Circulating precooling method and system for low-temperature liquefied hydrocarbon discharging header pipe boil-off gas

Abstract

The invention discloses a low-temperature liquefied hydrocarbon discharging header pipe boil-off gas circulating precooling method and a system thereof.The system comprises a discharging facility which is connected with a feeding pipe of a liquefied hydrocarbon storage tank through a discharging header pipe, and the liquefied hydrocarbon storage tank is provided with a liquid discharging pipe and a boil-off gas discharging pipe; a boil-off gas discharging pipe of the liquefied hydrocarbon storage tank is connected with a compressor through a first liquid separation tank, a first outlet pipeline of the compressor is connected with a downstream boil-off gas pipeline, and a second outlet pipeline of the compressor is connected with the boil-off gas discharging pipe through a main discharging pipe, a communication crossover line, a gas return pipe or a cold insulation circulation pipeline. According to the method, the low-temperature evaporation gas is used for precooling the discharging header pipe, the mode that liquid-phase low-temperature liquefied hydrocarbon is directly adopted for precooling the discharging header pipe in a traditional scheme is replaced, and the problem that the temperature difference between the bottom and the top of the discharging header pipe is too large, and consequently the pipe is bent and deformed is solved.

Description

Technical Field

[0001] The invention relates to a pipeline low-temperature pre-cooling collar, in particular to a low-temperature liquefied hydrocarbon unloading main pipe evaporation gas circulation pre-cooling method and system thereof. Background Art

[0002] Large-scale cryogenic liquefied hydrocarbon storage areas and LNG receiving stations are increasingly being constructed in my country's coastal regions. These stations primarily receive LNG, cryogenic ethylene, ethane, propane, propylene, and other cryogenic liquefied hydrocarbons transported by ocean-going vessels as feedstock or fuel.

[0003] As high-quality port resources become increasingly scarce, some project sites face the challenge of long distances between unloading terminals in the water and land-based storage areas. This often refers to the pre-cooling schemes for the unloading mainline at conventional LNG receiving stations, drawing on some LNG circulation and cold storage solutions. This is particularly true for peak-shaving LNG receiving stations with low unloading frequencies. Continuous operation introduces significant heat and requires extensive boil-off gas processing, increasing operating costs. In practice, to conserve energy, companies often intermittently operate the cold storage cycle, using cryogenic liquefied hydrocarbons only to pre-cool the unloading mainline before a ship arrives. During the outage, the unloading main pipe has returned to normal temperature. When the low-temperature liquefied hydrocarbon circulation is used again to pre-cool the unloading main pipe, the following problems arise: 1) The temperature of the low-temperature liquefied hydrocarbon from the storage tank cannot be adjusted. When it is directly introduced into the unloading main pipe, the cooling rate is uncontrollable; 2) When liquid low-temperature liquefied hydrocarbon flows into the unloading main pipe, the action of gravity will cause a significant temperature difference between the top and bottom of the pipeline (the bottom temperature is low and the top temperature is high). When the temperature difference exceeds the safe allowable range, local high stress problems are likely to occur. In severe cases, it may even cause the pipeline to bend and deform, increasing the hidden dangers to the safe operation of the unloading main pipe and the risks of pre-cooling operations.

[0004] CN116928582A discloses a nitrogen circulation pipeline precooling system and method for an LNG receiving station. The system utilizes nitrogen after pipeline precooling and adopts a mixed method of nitrogen circulation and liquid nitrogen injection to precool the pipeline. To ensure pressure, an external blower is required to pressurize and circulate the nitrogen and provide power for nitrogen circulation. This solution is only applicable to precooling operations of unloading pipelines and low-pressure external transmission pipelines in LNG receiving station projects before they are put into production.

[0005] Therefore, in order to solve the problem of cold circulation operation of the unloading main pipes of the peak-shaving LNG receiving stations and low-temperature liquefied hydrocarbon storage areas with low unloading frequency and small turnover, how to make the most of existing facilities and conditions to design a low-cost pre-cooling solution for the low-unloading main pipe has become a technical difficulty in the design of such projects. Summary of the Invention

[0006] The purpose of the present invention is to solve the above-mentioned technical problems and provide a low-temperature liquefied hydrocarbon unloading main pipe evaporation gas circulation precooling system that is suitable for solving the cold circulation operation problem of the peak-shaving LNG receiving station and the low-temperature liquefied hydrocarbon storage area unloading main pipe in operation, effectively making full use of existing facilities and conditions, and having low transformation and operation costs and good safety and reliability.

[0007] Another object of the present invention is to provide a method for circulating precooling of boil-off gas from a cryogenic liquefied hydrocarbon unloading main, which has an extremely simple process, does not require the introduction of external gas, has good safety and reliability, and has low modification and operating costs.

[0008] The present invention includes a discharge facility connected to a feed pipe of a liquefied hydrocarbon storage tank via a discharge main pipe, the liquefied hydrocarbon storage tank is provided with a liquid discharge pipe and a vapor discharge pipe, the vapor discharge pipe of the liquefied hydrocarbon storage tank is connected to a compressor via a first liquid separator, a first outlet pipe of the compressor is connected to a downstream vapor discharge pipe, and a second outlet pipe of the compressor is connected to the vapor discharge pipe via a discharge main pipe, a connecting cross-line, a return air pipe or a pre-cooling circulation pipeline.

[0009] The liquid discharge pipe of the liquefied hydrocarbon storage tank is respectively connected to the low-temperature liquefied hydrocarbon transmission pipeline and the post-spray device on the second outlet pipeline of the compressor.

[0010] The post-spray device is a BOG temperature reduction valve.

[0011] The second outlet pipe of the compressor is connected to the unloading main pipe through the second liquid separator.

[0012] A pre-cooling switch valve is provided on the pipeline connecting the second liquid separation tank to the discharge main pipe, a feed switch valve is provided on the pipeline connecting the discharge main pipe to the feed pipe of the liquefied hydrocarbon storage tank, and a connecting switch valve is provided on the connecting jumper.

[0013] The liquid discharge pipe of the liquefied hydrocarbon storage tank is also connected to the evaporated gas discharge pipe via a front spray device.

[0014] The front spray device is a BOG temperature reduction valve.

[0015] The unloading facility includes at least one or more unloading arms connected in parallel.

[0016] A temperature sensor is provided on the unloading main pipe, and a circulating gas flow control valve is provided on the pipeline connecting the compressor and the unloading main pipe.

[0017] The feed pipe of the liquefied hydrocarbon storage tank includes an upper feed pipe and a lower feed pipe connected in parallel.

[0018] A return liquid pipe is also connected between the pre-cooling circulation pipeline and the unloading main pipe, and a return liquid valve is provided on the return liquid pipe. Considering that the pre-cooling circulation pipeline does not have an air return function when it is set, a return liquid pipe is set to keep the pre-cooling circulation pipeline in a cold state during unloading. During unloading, part of the material can pass through the pre-cooling circulation pipeline and the return liquid pipe and then flow into the unloading main pipe, thereby preventing the pre-cooling circulation pipeline from returning to normal temperature.

[0019] The unloading main pipe is provided with multiple pairs of temperature sensors, and each pair of temperature sensors is arranged symmetrically on the upper and lower parts of the pipeline. A circulating gas flow control valve is provided on the pipeline connecting the compressor and the unloading main pipe. The pre-cooling speed of the top and bottom of the unloading main pipe can be monitored in real time through the temperature sensor, and then the flow of the evaporative gas for pre-cooling sent into the unloading main pipe is adjusted through the circulating gas flow control valve, and the pre-cooling temperature of the evaporative gas is adjusted through the post-spray device to meet the cooling speed requirement of the unloading main pipe pre-cooling.

[0020] The present invention provides a method for circulating precooling boil-off gas from a low-temperature liquefied hydrocarbon unloading main pipe, using the above-mentioned system. The process includes: feeding boil-off gas from a liquefied hydrocarbon storage tank into a first liquid separator via a boil-off gas discharge pipe for gas-liquid separation; the separated boil-off gas is pressurized by a compressor and then divided into two streams. One stream of boil-off gas is introduced into a downstream boil-off gas pipeline via a first outlet pipe of the compressor; the other stream of boil-off gas is cooled by atomized and sprayed low-temperature liquefied hydrocarbons through a second outlet pipe of the compressor, and then introduced into a precooling pipeline of the unloading main pipe. The boil-off gas is then passed through a connecting crossover line, a return gas pipe, or a cold-keeping circulation pipeline together with boil-off gas from the liquefied hydrocarbon storage tank, separated by gas and liquid in the first liquid separator, and then returned to the compressor. The low-temperature liquefied hydrocarbon in the liquefied hydrocarbon storage tank is drawn out through the liquid discharge pipe and divided into two streams. The first stream of low-temperature liquefied hydrocarbon is sent to the low-temperature liquefied hydrocarbon external transmission pipeline, and the second stream of low-temperature liquefied hydrocarbon is sent to the second outlet pipe of the compressor for atomization and spraying to cool the other stream of evaporation gas.

[0021] The other evaporation gas is sprayed into the low-temperature liquefied hydrocarbon by atomization through the second outlet pipe of the compressor and then cooled down by the second liquid separator for gas-liquid separation. The separated evaporation gas is introduced into the pre-cooling pipe of the discharge main pipe.

[0022] The low-temperature liquefied hydrocarbon in the liquefied hydrocarbon storage tank is drawn out through the liquid discharge pipe and divided into three streams. The first stream of low-temperature liquefied hydrocarbon is sent to the low-temperature liquefied hydrocarbon external transmission pipeline, the second stream of low-temperature liquefied hydrocarbon is sent to the second outlet pipe of the compressor and atomized and sprayed to cool the other stream of evaporation gas; and the third stream is sent to the evaporation gas discharge pipe and atomized and sprayed to cool the evaporation gas before entering the compressor.

[0023] After the pre-cooling of the unloading main pipe is completed, the pre-cooling switch valve on the pipeline connecting the second liquid separation tank to the unloading main pipe and the connecting switch valve on the connecting jumper are closed, and the feed switch valve on the pipeline connecting the unloading main pipe and the feed pipe of the liquefied hydrocarbon storage tank is opened. The low-temperature liquefied hydrocarbon from the unloading facility is fed into the liquefied hydrocarbon storage tank through the feed pipe of the liquefied hydrocarbon storage tank on the unloading main pipe.

[0024] The cryogenic liquefied hydrocarbon is LNG, cryogenic ethylene, ethane, propane or propylene.

[0025] This application addresses the low frequency of ship unloading at peak-shaving LNG receiving stations or cryogenic liquefied hydrocarbon storage areas, the long unloading main pipeline, and the periodic pre-cooling characteristics. By utilizing the boil-off gas from the cryogenic liquefied hydrocarbon storage tank after being pressurized by a compressor, a portion of cryogenic liquid liquefied hydrocarbon is simultaneously introduced. The atomized cryogenic liquid liquefied hydrocarbon is sprayed into the system through an additional spray device. The boil-off gas, which is then cooled and pressurized, is then used to pre-cool the unloading main pipeline. The boil-off gas is then recycled back to the boil-off gas treatment system and re-enters the compressor inlet, achieving the purpose of circulating the cryogenic boil-off gas to pre-cool the unloading main pipeline. This solution has the following advantages: 1. This system pre-cools the discharge main pipe by extracting some low-temperature evaporation gas, replacing the traditional method of directly using liquid-phase cryogenic liquefied hydrocarbons to pre-cool the discharge main pipe. This system also addresses the potential for insufficient evaporation gas cooling capacity by introducing some lower-temperature cryogenic liquid liquefied hydrocarbons, which are then sprayed in through atomization to rapidly cool the evaporation gas. Both the evaporation gas used for pre-cooling the pipe and the cryogenic liquid liquefied hydrocarbons used to cool the evaporation gas are sourced from existing cryogenic liquefied hydrocarbon storage tanks, eliminating the need for external gas. This system offers energy savings, low costs, and excellent safety and reliability. It is suitable for receiving stations using various cryogenic liquefied hydrocarbons, such as LNG, cryogenic ethylene, ethane, propane, or propylene, as feedstock or fuel.

[0026] 2. By controlling the amount of cryogenic liquid and liquefied hydrocarbons sprayed in through atomization, the cooling temperature of the BOG can be flexibly adjusted. Furthermore, compared to liquid-phase precooling, gas-phase precooling offers better uniformity in the pipeline and controllable precooling efficiency. This replaces the traditional approach of directly precooling the discharge main with liquid-phase cryogenic liquefied hydrocarbons. This effectively avoids pipe bending and deformation caused by a large temperature difference between the bottom and top of the discharge main, providing excellent safety and reliability. The front and rear spray devices described in the present invention can be various atomizers or valves with atomization functions, preferably a BOG temperature reduction valve, which combines flow control and temperature atomization mixing effects, allowing for flexible adjustment of the injection amount of cryogenic liquid and liquefied hydrocarbons to achieve rapid and uniform BOG cooling. Furthermore, since the temperature of the BOG returning after precooling the discharge main increases, a front spray device is provided in front of the buffer tank V01 at the compressor inlet to spray atomized cryogenic liquefied hydrocarbons for cooling, thereby meeting the intake temperature requirements of the compressor.

[0027] 3. Since part of the evaporated gas is pressurized by the compressor in the original facility, the existing compressor is cleverly used to provide driving force for gas phase pre-cooling. Therefore, there is no need to set up complex pressurization and control equipment such as blowers. The pre-cooling gas volume and temperature are adjustable, and the gas is directly recycled after the pre-cooling unloading main pipe, with low investment, transformation and operation costs.

[0028] 4. Only a connecting cross-line is added to achieve the circulation of pre-cooled evaporative gas, without the need for additional piping configurations. By setting corresponding switch valves and corresponding controls, the unloading main pipe can be pre-cooled before each intermittent unloading, and normal unloading can be carried out after pre-cooling is completed. The operation is simple and convenient. The present invention can effectively control the cooling rate of the unloading main pipe, and the temperature difference between the top and bottom of the pipe meets the regulatory requirements. It is simple, safe, and reliable, effectively achieving energy conservation and consumption reduction, and can meet the needs of peak-shaving LNG receiving stations and low-temperature liquefied hydrocarbon storage areas for periodic pre-cooling of the unloading main pipe. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is the system diagram of the present invention, i.e. the pre-cooling state flow chart of Example 1.

[0030] Figure 2 This is a schematic diagram of the unloading state of Example 1.

[0031] Figure 3 This is a flow chart of the pre-cooling state of Example 2 of the present invention.

[0032] Figure 4 This is a schematic diagram of the unloading state of Example 2.

[0033] Among them, there are liquefied hydrocarbon storage tank T01; unloading arm UA01, unloading arm UA02, unloading arm UA03, unloading arm UA04, return air arm RGA01, first liquid separation tank V01, second liquid separation tank V02, compressor K01, compressor first outlet pipe K01-1, compressor second outlet pipe K01-2, return air arm switch valve KV001, unloading arm switch valve KV002A, unloading arm switch valve KV002B, unloading arm switch valve KV002C, unloading arm switch valve KV002D, connecting switch valve KV003, feed switch valve KV004, pre-cooling switch valve KV005, switch valve KV006, return liquid valve KV007, upper feed valve HV001, lower feed valve HV002, circulating gas flow control valve FV001; tank pump P01.

[0034] Feed pipe 1, upper feed pipe 1-1, lower feed pipe 1-2, liquid discharge pipe 2, evaporated gas discharge pipe 3, unloading main pipe 4, return gas pipe 5, connecting cross-line 6, evaporated gas downstream pipeline 7, low-temperature liquefied hydrocarbon transmission pipeline 8, pre-cooling circulation pipe 10, return liquid pipe 11. DETAILED DESCRIPTION

[0035] System Example 1: As Figure 1 shown in the embodiment, the unloading facilities located at the water terminal include four unloading arms UA01, UA02, UA03 and UA04 and a gas return arm RGA01, and the gas return arm RGA01 is connected to the gas return pipe 5. The land storage facilities include a liquefied hydrocarbon storage tank T01, wherein the liquefied hydrocarbon storage tank T01 is provided with a feed pipe 1, a liquid discharge pipe 2 and a boil-off gas discharge pipe 3, and the four unloading arms UA01, UA02, UA03 and UA04 are connected to the feed pipe 1 of the liquefied hydrocarbon storage tank T01 through the unloading main pipe 4.

[0036] The boil-off gas discharge pipe 3 of the liquefied hydrocarbon storage tank T01 is connected to the compressor K01 through the first liquid separation tank V01, the first outlet pipeline K01-1 of the compressor K01 is connected to the downstream boil-off gas pipeline 7, and the second outlet pipeline K01-2 is connected to the boil-off gas discharge pipe 3 through the second liquid separation tank V02, the unloading main pipe 4, the cross-line 6 and the gas return pipe 5.

[0037] The liquid discharge pipe 2 of the liquefied hydrocarbon storage tank T01 is connected to the low-temperature liquefied hydrocarbon pipeline 8, the front spray device (i.e. BOG temperature reducing valve TV001) on the boil-off gas discharge pipe 3 and the rear spray device (i.e. BOG temperature reducing valve TV002) on the second outlet pipeline K01-2 of the compressor K01, respectively, and part of the low-temperature liquefied hydrocarbon is introduced as a cooling medium to cool the boil-off gas before and after the compressor K01.

[0038] Further, according to the needs, valves are reasonably arranged on each pipeline: the four unloading arms UA01, UA02, UA03 and UA04 are respectively provided with four unloading arm on-off valves KV002A, KV002B, KV002C and KV002D, and the gas return arm RGA01 is provided with a gas return arm on-off valve KV001. The second liquid separation tank V02 is provided with a pre-cooling on-off valve KV005 on the pipeline connected to the unloading main pipe, the unloading main pipe is provided with a feed on-off valve KV004 on the pipeline connected to the feed pipe of the liquefied hydrocarbon storage tank, and the cross-line 6 is provided with a cross-line on-off valve KV003.

[0039] The unloading header 4 can be provided with multiple temperature sensors (such as surface thermometers), preferably multiple pairs of temperature sensors symmetrically arranged on the pipeline (see the skin thermometers TI-01A / B... TI-N-A / B provided on the unloading header 4 in the figure), and a circulating gas flow control valve FV001 is provided on the pipeline connecting the compressor and the unloading header (in this embodiment, the second outlet pipeline K01-2). The pre-cooling speed and temperature difference at the top and bottom of the unloading header 4 can be monitored in real time through the temperature sensors, and then the flow of evaporated gas sent to the unloading header 4 for pre-cooling is adjusted through the circulating gas flow control valve FV001, and the pre-cooling temperature of the evaporated gas is adjusted through the post-spray device (i.e. BOG temperature reducing valve TV002) to meet the cooling speed requirement of the pre-cooling of the unloading header 4.

[0040] The feed pipe 1 of the liquefied hydrocarbon storage tank T01 includes parallel upper feed pipe 1-1 and lower feed pipe 1-2, and upper feed valve HV001 and lower feed valve HV002 are respectively provided, and upper feed and / or lower feed can be selected.

[0041] The BOG temperature reducing valve TV001 and the BOG temperature reducing valve TV002 are integrated custom automatic control angle valves with atomizing spray holes.

[0042] Method embodiment 1: The following takes low-temperature liquefied hydrocarbon as LNG as an example to illustrate the pre-cooling and normal unloading process of the LNG receiving station before unloading: Unloading header pre-cooling: see Figure 1 , for pre-cooling the 24-46 inch, 500m-6000m long unloading header of the LNG receiving station, the end of the unloading header 4 at the water terminal side needs to be pre-cooled to 40-50℃ higher than the normal pressure boiling point of low-temperature liquefied hydrocarbon. In this embodiment, the low-temperature liquefied hydrocarbon is LNG, and the unloading header 4 should be pre-cooled to -110~-120℃ to have the condition of direct introduction of LNG.

[0043] Evaporation gas (BOG) in liquefied hydrocarbon storage tank T01 is led out through evaporation gas outlet pipe 3 and mixed with evaporation gas circulated back after being warmed up, and then the LNG (temperature about -160℃) atomized and sprayed out by the front spray device (i.e. BOG temperature reducing valve TV001) is cooled to -120~ -110℃ to meet the low temperature operation requirements of compressor K01, and after gas-liquid separation in the first separation tank V01, the evaporation gas is sent to compressor K01 to be pressurized to 0.8~0.9MPa, and is divided into two streams, one of which is 3000~6000Nm3 / h of pressurized evaporation gas (the other stream) sent to the second outlet pipeline K01-2 of compressor K01, and its flow rate can be adjusted by the circulation gas flow control valve FV001 located on the second outlet pipeline K01-2, and this part of evaporation gas is cooled to -130~ -150℃ by the LNG (temperature about -160℃) atomized and sprayed out by the rear spray device (i.e. BOG temperature reducing valve TV002), and then sent to the second liquid separation tank V02 for liquid separation, and the gas phase enters the pre-cooling pipeline of the discharge main pipe 4 through the pre-cooling switch valve KV005, and then circulates back to the evaporation gas outlet pipe 3 before the first liquid separation tank V01 through the connecting cross-line 6 and the gas return pipe 5, and is mixed with the evaporation gas from the liquefied hydrocarbon storage tank T01, and is cooled to -120~ -110℃ by the LNG (temperature about -160℃) atomized and sprayed out by the front spray device (i.e. BOG temperature reducing valve TV001) again, and is sent to the compressor K01 again, and the remaining part (one stream) of pressurized BOG enters the downstream process through the downstream evaporation gas pipeline 7.

[0044] At the same time, the LNG hydrocarbon in the liquefied hydrocarbon storage tank T01 is divided into three streams after the liquid outlet pipe 2, the first stream of LNG is sent to the LNG delivery pipeline 8, the second stream of LNG is sent to the second outlet pipeline K01-2 of the compressor for atomization and spraying, and is used to cool the evaporation gas of the pre-cooled discharge main pipe 4; and the third stream is sent to the evaporation gas outlet pipe 3 for atomization and spraying, and is used to cool the evaporation gas before entering the compressor K01 to meet the low temperature operation requirements of the compressor K01.

[0045] During the pre-cooling process of the discharge main pipe 4, the cooling speed is controlled at 5~10℃ / h, the temperature difference between the top and bottom of the pipeline is controlled within 50℃, and the pre-cooling circulating evaporation gas operating pressure is controlled between 0.01~0.5MPaG.

[0046] After the pre-cooling is completed, the normal discharge process is entered: Referring to Figure 2At this time, close the pre-cooling switch valve KV005, BOG cooling valve TV001, BOG cooling valve TV002, flow control valve FV001, and connecting switch valve KV003; open the unloading arm switch valve KV002A, unloading arm switch valve KV002B, unloading arm switch valve KV002C and unloading arm switch valve KV002D corresponding to the four unloading arms, and the return air switch valve KV001 on the return air arm RGA01, and at the same time open the feed switch valve KV004, upper feed valve HV001 and / or lower feed valve HV002.

[0047] LNG from the four unloading arms UA01, UA02, UA03, and UA04 flows into the unloading manifold 4. It then flows through the feed on-off valve KV004 and feed pipe 1, via the upper feed valve HV001 and / or lower feed valve HV002, into the liquefied hydrocarbon storage tank T01. A portion of the boil-off gas in the liquefied hydrocarbon storage tank T01 is drawn through the boil-off gas discharge pipe 3 and then returned to the return gas arm RGA01 via the return gas pipe 5.

[0048] The precooling method of the present invention can be used to precool the unloading main pipe safely and stably, avoiding potential risks and hidden dangers such as large temperature difference between the top and bottom of the pipe and uneven and controllable cooling rate caused by directly using traditional low-temperature liquid hydrocarbons to precool the unloading main pipe. At the same time, it can effectively achieve the purpose of energy saving, consumption reduction and operating cost reduction.

[0049] Method Example 2 See also Figure 3 The following takes cryogenic liquefied hydrocarbon, cryogenic ethane, as an example to describe in detail the pre-cooling and normal unloading process before unloading in a liquid export-type cryogenic ethane storage area without unloading return gas. In the system of this embodiment, the return gas arm RGA01 and the return gas pipe 5 are not provided. The second outlet pipeline K01-2 is connected to the evaporation gas discharge pipe 3 via the second liquid separator V02, the unloading main pipe 4, the connecting jumper 6, and the pre-cooling circulation pipe 10. A return liquid pipe 11 is further connected between the pre-cooling circulation pipeline 10 and the unloading main pipe 4. A return liquid valve KV007 is provided on the return liquid pipe 11. An on-off valve KV006 is provided on the pipeline connecting the pre-cooling circulation pipeline 10 and the BOG desuperheating valve TV001.

[0050] Precooling: See Figure 3 , used to pre-cool the 24-46 inch unloading main pipe in the low-temperature ethane storage area and 500m-6000m in length, the end of the unloading main pipe 4 on the water area side of the dock needs to be pre-cooled to 20-30°C higher than the boiling point of the low-temperature liquefied hydrocarbon at atmospheric pressure. In this embodiment, the low-temperature liquefied hydrocarbon is low-temperature ethane, so the unloading main pipe 4 should be pre-cooled to -60-70°C to meet the conditions for directly introducing low-temperature ethane.

[0051] The boil-off gas (BOG) in the liquefied hydrocarbon storage tank T01 is drawn out through the boil-off gas discharge pipe 3 and mixed with the heated boil-off gas after being recycled. It is then sprayed by the front spray device (i.e., the BOG temperature reduction valve TV001) and the low-temperature ethane (temperature is about -89°C) is cooled to -70~-80°C to meet the low-temperature operation requirements of the compressor K01. After gas-liquid separation in the first separation tank V01, the boil-off gas is sent to the compressor K01 to be pressurized to 1.7~1.9MPa, divided into two streams, and 3000~6000Nm 3 The pressurized BOG (the other stream) at a rate of / h is fed into the second outlet pipe K01-2 of compressor K01. Its flow rate is regulated by the flow control valve FV001 located on this second outlet pipe K01-2. This portion of BOG is cooled to -70--80°C by the low-temperature ethane (approximately -89°C) atomized and sprayed out by the post-spray device (i.e., BOG desuperheating valve TV002). After separation in the second liquid separator V02, it passes through the pre-cooling on / off valve KV005 and enters the pre-cooling pipeline of the discharge manifold 4. It is then circulated back through the pre-cooling circulation pipe 10 to the BOG discharge pipe 3 before the first liquid separator V01, where it mixes with the BOG in the liquefied hydrocarbon storage tank T01. It is then cooled again to -60--70°C by the low-temperature ethane (approximately -89°C) atomized and sprayed out by the pre-spray device (i.e., BOG desuperheating valve TV001), and is again fed into compressor K01. The remaining portion (the other stream) of pressurized BOG passes through the downstream BOG pipeline 7 and enters the downstream process.

[0052] At the same time, the cryogenic ethane in the cryogenic ethane storage tank T01 is divided into three streams after passing through the liquid discharge pipe 2. The first stream of cryogenic ethane is sent to the cryogenic liquefied hydrocarbon transmission pipeline 8, the second stream of cryogenic ethane is sent to the second outlet pipe K01-2 of the compressor and is atomized and sprayed out to cool the evaporation gas of another pre-cooling unloading main pipe 4; the third stream is sent to the evaporation gas discharge pipe 3 and is atomized and sprayed out to cool the evaporation gas before entering the compressor K01, so as to meet the low-temperature operation requirements of the compressor K01.

[0053] During the pre-cooling process of the discharge main pipe 4, the cooling rate is controlled at 5-10°C / h, the temperature difference between the top and bottom of the pipe is controlled within 50°C, and the operating pressure of the pre-cooling cycle BOG is controlled between 0.01-0.5 MPaG.

[0054] After pre-cooling is completed, the normal unloading process begins: See also Figure 4At this time, the pre-cooling switch valve KV005, the BOG temperature reducing valve TV001, the BOG temperature reducing valve TV002, the flow control valve FV001 and the switch valve KV006 on the pre-cooling circulation pipe 10 are closed; the discharge arm switch valves KV001A, KV001B, KV001C and KV001D corresponding to the four discharge arms are opened, and the communication switch valve KV003, the feed switch valve KV004, the liquid return valve KV007, the upper feed valve HV001 and / or the lower feed valve HV002 are opened.

[0055] The low-temperature ethane from the four discharge arms UA01, UA02, UA03 and UA04 is divided into two streams, except for part of the discharge header 4, the remaining part is introduced into the discharge header 4 through the communication cross-line 6 and the pre-cooling circulation pipe 10 through the liquid return pipe 11, and then is sent into the low-temperature ethane storage tank T01 through the feed switch valve KV004 and the feed pipe 1 through the upper feed valve HV001 and / or the lower feed valve HV002. The evaporated gas in the low-temperature ethane storage tank T01 is introduced through the evaporated gas discharge pipe 3, pressurized by the compressor and sent to the downstream.

Claims

1. A low-temperature liquefied hydrocarbon unloading main boil-off gas circulation pre-cooling system, comprising a discharge facility connected to a feed pipe of a liquefied hydrocarbon storage tank via a discharge main, the liquefied hydrocarbon storage tank being provided with a liquid discharge pipe and a boil-off gas discharge pipe, the boil-off gas discharge pipe of the liquefied hydrocarbon storage tank being connected to a compressor via a first liquid separator, characterized in that: The first outlet pipe of the compressor is connected to the downstream evaporation gas pipe, and the second outlet pipe of the compressor is connected to the evaporation gas discharge pipe via the unloading main pipe, the connecting jump line, the return air pipe or the pre-cooling circulation pipeline.

2. The low-temperature liquefied hydrocarbon unloading main boil-off gas circulation pre-cooling system according to claim 1, characterized in that: The liquid discharge pipe of the liquefied hydrocarbon storage tank is respectively connected to the low-temperature liquefied hydrocarbon transmission pipeline and the post-spray device on the second outlet pipeline of the compressor.

3. The low-temperature liquefied hydrocarbon unloading main boil-off gas circulation pre-cooling system according to claim 2, characterized in that: The post-spray device is a BOG temperature reduction valve.

4. The low-temperature liquefied hydrocarbon unloading main boil-off gas circulation pre-cooling system according to claim 2, characterized in that: The second outlet pipe of the compressor is connected to the unloading main pipe through the second liquid separator.

5. The low-temperature liquefied hydrocarbon unloading main boil-off gas circulation pre-cooling system according to claim 4, characterized in that: A pre-cooling switch valve is provided on the pipeline connecting the second liquid separation tank to the discharge main pipe, a feed switch valve is provided on the pipeline connecting the discharge main pipe to the feed pipe of the liquefied hydrocarbon storage tank, and a connecting switch valve is provided on the connecting jumper.

6. The low-temperature liquefied hydrocarbon unloading main boil-off gas circulation pre-cooling system according to any one of claims 1 to 5, characterized in that: The liquid discharge pipe of the liquefied hydrocarbon storage tank is also connected to the evaporated gas discharge pipe via a front spray device.

7. The low-temperature liquefied hydrocarbon unloading main boil-off gas circulation pre-cooling system according to claim 6, characterized in that: The front spray device is a BOG temperature reduction valve.

8. The low-temperature liquefied hydrocarbon unloading main boil-off gas circulation pre-cooling system according to any one of claims 1 to 5, characterized in that: The unloading facility includes at least one or more unloading arms connected in parallel.

9. The low-temperature liquefied hydrocarbon unloading main boil-off gas circulation pre-cooling system according to any one of claims 1 to 5, characterized in that: A liquid return pipe is further connected between the pre-cooling circulation pipeline and the unloading main pipe, and a liquid return valve is provided on the liquid return pipe.

10. The low-temperature liquefied hydrocarbon unloading main boil-off gas circulation pre-cooling system according to any one of claims 1 to 5, characterized in that: The unloading main pipe is provided with multiple pairs of temperature sensors, and each pair of temperature sensors is symmetrically arranged on the pipeline. A circulating air flow control valve is provided on the pipeline connecting the compressor and the unloading main pipe.

11. A method for circulating precooling of boil-off gas from a cryogenic liquefied hydrocarbon unloading main, characterized in that: The process includes sending boil-off gas from a liquefied hydrocarbon storage tank into a first liquid separator through a boil-off gas discharge pipe for gas-liquid separation. The separated boil-off gas is pressurized by a compressor and then divided into two streams. One stream of boil-off gas is introduced into a downstream boil-off gas pipeline through a first outlet pipe of the compressor, and the other stream of boil-off gas is cooled by atomized low-temperature liquefied hydrocarbons sprayed into the second outlet pipe of the compressor and then introduced into a pre-cooling pipeline of a discharge main pipe. The gas is then separated by gas and liquid in the first liquid separator together with the boil-off gas from the liquefied hydrocarbon storage tank through a connecting cross-line, a return gas pipe or a cold-keeping circulation pipeline and then returned to the compressor. The low-temperature liquefied hydrocarbon in the liquefied hydrocarbon storage tank is drawn out through the liquid discharge pipe and divided into two streams. The first stream of low-temperature liquefied hydrocarbon is sent to the low-temperature liquefied hydrocarbon external transmission pipeline, and the second stream of low-temperature liquefied hydrocarbon is sent to the second outlet pipe of the compressor for atomization and spraying to cool the other stream of evaporation gas.

12. The method for circulating precooling of boil-off gas from a cryogenic liquefied hydrocarbon unloading main as claimed in claim 11, characterized in that: The other evaporation gas is sprayed into the low-temperature liquefied hydrocarbon by atomization through the second outlet pipe of the compressor and then cooled down by the second liquid separator for gas-liquid separation. The separated evaporation gas is introduced into the pre-cooling pipe of the discharge main pipe.

13. The method for circulating precooling of boil-off gas from a cryogenic liquefied hydrocarbon unloading main according to claim 11 or 12, wherein: The low-temperature liquefied hydrocarbon in the liquefied hydrocarbon storage tank is drawn out through the liquid discharge pipe and divided into three streams. The first stream of low-temperature liquefied hydrocarbon is sent to the low-temperature liquefied hydrocarbon external transmission pipeline, the second stream of low-temperature liquefied hydrocarbon is sent to the second outlet pipe of the compressor and atomized and sprayed to cool the other stream of evaporation gas; and the third stream is sent to the evaporation gas discharge pipe and atomized and sprayed to cool the evaporation gas before entering the compressor.

14. The method for circulating precooling of boil-off gas from a cryogenic liquefied hydrocarbon unloading main according to claim 11 or 12, wherein: After the pre-cooling of the unloading main pipe is completed, the pre-cooling switch valve on the pipeline connecting the second liquid separation tank to the unloading main pipe and the connecting switch valve on the connecting jumper are closed, and the feed switch valve on the pipeline connecting the unloading main pipe and the feed pipe of the liquefied hydrocarbon storage tank is opened. The low-temperature liquefied hydrocarbon from the unloading facility is fed into the liquefied hydrocarbon storage tank through the feed pipe of the liquefied hydrocarbon storage tank on the unloading main pipe.

15. The method for circulating precooling of boil-off gas from a cryogenic liquefied hydrocarbon unloading main as claimed in claim 11, wherein: The cryogenic liquefied hydrocarbon is LNG, cryogenic ethylene, ethane, propane or propylene.

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