Refrigeration – Cryogenic treatment of gas or gas mixture – Liquefaction
Patent
1995-08-30
1997-07-29
Capossela, Ronald C.
Refrigeration
Cryogenic treatment of gas or gas mixture
Liquefaction
F25J 300
Patent
active
056512694
DESCRIPTION:
BRIEF SUMMARY
The liquefaction of natural gas is an important industrial process which enables natural gas to be transported by tanker over long distances or stored in liquid form.
The methods currently used to produce liquefaction of a "natural gas" involve passing this natural gas through exchangers and cooling it using an outside cooling cycle. U.S. Pat. No. 3,735,600 and U.S. Pat. No. 3,433,026 describe liquefaction methods during which the gas is fed through one or several heat exchangers to liquefy it. Throughout this text, by "natural gas" is meant a mixture formed for the greater part of methane but possibly containing other hydrocarbons and nitrogen, in whatever form it occurs (gaseous, liquid or two-phase). At the start, natural gas is most often in gaseous form and may take on different forms during the liquefaction process, liquid and gaseous, which may coexist at a given instant.
In such processes, an external cooling cycle is undertaken using a mixture of fluids as a cooling fluid. On evaporation, such a mixture is likely to cool and liquefy the gas under pressure. After evaporation, the mixture is compressed and condensed by a process of heat exchange with an ambient medium such as water or air.
Such methods are complex and involve the use of high exchange surfaces areas as well as high compression forces. As a result, they tend to require high capital investment.
It has been discovered, and this is the objective of the present invention, that after a first cooling stage, it is possible to cool and liquefy natural gas directly from a "dense" phase by expansion in a turbine. The expression "dense phase" denotes a phase which can be obtained from an initially gaseous phase by means of an isobar evolution without any phase transition, which leads as a result of isentropic expansion to a liquid phase without any phase transition. At least a part of the liquefaction process takes place without a transition phase, i.e. the change from the gaseous phase to the liquid phase occurs continuously without any transformation during which two different phases would exist at the same time. Natural gas is brought to the "dense phase" before expansion by applying pressure at a level at least greater than the critical pressure of methane and by lowering the temperature of the "natural gas".
The present invention relates to a method of liquefying a natural gas. It is characterised in that it comprises at least one step during which at least a part of this gas is liquefied by expansion with mechanical energy, this expansion causing it to change from the state of a dense phase to the state of a liquid phase.
The changeover between these two states occurs without any phase transition, i.e. without two different phases existing at the same time.
The method consists of, for example, at least the following two steps:
a) the natural gas is cooled at a pressure at least greater than or equal to the critical pressure of methane and at a temperature that will cause the natural gas to take on the form of the dense phase at the end of this cooling process,
b) at least a fraction of this dense phase obtained from step a) is expanded and liquefied in a device designed to reduce the pressure of the natural gas by expansion with mechanical energy, the change from the state of dense phase to a state of liquid phase occurring without any transition phase, in order to form liquefied natural gas at least in part.
The pressure level of the liquefied natural gas at the end of step b) is virtually atmospheric pressure.
The expansion process of the liquid phase obtained during step b) continues until a gaseous fraction appears and the process can then move on to the following steps:
the liquid fraction and the gaseous fraction are separated during a step c),
the gaseous fraction resulting from step c) is put through a heat exchange process with a non-expanded fraction of the natural gas during a step d), the non-expanded fraction being expanded after this heat exchange process during a step e) when a liquid-vapour mixture is formed and separated i
REFERENCES:
patent: 2903858 (1959-09-01), Bocquet
Prevost Isabelle
Rojey Alexandre
Capossela Ronald C.
Institut Francais du Pe'trole
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