Reducing the amount of components having low boiling points in l

Details Reducing the amount of components having low boiling points in l Reducing the amount of components having low boiling points in l

1998-08-05

2000-01-18

Capossela, Ronald

Refrigeration

Cryogenic treatment of gas or gas mixture

Separation of gas mixture

62613, 62619, F25J 300

Patent

active

060148691

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to a method of reducing the amount of components having low boiling points in liquefied natural gas. The components having low boiling points are generally nitrogen, helium and hydrogen, these components are also called `light components`. In such a method the liquefied natural gas is liquefied at liquefaction pressure, and subsequently the pressure of the liquefied natural gas is reduced and separated to obtain liquefied natural gas having a reduced content of components having a low boiling point at a low pressure, which liquefied natural gas can be further treated or stored. Thus this method serves two ends, first reducing the pressure of the liquefied natural gas to the low pressure, and second separating a gas stream including components having low boiling points from the liquefied natural gas, thus ensuring that the remaining liquefied natural gas has a sufficiently low content of components having low boiling points. In general the contents of low boiling point components, in particular nitrogen, is reduced from between 2 to over 15 mol % to less than 1 mol %. Such a method is sometimes called an end flash method.
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.
International patent application publication No. WO 93/08 436 relates to a method of reducing the amount of components having low boiling points in liquefied natural gas, which method comprises the steps of:
(a) passing the liquefied natural gas at liquefaction pressure or at an intermediate pressure through the hot side of an external heat exchanger to obtain cooled liquefied natural gas, allowing the cooled liquefied natural gas to expand to a low pressure to obtain expanded fluid, and introducing the expanded fluid into the upper part of a fractionation column provided with a contacting section arranged between the upper part and the lower part of the fractionation column;
(b) passing a liquefied natural gas fraction withdrawn from the fractionation column through the cold side of the external heat exchanger to obtain heated two-phase fluid;
(c) introducing the heated two-phase fluid into the lower part of the fractionation column and allowing the vapour to flow upwards through the contacting section;
(d) allowing the liquid of the expanded fluid introduced in the upper part of the fractionation column to flow downwards through the contacting section; and
(e) withdrawing from the lower part of the fractionation column a liquid product stream having a reduced content of components having low boiling points, and withdrawing from the upper part of the fractionation column a gas stream which is enriched in components having low boiling points, wherein the expansion from liquefaction pressure to intermediate pressure is done dynamically and wherein the expansion from the intermediate pressure to low pressure is done statically.
The intermediate pressure is in between the liquefaction pressure and the low pressure, and it is so selected that evaporation during the dynamic expansion is substantially avoided.
In the known method, a fraction is withdrawn from the fractionation column which is heated in the external heat exchanger to provided vapour for stripping. The fraction is a normal side stream which is removed from the fractionation column at a level within the contacting section, which contacting section is arranged below the level at which the expanded fluid is introduced in the upper part of a fractionation column. For example if the contacting section comprises contacting trays, the fraction is removed from a level between adjacent contacting trays. Consequently the fraction has been in intimate contact with vapour rising through the fractionation column before it is removed from the fractionation column. A result of this intimate contact is that matter and heat are exchanged bet

REFERENCES:
patent: 3837172 (1974-09-01), Markbreiter et al.
patent: 3915680 (1975-10-01), Crawford et al.
patent: 4273566 (1981-06-01), Schwarz
patent: 4453958 (1984-06-01), Gulsby et al.
patent: 4479871 (1984-10-01), Pahade
Search Report dated May 20, 1997.

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