Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2000-02-23
2001-04-10
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S654000, C062S906000, C062S924000, C261S112200
Reexamination Certificate
active
06212907
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to cryogenic rectification of air for the separation of air into its components and is particularly useful for operating a cryogenic rectification column at increased capacity for carrying out the rectification.
BACKGROUND ART
It is desirable to operate an air separation plant beyond the design capacity of the plant in order to produce extra products from the plant if such increased capacity operation can be carried out economically.
Most components of an air separation plant can be designed or modified to accept an increased flowrate. For example, an upstream blower can be used to boost the capacity of a compressor. Heat exchangers can be operated at increased flowrates simply by accepting an increased pressure drop. The capacity of air prepurifiers can also be increased by operating at increased pressure drop provided that fluidization of the adsorbent particles is avoided. However, it is more difficult to increase the capacity of the distillation columns in an air separation plant because they are capacity limited by the phenomenon of flooding. Flooding occurs in process equipment whenever there is vertical countercurrent two-phase flow and the flowrates are such that they exceed the capacity of the equipment. In both packed and trayed columns, the approach to flooding is characterized by a rapidly increasing pressure drop, by a loss of separation performance and by unstable operation. The onset of flooding in the columns is usually the limiting bottleneck encountered when attempting to increase the capacity of an air separation plant beyond its design capacity.
In general it is well established that distillation column capacity can be increased by changing the column pressure. Raising the pressure increases the vapor density, allowing an increase in the mass flowrate of vapor. However, increasing the pressure lowers the relative volatility thus making the distillation separation more difficult. The vapor mass flowrate capacity increases as the 0.4 or 0.5 power of the operating pressure for packed and trayed columns respectively.
The disadvantage of this solution to the flooding problem is that an increase in the column operating pressure translates into a substantial increase in the discharge pressure of the main air compressor, and in increased power costs. A pressure increase is particularly disadvantageous in the upper (or lower pressure) column of a double column plant since any increase in pressure must typically be multiplied by three as it is propagated across the main condenser/reboiler, because of the differences in the vapor pressure/temperature relationships of oxygen and nitrogen.
A solution to the problem is to increase the flowrates through the columns beyond the design point but not as far as the flood point. Typically packed columns are designed at about 80 percent of the flood point. Unfortunately, using conventional structured packing, flowrates can be increased only slightly beyond the design point because pressure drop fluctuations become so large that the columns become unstable.
Accordingly it is an object of this invention to provide a method for operating a cryogenic rectification column to carry out the separation of the components of air at increased capacity while avoiding flooding.
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, which is:
A method for operating a cryogenic rectification column comprising:
(A) passing a mixture comprising a more volatile component of air and a less volatile component of air into a column, said column containing a height of packing comprising packing sheets which have a bottom portion, a middle portion, and a top portion, with the structure of each packing sheet in the bottom portion being different from the structure of the packing sheet in the middle portion and being the same as the structure of the packing sheet in the top portion;
(B) carrying out cryogenic rectification within the column wherein vapor flows upward through the height of packing sheets and liquid flows downward through the height of packing sheets whereby the said more volatile component concentrates in the upflowing vapor and the said less volatile component concentrates in the downflowing liquid;
(C) passing the upflowing vapor upward through the height of packing within the column at a flowrate so as to have a pressure drop within the column of at least 0.7 inches of water per foot of packing height; and
(D) withdrawing more volatile component from the upper portion of the column and withdrawing less volatile component from the lower portion of the column.
The term “column” as used herein 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 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.
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. Distillation is the separation process whereby heating of a liquid mixture can be used to concentrate the more volatile component(s) in the vapor phase and thereby 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 more 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 can be adiabatic or nonadiabatic 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 rectification carried out, at least in part, at temperatures below 150° K.
As used herein, the term “packing” means any solid or hollow body of predetermined configuration, size and shape used as column internals to provide surface area for the liquid to allow mass transfer at the liquid-vapor interface during countercurrent flow of the two phases.
As used herein, the term “structured packing” means diagonally cross-corrugated packing wherein individual members have specific orientation relative to each other and to the column axis.
As used herein, the terms “upper portion” and “lower portion” of a column mean those sections of the column respectively above and below the mid point of the column.
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Hanley et al., “A Unified Model for Countercurrent Vapor/Liquid Packed Columns. 1. Pressure Drop”, American Chemical Society (1994) pp 1208-1221.
Billingham John Fredric
Lockett Michael James
Seiler Daniel Mark
Capossela Ronald
Ktorides Stanley
Praxair Technology Inc.
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