Method of liquefaction of natural gas

• US 5,291,736 A
• Filed: 09/30/1992
• Issued: 03/08/1994
• Est. Priority Date: 09/30/1991
• Status: Expired due to Term

• Current Assignee: Compagnie Francaise D'Etudes et de Construction Technip
• Original Assignee: Compagnie Francaise D'Etudes et de Construction Technip
• Inventors: Paradowski, Henri
• Primary Examiner(s): Capossela, Ronald C.
• Application Number:
  US07/954,318
• Time in Patent Office: 524 Days
• Field of Search: 62/20, 62/23, 62/28, 62/40
• US Class Current: 62/613
• CPC Class Codes: F25J 1/0022 Hydrocarbons, e.g. natural gas
• CPC Class Codes: F25J 1/0035 by gas expansion with extraction of work
• CPC Class Codes: F25J 1/0052 by vaporising a liquid refrigerant stream
• CPC Class Codes: F25J 1/0055 originating from an incorporated cascade
• CPC Class Codes: F25J 1/0214 as a dual level refrigeration cascade with at least one MCR cycle
• CPC Class Codes: F25J 1/0216 using a C3 pre-cooling cycle
• CPC Class Codes: F25J 1/0239 Purification or treatment step being integrated between two refrigeration cycles of a refrigeration cascade, i.e. first cycle providing feed gas cooling and second cycle providing overhead gas cooling
• CPC Class Codes: F25J 1/0292 Refrigerant compression by cold or cryogenic suction of the refrigerant gas
• CPC Class Codes: F25J 2200/04 in a dual pressure main column system
• CPC Class Codes: F25J 2200/72 Refluxing the column with at least a part of the totally condensed overhead gas
• CPC Class Codes: F25J 2200/74 Refluxing the column with at least a part of the partially condensed overhead gas
• CPC Class Codes: F25J 2200/78 Refluxing the column with a liquid stream originating from an upstream or downstream fractionator column
• CPC Class Codes: F25J 2205/04 in the feed line, i.e. upstream of the fractionation step
• CPC Class Codes: F25J 2220/60 Separating impurities from natural gas, e.g. mercury, cyclic hydrocarbons
• CPC Class Codes: F25J 2220/66 Separating acid gases, e.g. CO2, SO2, H2S or RSH
• CPC Class Codes: F25J 2235/60 the fluid being (a mixture of) hydrocarbons
• CPC Class Codes: F25J 2240/02 Expansion of a process fluid in a work-extracting turbine (i.e. isentropic expansion), e.g. of the feed stream
• CPC Class Codes: F25J 2270/12 External refrigeration with liquid vaporising loop
• CPC Class Codes: F25J 2270/60 Closed external refrigeration cycle with single component refrigerant [SCR], e.g. C1-, C2- or C3-hydrocarbons
• CPC Class Codes: F25J 2270/66 Closed external refrigeration cycle with multi component refrigerant [MCR], e.g. mixture of hydrocarbons
• CPC Class Codes: F25J 3/0209 : Natural gas or substitute natural gas
• CPC Class Codes: F25J 3/0233 : separation of CnHm with 1 carbon atom or more
• CPC Class Codes: F25J 3/0242 : separation of CnHm with 3 carbon atoms or more

Abstract

A method of liquefying natural gas, wherein the gas (1) is cooled and separated into a liquid phase (6) and a gaseous phase (8) which is expanded (9) and added to the liquid phase in the column (7), at the head of which the gas enriched with methane (21) is separated and recompressed (27) and carried to the liquefaction (32, 33, 34) whereas the liquid phase from the bottom of column (7) is expanded and rectified in column (14); the head effluent (19) being condensed (20) and conveyed as a reflux (25) to column (7); the pressure in column (7) being higher than that of column (14); the C₃ + hydrocarbons from the bottom (16) being separated and the methane liquefaction (33, 34) being conventional.

Claims

1. Method of liquefaction of natural gas, comprising the steps of

cooling a natural gas containing methane and a hydrocarbon heavier than methane under a pressure P₁ so as to form at least one gaseous phase G₁,

expanding the gaseous phase G₁ to lower its pressure and to bring it to a pressure P₂ lower than the pressure P₁,

carrying the product of the expansion under the pressure P₂ into a first contact fractionating zone,

drawing off residual gas G₂ enriched with methane from the head of the first fractionating zone,

drawing off a liquid phase L₂ from the bottom of the first fractionating zone,

conveying the liquid phase L₂ into a second zone for fractionating through distillation,

drawing off at least one liquid phase L₃ enriched with hydrocarbons heavier than methane from the bottom of the second fractionating zone,

drawing off a gaseous phase G₃ from the head of said second fractionating zone,

condensing at least one part of the gaseous phase G₃ drawn off from the head of the second fractionating zone to produce a condensed phase L₄,

raising the pressure of at least one portion of the condensed phase L₄,

carrying said at least one portion of the condensed phase L₄ to the first fractionating zone as a reflux,

cooling the residual gas G₂ under a pressure at least equal to the pressure P₂ in a methane liquefaction zone so as to obtain a liquid rich in methane, and

operating the second fractionating zone under a pressure P₄ which is lower than the pressure P₂ of the first fractionating zone.

2. Method according to claim 1, further comprising the steps of

effecting the expansion of the gaseous phase G₁ in a turboexpander,

effecting an increase in the pressure of the residual gas from the pressure P₂ to a pressure P₃ in a turbocompressor and

using the energy supplied by the expansion of the gaseous phase G₁ for actuating the turbocompressor.

3. Method according to claim 1, wherein the pressure P₁ is greater than about 5 MPa, the pressure P₂ is from about 0.3 P₁ with P₂ being between 3.5 and 7 MPa and the pressure P₄ from about 0.3 P₂ to about 0.9 P₂, with P₄ between about 0.5 and about 4.5 MPa.

4. Method according to claim 3, wherein P₁ is greater than about 6 MPa, P₂ is between 4.5 and 6 MPa and P₄ is between 2.5 and 3.5 MPa.

5. Method according to claim 2, further comprising directing at least one portion of the residual gas G₂ to exchange heat with the natural gas to thereby contribute to the cooling of the natural gas, said at least one portion of residual gas G₂ exchanging heat with the natural gas prior to the raising of the pressure of said residual gas G₂ from pressure P₂ to pressure P₃.

6. Method according to claim 1, further comprising directing at least one part of the residual gas G₂ to exchange heat with at least one part of the gaseous phase G₃ to cool the gaseous phase G₃ and produce the condensed phase L₄.

7. Method according to claim 1, further comprising conducting the liquefaction of methane through indirect contact with one or several fractions of a multicomponent fluid, said multicomponent fluid being vaporized and circulating in a closed circuit comprising a compression zone, a cooling zone with liquefaction yielding one or several condensates and a zone for the vaporization of said condensates to reconstitute said multicomponent fluid.

8. Method according to claim 1, further comprising forming at least one liquid phase L₁ during the initial cooling of the gas in addition to the gaseous phase G₁ and carrying the liquid phase L₁ after an expansion of the liquid phase L₁ into said first fractionating zone.

9. Method according to claim 1, further comprising condensing the gaseous phase G₃ and conveying one portion thereof to the second fractionating zone as an internal reflux and the the remaining portion to the first fractionating zone as a reflux.