Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
1999-06-28
2001-02-06
Doerrler, William (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S647000
Reexamination Certificate
active
06182471
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to cryogenic rectification and, more particularly, to cryogenic rectification to produce oxygen at a non-constant production rate.
BACKGROUND ART
During the course of operation of a cryogenic rectification plant producing oxygen, the demand for the oxygen product may change. This change in oxygen product demand requires a corresponding change in the production of oxygen product. Without such a change the system would operate inefficiently. Moreover, the faster the system changes to accommodate the change in demand, the better will be the overall performance of the system.
Accordingly, it is an object of this invention to provide a cryogenic rectification system for producing oxygen whose operation can change quickly to match the delivery of oxygen product to a change, i.e. an increase or decrease, in the demand of the oxygen product from the system.
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:
Apparatus for producing oxygen by cryogenic rectification at a non-constant production rate comprising:
(A) a double column system comprising a higher pressure column and a lower pressure column;
(B) an oxygen-enriched liquid storage tank, means for passing fluid from the oxygen-enriched liquid storage tank to the lower pressure column, and means for passing fluid from the lower portion of the higher pressure column into the oxygen-enriched liquid storage tank;
(C) a product boiler, means for passing feed air to the product boiler, and means for passing feed air from the product boiler to the higher pressure column;
(D) a liquid oxygen storage tank and means for passing fluid from the liquid oxygen storage tank to the product boiler;
(E) means for passing fluid from the lower portion of the lower pressure column to the liquid oxygen storage tank, and means for passing fluid from the lower portion of the lower pressure column to the product boiler; and
(F) means for recovering fluid from the product boiler as product oxygen.
Another aspect of the invention is:
Process for producing oxygen by cryogenic rectification comprising:
(A) at least partially condensing a flow of feed air, passing the resulting feed air into the higher pressure column of a double column comprising a higher pressure column and a lower pressure column, and separating the feed air by cryogenic rectification within the higher pressure column into nitrogen-enriched vapor and oxygen-enriched liquid;
(B) passing nitrogen-enriched fluid and oxygen-enriched fluid from the higher pressure column into the lower pressure column, and separating the fluids passed into the lower pressure column by cryogenic rectification into nitrogen vapor and oxygen liquid;
(C) passing oxygen liquid from the lower portion of the lower pressure column in indirect heat exchange with the said at least partially condensing feed air to produce oxygen vapor;
(D) recovering the oxygen vapor as product oxygen, the improvement enabling product oxygen recovery at a non-constant rate comprising:
(E) increasing the production rate of product oxygen during a portion of the process by passing additional oxygen liquid from a liquid oxygen storage tank in indirect heat exchange with the at least partially condensing feed air and increasing the flow of the at least partially condensing feed air, and passing oxygen-enriched liquid from the higher pressure column into an oxygen-enriched liquid storage tank; and
(F) decreasing the production rate of product oxygen during another portion of the process by passing oxygen liquid from the lower portion of the lower pressure column into the liquid oxygen storage tank and decreasing the flow of the at least partially condensing feed air, and passing oxygen-enriched liquid from the oxygen-enriched liquid storage tank into the lower pressure column.
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 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 or the vapor and liquid phases on a series of vertically spaced trays or plates mounted within the column and/or on packing elements. 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 end in heat exchange relation with the lower end 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 may be adiabatic and can include integral or differential 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 “argon column” means a column which processes a feed comprising argon and produces a product having an argon concentration which exceeds that of the feed.
As used herein the term “sump” means the bottom portion of a distillation column below the trays or packing elements in which liquid is accumulated.
As used herein the term “level controller” means a mechanical, pneumatic or electronic device or a mathematical algorithm programmed in a computer used for feedback control of the liquid level within a storage volume such as a tank or a column sump.
As used herein, the term “feed air” means a mixture comprising primarily nitrogen, oxygen and argon, such as ambient air.
As used herein, the term “product oxygen” means a fluid having an oxygen concentration within the range of from 90 to 99.99 mole percent.
As used herein, the terms “upper portion” and “lower portion” means those sections of a column respectively above and below the mid point of the column.
As used herein, the term “product boiler” means a heat exchanger wherein feed air is at least partially condensed by indirect heat exchange with vaporizing liquid oxygen. The product boiler may be a separate or stand alone heat exchanger or may be incorporated into a larger heat exchanger.
REFERENCES:
patent: 5084081 (1992-01-01), Rohde
patent: 5224336 (1993-07-01), Agrawal et al.
patent: 5265429 (1993-11-01), Dray
patent: 5406800 (1995-04-01), Bonaquist
patent: 5408831 (1995-04-01), Guillard et al.
patent: 5437160 (1995-08-0
Doerrler William
Ktorides Stanley
Praxair Technology Inc.
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