Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2000-09-07
2002-04-30
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S905000, C261S112200
Reexamination Certificate
active
06378332
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to structured packing which is particularly useful for use in carrying out rectification, especially cryogenic rectification.
BACKGROUND ART
Cross-corrugated, sheet metal structured packings have gained a considerable portion of the distillation column internals business since coming into widespread commercial use approximately fifteen years ago. These structured packings are commonly recognized as being more efficient than random packings, based on their lower pressure drop per theoretical stage of separation. They have also replaced trays in some applications, primarily because of their higher capacity in a given column diameter, at least at low to medium liquid rates.
One industry where structured packings have had a major impact is the cryogenic air separation industry. The primary advantage of structured packings in cryogenic air separation is that the pressure drop over the columns may be an order of magnitude lower than when using the previous column internals of choice in cryogenic air separation columns, which were trays. This reduces the operating pressure of the columns, which in turn drops the pressure to which air must be compressed and thus the power requirements for the plant.
Meier (U.S. Pat. No. 4,296,050) describes the majority of structured packings sold commercially to this point, which are cross-corrugated sheets containing both apertures and fluting (a form of surface texture). Since the nearly twenty years since this patent, numerous attempts to improve on the basic structure of the structured packing have been published. However, none appear to have had a significant commercial impact until Billingham et al. (U.S. Pat. No. 5,632,934). It was recognized in this patent that flooding of structured packings was initiated at the interface between layers and that this bottleneck could be removed by reducing the pressure drop of gas in the base of each brick. Since the issuance of this patent, two products have been commercialized that use the ideas put forward in this patent.
One problem with structured packings is that the amount of mass transfer per unit height does not scale proportional to the surface area density, a. (Area density is also termed the specific surface area and is the surface area of the packing per unit volume of the packing.) In other words, doubling the amount of surface area per unit volume does not halve the height required to effect a given separation. This is particularly true at high area densities (a<400 m
2
/m
3
).
Accordingly, it is an object of this invention to provide corrugated structured packing, having an improvement over conventional corrugated structured packing, which enables separation such as cryogenic rectification to be carried out over a reduced column height.
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 corrugated structured packing sheet having a plurality of high corrugations each high corrugation having a corrugation height equal to the crimp height, and having a plurality of low corrugations each low corrugation having a corrugation height less than the crimp height.
Another aspect of the invention is:
A module comprising a plurality of vertically oriented corrugated structured packing sheets, each structured packing sheet having a plurality of high corrugations each high corrugation having a corrugation height equal to the crimp height of the packing sheet and having a plurality of low corrugations each low corrugation having a corrugation height less than the crimp height, said corrugated structured packing sheets stacked side by side with the direction of the corrugations reversed in neighboring sheets, said neighboring sheets contacting each other at their high corrugations and being spaced from each other at their low corrugations.
A further aspect of the invention is:
A method for carrying out rectification comprising:
(A) passing a feed mixture comprising a more volatile component and a less volatile component into a column containing a plurality of modules, each module comprising a plurality of vertically oriented corrugated structured packing sheets, each structured packing sheet having a plurality of high corrugations each high corrugation having a corrugation height equal to the crimp height of the packing sheet and having a plurality of low corrugations each low corrugation having a corrugation height less than the crimp height, said corrugated structured packing sheets stacked side by side with the direction of the corrugations reversed in neighboring sheets, said neighboring sheets contacting each other at their high corrugations and being spaced from each other at their low corrugations;
(B) carrying out rectification within the column wherein vapor flows upward through the modules and liquid flows downward through the modules whereby the said more volatile component concentrates in the upflowing vapor and the said less volatile component concentrates in the downflowing liquid; and
(C) withdrawing first fluid from the upper portion of the column, said first fluid having a concentration of more volatile component which exceeds that of the feed mixture, and withdrawing second fluid from the lower portion of the column, said second fluid having a concentration of less volatile component which exceeds that of the feed mixture.
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 mea
Billingham John Fredric
Lockett Michael James
Capossela Ronald
Ktorides Stanley
Praxair Technology Inc.
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