Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
1999-04-30
2001-03-06
Doerrler, William (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
Reexamination Certificate
active
06196022
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a process for recovering high-purity oxygen by low-temperature separation of air in a rectification system having a high-pressure column and a low-pressure column, comprising introducing feed air into the high-pressure column, withdrawing an oxygen-containing liquid fraction from the high-pressure column and feeding said withdrawn fraction into the low-pressure column and passing gaseous nitrogen from the low-pressure column to a top condenser having a condensing side and an evaporating side in indirect heat exchange with an evaporating liquid in said evaporation side, so as to at least partially condense said gaseous nitrogen.
A process with these steps is known from DE 3528374 A1. In this two-column process, the low-pressure column has a top condenser in which gaseous top nitrogen is condensed and is recycled as reflux to the low-pressure column. This type of reflux production for the low-pressure column permits the withdrawal of a portion of the nitrogen produced in the double column as a pressurized product. The oxygen-concentrated liquid that accumulates as a bottom product in the low-pressure column is directed entirely to the evaporation side of the top condenser of the low-pressure column and is withdrawn as residual gas.
SUMMARY OF THE INVENTION
An object of the invention is to provide a process and apparatus to recover a high-purity oxygen product, as well as a pressurized nitrogen product, in a modified process of the above-mentioned type.
Upon further study of the specification to appended claims, the objects and advantages of the invention will become apparent.
These objects are achieved in that (A) the oxygen-containing liquid fraction that is fed to the low-pressure column is removed from at least one theoretical or actual plate above the bottom of the high pressure column, in that (B) the bottom liquid from the high pressure column is directed into the evaporation chamber of the top condenser of the low-pressure column, and in that (C) a high-purity oxygen is removed from the lower area of the low-pressure column.
In the production of high-purity oxygen, the reduction of nitrogen and argon contents in the oxygen product is relatively uncritical since it can be achieved by a correspondingly large number of plates in the lower section of the low-pressure column. These conventional measures do not, however, keep all less volatile contaminants from collecting in the oxygen product, i.e., air components having boiling points higher than oxygen and which were not removed by pre-cleaning the air upstream of the rectification system. Such less volatile air components include, for example, krypton, xenon, and hydrocarbons. It is also known to remove such contaminants in one or more subsequent rectification steps (see, for example, EP-299364-B1).
The process according to the present invention makes it unnecessary to employ additional rectification columns and uses the lower part of the high pressure column or an additional mass transfer section in the lower part of the high pressure column to separate the less volatile contaminants. The oxygen-containing liquid fraction that is directed into the low-pressure column is not removed from the bottom of the high pressure column, but rather from an intermediate point that is located above the bottom, especially above the air feed point into the high pressure column. Between the feedpoint and the intermediate point is located a mass transfer section comprising at least one theoretical or actual plate. This section preferably comprises 1 to 10, preferably 2 to 5 theoretical or actual plates, which are arranged between the air feed or the high pressure column bottom, on the one hand, and the point of removal of the oxygen-containing liquid fraction, on the other. (If, in this section, only actual plates are used as mass transfer elements, the data apply to actual plate numbers; if packing and filling materials or combinations of various types of mass transfer elements are used, the data can be used as theoretical plate numbers.)
By drawing off the feedstock for the low pressure column above the air feed, less volatile components of air such as hydrocarbons, krypton, and xenon are kept away from the low-pressure column. At the bottom of the column, a high-purity oxygen product is removed (total purity 99.5 to 99.999 vol %, preferably 99.8 to 99.999 vol %; proportion of less volatile components: 1 to 10 ppm, preferably 3 to 5 ppm). The high-purity oxygen can be drawn off in liquid and/or gaseous form directly at the bottom of the low-pressure column.
In the process according to the invention, the operating pressures of the columns can be, for example, 6 to 20, preferably 7 to 16 bar in the high pressure column and, for example, 3 to 8, preferably 3 to 6 bar in the low-pressure column.
The top condenser of the low-pressure column is operated at least in part with bottom liquid from the high pressure column as a refrigerant. Reflux for the high pressure column is usually produced by a condenser-evaporator via which the top of the high pressure column and the bottom of the low-pressure column are connected in heat exchange.
Especially for removing argon, a residual fraction can be removed from an intermediate point on the low-pressure column. This residual fraction, preferably an impure nitrogen fraction containing argon, is removed above the point where the oxygen-enriched liquid fraction is fed into the high pressure column.
Process cold can be produced by engine expansion pressure reduction of one or more of the following fractions:
Residual gas from the evaporation chamber of the top condenser of the low-pressure column
Vapor from the middle range of the low-pressure column (for example, the above-mentioned residual fraction)
Partial current of the volume of feed air
Nitrogen from the high pressure column or from the low-pressure column.
In the case of engine expansion pressure reduction of air, the turbine waste gas is fed preferably to the high pressure column or removed from the process, for example by being mixed with another residual current. In any case, the engine expanded air must not be fed to the low-pressure column since this would result in renewed contamination by less volatile components.
Using internal compression, the high-purity oxygen product can be bought to a pressure that is higher than the low-pressure column pressure, by having at least a portion of the oxygen product withdrawn in liquid form from the low-pressure column and evaporated under a pressure that is higher than the operating pressure of the low-pressure column. As a heating agent during evaporation, for example, correspondingly highly compressed air can be used.
To recover pressurized nitrogen, it is advantageous if a nitrogen fraction is removed in liquid form from the low-pressure column or its top condenser and the pressure of the nitrogen fraction in liquid state is increased to a value that is higher than the operating pressure of the low-pressure column. In this way—optionally in addition to direct removal of nitrogen from the high pressure column—gaseous nitrogen can be obtained under a pressure that is higher than the operating pressure of the low-pressure column. The liquid nitrogen that is pressurized can be returned to the high pressure column or evaporated in indirect heat exchange while bypassing the high pressure column.
If this pressurized nitrogen is to be obtained in especially high purity, the nitrogen fraction is removed from at least one theoretical or actual plate below the top of the low-pressure column, and at least a portion of the liquid nitrogen fraction is evaporated by indirect heat exchange under a pressure that is higher than the operating pressure of the low-pressure column and is withdrawn as a high-purity pressurized nitrogen product. As a heating agent in the case of indirect heat exchange, for example, a gas from the upper area of the high pressure column and/or a gas from the lower area of the low-pressure column can be used. Further details of this heat exch
Dietrich Rottmann
Horst Corduan
Doerrler William
Linde Aktiengesellschaft
Millen White Zelano & Branigan P.C.
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