Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2002-12-11
2003-09-30
Bennett, Henry (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
Reexamination Certificate
active
06626008
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to the cryogenic rectification of feed air and, more particularly, to the cryogenic rectification of feed air to produce low purity oxygen.
BACKGROUND ART
The demand for low purity oxygen is increasing in applications such as glassmaking, steelmaking and energy production. Low purity oxygen is generally produced in large quantities by the cryogenic rectification of feed air. However, conventional cryogenic rectification systems for producing low purity oxygen are relatively inefficient.
Accordingly it is an object of this invention to provide a cryogenic rectification system which can more efficiently produce low purity oxygen.
SUMMARY OF THE INVENTION
The above and other objects, which will become apparent to one skilled in the art upon a reading of this disclosure, are attained by the present invention, one aspect of which is:
A method for producing low purity oxygen comprising:
(A) passing feed air into a higher pressure column and separating the feed air within the higher pressure column by cryogenic rectification into nitrogen-enriched fluid and oxygen-enriched fluid;
(B) passing nitrogen-enriched fluid and oxygen-enriched fluid from the higher pressure column into a lower pressure column and producing oxygen-richer fluid by cryogenic rectification within the lower pressure column;
(C) passing oxygen-richer fluid into the upper portion of an auxiliary column and producing low purity oxygen fluid in the lower portion of the auxiliary column;
(D) withdrawing reboiling fluid from an intermediate level of the higher pressure column, increasing the pressure of the withdrawn reboiling fluid by cold compression, and vaporizing a portion of the low purity oxygen fluid by indirect heat exchange with the increased pressure withdrawn reboiling fluid; and
(E) recovering low purity oxygen fluid from the lower portion of the auxiliary column as product low purity oxygen.
Another aspect of the invention is:
Apparatus for producing low purity oxygen comprising:
(A) a higher pressure column and means for passing feed air into the higher pressure column;
(B) a lower pressure column and means for passing fluid from the higher pressure column into the lower pressure column;
(C) an auxiliary column having a bottom reboiler, and means for passing fluid from the lower portion of the lower pressure column into the upper portion of the auxiliary column;
(D) a cold compressor, means for passing fluid from an intermediate level of the higher pressure column to the cold compressor, and means for passing fluid from the cold compressor to the auxiliary column bottom reboiler; and
(E) means for recovering product low purity oxygen from the lower portion of the auxiliary column.
A further aspect of the invention is:
A method for producing low purity oxygen comprising:
(A) cooling feed air in a main heat exchanger to produce cooled feed air, passing a portion of the cooled feed air into a higher pressure column, and separating the feed air within the higher pressure column by cryogenic rectification into nitrogen-enriched fluid and oxygen-enriched fluid;
(B) passing nitrogen-enriched fluid and oxygen-enriched fluid from the higher pressure column into a lower pressure column and producing oxygen-richer fluid by cryogenic rectification within the lower pressure column;
(C) passing oxygen-richer fluid into the upper portion of an auxiliary column and producing low purity oxygen fluid in the lower portion of the auxiliary column;
(D) increasing the pressure of another portion of the cooled feed air by cold compression, and vaporizing a portion of the low purity oxygen fluid by indirect heat exchange with the increased pressure feed air portion; and
(E) recovering low purity oxygen fluid from the lower portion of the auxiliary column as product low purity oxygen.
Yet another aspect of the invention is:
Apparatus for producing low purity oxygen comprising:
(A) a main heat exchanger, a higher pressure column, means for passing feed air to the main heat exchanger, and means for passing feed air from the main heat exchanger to the higher pressure column;
(B) a lower pressure column and means for passing fluid from the higher pressure column into the lower pressure column;
(C) an auxiliary column having a bottom reboiler, and means for passing fluid from the lower portion of the lower pressure column into the upper portion of the auxiliary column;
(D) a cold compressor, means for passing feed air from the main heat exchanger to the cold compressor, and means for passing feed air from the cold compressor to the auxiliary column bottom reboiler; and
(E) means for recovering product low purity oxygen from the lower portion of the auxiliary column.
As used herein, the term “feed air” means a mixture comprising primarily oxygen and nitrogen, such as ambient air.
As used herein, the term “column” means a distillation or fractionation column or zone, i.e. a contacting column or zone, wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, as for example, by contacting of the vapor and liquid phases on a series of vertically spaced trays or plates mounted within the column and/or on packing elements such as structured or random packing. For a further discussion of distillation columns, see the Chemical Engineer's Handbook, fifth edition, edited by R. H. Perry and C. H. Chilton, McGraw-Hill Book Company, New York, Section 13,
The Continuous Distillation Process.
The term “double column” is used to mean a higher pressure column having its upper portion in heat exchange relation with the lower portion of a lower pressure column. A further discussion of double columns appears in Ruheman “The Separation of Gases”, Oxford University Press, 1949, Chapter VII, Commercial Air Separation.
Vapor and liquid contacting separation processes depend on the difference in vapor pressures for the components. The high vapor pressure (or more volatile or low boiling) component will tend to concentrate in the vapor phase whereas the low vapor pressure (or less volatile or high boiling) component will tend to concentrate in the liquid phase. Partial condensation is the separation process whereby cooling of a vapor mixture can be used to concentrate the volatile component(s) in the vapor phase and thereby the less volatile component(s) in the liquid phase. Rectification, or continuous distillation, is the separation process that combines successive partial vaporizations and condensations as obtained by a countercurrent treatment of the vapor and liquid phases. The countercurrent contacting of the vapor and liquid phases is generally adiabatic and can include integral (stagewise) or differential (continuous) contact between the phases. Separation process arrangements that utilize the principles of rectification to separate mixtures are often interchangeably termed rectification columns, distillation columns, or fractionation columns. Cryogenic rectification is a rectification process carried out at least in part at temperatures at or below 150 degrees Kelvin (K).
As used herein, the term “indirect heat exchange” means the bringing of two fluids into heat exchange relation without any physical contact or intermixing of the fluids with each other.
As used herein, the term “subcooling” means cooling a liquid to be at a temperature lower than the saturation temperature of that liquid for the existing pressure.
As used herein, the term “top” when referring to a column means that section of the column above the column mass transfer internals, i.e. trays or packing.
As used herein, the term “bottom” when referring to a column means that section of the column below the column mass transfer internals, i.e. trays or packing.
As used herein, the term “reboiler” means a heat exchange device that generates column upflow vapor from column liquid. A reboiler may be located within or outside of the column. A bottom reboiler generates column upflow vapor from liquid from the bottom of a column. An intermediate reboiler generates column upflow vapor from liqui
Prosser Neil Mark
Shah Minish Mahendra
Bennett Henry
Drake Malik N.
Ktorides Stanley
Praxair Technology Inc.
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