Refrigeration – Cryogenic treatment of gas or gas mixture – Liquefaction
Patent
1997-12-23
1999-04-13
Capossela, Ronald
Refrigeration
Cryogenic treatment of gas or gas mixture
Liquefaction
62619, F25J 100
Patent
active
058932747
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method of liquefying and treating natural gas containing components having low boiling points. The components having low boiling points are generally nitrogen, helium and hydrogen, these components are also called "light components". In this method the liquefied gas is liquefied at liquefaction pressure, and subsequently the pressure of the liquefied gas is reduced to obtain liquefied gas having a reduced content of components having a low boiling point at a low pressure, which liquefied gas can be further treated or stored. The treating part of the method is sometimes called an end flash method. Such an end flash method serves two ends, first reducing the pressure of the liquefied gas to the low pressure, and second separating a gaseous stream including components having low boiling points from the liquefied gas, thus ensuring that the remaining liquefied gas has a sufficiently low content of components having low boiling points.
The liquefaction pressure of natural gas is generally in the range of from 3.0 to 6.0 MPa. The low pressure is below the liquefaction pressure, for example the low pressure is less than 0.3 MPa and suitably the low pressure is about atmospheric pressure, between 0.10 and 0.15 MPa.
Known is a method of liquefying and treating a natural gas containing components having low boiling points which method comprises the steps of: side of a main heat exchanger; cold side of the main heat exchanger, allowing the cooled refrigerant to evaporate at the refrigerant pressure in the cold side of the main heat exchanger to obtain vaporous refrigerant at refrigerant pressure, and removing vaporous refrigerant from the cold side of the main heat exchanger; side of the main heat exchanger; a low pressure to obtain expanded fluid; stream having a reduced content of components having low boiling points; and is enriched in components having low boiling points.
A different method of liquefying and treating a natural gas containing components having low boiling points is described in UK patent specification No. 1 572 899. This method comprises the steps of: side of a main heat exchanger; cold side of the main heat exchanger, allowing the cooled refrigerant to evaporate at the refrigerant pressure in the cold side of the main heat exchanger to obtain vaporous refrigerant at refrigerant pressure, and removing vaporous refrigerant from the cold side of the main heat exchanger; side of the main heat exchanger; arranged in the lower part of a fractionation column to obtain cooled liquefied gas; a low pressure to obtain expanded fluid; product stream having a reduced content of components having low boiling points; and stream which is enriched in components having low boiling points.
In the latter method the heat exchanger in which the liquefied gas is cooled is formed by the lower part of the fractionation column, and the hot side of the heat exchanger comprises a tube bundle arranged in the lower part of the fractionation column. The liquid in the lower part of the fractionation column cools the liquefied gas passing through the tube bundle. It will therefore be understood that withdrawing the liquid stream from the bottom of the fractionation column in step (g) has to be conducted at such a rate that the tube bundle of the heat exchanger remains submerged in liquid.
Such a heat exchanger is a so-called internal reboiler. An internal reboiler, however, cannot be designed separately from the fractionation column, and consequently the allowable heat transfer area per unit of column height is affected by the required dimensions of the fractionation column. Since the heat transfer area has an effect on the process design, mechanical limitations affect the process design and this may lead to a process design that is not optimal.
It is an object of the present invention to overcome the above-mentioned drawbacks. It is a further object of the present invention to obtain a larger temperature drop in the expanding liquefied gas and, consequently, to obtain a better overall liquefac
REFERENCES:
patent: 3735600 (1973-05-01), Dowdell et al.
patent: 4970867 (1990-11-01), Herron et al.
Klein Nagelvoort Robert
Vink Kornelis Jan
Capossela Ronald
Shell Research Limited
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