Refrigerant Recovery in Natural Gas Liquefaction Processes

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.

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 refrigeration circuit including a main heat exchanger through which natural gas can be fed to be liquefied and/or subcooled by indirect heat exchange with the circulating mixed refrigerant;
• a distillation column for receiving vaporized mixed refrigerant from the closed-loop refrigeration circuit and operable to separate the vaporized mixed refrigerant into an overhead vapor enriched in methane and a bottoms liquid enriched in heavier components of the mixed refrigerant;
• means for providing reflux to the distillation column;
• conduits for transferring vaporized mixed refrigerant from the closed-loop refrigeration circuit to the distillation column, for withdrawing from the distillation column and removing from the liquefaction system a methane enriched stream formed from the overhead vapor, and for reintroducing bottoms liquid from the distillation column into the closed-loop refrigeration circuit.