阅读说明：本技术 一种适应产气量波动的bog再液化系统及其工作方法 (BOG reliquefaction system adaptive to gas production fluctuation and working method thereof ) 是由 蒋庆峰 宋肖 谷家扬 蒋志勇 陈智同 李文娟 易冰 于 2021-08-19 设计创作，主要内容包括：本发明公开了一种适应产气量波动的BOG再液化系统以及工作方法。本发明所述的BOG再液化系统包括预冷系统、压缩系统以及过冷系统；所述预冷系统包括BOG泵、与BOG泵出口连接的第一多股流换热器,所述第一多股流换热器包括常压BOG预热流道、高压BOG预冷流道、BOG过冷流道以及低温LNG流道,所述常压BOG预热流道从中下部设置有第一流道,所述常压BOG预热流道从中部设置有第二流道。本发明在保证下游管网正常使用情况下,可针对基荷非卸船期、基荷卸船期、调峰非卸船期和调峰卸船期式中输出负荷变化,进行相应工况的优化调整,保证系统的精细化运行管理,从装置构成、能耗和运营成本等角度对BOG回收系统进行多工况下变负荷设计,适用于工况波动较大的接收站。(The invention discloses a BOG reliquefaction system adaptive to gas production fluctuation and a working method. The BOG reliquefaction system comprises a precooling system, a compression system and a supercooling system; the precooling system comprises a BOG pump and a first multi-stream heat exchanger connected with the outlet of the BOG pump, the first multi-stream heat exchanger comprises a normal-pressure BOG preheating flow channel, a high-pressure BOG precooling flow channel, a BOG supercooling flow channel and a low-temperature LNG flow channel, the normal-pressure BOG preheating flow channel is provided with a first flow channel from the middle lower part, and the normal-pressure BOG preheating flow channel is provided with a second flow channel from the middle part. Under the condition of ensuring normal use of a downstream pipe network, the invention can carry out optimization adjustment on corresponding working conditions aiming at output load changes in a base load non-unloading period, a base load unloading period, a peak-adjusting non-unloading period and a peak-adjusting unloading period, ensures refined operation management of the system, carries out variable load design on the BOG recovery system under multiple working conditions from the aspects of device composition, energy consumption, operation cost and the like, and is suitable for a receiving station with larger working condition fluctuation.)

1. A BOG reliquefaction system adapting to gas production fluctuation is characterized by comprising a precooling system (100), a compression system (200) and a supercooling system (300); the precooling system (100) comprises a BOG pump (26) and a first multi-stream heat exchanger (12) connected with an outlet of the BOG pump (26), the first multi-stream heat exchanger (12) comprises a normal-pressure BOG preheating flow channel (121), a high-pressure BOG precooling flow channel (122), a BOG supercooling flow channel (123) and a low-temperature LNG flow channel (124), the normal-pressure BOG preheating flow channel (121) is provided with a first flow channel (1211) from the middle lower part, and the normal-pressure BOG preheating flow channel (121) is provided with a second flow channel (1212) from the middle part;

the compression system (200) comprises a low-pressure BOG compression cooling unit and a high-pressure BOG compression cooling unit, the low-pressure BOG compression cooling unit comprises a low-pressure BOG compressor (5) and an inter-stage cooler (6), and the high-pressure BOG normal-temperature cooling unit comprises a high-pressure BOG compressor (9) and an after-cooler (10); the first flow channel (1211) is connected with an inlet of the after cooler (10), the second flow channel (1212) is communicated with an inlet of the interstage cooler (6), an outlet of the after cooler (10) and an outlet of the normal-pressure BOG pre-heating flow channel (121) are communicated with an inlet of the low-pressure BOG compressor (5), an outlet of the low-pressure BOG compressor (5) is communicated with an inlet of the interstage cooler (6), an outlet of the interstage cooler (6) is communicated with an inlet of the high-pressure BOG compressor (9), an outlet of the high-pressure BOG compressor (9) is communicated with an inlet of the after cooler (10), and an outlet of the after cooler (10) is communicated with an inlet of the high-pressure BOG pre-heating flow channel (122);

the subcooling system (300) comprises a second multi-stream heat exchanger (13), a gas-liquid separation tank (14), an economizer (18), a cooling medium compressor (19) and a heat exchanger (20); the second multi-stream heat exchanger (13) comprises a BOG liquefaction flow channel (131), a BOG backflow flow channel (132) and a cooling medium flow channel (133), wherein an inlet of the BOG liquefaction flow channel (131) is communicated with an outlet of a high-pressure BOG precooling flow channel (122), a gas-liquid separation tank (14) is arranged between the BOG liquefaction flow channel (131) and the BOG backflow flow channel (132), an economizer (18) is arranged at an outlet of the BOG backflow flow channel (132), a first outlet of the economizer (18) is communicated with an inlet of a BOG supercooling flow channel (123), a second outlet of the economizer (18) is communicated with the low-pressure BOG compressor (5), an outlet of the cooling medium flow channel (133) is communicated with an inlet of the cooler (20), and an outlet of the cooler (20) is communicated with an inlet of the low-temperature LNG flow channel (124).

2. BOG reliquefaction system according to claim 1, characterized in that a sixth three-way valve (8) is arranged between the outlet of the interstage cooler (6) and the inlet of the high pressure BOG compressor (9), and the outlet of the interstage cooler (6) is in communication with the outlet of the after cooler (10) via the sixth three-way valve (8).

3. The fluctuation in gas production compliant BOG reliquefaction system according to claim 2, wherein an outlet of the interstage cooler (6) and an outlet of the after cooler (10) are communicated with a third three-way valve (3) through a fifth three-way valve (7), an outlet of the atmospheric BOG preheat flow path (121) is communicated with a second three-way valve (2), and an outlet of the second three-way valve (2) is communicated with an inlet of the third three-way valve (3).

4. The fluctuation in gas production compliant BOG reliquefaction system according to claim 2, characterized in that an outlet of the sixth three-way valve (8) is in communication with an inlet of the high-pressure BOG pre-cooling flow passage (122).

5. BOG reliquefaction system according to claim 3 adapted to fluctuations in gas production, characterized in that the outlet of the third three-way valve (3) communicates with the inlet of a fourth three-way valve (4), the outlet of the fourth three-way valve (4) communicates with the inlet of a low pressure BOG compressor (5).

6. BOG reliquefaction system according to claim 1 adapted to fluctuations in gas production, characterized in that the outlet of the BOG return flow channel (132) is provided with an eighth three-way valve (15), and that two outlets of the eighth three-way valve (15) are in communication with two inlets of the economizer (18) via a first expansion valve (16) and a second expansion valve (17), respectively.

7. The fluctuation in gas production compliant BOG reliquefaction system according to claim 6, characterized in that the cooling medium flow passage (133) includes a third flow passage (1331), a fourth flow passage (1332) and a fifth flow passage (1333), outlets of the third flow passage (1331), the fourth flow passage (1332) and the fifth flow passage (1333) are respectively communicated with an inlet of a cooling medium compressor (19), and an outlet of the cooling medium compressor (19) is communicated with an inlet of a cooler (20); and the outlet of the cooler (20) is provided with a ninth three-way valve (21) connected with the inlet of the third flow passage (1331) and a thirteenth three-way valve (22) respectively connected with the inlet of the fourth flow passage (1332) and the inlet of the fifth flow passage (1333).

8. BOG reliquefaction system according to claim 7 adapted to gas production fluctuations characterized in that the inlet of the third flow channel (1331) is provided with a fifth expansion valve (25), the inlet of the fourth flow channel (1332) is provided with a fourth expansion valve (24) and the inlet of the fifth flow channel (1333) is provided with a third expansion valve (23).

9. The method of operating a BOG reliquefaction system according to claim 1, wherein the method comprises the steps of:

and (3) during the base load non-ship unloading period: when the LNG receiving station is in a base load non-unloading period, a supercooling system and an LNG cooling flow channel where a high-pressure BOG compressor (9), an after cooler (10) and a cooling medium compressor (19) are located in the system are all closed, and a BOG single-stage processing flow and a BOG after-processing flow are selected as a circulation flow of the system;

and (3) during the ship unloading period of the base load: when the LNG receiving station is in a basic loading and unloading period, a supercooling system in which a high-pressure BOG compressor (9), an after cooler (10) and a cooling medium compressor (19) are positioned is closed, and a BOG single-stage processing flow, a BOG post-processing flow and LNG preheating processing are selected as a circulation flow of the system;

and (3) during the peak-shaving non-ship-unloading period: when the LNG receiving station is in a peak-shaving non-ship-unloading period, a cooling LNG cooling flow channel (124) in the system is closed, and a BOG multi-stage processing flow, a BOG post-processing flow and a cooling medium circulation flow are selected as a circulation flow of the system;

adjusting the peak and unloading the ship: when the LNG receiving station is in the peak-shaving ship unloading period, all the processes in the system are started, and the circulation process of the system selects a BOG multi-stage processing process, a BOG post-processing process, a cooling medium circulation process and an LNG preheating processing process;

the BOG single-stage processing flow comprises the following steps: BOG enters a normal-pressure BOG preheating flow channel (121) in a first multi-flow heat exchanger (12) through a BOG pump (26), then flows in a middle part, passes through an interstage cooler (6) after passing through a part of BOG of a second flow channel (1212), cools the BOG after passing through a low-pressure BOG compressor (5), then passes through a fifth three-way valve (7), a third three-way valve (3) and a fourth three-way valve (4) to be mixed with the BOG sent out from the other part through an outlet of the BOG preheating flow channel (121), then enters a low-pressure BOG compressor (5) to be compressed, the compressed BOG exchanges heat in the interstage cooler (6), then passes through an outlet of the interstage cooler (6), passes through a sixth three-way valve (8) and a seventh three-way valve (11), then completely enters a high-pressure BOG precooling flow channel (122) in the first multi-flow heat exchanger (12), exchanges heat with the normal-pressure BOG preheating flow channel (121), and then enters a BOG liquefying flow channel (131) in a second multi-flow heat exchanger (13), the liquefied BOG enters a gas-liquid separation tank (14) after being subjected to heat exchange with a BOG backflow flow passage (132), and the liquefied BOG directly enters an LNG storage tank for storage;

the BOG multi-stage processing flow comprises the following steps: BOG enters a normal-pressure BOG preheating flow passage (121) in a first multi-flow heat exchanger (12) through a BOG pump (26), then is divided in a second flow passage (1211) in the middle part and a second flow passage (1212) in the middle-lower part, the BOG in the middle part flows through an interstage cooler (6) to cool the BOG after passing through a low-pressure BOG compressor (5), the BOG in the middle-lower part flows through an after cooler (10) to cool the BOG after passing through a high-pressure BOG compressor (9), the BOG in the middle part flows and the BOG in the middle-lower part flows are mixed with the rest BOG in a third three-way valve (3), then enters the low-pressure BOG compressor (5) through a fourth three-way valve (4) to be compressed, the compressed BOG exchanges heat in the interstage cooler (6), then completely enters the high-pressure BOG compressor (9) to be compressed, the compressed BOG exchanges heat in the after being cooled in the after cooler (10), and then completely enters the first multi-flow heat exchanger (12) through a seventh three-way valve (11) from an outlet of the after being exchanged heat exchanger, the heat-exchanged BOG enters a BOG liquefaction flow channel (131) in the second multi-stream heat exchanger (13), exchanges heat with a BOG backflow flow channel (132) and then enters a gas-liquid separation tank (14), and the liquefied BOG directly enters an LNG storage tank for storage;

the BOG post-processing flow comprises the following steps: at a gas outlet of the gas-liquid separation tank (14), unliquefied BOG is subjected to heat exchange with a BOG liquefaction flow channel (131) through a BOG backflow flow channel (132), then is distributed through an eighth three-way valve (15), is expanded through a first expansion valve (16) and a second expansion valve (17), and then enters an economizer (18) for heat exchange, and BOG fluid with lower temperature passes through a BOG sub-cooling flow channel (123) in a first multi-stream heat exchanger (12), is mixed with BOG fluid with higher temperature in a first three-way valve (1), and then sequentially passes through a second three-way valve (2), a third three-way valve (3) and a fourth three-way valve (4) and then enters a low-pressure BOG compressor (5) for recycling;

the cooling medium circulation process comprises the following steps: high-pressure cooling medium compressed in a cooling medium compressor (19) is cooled by a cooler (20) and then divided into three parts, the three parts respectively pass through a third expansion valve (23), a fourth expansion valve (24) and a fifth expansion valve (25) to cool the upper part, the middle part and the lower part of a cooling medium flow channel (133) in a second multi-flow heat exchanger (13), and the heated cooling medium enters the cooling medium compressor (19) and then enters the next cycle;

the LNG preheating treatment process comprises the following steps: the LNG flow channel (30) exchanges heat sequentially through the cooler (20) and the low-temperature LNG flow channel (124) of the first multi-stream heat exchanger (12).

Technical Field

The invention relates to a BOG reliquefaction system, in particular to a BOG reliquefaction system adaptive to gas production fluctuation and a working method thereof.

Background

Because the demand of natural gas in China presents a huge peak-valley difference in light and busy seasons at present, the BOG production amount of the LNG receiving station under different working conditions fluctuates greatly. For example, during the export period of baseload non-unloading, the export volume of LNG is about 51.56t/h, and the production volume of BOG is only about 6.26 t/h; in the export period of the baseload unloading ship, the LNG export amount is about 51.56t/h, and the BOG production amount is about 25 t/h; during the peak-shaving non-unloading ship export period, the LNG export amount is about 585t/h, and the BOG production amount is about 44 t/h; during the peak-shaving and ship-unloading export period, the LNG export amount is about 585t/h, and the BOG production amount reaches 62.84 t/h.

The fluctuation of the BOG production directly affects the working condition of a heat exchanger in a reliquefaction system, the alternating change of the output leads to the alternating change of the inlet temperature of the heat exchanger, and the heat exchanger is enabled to frequently bear the thermal shock effect, which is mainly characterized in that the temperature of a medium in a tube pass (or a shell pass) is sharply reduced in a period of time, the temperature difference of the cold side and the hot side is large, and the transient change is severe. The frequent thermal shock causes the structural fatigue failure of the pipe fitting and the joint, the accumulated fatigue damage generates cracks, and the leakage of equipment is seriously even caused, thereby affecting the normal work of the equipment.

Generally, because the flexible operation capacity of the conventional BOG reliquefaction system to variable working conditions is limited, the LNG receiving station only recovers a small part of BOG, and most of the BOG is discharged through combustion of the flare system, so that huge energy waste and economic loss are caused.

In addition, a common BOG reliquefaction system generally adopts a combination form of a compressor and an aftercooler to compress BOG and then cool the BOG to obtain LNG, but in general, the temperature of BOG is about-160 ℃, low-temperature BOG is directly compressed, a special low-temperature compressor needs to be configured, and when the working temperature reaches ultralow temperature of-160 ℃, a series of problems such as equipment low-temperature materials, cylinder structures under low-temperature conditions, fitting matching and the like are encountered.

Disclosure of Invention

The purpose of the invention is as follows: in order to solve the problems in the prior art, the invention provides a BOG reliquefaction system which is suitable for gas production fluctuation and can be suitable for reliquefaction of BOG under different working conditions. The invention also provides a working method of the BOG reliquefaction system suitable for the fluctuation of the gas production rate.

The technical scheme is as follows: the BOG reliquefaction system suitable for the fluctuation of the gas production rate comprises a precooling system, a compression system and a supercooling system; the precooling system comprises a BOG pump and a first multi-stream heat exchanger connected with the outlet of the BOG pump, the first multi-stream heat exchanger comprises a normal-pressure BOG preheating flow channel, a high-pressure BOG precooling flow channel, a BOG supercooling flow channel and a low-temperature LNG flow channel, the normal-pressure BOG preheating flow channel is provided with a first flow channel from the middle lower part, and the normal-pressure BOG preheating flow channel is provided with a second flow channel from the middle part;

the compression system comprises a low-pressure BOG compression cooling unit and a high-pressure BOG compression cooling unit, the low-pressure BOG compression cooling unit comprises a low-pressure BOG compressor and an interstage cooler, and the high-pressure BOG normal-temperature cooling unit comprises a high-pressure BOG compressor and an aftercooler; the first flow channel is connected with an inlet of an after cooler, the second flow channel is communicated with an inlet of an interstage cooler, an outlet of the after cooler and an outlet of a normal-pressure BOG preheating flow channel are all communicated with an inlet of a low-pressure BOG compressor, an outlet of the low-pressure BOG compressor is communicated with an inlet of the interstage cooler, an outlet of the interstage cooler is communicated with an inlet of a high-pressure BOG compressor, an outlet of the high-pressure BOG compressor is communicated with an inlet of the after cooler, and an outlet of the after cooler is communicated with an inlet of a high-pressure BOG precooling flow channel;

the subcooling system comprises a second multi-stream heat exchanger, a gas-liquid separation tank, an economizer cooling medium compressor and a heat exchanger; the second multi-stream heat exchanger comprises a BOG liquefaction flow channel, a BOG backflow flow channel and a cooling medium flow channel, wherein an inlet of the BOG liquefaction flow channel is communicated with an outlet of a high-pressure BOG precooling flow channel, a gas-liquid separation tank is arranged between the BOG liquefaction flow channel and the BOG backflow flow channel, an economizer is arranged at an outlet of the BOG backflow flow channel, a first outlet of the economizer is communicated with an inlet of the BOG supercooling flow channel, a second outlet of the economizer is communicated with a low-pressure BOG compressor, an outlet of the cooling medium flow channel is communicated with an inlet of a cooler, and an outlet of the cooler is communicated with an inlet of a low-temperature LNG flow channel.

As a preferable structure of the present invention, a sixth three-way valve is provided between the outlet of the interstage cooler and the inlet of the high-pressure BOG compressor, and the outlet of the interstage cooler is communicated with the outlet of the after cooler through the sixth three-way valve.

As a preferable structure of the present invention, an outlet of the interstage cooler and an outlet of the after cooler are communicated with a third three-way valve through a fifth three-way valve, an outlet of the atmospheric BOG preheating flow passage is communicated with a second three-way valve, and an outlet of the second three-way valve is communicated with an inlet of the third three-way valve.

As a preferable structure of the present invention, an outlet of the sixth three-way valve is communicated with an inlet of the high-pressure BOG precooling flow passage.

As a preferable structure of the present invention, an outlet of the third three-way valve communicates with an inlet of a fourth three-way valve, and an outlet of the fourth three-way valve communicates with an inlet of the low-pressure BOG compressor.

As a preferable structure of the present invention, an outlet of the BOG return flow passage is provided with an eighth three-way valve, and two outlets of the eighth three-way valve are respectively communicated with two inlets of the economizer through a first expansion valve and a second expansion valve.

As a preferred structure of the present invention, the cooling medium channel includes a third channel, a fourth channel and a fifth channel, outlets of the third channel, the fourth channel and the fifth channel are respectively communicated with an inlet of a cooling medium compressor, and an outlet of the cooling medium compressor is communicated with an inlet of a cooler; and the outlet of the cooler is provided with a ninth three-way valve connected with the inlet of the third flow passage and a thirteenth three-way valve respectively connected with the inlet of the fourth flow passage and the inlet of the fifth flow passage.

In a preferred configuration of the present invention, an inlet of the third flow passage is provided with a fifth expansion valve, an inlet of the fourth flow passage is provided with a fourth expansion valve, and an inlet of the fifth flow passage is provided with a third expansion valve.

The working method of the BOG reliquefaction system adapting to the fluctuation of the gas production rate comprises the following steps:

and (3) during the base load non-ship unloading period: when the LNG receiving station is in a base load non-unloading period, a high-pressure BOG compressor, an after cooler, a supercooling system with a cooling medium compressor and an LNG cooling flow channel in the system are all closed, and a BOG single-stage processing flow and a BOG post-processing flow are selected as a circulation flow of the system;

and (3) during the ship unloading period of the base load: when the LNG receiving station is in a basic loading and unloading period, a high-pressure BOG compressor, an after cooler and a supercooling system where a cooling medium compressor is located in the system are all closed, and a BOG single-stage processing flow, a BOG post-processing flow and LNG preheating processing are selected as a circulation flow of the system;

and (3) during the peak-shaving non-ship-unloading period: when the LNG receiving station is in a peak-shaving non-ship-unloading period, a cooling LNG cooling flow channel in the system is closed, and a BOG multi-stage processing flow, a BOG post-processing flow and a cooling medium circulation flow are selected as a circulation flow of the system;

adjusting the peak and unloading the ship: when the LNG receiving station is in the peak-shaving ship unloading period, all the processes in the system are started, and the circulation process of the system selects a BOG multi-stage processing process, a BOG post-processing process, a cooling medium circulation process and an LNG preheating processing process;

the BOG single-stage processing flow comprises the following steps: BOG enters a normal-pressure BOG preheating flow channel in a first multi-stream heat exchanger through a BOG pump, then flows in a middle part, part of BOG passing through a second flow channel passes through an interstage cooler, after being cooled, the BOG passing through a low-pressure BOG compressor is mixed with the BOG sent out from the other part through an outlet of the BOG preheating flow channel through a fifth three-way valve, a third three-way valve and a fourth three-way valve, the mixture enters the low-pressure BOG compressor for compression, after exchanging heat with the normal-pressure BOG preheating flow channel, the compressed BOG passes through an outlet of the interstage cooler, and enters a high-pressure BOG pre-cooling flow channel in the first multi-stream heat exchanger through a sixth three-way valve and a seventh three-way valve, after exchanging heat with the normal-pressure BOG preheating flow channel, the BOG liquefaction flow channel in the second multi-stream heat exchanger, after exchanging heat with a BOG backflow flow channel, the BOG enters a gas-liquid separation tank, and the liquefied BOG directly enters an LNG storage tank for storage;

the BOG multi-stage processing flow comprises the following steps: and the BOG enters a normal-pressure BOG preheating flow channel in the first multi-stream heat exchanger through a BOG pump and then is divided in a second flow channel in the middle part and a second flow channel in the middle-lower part, the BOG in the middle part of the flow is cooled by the interstage cooler after passing through the low-pressure BOG compressor, and the BOG in the middle-lower part of the flow is cooled by the aftercooler after passing through the high-pressure BOG compressor. After being mixed by a third three-way valve, the BOG of the middle part, the middle part and the lower part flows and the rest BOG enter a low-pressure BOG compressor 5 through a fourth three-way valve for compression, the compressed BOG completely enters a high-pressure BOG compressor for compression after heat exchange in an interstage cooler, the compressed BOG completely enters a first multi-stream heat exchanger for heat exchange through a seventh three-way valve from an outlet of the aftercooler after heat exchange in the aftercooler, enters a BOG liquefaction flow channel in a second multi-stream heat exchanger after heat exchange, enters a gas-liquid separation tank after heat exchange with a BOG backflow flow channel, and the liquefied BOG directly enters an LNG storage tank for storage;

the BOG post-processing flow comprises the following steps: at a gas outlet of the gas-liquid separation tank, the unliquefied BOG is subjected to heat exchange with the BOG liquefaction flow channel through a BOG backflow flow channel and then is shunted through an eighth three-way valve, and is expanded through a first expansion valve and a second expansion valve respectively and then enters an economizer for heat exchange, a BOG fluid with lower temperature is mixed with a BOG fluid with higher temperature through a BOG sub-cooling flow channel in a first multi-stream heat exchanger, and then the BOG fluid sequentially passes through the second three-way valve, a third three-way valve and a fourth three-way valve and then enters a low-pressure BOG compressor for recycling;

the cooling medium circulation process comprises the following steps: cooling the high-pressure cooling medium compressed in the cooling medium compressor by the cooler and then dividing the high-pressure cooling medium into three parts, cooling the upper part, the middle part and the lower part of a cooling medium flow passage in the second multi-flow heat exchanger by each part through a third expansion valve, a fourth expansion valve and a fifth expansion valve respectively, and enabling the heated cooling medium to enter the cooling medium compressor and then enter the next circulation;

the LNG preheating treatment process comprises the following steps: the LNG runner carries out heat exchange through the cooler and the low-temperature LNG runner of the first multi-stream heat exchanger in sequence.

Has the advantages that: (1) under the condition of ensuring the use of a downstream pipe network, the invention can carry out optimization adjustment on corresponding working conditions aiming at output load changes in a base load non-ship unloading period, a base load ship unloading period, a peak regulation non-ship unloading period and a peak regulation ship unloading period mode, ensures the refined operation management of the system, carries out variable load design on the BOG recovery system under multiple working conditions from the aspects of device composition, energy consumption, operation cost and the like, and is suitable for a receiving station with larger working condition fluctuation. (2) According to the invention, through the design of the BOG single-stage/multi-stage treatment system, the BOG flow and temperature entering the multi-flow heat exchanger can be flexibly adjusted according to the change of the actual upstream BOG under the working condition of impact load, the load of the heat exchanger is prevented from being directly influenced by the working condition of the upstream BOG, the destructive thermal strain of an internal structure caused by the severe change of the local temperature in the heat exchanger is relieved, and the service life of the heat exchanger is ensured. (3) Compared with the method that the interstage cooler and the aftercooler are cooled by cooling media (air or water), the method has the advantages that the cooling effect can be greatly improved by adopting the BOG shunting mode for cooling, and the liquefaction rate of the downstream in the second multi-stream heat exchanger is ensured. (4) According to the invention, the normal-pressure BOG pre-cooling flow channel and the high-pressure BOG pre-heating flow channel are arranged in the first multi-stream heat exchanger, so that not only can the cooling energy of BOG be fully utilized and the irreversible loss of the system be reduced, but also the normal-temperature BOG compressor is adopted to replace the conventional low-temperature BOG compressor, and the system cost is reduced. (5) According to the invention, the BOG reflux flow channel in the second multi-stream heat exchanger is provided with the flow splitting, the gas is divided into two parts, one path of gas is subcooled through the economizer and then is subjected to heat exchange through the BOG subcooling flow channel in the second multi-stream heat exchanger, and the liquefaction efficiency of the system is further improved.

Drawings

FIG. 1 is a schematic diagram of the configuration of a BOG reliquefaction system adapted to fluctuations in gas production according to the present invention.

Detailed Description

The BOG reliquefaction system adapting to the gas production fluctuation comprises a precooling system 100, a compression system 200 and a supercooling system 300.

The pre-cooling system 100 includes a first multi-stream heat exchanger 12, and the first multi-stream heat exchanger 12 includes 4 flow channels, which are a normal-pressure BOG pre-heating flow channel 121, a high-pressure BOG pre-cooling flow channel 122, a BOG sub-cooling flow channel 123, and a low-temperature LNG flow channel 124. The compression system 200 comprises a low-pressure BOG compression cooling unit and a high-pressure BOG compression cooling unit, wherein the low-pressure BOG compression cooling unit consists of a low-pressure BOG compressor 5 and an interstage cooler 6, and the high-pressure BOG normal-temperature cooling unit consists of a high-pressure BOG compressor 9 and an aftercooler 10. The subcooling system is composed of a second multi-flow heat exchanger 13, a gas-liquid separation tank 14, an economizer 18, an eighth three-way valve 15, a first expansion valve 16, a second expansion valve 17, a cooling medium compressor 19, and a cooler 20. The second multi-stream heat exchanger 13 includes 3 flow paths, which are a BOG liquefaction flow path 131, a BOG reflux flow path 132, and a cooling medium flow path 133, respectively.

An inlet of the normal-pressure BOG preheating flow passage 121 is connected to an outlet of the BOG pump 26, the normal-pressure BOG preheating flow passage 121 branches off two flow passages from a lower portion and a middle portion thereof, specifically, a first flow passage 1211 branched from the lower portion and a second flow passage 1212 branched from the middle portion, an outlet of the normal-pressure BOG preheating flow passage 121 is sequentially connected to a second three-way valve 2, a third three-way valve 3 and a fourth three-way valve 4, and an outlet of the fourth three-way valve 4 is connected to the low-pressure BOG compressor 5. Specifically, the outlet of the first flow passage 1211 led out from the middle-lower portion is connected to the cold-side inlet of the after-cooler 10, and the outlet of the second flow passage 1212 led out from the middle portion is connected to the cold-side inlet of the inter-stage cooler 6; and after heat exchange is carried out by the aftercooler 10 and the interstage cooler 6, a cold side outlet of the aftercooler 10 and a cold side outlet of the interstage cooler 6 are connected with the third three-way valve 3 through a fifth three-way valve 7, namely, the cold side outlet of the aftercooler 10 is connected with one inlet of the fifth three-way valve 7, the cold side outlet of the interstage cooler 6 is connected with the other inlet of the fifth three-way valve 7, an outlet of the fifth three-way valve 7 is connected with an inlet of the third three-way valve 3, the third three-way valve 3 is connected with the low-pressure BOG compressor 5 through a fourth three-way valve 4, and one outlet of the fourth three-way valve 4 is used as an additionally arranged bypass outlet and used for system buffering. An outlet of the low-pressure BOG compressor 5 is connected with a hot side inlet of the interstage cooler 6, a hot side outlet of the interstage cooler 6 is connected with an inlet of the high-pressure BOG compressor 9 through a sixth three-way valve 8, an outlet of the sixth three-way valve 8 is connected with an inlet of a seventh three-way valve 11 as an additionally arranged bypass, an outlet of the high-pressure BOG compressor 9 is connected with a hot side inlet of the aftercooler 10, a hot side outlet of the aftercooler 10 is connected with an inlet of the seventh three-way valve 11, an outlet of the seventh three-way valve 11 is connected with an inlet of the high-pressure BOG precooling flow passage 122, namely fluid sent out from the hot side outlet of the aftercooler 10 is mixed with bypass fluid of the sixth three-way valve 8 and then sent into the seventh three-way valve 11 and the high-pressure BOG precooling flow passage 122 in the first multi-stream heat exchanger 12.

The outlet of the high-pressure BOG precooling flow passage 122 is connected with a BOG liquefaction flow passage 131 in the second multi-stream heat exchanger 13, the inlet of the BOG supercooling flow passage 123 is connected with a cold end outlet in the economizer 18, the outlet of the BOG supercooling flow passage 123 is provided with a first three-way valve 1, and is sequentially communicated with a second three-way valve 2, a third three-way valve 3 and a fourth three-way valve 4 through the outlet of the first three-way valve 1, so as to be communicated with the inlet of the low-pressure BOG compressor 5. The inlet of the low-temperature LNG flow path 124 is connected to the outlet of the heat exchanger 20 in the subcooling system 300, and the outlet of the low-temperature LNG flow path 124 is connected to the LNG vaporizer or the user terminal.

The inlet of the low-pressure compressor 5 is respectively communicated with a BOG supercooling flow channel 123 which is subjected to self-expansion heat exchange by the economizer 18, a normal-pressure BOG preheating flow channel 121, and a BOG flow channel which is heated by the interstage cooler 6 and the aftercooler 10 through a first three-way valve 1, a second three-way valve 2, a third three-way valve 3 and a fourth three-way valve 4 which are arranged in series, and the outlet of the low-pressure compressor 5 is communicated with the inlet connected with the interstage heat exchanger 6.

An inlet of a BOG liquefaction flow passage 131 of the second multi-stream heat exchanger 13 is communicated with an outlet of a high-pressure BOG precooling flow passage 122 of the first multi-stream heat exchanger 12, an outlet of the BOG liquefaction flow passage 131 is connected with an inlet of the gas-liquid separation tank 14, an inlet of a BOG backflow flow passage 132 is connected with a gas outlet of the gas-liquid separation tank 14, an outlet of the BOG backflow flow passage 132 is communicated with an inlet of an eighth three-way valve 15, two outlets of the eighth three-way valve 15 are respectively provided with a first expansion valve 16 and a second expansion valve 17 and are respectively communicated with a cold end inlet and a hot end inlet which are connected with the economizer 18 through the first expansion valve 16 and the second expansion valve 17, a cold end in the outlet of the economizer 18 is connected with an inlet of a BOG supercooled flow passage 123, and a hot end is connected with the fourth three-way valve 4 through the first three-way valve 1, the second three-way valve 2 and the third three-way valve 3.

The cooling medium channel 133 is divided into three channels, which are respectively connected to the upper portion, the middle portion, and the lower portion of the cooling medium channel 133 in the second multi-stream heat exchanger 13, and include a third channel 1331, a fourth channel 1332, and a fifth channel 1333. Outlets of the third flow passage 1331, the fourth flow passage 1332 and the fifth flow passage 1333 are respectively communicated with a cooling medium compressor 19, an outlet of the cooling medium compressor 19 is connected with a hot end inlet of the cooler 20, a hot end outlet of the cooler 20 is respectively communicated with three stages of flow passages of the cooling medium flow passage 133 through a ninth three-way valve 21 and a thirteenth three-way valve 22 which are arranged in series, specifically, a hot end outlet of the cooler 20 is provided with a ninth three-way valve 21 and a thirteenth three-way valve 22 in series, one inlet of the ninth three-way valve 21 is connected with a hot end outlet of the cooler 20, one outlet of the ninth three-way valve 21 is connected with an inlet of the thirteenth three-way valve 22, the other outlet of the ninth three-way valve 21 is communicated with an inlet of the third flow passage 1331, a fifth expansion valve 25 is arranged between the ninth three-way valve 21 and the inlet of the third flow passage 1331, one outlet of the thirteenth three-way valve 22 is connected with an inlet of the fourth flow passage 1332, a fourth expansion valve 24 is provided between the thirteenth valve 22 and the inlet of the fourth flow passage 1332, the other outlet of the thirteenth valve 22 is communicated with the inlet of the fifth flow passage 1333, a third expansion valve 23 is provided between the thirteenth valve 22 and the inlet of the fifth flow passage 1333, and the outlets of the three flow passages are mixed and then connected to the refrigerant compressor 19, thereby forming a circulation circuit. The cold side outlet of the cooler 20 is connected to the inlet of the low temperature LNG flow path 124 in the first multi-stream heat exchanger 12 and the cold side inlet of the cooler 20 is connected to the LNG flow path.

The working method of the invention comprises the following steps: BOG treatment process, supercooling medium process and LNG preheating process.

The BOG processing flow comprises a BOG single-stage processing flow, a BOG multi-stage processing flow and a BOG post-processing flow.

BOG single-stage treatment process: BOG enters the atmospheric BOG preheating flow channel 121 in the first multi-stream heat exchanger 12 through the BOG pump 26, then flows in the middle part, passes through the interstage cooler 6 after passing through a part of BOG of the second flow channel 1221, cools the BOG after passing through the low-pressure BOG compressor 5, then is mixed with the BOG sent out from the other part through the outlet of the BOG preheating flow channel 121 through the fifth three-way valve 7, the third three-way valve 3 and the fourth three-way valve 4, then enters the low-pressure BOG compressor 5 for compression, after heat exchange is carried out on the compressed BOG in the interstage cooler 6, passes through the outlet of the interstage cooler 6, completely enters the high-pressure BOG precooling flow channel 122 in the first multi-stream heat exchanger 12 through the sixth three-way valve 8 and the seventh three-way valve 11, enters the BOG liquefying flow channel 131 in the second multi-stream heat exchanger 13 after heat exchange with the atmospheric BOG preheating flow channel 121, and enters the gas-liquid separation tank 14 after heat exchange with the BOG return flow channel 132, and directly feeding the liquefied BOG into an LNG storage tank for storage.

BOG multistage processing flow: after entering the normal-pressure BOG preheating flow channel 121 in the first multi-stream heat exchanger 12 through the BOG pump 26, the BOG is split in the middle first flow channel 1211 and the middle and lower second flow channels 1212, the BOG in the middle part stream is cooled by the interstage cooler 6 after passing through the low-pressure BOG compressor 5, and the BOG in the middle and lower part stream is cooled by the aftercooler 10 after passing through the high-pressure BOG compressor 9. After the BOG and the rest BOG of the middle part, the middle part and the lower part flow are mixed in the third three-way valve 3, the mixture enters the low-pressure BOG compressor 5 through the fourth three-way valve 4 to be compressed, the compressed BOG is subjected to heat exchange in the interstage cooler 6 and then completely enters the high-pressure BOG compressor 9 to be compressed, the compressed BOG is subjected to heat exchange in the aftercooler 10 and then completely enters the first multi-strand heat exchanger 12 through the seventh three-way valve 11 from the outlet of the aftercooler 10 to be subjected to heat exchange, and the post-process is consistent with the single-stage processing of the BOG.

BOG post-treatment process: at the gas outlet of the gas-liquid separation tank 14, the unliquefied BOG is heat exchanged with a BOG liquefaction flow passage 131 through a BOG return flow passage 132, then is branched through an eighth three-way valve 15, is expanded through a first expansion valve 16 and a second expansion valve 17, and then enters an economizer 18 for heat exchange, and the BOG fluid with lower temperature passes through a BOG sub-cooling flow passage 123 in the first multi-stream heat exchanger 12, then is mixed with the BOG fluid with higher temperature in the first three-way valve 1, and then sequentially passes through the second three-way valve 2, the third three-way valve 3 and the fourth three-way valve 4, and then enters the low-pressure BOG compressor 5 for recirculation.

Cooling medium circulation flow: the high-pressure cooling medium compressed in the cooling medium compressor 19 is cooled by the cooler 20 and then divided into three parts, each part cools the upper part, the middle part and the lower part of the cooling medium channel 133 in the second multi-flow heat exchanger 13 by the third expansion valve 23, the fourth expansion valve 24 and the fifth expansion valve 25, and the heated cooling medium enters the cooling medium compressor 19 and then enters the next cycle.

LNG preheating process flow: the LNG flow path 30 is heat exchanged via the chiller 20 and the cryogenic LNG flow path 124 of the first multi-stream heat exchanger 12 in sequence.

The operation conditions of the system comprise a base load non-unloading ship export period, a base load unloading ship export period, a peak-regulating non-unloading ship export period and a peak-regulating unloading ship export period.

And (3) during the base load non-ship unloading period: when the LNG receiving station is in a base load non-unloading period, the supercooling system and the LNG cooling flow channel where the high-pressure BOG compressor 9, the after cooler 10 and the cooling medium compressor 19 are located in the system are all closed, and a BOG single-stage processing flow and a BOG post-processing flow are selected.

And (3) during the ship unloading period of the base load: when the LNG receiving station is in the baseload unloading period, the subcooling system in which the high-pressure BOG compressor 9, the aftercooler 10 and the cooling medium compressor 19 are located is all turned off. And selecting a BOG single-stage processing flow, a BOG post-processing flow and an LNG preheating processing flow.

And (3) during the peak-shaving non-ship-unloading period: when the LNG receiving station is in the peak-shaving non-ship-unloading period, the cooling LNG cooling flow channel 124 in the system is closed, and the selected flow is a BOG multi-stage processing flow, a BOG post-processing flow and a cooling medium circulation flow.

Adjusting the peak and unloading the ship: when the LNG receiving station is in the peak-shaving ship unloading period, all the processes in the system are started, and a BOG multistage processing process, a BOG post-processing process, a cooling medium circulation process and an LNG preheating processing process are selected

Application example: and (3) during the base load non-ship unloading period: the LNG output is about 51.56t/h, and the BOG production is about 6.12-6.36 t/h. The subcooling system in which the high-pressure BOG compressor 9, the aftercooler 10 and the cooling medium compressor 19 are located and the LNG cooling flow path 124 are all closed. BOG enters a normal pressure BOG preheating flow channel 121 in the first multi-stream heat exchanger 12 through a BOG pump 26, in the flow channel, a part of BOG is led out from a bypass of a second flow channel 1212 to enter a cold side inlet of an interstage cooler 6 for preheating, the other part of BOG is led out from an outlet of the normal pressure BOG preheating flow channel 121, is respectively mixed with BOG mixed by a first three-way valve 1 and BOG at a cold side outlet of the interstage cooler 6 through a second three-way valve 2 and a third three-way valve 3, and enters a low pressure BOG compressor 5 for compression through a fourth three-way valve 4, the compressed BOG enters the cooler 6 for cooling, enters a high pressure precooling flow channel 122 of the first multi-stream heat exchanger 12 through a sixth three-way valve 8 and a seventh three-way valve 11 for cooling, enters a BOG liquefying flow channel 131 in the second multi-stream heat exchanger 13 after cooling for liquefying, enters a gas-liquid separation tank 14 for separating after liquefying, and the liquefied BOG is returned to a storage tank, the gas phase enters a BOG reflux flow channel 132 to exchange heat with a BOG liquefaction flow channel 133, the gas phase is split by an eighth three-way valve 15, the cold end and the hot end of an economizer 18 are connected through a first expansion valve 16 and a second expansion valve 17 respectively, the BOG at the outlet of the cold end of the economizer enters a BOG supercooling flow channel 123 of a first multi-stream heat exchanger 12 to exchange heat, the BOG at the outlet of the hot end of the economizer is mixed with the gas of the BOG supercooling flow channel through a first three-way valve 1, is mixed with the gas of a normal-pressure BOG preheating flow channel 121 at a second three-way valve 2, is mixed with the gas which is subjected to heat exchange by an interstage cooler 6 at a third three-way valve 3 and enters the next cycle.

And (3) during the ship unloading period of the base load: the LNG output is about 51.56t/h, and the BOG production is about 23 t/h. High pressure BOG in a system

The compressor 9 and the aftercooler 10 are switched off and the supercooling system in which the refrigerant compressor 19 is located is switched off. The BOG cycle coincides with the baseload non-ship-off period. And adding LNG preheating treatment, wherein LNG exchanges heat with a refrigerating medium through the cooler 20, and exchanges heat through the LNG low-temperature flow channel 124 of the first multi-flow heat exchanger 12.

And (3) during the peak-shaving non-ship-unloading period: the LNG output is about 585t/h, and the BOG production is about 44 t/h. In-system LNG

The cooling flow passage 124 is closed. In the BOG circulation period, compared with the baseload ship unloading period, the flow is divided at the position of the normal-pressure BOG preheating flow passage 121 close to the inlet, the aftercooler 10 is cooled, and the BOG compressor is additionally provided with a first-stage compression cooling, namely, high-pressure BOG

Compressor 9 and aftercooler 10, i.e. BOG multi-stage process flow. And a cooling medium circulation is newly added, the high-pressure cooling medium compressed in the cooling medium compressor 19 is cooled by the cooler 20 and then divided into three parts, each part is used for cooling the upper part, the middle part and the lower part of the cooling medium channel 133 in the second multi-stream heat exchanger 13 by the third expansion valve 23, the fourth expansion valve 24 and the fifth expansion valve 25 respectively, and the heated cooling medium enters the cooling medium compressor 19 and then enters the next circulation.

Adjusting the peak and unloading the ship: the LNG output is about 585t/h, and the BOG production is about 60.39-63.46 t/h. All the flows of the BOG treatment flow, the supercooling medium flow and the LNG preheating flow in the system are started.