Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
1999-04-08
2001-02-13
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S646000, C062S900000
Reexamination Certificate
active
06185960
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for the production of a pressurized gaseous product by the low-temperature separation of air, which is at times carried out in a gas operation and at times in a combined operation,
whereby in the gas operation and the combined operation
purified feed air is cooled under a pressure, partially liquefied, and subjected to rectification so as to obtain gaseous and liquid fractions,
extremely cold liquid from at least one of the liquid fractions from the rectification is evaporated under elevated pressure by indirect heat exchange with feed air, heated, and obtained as a pressurized gaseous product,
whereby in combined operation
the cold that is required for this purpose is generated in an air-refrigeration cycle, by air being pressurized in the refrigeration cycle and work expanded, whereby heat is removed from the air, and the work expanded air is partially reheated countercurrently to the feed air that is to be cooled, and the resultant reheated air is then repressurized,
extremely cold liquid is produced and at least partially stored.
In addition, the invention relates to apparatus for conducting the process with
a main compressor for feed air, whereby the exhaust pressure of the main air compressor also provides a working pressure for a subsequent purification unit,
a pure-air line from the purification unit to a compressor station for the air in the refrigeration cycle and for the air supply for rectification,
and a pressure-side conduit from the compressor station, which, on the one hand, empties into a line of the refrigeration cycle having at least one expansion turbine and, on the other hand, into a branch for throttle air to the columns.
BACKGROUND OF THE INVENTION
A process for the production of pressurized gaseous oxygen (DGOX) and small amounts of liquid oxygen (LOX) is known from publication EP 0 044 679 A1: Cold for air separation and the production of liquid product is furnished by an air refrigeration cycle. Said cycle is provided with two compressor stages in series: for compression of an air stream in the first stage to an intermediate pressure allowing work expansion of a partial stream of this air down to a lower pressure and a second compressor stage to compress the other partial stream of air to an even higher pressure allowing for throttle depressurization to the same low pressure. After the partial flows are combined and a liquid phase that is formed is branched off, the gas phase is recycled for compression, and the liquid phase is fed to the rectification after being divided into two throttled streams. In such a process, the refrigeration cycle cannot be turned off, and a returning of the refrigeration output results in energy-wasteful operation.
SUMMARY OF THE INVENTION
Objects of the invention include a process and a device of the above-mentioned type with energy-favorable production of the pressurized gaseous product and the liquid product, respectively, in variable amounts and with high availability of the production of the pressurized product.
Upon further study of the specification and appended claims, other objects and advantages will become apparent.
A characteristic feature of the process according to the invention is that during gas operation, the air throughput in the refrigeration cycle is reduced to zero and extremely cold stored liquid is used to compensate for cold losses that can no longer be covered by the refrigeration cycle. This makes it possible to produce pressurized gaseous product even in the case of a full liquid product tank by, for example, stored liquid product being fed to a heat exchanger countercurrently to the air that is used, whereby this air is cooled, partially liquefied, and fed to rectification, or the stored liquid is fed directly to rectification.
Extremely cold liquid of at least one liquid fraction from the rectification, for example, liquid nitrogen (LIN), liquid oxygen (LOX), or liquid air can be temporarily stored in a tank to compensate for cold losses in gas operation, whereby working tanks and/or product tanks are used as tanks for storing these fractions. In most cases, the use of product tanks is the most advantageous solution, while liquid air likely requires a working tank since liquid air in most cases plays no role as a product.
By “extremely cold liquid”, is preferably meant a liquid having a temperature at least as low as about the temperature of liquid oxygen at the prevailing pressure under which it is stored; for example, liquid oxygen at atmospheric pressure without subcooling has a temperature of about −183° C., and at higher pressures commensurately higher.
When at least two tanks are used, at times alternate storage can be performed, whereby, on the one hand, in the event of increased high-pressure oxygen (DGOX) demand, in addition to the LOX from the rectification, temporarily stored LOX is removed from one tank, compressed, evaporated countercurrently and heated, and then released as a DGOX product, and in this case, cold is recovered countercurrently and used for production and intermediate storage of LIN product, whereby, on the other hand, in the event of low DGOX demand, correspondingly little LOX is released from the rectifying system as DGOX and instead more LOX is intermediately stored. The advantage lies in the fact that at times more DGOX is supplied than would be possible according to the design of the air separation by virtue of the fact that stored LOX is removed, and the cold content of the LOX corresponding to LIN are stored.
For rectification, a two-column process can be used, whereby cooling of the top of the pressurized column is done with an intermediate liquid from the low-pressure column, and heating of the bottom of the low-pressure column is ensured by indirect heat exchange with air. The two-column process is known from DE 196 09 490 A1 and is especially suitable if only a low oxygen purity is necessary.
Alternatively, a three-column process can also be used as a rectifying system, whereby a double column with a high-pressure part and a low-pressure part and an additional column under intermediate pressure are used. The three-column process is known from DE 195 37 913 A1. Even in the case of oxygen purities of >99.5 mol %, energy savings are possible with this process.
When pressurized gaseous product is recovered by evaporating and heating pressurized liquid, also called inner compression, countercurrently with hot air, air in the upper pressure level of the compression in the refrigeration cycle can be used or the air can be further compressed starting from this pressure level.
Work expansion can be carried out in at least one expansion turbine, whereby the power at the shaft of such a turbine is used in driving either a flow-generating generator or a booster, whereby the booster is used, for example, to further compress the air in the refrigeration cycle. In both cases, the energy of the expansion turbine is used advantageously.
A characteristic feature of the apparatus according to the invention is that the compressor station is designed with at least two compressors that are arranged in parallel and that are designed such that in gas operation, only one of the compressors is in operation, whereby this compressor outputs throttle air, and the refrigeration cycle is not exposed to air, while in operation with the production of pressurized product and liquid product, at least two compressors that are arranged in parallel arc in operation, and in addition to yielding throttle air, the refrigeration cycle is also exposed to air. Such a compressor station has several advantages. For gas operation, a compressor is operated at its energy-favorable working point; in the case of additional production of liquid product, multiple compressors, for example two, are operated near their optimal working point. In addition, multiple compressors simultaneously ensure machine redundancy, which correspondingly increases supply security in gas operation. Another advantage of the invention lies in the fact that, with a
Capossela Ronald
Linde Aktiengesellschaft
Millen White Zelano & Branigan P.C.
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