Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
1999-04-07
2001-01-09
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S939000
Reexamination Certificate
active
06170291
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a method of and apparatus for the separation of air.
The separation of air by rectification is very well known indeed. Rectification is a method in which mass exchange is effected between a descending stream of liquid and an ascending stream of vapour such that the ascending stream of vapour is enriched in a more volatile component (nitrogen) of the mixture to be separated and the descending stream of liquid is enriched in a less volatile component (oxygen) of the mixture to be separated.
It is known to separate air in a double rectification column comprising a higher pressure rectification column which receives a stream of purified, compressed, vaporous air at a temperature suitable for its separation by rectification, and a lower pressure rectification column which receives a stream of oxygen-enriched liquid air for separation from the higher pressure rectification column, and which is in heat exchange relationship with the higher pressure rectification column through a condenser-reboiler, of which the condenser provides liquid nitrogen reflux for the separation and the reboiler provides an upward flow of nitrogen vapour in the lower pressure rectification column.
The double rectification column may be operated so as to produce an oxygen fraction at the bottom of the lower pressure column and a nitrogen fraction at the top of the lower pressure column. The oxygen fraction may be essentially pure, containing less than 0.5% by volume of impurities, or may be impure containing up to 50% by volume of impurities.
There is a net requirement for refrigeration to be provided to the air separation plant. At least part of this requirement arises from the operation of the double rectification column at cryogenic temperatures. Particularly if none of the products of the air separation is taken in liquid state, the requirements for refrigeration are typically met by raising the pressure of a part of the air to at least 2 bar above the operating pressure at the top of the higher pressure column and expanding it with the performance of external work in an expansion turbine which exhausts into the lower pressure column. Typically, the turbine is coupled to a booster-compressor which raises the pressure of the air to above that at the top of the higher pressure column.
An air separation plant typically consumes a considerable amount of power. Its is therefore desirable for the air separation plant to have a configuration which enables power consumption to be minimised without unduly increasing its capital cost. In order to minimise the power consumption much attention in the art has recently been focused upon operating the lower pressure rectification column with two reboilers, one operating at a higher temperature and being heated by a flow of the air to be separated, and the other operating at a lower temperature and being heated by a flow of nitrogen separated in the higher pressure rectification column. A disadvantage of such plant is that the requirement for a second reboiler adds to its capital cost.
U.S. Pat. No. 5,337,570 provides examples of a yet further kind of air separation plant. There is a first condenser-reboiler which condenses a part of the top nitrogen fraction separated in the higher pressure column. The condensation is effected by indirect heat exchange with a stream of a bottom oxygen-enriched liquid fraction formed in the higher pressure column. As a result, the stream of the bottom oxygen-enriched liquid fraction is partially reboiled. Resulting vapour and residual liquid are fed to the lower pressure rectification column. The plant employs a single generator-loaded expansion turbine exhausting into the lower pressure column. The air to be separated is compressed in a main, plural stage, compressor. The main air feed to the higher pressure rectification column is taken from a lower pressure stage than the feed to the expansion turbine.
It is an aim of the present invention to provide a method and apparatus for separating air by rectification which are able to be operated at a favourable net power consumption without imposing on the apparatus an unacceptably high capital cost and without the need to have two reboilers associated with the lower pressure rectification column.
SUMMARY OF THE INVENTION
According to the present invention there is provided a method of separating air by rectification, including compressing the air to a first pressure; without further compression cooling in a main heat exchanger a first flow of the compressed air to a temperature suitable for its separation by rectification and introducing the first flow into the higher pressure column of a double rectification column comprising, in addition to the higher pressure column, a lower pressure column, in which a bottom oxygen fraction having an oxygen content in the range of about 50 to about 96 mole percent is formed; expanding with the performance of external work a second flow of the compressed air; introducing the expanded second flow into the lower pressure column, and taking an impure oxygen product from the said bottom fraction, wherein the external work is the generation of electrical power, characterised in that the double rectification column additionally includes a condenser-reboiler placing the higher pressure column in heat exchange relationship with the lower pressure column, and the expansion of the second flow of the compressed air takes place without further compression of the second flow upstream thereof.
The present invention also provides apparatus for separating air by rectification, including a double rectification column comprising a higher pressure column and a lower pressure column, at least one air compressor for compressing the air to a first pressure, a main heat exchanger for cooling the first flow of the compressed air to a temperature suitable for its separation by rectification, an inlet to the higher pressure column for the first flow, an expansion turbine for expanding with the performance of external work a second flow of the compressed air having an inlet for the second flow of the compressed air and an outlet communicating with the lower pressure column, the expansion turbine being loaded by an electrical generator, and an outlet from the lower pressure column for an impure oxygen product formed of a bottom fraction having an oxygen content in the range of about 50 to about 96 mole percent, characterised in that there is no additional compression means for raising the pressure of either the said first flow or the said second flow of the compressed air above the first pressure, and the double rectification column additionally includes a condenser-reboiler able to place the higher pressure column in heat exchange relationship with the lower pressure column.
The method and apparatus according to the invention offer a number of advantages. First, they enable a particularly large proportion of the air to be expanded with the performance of external work and introduced into the lower pressure column. This makes it possible to operate the lower pressure column relatively efficiently and with a relatively low vapour traffic below the level at which the expanded air is introduced. In addition, the load on the condenser-reboiler is reduced. The effective diameter of the lower pressure column may be reduced in the lower part of the lower pressure column, thereby making possible a reduction in the total area of liquid-vapour contact surfaces. The size of the condenser-reboiler may also be reduced. Although operation of the method and apparatus according to the invention in such a manner has the effect of widening the temperature difference in the main heat exchanger between flow being cooled and flow being warmed, this disadvantage is more than compensated for by the relatively high efficiency with which the lower pressure column can be operated, particularly because a wider temperature difference in the main heat exchanger permits either the pressure drop in, or the heat transfer area per unit volume of the main heat e
Capossela Ronald
Pace Salvatore P.
The BOC Group plc
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