Refrigeration – Cryogenic treatment of gas or gas mixture – Liquefaction
Patent
1997-01-21
1998-06-23
Kilner, Christopher
Refrigeration
Cryogenic treatment of gas or gas mixture
Liquefaction
62912, F25J 300
Patent
active
057689121
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates generally to liquefaction processes and in particular to liquefaction of gaseous products including natural gas. The invention particularly relates to the initial liquefaction of natural gas from the field. More particularly the present invention relates to a method and process for operating a liquefaction plant in a more efficient and economical manner. Even more particularly, the present invention relates to the use of nitrogen as the refrigerant in the liquefaction of natural gas, more particularly, to a modification of or an improvement in the nitrogen expander cycle process which is used in the liquefaction of natural gas feed stock whereby the supply of nitrogen that is used to effect cooling of the natural gas feed is divided into two or more portions in which each portion effects cooling of the natural gas in a different operation and/or in different parts of the installation in which the overall process is conducted and at different temperatures and pressures. The present invention particularly relates to split nitrogen flow cycles whereby the different portions of the nitrogen refrigerant are passed through different expanders which are arranged in parallel with each other.
DESCRIPTION OF RELATED ART
Although the present invention will be described with particular reference to process cycles for the liquefaction of natural gas in which nitrogen is used as the refrigerant, it is to be noted that the scope of the present invention is not restricted to the described embodiment or embodiments but rather the scope of the present invention is more extensive so as to include other methods and applications of the process using nitrogen, and to the use of other gases in the improved application or in other applications than those specifically described.
Natural gas which is obtained in the form of a gas from gas and oil fields occurring in nature, is discharged from the earth to form a natural gas teed which requires processing before it can be used commercially. The natural gas feed enters a processing facility and is processed through a variety of operations in different installations to finally emerge as liquid natural gas (LNG) in a form which is suitable for use. The liquid gas is subsequently stored and transported to another suitable site for revaporisation and subsequent use. In the processing of the natural gas feed the gas emerging from the naturally occurring field must be first pretreated to remove or reduce the concentrations of impurities or contaminants, such as for example carbon dioxide and water or the like, before it is cooled to form LNG in order to reduce or eliminate the chances of blockage to equipment used in the processing occurring and to overcome other processing difficulties. One example of the impurities and/or contaminants are acid gases such as carbon dioxide and hydrogen sulphide. After the acid gas is removed in an acid gas removal installation, the feed gas stream is dried to remove all traces of water. Mercury is also removed from the natural feed gas prior to cooling. Once all of the contaminants or unwanted or undesirable materials are removed from the feed gas stream it undergoes subsequent processing, such as cooling, to produce LNG.
Cooling of the natural gas feed may be accomplished by a number of different cooling process cycles, such as for example, the cascade cycle where refrigeration is provided by three different refrigerant cycles, i.e. by using methane, ethylene and propane in sequence. Another cooling process cycle uses a propane precooled, mixed refrigerant cycle which involves the use of a multicomponent mixture of hydrocarbons, e.g. propane/ethane/methane and/or nitrogen in one cycle and a separate propane refrigeration cycle in another cycle to provide precooling of the mixed refrigerant and natural gas. A further cooling process involves the use of a nitrogen expander cycle in which, in its simplest form, a closed loop is employed in which nitrogen gas is first compressed and cooled to ambi
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Kilner Christopher
Kondracki Edward J.
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