Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2002-10-31
2003-09-02
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S925000
Reexamination Certificate
active
06612129
ABSTRACT:
DESCRIPTION
The invention relates to a process which is used to produce krypton and/or xenon by low-temperature fractionation of air.
The basic principles of the low-temperature fractionation of air in general and the structure of rectification systems for nitrogen-oxygen separation specifically are described in the monograph “Tieftemperaturtechnik” [Cryogenic Engineering] by Hausen/Linde (2nd Edition, 1985) and in an article by Latimer in Chemical Engineering Progress (Vol. 63, No. 2, 1967, page 35). The high-pressure column is operated under a higher pressure than the low-pressure column; the two columns are preferably in heat-exchanging relationship with one another, for example via a main condenser, in which top gas from the high-pressure column is liquefied against evaporating bottom liquid from the low-pressure column. The rectification system of the invention may be designed as a conventional double column system, but may also be designed as a three-column or multicolumn system. In addition to the columns for nitrogen-oxygen separation, there may also be further apparatus for producing other air components, in particular noble gases, for example an argon production apparatus.
A process for producing krypton and/or xenon by low-temperature fractionation of air and a corresponding apparatus are known from DE 10000017 A1. In this process, a krypton- and xenon-containing fraction, specifically the bottom liquid, from the high-pressure column of the double column for nitrogen-oxygen separation is passed, without any measures which change the concentrations, into a further column which is used to produce krypton-xenon.
DE 2605305 A shows a process and an apparatus for producing krypton and/or xenon by low-temperature fractionation of air of the type described in the introduction. In this document, the first condenser-evaporator is heated by condensing top gas from a crude argon column and, at the same time, forms the bottom heating of the krypton-xenon enrichment column. All the vapour which rises in the krypton-xenon enrichment column is produced in the first condenser-evaporator.
An object of the invention is to further improve the production of krypton and xenon, and in particular to carry out this production in a particularly economic way.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
These objects are achieved by introducing a liquid from the lower region of the krypton-xenon enrichment column into a second condenser-evaporator, which is separate from the first condenser-evaporator.
In the invention, therefore, there is a separate heat exchanger, the “second condenser-evaporator”, in which rising vapour for the krypton-xenon enrichment column is produced independently of the first condenser-evaporator, and in this way relatively low-volatility constituents are concentrated further. The second condenser-evaporator is preferably designed for bottom heating of the krypton-xenon enrichment column. It may be arranged inside this column or in a separate vessel.
The second condenser-evaporator leads to a less high oxygen concentration being established in the first condenser-evaporator, so that, on account of the correspondingly reduced temperature difference, the overall size of the first condenser-evaporator can be reduced. Moreover, there is less intensive concentration of relatively low-volatility constituents in the first condenser-evaporator, which is undesirable at this location for operational reasons. Within the context of the invention, the choice of heating means for the second condenser-evaporator can be selected as desired. In principle, any suitable process fraction can be used, for example, nitrogen, perhaps from the high-pressure column, any other fraction from the high-pressure column, a part-stream of the charge air or a fraction from a crude argon column which is connected to the low-pressure column, in particular crude argon from the top of a crude argon column of this type.
The “purge liquid” of the first condenser-evaporator serves as a charge fraction for the krypton-xenon enrichment column. In the present context, the term “krypton-xenon enrichment column” is understood as meaning a countercurrent mass transfer column in which a fraction which has a higher concentration of krypton and/or xenon than each of the charge fractions of this column is produced. By way of example, the krypton-xenon concentrate has a higher molar level of krypton and/or xenon than the “purge liquid” which is fed into the krypton-xenon enrichment column. This column may, for example, be designed as a transfer column, as described in DE 1000017 A1, and/or may at the same time be used to expel methane.
It is preferable for the purge liquid to be introduced in the lower region, for example directly above the bottom. In this case, a liquid is added to the top of the krypton-xenon enrichment column, in order to force the krypton which is present in the rising vapour downwards and to force methane upwards. This liquid may, for example, be removed from the high-pressure column, for example from the bottom of this column or a few plates above it. A possible alternative or additional source is the evaporation space of the top condenser of a pure argon column. In the bottom of the krypton-xenon enrichment column, the liquid flowing down can be boiled by means of a bottom evaporator. This allows the krypton and xenon contents of the krypton-xenon concentrate to be increased further. The bottom evaporator can be operated, for example, with compressed air or with compressed nitrogen from the top of the high-pressure column.
In the invention, an intermediate step, in the form of a partial evaporation in the first condenser-evaporator, may be carried out between the extraction of the krypton- and xenon-containing fraction from the high-pressure column and the feeding of this fraction into the krypton-xenon enrichment column. This step is used to concentrate krypton and/or xenon even before the krypton-xenon enrichment column is reached. As a further effect, all the other components with a lower volatility than oxygen are guided with the purge liquid out of the partial evaporation into the krypton-xenon enrichment column and are in this way kept away from other parts of the installation, in particular the low-pressure column.
The krypton-xenon concentrate which is produced in the krypton-xenon enrichment column has a krypton content of, for example, 600 to 5 000 ppm, preferably 1 200 to 4 000 ppm, a xenon content of, for example, 60 to 500 ppm, preferably 120 to 400 ppm. Otherwise, it consists mainly of oxygen and typically up to about 10 mol % of nitrogen.
The invention can particularly advantageously be implemented as part of an air fractionation plant with argon production in which an argon-containing fraction from the low-pressure column is introduced into a crude argon rectification stage. The crude argon rectification stage is used in particular for argon-oxygen separation and may be carried out in one or more columns (cf. for example EP 377117 B2 or EP 628777 B1). The cooling of the crude argon rectification stage which is in any case required is, in the context of the invention, effected by the krypton- and xenon-containing fraction, an argon-enriched vapour from the crude argon rectification coming into indirect heat exchange with the evaporating krypton- and xenon-containing fraction in the first condenser-evaporator. The partial evaporation as part of the krypton-xenon production therefore simultaneously serves to produce reflux and/or liquid product in the crude argon rectification stage.
In many cases, there is a liquid charge-air stream, for example in the internal compression of one or more products. The liquefied air is often split between high-pressure column and low-pressure column, for example by being introduced into a vessel which is arranged inside the high-pressure column and part of the liquid being removed again from this vessel and passed to the lo
Capossela Ronald
Linde Aktiengesellschaft
Millen White Zelano & Branigan P.C.
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