Gas and liquid contact apparatus – Contact devices – Wet baffle
Reexamination Certificate
1999-03-15
2002-03-19
Bushey, C. Scott (Department: 1724)
Gas and liquid contact apparatus
Contact devices
Wet baffle
Reexamination Certificate
active
06357728
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH FOR DEVELOPMENT
Not Applicable
BACKGROUND OF THE INVENTION
The present invention relates to corrugated structured packing and methods for installing such packing in an exchange column to provide optimum performance. The structured packing has particular application in exchange columns, especially in cryogenic air separation processes, although it also may be used in other heat and/or mass transfer processes that can utilize structured packing.
The term, “column”, as used herein, means a distillation or fractionation column or zone, ie., a column or zone wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, such as by contacting of the vapor and liquid phases on packing elements or on a series of vertically-spaced trays or plates mounted within the column.
The term “column section” (or “section”) means a zone in a column filling the column diameter. The top or bottom of a particular section or zone ends at the liquid and vapor distributors respectively.
The term “packing” means solid or hollow bodies of predetermined size, shape, and configuration used as column internals to provide surface area for the liquid to allow mass transfer at the liquid-vapor interface during countercurrent flow of two phases. Two broad classes of packings are “random” and “structured”.
“Random packing” means packing wherein individual members do not have any particular orientation relative to each other or to the column axis. Random packings are small, hollow structures with large surface area per unit volume that are loaded at random into a column.
“Structured packing” means packing wherein individual members have specific orientation relative to each other and to the column axis. Structured packings usually are made of thin metal foil, expanded metal or woven wire screen stacked in layers or as spiral windings.
The term “surface area density” means the surface area of the structured packing per unit volume of the structured packing, and usually is expressed in terms of m
2
/m
3
of the volume occupied by the packing.
In processes such as distillation or direct contact cooling, it is advantageous to use structured packing to promote heat and mass transfer between counter-flowing liquid and vapor streams. Structured packing, when compared with random packing or trays, offers the benefits of higher efficiency for heat and mass transfer with lower pressure drop. It also has more predictable performance than random packing.
Cryogenic separation of air is carried out by passing liquid and vapor in countercurrent contact through a distillation column. A vapor phase of the mixture ascends with an ever increasing concentration of the more volatile components (e.g., nitrogen) while a liquid phase of the mixture descends with an ever increasing concentration of the less volatile components (e.g., oxygen). Various packings or trays may be used to bring the liquid and gaseous phases of the mixture into contact to accomplish mass transfer between the phases.
There are many processes for the separation of air by cryogenic distillation into its components (i.e., nitrogen, oxygen, argon, etc.). A typical cryogenic air separation unit
10
is shown schematically in FIG.
1
. High pressure feed air
1
is fed into the base of a high pressure column
2
. Within the high pressure column, the air is separated into nitrogen-enriched vapor and oxygen-enriched liquid. The oxygen-enriched liquid
3
is fed from the high pressure column
2
into a low pressure column
4
. Nitrogen-enriched vapor
5
is passed into a condenser
6
where it is condensed against boiling oxygen which provides reboil to the low pressure column. The nitrogen-enriched liquid
7
is partly tapped
8
and is partly fed
9
into the low pressure column as liquid reflux. In the low pressure column, the feeds (
3
,
9
) are separated by cryogenic distillation into oxygen-rich and nitrogen-rich components. Structured packing
11
may be used to bring into contact the liquid and gaseous phases of the oxygen and nitrogen to be separated. The nitrogen-rich component is removed as a vapor
12
, and the oxygen-rich component is removed as a vapor
13
. Alternatively, the oxygen-rich component can be removed from a location in the sump surrounding reboiler/condenser
6
as a liquid. A waste stream
14
also is removed from the low pressure column. The low pressure column can be divided into multiple sections. Three such sections with structured packing
11
are shown in
FIG. 1
by way of example.
The most commonly used structured packing consists of corrugated sheets of metal or plastic foils (or corrugated mesh cloths) stacked vertically. These foils may have various forms of apertures and/or surface texture features aimed at improving the heat and mass transfer efficiency. An example of this type of structured packing is disclosed in U.S. Pat. No. 4,296,050 (Meier). It also is well-known in the prior art that mesh type packing helps spread liquid efficiently and gives good mass transfer performance, but mesh type packing is much more expensive than most foil type packing.
The separation performance of structured packing often is given in terms of height equivalent to a theoretical plate (HETP). The term “HETP” means the height of packing over which a composition change is achieved which is equivalent to the composition change achieved by a theoretical plate. The term “theoretical plate” means a contact process between vapor and liquid such that the existing vapor and liquid streams are in equilibrium. The smaller the HETP of a particular packing for a specific separation, the more efficient the packing because the height of packing being utilized decreases with the HETP.
U.S. Pat. No 4,836,836 (Bennett et al.) teaches the use of structured packing in cryogenic distillation wherein the power benefits relative to the use of distillation trays is discussed. The teachings of this patent can be applied to all of the packed sections of an air separation plant, although it is most useful in those sections separating argon and oxygen.
U.S. Pat. No. 5,613,374 (Rohde et aL) teaches the use of packing with a surface area density greater than 1,000 m
2
/m
3
as optimal for the low temperature separation of air using at least one rectification column. While the most commonly available structured packings have a surface area density in the broad range of 125-750 m
2
/m
3
, Rohde et al. teaches a preferred range of 1000-1500 m
2
/m
3
. Although structured packing of such high surface area density could provide an advantage of high mass transfer efficiency leading to a reduction in the heights of distillation columns, there also are several disadvantages. First, the additional surface area would increase the cost of the packing. Second, the high pressure drop associated with the high surf- area density would lead to a reduction in capacity, which would result in a significant increase in the diameter of the distillation columns used for this separation, which would further increase the cost of the system. Finally, the increased diameter, together with the high surface area density, also would limit severely the ability of distillation columns to operate in a turndown mode because of the unavailability of enough liquid during turndown to keep the large surface area of the packing well wetted. The ability to turndown the production capacity of a plant without losing efficiency is a critical requirement of most modern distillation plants.
U.S. Pat. No. 5,100,448 (Lockett et al.) teaches the use of variable surface area density packing within a single distillation column of constant diameter. Different sections within a column can be under very different loading conditions in terms of vapor and liquid velocities, especially if there are vapor or liquid draws or feeds in between the sections. If the surface area density of the packing used in all sections is the same, then the column will be unevenly loaded relative to the max
Klotz Herbert Charles
Meski George Amir
Sunder Swaminathan
Air Products and Chemicals Inc.
Bushey C. Scott
Jones II Willard
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