阅读说明：本技术 一种油田伴生气生产轻烃和lng的工艺及系统 (Process and system for producing light hydrocarbon and LNG (liquefied Natural gas) from oilfield associated gas ) 是由 王弯弯 苏柯洋 宋玲 于 2021-02-04 设计创作，主要内容包括：本发明涉及一种油田伴生气生产轻烃和LNG的工艺及系统,包括洗涤脱重烃、预冷、NGL回收、液化和制冷等步骤。将NGL回收与LNG液化设计为一个工艺,与独立的生产工艺相比较,能够去掉重复使用的换热器,降低两个单元之间的压降,降低了能耗,同时减少了设备,节约了占地面积,降低了投资。采用本发明工艺,丙烷回收率可达到98%以上。(The invention relates to a process and a system for producing light hydrocarbon and LNG (liquefied natural gas) from oilfield associated gas, which comprises the steps of washing and heavy hydrocarbon removal, precooling, NGL (natural gas) recovery, liquefaction, refrigeration and the like. Compared with an independent production process, the NGL recovery and LNG liquefaction process is designed into one process, and a heat exchanger which is repeatedly used can be omitted, so that the pressure drop between the two units is reduced, the energy consumption is reduced, the equipment is reduced, the occupied area is saved, and the investment is reduced. By adopting the process, the recovery rate of the propane can reach more than 98 percent.)

1. A process for producing light hydrocarbon and LNG by using oilfield associated gas is characterized by comprising the following steps: the oil field associated gas enters a washing tower after being subjected to decarburization and dehydration, is completely or partially cooled by a cooling box after being contacted with washing liquid, is subjected to gas-liquid separation after being cooled, the separated liquid phase is used as the washing liquid, and the separated gas phase is an LNG product; the heavy hydrocarbon discharged from the washing tower enters a de-ethane tower for rectification, then enters a liquefied gas tower for rectification after heat exchange with light hydrocarbon discharged from the liquefied gas tower, the gas discharged from the liquefied gas tower is cooled and separated, one part of the separated liquid phase returns to the liquefied gas tower, and the rest part is an LPG product; and performing heat exchange on the product discharged from the bottom of the liquefied gas tower to obtain a light hydrocarbon product.

2. The process for producing light hydrocarbons and LNG as claimed in claim 1, wherein the gas discharged from the de-ethanizer is condensed and separated into gas and liquid phases, the separated liquid phase is returned to the de-ethanizer, and the gas phase is used as fuel gas.

3. The process for producing light hydrocarbons and LNG from oilfield associated gas as claimed in claim 2, further comprising the steps of: the propane refrigerant is pressurized by a compressor unit, and then is cooled by a propane cooler to carry out gas-liquid separation, the separated gas returns to the inlet of the compressor unit, the liquid phase fluid is divided into two paths, one path of liquid phase propane enters a propane low-temperature separator to carry out gas-liquid separation, the separated liquid phase provides cold energy for the cold box and then returns to the propane low-temperature separator, and the separated gas phase propane and the gas phase propane returning to the propane low-temperature separator after heat exchange return to the compressor unit; and the other path of liquid-phase propane provides cold energy for cooling the gas discharged by the ethane-removing tower, and the gas-phase propane after heat exchange returns to the compressor unit.

4. The process for producing light hydrocarbons and LNG from oilfield associated gas as claimed in claim 3, further comprising the steps of: and the gas phase separated by cooling the gas discharged from the ethane-removing tower is discharged after being reheated by the propane cooler and used as fuel gas.

5. The process for producing light hydrocarbons and LNG from oilfield associated gas as claimed in claim 1, 2, 3 or 4, further comprising the steps of: and the mixed refrigerant enters the cold box after being pressurized by another compressor unit, returns to the cold box for further deep cooling after being precooled and throttled and reduced in pressure, returns to the cold box for reheating after being throttled and reduced in pressure, and then goes out of the cold box and returns to the other compressor unit.

6. A system for producing light hydrocarbon and LNG by using oilfield associated gas comprises a cryogenic scrubber (1) with an oilfield associated gas inlet, a cryogenic separator (15) and a cold box (14); the device is characterized in that an outlet port of the low-temperature washing tower (1) is connected with an inlet of a cold box (14), an outlet of the cold box (14) corresponding to the inlet is connected with an inlet of a low-temperature separator (15), a first outlet of the low-temperature separator (15) is connected with the low-temperature washing tower (1) through a pipeline with a liquid-phase circulating pump (16), a second outlet of the low-temperature separator (15) is connected with the other inlet of the cold box (14) through a pipeline, and an outlet of the cold box (14) corresponding to the other inlet is connected with an LNG product output pipe; the low-temperature washing tower (1) is connected with a demethanizer (2), the demethanizer (2) is connected with a liquefied gas tower feeding preheater (7) or the demethanizer (2) is connected with the liquefied gas tower feeding preheater (7) through a demethanizer reboiler (6); the liquefied gas tower feeding preheater (7) is connected with the feeding end of the liquefied gas tower (8); one discharge end of the liquefied gas tower (8) is connected with the liquefied gas tower feeding preheater (7) or one discharge end of the liquefied gas tower (8) is connected with the liquefied gas tower feeding preheater (7) through a liquefied gas tower reboiler (12); the feeding preheater (7) of the liquefied gas tower is directly connected with a stable light hydrocarbon product output pipe or is connected with the stable light hydrocarbon product output pipe through a stable light hydrocarbon product cooler (13); the other discharge end of the liquefied gas tower (8) is connected with a first condensation separation device, the LPG output end of the first condensation separation device is connected with an LPG product output pipe, and the LPG output end is also connected with the liquefied gas tower (8); the system also includes a second condensate separation unit connected to the demethanizer (2), the liquid phase outlet of the second condensate separation unit being connected to the demethanizer (2) via a demethanizer reflux pump (5).

7. The system for producing light hydrocarbons and LNG from oilfield associated gas of claim 6, wherein: the first condensation separation device comprises a liquefied gas overhead condenser (9) and a liquefied gas overhead separator (10) which are connected with each other; the LPG output end is the output end of a liquefied gas tower top separator (10).

8. The system for producing light hydrocarbons and LNG from oilfield associated gas of claim 6, wherein: the second condensation separation device comprises a demethanized ethane overhead condenser (3) and a demethanized ethane overhead separator (4) which are connected with each other; the demethanizer (2) is connected with one inlet of the demethanizer overhead condenser (3), the outlet of the demethanizer overhead condenser (3) corresponding to the inlet is connected with the demethanizer overhead separator (4), and the liquid phase outlet of the demethanizer overhead separator (4) is connected with the demethanizer (2) through a demethanizer reflux pump (5).

9. The system for producing light hydrocarbons and LNG from oilfield associated gas of claim 6, wherein: the system further comprises a first compressor string (18); the first compressor unit (18) is connected with one inlet of the cold box (14), and the outlet of the cold box (14) corresponding to the inlet is connected with the first compressor unit (18).

10. The system for producing light hydrocarbons and LNG from oilfield associated gas of claim 6, 7, 8 or 9, wherein: the system further comprises a second compressor group (19); a propane cooler (20) is connected in the second compressor unit (19); a gas-phase outlet of the second condensation and separation device is connected with an inlet of the propane cooler (20) through a fuel gas pipe, and outlets of the propane cooler (20) corresponding to the inlet are connected with a fuel gas output pipe; the second compressor set (19) is connected with one inlet of the second condensation and separation device, and the outlet of the second condensation and separation device corresponding to the inlet is connected with the second compressor set (19); the second compressor unit (19) is also connected with a propane low-temperature separator (17), a liquid-phase propane outlet of the propane low-temperature separator (17) is connected with an inlet of the cold box (14), an outlet of the cold box (14) corresponding to the inlet is connected with the propane low-temperature separator (17), and a gas-phase outlet of the propane low-temperature separator (17) is connected with the second compressor unit (19).

Technical Field

The invention relates to the field of natural gas, in particular to a process and a system for producing light hydrocarbon and LNG from oilfield associated gas.

Background

The oil field associated gas is an important product in the process of petroleum production, is a mixed gas containing various alkanes, mainly contains methane and ethane, contains a small amount of propane, butane or pentane, belongs to clean energy, and has huge loss and environmental pollution if not recovered. The oil field associated gas resource of China is very goodIt is considerable. At present, most of domestic small oil fields adopt a torch combustion process for treating the oilfield associated gas, and although the treatment method reduces the harm of the oilfield associated gas to a certain extent, on one hand, the treatment method causes resource waste, and on the other hand, although the paraffin ratio is reduced after combustion, but CO is reduced₂The ratio rises. Based on this, the recycling of the associated gas has great market value and social benefit, and is vital to ensure that the oil gas has higher comprehensive utilization efficiency and strengthen the research of the associated gas recycling technology.

At present, the treatment mode after the recovery of associated gas mainly comprises the following aspects: 1) generating power by using the associated gas; 2) the method is used for recovering light hydrocarbon, and the associated gas of the oil field is deethanized to prepare light hydrocarbon and liquefied gas; for example, Tony company develops a cold liquid reflux technology, can efficiently recover LPG from associated gas in an oil field, and has over 95 percent of propane recovery rate and lower energy consumption. Peru PTP company adopts expansion refrigeration technology, and the recovery rate of C3+ C4 is 98%. 3) The LNG or CNG is processed on site and is externally applied as energy of novel environment-friendly automobiles, and the LNG or CNG mainly relates to offshore oil fields. 4) And recovering the methane, and preparing the acetylene by using the methane. 5) And recovering natural gas in the oilfield associated gas to be used as a raw material for synthesizing ammonia.

In the field of associated gas recovery technology, conventional typical processes represented by condensation fractionation, cold oil absorption, and self-produced condensate refrigeration are currently formed, but these processes produce LPG, NGL, and dry gas only, and the recovered dry gas is used as fuel, pipeline, or CNG.

Chinese invention patent ZL201510538190.7 discloses a system and a method for preparing two mixed refrigerants by using products of LNG/LPG/NGL recovered from petroleum associated gas, the system and the method utilize self products (dry gas, LPG and nitrogen-rich tail gas) as raw materials, and the system and the method are mixed according to a specific proportion to prepare two mixed refrigerants of shallow cooling and deep cooling so as to realize the purposes of shallow cooling and deep cooling, and an LPG cooling box and an LNG cooling box are independently arranged. Because two cold boxes are adopted for refrigeration, the process flow is complex, and the equipment investment cost is high.

The invention patent ZL201510145834.6 in China discloses a fine heavy hydrocarbon removal system for recovering LNG/LPG/NGL from petroleum associated gas, which adopts light components of condensate C1-C6 obtained through low-temperature separation as an absorbent to absorb heavy hydrocarbons above C5+ in the high-temperature associated gas, so that the heavy hydrocarbons above C5+ in the associated gas are lower than 70ppm and the fine heavy hydrocarbon removal system is used for recovering LNG/LPG/NGL from the petroleum associated gas. The system is a heavy hydrocarbon removal system for recovering LNG or LPG or NGL from petroleum associated gas, and the LPG, stable light hydrocarbon and LNG can not be obtained by the system. In addition, the system adopts grading temperature reduction, and the cold box is more complicated.

From the above, at present, no process or system for simultaneously producing light hydrocarbon (LPG and stable light hydrocarbon) and LNG by using oilfield associated gas is available at home and abroad.

Disclosure of Invention

The invention aims to solve the technical problem of providing a process and a system for producing light hydrocarbon and LNG by using oilfield associated gas, wherein the oilfield associated gas is used for simultaneously recovering the light hydrocarbon (LPG and stable light hydrocarbon) and the LNG.

The technical scheme of the invention is as follows:

a process for producing light hydrocarbon and LNG by using oilfield associated gas is characterized by comprising the following steps: the oil field associated gas enters a washing tower after being subjected to decarburization and dehydration, is completely or partially cooled by a cooling box after being contacted with washing liquid, is subjected to gas-liquid separation after being cooled, the separated liquid phase is used as the washing liquid, and the separated gas phase is an LNG product; the heavy hydrocarbon discharged from the washing tower enters a de-ethane tower for rectification, then enters a liquefied gas tower for rectification after heat exchange with light hydrocarbon discharged from the liquefied gas tower, the gas discharged from the liquefied gas tower is cooled and separated, one part of the separated liquid phase returns to the liquefied gas tower, and the rest part is an LPG product; and performing heat exchange on the product discharged from the bottom of the liquefied gas tower to obtain a light hydrocarbon product.

Preferably, the gas discharged from the demethanizer is condensed and separated into a gas phase and a liquid phase, the separated liquid phase is refluxed to the demethanizer, and the gas phase is used as a fuel gas.

Further preferably, the method further comprises the following steps: the propane refrigerant is pressurized by a compressor unit, and then is cooled by a propane cooler to carry out gas-liquid separation, the separated gas returns to the inlet of the compressor unit, the liquid phase fluid is divided into two paths, one path of liquid phase propane enters a propane low-temperature separator to carry out gas-liquid separation, the separated liquid phase provides cold energy for the cold box and then returns to the propane low-temperature separator, and the separated gas phase propane and the gas phase propane returning to the propane low-temperature separator after heat exchange return to the compressor unit; and the other path of liquid-phase propane provides cold energy for cooling the gas discharged by the ethane-removing tower, and the gas-phase propane after heat exchange returns to the compressor unit.

Further preferably, the method further comprises the following steps: and the gas phase separated by cooling the gas discharged from the ethane-removing tower is discharged after being reheated by the propane cooler and used as fuel gas.

Preferably, the method further comprises the following steps: and the mixed refrigerant enters the cold box after being pressurized by another compressor unit, returns to the cold box for further deep cooling after being precooled and throttled and reduced in pressure, returns to the cold box for reheating after being throttled and reduced in pressure, and then goes out of the cold box and returns to the other compressor unit.

A system for producing light hydrocarbon and LNG from oilfield associated gas comprises a low-temperature washing tower with an oilfield associated gas inlet, a low-temperature separator and a cold box; the device is characterized in that an outlet port of the low-temperature washing tower is connected with one inlet of a cold box, an outlet of the cold box corresponding to the inlet is connected with an inlet of a low-temperature separator, a first outlet of the low-temperature separator is connected with the low-temperature washing tower through a pipeline with a liquid-phase circulating pump, a second outlet of the low-temperature separator is connected with the other inlet of the cold box through a pipeline, and an outlet of the cold box corresponding to the other inlet is connected with an LNG product output pipe; the low-temperature washing tower is connected with a demethanizer ethane tower which is connected with a liquefied gas tower feeding preheater or the demethanizer ethane tower is connected with a liquefied gas tower feeding preheater through a demethanizer ethane tower reboiler; the liquefied gas tower feeding preheater is connected with the feeding end of the liquefied gas tower; one discharge end of the liquefied gas tower is connected with the liquefied gas tower feeding preheater or one discharge end of the liquefied gas tower is connected with the liquefied gas tower feeding preheater through a liquefied gas tower reboiler; the liquefied gas tower feeding preheater is directly connected with a stable light hydrocarbon product output pipe or is connected with the stable light hydrocarbon product output pipe through a stable light hydrocarbon product cooler; the other discharge end of the liquefied gas tower is connected with a first condensation separation device, the LPG output end of the first condensation separation device is connected with an LPG product output pipe, and the LPG output end is also connected with the liquefied gas tower; the system also includes a second condensate separation unit coupled to the demethanizer, the liquid phase outlet of the second condensate separation unit coupled to the demethanizer via a demethanizer reflux pump.

Preferably, the first condensation and separation device comprises a liquefied gas overhead condenser and a liquefied gas overhead separator which are connected with each other; the LPG output end is the output end of the liquefied gas tower top separator.

Preferably, said second condensate separation unit comprises a demethanizer overhead condenser and a demethanizer overhead separator connected to each other; the demethanizer is connected with one inlet of the demethanizer overhead condenser, the demethanizer overhead condenser and the outlet corresponding to the inlet are connected with the demethanizer overhead separator, and the liquid phase outlet of the demethanizer overhead separator is connected with the demethanizer through a demethanizer reflux pump.

Preferably, the system further comprises a first compressor string; the first compressor unit is connected with one inlet of the cold box, and an outlet of the cold box corresponding to the inlet is connected with the first compressor unit.

Preferably, the system further comprises a second compressor train; a propane cooler is connected in the second compressor unit; the gas-phase outlet of the second condensation separation device is connected with an inlet of the propane cooler through a fuel gas pipe, and an outlet of the propane cooler corresponding to the inlet is connected with a fuel gas output pipe; the second compressor set is connected with an inlet of the second condensation and separation device, and an outlet of the second condensation and separation device corresponding to the inlet is connected with the second compressor set; the second compressor unit is also connected with a propane low-temperature separator, a liquid-phase propane outlet of the propane low-temperature separator is connected with an inlet of the cold box, an outlet of the cold box corresponding to the inlet is connected with the propane low-temperature separator, and a gas-phase outlet of the propane low-temperature separator is connected with the second compressor unit.

The invention has the positive effects that:

the invention provides a process for producing light hydrocarbon and LNG from oilfield associated gas, aiming at the defect that the oilfield associated gas can not simultaneously produce light hydrocarbon (LPG and stable light hydrocarbon) and LNG in the prior art, and the LPG, the stable light hydrocarbon and the LNG with high added values are obtained. Compared with the method for simply recovering light hydrocarbon or producing LNG, the method has the advantages of low investment cost, large operation flexibility, capability of removing a heat exchanger which is repeatedly used, reduction of pressure drop between two units, and full utilization of heat energy and cold energy.

Compared with an independent production process, the invention designs the NGL recovery and the LNG liquefaction into one process, can remove a repeatedly used heat exchanger, reduces the pressure drop between two units, reduces the energy consumption, reduces equipment, saves the occupied area and reduces the investment.

The process has high operation flexibility. When the production capacity is reduced, the efficiency of the mixed refrigerant cycle can be maintained by changing the composition of the mixed refrigerant or reducing the suction pressure; when the composition of raw gas to be liquefied is changed, the natural gas can be efficiently liquefied by adjusting the composition of mixed refrigerant or the suction and discharge pressure of a mixed refrigerant compressor;

by adopting the process, the recovery rate of the propane can reach more than 98 percent.

Drawings

FIG. 1 is a process flow diagram of an embodiment of the invention.

In the figure: the system comprises a 1-low-temperature washing tower, a 2-demethanization ethane tower, a 3-demethanization ethane tower top condenser, a 4-demethanization ethane tower top separator, a 5-demethanization ethane tower reflux pump, a 6-demethanization ethane tower reboiler, a 7-liquefied gas tower feeding preheater, an 8-liquefied gas tower, a 9-liquefied gas tower top condenser, a 10-liquefied gas tower top separator, an 11-liquefied gas tower reflux pump, a 12-liquefied gas tower reboiler, a 13, a stable light hydrocarbon product cooler, a 14-cold box, a 15-low-temperature separator, a 16-liquid phase circulating pump, a 17-propane low-temperature separator, a 19-second compressor unit and a 20-propane cooler.

Detailed Description

The invention is further illustrated with reference to the following figures and examples.

Referring to fig. 1, a system embodiment of the present invention includes a cryogenic scrubber 1, a demethanizer 2, a liquefied gas column 8, a cryogenic separator 15, a demethanizer overhead condenser 3, and a cold box 14. The low-temperature washing tower 1 is provided with an air inlet port used as an inlet of oilfield associated gas (which is subjected to decarburization and dehydration before entering the system), an air outlet port, a washing liquid inlet end and a heavy hydrocarbon outlet, the air outlet port is connected with the first inlet of the cold box 14 through a pipeline, and the air outlet port of the low-temperature washing tower 1 can also be connected with a fuel gas pipe through a pipeline. The first outlet of the cold box 14 is connected with the inlet of the low-temperature separator 15 through a pipeline, the first outlet of the low-temperature separator 15 is connected with the washing liquid inlet end through a pipeline with a liquid-phase circulating pump 16, the second outlet of the low-temperature separator 15 is connected with the fourth inlet of the cold box 14 through a pipeline, and the fourth outlet of the cold box 14 is connected with an LNG product output pipe.

The ethane removing tower 2 is provided with a heavy hydrocarbon inlet, a methane and ethane containing gas outlet and a reflux port, and is also provided with a bottom end discharge port. The heavy hydrocarbon export passes through the tube coupling the heavy hydrocarbon entry, contains methane and ethane gas outlet connection and removes the first entry of ethane tower top condenser 3, and the first exit linkage of removing the ethane tower top condenser 3 removes the ethane tower top separator 4, and the liquid phase export of removing the ethane tower top separator 4 passes through the ethane tower reflux pump 5 of removing the methyl and connects the reflux opening. The demethanizer overhead condenser 3 and the demethanizer overhead separator 4 constitute a second condensate separation unit. The demethanizer 2 may also be connected to a demethanizer reboiler 6.

The bottom discharge port of the demethanizer 2 or the product outlet of the demethanizer reboiler 6 is connected with the feed port of the liquefied gas tower 8 through a liquefied gas tower feeding preheater 7, the upper end discharge port of the liquefied gas tower 8 is connected with a liquefied gas tower top separator 10 through a liquefied gas tower top condenser 9, and the liquefied gas tower top condenser 9 and the liquefied gas tower top separator 10 form a first condensation separation device.

The discharge port of the liquefied gas tower top separator 10 is connected with a liquefied gas tower reflux pump 11 through a pipeline, and the outlet end of the liquefied gas tower reflux pump 11 is respectively connected with the reflux port of the liquefied gas tower 8 and an LPG product output pipe through pipelines. The liquefied gas overhead separator 10 is also connected with a blow-down pipe.

The liquefied gas tower 8 can also be connected with a liquefied gas tower reboiler 12, and a bottom discharge hole of the liquefied gas tower 8 or a product outlet of the liquefied gas tower reboiler 12 is connected with a stable light hydrocarbon product output pipe through a liquefied gas tower feeding preheater 7 and an optional stable light hydrocarbon product cooler 13.

Embodiments of the present system also include a first compressor string 18, with the separator of the first compressor string 18 being piped to the second inlet of the cold box 14, and the second outlet of the cold box 14 being piped to the surge tank of the first compressor string 18.

Embodiments of the system further comprise a second compressor string 19, with a propane cooler 20 connected between the cooler and the separator of the second compressor string 19. The separator of the second compressor unit 19 is connected with the second inlet of the ethane-demethylating overhead condenser 3 through a pipeline, and the second outlet of the ethane-demethylating overhead condenser 3 is connected with the buffer tank of the second compressor unit 19 through a pipeline; the separator of the second compressor unit 19 is further connected with a propane low-temperature separator 17 through a pipeline, a liquid-phase propane outlet of the propane low-temperature separator 17 is connected with a third inlet of the cold box 14 through a pipeline, and a third outlet of the cold box 14 is connected with a gas-phase inlet of the propane low-temperature separator 17 through a pipeline. The gas phase outlet of the propane low-temperature separator 17 is connected with the buffer tank of the second compressor unit 19.

The gas phase outlet of the demethanizer overhead separator 4 is connected with an inlet of the propane cooler 20 through the fuel gas pipe, and the outlet of the propane cooler 20 corresponding to the inlet is connected with a fuel gas output pipe through a pipeline.

The embodiment of the method of the invention is as follows:

and (3) the oil field associated gas enters from the lower part of the low-temperature washing tower 1 after being subjected to decarburization and dehydration, reversely contacts with washing liquid entering from the upper part of the tower, and is washed to remove heavy hydrocarbon in the purified natural gas. The pressure of the low-temperature scrubber 1 was 4.8 MPaG. The heavy hydrocarbon enters the ethane-removing tower 2 from the bottom of the low-temperature washing tower 1 through decompression. The natural gas at the top of the low-temperature washing tower 1 can completely enter the cold box, and part of the natural gas can enter the cold box if more fuel gas is required to be output. In this embodiment, part of the gas enters the cold box, and the part of the gas which does not enter the cold box is mixed with the gas at the top of the demethanizer overhead separator 4, reheated by the propane cooler 20 and discharged as fuel gas. The natural gas from the top of the low-temperature washing tower 1 is pre-cooled in a cold box 14, the pre-cooled natural gas enters a low-temperature separator 15 after being pre-cooled to-55 ℃, a separated low-temperature liquid phase is sent to the low-temperature washing tower 1 through a liquid phase circulating pump 16 to be used as washing liquid, separated gas-phase natural gas returns to the cold box 14 to be further cooled to-162 ℃ under the action of refrigerant, and the gas-phase natural gas is decompressed to be used as an LNG product.

The heavy hydrocarbon from the bottom of the low-temperature washing tower 1 enters a demethanizer 2 and is rectified under the action of a reboiler 6 of the demethanizer, the pressure of the demethanizer 2 is 2.5MPaG, the gas containing methane and ethane is separated from the top of the tower and then enters a condenser 3 of the demethanizer top to be cooled to-32 ℃ (the condenser of the demethanizer top can adopt a plate heat exchanger or a shell-and-tube heat exchanger), then the gas enters a separator 4 of the demethanizer top to be separated, the separated liquid phase is refluxed into the demethanizer 2 through a reflux pump 5 of the demethanizer, and the gas at the top of the separator 4 of the demethanizer top is reheated by a propane cooler 20 and then discharged to be used as fuel gas. The reboiler 6 of the ethane eliminating tower adopts an electric heater or a heat conducting oil heater.

The heavy hydrocarbon at the bottom of the ethane removing tower 2 is subjected to heat exchange with stable light hydrocarbon at the bottom of a liquefied gas tower through a liquefied gas tower feeding preheater 7, the heavy hydrocarbon is heated to 80 ℃ and enters the liquefied gas tower 8, rectification is performed by utilizing a heat source provided by a liquefied gas tower reboiler 12, the pressure of the liquefied gas tower 8 is 1.5MPaG, LPG is separated from the top of the liquefied gas tower, the LPG is cooled to 55 ℃ through a liquefied gas tower top condenser 9 (a cold source can be circulating water or air) and then enters a liquefied gas tower top separator 10, a trace amount of gas phase separated from the top of the liquefied gas tower top separator 10 is sent to an air-discharging subsystem, LPG at the bottom of the liquefied gas tower top separator 10 is pressurized through a liquefied gas tower reflux pump 11, a part of the LPG is sent to the liquefied gas tower 8, a part of the LPG is used as a product, and. The stable light hydrocarbon at the bottom of the liquefied gas tower 8 is cooled to 130 ℃ after heat exchange with the heavy hydrocarbon by the liquefied gas tower feeding preheater 7, and is further cooled to 45 ℃ to be discharged by the stable light hydrocarbon product cooler 13 (the cold source can be circulating water or air) to be used as a stable light hydrocarbon product.

The mixed refrigerant refrigerating system adopts closed circulation and is an independent closed system. Methane, ethane, propane, nitrogen and ethylene are used as mixed refrigerant, the mixed refrigerant is pressurized to 1.9MPaG by a first compressor unit 18, enters a cold box 14, is pre-cooled to-40 ℃, returns to the cold box 14 after throttling and pressure reduction, is further deep-cooled to-150 ℃, returns to the cold box 14 after throttling and pressure reduction for reheating, and exits from the cold box 14 to 30 ℃, and is subjected to stepwise condensation and throttling expansion in the cold box 14 to obtain refrigerating capacity of different temperature levels. The above process provides cold for the liquefaction of natural gas.

Propane is used as a refrigerant, after the refrigerant is pressurized to 1.3MPaG by a second compressor unit 19, the refrigerant enters a separator in the compressor unit for gas-liquid separation after being cooled by a cooler in the propane compressor unit, gas discharged by the separator in the compressor unit returns to an inlet of the compressor unit, liquid phase fluid is divided into two paths, one path of liquid phase propane is decompressed and enters a propane low-temperature separator 17 for gas-liquid separation, the separated liquid phase propane provides precooling cold energy for a cold box 14, and gas phase propane after the separator and the gas phase propane after heat exchange return to the second compressor unit 19; the other path of liquid-phase propane provides cold energy for the condenser 3 at the top of the ethane-removing tower, and the propane after heat exchange returns to the second compressor unit 19.