Refrigerant Recovery in Natural Gas Liquefaction Processes

Patent metadata

Field	Value
Publication number	US-2016102908-A1
Application number	US-201414511774-A
Country	US
Kind code	A1
Filing date	Oct 10, 2014
Priority date	Oct 10, 2014
Publication date	Apr 14, 2016
Grant date	—

Abstract

Described herein is a method of removing refrigerant from a natural gas liquefaction system in which vaporized mixed refrigerant is withdrawn from the closed-loop refrigeration circuit and introduced into a distillation column so as to be separated into an overhead vapor enriched in methane and a bottoms liquid enriched in heavier components. Overhead vapor is withdrawn from the distillation column to form a methane enriched stream that is removed from the liquefaction system, and bottoms liquid is reintroduced from the distillation column into the closed-loop refrigeration circuit. Also described are methods of altering the rate of production in a natural gas liquefaction system in which refrigerant is removed as described above, and a natural gas liquefaction systems in which such methods can be carried out.

First claim

1 . A method of removing refrigerant from a natural gas liquefaction system that uses a mixed refrigerant to liquefy and/or subcool natural gas, the mixed refrigerant comprising a mixture of methane and one or more heavier components, and the liquefaction system comprising a closed-loop refrigeration circuit in which the mixed refrigerant is circulated when the liquefaction system is in use, the closed-loop refrigeration circuit including a main heat exchanger through which natural gas is fed to be liquefied and/or subcooled by indirect heat exchange with the circulating mixed refrigerant, the method comprising:
(a) withdrawing vaporized mixed refrigerant from the closed-loop refrigeration circuit; (b) introducing the vaporized mixed refrigerant into a distillation column and providing reflux to the distillation column so as to separate the vaporized mixed refrigerant into an overhead vapor enriched in methane and a bottoms liquid enriched in heavier components; (c) withdrawing overhead vapor from the distillation column to form a methane enriched stream that is removed from the liquefaction system; and (d) reintroducing bottoms liquid from the distillation column into the closed-loop refrigeration circuit, and/or storing bottoms liquid such that it can subsequently be reintroduced into the closed-loop refrigeration circuit.

2 . The method of claim 1 , wherein the heavier components comprise one or more heavier hydrocarbons.

3 . The method of claim 1 , wherein the mixed refrigerant further comprises nitrogen, the overhead vapor in step (b) is enriched in nitrogen and methane, and the methane enriched stream in step (c) is a nitrogen and methane enriched stream.

4 . The method of claim 1 , wherein in step (b) reflux to the distillation column is provided by a reflux stream of condensate obtained by cooling and condensing at least a portion of the overhead vapor in an overhead condenser by indirect heat exchange with a coolant.

5 . The method of claim 4 , wherein the coolant comprises a liquefied natural gas stream taken from liquefied natural gas that is being or has been produced by the liquefaction system.

6 . The method of claim 1 , wherein in step (b) reflux to the distillation column is provided by a reflux stream of liquid introduced into the top of the distillation column.

7 . The method of claim 6 , wherein the reflux stream of liquid comprises a stream of liquefied natural gas taken from liquefied natural gas that is being or has been produced by the liquefaction system.

8 . The method of claim 1 , wherein the methane enriched stream formed in step (c) is flared, used as fuel and/or added to a natural gas feed that is to be liquefied by the liquefaction system.

9 . The method of claim 1 , wherein in step (d) the bottoms liquid is stored in the bottom of the distillation column and/or is withdrawn from the distillation column and stored in a separate storage vessel prior to being reintroduced into the closed-loop refrigeration circuit.

10 . The method of claim 1 , wherein in step (a) the vaporized mixed refrigerant is withdrawn from a cold end of and/or from an intermediate location of the main heat exchanger.

11 . The method of claim 1 , wherein the main heat exchanger is a coil-wound heat exchanger.

12 . The method of claim 11 , wherein in step (a) the vaporized mixed refrigerant is withdrawn from the shell-side of the coil-wound heat exchanger.

13 . The method of claim 1 , wherein the method is carried out in response to a shutdown of or turn-down in the rate of natural gas liquefaction and/or subcooling by the liquefaction system.

14 . A method of altering the rate of production of liquefied or subcooled natural gas in a natural gas liquefaction system that uses a mixed refrigerant to liquefy and/or subcool the natural gas, the liquefaction system comprising a closed-loop refrigeration circuit in which the mixed refrigerant is circulated, the mixed refrigerant comprising a mixture of methane and one or more heavier components, and the closed-loop refrigeration circuit including a main heat exchanger through which natural gas is fed to be liquefied and/or subcooled by indirect heat exchange with the circulating mixed refrigerant, the method comprising:
a first period of time during which natural gas is fed through the main heat exchanger at a first feed rate and mixed refrigerant is circulated in the closed-loop refrigeration circuit at a first circulation rate so as to produce liquefied or subcooled natural gas at a first production rate; a second period of time during which the production of liquefied or subcooled natural gas is stopped, or the rate of production of liquefied or subcooled natural gas is reduced to a second production rate, by stopping the feed of natural gas through the main heat exchanger or reducing the feed rate thereof to a second feed rate, stopping the circulation of the mixed refrigerant in the closed-loop refrigeration circuit or reducing the circulation rate thereof to a second circulation rate, and removing refrigerant from the liquefaction system, wherein the method of removing refrigerant from the liquefaction system comprises: (a) withdrawing vaporized mixed refrigerant from the closed-loop refrigeration circuit; (b) introducing the vaporized mixed refrigerant into a distillation column and providing reflux to the distillation column so as to separate the vaporized mixed refrigerant into an overhead vapor enriched in methane and bottoms liquid enriched in heavier components; (c) withdrawing overhead vapor from the distillation column to form a methane enriched stream that is removed from the liquefaction system; and (d) reintroducing bottoms liquid from the distillation column into the closed-loop refrigeration circuit, and/or storing bottoms liquid such that it can subsequently be reintroduced into the closed-loop refrigeration circuit.

15 . The method of claim 14 , wherein the method further comprises, after the second period of time:
a third period of time during which the rate of production of liquefied or subcooled natural gas is increased to a third production rate, by increasing the feed of natural gas through the main heat exchanger to a third feed rate, adding refrigerant to the liquefaction system, and increasing the circulation of the mixed refrigerant to a third circulation rate, wherein the step of adding refrigerant to the liquefaction system comprises introducing methane into the closed-loop refrigeration circuit and, if bottoms liquid has not already been reintroduced into the closed-loop refrigeration circuit in step (d) of the second time period, reintroducing stored bottoms liquid to the closed-loop refrigeration circuit.

16 . The method of claim 15 , wherein the third production rate of liquefied or subcooled natural gas, third feed rate of natural gas and third circulation rate of mixed refrigerant are the same as or less than the first production rate, first feed rate and first circulation rate, respectively.

17 . The method of claim 15 , wherein the methane that is introduced into the closed-loop refrigeration circuit is obtained from the natural gas supply that provides natural gas for liquefaction in the liquefaction system.

18 . A natural gas liquefaction system that uses a mixed refrigerant, comprising a mixture of methane and one or more heavier components, to liquefy and/or subcool natural gas, the liquefaction system comprising:
a closed-loop refrigeration circuit for containing and circulating a mixed refrigerant when the liquefaction system is in use, the closed-loop r

Assignees

• Air Prod & Chem

Inventors

• Johnston Brian Keith
• Krishnamurthy Gowri
• Roberts Mark Julian

Classifications

• F25J1/0214

  as a dual level refrigeration cascade with at least one MCR cycle · CPC title

• F25J3/0233

  separation of CnHm with 1 carbon atom or more · CPC title

• F25J2290/62

  Details of storing a fluid in a tank · CPC title

• F25J1/0249

  Controlling refrigerant inventory, i.e. composition or quantity (charging or discharging refrigerants in cooling systems F25B45/00) · CPC title

• F25J1/0092

  Mixtures of hydrocarbons comprising possibly also minor amounts of nitrogen · CPC title