Gas and liquid contact apparatus – Contact devices – Porous mass
Reexamination Certificate
2002-06-28
2003-02-04
Bushey, C. Scott (Department: 1724)
Gas and liquid contact apparatus
Contact devices
Porous mass
C261S112200, C261SDIG007
Reexamination Certificate
active
06513795
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH FOR DEVELOPMENT
Not Applicable
BACKGROUND OF THE INVENTION
The present invention relates to mixed-resistance structured packing and methods for assembling such packing in an exchange column. The mixed-resistance 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 expanded metal or woven wire screen stacked in layers or as spiral windings.
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 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 roughening features aimed at improving the heat and mass transfer efficiency. An example of this type of 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 is often 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.
The efficiency of distillation columns with structured packing shows a dependency on their diameter when all the other geometric and process factors are held constant. While performing equivalent separations at different scales, as the diameter increases from a small fraction of a meter to several meters, the HETP increases first and then tends to level out. This may be explained by a combination of two factors—the flow characteristics and the mixing characteristics of structured packing columns.
In terms of flow characteristics, even when the initial liquid and vapor distribution into a packed section of a column is highly uniform, the distribution changes as the liquid and vapor flow in countercurrent contact through the packed section, resulting in variations in the liquid to vapor (L/V) ratio across the cross section of the column. Also, it is known that a significant flow of liquid occurs at the column wall, thereby reducing the liquid loading in the packing in an annular region of the packing adjacent the wall. The vapor flow, although not completely uniform, is more uniform within the packing than is the liquid flow.
Thus, there usually is a systematic variation in the L/V ratio across the cross section of a typical cylindrical packed column as shown schematically in FIG.
2
. Referring to
FIG. 2
, in a typical cylindrical packed column
22
, there is an annular space
19
between the column inner wall
40
and the packing, which is disposed between the parallel broken lines
16
(representing the perimeter of a cylindrical layer of packing). The column axis is represented by broken line
15
. Broken line
17
represents the “nominal” L/V ratio for theoretical or ideal conditions where there would be no variation in the L/V ratio across the cross section of the column. Solid line
18
is a schematic representation of the non-uniform L/V ratio (relative to nominal) across the cross section of a typical cylindrical packed column. The L/V ratio is much higher near the column inner wall because of excessive liquid flowing down the column inner wall (as indicated by the steep slope of line
18
above annular space
19
in FIG.
2
).
The general pattern of the actual L/V ratio illustrated by line
18
in
FIG. 2
may vary considerably depending on the details of the packing, the mixture being separated, and the process conditions.
Further, it is well known that maldistribution can result in degradation of the separation efficiency of the column unless it is mitigated by repeated mixing of the liquid and vapor phases within the column. This
Bushey C. Scott
Jones III Willard
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