Refrigeration – Cryogenic treatment of gas or gas mixture – Liquefaction
Patent
1996-06-03
1997-12-30
Capossela, Ronald C.
Refrigeration
Cryogenic treatment of gas or gas mixture
Liquefaction
62619, 62623, F25J 300
Patent
active
057017611
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Natural gas liquefaction is an important industrial process enabling natural gas to be transported over long distances by methane tanker or stored in liquid form.
With currently used methods, a "natural gas" is liquefied by passing this natural gas through exchangers and cooling it by means of an external cooling cycle. Patents U.S. Pat. Nos. 3,735,600 and 3,433,026 describe liquefaction methods during which the gas is fed through one or several heat exchangers in order to produce liquefaction. The terms "natural gas" are used here to denote a mixture formed mostly of methane but possibly also containing other hydrocarbons and nitrogen, irrespective of the form in which it occurs (gaseous, liquid or di-phase). Natural gas mostly starts off in gaseous form and its pressure and temperature values during the liquefaction process are such that it may exist in different forms, with a liquid and gaseous phase occurring together at a given moment, for example.
An external cooling cycle is carried out as part of such methods, using a mixture of fluids as a cooling fluid. On evaporation, such a mixture is likely to cool and liquefy the pressurised natural gas. After evaporation, the mixture is compressed and condensed by a process of heat exchange with an ambient medium such as water or air.
In addition, in most of the processes that make use of a cooling mixture, the vapour fraction leaving the separator is liquefied by incorporating a cascade effect, whereby the increasingly lighter liquid fractions produced by each of the steps carried out to partially condense the cooling mixture are used to cool the natural gas and provide the cooling means required during the successive steps of condensing the vapour fraction.
Such methods are complex and require large exchange surfaces. They also require high compression capabilities and hence high capital investment.
The prior art also describes methods that operate by compressing and expanding a permanent gas such as nitrogen. These methods have the particular advantage of being simple in design. However, their performance is limited and as a result they are not especially suitable for natural gas liquefaction units on an industrial scale.
SUMMARY OF THE INVENTION
It has been discovered, and this is one of the objectives of the present invention, that it is possible to simplify the design of a liquefaction process, particularly one used to liquefy a natural gas, by using a cooling mixture without fully condensing it during the cycle and by substituting for the final step when the mixture is cooled a process of expanding the vapour phase produced from a first stage of condensing the mixture and of mixing it with an expanded liquid fraction to obtain a cooling mixture that can be used to liquefy the natural gas, by contact and heat exchange, for example.
By mixing the expanded liquid fraction with the expanded vapour fraction, the temperature at which the liquid fraction starts to evaporate at the low pressure level of the cycle can be lowered.
Unlike the prior art, the vapour fraction is not fully condensed but only partially condensed so that, at the lowest temperature of the cycle, it assumes the form of a mixture comprising a vapour fraction and a liquid fraction in variable proportions.
The process can be optimised by expanding the vapour phase through a turbine, recuperating the mechanical expansion power.
The invention relates to a method for liquefying a pressurised natural gas consisting of a cooling cycle using a mixture of cooling fluids, during which the following steps are carried out: and cooling using an external cooling fluid, for example, to obtain a vapour fraction and a liquid fraction, expanded separately to obtain respectively a light fluid M1 consisting mostly of a vapour phase and a heavy fluid M2 consisting mostly of a liquid phase, mixture being formed before being passed through a process of heat exchange with the natural gas, and process of heat exchange with the low-temperature mixture obtained during step c).
Advan
REFERENCES:
patent: 3645106 (1972-02-01), Gaumer, Jr. et al.
patent: 3932154 (1976-01-01), Coers et al.
patent: 4094655 (1978-06-01), Krieger
patent: 4141707 (1979-02-01), Springmann
Prevost Isabelle
Rojey Alexandre
Capossela Ronald C.
Institut Francais du Pe'trole
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