Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Halogenous component
Reexamination Certificate
2000-09-05
2003-05-20
Nguyen, Ngoc-Yen (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Halogenous component
C423S245100, C423S245300, C423S262000, C095S045000, C095S048000, C095S049000, C095S050000
Reexamination Certificate
active
06565821
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for purifying a gas stream containing krypton (Kr) and/or xenon (Xe), which gas stream contains, in addition, fluorine- and/or fluorosulphur-containing impurities and possibly other impurities that have to be separated from the said gas stream, such as oxygen and/or hydrocarbons (C
n
H
m
), the said gas stream then being able to be separated by cryogenic distillation, in order to produce high-purity krypton or xenon, or else possibly being a waste gas from such a cryogenic distillation
BACKGROUND OF THE INVENTION
The production of xenon or krypton is usually carried using atmospheric air, which air is separated by cryogenic distillation, so as to obtain, on the one hand, the conventionally recovered compounds, namely nitrogen, oxygen and/or argon, and, on the other hand, a gas mixture essentially containing xenon, krypton and impurities chosen from hydrocarbons (C
n
H
m
), oxygen and fluorocompounds or fluorosulphur compounds, such as CF
4
, C
2
F
6
or SF
6
for example.
Next, this gas mixture, essentially containing xenon, krypton and impurities, is:
either immediately separated and purified in order to obtain xenon on the one hand and krypton on the other hand;
or put into containers, such as gas bottles, and stored, before being separated and purified, as in the first case, later on.
At the present time, there are several known techniques which can be used to remove the fluorine- or fluorosulphur-containing impurities, such as the compounds CF
4
, C
2
F
6
or SF
6
, which may be contained in a gas or a gas mixture.
Among these known techniques, mention may be made of chemisorption, the plasma technique and hot catalytic destruction.
Thus, the technique of chemisorption of the fluorine-containing impurities on an adsorbent material of the phyllosilicate type, as described in document EP-A-863,375, is difficult to use on an industrial scale because of the complexity of the process and raises certain reliability problems.
Other documents propose to eliminate the fluorocompounds, often called PFC (standing for PerFluoroCompounds), by means of polymer membranes.
Thus, document EP-A-754,487 discloses a very effective process for removing by permeation, by means of one or more polymer membranes, the PFCs contained in a gas stream consisting of a carrier gas, such as air, oxygen, nitrogen, helium, CO
2
, xenon, CO, water vapour, hydrogen, krypton, neon or argon, contaminated with PFCs. This process is particularly suitable for removing the PFCs contained in a gas stream coming from a semiconductor fabrication process.
Moreover, document WO-A-90/15662 describes the use of a permselective membrane for separating a gas mixture, particularly a membrane consisting of an amorphous polymer of the perfluoro-2,2-dimethyl-1,3-dioxol type. The gas mixture may be air—a gas mixture of the nitrogen/oxygen type—which possibly contains one or more organic compounds, such as fluorocarbon compounds or any other volatile compound, such as Freon.
Furthermore, document EP-A-358,915 describes a membrane system for gas separation, in which any degradation of the membrane is minimized or prevented by removing the heavy-hydrogen-type impurities by adsorption of the latter on a bed of active carbon lying upstream of the membrane.
Moreover, documents JP-A-61,187,918 and JP-A-4,322,716 relate to the use of a membrane formed from a fluorocopolymer to remove water vapour from a wet gas, such as HCl, CF
4
, C
2
F
6
or the like, i.e. to dry such a wet gas.
More generally, mention may also be made of document JP-A-4,016,213 which describes the selective permeation of light alcohols, such as ethanol, contained in a gas mixture through a membrane consisting of a heat-resistant polymer, document JP-A-61,187,918 which teaches the use of a membrane formed from a fluorocopolymer to separate a gas used in a welding operation and document U.S. Pat. No. 5,383,957 which relates to a membrane system that can be used to produce nitrogen from air.
Furthermore, there are other, more or less specific, publications reporting processes which use membranes to separate a gas mixture, namely documents
US-A-4,957,513,
US-A-4,941,893,
US-A-4,178,224,
US-A-4,424,067,
EP-A-239,190,
JP-A-60,022,902,
US-A-4,701,187,
US-A-4,880,441,
US-A-4,881,953,
US-A-4,988,371,
US-A-5,051,114,
US-A-5,064,447,
US-A-5,256,295,
US-A-5,281,253,
US-A-5,282,964,
US-A-5,282,969,
US-A-5,290,341
and
WO-A-95/18674
However, these various techniques are often complex to use on an industrial scale, sometimes generating reliability and safety problems, and requiring, in some cases, considerable investment, and often high operating costs.
In addition, some of these processes cannot be applied to the purification of gas mixtures essentially containing xenon and/or krypton and/or do not allow effective removal of all the impurities likely to be present in the gas stream, namely mainly fluorocompounds or fluorosulphur compounds.
SUMMARY OF THE INVENTION
The object of the present invention is therefore to propose an improved process making it possible to achieve effective removal of the fluorine- or fluorosulphur-containing impurities which are contained in a xenon and/or krypton gas stream, which process is easy to employ and of acceptable costs from the industrial standpoint.
In other words, the present invention aims to provide a process for separating and purifying a gas stream containing krypton and xenon, as well as impurities, so as to remove the said impurities (fluorocompounds or fluorosulphur compounds) effectively.
Furthermore, in the case of a krypton/xenon mixture that has to be separated into each of these components, the invention also aims to provide, as an alternative, a process for the effective separation of the krypton/xenon mixture thus obtained and this separation being carried out in order to produce, on the one hand, high-purity xenon and, on the other hand, high-purity krypton, combined with effective removal of the said fluorocompounds or fluorosulphur compounds.
The invention therefore relates to a process for removing at least some of the gaseous fluorocompounds and/or fluorosulphur compounds present in a gas feed stream containing xenon and krypton, in which:
(i) the gas feed stream containing xenon and/or krypton and at least the said gaseous fluorocompounds and/or fluorosulphur compounds are brought into contact with at least one first membrane;
(ii) a production gas containing xenon and/or krypton stripped of at least some of the said gaseous fluorocompounds and/or fluorosulphur compounds is recovered on the output side of at least the said first membrane.
Within the context of the invention, the term “membrane” denotes any permeation means, especially membranes taken as they are, but also membrane modules, especially membrane modules based on hollow fibres, or else ceramic or similar membranes.
Moreover, within the context of the invention, the term “output side” should be understood to mean that side of the membrane from which the gas produced is recovered, that is to say the gas substantially purified of fluorine- or fluorosulphur-containing impurities. In the case of a membrane operating by conventional permeability, the “output side” is the permeate side, whereas in the case of a membrane operating by reverse permeability, the output side is the retentate side.
By analogy, within the context of the invention the term “waste gas side” should be understood to mean the opposite side of the membrane to the one where the gas produced is recovered, that is to say the side of the membrane via which a gas enriched with fluorine- or fluorosulphur-containing impurities leaves. In the case of a membrane operating by conventional permeability, the “waste gas side” is the retentate side, whereas in the case of a membrane operating by reverse permeability, the “waste gas side” is the permeate side.
According to a first variant, the invention also relates to a process for removing at least some of the gaseous fluorocompounds and/or fluorosulphur compounds present in a feed gas formed
L'Air Liquide - Societe Anonyme à Directoire et Conseil de
Nguyen Ngoc-Yen
Young & Thompson
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