CA2909616A1 - Method and apparatus for producing a liquefied hydrocarbon stream - Google Patents

Method and apparatus for producing a liquefied hydrocarbon stream

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Publication number
CA2909616A1
CA2909616A1 CA2909616A CA2909616A CA2909616A1 CA 2909616 A1 CA2909616 A1 CA 2909616A1 CA 2909616 A CA2909616 A CA 2909616A CA 2909616 A CA2909616 A CA 2909616A CA 2909616 A1 CA2909616 A1 CA 2909616A1
Authority
CA
Canada
Prior art keywords
stream
vapour
fraction
gas
condensed
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.)
Granted
Application number
CA2909616A
Other languages
French (fr)
Other versions
CA2909616C (en
Inventor
Jan Van Amelsvoort
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.)
Shell Internationale Research Maatschappij BV
Original Assignee
Shell Internationale Research Maatschappij BV
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 Shell Internationale Research Maatschappij BV filed Critical Shell Internationale Research Maatschappij BV
Publication of CA2909616A1 publication Critical patent/CA2909616A1/en
Application granted granted Critical
Publication of CA2909616C publication Critical patent/CA2909616C/en
Active legal-status Critical Current
Anticipated expiration legal-status Critical

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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
    • F25J1/0002—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
    • 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
    • F25J1/0032—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 the feed stream itself or separated fractions from it, i.e. "internal refrigeration"
    • F25J1/004—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 the feed stream itself or separated fractions from it, i.e. "internal refrigeration" by flash gas recovery
    • 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
    • F25J1/0032—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 the feed stream itself or separated fractions from it, i.e. "internal refrigeration"
    • F25J1/0042—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 the feed stream itself or separated fractions from it, i.e. "internal refrigeration" by liquid expansion 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
    • F25J1/003—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
    • F25J1/0032—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 the feed stream itself or separated fractions from it, i.e. "internal refrigeration"
    • F25J1/0045—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 the feed stream itself or separated fractions from it, i.e. "internal refrigeration" by vaporising a liquid return 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
    • 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
    • F25J1/0052—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 vaporising a liquid refrigerant 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
    • 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
    • F25J1/0052—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 vaporising a liquid refrigerant stream
    • F25J1/0055—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 vaporising a liquid refrigerant stream originating from an incorporated cascade
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    • F25J1/0214—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 multi-component refrigerant [MCR] fluid in a closed vapor compression cycle as a dual level refrigeration cascade with at least one MCR cycle
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
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    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • F25J1/0211—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 multi-component refrigerant [MCR] fluid in a closed vapor compression cycle
    • F25J1/0214—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 multi-component refrigerant [MCR] fluid in a closed vapor compression cycle as a dual level refrigeration cascade with at least one MCR cycle
    • F25J1/0215—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 multi-component refrigerant [MCR] fluid in a closed vapor compression cycle as a dual level refrigeration cascade with at least one MCR cycle with one SCR cycle
    • F25J1/0216—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 multi-component refrigerant [MCR] fluid in a closed vapor compression cycle as a dual level refrigeration cascade with at least one MCR cycle with one SCR cycle using a C3 pre-cooling 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
    • F25J1/0228—Coupling of the liquefaction unit to other units or processes, so-called integrated processes
    • F25J1/0229—Integration with a unit for using hydrocarbons, e.g. consuming hydrocarbons as feed stock
    • F25J1/023—Integration with a unit for using hydrocarbons, e.g. consuming hydrocarbons as feed stock for the combustion as fuels, i.e. integration with the fuel gas system
    • 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
    • F25J1/0244—Operation; Control and regulation; Instrumentation
    • F25J1/0254—Operation; Control and regulation; Instrumentation controlling particular process parameter, e.g. pressure, temperature
    • F25J1/0255—Operation; Control and regulation; Instrumentation controlling particular process parameter, e.g. pressure, temperature controlling the composition of the feed or liquefied gas, e.g. to achieve a particular heating value of natural gas
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
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    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • F25J1/0243—Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
    • F25J1/0257—Construction and layout of liquefaction equipments, e.g. valves, machines
    • F25J1/0262—Details of the cold heat exchange system
    • F25J1/0264—Arrangement of heat exchanger cores in parallel with different functions, e.g. different cooling streams
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
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    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • 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
    • 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
    • F25J1/0274—Retrofitting or revamping of an existing liquefaction unit
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    • F25J1/0279—Compression of refrigerant or internal recycle fluid, e.g. kind of compressor, accumulator, suction drum etc.
    • F25J1/0281—Compression of refrigerant or internal recycle fluid, e.g. kind of compressor, accumulator, suction drum etc. characterised by the type of prime driver, e.g. hot gas expander
    • F25J1/0283—Gas turbine as the prime mechanical driver
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    • F25J3/0635—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by partial condensation characterised by the separated product stream separation of CnHm with 1 carbon atom or more
    • 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
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/06—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by partial condensation
    • F25J3/063—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by partial condensation characterised by the separated product stream
    • F25J3/066—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by partial condensation characterised by the separated product stream separation of 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
    • F25J2200/00—Processes or apparatus using separation by rectification
    • F25J2200/70—Refluxing the column with a condensed part of the feed stream, i.e. fractionator top is stripped or self-rectified
    • 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
    • F25J2200/00—Processes or apparatus using separation by rectification
    • F25J2200/78—Refluxing the column with a liquid stream originating from an upstream or downstream fractionator column
    • 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
    • F25J2210/00—Processes characterised by the type or other details of the feed stream
    • F25J2210/06—Splitting of the feed stream, e.g. for treating or cooling in different ways
    • 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
    • F25J2215/00—Processes characterised by the type or other details of the product stream
    • F25J2215/04—Recovery of liquid products
    • 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
    • F25J2220/64—Separating heavy hydrocarbons, e.g. NGL, LPG, C4+ hydrocarbons or heavy condensates in general
    • 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
    • F25J2230/20—Integrated compressor and process expander; Gear box arrangement; Multiple compressors on a common shaft
    • 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
    • F25J2230/60—Processes or apparatus involving steps for increasing the pressure of gaseous process streams the fluid being hydrocarbons or a mixture of hydrocarbons
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2235/00—Processes or apparatus involving steps for increasing the pressure or for conveying of liquid process streams
    • F25J2235/60—Processes or apparatus involving steps for increasing the pressure or for conveying of liquid process streams the fluid being (a mixture of) hydrocarbons
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2240/00—Processes or apparatus involving steps for expanding of process streams
    • F25J2240/30—Dynamic liquid or hydraulic expansion with extraction of work, e.g. single phase or two-phase turbine
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2240/00—Processes or apparatus involving steps for expanding of process streams
    • F25J2240/80—Hot exhaust gas turbine combustion engine
    • 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
    • F25J2245/00—Processes or apparatus involving steps for recycling of process streams
    • F25J2245/02—Recycle of a stream in general, e.g. a by-pass 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
    • F25J2245/00—Processes or apparatus involving steps for recycling of process streams
    • F25J2245/90—Processes or apparatus involving steps for recycling of process streams the recycled stream being boil-off gas from storage
    • 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
    • F25J2260/00—Coupling of processes or apparatus to other units; Integrated schemes
    • F25J2260/60—Integration in an installation using hydrocarbons, e.g. for fuel purposes
    • 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
    • F25J2270/00—Refrigeration techniques used
    • F25J2270/12—External refrigeration with liquid vaporising loop
    • 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
    • F25J2280/00—Control of the process or apparatus
    • F25J2280/02—Control in general, load changes, different modes ("runs"), measurements

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  • Oil, Petroleum & Natural Gas (AREA)
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Abstract

A cryogenic hydrocarbon composition, obtained by subjecting a raw liquefied hydrocarbon stream to a pressure reduction step, is first separated into a vaporous reject stream and a liquid stream. The liquid stream is discharged in the form of the liquefied hydrocarbon stream. The vaporous reject stream is recompressed, partially condensed by indirectly heat exchanging the compressed vapour stream against an auxiliary refrigerant stream, and separated. The condensed fraction is revaporized and combusted in a gas turbine. The vapour fraction, which generally has a higher nitrogen content and a lower heating value than the condensed fraction, is combusted in a combustion device other than a gas turbine. The auxiliary refrigerant stream is formed by a slip stream of the liquefied hydrocarbon stream.

Description

METHOD AND APPARATUS FOR PRODUCING A LIQUEFIED
HYDROCARBON STREAM
The present invention relates to a method and apparatus for producing a liquefied hydrocarbon stream.
Liquefied natural gas (LNG) forms an economically important example of such a cryogenic hydrocarbon stream.
Natural gas is a useful fuel source, as well as a source of various hydrocarbon compounds. It is often desirable to liquefy natural gas in a liquefied natural gas plant at or near the source of a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form because it occupies a smaller volume and does not need to be stored at high pressure.
WO 2006/120127 describes an LNG separation process and installation. Liquefied natural gas in liquid form is sent to a separation unit, wherein a stream of LNG
purified of nitrogen, and a nitrogen-enriched vapour are produced. The separation unit employs two columns. An LNG stream which has been liquefied in a liquefier is first separated in a first column operating at about 1.25 bar producing a nitrogen-depleted liquid and an overhead gas stream. The overhead gas stream is recompressed to about 4 bar and passed to a second column, where any remaining methane is recondensed. The recondensed methane is withdrawn as liquid from the second column and mixed with the nitrogen-depleted liquid from the first column to form the stream of LNG purified of nitrogen.
Gaseous nitrogen is withdrawn from the top of the second column, allowing for the nitrogen contained in the natural gas to be utilized at commercial purity.
- 2 -The refrigeration or said recondensing of the methane in the second column is provided by a nitrogen cycle independent of the liquefier, which employs a refrigerant fluid is of which the nitrogen content is greater than 80 mol%.
A drawback of this LNG separation process is that an independent refrigeration cycle is required which involves both capital expenditure as well as operational expenditure. Moreover, as the recondensed methane is added to the purified LNG stream, it becomes increasingly demanding to maintain the nitrogen level in the purified LNG stream below the specification required for commercial LNG.
The present invention provides a method of producing a liquefied hydrocarbon stream, comprising:
- providing a cryogenic hydrocarbon composition comprising a nitrogen- and methane-containing liquid phase at an initial pressure of between 1 and 2 bar absolute;
- phase separating the cryogenic hydrocarbon composition, in an end flash separator at a first separation pressure of between 1 and 2 bar absolute, into a vaporous reject stream and a liquid stream;
- discharging the liquid stream from the end flash separator in the form of the liquefied hydrocarbon stream;
- compressing the vaporous reject stream in an end-flash compressor to a pressure of above 2 bar absolute, thereby obtaining a compressed vapour stream;
- forming a partially condensed intermediate stream from the compressed vapour, said partially condensed intermediate stream comprising a condensed fraction and a vapour fraction, by partially condensing the compressed
- 3 -vapour stream, wherein said partially condensing comprises indirectly heat exchanging the compressed vapour stream against an auxiliary refrigerant stream formed by a slip stream of the liquefied hydrocarbon stream whereby passing heat from at least part of the compressed vapour stream to the auxiliary refrigerant stream;
separating the condensed fraction from the vapour fraction in a gas/liquid separator, at a second separation pressure;
discharging the vapour fraction from the gas/liquid separator, said vapour fraction having a first heating value;
combusting the vapour fraction in a combustion device other than a gas turbine;
discharging the condensed fraction from the gas/liquid separator;
revaporizing the condensed fraction whereby transforming the condensed fraction into a fully vaporized stream having a second heating value that is higher than the first heating value;
combusting the fully vaporized stream in a gas turbine.
In another aspect, the present invention provides an apparatus for producing a liquefied hydrocarbon stream, comprising:
a cryogenic feed line connected to a source of a cryogenic hydrocarbon composition comprising nitrogen and a methane-containing liquid phase an end-flash separator arranged to receive the cryogenic hydrocarbon composition and to separate the cryogenic hydrocarbon composition into a liquid stream and vaporous reject stream;
-4--a liquid hydrocarbon product line fluidly connected to a bottom part of the end-flash separator to discharge said liquid stream in the form of the liquefied hydrocarbon stream from the end-flash separator;
a vapour reject line fluidly connected to an overhead part of the end-flash separator to discharge said vaporous reject stream from the end-flash separator;
an end-flash compressor arranged in the vapour reject line to compress the vaporous reject stream, thereby obtaining a compressed vapour stream;
a condenser arranged in the vapour reject line downstream of the end-flash compressor, arranged to receive the compressed vapour stream and to form a partially condensed intermediate stream from the compressed vapour stream, said partially condensed intermediate stream comprising a condensed fraction and a vapour fraction, which condenser is arranged to establish indirect heat exchanging contact between at least part of the compressed vapour stream and an auxiliary refrigerant stream;
an auxiliary refrigerant feed line extending between the liquid hydrocarbon product line and the condenser, to supply a slip stream of the liquid hydrocarbon product stream to the condenser;
a gas/liquid separator arranged downstream of the condenser and arranged to receive the condensed fraction and vapour fraction;
a vapour fraction discharge line fluidly connected with an overhead part of the gas/liquid separator arranged to receive the vapour fraction from the gas/liquid separator;
a combustion device other than a gas turbine fluidly connected with the gas/liquid separator by means of the
- 5 -vapour fraction discharge line to receive and combust the discharged vapour fraction;
a condensed fraction discharge line fluidly connected with a bottom part of the gas/liquid separator arranged to receive the condensed fraction from the gas/liquid separator;
a gas turbine fluidly connected with the gas/liquid separator by means of the condensed fraction discharge line to receive and combust the discharged condensed fraction;
a revaporizer arranged in the condensed fraction discharge line between the gas/liquid separator and the gas turbine and arranged to transform the condensed fraction into a fully vaporized stream prior to combustion in the gas turbine.
The invention will be further illustrated hereinafter, using examples and with reference to the drawing in which;
Fig. 1 schematically represents a process flow scheme representing a method and apparatus according to an embodiment of the invention;
Fig. 2 schematically represents an embodiment of a pressure reduction system for use in the invention;
Fig. 3 schematically represents a process flow scheme representing a method and apparatus according to another embodiment of the invention;
Fig. 4 schematically represents a process flow scheme representing a method and apparatus according to still another embodiment of the invention;
Fig. 5 schematically represents a process flow scheme wherein the embodiment of Fig. 3 is applied in combination with a selected liquefier;
- 6 -Fig. 6 schematically represents a process flow scheme wherein the embodiment of Fig. 4 is applied in combination with the selected liquefier; and Fig. 7 schematically represents a process flow scheme wherein the embodiment of Fig. 4 is applied with a specific type of end-flash separator.
In these figures, same reference numbers will be used to refer to same or similar parts. Furthermore, a single reference number will be used to identify a conduit or line as well as the stream conveyed by that line.
The present description concerns producing of a liquefied hydrocarbon stream, such as for instance a liquefied natural gas stream. A cryogenic hydrocarbon composition is first separated into a vaporous reject stream and a liquid stream. The liquid stream is discharged in the form of the liquefied hydrocarbon stream. The vaporous reject stream is recompressed, partially condensed by indirectly heat exchanging the compressed vapour stream against an auxiliary refrigerant stream, and separated. The condensed fraction is revaporized and combusted in a gas turbine. This fuel gas vapour stream is identified as high quality fuel gas stream.
The vapour fraction, which generally has a higher nitrogen content and a lower heating value than the condensed fraction, is combusted in a combustion device other than a gas turbine. In the context of the present description and compared to the condensed fraction, this fuel is referred to as low quality fuel gas. Low quality in this context means having a heating value that is lower compared to the heating value of the high quality fuel gas vapour stream, which is combusted in the gas turbine.
- 7 -The auxiliary refrigerant stream is advantageously formed by a slip stream of the liquefied hydrocarbon stream. No high degree of separation between the methane and the nitrogen in the intermediate condensed stream formed from the vaporous reject stream is required, as both the vapour fraction and the condensed fraction are combusted. Therefore the vapour fraction does not have to be free from methane while the condensed fraction is bound to less stringent requirements for its nitrogen content than if it would be added to the liquefied hydrocarbon stream.
The proposed method and apparatus thus do not require a full nitrogen rejection unit, since a combustible fuel gas stream is produced instead of a ventable nitrogen stream.
The proposed method and apparatus can be advantageously applied for instance if the raw liquefied stream comprises in the range of from 1 mol% to 7 mol%
nitrogen. However, most benefit is enjoyed in cases wherein the raw liquefied stream comprises more than 3 mol% of nitrogen, as in such cases a relatively high flow rate of vaporous reject gas is generated in order to maintain the liquid stream from which the liquefied hydrocarbon stream is derived within specification with regard to maximum content of lower boiling constituents such as nitrogen in commercially tradable liquefied natural gas. The high flow rate of vaporous reject gas generally contains too much nitrogen for use as fuel in gas turbines, and it usually exceeds plant fuel requirements if gas turbines are used to drive the refrigeration cycles in the liquefier.
More than 30 mol% of the vaporous reject stream and/or more than 30 mol% of the partially condensed
- 8 -intermediate stream may consist of nitrogen. Such nitrogen content would be too high to meet the fuel gas requirements of most gas turbines. The proposed method and apparatus may then be advantageously employed to recondense a fraction of the vaporous reject stream, to obtain a condensed fraction of which less than 30 mol%
consists of nitrogen so that, after revaporization, it can be used to fuel a gas turbine.
If the nitrogen content is still too high for the selected gas turbine, the condensed fraction (preferably after revaporization) may be blended with other fuel gas to bring the fuel on specification. In such cases the invention provides the benefit that the blending requirements are less demanding than if the fuel gas had more than 30 mol% of nitrogen.
The revaporized condensed fraction may have to be subjected to compression in order to meet a pre-determined gas turbine fuel gas pressure specification.
Alternatively, the condensed fraction may be pressurized, e.g. by means of a liquid pump, before revaporizing so that the condensed fraction can be revaporized at a pressure that is already sufficiently high to meet the fuel pressure specification of the gas turbine in which the revaporized condensed fraction will be combusted.
The cryogenic hydrocarbon composition may be obtained by subjecting a raw liquefied hydrocarbon stream to a pressure reduction step.
The cryogenic hydrocarbon composition may be sourced from a liquefier. Such liquefier may comprise a refrigerant circuit for cycling a refrigerant stream.
The refrigerant circuit may comprise a refrigerant compressor coupled to a refrigerant compressor driver, and arranged to compress the refrigerant stream; and a
9 cryogenic heat exchanger arranged to establish an indirect heat exchanging contact between a hydrocarbon stream and the refrigerant stream of the refrigerant circuit, whereby a raw liquefied stream is formed out of the hydrocarbon stream comprising a subcooled hydrocarbon stream. The liquefier may further comprise a pressure reduction system arranged downstream of the cryogenic heat exchanger and in fluid communication therewith, to receive the raw liquefied stream and to reduce pressure of the raw liquefied stream. A rundown line may fluidly connect the pressure reduction system with the cryogenic heat exchanger to establish fluid communication for the raw liquefied stream to pass from the cryogenic heat exchanger to the pressure reduction system, wherein the end-flash separator is arranged downstream of the pressure reduction system and in fluid communication therewith to receive the cryogenic hydrocarbon composition from the pressure reduction system.
Suitably, the gas turbine in which the condensed fraction is revaporized and combusted is the refrigerant compressor driver of the refrigerant circuit in the liquefier. The gas turbine is preferably selected from the group consisting of aeroderivative gas turbines.
Accordingly, the method may suitably comprise cycling a refrigerant stream in the liquefier, comprising driving a refrigerant compressor and compressing said refrigerant stream in the refrigerant compressor. A hydrocarbon stream may be condensed and subcooled, comprising indirectly heat exchanging said hydrocarbon stream against the refrigerant stream in the liquefier, thereby forming a raw liquefied stream at a liquefaction pressure of higher than 2 bara. The raw liquefied stream may be passed through a pressure reduction step, thereby
- 10 -obtaining the cryogenic hydrocarbon composition comprising nitrogen and a methane-containing liquid phase. Suitably, the refrigerant compressor is driven by the mentioned gas turbine in which the fully vaporized condensed fraction is combusted.
The slip stream for the auxiliary refrigerant stream is preferably formed of a part of the liquefied hydrocarbon stream. An advantage of employing a slip stream of the liquefied hydrocarbon stream for this purpose is that it can relatively easily be implemented on an already existing plant without the need to interrupt or modify any part relating to the source of the cryogenic hydrocarbon composition. Moreover, it is the coldest stream readily available in the plant, without the need for providing a dedicated refrigeration cycle, and there is generally plenty of it.
An auxiliary refrigerant return line suitably extends between the condenser and the end-flash separator.
Herewith, the auxiliary refrigerant stream containing heat from the at least part of the compressed vapour stream can be passed to and into the end-flash separator, such that a half-open refrigeration cycle is formed.
Preferably, a pump is configured in the auxiliary refrigerant line wherein the slip stream of the liquefied hydrocarbon stream can be pumped to the condenser.
Figure 1 illustrates an embodiment of the invention.
A cryogenic hydrocarbon composition comprising a nitrogen- and methane-containing liquid phase is conveyed in a cryogenic feed line 8. The source of the cryogenic hydrocarbon composition is not a limitation of the invention in its broadest definition, but for the sake of completeness one embodiment is illustrated wherein the
- 11 -cryogenic hydrocarbon composition is sourced from a liquefier 100.
Such a liquefier 100 would typically be provided upstream of the cryogenic feed line 8. The liquefier 100 may be in fluid communication with the cryogenic feed line 8 via a pressure reduction system 5, which communicates with the liquefier 100 via a rundown line 1.
The pressure reduction system 5 is arranged downstream of the cryogenic heat exchanger 180 and arranged to receive and reduce the pressure of a raw liquefied stream from the main cryogenic heat exchanger 5.
The pressure reduction system 5 may comprise a dynamic unit, such as an expander turbine, a static unit, such as a Joule Thomson valve, or a combination thereof.
An example of a pressure reduction system 5 with a Joule Thomson valve 7 in series with an expander turbine 6 is shown in Fig. 2. If an expander turbine is used, it may optionally be drivingly connected to a power generator.
Many arrangements are possible and known to the person skilled in the art.
In the example embodiment shown in Fig. 1, liquefier 100 comprises a refrigerant circuit 101 for cycling a refrigerant. The refrigerant circuit 101 comprises a refrigerant compressor 160 coupled to a refrigerant compressor driver 190 in a mechanical driving engagement.
The refrigerant compressor 160 is arranged to compress a spent refrigerant stream 150 and to discharge the refrigerant, in a pressurized condition, into a compressed refrigerant line 120. At least one reject heat exchanger 124 is normally provided in the compressed refrigerant line 120 of the refrigerant circuit 101. The reject heat exchanger 124 is arranged to reject heat from the pressurized refrigerant stream carried in the
- 12 -compressed refrigerant line 120 to the ambient, either to the air or to a body of water such as a lake, a river, or the sea.
The liquefier 100 typically comprises a refrigerant refrigerator arranged to refrigerate the pressurized refrigerant from the compressed refrigerant line 120 from which heat has been rejected in the reject heat exchanger 124. Herewith a refrigerated refrigerant stream is obtained in a refrigerated refrigerant line 131.
The liquefier 100 further comprises a cryogenic heat exchanger 180 connected to the refrigerant compressor 160 discharge outlet via the compressed refrigerant line 120.
In the embodiment of Figure 1, the cryogenic heat exchanger 180 also fulfils the function of the refrigerant refrigerator discussed in the previous paragraph, but this is not a requirement of the invention. The cryogenic heat exchanger is generally arranged to establish an indirect heat exchanging contact between a hydrocarbon stream 110 and the refrigerant of the refrigerant circuit 101.
A spent refrigerant line 150 connects the cryogenic heat exchanger 180 with a main suction end of the refrigerant compressor 160. The refrigerated refrigerant line 131 is in fluid communication with the spent refrigerant line 150, via a cold side of the cryogenic heat exchanger 180. The hydrocarbon stream 110 flows through a warm side of the cryogenic heat exchanger 180.
The cold side and the warm side are in heat exchanging contact with each other.
A main refrigerant return line 133 establishes fluid communication between the refrigerated refrigerant line 131 and the cold side of the cryogenic heat exchanger 180. The main refrigerant return line 133 is in fluid
- 13 -communication with the spent refrigerant line 150, via said cold side and in heat exchanging arrangement with the hot side. A main refrigerant control valve 134 is configured in the main refrigerant return line 133.
The cryogenic heat exchanger 180 receives the refrigerant stream in a depressurized condition from the main refrigerant return line 133 via the main refrigerant control valve 134, and discharges into the refrigerant compressor 160. Thus, the cryogenic heat exchanger 180 forms part of the refrigerant circuit 101.
The cryogenic heat exchanger 180 may be provided in any suitable form, including a printed circuit type, a plate fin type, optionally in a cold box configuration, or a tube-in-shell type heat exchanger such as a coil wound heat exchanger or a spool wound heat exchanger.
A specific non-limiting example of the liquefier and its refrigerant circuit based on a tube-in-shell type heat exchanger and including the refrigerant compressor and the cryogenic heat exchanger is shown in Figures 5 and 6. These figures will be described in detail later below.
Back to the invention, an end-flash separator 50 is arranged to receive the cryogenic hydrocarbon composition 8, optionally downstream of the pressure reduction system 5 and in fluid communication therewith, if such system is provided. Depending on the separation requirements, the end flash separator 50 may be provided in the form of a simple drum which separates vapour from liquid phases in a single equilibrium stage (such as depicted in Fig. 1), or a more sophisticated distillation column. Non-limiting examples of possibilities are disclosed in US
Patents 5,421,165; 5,893,274; 6,014,869; 6,105,391; and pre-grant publication US 2008/0066492.
- 14 -A liquid hydrocarbon product line 90 is fluidly connected to a bottom part of the end-flash separator 50.
The liquid hydrocarbon product line 90 connects the end-flash separator 50 to a cryogenic storage tank 210. An optional cryogenic pump (not shown) may be present in the liquid hydrocarbon product line 90, to assist the transport of any liquid hydrocarbon product that is being discharged from the end-flash separator 50 to the cryogenic storage tank 210.
A vapour reject line 64 is fluidly connected to an overhead part of the end-flash separator 50. An end-flash compressor 260 is arranged in the vapour reject line 64, to compress the vaporous reject stream from the end-flash separator 50. A condenser 35 is arranged in the vapour reject line 64, downstream of the end-flash compressor 260. This part of the vapour reject line will be referred to as compressed vapour stream line 70.
The condenser 35 is arranged to receive the compressed vapour stream, and to form a partially condensed intermediate stream from the compressed vapour stream. The condenser is arranged to establish indirect heat exchanging contact between at least part of the compressed vapour stream, and an auxiliary refrigerant stream.
An aftercooler 69 may be provided in the compressed vapour stream line 70 between the end-flash compressor 260 and the condenser 35. The aftercooler is arranged to reject heat from the compressed vapour to the ambient (for instance by heat exchanging against an ambient air stream or an ambient water stream). Such an aftercooler is recommended in embodiments where the temperature of the compressed vapour stream as it is discharged from the end-flash compressor exceeds the temperature of the
- 15 -ambient air and/or ambient water so that at least part of the heat added to the vapour in the end-flash compressor can be rejected to the ambient.
A cold recovery heat exchanger 65 may optionally be provided in the vapour reject line 64 resulting in that the reject vapour is fed into the end flash compressor 260 at an end-flash compressor suction temperature that is higher than the temperature at which the reject vapour is discharged from the end-flash separator 50 into the vapour reject line 64. Herewith the cold vested in the reject vapour in vapour reject 64 is preserved in a cold recovery stream 66, by heat exchanging against the cold recovery stream 66 prior to compressing the reject vapour to the end-flash compressor 260.
In one embodiment, the cold recovery stream 66 may comprise or consist of a side stream sourced from the hydrocarbon stream 110 in the liquefier 100. The resulting cooled side stream may for instance be combined with the cryogenic hydrocarbon composition in the cryogenic feed line 8. Thus, the cold recovery heat exchanging in the cold recovery heat exchanger 65 supplements the production rate of the cryogenic hydrocarbon composition.
In another embodiment, the cold recovery stream 66 may comprise or consist of a refrigerant stream being cycled in the liquefier 100 whereby the refrigerant stream (or a slipstream thereof) is condensed or sub-cooled. For instance, a slip stream of the compressed refrigerant can be drawn from the compressed refrigerant line 120 and be refrigerated by vapour reject line 64.
In still another embodiment, the cold recovery stream 66 may comprise or consist of the aftercooled reject vapour in the compressed vapour stream line 70,
- 16 -preferably in the part of the compressed vapour stream line 70 that extends between the aftercooler 69 and the condenser 35 through which the compressed vapour is passed from the end-flash compressor 260 to the condenser 35. Herewith the duty required from the auxiliary refrigerant stream 132 in the condenser 35 would be reduced.
The auxiliary refrigerant stream is supplied from an auxiliary refrigerant feed line 132 that extends between the liquid hydrocarbon product line 90 and the condenser 35. In the example as shown, the liquefied hydrocarbon product line 90 is split into the auxiliary refrigerant feed line 132 and a main product line 91. An auxiliary refrigerant return line 138 extends between the condenser 35 and the end-flash separator 50, and is arranged to return the auxiliary refrigerant containing heat from the compressed vapour stream back to the end-flash separator 50. An auxiliary refrigerant control valve 135 is arranged in the auxiliary refrigerant feed line 132. An optional auxiliary refrigerant pump (not shown) may optionally be provided in the auxiliary refrigerant feed line 132.
A gas/liquid separator 33 is arranged downstream of the condenser 35. A vapour fraction discharge line 80 is fluidly connected with an overhead part of the gas/liquid separator 33 and a condensed fraction discharge line 40 is fluidly connected with a bottom part of the gas/liquid separator 33. A gas turbine 320 is fluidly connected with the gas/liquid separator, by means of the condensed fraction discharge line 40. A combustion device 220 other than a gas turbine is fluidly connected with the gas/liquid separator by means of the vapour fraction discharge line 80.
- 17 -The combustion device 220 may comprise multiple combustion units. It may include, for example, one or more of a furnace, a boiler, an incinerator, a dual fuel diesel engine, or cross-combinations thereof. A boiler and a duel fuel diesel engine may advantageously be coupled to an electric power generator.
A revaporizer 285 is arranged in the condensed fraction discharge line 40, between the gas/liquid separator 33 and the gas turbine 320. The revaporizer is arranged to bring the condensed fraction in the condensed fraction discharge line 40 in indirect heat exchanging contact with a heating fluid 286 whereby during operation heat is transferred from the heating fluid 286 to the condensed fraction in the condensed fraction discharge line 40. Optionally, a fuel gas compressor 360 is arranged in the condensed fraction discharge line 40 between the revaporizer 285 and the gas turbine 320.
A cold recovery heat exchanger 85 may optionally be provided in the vapour fraction discharge line 80 to recover cold vested in the vapour fraction prior to combusting it in the combustion device 220. The cold recovery heat exchanger 85 is arranged to bring the vapour fraction in the vapour fraction discharge line 80 in indirect heat exchanging contact with a cold recovery stream 86. During operation heat is transferred from the cold recovery stream 86 to the vapour fraction in the vapour fraction discharge line 80. This cold recovery heat exchanger 85 may be referred to as second cold recovery heat exchanger in embodiments wherein the cold recovery heat exchanger 65 is provided in the vapour reject line 64. In such embodiments, the cold recovery heat exchanger 65 in the vapour reject line 64 may be referred to as first cold recovery heat exchanger.
- 18 -The apparatus described above may be used in a method described as follows.
A cryogenic hydrocarbon composition 8 comprising a nitrogen- and methane-containing liquid phase is provided at an initial pressure of between 1 and 2 bar absolute, and an initial temperature. Providing of the cryogenic hydrocarbon composition 8 may comprise passing a hydrocarbon stream 110 through the liquefier 100. The hydrocarbon stream 110 may be condensed and subcooled in the liquefier 100. The condensing and subcooling of the hydrocarbon stream 110 preferably involves indirectly heat exchanging the hydrocarbon stream 110 against the refrigerant in the liquefier 100. The thus formed subcooled liquefied hydrocarbons stream is referred to as the raw liquefied stream. Thus the raw liquefied stream is formed out of the hydrocarbon stream by condensing and subsequently subcooling the hydrocarbon stream.
For example, in such a liquefier 100, the hydrocarbon stream 110 comprising a hydrocarbon-containing feed vapour may be heat exchanged, for example in the cryogenic heat exchanger 180, against a main refrigerant stream, thereby liquefying the feed vapour of the feed stream to provide the raw liquefied stream within the rundown line 1. The desired cryogenic hydrocarbon composition 8 may then be obtained from the raw liquefied stream 1. The raw liquefied stream may be discharged in the rundown line 1 from the liquefier 100. The cryogenic hydrocarbon composition 8 may be obtained from the raw liquefied stream, for instance by passing the raw liquefied stream through a pressure reduction step in pressure reduction system 5. In this pressure reduction step, the pressure may be reduced from the liquefaction pressure to the initial pressure.
- 19 -The cryogenic hydrocarbon composition 8 is subsequently phase separated, at a first separation pressure of between 1 and 2 bar absolute, into a vaporous reject stream 64 and a liquid stream 90. Suitably, this phase separating is performed in the end-flash separator 50. The vaporous reject stream 64 comprises a majority of, preferably all of, any flash vapour that has been generated during the pressure reduction step. The liquid stream 90 is discharged in the form of the liquefied hydrocarbon stream, which may be a liquefied natural gas stream provided that the methane content is at least 81 mol%. The liquid stream 90 is typically conveyed to the cryogenic storage tank 210.
The vaporous reject stream 64 is discharged from the end-flash separator 50, and subsequently compressed in the end-flash compressor 260 to a pressure of above 2 bar absolute, thereby obtaining a compressed vapour stream 70.
The compressed vapour stream 70 is passed to the condenser 35. If an aftercooler 69 is provided in the compressed vapour stream line 70, the compressed vapour stream 70 is passed through the aftercooler 69 as it is being passed to the condenser 35. In the aftercooler 69, heat is rejected from the compressed vapour to the ambient (for instance by heat exchanging against an ambient air stream or an ambient water stream). The compressed vapour 70 is discharged from the optional aftercooler 69 at an aftercooled temperature that is close to ambient temperature, for instance 2 C above ambient temperature. Ambient temperature is considered to be the temperature of the ambient medium (air or water) to which the heat is rejected.
- 20 -In the condenser 35, a partially condensed intermediate stream is formed from the compressed vapour 70, by partially condensing the compressed vapour stream.
The partially condensed intermediate stream comprises a condensed fraction 40 and a vapour fraction 80. The partially condensing comprises indirect heat exchanging of the compressed vapour stream 70 against an auxiliary refrigerant stream 132, formed by a slip stream of the liquid stream 90. During the indirect heat exchanging, heat passes from, at least part of, the compressed vapour stream 70 to the auxiliary refrigerant stream 132. It is conceived that only about 0.2 % of the liquid stream 90 is needed as the auxiliary refrigerant stream 132.
Generally, between 0.05 and 0.40 % of the liquid stream 90 may be needed as the auxiliary refrigerant stream 132.
The condensed fraction 40 is then separated from the vapour fraction 80 in the gas/liquid separator 33, at a second separation pressure. Preferably, the vapour fraction and the condensed fraction co-exist in the gas/liquid separator 33, and are separated in a single thermodynamic equilibrium state between said vapour fraction and the condensed fraction residing inside the gas/liquid separator 33. This can generally be achieved if the gas/liquid separator 33 is embodied in the form of simple drum with no gas/liquid contacting internals such as trays or packing, thus essentially representing one single theoretical stage.
The vapour fraction 80 is discharged from the gas/liquid separator 33, typically as a vapour phase in its dew point. The vapour fraction 80, as it is being discharged from the gas/liquid separator 33, has a first heating value. The vapour fraction 80 is combusted in the combustion device 220.
- 21 -The condensed fraction 40 is also discharged from the gas/liquid separator 33, but as liquid phase at its bubble point. The condensed fraction 40 is subsequently revaporized in revaporizer 285. The revaporizing comprises bringing the condensed fraction 40 in indirect heat exchanging contact with the heating fluid 286, whereby heat is transferred from the heating fluid 286 to the condensed fraction 40. During revaporizing the condensed fraction 40 is transformed into a fully vaporized stream having a second heating value. After revaporizing, the condensed fraction 40 is fully in vapour phase. The fully vaporized stream resulting from the condensed fraction 40 is combusted in a gas turbine 320.
The auxiliary cooling duty transferred by the auxiliary refrigerant stream 132 in the condenser 35 can be altered by manipulating the auxiliary refrigerant control valve 135. Various control strategies are possible. For example, the auxiliary refrigerant control valve 135 is functionally coupled to an optional level controller 37 arranged in the gas/liquid separator 33 to establish a constant liquid level in gas/liquid separator 33 by regulating the amount of partial condensation of the compressed vapour 70 that occurs in the condenser 35.
Another example involves the auxiliary refrigerant control valve 135 is functionally coupled to an optional temperature sensor (not shown) arranged between the condenser 35 and the gas/liquid separator 33 to establish a constant temperature of the partially condensed intermediate stream.
A suitable set point for the temperature sensor can be derived from a desired nitrogen content of the condensed fraction 40 that is consisted with the fuel gas
- 22 -composition specification of the gas turbine 320. The remaining nitrogen stays in the vapour fraction. The rundown temperature of raw liquefied stream in the rundown line 1 can be regulated such as to ensure that the total available heating power in the reject vapour 64 and/or the partially condensed intermediate stream meets the combined fuel gas requirement of the combustion device(s) 220 and the gas turbine(s) 320. For instance, if there is too much heating power in the reject vapour 64, the rundown temperature can be lowered to reduce the amount of methane that is flashed in the pressure reduction step in pressure reduction system 5. The partitioning of the nitrogen over the vapour fraction 80 and the condensed fraction 40 is regulated by the auxiliary cooling duty.
The nitrogen content of the liquid stream 90 may be kept within specification over the range of rundown temperatures anticipated during operation by the correct choice and dimensioning of the end-flash separator 50 at the design stage.
The first and second heating values define the amount of heat that can be released by combustion of a mole of the fuel gas. This can be either the so-called "high"
heating value as the "low" heating value as long as the same conditions are used for comparing the two heating values. Preferably the "low" heating value is used to compare the two heating values, as this is the closest to the combustion conditions used in the invention. The heating value may be determined using ASTM D3588-98 applied regardless of the composition of the vapour fraction 80 and/or the condensed fraction 40. As a result of the separation in the cooled gas/liquid separator 33, the second heating value (belonging to the
- 23 -condensed fraction 40) is higher than the first heating value (belonging to the vapour fraction 80). However, as the partially condensed intermediate stream essentially consists of two components, methane and nitrogen, the first and second heating values uniquely map onto nitrogen content of the vapour fraction 80 and the condensed fraction 40, respectively.
The vapour fraction 80 is combusted the combustion device 220 preferably at a first fuel gas pressure that is not higher than the second separation pressure. This way a compressor can be avoided as the pressure in the vapour fraction 80 does not have to be increased.
Preferably, the vapour fraction 80 is combusted in the combustion device at a pressure of between 2 and 15 bara, more preferably at a pressure of between 2 and 6 bara.
The condensed fraction 40 may have to be pressurized to a second fuel gas pressure that is higher than the second separation pressure. If a fuel gas compressor 360 is arranged in the condensed fraction discharge line 40 between the revaporizer 285 and the gas turbine 320, the fully vaporized stream may optionally be compressed in such fuel gas compressor 360 to the second fuel gas pressure before combusting the fully vaporized stream in the gas turbine 320. The second fuel gas pressure is generally higher than the second separation pressure, and preferably adapted to meet fuel gas pressure requirements imposed by the selected gas turbine 320.
A pressure reduction valve 245 may optionally be arranged in the condensed fraction discharge line 40 between the gas/liquid separator 33 and the revaporizer 285. This allows for flashing some of the condensed fraction 40 prior to passing the condensed fraction 40 through the revaporizer 285, by first passing the
- 24 -condensed fraction 40 through the pressure reduction valve 245 and subsequently performing the indirectly heat exchanging of the condensed fraction 40 against the heating fluid 286. Herewith a lower temperature of the heating fluid 286 can be achieved when it is discharged from the revaporizer 285 and/or more of the cold vested in the condensed fraction 40 can be recovered into the heating fluid 286. The pressure reduction valve 245 controls the discharge temperature of the vaporized stream being discharged from the revaporizer 285. The pressure reduction valve 245 may suitably be functionally coupled to a first temperature sensor 247, arranged in the condensed fraction discharge line 40 downstream of the revaporizer 285, whereby the valve setting is controlled in response to a first temperature signal generated in the first temperature sensor 247. If the optional fuel gas compressor 390 is provided, the first temperature sensor is suitably arranged in the condensed fraction discharge line 40 between the revaporizer 285 and the optional fuel gas compressor 390. A first target temperature setting for this control loop may be set at a few degrees below, e.g. 2 C below, the temperature of the heating fluid 286 at the inlet of the revaporizer 285. Preferably, the temperature of the condensed fraction 40 at the outlet of the revaporizer is between ambient temperature and 10 C below ambient temperature to effectively obtain the most benefit from the cold vested in the condensed fraction 40.
The second separation pressure is preferably higher than the first separation pressure. The second separation pressure may suitably be between 2 and 22 bara, preferably between 5 and 22 bara, more preferably between 5 and 15 bara. A second separation pressure in
- 25 -the higher end of the range of 2 to 22 bara helps the partial condensation of the compressed stream 70 and to provide clearance for a higher pressure drop in the optional pressure reduction valve 245 and/or to maintain a higher pressure even after the pressure reduction valve 245, which saves on fuel gas compression duty in the optional fuel gas compressor 360. The lower end of the range helps the separation efficiency in the gas/liquid separator 33 and causes less over compressing of the vapor fraction 80 which is to be combusted in the combustion device 220 at a relatively low pressure of typically less than 15 bara. The proposed range of between 5 and 15 bara for the second separation pressure strikes a proper balance between the beneficial and the adverse effects summarized earlier in this paragraph.
A typical pressure drop of between 1.0 and 4.0 bar over the optional pressure reduction valve 245 has been found adequate in typical cases.
In some embodiments, the second separation pressure is in the range of from 5 to 8 bara, which pressure most often meets the requirements of a low-pressure fuel gas stream suitable for conveying the vapour fraction 80 to the combustion device 220 without need for further compression. A higher pressure may be selected if the combustion device 220 is at a relatively large distance from the first gas/liquid phase separator and/or when the vapour fraction 80 is intended to pass through one or more cold recovery heat exchangers 85. In such circumstances an additional pressure drop may be expected in the course of conveying the off gas to the combustion device 220. In one embodiment, the second separation pressure is about 6.5 bara.
- 26 -The cryogenic hydrocarbon composition 8 may be obtained from natural gas or petroleum reservoirs or coal beds. As an alternative the cryogenic hydrocarbon composition 8 may also be obtained from another source, including as an example a synthetic source such as a Fischer-Tropsch process. Preferably the cryogenic hydrocarbon composition 8 comprises at least 50 mol%
methane, more preferably at least 80 mol% methane. A
preferred initial temperature of lower than -130 C may be achieved by passing a hydrocarbon stream 110 through a liquefaction system 100. An embodiment of passing the hydrocarbon stream 110 through the liquefaction system 100 will be described in more detail below.
A refrigerant is cycled in the refrigerant circuit 101 of the liquefier 100. Cycling comprises driving the refrigerant compressor 160, and compressing the refrigerant stream in the refrigerant compressor 160. A
hydrocarbon stream 110 is condensed and subcooled. The condensing and subcooling involves indirectly heat exchanging the hydrocarbon stream 110 against the refrigerant in the liquefier 100. The thus formed subcooled liquefied hydrocarbons stream is referred to as raw liquefied stream. Thus the raw liquefied stream is formed out of the hydrocarbon stream by condensing and subsequently subcooling the hydrocarbon stream.
The hydrocarbon stream 110 in any of the examples disclosed herein may be obtained from natural gas or petroleum reservoirs or coal beds. As an alternative the cryogenic hydrocarbon composition 8 may also be obtained from another source, including as an example a synthetic source such as a Fischer-Tropsch process. Preferably the cryogenic hydrocarbon stream 110 comprises at least 50 mol% methane, more preferably at least 80 mol%
- 27 -methane. The resulting liquid hydrocarbon product conveyed in the liquid hydrocarbon product line 90 and/or stored in the cryogenic storage tank 210 is preferably liquefied natural gas (LNG).
Depending on the source, the hydrocarbon stream 110 may contain varying amounts of components other than methane and nitrogen, including one or more non-hydrocarbon components other than water, such as 002, Hg, H2S and other sulphur compounds; and one or more hydrocarbons heavier than methane such as in particular ethane, propane and butanes, and, possibly lesser amounts of pentanes and aromatic hydrocarbons. Hydrocarbons with a molecular mass of at least that of propane may herein be referred to as C3+ hydrocarbons, and hydrocarbons with a molecular mass of at least that of ethane may herein be referred to as C2+ hydrocarbons.
If desired, the hydrocarbon stream 110 may have been pre-treated to reduce and/or remove one or more of undesired components such as CO2 and H2S, or have undergone other steps such as pre-pressurizing or the like. Such steps are well known to the person skilled in the art, and their mechanisms are not further discussed here. The composition of the hydrocarbon stream 110 thus varies depending upon the type and location of the gas and the applied pre-treatment(s).
The raw liquefied stream is discharged in the rundown line 1 from the liquefier 100. The raw liquefied stream may comprise in the range of from 1 mol% to 7 mol%
nitrogen and more than 81 mol% of methane. The temperature of the raw liquefied stream in the rundown line 1 may be anywhere between -165 C and -120 C. A
cryogenic hydrocarbon composition 8 is obtained from the raw liquefied stream by passing the raw liquefied stream
- 28 -through a pressure reduction step in pressure reduction system 5, whereby reducing the pressure from the liquefaction pressure to an initial pressure of between 1 and 2 bar absolute. Flash vapour is usually generated during such pressure reduction step.
The cryogenic hydrocarbon composition 8 comprises a nitrogen- and methane-containing liquid phase, and is usually at a temperature lower than -130 C.
In many cases, the temperature of the raw liquefied stream in the rundown line 1 may be in the range of from -160 C to -145 C. Within this more narrow range the cooling duty needed in the liquefaction system 100 is lower than when lower temperatures are desired, while the amount of sub-cooling at the pressure of above 15 bara is sufficiently high to avoid excessive production of flash vapours upon reducing the pressure to the initial pressure of between 1 and 2 bara.
The liquefaction system 100 in the present specification has so far been depicted very schematically. It can represent any suitable hydrocarbon liquefaction system and/or process, in particular any natural gas liquefaction process producing liquefied natural gas, and the invention is not limited by the specific choice of liquefaction system. Examples of suitable liquefaction systems employ single refrigerant cycle processes (usually single mixed refrigerant - SMR -processes, such as PRICO described in a paper by K R
Johnsen and P Christiansen, presented at Gastech 1998 (Dubai), but also possible is a single component refrigerant such as for instance the BHP-cLNG process also described in the afore-mentioned paper by Johnsen and Christiansen); double refrigerant cycle processes (for instance the much applied Propane-Mixed-Refrigerant
- 29 -process, often abbreviated C3MR, such as described in for instance US Patent 4,404,008, or for instance double mixed refrigerant - DMR - processes of which an example is described in US Patent 6,658,891, or for instance two-cycle processes wherein each refrigerant cycle contains a single component refrigerant); and processes based on three or more compressor trains for three or more refrigeration cycles of which an example is described in US Patent 7,114,351.
Other examples of suitable liquefaction systems are described in: US Patent 5,832,745 (Shell SMR); US Patent 6,295,833; US Patent 5,657,643 (both are variants of Black and Veatch SMR); US Pat. 6,370,910 (Shell DMR).
Another suitable example of DMR is the so-called Axens LIQUEFIN process, such as described in for instance the paper entitled "LIQUEFIN: AN INNOVATIVE PROCESS TO REDUCE
LNG COSTS" by P-Y Martin et al, presented at the 22nd World Gas Conference in Tokyo, Japan (2003). Other suitable three-cycle processes include for example US
Pat. 6,962,060; WO 2008/020044; US Pat. 7,127,914;
DE3521060A1; US Pat. 5,669,234 (commercially known as optimized cascade process); US Pat. 6,253,574 (commercially known as mixed fluid cascade process); US
Pat. 6,308,531; US application publication 2008/0141711;
Mark J. Roberts et al "Large capacity single train AP-X(TM) Hybrid LNG Process", Gastech 2002, Doha, Qatar (13-16 October 2002). These suggestions are provided to demonstrate wide applicability of the invention, and are not intended to be an exclusive and/or exhaustive list of possibilities. Not all examples listed above employ (aeroderivative) gas turbines as primary refrigerant compressor drivers. It will be clear that any drivers other than gas turbines can be replaced for a gas turbine
- 30 -to enjoy the certain preferred benefits of the present invention.
An example, wherein in the liquefaction system 100 is based on, for instance C3MR or Shell DMR, is briefly illustrated in Figures 5 and 6. In both cases the cryogenic heat exchanger 180 in the liquefaction system 100 is selected to be a coil wound heat exchanger, comprising a warm side comprising all the tubes, including lower and upper hydrocarbon product tube bundles (181 and 182, respectively), lower and upper LMR
tube bundles (183 and 184, respectively) and an HMR tube bundle 185. The cold side is formed by the shell side of the cryogenic heat exchanger 180.
The lower and upper hydrocarbon product tube bundles 181 and 182 fluidly connect the hydrocarbon stream line 110 with the rundown line 1. At least one refrigerated hydrocarbon pre-cooling heat exchanger 115 may be provided in the hydrocarbon stream line 110, upstream of the cryogenic heat exchanger 180.
The refrigerant provided in the refrigerant circuit 101 will be referred to as "main refrigerant" to distinguish it from other refrigerants that may used in the liquefaction system 100 such as a pre-cooling refrigerant 127 which may provide cooling duty to the refrigerated hydrocarbon pre-cooling heat exchanger 115.
The main refrigerant in the present embodiment is a mixed refrigerant.
The refrigerant circuit 101 comprises a spent refrigerant line 150, connecting the cryogenic heat exchanger 180 (in this case a shell side 186 of the cryogenic heat exchanger 180) with a main suction end of the refrigerant compressor 160, and a compressed refrigerant line 120 connecting the refrigerant
- 31 -compressor 160 discharge outlet with an MR separator 128.
One or more heat exchangers are provided in the compressed refrigerant line 120, including in the present example at least one reject heat exchanger 124. The MR
separator 128 is in fluid connection with the lower LMR
tube bundle 183 via a light refrigerant fraction line 121, and with the HMR tube bundle via a heavy refrigerant fraction line 122.
The at least one refrigerated hydrocarbon pre-cooling heat exchanger 115 and the at least one refrigerated main refrigerant pre-cooling heat exchanger 125 are refrigerated by the pre-cooling refrigerant (via lines 127 and 126, respectively). The same pre-cooling refrigerant may be shared from the same pre-cooling refrigerant cycle. Moreover, the at least one refrigerated hydrocarbon pre-cooling heat exchanger 115 and the at least one refrigerated main refrigerant pre-cooling heat exchanger 125 may be combined into one pre-cooling heat exchanger unit (not shown). Reference is made to US Pat. 6,370,910 as a non-limiting example.
At a transition point between the upper (182, 184) and lower (181, 183) tube bundles, the HMR tube bundle 185 is in fluid connection with an HMR line 141. The HMR
line 141 is in fluid communication with the shell side 186 of the cryogenic heat exchanger 180 via a first HMR
return line 143, in which an HMR control valve 144 is configured. Via the said shell side 186, and in heat exchanging arrangement with each of one of the lower hydrocarbon product tube bundle 181 and the lower LMR
tube bundle 183 and the HMR tube bundle 185, first HRM
return line 143 is fluidly connected to the spent refrigerant line 150.
- 32 -Above the upper tube bundles 182 and 184, near the top of the cryogenic heat exchanger 180, the LMR tube bundle 184 is in fluid connection with the refrigerated refrigerant line 131. A main refrigerant return line 133 establishes fluid communication between the refrigerated refrigerant line 131 and the shell side 186 of the cryogenic heat exchanger 180. A main refrigerant control valve 134 is configured in the main refrigerant return line 133. The main refrigerant return line 133 is in fluid communication with the spent refrigerant line 150, via said shell side 186 and in heat exchanging arrangement with each of one of the upper and lower hydrocarbon product tube bundles 182 and 181, respectively, and each one of the LMR tube bundles 183 and 184, and the HMR tube bundle 185.
The line-up around the end flash separator 50 and the gas/liquid separator 33 as shown in Figure 5 corresponds to the line-up shown in Fig. 3. The line-up around the end flash separator 50 and the gas/liquid separator 33 as shown in Figure 6 corresponds to the line-up shown in Fig. 4. In both cases, the auxiliary refrigerant line, on an upstream end thereof, fluidly connects with the liquid hydrocarbon product line 90, and on a downstream end thereof with the condenser 35. The optional auxiliary refrigerant pump 96 is configured in the auxiliary refrigerant feed line 132. The auxiliary refrigerant return line 138, on an upstream end thereof, fluidly connects with the auxiliary refrigerant feed line 132 via the condenser 35. In the embodiments of Figures 5 and 6, the auxiliary refrigerant return line 138, on a downstream end thereof, ultimately connects with the end-flash separator 50.
- 33 -Employing a slip stream from the liquid hydrocarbon product stream has as advantage that the amount of additional equipment to be installed is minimal. For instance, no additional auxiliary refrigerant compressor and auxiliary refrigerant condenser would be needed, which would be the case if a separate independent auxiliary refrigerant cycle would be proposed.
The refrigerant in the liquefier 100 is cycled in the refrigerant circuit 101, whereby spent refrigerant 150 is compressed in the refrigerant compressor 160 to form a compressed refrigerant 120 out of the spent refrigerant 150. Heat is removed from the compressed refrigerant discharged from the refrigerant compressor 160, via the one or more heat exchangers that are provided in the compressed refrigerant line 120 including the least one reject heat exchanger 124. This results in a partially condensed compressed refrigerant, which is phase separated in the MR separator 128 into a light refrigerant fraction 121 consisting of the vaporous constituents of the partially condensed compressed refrigerant, and a heavy refrigerant fraction 122 consisting of the liquid constituents of the partially condensed compressed refrigerant.
The light refrigerant fraction 121 is passed via successively the lower LMR bundle 183 and the upper LMR
bundle 184 through the cryogenic heat exchanger 180, while the heavy refrigerant fraction 122 is passed via the HMR bundle 185 through the cryogenic heat exchanger 180 to the transition point. While passing through these respective tube bundles, the respective light- and heavy refrigerant fractions are cooled against the light and heavy refrigerant fractions that are evaporating in the shell side 186 again producing spent refrigerant 150
- 34 -which completes the cycle. Simultaneously, the hydrocarbon stream 110 passes through the cryogenic heat exchanger 180 via successively the lower hydrocarbon bundle 181 and the upper hydrocarbon bundle 182 and is being liquefied evaporating heavy refrigerant fraction and sub-cooled against the evaporating light refrigerant fraction.
Preferably, the vaporizer 285 is arranged in the compressed vapour line 70 between the end-flash compressor 260 and upstream of the gas/liquid separator 33, such that the heating fluid 286 can be provided in the form of the compressed vapour stream 70. Examples of such embodiments, wherein the condensed fraction 40 is indirectly heat exchanged against at least a part of the compressed vapour 70 stream whereby fully vaporizing the condensed fraction 40, are shown in Figures 3 and 4. As a result, the cooling duty required from the auxiliary refrigerant 132 in condenser 35 to generate the same degree of condensation is smaller.
Also illustrated in Figs. 3 and 4 is that the end flash compressor 260 and the optional fuel gas compressor 360 may share a single compressor driver 290. These compressors may be embodied as two separate compressor casings on a common drive shaft, or they may actually be two compressor stages within a single casing.
Figure 4 shows a special group of embodiments wherein a stream splitter 75 is provided in the compressed vapour stream line 70, whereby the compressed vapour stream line 70 is divided over a first branch 71 and a second branch 72. The first branch 71 is arranged to convey a first compressed vapour part stream to the gas/liquid separator 33, and the second branch 72 is arranged to convey a second compressed vapour part stream to the same
- 35 -gas/liquid separator 33. The stream splitter 70 merely divides the incoming compressed vapour stream 70 into two part streams of equal composition and phase. The stream splitter 70 may be a pipe junction in the form a simple T-junction, preferably in conjunction with a split ratio control valve 76 in one of the first and second branches.
The condenser 35 is arranged in the first branch 71.
Advantageously, the optional cold recovery heat exchanger 85 is also arranged in the first branch 71 such that the first compressed vapour part stream is employed as the cold recovery fluid 86 generally referenced in Figure 1.
The revaporizer 285 may be arranged in the second branch 72, such that the second compressed vapour part stream is employed as the heating fluid 286 generally referenced in Figure 1.
In the embodiment illustrated in Fig. 4, the compressed vapour stream 70 is split into a first compressed vapour part stream and a second compressed vapour part stream. The first compressed vapour part stream is discharged from the stream splitter 75 into and conveyed to the gas/liquid separator 33 in the first branch 71, while the second compressed vapour part stream is discharged from the stream splitter 75 into and conveyed to the gas/liquid separator 33 in the second branch 72. The first compressed vapour part stream and second compressed vapour part stream both have the same composition and phase as the compressed vapour 70. The part of the compressed vapour stream 70 from which heat is passed to the auxiliary refrigerant stream 132 is formed by the first compressed vapour part stream.
However, prior to passing the heat from the first compressed vapour part stream to the auxiliary refrigerant stream 132, the first compressed vapour part
- 36 -stream is indirectly heat exchanged against the vapour fraction 80 from the gas/liquid separator 33 in the cold recovery heat exchanger 85. Downstream of this heat exchanging, the vapour fraction 80 is warmer than between the gas/liquid separator 33 and the cold recovery heat exchanger 85. It may then be combusted in the combustion device 220 as explained hereinbefore.
Still referring to Figure 4, the part of the compressed vapour stream that is indirectly heat exchanged in the revaporizer 285 against the condensed fraction 40 is formed by the second compressed vapour part stream. This way, the partially condensed intermediate stream that is fed into the gas/liquid separator 33 is formed by the combination of the first and second compressed part streams.
The splitting of the compressed vapour is preferably performed with an adjustable split ratio. The split ratio corresponds to the quotient of mass flow rates in the second branch 71 and the compressed vapour line 70.
The split ratio may be adjusted in response to a temperature signal representative of the temperature of the vapour fraction 80 from the gas/liquid separator 33 being discharged from the cold recovery heat exchanger 85 before being combusted. This temperature is preferably maintained at a pre-determined target value by adjusting the split ratio, and this way it will be possible to achieve a certain degree of cold recovery from the vapour fraction 80 regardless of variations in the flow rate of the vapour fraction 80. The flow rate of first compressed vapour part stream, which functions as the cold recovery fluid, is effectively assimilated to the available flow rate of the vapour fraction 80. To this end, a second temperature sensor 77 may be provided in
- 37 -the vapour fraction line 80 between the cold recovery heat exchanger 85 and the combustion device 220, which is electronically coupled to the split ratio control valve 76 such that the valve setting of the split ratio control valve 76 is controlled using the signal representative of the temperature generated in the second temperature sensor 77. A second target temperature setting for this control loop may be set at a few degrees below, e.g. 2 C
below, the temperature of the cold recovery fluid 86 at the inlet of the cold recovery heat exchanger 85. If the temperature of the vapour fraction 80 at the outlet of the cold recovery heat exchanger 85 is still lower than the second target temperature, the split ratio may be adjusted to be increased (for instance by reducing the flow opening in the split ratio control valve 76).
Additional control strategies known to the person skilled in the art may be implemented to avoid pinching of the cold recovery heat exchanger 85 when it is regulated on the outlet temperature.
Preferably, the temperature of the vapour fraction 80 at the outlet of the cold recovery heat exchanger 85 is between ambient temperature and at most 10 C below ambient temperature, to obtain the most cold recovery out of the vapour fraction 80.
Material and heat balance calculations have been performed using Pro2 simulation software, to demonstrate the feasibility of the proposed methods and apparatuses.
Tables 1 to 4 show results for embodiments based on Figure 7. The embodiment of Figure 7 implements an extended end-flash separator system as previously been described in US Pat. 6,014,869, the contents of which are herein incorporated by reference. The end-flash separator 50 in this case comprises a gas/liquid
- 38 -contacting device (for instance in the form of packing or a collection of contacting trays), a lower inlet device 52 connected to a reboiler 55 and an upper inlet device 53. The cold recovery stream 66 consists of a side stream of natural gas with the same composition as the raw liquefied stream 1. It should be noted that the majority of the auxiliary refrigerant feed line 132 and the auxiliary refrigerant return line 138 have not been drawn for clarity reasons. Only the end parts near the condenser 35 and near the end-flash separator 50 have been indicated, and it should be understood that the ends parts of the auxiliary refrigerant feed line 132 are mutually connected, as well as the end parts of the auxiliary refrigerant feed line 138.
The cryogenic hydrocarbon composition 8 is assumed to consist of the cold recovery stream 66 and the raw liquefied stream 1 coming from the pressure reduction system 5. The remainder of Figure 7 corresponds to Figure 4.
For the calculations it is further assumed that the reject vapour stream 64 between the cold recovery heat exchanger 65 and the end flash compressor 260 contains the vapour from the end-flash separator 50 together with a boil-off gas stream from the cryogenic storage tank 210. It is further assumed that vapour bypass control valve 77, vapour recycle control valve 88, recycle valve 14, and external stripping vapour flow control valve 73 are closed and in no-flow condition.
Tables 1 and 2 correspond to one calculation wherein the second separation pressure falls in the range of from 4 to 8 bara. This is referred to as the low pressure case. Tables 3 and 4 correspond to another calculation, which is referred to as the high pressure case. In the Table 1 (low pressure case) Stream Nr. 1 la lb 8 60 66 90 138 Pressure 74.8 74.2 9.88 1.05 1.05 9.88 1.12 1.50 (bara) Temperature (0c) Flow rate 211 211 211 218 19.4 6.65 198 0.30 (kg/s) Nitrogen 3.93 3.93 3.93 3.93 41.4 3.93 1.09 1.09 (mol.%) Methane 95.7 95.7 95.7 95.7 58.6 95.7 98.5 98.5 (mol.%) c2+
0.39 0.39 0.39 0.39 0.00 0.39 0.42 0.42 (mol.%) Table 2 (low pressure case, continued) Stream Nr. 8 40 64 70 71 71a 71b 72 80 90 Pressure 1.05 6.50 0.85 7.50 7.50 7.00 6.50 7.00 6.50 1.12 2.00 1.50 1.00 3.00 (bara) Temperature -167 -162 -27 +31 +31 -131 -142 -169 -162 -163 (0c) Flow rate 218 19.6 23.7 23.7 3.48 3.48 3.48 20.3 4.12 198 0.30 0.30 3.20 19.6 (kg/s) Nitrogen 3.93 29.0 36.8 36.8 36.8 36.8 36.8 36.8 86.4 1.09 1.09 1.09 17.3 29.0 (mol.%) Methane 95.7 71.0 63.2 63.2 63.2 63.2 63.2 63.2 13.6 98.5 98.5 98.5 82.7 71.0 (mol.%) c2+
0.39 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.42 0.42 0.42 0.00 0.00 (mol.%) =
=

Table 3 (high pressure case) Stream Nr. 1 la lb 8 60 66 90 138 Pressure 74.8 74.2 9.88 1.05 1.05 9.73 1.12 1.50 (bara) Temperature (0c) Flow rate 210 210 210 217 19.4 6.58 198 0.49 (kg/s) Nitrogen 3.93 3.93 3.93 3.93 41.6 3.93 1.11 1.11 (mol.%) Methane 95.7 95.7 95.7 95.7 58.4 95.7 98.5 98.5 (mol.%) c2+
0.39 0.39 0.39 0.39 0.00 0.39 0.42 0.42 (mol.%) =
=

Table 4 (high pressure case, continued) Stream Nr. 8 40 64 70 71 71a 71b 72 80 90 Pressure 1.05 16.5 0.85 17.5 17.5 17.0 16.5 17.0 16.5 1.12 2.00 1.50 1.00 9.50 (bara) Temperature -167 -135 -27 +31 +31 -125 -127 -140 -135 -163 (0c) Flow rate 217 12.5 23.5 23.5 8.18 8.18 8.18 15.3 10.9 198 0.49 0.49 4.32 12.5 (kg/s) Nitrogen 3.93 19.3 36.9 36.9 36.9 36.9 36.9 36.9 62.9 1.09 1.11 1.11 18.9 19.3 (mol.%) Methane 95.7 80.7 63.1 63.1 63.1 63.1 63.1 63.1 37.1 98.5 98.5 98.5 81.1 80.7 (mol.%) c2+
0.39 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.42 0.42 0.42 0.00 0.00 (mol.%) =
=

other calculation the second separation pressure falls in the range of from 10 to 20 bara. This affects the pressure drop that is available over the pressure reduction valve 245, which in turn affects the cooling duty that is available in revaporizer 285. This, of course, at the cost of extra compression power. It can be seen that the temperature under which the phase separation in the gas/liquid separator 33 is performed can be higher in this pressure range. Notwithstanding, a larger amount of the liquid hydrocarbon stream has to be used as the auxiliary refrigerant stream.
The low pressure case as calculated in the present example yields a low quality fuel gas that is discharged from the cold recovery heat exchanger 85 at a pressure of 5.00 bara and a temperature of 22 C; and a revaporized condensed fraction that is discharged from the revaporizer 285 at a pressure of 3.00 bara and a temperature of 25 C. The latter may be utilized as high quality fuel gas.
The high pressure case as calculated in the present example yields a low quality fuel gas that is discharged from the cold recovery heat exchanger 85 at a pressure of 5.00 bara and a temperature of 28 C; and a revaporized condensed fraction that is discharged from the revaporizer 285 at a pressure of 9.00 bara and a temperature of 19 C. The latter may be utilized as high quality fuel gas.
In both the low pressure case and the high pressure case, the ultimate composition of the liquefied hydrocarbon inventory as stored in the cryogenic storage tank 210 is 0.83 mol.% nitrogen; 98.74 mol.% methane and 0.43 mol.% C2+, whereby C2+ indicates all hydrocarbons having a mass corresponding to that of ethane, and upward. The liquefied hydrocarbon stream being passed through the main product line 91 to the cryogenic storage tank 210 has slightly more nitrogen than the liquefied hydrocarbon inventory as stored in the cryogenic storage tank 210.
In any of the examples above, a preferred range of liquefaction pressure, at which raw liquefied stream is discharged in the rundown line 1 from the liquefier 100, is from 15 bara to 120 bara, more preferably from 15 bara to 90 bara or from 45 bara to 120 bara. The most preferably range for the liquefaction pressure is from 45 bara to 90 bara. In case that the raw liquefied stream consists for at least 80 mol% of methane and nitrogen, a preferred temperature range for the raw liquefied stream in the rundown line 1 may be from -165 C to -120 C.
In any of the examples above, the vapour fraction 80 is envisaged to contain in the range of from 30 mol% to 90 mol% of nitrogen, preferably in the range of from 30 mol% to 80 mol% of nitrogen or in the range of from 45 mol% to 90 mol% of nitrogen, preferably in the range of from 45 mol% to 80 mol% of nitrogen, most preferably from 50 mol% to 80 mol% of nitrogen. To achieve a content of nitrogen of between 50 mol% and 80 mol%, such as about 60 mol%, sufficient methane must be recondensed from the compressed vapour stream 70. This may for instance be done using a pressure of the compressed vapour stream 70 of between 4 and 8 bara, and achieving a temperature of the partially condensed intermediate stream of in the range of from -150 C to -135 C. The temperature range may have higher end points if the pressure is higher than 8 bara.
Furthermore, the condensed fraction 40 generally contains up to 30 mol% of nitrogen, and not less than mol%, preferably not less than 10 mol%. Striving for lower values would cost more auxiliary cooling duty whereas that is not needed for typical gas turbines and particularly not for aero derivative gas turbines.
5 Compressors forming part of the hydrocarbon liquefaction process in the liquefaction system 100, particularly any refrigerant compressor including refrigerant compressor 160, may be driven by any type of suitable compressor driver 190, including any selected from the group consisting of gas turbine; steam turbine;
and electric motor; and inter combinations thereof. This generally applies also to refrigerant compressor driver 190.
The gas turbine may be selected from the group of so-called industrial gas turbines, or the group of so-called aeroderivative gas turbines. The group of aeroderivative gas turbines includes: Rolls Royce Trent 60, RB211, or 6761, and General Electric LMS100TM, LM6000, LM5000 and LM2500, and variants of any of these (e.g. LM2500+).
Suitably, the gas turbine 320 in which the condensed fraction 40 is ultimately combusted is the refrigerant compressor driver 190 that is in driving engagement with the refrigerant compressor 160. The gas turbine 320 may drive the refrigerant compressor 160.
Typically, the second fuel gas pressure is selected in a range between 15 and 75 bara, more preferably in a range of between 45 and 75 bara. The usual prescribed fuel gas pressure for most conventional types of industrial gas turbines is between around 15 and around 25 bara, on average. However, the latest generation of industrial gas turbine requires relatively high pressure fuel gas, such as in the range of from 35 to 45 bara.
The range of between 45 and 75 bara is recommended to meet fuel gas pressure requirements of typical aeroderivative gas turbines.
The nitrogen content of the liquid stream 90 typically does not exceed a desired maximum of about 1.1 mol%. In some embodiments, the amount of nitrogen in the liquid hydrocarbon stream 90 is between 0.5 and 1 mol%, preferably as close to 1.0 mol% as possible yet not exceeding said maximum of about 1.1 mol%.
It is a known phenomenon that boil-off gas results from thermal evaporation caused by heat added to the liquefied product, for instance in the form of heat leakage into storage tanks, LNG piping, and heat input from plant LNG pumps. In any of the examples and embodiments illustrated herein, boil-off gas may optionally be injected into the vapour reject line 64, either upstream or downstream of the end-flash compressor 260 to be subject to the phase separation in the gas/liquid separator 33. This may suitably comprise collecting boil-off gas from the cryogenic storage tank 210, possibly via a boil-off gas supply line 230 as is illustrated for example in Figure 5. Boil-off gas results from adding heat to at least part of the liquefied hydrocarbons, whereby a part of the methane-containing liquid phase in the liquefied hydrocarbons evaporates to form said boil-off gas.
The person skilled in the art will understand that the present invention can be carried out in many various ways without departing from the scope of the appended claims.

Claims (17)

1. Method of producing a liquefied hydrocarbon stream, comprising:
- providing a cryogenic hydrocarbon composition comprising a nitrogen- and methane-containing liquid phase at an initial pressure of between 1 and 2 bar absolute;
- phase separating the cryogenic hydrocarbon composition, in an end flash separator at a first separation pressure of between 1 and 2 bar absolute, into a vaporous reject stream and a liquid stream;
discharging the liquid stream from the end flash separator in the form of the liquefied hydrocarbon stream;
compressing the vaporous reject stream in an end-flash compressor to a pressure of above 2 bar absolute, thereby obtaining a compressed vapour stream;
forming a partially condensed intermediate stream from the compressed vapour, said partially condensed intermediate stream comprising a condensed fraction and a vapour fraction, by partially condensing the compressed vapour stream, wherein said partially condensing comprises indirectly heat exchanging the compressed vapour stream against an auxiliary refrigerant stream formed by a slip stream of the liquefied hydrocarbon stream whereby passing heat from at least part of the compressed vapour stream to the auxiliary refrigerant stream;
separating the condensed fraction from the vapour fraction in a gas/liquid separator, at a second separation pressure;

discharging the vapour fraction from the gas/liquid separator, said vapour fraction having a first heating value;
combusting the vapour fraction in a combustion device other than a gas turbine;
discharging the condensed fraction from the gas/liquid separator;
revaporizing the condensed fraction whereby transforming the condensed fraction into a fully vaporized stream having a second heating value that is higher than the first heating value;
combusting the fully vaporized stream in a gas turbine.
2. The method of claim 1, further comprising passing the auxiliary refrigerant stream containing said heat from the at least part of the compressed vapour stream to and into the end-flash separator.
3. The method of any one of the preceding claims, wherein said revaporizing of the condensed fraction comprises passing the condensed fraction through a pressure reduction valve and subsequently indirectly heat exchanging the condensed fraction against at least part of the compressed vapour stream whereby fully vaporizing the condensed fraction.
4. The method of claim 3, further comprising:
splitting the compressed vapour into a first compressed vapour part stream and a second compressed vapour part stream whereby first compressed vapour part stream and second compressed vapour part stream both have the same composition and phase as the compressed vapour;
wherein the part of the compressed vapour stream from which heat is passed to the auxiliary refrigerant stream is formed by said first compressed vapour part stream, and wherein said part of the compressed vapour stream that is indirectly heat exchanged against the condensed fraction is formed by the second compressed vapour part stream.
5. The method of claim 4, wherein said forming of the partially condensed intermediate stream from the compressed vapour further comprises indirectly heat exchanging of the first compressed vapour part stream against the vapour fraction from the gas/liquid separator prior to said combusting of the vapour fraction in said combustion device.
6. The method of claim 5, wherein said splitting of the compressed vapour is performed with an adjustable split ratio, said method further comprising adjusting the split ratio in response to a temperature signal representative of the temperature of the vapour fraction from the gas/liquid separator being discharged from said indirectly heat exchanging against the first compressed vapour part stream whereby maintaining said temperature of the vapour fraction at a pre-determined target value.
7. The method of any one of the preceding claims, wherein after revaporizing the condensed fraction and before combusting the fully vaporized stream, the fully vaporized stream is compressed in a fuel gas compressor to a second fuel gas pressure of higher than the second separation pressure, and between 15 and 75 bara, preferably to a second fuel gas pressure of between 45 and 75 bara.
8. The method of any one of the preceding claims, wherein the vapour fraction is combusted in said combustion device at a first fuel gas pressure not higher than the second separation pressure, preferably wherein the vapour fraction is combusted in said combustion device at a pressure of between 2 and 15 bara.
9. The method of any one of the preceding claims, wherein the second separation pressure is between 2 and 22 bara, preferably between 5 and 22 bara, more preferably between 5 and 15 bara.
10. The method of any one of the preceding claims, wherein the liquid stream and the liquefied hydrocarbon stream contain less than 1.1 mol% of nitrogen.
11. The method of any one of the preceding claims, wherein more than 30 mol% of the partially condensed intermediate stream consists of nitrogen, and less than 30 mol% of the condensed fraction being discharged from the gas/liquid separator consists of nitrogen.
12. The method of any one of the preceding claims, wherein the vapour fraction and the condensed fraction co-exist and are separated in one thermodynamic equilibrium state between said vapour fraction and the condensed fraction.
13. An apparatus for producing a liquefied hydrocarbon stream, comprising:
a cryogenic feed line connected to a source of a cryogenic hydrocarbon composition comprising nitrogen and a methane-containing liquid phase;
an end-flash separator arranged to receive the cryogenic hydrocarbon composition and to separate the cryogenic hydrocarbon composition into a liquid stream and vaporous reject stream;
a liquid hydrocarbon product line fluidly connected to a bottom part of the end-flash separator to discharge said liquid stream in the form of the liquefied hydrocarbon stream from the end-flash separator;

a vapour reject line fluidly connected to an overhead part of the end-flash separator to discharge said vaporous reject stream from the end-flash separator;
an end-flash compressor arranged in the vapour reject line to compress the vaporous reject stream, thereby obtaining a compressed vapour stream;
a condenser arranged in the vapour reject line downstream of the end-flash compressor, arranged to receive the compressed vapour stream and to form a partially condensed intermediate stream from the compressed vapour stream, said partially condensed intermediate stream comprising a condensed fraction and a vapour fraction, which condenser is arranged to establish indirect heat exchanging contact between at least part of the compressed vapour stream and an auxiliary refrigerant stream;
an auxiliary refrigerant feed line extending between the liquid hydrocarbon product line and the condenser, to supply a slip stream of the liquid hydrocarbon product stream to the condenser;
a gas/liquid separator arranged downstream of the condenser and arranged to receive the condensed fraction and vapour fraction;
a vapour fraction discharge line fluidly connected with an overhead part of the gas/liquid separator arranged to receive the vapour fraction from the gas/liquid separator;
a combustion device other than a gas turbine fluidly connected with the gas/liquid separator by means of the vapour fraction discharge line to receive and combust the discharged vapour fraction;
a condensed fraction discharge line fluidly connected with a bottom part of the gas/liquid separator arranged to receive the condensed fraction from the gas/liquid separator;
a gas turbine fluidly connected with the gas/liquid separator by means of the condensed fraction discharge line to receive and combust the discharged condensed fraction;
a revaporizer arranged in the condensed fraction discharge line between the gas/liquid separator and the gas turbine and arranged to transform the condensed fraction into a fully vaporized stream prior to combustion in the gas turbine.
14. The apparatus of claim 13, further comprising:
an auxiliary refrigerant return line extending between the condenser and the end-flash separator and arranged to return the auxiliary refrigerant containing heat from the compressed vapour stream to the end-flash separator.
15. The apparatus of any one of claims 13 to 14, wherein the gas/liquid separator consists of a drum free from internals forming a vapour/liquid contacting section.
16. The apparatus of any one of claims 13 to 15, wherein the vapour reject line downstream of the end flash compressor is a compressed vapour stream line, said apparatus further comprising:
- a pressure reduction valve arranged in the condensed fraction discharge line between the gas/liquid separator and the revaporizer, - a stream splitter configured in the compressed vapour line, for dividing the compressed vapour line over a first branch and a second branch, whereby the first branch is arranged between the stream splitter and the gas/liquid separator, and whereby the second branch is arranged between the stream splitter and the gas/liquid separator, whereby the condenser is configured in the first branch and wherein the revaporizer is configured in the second branch.
17. The apparatus of claim 16, further comprising a cold recovery heat exchanger configured in the vapour fraction discharge line upstream of the combustion device, which cold recovery heat exchanger is arranged in the first branch in addition to the condenser.
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