WO2023180391A1 - Method for liquefying a methane-rich feed gas, and corresponding facility - Google Patents

Method for liquefying a methane-rich feed gas, and corresponding facility Download PDF

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Publication number
WO2023180391A1
WO2023180391A1 PCT/EP2023/057348 EP2023057348W WO2023180391A1 WO 2023180391 A1 WO2023180391 A1 WO 2023180391A1 EP 2023057348 W EP2023057348 W EP 2023057348W WO 2023180391 A1 WO2023180391 A1 WO 2023180391A1
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Prior art keywords
gas
cooling
flow
recycled
stream
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PCT/EP2023/057348
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French (fr)
Inventor
Laurent Benoit
Gabrielle MENARD
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Engie
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Publication of WO2023180391A1 publication Critical patent/WO2023180391A1/en

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    • 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
    • F25J1/0002—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"
    • F25J1/0042—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
    • F25J1/0047—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using an "external" refrigerant stream in a closed vapor compression cycle
    • F25J1/005—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using an "external" refrigerant stream in a closed vapor compression cycle by expansion of a gaseous refrigerant stream with extraction of work
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/003—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
    • F25J1/0087—Propane; Propylene
    • 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
    • F25J1/0095—Oxides of carbon, e.g. CO2
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • F25J1/0203—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a single-component refrigerant [SCR] fluid in a closed vapor compression cycle
    • F25J1/0208—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a single-component refrigerant [SCR] fluid in a closed vapor compression cycle in combination with an internal quasi-closed refrigeration loop, e.g. with deep flash recycle 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
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • F25J1/0203—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a single-component refrigerant [SCR] fluid in a closed vapor compression cycle
    • F25J1/0208—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a single-component refrigerant [SCR] fluid in a closed vapor compression cycle in combination with an internal quasi-closed refrigeration loop, e.g. with deep flash recycle loop
    • F25J1/0209—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a single-component refrigerant [SCR] fluid in a closed vapor compression cycle in combination with an internal quasi-closed refrigeration loop, e.g. with deep flash recycle loop as at least a three level refrigeration cascade
    • F25J1/021—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a single-component refrigerant [SCR] fluid in a closed vapor compression cycle in combination with an internal quasi-closed refrigeration loop, e.g. with deep flash recycle loop as at least a three level refrigeration cascade using a deep flash recycle 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
    • 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
    • F25J1/0221—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 the cold stored in an external cryogenic component in an open refrigeration 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
    • 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
    • F25J1/0225—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 other external refrigeration means not provided before, e.g. heat driven absorption chillers
    • F25J1/0227—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 other external refrigeration means not provided before, e.g. heat driven absorption chillers within a refrigeration cascade
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • F25J1/0243—Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
    • F25J1/0257—Construction and layout of liquefaction equipments, e.g. valves, machines
    • F25J1/0262—Details of the cold heat exchange system
    • 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
    • F25J2210/42—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
    • F25J2210/00—Processes characterised by the type or other details of the feed stream
    • F25J2210/66—Landfill or fermentation off-gas, e.g. "Bio-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
    • F25J2270/00—Refrigeration techniques used
    • F25J2270/90—External refrigeration, e.g. conventional closed-loop mechanical refrigeration unit using Freon or NH3, unspecified external refrigeration
    • 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
    • F25J2270/90—External refrigeration, e.g. conventional closed-loop mechanical refrigeration unit using Freon or NH3, unspecified external refrigeration
    • F25J2270/908—External refrigeration, e.g. conventional closed-loop mechanical refrigeration unit using Freon or NH3, unspecified external refrigeration by regenerative chillers, i.e. oscillating or dynamic systems, e.g. Stirling refrigerator, thermoelectric ("Peltier") or magnetic refrigeration
    • 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
    • F25J2290/34—Details about subcooling of liquids

Definitions

  • the present invention relates to a process for liquefying a feed gas rich in methane, comprising purifying the feed gas to obtain a purified gas, pre-cooling the purified gas to obtain a gas precooled, liquefying the precooled gas to obtain a liquid stream, subcooling the liquid stream to obtain a subcooled liquid stream, and expanding the subcooled liquid stream to obtain a liquefied gas.
  • the invention also relates to an installation adapted to implement such a method.
  • the gas to be treated is, for example, biogas (from the fermentation of organic materials).
  • the market concerned is for example that of retail LNG (Liquefied Natural Gas), with final storage of LNG produced at a pressure lower than 3 bars (10 5 Pa) absolute.
  • This market requires relatively low liquefied gas production capacities, typically less than 20 tonnes of gas to be liquefied per day, or a mechanical power consumption of less than 1 MW.
  • Liquefaction of the feed gas consists of cooling it to a sufficiently cold temperature so that it can remain in liquid form at thermodynamic equilibrium for pressures making it transportable by state storage techniques. art, that is to say at pressures less than 20 bars absolute and more often less than 3 bars absolute. At these pressures, the thermodynamic equilibrium of a methane-rich gas is reached at temperatures below -100°C and more often below -140°C.
  • the feed gas To cool the feed gas to these temperatures, it is typically pretreated to remove any compounds that may crystallize at these cryogenic temperatures. Then, the feed gas can also be compressed to a pressure higher than its initial pressure. Then, cooling is generally carried out in three main stages summarized below.
  • the gas is first pre-cooled, but not enough to condense at the pressure considered.
  • This pressure is, except for pressure losses, the pressure at the outlet of the pretreatment and is generally higher than the final storage pressure of the liquefied gas.
  • the gas then undergoes a condensation or liquefaction stage itself, during which it effectively liquefies and remains, except for pressure losses, at the cooling inlet pressure.
  • the gas finally undergoes a subcooling stage which continues the cooling of the liquid and, after a final expansion allowing the storage pressure to be reached, the remaining liquid is collected and stored.
  • cryogenic temperatures typically below -80°C, and are very expensive. Indeed, on the one hand, the heat extracted, in other words the cooling provided, at these low temperatures requires a lot of energy and, on the other hand, the equipment adapted to these cryogenic temperatures is much more specific and expensive than that designed for lower temperatures, such as pre-cooling.
  • the feed gas is already at high pressure, greater than 40 bar absolute, because the gas comes either directly from a geological reservoir or of a network.
  • subcooling is continued down to very low temperatures, around -150°C, in order to generate less than 10% by volume of steam during final expansion.
  • the cold of the steam possibly produced is generally recovered and used to cool the feed gas over the entire cooling range, ie during pre-cooling, liquefaction and sub-cooling.
  • - cooling in order to reduce the overall energy consumption of the cycle or cycles cooling refrigerants.
  • this does not reduce the cost of the most expensive equipment, the contribution of which is significant in small capacity installations.
  • An aim of the invention is therefore to propose a liquefaction process making it possible to reduce the overall production cost, in particular for capacities of less than 20 tonnes per day.
  • the subject of the invention is a process for liquefying a feed gas comprising at least 40% by volume of methane, the process comprising the following steps:
  • the process comprises one or more of the following characteristics, taken in isolation or in all technically possible combinations:
  • the subcooling of the liquid stream comprises: a first subcooling of the liquid stream by heat exchange with the third stream of recycled gas, to obtain an intermediate subcooled liquid stream and the second flow recycled gas; and a second subcooling of the intermediate subcooled liquid stream to obtain the subcooled liquid stream;
  • the second sub-cooling of the intermediate sub-cooled liquid stream is carried out by exchange with a flow of liquid nitrogen, the second sub-cooling producing the sub-cooled liquid stream and a flow of vaporized nitrogen, the second pre-cooling of the first pre-cooled gas being further produced by heat exchange the flow of vaporized nitrogen;
  • the purified gas undergoes the first pre-cooling in a first pre-cooling unit including a pre-cooling refrigeration cycle, by heat exchange with a refrigerant fluid to form the first pre-cooled gas without heat exchange with the first flow of recycled gas, the refrigerant being produced by the pre-cooling refrigeration cycle;
  • the liquid stream leaving the liquefaction unit has a temperature between -113°C and -90°C;
  • the second pre-cooled gas is liquefied by the liquefaction unit with sub-cooling less than or equal to 5°C;
  • the refrigeration liquefaction cycle is a Stirling cycle or an inverse Brayton cycle
  • the expansion of the subcooled liquid stream is carried out in at least one Joule-Thomson valve or by expansion turbine.
  • the invention also relates to an installation adapted to implement a process as described above, comprising:
  • a mixer for mixing the feed gas with the first flow of recycled gas and obtaining the gas to be treated, and at least one compressor adapted to compress the gas to be treated to the treatment pressure,
  • a purification unit adapted to purify the gas to be treated and obtain the purified gas
  • a first pre-cooling unit adapted to pre-cool the purified gas and obtain the first pre-cooled gas
  • a second pre-cooling unit adapted to pre-cool the first pre-cooled gas by heat exchange with at least the second stream of recycled gas and to obtain the second pre-cooled gas and the first stream of recycled gas
  • liquefaction unit for liquefying the second pre-cooled gas and obtaining the liquid stream
  • the liquefaction unit including the liquefaction refrigeration cycle
  • subcooling unit adapted to subcool the liquid stream to the subcooling temperature by heat exchange with at least the third recycled gas flow and to obtain the subcooled liquid flow and the second flow recycled gas
  • the subcooling unit and the expansion unit being configured so that the third stream of recycled gas represents a fraction, relative to the subcooled liquid stream, less than 35 mol%.
  • FIG. 1 is a schematic view of an installation according to the invention adapted to implement a method according to the invention.
  • FIG. 2 is a schematic view of an installation according to the invention constituting a variant of the installation shown in Figure 1.
  • upstream and downstream generally extend in relation to the normal direction of circulation of a fluid.
  • 1 Nm 3 /h means in this document one cubic meter per hour at a pressure of 101325 Pa and a temperature of 0°C.
  • an installation 10 is described.
  • the installation is suitable for liquefying a feed gas 12 comprising at least 40% by volume of methane and obtaining a liquefied gas 14 (that is to say a liquid), for example with a view to marketing it on the market.
  • a feed gas 12 comprising at least 40% by volume of methane
  • a liquefied gas 14 that is to say a liquid
  • retail LNG Liquefied Natural Gas
  • the feed gas 12 is for example at low pressure, close to atmospheric pressure.
  • the feed gas 12 is at a temperature close to ambient temperature, that is to say much hotter than its bubble temperature at atmospheric pressure (101325 Pa).
  • the feed gas 12 is for example a biogas.
  • the liquefied gas 14 is advantageously stored at a pressure of less than 3 bar absolute (300 kPa).
  • the installation 10 comprises a mixer 16 for mixing the feed gas 12 with a first stream of recycled gas 18 and obtaining a gas to be treated 20.
  • the installation 10 comprises at least one compressor 22 for compressing the gas to be treated 20, for example followed by a cooler 24 and a purification unit 26 adapted to purify the gas to be treated 20 and obtain a purified gas 28.
  • the installation 10 comprises a first pre-cooling unit 30 adapted to pre-cool the purified gas 28 and obtain a first pre-cooled gas 32, the first pre-cooling unit including in the example a pre-cooling refrigeration cycle. cooling 34.
  • cooling cycle we mean a set of pipes and elements (not shown), such as compressors or turbines, adapted to subject a fluid to a series of transformations with the aim of generating cold at a location. of the cycle, in a manner known in itself.
  • the installation 10 comprises a second pre-cooling unit 36 adapted to pre-cool the first pre-cooled gas 32 by heat exchange with a second stream of recycled gas 38 and to obtain a second pre-cooled gas 40 and the first recycled gas flow 18.
  • the installation 10 comprises a liquefaction unit 42 for liquefying the second pre-cooled gas 40 and obtaining a stream of liquid 44, the liquefaction unit including a refrigeration liquefaction cycle 46.
  • the installation 10 comprises a sub-cooling unit 48 adapted to sub-cool the liquid stream 44 to a sub-cooling temperature by heat exchange with at least a third recycled gas flow 50 and to obtain a liquid stream subcooled 52 and the second flow of recycled gas 38.
  • the installation 10 includes an expansion unit 54 for expanding the stream of sub-cooled liquid 52 and obtaining the liquefied gas 14, for example received in a storage 56, and the third flow of recycled gas 50.
  • the compressor 22 is adapted to compress the gas to be treated 20 to a treatment pressure of between 19 and 70 bar absolute, which makes the gas to be treated 20, after purification, liquefiable at cryogenic temperatures, nevertheless remaining above - 113°C .
  • the treatment pressure is advantageously less than 45 bar absolute.
  • the purification unit 26 is adapted to remove from the gas to be treated the compounds likely to crystallize downstream.
  • the purification unit 26 is conventionally adapted to eliminate volatile compounds and heavy hydrocarbons (called “C6+”), for example using activated carbons (not shown and known per se).
  • C6+ volatile compounds and heavy hydrocarbons
  • the purification unit 26 includes for example a condensation system (not shown).
  • a membrane system (not shown) is for example used.
  • molecular sieves can be used (not shown).
  • the purified gas 28 contains at least 90%, or even 99%, of methane by volume.
  • the first pre-cooling unit 30 comprises for example a heat exchanger 58 adapted to carry out a heat exchange between the purified gas 28 and a refrigerating fluid 60 produced by the pre-cooling refrigeration cycle 34, without heat exchange with the first recycled gas flow 18.
  • the pre-cooling refrigeration cycle 34 is disjoint from the liquefaction refrigeration cycle 46.
  • disjoint we mean that the two refrigeration cycles do not share a refrigerating fluid which would be common to them.
  • the pre-cooling refrigeration cycle 34 used is for example a brine cycle, a CO2 cycle, an ammonia cycle, a freon cycle, or a propane cycle, known in themselves and which will not be described. in detail.
  • the temperature of the first pre-cooled gas 32 is between -40°C and -15°C.
  • the second pre-cooling unit 36 comprises for example a heat exchanger 62 to carry out the heat exchange with the second flow of recycled gas 38.
  • the liquefaction unit 42 comprises for example a heat exchanger 64 adapted to carry out a heat exchange between the second pre-cooled gas 40 and a refrigerating fluid 66 produced by the refrigerating liquefaction cycle 46, without heat exchange with the second flow of recycled gas 38.
  • the liquefaction refrigeration cycle 46 is adapted to provide all the cold necessary to the liquefaction unit 42.
  • the refrigeration liquefaction cycle 46 is for example a Stirling cycle.
  • the refrigeration liquefaction cycle 46 is for example an inverse Brayton cycle, also known in itself to those skilled in the art.
  • the subcooling unit 48 and the expansion unit 54 are configured so that the third stream of recycled gas 50 represents a fraction, relative to the stream of subcooled liquid 52, less than 35 molar%, and preferably between 10% and 30 mol%. This is particularly possible by sufficiently lowering the temperature of the subcooled liquid stream 52.
  • the sub-cooling unit 48 comprises a first heat exchanger 68, a second heat exchanger 70 and a sub-cooling refrigeration cycle 72.
  • the first heat exchanger 68 is adapted to carry out a first sub-cooling of the liquid stream 44 by heat exchange with the third flow of liquid. recycled gas 50, and to obtain an intermediate subcooled liquid stream 74 and the second stream of recycled gas 38.
  • the second heat exchanger 70 is adapted to carry out a second sub-cooling of the intermediate sub-cooled liquid stream 74 to obtain the sub-cooled liquid stream 52, by heat exchange with a refrigerating fluid 76 produced by the sub-cooling refrigeration cycle. -cooling 72, without heat exchange with the third flow of recycled gas 50.
  • the expansion unit 54 advantageously comprises an expansion member 78 for expanding the stream of sub-cooled liquid 52 and obtaining a relaxed sub-cooled stream 80, for example at a pressure less than 3 bar absolute.
  • the expansion unit 54 comprises for example a flash drum 82 to separate the expanded subcooled stream 80 into the liquefied gas 14 and a vapor forming the third stream of recycled gas 50.
  • the expansion member 78 is for example a Joule-Thomson valve or an expansion turbine.
  • the feed gas 12 and the first flow of recycled gas 18 are mixed by the mixer 16 to form the gas to be treated 20.
  • the gas to be treated 20 is compressed in the compressor 22, then cooled to approximately ambient temperature, for example 20°C, in the cooler 24. Then the gas to be treated 20 is purified in the purification unit 26 to form the purified gas 28.
  • the purified gas 28 undergoes a first pre-cooling in the first pre-cooling unit 30, by heat exchange with the refrigerant fluid 60, to form the first pre-cooled gas 32.
  • the first pre-cooled gas 32 undergoes a second pre-cooling in the second pre-cooling unit 36, by heat exchange with the second flow of recycled gas 38, to form the second pre-cooled gas 40.
  • the second flow of recycled gas 38 heats up and becomes the first flow of recycled gas 18.
  • the second pre-cooled gas 40 is liquefied in the liquefaction unit 42 and forms the liquid stream 44.
  • the second pre-cooling unit 36 does not carry out liquefaction.
  • Liquefaction is entirely carried out by liquefaction unit 42.
  • the second pre-cooled gas 40 is liquefied by the liquefaction unit 42, with subcooling advantageously less than or equal to 5°C, for example approximately 3°C.
  • the temperature of the liquid stream 44 leaving the liquefaction unit 42 is for example 3°C below the bubble temperature of the second precooled gas 40.
  • the temperature of the liquid stream 44 leaving the liquefaction unit is preferably between -90°C and -113°C.
  • the refrigeration liquefaction cycle 46 advantageously provides all the cold allowing the liquefaction of the second pre-cooled gas 40.
  • the liquid stream 44 is then subcooled in the subcooling unit 48 to form the subcooled liquid stream 52, by heat exchange with at least the third recycled gas flow 50, i.e. i.e. the steam coming from the flash tank 82.
  • the third flow of recycled gas 50 heats up and becomes the second flow of recycled gas 38.
  • the liquid stream 44 undergoes a first subcooling in the first heat exchanger 68 by heat exchange with the third flow of recycled gas 50, then a second subcooling in the second heat exchanger 70 by heat exchange. heat with the refrigerant fluid 76 to form the subcooled liquid stream 52.
  • the subcooling applied makes it possible to reduce the evaporation rate at the outlet of the flash tank 82 to a value less than 35 mol%.
  • the evaporation rate remains greater than or equal to 20 mol%.
  • the subcooled liquid stream 52 is expanded in the expansion member 78 to form the expanded subcooled stream 80, which is received in the flash tank 82.
  • the liquefied gas 14 is for example recovered at the bottom of the flash tank. flash 82 and sent to storage 56.
  • the steam from the flash drum 82 is recycled in the gas to be treated 20.
  • This steam forms the third flow of recycled gas 50, which first becomes the second flow of recycled gas 38 after its passage through the sub-unit. cooling 48, then becomes the first flow of recycled gas 18 after its passage into the second pre-cooling unit 36.
  • This vapor does not pass into the liquefaction unit 42, or in any case this vapor does not release cold to the liquefaction unit 42.
  • this vapor does not pass into the first pre-cooling unit 30 either, or in any case does not transfer cold to the first pre-cooling unit 30.
  • this steam can release part of its cold into the pre-cooling unit 30, in particular depending on the size of the installation 10. For example, if the production of liquefied gas 14 is less than 20 tonnes per day, the cold recovery in the first pre-cooling unit 30 coming from the first flow of recycled gas 18 will be avoided, as shown in Figure 1.
  • the production of liquefied gas 14 is greater than or equal to 20 tonnes per day for example, this recovery will be preferred.
  • the first stream of recycled gas 18 is passed through the first pre-cooling unit 30.
  • an installation 100 according to the invention constituting a variant of the installation 10.
  • the installation 100 is similar to the installation 10 shown in Figure 1. Similar elements bear the same numerical references and will not be described again. Only the differences will be described in detail below.
  • the first pre-cooling of the purified gas 28 is carried out by heat exchange with a refrigeration liquefaction cycle 146.
  • the liquefaction unit 42 and the first pre-cooling unit 30 share the same refrigeration cycle 146, which provides the cold used for the first pre-cooling and for liquefaction.
  • the refrigerating fluid 66 gives up cold to the second pre-cooled gas 40 in the heat exchanger 64 of the liquefaction unit 42, and becomes the refrigerating fluid 60.
  • the refrigerating fluid 60 gives up cold to the purified gas 28 in the exchanger thermal 58 of the first pre-cooling unit 30.
  • the second pre-cooling unit 36 of installation 100 does not receive cold from the refrigeration cycle 146.
  • the second heat exchanger 70 does not receive cold from a refrigeration cycle dedicated to sub-cooling, but from an open loop 172 with liquid nitrogen.
  • a flow of liquid nitrogen 174 (coming from a source not shown, such as a liquid nitrogen storage) transfers cold to the stream of intermediate subcooled liquid 74 in the second heat exchanger 70 of the sub-cooling unit 48, and vaporizes to become a flow of nitrogen gas 176.
  • the flow of nitrogen gas 176 then transfers cold to the first pre-cooled gas 32 in the heat exchanger 62, and becomes a flow of nitrogen 178.
  • a nitrogen make-up 180 is made in the gaseous nitrogen flow 176 before the latter enters the second pre-cooling unit 36.
  • This make-up 180 has a temperature lower than that of the flow of nitrogen. nitrogen gas 176 before topping up.
  • the supplement 180 is advantageously produced in liquid form.
  • the second pre-cooling is carried out by heat exchange, on the one hand, with the second flow of recycled gas 38 and, on the other hand, advantageously with the flow of gaseous nitrogen 176, possibly increased by the make-up 180.
  • installation 100 is similar to that of installation 10.
  • the second heat exchanger 70 of the sub-cooling unit 48 is absent, as well as the sub-cooling refrigeration cycle 72 (figure 1) or the open loop 172 (figure 2).
  • the sub-cooling carried out by the sub-cooling unit 48 is then carried out only in the first exchanger 68, by heat exchange with the third flow of recycled gas 50.
  • the treatment pressure is 40 bars absolute (pressure of the gas to be treated after compression by compressor 22).
  • the second pre-cooled gas 40 has a temperature of -53.5°C.
  • the liquid stream 44, leaving the liquefaction unit 42, has a temperature of -90°C.
  • the subcooling unit 48 and the expansion unit 54 are configured to obtain an evaporation rate of between 20% and 30 molar%.
  • Case 2 (example according to the invention): with recovery of the cold contained in the flash gas (the third gas flow 50 passes through the exchangers 68 and 62), but no open loop 172 with liquid nitrogen. Case 2 corresponds substantially to Figure 1;
  • Case 4 (example according to the invention): with recovery of the cold contained in the flash gas (the third gas flow 50 passes through the exchangers 68 and 62), and presence of the loop 172 open to liquid nitrogen. Case 4 corresponds substantially to Figure 2.
  • Case 1 represents a simple process, namely just liquefaction without subcooling and without cold recovery on the flash. We then seek to evaluate the energy gains resulting from the progressive addition of cold recovery systems using flash gas and liquid nitrogen sub-cooling systems.
  • case 4 makes it possible to reduce the size of the liquefaction unit by 44%, resulting in a significant reduction in the overall production cost of liquefied gas.
  • the process makes it possible to reduce the overall production cost of liquefied gas 14, in particular for production capacities of less than 20 tonnes per day.
  • the treatment pressure between 19 and 70 bar absolute, is sufficiently high so that the liquefaction temperature is not too low, ie preferably greater than -90°C.
  • the equipment used is less specific and less expensive.
  • the weight of the investment being significant for small capacities, this has a favorable impact on the unit production cost.
  • the energy spent to provide cold is also lower when the temperature of the fluid to be cooled is lower.
  • the processing pressure remains relatively low and allows a sufficiently low subcooling temperature, which maintains the volume fraction of flash gas recycled upstream of the compressor in a reasonable proportion, which reduces the energy spent for compressing the gas to be treated 20.
  • the lower pressure also allows savings on equipment, which does not have to withstand very high pressures.
  • the cold present in the recycled flash gas (third recycled gas flow 50) is used specifically to amplify the pre-cooling of the gas to be treated and its sub-cooling. This cold is not used in the liquefaction unit 42. This reduces the cooling range of the liquefaction unit 42, and reduces the size of the liquefaction refrigeration cycles 46, 146. In addition, this avoids having to to modify the refrigeration liquefaction cycles themselves to integrate a flow of recycled gas.
  • the sub-cooling is advantageously carried out with liquid nitrogen, the cold of which is for example also used to amplify the pre-cooling.

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Abstract

The invention relates to a method for liquefying a methane-rich feed gas (12), the method comprising: - a step of mixing the feed gas with recycled gas (18), a step of compressing the gas to a pressure of between 19 and 70 bars absolute, and a step of purifying the gas; - a first step of pre-cooling the gas to a temperature of between -40°C and -15°C, and a second step of pre-cooling the gas by heat exchange with the recycled gas; - a step of liquefying the gas by heat exchange only with a liquefaction refrigeration cycle (46); - a step of sub-cooling the gas by heat exchange with at least the recycled gas; and - expanding the sub-cooled liquid (52) to obtain a liquefied gas (14) and wherein the recycled gas represents a mole fraction of less than 35%.

Description

Procédé de liquéfaction d’un gaz d’alimentation riche en méthane, et installation correspondante Process for liquefying a feed gas rich in methane, and corresponding installation
La présente invention concerne un procédé de liquéfaction d’un gaz d’alimentation (en anglais feed gas) riche en méthane, comprenant une épuration du gaz d’alimentation pour obtenir un gaz épuré, un pré-refroidissement du gaz épuré pour obtenir un gaz prérefroidi, une liquéfaction du gaz pré-refroidi pour obtenir un courant de liquide, un sous- refroidissement du courant de liquide pour obtenir un courant de liquide sous-refroidi, et une détente du courant de liquide sous-refroidi pour obtenir un gaz liquéfié. The present invention relates to a process for liquefying a feed gas rich in methane, comprising purifying the feed gas to obtain a purified gas, pre-cooling the purified gas to obtain a gas precooled, liquefying the precooled gas to obtain a liquid stream, subcooling the liquid stream to obtain a subcooled liquid stream, and expanding the subcooled liquid stream to obtain a liquefied gas.
L’invention concerne également une installation adaptée pour mettre en œuvre un tel procédé. The invention also relates to an installation adapted to implement such a method.
Le gaz à traiter est par exemple du biogaz (issu de la fermentation de matières organiques). Le marché concerné est par exemple celui du GNL (Gaz Naturel Liquéfié) de détail, avec un stockage final du GNL produit à une pression inférieure à 3 bars (105 Pa) absolus. Ce marché requiert des capacités de production de gaz liquéfié relativement faibles, typiquement inférieures à 20 tonnes de gaz à liquéfier par jour, ou une puissance mécanique consommée inférieure à 1 MW. The gas to be treated is, for example, biogas (from the fermentation of organic materials). The market concerned is for example that of retail LNG (Liquefied Natural Gas), with final storage of LNG produced at a pressure lower than 3 bars (10 5 Pa) absolute. This market requires relatively low liquefied gas production capacities, typically less than 20 tonnes of gas to be liquefied per day, or a mechanical power consumption of less than 1 MW.
La liquéfaction du gaz d’alimentation consiste à le refroidir jusqu’à une température suffisamment froide pour qu’il puisse demeurer sous forme liquide à l’équilibre thermodynamique pour des pressions le rendant transportable par des techniques de stockage de l’état de l’art, c’est-à-dire à des pressions inférieures à 20 bars absolus et plus souvent inférieures à 3 bars absolus. A ces pressions, l’équilibre thermodynamique d’un gaz riche en méthane est atteint à des températures inférieures à -100°C et plus souvent inférieures à -140°C. Liquefaction of the feed gas consists of cooling it to a sufficiently cold temperature so that it can remain in liquid form at thermodynamic equilibrium for pressures making it transportable by state storage techniques. art, that is to say at pressures less than 20 bars absolute and more often less than 3 bars absolute. At these pressures, the thermodynamic equilibrium of a methane-rich gas is reached at temperatures below -100°C and more often below -140°C.
Pour refroidir le gaz d’alimentation à ces températures, celui-ci est généralement prétraité pour éliminer tout composé susceptible de cristalliser à ces températures cryogéniques. Puis, le gaz d’alimentation peut également être comprimé à une pression supérieure à sa pression initiale. Ensuite, le refroidissement est généralement réalisé en trois étapes principales résumées ci-après. To cool the feed gas to these temperatures, it is typically pretreated to remove any compounds that may crystallize at these cryogenic temperatures. Then, the feed gas can also be compressed to a pressure higher than its initial pressure. Then, cooling is generally carried out in three main stages summarized below.
Le gaz est d’abord pré-refroidi, mais pas suffisamment pour se condenser à la pression considérée. Cette pression est, aux pertes de charge près, la pression en sortie du prétraitement et est généralement supérieure à la pression de stockage finale du gaz liquéfié. The gas is first pre-cooled, but not enough to condense at the pressure considered. This pressure is, except for pressure losses, the pressure at the outlet of the pretreatment and is generally higher than the final storage pressure of the liquefied gas.
Le gaz subit ensuite une étape de condensation ou de liquéfaction proprement dite, pendant laquelle il se liquéfie effectivement et reste, aux pertes de charge près, à la pression d’entrée de refroidissement. Le gaz subit enfin une étape de sous-refroidissement (en anglais subcooling) qui poursuit le refroidissement du liquide et, après une détente finale permettant d’atteindre la pression de stockage, le liquide qui subsiste est recueilli et stocké. The gas then undergoes a condensation or liquefaction stage itself, during which it effectively liquefies and remains, except for pressure losses, at the cooling inlet pressure. The gas finally undergoes a subcooling stage which continues the cooling of the liquid and, after a final expansion allowing the storage pressure to be reached, the remaining liquid is collected and stored.
Les deux dernières étapes sont réalisées à des températures cryogéniques, typiquement inférieures à -80°C, et sont très coûteuses. En effet, d’une part, la chaleur extraite, autrement dit le refroidissement apporté, à ces températures basses requiert beaucoup d’énergie et, d’autre part, les équipements adaptés à ces températures cryogéniques sont nettement plus spécifiques et onéreux que ceux conçus pour des températures moins basses, comme celle du pré-refroidissement. The last two steps are carried out at cryogenic temperatures, typically below -80°C, and are very expensive. Indeed, on the one hand, the heat extracted, in other words the cooling provided, at these low temperatures requires a lot of energy and, on the other hand, the equipment adapted to these cryogenic temperatures is much more specific and expensive than that designed for lower temperatures, such as pre-cooling.
Il serait possible d’augmenter la température du liquide en sortie de l’échangeur de sous-refroidissement, juste avant la détente finale. Malheureusement, lorsque le liquide est moins sous-refroidi, la détente produit une grande quantité de vapeur finale (en anglais end-flash gas). Tout ou partie de cette vapeur finale est le plus souvent réinjectée en entrée du procédé de liquéfaction, mais ceci impose de recomprimer la vapeur. Ainsi, élever la température du liquide sous-refroidi augmente notablement le débit de gaz à liquéfier et les besoins du procédé en compression. It would be possible to increase the temperature of the liquid leaving the subcooling exchanger, just before the final expansion. Unfortunately, when the liquid is less subcooled, the expansion produces a large quantity of end-flash gas. All or part of this final vapor is most often reinjected into the liquefaction process, but this requires the vapor to be recompressed. Thus, raising the temperature of the subcooled liquid significantly increases the flow rate of gas to be liquefied and the compression requirements of the process.
Pour des installations de grande capacité, supérieure à 20 tonnes par jour, la plupart du temps, le gaz d’alimentation est déjà à haute pression, supérieure à 40 bars absolus, car le gaz est issu soit directement d’un réservoir géologique, soit d’un réseau. Généralement, le sous-refroidissement est poursuivi jusqu’à des températures très basses, d’environ -150°C, afin de générer moins de 10% en volume de vapeur lors de la détente finale. For large capacity installations, greater than 20 tonnes per day, most of the time, the feed gas is already at high pressure, greater than 40 bar absolute, because the gas comes either directly from a geological reservoir or of a network. Generally, subcooling is continued down to very low temperatures, around -150°C, in order to generate less than 10% by volume of steam during final expansion.
Pour les installations de plus petite capacité, inférieure à 20 tonnes par jour, par exemple les applications biogaz-biométhane et reliquéfaction de vapeurs de GNL stocké ou « BOG » (de l’anglais Boil-Off Gas), il est connu de réaliser une compression du gaz d’alimentation à plus haute pression, au-delà de 80, voire 120, bars absolus, combinée à un refroidissement plus modéré qui s’arrête à -80°C, voire -50°C. L’avantage de relever la température finale de sous-refroidissement est de limiter le coût des équipements de liquéfaction et de sous-refroidissement, mais au prix, d’une part, de la production d’une grande quantité de vapeur lors de la détente finale (plus de 45% en volume) et, d’autre part, d’une compression initiale du gaz d’alimentation qui requiert des compresseurs onéreux et impose des équipements spécifiquement adaptés aux très hautes pressions. For smaller capacity installations, less than 20 tonnes per day, for example biogas-biomethane applications and reliquefaction of stored LNG vapors or “BOG” (from English Boil-Off Gas), it is known to carry out a compression of the feed gas at higher pressure, beyond 80, or even 120, absolute bars, combined with more moderate cooling which stops at -80°C, or even -50°C. The advantage of raising the final subcooling temperature is to limit the cost of liquefaction and subcooling equipment, but at the cost, on the one hand, of the production of a large quantity of steam during expansion. final (more than 45% by volume) and, on the other hand, an initial compression of the feed gas which requires expensive compressors and requires equipment specifically adapted to very high pressures.
En outre, dans les procédés actuels, le froid de la vapeur éventuellement produite est en général récupéré et utilisé pour refroidir le gaz d’alimentation sur la totalité de la plage de refroidissement, i.e. lors du pré-refroidissement, de la liquéfaction et du sous- refroidissement, afin de réduire la consommation d’énergie globale du cycle ou des cycles frigorifiques de refroidissement. Toutefois, cela ne réduit pas le coût des équipements les plus onéreux, dont la contribution est importante dans les installations de petite capacité. Furthermore, in current processes, the cold of the steam possibly produced is generally recovered and used to cool the feed gas over the entire cooling range, ie during pre-cooling, liquefaction and sub-cooling. - cooling, in order to reduce the overall energy consumption of the cycle or cycles cooling refrigerants. However, this does not reduce the cost of the most expensive equipment, the contribution of which is significant in small capacity installations.
Ainsi, le coût de la liquéfaction d’un gaz riche en méthane reste élevé, notamment à petite échelle. Thus, the cost of liquefaction of a gas rich in methane remains high, particularly on a small scale.
Un but de l’invention est donc de proposer un procédé de liquéfaction permettant de réduire le coût de production global, en particulier pour des capacités inférieures à 20 tonnes par jour. An aim of the invention is therefore to propose a liquefaction process making it possible to reduce the overall production cost, in particular for capacities of less than 20 tonnes per day.
A cet effet, l’invention a pour objet un procédé de liquéfaction d’un gaz d’alimentation comprenant au moins 40% en volume de méthane, le procédé comprenant les étapes suivantes : To this end, the subject of the invention is a process for liquefying a feed gas comprising at least 40% by volume of methane, the process comprising the following steps:
- mélange du gaz d’alimentation avec un premier flux de gaz recyclé pour obtenir un gaz à traiter, et compression du gaz à traiter à une pression de traitement comprise entre 19 et 70 bars absolus, - mixing the feed gas with a first flow of recycled gas to obtain a gas to be treated, and compression of the gas to be treated at a treatment pressure of between 19 and 70 bar absolute,
- épuration du gaz à traiter pour obtenir un gaz épuré, - purification of the gas to be treated to obtain a purified gas,
- premier pré-refroidissement du gaz épuré pour obtenir un premier gaz pré-refroidi ayant une température inférieure ou égale à -15°C et supérieure ou égale à -40°C, - first pre-cooling of the purified gas to obtain a first pre-cooled gas having a temperature less than or equal to -15°C and greater than or equal to -40°C,
- deuxième pré-refroidissement du premier gaz pré-refroidi par échange de chaleur avec au moins un deuxième flux de gaz recyclé pour obtenir un deuxième gaz pré-refroidi et le premier flux de gaz recyclé, - second pre-cooling of the first pre-cooled gas by heat exchange with at least a second flow of recycled gas to obtain a second pre-cooled gas and the first flow of recycled gas,
- liquéfaction du deuxième gaz pré-refroidi pour obtenir un courant de liquide, par échange de chaleur uniquement avec un cycle frigorifique de liquéfaction, - liquefaction of the second pre-cooled gas to obtain a liquid stream, by heat exchange only with a refrigeration liquefaction cycle,
- sous-refroidissement du courant de liquide par échange de chaleur avec au moins un troisième flux de gaz recyclé pour obtenir un courant de liquide sous-refroidi et le deuxième flux de gaz recyclé, le courant de liquide sous-refroidi étant à une température de sous-refroidissement, et - subcooling of the liquid stream by heat exchange with at least a third recycled gas flow to obtain a subcooled liquid flow and the second recycled gas flow, the subcooled liquid flow being at a temperature of subcooling, and
- détente du courant de liquide sous-refroidi pour obtenir un gaz liquéfié et le troisième flux de gaz recyclé, ladite détente et la température de sous-refroidissement étant adaptées pour que le troisième flux de gaz recyclé représente une fraction, par rapport au courant de liquide sous-refroidi, inférieure à 35% molaire. - expansion of the subcooled liquid stream to obtain a liquefied gas and the third stream of recycled gas, said expansion and the subcooling temperature being adapted so that the third stream of recycled gas represents a fraction, relative to the current of subcooled liquid, less than 35 mol%.
Selon des modes particuliers de réalisation, le procédé comprend l’une ou plusieurs des caractéristiques suivantes, prise(s) isolément ou selon toutes les combinaisons techniquement possibles : According to particular embodiments, the process comprises one or more of the following characteristics, taken in isolation or in all technically possible combinations:
- le sous-refroidissement du courant de liquide comprend : un premier sous- refroidissement du courant de liquide par échange de chaleur avec le troisième flux de gaz recyclé, pour obtenir un courant de liquide sous-refroidi intermédiaire et le deuxième flux de gaz recyclé ; et un deuxième sous-refroidissement du courant de liquide sous-refroidi intermédiaire pour obtenir le courant de liquide sous-refroidi ; - the subcooling of the liquid stream comprises: a first subcooling of the liquid stream by heat exchange with the third stream of recycled gas, to obtain an intermediate subcooled liquid stream and the second flow recycled gas; and a second subcooling of the intermediate subcooled liquid stream to obtain the subcooled liquid stream;
- le deuxième sous-refroidissement du courant de liquide sous-refroidi intermédiaire est réalisé par échange avec un flux d’azote liquide, le deuxième sous-refroidissement produisant le courant de liquide sous-refroidi et un flux d’azote vaporisé, le deuxième prérefroidissement du premier gaz pré-refroidi étant réalisé en outre par échange de chaleur le flux d’azote vaporisé ; - the second sub-cooling of the intermediate sub-cooled liquid stream is carried out by exchange with a flow of liquid nitrogen, the second sub-cooling producing the sub-cooled liquid stream and a flow of vaporized nitrogen, the second pre-cooling of the first pre-cooled gas being further produced by heat exchange the flow of vaporized nitrogen;
- le gaz épuré subit le premier pré-refroidissement dans une première unité de prérefroidissement incluant un cycle frigorifique de pré-refroidissement, par échange de chaleur avec un fluide réfrigérant pour former le premier gaz pré-refroidi sans échange de chaleur avec le premier flux de gaz recyclé, le fluide frigorifique étant produit par le cycle frigorifique de pré-refroidissement ; - the purified gas undergoes the first pre-cooling in a first pre-cooling unit including a pre-cooling refrigeration cycle, by heat exchange with a refrigerant fluid to form the first pre-cooled gas without heat exchange with the first flow of recycled gas, the refrigerant being produced by the pre-cooling refrigeration cycle;
- le cycle frigorifique de pré-refroidissement et le cycle frigorifique de liquéfaction sont disjoints ; - the pre-cooling refrigeration cycle and the liquefaction refrigeration cycle are separate;
- la pression de traitement est inférieure à 45 bars absolus ; - the treatment pressure is less than 45 bar absolute;
- le courant de liquide en sortie de l’unité de liquéfaction a une température comprise entre -113°C et -90°C ; - the liquid stream leaving the liquefaction unit has a temperature between -113°C and -90°C;
- le deuxième gaz pré-refroidi est liquéfié par l’unité de liquéfaction avec un sous- refroidissement inférieur ou égal à 5°C ; - the second pre-cooled gas is liquefied by the liquefaction unit with sub-cooling less than or equal to 5°C;
- le cycle frigorifique de liquéfaction est un cycle de Stirling ou un cycle de Brayton inverse ; et - the refrigeration liquefaction cycle is a Stirling cycle or an inverse Brayton cycle; And
- la détente du courant de liquide sous-refroidi est réalisée dans au moins une vanne Joule-Thomson ou par turbine à détente. - the expansion of the subcooled liquid stream is carried out in at least one Joule-Thomson valve or by expansion turbine.
L’invention a aussi pour objet une installation adaptée pour mettre en œuvre un procédé tel que décrit ci-dessus, comprenant : The invention also relates to an installation adapted to implement a process as described above, comprising:
- un mélangeur pour mélanger le gaz d’alimentation avec le premier flux de gaz recyclé et obtenir le gaz à traiter, et au moins un compresseur adapté pour comprimer le gaz à traiter à la pression de traitement, - a mixer for mixing the feed gas with the first flow of recycled gas and obtaining the gas to be treated, and at least one compressor adapted to compress the gas to be treated to the treatment pressure,
- une unité d’épuration adaptée pour épurer le gaz à traiter et obtenir le gaz épuré,- a purification unit adapted to purify the gas to be treated and obtain the purified gas,
- une première unité de pré-refroidissement adaptée pour pré-refroidir le gaz épuré et obtenir le premier gaz pré-refroidi, - a first pre-cooling unit adapted to pre-cool the purified gas and obtain the first pre-cooled gas,
- une deuxième unité de pré-refroidissement adaptée pour pré-refroidir le premier gaz pré-refroidi par échange de chaleur avec au moins le deuxième flux de gaz recyclé et pour obtenir le deuxième gaz pré-refroidi et le premier flux de gaz recyclé, - a second pre-cooling unit adapted to pre-cool the first pre-cooled gas by heat exchange with at least the second stream of recycled gas and to obtain the second pre-cooled gas and the first stream of recycled gas,
- une unité de liquéfaction pour liquéfier le deuxième gaz pré-refroidi et obtenir le courant de liquide, l’unité de liquéfaction incluant le cycle frigorifique de liquéfaction, - une unité de sous-refroidissement adaptée pour sous-refroidir le courant de liquide à la température de sous-refroidissement par échange de chaleur avec au moins le troisième flux de gaz recyclé et pour obtenir le courant de liquide sous-refroidi et le deuxième flux de gaz recyclé, et - a liquefaction unit for liquefying the second pre-cooled gas and obtaining the liquid stream, the liquefaction unit including the liquefaction refrigeration cycle, - a subcooling unit adapted to subcool the liquid stream to the subcooling temperature by heat exchange with at least the third recycled gas flow and to obtain the subcooled liquid flow and the second flow recycled gas, and
- une unité de détente pour détendre le courant de liquide sous-refroidi et obtenir le gaz liquéfié et le troisième flux de gaz recyclé, l’unité de sous-refroidissement et l’unité de détente étant configurées pour que le troisième flux de gaz recyclé représente une fraction, par rapport au courant de liquide sous-refroidi, inférieure à 35% molaire. - an expansion unit for expanding the subcooled liquid stream and obtaining the liquefied gas and the third stream of recycled gas, the subcooling unit and the expansion unit being configured so that the third stream of recycled gas represents a fraction, relative to the subcooled liquid stream, less than 35 mol%.
L’invention sera mieux comprise à la lecture de la description qui va suivre, donnée uniquement à titre d’exemple et faite en se référant au dessin annexé, sur lequel : The invention will be better understood on reading the description which follows, given solely by way of example and made with reference to the appended drawing, in which:
- la figure 1 est une vue schématique d’une installation selon l’invention adaptée pour mettre en œuvre un procédé selon l’invention ; et - Figure 1 is a schematic view of an installation according to the invention adapted to implement a method according to the invention; And
- la figure 2 est une vue schématique d’une installation selon l’invention constituant une variante de l’installation représentée sur la figure 1 . - Figure 2 is a schematic view of an installation according to the invention constituting a variant of the installation shown in Figure 1.
Dans tout ce qui suit, on désignera par les mêmes références un courant circulant dans une conduite et la conduite qui le transporte. Les termes « amont » et « aval » s’étendent généralement par rapport au sens normal de circulation d’un fluide. In all that follows, we will designate by the same references a current circulating in a pipe and the pipe which transports it. The terms “upstream” and “downstream” generally extend in relation to the normal direction of circulation of a fluid.
1 Nm3/h signifie dans le présent document un mètre cube par heure à une pression de 101325 Pa et une température de 0°C. 1 Nm 3 /h means in this document one cubic meter per hour at a pressure of 101325 Pa and a temperature of 0°C.
En référence à la figure 1 , on décrit une installation 10 selon l’invention. L’installation est adaptée pour liquéfier un gaz d’alimentation 12 comprenant au moins 40% en volume de méthane et obtenir un gaz liquéfié 14 (c’est-à-dire un liquide), par exemple en vue de sa commercialisation sur le marché du GNL (Gaz Naturel Liquéfié) de détail. With reference to Figure 1, an installation 10 according to the invention is described. The installation is suitable for liquefying a feed gas 12 comprising at least 40% by volume of methane and obtaining a liquefied gas 14 (that is to say a liquid), for example with a view to marketing it on the market. retail LNG (Liquefied Natural Gas).
Le gaz d’alimentation 12 est par exemple à basse pression, proche de la pression atmosphérique. Le gaz d’alimentation 12 est à une température proche de la température ambiante, c’est-à-dire largement plus chaude que sa température de bulle à pression atmosphérique (101325 Pa). The feed gas 12 is for example at low pressure, close to atmospheric pressure. The feed gas 12 is at a temperature close to ambient temperature, that is to say much hotter than its bubble temperature at atmospheric pressure (101325 Pa).
Le gaz d’alimentation 12 est par exemple un biogaz. The feed gas 12 is for example a biogas.
Le gaz liquéfié 14 est avantageusement stocké à une pression inférieure à 3 bars absolus (300 kPa). The liquefied gas 14 is advantageously stored at a pressure of less than 3 bar absolute (300 kPa).
Dans l’exemple, l’installation 10 comprend un mélangeur 16 pour mélanger le gaz d’alimentation 12 avec un premier flux de gaz recyclé 18 et obtenir un gaz à traiter 20. L’installation 10 comprend au moins un compresseur 22 pour comprimer le gaz à traiter 20, par exemple suivi d’un refroidisseur 24 et d’une unité d’épuration 26 adaptée pour épurer le gaz à traiter 20 et obtenir un gaz épuré 28. L’installation 10 comprend une première unité de pré-refroidissement 30 adaptée pour pré-refroidir le gaz épuré 28 et obtenir un premier gaz pré-refroidi 32, la première unité de pré-refroidissement incluant dans l’exemple un cycle frigorifique de pré-refroidissement 34. In the example, the installation 10 comprises a mixer 16 for mixing the feed gas 12 with a first stream of recycled gas 18 and obtaining a gas to be treated 20. The installation 10 comprises at least one compressor 22 for compressing the gas to be treated 20, for example followed by a cooler 24 and a purification unit 26 adapted to purify the gas to be treated 20 and obtain a purified gas 28. The installation 10 comprises a first pre-cooling unit 30 adapted to pre-cool the purified gas 28 and obtain a first pre-cooled gas 32, the first pre-cooling unit including in the example a pre-cooling refrigeration cycle. cooling 34.
Par « cycle frigorifique », on entend un ensemble de canalisations et d’éléments (non représentés), tels que des compresseurs ou des turbines, adaptés pour faire subir à un fluide une série de transformations dans le but de générer du froid à un endroit du cycle, de manière connue en elle-même. By “refrigeration cycle”, we mean a set of pipes and elements (not shown), such as compressors or turbines, adapted to subject a fluid to a series of transformations with the aim of generating cold at a location. of the cycle, in a manner known in itself.
L’installation 10 comprend une deuxième unité de pré-refroidissement 36 adaptée pour pré-refroidir le premier gaz pré-refroidi 32 par échange de chaleur avec un deuxième flux de gaz recyclé 38 et pour obtenir un deuxième gaz pré-refroidi 40 et le premier flux de gaz recyclé 18. The installation 10 comprises a second pre-cooling unit 36 adapted to pre-cool the first pre-cooled gas 32 by heat exchange with a second stream of recycled gas 38 and to obtain a second pre-cooled gas 40 and the first recycled gas flow 18.
L’installation 10 comprend une unité de liquéfaction 42 pour liquéfier le deuxième gaz pré-refroidi 40 et obtenir un courant de liquide 44, l’unité de liquéfaction incluant un cycle frigorifique de liquéfaction 46. The installation 10 comprises a liquefaction unit 42 for liquefying the second pre-cooled gas 40 and obtaining a stream of liquid 44, the liquefaction unit including a refrigeration liquefaction cycle 46.
L’installation 10 comprend une unité de sous-refroidissement 48 adaptée pour sous- refroidir le courant de liquide 44 à une température de sous-refroidissement par échange de chaleur avec au moins un troisième flux de gaz recyclé 50 et pour obtenir un courant de liquide sous-refroidi 52 et le deuxième flux de gaz recyclé 38. The installation 10 comprises a sub-cooling unit 48 adapted to sub-cool the liquid stream 44 to a sub-cooling temperature by heat exchange with at least a third recycled gas flow 50 and to obtain a liquid stream subcooled 52 and the second flow of recycled gas 38.
L’installation 10 comprend une unité de détente 54 pour détendre le courant de liquide sous-refroidi 52 et obtenir le gaz liquéfié 14, par exemple reçu dans un stockage 56, et le troisième flux de gaz recyclé 50. The installation 10 includes an expansion unit 54 for expanding the stream of sub-cooled liquid 52 and obtaining the liquefied gas 14, for example received in a storage 56, and the third flow of recycled gas 50.
Le compresseur 22 est adapté pour comprimer le gaz à traiter 20 à une pression de traitement comprise entre 19 et 70 bars absolus, qui rend le gaz à traiter 20, après épuration, liquéfiable à des températures cryogéniques, restant néanmoins supérieures à - 113°C. The compressor 22 is adapted to compress the gas to be treated 20 to a treatment pressure of between 19 and 70 bar absolute, which makes the gas to be treated 20, after purification, liquefiable at cryogenic temperatures, nevertheless remaining above - 113°C .
La pression de traitement est avantageusement inférieure à 45 bars absolus. The treatment pressure is advantageously less than 45 bar absolute.
L’unité d’épuration 26 est adaptée pour retirer du gaz à traiter 20 les composés susceptibles de cristalliser en aval. L’unité d’épuration 26 est classiquement adaptée pour éliminer les composés volatils et les hydrocarbures lourds (dits « C6+ »), par exemple grâce à des charbons actifs (non représentés et connus en eux-mêmes). Pour abaisser le taux d’eau jusqu’à quelques milliers de ppmv (partie par million, en volume), l’unité d’épuration 26 comprend par exemple un système de condensation (non-représenté). Pour abaisser le taux de CO2 à moins de 2,5%mol, un système membranaire (non représenté) est par exemple utilisé. Pour abaisser le taux de CO2 en dessous de 50 ppmv et le taux d’eau en dessous de 2 ppmv, des tamis moléculaires peuvent être utilisés (non représentés). Le gaz épuré 28 comporte au moins 90%, voire 99%, de méthane en volume.The purification unit 26 is adapted to remove from the gas to be treated the compounds likely to crystallize downstream. The purification unit 26 is conventionally adapted to eliminate volatile compounds and heavy hydrocarbons (called “C6+”), for example using activated carbons (not shown and known per se). To lower the water rate to a few thousand ppmv (part per million, by volume), the purification unit 26 includes for example a condensation system (not shown). To lower the CO2 level to less than 2.5% mol, a membrane system (not shown) is for example used. To lower the CO2 level below 50 ppmv and the water level below 2 ppmv, molecular sieves can be used (not shown). The purified gas 28 contains at least 90%, or even 99%, of methane by volume.
La première unité de pré-refroidissement 30 comprend par exemple un échangeur thermique 58 adapté pour réaliser un échange de chaleur entre le gaz épuré 28 et un fluide frigorifique 60 produit par le cycle frigorifique de pré-refroidissement 34, sans échange de chaleur avec le premier flux de gaz recyclé 18. The first pre-cooling unit 30 comprises for example a heat exchanger 58 adapted to carry out a heat exchange between the purified gas 28 and a refrigerating fluid 60 produced by the pre-cooling refrigeration cycle 34, without heat exchange with the first recycled gas flow 18.
Dans cet exemple, le cycle frigorifique de pré-refroidissement 34 est disjoint du cycle frigorifique de liquéfaction 46. Par « disjoint », on entend que les deux cycles frigorifiques ne partagent pas un fluide frigorifique qui leur serait commun. In this example, the pre-cooling refrigeration cycle 34 is disjoint from the liquefaction refrigeration cycle 46. By “disjoint” we mean that the two refrigeration cycles do not share a refrigerating fluid which would be common to them.
Le cycle frigorifique de pré-refroidissement 34 utilisé est par exemple un cycle à eau glycolée, un cycle au CO2, un cycle à ammoniac, un cycle au fréon, ou un cycle au propane, connus en eux-mêmes et qui ne seront pas décrits en détail. The pre-cooling refrigeration cycle 34 used is for example a brine cycle, a CO2 cycle, an ammonia cycle, a freon cycle, or a propane cycle, known in themselves and which will not be described. in detail.
La température du premier gaz pré-refroidi 32 est comprise entre -40°C et -15°C.The temperature of the first pre-cooled gas 32 is between -40°C and -15°C.
La deuxième unité de pré-refroidissement 36 comprend par exemple un échangeur thermique 62 pour réaliser l’échange de chaleur avec le deuxième flux de gaz recyclé 38. The second pre-cooling unit 36 comprises for example a heat exchanger 62 to carry out the heat exchange with the second flow of recycled gas 38.
L’unité de liquéfaction 42 comprend par exemple un échangeur thermique 64 adapté pour réaliser un échange de chaleur entre le deuxième gaz pré-refroidi 40 et un fluide frigorifique 66 produit par le cycle frigorifique de liquéfaction 46, sans échange de chaleur avec le deuxième flux de gaz recyclé 38. The liquefaction unit 42 comprises for example a heat exchanger 64 adapted to carry out a heat exchange between the second pre-cooled gas 40 and a refrigerating fluid 66 produced by the refrigerating liquefaction cycle 46, without heat exchange with the second flow of recycled gas 38.
Dans l’exemple, le cycle frigorifique de liquéfaction 46 est adapté pour apporter tout le froid nécessaire à l’unité de liquéfaction 42. In the example, the liquefaction refrigeration cycle 46 is adapted to provide all the cold necessary to the liquefaction unit 42.
Le cycle frigorifique de liquéfaction 46 est par exemple un cycle de Stirling. The refrigeration liquefaction cycle 46 is for example a Stirling cycle.
Par « cycle de Stirling », on entend ici un cycle frigorifique mis en œuvre par une machine de Stirling connue en elle-même de l’homme du métier. By “Stirling cycle”, we mean here a refrigeration cycle implemented by a Stirling machine known in itself to those skilled in the art.
En variante, le cycle frigorifique de liquéfaction 46 est par exemple un cycle Brayton inverse, également connu en lui-même de l’homme du métier. Alternatively, the refrigeration liquefaction cycle 46 is for example an inverse Brayton cycle, also known in itself to those skilled in the art.
L’unité de sous-refroidissement 48 et l’unité de détente 54 sont configurées pour que le troisième flux de gaz recyclé 50 représente une fraction, par rapport au courant de liquide sous-refroidi 52, inférieure à 35% molaire, et de préférence comprise entre 10% et 30% molaire. Ceci est notamment possible en abaissant suffisamment la température du courant de liquide sous-refroidi 52. The subcooling unit 48 and the expansion unit 54 are configured so that the third stream of recycled gas 50 represents a fraction, relative to the stream of subcooled liquid 52, less than 35 molar%, and preferably between 10% and 30 mol%. This is particularly possible by sufficiently lowering the temperature of the subcooled liquid stream 52.
Toujours dans l’exemple représenté sur la figure 1 , l’unité de sous-refroidissement 48 comprend un premier échangeur thermique 68, un deuxième échangeur thermique 70 et un cycle frigorifique de sous-refroidissement 72. Still in the example shown in Figure 1, the sub-cooling unit 48 comprises a first heat exchanger 68, a second heat exchanger 70 and a sub-cooling refrigeration cycle 72.
Le premier échangeur thermique 68 est adapté pour réaliser un premier sous- refroidissement du courant de liquide 44 par échange de chaleur avec le troisième flux de gaz recyclé 50, et pour obtenir un courant de liquide sous-refroidi intermédiaire 74 et le deuxième flux de gaz recyclé 38. The first heat exchanger 68 is adapted to carry out a first sub-cooling of the liquid stream 44 by heat exchange with the third flow of liquid. recycled gas 50, and to obtain an intermediate subcooled liquid stream 74 and the second stream of recycled gas 38.
Le deuxième échangeur thermique 70 est adapté pour réaliser un deuxième sous- refroidissement du courant de liquide sous-refroidi intermédiaire 74 pour obtenir le courant de liquide sous-refroidi 52, par échange de chaleur avec un fluide frigorifique 76 produit par le cycle frigorifique de sous-refroidissement 72, sans échange de chaleur avec le troisième flux de gaz recyclé 50. The second heat exchanger 70 is adapted to carry out a second sub-cooling of the intermediate sub-cooled liquid stream 74 to obtain the sub-cooled liquid stream 52, by heat exchange with a refrigerating fluid 76 produced by the sub-cooling refrigeration cycle. -cooling 72, without heat exchange with the third flow of recycled gas 50.
L’unité de détente 54 comprend avantageusement un organe de détente 78 pour détendre le courant de liquide sous-refroidi 52 et obtenir un courant sous-refroidi détendu 80, par exemple à une pression inférieure à 3 bars absolus. L’unité de détente 54 comprend par exemple un ballon de flash 82 pour séparer le courant sous-refroidi détendu 80 en le gaz liquéfié 14 et une vapeur formant le troisième flux de gaz recyclé 50. The expansion unit 54 advantageously comprises an expansion member 78 for expanding the stream of sub-cooled liquid 52 and obtaining a relaxed sub-cooled stream 80, for example at a pressure less than 3 bar absolute. The expansion unit 54 comprises for example a flash drum 82 to separate the expanded subcooled stream 80 into the liquefied gas 14 and a vapor forming the third stream of recycled gas 50.
L’organe de détente 78 est par exemple une vanne de Joule-Thomson ou une turbine à détente. The expansion member 78 is for example a Joule-Thomson valve or an expansion turbine.
Un procédé selon l’invention, mis en œuvre par l’installation 10, va maintenant être décrit rapidement. A method according to the invention, implemented by the installation 10, will now be described quickly.
Le gaz d’alimentation 12 et le premier flux de gaz recyclé 18 (i.e. la vapeur issue du ballon de flash 82, après réchauffages successifs dans l’unité de sous-refroidissement 48 puis dans la deuxième unité de pré-refroidissement 36) sont mélangés par le mélangeur 16 pour former le gaz à traiter 20. The feed gas 12 and the first flow of recycled gas 18 (i.e. the steam from the flash tank 82, after successive reheating in the sub-cooling unit 48 then in the second pre-cooling unit 36) are mixed by the mixer 16 to form the gas to be treated 20.
Le gaz à traiter 20 est comprimé dans le compresseur 22, puis refroidi environ à la température ambiante, par exemple 20°C, dans le refroidisseur 24. Puis le gaz à traiter 20 est épuré dans l’unité d’épuration 26 pour former le gaz épuré 28. The gas to be treated 20 is compressed in the compressor 22, then cooled to approximately ambient temperature, for example 20°C, in the cooler 24. Then the gas to be treated 20 is purified in the purification unit 26 to form the purified gas 28.
Le gaz épuré 28 subit un premier pré-refroidissement dans la première unité de prérefroidissement 30, par échange de chaleur avec le fluide réfrigérant 60, pour former le premier gaz pré-refroidi 32. The purified gas 28 undergoes a first pre-cooling in the first pre-cooling unit 30, by heat exchange with the refrigerant fluid 60, to form the first pre-cooled gas 32.
Le premier gaz pré-refroidi 32 subit un deuxième pré-refroidissement dans la deuxième unité de pré-refroidissement 36, par échange de chaleur avec le deuxième flux de gaz recyclé 38, pour former le deuxième gaz pré-refroidi 40. Le deuxième flux de gaz recyclé 38 se réchauffe et devient le premier flux de gaz recyclé 18. The first pre-cooled gas 32 undergoes a second pre-cooling in the second pre-cooling unit 36, by heat exchange with the second flow of recycled gas 38, to form the second pre-cooled gas 40. The second flow of recycled gas 38 heats up and becomes the first flow of recycled gas 18.
Le deuxième gaz pré-refroidi 40 est liquéfié dans l’unité de liquéfaction 42 et forme le courant de liquide 44. The second pre-cooled gas 40 is liquefied in the liquefaction unit 42 and forms the liquid stream 44.
Dit autrement, la deuxième unité 36 de pré-refroidissement ne réalise pas de liquéfaction. La liquéfaction est entièrement réalisée par l’unité de liquéfaction 42. In other words, the second pre-cooling unit 36 does not carry out liquefaction. Liquefaction is entirely carried out by liquefaction unit 42.
Le deuxième gaz pré-refroidi 40 est liquéfié par l’unité de liquéfaction 42, avec un sous-refroidissement avantageusement inférieur ou égal à 5°C, par exemple d’environ 3°C. Dit autrement, la température du courant de liquide 44 en sortie de l’unité de liquéfaction 42 est par exemple de 3°C en dessous de la température de bulle du deuxième gaz prérefroidi 40. La température du courant de liquide 44 en sortie de l’unité de liquéfaction 42 est de préférence comprise entre -90°C et -1 13°C. The second pre-cooled gas 40 is liquefied by the liquefaction unit 42, with subcooling advantageously less than or equal to 5°C, for example approximately 3°C. In other words, the temperature of the liquid stream 44 leaving the liquefaction unit 42 is for example 3°C below the bubble temperature of the second precooled gas 40. The temperature of the liquid stream 44 leaving the liquefaction unit The liquefaction unit 42 is preferably between -90°C and -113°C.
Le cycle frigorifique de liquéfaction 46 apporte avantageusement tout le froid permettant la liquéfaction du deuxième gaz pré-refroidi 40. The refrigeration liquefaction cycle 46 advantageously provides all the cold allowing the liquefaction of the second pre-cooled gas 40.
Le courant de liquide 44 est ensuite sous-refroidi dans l’unité de sous- refroidissement 48 pour former le courant de liquide sous-refroidi 52, par échange de chaleur avec au moins le troisième flux de gaz recyclé 50, c’est-à-dire la vapeur issue du ballon de flash 82. Le troisième flux de gaz recyclé 50 se réchauffe et devient le deuxième flux de gaz recyclé 38. The liquid stream 44 is then subcooled in the subcooling unit 48 to form the subcooled liquid stream 52, by heat exchange with at least the third recycled gas flow 50, i.e. i.e. the steam coming from the flash tank 82. The third flow of recycled gas 50 heats up and becomes the second flow of recycled gas 38.
Dans l’exemple, le courant de liquide 44 subit un premier sous-refroidissement dans le premier échangeur thermique 68 par échange de chaleur avec le troisième flux de gaz recyclé 50, puis un deuxième sous-refroidissement dans le deuxième échangeur thermique 70 par échange de chaleur avec le fluide réfrigérant 76 pour former le courant de liquide sous-refroidi 52. In the example, the liquid stream 44 undergoes a first subcooling in the first heat exchanger 68 by heat exchange with the third flow of recycled gas 50, then a second subcooling in the second heat exchanger 70 by heat exchange. heat with the refrigerant fluid 76 to form the subcooled liquid stream 52.
Le sous-refroidissement appliqué permet de réduire le taux d’évaporation en sortie du ballon de flash 82 à une valeur inférieure à 35% molaire. Avantageusement, le taux d’évaporation reste supérieur ou égal à 20% molaire. The subcooling applied makes it possible to reduce the evaporation rate at the outlet of the flash tank 82 to a value less than 35 mol%. Advantageously, the evaporation rate remains greater than or equal to 20 mol%.
Le courant de liquide sous-refroidi 52 est détendu dans l’organe de détente 78 pour former le courant sous-refroidi détendu 80, qui est reçu dans le ballon de flash 82. Le gaz liquéfié 14 est par exemple récupéré en pied du ballon de flash 82 et envoyé dans le stockage 56. The subcooled liquid stream 52 is expanded in the expansion member 78 to form the expanded subcooled stream 80, which is received in the flash tank 82. The liquefied gas 14 is for example recovered at the bottom of the flash tank. flash 82 and sent to storage 56.
La vapeur issue du ballon de flash 82 est recyclée dans le gaz à traiter 20. Cette vapeur forme le troisième flux de gaz recyclé 50, qui devient d’abord le deuxième flux de gaz recyclé 38 après son passage dans l’unité de sous-refroidissement 48, puis devient le premier flux de gaz recyclé 18 après son passage dans la deuxième unité de prérefroidissement 36. The steam from the flash drum 82 is recycled in the gas to be treated 20. This steam forms the third flow of recycled gas 50, which first becomes the second flow of recycled gas 38 after its passage through the sub-unit. cooling 48, then becomes the first flow of recycled gas 18 after its passage into the second pre-cooling unit 36.
Cette vapeur ne passe pas dans l’unité de liquéfaction 42, ou en tous cas cette vapeur ne cède pas de froid à l’unité de liquéfaction 42. This vapor does not pass into the liquefaction unit 42, or in any case this vapor does not release cold to the liquefaction unit 42.
Dans l’exemple représenté sur la figure 1 , cette vapeur ne passe pas non plus dans la première unité de pré-refroidissement 30, ou en tous cas ne cède pas de froid à la première unité de pré-refroidissement 30. In the example shown in Figure 1, this vapor does not pass into the first pre-cooling unit 30 either, or in any case does not transfer cold to the first pre-cooling unit 30.
Selon une variante non représentée, cette vapeur peut céder une partie de son froid dans l’unité de pré-refroidissement 30, notamment en fonction de la taille de l’installation 10. Par exemple, si la production de gaz liquéfié 14 est inférieure à 20 tonnes par jour, la récupération de froid dans la première unité de pré-refroidissement 30 en provenance du premier flux de gaz recyclé 18 sera évitée, comme représenté sur la figure 1 . According to a variant not shown, this steam can release part of its cold into the pre-cooling unit 30, in particular depending on the size of the installation 10. For example, if the production of liquefied gas 14 is less than 20 tonnes per day, the cold recovery in the first pre-cooling unit 30 coming from the first flow of recycled gas 18 will be avoided, as shown in Figure 1.
En revanche, si la production de gaz liquéfié 14 est supérieure ou égale à 20 tonnes par jour par exemple, cette récupération sera préférée. Pour la réaliser, on fait par exemple passer le premier flux de gaz recyclé 18 dans la première unité de pré-refroidissement 30. On the other hand, if the production of liquefied gas 14 is greater than or equal to 20 tonnes per day for example, this recovery will be preferred. To achieve this, for example, the first stream of recycled gas 18 is passed through the first pre-cooling unit 30.
En référence à la figure 2, on décrit une installation 100 selon l’invention constituant une variante de l’installation 10. L’installation 100 est analogue à l’installation 10 représentée sur la figure 1. Les éléments similaires portent les mêmes références numériques et ne seront pas décrits à nouveau. Seules les différences seront décrites en détail ci-après. With reference to Figure 2, an installation 100 according to the invention is described constituting a variant of the installation 10. The installation 100 is similar to the installation 10 shown in Figure 1. Similar elements bear the same numerical references and will not be described again. Only the differences will be described in detail below.
Dans l’installation 100, le premier pré-refroidissement du gaz épuré 28 est réalisé par échange de chaleur avec un cycle frigorifique de liquéfaction 146. Dit autrement, l’unité de liquéfaction 42 et la première unité de pré-refroidissement 30 partagent un même cycle frigorifique 146, qui apporte le froid utilisé pour le premier pré-refroidissement et pour la liquéfaction. In the installation 100, the first pre-cooling of the purified gas 28 is carried out by heat exchange with a refrigeration liquefaction cycle 146. In other words, the liquefaction unit 42 and the first pre-cooling unit 30 share the same refrigeration cycle 146, which provides the cold used for the first pre-cooling and for liquefaction.
Le fluide frigorifique 66 cède du froid au deuxième gaz pré-refroidi 40 dans l’échangeur thermique 64 de l’unité de liquéfaction 42, et devient le fluide frigorifique 60. Le fluide frigorifique 60 cède du froid au gaz épuré 28 dans l’échangeur thermique 58 de la première unité de pré-refroidissement 30. The refrigerating fluid 66 gives up cold to the second pre-cooled gas 40 in the heat exchanger 64 of the liquefaction unit 42, and becomes the refrigerating fluid 60. The refrigerating fluid 60 gives up cold to the purified gas 28 in the exchanger thermal 58 of the first pre-cooling unit 30.
Comme dans l’installation 10, la deuxième unité de pré-refroidissement 36 de l’installation 100 ne reçoit pas de froid du cycle frigorifique 146. As in installation 10, the second pre-cooling unit 36 of installation 100 does not receive cold from the refrigeration cycle 146.
En variante ou en complément, dans l’unité de sous-refroidissement 48, le deuxième échangeur thermique 70 ne reçoit pas de froid d’un cycle frigorifique dédié au sous- refroidissement, mais d’une boucle ouverte 172 à l’azote liquide. Alternatively or in addition, in the sub-cooling unit 48, the second heat exchanger 70 does not receive cold from a refrigeration cycle dedicated to sub-cooling, but from an open loop 172 with liquid nitrogen.
Dans la boucle ouverte 172, un flux d’azote liquide 174 (provenant d’une source non représentée, telle qu’un stockage d’azote liquide) cède du froid au courant de liquide sous- refroidi intermédiaire 74 dans le deuxième échangeur thermique 70 de l’unité de sous- refroidissement 48, et se vaporise pour devenir un flux d’azote gazeux 176. In the open loop 172, a flow of liquid nitrogen 174 (coming from a source not shown, such as a liquid nitrogen storage) transfers cold to the stream of intermediate subcooled liquid 74 in the second heat exchanger 70 of the sub-cooling unit 48, and vaporizes to become a flow of nitrogen gas 176.
Avantageusement, le flux d’azote gazeux 176 cède ensuite du froid au premier gaz pré-refroidi 32 dans l’échangeur thermique 62, et devient un flux d’azote 178. Advantageously, the flow of nitrogen gas 176 then transfers cold to the first pre-cooled gas 32 in the heat exchanger 62, and becomes a flow of nitrogen 178.
Selon un mode de réalisation particulier, un appoint 180 d’azote est réalisé dans le flux d’azote gazeux 176 avant l’entrée de ce dernier dans la deuxième unité de prérefroidissement 36. Cet appoint 180 a une température inférieure à celle du flux d’azote gazeux 176 avant l’appoint. L’appoint 180 est avantageusement réalisée sous forme liquide. Ainsi, le deuxième pré-refroidissement est réalisé par échange de chaleur, d’une part, avec le deuxième flux de gaz recyclé 38 et, d’autre part, avantageusement avec le flux d’azote gazeux 176, éventuellement augmenté par l’appoint 180. According to a particular embodiment, a nitrogen make-up 180 is made in the gaseous nitrogen flow 176 before the latter enters the second pre-cooling unit 36. This make-up 180 has a temperature lower than that of the flow of nitrogen. nitrogen gas 176 before topping up. The supplement 180 is advantageously produced in liquid form. Thus, the second pre-cooling is carried out by heat exchange, on the one hand, with the second flow of recycled gas 38 and, on the other hand, advantageously with the flow of gaseous nitrogen 176, possibly increased by the make-up 180.
Le fonctionnement de l’installation 100 est analogue à celui de l’installation 10.The operation of installation 100 is similar to that of installation 10.
Selon une autre variante encore (non représentée), le deuxième échangeur thermique 70 de l’unité de sous-refroidissement 48 est absent, ainsi que le cycle frigorifique de sous-refroidissement 72 (figure 1 ) ou la boucle ouverte 172 (figure2). Le sous- refroidissement réalisé par l’unité de sous-refroidissement 48 se fait alors uniquement dans le premier échangeur 68, par échange de chaleur avec le troisième flux de gaz recyclé 50. According to yet another variant (not shown), the second heat exchanger 70 of the sub-cooling unit 48 is absent, as well as the sub-cooling refrigeration cycle 72 (figure 1) or the open loop 172 (figure 2). The sub-cooling carried out by the sub-cooling unit 48 is then carried out only in the first exchanger 68, by heat exchange with the third flow of recycled gas 50.
Exemples Examples
Ces exemples comprennent l’une ou plusieurs des caractéristiques suivantes, selon toutes les combinaisons possibles. These examples include one or more of the following characteristics, in any possible combination.
La pression de traitement est de 40 bars absolus (pression du gaz à traiter après compression par le compresseur 22). The treatment pressure is 40 bars absolute (pressure of the gas to be treated after compression by compressor 22).
Le deuxième gaz pré-refroidi 40 a une température de -53,5°C. The second pre-cooled gas 40 has a temperature of -53.5°C.
Le courant de liquide 44, en sortie de l’unité de liquéfaction 42, a une température de -90°C. The liquid stream 44, leaving the liquefaction unit 42, has a temperature of -90°C.
L’unité de sous-refroidissement 48 et l’unité de détente 54 sont configurées pour obtenir un taux d’évaporation compris entre 20% et 30% molaire. The subcooling unit 48 and the expansion unit 54 are configured to obtain an evaporation rate of between 20% and 30 molar%.
Les deux tableaux suivants définissent quatre cas et permettent de les comparer entre eux : The following two tables define four cases and allow them to be compared with each other:
- cas 1 (contre-exemple) : pas de récupération du froid contenu dans le gaz de flash (le troisième flux de gaz 50 ne passe pas dans les échangeurs 68 et 62), ni boucle ouverte 172 à l’azote liquide ; - case 1 (counter-example): no recovery of the cold contained in the flash gas (the third gas flow 50 does not pass through the exchangers 68 and 62), nor open loop 172 with liquid nitrogen;
- cas 2 (exemple selon l’invention) : avec récupération du froid contenu dans le gaz de flash (le troisième flux de gaz 50 passe dans les échangeurs 68 et 62), mais pas de boucle ouverte 172 à l’azote liquide. Le cas 2 correspond sensiblement à la figure 1 ; - case 2 (example according to the invention): with recovery of the cold contained in the flash gas (the third gas flow 50 passes through the exchangers 68 and 62), but no open loop 172 with liquid nitrogen. Case 2 corresponds substantially to Figure 1;
- cas 3 (contre-exemple) : pas de récupération du froid contenu dans le gaz de flash (le troisième flux de gaz 50 ne passe pas dans les échangeurs 68 et 62), mais présence de la boucle ouverte 172 à l’azote liquide ; et - case 3 (counter-example): no recovery of the cold contained in the flash gas (the third gas flow 50 does not pass through the exchangers 68 and 62), but presence of the open loop 172 with liquid nitrogen ; And
- cas 4 (exemple selon l’invention) : avec récupération du froid contenu dans le gaz de flash (le troisième flux de gaz 50 passe dans les échangeurs 68 et 62), et présence de la boucle ouverte 172 à l’azote liquide. Le cas 4 correspond sensiblement à la figure 2.
Figure imgf000014_0001
Figure imgf000014_0002
- case 4 (example according to the invention): with recovery of the cold contained in the flash gas (the third gas flow 50 passes through the exchangers 68 and 62), and presence of the loop 172 open to liquid nitrogen. Case 4 corresponds substantially to Figure 2.
Figure imgf000014_0001
Figure imgf000014_0002
Le cas 1 représente un procédé simple, à savoir juste une liquéfaction sans sous- refroidissement et sans récupération de froid sur le flash. On cherche ensuite à évaluer les gains énergétiques issus de l’ajout progressif de systèmes de récupération du froid sur le gaz de flash et de système de sous-refroidissement à l’azote liquide. Case 1 represents a simple process, namely just liquefaction without subcooling and without cold recovery on the flash. We then seek to evaluate the energy gains resulting from the progressive addition of cold recovery systems using flash gas and liquid nitrogen sub-cooling systems.
Pour évaluer ce gain, en première approche, on considère la seule charge thermique de l’unité de liquéfaction 42, car cette partie est la plus coûteuse du procédé. De plus, on utilise la charge thermique plutôt que la puissance mécanique consommée par le cycle de liquéfaction, car la puissance mécanique dépend du type de cycle utilisé (Brayton inverse, MR, Stirling, ... etc.), or on s’intéresse au gain énergétique indépendamment du type de cycle de liquéfaction. To evaluate this gain, as a first approach, we consider only the thermal load of the liquefaction unit 42, because this part is the most expensive of the process. In addition, we use the thermal load rather than the mechanical power consumed by the liquefaction cycle, because the mechanical power depends on the type of cycle used (reverse Brayton, MR, Stirling, etc.), but we are interested to the energy gain independently of the type of liquefaction cycle.
En conclusion, le cas 4 permet de réduire la taille de l’unité de liquéfaction de 44%, d’où une réduction significative du coût de production global du gaz liquéfié. In conclusion, case 4 makes it possible to reduce the size of the liquefaction unit by 44%, resulting in a significant reduction in the overall production cost of liquefied gas.
Grâce aux caractéristiques décrites ci-dessus, le procédé permet de réduire le coût de production global du gaz liquéfié 14, en particulier pour des capacités de production inférieures à 20 tonnes par jour. En effet, la pression de traitement, comprise entre 19 et 70 bars absolus, est suffisamment élevée pour que la température de liquéfaction ne soit pas trop basse, i.e. de préférence supérieure à -90°C. Ainsi, les équipements mis en œuvre sont moins spécifiques et moins onéreux. Le poids de l’investissement étant important pour les petites capacités, ceci a un impact favorable sur le coût de production unitaire. De plus, l’énergie dépensée pour apporter du froid est également plus faible lorsque la température du fluide à refroidir est moins basse. Thanks to the characteristics described above, the process makes it possible to reduce the overall production cost of liquefied gas 14, in particular for production capacities of less than 20 tonnes per day. Indeed, the treatment pressure, between 19 and 70 bar absolute, is sufficiently high so that the liquefaction temperature is not too low, ie preferably greater than -90°C. Thus, the equipment used is less specific and less expensive. The weight of the investment being significant for small capacities, this has a favorable impact on the unit production cost. In addition, the energy spent to provide cold is also lower when the temperature of the fluid to be cooled is lower.
Toutefois, la pression de traitement reste relativement basse et autorise une température de sous-refroidissement suffisamment basse, qui maintient la fraction volumique de gaz de flash recyclé en amont du compresseur dans une proportion raisonnable, ce qui réduit l’énergie dépensée pour la compression du gaz à traiter 20. De plus, la pression moins élevée permet aussi des économies sur les équipements, qui n’ont pas à résister à de très hautes pressions. However, the processing pressure remains relatively low and allows a sufficiently low subcooling temperature, which maintains the volume fraction of flash gas recycled upstream of the compressor in a reasonable proportion, which reduces the energy spent for compressing the gas to be treated 20. In addition, the lower pressure also allows savings on equipment, which does not have to withstand very high pressures.
En outre, le froid présent dans le gaz de flash recyclé (troisième flux de gaz recyclé 50) est utilisé spécifiquement pour amplifier le pré-refroidissement du gaz à traiter et son sous-refroidissement. Ce froid n’est pas utilisé dans l’unité de liquéfaction 42. Ceci réduit la plage de refroidissement de l’unité de liquéfaction 42, et réduit la taille des cycles frigorifiques de liquéfaction 46, 146. De plus, ceci évite d’avoir à modifier les cycles frigorifiques de liquéfaction eux-mêmes pour intégrer un flux de gaz recyclé. In addition, the cold present in the recycled flash gas (third recycled gas flow 50) is used specifically to amplify the pre-cooling of the gas to be treated and its sub-cooling. This cold is not used in the liquefaction unit 42. This reduces the cooling range of the liquefaction unit 42, and reduces the size of the liquefaction refrigeration cycles 46, 146. In addition, this avoids having to to modify the refrigeration liquefaction cycles themselves to integrate a flow of recycled gas.
Le sous-refroidissement est avantageusement réalisé à l’azote liquide, dont le froid est par exemple utilisé aussi pour amplifier le pré-refroidissement. The sub-cooling is advantageously carried out with liquid nitrogen, the cold of which is for example also used to amplify the pre-cooling.
L’utilisation du froid du gaz de flash, et éventuellement de l’azote liquide, n’est pas « étalée » tout au long de la plage de refroidissement comme dans certaines solutions de l’art antérieur, mais au contraire concentrée sur le pré-refroidissement et le sous- refroidissement, ce qui permet de réduire spécifiquement les parties les plus coûteuses du procédé de refroidissement, notamment l’unité de liquéfaction 42. The use of cold flash gas, and possibly liquid nitrogen, is not “spread” throughout the cooling range as in certain solutions of the prior art, but on the contrary concentrated on the pre -cooling and sub-cooling, which makes it possible to specifically reduce the most expensive parts of the cooling process, in particular the liquefaction unit 42.

Claims

REVENDICATIONS
1. Procédé de liquéfaction d’un gaz d’alimentation (12) comprenant au moins 40% en volume de méthane, le procédé comprenant les étapes suivantes : 1. Process for liquefying a feed gas (12) comprising at least 40% by volume of methane, the process comprising the following steps:
- mélange du gaz d’alimentation (12) avec un premier flux de gaz recyclé (18) pour obtenir un gaz à traiter (20), et compression du gaz à traiter (20) à une pression de traitement comprise entre 19 et 70 bars absolus, - mixing the feed gas (12) with a first flow of recycled gas (18) to obtain a gas to be treated (20), and compression of the gas to be treated (20) at a treatment pressure of between 19 and 70 bars absolute,
- épuration du gaz à traiter (20) pour obtenir un gaz épuré (28), - purification of the gas to be treated (20) to obtain a purified gas (28),
- premier pré-refroidissement du gaz épuré (28) pour obtenir un premier gaz prérefroidi (32) ayant une température inférieure ou égale à -15°C et supérieure ou égale à - 40°C, - first pre-cooling of the purified gas (28) to obtain a first pre-cooled gas (32) having a temperature less than or equal to -15°C and greater than or equal to - 40°C,
- deuxième pré-refroidissement du premier gaz pré-refroidi (32) par échange de chaleur avec au moins un deuxième flux de gaz recyclé (38) pour obtenir un deuxième gaz pré-refroidi (40) et le premier flux de gaz recyclé (18), - second pre-cooling of the first pre-cooled gas (32) by heat exchange with at least a second flow of recycled gas (38) to obtain a second pre-cooled gas (40) and the first flow of recycled gas (18) ),
- liquéfaction du deuxième gaz pré-refroidi (40) pour obtenir un courant de liquide, (44) par échange de chaleur uniquement avec un cycle frigorifique de liquéfaction (46 ; 146), - liquefaction of the second pre-cooled gas (40) to obtain a liquid stream, (44) by heat exchange only with a refrigeration liquefaction cycle (46; 146),
- sous-refroidissement du courant de liquide (44) par échange de chaleur avec au moins un troisième flux de gaz recyclé (50) pour obtenir un courant de liquide sous-refroidi (52) et le deuxième flux de gaz recyclé (38), le courant de liquide sous-refroidi (52) étant à une température de sous-refroidissement, et - subcooling of the liquid stream (44) by heat exchange with at least a third recycled gas flow (50) to obtain a subcooled liquid flow (52) and the second recycled gas flow (38), the subcooled liquid stream (52) being at a subcooling temperature, and
- détente du courant de liquide sous-refroidi (52) pour obtenir un gaz liquéfié (14) et le troisième flux de gaz recyclé (50), ladite détente et la température de sous- refroidissement étant adaptées pour que le troisième flux de gaz recyclé (50) représente une fraction, par rapport au courant de liquide sous-refroidi (52), inférieure à 35% molaire. - expansion of the subcooled liquid stream (52) to obtain a liquefied gas (14) and the third flow of recycled gas (50), said expansion and the subcooling temperature being adapted so that the third flow of recycled gas (50) represents a fraction, relative to the subcooled liquid stream (52), less than 35 mol%.
2. Procédé selon la revendication 1 , dans lequel le sous-refroidissement du courant de liquide (44) comprend : 2. Method according to claim 1, in which the subcooling of the liquid stream (44) comprises:
- un premier sous-refroidissement du courant de liquide (44) par échange de chaleur avec le troisième flux de gaz recyclé (50), pour obtenir un courant de liquide sous-refroidi intermédiaire (74) et le deuxième flux de gaz recyclé (38), et - a first subcooling of the liquid stream (44) by heat exchange with the third recycled gas flow (50), to obtain an intermediate subcooled liquid flow (74) and the second recycled gas flow (38 ), And
- un deuxième sous-refroidissement du courant de liquide sous-refroidi intermédiaire (74) pour obtenir le courant de liquide sous-refroidi (52). - a second subcooling of the intermediate subcooled liquid stream (74) to obtain the subcooled liquid stream (52).
3. Procédé selon la revendication 2, dans lequel le deuxième sous-refroidissement du courant de liquide sous-refroidi intermédiaire (74) est réalisé par échange avec un flux d’azote liquide (174), le deuxième sous-refroidissement produisant le courant de liquide sous-refroidi (52) et un flux d’azote vaporisé (176), le deuxième pré-refroidissement du premier gaz pré-refroidi (32) étant réalisé en outre par échange de chaleur le flux d’azote vaporisé. (176) 3. Method according to claim 2, wherein the second subcooling of the intermediate subcooled liquid stream (74) is carried out by exchange with a flow liquid nitrogen (174), the second sub-cooling producing the sub-cooled liquid stream (52) and a stream of vaporized nitrogen (176), the second pre-cooling of the first pre-cooled gas (32) being further achieved by heat exchange the flow of vaporized nitrogen. (176)
4. Procédé selon l’une quelconque des revendications 1 à 3, dans lequel le gaz épuré (28) subit le premier pré-refroidissement dans une première unité de prérefroidissement (30) incluant un cycle frigorifique de pré-refroidissement (34), par échange de chaleur avec un fluide réfrigérant (60) pour former le premier gaz pré-refroidi (32) sans échange de chaleur avec le premier flux de gaz recyclé (18), le fluide frigorifique (60) étant produit par le cycle frigorifique de pré-refroidissement (34), le cycle frigorifique de prérefroidissement (34) et le cycle frigorifique de liquéfaction (46) étant disjoints. 4. Method according to any one of claims 1 to 3, in which the purified gas (28) undergoes the first pre-cooling in a first pre-cooling unit (30) including a pre-cooling refrigeration cycle (34), by heat exchange with a refrigerant fluid (60) to form the first pre-cooled gas (32) without heat exchange with the first recycled gas stream (18), the refrigerant fluid (60) being produced by the pre-cooled refrigeration cycle -cooling (34), the pre-cooling refrigeration cycle (34) and the liquefaction refrigeration cycle (46) being disjointed.
5. Procédé selon l’une quelconque des revendications 1 à 4, dans lequel la pression de traitement est inférieure à 45 bars absolus. 5. Method according to any one of claims 1 to 4, in which the treatment pressure is less than 45 bar absolute.
6. Procédé selon l’une quelconque des revendications 1 à 5, dans lequel le courant de liquide (44) en sortie de l’unité de liquéfaction (42) a une température comprise entre - 113°C et -90°C. 6. Method according to any one of claims 1 to 5, in which the liquid stream (44) leaving the liquefaction unit (42) has a temperature between - 113°C and -90°C.
7. Procédé selon l’une quelconque des revendications 1 à 6, dans lequel le deuxième gaz pré-refroidi (40) est liquéfié par l’unité de liquéfaction (42) avec un sous- refroidissement inférieur ou égal à 5°C. 7. Method according to any one of claims 1 to 6, in which the second pre-cooled gas (40) is liquefied by the liquefaction unit (42) with sub-cooling less than or equal to 5°C.
8. Procédé selon l’une quelconque des revendications 1 à 7, dans lequel le cycle frigorifique de liquéfaction (46 ; 146) est un cycle de Stirling ou un cycle de Brayton inverse. 8. Method according to any one of claims 1 to 7, in which the refrigeration liquefaction cycle (46; 146) is a Stirling cycle or an inverse Brayton cycle.
9. Procédé selon l’une quelconque des revendications 1 à 8, dans lequel la détente du courant de liquide sous-refroidi (52) est réalisée dans au moins une vanne Joule- Thomson ou par turbine à détente. 9. Method according to any one of claims 1 to 8, in which the expansion of the subcooled liquid stream (52) is carried out in at least one Joule-Thomson valve or by expansion turbine.
10. Installation (10 ; 100) adaptée pour mettre en œuvre un procédé selon l’une quelconque des revendications 1 à 9, comprenant : 10. Installation (10; 100) adapted to implement a method according to any one of claims 1 to 9, comprising:
- un mélangeur (16) pour mélanger le gaz d’alimentation (12) avec le premier flux de gaz recyclé (18) et obtenir le gaz à traiter (20), et au moins un compresseur (22) adapté pour comprimer le gaz à traiter (20) à la pression de traitement, - a mixer (16) for mixing the feed gas (12) with the first flow of recycled gas (18) and obtaining the gas to be treated (20), and at least one compressor (22) adapted to compress the gas to be treat (20) at the treatment pressure,
- une unité d’épuration (26) adaptée pour épurer le gaz à traiter (20) et obtenir le gaz épuré (28), - une première unité de pré-refroidissement (30) adaptée pour pré-refroidir le gaz épuré (28) et obtenir le premier gaz pré-refroidi (32), - a purification unit (26) adapted to purify the gas to be treated (20) and obtain the purified gas (28), - a first pre-cooling unit (30) adapted to pre-cool the purified gas (28) and obtain the first pre-cooled gas (32),
- une deuxième unité de pré-refroidissement (36) adaptée pour pré-refroidir le premier gaz pré-refroidi (32) par échange de chaleur avec au moins le deuxième flux de gaz recyclé (38) et pour obtenir le deuxième gaz pré-refroidi (40) et le premier flux de gaz recyclé (18), - a second pre-cooling unit (36) adapted to pre-cool the first pre-cooled gas (32) by heat exchange with at least the second recycled gas flow (38) and to obtain the second pre-cooled gas (40) and the first stream of recycled gas (18),
- une unité de liquéfaction (42) pour liquéfier le deuxième gaz pré-refroidi (40) et obtenir le courant de liquide (44), l’unité de liquéfaction (42) incluant le cycle frigorifique de liquéfaction (46 ; 146), - une unité de sous-refroidissement (48) adaptée pour sous-refroidir le courant de liquide (44) à la température de sous-refroidissement par échange de chaleur avec au moins le troisième flux de gaz recyclé (50) et pour obtenir le courant de liquide sous-refroidi (52) et le deuxième flux de gaz recyclé (38), et - a liquefaction unit (42) for liquefying the second pre-cooled gas (40) and obtaining the liquid stream (44), the liquefaction unit (42) including the liquefaction refrigeration cycle (46; 146), - a subcooling unit (48) adapted to subcool the liquid stream (44) to the subcooling temperature by heat exchange with at least the third recycled gas stream (50) and to obtain the current of subcooled liquid (52) and the second stream of recycled gas (38), and
- une unité de détente (54) pour détendre le courant de liquide sous-refroidi (52) et obtenir le gaz liquéfié (14) et le troisième flux de gaz recyclé (50), l’unité de sous- refroidissement et l’unité de détente étant configurées pour que le troisième flux de gaz recyclé (50) représente une fraction, par rapport au courant de liquide sous-refroidi (52), inférieure à 35% molaire. - an expansion unit (54) for expanding the subcooled liquid stream (52) and obtaining the liquefied gas (14) and the third stream of recycled gas (50), the subcooling unit and the unit expansion being configured so that the third stream of recycled gas (50) represents a fraction, relative to the stream of sub-cooled liquid (52), less than 35 mol%.
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