Refrigeration – Cryogenic treatment of gas or gas mixture – Liquefaction
Patent
1998-07-13
1999-06-29
Doerrler, William
Refrigeration
Cryogenic treatment of gas or gas mixture
Liquefaction
62912, F25J 300
Patent
active
059162600
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a liquefaction process, and more particularly relates to a natural gas liquefaction process.
Natural gas is obtained from gas, gas/condensate and oil fields occurring in nature, and generally comprises a mixture of compounds, the most predominant of which is methane. Usually, natural gas contains at least 95% methane and other low boiling hydrocarbon (although it may contain less): the remainder of the composition comprises mainly nitrogen and carbon dioxide. The precise composition varies widely, and may include a variety of other impurities including hydrogen sulphide and mercury.
Natural gas may be "lean" gas or "rich" gas. These terms do not have a precise meaning, but it is generally understood in the art that a lean gas will tend to have less higher hydrocarbons than a rich gas. Thus, a lean gas may contain little or no propane, butane or pentane, whereas a rich gas would contain at least some of these materials.
Since natural gas is a mixture of gases, it liquefies over a range of temperatures; when liquefied, natural gas is called "LNG" (liquefied natural gas). Typically, natural gas compositions will liquefy, at atmospheric pressure, in the temperature range -165.degree. C. to -155.degree. C. The critical temperature of natural gas is about -90.degree. C. to -80.degree. C., which means that in practice it cannot be liquefied purely by the application of pressure: it must be also be cooled below the critical temperature.
Natural gas is often liquefied before being transported to its point of end use. Liquefaction enables the volume of natural gas to be reduced by a factor of about 600. The capital costs, and running costs, of the apparatus required to liquefy the natural gas is very high, but not as high as the cost of transporting unliquefied natural gas.
The liquefaction of natural gas can be carried out by cooling the gas in countercurrent heat exchange relationship with a gaseous refrigerant, rather than with the liquid refrigerants used in conventional liquefaction methods, such as the cascade or propane-precooled mixed refrigerant processes. At least part of the refrigerant is passed through a refrigeration cycle which involves at least one compression step and at least one expansion step. Before the compression step, the refrigerant is usually at ambient temperature (ie the temperature of the surrounding atmosphere). During the compression step, the refrigerant is compressed to a high pressure, and is warmed by the compression process. The compressed refrigerant is then cooled with the ambient air, or with water if there is a water supply available, to return the refrigerant back to ambient temperature. The refrigerant is then expanded in order to cool it further. There are basically two methods of achieving the expansion. One method involves a throttling process, which may take place through a J-T valve (Joule-Thomson valve), wherein the refrigerant is expanded substantially isenthalpically. The other method involves a substantially isentropic expansion, which may take place through a nozzle, or, more usually, through an expander or turbine. The substantially isentropic expansion of the refrigerant is known in the art as "work expansion". When the refrigerant is expanded through a turbine, work may be recovered from the turbine: this work can be used to contribute to the energy required to compress the refrigerant.
It is generally recognised that work expansion is more efficient than throttling (a greater temperature drop can be achieved for the same pressure reduction), but the equipment is more expensive. As a result most processes usually use only work expansion, or a mixture of work expansion and throttling.
When natural gas of a particular composition is cooled at a constant pressure, then for any given temperature of the gas there will be a particular value for the rate of change of enthalpy (Q) of the gas. The temperature (T) can be plotted against Q to produce a "cooling curve" for natural gas. The cooling curve is highly dependent upon pressure: if the pre
REFERENCES:
patent: 3516262 (1970-06-01), Bernstein
patent: 3677019 (1972-07-01), Olszewski
patent: 5768912 (1998-06-01), Dubar
BHP Petroleum Pty Ltd.
Doerrler William
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