CN101392980B - Re-liquefying method for shipping liquefaction ethylene/ethane vaporized gas - Google Patents

Abstract

The invention relates to a reliquefying method for shipped liquefied ethylene/ethane vapor. The method comprises the following steps of two stage compression of the ethylene/ethane vapor, successive cooling of the ethylene/ethane vapor by seawater and a refrigerant, phase-change latent heat cooling of ethylene/ethane, and re-liquefied storage of the ethylene/ethane vapor in a tank. The method utilizes phase-change latent heat of the ethylene/ethane and the refrigerant for heat exchange, supplies the cooling capacity through cycle refrigeration of the refrigerant, and re-liquefies and recoversthe ethylene/ethane vapor. In the refrigeration cycle of the method, low-temperature return flow is mixed and returned to a cold box so as to cool the refrigerant, so that the method fully utilizes the cooling capacity of the return flow, and improves the recovery utilization rate of energy; and the temperature difference of seawater of the technological method before and after heat exchange accords with the environmental protection rule.

Description

A kind of reliquefaction method of shipping liquefied ethylene or ethane boil-off gas

technical field

The invention relates to the technical field of reliquefaction of boil-off gas (BOG) of liquefied petrochemical products, and specifically refers to a method for reliquefaction of BOG generated by heat during transportation and unloading when liquefied ethylene or ethane is shipped.

Background technique

The ethylene industry is a symbol to measure the scale and development of a country's petrochemical industry. No matter whether it is a developed country or a developing country, all conditions will try their best to promote the construction and development of the ethylene industry. With the sustained and rapid development of my country's petrochemical industry and national economy, the demand for ethylene continues to increase. In 2007, my country's ethylene consumption was 21.69 million tons, while ethylene production was 10.47 million tons, and the self-sufficiency rate was only 48.3%. It is estimated that my country's ethylene production capacity will reach 16 million tons in 2010, and the ethylene demand equivalent will reach 27.35 million tons. The satisfaction rate is 58.5%, and it still needs to rely on a large amount of imported ethylene to meet the demand.

The cost of cracking raw materials accounts for a large proportion of the cost of ethylene, and the regions rich in natural gas in the world use ethane and propane in cheap natural gas as cracking raw materials to produce ethylene. The Middle East (such as Saudi Arabia) is rich in resources. The local ethane price is US$37.5/t, and 85% of the ethylene raw materials are ethane and propane, making its ethylene production cost as low as US$100/t. However, my country's ethylene industry is limited by resources. 90% of domestic ethylene raw materials come from refineries, and the raw materials are heavy, mostly naphtha. The production cost of ethylene is as high as 530 US dollars/t. In order to meet the demand for ethylene raw materials, my country's dependence on foreign oil was further increased in 2010. Selecting appropriate imported ethane to supplement the shortage of ethylene raw materials has significant social and economic significance for reducing domestic ethylene production costs and alleviating the shortage of oil resources.

In the world, low-temperature tankers are often used for commercial shipping of ethylene. For example, developed countries such as the United States import liquid ethylene from the Middle East and ship it back to China. The volume of liquefied ethylene at -104°C under normal pressure is about 1/490 of that of gaseous ethylene under normal pressure, and the volume after liquefaction is greatly reduced, which is more beneficial to the storage and transportation of ethylene. However, due to the intrusion of the low-temperature ethylene storage tank by the heat of the external environment during sea transportation, part of the mechanical energy of the submerged pump in the ethylene liquid cargo tank is converted into heat energy, which will cause the liquid ethylene in the tank to vaporize and produce flash steam, that is, BOG (boiloff -gas) gas. The generation of boil-off gas will increase the temperature of the liquid cargo ethylene in the cargo tank, resulting in an increase in the pressure of the cargo tank. Excessive cargo tank pressure can damage the structure of the cargo tank and pose a danger to its maintenance system. But if it is allowed to be discharged into the air, it is not only dangerous, but also economically uneconomical. Therefore, considering the safety and economy of ship transportation, it is necessary to re-liquefy the evaporated gas to recover this part of BOG. Ethane has similar properties to ethylene. Under normal pressure, liquid ethane with the same weight as ethane gas has a temperature of -88°C and a volume of only 1/435. In the sea transportation of ethane, there are also problems with the sea transportation of ethylene, and it is also necessary to re-liquefy and recover the evaporated ethane gas.

The process of reliquefaction of the steam generated during the transportation of liquid cargo, the current literature "Reliquefaction Principle and Reliquefaction Device Overview of Cryogenic Liquefied Gas Carrier" (Dai Gan, Li Pinyou. Reliquefaction Principle and Reliquefaction Device Overview of Cryogenic Liquefied Gas Carrier. Deep There are related technical reports in Cold Technology, 2005(1):6-9). Relevant technologies include: the reliquefaction device on multi-purpose cryogenic liquefied gas ship mentioned in "Reliquefaction Principle and Reliquefaction Device Overview of Cryogenic Liquefied Gas Carrier", which is mainly used to liquefy ethylene, petroleum gas and ammonia to meet different needs Cargo requirements. The device adopts a cascade direct refrigeration cycle, that is, an open cycle with liquid cargo as the refrigerant medium and a closed refrigeration cycle with R22 refrigerant connected in series. In the refrigerant closed cycle of the device, the refrigerant gas is mixed with the return gas of the subsequent process, compressed by the compressor, and then directly enters the refrigerant condenser for cooling. The refrigerant is cooled entirely by seawater, and the heat exchange load is relatively large. The cooled refrigerant is divided into two streams, one is decompressed by the throttle valve and then enters the heat exchanger to cool the other refrigerant, and the refrigerant gas after heat exchange is used as return gas. After cooling, another stream of refrigerant enters the condensing evaporator to cool the BOG gas, and the refrigerant gas after heat exchange enters the primary compressor to form a cycle.

In the closed refrigeration cycle of the R22 refrigerant in the reliquefaction device introduced in this article, R22 is used as the refrigerant. Because of its ozone depletion index of 0.055, R22 has been listed as one of the controlled substances in the Montreal Protocol, and has been internationally recognized. Chlorine-containing refrigerants such as R22 and R502 have stipulated a ban period. In addition, in the R22 condenser in the reliquefaction process of the device, the cooling of the refrigerant basically relies on seawater, and the heat exchange load is large, which increases the investment cost. Moreover, due to the restriction of the temperature of the imported seawater, the cooling temperature of the refrigerant is also restricted.

Contents of the invention

The purpose of the present invention is to solve the shortcomings in the above-mentioned prior art, and provide a process for reliquefying BOG produced during the shipping process of liquefied ethylene or ethane. The method utilizes the latent heat of phase change between ethylene or ethane and the refrigerant for heat exchange, the cooling capacity is provided by the refrigerant cycle refrigeration, and the ethylene or ethane BOG is reliquefied and recovered. In the refrigerant R404A refrigeration cycle of this method, the low-temperature backflow gas is mixed and then returned to the cold box to cool the refrigerant, which fully utilizes the cooling capacity of the backflow gas, improves the recovery and utilization rate of energy, and the process method The temperature difference before and after seawater heat exchange complies with environmental regulations.

The object of the present invention is achieved through the following technical solutions, a method for reliquefaction of shipping liquefied ethylene or ethane boil-off gas, comprising the following specific steps:

Two-stage compression of ethylene or ethane BOG in the first step

The BOG gas from the ethylene or ethane liquid cargo tank on board enters the primary compressor for compression, mixes with the low-temperature reflux gas, and then enters the secondary compressor for compression to obtain high-temperature and high-pressure ethylene or ethane gas.

Second step cooling of ethylene or ethane BOG

The high-temperature and high-pressure ethylene or ethane gas after two-stage compression first enters the seawater heat exchanger, is cooled by heat exchange with seawater, and then enters the ethylene or ethane condenser, and is cooled to a lower temperature by heat exchange with refrigerant.

Phase-change latent heat cooling of ethylene or ethane in the third step

The ethylene or ethane from the ethylene or ethane condenser is divided into two streams, one stream enters the flash tower for adiabatic flash evaporation, and the ethylene or ethane gas phase output from the top of the flash tower is used as low-temperature reflux gas. The ethylene or ethane liquid phase output at the bottom is used as a cold source, and enters the heat exchanger to use its own latent heat of phase change to cool another ethylene or ethane to further reduce the temperature, and the ethylene or ethane gas after heat exchange is also As low temperature reflux gas.

The fourth step is the tank storage of ethylene or ethane BOG reliquefaction

The low-temperature ethylene or ethane cooled by the heat exchanger enters another flash tower for adiabatic flash evaporation, and the ethylene or ethane gas phase output from the top of the flash tower returns to the first-stage compressor in the first step, and the first-stage compressor at the bottom of the tower Ethylene or ethane, that is, the liquid phase output after the BOG of the liquid cargo tank is reliquefied, enters the ethylene or ethane liquid cargo tank for storage.

In the first step of the reliquefaction method for shipping liquefied ethylene or ethane boil-off gas, the refrigerant is R404A.

In the first step of the reliquefaction method for shipping liquefied ethylene or ethane boil-off gas, the refrigerant is provided by a refrigeration cycle, and the working process of the refrigeration cycle includes the following steps:

Two-stage compression of refrigerant gas in the first step

After the refrigerant gas is compressed by the first-stage compressor, it enters the seawater heat exchanger, and part of the heat is removed by exchanging heat with seawater, and then enters the second-stage compressor for compression to obtain high-temperature and high-pressure refrigerant gas. After the seawater under normal pressure is boosted by the pump, it is divided into two parts to provide cooling capacity for the refrigerant. One part enters the seawater heat exchanger at the outlet of the first-stage compressor, and the other part enters the seawater heat exchanger at the outlet of the second-stage compressor.

Cooling of the refrigerant in the second step

After two-stage compression, the high-temperature and high-pressure refrigerant gas first enters the seawater heat exchanger, cools down, and then enters the cold box, and further cools down by exchanging heat with the returning low-temperature refrigerant gas.

Phase-change latent heat cooling of the third-step refrigerant

The refrigerant coming out of the cold box is divided into two streams. One stream enters the flash tower for adiabatic flash evaporation. The material is used as a cold source, using its own latent heat of evaporation to cool another refrigerant to further reduce its temperature. The refrigerant gas after heat exchange also enters the cold box as low-temperature reflux gas to provide cooling capacity for the compressed refrigerant.

The fourth step is the reflux cooling of the refrigerant

The low-temperature refrigerant cooled by the latent heat of evaporation of the liquid-phase refrigerant itself enters another flash tower for adiabatic flash evaporation, and the refrigerant gas phase from the top of the flash tower enters the cold box as a low-temperature reflux gas and becomes compressed refrigerant Provide cooling capacity; the refrigerant liquid phase discharge at the bottom of the tower enters the ethylene or ethane condenser to cool the ethylene or ethane BOG. The refrigerant that provides the cooling capacity turns into a gas phase, which is also used as a low-temperature reflux gas.

After the four streams of low-temperature return air described in the third and fourth steps are mixed, they enter the cold box to provide cooling capacity for the compressed refrigerant, and the return air after heat exchange enters the first-stage compressor to form a closed refrigeration system. agent circulation system.

In the first step, the temperature range of the primary compressor feed is -102°C to -85°C, the pressure range is 0.11MPa to 0.12MPa, and the pressure is raised to 0.52MPa to 0.56MPa; the pressure of the second stage compressor is raised to 1.72MPa ~1.86MPa.

In the third step of the above reliquefaction method for shipping liquefied ethylene or ethane evaporated gas, the pressure range of the flash tower is 0.52MPa-0.56MPa.

In the fourth step of the above reliquefaction method for shipping liquefied ethylene or ethane evaporated gas, the pressure range of the flash tower is 0.11MPa-0.12MPa.

Compared with the prior art, the present invention has the following advantages and beneficial effects:

1. In the ethylene or ethane BOG reliquefaction system of the present invention, after the feed is boosted by the first-stage compressor, it is mixed with the low-temperature return gas of the subsequent process to reduce the inlet temperature of the second-stage compressor, thereby increasing the pressure of the second-stage compressor. The compression ratio of the compressor can reduce the power consumption of the compressor and save the operating cost of the process.

2. The refrigerant of the present invention is preferably R404A. R404A is an environmentally friendly refrigerant with zero ozone depletion index, non-toxic, high safety, small temperature glide, and belongs to near azeotropic refrigerant. The pressure and volume refrigeration capacity are quite comparable to R22.

3. In the fourth step of the refrigerant refrigerating cycle system of the present invention, multiple streams of low-temperature backflow air are mixed and then enter the cold box to provide cooling capacity for the compressed refrigerant; this process fully utilizes the cooling capacity of the backflow air, The temperature of the refrigerant is further lowered to realize the recovery and utilization of the energy of the low-temperature refrigerant; at the same time, the consumption of the circulating refrigerant is also saved, thereby saving the investment cost.

4. According to the current situation of my country's ethylene industry, the present invention can not only improve the composition of my country's ethylene raw materials, reduce energy consumption and production costs, but also can Alleviating the shortage of my country's oil resources, it has significant social and economic value. The energy utilization of the ethylene or ethane BOG reliquefaction process of the present invention is reasonable, so the liquefaction efficiency is high, and at the same time, the circulation amount of the refrigerant is small, the refrigeration effect is good, and the operation cost is low.

Description of drawings

Fig. 1 is a process flow diagram of the present invention.

Detailed ways

The present invention will be further described in detail below in conjunction with the embodiments and the accompanying drawings, but the embodiments of the present invention are not limited thereto.

As shown in Fig. 1, what the following specific examples describe is a liquid ethylene or ethane carrier with a capacity of 6400M ³ adopting the method of the present invention to carry out ethylene or ethane BOG reliquefaction. According to the design requirements of liquid cargo tanks, the daily BOG evaporation rate of the ethylene or ethane liquid cargo tanks of the carrier is about 0.2-0.38% of the total liquid cargo. The amount of BOG produced by the loading or unloading of alkane; the BOG reliquefaction system of the carrier must be able to handle the total amount of BOG of 1,500kg/h, so as to maintain the constant pressure of the liquid cargo tank with a capacity of ^6400M3 ; and the The BOG reliquefaction system should also have the capability that the temperature rise of the ethylene or ethane in the liquid cargo tank should not exceed 4°C within 90 hours, that is, the temperature of the liquid cargo tank for ethylene can change from -102°C to -98°C within 90 hours. ability. The unit equipment of the present invention includes a seawater pump, a BOG compressor and a refrigerant compressor, a heat exchanger and a flash tower. The specific working steps and process conditions are as follows:

Embodiment one: reliquefaction of liquid ethylene BOG

Two-stage compression of ethylene BOG in the first step

The 1500Kg/hr, -102.0°C, 0.11MPa ethylene BOG gas evaporated from the ship's ethylene liquid cargo tank enters the primary compressor 1 and is compressed to 0.52MPa, and the temperature rises from -102°C to -0.7°C. The ethylene BOG from the outlet of the first-stage compressor is mixed with the return gas S3 and S5 of the subsequent process, cooled to -13.1°C, and entered into the second-stage compressor 2 for compression, the pressure rises to 1.72MPa, and the temperature is 83.9°C.

The second step is the cooling of ethylene BOG

The cooling device includes a seawater heat exchanger 3 and an ethylene condenser 4; the high-temperature and high-pressure ethylene gas compressed in two stages first enters the seawater heat exchanger 3 and exchanges heat with seawater. The temperature drops from 83.9°C to 38°C, and the pressure drops to 1.69 MPa. Then enter the ethylene condenser 4 to exchange heat with refrigerant S12, the temperature drops from 38°C to -38°C, and the pressure drops to 1.59MPa. The 5400Kg/hr seawater under normal pressure is pumped up to 0.50MPa, and then enters the seawater heat exchanger 3, the temperature rises from 32°C to 40.3°C, and the pressure drops to 0.30MPa.

The third step is the phase change latent heat cooling of ethylene

The ethylene coming out of the ethylene condenser 4 enters the splitter and is divided into two strands, one of which S1 enters the flash tower 5 at 418Kg/hr for adiabatic flash evaporation, and the 86Kg/hr separated from the top of the tower has a temperature and pressure of -70 ℃, 0.52MPa ethylene gas as low-temperature reflux gas S3. The ethylene liquid phase output S4 at the bottom of the tower is 332Kg/hr, and enters the heat exchanger 6 to use its own latent heat of evaporation to cool another 1830Kg/hr ethylene BOG S2 to -66°C, the pressure drops to 1.52MPa, and the ethylene gas after heat exchange It is also used as low-temperature reflux gas S5.

Step 4 Ethylene BOG reliquefaction into tank storage

The 1830Kg/hr low-temperature ethylene cooled by the heat exchanger 6 enters another flash tower 7 for adiabatic flash evaporation, and the gas separated from the top of the tower is 330Kg/hr, the temperature and pressure are respectively -102°C and 0.11MPa, and return The inlet of the primary compressor is mixed with the ethylene BOG gas evaporated from the ethylene liquid cargo tank and compressed. The output of the liquid phase at the bottom of the tower is 1500Kg/hr and enters the ethylene liquid cargo tank for storage.

In the above method, the second step refrigerant R404A is provided by a refrigeration cycle, and the steps of the refrigeration cycle include:

Two-stage compression of refrigerant gas in the first step

7673Kg/hr, 30.3°C, 0.13MPa refrigerant gas enters the primary compressor 8, is compressed to 0.59MPa, the temperature rises from 30.3°C to 96°C, enters the seawater heat exchanger 9 to exchange heat with seawater, and the temperature drops to 38°C ℃, the pressure drop is 0.56MPa. Then enter the secondary compressor 10, compress to 1.91MPa, and the temperature is 95.8°C. Seawater under normal pressure of 56556Kg/hr is pumped up to 0.50MPa, enters the splitter and is divided into two streams, one enters the seawater heat exchanger 9, the flow rate is 13117Kg/hr, the temperature rises from 32°C to 39.6°C, and the pressure drops to 0.30MPa. Another stream enters the seawater heat exchanger 11, and the flow rate is 43439Kg/hr.

Cooling of the refrigerant in the second step

The cooling device includes a seawater heat exchanger 11 and a cold box 12 . The high-temperature and high-pressure refrigerant gas compressed in two stages first enters the seawater heat exchanger 11 to exchange heat with seawater, the temperature drops from 95.8°C to 38°C, and the pressure drops to 1.88MPa. The temperature of seawater rose from 32°C to 39.6°C, and the pressure dropped to 0.30MPa. The refrigerant cooled by seawater enters the cold box 12 again, and the temperature drops to 2°C by exchanging heat with the returning low-temperature refrigerant gas S13 in the fourth step.

Phase-change latent heat cooling of the third-step refrigerant

The refrigerant coming out of the cold box 12 enters the splitter and is divided into two streams, and one stream of S6 enters the flash tower 13 at 2087Kg/hr for adiabatic flash evaporation. The gas separated from the top of the tower is 610Kg/hr, the temperature and pressure are -41.5°C and 0.13MPa respectively, and it is used as low-temperature reflux gas S8. The refrigerant liquid phase discharge S9 at the bottom of the tower is 1477Kg/hr, enters the heat exchanger 14, and uses its own latent heat of evaporation to cool another 5586Kg/hr refrigerant S7 to -38°C, and the pressure drops to 1.78MPa. The refrigerant gas after heat exchange is used as low-temperature return gas S10.

The fourth step is the reflux cooling of the refrigerant

The 5586Kg/hr low-temperature refrigerant cooled by the heat exchanger 14 enters another flash tower 15 for adiabatic flash evaporation. The gas coming out from the top of the flash tower is 144Kg/hr, and the temperature and pressure are respectively -41.62°C, 0.13MPa, as low-temperature reflux gas S11. The liquid phase discharge of refrigerant at the bottom of the tower is 5442Kg/hr, and the compressed ethylene BOG is cooled from 38°C to -38°C in the heat exchanger 4 by using its own latent heat of evaporation, and the refrigerant gas after heat exchange is used as low-temperature reflux gas S14. The above four streams of low-temperature return gas S8, S10, S11 and S14 are mixed to form return low-temperature refrigerant gas S13, the temperature and pressure are respectively -40.8°C, and 0.13MPa enters the cold box, which is used as a cold source to cool the compressed refrigerant gas. The temperature of the refrigerant gas that provides the cooling capacity rises from -40.8°C to 30.3°C, and then enters the first-stage compressor for compression 8, forming a refrigerant cycle system.

Embodiment two: reliquefaction of liquid ethane BOG

Step 1 Two-stage compression of ethane gas

The 1480Kg/hr, -85.0°C, 0.12MPa ethane BOG evaporated from the ship's ethane liquid cargo tank enters the primary compressor 1 and is compressed to 0.56MPa, and the temperature rises from -85.0°C to 2.8°C. The ethane BOG from the outlet of the primary compressor is mixed with the return gas S3 and S5 of the subsequent process, cooled to -6.1°C, and enters the secondary compressor 2, the pressure rises to 1.86MPa, and the temperature is 75.4°C.

The second step cooling of ethane BOG

The cooling device includes a seawater heat exchanger 3 and an ethane condenser 4; the high-temperature and high-pressure ethane gas compressed in two stages first enters the seawater heat exchanger 3 and exchanges heat with seawater. The temperature drops from 75.4°C to 40°C, and the pressure drop It is 1.82MPa. Then enter the ethane condenser 4 to exchange heat with refrigerant S12, the temperature drops from 40°C to -20°C, and the pressure drops to 1.75MPa. The 5044Kg/hr seawater under normal pressure is pumped up to 0.50MPa, then enters the seawater heat exchanger 3, the temperature rises from 32°C to 40°C, and the pressure drops to 0.30MPa.

Step 3 Phase change latent heat cooling of ethane

The ethane that comes out from ethane condenser 4 enters splitter and is divided into two strands, wherein stream S1 enters flash tower 5 with 400Kg/hr and carries out adiabatic flash evaporation, and the 80Kg/hr that separates from tower top, temperature, pressure are respectively -49.7°C, 0.56MPa ethane gas as low-temperature reflux gas S3. The output S4 of the ethane liquid phase at the bottom of the tower is 320Kg/hr, and enters the heat exchanger 6 to cool another stream of ethane BOG S2 of 1877Kg/hr to -45°C by using its own latent heat of evaporation, and the pressure drops to 1.69MPa. After heat exchange The ethane gas is also used as low-temperature reflux gas S5.

Step 4: Tank storage of ethane BOG reliquefaction

The 1877Kg/hr low-temperature ethane cooled by the heat exchanger 6 enters another flash tower 7 for adiabatic flash evaporation, and the gas separated from the top of the tower is 397Kg/hr, and the temperature and pressure are respectively -85°C and 0.12MPa. Returning to the inlet of the primary compressor, it is mixed with the ethane BOG gas evaporated from the ethane liquid cargo tank and compressed. The output of the liquid phase at the bottom of the tower is 1480Kg/hr and enters the ethane liquid cargo tank for storage.

In the above method, the refrigerant R404A in the second step is provided by a refrigeration cycle, and the steps of the refrigeration cycle are the same as those in the first embodiment.

The specific implementation described above is a preferred embodiment of the present invention, and cannot limit the claims of the present invention. Any other changes or other equivalent replacement methods that do not deviate from the technical solution of the present invention are included in the present invention. within the scope of protection.

Claims (6)

1. A method for reliquefaction of shipping liquefied ethylene or ethane boil-off gas, characterized in that it comprises the following steps: Two-stage compression of ethylene or ethane boil-off gas in the first step The evaporated gas of ethylene or ethane from the ethylene or ethane liquid cargo tank on board enters the primary compressor for compression, mixes with the low-temperature reflux gas, and then enters the secondary compressor for compression to obtain high-temperature and high-pressure ethylene or ethane gas; The second step is the cooling of ethylene or ethane boil-off gas After two-stage compression, the high-temperature and high-pressure ethylene or ethane gas first enters the seawater heat exchanger, is cooled by heat exchange with seawater, and then enters the ethylene or ethane condenser, and is cooled to a lower temperature by heat exchange with the refrigerant; Phase-change latent heat cooling of ethylene or ethane in the third step The ethylene or ethane gas from the ethylene or ethane condenser is divided into two streams, one stream enters the flash tower for adiabatic flash evaporation, and the ethylene or ethane gas phase output from the top of the flash tower is used as low-temperature reflux gas, The ethylene or ethane liquid phase output at the bottom of the tower is used as a cold source, and enters the heat exchanger to use its own phase change latent heat to cool another ethylene or ethane to further reduce the temperature. The ethylene or ethane gas after heat exchange Also as low temperature reflux gas; The fourth step is the tank storage of ethylene or ethane vaporized gas reliquefaction The low-temperature ethylene or ethane cooled by the heat exchanger enters another flash tower for adiabatic flash evaporation, and the ethylene or ethane gas phase output from the top of the flash tower returns to the first-stage compressor in the first step, and the first-stage compressor at the bottom of the tower Ethylene or ethane, that is, the liquid phase output after reliquefaction of the evaporated gas of the liquid cargo tank, enters the ethylene or ethane liquid cargo tank for storage; 2. The method for reliquefaction of ship-shipped liquefied ethylene or ethane boil-off gas according to claim 1, characterized in that: in the first step, the temperature range of the feedstock of the primary compressor is -102°C to - 85°C, the pressure range is 0.11MPa-0.12MPa, and the pressure is increased to 0.52MPa-0.56MPa; the secondary compressor is increased to 1.72MPa-1.86MPa. 3. The method for reliquefaction of ship-shipped liquefied ethylene or ethane evaporated gas according to claim 1, characterized in that: in the second step, the refrigerant is R404A. 4. The reliquefaction method of shipping liquefied ethylene or ethane boil-off gas according to claim 1, characterized in that: in the second step, the refrigerant is provided by a refrigeration cycle, and the refrigeration cycle consists of the following steps accomplish: Two-stage compression of refrigerant gas in the first step After the refrigerant gas is compressed by the primary compressor, it enters the seawater heat exchanger, and part of the heat is removed by exchanging heat with seawater, and then enters the secondary compressor for compression to obtain high-temperature and high-pressure refrigerant gas; seawater under normal pressure passes through the pump After the pressure is raised, it is divided into two parts to provide cooling capacity for the refrigerant; one part enters the seawater heat exchanger at the outlet of the first-stage compressor, and the other part enters the seawater heat exchanger at the outlet of the second-stage compressor; Cooling of the refrigerant in the second step After two-stage compression, the high-temperature and high-pressure refrigerant gas first enters the seawater heat exchanger at the outlet of the second-stage compressor, cools down, and then enters the cold box, and further cools down by exchanging heat with the returning low-temperature refrigerant gas; Phase-change latent heat cooling of the third-step refrigerant The refrigerant coming out of the cold box is divided into two streams. One stream enters the flash tower for adiabatic flash evaporation. The material is used as a cold source, using its own latent heat of evaporation to cool another refrigerant to further reduce its temperature; the refrigerant gas after heat exchange also enters the cold box as a low-temperature return gas to provide cooling capacity for the compressed refrigerant; The fourth step is the reflux cooling of the refrigerant The low-temperature refrigerant cooled by the latent heat of evaporation of the liquid-phase refrigerant itself enters another flash tower for adiabatic flash evaporation, and the refrigerant gas phase from the top of the flash tower enters the cold box as a low-temperature reflux gas and becomes compressed refrigerant Provide cooling capacity; the refrigerant liquid phase discharge at the bottom of the tower enters the ethylene or ethane condenser to cool the ethylene or ethane evaporated gas; and the refrigerant that provides cooling capacity becomes a gas phase, which is also used as a low-temperature reflux gas; After the four streams of low-temperature return air in the third and fourth steps are mixed, they enter the cold box to provide cooling capacity for the compressed refrigerant, and the return air after heat exchange enters the first-stage compressor to form a closed refrigerant. circulation system. 5. The reliquefaction method for shipping liquefied ethylene or ethane boil-off gas according to claim 4, characterized in that: in the third step of the reliquefaction method for shipping liquefied ethylene or ethane boil-off gas, the The pressure range of the flash tower is 0.52MPa～0.56MPa. 6. The reliquefaction method for shipping liquefied ethylene or ethane boil-off gas according to claim 4, characterized in that: in the fourth step of the reliquefaction method for shipping liquefied ethylene or ethane boil-off gas, the The pressure range of the flash tower is 0.11MPa～0.12MPa.

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