Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
1999-12-01
2001-10-16
Doerrler, William (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
Reexamination Certificate
active
06301929
ABSTRACT:
The invention starts from a process for producing pressurized oxygen in which compressed and prepurified feed air is introduced into the rectifying system and a first oxygen fraction is taken off from the low-pressure column, brought to an elevated pressure in the liquid state, vaporized and removed as gaseous pressurized oxygen product.
Processes of this type for producing gaseous pressurized oxygen has long been known (see for example DE 880893). The pressure elevation in the liquid product with subsequent vaporization is frequently called “internal compression”. DE 19529681 A and EP 716280 A show relatively recent examples of such processes.
The object underlying the invention is, in a process of this type and in a corresponding apparatus, to produce in addition to the pressurized oxygen product, a krypton- and xenon-enriched product in an economically expedient manner.
In the methods for krypton/xenon production known to date, the bottoms fraction of the low-pressure column (the second oxygen fraction) is introduced into a krypton-xenon enrichment column (methane ejection column), to the top of which is applied low-krypton/xenon liquid oxygen. By this means the methane which collects in the bottoms of the low-pressure column can be removed from the process via the gaseous overhead product of the methane ejection column. The bottoms product of the methane ejection column contains only extremely low amounts of methane and is enriched in krypton and xenon. It can either be withdrawn directly as krypton/xenon preconcentrate from the methane ejection column or recirculated into the low-pressure column and from there withdrawn as preconcentrate. This mode of operation is known per se and is described for example in Hausen/Linde, Tieftemperaturtechnik [Cryogenics], 2nd edition, 1985, pages 337 ff. and in DE 4332870 A1.
In the present invention, the krypton/xenon enrichment column (which if appropriate acts as methane ejection column) is operated at an elevated pressure which preferably approximately corresponds to the desired product pressure in the pressurized oxygen. The operating pressure of the krypton-xenon enrichment column is, for example, 1.5 to 10 bar, preferably 2.5 to 7 bar. The liquid oxygen from which the pressurized oxygen product is formed (the first oxygen fraction) is not withdrawn as is customary at its bottom, but above a mass-transfer section which retains krypton and xenon in the bottoms of the low-pressure column. The mass-transfer section forms the low-krypton/xenon reflux liquid for the krypton-xenon enrichment column. With regard to the production of pressurized oxygen, the oxygen is vaporized, instead of the indirect evaporation which is customary in internal compression processes, by direct heat exchange with the vapour ascending in the krypton-xenon enrichment column. The vaporized first oxygen fraction is withdrawn as overhead vapour of the krypton-xenon enrichment column, heated to ambient temperature and removed as pressurized oxygen product. The mass-transfer section below the takeoff of the first oxygen fraction is formed by at least one, preferably 1 to 5, most preferably 1 to 3, rectifying plates which are disposed directly above the low-pressure column bottoms.
Preferably, in the invention a two- or multicolumn system is used for the nitrogen-oxygen separation, which system, in addition to the low-pressure column also has a high-pressure column which is operated at a higher pressure than the low-pressure column. Preferably, high-pressure column and low-pressure column are thermally coupled via a shared condenser-evaporator (main condenser), in which nitrogen-rich vapour of the high-pressure column is condensed against a vaporizing oxygen-rich liquid from the low-pressure column. However, the invention can also be implemented with a single-column system in which the low-pressure column is formed by an individual column. The use of the term low-pressure column does not necessarily mean that this column is operated at about atmospheric pressure. Not only in the case of single-column processes, but also with double-column and multicolumn processes, the low-pressure column can also be operated at elevated pressure. The operating pressure of the low-pressure column is for example 1.1 to 4 bar, preferably 1.1 to 2.0 bar. The krypton-xenon enrichment column is operated below the critical pressure of oxygen, depending on the product pressure for example at 2 to 10 bar, preferably at 5 to 6 bar.
The first oxygen fraction is not taken off directly at the bottom of the low-pressure column, but at least one actual or theoretical plate above the bottom or above the takeoff of the second oxygen fraction. (In the event that in the respective section only actual plates are used as mass-transfer elements, the specifications apply as actual numbers of plates; if arranged packing, random packing or combinations of different types of mass-transfer elements are used, the specifications must be employed as theoretical numbers of plates.) For pressure elevation in the liquid state, any known means or a combination of different known means can be used.
In comparison with a simple combination of known internal compression processes, in which the first oxygen fraction is withdrawn from bottoms of the low-pressure column, with known processes for krypton/xenon production using a krypton/xenon enrichment column (methane ejection column), in the process of the invention, the yield of krypton and/or xenon is increased by 20 to 25%.
The second oxygen fraction, before it is introduced into the krypton-xenon enrichment column, must be brought to its operating pressure. Preferably, the second oxygen fraction, before it is introduced into the krypton-xenon enrichment column, is brought, however, in the liquid state to an elevated pressure and thereafter introduced in the liquid state into the krypton-xenon enrichment column.
Especially in the case of liquid introduction of the second oxygen fraction into the krypton-xenon enrichment column, this requires a bottoms evaporator. It is expedient if this is operated by indirect heat exchange with a partial stream of the feed air. Preferably, the feed air condenses at least partially in the bottoms evaporator. The condensate produced in the indirect heat exchange is introduced, for example, into one of the columns of the rectifying system, preferably into the low-pressure column.
Preferably, the feed air used as heating medium is brought upstream of the bottoms evaporator to a pressure which is higher than the highest operating pressure of the rectifying system columns. This pressure is chosen so that the condensation temperature of the feed air in the bottoms evaporator is, for example, about 1 to 2 K above the evaporation temperature of the bottoms liquid of the krypton-xenon enrichment column. This can be effected, for example, by all of the feed air being compressed to a very high pressure (for example to above the high-pressure column pressure in the case of a double-column system) or by the partial stream used as heating medium being recompressed from a lower level (for example high-pressure column pressure) to this high pressure.
REFERENCES:
patent: 3222879 (1965-12-01), Stoklosinski
patent: 5067976 (1991-11-01), Agrawal et al.
patent: 5122173 (1992-06-01), Agrawal et al.
patent: 5309719 (1994-05-01), Agrawal et al.
patent: 4332870 (1995-03-01), None
patent: 8706684 (1987-11-01), None
Doerrler William
Drake Malik N.
Linde Aktiengesellschaft
Millen White Zelano & Branigan P.C.
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