Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2000-09-29
2002-03-19
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S654000, C062S913000
Reexamination Certificate
active
06357259
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of separating air to produce a gaseous product in accordance with the demand cycle. More particularly, the present invention relates to such a method in which during periods of high demand air is liquefied and stored against vaporizing a product stream and during periods of low demand at least part of the product to be vaporized during the high demand period is stored. Even more particularly, the present invention relates to such a method in which liquid product is continually produced to decrease the magnitude of variation in the air flow rate to compression machinery.
There are various processes and apparatus that have been provided in the prior art to separate air and thereby produce gaseous products in accordance with a demand cycle. During a demand cycle, demand cyclically swings between periods of high and low demand. In accordance with such demand, more gaseous product is produced during the high demand period than during the low demand period. This type of production requirement is often required in industries having a cyclical demand for oxygen, such as in the production of steel.
In an example of an air separation plant designed to function in accordance with a demand cycle, air after having been filtered, is compressed. The heat of compression is removed and the air is further purified of moisture, carbon dioxide and etc. Thereafter, part of the air is cooled to around its dew point temperature and is introduced into the bottom of a double distillation column system. In a double distillation column system, higher and lower pressure columns are connected to one another in a heat transfer relationship to produce liquid oxygen as a bottoms product of the lower pressure column. Another part of the air to be separated is compressed in a booster compressor and is then divided so that part of the air is partly cooled and turbo expanded to produce plant refrigeration and another part of the air is fully cooled.
During periods of high demand, liquid oxygen that has been previously produced during a low demand period and stored within a storage tank is pressurized by being pumped and then fully vaporized within the main heat exchanger against condensing the other part of the air that had been boosted in pressure and fully cooled. The resultant condensed air stream is in part stored and in part introduced into the higher pressure distillation column. During the low demand period, the previously stored liquid air is supplied to the higher pressure distillation column.
A practical difficulty in effectuating a process such as been described above is that during periods of high demand, more air must be further compressed and therefore liquefied in order to vaporize the increase in the flow rate of product. Inlet flow to the booster compressor can vary by as much as 50%. Most known compressors are not able to accommodate such a variation of inlet flow without recirculation of the outlet flow. Thus, in order to accommodate a 50% of design flow rate, part of the outlet flow of the compressor must be recirculated back to the inlet. The compressor must be sized, however, to produce the requisite outlet flow. As a result, the booster compressor has an over capacity when flow rates are 50% and therefore, practically, a larger compressor is used than would theoretically be necessary. This is not efficient from standpoints of both equipment cost and electrical power usage.
As will be discussed, the present invention operates a demand cycle so that there are not excessive swings of air flow rate to the booster compressor to allow for more efficient compressor utilization.
SUMMARY OF THE INVENTION
The present invention relates to a method for separating air to produce a gaseous product enriched in a component of the air and in accordance with the demand cycle having high and low periods of demand. In accordance with the method, first and second liquid streams are produced by the cryogenic rectification of the air. The first liquid stream is enriched in the component of the air that will eventually form the product. It is to be noted that the term “cryogenic rectification” as used herein and in the claims encompasses a process in which air is compressed and cooled to around its dew point and then is distilled in one or more distillation columns.
During the period of low demand, a supply of stored product is formed from at least part of the liquid stream. During the high period of demand, a product stream is formed from at least part of the stored liquid product and is pressurized. Thereafter the product stream is vaporized to produce the gaseous product. At the same time, a first further compressed air stream, passing in indirect heat exchange with the product stream, is condensed. It is to be noted that the term “condensing” as used herein and in the claims encompasses not only a process in which a substance changes state from a vapor to a liquid, but also to processes in which a super critical fluid is depressurized, after having been fully cooled, to produce a liquid. During at least the low period of demand, refrigeration is produced by expanding a second further compressed air stream with performance of work, thereby to refrigerate the low temperature rectification process and to permit production of the first and second liquid streams and also, producing a liquid product composed of the second liquid stream. As a result, the flow rate of the air to be further compressed during the low period of demand is greater than that that would otherwise have been required had the liquid product not been produced.
The advantage of the foregoing method is that increasing the air flow rate of the air to be further compressed, while resulting in an increase in compression requirements, actually reduces the required percentage increase between high and low demand periods that would otherwise have occurred. As will be discussed, product production can be made to vary as can the amount of further compressed air that serves refrigeration purposes. In such a manner, the air that is further compressed can serve in vaporizing the pressurized liquid product and thus air flow swings to the main air compressor can be further reduced. In fact, a process can be carried out in which the air flow rate to the booster compressor remains unchanged during periods of both high and low demand.
BRIEF DESCRIPTION OF THE DRAWINGS
While the specifications concludes with claims distinctly pointing out the subject matter that applicants regard as their invention, it is believed that the invention will be better understood when taken in connection with the accompanying sole FIGURE which is a schematic illustration of an apparatus for carrying out a method in accordance with the present invention.
DETAILED DESCRIPTION
With reference to the FIGURE, air separation apparatus
1
in accordance with the present invention is illustrated that can be used to produce both gaseous nitrogen and gaseous oxygen products.
In accordance with a method of operation of air separation apparatus
1
, air after having been filtered in a filter
10
is compressed by a main air compressor
12
. The heat of compression is removed from the resultant compressed air by an after-cooler
14
. The air is further purified by removal of impurities such as carbon dioxide and moisture by a known prepurification unit
16
. The air is further compressed by a booster compressor
18
to form a further compressed air stream
20
. Further compressed air stream
20
is introduced into a main heat exchanger
22
where it is cooled against other warming streams passing countercurrently through main heat exchanger
22
. Although main heat exchanger
22
is illustrated as a single unit, in practice, the heat exchanger might be a heat exchanger complex of several heat exchangers.
Further compressed air stream
20
after having been partly cooled, that is cooled to a temperature that is between the warm and cold ends of main heat exchanger
22
, is divided into first and second further compressed air stre
Higginbotham Paul
Naumovitz Joseph P.
Straub Joseph
Sundaram Niranjan
Wilson Leighton B.
Capossela Ronald
Pace Salvatore P.
The BOC Group Inc.
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