Classifications

• • 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
  • F25J1/02—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
  • F25J1/0262—Details of the cold heat exchange system
  • F25J1/0264—Arrangement of heat exchanger cores in parallel with different functions, e.g. different cooling streams
  • F25J1/0265—Arrangement of heat exchanger cores in parallel with different functions, e.g. different cooling streams comprising cores associated exclusively with the cooling of a refrigerant stream, e.g. for auto-refrigeration or economizer
  • F25J1/0267—Arrangement of heat exchanger cores in parallel with different functions, e.g. different cooling streams comprising cores associated exclusively with the cooling of a refrigerant stream, e.g. for auto-refrigeration or economizer using flash gas as heat sink
• • 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
• • 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
  • F17C13/00—Details of vessels or of the filling or discharging of vessels
• • 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
  • F25J1/003—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production
  • F25J1/0047—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using an "external" refrigerant stream in a closed vapor compression cycle
  • F25J1/005—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using an "external" refrigerant stream in a closed vapor compression cycle by expansion of a gaseous refrigerant stream with extraction of work
• • 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
  • F25J1/006—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the refrigerant fluid used
  • F25J1/007—Primary atmospheric gases, mixtures thereof
  • F25J1/0072—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
  • 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
• • 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
  • F25J1/02—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
  • F25J1/0203—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 a single-component refrigerant [SCR] fluid in a closed vapor compression cycle
  • F25J1/0204—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 a single-component refrigerant [SCR] fluid in a closed vapor compression cycle as a single flow SCR cycle
• • 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
  • F25J1/02—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
  • F25J1/0262—Details of the cold heat exchange system
  • F25J1/0264—Arrangement of heat exchanger cores in parallel with different functions, e.g. different cooling streams
  • F25J1/0265—Arrangement of heat exchanger cores in parallel with different functions, e.g. different cooling streams comprising cores associated exclusively with the cooling of a refrigerant stream, e.g. for auto-refrigeration or economizer
• • 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
  • F25J1/02—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
  • F25J1/0279—Compression of refrigerant or internal recycle fluid, e.g. kind of compressor, accumulator, suction drum etc.
  • F25J1/0285—Combination of different types of drivers mechanically coupled to the same refrigerant compressor, possibly split on multiple compressor casings
  • F25J1/0288—Combination of different types of drivers mechanically coupled to the same refrigerant compressor, possibly split on multiple compressor casings using work extraction by mechanical coupling of compression and expansion of the refrigerant, so-called companders
• • 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
  • F25J2220/00—Processes or apparatus involving steps for the removal of impurities
  • F25J2220/60—Separating impurities from natural gas, e.g. mercury, cyclic hydrocarbons
  • F25J2220/62—Separating low boiling components, e.g. He, H2, N2, Air
• • 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
  • F25J2230/00—Processes or apparatus involving steps for increasing the pressure of gaseous process streams
  • F25J2230/30—Compression of the feed stream
• • 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

Definitions

• the invention relates to the field of re-liquefaction of boil-off gases from liquid natural gas (LNG). More specifically, the invention relates to a method and an apparatus for pre-heating LNG boil-off gas (BOG) stream flowing from a reservoir in a reliquefaction system, prior to compression, and a method and an apparatus for cooling an LNG boil- off gas (BOG) stream in a reliquefaction plant.
• LNG liquid natural gas
• LNG RS LNG reliquefaction systems
• the new LNG RS opened the possibility to collect, cool down and reliquefy all BOG and hence preserve the total cargo volume throughout the laden and ballast voyages.
• a method of A method of pre-heating LNG boil-off gas (BOG) stream flowing from a reservoir in a reliquefaction system, prior to compression comprising heat exchanging the BOG stream in a first heat exchanger, against a 0 second coolant stream having a higher temperature than the BOG stream, the method being characterized in that the second coolant stream is obtained by selectively splitting a first coolant stream into said second coolant stream and a third coolant stream, said third coolant stream being flowed into a first coolant passage in a reliquefaction system cold box, whereby the BOG has reached near-ambient temperatures prior to 5 compression and heat exchange with low temperature BOG is done by optimising the split of the coolant in the first heat exchanger in order to minimize exergy losses, and thermal stresses in the cold box are reduced.
• BOG LNG boil-off gas
• BOG LNG boil-off gas
• the pressure of the reliquefied BOG between the cold box and the reservoir is controlled independently of the BOG compressor discharge pressure and the reservoir pressure, and the amount of vent gas generated and the vent gas composition thus may be controlled.
• an apparatus for cooling an LNG boil-off gas (BOG) in a reliquefaction system comprising a closed-loop coolant circuit for heat exchange between a coolant and the BOG; a BOG compressor having an inlet side fluidly connected to an LNG reservoir; a cold box having a BOG flowpath with a BOG inlet fluidly connected to the BOG compressor outlet side; said BOG flowpath having outlet for substantially re-liquefied BOG, fluidly connected to the reservoir; said cold box further comprising coolant flowpaths for heat exchange between the BOG and the coolant; characterized by a first heat exchanger in the fluid connection between the reservoir and the BOG compressor inlet side, said first heat exchanger having a coolant path fluidly connected to the closed-loop coolant circuit, at a point downstream of the coolant circuit's compander aftercooler but upstream of the coolant flow paths in the cold box, whereby the BOG compressor receives BOG with temperatures near or at the system ambient temperatures.
• BOG LNG boil-off gas
• the invention provides a separator in fluid connection with the cold box outlet and with the reservoir, a first valve in the cold box outlet line and a second valve in a line connected to the reservoir, said separator also comprising a vent line (11), whereby the pressure in the separator may be controlled, and the amount of vent gas and the vent gas composition thus maybe adjusted.
• the figure shows schematic a cargo tank 74, holding a volume of LNG 72.
• BOG evaporating from the LNG, enters a line 1 which is connected to a first heat exchanger HlO. In this heat exchanger, the BOG is heated up to near- ambient temperatures, as will be described later.
• the BOG enters the first stage BOG compressor CI l via line 2.
• the BOG compressor is shown as a three-stage centrifugal compressor CI l, Cl 2, C13, interconnected via lines 3 — 7 via intercoolers Hl 1, H12 and aftercooler Hl 3 as shown in the figure, but other compressor types may be equally applicable.
• the pre-heating ensures that the heat generated by the compression may be rejected through cooling water in the intercoolers Hl 1, H12 and the aftercooler H13.
• Pressurized BOG is then, via a line 8, fed into a second heat exchanger (or "cold box") H20 where it is heat exchanged against a coolant, as will be described later.
• the coolant is preferably nitrogen (N 2 ).
• substantially reliquefied BOG exits the cold box H20 via a lines 9, 10 connected to a separator FlO.
• the separator is provided with a vent line 11.
• a throttling valve VlO is arranged in the lines 9, 10 between the cold box and the separator, for expanding the reliquefied BOG.
• reliquefied BOG is fed into the LNG 72 in the cargo tank 74 via lines 12, 13, as shown in figure 1.
• a valve Vl 1 is arranged in the lines between the separator FlO and the tank 74, the purpose of which will be described later.
• the closed N 2 -Brayton cooling cycle is here represented by a 3 -stage compressor C21, C22, C23 with intercoolers H21, H22, aftercooler H23, interconnected via lines 51 - 55 as shown in the figure, and a single expander stage E20.
• Pressurized coolant (N 2 ) exits the compressor and the aftercooler H23 via a line 56 connected to a three-way valve Vl 2.
• the three-way valve V12 is controllable to selectively split the high-pressure N 2 stream flowing in the line 56 into two different streams in respective lines 57, 59, as further detailed below.
• a first outlet of the three-way valve V12 is connected to a coolant inlet in the first heat exchanger HlO via a line 59.
• a line 60 connects the coolant outlet of the first heat exchanger HlO with the second heat exchanger's H20 middle section, via line 61, as shown in figure 1.
• a line 57 connects a second outlet of the three-way valve V12 to the inlet of a first coolant passage 82 in the second heat exchanger H20 upper section.
• first coolant passage 82 outlet is connected via a line 58 to an entry point on the line 60 described above.
• a line 61 connects this entry point to a the inlet of a second coolant passage 84 in the cold box, in the vicinity of the cold box' middle section, as illustrated by figure 1.
• Coolant flows through the second coolant passage 84 and into an expander E20 via a line 62.
• the expanded coolant enters the second heat exchanger (cold box) Q H20 lower section via a line 63 connected to the inlet of a third coolant passage 86 before it exits the heat exchanger and flows back to the compressor C21, C22, C23 via the line 50.
• the flow split here described as a three-way valve Vl 2 can equally be performed by other flow control configurations, such as normal single line control valves, orifices, etc.
• the important aspect is that the flow split can be controlled in order s to cope with varying BOG flow conditions.
• a heat exchanger HlO to ensure that most of the low-temperature duty which can be extracted from the BOG in the ship's vapor header line 1, remains preserved within the reliquefaction system,
• a BOG compressor C 11 , C 12, C 13 working under ambient, or near-ambient conditions, with rejection of its heat of compression Hl 1, H12, H13 to the s ambience;
• This pressure control must be seen in association with flow control through the separator vent line 11 (flow control valve not shown in figure 1).
• the vent flow, as well as the composition of the condensate which is returned to 5 tanks 74 can be controlled according to the operator preferences. Minimizing the vent gas flow results in higher required reliquefaction power input and vice versa. Adjustments of the separator pressure will therefore allow the operator to select the most favourable conditions for economic optimization of the LNG RS operation.
• This other stream will typically be a fraction of the warm high-pressure N 2 -stream 59 as shown in figure 1.
• Other alternatives such as using the entire N 2 -stream (not only a part of it), or the BOG-stream from downstream the BOG compressor's aftercooler are also possible.
• the process of figure 1 will probably be the most beneficial, given the limitations and characteristics of commonly employed equipment for such processes. Consequently, only the process of figure 1, involving a split of the high- pressure N 2 -stream 56 downstream the N 2 -compander's aftercooler H23 into two different streams 57, 59, will be discussed next.
• the BOG pre-heater control is based on controlling the coolant flow (N 2 ) on the secondary side.
• the energy which is transferred between the compressed N 2 and the BOG in the first heat exchanger HlO (pre-heater) will depend on the BOG flow and temperature, and consequently be a more or less fixed value [kW] as long as the BOG flow is constant. This means that the temperature of the N 2 flow exiting the pre-heater HlO will vary with the N 2 flow rate.
• the three-way valve V12 or equivalent flow split constellations in the N 2 stream upstream the pre-heater HlO can be used for two different purposes:
• the freedom represented by the flow split (three-way valve V 12) can be used to ensure a very efficient heat exchange (low LMTD [log mean temp difference], and consequently low exergy losses) in the upper parts of the cold box H20.
• the heating and cooling curves can in theory be designed to be parallel with a constant temperature difference between streams at any temperature in the upper (warm) parts of the cold box. Since the Brayton cycle is based on the concept that pressurized N 2 has a higher heat capacity than low pressure N 2 , the heating curves can only be made parallel if the high pressure mass flow is smaller than the cold, low pressure flow.
• the split of the high pressure stream will consequently cause a very efficient heat exchange in the upper parts of the cold box, and since the branch flow also is cooled independently in the BOG pre-heater, the energy penalty which otherwise would have been associated with the mixing of the two high pressure N2 streams at a lower temperature is reduced to a minimum.
• the flow split will typically be controlled based on the BOG compressor suction temperature.
• the cold box is normally made in aluminium and is sensitive to thermal stress.
• a safety control function which changes the flow through the pre-heater based on undesirable conditions, the temperature of all streams entering the cold box can be carefully controlled. This would not have been possible if the pre-heater was a low pressure BOG vs. high pressure BOG heat exchanger, as the high temperature BOG outlet temperature would change synchronously with the fluctuation in the low pressure incoming BOG.
• the split ratio defining the flows of streams 57 and 59 will be adjusted in order to extract as much low temperature duty as possible from the low temperature BOG.
• this configuration also opens for controlling the split ratio with respect to the temperature of the nitrogen stream 61 entering the cold box' middle section. Doing so, conditions which may expose the main heat exchanger H20 to damaging thermal stresses can easily be eliminated.
• the heat exchangers HlO and H20 can be combined in one single multi-pass heat exchanger.
• the main heat exchanger (cold box) H20 typically will be a plate-fm heat exchanger, which to some extent is sensitive to both rapid temperature fluctuations and large local temperature approaches, it can be feasible to extract some of the heat transfer to an external heat exchanger of a more robust type, as shown at the pre-heater HlO in figure 1.
• the heat exchanger configuration shown in figure 1 will also dampen the temperature fluctuations of the flow 61 entering the main heat exchanger's H20 middle section, since the N 2 -coolant stream will be very large compared to the BOG flow. This will ensure a much safer operation with respect to thermal stresses in the cold box.
• the main incentive for employing ambient temperature BOG compression is the possibility this offers for rejecting heat to the ambience. While today's commonly used BOG compressors preserves the compression heat within the BOG stream, the compression heat can now be delivered to an external source operating at ambient or near ambient temperatures (e.g. cooling water).
• ambient or near ambient temperatures e.g. cooling water
• Ambient temperature compression also offers other benefits. Since an aftercooler Hl 3 as shown in figure 1 typically will be associated with this system, the temperature of the compressed stream 8 entering the cold box is stabilized relative to the heat rejection source's temperature. After- and intercooling also represent major advantages with respect to operation in recycle and/or anti surge modes, where the external cooling media ensures stable operation, normally without any additional temperature control.
• Ambient temperature BOG compression is especially favourable for LNG vessels where boil-off rates, compositions, temperatures and pressures may vary considerably with the type of voyage (ballast or laden voyages) and cargo. Inter- and aftercooling towards ambient conditions will stabilize the compression conditions and ease capacity control (recycling, etc.)
• a "higher" pressure ratio over the BOG compressors Cl 1,Cl 2,Cl 3 will in this context relate to a higher cold box inlet pressure in the line 8 than what is strictly necessary to provide a sufficient differential pressure for forcing the LNG back to the cargo tanks.
• the pressure in this zone can then be controlled independently of the BOG compressor discharge pressure and the cargo tank pressure. Accordingly, some of the overall system's capacity control can be performed by pressure adjustments in this region. It will consequently enable the operator or the automated control system to adjust both the amount of vent gas generated as well as the vent gas composition in order to operate under the most economically favourable conditions during all LNG price fluctuations.
• a dedicated line can also be placed in order to bypass the separator under conditions where reliquefied BOG is so much subcooled that the separation pressure otherwise will drop below a defined minimum value.
• the pressure differential between the main heat exchanger H20 and the separator FlO ensures that the separator can be placed more independent of the main heat exchanger.
• a higher BOG compressor discharge pressure will increase the gain (either in form of a higher adiabatic temperature change or reduced flash gas generation) during the throttling processes down to tank pressure.
• the purpose of the three-way valve V12 is to selectively control the flow split between (i) the line 59 connected to the first heat exchanger HlO and (ii) the line 57 connected to the cold box H20.
• the three-way valve V12 described above may be replaced by e.g. a controllable choke valve in the line 60, downstream of the first heat exchanger HlO, and a fixed-dimension restriction in the line 57.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• General Engineering & Computer Science (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• General Chemical & Material Sciences (AREA)
• Oil, Petroleum & Natural Gas (AREA)
• Filling Or Discharging Of Gas Storage Vessels (AREA)
• Separation By Low-Temperature Treatments (AREA)

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Cited By (17)

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• 2007
  • 2007-04-02 EP EP07747584.6A patent/EP2005094B1/en active Active
  • 2007-04-02 KR KR1020087024494A patent/KR101290032B1/ko active IP Right Grant
  • 2007-04-02 ES ES07747584T patent/ES2766767T3/es active Active
  • 2007-04-02 JP JP2009504142A patent/JP5280351B2/ja active Active
  • 2007-04-02 CN CN2007800184395A patent/CN101449124B/zh active Active
  • 2007-04-02 NO NO20084544A patent/NO345489B1/no unknown
  • 2007-04-02 US US12/295,403 patent/US20090113929A1/en not_active Abandoned
  • 2007-04-02 WO PCT/NO2007/000123 patent/WO2007117148A1/en active Application Filing

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