Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
1999-08-31
2001-01-16
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S905000, C062S646000
Reexamination Certificate
active
06173586
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 oxygen.
BACKGROUND ART
In the cryogenic rectification of feed air into nitrogen and oxygen products, the oxygen is typically produced at a purity of about 99.5 mole percent. Because of the relative volatilities of the components of air, the argon in the feed air tends to concentrate with the oxygen rather than with the nitrogen. Accordingly, the remainder of the typical oxygen product stream from a conventional cryogenic air separation process is comprised primarily of argon.
For most uses, the presence of this small amount of argon in the oxygen stream is not a problem. However, in some situations, such as in the use of oxygen in the production of chemicals such as ethylene oxide, the argon, owing to its inertness, undergoes a buildup within the chemical reactor requiring a periodic venting of the reactor so as to avoid retarding the production reaction. This periodic venting causes a loss of valuable products.
The problem of production reaction burden due to argon buildup can be addressed by increasing the purity of the oxygen input to the reactor, and systems for producing oxygen of higher than conventional purity are known. However, such systems generally can produce only relatively small quantities of elevated purity oxygen. Moreover, such systems are generally not readily adaptable to existing cryogenic rectification systems designed to produce oxygen of conventional purity.
Accordingly, it is an object of this invention to provide an improved cryogenic rectification system for the production of very high purity oxygen.
It is another object of this invention to provide an improved cryogenic rectification system for the production of very high purity oxygen which can be easily retrofitted to existing systems designed to produce oxygen of conventional purity.
SUMMARY OF THE INVENTION
The above and other objects, which will become apparent to those 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 very high purity oxygen by the cryogenic rectification of feed air 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 having a diaphragm in its lower portion, and producing oxygen-rich liquid by cryogenic rectification within the lower pressure column;
(C) passing oxygen-rich liquid from the lower pressure column above the diaphragm into an upgrader column, and producing oxygen-richer liquid by cryogenic rectification within the upgrader column;
(D) passing oxygen-richer liquid from the lower portion of the upgrader column into the lower pressure column below the diaphragm, and at least partially vaporizing the oxygen-richer liquid to produce oxygen-richer fluid; and
(E) recovering oxygen—richer fluid from the lower pressure column as product very high purity oxygen.
Another aspect of the invention is:
Apparatus for producing very high purity oxygen by the cryogenic rectification of feed air comprising:
(A) a higher pressure column and means for passing feed air into the higher pressure column;
(B) a lower pressure column, means for passing fluid from the higher pressure column into the lower pressure column, and a diaphragm in the lower portion of the lower pressure column;
(C) an upgrader column, means for passing liquid from the lower pressure column above the diaphragm to the upper portion of the upgrader column, and means for passing vapor from the lower pressure column below the diaphragm to the lower portion of the upgrader column;
(D) means for passing vapor from the upper portion of the upgrader column to the lower pressure column above the diaphragm, and means for passing liquid from the lower portion of the upgrader column to the lower pressure column below the diaphragm; and
(E) means for recovering very high purity oxygen from the lower pressure column below the diaphragm.
As used herein, the term “feed air” means a mixture comprising primarily oxygen, nitrogen and argon, 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 terms “turboexpansion” and “turboexpander” mean respectively method and apparatus for the flow of high pressure gas through a turbine to reduce the pressure and the temperature of the gas thereby generating refrigeration.
As used herein, the terms “upper portion” and “lower portion” mean those sections of a column respectively above and below the midpoint of the column.
As used herein, the term “tray” means a contacting stage, which is not necessarily an equilibrium stage, and may mean other contacting apparatus such as packing having a separation capability equivalent to one tray.
As used herein, the term “equilibrium stage” means a vapor-liquid contacting stage whereby the vapor and liquid leaving the stage are in mass transfer equilibrium, e.g. a tray having 100 percent efficiency or a packing element height equivalent to one theoretical plate (HETP).
As used herein the term “very high purity oxygen” means a fluid having an oxygen concentration of at least 99.9 mole percent.
As used herein, the term “diaphragm” means a device which prevents, or substantially prevents, the flow of material across it
REFERENCES:
patent: 4091633 (1978-05-01), Linde
patent: 4615716 (1986-10-01), Cormier et al.
patent: 4838913 (1989-06-
Billingham John Fredric
Bonaquist Dante Patrick
Capossela Ronald
Ktorides Stanley
Praxair Technology Inc.
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