CN109458788B - BOG self-circulation re-liquefaction recovery heat exchange system and method for LNG storage tank - Google Patents

BOG self-circulation re-liquefaction recovery heat exchange system and method for LNG storage tank Download PDF

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CN109458788B
CN109458788B CN201811499592.0A CN201811499592A CN109458788B CN 109458788 B CN109458788 B CN 109458788B CN 201811499592 A CN201811499592 A CN 201811499592A CN 109458788 B CN109458788 B CN 109458788B
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bog
heat exchange
channel
lng
circulation
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CN109458788A (en
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韩凤翚
王哲
李文华
陈海泉
孙玉清
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Dalian Maritime University
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Dalian Maritime University
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/0002—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the fluid to be liquefied
    • F25J1/0022—Hydrocarbons, e.g. natural gas
    • F25J1/0025—Boil-off gases "BOG" from storages
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/0002—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the fluid to be liquefied
    • F25J1/0022—Hydrocarbons, e.g. natural gas
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    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C13/00—Details of vessels or of the filling or discharging of vessels
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    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • F25J1/0221—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using the cold stored in an external cryogenic component in an open refrigeration loop
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    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • F25J1/0243—Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
    • F25J1/0257—Construction and layout of liquefaction equipments, e.g. valves, machines
    • F25J1/0261—Details of cold box insulation, housing and internal structure
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    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • F25J1/0243—Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
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    • F25J1/0262—Details of the cold heat exchange system
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    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • F25J1/0243—Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
    • F25J1/0257—Construction and layout of liquefaction equipments, e.g. valves, machines
    • F25J1/0275—Construction and layout of liquefaction equipments, e.g. valves, machines adapted for special use of the liquefaction unit, e.g. portable or transportable devices
    • F25J1/0277—Offshore use, e.g. during shipping
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J5/00—Arrangements of cold exchangers or cold accumulators in separation or liquefaction plants
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J5/00—Arrangements of cold exchangers or cold accumulators in separation or liquefaction plants
    • F25J5/002—Arrangements of cold exchangers or cold accumulators in separation or liquefaction plants for continuously recuperating cold, i.e. in a so-called recuperative heat exchanger
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C2221/00—Handled fluid, in particular type of fluid
    • F17C2221/03—Mixtures
    • F17C2221/032—Hydrocarbons
    • F17C2221/033—Methane, e.g. natural gas, CNG, LNG, GNL, GNC, PLNG
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    • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C2227/00—Transfer of fluids, i.e. method or means for transferring the fluid; Heat exchange with the fluid
    • F17C2227/03—Heat exchange with the fluid
    • F17C2227/0337—Heat exchange with the fluid by cooling
    • F17C2227/0341—Heat exchange with the fluid by cooling using another fluid
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C2265/00—Effects achieved by gas storage or gas handling
    • F17C2265/03—Treating the boil-off
    • F17C2265/032—Treating the boil-off by recovery
    • F17C2265/033—Treating the boil-off by recovery with cooling
    • F17C2265/034—Treating the boil-off by recovery with cooling with condensing the gas phase
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2235/00—Processes or apparatus involving steps for increasing the pressure or for conveying of liquid process streams
    • F25J2235/42—Processes or apparatus involving steps for increasing the pressure or for conveying of liquid process streams the fluid being nitrogen
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2235/00—Processes or apparatus involving steps for increasing the pressure or for conveying of liquid process streams
    • F25J2235/60—Processes or apparatus involving steps for increasing the pressure or for conveying of liquid process streams the fluid being (a mixture of) hydrocarbons
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2240/00—Processes or apparatus involving steps for expanding of process streams
    • F25J2240/02—Expansion of a process fluid in a work-extracting turbine (i.e. isentropic expansion), e.g. of the feed stream
    • F25J2240/12—Expansion of a process fluid in a work-extracting turbine (i.e. isentropic expansion), e.g. of the feed stream the fluid being nitrogen
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
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    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2290/00—Other details not covered by groups F25J2200/00 - F25J2280/00
    • F25J2290/30—Details about heat insulation or cold insulation
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2290/00—Other details not covered by groups F25J2200/00 - F25J2280/00
    • F25J2290/62—Details of storing a fluid in a tank

Abstract

The invention relates to a BOG self-circulation re-liquefaction recovery heat exchange system and a BOG self-circulation re-liquefaction recovery heat exchange method. The system comprises a BOG liquefaction unit, a circulating heat exchange network and an LN2 regeneration unit, wherein the BOG liquefaction unit and the LN2 regeneration unit are connected with each other through the circulating heat exchange network for circulating heat exchange and regeneration, and the circulating heat exchange network is an integrated composite circulating heat exchange device formed by packaging and integrating a multi-channel precooler, a multi-channel subcooler and a multi-channel condenser. The system has the characteristics of compact structure, no freezing, high-efficiency liquefaction and the like, the system and the method not only can re-liquefy and recycle the BOG according to the design requirement, but also can supply cold through LN2 repeated circulation, the required power device is less, the liquefaction circulation can be driven by the self pressure in the early stage of the BOG, and an additional refrigerator is not required in the LN2 circulation and regeneration process, so that the energy consumption of related equipment is reduced, and the system and the method have the advantages of simple structure, safety and energy conservation, and meet the requirement of re-liquefying and recycling the BOG with large flow.

Description

BOG self-circulation re-liquefaction recovery heat exchange system and method for LNG storage tank
Technical Field
The invention relates to the field of low-temperature gas liquefaction, and in particular relates to a BOG self-circulation re-liquefaction recovery heat exchange system and method.
Background
Fossil fuels are well known to include coal, oil, and natural gas. To date, coal has been the primary energy source to meet global energy demands. In global energy demand, the coal fraction is 42% and the natural gas fraction is only 21%. However, natural gas is considered a clean energy source for providing power or heat by combustion, which greatly reduces emissions of particulate matter, sulfur oxides (SOx) and nitrogen oxides (NOx) waste materials, and has a greenhouse gas emission of only about 40% of that of diesel fuel under equivalent thermal mass conditions, compared to the high emission values of coal and petroleum.
The natural gas has a boiling point of about-162 ℃ and can be stored in a volume reduced by more than 620 times by liquefying to form LNG. Natural gas reserves are commonly transported and stored in liquid form over long distances due to their maldistribution worldwide. Up to now, there are about 124 LNG receiving stations operating worldwide, and rising at a rate of 1% per year. If estimated for about 4 tanks per receiving station, there will be about 600 total tanks for ultra-large LNG on land. Furthermore, the total number of small storage tanks for sea and cargo transportation is even greater. Due to the influence of environmental factors, artificial factors such as filling operation and the like, the LNG heated part is gasified to become BOG low-temperature gas which floats on the top of the storage tank, the internal pressure of the storage tank is gradually increased, and even the critical value of the storage tank is reached, so that serious consequences can be caused if the LNG heated part is not processed.
Currently, the traditional BOG treatment method is to empty or burn by using a torch, so that energy is wasted, and potential safety hazards and environmental pollution are brought. Therefore, there is a strong need for a reliable, simple, continuous-cycle BOG reliquefaction system based on economic and safety considerations for LNG storage tanks. In view of the complex flow of the traditional reliquefaction system, an additional refrigerator and a gas-liquid separator are required to be added or the structure of a liquefied natural gas storage tank is changed, other equipment is additionally arranged, and the like, and meanwhile, the defects of low BOG liquefaction efficiency, freezing of heat exchange equipment, mismatching of additional refrigeration capacity, excessively high manufacturing cost and the like exist, so that the reliquefaction system is difficult to popularize and use.
Disclosure of Invention
In order to solve the problems, the invention discloses a BOG self-circulation re-liquefaction recovery heat exchange system and a BOG self-circulation re-liquefaction recovery heat exchange method based on liquid nitrogen LN2 cooling, which enable BOG to repeatedly circulate in a specially-made circulation heat exchange network by means of self pressure and then return to a storage tank through an LNG pump; the gasified nitrogen is re-liquefied into LN2 through an expander and a throttle valve, and the nitrogen is recycled. The invention overcomes the defect of low liquefaction efficiency of the traditional BOG, LN2 has no extra mechanical work input in the expansion throttling process, and can be recycled without extra lubrication, and the whole system only needs an initial LN2 pump and an LNG pump at the tail end as power sources to drive the circulation, thereby being safe and energy-saving, reducing the energy consumption of related equipment, simultaneously taking the characteristics of no freezing and compact structure into consideration, and meeting the requirements of large-flow BOG re-liquefaction recovery.
In order to achieve the above purpose, the present invention adopts the following technical scheme:
the BOG self-circulation re-liquefaction recovery heat exchange system for the LNG storage tank comprises a BOG liquefaction unit, a circulation heat exchange network and an LN2 regeneration unit, wherein the BOG liquefaction unit and the LN2 regeneration unit are connected with circulation heat exchange and regeneration through the circulation heat exchange network; wherein:
the BOG liquefaction circulation unit comprises an LNG storage tank, a pressure switch valve P-1 and an LNG return pump P-LNG; the LNG storage tank is communicated with the inlet of the circulating heat exchange network through an LNG pressure switching valve P-1, and is communicated with the outlet of the circulating heat exchange network through an LNG return pump P-LNG; the superheated BOG in the LNG storage tank enters a circulating heat exchange network through a safety pressure switch valve P-1, liquefied LNG after being completely condensed through the circulating heat exchange network is returned to the LNG storage tank for continuous storage through an LNG return pump P-LNG connected with an outlet of the circulating heat exchange network;
the circulating heat exchange network comprises a multi-channel precooler 1, a multi-channel subcooler 2, a multi-channel condenser 3 and pipelines correspondingly connected, wherein the multi-channel precooler 1, the multi-channel subcooler 2 and the multi-channel condenser 3 are multi-stream heat exchangers, the multi-channel precooler 1 is connected with a pressure switch valve P-1 and is used for receiving overheated BOG of an LNG storage tank, the multi-channel precooler 1 is communicated with the multi-channel subcooler 2 to form a BOG precooling circulation loop, the multi-channel precooler 1 is communicated with the multi-channel condenser 3 to form a BOG condensation liquefaction circulation loop, meanwhile, the multi-channel precooler 2 and the multi-channel condenser 3 respectively receive two LN2 of the LN2 circulation regeneration unit as cold sources, the two LN2 enters the BOG precooling circulation loop and the BOG condensation liquefaction circulation loop to exchange heat with the BOG, the overheated vapor LN2 after the LN2 outlet of the multi-channel precooler 2 and the multi-channel condenser 3 exchanges heat flows back to the LN2 circulation regeneration unit, and the LNG outlet of the multi-channel condenser 3 is connected with an LNG return pump P-LNG;
The LN2 circulating regeneration unit comprises an LN2 storage tank, an LN2 liquid pump P-LN2, an expansion machine 4, a J-T throttle valve and a low-temperature pipeline which are connected, wherein the LN2 storage tank is connected with the LN2 liquid pump P-LN2, the LN2 liquid pump P-LN2 is connected with a distributor, two LN2 after being distributed by the distributor respectively enter a multi-channel subcooler 2 and a multi-channel condenser 3 of a circulating heat exchange network, two superheated steam LN2 flowing out of the multi-channel subcooler 2 and the multi-channel condenser 3 after heat exchange are collected and enter the expansion machine 4, the expansion machine 4 is connected with the LN2 storage tank through the J-T throttle valve, and the LN2 regenerated by the expansion machine 4 is returned to the LN2 storage tank.
Further, the multi-channel precooler 1, the multi-channel subcooler 2 and the multi-channel condenser 3 of the circulating heat exchange network are plate-fin type, plate type, winding pipe type or shell-and-tube type multi-stream heat exchangers.
Furthermore, the circulating heat exchange network is an integrated composite circulating heat exchange device formed by packaging and integrating the three parts of the multi-channel precooler 1, the multi-channel subcooler 2 and the multi-channel condenser 3 in the same heat exchanger.
Further, fin structures are adopted in the heat exchange channels in the multi-channel precooler 1, the multi-channel subcooler 2 and the multi-channel condenser 3, flat fins or perforated fins are preferably selected as the heat exchange channels in consideration of the relatively high viscosity of LN2 liquid, and zigzag fins or corrugated fins are preferably selected as the heat exchange channels for the superheated and supercooled vapor of nitrogen and BOG.
Further, the arrangement of the multi-channel precooler 1, the multi-channel subcooler 2 and the multi-channel condenser 3 is configured by adopting a single-layer, double-layer or combined mode of cold and hot fluid, namely, a layer of cold (or hot) channels is sandwiched between two adjacent layers of hot (or cold) channels, and the two layers of cold (or hot) channels are repeatedly stacked.
Further, the multi-channel precooler 1 is a single multi-flow plate-fin heat exchanger, the multi-channel subcooler 2 and the multi-channel condenser 3 are combined into a heat exchanger with an integrated structure, industrial plate-fin heat exchanger fins are selected in the heat exchange channels, and vacuum heat insulation layers K are respectively arranged on two sides of the multi-channel precooler 1, the multi-channel subcooler 2 and the multi-channel condenser 3 and used for isolating heat transfer between the heat exchanger and the outside.
Further, vacuum heat insulation layers K are respectively arranged on two sides of the subcooler and the condenser and used for isolating heat transfer between the heat exchanger and the outside and between the heat exchanger.
The BOG self-circulation re-liquefaction recovery heat exchange method for the LNG storage tank is characterized by comprising the following steps of:
(1) Superheated BOG precooling
Due to the influence of factors such as environment, manual operation and the like, BOG in an LNG storage tank is continuously increased, after the pressure in the LNG storage tank reaches a safe limit value, an LNG storage tank pressure switch valve P-1 is activated, overheated BOG is discharged into a multi-channel precooler 1 of a circulating heat exchange network through the pressure of the overheated BOG, the overheated BOG performs precooling heat exchange by utilizing a first reflux supercooled BOG which flows back to the multi-channel precooler 1 after heat exchange between the multi-channel subcooler 2 and LN2, and then the overheated BOG enters the multi-channel subcooler 2 and flows back to the multi-channel precooler 1 after heat exchange between the overheated BOG and LN2 as the first reflux supercooled BOG;
(2) BOG condensation liquefaction
The first reflux supercooled BOG enters a multichannel condenser 3 to be condensed, heat exchanged and liquefied with LN2 after being backheated by the multichannel precooler 1, so that the first liquefaction cycle is completed; the non-liquefied BOG flows back to the multi-channel precooler 1 again, and as the start of the next liquefaction cycle, the pre-cooled initial overheated BOG flows back to the multi-channel condenser 3 again for continuous condensation;
(3) Reflux after BOG liquefaction
After repeated precooling and liquefaction circulation of the circulating heat exchange network are carried out for n times, the circulation times n is more than or equal to 2, and finally BOG is liquefied into LNG, and the LNG is conveyed back into an LNG storage tank through an LNG pump P-LNG, so that the liquefaction recovery circulation of the BOG is completed;
in the above-mentioned BOG liquefaction recovery cycle, the BOG is liquefied and recovered through the circulation heat exchange network, the sensible heat of the BOG is absorbed by the first LN2 in the multichannel subcooler 2, and the remaining BOG liquefaction latent heat is continuously absorbed by the second LN2 in the multichannel condenser 3;
(4) LN2 cooling cycle
Liquid nitrogen LN2 in the LN2 storage tank is divided into two streams, after being pressurized by an LN2 liquid pump P-LN2, one stream is introduced into the multichannel subcooler 2 to absorb the sensible heat of BOG, and the other stream is introduced into the multichannel condenser 3 to absorb the latent heat of BOG; two LN2 heat exchanges to become nitrogen steam which is collected in the same low-temperature pipeline; then the LN2 is cooled by expansion of the expander 4, and is reconverted into liquid by the J-T throttle valve and returned to the LN2 storage tank, thereby completing the cooling regeneration cycle of the LN 2.
Further, according to the required BOG flow, the circulation times n of the circulation heat exchange network can be designed, and the relation between the circulation heat exchange times n of the circulation heat exchange network and the BOG flow required to be liquefied is as follows:
Figure BDA0001897849910000051
wherein: m is BOG mass flow, r is BOG liquefied latent heat, c p Is the specific heat capacity of BOG, t LNG-0 Temperature of LNG flowing out of the outlet of the circulation network, t BOG-0 Initial superheat BOG temperature for the loop network entry, t BOG-3 T is the temperature of the first reflux supercooled BOG in the multi-channel precooler after heat exchange with LN2 by the multi-channel subcooler LN2-2 And t LN2-3 Liquid nitrogen at inlet end and outlet end of multichannel subcooler and temperature of superheated steam of liquid nitrogen at inlet end and outlet end of multichannel subcooler respectively LN2-4 And t LN2-5 The temperature of the liquid nitrogen and the superheated steam thereof at the inlet and the outlet of the multichannel condenser are controlled by LN2 circulation flow.
Further, the BOG liquefaction condensation cycle number n can be calculated according to energy conservation in the heat exchange network, and the relationship among the heat exchange quantity Q, the mass flow m and the cycle heat exchange number n of the BOG and the LN2 is as follows:
Figure BDA0001897849910000061
Figure BDA0001897849910000062
wherein: m is m BOG And m LN2 Mass flow rates, h, of BOG and LN2, respectively BOG0 And h LNG0 Enthalpy values of the recycle network inlet and outlet superheated BOG and LNG respectively, h LN2-2 And h LN2-3 Liquid nitrogen at inlet end and outlet end of multichannel subcooler and enthalpy value of superheated steam of liquid nitrogen, h LN2-4 And h LN2-5 The enthalpy values of liquid nitrogen and superheated steam of the liquid nitrogen are respectively the inlet end and the outlet end of the multichannel condenser, delta h is enthalpy difference, and n represents the number of circulating heat exchange times in the heat exchange network.
Further, the liquid nitrogen LN2 is divided into two streams, the flow rates of the first LN2 stream and the second LN2 stream are specifically selected according to the temperature, physical properties of the BOG, the cycle times n of different heat exchange networks and the external environment working conditions, and the calculation relation formula is as follows:
Figure BDA0001897849910000063
wherein: m is m BOG And m LN2 Mass flow rates, m, of BOG and LN2, respectively LN2-2 And m LN2-4 Mass flow rates, T, of two LN2 fluids respectively BOG-2n T is the temperature of the BOG superheated steam subjected to heat exchange by the multi-channel precooler in the n-1 th cycle BOG-2n+1 For the temperature of the BOG subcooled steam of the nth reflux multi-channel precooler, T LN2-2 And T LN2-3 Liquid nitrogen at inlet end and outlet end of multichannel subcooler and temperature of superheated steam of liquid nitrogen at inlet end and outlet end of multichannel subcooler respectively, T LN2-4 And T LN2-5 The temperature of liquid nitrogen and superheated steam thereof respectively at the inlet end and the outlet end of the multichannel condenser, r BOG And r LN2 The latent heat of liquefaction of BOG and the latent heat of vaporization of LN2, c p N represents the number of heat exchange cycles in the heat exchange network for the specific heat capacity of the corresponding fluid.
Compared with the prior art, the invention has the beneficial effects that:
1. because the BOG exchanges heat with the self-supercooled gas in the precooler for a plurality of times, the sensible heat of the BOG is completely transferred to the self-supercooled steam, and the phenomenon of freezing caused by directly precooling and liquefying the BOG by using a refrigerator in traditional heat exchange equipment is avoided;
2. the unique design of the circulating heat exchange network is that the BOG in the multi-channel precooler and the supercooled steam thereof have smaller heat exchange temperature difference, thereby being beneficial to complete precooling of the BOG and obtaining relatively higher cold energy recovery efficiency; in the condenser, LN2 and BOG supercooled steam adopt large temperature difference heat exchange, so that LN2 can completely absorb the latent heat of phase change of BOG, and relatively high BOG liquefaction efficiency is facilitated;
3. the system of the invention is also added with a multichannel subcooler, comprehensively utilizes two LN2 beams in the multichannel subcooler and the multichannel condenser to cool and reliquefy BOG under the temperature gradients of different temperature differences, does not directly liquefy BOG by utilizing the traditional single LN2 beam, and can add various degrees of freedom by distributing different flow to the multichannel precooler and the multichannel condenser so as to meet BOG recycling requirements of different occasions and different LNG storage tanks;
4. the LN2 regeneration cycle ensures continuous supply of the system cold energy while no power devices such as an additional refrigerator and the like are added, and meets the energy-saving generation and transportation requirements;
In summary, the system has the characteristics of compact structure, no freezing, high-efficiency liquefaction and the like, the system and the method not only can re-liquefy and recycle the BOG according to the design requirement, but also can supply cold through LN2 repeated circulation, the required power device is less, the BOG can drive the liquefaction circulation through self pressure in the early stage, an additional refrigerator is not required in the LN2 circulation and the regeneration process, the energy consumption of related equipment is reduced, and the system and the method have the advantages of simple structure, safety and energy conservation, and meet the requirement of re-liquefying and recycling the BOG with large flow.
Drawings
In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings that are required to be used in the embodiments or the description of the prior art will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments described in the present application, and other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
Fig. 1 is a schematic structural diagram of a BOG self-circulation re-liquefaction recovery heat exchange system for an LNG storage tank according to embodiment 1 of the present invention;
FIG. 2 is a schematic diagram of the apparatus structure of the cyclical heat exchange network of FIG. 1;
FIG. 3 is a schematic diagram of the arrangement of heat exchanging channels of a multi-channel precooler of the circulating heat exchanging network in embodiment 1 of the present invention;
FIG. 4 is a schematic diagram of the arrangement of heat exchanging channels of the multi-channel subcooler and the multi-channel condenser of the circulating heat exchanging network in embodiment 1 of the present invention;
in the figure: LNG, LNG storage tank 1, multichannel precooler, 2, multichannel subcooler, 3, multichannel condenser, 4, expander, P-LNG, LNG liquid pump, P-LN2, LN2 liquid pump, LN2, liquid nitrogen storage tank, J-T, choke valve, P-1, pressure switch valve.
Detailed Description
It should be noted that, without conflict, the embodiments of the present invention and features of the embodiments may be combined with each other. The invention will be described in detail below with reference to the drawings in connection with embodiments.
For the purpose of making the objects, technical solutions and advantages of the embodiments of the present invention more apparent, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention, and it is apparent that the described embodiments are only some embodiments of the present invention, not all embodiments. The following description of at least one exemplary embodiment is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of exemplary embodiments according to the present invention. As used herein, the singular is also intended to include the plural unless the context clearly indicates otherwise, and furthermore, it is to be understood that the terms "comprises" and/or "comprising" when used in this specification are taken to specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof.
The relative arrangement of the components and steps, numerical expressions and numerical values set forth in these embodiments do not limit the scope of the present invention unless it is specifically stated otherwise. Meanwhile, it should be clear that the dimensions of the respective parts shown in the drawings are not drawn in actual scale for convenience of description. Techniques, methods, and apparatus known to those of ordinary skill in the relevant art may not be discussed in detail, but are intended to be part of the specification where appropriate. In all examples shown and discussed herein, any specific values should be construed as merely illustrative, and not a limitation. Thus, other examples of the exemplary embodiments may have different values. It should be noted that: like reference numerals and letters denote like items in the following figures, and thus once an item is defined in one figure, no further discussion thereof is necessary in subsequent figures.
In the description of the present invention, it should be understood that the azimuth or positional relationships indicated by the azimuth terms such as "front, rear, upper, lower, left, right", "lateral, vertical, horizontal", and "top, bottom", etc., are generally based on the azimuth or positional relationships shown in the drawings, merely to facilitate description of the present invention and simplify the description, and these azimuth terms do not indicate and imply that the apparatus or elements referred to must have a specific azimuth or be constructed and operated in a specific azimuth, and thus should not be construed as limiting the scope of protection of the present invention: the orientation word "inner and outer" refers to inner and outer relative to the contour of the respective component itself.
Spatially relative terms, such as "above … …," "above … …," "upper surface at … …," "above," and the like, may be used herein for ease of description to describe one device or feature's spatial location relative to another device or feature as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as "above" or "over" other devices or structures would then be oriented "below" or "beneath" the other devices or structures. Thus, the exemplary term "above … …" may include both orientations of "above … …" and "below … …". The device may also be positioned in other different ways (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.
In addition, the terms "first", "second", etc. are used to define the components, and are only for convenience of distinguishing the corresponding components, and the terms have no special meaning unless otherwise stated, and therefore should not be construed as limiting the scope of the present invention.
Example 1
As shown in fig. 1, the BOG self-circulation re-liquefaction recovery heat exchange system for the LNG storage tank comprises a BOG liquefaction unit, a circulation heat exchange network and an LN2 regeneration unit, wherein the BOG liquefaction unit and the LN2 regeneration unit are connected with circulation heat exchange and regeneration through the circulation heat exchange network; wherein:
the BOG liquefaction circulation unit comprises an LNG storage tank LNG, a pressure switch valve P-1 and an LNG return pump P-LNG; the LNG storage tank LNG is communicated with the inlet of the circulating heat exchange network through an LNG pressure switching valve P-1, and is communicated with the outlet of the circulating heat exchange network through an LNG return pump P-LNG; the superheated BOG in the LNG storage tank enters a circulating heat exchange network through a safety pressure switch valve P-1, liquefied LNG after being completely condensed through the circulating heat exchange network is returned to the LNG storage tank for continuous storage through an LNG return pump P-LNG connected with an outlet of the circulating heat exchange network;
as shown in fig. 2, the circulating heat exchange network is an integrated composite circulating heat exchange device formed by packaging and integrating three parts of the multi-channel precooler 1, the multi-channel subcooler 2 and the multi-channel condenser 3 in the same heat exchanger, and is convenient to connect and use. The multi-channel precooler 1, the multi-channel subcooler 2 and the multi-channel condenser 3 are all plate-fin type, plate type, winding pipe type or shell-and-tube type multi-flow heat exchangers. The multi-channel precooler 1 is connected with a pressure switch valve P-1, receives overheated BOG of an LNG storage tank, the multi-channel precooler 1 is communicated with the multi-channel subcooler 2 to form a BOG precooling circulation loop, the multi-channel precooler 1 is communicated with the multi-channel condenser 3 to form a BOG condensation liquefaction circulation loop, meanwhile, the multi-channel precooler 2 and the multi-channel condenser 3 respectively receive two LN2 of an LN2 circulation regeneration unit as cold sources, the two LN2 are respectively used as cold sources and enter the BOG precooling circulation loop and the BOG condensation liquefaction circulation loop to exchange heat with the BOG, overheated steam LN2 after the LN2 outlet of the multi-channel precooler 2 and the LN2 outlet of the multi-channel condenser 3 exchange heat flows back to the LN2 circulation regeneration unit, and the LNG outlet of the multi-channel condenser 3 after condensation liquefaction is connected with an LNG return pump P-LNG;
The heat exchange channels in the multichannel precooler 1, the multichannel subcooler 2 and the multichannel condenser 3 all adopt fin structures, and in consideration of the fact that LN2 liquid has high viscosity, the heat exchange channels preferably adopt straight fins or perforated fins, and the heat exchange channels of overheated and supercooled vapor of nitrogen and BOG preferably adopt zigzag fins or corrugated fins. And the arrangement of the multi-channel precooler 1, the multi-channel subcooler 2 and the multi-channel condenser 3 is configured in a single-layer, double-layer or combined manner of cold and hot fluid, i.e. one cold (or hot) channel is sandwiched between two adjacent hot (or cold) channels, and stacking is repeated.
As shown in fig. 3, the multi-channel precooler 1 is a single multi-flow plate-fin heat exchanger, and comprises 5 heat exchange fluids BOG-1, BOG-3, BOG-5, BOG-7 and BOG-9, 12 layers of heat exchange channels, wherein the arrangement mode of the heat exchange channels adopts a cold and hot fluid single-layer, double-layer or combined mode, namely, one layer of cold (or hot) channels is clamped between two adjacent layers of hot (or cold) channels, and the heat exchange channels are repeatedly stacked; a vacuum heat insulation layer K is respectively arranged on two sides of the multi-channel precooler 1, and sealing heads, distributors and the like are arranged on two ends of an inlet and an outlet of the multi-channel precooler 1.
As shown in fig. 4, the multi-channel subcooler 2 and the multi-channel condenser 3 are combined into an integrally structured heat exchanger, and the arrangement mode of the heat exchange channels is configured by adopting a single-layer, double-layer or combined mode of cold and hot fluid, namely, a layer of cold (or hot) channels is sandwiched between two adjacent layers of hot (or cold) channels, and the heat exchange channels are repeatedly stacked; the multichannel subcooler 2 comprises 2 strands of heat exchange fluid BOG-2 and LN2-2,3 layers of heat exchange channels, and heat exchange fins are arranged in the channels; the multi-channel condenser 3 comprises 5 heat exchange fluids BOG-4, BOG-6, BOG-8, BOG-10 and LN2-4, 17 layers of heat exchange channels, heat exchange fins are arranged in the heat exchange channels, and vacuum heat insulation layers K are respectively arranged on two sides of the multi-channel subcooler 2 and the multi-channel condenser 3 and used for isolating heat transfer between the heat exchanger and the outside as well as between the heat exchanger; and the two ends of the inlet and the outlet are provided with a sealing head, a distributor and the like.
As shown in fig. 1, the LN2 circulation regeneration unit comprises an LN2 storage tank LN2, an LN2 liquid pump P-LN2, an expansion machine 4, a J-T throttle valve and a connected low-temperature pipeline, wherein the LN2 storage tank is connected with the LN2 liquid pump P-LN2, the LN2 liquid pump P-LN2 is connected with a distributor, two LN2 distributed by the distributor respectively enter a multi-channel subcooler 2 and a multi-channel condenser 3 of a circulation heat exchange network, two superheated steam LN2 flowing out of the multi-channel subcooler 2 and the multi-channel condenser 3 after heat exchange is collected and enters the expansion machine 4, and the expansion machine 4 is connected with the LN2 storage tank through the J-T throttle valve, so that the LN2 regenerated by the expansion machine 4 is returned to the LN2 storage tank.
In the BOG self-circulation re-liquefaction recovery heat exchange system for the LNG storage tank, BOG gas heat is finally absorbed by two liquid nitrogen fluids, namely LN2-2 in the multichannel subcooler 2 and LN2-4 in the multichannel condenser 3 in the circulation heat exchange network, under different temperature gradient conditions. The circulating heat exchange network adopts a plurality of circulations to pre-cool and liquefy the BOG, the BOG-1 at the inlet is BOG gas, and the LNG-0 at the outlet is LNG liquid.
The BOG self-circulation re-liquefaction recovery heat exchange method for the LNG storage tank is characterized by comprising the following steps of:
(1) Superheated BOG precooling
Due to the influence of factors such as environment, manual operation and the like, BOG in the LNG storage tank is continuously increased, the pressure in the LNG storage tank reaches a safe limit value, then the pressure switch valve P-1 of the LNG storage tank is activated, overheated BOG-0 in the LNG storage tank is converted into BOG-1 through the pressure switch valve P-1 and is discharged into the multi-channel precooler 1 of the circulating heat exchange network, and the BOG-1 is used as hot fluid to be precooled by the first reflux supercooled BOG-3 in the multi-channel precooler 1 after heat exchange between the multi-channel precooler 2 and the LN 2-2; the pre-cooled liquid is changed into BOG-2 and flows out of the multi-channel pre-cooler 1, and then enters the subcooler 2 continuously to be used as hot fluid, and the hot fluid exchanges heat with LN2-2 and flows back to the first reflux supercooled BOG-3 converted into the multi-channel pre-cooler 1;
(2) BOG condensation liquefaction
The first reflux supercooled BOG-3 is changed into BOG-4 after heat exchange with the BOG-1 in the multi-channel precooler 1, and enters the multi-channel condenser 3 as a hot fluid to be condensed, heat exchanged and liquefied with LN2-4, so that the first liquefaction cycle is completed; non-liquefied BOG (BOG-5, BOG-7, BOG-9) in the multi-channel condenser 3 flows back to the multi-channel precooler 1 again as the start of the next liquefaction cycle, and the pre-cooled initial overheated BOG flows back to the multi-channel condenser 3 again for continuous condensation;
(3) Reflux after BOG liquefaction
After repeated pre-cooling and liquefaction circulation for 4 times through a circulation heat exchange network, the final BOG is liquefied into LNG-0 in a condenser 3, and the LNG is conveyed back into an LNG storage tank through an LNG pump P-LNG, so that the liquefaction recovery circulation of the BOG is completed;
in the above-mentioned BOG liquefaction recovery cycle, the circulating power is provided by the initial pressure of the superheated BOG and the LNG return pump P-LNG, the BOG is liquefied and recovered by the circulating heat exchange network, the sensible heat of the BOG is absorbed by the first LN2 in the multichannel subcooler 2, and the remaining BOG liquefaction latent heat is continuously absorbed by the second LN2 in the multichannel condenser 3.
(4) LN2 cooling cycle
After the LN2 used for liquefying the BOG is gasified by absorbing heat, the generated nitrogen is required to be liquefied and regenerated so as to ensure the circulating operation of the whole working system. Liquid nitrogen LN2-0 in the LN2 storage tank is changed into LN2-1 after being pressurized by a liquid pump P-LN2, the LN2-1 is divided into two liquid nitrogen flows LN2-2 and LN2-4 by a distributor, wherein LN2-2 is introduced into the multichannel subcooler 2 to absorb sensible heat of BOG, and LN2-4 is introduced into the multichannel condenser 3 to absorb latent heat of BOG; the two LN2 heat exchanges to become nitrogen vapor LN2-3 and LN2-5, and the nitrogen vapor LN2-3 and LN2-5 are collected into the same low-temperature pipeline to become LN2-6; then the LN2 is expanded, cooled and converted into LN2-7 through the expander 4, and converted into liquid nitrogen LN2-8 again through the J-T throttle valve and returned into the LN2 storage tank, thereby completing the cooling regeneration cycle of LN 2.
In the BOG self-circulation re-liquefaction recovery heat exchange method for the LNG storage tank, according to the required BOG flow, the circulation times n of the circulation heat exchange network can be designed, and the relation between the circulation heat exchange times n of the circulation heat exchange network and the BOG flow required to be liquefied is as follows:
Figure BDA0001897849910000131
/>
wherein: m is BOG mass flow, r is BOG liquefied latent heat, c p And t is the temperature of each corresponding fluid in the heat exchange network in fig. 1, wherein the temperature of the liquid nitrogen and the superheated steam thereof is controlled by LN2 circulation flow, namely, the temperature of the liquid nitrogen and the superheated steam thereof in the heat exchange network is controlled by controlling the flow of the liquid nitrogen in the heat exchange network and the ratio of two liquid nitrogen fluids.
The BOG liquefaction condensation cycle number n can be calculated according to energy conservation in a heat exchange network, and the relation among the heat exchange quantity Q, the mass flow m and the cycle heat exchange number n of the BOG and the LN2 is as follows:
Figure BDA0001897849910000141
wherein: m is m BOG And m LN2 The mass flow rates are BOG and LN2 respectively, h is the enthalpy value of each corresponding fluid in the heat exchange network in FIG. 1, deltah is the enthalpy difference, and n represents the number of circulating heat exchange times in the heat exchange network.
The liquid nitrogen LN2 is divided into two streams, the flow rates of the first stream LN2-2 and the second stream LN2-4 are specifically selected according to the temperature and physical properties of the BOG, the circulation times of different heat exchange networks and the external environment working conditions, and the calculation relational expression is as follows:
Figure BDA0001897849910000142
Wherein: m is m BOG And m LN2 The mass flow rates of BOG and LN2, respectively, T is the temperature of each corresponding fluid in the heat exchange network of FIG. 1, r BOG And r LN2 Liquefaction dives of BOG respectivelyHeat and latent heat of vaporization of LN2, c p N represents the number of heat exchange cycles in the heat exchange network for the specific heat capacity of the corresponding fluid.
In the BOG self-circulation re-liquefaction recovery heat exchange network and the device for the LNG storage tank in the embodiment, the circulation frequency n of the circulation heat exchange network is set to be 4 times, in the multi-channel precooler 1, the initial BOG is designed to be 4 layers of channels as a hot fluid passage, and the supercooled BOG circulated each time is set to be 2 layers of channels and circulated 4 times as a cold fluid passage, and the total number of channels is 8 layers. The arrangement of the channels of the multi-channel precooler 1 is shown in fig. 3, and the arrangement is a sandwich arrangement of cold fluid and hot fluid, namely, 2 layers of cold fluid channels in the same circulation wrap 1 layer of hot fluid channels. In the multichannel subcooler 2, the first LN2-2 cold fluid is distributed as a 2-layer channel, the pre-cooled BOG is distributed as a 1-layer channel as a hot fluid, and the cold and hot fluids are arranged alternately with each other. In the multichannel condenser 3, BOG supercooled vapor for each cycle is set as a heat fluid to 2-layer channels for 4 cycles, totaling 8-layer channels; while the second LN2-4 stream is set as a 9-layer channel as a cold fluid, alternately arranged and surrounding each BOG supercooling passage. The integrated structure of the multi-channel subcooler 2 and the multi-channel condenser 3 is shown in fig. 4.
As shown in the schematic diagram of the circulating heat exchange network in FIG. 1, partial LNG is gasified into BOG-0 by the LNG storage tank due to factors such as filling, operation, environmental heat leakage and the like, so that the pressure in the LNG storage tank rises, when the pressure in the LNG storage tank rises to a safe preset limit value, the pressure control valve P-1 is opened, and the BOG-1 with certain pressure and temperature is sent into the circulating heat exchange network.
First, during the BOG liquefaction cycle: the BOG-1 transfers high-temperature sensible heat to self supercooling steam (BOG-3, BOG-5, BOG-7, … and BOG-2n+1) through the multi-channel precooler 1, then changes into BOG-2, enters the multi-channel precooler 2, exchanges heat with a first stream of liquid nitrogen LN2-2, returns to the multi-channel precooler 1 as a first reflux supercooling fluid after being cooled by the liquid nitrogen LN2-2 to become BOG-3, and continuously precools the BOG-1; after precooling the BOG-1, the supercooled steam (BOG-3, BOG-5, BOG-7, …, BOG-2n+1) is changed into superheated steam (BOG-4, BOG-6, BOG-8, …, BOG-2 n) which continuously flows into the multi-channel condenser 3 in a circulating way; in the multi-channel condenser 3, a second liquid nitrogen LN2-4 circulates and liquefies superheated steam BOG-4, BOG-6, BOG-8, … and BOG-2n, and after heat exchange, the BOG-4 is changed into BOG-5 and returns to the multi-channel precooler 1 again, so that the first circulation of the heat exchange network is completed; BOG-5 becomes a second reflux subcooled fluid which is returned to the multi-channel precooler 1 to continue precooling BOG-1, thereby opening the next cycle.
In the manner described above, BOG-1 releases the sensible heat of temperature to its own subcooled vapor (BOG-3, BOG-5, BOG-7, …, BOG-2n+1), which releases heat to the first liquid nitrogen fluid LN2-2 through the multichannel subcooler 2 and continues to release the latent heat of phase change of BOG to the second liquid nitrogen fluid LN2-4 in the condenser 3. In the multi-channel precooler 1 and the multi-channel condenser 3, after the circulation is carried out for n times, LNG-0 is finally output from the outlet of the condenser 3 as a reliquefaction product of BOG, and the reliquefaction product is returned to an LNG storage tank through a liquid natural gas pump P-LNG, so that the BOG self-circulation reliquefaction recovery heat exchange process is completed.
Secondly, during LN2 cyclic regeneration: leading out liquid nitrogen LN2-0 from a liquid nitrogen storage tank LN2, pressurizing by a pump P-LN2 to become LN2-1, dividing the liquid nitrogen LN2-0 into two parts according to the specific working condition requirement, leading one part of liquid nitrogen LN2-2 into a multi-channel subcooler 2 in a heat exchange network for subcooling the BOG-2, changing the LN2-2 into LN2-3 after heat exchange, and flowing out of the multi-channel subcooler 2; the other liquid nitrogen LN2-4 is introduced into the multi-channel condenser 3 for condensing BOG-4, BOG-6, BOG-8, … and BOG-2n, and flows out of the multi-channel condenser 3 as LN2-5 after heat exchange. After the heat exchange, LN2-3 and LN2-5 nitrogen with certain pressure are converged into LN2-6, and are introduced into the expander 4 to be expanded at low temperature and cooled into LN2-7 fluid, and then the LN2-7 fluid is re-liquefied into liquid nitrogen LN2-8 through the J-T throttle valve and returned to the liquid nitrogen storage tank LN 2.
In this embodiment, the circulating heat exchange network adopts a multi-channel plate-fin heat exchanger, as shown in fig. 3 and fig. 4, and the specific structure and the pipeline arrangement are described as follows:
the multi-pass plate-fin precooler exchanges heat for 1 hot fluid BOG-1 and 4 cold fluids BOG-3, BOG-5, BOG-7, BOG-9, as shown in fig. 3. The hot fluid BOG-1 is distributed into 4 layers of channels, fins of each layer of channels are saw-tooth fins, the fin height is 9.5mm, the fin width is 1.4mm, the fin thickness is 0.2mm, the saw-tooth pitch is 3mm, BOG-1 in the 4 layers of channels is converged by guide vanes to corresponding end sockets to be discharged, and the model of the guide vanes is 95D4205; the subcooled steam BOG-3, the LNG-5, the LNG-7 and the LNG-9 are divided into 4 heat exchange circulation loops, each loop is divided into 2 layers, 8 layers of heat exchange channels are combined, fins of each layer of channels are saw-tooth fins, the height of each fin is 6.5mm, the width of each fin is 1.4mm, the thickness of each fin is 0.2mm, the pitch of each fin is 3mm, BOG in the 8 layers of channels is converged to the corresponding end socket by a guide vane and discharged, and the model of the guide vane is 65D4205. In addition, in order to reduce the size of the heat exchanger and facilitate the distribution of the channel arrangement, the heat exchange layer of the hot fluid is designed into a symmetrical structure, and the proportion is 1 to 1, namely: the 4 channels include BOG-3 and BOG-5, and the 4 layer channels include BOG-7 and BOG-9. For proper heat preservation, two ends are respectively provided with a heat insulation layer K. Each layer of channel consists of a seal head, a seal strip, a side plate, a guide vane and heat exchange fins. The 4 heat exchange loops BOG-3, BOG-5, BOG-7 and BOG-9 in the multi-channel precooler 1 adopt sequential heat exchange instead of simultaneous heat exchange, so that the cold energy recovery efficiency can be improved to a greater extent.
The multi-channel plate-fin condenser 3 exchanges heat for 1 cold fluid LN2-4 and 4 hot fluids BOG-4, BOG-6, BOG-8, BOG-10, as shown in FIG. 4. Wherein superheated steam BOG-4, BOG-6, BOG-8 and BOG-10 are divided into 4 heat exchange circulation loops, each loop is divided into 2 layers, 8 layers of heat exchange channels are combined, fins of each layer of channels are perforated fins, the height of each fin is 6.5mm, the width of each fin is 1.4mm, the fin thickness is 0.2mm, BOG condensate in each layer of channels is converged to a corresponding end socket by a guide vane and discharged, and the model of the guide vane is 65D4205. In addition, in order to reduce the size of the heat exchanger and facilitate the distribution of the channel arrangement, the heat exchange layer of the hot fluid is designed into a symmetrical structure, and the proportion is 1 to 1, namely: the 4-layer channels include BOG-4 and BOG-6, and the 4-layer channels include BOG-8 and BOG-10. 1 strand of cold fluid in the multi-channel plate-fin condenser 3 is distributed into 9 layers of heat exchange channels, fins of each layer of channels are flat fins, the fin height is 6.5mm, the fin width is 2mm, the fin thickness is 0.3mm, and cold fluid in the 9 layers of channels is converged to corresponding sealing heads by guide vanes to enter and discharge, and the model of the guide vanes is 65D4205. The 4 heat exchange circulation loops BOG-4, BOG-6, BOG-8 and BOG-10 in the multi-channel plate-fin condenser 3 also exchange heat sequentially instead of simultaneously so as to further improve the cold energy recovery efficiency.
In order to reduce the constructional size, the multichannel subcooler 2 is integrated inside the multichannel condenser 3 and is separated by 2 insulating layers K. The multichannel subcooler 2 is provided with 2 liquid nitrogen LN2-2 heat exchange layers and 1 BOG-2 channel layer for heat exchange, and the structure is the same as the structure and is not repeated. Each layer of channels consists of a seal head, a seal strip, a side plate, a guide vane and heat exchange fins, and the arrangement condition of the specific channels is shown in fig. 4. Finally, a plurality of heat exchange channels are connected in series through corresponding pipelines to form compact packaged integral heat exchange equipment.
According to the invention, BOG in the LNG storage tank is subjected to pressure regulation and control, is introduced into the circulating heat exchange network for efficient reliquefaction and recovery, and LN2 in the system is regenerated through ingenious design so as to continuously supply cold energy to the circulating heat exchange network in a circulating way. The invention has simple structure and wide application, can be custom-designed according to the special requirements of different LNG storage tanks in different occasions, and can meet the BOG reliquefaction requirements of various large, medium and small LNG storage tanks. Meanwhile, the heat exchange network enables the BOG to be completely liquefied in multiple circulation through ingenious design, and the cooling medium is LN2 and self-supercooled steam of the BOG, so that the heat exchange network cannot be frozen. In addition, the regeneration cycle process of LN2 ensures continuous supply of the cold energy of the system without adding power devices such as an additional refrigerator and the like. Therefore, the invention provides a compact and efficient heat exchange network and equipment for the BOG re-liquefaction recovery process in the LNG storage tank.
Finally, it should be noted that: the above embodiments are only for illustrating the technical solution of the present invention, and not for limiting the same; although the invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical scheme described in the foregoing embodiments can be modified or some or all of the technical features thereof can be replaced by equivalents; such modifications and substitutions do not depart from the spirit of the invention.

Claims (5)

  1. The BOG self-circulation re-liquefaction recovery heat exchange system for the LNG storage tank is characterized by comprising a BOG liquefaction unit, a circulation heat exchange network and an LN2 regeneration unit, wherein the BOG liquefaction unit and the LN2 regeneration unit are connected with circulation heat exchange and regeneration through the circulation heat exchange network; wherein:
    the BOG liquefaction circulation unit comprises an LNG storage tank (LNG), a pressure switch valve (P-1) and an LNG return pump (P-LNG); the LNG storage tank (LNG) is communicated with the inlet of the circulating heat exchange network through an LNG pressure switching valve (P-1), and is communicated with the outlet of the circulating heat exchange network through an LNG return pump (P-LNG); overheated BOG in an LNG storage tank (LNG) enters a circulating heat exchange network through a safety pressure switch valve (P-1), liquefied LNG after being completely condensed through the circulating heat exchange network returns to the LNG storage tank (LNG) through an LNG return pump (P-LNG) connected with an outlet of the circulating heat exchange network for continuous storage;
    The circulating heat exchange network comprises a multi-channel precooler (1), a multi-channel subcooler (2), a multi-channel condenser (3) and pipelines correspondingly connected, wherein the multi-channel precooler (1), the multi-channel subcooler (2) and the multi-channel condenser (3) are multi-stream heat exchangers, the multi-channel precooler (1) is connected with a pressure switch valve (P-1) to receive superheated BOG of an LNG storage tank, the multi-channel precooler (1) is communicated with the multi-channel subcooler (2) to form a BOG precooling circulation loop, the multi-channel precooler (1) is communicated with the multi-channel condenser (3) to form a BOG condensation liquefaction circulation loop, simultaneously, the multi-channel precooler (2) and the multi-channel condenser (3) respectively receive two LN2 of the LN2 circulation regeneration unit as cold sources, the BOG precooling circulation loop and the BOG condensation liquefaction circulation loop exchange heat with the BOG, the superheated LN2 after heat exchange is carried out by the multi-channel precooler (2) and an LN2 outlet of the multi-channel condenser (3) returns to the 2 circulation regeneration unit, and the LNG (LNG) returns to the LNG pump (P) after the multi-channel condenser (3) is connected with the LNG outlet;
    the LN2 circulating regeneration unit comprises an LN2 storage tank (LN 2), an LN2 liquid pump (P-LN 2), an expander (4), a J-T throttle valve and a low-temperature pipeline connected with the LN2 storage tank (LN 2), the LN2 liquid pump (P-LN 2) is connected with a distributor, two LN2 distributed by the distributor respectively enter a multi-channel subcooler (2) and a multi-channel condenser (3) of a circulating heat exchange network, two superheated steam LN2 flowing out of the multi-channel subcooler (2) and the multi-channel condenser (3) after heat exchange is collected into the expander (4), the expander (4) is connected with the LN2 storage tank (LN 2) through the J-T throttle valve, and the LN2 regenerated by the expander (4) is returned to the LN2 storage tank (LN 2);
    The multi-channel precooler (1), the multi-channel subcooler (2) and the multi-channel condenser (3) of the circulating heat exchange network are plate-fin type, plate type, winding pipe type or shell-and-tube type multi-flow heat exchangers;
    the circulating heat exchange network is an integrated composite circulating heat exchange device formed by packaging and integrating a multi-channel precooler (1), a multi-channel subcooler (2) and a multi-channel condenser (3) into one heat exchanger;
    the heat exchange channels in the multichannel precooler (1), the multichannel subcooler (2) and the multichannel condenser (3) all adopt fin structures, the heat exchange channels are preferably flat fins or perforated fins in consideration of the large viscosity of LN2 liquid, and the heat exchange channels of overheated and supercooled vapor of nitrogen and BOG are preferably saw-tooth type or corrugated type fins;
    the arrangement mode of the multi-channel precooler (1), the multi-channel subcooler (2) and the multi-channel condenser (3) adopts a cold-hot fluid single-layer, double-layer or combined mode to configure, namely, one layer of cold channels are clamped between two layers of adjacent hot channels or one layer of hot channels are clamped between two layers of adjacent cold channels, and the two layers of adjacent cold channels are repeatedly stacked.
  2. 2. The BOG self-circulation re-liquefaction recovery heat exchange system for the LNG storage tank according to claim 1, wherein the multi-channel precooler (1) is a single multi-flow plate-fin heat exchanger, the multi-channel subcooler (2) and the multi-channel condenser (3) are combined into a heat exchanger with an integrated structure, industrial plate-fin heat exchanger fins are selected in a heat exchange channel, and vacuum heat insulation layers (K) are respectively arranged on two sides of the multi-channel precooler (1), the multi-channel subcooler (2) and the multi-channel condenser (3) and used for isolating the heat exchanger from the outside and heat transfer between the heat exchangers.
  3. The BOG self-circulation re-liquefaction recovery heat exchange method for the LNG storage tank is characterized by comprising the following steps of:
    (1) Superheated BOG precooling
    Activating a pressure switch valve (P-1) after the pressure in an LNG storage tank (LNG) reaches a safe limit value, discharging overheated BOG into a multi-channel precooler (1) of a circulating heat exchange network through the pressure of the overheated BOG, performing precooling heat exchange by using a first reflux supercooling BOG which flows back to the multi-channel precooler (1) after heat exchange between the multi-channel precooler (2) and LN2, and then entering the multi-channel precooler (2) to be used as the first reflux supercooling BOG in the multi-channel precooler (1) after heat exchange between the overheated BOG and the LN 2;
    (2) BOG condensation liquefaction
    The first reflux supercooled BOG enters a multichannel condenser (3) to be condensed, heat-exchanged and liquefied with LN2 after being regenerated by the multichannel precooler (1), so that the first liquefaction cycle is completed; the non-liquefied BOG flows back to the multi-channel precooler (1) again, and as the start of the next liquefaction cycle, the pre-cooled initial overheated BOG flows back to the multi-channel condenser (3) again for continuous condensation;
    (3) Reflux after BOG liquefaction
    After repeated precooling and liquefaction circulation of the circulation heat exchange network are carried out for n times, the circulation times n is more than or equal to 2, and finally the BOG is liquefied into LNG, and is conveyed back into an LNG storage tank (LNG) through an LNG pump (P-LNG), so that the liquefaction recovery circulation of the BOG is completed;
    (4) LN2 cooling cycle
    Liquid nitrogen LN2 in the LN2 storage tank (LN 2) is divided into two streams, after being pressurized by an LN2 liquid pump (P-LN 2), one stream is introduced into the multichannel subcooler (2) to absorb the sensible heat of the BOG, and the other stream is introduced into the multichannel condenser (3) to absorb the latent heat of the BOG; two LN2 heat exchanges to become nitrogen steam which is collected in the same low-temperature pipeline; then the expansion machine (4) is used for expansion and temperature reduction, and the expansion machine is used for converting the expansion water into liquid again through the J-T throttle valve and returning the liquid to the LN2 storage tank, so that the cooling regeneration cycle of LN2 is completed;
    the relationship between the number of circulating heat exchange times n of the circulating heat exchange network and the BOG flow required to be liquefied is as follows:
    Figure FDA0004187136800000041
    wherein: m is BOG mass flow, r is BOG liquefied latent heat, c p Is the specific heat capacity of BOG, t LNG-0 Temperature of LNG flowing out of the outlet of the circulation network, t BOG-0 Initial superheat BOG temperature for the loop network entry, t BOG-3 T is the temperature of the first reflux supercooled BOG in the multi-channel precooler after heat exchange with LN2 by the multi-channel subcooler LN2-2 And t LN2-3 Liquid nitrogen at inlet end and outlet end of multichannel subcooler and temperature of superheated steam of liquid nitrogen at inlet end and outlet end of multichannel subcooler respectively LN2-4 And t LN2-5 The temperature of the liquid nitrogen and the superheated steam thereof at the inlet and the outlet of the multichannel condenser are controlled by LN2 circulation flow.
  4. 4. The BOG self-circulation re-liquefaction recovery heat exchange method for the LNG storage tank according to claim 3, wherein the relationship between the number of BOG liquefaction condensation cycles n and the heat exchange amounts Q, mass flow rates m and the number of cycle heat exchange times n of BOG and LN2 is as follows:
    Figure FDA0004187136800000042
    Figure FDA0004187136800000043
    wherein: m is m BOG And m LN2 Mass flow rates, h, of BOG and LN2, respectively BOG-0 And h LNG-0 Enthalpy values h of the superheated BOG and LNG at the inlet and outlet of the circulation network, respectively LN2-2 And h LN2-3 Liquid nitrogen at inlet end and outlet end of multichannel subcooler and enthalpy value of superheated steam of liquid nitrogen, h LN2-4 And h LN2-5 The enthalpy values of liquid nitrogen and superheated steam thereof respectively at the inlet end and the outlet end of the multichannel condenser are respectively shown as delta h, delta h is enthalpy difference, n represents the number of circulating heat exchange in a heat exchange network, delta h is shown as delta h BOG-2n Is the enthalpy difference of the bog fluid of the nth circulation in the multichannel precooler, delta h sup Is the enthalpy difference of fluid in the multichannel precooler, deltah LN2-2n+1 Is the enthalpy difference of the n-th circulating LN2 fluid in the multichannel precooler.
  5. 5. The BOG self-circulation re-liquefaction recovery heat exchange method for the LNG storage tank according to claim 3, wherein the liquid nitrogen LN2 is divided into two streams, the flow rates of the first LN2 stream and the second LN2 stream are specifically selected according to the temperature, physical properties of the BOG, the circulation times n of different heat exchange networks and external environment conditions, and the calculation relation is as follows:
    Figure FDA0004187136800000051
    m LN2-2 c p,LN2 (T LN2-2 -T LN2-3 )+m LN2-4 c p,LN2 (T LN2-4 -T LN2-5 )+r LN2 (m LN2-2 +m LN2-4 )
    Wherein: m is m BOG And m LN2 Mass flow rates, m, of BOG and LN2, respectively LN2-2 And m LN2-4 Mass flow rates, T, of two LN2 fluids respectively BOG-2n T is the temperature of the BOG superheated steam subjected to heat exchange by the multi-channel precooler in the n-1 th cycle BOG-2n+1 For the temperature of the BOG subcooled steam of the nth reflux multi-channel precooler, T LN2-2 And T LN2-3 Liquid nitrogen at inlet end and outlet end of multichannel subcooler and temperature of superheated steam of liquid nitrogen at inlet end and outlet end of multichannel subcooler respectively, T LN2-4 And T LN2-5 The temperature of liquid nitrogen and superheated steam thereof respectively at the inlet end and the outlet end of the multichannel condenser, r BOG And r LN2 The latent heat of liquefaction of BOG and the latent heat of vaporization of LN2, c p C is the specific heat capacity of the corresponding fluid p，BOG C is the specific heat capacity of BOG p，LN2 And n represents the number of circulating heat exchange times in the heat exchange network, wherein the specific heat capacity is LN 2.
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