Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
1998-10-30
2001-01-30
Doerrler, William (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S653000
Reexamination Certificate
active
06178775
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for separating air to produce an oxygen product. More particularly, the present invention relates to such a method and apparatus in which air is separated in double column arrangement having higher and lower pressure columns. Even more particularly, the present invention relates to such a method and apparatus in which the lower pressure column is reboiled with compressed nitrogen vapor from the higher pressure column and the vapor rate is increased at an intermediate location thereof by generation of vaporized liquid.
Air is commonly separated in a double column arrangement having higher and lower pressure columns. Prior to separation, air is filtered and compressed. After removing the heat of compression, the air is purified by removing impurities such as carbon dioxide, moisture and heavy hydrocarbons. The resultant compressed and purified air stream is then cooled in a main heat exchanger to a temperature suitable for its rectification and introduced into double column arrangement. Liquid oxygen is produced as a column bottoms of the lower pressure column. An oxygen product is extracted as a liquid stream that may be pumped to pressurize the liquid. The liquid is then vaporized in the main heat exchanger against cooling the incoming air.
In order to reboil the lower pressure column, a condenser reboiler can be provided to condense incoming air against boiling the liquid oxygen. The air may be partially or fully condensed and is introduced into the higher pressure column. Examples of this can be found in U.S. Pat. No. 5,626,036 and WO 885893. In both of these patents the air is partially condensed to reboil the lower pressure column. Such partial condensation is advantageous in that the majority of the air may be compressed in the main compressor to a pressure below 4 bar absolute. This minimum compression will produce a minimum amount of boiling in the lower pressure column so that a liquid product may be withdrawn. Additionally, in both of these patents, an increase in the vapor rate is effected at an intermediate location of the lower pressure column by means of an intermediate reboiler in which nitrogen vapor constitutes the coolant. The condensate of such intermediate reboiler is returned to both the higher and lower pressure columns as reflux.
As will be discussed, the present invention produces greater efficiency than such prior art patents.
SUMMARY OF THE INVENTION
The present invention provides air separation method separating air to produce an oxygen product. In accordance with the method, compressed and purified air is cooled to a temperature suitable for its rectification. The cooled and compressed air is then introduced into a double rectification column system having a higher pressure column and a lower pressure column. The compressed and purified air is then rectified in the double rectification system so that a nitrogen-rich tower overhead and an oxygen-rich liquid column bottoms are produced within the higher pressure column. An oxygen liquid column bottoms is produced within the lower pressure column. The lower pressure column is reboiled by cold compressing a first nitrogen stream composed of the nitrogen-rich tower overhead and introducing the first nitrogen stream into a reboiler associated with a bottom region of the lower pressure column, thereby to form a nitrogen liquid stream. The lower pressure column is reboiled at an intermediate location thereof with a second nitrogen rich stream composed of the nitrogen-rich tower overhead, thereby to form an additional nitrogen liquid stream. The lower and higher pressure columns are refluxed with liquid nitrogen contained within the nitrogen liquid stream and the additional nitrogen rich liquid stream. A product stream composed of the oxygen liquid column bottoms is extracted from the lower pressure column and is fully warmed through indirect heat exchange with the compressed and purified air, thereby to form the oxygen product.
In another aspect, the present invention provides an apparatus for separating air to produce an oxygen product. In accordance with this aspect of the present invention, a main heat exchanger is provided for cooling compressed and purified air to temperature suitable for its rectification. A double rectification column system is also provided. The double rectification system has a higher and lower pressure column configured to rectify the air to produce a nitrogen-rich tower overhead and an oxygen-rich liquid column bottoms. An oxygen liquid column bottoms is produced within a lower pressure column. The main heat exchanger is connected to the double rectification column system so that the compressed and purified air is introduced therein. A lower reboiler is located within a bottom region of the lower pressure column. A cold compressor is interposed between the lower reboiler and the higher pressure column to compress a first nitrogen stream composed of the nitrogen-rich tower overhead and to introduce the first nitrogen stream into the lower reboiler to form a nitrogen liquid stream. An intermediate reboiler is associated with an intermediate region of the lower pressure column and connected to the higher pressure column so that a rich liquid stream, composed of the oxygen-rich column bottoms, indirectly exchangers heat with a second nitrogen rich stream composed of the nitrogen rich tower overhead, thereby to form an additional nitrogen liquid steam and a partially vaporized rich liquid stream. The lower and intermediate reboilers and the higher and lower pressure columns are all associated with one another so that the liquid nitrogen contained within the nitrogen liquid stream and the additional nitrogen liquid stream reflux the higher and lower pressure columns and the vaporized rich liquid stream is introduced into an intermediate location of the lower pressure column. The lower pressure column is connected to the main heat exchanger so that product stream composed of the oxygen liquid column bottoms as fully warmed through a direct heat exchange with the cooled and compressed air, thereby to form the oxygen product.
In a conventional double column arrangement, in which nitrogen is used to reboil the lower pressure column, the lower pressure column pressure and the higher pressure column pressure are tied to one another because the nitrogen must be at a sufficient pressure to vaporize oxygen against its own condensation, In the present invention, since cold compression is provided, that is, compression at the rectification temperature of the air, the higher pressure column may be made to operate at a lower pressure than otherwise would be required. Therefore, the main air compressor may be made to operate at a lower pressure and thus utilize less energy. At the same time, since vaporized rich liquid is being introduced into an intermediate location of the lower pressure column, boil up is increased within the lower pressure column to approximate a more ideal case. It has been calculated by the inventors therein that the present invention allows overall power requirements of an air reboiled plant to be reduced by about 2.5%.
BRIEF DESCRIPTION OF THE DRAWINGS
While the specification concludes with claims distinctly pointing out the subject mater 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 representation of an apparatus used in carrying out a method in accordance with the present invention.
REFERENCES:
patent: 5341646 (1994-08-01), Langston et al.
patent: 5379598 (1995-01-01), Mostello
patent: 5379599 (1995-01-01), Mostello
patent: 5586451 (1996-12-01), Koeberle et al.
patent: 5845517 (1998-12-01), Attlfellner
Higginbotham Paul
Naumovitz Joseph P.
Doerrler William
Pace Salvatore P.
The BOC Group Inc.
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