Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2000-02-25
2001-11-13
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S651000
Reexamination Certificate
active
06314755
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for the low-temperature fractionation of air and particularly to a low-temperature fractionation including at least one pressure column and at least one compressor.
BACKGROUND OF THE INVENTION
A process of low-temperature fractionation and a corresponding apparatus are disclosed by EP 810412A. There, the nitrogen-rich fraction is recompressed by means of a compressor prior to its mixing.
Another process is disclosed by DE-38 14 187-C2. Here, impure nitrogen from an intermediate point from the low-pressure column is recycled upstream of the first stage of the air compressor. Still another process is shown in U.S. Pat. No. 4,848,996, where the impure nitrogen is taken off at the top of the low-pressure column and the feed air is added at an intermediate stage of the air compressor.
Recycling the nitrogen-rich fraction into the feed air is advantageous per se and increases the product yield. Nevertheless, the process can be improved.
SUMMARY OF THE INVENTION
One feature of the invention is a process and a corresponding apparatus for low-temperature air fractionation which are particularly expedient economically and, in particular, require relatively low capital costs.
This feature is achieved by the fact that the mixture of feed air and nitrogen-rich fraction is further compressed in a second compressor to a second pressure p
2
which is higher than the first pressure P
1
.
In this manner the expenditure on compression of the feed air and on recompression of the nitrogen-rich fraction is kept comparatively small by, desirably, only compressing the feed air at the first compressor rather than a combination of the feed air and nitrogen-rich fraction. The first and second compressors can be of a single- or multistage design. They can be driven independently of one another or be coupled to one another via a shared shaft or a gear. Preferably, the first pressure p
1
is near the operating pressure of the low-pressure column, that is to say the difference between the two said pressures is no more than about 0.5 bar.
Preferred ranges of values for the outlet pressures of the two compressors are:
first compressor (p
1
): about 2 to about 12 bar, preferably about 3 to about 4 bar
second compressor (p
2
): about 6 to about 40 bar, preferably about 9 to about 13 bar.
The specific values depend in the individual case on the desired delivery pressure of the product or one of the products (for example nitrogen) which are generated in the gaseous state in one of the columns, or on the pressure of one or more product streams (oxygen and/or nitrogen) which are withdrawn in the liquid state from one of the columns and, after pressure boosting, are vaporized in the liquid state at delivery pressure.
The “nitrogen-rich fraction” can be formed by pure nitrogen or by a mixture of atmospheric gases whose nitrogen content is, for example, greater than about 50 mol %. They can be taken off overhead or from an intermediate point of the low-pressure column.
It is expedient if the nitrogen-rich fraction is warmed at least in part by indirect heat exchange with the feed air, for example downstream of the second compressor.
The advantages of the process of the invention become noticeable particularly when a nitrogen fraction is withdrawn from the upper region of the pressure column, warmed and taken off as pressurized nitrogen product. The expedient form of the recycling of a nitrogen-rich fraction from the low-pressure column into the feed air effects a particularly high yield of pressurized nitrogen product with relatively low equipment cost.
Cold can be generated in the process by work-expansion of a further nitrogen-rich fraction from the low-pressure column. This can be taken off, for example, from the upper region, for example from the top, of the low-pressure column. It is expedient if the nitrogen-rich fraction to be recycled and the nitrogen-rich fraction to be work-expanded are taken off from the low-pressure column and, if appropriate, warmed, together. The further nitrogen-rich fraction can, for example, be separated off from the stream to be recycled at an intermediate temperature of the main heat exchanger for cooling feed air.
Alternatively or additionally, an oxygen-containing fraction from the lower region of the low-pressure column can be work-expanded, in particular in the same expansion machine. For this purpose an oxygen-containing fraction is taken off, for example, from the bottom of the low-pressure column or from the vaporization space of the bottom of the boiler of the low-pressure column (main condenser), warmed in the main heat exchanger to an intermediate temperature and fed to an expansion machine. If, in addition, a further nitrogen-rich fraction is work-expanded, this is preferably mixed with the oxygen-containing fraction immediately upstream of the work-expansion and the two fractions to be expanded are introduced together into the same expansion machine (preferably expansion turbine).
The invention further relates to an apparatus for the low-temperature fractionation of air.
The process of the invention is particularly suitable for processes in which the operating pressure at the top of the pressure column is about 5.7 to about 29.7 bar, preferably about 8.7 to about 12.7 bar, and the operating pressure at the top of the low-pressure column is about 1.8 to about 11.8 bar, preferably about 2.8 to about 3.8 bar. Process cold can be generated in the process by work-expansion of a process stream. It is expedient in this case to expand a residual gas stream from the low-pressure column which is taken off, for example, together with the nitrogen-rich fraction from the low-pressure column, warmed to an intermediate temperature and fed to an expansion machine.
REFERENCES:
patent: 4555256 (1985-11-01), Skolaude et al.
patent: 4964901 (1990-10-01), Rhode
patent: 5037462 (1991-08-01), Schweigert
patent: 5379598 (1995-01-01), Mostello
patent: 38 14 187 A1 (1989-11-01), None
patent: 0 446 004 A1 (1991-09-01), None
patent: 0 810 412 A2 (1997-12-01), None
Schoenecker Herbert
Voit Jurgen
Capossela Ronald
Linde Aktiengesellschaft
Millen White Zelano & Branigan P.C.
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