Process and apparatus for liquefying natural gas

• US 4,012,212 A
• Filed: 07/07/1975
• Issued: 03/15/1977
• Est. Priority Date: 07/07/1975
• Status: Expired due to Term

• Current Assignee: The Lummus Company
• Original Assignee: The Lummus Company
• Inventors: Kniel, Ludwig
• Primary Examiner(s): Sofer, Jack
• Application Number:
  US05/593,222
• Time in Patent Office: 617 Days
• Field of Search: 62/26, 62/28, 62/23, 62/31
• US Class Current: 62/621
• 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 1/0037 of a return stream
• CPC Class Codes: F25J 1/004 by flash gas recovery F25J1/0267 takes precedence
• CPC Class Codes: F25J 1/0202 in a quasi-closed internal refrigeration loop F25J1/0208, F25J1/0219, F25J1/0224 take precedence
• CPC Class Codes: F25J 1/0238 Purification or treatment step is integrated within one refrigeration cycle only, i.e. the same or single refrigeration cycle provides feed gas cooling (if present) and overhead gas cooling
• CPC Class Codes: F25J 1/0247 start-up of the process
• CPC Class Codes: F25J 1/0259 Modularity and arrangement of parts of the liquefaction unit and in particular of the cold box, e.g. pre-fabrication, assembling and erection, dimensions, horizontal layout "plot"
• CPC Class Codes: F25J 1/0278 Unit being stationary, e.g. on floating barge or fixed platform
• CPC Class Codes: F25J 2200/02 in a single pressure main column system
• 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 2205/30 using a washing, e.g. "scrubbing" or bubble column for purification purposes
• CPC Class Codes: F25J 2205/50 using absorption, i.e. with selective solvents or lean oil, heavier CnHm and including generally a regeneration step for the solvent or lean oil
• CPC Class Codes: F25J 2220/64 Separating heavy hydrocarbons, e.g. NGL, LPG, C4+ hydrocarbons or heavy condensates in general
• CPC Class Codes: F25J 2220/66 Separating acid gases, e.g. CO2, SO2, H2S or RSH
• CPC Class Codes: F25J 2220/68 Separating water or hydrates
• CPC Class Codes: F25J 2230/60 the fluid being hydrocarbons or 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 2245/02 : Recycle of a stream in general, e.g. a by-pass stream
• CPC Class Codes: F25J 2270/06 : with multiple gas expansion loops
• CPC Class Codes: F25J 2270/90 : External refrigeration, e.g. conventional closed-loop mechanical refrigeration unit using Freon or NH3, unspecified external refrigeration
• CPC Class Codes: F25J 2280/10 : Control for or during start-up and cooling down of the installation
• CPC Class Codes: F25J 2290/42 : Modularity, pre-fabrication of modules, assembling and erection, horizontal layout, i.e. plot plan, and vertical arrangement of parts of the cryogenic unit, e.g. of the cold box
• CPC Class Codes: F25J 2290/72 : Processing device is used off-shore, e.g. on a platform or floating on a ship or barge
• 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 3/0247 : separation of CnHm with 4 carbon atoms or more

Abstract

There is disclosed a novel process and apparatus for liquefying a hydrocarbon gas under a pressure greater than the critical pressure thereof wherein the gas is first expanded to below the critical pressure thereof to permit the facile separation of C₅ and heavier hydrocarbons with the lighter components being re-pressurized to above the critical pressure thereof by such expanding gas prior to the further cooling of such re-compressed lighter components for eventual liquefaction.

Claims

1. A method of liquefying natural gas having a pressure above the critical pressure thereof, which comprises:

a. expanding said natural gas to a pressure below the critical pressure thereof;

b. introducing the expanded gaseous stream of step (a) into a fractionating zone to remove as a liquid a C₅ + hydrocarbon stream;

c. compressing the resulting gas stream recovered from step (b) to a pressure above the critical pressure thereof;

d. cooling the cooled gas stream of step (c);

e. expanding said cooled gas stream from step (d) to effect liquefaction of a major portion of said cooled gas stream of step (d), and

f. recovering said liquified major portion.

2. The process as defined in claim 1 wherein the work necessary to compress the gas stream in step (c) is obtained by the energy of expansion of step (a).

3. The process as defined in claim 1 wherein said C₅ + hydrocarbon stream includes minor portions of light hydrocarbons and additionally comprising the step of separating such lighter hydrocarbons from C₅ and heavier hydrocarbons.

4. The process as defined in claim 3 wherein said lighter hydrocarbons are returned to step (c).

5. The process as defined in claim 1 wherein cooling of said compressed gas is effected by passage in indirect heat transfer relationship with a compressed gaseous refrigerant after the expansion of said compressed gaseous refrigerant.

6. The process as defined in claim 5 wherein the work necessary to compress the gas stream in step (c) is obtained by the energy of expension of step (a).

7. The process as defined in claim 1 wherein liquefied natural gas recovered from step (f) is passed to a storage area.

8. The process as defined in claim 1 wherein said compressed gas is expanded by throttling across a valve to a pressure of about atmospheric pressure.

9. In an apparatus for liquefying natural gas having a pressure above the critical pressure thereof, which comprises:

expander means for reducing the pressure of said natural gas below that of the critical pressure thereof;

separation means for separating a C₅ + hydrocarbon stream from said expanded natural gas to obtain a resulting gas stream of C₄ or less;

compressor means for compressing the resulting gas stream to a pressure above the critical pressure thereof;

means for cooling said resulting gas stream;

means for expanding said resulting gas stream to effect liquefaction of a major portion thereof; and

means for recovering a major portion of said resulting gas stream.

10. The apparatus as defined in claim 9 wherein said compressor means is driven by said expander means.

11. The apparatus as defined in claim 9 wherein said separation means includes fractionator means for separating C₅ and heavier hydrocarbons from lighter hydrocarbons.

12. The apparatus as defined in claim 9 wherein said cooling means is comprised of a compressor means, an expander means and heat exchanger means wherein a gaseous refrigerant is compressed, expanded and passed through said heat exchanger means.

13. The apparatus as defined in claim 9 wherein said apparatus is disposed on a vessel and wherein storage means for liquefied natural gas is provided at a location remote from said vessel.