Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2000-01-27
2001-04-24
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S925000
Reexamination Certificate
active
06220054
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a method and plant for separating air. In particular, it relates to the separation of a krypton-xenon concentrate from air.
The main components of air are oxygen and nitrogen. Oxygen and nitrogen are separated from air by rectification at cryogenic temperatures. The air is pre-purified by adsorption to remove impurities such as water vapour and carbon dioxide which freeze at cryogenic temperatures. The purified air is cooled to a temperature suitable for its rectification. The rectification is typically conducted in a double rectification column comprising a higher pressure column, a lower pressure column and a condenser-reboiler placing the higher pressure column in heat exchange relationship with the lower pressure column. The higher pressure column serves to provide liquid nitrogen reflux for the lower pressure column and to produce an oxygen-enriched air fraction which is separated in the lower pressure column. If an argon product is required, a stream of argon-enriched oxygen vapour, typically containing some 5 to 15% by volume of oxygen, is withdrawn from an intermediate region of the lower pressure column and is separated in a further rectification column.
Whereas nitrogen, oxygen and argon make up more than 99% by volume of ambient air, the proportions of krypton and xenon in it are small indeed. Thus, each million volumes of air contains about 1.1 volumes of krypton but only 0.08 volumes of xenon. Nonetheless, krypton and xenon are both valuable industrial products. Both gases are used in lighting in view of their fluorescent properties. In addition, xenon is used in flash photography and X-ray photography. Xenon also has anaesthetic properties and was used in the former Soviet Union as an anaesthetic gas for maintaining a state of anaesthesia.
Because krypton and xenon have a low volatility in comparison to oxygen, they tend to accumulate in the liquid oxygen fraction which is obtained at the bottom of the lower pressure column of the double rectification column during separation of air to form an oxygen product. Nonetheless, the proportion of krypton and xenon in this bottom fraction is still very small and a number of purification steps need to be formed in order to obtain relatively pure krypton and xenon products. The first of these purification steps involves continuously withdrawing a stream of krypton-xenon concentrate from the bottom of the lower pressure column and separating it in a further rectification column which is provided with both a reboiler and a condenser. According to W H Isalski at page 97 of his textbook “
Separation of Gases
” (Clarendon Press, Oxford (1989)) the bottom fraction obtained from the further concentration of the liquid oxygen in krypton and xenon still only contains from 0.1 to 0.2% by volume of krypton and xenon. The bottom fraction is subjected to so-called secondary and tertiary separations in order to provide krypton and xenon products. Frequently, a significant proportion of the krypton/xenon content of the air is lost in gaseous oxygen product. (Even more would be lost were oxygen product to be taken in liquid state).
There is therefore a need to improve upon the conventional method and plant for obtaining the krypton-xenon product by making it possible both to achieve relatively easily a greater proportion of krypton and xenon in the concentrate and to improve the recovery of krypton and xenon. It is the aim of the method and plant according to the invention to meet this need.
SUMMARY OF THE INVENTION
According to the present invention there is provided a method of separating air, comprising subjecting air to a main rectification which includes an argon-oxygen separation and in which a first liquid oxygen fraction concentrated in krypton is formed, taking a vaporous argon-oxygen stream depleted of krypton and xenon from the main rectification, subjecting the vaporous argon-oxygen stream to further rectification and thereby obtaining a second liquid oxygen fraction containing argon impurity, taking a stream of the first liquid oxygen fraction as a krypton-xenon concentrate, and purifying the second liquid oxygen fraction separately from the main rectification and thereby obtaining a purified oxygen product.
The invention also provides plant for separating air, comprising a main rectification column for performing a main rectification of air so as to obtain a first liquid oxygen fraction concentrated in krypton and xenon, the main rectification column having an argon-oxygen separation section; a first outlet from the main rectification column for a vaporous argon-oxygen stream depleted of krypton and xenon; a further rectification column for obtaining from the vaporous argon-oxygen stream a second liquid oxygen fraction containing argon impurity; at least one additional liquid-vapour contact column or column section for purifying the second liquid oxygen fraction separately from the main rectification and for thereby obtaining a purified oxygen product; and a second outlet from the main rectification column for a stream of the first liquid oxygen fraction as a krypton-xenon concentrate.
By forming a purified oxygen product separately from the main rectification it becomes possible in the method and plant according to the present invention to reduce the krypton/xenon losses in the purified oxygen product and obtain a higher recovery of krypton/xenon than would be the case with conventional methods. Indeed, the purified oxygen product has a lower concentration of krypton/xenon than would be possible if the oxygen product were simply taken from the bottom of the lower pressure column of a double rectification column in a conventional manner.
Another advantage of the method and plant according to the invention is their simplicity.
The main rectification is preferably conducted in a double rectification column comprising a higher pressure column, a lower pressure column, and a condenser-reboiler placing the higher pressure column in heat exchange relationship with the lower pressure column. In one alternative, the main rectification may be performed in a single rectification column, for example, as shown in GB-A-1 258 568.
Preferably, no (gaseous) oxygen product is taken from the main rectification apart from any oxygen recovered from the krypton-xenon concentrate. It is, however, within the scope of the invention to take additional oxygen product in vapour state directly from the main rectification. Such a practice entails enhanced losses of krypton and xenon but these losses are not as great as they would be were all the oxygen product to be taken directly from the main rectification.
The further rectification is preferably conducted in one or more further rectification columns so as to give either a crude argon fraction containing oxygen impurity or an oxygen-free argon fraction in addition to the second liquid oxygen fraction.
The purification of the second liquid oxygen fraction preferably comprises countercurrent contact between a stripping gas and a flow of the second liquid oxygen fraction. Following this countercurrent contact the stripping gas is preferably introduced into the further rectification column or one of the further rectification columns and at least part of the stripped second liquid oxygen fraction taken as product.
The purification of the second liquid oxygen fraction is preferably performed in a stripping column separate from the further rectification column or columns. An alternative arrangement, however, is to perform the purification in a discrete stripping section of the further rectification column or columns. Such an arrangement is often not preferred because it adds to the total height of the further rectification column or columns. Another alternative arrangement is to house the stripping section in the main rectification column, albeit in such a way that the second liquid oxygen fraction is kept separate from the first liquid oxygen fraction. In a typical example of such an arrangement, the stripping section is provided in an annular region of
Capossela Ronald
Cohen Joshua L.
Pace Salvatore P.
The BOC Group plc
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