Refrigeration – Processes – Circulating external gas
Patent
1988-02-29
1989-02-21
Capossela, Ronald C.
Refrigeration
Processes
Circulating external gas
55 66, F25J 304
Patent
active
048054127
DESCRIPTION:
BRIEF SUMMARY
This invention relates to air separation and in particular to apparatus and method for reducing the loss of krypton and xenon during air separation by cryogenic (low-temperature) distillation.
The separation of air into two main constituents (nitrogen and oxygen) has long been practised. Methods and apparatus employed on a large scale make use of the lower boiling point of nitrogen (77.degree. K.) as compared to that of oxygen (90.degree. K.). Thus from a liquid mixture of nitrogen and oxygen, the nitrogen will boil away preferentially, to be captured as required, the residual liquid being oxygen rich. The residual liquid also contains some trace constituents of the original air, including krypton (boiling point 120.degree. K.) and xenon (boiling point 165.degree. K.). These rare gases both have significant commercial value, for instance for inclusion in electric lamp bulbs and other illuminating devices. The efficient recovery from the oxygen fraction of krypton in particular has been the subject of many studies.
When the oxygen fraction is delivered from the primary air separation plant mainly or wholly in the gas phase, one known method and apparatus is to wash the gaseous oxygen with liquid oxygen having a krypton content below that in equilibrium with the gas phase. The washings are partially concentrated by distillation.
A general disclosure of this approach is that shown in British Patent Specification No. 390,069 (FIG. 2); the washing column 4 and concentrating column 6 are hereinafter termed the primary krypton column. A double column with oxygen recovery is described in U.S. Pat. No. 4,401,448 (La Clair).
The concentration of krypton which can be attained with safety is limited by the desirability of restricting the concentration of accompanying hydrocarbons. The method is suited to producing a partial concentrate having a molar content of 0.1-5% krypton plus associated xenon which is hereinafter termed primary krypton concentrate. This is the first step and a major step toward complete separation of krypton and xenon from the oxygen.
A specific embodiment of the known art is shown in FIG. 1. This shows a conventional double column comprising medium-pressure column 10 and low-pressure column 12. A condenser-reboiler 14 is positioned in heat-transfer relationship with both columns; the unit 14, which conventionally is of either the multi-tubular type or the extended-surface type, acts as the main condenser. It will be understood that in alternative embodiments the columns may be positioned alongside each other, with the condenser-reboiler 14 at the head of the column 10 or outside it. Column 12 includes vapour-liquid contacting trays 13.
In use, chilled compressed air enters column 10 through inlet conduit 16. Liquid oxygen containing krypton and xenon collects in the main condenser bath 18 formed around condenser-reboiler 14. Nitrogen vapour from medium-pressure column 10 condenses in unit 14 giving up its latent heat and thereby vaporizing main condenser bath liquid. Waste nitrogen is exhausted at outlet 52 and is preferably passed through heat exchangers. From above the bath 18, vapour is withdrawn through conduit 118 and fed to the primary krypton column 20. Since krypton has a higher boiling point than oxygen, the oxygen vapour contains less krypton than does an equal mass of bath liquid.
The primary krypton column 20 houses trays which effect vapour and liquid contact. Thus vapour from conduit 118 ascends through tray set 22 and in doing so meets descending liquid oxygen which washes out the less volatile xenon and most of the krypton. The now kryptonlean vapour continues to ascend and is then separated into two streams 26,28; stream 26 is removed as gaseous oxygen plant product, after passing through heat exchangers or regenerators, and stream 28 enters cooling condenser 30 where it liquifies and is fed through inlet 32 back into the primary krypton column 20 to provide the aforementioned descending wash liquid.
The washings containing krypton and xenon descend into the lower section 34 and
REFERENCES:
patent: 3609983 (1971-10-01), Lofredo et al.
patent: 3779028 (1973-12-01), Schuftan et al.
patent: 4401448 (1983-08-01), La Clair
patent: 4433990 (1984-02-01), Olszewski
patent: 4568528 (1986-02-01), Cheung
patent: 4574006 (1986-03-01), Cheung
patent: 4647299 (1987-03-01), Cheung
Colley, deceased Charles R.
Passant, executrix by Muriel E.
BOC Cryoplants Limited
Capossela Ronald C.
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