EP2205920A2 - Method for liquefying natural gas with high pressure fractioning - Google Patents

Method for liquefying natural gas with high pressure fractioning

Info

Publication number
EP2205920A2
EP2205920A2 EP08870150A EP08870150A EP2205920A2 EP 2205920 A2 EP2205920 A2 EP 2205920A2 EP 08870150 A EP08870150 A EP 08870150A EP 08870150 A EP08870150 A EP 08870150A EP 2205920 A2 EP2205920 A2 EP 2205920A2
Authority
EP
European Patent Office
Prior art keywords
liquid
column
ethane
methane
natural gas
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
EP08870150A
Other languages
German (de)
French (fr)
Other versions
EP2205920B1 (en
Inventor
Béatrice Fischer
Gilles Ferschneider
Anne-Claire Lucquin
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.)
LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Original Assignee
IFP Energies Nouvelles IFPEN
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 IFP Energies Nouvelles IFPEN filed Critical IFP Energies Nouvelles IFPEN
Publication of EP2205920A2 publication Critical patent/EP2205920A2/en
Application granted granted Critical
Publication of EP2205920B1 publication Critical patent/EP2205920B1/en
Active legal-status Critical Current
Anticipated expiration legal-status Critical

Links

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
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/0228—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream
    • F25J3/0238—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream separation of CnHm with 2 carbon atoms 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
    • 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"
    • F25J1/0045—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
    • F25J1/006—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the refrigerant fluid used
    • F25J1/008—Hydrocarbons
    • F25J1/0085—Ethane; Ethylene
    • 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
    • F25J1/0211—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
    • 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
    • F25J1/0231—Integration with a unit for using hydrocarbons, e.g. consuming hydrocarbons as feed stock for the working-up of the hydrocarbon feed, e.g. reinjection of heavier hydrocarbons into the liquefied 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
    • F25J1/02—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
    • F25J1/0235—Heat exchange integration
    • F25J1/0237—Heat exchange integration integrating refrigeration provided for liquefaction and purification/treatment of the gas to be liquefied, e.g. heavy hydrocarbon removal from 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
    • F25J1/02—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
    • F25J1/0235—Heat exchange integration
    • F25J1/0237—Heat exchange integration integrating refrigeration provided for liquefaction and purification/treatment of the gas to be liquefied, e.g. heavy hydrocarbon removal from natural gas
    • F25J1/0238—Purification or treatment step is integrated within one refrigeration cycle only, i.e. the same or single refrigeration cycle provides feed gas cooling (if present) and overhead gas cooling
    • 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
    • F25J1/0245—Different modes, i.e. 'runs', of operation; Process control
    • F25J1/0249—Controlling refrigerant inventory, i.e. composition or quantity
    • F25J1/025—Details related to the refrigerant production or treatment, e.g. make-up supply from feed gas itself
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • F25J1/0243—Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
    • F25J1/0279—Compression of refrigerant or internal recycle fluid, e.g. kind of compressor, accumulator, suction drum etc.
    • F25J1/0292—Refrigerant compression by cold or cryogenic suction of the refrigerant 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
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/0204—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the feed stream
    • F25J3/0209—Natural gas or substitute 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
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/0228—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream
    • F25J3/0233—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream 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
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/0228—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream
    • F25J3/0242—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream separation of CnHm with 3 carbon atoms 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
    • F25J2200/00—Processes or apparatus using separation by rectification
    • F25J2200/02—Processes or apparatus using separation by rectification in a single pressure main column 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
    • F25J2200/00—Processes or apparatus using separation by rectification
    • F25J2200/30—Processes or apparatus using separation by rectification using a side column in a single pressure column 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
    • F25J2200/00—Processes or apparatus using separation by rectification
    • F25J2200/50—Processes or apparatus using separation by rectification using multiple (re-)boiler-condensers at different heights of the 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
    • F25J2200/00—Processes or apparatus using separation by rectification
    • F25J2200/74—Refluxing the column with at least a part of the partially condensed overhead 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
    • F25J2215/00—Processes characterised by the type or other details of the product stream
    • F25J2215/62—Ethane or ethylene
    • 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
    • F25J2260/02—Integration in an installation for exchanging heat, e.g. for waste heat 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
    • 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
    • F25J2290/00—Other details not covered by groups F25J2200/00 - F25J2280/00
    • F25J2290/12—Particular process parameters like pressure, temperature, ratios

Definitions

  • the present invention relates to the field of liquefaction of a natural gas.
  • Natural gas is often produced in places far from where it is used.
  • One method used for transport is to liquefy natural gas around -160 0 C, then to transport it by boat in liquid form at atmospheric pressure.
  • natural gas Before being liquefied, natural gas must undergo various treatments, on the one hand to adjust its composition for sale (sulfur and carbon dioxide content, calorific value), and on the other hand to allow its liquefaction .
  • the fractionation of the natural gas produced by distillation makes it possible to eliminate the hydrocarbons that are too heavy and that risk blocking the pipes and the heat exchangers of the liquefaction plant by crystallization.
  • the fractionation by distillation makes it possible to separately recover compounds such as ethane, propane or butane which can be recovered separately, for example at the sale or as refrigerant fluids used in the liquefaction process.
  • the liquefaction is carried out at a pressure approximately equal to the operating pressure of the fractionation column.
  • the present invention proposes to modify the fractionation step to increase the operating pressure of the fractionation and, consequently, to increase the pressure at which the natural gas is liquefied in order to improve the overall efficiency of the liquefaction process.
  • the invention defines a method for liquefying a natural gas, in which the following steps are carried out: a) the natural gas is cooled, b) the cooled natural gas is introduced into a fractionation column so as to separate a gas phase rich in methane and a liquid phase rich in compounds heavier than ethane, c) withdrawing said liquid phase at the bottom of the fractionation column and said gas phase is discharged at the top of the separation column; d) said gas phase is partially liquefied so as to produce a condensate and a gaseous flow, said condensate being recycled at the top of the separation column; fractionation column as reflux, e) liquefies said gas stream, and wherein the operating conditions of the fractionation column are chosen so that said liquid phase comprises a molar amount of methane of between 10% and 150% the molar amount of ethane of said phase.
  • the operating conditions of the fractionation column can be chosen so that said liquid phase comprises a molar amount of methane of between 40% and 70% of the molar amount of ethane.
  • the molar quantity of methane of said liquid phase can be adjusted by modifying the power of a reboiler located at the bottom of the fractionation column.
  • step g) the portion of liquid can be withdrawn at a level located between the feed point and the head of the separation column.
  • step h) it is possible to vaporize a portion of said liquid portion so as to obtain said liquid stream comprising more than 95 mol% of ethane, said vaporized portion being introduced into the separation column.
  • Liquid reflux can be introduced at the top of the separation column at a temperature of between -10 ° C. and -40 ° C.
  • step a) the natural gas can be cooled by heat exchange with a refrigerant circulating in a refrigeration circuit and the methane-rich gaseous fraction obtained in step f) can be partially condensed by heat exchange with a portion of said refrigerant fluid, so as to obtain said liquid reflux introduced at the top of the separation column.
  • the refrigerant portion may be subcooled by heat exchange with a liquid withdrawn from the fractionation column.
  • step e) the gas flow can be cooled by heat exchange at a pressure greater than 50 bar.
  • FIG. 1 schematizes a method according to the prior art
  • FIGS. 2 and 3 schematize two methods according to the invention.
  • the natural gas to be liquefied arrives through the conduit. Natural gas may have been previously purified to remove acid compounds, water and possibly mercury.
  • the natural gas flowing in the duct 1 is cooled in the heat exchanger E1 at a temperature of between 0 ° C. and -60 ° C.
  • the refrigeration is carried out by means of the refrigeration closed circuit 100 which operates by compression and expansion of a refrigerant fluid, for example composed of a mixture of ethane and propane.
  • the partially liquefied natural gas in E1 is introduced via line 1 into fractionation column 2, reboiled using heat exchanger 9.
  • the vapor discharged at the top of column 2 through line 3 is partially condensed in the heat exchanger E1, before being introduced into the reflux tank 4.
  • the gaseous fraction discharged at the top of the flask 4 is sent through line 5 into the heat exchanger E2 to be liquefied.
  • the liquid natural gas is removed from E2 by the pipe 5 '.
  • the refrigeration is carried out by means of the refrigeration closed circuit 200 which operates by compression and expansion of a cooling fluid, for example composed of a mixture of nitrogen, methane and ethane.
  • the liquid obtained at the bottom of the flask 4 is introduced by the pump 6 and the duct 7 at the top of the column 2 as reflux.
  • the liquid obtained at the bottom of column 2 is discharged through line 8.
  • the liquid obtained at the bottom of the column 2 through the conduit 8 is cooled in the exchanger 10, for example by water or air, and then expanded in the expansion element V.
  • the cooled and expanded liquid is introduced into the deethanizer column 1 1, reboiled by the heat exchanger 16.
  • the column 11 operates at a pressure between 20 and 35 bar.
  • the gaseous fraction obtained at the top of the column 11 is partially condensed at a temperature of between 0 ° C. and 10 ° C. in the heat exchanger 12, by heat exchange with a portion of liquid withdrawn laterally from the column 2.
  • the condensates are separated from the gas phase in the flask 13.
  • the gaseous phase discharged at the top of the flask 13 consists mainly of methane and ethane. It can be sent to the fuel gas network or liquefaction in the pipe 5.
  • the condensate recovered at the bottom of the separation tank 13 are sent, at a temperature preferably between 0 0 C and 1 0 0 C, by the pump 14 at the top of column 11 as reflux.
  • a fraction of the condensates, which consist mainly of ethane is withdrawn through line 30 to be used, for example, in the composition of refrigerant fluids circulating in circuits 100 or 200. Hydrocarbons heavier than methane are discharged in liquid form. at the bottom of the column 11 by the conduit 17.
  • Figures 2 and 3 which schematize two implementations of the invention, take the same elements of Figure 1 by applying different operating conditions. The references of Figures 2 and 3 identical to those of Figure 1 designate the same elements.
  • the operating conditions of column 2 are chosen so that the methane content of the stream discharged through line 8 is between 10% and 150 mol%, preferably between 40% and 70% molar, of the ethane content of this stream.
  • the temperature or operating pressure of column 2 can be changed.
  • column 2 operates at a pressure of between 40 and 60 bar.
  • the pressure of the column 2 can be adjusted by means of a valve arranged upstream of the column 2, for example on the conduit 1 or the.
  • the operating temperature of the column 2 can be adjusted by modifying the power of the reboiling, that is to say that increases or decreases the amount of heat that the reboiler 9 brings to the bottom of the column 2.
  • the adjustment of the power of the reboiler 9, the flow of gas discharged through line 3 and the flow of liquid discharged through line 8 are modified.
  • the power of the reboiler 9 is reduced so as to increase the amount of methane contained in the liquid at the bottom of the column 2 and, consequently, the liquid flow 8 increases.
  • the separation column 11 may be a distillation column equipped with trays .
  • the heat exchanger 12 can perform cooling at a low temperature, preferably between -10 ° C. and -40 ° C.
  • the condensates recovered at the bottom of the separation tank 13 are sent, at a temperature preferably between -10 ° C. and -40 ° C., by the pump 14 at the top of the column 11 as reflux.
  • a portion of the refrigerant fluid of the first refrigeration circuit 100 is withdrawn through line 101, which is relaxed in the valve V1 before exchanging heat in 12 with the effluent discharged at the top of the column 11.
  • a portion of the refrigerant fluid is taken from the first refrigeration circuit 100 via the pipe 101
  • This fluid is cooled by heat exchange in 9 'with a liquid portion withdrawn laterally from the column 2.
  • the liquid portion is withdrawn between the feed point through the conduit 1 of the column 2 and the bottom of the column 2.
  • the cooling fluid can be cooled to a temperature of between -10 ° C. and 20 ° C.
  • the cooled refrigerant mixture is expanded in the organ e V1 so as to be partially vaporized at a temperature between -10 ° C and -40 0 C.
  • the partially vaporized fluid is introduced into the exchanger 12 to cool and partially liquefy the gaseous fraction discharged at the top of the column 11.
  • the refrigerant from the exchanger 12 is returned through the conduit 103 to one of the compressor balloons of the first refrigerant circuit.
  • the power of the exchanger 19 can be adjusted so as to maintain the liquid at the bottom of the column 20 at a temperature of between 10 ° C. and 20 ° C.
  • the vaporized fraction is discharged at the top of the column 20 to be reintroduced into the column 11.
  • the column 20 is operated so as to obtain a liquid cut comprising more than 92 mol% of ethane, preferably more than 95% molar of ethane.
  • the liquid rich in ethane can be used to form the refrigerant mixtures used in the circuits 100 and 200.
  • a hydrocarbon-enriched liquid heavier than ethane which can be sent via line 17 to a depropanizer column, is discharged.
  • a fraction enriched in propane which can be used to form the refrigerant mixtures used in the circuits 100 and 200.
  • the scheme of FIG. 1 is carried out according to the prior art.
  • the pretreated and dried natural gas circulates in the duct 1 "at a rate of 35,000 kmol / h, with the following composition:
  • the gas is cooled in E1 to a temperature of -30 ° C. and then introduced into the fractionation column 2.
  • the liquefaction is thus carried out in E2 at a pressure of 40 bar.
  • a total of 162.4 MW of power is required for the two refrigerant cycle compressors.
  • Example 1 the deethanization column 11 has no side column.
  • the flow obtained at the top of the column 1 is cooled only by heat exchange with a side withdrawal of the fractionation column 2, and therefore does not increase the cooling capacity necessary for the operation of the process.
  • the gas to be treated has a composition and a flow rate identical to that of Example 1.
  • the gas is cooled in E1 to a temperature of -30 ° C. and then introduced into the fractionation column 2.
  • the liquefaction is thus carried out in E2 at a pressure of 53 bars.
  • a power of 148.3 MW is required in total for the compressors of the two refrigerant cycles, a gain of about 9% compared with Example 1.
  • the counterpart of this efficiency gain lies in the difficulty in recovering a stream enriched with ethane, which is necessary to make the additional heat-transfer fluid of the refrigeration circuits 100 and 200.
  • a simple distillation in the separation column 11 makes it possible to obtain at the top a mixture of C1 and C2 that can be used in the second refrigeration cycle 200, but not in the first cycle 100 which uses a mixture of C2 and C3.
  • the invention proposes, in Example 2, to implement the lateral stripping column 20.
  • the flow at the top of the column 11 is cooled to a temperature of -20 ° C. by heat exchange with a portion of the coolant of the first refrigeration circuit 100.
  • the effluent discharged at the top of the flask 13 must be liquefied.
  • Example 2 the procedure according to the invention of Example 2 is much more attractive than the procedure of the invention.
  • example 1 it saves about 8% energy or increase the liquefaction capacity of about 8% with the same gas turbines.

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Abstract

The invention relates to a method for liquefying a natural gas that comprises the following steps: cooling a natural gas; feeding the cooled natural gas into a fractioning column so as to separate it into a methane-rich gaseous phase and a liquid phase rich in compounds heavier than ethane; and liquefying the methane-rich flow in order to obtain the liquid natural gas. According to the invention, the method comprises selecting the operational conditions of the fractioning column so that said liquid phase includes a methane molar amount representing 10% to 150% of the ethane molar amount.

Description

PROCÉDÉ DE LIQUÉFACTION D'UN GAZ NATUREL AVEC FRACTIONNEMENT À HAUTE PRESSION METHOD FOR LIQUEFACTING A NATURAL GAS WITH HIGH PRESSURE FRACTIONATION
La présente invention concerne le domaine de la liquéfaction d'un gaz naturel.The present invention relates to the field of liquefaction of a natural gas.
Le gaz naturel est souvent produit dans des endroits éloignés de son lieu d'utilisation. Une méthode utilisée pour le transport est de liquéfier le gaz naturel aux alentours de -1600C, puis de le transporter par bateau sous forme liquide à pression atmosphérique.Natural gas is often produced in places far from where it is used. One method used for transport is to liquefy natural gas around -160 0 C, then to transport it by boat in liquid form at atmospheric pressure.
Avant d'être liquéfié, le gaz naturel doit subir divers traitements, d'une part pour ajuster sa composition en vue de la vente (teneur en soufre et en dioxyde de carbone, valeur calorifique), et d'autre part pour permettre sa liquéfaction. En particulier, le fractionnement du gaz naturel réalisé par distillation permet d'éliminer les hydrocarbures trop lourds qui risquent de boucher par cristallisation les conduites et les échangeurs de chaleur de l'usine de liquéfaction. De plus, le fractionnement par distillation permet de récupérer séparément des composés tels que l'éthane, le propane ou le butane qui peuvent être valorisés séparément, par exemple à la vente ou en tant que fluides réfrigérants mis en oeuvre dans le procédé de liquéfaction.Before being liquefied, natural gas must undergo various treatments, on the one hand to adjust its composition for sale (sulfur and carbon dioxide content, calorific value), and on the other hand to allow its liquefaction . In particular, the fractionation of the natural gas produced by distillation makes it possible to eliminate the hydrocarbons that are too heavy and that risk blocking the pipes and the heat exchangers of the liquefaction plant by crystallization. In addition, the fractionation by distillation makes it possible to separately recover compounds such as ethane, propane or butane which can be recovered separately, for example at the sale or as refrigerant fluids used in the liquefaction process.
En général, la liquéfaction est effectuée à une pression environ égale à la pression de fonctionnement de la colonne de fractionnement.In general, the liquefaction is carried out at a pressure approximately equal to the operating pressure of the fractionation column.
La présente invention propose de modifier l'étape de fractionnement pour augmenter la pression d'opération du fractionnement et, en conséquence, augmenter la pression à laquelle le gaz naturel est liquéfié dans le but d'améliorer l'efficacité globale du procédé de liquéfaction.The present invention proposes to modify the fractionation step to increase the operating pressure of the fractionation and, consequently, to increase the pressure at which the natural gas is liquefied in order to improve the overall efficiency of the liquefaction process.
De manière générale, l'invention définit un procédé de liquéfaction d'un gaz naturel, dans lequel on effectue les étapes suivantes : a) on refroidit le gaz naturel, b) on introduit le gaz naturel refroidi dans une colonne de fractionnement de manière à séparer une phase gazeuse riche en méthane et une phase liquide riche en composés plus lourds que l'éthane, c) on soutire ladite phase liquide en fond de la colonne de fractionnement et on évacue ladite phase gazeuse en tête de la colonne de séparation, d) on liquéfie partiellement ladite phase gazeuse de manière à produire un condensât et un flux gazeux, ledit condensât étant recyclé en tête de la colonne de fractionnement à titre de reflux, e) on liquéfie ledit flux gazeux, et dans lequel on choisit les conditions opératoires de la colonne de fractionnement de manière à ce que ladite phase liquide comporte une quantité molaire de méthane comprise entre 10 % et 150 % de la quantité molaire d'éthane de ladite phase.In general, the invention defines a method for liquefying a natural gas, in which the following steps are carried out: a) the natural gas is cooled, b) the cooled natural gas is introduced into a fractionation column so as to separate a gas phase rich in methane and a liquid phase rich in compounds heavier than ethane, c) withdrawing said liquid phase at the bottom of the fractionation column and said gas phase is discharged at the top of the separation column; d) said gas phase is partially liquefied so as to produce a condensate and a gaseous flow, said condensate being recycled at the top of the separation column; fractionation column as reflux, e) liquefies said gas stream, and wherein the operating conditions of the fractionation column are chosen so that said liquid phase comprises a molar amount of methane of between 10% and 150% the molar amount of ethane of said phase.
Selon l'invention, on peut choisir les conditions opératoires de la colonne de fractionnement de manière à ce que ladite phase liquide comporte une quantité molaire de méthane comprise entre 40 % et 70 % de la quantité molaire d'éthane. On peut ajuster la quantité molaire de méthane de ladite phase liquide en modifiant la puissance d'un rebouilleur situé en fond de la colonne de fractionnement.According to the invention, the operating conditions of the fractionation column can be chosen so that said liquid phase comprises a molar amount of methane of between 40% and 70% of the molar amount of ethane. The molar quantity of methane of said liquid phase can be adjusted by modifying the power of a reboiler located at the bottom of the fractionation column.
Selon l'invention, on peut, en outre, effectuer les étapes suivantes : f) on introduit ladite phase liquide dans une colonne de séparation pour séparer une fraction gazeuse riche en méthane et une fraction liquide comportant des hydrocarbures plus lourds que l'éthane, g) on soutire une portion liquide de la colonne de séparation, h) on extrait, de ladite portion liquide, un flux liquide comportant plus de 95 % molaire d'éthane.According to the invention, it is also possible to carry out the following steps: f) introducing said liquid phase into a separation column in order to separate a gaseous fraction rich in methane and a liquid fraction comprising hydrocarbons heavier than ethane, g) withdrawing a liquid portion from the separation column, h) extracting from said liquid portion, a liquid stream comprising more than 95 mol% of ethane.
A l'étape g), on peut soutirer la portion de liquide à un niveau situé entre le point d'alimentation et la tête de la colonne de séparation. A l'étape h), on peut vaporiser une partie de ladite portion liquide de manière à obtenir ledit flux liquide comportant plus de 95 % molaire d'éthane, ladite partie vaporisée étant introduite dans la colonne de séparation.In step g), the portion of liquid can be withdrawn at a level located between the feed point and the head of the separation column. In step h), it is possible to vaporize a portion of said liquid portion so as to obtain said liquid stream comprising more than 95 mol% of ethane, said vaporized portion being introduced into the separation column.
On peut introduire en tête de la colonne de séparation un reflux liquide à une température comprise entre -100C et -400C.Liquid reflux can be introduced at the top of the separation column at a temperature of between -10 ° C. and -40 ° C.
A l'étape a), on peut refroidir le gaz naturel par échange de chaleur avec un fluide réfrigérant circulant dans un circuit de réfrigération et on peut condenser partiellement ladite fraction gazeuse riche en méthane obtenue à l'étape f) par échange de chaleur avec une portion dudit fluide réfrigérant, de manière à obtenir ledit reflux liquide introduit en tête de la colonne de séparation.In step a), the natural gas can be cooled by heat exchange with a refrigerant circulating in a refrigeration circuit and the methane-rich gaseous fraction obtained in step f) can be partially condensed by heat exchange with a portion of said refrigerant fluid, so as to obtain said liquid reflux introduced at the top of the separation column.
On peut sous-refroidir la portion de fluide réfrigérant par échange de chaleur, avec un liquide soutiré de la colonne de fractionnement.The refrigerant portion may be subcooled by heat exchange with a liquid withdrawn from the fractionation column.
A l'étape e), on peut refroidir le flux gazeux par échange de chaleur à une pression supérieure à 50 bars.In step e), the gas flow can be cooled by heat exchange at a pressure greater than 50 bar.
D'autres caractéristiques et avantages de l'invention seront mieux compris et apparaîtront clairement à la lecture de la description faite ci-après en se référant aux dessins parmi lesquels :Other features and advantages of the invention will be better understood and will become clear from reading the description given below with reference to the drawings among which:
- la figure 1 schématise un procédé selon l'art antérieur, - Les figures 2 et 3 schématisent deux procédés selon l'invention.FIG. 1 schematizes a method according to the prior art; FIGS. 2 and 3 schematize two methods according to the invention.
Sur la figure 1 , le gaz naturel à liquéfier arrive par le conduit l'. Le gaz naturel peut avoir été préalablement purifié pour enlever les composés acides, l'eau et éventuellement le mercure. Le gaz naturel circulant dans le conduit l' est refroidi dans l'échangeur de chaleur E1 à une température comprise entre 0°C et -60°C. Dans E1 , la réfrigération est effectuée au moyen du circuit fermé de réfrigération 100 qui fonctionne par compression et détente d'un fluide réfrigérant, par exemple composé d'un mélange d'éthane et de propane.In Figure 1, the natural gas to be liquefied arrives through the conduit. Natural gas may have been previously purified to remove acid compounds, water and possibly mercury. The natural gas flowing in the duct 1 is cooled in the heat exchanger E1 at a temperature of between 0 ° C. and -60 ° C. In E1, the refrigeration is carried out by means of the refrigeration closed circuit 100 which operates by compression and expansion of a refrigerant fluid, for example composed of a mixture of ethane and propane.
Le gaz naturel partiellement liquéfié dans E1 est introduit par le conduit 1 dans la colonne de fractionnement 2, rebouillie à l'aide de l'échangeur de chaleur 9. La vapeur évacuée en tête de la colonne 2 par le conduit 3 est partiellement condensée dans l'échangeur de chaleur E1 , avant d'être introduite dans le ballon de reflux 4.The partially liquefied natural gas in E1 is introduced via line 1 into fractionation column 2, reboiled using heat exchanger 9. The vapor discharged at the top of column 2 through line 3 is partially condensed in the heat exchanger E1, before being introduced into the reflux tank 4.
La fraction gazeuse évacuée en tête du ballon 4 est envoyée par le conduit 5 dans l'échangeur de chaleur E2 pour être liquéfiée. Le gaz naturel liquide est évacué de E2 par le conduit 5'. Dans E2, la réfrigération est effectuée au moyen du circuit fermé de réfrigération 200 qui fonctionne par compression et détente d'un fluide réfrigérant, par exemple composé d'un mélange d'azote, de méthane et d'éthane.The gaseous fraction discharged at the top of the flask 4 is sent through line 5 into the heat exchanger E2 to be liquefied. The liquid natural gas is removed from E2 by the pipe 5 '. In E2, the refrigeration is carried out by means of the refrigeration closed circuit 200 which operates by compression and expansion of a cooling fluid, for example composed of a mixture of nitrogen, methane and ethane.
Le liquide obtenu en fond du ballon 4 est introduit par la pompe 6 et le conduit 7 en tête de la colonne 2 à titre de reflux. Le liquide obtenu en fond de la colonne 2 est évacué par le conduit 8.The liquid obtained at the bottom of the flask 4 is introduced by the pump 6 and the duct 7 at the top of the column 2 as reflux. The liquid obtained at the bottom of column 2 is discharged through line 8.
Le liquide obtenu en fond de la colonne 2 par le conduit 8 est refroidi dans l'échangeur 10, par exemple par de l'eau ou de l'air, puis détendu dans l'organe de détente V. Le liquide refroidi et détendu est introduit dans la colonne de dééthanisation 1 1 , rebouilli par l'échangeur de chaleur 16. En général, la colonne 11 fonctionne à une pression comprise entre 20 et 35 bars. La fraction gazeuse obtenue en tête de la colonne 1 1 est partiellement condensée à une température comprise entre 00C et 100C dans l'échangeur de chaleur 12, par échange de chaleur avec une portion de liquide soutiré latéralement de la colonne 2.The liquid obtained at the bottom of the column 2 through the conduit 8 is cooled in the exchanger 10, for example by water or air, and then expanded in the expansion element V. The cooled and expanded liquid is introduced into the deethanizer column 1 1, reboiled by the heat exchanger 16. In general, the column 11 operates at a pressure between 20 and 35 bar. The gaseous fraction obtained at the top of the column 11 is partially condensed at a temperature of between 0 ° C. and 10 ° C. in the heat exchanger 12, by heat exchange with a portion of liquid withdrawn laterally from the column 2.
Les condensats sont séparés de la phase gazeuse dans le ballon 13. La phase gazeuse évacuée en tête du ballon 13 est constituée principalement de méthane et d'éthane. Elle peut être envoyée au réseau de gaz combustible ou à la liquéfaction dans le conduit 5. Les condensats récupérés en fond du ballon de séparation 13 sont envoyés, à une température de préférence comprise entre 00C et 1 O0C, par la pompe 14 en tête de la colonne 11 à titre de reflux. Une fraction des condensats, qui sont principalement constitués d'éthane, est prélevée par le conduit 30 pour être utilisée par exemple dans la composition des fluides réfrigérants circulant dans les circuits 100 ou 200. Les hydrocarbures plus lourds que le méthane sont évacués sous forme liquide en fond de la colonne 11 par le conduit 17. Les figures 2 et 3, qui schématisent deux mises en oeuvre de l'invention, reprennent des mêmes éléments de la figure 1 en appliquant des conditions opératoires différentes. Les références des figures 2 et 3 identiques à celles de la figure 1 désignent les mêmes éléments.The condensates are separated from the gas phase in the flask 13. The gaseous phase discharged at the top of the flask 13 consists mainly of methane and ethane. It can be sent to the fuel gas network or liquefaction in the pipe 5. The condensate recovered at the bottom of the separation tank 13 are sent, at a temperature preferably between 0 0 C and 1 0 0 C, by the pump 14 at the top of column 11 as reflux. A fraction of the condensates, which consist mainly of ethane, is withdrawn through line 30 to be used, for example, in the composition of refrigerant fluids circulating in circuits 100 or 200. Hydrocarbons heavier than methane are discharged in liquid form. at the bottom of the column 11 by the conduit 17. Figures 2 and 3, which schematize two implementations of the invention, take the same elements of Figure 1 by applying different operating conditions. The references of Figures 2 and 3 identical to those of Figure 1 designate the same elements.
Selon l'invention, en référence aux figures 2 et 3, les conditions opératoires de la colonne 2 sont choisies de manière à ce que la teneur de méthane du flux évacué par le conduit 8 soit comprise entre 10 % et 150 % molaire, de préférence entre 40 % et 70 % molaire, de la teneur en éthane de ce flux. Par exemple, on peut modifier la température ou la pression opératoire de la colonne 2. En général, la colonne 2 fonctionne à une pression comprise entre 40 et 60 bars. La pression de la colonne 2 peut être ajustée au moyen d'une vanne disposée en amont de la colonne 2 par exemple sur le conduit 1 ou l'. La température de fonctionnement de la colonne 2 peut être ajustée en modifiant la puissance du rebouillage, c'est-à-dire qu'on augmente ou on diminue la quantité de chaleur que le rebouilleur 9 apporte en fond de la colonne 2. En conséquence de l'ajustement de la puissance du rebouilleur 9, le débit de gaz évacué par le conduit 3 et le débit de liquide évacué par le conduit 8 sont modifiés. En général, on réduit la puissance du rebouilleur 9 de manière à augmenter la quantité de méthane contenu dans le liquide en fond de la colonne 2 et, en conséquence, le débit de liquide 8 augmente.According to the invention, with reference to FIGS. 2 and 3, the operating conditions of column 2 are chosen so that the methane content of the stream discharged through line 8 is between 10% and 150 mol%, preferably between 40% and 70% molar, of the ethane content of this stream. For example, the temperature or operating pressure of column 2 can be changed. In general, column 2 operates at a pressure of between 40 and 60 bar. The pressure of the column 2 can be adjusted by means of a valve arranged upstream of the column 2, for example on the conduit 1 or the. The operating temperature of the column 2 can be adjusted by modifying the power of the reboiling, that is to say that increases or decreases the amount of heat that the reboiler 9 brings to the bottom of the column 2. Accordingly the adjustment of the power of the reboiler 9, the flow of gas discharged through line 3 and the flow of liquid discharged through line 8 are modified. In general, the power of the reboiler 9 is reduced so as to increase the amount of methane contained in the liquid at the bottom of the column 2 and, consequently, the liquid flow 8 increases.
L'envoi d'une quantité importante de méthane en fond de la colonne 2 permet d'avoir une masse spécifique vapeur plus faible à pression identique, donc un ratio de masse spécifique plus élevé. Par conséquent, le fait d'envoyer une quantité importante de méthane en fond de la colonne 2 selon l'invention permet d'opérer la liquéfaction à une pression plus élevée, ce qui diminue la puissance nécessaire pour effectuer la liquéfaction.Sending a large quantity of methane at the bottom of column 2 makes it possible to have a lower specific steam mass at identical pressure, and therefore a higher specific mass ratio. Therefore, the fact of sending a large amount of methane at the bottom of the column 2 according to the invention makes it possible to operate the liquefaction at a higher pressure, which decreases the power required to effect the liquefaction.
Selon l'invention, compte tenu que le liquide évacué en fond de la colonne 2 comporte une portion importante de méthane, on applique des conditions de fonctionnement particulières à la colonne de séparation 11. La colonne 11 peut être une colonne de distillation munie de plateaux. On peut imposer une température en tête de la colonne 11 relativement basse, de préférence comprise entre -1O0C et -400C, de manière à améliorer la séparation entre le méthane et les hydrocarbures plus lourds que l'éthane. En référence aux figures 2 et 3, l'échangeur de chaleur 12 peut effectuer un refroidissement à basse température, de préférence comprise entre -100C et -40°C. Les condensats récupérés en fond du ballon de séparation 13 sont envoyés, à une température de préférence comprise entre -10°C et -400C, par la pompe 14 en tête de la colonne 11 à titre de reflux.According to the invention, given that the liquid discharged at the bottom of the column 2 comprises a large portion of methane, particular operating conditions are applied to the separation column 11. The column 11 may be a distillation column equipped with trays . We can impose a temperature at the head of the relatively low column 11, preferably between -10 ° C. and -40 ° C., so as to improve the separation between methane and heavier hydrocarbons than ethane. With reference to FIGS. 2 and 3, the heat exchanger 12 can perform cooling at a low temperature, preferably between -10 ° C. and -40 ° C. The condensates recovered at the bottom of the separation tank 13 are sent, at a temperature preferably between -10 ° C. and -40 ° C., by the pump 14 at the top of the column 11 as reflux.
Pour effectuer le refroidissement à basse température dans l'échangeur 12, on peut utiliser une portion du fluide réfrigérant du premier circuit de réfrigération 100. En référence à la figure 2, on prélève une portion du fluide réfrigérant, par le conduit 101 , qui est détendue dans la vanne V1 avant d'échanger de la chaleur dans 12 avec l'effluent évacué en tête de la colonne 11. En référence à la figure 3, on prélève une portion du fluide réfrigérant du premier circuit de réfrigération 100 par le conduit 101. On refroidit ce fluide par échange de chaleur dans 9' avec une portion de liquide soutirée latéralement de la colonne 2. Par exemple, la portion de liquide est soutirée entre le point d'alimentation par le conduit 1 de la colonne 2 et le fond de la colonne 2. Dans l'échangeur de chaleur 9', le fluide réfrigérant peut être refroidi à une température comprise entre -100C et 2O0C. Le mélange réfrigérant refroidi est détendu dans l'organe V1 de manière à être partiellement vaporisé à une température comprise entre -10°C et -400C. Le fluide partiellement vaporisé est introduit dans l'échangeur 12 pour refroidir et partiellement liquéfier la fraction gazeuse évacuée en tête de la colonne 11. En référence aux figures 2 et 3, le fluide réfrigérant issu de l'échangeur 12 est renvoyé par le conduit 103 vers un des ballons dévésiculeurs du compresseur du premier circuit réfrigérant.To carry out the cooling at low temperature in exchanger 12, it is possible to use a portion of the refrigerant fluid of the first refrigeration circuit 100. Referring to FIG. 2, a portion of the refrigerant fluid is withdrawn through line 101, which is relaxed in the valve V1 before exchanging heat in 12 with the effluent discharged at the top of the column 11. With reference to FIG. 3, a portion of the refrigerant fluid is taken from the first refrigeration circuit 100 via the pipe 101 This fluid is cooled by heat exchange in 9 'with a liquid portion withdrawn laterally from the column 2. For example, the liquid portion is withdrawn between the feed point through the conduit 1 of the column 2 and the bottom of the column 2. In the heat exchanger 9 ', the cooling fluid can be cooled to a temperature of between -10 ° C. and 20 ° C. The cooled refrigerant mixture is expanded in the organ e V1 so as to be partially vaporized at a temperature between -10 ° C and -40 0 C. The partially vaporized fluid is introduced into the exchanger 12 to cool and partially liquefy the gaseous fraction discharged at the top of the column 11. Referring to Figures 2 and 3, the refrigerant from the exchanger 12 is returned through the conduit 103 to one of the compressor balloons of the first refrigerant circuit.
Selon l'invention, en référence aux figures 2 et 3, on peut effectuer un soutirage latéral de la colonne 11 afin d'extraire une coupe enrichie en éthane. On soutire du liquide de la colonne 11 par le conduit 18 à un niveau situé entre le point d'alimentation de la colonne 11 par le conduit 8 et le point d'introduction du reflux. Le conduit 18 effectue un soutirage au niveau d'un plateau situé de préférence au moins deux plateaux au-dessus du point d'alimentation. Le liquide soutiré est introduit par le conduit 18 dans la colonne latérale 20, dénommée "colonne de stripage". La colonne 20 fonctionne à une pression sensiblement égale à la pression de la colonne 11 , aux pertes de charge près. La colonne 20 est rebouillie à l'aide de l'échangeur de chaleur 19, de manière à vaporiser le méthane présent dans le liquide soutiré. On récupère en fond de la colonne 20 une coupe enrichie en éthane et comportant une très faible proportion de méthane et de propane. Selon l'invention, on peut ajuster la puissance de l'échangeur 19 de manière à maintenir le liquide en fond de la colonne 20 à une température comprise entre 10°C et 20°C. La fraction vaporisée est évacuée en tête de la colonne 20 pour être réintroduite dans la colonne 11. De préférence, on opère la colonne 20 de manière à obtenir une coupe liquide comportant plus de 92 % molaire d'éthane, de préférence plus de 95 % molaire d'éthane. Le liquide riche en éthane peut être utilisé pour constituer les mélanges réfrigérants mis en oeuvre dans les circuits 100 et 200.According to the invention, with reference to FIGS. 2 and 3, it is possible to carry out a lateral withdrawal of the column 11 in order to extract a cup enriched in ethane. Liquid is withdrawn from the column 11 through the conduit 18 at a level located between the feed point of the column 11 through the conduit 8 and the point of introduction of reflux. The duct 18 carries out a withdrawal at a plateau located from preferably at least two trays above the feed point. The withdrawn liquid is introduced through the conduit 18 into the side column 20, called "stripping column". The column 20 operates at a pressure substantially equal to the pressure of the column 11, with the pressure losses close. The column 20 is reboiled with the aid of the heat exchanger 19, so as to vaporize the methane present in the liquid withdrawn. At the bottom of column 20 is recovered a cup enriched in ethane and having a very low proportion of methane and propane. According to the invention, the power of the exchanger 19 can be adjusted so as to maintain the liquid at the bottom of the column 20 at a temperature of between 10 ° C. and 20 ° C. The vaporized fraction is discharged at the top of the column 20 to be reintroduced into the column 11. Preferably, the column 20 is operated so as to obtain a liquid cut comprising more than 92 mol% of ethane, preferably more than 95% molar of ethane. The liquid rich in ethane can be used to form the refrigerant mixtures used in the circuits 100 and 200.
En fond de la colonne 11 , on évacue un liquide enrichi en hydrocarbures plus lourds que l'éthane, qui peut être envoyé par le conduit 17 vers une colonne de dépropanisation. Ainsi, on peut extraire une coupe enrichie en propane qui peut être utilisée pour constituer les mélanges réfrigérants mis en oeuvre dans les circuits 100 et 200. At the bottom of the column 11, a hydrocarbon-enriched liquid heavier than ethane, which can be sent via line 17 to a depropanizer column, is discharged. Thus, it is possible to extract a fraction enriched in propane which can be used to form the refrigerant mixtures used in the circuits 100 and 200.
Les exemples numériques présentés ci-après permettent d'illustrer le fonctionnement du procédé selon l'invention.The numerical examples presented below make it possible to illustrate the operation of the method according to the invention.
Exemple 1 :Example 1
On opère le schéma de la figure 1 selon l'art antérieur. Le gaz naturel prétraité et séché circule dans le conduit 1" à un débit de 35 000 kmole/h, avec la composition suivante :The scheme of FIG. 1 is carried out according to the prior art. The pretreated and dried natural gas circulates in the duct 1 "at a rate of 35,000 kmol / h, with the following composition:
Le gaz est refroidi dans E1 à une température de -300C, puis introduit dans la colonne de fractionnement 2.The gas is cooled in E1 to a temperature of -30 ° C. and then introduced into the fractionation column 2.
Pour pouvoir distiller le gaz dans la colonne 2, il faut rester suffisamment en dessous des conditions critiques. Un critère couramment employé par l'homme du métier est que le rapport des masses spécifiques des phases liquides et vapeur au fond de la colonne 2 doit rester supérieur à une certaine valeur pour pouvoir opérer. Des valeurs entre 3 et 6 sont utilisées par l'homme du métier. Nous utilisons dans cet exemple 1 une valeur de 4,5.To be able to distill the gas in column 2, it is necessary to remain sufficiently below the critical conditions. One criterion commonly used by those skilled in the art is that the ratio of specific masses of the liquid and vapor phases at the bottom of column 2 must remain above a certain value in order to operate. Values between 3 and 6 are used by those skilled in the art. We use in this example 1 a value of 4.5.
La colonne 2 fonctionne à 40,5 bars, le condenseur 4 fonctionne à -600C, et le rapport C1/C2 en fond de la colonne 2 est de 1%.Column 2 operates at 40.5 bars, condenser 4 operates at -60 ° C., and the ratio C1 / C2 at the bottom of column 2 is 1%.
Dans ces conditions, on obtient en fond de la colonne 2 une masse spécifique de liquide de 404,8 kg/m3 et une masse spécifique de vapeur de 88,95 kg/m3. Ainsi le rapport des masses spécifiques des phases liquide et vapeur au fond de la colonne 2 est de 4,55.Under these conditions, at the bottom of column 2, a specific liquid mass of 404.8 kg / m 3 and a specific mass of 88.95 kg / m 3 . Thus the ratio of the specific masses of the liquid and vapor phases at the bottom of column 2 is 4.55.
La liquéfaction est donc effectuée dans E2 à une pression de 40 bars. Pour l'ensemble de la liquéfaction, une puissance de 162,4 MW est nécessaire au total pour les compresseurs des deux cycles à mélange réfrigérant.The liquefaction is thus carried out in E2 at a pressure of 40 bar. For the entire liquefaction, a total of 162.4 MW of power is required for the two refrigerant cycle compressors.
Dans l'exemple 1 , la colonne de dééthanisation 11 ne comporte pas de colonne latérale. De plus, le flux obtenu en tête de la colonne 1 est refroidi uniquement par échange de chaleur avec un soutirage latéral de la colonne de fractionnement 2, et donc n'augmente pas la puissance frigorifique nécessaire au fonctionnement du procédé.In Example 1, the deethanization column 11 has no side column. In addition, the flow obtained at the top of the column 1 is cooled only by heat exchange with a side withdrawal of the fractionation column 2, and therefore does not increase the cooling capacity necessary for the operation of the process.
Exemple 2 :Example 2
On opère le schéma 2 selon l'invention.Scheme 2 according to the invention is operated.
Le gaz à traiter a une composition et un débit identique à celui de l'exemple 1.The gas to be treated has a composition and a flow rate identical to that of Example 1.
Le gaz est refroidi dans E1 à une température de -300C, puis introduit dans la colonne de fractionnement 2.The gas is cooled in E1 to a temperature of -30 ° C. and then introduced into the fractionation column 2.
La colonne 2 fonctionne à 53,5 bars, le condenseur 4 fonctionne à -6O0C, et le rapport C1/C2 en fond de la colonne 2 est de 55 %. Dans ces conditions, on obtient en fond de la colonne 2 une masse spécifique de liquide de 405,6 kg/m3 et une masse spécifique de vapeur de 87,7 kg/m3. Ainsi le rapport des masses spécifiques des phases liquide et vapeur au fond de la colonne 2 est de 4,6.Column 2 operates at 53.5 bars, condenser 4 operates at -60 ° C., and the ratio C1 / C2 at the bottom of column 2 is 55%. Under these conditions, a liquid density mass of 405.6 kg / m 3 and a specific vapor mass of 87.7 kg / m 3 are obtained at the bottom of column 2. Thus the ratio of the specific masses of the liquid and vapor phases at the bottom of column 2 is 4.6.
La liquéfaction est donc effectuée dans E2 à une pression de 53 bars. Pour l'ensemble de la liquéfaction, une puissance de 148,3 MW est nécessaire au total pour les compresseurs des deux cycles à mélange réfrigérant, soit un gain d'environ 9% par rapport à l'exemple 1.The liquefaction is thus carried out in E2 at a pressure of 53 bars. For all the liquefaction, a power of 148.3 MW is required in total for the compressors of the two refrigerant cycles, a gain of about 9% compared with Example 1.
La contrepartie de ce gain d'efficacité réside dans la difficulté pour récupérer un flux enrichi en éthane, nécessaire pour effectuer l'appoint de fluide caloporteur des circuits de réfrigération 100 et 200. En effet, une simple distillation dans la colonne de séparation 11 permet d'obtenir en tête un mélange de C1 et C2 utilisable dans le deuxième cycle de réfrigération 200, mais pas dans le premier cycle 100 qui met en oeuvre un mélange de C2 et de C3. L'invention propose, dans l'exemple 2, de mettre en oeuvre la colonne de stripage latérale 20. Le flux en tête de la colonne 11 est refroidi à une température de -200C par échange de chaleur avec une portion du fluide caloporteur du premier circuit de réfrigération 100. De plus, l'effluent évacué en tête du ballon 13 doit être liquéfié. Ces échanges de chaleur supplémentaires entraînent une perte d'efficacité d'environ 1 % par rapport à l'exemple 1. Au final, le mode opératoire selon l'invention de l'exemple 2 est beaucoup plus attractif que le mode opératoire de l'exemple 1 : il permet d'économiser environ 8% d'énergie ou d'augmenter la capacité de liquéfaction d'environ 8 % avec les mêmes turbines à gaz. The counterpart of this efficiency gain lies in the difficulty in recovering a stream enriched with ethane, which is necessary to make the additional heat-transfer fluid of the refrigeration circuits 100 and 200. In fact, a simple distillation in the separation column 11 makes it possible to obtain at the top a mixture of C1 and C2 that can be used in the second refrigeration cycle 200, but not in the first cycle 100 which uses a mixture of C2 and C3. The invention proposes, in Example 2, to implement the lateral stripping column 20. The flow at the top of the column 11 is cooled to a temperature of -20 ° C. by heat exchange with a portion of the coolant of the first refrigeration circuit 100. In addition, the effluent discharged at the top of the flask 13 must be liquefied. These additional heat exchanges lead to a loss of efficiency of approximately 1% compared with Example 1. In the end, the procedure according to the invention of Example 2 is much more attractive than the procedure of the invention. example 1: it saves about 8% energy or increase the liquefaction capacity of about 8% with the same gas turbines.

Claims

REVENDICATIONS
1) Procédé de liquéfaction d'un gaz naturel, dans lequel on effectue les étapes suivantes : a) on refroidit le gaz naturel, b) on introduit le gaz naturel refroidi dans une colonne de fractionnement de manière à séparer une phase gazeuse riche en méthane et une phase liquide riche en composés plus lourds que l'éthane, c) on soutire ladite phase liquide en fond de la colonne de fractionnement et on évacue ladite phase gazeuse en tête de la colonne de séparation, d) on liquéfie partiellement ladite phase gazeuse de manière à produire un condensât et un flux gazeux, ledit condensât étant recyclé en tête de la colonne de fractionnement à titre de reflux, e) on liquéfie ledit flux gazeux, et dans lequel on choisit les conditions opératoires de la colonne de fractionnement de manière à ce que ladite phase liquide comporte une quantité molaire de méthane comprise entre 10 % et 150 % de la quantité molaire d'éthane contenu dans ladite phase liquide.1) Process for liquefying a natural gas, in which the following steps are carried out: a) the natural gas is cooled, b) the cooled natural gas is introduced into a fractionation column so as to separate a gas phase rich in methane and a liquid phase rich in compounds heavier than ethane, c) said liquid phase is withdrawn at the bottom of the fractionation column and said gas phase is discharged at the top of the separation column, d) said gas phase is partially liquefied so as to produce a condensate and a gas flow, said condensate being recycled to the top of the fractionation column as reflux, e) said gas stream is liquefied, and wherein the operating conditions of the fractionation column are selected so as to said liquid phase comprises a molar amount of methane of between 10% and 150% of the molar amount of ethane contained in said liquid phase.
2) Procédé selon la revendication 1 , dans lequel on choisit les conditions opératoires de la colonne de fractionnement de manière à ce que ladite phase liquide comporte une quantité molaire de méthane comprise entre 40 % et 70 % de la quantité molaire d'éthane contenu dans ladite phase liquide.2) Process according to claim 1, wherein the operating conditions of the fractionating column are chosen so that said liquid phase comprises a molar amount of methane of between 40% and 70% of the molar amount of ethane contained in said liquid phase.
3) Procédé selon l'une des revendications 1 et 2, dans lequel on ajuste la quantité molaire de méthane par rapport à la quantité d'éthane de ladite phase liquide en modifiant la puissance d'un rebouilleur situé en fond de la colonne de fractionnement. 4) Procédé selon l'une des revendications 1 à 3, dans lequel on effectue, en outre, les étapes suivantes : f) on introduit ladite phase liquide dans une colonne de séparation pour séparer une fraction gazeuse riche en méthane et une fraction liquide comportant des hydrocarbures plus lourds que l'éthane, g) on soutire une portion liquide de la colonne de séparation, h) on extrait, de ladite portion liquide, un flux liquide comportant plus de 95 % molaire d'éthane.3) Method according to one of claims 1 and 2, wherein the molar amount of methane is adjusted relative to the amount of ethane of said liquid phase by modifying the power of a reboiler located at the bottom of the fractionation column . 4) Method according to one of claims 1 to 3, wherein is carried out, in addition, the following steps: f) said liquid phase is introduced into a separation column to separate a gaseous fraction rich in methane and a liquid fraction comprising heavier hydrocarbons than ethane, g) withdrawing a liquid portion of the separation column, h) extracting from said liquid portion, a liquid stream comprising more than 95 mol% of ethane.
5) Procédé selon la revendication 4, dans lequel à l'étape g), on soutire la portion de liquide à un niveau situé entre le point d'alimentation et la tête de la colonne de séparation.5) The method of claim 4, wherein in step g), the liquid portion is withdrawn at a level between the feed point and the head of the separation column.
6) Procédé selon l'une des revendications 4 et 5, dans lequel à l'étape h), on vaporise une partie de ladite portion liquide de manière à obtenir ledit flux liquide comportant plus de 95 % molaire d'éthane, ladite partie vaporisée étant introduite dans la colonne de séparation.6) Process according to one of claims 4 and 5, wherein in step h), a portion of said liquid portion is vaporized so as to obtain said liquid flow comprising more than 95 mol% of ethane, said vaporized portion being introduced into the separation column.
7) Procédé selon l'une des revendications 4 à 6, dans lequel on introduit en tête de la colonne de séparation un reflux liquide à une température comprise entre -1O0C et -400C.7) Method according to one of claims 4 to 6, wherein is introduced at the head of the separation column a liquid reflux at a temperature between -1O 0 C and -40 0 C.
8) Procédé selon la revendication 7, dans lequel à l'étape a) on refroidit le gaz naturel par échange de chaleur avec un fluide réfrigérant circulant dans un circuit de réfrigération et dans lequel on condense partiellement ladite fraction gazeuse riche en méthane obtenue à l'étape f) par échange de chaleur avec une portion dudit fluide réfrigérant, de manière à obtenir ledit reflux liquide introduit en tête de la colonne de séparation. 9) Procédé selon la revendication 8, dans lequel on sous-refroidit la portion de fluide réfrigérant par échange de chaleur, avec un liquide soutiré de la colonne de fractionnement.8) The method of claim 7, wherein in step a) the natural gas is cooled by heat exchange with a refrigerant circulating in a refrigeration circuit and wherein said partially condensed methane-rich gas fraction is partially condensed. step f) by heat exchange with a portion of said coolant, so as to obtain said liquid reflux introduced at the top of the separation column. 9) The method of claim 8, wherein the subcooling portion of coolant by heat exchange, with a liquid withdrawn from the fractionation column.
10) Procédé selon l'une des revendications précédentes, dans lequel à l'étape e) on refroidit le flux gazeux par échange de chaleur à une pression supérieure à 50 bars. 10) Method according to one of the preceding claims, wherein in step e) the gas stream is cooled by heat exchange at a pressure greater than 50 bar.
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