flash absorption tower 3, the demethanizer 5, and the deethanizer 6, an absorption-distillation process is realized, which improves the overall recovery rate of light hydrocarbons such as ethane and significantly reduces the energy consumption of the demethanizer 5 to 65% of similar processes;

[0070] (4) It has good operational stability and strong adaptability to LNG raw materials. The ethane recovery rate is not limited by the ethane content in the LNG raw materials, and it has strong tolerance to fluctuations in LNG composition. It is insensitive to production operation fluctuations. Since this technology adjusts the operating point to a gentle range, it overcomes the defect of similar technologies that are overly sensitive to key temperature points such as the feed of the demethanizer 5. Even if the operation of the device fluctuates greatly, the impact on key equipment is small, which is conducive to the smooth and safe operation of the device.

[0071] (5) The structure is novel. There is no need to set up a raw material booster pump facility. The LNG-rich liquid pumped out of the LNG storage tank directly enters the light hydrocarbon separation unit. While simplifying the process, the pressure level of most system equipment and pipelines can be maintained at 150#, saving engineering construction investment and reducing the danger of process facilities;

[0072] (6) The demethanizer 5 and the deethanizer 6 operate at a low operating pressure with high separation efficiency. Therefore, the number of equipment plates is small, the pressure level is low, and the load of top condensation and bottom reboiling is small, which solves the problem of high heat source energy consumption of similar devices and significantly reduces operating costs and equipment investment;

[0073] (7) The bottom reboilers of the demethanizer 5 and the deethanizer 6 can use steam, circulating water, electric current and environmental heat sources. When seawater heat source is used, it can rely on the seawater facilities of the LNG receiving station. The heat source energy consumption is only the power consumption of the seawater pump, which significantly improves the electrification rate of the overall device (up to 97%), reduces carbon emissions, and has the potential for zero emissions;

[0074] (8) The methane-poor condensate after the removal of light hydrocarbons is integrated with the top condenser of the deethanizer 6 to meet the cooling demand of the deethanizer 6 while reducing the heat load of the downstream LNG vaporizer, which can significantly reduce the energy consumption of the seawater pump of the LNG receiving station.

[0075] In the present application, unless otherwise clearly specified and limited, a first feature being "above" or "below" a second feature may include that the first and second features are in direct contact, or may include that the first and second features are not in direct contact but are in contact through another feature between them. Moreover, a first feature being "above", "above" and "above" a second feature includes that the first feature is directly above and obliquely above the second feature, or simply indicates that the first feature is higher in level than the second feature. A first feature being "below", "below" and "below" a second feature includes that the first feature is directly below and obliquely below the second feature, or simply indicates that the first feature is lower in level than the second feature.

[0076] In the description of the present application, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise" and "counterclockwise" indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present application.

[0077] In this application, unless otherwise clearly specified and limited, the terms "connection", "fixation", etc. should be understood in a broad sense. For example, "fixation" can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0078] In addition, the descriptions of "first", "second", etc. in this application are only for descriptive purposes and cannot be understood as indicating or implying their relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" or "second" may explicitly or implicitly include one or more features. In the description of this application, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0079] Although the embodiments of the present application have been shown and described, those skilled in the art will appreciate that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present application, and that the scope of the present application is defined by the claims and their equivalents.

Claims

1. A system for separating light hydrocarbons using cold energy of liquefied natural gas, characterized in that: It includes a condensing unit, a flash absorption tower, a feed pump, a demethanizer and a deethanizer, wherein: The condensing unit is connected to the flash absorption tower and is used to heat the LNG rich liquid directly delivered from the LNG receiving station without being pressurized, and provide the heated LNG rich liquid to the flash absorption tower; The feed pump is connected to the bottom outlet of the flash absorption tower and the feed inlet of the demethanizer respectively, and is used to supply the bottom product of the flash absorption tower to the demethanizer; The bottom outlet of the demethanizer is connected to the feed inlet of the deethanizer to supply the bottom product of the demethanizer to the deethanizer, and the working pressure of the demethanizer is 0.5-2.5 MPag; The LPG product produced by the deethanizer is discharged from the bottom of the tower to the LPG storage and transportation facilities outside the boundary area, and the gaseous ethane produced by the deethanizer is discharged from the top of the tower. The working pressure of the deethanizer is 0.1-2.0MPag.

2. The system for separating light hydrocarbons by utilizing cold energy of liquefied natural gas according to claim 1, characterized in that: The condensing unit includes a first condenser and a second condenser; the first condenser is connected to the LNG receiving station and is used to perform primary heating on the LNG rich liquid; the second condenser is respectively connected to the first condenser and the flash absorption tower and is used to perform secondary heating on the LNG rich liquid and then provide it to the flash absorption tower.

3. The system for separating light hydrocarbons by utilizing cold energy of liquefied natural gas according to claim 2, characterized in that: The top of the flash absorption tower is connected to the first condenser, so as to provide part of the generated lean gas to the first condenser without increasing pressure; It also includes: a first condensate tank, a condensate pump, a second condensate tank and a lean liquid pump, wherein: The first condensate tank is connected to the first condenser, and the LNG rich liquid in the first condenser condenses the lean gas to obtain LNG lean liquid, and the obtained LNG lean liquid enters the first condensate tank for storage; The condensate pump is connected to the first condensate tank and the second condensate tank respectively to pressurize the LNG lean liquid and then transport it to the second condensate tank for storage; The second condensate tank is connected to the flash absorption tower and the lean liquid pump respectively, and is used to transport the LNG lean liquid to the flash absorption tower as absorption liquid; The lean liquid pump is connected to the demethanizer and is used to pressurize the LNG lean liquid and transport it to the demethanizer as top reflux.

4. The system for separating light hydrocarbons by utilizing cold energy of liquefied natural gas according to claim 3, characterized in that: A branch pipe is connected to the pipeline connecting the top of the flash absorption tower and the first condenser. The branch pipe is connected to an external flare to provide the remaining lean gas to the flare.

5. The system for separating light hydrocarbons by utilizing cold energy of liquefied natural gas according to claim 3, characterized in that: The second condenser is connected to the top of the demethanizer and is used to receive the gaseous methane generated at the top of the demethanizer; Also included are the inlet surge tank and booster; The top of the flash absorption tower is connected to the inlet buffer tank, so as to provide the generated lean gas to the inlet buffer tank; The inlet buffer tank is connected to the supercharger; the supercharger is connected to the second condenser, and is used to supercharge the lean gas and provide it to the second condenser; The second condensate tank is connected to the second condenser, and the LNG rich liquid in the second condenser condenses the lean gas and the gaseous methane to obtain LNG lean liquid, and the obtained LNG lean liquid enters the second condensate tank for storage.

6. The system for separating light hydrocarbons by utilizing cold energy of liquefied natural gas according to claim 5, characterized in that: It also includes a third condenser and a third condensate tank; the third condenser is connected to the top of the deethanizer and the third condensate tank respectively, and is used to condense the gaseous ethane produced at the top of the deethanizer into liquid ethane, and provide the liquid ethane to the third condensate tank; the third condensate tank is connected to the deethanizer, and is used to transport the liquid ethane to the deethanizer as reflux.

7. The system for separating light hydrocarbons by utilizing cold energy of liquefied natural gas according to claim 6, characterized in that: The lean liquid pump is connected to the third condenser and is used to transport the LNG lean liquid to the third condenser as a cold source.

8. The system for separating light hydrocarbons by utilizing cold energy of liquefied natural gas according to claim 7, characterized in that: The lean liquid pump is connected to the LNG receiving station and is used to transport the remaining LNG lean liquid back to the LNG receiving station.

9. The system for separating light hydrocarbons by utilizing cold energy of liquefied natural gas according to any one of claims 1 to 8, characterized in that: It also includes a first reboiler and a second reboiler, wherein the first reboiler is connected to the demethanizer for heating the demethanizer; and the second reboiler is connected to the deethanizer for heating the deethanizer.

10. The system for separating light hydrocarbons by utilizing cold energy of liquefied natural gas according to claim 9, characterized in that: The first reboiler and the second reboiler use steam, circulating hot water, electric current and environmental heat source as heat sources.

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