Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2000-05-03
2001-09-11
Doerrler, William (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S651000, C062S652000, C062S653000
Reexamination Certificate
active
06286336
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to cryogenic air separation and is particularly useful for the production of elevated pressure product or products using cryogenic air separation.
BACKGROUND ART
In the practice of cryogenic air separation for the production of nitrogen product and/or oxygen product, it is at times desired to produce such product or products at an elevated pressure. One very effective way for producing elevated pressure product using cryogenic air separation is to operate the column system used to carry out the cryogenic air separation at an elevated pressure. Unfortunately, such operating practice generally results in lower recovery or yield of such elevated pressure product(s).
Accordingly, it is an object of this invention to provide an improved cryogenic air separation system which can be used to effectively produce one or more products at elevated pressure without significantly compromising the recovery of such product(s).
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 carrying out cryogenic air separation comprising:
(A) passing feed air into a higher pressure column and separating the feed air by cryogenic rectification within the higher pressure column 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 by cryogenic rectification within the lower pressure column nitrogen-rich vapor and oxygen-rich fluid;
(C) withdrawing nitrogen-rich vapor from the upper portion of the lower pressure column, compressing the withdrawn nitrogen-rich vapor, condensing the compressed nitrogen-rich vapor by indirect heat exchange with intermediate liquid from the lower pressure column to produce nitrogen-rich liquid, and passing the nitrogen-rich liquid into the upper portion of the lower pressure column; and
(D) recovering at least one of nitrogen-rich vapor, oxygen-rich fluid and nitrogen-enriched fluid as product.
Another aspect of the invention is:
Apparatus for carrying out cryogenic air separation comprising:
(A) a higher pressure column, a lower pressure column having an intermediate reboiler, and means for passing feed air into the higher pressure column;
(B) means for passing fluid from the higher pressure column into the lower pressure column;
(C) a compressor, means for passing fluid from the upper portion of the lower pressure column to the compressor, means for passing fluid from the compressor to the intermediate reboiler, and means for passing fluid from the intermediate reboiler to the upper portion of the lower pressure column; and
(D) means for recovering product from at least one of the upper portion of the lower pressure column, the lower portion of the lower pressure column, and the upper portion of the higher pressure column.
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 “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 “feed air” means a mixture comprising primarily nitrogen and oxygen, such as ambient air.
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 of 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 which may be structured packing and/or random packing elements. For a further discussion of distillation columns see the Chemical Engineers' Handbook, Fifth Edition, edited by R. H. Perry and C. H. Chilton, McGraw-Hill Book Company, New York, Section 13, “Distillation” B. D. Smith et al., page 13-3, 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. Distillation is the separation process whereby heating of a liquid mixture can be used to concentrate the volatile component(s) in the vapor phase and the less volatile component(s) 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 adiabatic and includes 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.
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 “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 “intermediate” when referring to a column means that section of the column above the bottom and below the top.
As used herein, the term “intermediate liquid” means liquid from the intermediate section of a column.
As used herein, the term “intermediate reboiler” means a heat exchanger wherein intermediate liquid of a column is vaporized for upflow within the column. An intermediate reboiler may be physically within or outside of the column.
As used herein, the term “cold compression” means the method of mechanically raising the pressure of a gas stream that is lower in temperature than the ambient level feeds to the cryogenic separation system. The mechanical energy of cold compression must be balanced by refrigeration.
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Doerrler William
Ktorides Stanley
Praxair Technology Inc.
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