CN107560321B - BOG recovery and nitrogen liquefaction system and technological method - Google Patents
BOG recovery and nitrogen liquefaction system and technological method Download PDFInfo
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- CN107560321B CN107560321B CN201710835492.XA CN201710835492A CN107560321B CN 107560321 B CN107560321 B CN 107560321B CN 201710835492 A CN201710835492 A CN 201710835492A CN 107560321 B CN107560321 B CN 107560321B
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Abstract
The invention discloses a BOG recycling and nitrogen liquefying system and a process method, wherein the system carries out secondary cooling on nitrogen prepared by a PSA nitrogen preparing device through LNG cold energy to liquefy the nitrogen and store the liquefied nitrogen; in the BOG liquefaction system, BOG is pressurized by a compressor and then exchanges heat with liquid nitrogen to liquefy the BOG, and the liquid nitrogen absorbs heat to be gasified and stored in a nitrogen storage tank, so that the BOG can be used for station sweeping; the liquefied BOG enters a high-pressure pump for external transportation or is throttled and decompressed, enters a separator, the liquid phase is injected into the bottom of an LNG storage tank for storage, and the gas phase enters a BOG compression cycle. The method solves the problem of difficult BOG recovery under the condition of low external transmission capacity of the LNG receiving station, meets the requirements of a station yard low-temperature purging process, improves the energy utilization efficiency by utilizing LNG cold energy, and reduces the equipment investment by using the existing PSA nitrogen making equipment of the LNG station yard.
Description
Technical Field
The invention relates to the technical field of LNG storage and transportation, in particular to a BOG recovery and nitrogen liquefaction system and a process method.
Background
With the increasingly prominent atmospheric pollution problems, the demands of China for clean energy are becoming urgent. LNG (liquefied natural gas) is used as a clean energy source of natural gas in a storage and transportation mode, and is widely used for coastal city gas guarantee and peak shaving. LNG receiving stations in China are widely distributed in coastal cities, and a large amount of BOGs (flash evaporation gas) generated in the operation process of the LNG receiving stations threatens the safety of the LNG receiving stations. BOG is the gas that LNG is heated and gasified and forms, because of the insulation material restriction, unavoidable infiltration heat from the external world causes LNG to be heated and evaporated. With the continuous invasion of heat, the gas phase mass is increased, the pressure bearing of the pressure vessel is increased, if the BOG is released before the pressure of the vessel can be borne, the equipment is greatly damaged, and the tank body is damaged by overpressure. When the gas phase space reaches a certain pressure, a safety relief valve is triggered, and the redundant flash gas in the tank is directly discharged to a torch for emptying and burning. The total amount of BOGs in a large LNG station is large, and direct discharge causes energy waste and environmental pollution.
The BOG processing method for realizing application in the LNG receiving station in China at present mainly comprises the following steps:
(1) The direct compression process is that BOG gas is directly pressurized to the pressure of a gas transmission main line through a high-pressure compressor unit for transmission, and the process equipment is few in flow and simple, but the process energy consumption is high and the safety is poor.
(2) And the recondensing process is that BOG gas is pressurized by a compressor, mixed with LNG pumped out of a tank, liquefied by supercooled LNG, pressurized by a high-pressure pump in a second stage, and gasified by a gasifier and sent into a pipe network. The energy consumption of the process is far lower than that of a direct compression method, but the recovery amount of BOG is limited by the flow of the externally-transmitted LNG, and for a city pipe network with nonuniform downstream gas consumption, the LNG can not fully condense the BOG when the LNG is in low-grade gas consumption, and only is discharged. The general BOG compressor is not used, long-time maintenance is required under unexpected conditions, and a large amount of BOG is wasted.
The novel BOG treatment process based on recondensing theory comprises the following steps:
(1) The mixed refrigerant BOG recondensing process uses multicomponent light hydrocarbon as refrigerant and uses refrigeration cycle to condense and recycle BOG, but the process has more heat exchange equipment and complex flow, and the refrigerant proportioning needs additional equipment.
(2) The multi-stage compression BOG recondensing process reduces compression power consumption and improves operation flexibility by utilizing a multi-stage compression technology, but the process cannot handle BOG recovery under a stop-transport condition, and has limitations under the condition that BOG is produced in a large amount, such as a ship unloading condition.
(3) The BOG recondensing process with the expander utilizes gasified high-pressure natural gas to drive the output shaft of the expander to work and supply the output shaft to the high-pressure pump, so that the purposes of LNG pressurization and almost no consumption of additional energy are achieved, but the process has higher requirements on a process pipeline and pressurization equipment due to a higher pressure range, and the feasibility under a fluctuation working condition needs to be further verified.
In view of the above state of the art, it is necessary to provide a BOG auxiliary recovery method that has low energy consumption, high yield, moderate equipment investment, can adapt to fluctuating working conditions, and can meet other requirements of a large LNG yard, so as to achieve energy saving and emission reduction, improve process operation flexibility, and have high energy utilization efficiency.
Disclosure of Invention
The invention aims to overcome the defects of the background technology, and provides a BOG recycling and nitrogen liquefying system which is energy-saving, emission-reducing and high in energy utilization efficiency.
In order to achieve the above purpose, the BOG recycling and nitrogen liquefying system comprises an LNG storage tank, an in-tank pump, a first LNG distributor, a recondenser, a first LNG mixer, a high-pressure pump, a second LNG distributor, a first heat exchanger, a second LNG mixer, an LNG vaporizer, a PSA nitrogen making device, a first valve, a nitrogen mixer, a nitrogen compressor, a second heat exchanger, a nitrogen separator, a liquid nitrogen tank, a second valve, a BOG-liquid nitrogen heat exchanger, a nitrogen tank, a third valve, a liquid nitrogen vaporizer, a BOG compressor, a BOG distributor, a fourth valve, a fifth valve, a gas-liquid separator and a sixth valve;
the pump in the tank is arranged in the LNG storage tank, and the outlet A of the pump in the tank is connected with the B of the first LNG distributor 3 An inlet, B of the first LNG dispenser 1 D of the first LNG mixer of exit linkage 1 An inlet, B of the first LNG dispenser 2 C with outlet connected to recondensor 1 An inlet, C of the recondensor 3 D of the first LNG mixer of exit linkage 2 An inlet, the outlet of the first LNG mixer is connected with E of the second LNG distributor through a high-pressure pump 3 An inlet, E of the second LNG dispenser 1 F with outlet connected with the first heat exchanger 1 An inlet of the first heat exchanger F 2 G of the outlet connection second LNG mixer 1 An inlet;
e of the second LNG dispenser 2 L with outlet connected with the second heat exchanger 1 An inlet, L of the second heat exchanger 2 G of the outlet connection second LNG mixer 2 An inlet, the outlet of the second LNG mixer is connected to the H of the LNG vaporizer 1 Inlet, H of the LNG vaporizer 2 The outlet is connected with an external gas transmission pipe network;
i of the PSA nitrogen making device 1 The outlet is divided into two branches, one branch is connected with F of the first heat exchanger through a first valve 3 The other path of the inlet is connected with an external purging pipeline through a sixth valve; f of the first heat exchanger 4 J with outlet connected to nitrogen mixer 1 Inlet, J of the nitrogen mixer 3 K with outlet connected with nitrogen compressor 1 Inlet, K of the nitrogen compressor 2 L with outlet connected with the second heat exchanger 3 An inlet, L of the second heat exchanger 4 M with outlet connected with nitrogen separator 3 Inlet, M of the nitrogen separator 2 The gas phase outlet is connected with J of a nitrogen mixer 2 Inlet, M of the nitrogen separator 1 N of liquid phase outlet connection liquid nitrogen tank 1 Inlet, N of the liquid nitrogen tank 2 The outlet is divided into two branches, one branch is connected with O of the BOG-liquid nitrogen heat exchanger through a second valve 3 The other path of the liquid nitrogen gasifier is connected with the P of the liquid nitrogen gasifier through a third valve 1 An inlet; o of the BOG-liquid nitrogen heat exchanger 4 Q with outlet connected with nitrogen tank 1 An inlet;
r of the LNG storage tank 1 The outlet is connected with the inlet of the BOG compressor, and the outlet of the BOG compressor is connected with the T of the BOG distributor 3 Inlet, T of the BOG distributor 1 C with outlet connected to recondensor 2 An inlet; t of the BOG dispenser 2 O with outlet connected with BOG-liquid nitrogen heat exchanger 1 Inlet, O of BOG-liquid nitrogen heat exchanger 2 The outlet is divided into two branches, and one branch is connected with D of the first LNG mixer through a fifth valve 3 The other path of the gas-liquid separator is connected with the V of the gas-liquid separator through a fourth valve 3 Inlet, V of the gas-liquid separator 1 The gas phase outlet is connected with the inlet of the BOG compressor, and the V of the gas-liquid separator 2 R of liquid phase outlet connection LNG storage tank 2 An inlet.
In the technical scheme, the LNG vaporizer further comprises a first seawater pump and a second seawater pump, and the LNG vaporizer comprises H 3 The inlet is connected with seawater through a first seawater pump, and the LNG vaporizer is also provided with H for discharging seawater 4 An outlet; p of the liquid nitrogen gasifier 3 The inlet is connected with the sea water through a second sea water pump, and the liquid nitrogen gasifier is also provided with P for discharging the sea water 4 An inlet.
In the technical scheme, the Q of the nitrogen tank 2 P of outlet, liquid nitrogen gasifier 2 The outlet is connected with the purging pipeline after being converged.
In the technical scheme, the LNG storage tank further comprises a torch, wherein an air inlet of the torch is connected with R of the LNG storage tank through a branch 1 The outlet is connected.
In the above technical solution, L of the second heat exchanger 4 M of outlet and nitrogen separator 3 A first throttle valve is arranged on the pipeline between the inlets.
In the above technical scheme, V of the gas-liquid separator 3 O of inlet and BOG-liquid nitrogen heat exchanger 2 A second throttle valve is arranged on the pipeline between the outlets.
The invention also provides a process method for BOG recovery and nitrogen liquefaction, which comprises the following steps:
step 1: LNG normal export flow: BOG in the LNG storage tank is pressurized to 0.5-0.9 MPa by a BOG compressor at-150 to-100 ℃ and 115kPa, and is divided into two paths by a BOG distributor, wherein only the T of the BOG distributor exists at the moment 1 Outlet communication, BOG enters C of recondenser 2 An inlet, and the supercooled LNG pressurized to 0.5-0.9 MPa by the pump in the tank passes through B of the first LNG distributor 2 C outlet into recondenser 1 An inlet for contacting the BOG with the subcooled LNG in a recondenser, mixing and condensing the liquid, the condensed liquid passing through C of the recondenser 3 D outlet into first LNG Mixer 2 Inlet and B through a first LNG distributor 1 LNG flowing out of the outlet is converged, sent to an inlet of a high-pressure pump, pressurized to 9MPa and 153 ℃ by the high-pressure pump, split into two paths by a second LNG distributor, precooled nitrogen by a first heat exchanger, heated to-149.5 ℃ by one path, liquefied by a second heat exchanger, heated to-30.6 ℃ by the other path, mixed by a second LNG mixer, sent to an LNG gasifier to exchange heat with seawater and gasify to natural gas, and then sent to a gas transmission pipe network; h for pressurizing seawater to enter LNG gasifier through first seawater pump 3 Inlet, exchange heat with LNG and pass through H of LNG vaporizer 4 An outlet vent;
step 2: nitrogen liquefaction storage flow: nitrogen gas with the temperature of 20-25 ℃ and the pressure of 0.6-0.7 MPa, which is prepared by the PSA nitrogen preparation device, enters a first heat exchanger to exchange heat with LNG with the temperature of-153 ℃ and the pressure of 9MPa, is precooled and cooled to-150 ℃, and then is separated with M of a nitrogen separator 2 The low-temperature nitrogen mixing temperature of-175 ℃ and 0.69MPa flowing out of the gas phase outlet is reduced to-160 ℃, the mixture is pressurized to 2.95MPa and-85 ℃ by a nitrogen compressor, and enters a second heat exchanger and is at-153 ℃ and 9MPaLNG exchanges heat, is cooled to-150 ℃, is depressurized to 0.69MPa and-175 ℃ through a first throttle valve, flows into a nitrogen separator to separate out an air liquid phase, liquid nitrogen separated by the nitrogen separator enters a liquid nitrogen tank for storage, low-temperature nitrogen separated by the nitrogen separator and nitrogen precooled by a first heat exchanger are converged in a nitrogen mixer and enter a nitrogen compressor for nitrogen compression circulation, and the cold quantity of the LNG is continuously utilized for liquefying the nitrogen;
step 3: BOG liquefaction output and tank returning flow: pressurizing BOG of 115kPa at-150 to-100 ℃ in an LNG storage tank to 0.5-0.9 MPa by a BOG compressor, dividing the BOG into two paths by a BOG distributor, and enabling one path of the BOG to enter a recondenser to carry out the LNG normal output flow in the step 1; the other path of the liquid nitrogen enters a BOG-liquid nitrogen heat exchanger, exchanges heat with liquid nitrogen flowing out of a liquid nitrogen tank at minus 175 ℃ and 0.69MPa and cools to BOG condensate at minus 148 ℃, the nitrogen temperature after liquid nitrogen gasification is minus 40 ℃ and 0.68MPa enters a nitrogen tank, and the BOG condensate passes through O of the BOG-liquid nitrogen heat exchanger 2 The outlet is divided into two paths, one path is sent to the high-pressure pump through a fifth valve to carry out the LNG normal output flow in the step 1, the other path is depressurized through a second throttling valve to enter the gas-liquid separator, the liquid phase separated by the gas-liquid separator flows into the LNG storage tank, and the gas phase separated by the gas-liquid separator enters the inlet of the BOG compressor to continuously carry out the BOG liquefaction output and tank returning.
In the above technical scheme, in the step 1, the mass ratio of the LNG distributed by the second LNG distributor into the first heat exchanger and the second heat exchanger is 1:70-72.
In the above technical scheme, the step 2 further includes a nitrogen purging process, wherein the nitrogen source of the nitrogen purging process is three parts, and the first part is that nitrogen generated by the PSA nitrogen making device enters the purging pipeline through a sixth valve; the second part is that liquid nitrogen in the liquid nitrogen tank passes through a third valve and is gasified into nitrogen by a liquid nitrogen gasifier and then enters a purging pipeline; the third part is that nitrogen in the nitrogen tank directly enters the purging pipeline.
In the above technical solution, if both the step 1 and the step 2 cannot be implemented, R of the LNG tank 1 The outlet will be in communication with the inlet of the flare and the BOG will be fed into the flare for blow-down combustion.
Compared with the prior art, the invention has the following advantages:
firstly, the invention uses the cold energy carried by surplus LNG to liquefy and condense the nitrogen generated by a PSA (pressure swing adsorption separation) nitrogen making device, stores the nitrogen in a liquid nitrogen tank for standby, carries out primary precooling and LNG heat exchange, is pressurized by a nitrogen compressor, carries out heat exchange with the LNG to realize secondary cooling, generates temperature drop through throttling, and separates low-temperature liquid nitrogen from a separator to enter a storage tank for storage; the nitrogen of the separator and the nitrogen of the primary pre-cooling are converged and enter a nitrogen compression cycle, when the LNG output is low or the BOG output is large, low-temperature liquid nitrogen in a liquid nitrogen tank is prepared for BOG liquefaction and condensation, and the gas nitrogen after heat exchange is stored in a nitrogen storage tank; the condensed BOG is pressurized and externally conveyed by a high-pressure pump or is depressurized to LNG storage pressure by a throttle valve, condensate separated by a separator is injected from the bottom of a storage tank and stored, and gas-phase BOG returns to a gas-phase main pipe to enter BOG compression circulation, so that a PSA pressure swing adsorption nitrogen making device commonly arranged in an LNG station in China is fully utilized, and equipment investment required for station process improvement is reduced.
Secondly, the nitrogen prepared by the PSA nitrogen preparation device is subjected to secondary cooling by LNG cold energy so as to be liquefied and stored; in the BOG liquefaction system, BOG is pressurized by a compressor and then exchanges heat with liquid nitrogen to liquefy the BOG, and the liquid nitrogen absorbs heat to be gasified and stored in a nitrogen storage tank, so that the BOG can be used for station sweeping; the liquefied BOG enters a high-pressure pump for external transportation or is throttled and decompressed, enters a separator, the liquid phase is injected into the bottom of an LNG storage tank for storage, and the gas phase enters a BOG compression cycle. The method solves the problem of difficult BOG recovery under the condition of low external transmission capacity of the LNG receiving station, meets the requirements of a station yard low-temperature purging process, improves the energy utilization efficiency by utilizing LNG cold energy, and reduces the equipment investment by using the existing PSA nitrogen making equipment of the LNG station yard.
Thirdly, the cold storage medium adopted by the invention is inert gas nitrogen, and compared with the light hydrocarbon mixture type refrigeration medium, the cold storage medium hardly brings danger in the compression and heat exchange process; and nitrogen-liquid nitrogen is used as a cold storage medium instead of pure nitrogen, and the total amount of the cold storage medium can be reduced by utilizing the latent heat instead of sensible heat.
Fourthly, the storage and the discharge of the liquid nitrogen and the nitrogen do not cause greenhouse effect and environmental pollution, and compared with alkane and fluorochlorohydrocarbon cold-carrying media, the invention has the characteristics of cleanness and environmental protection;
fifth, the BOG and nitrogen liquefaction process is simple, the cost is low, the operation is convenient, and the requirement of the station nitrogen purging process is combined, so that the station process is integrated without the need of external liquid nitrogen.
And the invention can adapt to fluctuation working conditions, store surplus cold energy at peak of gas consumption, release cold energy at valley of gas consumption, realize condensation recovery under high load of BOG, and reduce energy waste and environmental pollution.
Drawings
FIG. 1 is a schematic diagram of a BOG recovery and nitrogen liquefaction system;
Detailed Description
The following describes the embodiments of the present invention in detail with reference to examples, but they are not to be construed as limiting the invention. While at the same time becoming clearer and more readily understood by way of illustration of the advantages of the present invention.
The BOG recovery and nitrogen liquefaction system comprises an LNG storage tank 1, an in-tank pump 2, a first LNG distributor 3, a recondenser 4, a first LNG mixer 5, a high-pressure pump 6, a second LNG distributor 7, a first heat exchanger 8, a second LNG mixer 9, an LNG vaporizer 10, a PSA nitrogen making device 11, a first valve 12, a nitrogen mixer 13, a nitrogen compressor 14, a second heat exchanger 15, a nitrogen separator 17, a liquid nitrogen tank 18, a second valve 19, a BOG-liquid nitrogen heat exchanger 20, a nitrogen tank 21, a third valve 22, a liquid nitrogen vaporizer 23, a BOG compressor 24, a BOG distributor 25, a fourth valve 26, a fifth valve 27, a gas-liquid separator 29, a sixth valve 33 and a torch 30;
the in-tank pump 2 is arranged in the LNG storage tank 1, and an A outlet 2.1 of the in-tank pump 2 is connected with a B of the first LNG distributor 3 3 Inlet 3.3, B of said first LNG dispenser 3 1 Outlet 3.1 is connected to D of the first LNG mixer 5 1 Inlet 5.1, B of said first LNG dispenser 3 2 Outlet 3.2 is connected to C of recondensor 4 1 Inlet 4.1, C of the recondensor 4 3 Outlet 4.3 is connected to D of the first LNG mixer 5 2 An inlet 5.2, the outlet of the first LNG mixer 5 is connected with E of a second LNG distributor 7 through a high-pressure pump 6 3 Inlet 7.3, E of said second LNG distributor 7 1 The outlet 7.1 is connected with F of the first heat exchanger 8 1 Inlet 8.1, F of said first heat exchanger 8 2 Outlet 8.2 is connected to G of the second LNG mixer 9 1 An inlet 9.1;
e of the second LNG dispenser 7 2 The outlet 7.2 is connected with the L of the second heat exchanger 15 1 An inlet 15.1, L of the second heat exchanger 15 2 Outlet 15.2 is connected to G of the second LNG mixer 9 2 An inlet 9.2, the outlet of the second LNG mixer 9 is connected to the H of the LNG vaporizer 10 1 Inlet 10.1, H of the LNG vaporizer 10 2 The outlet 10.2 is connected with an external gas transmission pipe network 35;
i of the PSA nitrogen production apparatus 11 1 The outlet 11.1 is divided into two branches, one branch is connected with F of the first heat exchanger 8 through a first valve 12 3 The inlet 8.3, the other path is connected with an external purging pipeline 34 through a sixth valve 33; f of the first heat exchanger 8 4 Outlet 8.4 is connected to J of nitrogen mixer 13 1 Inlet 13.1, J of the Nitrogen Mixer 13 3 Outlet 13.3K connected to nitrogen compressor 14 1 Inlet 14.1, K of said nitrogen compressor 14 2 The outlet 14.2 is connected to the L of the second heat exchanger 15 3 An inlet 15.3, L of the second heat exchanger 15 4 The outlet 15.4 is connected with M of the nitrogen separator 17 3 Inlet 17.3, L of said second heat exchanger 15 4 M of outlet 15.4 and nitrogen separator 17 3 A first throttle valve 16 is arranged on the pipeline between the inlets 17.3, M of the nitrogen separator 17 2 Gas phase outlet 17.2 is connected to J of nitrogen mixer 13 2 Inlet 13.2, M of the nitrogen separator 17 1 The liquid phase outlet 17.1 is connected with N of a liquid nitrogen tank 18 1 Inlet 18.1, N of said liquid nitrogen tank 18 2 The outlet 18.2 is divided into two branches, one branch is connected with O of the BOG-liquid nitrogen heat exchanger 20 through the second valve 19 3 Inlet 20.3, another path is connected with P of liquid nitrogen gasifier 23 through third valve 22 1 O of BOG-liquid nitrogen heat exchanger 20 described in inlet 23.1 4 An outlet20.4Q connected to Nitrogen tank 21 1 Inlet 21.1, Q of said nitrogen tank 21 2 P of outlet 21.2, liquid nitrogen gasifier 23 2 The outlet 23.2 is connected with a purging pipeline 34 after being converged, the outlet material flow of the liquid nitrogen storage tank 18 flows to different equipment according to the station requirements, when the cooling capacity is required to be supplemented for the liquefaction of the BOG, the second valve 19 is opened to flow to the BOG-liquid nitrogen heat exchanger, and the gasified liquid nitrogen enters the nitrogen storage tank 21 for storage for standby; when a large amount of nitrogen is needed for station purging, the third valve 22 is opened to enable liquid nitrogen to flow to the liquid nitrogen vaporizer 23, so that a rapid vaporizing supplemental nitrogen source is realized, and the liquid nitrogen flows to a purging pipeline. The tank volumes of the liquid nitrogen storage tank 18 and the nitrogen storage tank 21 are required to be designed and calculated according to the site scale and the BOG production amount, and the nitrogen amount for purging so as to meet the process requirements of LNG with different scales.
R of the LNG tank 1 1 Outlet 1.1 is connected to the inlet of BOG compressor 24, the outlet of BOG compressor 24 is connected to T of BOG distributor 25 3 Inlet 25.3, T of said BOG dispenser 25 1 Outlet 25.1 is connected to C of recondensor 4 2 Inlet 4.2; t of the BOG dispenser 25 2 Outlet 25.2 is connected with O of BOG-liquid nitrogen heat exchanger 20 1 Inlet 20.1, O of said BOG-liquid nitrogen heat exchanger 20 2 The outlet 20.2 is divided into two branches, one branch is connected with the D of the first LNG mixer 5 through a fifth valve 27 3 Inlet 5.3, the other path is connected with V of the gas-liquid separator 29 through the fourth valve 26 3 Inlet 29.3, V of said gas-liquid separator 29 3 Inlet 29.3 and O of BOG-liquid nitrogen heat exchanger 20 2 A second throttle valve 28 is arranged on the pipeline between the outlets 20.2, and the V of the gas-liquid separator 29 1 The gas phase outlet 29.1 is connected to the inlet of the BOG compressor 24, V of the gas-liquid separator 29 2 Liquid phase outlet 29.2 is connected with R of LNG storage tank 1 2 Inlet 1.2, the inlet of the torch 30 is connected with R of the LNG storage tank 1 through a branch 1 The outlet 1.1 is connected.
In the above technical solution, the LNG vaporizer 10 further includes a first seawater pump 31, a second seawater pump 32, and H 3 The inlet 10.3 is connected to the sea water by a first sea water pump 31, said LNG vaporizer 10 being further provided with H for discharging sea water 4 An outlet 10.4; p of the liquid nitrogen gasifier 23 3 The inlet 23.3 is connected to the sea water by a second sea water pump 32, said liquid nitrogen gasifier 23 is further provided with P for discharging sea water 4 Inlet 23.4.
The invention relates to a technological method for BOG recovery and nitrogen liquefaction, which comprises the following steps:
step 1: LNG normal export flow: BOG in the LNG storage tank 1 is pressurized to 0.5-0.9 MPa by a BOG compressor 24 at-150 to-100 ℃ and 115kPa, and is divided into two paths by a BOG distributor 25, wherein only the T of the BOG distributor 25 is present at the moment 1 Outlet 25.1 communicates with BOG entering C of recondensor 4 2 Inlet 4.2, while subcooled LNG pressurized to 0.5-0.9 MPa by in-tank pump 2 is passed through B of first LNG distributor 3 2 Outlet 3.2C into recondensor 4 1 Inlet 4.1, bog is contacted with subcooled LNG in recondenser 4, mixed and condensed into a liquid (the saturated vapor pressure of which meets the required cavitation margin requirement for the high pressure pump inlet), which passes through recondenser 4C 3 Outlet 4.3D into the first LNG mixer 5 2 Inlet 5.2 and B through the first LNG distributor 3 1 The LNG flowing out of the outlet 3.1 is converged and sent to an inlet of a high-pressure pump 6, the pressure is increased to 9MPa and 153 ℃ through the high-pressure pump 6, the LNG is divided into two paths through a second LNG distributor 7, one path of the LNG is precooled through a first heat exchanger 8, the temperature of the LNG is increased to-149.5 ℃, the other path of the LNG is liquefied through a second heat exchanger 15, the temperature of the LNG is increased to-30.6 ℃ (the LNG mass ratio of the LNG distributed by the second LNG distributor 7 to enter the first heat exchanger 8 and the second heat exchanger 15 is 1:70-72, the cold energy required by the first-stage cooling is smaller, the phase-change latent heat of the second-stage cooling nitrogen consumes more cold energy), and the two paths of LNG are mixed in a second LNG mixer 9 and then sent to an LNG gasifier 10 to be gasified with seawater through heat exchange to form natural gas which enters a gas pipe network 35; the seawater is pressurized by the first seawater pump 31 to enter the H of the LNG vaporizer 10 3 Inlet 10.3, exchanges heat with LNG and passes through H of LNG vaporizer 10 4 An outlet 10.4;
step 2: nitrogen liquefaction storage flow: nitrogen (oxygen content is lower than 1%) of 20-25 ℃ and 0.6-0.7 MPa prepared by the PSA nitrogen preparation device 11 enters the first heat exchanger 8 to exchange heat with LNG of-153 ℃ and 9MPa, precooled and cooled to-150 ℃, and then enters M of the nitrogen separator 17 2 Low temperature nitrogen at-175 ℃ and 0.69MPa flowing out from the gas phase outlet 17.2The gas mixing temperature is reduced to-160 ℃, the mixture is pressurized to 2.95MPa and-85 ℃ by a nitrogen compressor 14, enters a second heat exchanger 15 to exchange heat with LNG at-153 ℃ and 9MPa, is cooled to-150 ℃, is depressurized to 0.69MPa and-175 ℃ by a first throttle valve 16, flows into a nitrogen separator 17 to separate out gas-liquid phase, liquid nitrogen separated by the nitrogen separator 17 enters a liquid nitrogen tank 18 to be stored, low-temperature nitrogen separated by the nitrogen separator 17 and nitrogen precooled by the first heat exchanger 8 are converged in a nitrogen mixer 13 to enter the nitrogen compressor 14 to carry out nitrogen compression circulation, and the cold quantity of LNG is continuously utilized to liquefy the nitrogen;
step 3: BOG liquefaction output and tank returning flow: the BOG in the LNG storage tank 1 is pressurized to 0.5-0.9 MPa by a BOG compressor 24 at the temperature of-150 to-100 ℃ and 115kPa, and then is divided into two paths by a BOG distributor 25, and one path enters a recondenser 4 to carry out the LNG normal output flow described in the step 1; the other path of the liquid nitrogen enters a BOG-liquid nitrogen heat exchanger 20, and exchanges heat with liquid nitrogen flowing out of a liquid nitrogen tank 18 at minus 175 ℃ and 0.69MPa to cool the liquid nitrogen to BOG condensate at minus 148 ℃, meanwhile, the pressure needs to meet the NPSH (the required cavitation allowance) requirement of a high-pressure pump inlet (in this example, NPSH is 0.1 MPa), the nitrogen temperature after liquid nitrogen gasification is minus 40 ℃ and 0.68MPa enters a nitrogen tank 21, and the BOG condensate passes through O of the BOG-liquid nitrogen heat exchanger 20 2 The outlet 20.2 is divided into two paths, one path is sent to the high-pressure pump 6 through the fifth valve 27 to carry out the LNG normal output flow in the step 1, the other path is depressurized through the second throttle valve 28 to enter the gas-liquid separator 29, the liquid phase separated by the gas-liquid separator 29 flows into the LNG storage tank 1, the gas phase separated by the gas-liquid separator 29 enters the inlet of the BOG compressor 24 to continuously carry out the BOG liquefaction output and tank return, wherein the mass ratio of consumed liquid nitrogen to condensed BOG is 2.2-2.3 (BOG is output after condensed): 2.6 to 2.7 (the BOG is condensed and then decompressed and returned to the tank).
In the above technical solution, the step 2 further includes a nitrogen purging process, wherein the nitrogen source of the nitrogen purging process is three parts, and the first part is that nitrogen generated by the PSA nitrogen making device 11 enters the purging pipeline 34 through the sixth valve 33; the second part is that the liquid nitrogen in the liquid nitrogen tank 18 passes through a third valve 22, is gasified into nitrogen by a liquid nitrogen gasifier 23 and then enters a purging pipeline 34; the third part is that the nitrogen in the nitrogen tank 21 directly enters the purge line 34. The three kinds of materialsThe sources of nitrogen are mutually supplemented, and no sequence exists. If both of the steps 1 and 2 cannot be implemented, R of the LNG tank 1 1 The outlet 1.1 will be in communication with the inlet of the flare 30 and the BOG will be fed to the flare for flare.
According to the invention, nitrogen generated by a PSA (pressure swing adsorption separation) nitrogen making device is liquefied and condensed by utilizing cold energy carried by surplus LNG, stored in a liquid nitrogen tank for standby, subjected to primary pre-cooling and LNG heat exchange, pressurized by a nitrogen compressor, subjected to heat exchange with LNG to realize secondary cooling, subjected to throttling to generate temperature drop, and separated out of low-temperature liquid nitrogen by a separator and stored in a storage tank; the nitrogen of the separator and the nitrogen of the primary pre-cooling are converged and enter a nitrogen compression cycle, when the LNG output is low or the BOG output is large, low-temperature liquid nitrogen in a liquid nitrogen tank is prepared for BOG liquefaction and condensation, and the gas nitrogen after heat exchange is stored in a nitrogen storage tank; the condensed BOG is pressurized and externally conveyed by a high-pressure pump or is depressurized to LNG storage pressure by a throttle valve, condensate separated by a separator is injected from the bottom of a storage tank and stored, and gas-phase BOG returns to a gas-phase main pipe to enter BOG compression circulation, so that a PSA pressure swing adsorption nitrogen making device commonly arranged in an LNG station in China is fully utilized, and equipment investment required for station process improvement is reduced.
The foregoing is merely exemplary embodiments of the present invention, and it should be noted that any changes and substitutions that would be easily recognized by those skilled in the art within the technical scope of the present invention should be included in the scope of the present invention.
Claims (10)
1. The BOG recycling and nitrogen liquefying system comprises an LNG storage tank (1), an in-tank pump (2), a first LNG distributor (3), a recondenser (4), a first LNG mixer (5), a high-pressure pump (6), a second LNG distributor (7), a first heat exchanger (8), a second LNG mixer (9), an LNG gasifier (10), a PSA nitrogen making device (11), a first valve (12), a nitrogen mixer (13), a nitrogen compressor (14), a second heat exchanger (15), a nitrogen separator (17), a tank (18), a second valve (19), a BOG-liquid nitrogen heat exchanger (20), a nitrogen tank (21), a third valve (22), a liquid nitrogen gasifier (23), a BOG compressor (24), a BOG distributor (25), a fourth valve (26), a fifth valve (27), a gas-liquid separator (29) and a sixth valve (33);
the in-tank pump (2) is arranged in the LNG storage tank (1), and an A outlet (2.1) of the in-tank pump (2) is connected with a B of the first LNG distributor (3) 3 An inlet (3.3), B of the first LNG dispenser (3) 1 The outlet (3.1) is connected with the D of the first LNG mixer (5) 1 An inlet (5.1) B of the first LNG dispenser (3) 2 The outlet (3.2) is connected with C of the recondensor (4) 1 An inlet (4.1) for C of the recondensor (4) 3 The outlet (4.3) is connected with the D of the first LNG mixer (5) 2 An inlet (5.2), the outlet of the first LNG mixer (5) is connected with E of a second LNG distributor (7) through a high-pressure pump (6) 3 An inlet (7.3) for E of the second LNG dispenser (7) 1 The outlet (7.1) is connected with F of the first heat exchanger (8) 1 An inlet (8.1), F of the first heat exchanger (8) 2 The outlet (8.2) is connected with G of the second LNG mixer (9) 1 An inlet (9.1);
e of the second LNG distributor (7) 2 The outlet (7.2) is connected with the L of the second heat exchanger (15) 1 An inlet (15.1), L of the second heat exchanger (15) 2 The outlet (15.2) is connected with G of the second LNG mixer (9) 2 An inlet (9.2), the outlet of the second LNG mixer (9) being connected to the H of the LNG vaporizer (10) 1 An inlet (10.1) for H of the LNG vaporizer (10) 2 The outlet (10.2) is connected with an external gas transmission pipe network (35); the second LNG distributor (7) distributes LNG entering the first heat exchanger (8) and the second heat exchanger (15) in a mass ratio of 1:70-72;
i of the PSA nitrogen production device (11) 1 The outlet (11.1) is divided into two branches, one branch is connected with F of the first heat exchanger (8) through a first valve (12) 3 An inlet (8.3), and the other path is connected with an external purging pipeline (34) through a sixth valve (33); f of the first heat exchanger (8) 4 The outlet (8.4) is connected with J of a nitrogen mixer (13) 1 An inlet (13.1) J of the nitrogen mixer (13) 3 The outlet (13.3) is connected with K of the nitrogen compressor (14) 1 An inlet (14.1), K of the nitrogen compressor (14) 2 The outlet (14.2) is connected with the L of the second heat exchanger (15) 3 An inlet (15.3), L of the second heat exchanger (15) 4 The outlet (15.4) is connected with M of the nitrogen separator (17) 3 An inlet (17.3), M of the nitrogen separator (17) 2 The gas phase outlet (17.2) is connected with J of the nitrogen mixer (13) 2 An inlet (13.2), M of the nitrogen separator (17) 1 The liquid phase outlet (17.1) is connected with N of a liquid nitrogen tank (18) 1 An inlet (18.1), N of the liquid nitrogen tank (18) 2 The outlet (18.2) is divided into two branches, one branch is connected with O of the BOG-liquid nitrogen heat exchanger (20) through a second valve (19) 3 An inlet (20.3) and the other path is connected with P of the liquid nitrogen gasifier (23) through a third valve (22) 1 An inlet (23.1); o of the BOG-liquid nitrogen heat exchanger (20) 4 The outlet (20.4) is connected with Q of the nitrogen tank (21) 1 An inlet (21.1);
r of the LNG storage tank (1) 1 The outlet (1.1) is connected with the inlet of the BOG compressor (24), and the outlet of the BOG compressor (24) is connected with the T of the BOG distributor (25) 3 An inlet (25.3) for T of the BOG dispenser (25) 1 The outlet (25.1) is connected to C of the recondensor (4) 2 An inlet (4.2); t of the BOG dispenser (25) 2 The outlet (25.2) is connected with O of the BOG-liquid nitrogen heat exchanger (20) 1 An inlet (20.1) for O of the BOG-liquid nitrogen heat exchanger (20) 2 The outlet (20.2) is divided into two branches, and one branch is connected with the D of the first LNG mixer (5) through a fifth valve (27) 3 An inlet (5.3) and the other path is connected with V of the gas-liquid separator (29) through a fourth valve (26) 3 An inlet (29.3) for V of the gas-liquid separator (29) 1 The gas phase outlet (29.1) is connected with the inlet of the BOG compressor (24), and the V of the gas-liquid separator (29) 2 The liquid phase outlet (29.2) is connected with R of the LNG storage tank (1) 2 An inlet (1.2).
2. The BOG recovery and nitrogen liquefaction system of claim 1, wherein: also comprises a first seawater pump (31) and a second seawater pump (32), and the LNG vaporizer (10) comprises H 3 The inlet (10.3) is connected with the sea water through a first sea water pump (31), the LNG vaporizer (10) is also provided with H for discharging the sea water 4 An outlet (10.4); p of the liquid nitrogen gasifier (23) 3 The inlet (23.3) is connected with the sea water through a second sea water pump (32), and the liquid nitrogen gasifier (23) is also provided with P for discharging the sea water 4 An inlet (23.4).
3. The BOG recovery and nitrogen liquefaction system of claim 2, wherein: q of the nitrogen tank (21) 2 P of outlet (21.2), liquid nitrogen gasifier (23) 2 The outlet (23.2) is connected with a purging pipeline (34) after being converged.
4. The BOG recovery and nitrogen liquefaction system of claim 3, wherein: the utility model also comprises a torch (30), wherein the air inlet of the torch (30) is connected with the R of the LNG storage tank (1) through a branch 1 The outlet (1.1) is connected.
5. The BOG recovery and nitrogen liquefaction system of claim 4, wherein: l of the second heat exchanger (15) 4 M of outlet (15.4) and nitrogen separator (17) 3 A first throttle valve (16) is arranged on the pipeline between the inlets (17.3).
6. The BOG recovery and nitrogen liquefaction system of claim 5, wherein: v of the gas-liquid separator (29) 3 O of inlet (29.3) and BOG-liquid nitrogen heat exchanger (20) 2 A second throttle valve (28) is arranged on the pipeline between the outlets (20.2).
7. A process for utilizing the BOG recovery and nitrogen liquefaction system of claim 6, comprising the steps of:
step 1: LNG normal export flow: BOG in the LNG storage tank (1) is pressurized to 0.5-0.9 MPa by a BOG compressor (24) at-150 to-100 ℃ and 115kPa, and is divided into two paths by a BOG distributor (25), wherein only the T of the BOG distributor (25) is arranged at the moment 1 The outlet (25.1) is communicated with and BOG enters the C of the recondensor (4) 2 An inlet (4.2) for passing the supercooled LNG pressurized to 0.5-0.9 MPa by the in-tank pump (2) through the B of the first LNG distributor (3) 2 The outlet (3.2) enters C of the recondensor (4) 1 An inlet (4.1) for contacting, mixing and condensing the BOG with the subcooled LNG in the recondenser (4) to form a liquid, the liquid passing through C of the recondenser (4) 3 The outlet (4.3) enters the first LD of NG Mixer (5) 2 Inlet (5.2) and B through the first LNG distributor (3) 1 LNG flowing out of the outlet (3.1) is converged and sent to an inlet of a high-pressure pump (6), the LNG is pressurized to 9MPa to minus 153 ℃ through the high-pressure pump (6), the LNG is divided into two paths through a second LNG distributor (7), one path of the LNG is precooled through a first heat exchanger (8), the temperature of the LNG is increased to minus 149.5 ℃, the other path of the LNG is liquefied through a second heat exchanger (15), the temperature of the LNG is increased to minus 30.6 ℃, and the two paths of LNG are mixed in a second LNG mixer (9) and then sent to an LNG gasifier (10) to be gasified with seawater through heat exchange to natural gas and enter a gas conveying pipe network (35); the seawater is pressurized by a first seawater pump (31) and enters the H of the LNG gasifier (10) 3 An inlet (10.3) exchanges heat with LNG and then passes through H of the LNG gasifier (10) 4 An outlet (10.4) for discharging;
step 2: nitrogen liquefaction storage flow: nitrogen with the temperature of 20-25 ℃ and the pressure of 0.6-0.7 MPa, which is prepared by the PSA nitrogen preparation device (11), enters the first heat exchanger (8) to exchange heat with LNG with the temperature of-153 ℃ and the pressure of 9MPa, precooled to be cooled to-150 ℃, and then cooled to M with the nitrogen separator (17) 2 The low-temperature nitrogen mixed temperature of 0.69MPa at minus 175 ℃ flowing out of the gas phase outlet (17.2) is reduced to minus 160 ℃, the temperature is increased to 2.95MPa and minus 85 ℃ by the nitrogen compressor (14), the mixture enters the second heat exchanger (15) to exchange heat with LNG at minus 153 ℃ and 9MPa, the mixture is cooled to minus 150 ℃, then the mixture is depressurized to 0.69MPa and minus 175 ℃ by the first throttle valve (16), the mixture flows into the nitrogen separator (17) to separate out gas and liquid phases, the liquid nitrogen separated by the nitrogen separator (17) enters the liquid nitrogen tank (18) for storage, the low-temperature nitrogen separated by the nitrogen separator (17) and the nitrogen precooled by the first heat exchanger (8) are converged in the nitrogen mixer (13) to enter the nitrogen compressor (14) for nitrogen compression circulation, and the cold liquefied nitrogen of the LNG is continuously utilized;
step 3: BOG liquefaction output and tank returning flow: BOG in the LNG storage tank (1) is pressurized to 0.5-0.9 MPa through a BOG compressor (24) at the temperature of-150 to-100 ℃ and 115kPa, and then is divided into two paths through a BOG distributor (25), and one path enters a recondenser (4) to carry out the LNG normal output flow described in the step 1; the other path of the liquid nitrogen enters a BOG-liquid nitrogen heat exchanger (20), exchanges heat with liquid nitrogen flowing out of a liquid nitrogen tank (18) at the temperature of minus 175 ℃ and the pressure of 0.69MPa, cools to BOG condensate at the temperature of minus 148 ℃, and enters a nitrogen tank (21) at the temperature of minus 40 ℃ and the pressure of 0.68MPa after the liquid nitrogen is gasified, and the BOG condensate passes through O of the BOG-liquid nitrogen heat exchanger (20) 2 The outlet (20.2) is divided intoAnd (2) sending one path of the Liquefied Natural Gas (LNG) to the high-pressure pump (6) through the fifth valve (27) to perform the normal LNG output flow in the step (1), and sending the other path of the liquefied natural gas to the gas-liquid separator (29) through the second throttle valve (28) in a decompression way, wherein the liquid phase separated by the gas-liquid separator (29) flows into the LNG storage tank (1), and the gas phase separated by the gas-liquid separator (29) enters the inlet of the BOG compressor (24) to continuously perform the liquefied output and tank return of the BOG.
8. The BOG recovery and nitrogen liquefaction method according to claim 7, wherein the step 2 further comprises a nitrogen purging process, wherein the nitrogen source of the nitrogen purging process is three parts, and the first part is nitrogen generated by the PSA nitrogen device (11) and enters the purging pipeline (34) through the sixth valve (33); the second part is that liquid nitrogen in a liquid nitrogen tank (18) passes through a third valve (22), is gasified into nitrogen by a liquid nitrogen gasifier (23) and then enters a purging pipeline (34); the third part is that nitrogen in the nitrogen tank (21) directly enters the purging pipeline (34).
9. The BOG recovery and nitrogen liquefaction method according to claim 7, wherein in the step 1, the second LNG distributor (7) distributes LNG into the first heat exchanger (8) and the second heat exchanger (15) in a mass ratio of 1:70-72.
10. The BOG recovery and nitrogen liquefaction process according to claim 9, characterized in that if both steps 1 and 2 are not possible, R of the LNG storage tank (1) 1 The outlet (1.1) is communicated with the air inlet of the torch (30), and the BOG enters the torch to be burnt.
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| CN110285318B (en) * | 2019-07-12 | 2021-01-05 | 青岛乐戈新能源科技有限公司 | System and method for reducing generation of flash gas of liquefied natural gas filling station |
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| FR2882129A1 (en) * | 2005-02-17 | 2006-08-18 | Inst Francais Du Petrole | LIQUEFIED NATURAL GAS REGASIFICATION INSTALLATION |
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| US9945604B2 (en) * | 2014-04-24 | 2018-04-17 | Air Products And Chemicals, Inc. | Integrated nitrogen removal in the production of liquefied natural gas using refrigerated heat pump |
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| CN207379164U (en) * | 2017-09-15 | 2018-05-18 | 长江大学 | BOG is recycled and nitrogen gas liquefaction system |
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