EP1861670A1 - System and method for cooling a bog stream - Google Patents

System and method for cooling a bog stream

Info

Publication number
EP1861670A1
EP1861670A1 EP06716763A EP06716763A EP1861670A1 EP 1861670 A1 EP1861670 A1 EP 1861670A1 EP 06716763 A EP06716763 A EP 06716763A EP 06716763 A EP06716763 A EP 06716763A EP 1861670 A1 EP1861670 A1 EP 1861670A1
Authority
EP
European Patent Office
Prior art keywords
bog
coolant
refrigeration system
stream
closed
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP06716763A
Other languages
German (de)
French (fr)
Other versions
EP1861670A4 (en
Inventor
Carl Jørgen RUMMELHOFF
Bjørn H. HAUKEDAL
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Wartsila Oil and Gas Systems AS
Original Assignee
Hamworthy KSE Gas Systems AS
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Hamworthy KSE Gas Systems AS filed Critical Hamworthy KSE Gas Systems AS
Publication of EP1861670A1 publication Critical patent/EP1861670A1/en
Publication of EP1861670A4 publication Critical patent/EP1861670A4/en
Withdrawn legal-status Critical Current

Links

Classifications

    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25JLIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02Processes 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
    • F25J1/0243Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
    • F25J1/0257Construction and layout of liquefaction equipments, e.g. valves, machines
    • F25J1/0275Construction and layout of liquefaction equipments, e.g. valves, machines adapted for special use of the liquefaction unit, e.g. portable or transportable devices
    • F25J1/0277Offshore use, e.g. during shipping
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25JLIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02Processes 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
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25JLIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/0002Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the fluid to be liquefied
    • F25J1/0022Hydrocarbons, e.g. natural gas
    • F25J1/0025Boil-off gases "BOG" from storages
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25JLIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/003Processes 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
    • F25J1/0047Processes 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
    • F25J1/005Processes 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
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25JLIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/006Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the refrigerant fluid used
    • F25J1/007Primary atmospheric gases, mixtures thereof
    • F25J1/0072Nitrogen
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25JLIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02Processes 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
    • F25J1/0203Processes 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
    • F25J1/0204Processes 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
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25JLIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02Processes 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
    • F25J1/0243Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
    • F25J1/0279Compression of refrigerant or internal recycle fluid, e.g. kind of compressor, accumulator, suction drum etc.
    • F25J1/0285Combination of different types of drivers mechanically coupled to the same refrigerant compressor, possibly split on multiple compressor casings
    • F25J1/0288Combination 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
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25JLIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2230/00Processes or apparatus involving steps for increasing the pressure of gaseous process streams
    • F25J2230/04Compressor cooling arrangement, e.g. inter- or after-stage cooling or condensate removal
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25JLIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2230/00Processes or apparatus involving steps for increasing the pressure of gaseous process streams
    • F25J2230/08Cold compressor, i.e. suction of the gas at cryogenic temperature and generally without afterstage-cooler
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25JLIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2230/00Processes or apparatus involving steps for increasing the pressure of gaseous process streams
    • F25J2230/20Integrated compressor and process expander; Gear box arrangement; Multiple compressors on a common shaft
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25JLIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2230/00Processes or apparatus involving steps for increasing the pressure of gaseous process streams
    • F25J2230/30Compression of the feed stream
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25JLIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2230/00Processes or apparatus involving steps for increasing the pressure of gaseous process streams
    • F25J2230/60Processes or apparatus involving steps for increasing the pressure of gaseous process streams the fluid being hydrocarbons or a mixture of hydrocarbons
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25JLIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2270/00Refrigeration techniques used
    • F25J2270/14External refrigeration with work-producing gas expansion loop
    • F25J2270/16External refrigeration with work-producing gas expansion loop with mutliple gas expansion loops of the same refrigerant
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25JLIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2290/00Other details not covered by groups F25J2200/00 - F25J2280/00
    • F25J2290/10Mathematical formulae, modeling, plot or curves; Design methods

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 system for cooling a boil-off gas stream, as set out in the introduction to the independent claim 1.
  • LNG liquid natural gas
  • a common technique for transporting natural gas from its extraction site is to liquefy the natural gas at or near this site, and transport the LNG to the market in specially designed storage tanks, often placed aboard a sea-going vessel.
  • the process of liquefying the natural gas involves compression and cooling of the gas to cryogenic temperatures (e.g. -160 0 C).
  • cryogenic temperatures e.g. -160 0 C.
  • the LNG carrier may thus transport a significant amount of liquefied gas to its destination.
  • the LNG is offloaded to special tanks onshore, before it is either transported by road or rail on LNG carrying vehicles or re-vaporized and transported by e.g. pipelines.
  • the Moss RSTM Concept is based on a closed nitrogen expansion cycle, extracting heat from the boil-off gas.
  • Boil-off gas (BOG) is removed from the cargo tanks by two conventional LD compressors operating in series.
  • the BOG is cooled and condensed to LNG in a cryogenic heat exchanger ("cold box"), to a temperature between the saturation temperature for compressed CH 4 and N 2 before being fed into a separator vessel where certain non-condensibles (mainly N 2 ) is removed.
  • the LNG coming out of the separator is pumped back to the cargo tanks, while the non-condensibles (i.e. gases) are sent to a flare or vent stack.
  • the present Moss RSTM concept is based on a nitrogen Brayton cycle with three-stage compression and one-stage expansion. Using only one expander reduces the complexity of the compander-unit (compressors and expander) to a minimum, but the internal temperature approach between hot and cold streams is inadequately large in the middle sec- tions of the cold-box. This is shown in Figure 1. As the rather large area between the hot and cold composite curves represent a plain exergy loss, process innovations are sought in order to minimize this area.
  • exergy losses can be reduced through the introduction of an additional expander.
  • a recognized method for implementing such a unit is to split the refrigerant stream at a given temperature level and hence let two expanders work in parallel as described in Norwegian patent application 2004 0306.
  • the present invention meets that need, in that it provides a method for cooling a boil-off gas (BOG) stream prior to compression in a boil-off reliquef action plant where the BOG stream following compression is reliquefied in heat exchange with a closed-loop refrigeration system comprising a coolant being compressed, before said reliquefied BOG being returned to a storage vessel, characterized by the following steps:
  • the invention also provides a system for cooling a boil-off gas (BOG) stream prior to compression in a boil-off reliquefaction plant, comprising a line for feeding BOG into a compressor prior to heat exchange with a closed-loop refrigeration system, said refrigeration system comprising compressors and expanders and a number of heat exchangers for heat exchange with the BOG stream, characterized in that the expanders are arranged in series.
  • BOG boil-off gas
  • Figure 1 shows a composite curve for the known one-expander Moss RSTM concept.
  • Figure 2 is a principle flow diagram showing the Nitrogen Brayton cycle with two expanders in series, according to the invention.
  • Figure 3 shows a composite curve for the nitrogen Brayton cycle with two expanders in series, (cf. Figure 2).
  • Figure 4 is a principle flow diagram illustrating the Nitrogen Brayton process with two expanders in series and nitrogen precooling according to the invention.
  • Figure 5 shows a composite curve for the nitrogen Brayton cycle with the process ac- cording to the invention (cf. Figure 4).
  • Figure 6 is a principle flow diagram of an embodiment of the invention, illustrating pre- and intercooling LD compressor with parallel split streams.
  • Figure 7 is a principle flow diagram of an embodiment of the invention, illustrating pre- and intercooling LD compressor with split stream in series.
  • Figure 8 is a principle flow diagram of an embodiment of the invention, showing points for pre- and intercooling with intermediate-pressure nitrogen.
  • the BOG is in most cases precooled before compression (by the compressor 11) prior to the cold-box. This is done in order to ensure a reasonable temperature profile in the cold-box and to achieve a more efficient compression in a reasonably sized LD- compressor-unit 11.
  • precooling has been described in several other patents, involving methods such as
  • the boil-off gas stream 10 from a reservoir (not shown) is compressed in a regular fashion in the compressor 11.
  • the boil-off stream 12 is thus routed through a compact heat exchanger (visualized in the figures as four separate heat exchangers) 5, 6, 7, 7b and heat exchanged against the closed-loop refrigeration system as will be described below.
  • the stream 13 exiting the heat exchanger (or series of heat exchangers) is completely re-liquefied with a careful tuning of the refrigeration system.
  • the person skilled in the art will appreciate that the heat exchangers 5, 6, 7, 7b as shown in the figures, may be combined into one compact heat exchanger.
  • FIG 4 shows a compressor system comprising three in-line compressors 2, 3, 4.
  • the compression may be achieved by one compression unit comprising three compressor wheels and two expander wheels connected via a common gear box.
  • the compressor system compresses the coolant (refrigerant), e.g. nitrogen, and feeds this stream 15 into the first heat ex- changer stage 5 where it is heat exchanged against the return coolant stream 20.
  • the coolant stream 16 is expanded in the expander 8 before being heat exchanged (stream 17) in the second heat exchanger 6 against the return coolant stream 20.
  • the heat exchanged stream 17B is the heat exchanged in the third heat exchanger 7, before the stream 18 is expanded in the expander 9.
  • the expanded coolant stream 19 is then heat exchanged in the fourth heat exchanger 7b, then routed (line 20) back to the compressor system 2, 3, 4 through the heat exchangers 7, 6, 5.
  • a BOG precooler 30 is included in the BOG feed line 10, upstream of the compressor 11.
  • the line 33 feeds a fraction of the coolant (e.g. nitrogen) from a take-off point on the return coolant stream 20 between the second 6 and third 7 heat exchanger stage to the precooler 30, and the (heat exchanged) coolant is returned to the return coolant stream 20 via line 32, at an entry point downstream of said take-off point.
  • the coolant e.g. nitrogen
  • the BOG flow rate through the cold-box is under normal operation modes kept at the absolute minimum. When no recycling of the LD-compressor is needed only direct boil-off from the cargo tanks are processed in the reliquefaction sys- tern.
  • the refrigerant (or coolant, e.g. nitrogen) flow rate through the low-temperature sections of the cold-box is the same as that running through the 3 nitrogen compressors 2, 3, 4, i.e. the maximum.
  • the fraction of the low-pressure nitrogen taken out from the cold-box can be de- signed with an optimal temperature, ensuring minimal exergy losses in the precooler 30. 4. Taking out some of the low-pressure refrigerant (e.g. nitrogen) will result in a better match for the cold-box' composite curves (i.e. reduced exergy losses) as this will reduce the local temperature approach in an area where this approach is generally large (ref. Figure 5).
  • some of the low-pressure refrigerant e.g. nitrogen
  • Another effect of splitting the low-pressure refrigerant stream is that it can be used, not only to precool the boil-off gas to the LD-compressor 30, but also to intercool the BOG between the two LD-compressor stages.
  • choosing such a solution offers more flexibility to adjust the temperature of the BOG entering the cold-box. This will, for different operational modes, reduce thermal stresses in the plate-fin heat exchanger, and open the possibility for reducing power under various operating conditions such as ballast voyages and voyages with nitrogen-rich LNG cargos.
  • FIG. 6 a preferred embodiment and a flexible solution for integrating both precoolers and intercoolers is shown in figure 6.
  • two splits ensure that the BOG temperature can be cooled down to the same low temperature in both the precooler 30' and the inter- cooler 30".
  • a BOG precooler 30' is included in the BOG feed line 10, upstream of the first compressor 11', while the BOG intercooler 30" is included in the BOG feed line downstream of the first compressor 11" and upstream of the second compressor 11'.
  • the lines 33', 33" feed a part of the coolant (e.g.
  • the coolant is fed from the similar take-off point in the return line 20 as described above via line 37 to the precooler 30', then via line 36 from the precooler 30' to the intercooler 30", before it is returned to the cold-box via line 38.
  • This embodiment is only possible when higher temperatures are allowed in the intercooler compared to the aftercooler. It is, however, possible to reach intercooler temperature levels close to those of the precooler, but this implies a high nitrogen flow rate, and hence high pre- and intercooling exergy losses.
  • An alternative solution is to feed the cold nitrogen stream to the precooler from the intermediate-pressure nitrogen stream between the two expansion stages.
  • This can in principle be done at any point between the two expanders, shown as points A, B, and C in Figure 8.
  • points A, B, and C In point A, the local temperature approach is small, and the nitrogen flow rate might be slightly increased as a consequence of this, but points A and C will on the other hand make the plate-fin heat exchanger design somewhat less complicated than point B.
  • the most suitable of the three points will be chosen as a result of economical considerations, control procedures, and energy demand of different LNG cargo compositions. For the already discussed two-expander solution, this will ensure that some of the expansion work is utilized for the nitrogen stream that is redirected to the precooler.

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Mechanical Engineering (AREA)
  • Thermal Sciences (AREA)
  • General Engineering & Computer Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • General Chemical & Material Sciences (AREA)
  • Oil, Petroleum & Natural Gas (AREA)
  • Ocean & Marine Engineering (AREA)
  • Filling Or Discharging Of Gas Storage Vessels (AREA)

Abstract

A system for cooling a boil-off gas (BOG) stream prior to compression in a boil-off reliquefaction plant, comprising a line (10) for feeding BOG into a compressor (11; 11") prior to heat exchange with a closed-loop refrigeration system. The refrigeration system comprising compressors (2, 3, 4) and expanders(8, 9) and a number of heat ex-changers for heat exchange with the BOG stream. The expanders (8, 9) are arranged in series. A precooler in the feed line (10) is fluidly connected (32, 33) to the closed-loop refrigeration system, whereby said BOG is pre-cooled in heat exchange with a portion of the coolant in the closed-loop refrigeration system prior to said compression.

Description

System and method for cooling a BOG stream
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 system for cooling a boil-off gas stream, as set out in the introduction to the independent claim 1.
A common technique for transporting natural gas from its extraction site, is to liquefy the natural gas at or near this site, and transport the LNG to the market in specially designed storage tanks, often placed aboard a sea-going vessel.
The process of liquefying the natural gas involves compression and cooling of the gas to cryogenic temperatures (e.g. -1600C). The LNG carrier may thus transport a significant amount of liquefied gas to its destination. At this destination, the LNG is offloaded to special tanks onshore, before it is either transported by road or rail on LNG carrying vehicles or re-vaporized and transported by e.g. pipelines.
LNG boils at slightly above -163 0C at atmospheric pressure, and is usually loaded, transported and offloaded at this temperature. This requires special materials, insulation and handling equipment in order to deal with the low temperature and the boil-off va- por. Due to heat leakage, the cargo (LNG) surface is constantly boiling, generating vaporized natural gas ("boil-off) - primarily methane - from the LNG.
Plants for the continuous re-liquefaction of this boil-off gas are well known. The re- liquefaction of boil-off gases on LNG carriers results in increased cargo deliveries and allows the operator to choose the most optimal carrier propulsion system. LNG carriers have traditionally been driven by steam turbines, and the boil-off gases from the LNG cargo have been used as fuel. This has been considered a costly solution.
One such alternative to using the boil-off gas as fuel is the Moss RS™ Concept, wherein the boil-off gas is liquefied and the resulting LNG is pumped back to the cargo tanks. The Moss RS™ Concept, described in Norwegian Patent No. 305525 Bl, is based on a closed nitrogen expansion cycle, extracting heat from the boil-off gas. Boil-off gas (BOG) is removed from the cargo tanks by two conventional LD compressors operating in series. The BOG is cooled and condensed to LNG in a cryogenic heat exchanger ("cold box"), to a temperature between the saturation temperature for compressed CH4 and N2 before being fed into a separator vessel where certain non-condensibles (mainly N2) is removed. The LNG coming out of the separator is pumped back to the cargo tanks, while the non-condensibles (i.e. gases) are sent to a flare or vent stack.
The patented Moss RS™ concept has so far been designed for implementation onboard LNGC vessels ranging up to gross volumes of 216 000 m3. However, as newer and larger vessels are designed, the onboard power supply systems are not enlarged proportionally with the ship's physical dimensions. This provokes changes in process design in order to present more energy efficient solutions for the re-liquefaction of LNG boil-off gas.
The present Moss RS™ concept is based on a nitrogen Brayton cycle with three-stage compression and one-stage expansion. Using only one expander reduces the complexity of the compander-unit (compressors and expander) to a minimum, but the internal temperature approach between hot and cold streams is inadequately large in the middle sec- tions of the cold-box. This is shown in Figure 1. As the rather large area between the hot and cold composite curves represent a plain exergy loss, process innovations are sought in order to minimize this area.
The exergy losses can be reduced through the introduction of an additional expander. A recognized method for implementing such a unit is to split the refrigerant stream at a given temperature level and hence let two expanders work in parallel as described in Norwegian patent application 2004 0306.
It is, however, an object of the invention to reduce the exergy losses, without splitting streams. It is a need for a more efficient system that will improve performance and reduce the power demand.
The present invention meets that need, in that it provides a method for cooling a boil-off gas (BOG) stream prior to compression in a boil-off reliquef action plant where the BOG stream following compression is reliquefied in heat exchange with a closed-loop refrigeration system comprising a coolant being compressed, before said reliquefied BOG being returned to a storage vessel, characterized by the following steps:
- feeding compressed coolant into a first heat exchanger,
- expanding the heat exchanged coolant, - feeding the expanded coolant into a second heat exchanger,
- expanding the heat exchanged coolant,
- feeding the expanded coolant into a third heat exchanger, - returning the coolant via heat exchangers to compressors.
The invention also provides a system for cooling a boil-off gas (BOG) stream prior to compression in a boil-off reliquefaction plant, comprising a line for feeding BOG into a compressor prior to heat exchange with a closed-loop refrigeration system, said refrigeration system comprising compressors and expanders and a number of heat exchangers for heat exchange with the BOG stream, characterized in that the expanders are arranged in series.
An embodiment of the invention will now be described in more detail, with reference to the accompanying drawings, where like parts have been given like reference numbers.
Figure 1 shows a composite curve for the known one-expander Moss RS™ concept.
Figure 2 is a principle flow diagram showing the Nitrogen Brayton cycle with two expanders in series, according to the invention.
Figure 3 shows a composite curve for the nitrogen Brayton cycle with two expanders in series, (cf. Figure 2).
Figure 4 is a principle flow diagram illustrating the Nitrogen Brayton process with two expanders in series and nitrogen precooling according to the invention.
Figure 5 shows a composite curve for the nitrogen Brayton cycle with the process ac- cording to the invention (cf. Figure 4).
Figure 6 is a principle flow diagram of an embodiment of the invention, illustrating pre- and intercooling LD compressor with parallel split streams.
Figure 7 is a principle flow diagram of an embodiment of the invention, illustrating pre- and intercooling LD compressor with split stream in series.
Figure 8 is a principle flow diagram of an embodiment of the invention, showing points for pre- and intercooling with intermediate-pressure nitrogen.
Implementing two expanders 8, 9 in series instead of in parallel, the exergy losses are reduced without splitting streams. The closed coolant (nitrogen) loop will then be simi- lar to that of Figure 2, where boil-off gas (BOG) from a reservoir (not shown) enters the compressor 11 through the line 10. The compressed stream 12 is routed through the heat exchanger 5, 6, 7 and heat exhanged against the closed-loop refrigeration system (this arrangement commonly referred to as a "cold-box"). With a careful tuning of the cold- box, the stream 13 exiting the heat exchanger(s) should be completely re-liquified.
Choosing two expanders in series, the exergy loss area is reduced by the introduction of a new local temperature pinch. This becomes clear when investigating the composite curve of Figure 3, showing representative data for the two expanders 8, 9 in series cycle, Note that the overall duty is reduced with more than 1 MW compared to the process of Figure 1, even though cooling capacity and temperature/pressure levels are maintained equal, seen from the BOG side.
The BOG is in most cases precooled before compression (by the compressor 11) prior to the cold-box. This is done in order to ensure a reasonable temperature profile in the cold-box and to achieve a more efficient compression in a reasonably sized LD- compressor-unit 11. The issue of precooling has been described in several other patents, involving methods such as
• direct cooling with a fraction of the condensed BOG stream • indirect cooling with a fraction of the condensed BOG stream
• indirect cooling with a fraction of the cold high pressure nitrogen stream, taken out before entering the nitrogen expander and throttled down to a lower pressure in a J-T expansion valve
To be able to liquefy most BOG compositions with a minimum of exergy losses, it is crucial to bring as much of the nitrogen refrigerant as possible down to the liquefier section of the cold-box. However, taking out nitrogen before (i.e. upstream of) the expander, will work against this principle. The same goes for the methods involving recycled liquefied BOG to be used in the precooling process. This will imply higher BOG flow rates at the low temperatures, and hence more circulated nitrogen will be necessary to cope with the increase in cooling demand at this temperature level.
In figure 4, the boil-off gas stream 10 from a reservoir (not shown) is compressed in a regular fashion in the compressor 11. The boil-off stream 12 is thus routed through a compact heat exchanger (visualized in the figures as four separate heat exchangers) 5, 6, 7, 7b and heat exchanged against the closed-loop refrigeration system as will be described below. The stream 13 exiting the heat exchanger (or series of heat exchangers) is completely re-liquefied with a careful tuning of the refrigeration system. The person skilled in the art will appreciate that the heat exchangers 5, 6, 7, 7b as shown in the figures, may be combined into one compact heat exchanger.
Turning now to the refrigeration system, figure 4 shows a compressor system comprising three in-line compressors 2, 3, 4. In a practical application, the compression may be achieved by one compression unit comprising three compressor wheels and two expander wheels connected via a common gear box. The compressor system compresses the coolant (refrigerant), e.g. nitrogen, and feeds this stream 15 into the first heat ex- changer stage 5 where it is heat exchanged against the return coolant stream 20. After the first heat exchanger step 5, the coolant stream 16 is expanded in the expander 8 before being heat exchanged (stream 17) in the second heat exchanger 6 against the return coolant stream 20. The heat exchanged stream 17B is the heat exchanged in the third heat exchanger 7, before the stream 18 is expanded in the expander 9. The expanded coolant stream 19 is then heat exchanged in the fourth heat exchanger 7b, then routed (line 20) back to the compressor system 2, 3, 4 through the heat exchangers 7, 6, 5.
As shown in figure 4, a BOG precooler 30 is included in the BOG feed line 10, upstream of the compressor 11. The line 33 feeds a fraction of the coolant (e.g. nitrogen) from a take-off point on the return coolant stream 20 between the second 6 and third 7 heat exchanger stage to the precooler 30, and the (heat exchanged) coolant is returned to the return coolant stream 20 via line 32, at an entry point downstream of said take-off point.
Thus, by using a fraction of the low-pressure nitrogen to precool the BOG stream, as in the invention, several favorable effects are seen:
1. The BOG flow rate through the cold-box is under normal operation modes kept at the absolute minimum. When no recycling of the LD-compressor is needed only direct boil-off from the cargo tanks are processed in the reliquefaction sys- tern.
2. The refrigerant (or coolant, e.g. nitrogen) flow rate through the low-temperature sections of the cold-box is the same as that running through the 3 nitrogen compressors 2, 3, 4, i.e. the maximum.
3. The fraction of the low-pressure nitrogen taken out from the cold-box can be de- signed with an optimal temperature, ensuring minimal exergy losses in the precooler 30. 4. Taking out some of the low-pressure refrigerant (e.g. nitrogen) will result in a better match for the cold-box' composite curves (i.e. reduced exergy losses) as this will reduce the local temperature approach in an area where this approach is generally large (ref. Figure 5).
As a direct consequence of these measures the power demand of the invented process as illustrated in Figure 4, can be reduced with additionally 100-150 kW. This reduction is reflected by the small step in the cold side composite curve, magnified in figure 5.
Another effect of splitting the low-pressure refrigerant stream is that it can be used, not only to precool the boil-off gas to the LD-compressor 30, but also to intercool the BOG between the two LD-compressor stages. This could potentially reduce the LD- compressor 11 work with around 50 kW (depending on amongst others the compressor efficiencies), but a slight increase in power demand to the nitrogen compander will equalize much of the power gained when considering the overall system. However, choosing such a solution offers more flexibility to adjust the temperature of the BOG entering the cold-box. This will, for different operational modes, reduce thermal stresses in the plate-fin heat exchanger, and open the possibility for reducing power under various operating conditions such as ballast voyages and voyages with nitrogen-rich LNG cargos.
Thus, a preferred embodiment and a flexible solution for integrating both precoolers and intercoolers is shown in figure 6. Here, two splits ensure that the BOG temperature can be cooled down to the same low temperature in both the precooler 30' and the inter- cooler 30". A BOG precooler 30' is included in the BOG feed line 10, upstream of the first compressor 11', while the BOG intercooler 30" is included in the BOG feed line downstream of the first compressor 11" and upstream of the second compressor 11'. The lines 33', 33" feed a part of the coolant (e.g. nitrogen) from a take-off point on the return coolant stream 20 between the second 6 and third 7 heat exchanger stage to the precooler 30' and the intercooler 30", respectively, and the (heat exchanged) coolant is returned to the return coolant stream 20 via lines 32', 32", respectively, at an entry point downstream of said take-off point.
It is also possible to choose only one split, as shown in figure 7. Here, the coolant is fed from the similar take-off point in the return line 20 as described above via line 37 to the precooler 30', then via line 36 from the precooler 30' to the intercooler 30", before it is returned to the cold-box via line 38. This embodiment is only possible when higher temperatures are allowed in the intercooler compared to the aftercooler. It is, however, possible to reach intercooler temperature levels close to those of the precooler, but this implies a high nitrogen flow rate, and hence high pre- and intercooling exergy losses.
An alternative solution is to feed the cold nitrogen stream to the precooler from the intermediate-pressure nitrogen stream between the two expansion stages. This can in principle be done at any point between the two expanders, shown as points A, B, and C in Figure 8. In point A, the local temperature approach is small, and the nitrogen flow rate might be slightly increased as a consequence of this, but points A and C will on the other hand make the plate-fin heat exchanger design somewhat less complicated than point B. The most suitable of the three points will be chosen as a result of economical considerations, control procedures, and energy demand of different LNG cargo compositions. For the already discussed two-expander solution, this will ensure that some of the expansion work is utilized for the nitrogen stream that is redirected to the precooler.

Claims

C l a i m s
1.
A method for cooling a boil-off gas (BOG) stream (10) prior to compression (11; 11', 11") in a boil-off reliquef action plant where the BOG stream following compression is reliquefied in heat exchange with a closed-loop refrigeration system comprising a coolant being compressed (2, 3, 4), before said reliquefied BOG being returned (13) to a storage vessel, c h a r a c t e r i z e d b y the following steps:
- feeding compressed coolant (15) into a first heat exchanger (5), - expanding (8) the heat exchanged coolant (16),
- feeding the expanded coolant (17) into a second heat exchanger (6),
- expanding (9) the heat exchanged coolant (18),
- feeding the expanded coolant (19) into a third heat exchanger (7),
- returning the coolant (20) via heat exchangers (6, 5) to compressors (2, 3, 4).
2.
The method of claim 1, wherein a portion of the coolant in the closed-loop refrigeration system is heat exchanged (30; 30') with the BOG stream prior to BOG compression.
3.
The method of claim 1, wherein said portion of the coolant is taken from the closed- loop refrigeration system between the third (7) and second (6) heat exchanger stage.
4. The method of claim 1, wherein said potion of the coolant is returned to the closed-loop refrigeration system between the third (7) and second (6) heat exchanger stage, following heat exchange with the .BOG.
5. The method of claim 1, wherein the BOG following compression (11") is heat exchanged (30") against said portion of the coolant in the closed-loop refrigeration system.
6. A system for cooling a boil-off gas (BOG) stream prior to compression in a boil-off reliquef action plant, comprising a line (10) for feeding BOG into a compressor (11; 11") prior to heat exchange with a closed-loop refrigeration system, said refrigeration system comprising compressors (2, 3, 4) and expanders (8, 9) and a number of heat exchangers for heat exchange with the BOG stream, c h a r a c t e r i z e d i n t h a t the expanders (8, 9) are arranged in series.
7.
The system of claim 6, wherein a precooler in said feed line (10) is fluidly connected (32, 33) to said closed-loop refrigeration system, whereby said BOG is pre-cooled in heat exchange with a portion of the coolant in the closed-loop refrigeration system prior to said compression.
EP06716763.5A 2005-03-14 2006-03-08 System and method for cooling a bog stream Withdrawn EP1861670A4 (en)

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NO20051315A NO20051315L (en) 2005-03-14 2005-03-14 System and method for cooling a BOG stream
PCT/NO2006/000090 WO2006098630A1 (en) 2005-03-14 2006-03-08 System and method for cooling a bog stream

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CN101137878A (en) 2008-03-05
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KR20070119686A (en) 2007-12-20
WO2006098630A1 (en) 2006-09-21
NO20051315D0 (en) 2005-03-14
EP1861670A4 (en) 2018-01-24
KR101194474B1 (en) 2012-10-24

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