Patent Application
20070033968
The invention relates to a process for obtaining krypton and/or xenon by cryogenic separation of air, in which krypton and xenon are enriched to form a krypton/xenon concentrate and krypton and/or xenon are obtained from the krypton/xenon concentrate by means of distillation.
The basic principles of cryogenic separation of air in general as well as the structure of rectification systems for nitrogen/oxygen separation in particular 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 at a higher pressure than the low-pressure column; the two columns preferably exchange heat with one another, for example via a main condenser, in which top gas of the high-pressure column is liquefied against evaporating bottom liquid of the low-pressure column. The rectification system of the invention may be designed as a conventional double column system or alternatively as a three-column system or a system comprising even more than three columns. In addition to the columns for nitrogen/oxygen separation, it is also possible for there to be further apparatuses for obtaining other air constituents, in particular noble gases, for example argon recovery.
To obtain pure krypton and xenon, it is customary for the heavy components of the bottom oxygen of a cryogenic air separation unit (which is formed for example in the bottom of a krypton/xenon enrichment column) to be enriched by a factor of 3000 to 5000 compared to the feed air. This enriched liquid (“krypton/xenon concentrate”), in addition to, for example, 3000 to 5000 vppm of krypton and 200 to 400 vppm of xenon, also contains several hundred vppm of hydrocarbons, CO2, N2O and traces of fluorine-containing components, such as for example CF4, SF6 and C2F6.
Halogenated carbons are also present in clean air in a concentration of in each case a few hundred mol ppt (parts per trillion=10−12). On account of the high level of enrichment for obtaining krypton and/or xenon, some halogenated compounds constitute disruptive impurities which are difficult to separate off. In particular the fully fluorinated compounds are highly inert and can only be broken down at very high temperatures.
In a cryogenic air separation unit, the feed air of which is purified in molecular sieve adsorbers, in principle only those halogenated compounds which can also pass through the molecular sieve adsorbers as constituents of the feed air pass into the bottom oxygen. The primary purpose of the molecular sieve adsorbers is to remove CO2 and H2O from the feed air. In addition to the compounds CF4, C2F6 and SF6 which are often cited in the literature, NF3, CClF3 and C3F8 have also been proven to break through an adsorber filled with the molecular sieve 13X, which is regularly used for air purification, even before CO2. They therefore represent possible impurities in the krypton/xenon concentrate.
The krypton/xenon concentrate produced in the bottom oxygen of the cryogenic air separation unit can be separated in situ in further process steps to form pure krypton and/or xenon. Alternatively, it is stored, for example in a liquid tank, and processed further elsewhere.
The krypton/xenon concentrate is generally evaporated and reacted at approximately 500° C. over a catalyst which contains platinum or palladium and on which methane is completely burnt to form CO2 and H2O, and in addition N2O is broken down to form nitrogen and oxygen. Only a small proportion of the fluorine-containing compounds are reacted. CO2 and moisture are removed from the resulting gas in molecular sieve adsorbers, and the gas is then fed to cryogenic separation, where krypton and/or xenon have to be separated not only from oxygen, argon and nitrogen, by distillation, but also from the abovementioned fluorinated compounds. The latter constitutes a particularly high level of outlay.
Therefore, EP 863375 A1 (=U.S. Pat. No. 6,063,353) has proposed an additional adsorption step with a fixed sorbent which contains phyllosilicates, in order to separate fluorine-containing and/or chlorine-containing impurities, in particular fluorohydrocarbons, CF4 and/or SF6, from the krypton/xenon concentrate.
The invention is based on the object of realizing the removal of halogen compounds in a particularly economical way.
This object is achieved by virtue of the fact that the krypton/xenon concentrate is passed over a bed which contains a catalyst based on a transition metal oxide, in particular based on TiO2 or ZrO2 and in which at least one fully halogenated compound, in particular at least one fully fluorinated compound, is reacted.
“Fully halogenated” compounds and “fully fluorinated” (“perfluorinated”) compounds do not contain any hydrogen atoms which would permit catalytic reaction under very much milder conditions.
Catalytic reactions of this type are previously unknown in cryogenic air separation and in the production of krypton and xenon. However, they are already used in semiconductor manufacture for converting used etching gases, where fully fluorinated (=perfluorinated) carbons, such as CF4, C2F6 or C3F8, or fully fluorinated compounds, such as NF3 and SF6, are reacted over catalysts based on TiO2 and ZrO2. The invention now employs a method of this type to remove fully halogenated compounds from the krypton/xenon concentrate.
Unlike catalysts which have previously been used for purifying krypton/xenon concentrate, TiO2 and ZrO2 are chemically inert with respect to the HF formed during the reaction. In addition, like the materials made from platinum or palladium on an aluminium base which have previously been used, they convert methane contained in the krypton/xenon concentrate to form CO2 and H2O; moreover, N2O is broken down into N2 and O2.
The reactions which take place during the conversion of the halogen compounds are generally hydrolysis reactions, and therefore apart from the destruction of C2F6 they do not require any oxygen but do necessitate the presence of H2O. However, this substance is already present in any case on account of the parallel reaction of methane described above.
The catalyst bed can be used instead of or in addition to the catalyst bed which has hitherto been customary for the conversion of methane. If it is used in addition to the previously customary catalyst bed, the krypton/xenon concentrate is, for example, first of all passed, at approximately 400 to 500° C., over a material containing platinum and/or palladium, and is then passed over the transition metal oxide at approximately 750° C. The two catalyst beds may be arranged in separate containers or in one common housing.
It is advantageous if the catalyst bed is operated at a temperature of 650° C. or more, in particular of 700° C. or more, typically at approximately 750° C.
TiO2 and ZrO2 can be stabilized in order to be able to perform their role at temperatures from 650° C. to 700° C. (cf. Solid State Technology, July 2001, pages 189-200).
During the conversion, at least one hydrogen halide, HF and/or HCl is produced. Downstream of the catalyst bed, it is preferable for the krypton/xenon concentrate to be fed to a purification stage for removing the hydrogen halide, in particular a water or lye scrub. The scrub can also be used to separate off SO3; furthermore, it serves to further cool the gas stream.
CO2 is generally also formed during the conversion. This component is removed again from the krypton/xenon concentrate by being passed through an adsorption bed downstream of the catalyst bed, as has hitherto already been customary. The adsorption bed is preferably located downstream of the purification stage for removing the hydrogen halide.
It is preferable to react at least one of the compounds CF4, C2F6, SF6, NF3, CClF3 and C3F8 in the catalyst bed; in particular, at least one of the following reactions takes place:
CF4+2H2O→CO2+4HF
C2F6+3H2O+0.5 02→2CO2+6HF
SF6+3H2O→SO3+6HF
NF3+1.5H2O→NOx+3HF
CF3Cl+2H2O→CO2+3HF+HCl
The invention also relates to an apparatus for obtaining krypton and/or xenon by cryogenic separation of air according to patent claims 8 to 10.
The invention and further details of the invention are explained in more detail below on the basis of an exemplary embodiment.
In the exemplary embodiment, the cryogenic air separation is carried out in a conventional Linde double column which comprises high-pressure column and low-pressure column. An oxygen fraction is removed from the bottom of the low-pressure column or the bottom of the crude argon condenser; this fraction also contains all the air components of relatively low volatility and is passed into a krypton/xenon enrichment column. A krypton/xenon concentrate is extracted from the bottom of this column, evaporated and fed to a catalyst bed operated at 750° C. After cooling, HF and HCl are removed downstream of the catalyst bed by a water scrub. The krypton/xenon concentrate then flows through an adsorption bed, in which water and CO2 as well as any SO2 which may have formed from SF6 are removed. Finally, the purified krypton/xenon concentrate is fed to a distillation device, in which oxygen, argon and nitrogen are separated off and then a pure krypton product and a pure xenon product are obtained.
Alternatively, the krypton/xenon concentrate from the double column or the krypton/xenon enrichment column can be collected in a liquid reservoir and transported to a spatially remote preparation plant which includes the catalyst bed according to the invention.
The entire disclosure[s] of all applications, patents and publications, cited herein and of corresponding German Application No. 10 2005 037 576.6, filed Aug. 9, 2005 are incorporated by reference herein.
The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.
From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102005037576.6 | Aug 2005 | DE | national |
