US3837821A - Elevating natural gas with reduced calorific value to distribution pressure - Google Patents

Elevating natural gas with reduced calorific value to distribution pressure Download PDF

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US3837821A
US3837821A US23913672A US3837821A US 3837821 A US3837821 A US 3837821A US 23913672 A US23913672 A US 23913672A US 3837821 A US3837821 A US 3837821A
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natural gas
intermediate pressure
compressed
methane
gas
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J Buffiere
M Grenier
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Air Liquide SA
LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
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Air Liquide SA
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C9/00—Methods or apparatus for discharging liquefied or solidified gases from vessels not under pressure
    • F17C9/02—Methods or apparatus for discharging liquefied or solidified gases from vessels not under pressure with change of state, e.g. vaporisation
    • F17C9/04—Recovery of thermal energy
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/0204—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the feed stream
    • F25J3/0209—Natural gas or substitute natural gas
    • F25J3/0214—Liquefied natural gas
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/0228—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream
    • F25J3/0233—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream separation of CnHm with 1 carbon atom or more
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/0228—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream
    • F25J3/0238—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream separation of CnHm with 2 carbon atoms or more
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/0228—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream
    • F25J3/0242—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream separation of CnHm with 3 carbon atoms or more
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/0228—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream
    • F25J3/0247—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream separation of CnHm with 4 carbon atoms or more
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C2221/00—Handled fluid, in particular type of fluid
    • F17C2221/03—Mixtures
    • F17C2221/032—Hydrocarbons
    • F17C2221/033—Methane, e.g. natural gas, CNG, LNG, GNL, GNC, PLNG
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C2223/00—Handled fluid before transfer, i.e. state of fluid when stored in the vessel or before transfer from the vessel
    • F17C2223/01—Handled fluid before transfer, i.e. state of fluid when stored in the vessel or before transfer from the vessel characterised by the phase
    • F17C2223/0107—Single phase
    • F17C2223/0123—Single phase gaseous, e.g. CNG, GNC
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C2223/00—Handled fluid before transfer, i.e. state of fluid when stored in the vessel or before transfer from the vessel
    • F17C2223/01—Handled fluid before transfer, i.e. state of fluid when stored in the vessel or before transfer from the vessel characterised by the phase
    • F17C2223/0146—Two-phase
    • F17C2223/0153—Liquefied gas, e.g. LPG, GPL
    • F17C2223/0161—Liquefied gas, e.g. LPG, GPL cryogenic, e.g. LNG, GNL, PLNG
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C2265/00—Effects achieved by gas storage or gas handling
    • F17C2265/01—Purifying the fluid
    • F17C2265/015—Purifying the fluid by separating
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2200/00—Processes or apparatus using separation by rectification
    • F25J2200/02—Processes or apparatus using separation by rectification in a single pressure main column system
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2200/00—Processes or apparatus using separation by rectification
    • F25J2200/04—Processes or apparatus using separation by rectification in a dual pressure main column system
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2200/00—Processes or apparatus using separation by rectification
    • F25J2200/70—Refluxing the column with a condensed part of the feed stream, i.e. fractionator top is stripped or self-rectified
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2200/00—Processes or apparatus using separation by rectification
    • F25J2200/72—Refluxing the column with at least a part of the totally condensed overhead gas
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2205/00—Processes or apparatus using other separation and/or other processing means
    • F25J2205/02—Processes or apparatus using other separation and/or other processing means using simple phase separation in a vessel or drum
    • 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
    • F25J2205/00—Processes or apparatus using other separation and/or other processing means
    • F25J2205/02—Processes or apparatus using other separation and/or other processing means using simple phase separation in a vessel or drum
    • F25J2205/04—Processes or apparatus using other separation and/or other processing means using simple phase separation in a vessel or drum in the feed line, i.e. upstream of the fractionation step
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2215/00—Processes characterised by the type or other details of the product stream
    • F25J2215/64—Propane or 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
    • F25J2245/00—Processes or apparatus involving steps for recycling of process streams
    • F25J2245/90—Processes or apparatus involving steps for recycling of process streams the recycled stream being boil-off gas from storage

Definitions

  • the present invention has for its object a method of production under high pressure of a gas rich in methane, to which is added a volatile gaseous fraction, in which the natural gas is brought up to an intermediate pressure and the volatile gaseous fraction is added to the natural gas at the said intermediate pressure.
  • the method of production according to the present invention is equally well applicable to the case of liquid natural gas in which the content of hydrocarbons heavier than methane is relatively high, as to the case of liquid natural gas having a low content of hydrocarbons heavier than methane, that is to say in which it would not be necessary to separate out the hydrocarbons heavier than methane from the natural gas.
  • liquid natural gas having a relativelyhigh content of hydrocarbons heavier than methane it is known that before sending the natural gas under high-pressure into a distribution system, it is desirable to eliminate from it the relatively heavy hydrocarbons such as propane, butanes and less volatile homologues, and also sometimes a part of the ethane.
  • the natural gas is obtained by re-vaporization of natural gas in the liquid state under low pressure, the liquid natural gas is subjected to a partial vaporization, followed by rectification of the residual liquid fraction, so as to separate out a residual liquid consisting of hydrocarbons of C and higher, with sometimes a part of the initial ethane, substantially free from methane.
  • the gas which is compressed and sent into a distribution system has sometimes however a calorific power which is too high for the use for which it is intended, whether this use is industrial or domestic.
  • This calorific power can be reduced by the addition of a noncombustible gas such as air or nitrogen, that is to say of a gaseous fraction which is more volatile than methane.
  • the liquid natural gas can be brought up to the pressure of the system, at least partly in the liquid phase, and therefore with a small expenditure of power
  • the more volatile gaseous fraction necessitates a compressor having a large output from the initial pressure of this gas, generally in the vicinity of atmospheric pressure, up to the high pressure of the distribution system and which therefore has a large number of compression stages.
  • This compressor is costly and necessitates close supervision and a high consumption of power.
  • vapours liberated in the storage tanks of liquefied natural gas resulting from the passage of heat through the lagging may also be recovered by recompressing them in the same compressor as the noncombustible gas when a non-combustible gas is added to the natural gas in order to reduce its calorific power, but this recompression requires an additional expenditure of power.
  • the method of the invention has for its object to remedy the disadvantages indicated above, and to permit the addition to the gas, freed from relatively little volatile hydrocarbons, of a more volatile gaseous fraction constituted by a non-combustible gas, to which may possibly be added the vapours evolved in the storage tanks of the natural gas in the liquid state, with a small expenditure of energy, by means of a compressor having a low ratio of the suction and delivery pressures and consequently comprising a small number of compression stages.
  • the volatile gaseous fraction is added to the natural gas in the liquid state, after which the mixture obtained is brought up to the pressure of the distribution system, reheated and vaporized.
  • the volatile gaseous fraction consists wholly or partly of vapours rich in methane in pressure equilibrium with the liquid natural gas.
  • the volatile gaseous fraction consists, wholly or partly. of a non-combustible gaseous fraction such as air or nitrogen, that is to say of a gaseous fraction more volatile than methane.
  • the volatile gaseous fraction consists, at least in part, of a vapour rich in methane and ethane obtained from liquid natural gas under low pressure, partly vaporized, from which there is separated a residual liquid fraction containing at least a part of the hydrocarbons heavier than methane, the said vapour rich in methane and ethane being re-liquefied by exchange of heat with the natural gas during the course of fractionated vaporization, and is then brought-up by a pump to the high-pressure of production and reheated.
  • the volatile gaseous fraction formed by vapours rich in methane in pressure equilibrium with the liquid natural gas and the non-combustible gaseous fraction intended to be added to the gas rich in methane is only compressed up to an intermediate pressure and is added to at least part of the said vapour before the re-liquefaction of this latter, the said intermediate pressure being such that the mixture of the said vapours and non-combustible gaseous fraction is wholly lequefled during the course of its heat exchange with the natural gas during the fractionated vaporization.
  • the intermediate pressure is lower than the highest level of pressure at which is effected the partial vaporization of the natural gas, and the residual liquid fraction is expanded to this intermediate pressure before being subjected to separation into a liquid containing at least part of the hydrocarbons heavier than methane and the said vapour rich in methane and ethane.
  • At least part of the volatile gaseous frac tion is compressed prior to the addition to the liquid natural gas.
  • the volatile gaseous fraction is cooled before it is added to the liquid natural gas.
  • the cooling is obtained by exchange of heat with a fraction of gas rich in methane to which is added a volatile gaseous fraction, at the pressure of the distribution system derived from the main flow before reheating and vaporization.
  • the liquid natural gas at the intermediate pressure is at a temperature below its boiling point after injection of the volatile gaseous fraction.
  • FIG. 1 of the accompanying drawings shows, by way of non-limitative example, an installation for the production of a gas rich in methane under high pressure, to which nitrogen is added so as to reduce its calorific power to the desired level, from liquid natural gas under low pressure, in which the content of hydrocarbons heavier than methane is relatively high.
  • the liquid natural gas coming in through the conduit 1 at a pressure of 2 bars absolute and a temperature of about 1 62 C. is delivered by the pump 2 at a pressure of about 40 bars through the conduit 3 to the exchanger 4. it is reheated in this latter to about 1 C in heat exchange with the gaseous fraction rich in methane arriving through the conduit 18, the origin of which will be indicated later.
  • the natural gas is subjected in the exchanger 10 to partial vaporization by indirect exchange of heat with I an external water circulation shown diagrammatically at 1 1. It then passes through the conduit 12 to the separator 13.
  • This column ensures the separation at its tank of a liquid fraction concentrated in hydrocarbons of C and higher, and at its head of a gaseous fraction containing substantially all the methane contained in the liquid coming from the separator, with the major part of the ethane. It is provided at its base with a water-circulation or steam-circulation boiler 34.
  • the volatile fraction evacuated at the head of the column 16 through the conduit 17 is sent for reliquefaction into the exchanger 4.
  • the mixture is broughtup to a pressure of 7 bars by the turbo-compressor 21 and is then sent into the conduit 22.
  • the mixture of volatile gases rich in methane and ethane, coming through the conduit 18 into the exchanger 4 at a temperature of about 75 C. is liquefied there and again cooled to about -l45 C in heat exchange with the natural gas during its reheating.
  • the mixture of volatile gases rich in methane and reliquefied passes through the conduit 25 into the pump 26 which delivers it at a pressure of about 39 bars absolute, equal to that existing in the separator 13. It can then be combined in the conduit 27 with the reliquefied gaseous fraction evacuated from the exchanger 6 by the conduit 28. The whole mixture is admitted to the pump 29 which delivers it at a pressure of 72 bars absolute into the conduit 30 leading to the exchanger 31. There it is heated to about 0 C. in heat exchange with an external water circulation shown at 32, and is then sent to the conduit 33 to a transport gasduct.
  • troughs 40 At the head there is collected in troughs 40 a liquid fraction rich in propane which is sent through a conduit 41 to the utilization sytem, and at its tank, a liquid frac tion rich in butane which passes to the utilization system through the conduit 39.
  • the partial vaporization of the liquefied natural gas can be effected by the method which formed the subject of French Pat. No. 2,501,013 of September 13, 1966 by the present Applicants, the gas eous nitrogen and the methane vapours being added to the re-vaporized fraction of the natural gas relatively rich in ethane.
  • a part of the natural gas can be re-liquefied for the purpose of storage after extraction of gasolene. only the other portion being sent to the transport gas ducts after addition of the non-combustible gas and when so required, of vapours liberated in the storage tanks.
  • FIG. 2 shows by way of non-limitative example, an installation for the production of a gas rich in methane under high pressure, to which nitrogen is added in order to reduce its calorific power to the desired level, from liquid natural gas under low pressure, in which the content of hydrocarbons heavier than methane is small.
  • the liquid natural gas coming-in through the conduit 101 is delivered by a pump 102 through the conduit 103 into a conduit 127, in which it is combined with a mixture of gas brought in through the conduit 125.
  • the mixture is composed of gaseous nitrogen, brought in through the conduit 119, and de-gasification vapours from the tanks of liquefied natural gas (not shown), brought in through the conduit 120.
  • This mixture is compressed by the compressor 121 to an intermediate pressure, and is then passed through the conduit 122 to the exchanger 104, in which it is cooled to the vicinity of the temperature of the liquid natural gas. it leaves this exchanger through the conduit 125.
  • the liquid gas rich in methane from the conduit 127 is compressed by a pump 129 to the final pressure of the transport system, and is completely vaporized and reheated in the exchanger 131, to a temperature of about 0 C, in heat exchange with an external water circulation shown at 132, after which it is sent through the conduit 133 to a transport gas-duct (not shown).
  • conduit 125 is delivered by the pump 102, at an intermediate pressure of 3.5 atmospheres absolute into the conduit 103, in which it is at a temperature of l 6 1 .4 C, and is then passed into the conduit 127 in which it is combined with a mixture of gaseous nitrogen (non-combustible gaseous fraction) and de-gasification vapours (vapours rich in methane in pressure equilibrium with the liquid natural gas) at 140 C. coming from the conduit 125.
  • gaseous nitrogen non-combustible gaseous fraction
  • de-gasification vapours vapours rich in methane in pressure equilibrium with the liquid natural gas
  • the liquid gas rich in methane to which is added a volatile fraction, obtained in the conduit 127, is at 157.8 C., that is to say at a temperature slightly lower than the boiling point.
  • This liquid gas is delivered by the pump 129 which brings it up to the pressure of the system, into the conduit 142 in which it is at 147 C.
  • This liquid gas is then brought into the heat exchanger 131 in which it is vaporized and reheated to 0 C. by exchange of heat with an external water circulation 132, and leaves the installation through the conduit 133.
  • a branch conduit 143 permits the vaporization in the heat exchanger 104 of a small portion of the liquid gas which is led through the conduit 144 to the conduit 133.
  • the mixture of volatile gases and vapours, at ambient temperature, brought in by the conduit 122 is cooled to l40 C. by heat exchange with the small part of liquid gas derived from the main conduit 142. This mixture leaves the exchanger through the conduit 125.
  • the non-combustible volatile gaseous fraction brought in through the conduit 120 is nitrogen in volumetric proportions of 6% with respect to the gas emitted at 133.
  • the vapours arriving through the conduit 120 in volumetric proportions of 0.5 percent with respect to the gase delivered at 133, are de-gasification vapours from tanks of natural gas; they are in principle composed of methane (and small proportions of ethane).
  • The. mixture of nitrogen and vapours rich in methane in pressure equilibrium with the liquid natural gas. is composed in the compressor 121 before passing into the conduit 122.
  • At least part of the volatile fraction added to the liquid natural gas may be derived from another installation, in which it is available at an intermediate pressure and at a suitable temperature; it is directly added to the liquid natural gas brought-up to this same intermediate pressure.
  • the method according to the present invention can be applicable to the case where a natural liquid gas, brought in by a tanker ship for example, is delivered directly into the distribution system, after prior addition of the vapours rich in methane in pressure equilibrium with the liquid natural gas.
  • said gaseous fraction comprises a non-combustible gas more volatile than methane, obtained from a separate source independently of said natural gas.
  • a method as claimed in claim 4 and liquefying said first portion of gas rich in methane and admixing it under said second intermediate pressure with said admixture in liquid phase prior to the compression thereof to said high pressure.
  • a method as claimed in claim 8 and cooling to a low temperature at least equal to the temperature of the liquid portion of compressed natural gas, and below ambient temperature. the compressed gaseous fraction. prior to its admixing with said portion of natural gas at said second intermediate pressure l

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Abstract

A method of production under high pressure of a gas rich in methane, to which is added a volatile gaseous fraction in which the natural gas is brought to an intermediate pressure and a volatile gaseous fraction is added to the natural gas at the said intermediate pressure, wherein the volatile gaseous fraction is added to the natural gas in the liquid state, after which the mixture obtained is brought to the pressure of the distribution system reheated and vaporized.

Description

United States Patent Buffiere et al.
[4 1 Sept. 24, 1974 ELEVATING NATURAL GAS WITH REDUCED CALORIFIC VALUE TO DISTRIBUTION PRESSURE Inventors: Jean-Pierre Buffiere, Saint Mande;
Maurice Grenier, Paris, both of France Assignee: LAir Liquide, Societe Anonyme pour letude et lexploitation des procedes Georges Claude, Paris,
France Filed: Mar. 29, 1972 Appl. No.: 239,136
Related U.S. Application Data Continuation of Serv No. 837,518, June 30, 1969.
References Cited UNITED STATES PATENTS Templer 62/51 3,271,965 9/1966 Maher 62/40 3,285,719 11/1966 Bodle 62/51 3,302,416 2/1967 Proctor 62/54 3,303,660 2/1967 Berg 62/54 3,365,898 l/1968 Van Kleef.... 62/54 3,420,068 1/1969 Petit 62/23 3,516,262 6/1970 Bernstein 62/28 3,656,312 4/1972 Streich 62/28 FOREIGN PATENTS OR APPLICATIONS 220,879 3/1958 Australia 62/53 Primary Examiner-Norman Yudkoff Assistant Examiner-Frank Sever Attorney, Agent, or FirmYoung & Thompson 5 7 ABSTRACT A method of production under high pressure of a gas rich in methane, to which is added a volatile gaseous fraction in which the natural gas is brought to an intermediate pressure and a volatile gaseous fraction is added to the natural gas at the said intermediate pressure, wherein the volatile gaseous fraction is added to the natural gas in the liquid state, after which the mixture obtained is brought to the pressure of the distribution system reheated and vaporized.
10 Claims, 2 Drawing Figures PATENTEUSEPZMBM 3.837.821
S'HEETEUF 2 Arrys.
ELEVATING NATURAL GAS WITH REDUCED CALORIFIC VALUE TO DISTRIBUTION PRESSURE This is a continuation of application Ser. No. 837,518, filed June 30, 1969.
The present invention has for its object a method of production under high pressure of a gas rich in methane, to which is added a volatile gaseous fraction, in which the natural gas is brought up to an intermediate pressure and the volatile gaseous fraction is added to the natural gas at the said intermediate pressure.
The method of production according to the present invention is equally well applicable to the case of liquid natural gas in which the content of hydrocarbons heavier than methane is relatively high, as to the case of liquid natural gas having a low content of hydrocarbons heavier than methane, that is to say in which it would not be necessary to separate out the hydrocarbons heavier than methane from the natural gas.
In the case of liquid natural gas having a relativelyhigh content of hydrocarbons heavier than methane, it is known that before sending the natural gas under high-pressure into a distribution system, it is desirable to eliminate from it the relatively heavy hydrocarbons such as propane, butanes and less volatile homologues, and also sometimes a part of the ethane. When the natural gas is obtained by re-vaporization of natural gas in the liquid state under low pressure, the liquid natural gas is subjected to a partial vaporization, followed by rectification of the residual liquid fraction, so as to separate out a residual liquid consisting of hydrocarbons of C and higher, with sometimes a part of the initial ethane, substantially free from methane.
The gas which is compressed and sent into a distribution system has sometimes however a calorific power which is too high for the use for which it is intended, whether this use is industrial or domestic. This calorific power can be reduced by the addition of a noncombustible gas such as air or nitrogen, that is to say of a gaseous fraction which is more volatile than methane.
However, while the liquid natural gas can be brought up to the pressure of the system, at least partly in the liquid phase, and therefore with a small expenditure of power, the more volatile gaseous fraction necessitates a compressor having a large output from the initial pressure of this gas, generally in the vicinity of atmospheric pressure, up to the high pressure of the distribution system and which therefore has a large number of compression stages. This compressor is costly and necessitates close supervision and a high consumption of power.
Similarly, the vapours liberated in the storage tanks of liquefied natural gas resulting from the passage of heat through the lagging may also be recovered by recompressing them in the same compressor as the noncombustible gas when a non-combustible gas is added to the natural gas in order to reduce its calorific power, but this recompression requires an additional expenditure of power.
The method of the invention has for its object to remedy the disadvantages indicated above, and to permit the addition to the gas, freed from relatively little volatile hydrocarbons, of a more volatile gaseous fraction constituted by a non-combustible gas, to which may possibly be added the vapours evolved in the storage tanks of the natural gas in the liquid state, with a small expenditure of energy, by means of a compressor having a low ratio of the suction and delivery pressures and consequently comprising a small number of compression stages.
According to the present invention the volatile gaseous fraction is added to the natural gas in the liquid state, after which the mixture obtained is brought up to the pressure of the distribution system, reheated and vaporized.
According to one form of embodiment of the present invention, the volatile gaseous fraction consists wholly or partly of vapours rich in methane in pressure equilibrium with the liquid natural gas.
According to a further form of embodiment of the present invention, the volatile gaseous fraction consists, wholly or partly. of a non-combustible gaseous fraction such as air or nitrogen, that is to say of a gaseous fraction more volatile than methane.
According to still another form of embodiment of the present invention, the volatile gaseous fraction consists, at least in part, of a vapour rich in methane and ethane obtained from liquid natural gas under low pressure, partly vaporized, from which there is separated a residual liquid fraction containing at least a part of the hydrocarbons heavier than methane, the said vapour rich in methane and ethane being re-liquefied by exchange of heat with the natural gas during the course of fractionated vaporization, and is then brought-up by a pump to the high-pressure of production and reheated.
According to one form of execution of the present invention, the volatile gaseous fraction formed by vapours rich in methane in pressure equilibrium with the liquid natural gas and the non-combustible gaseous fraction intended to be added to the gas rich in methane, is only compressed up to an intermediate pressure and is added to at least part of the said vapour before the re-liquefaction of this latter, the said intermediate pressure being such that the mixture of the said vapours and non-combustible gaseous fraction is wholly lequefled during the course of its heat exchange with the natural gas during the fractionated vaporization.
According to a still further form of execution of the present invention, the intermediate pressure is lower than the highest level of pressure at which is effected the partial vaporization of the natural gas, and the residual liquid fraction is expanded to this intermediate pressure before being subjected to separation into a liquid containing at least part of the hydrocarbons heavier than methane and the said vapour rich in methane and ethane.
According to another form of execution of the present invention, at least part of the volatile gaseous frac tion is compressed prior to the addition to the liquid natural gas.
In another form of embodiment of the present invention, the volatile gaseous fraction is cooled before it is added to the liquid natural gas.
According to still another form of embodiment of the present invention, the cooling is obtained by exchange of heat with a fraction of gas rich in methane to which is added a volatile gaseous fraction, at the pressure of the distribution system derived from the main flow before reheating and vaporization.
According to still a further form of embodiment of the present invention, the liquid natural gas at the intermediate pressure is at a temperature below its boiling point after injection of the volatile gaseous fraction.
FIG. 1 of the accompanying drawings shows, by way of non-limitative example, an installation for the production of a gas rich in methane under high pressure, to which nitrogen is added so as to reduce its calorific power to the desired level, from liquid natural gas under low pressure, in which the content of hydrocarbons heavier than methane is relatively high.
The liquid natural gas coming in through the conduit 1 at a pressure of 2 bars absolute and a temperature of about 1 62 C. is delivered by the pump 2 at a pressure of about 40 bars through the conduit 3 to the exchanger 4. it is reheated in this latter to about 1 C in heat exchange with the gaseous fraction rich in methane arriving through the conduit 18, the origin of which will be indicated later. After evacuation from the exchanger 4 by the conduit 5, it passes into the exchanger 6 which ensures its heating to the vicinity of its boiling point, in exchange with the methane vapours to be re-liquefied, coming from the separator 13. It is then sent through the conduit 7 into a condenser 8 at the head of a rectification column 37 which ensures the separation between the hydrocarbons of C and C present in the residual liquid fraction of the natural gas. The natural gas thus heated to its boiling point returns through the conduit 9 to the exchanger 10.
The natural gas is subjected in the exchanger 10 to partial vaporization by indirect exchange of heat with I an external water circulation shown diagrammatically at 1 1. It then passes through the conduit 12 to the separator 13.
The liquid collected in the lower portion of the separator 13, which still contains a large proportion of methane, passes through the conduit 14 and the pressure-reducing valve 15 at about 7 bars absolute, to the head of the exhaustion column 16. This column ensures the separation at its tank of a liquid fraction concentrated in hydrocarbons of C and higher, and at its head of a gaseous fraction containing substantially all the methane contained in the liquid coming from the separator, with the major part of the ethane. It is provided at its base with a water-circulation or steam-circulation boiler 34.
The volatile fraction evacuated at the head of the column 16 through the conduit 17 is sent for reliquefaction into the exchanger 4. There is previously added to it, through the conduit 22, a mixture of gaseous nitrogen coming from the conduit 19 and degasification vapours from the liquefied natural gas tanks (not shown), these vapours being combined with the nitrogen by the conduit 20. The mixture is broughtup to a pressure of 7 bars by the turbo-compressor 21 and is then sent into the conduit 22.
The mixture of volatile gases rich in methane and ethane, coming through the conduit 18 into the exchanger 4 at a temperature of about 75 C. is liquefied there and again cooled to about -l45 C in heat exchange with the natural gas during its reheating.
The mixture of volatile gases rich in methane and reliquefied passes through the conduit 25 into the pump 26 which delivers it at a pressure of about 39 bars absolute, equal to that existing in the separator 13. It can then be combined in the conduit 27 with the reliquefied gaseous fraction evacuated from the exchanger 6 by the conduit 28. The whole mixture is admitted to the pump 29 which delivers it at a pressure of 72 bars absolute into the conduit 30 leading to the exchanger 31. There it is heated to about 0 C. in heat exchange with an external water circulation shown at 32, and is then sent to the conduit 33 to a transport gasduct.
The liquid fraction rich in hydrocarbons of C and higher, evacuated by the conduit 35 from the tank of the column 16, is sent through the pressure-reducing valve 36 at 1.3 bars into the rectification column 37, comprising at its head the reflux condenser 8 already mentioned, and at its base the boiler 38 with external water circulation.
At the head there is collected in troughs 40 a liquid fraction rich in propane which is sent through a conduit 41 to the utilization sytem, and at its tank, a liquid frac tion rich in butane which passes to the utilization system through the conduit 39.
It will be understood that various modifications may be made to the installation for heating, extraction of gasolene and re-compression of natural gas which has been described above, without thereby departing from the scope of the invention.
In particular, the partial vaporization of the liquefied natural gas can be effected by the method which formed the subject of French Pat. No. 2,501,013 of September 13, 1966 by the present Applicants, the gas eous nitrogen and the methane vapours being added to the re-vaporized fraction of the natural gas relatively rich in ethane.
A part of the natural gas can be re-liquefied for the purpose of storage after extraction of gasolene. only the other portion being sent to the transport gas ducts after addition of the non-combustible gas and when so required, of vapours liberated in the storage tanks.
FIG. 2 shows by way of non-limitative example, an installation for the production of a gas rich in methane under high pressure, to which nitrogen is added in order to reduce its calorific power to the desired level, from liquid natural gas under low pressure, in which the content of hydrocarbons heavier than methane is small.
The liquid natural gas coming-in through the conduit 101 is delivered by a pump 102 through the conduit 103 into a conduit 127, in which it is combined with a mixture of gas brought in through the conduit 125. The mixture is composed of gaseous nitrogen, brought in through the conduit 119, and de-gasification vapours from the tanks of liquefied natural gas (not shown), brought in through the conduit 120. This mixture is compressed by the compressor 121 to an intermediate pressure, and is then passed through the conduit 122 to the exchanger 104, in which it is cooled to the vicinity of the temperature of the liquid natural gas. it leaves this exchanger through the conduit 125.
The liquid gas rich in methane from the conduit 127 is compressed by a pump 129 to the final pressure of the transport system, and is completely vaporized and reheated in the exchanger 131, to a temperature of about 0 C, in heat exchange with an external water circulation shown at 132, after which it is sent through the conduit 133 to a transport gas-duct (not shown).
A fraction of the liquid gas rich in methane, compressed by the pump 129, is led through a branch conduit 143 into the exchanger 104, and is then combined in the conduit 144 with the vaporized gas rich in methane from the conduit 133.
EXAMPLE.
A liquid natural gas at a temperature of l62C. and at a pressure of 1 atmosphere absolute, comprising, by volume:
9315 of methane. 0.40 7: of nitrogen. 5.95 7r of ethane. 0.40 7: of propane, 0.10 7! of butane.
is delivered by the pump 102, at an intermediate pressure of 3.5 atmospheres absolute into the conduit 103, in which it is at a temperature of l 6 1 .4 C, and is then passed into the conduit 127 in which it is combined with a mixture of gaseous nitrogen (non-combustible gaseous fraction) and de-gasification vapours (vapours rich in methane in pressure equilibrium with the liquid natural gas) at 140 C. coming from the conduit 125.
The liquid gas rich in methane to which is added a volatile fraction, obtained in the conduit 127, is at 157.8 C., that is to say at a temperature slightly lower than the boiling point. This liquid gas is delivered by the pump 129 which brings it up to the pressure of the system, into the conduit 142 in which it is at 147 C. This liquid gas is then brought into the heat exchanger 131 in which it is vaporized and reheated to 0 C. by exchange of heat with an external water circulation 132, and leaves the installation through the conduit 133. A branch conduit 143 permits the vaporization in the heat exchanger 104 of a small portion of the liquid gas which is led through the conduit 144 to the conduit 133. In the exchanger 104, the mixture of volatile gases and vapours, at ambient temperature, brought in by the conduit 122 is cooled to l40 C. by heat exchange with the small part of liquid gas derived from the main conduit 142. This mixture leaves the exchanger through the conduit 125.
The non-combustible volatile gaseous fraction brought in through the conduit 120 is nitrogen in volumetric proportions of 6% with respect to the gas emitted at 133. The vapours arriving through the conduit 120, in volumetric proportions of 0.5 percent with respect to the gase delivered at 133, are de-gasification vapours from tanks of natural gas; they are in principle composed of methane (and small proportions of ethane). The. mixture of nitrogen and vapours rich in methane in pressure equilibrium with the liquid natural gas. is composed in the compressor 121 before passing into the conduit 122.
It will of course be understood that the invention is not limited to the form of execution shown; it is capable of receiving other alternative forms available to those skilled in the art, according to the applications contemplated, without thereby departing from the spirit of the invention.
Thus, at least part of the volatile fraction added to the liquid natural gas may be derived from another installation, in which it is available at an intermediate pressure and at a suitable temperature; it is directly added to the liquid natural gas brought-up to this same intermediate pressure.
Similarly, the method according to the present invention can be applicable to the case where a natural liquid gas, brought in by a tanker ship for example, is delivered directly into the distribution system, after prior addition of the vapours rich in methane in pressure equilibrium with the liquid natural gas.
What we claim is:
1. In a method of reducing the calorific power of natural gas recompressed in liquid phase from a low storage pressure to a high distribution pressure, by adding thereto a gaseous fraction which is of lower calorific power than said natural gas, which is at least as volatile as a gas that is in equilibrium with said liquid natural gas under said low storage pressure, and which is obtained independently and separately from said liquid natural gas undergoing said recompression; the improvement in which said method is conducted at said low storage pressure and at high distribution pressure and at least one intermediate pressure between said high and low pressures, comprising:
a. compressing at least part of said natural gas in liquid phase from said low storage pressure to a first said intermediate pressure,
b. compressing a said gaseous fraction in vapor phase to a second said intermediate pressure,
c, thereafter admixing at least a portion of said compressed natural gas and said compressed gaseous fraction at said second intermediate pressure,
d. compressing said admixture in liquid phase from said second intermediate pressure to said high distribution pressure, and
e. vaporizing at least a part of said compressed admixture at said high pressure.
2. A method as claimed in claim 1, in which said gaseous fraction comprises a non-combustible gas more volatile than methane, obtained from a separate source independently of said natural gas.
3. A method as claimed in claim 2, said noncombustible gas being nitrogen.
4. A method as claimed in claim 1, in which said first intermediate pressure to which said part of said natural gas in liquid phase is compressed is higher than said second intermediate pressure to which said gaseous fraction in vapor phase is compressed, partially vaporizing said compressed part of said natural gas in liquid phase under said first intermediate pressure, then separating a first portion of gas rich in methane from a remaining liquid portion; expanding said remaining liquid portion to said second intermediate pressure, than separating a second portion of gas rich in methane from at least a residual liquid portion; and admixing at said second intermediate pressure said second portion of gas rich in methane and said compressed gaseous fraction.
5. A method as claimed in claim 4, and liquefying said first portion of gas rich in methane and admixing it under said second intermediate pressure with said admixture in liquid phase prior to the compression thereof to said high pressure.
6. A method as claimed in claim 1, and prior to said admixing under said second intermediate pressure, separating under at least a said intermediate pressure at least a portion of the hydrocarbons less volatile than methane from said compressed part of natural gas.
7. A method as claimed in claim 1, in which said portion of compressed natural gas admixed with said compressed gaseous fraction is in vapor phase prior to said admixture, and liquefying said admixture under said second intermediate pressure.
8. A method as claimed in claim 1, in which said portion of compressed natural gas admixed with said comeous fraction in vapor phase is compressed.
10. A method as claimed in claim 8, and cooling to a low temperature at least equal to the temperature of the liquid portion of compressed natural gas, and below ambient temperature. the compressed gaseous fraction. prior to its admixing with said portion of natural gas at said second intermediate pressure l

Claims (9)

  1. 2. A method as claimed in claim 1, in which said gaseous fraction comprises a non-combustible gas more volatile than methane, obtained from a separate source independently of said natural gas.
  2. 3. A method as claimed in claim 2, said non-combustible gas being nitrogen.
  3. 4. A method as claimed in claim 1, in which said first intermediate pressure to which said part of said natural gas in liquid phase is compressed is higher than said second intermediate pressure to which said gaseous fraction in vapor phase is compressed, partially vaporizing said compressed part of said natural gas in liquid phase under said first intermediate pressure, then separating a first portion of gas rich in methane from a remaining liquid portion; expanding said remaining liquid portion to said second intermediate pressure, than separating a second portion of gas rich in methane from at least a residual liquid portion; and admixing at said second intermediate pressure said second portion of gas rich in methane and said compressed gaseous fraction.
  4. 5. A method as claimed in claim 4, and liquefying said first portion of gas rich in methane and admixing it under said second intermediate pressure with said admixture in liquid phase prior to the compression thereof to said high pressure.
  5. 6. A method as claimed in claim 1, and prior to said admixing under said second intermediate pressure, separating under at least a said intermediate pressure at least a portion of the hydrocarbons less volatile than methane from said compressed part of natural gas.
  6. 7. A method as claimed in claim 1, in which said portion of compressed natural gas admixed with said compressed gaseous fraction is in vapor phase prior to said admixture, and liquefying said admixture under said second intermediate pressure.
  7. 8. A method as claimed in claim 1, in which said portion of compressed natural gas admixed with said compressed gaseous fraction is in liquid phase prior to said admixture, and dissolving said compressed gaseous fraction into said portion of the natural gas under said second intermediate pressure.
  8. 9. A method as claimed in claim 8, in which said first intermediate pressure to which said part of said natural gas in liquid phase is compressed, is substantially equal to said second intermediate pressure to which said gaseous fraction in vapor phase is compressed.
  9. 10. A method as claimed in claim 8, and cooling to a low temperature at least equal to the temperature of the liquid portion of compressed natural gas, and below ambient temperature, the compressed gaseous fraction, prior to its admixing with said portion of natural gas at said second intermediate pressure.
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