Variegated density mesh pad for mist removal and method of preparing same

A variegated density mist eliminator pad for use in gas-liquid contacting devices for the removal of mists from a mist-containing gas stream which pad comprises a plurality of fibrous elements such as a knitted or woven mesh having open areas for the flow of the gas stream therethrough and characterized by defined adjacent regions of different, i.e. high and low, fiber density with the direction of density variation generally perpendicular to the direction of gas flow, the high density region within the pad permitting enhanced drainage of coalesced liquid, and a method of preparing the high and low density regions through a nesting and non-nesting relationship of fibrous layers.

BACKGROUND OF THE INVENTION 
Mist eliminator mesh pads are typically pads composed of elements, such as 
knitted wire mesh, and are commonly placed in a gas-liquid contact 
apparatus to remove mists from a mist-containing gas stream. Typically, 
such mist eliminator mesh pads are composed of fibrous or filament 
elements, such as four to fifteen mil diameter stainless steel wire and 
are arranged from about three to twenty-four inches in thickness, have a 
density ranging from about four to fifteen pounds per cubic foot and range 
in diameter from about one to thirty feet depending upon the gas-liquid 
contact apparatus in which the pads are employed. Such mist eliminator 
mesh pads are generally effective in removing droplets as small as one to 
five micrometers from mist-containing gas streams. 
The capacity of a mist eliminator mesh pad in a gas-liquid contact 
apparatus, i.e. the maximum gas velocity of the gas stream through the 
mesh pad, is generally limited by the mesh pad's ability to drain rapidly 
the coalesced liquid collected by the mesh pad. One attempt to increase 
the capacity of mist eliminator mesh pads and to reduce the mesh pad's 
pressure drop has been the employment of drainage cylinders or ancillary 
rolls of wire mesh fixed to the bottom of conventional mist eliminator 
mesh pads. Such drainage cylinder or ancillary rolls provide for 
localized, separate regions of flow interruption and interception, 
therefore creating preferential drainage foci. (See for example U.S. Pat. 
No. 4,022,593, issued May 10, 1977, hereby incorporated by reference in 
its entirety.) 
In some limited cases, it has been the past practice to employ variable 
high and low density mesh pads in a vapor phase intercept pattern to 
enhance mist elimination performance. In such cases, the lower portion of 
the mesh pad is formed of a low density material to promote rapid and easy 
draining of coalesced liquid and to aid in working away precipitated 
material from the pad, while the upper portion of the pad is formed of a 
high density material to collect liquid particulates from the upwardly 
flowing, mist containing vapor stream. 
It is desirable to provide an improved mist eliminator mesh pad in order to 
improve the mesh pad capacity and to provide for reductions in pressure 
drop compared to conventional mesh pads. 
SUMMARY OF THE INVENTION 
The invention relates to mesh pads, to the method of preparing such mesh 
pads and of using such mesh pads in gas-liquid contact, such as mist 
eliminator, apparatus. Particularly, the invention relates to a variegated 
density, mist eliminator mesh pad having increased capacity and low 
pressure drop. 
An improved mesh pad has been discovered particularly for use in gas-liquid 
contact apparatus for the removal of mists from mist-containing gas 
streams which mesh pad provides for defined localized regions of higher 
density and consequently enhances drainage within the mesh pad itself with 
the direction of variegated density generally perpendicular to the 
direction of gas flow. The mesh pad is variegated in density, that is the 
density of the mesh pad varies in a defined, controlled manner within the 
mesh pad itself in a direction generally perpendicular to the direction of 
the gas flow. The improved mist eliminator mesh pads are characterized by 
a higher capacity as a result of the enhanced drainage from the mesh pad's 
high density regions as well as lower pressure drops when using the pad. 
The mist eliminator mesh pads are composed of a plurality of formed, 
fibrous elements, typically knitted or woven and having open areas for the 
flow of a gas stream therethrough and characterized within the mesh pad by 
defined adjacent areas of high and low density, with the high, localized 
density regions providing for the enhanced drainage of coalesced liquid 
from the mesh pad, while the low density regions provide a low-resistance 
path for gas flow and mist removal. 
The mesh pad of the invention may assume various forms such as a horizontal 
or vertical mesh pad comprised of mesh layers which are stacked or spiral 
wound, which pad is characterized by localized regions of lower or higher 
density where the direction of density variation is generally 
perpendicular to the direction of gas flow. One method of achieving 
density variation is to place two or more layers of fibrous elements in a 
close contacting, non-nesting or nesting arrangement. In the nesting 
position, the layer of fibrous elements, is placed in contact with another 
layer of fibrous elements, either the same or different, and the fibrous 
elements nest in a close mating, nesting, cooperative-type relationship to 
form a layer of higher density, that is, a layer of about twice the 
density of either layer alone. In a non-nesting position, each layer of 
the fibrous elements is not engaged in a full or substantial cooperative 
mating relationship and there is little, if any, change in the fiber 
density between the two adjacent layers. 
It has been discovered that by placing corrugated layers of fibrous 
elements, such as stainless steel woven or knitted mesh, together in 
either a nesting and non-nesting or a plurality of nesting and non-nesting 
positions, localized regions of higher density and thus enhanced drainage, 
can be formed. For example, in the preparation of one-piece spiral wound 
mesh pads, such mesh pads may be formed by winding the layer of corrugated 
mesh in a spiral fashion around a central axis. At fixed distances from 
the axis, a high density region is created by interwinding a layer of 
corrugated mesh that nests fully or substantially into the adjacent mesh 
layer to form a region of higher mesh density. At other intermittent fixed 
distances from the central axis, low density regions are created by 
interwinding layers of corrugated mesh that do not nest substantially or 
fully together so as to form a non-nesting, low density region. The result 
is a spiral wound mesh pad that is variegated in density, that is, the 
density of the mesh pad varies as desired with the radial distance from 
the mesh pad's central axis. Gas flow through such a mesh pad would be in 
the axial direction. Experimental tests with such variegated mesh pads 
have demonstrated a higher capacity of the mesh pad as a result of 
enhanced drainage from the mesh pad's higher density regions as well as a 
low pressure drop when such mesh pads are employed in a gas-liquid contact 
device for mist elimination. 
The concept of density variegation in mist eliminator mesh pads of the 
invention is not limited to those mesh pads which are formed by spiral 
winding, but applies to all mist eliminator mesh pads in which there are 
two or more adjacent areas of different mesh density and when the 
direction of density variation is generally perpendicular to the direction 
of gas flow. The fibrous or other elements and materials employed in the 
mesh pads may be the same or different, but typically comprise a metal 
wire mesh, such as knitted or woven stainless steel or may comprise 
polymeric fibers or filaments. Generally, the mist eliminator mesh pads 
would have areas of defined high and low density, either formed from the 
same or different fibrous elements and may be of the same or different 
thicknesses and different densities. For example, for stainless steel wire 
mesh pads, the density may range from densities of four to fifteen pounds 
per cubic foot for the low density, to eight to thirty pounds per cubic 
foot for the high density regions. 
The knitted, woven or other fabric-type material used to form mesh pads for 
mist eliminators generally comprises a double layer of fiber or filament 
material, typically metal wire or plastic filaments. A variety of means 
may be used to obtain density variation within the mesh pad in a direction 
generally perpendicular to the direction of gas flow. One such means, 
particularly useful for metal mesh pads, is to corrugate the double layer 
of knitted or woven material to form ridges or peaks and valleys. A high 
density region is formed when the peaks of one layer fit or "nest" into 
the valleys of the adjoining layer, while low density or regular regions 
are formed when the peaks of one layer hit the peaks of the adjoining 
layer and do not nest. The thickness of any particular high or low density 
area can be controlled by the number of cooperatively mating engagements 
of each layer made. 
In one embodiment, the mesh pad is formed as a cylindrical spiral by 
winding two layers of corrugated wire mesh about the axis of rotation. For 
the low density regions, the corrugations of the said layers are at 
different angles with respect to the said axis and do not nest together. 
For the high density regions, the corrugations of the two layers lie at 
the same angle with respect to the axis so that the layers nest together. 
In another embodiment, layers of corrugated wire mesh are maintained in a 
planar configuration with the plane of the layers parallel to the 
direction of the gas flow. Density variegation is accomplished by placing 
adjacent layers in either a nesting or non-nesting arrangement. The 
number, height and thickness of the layers may vary, as desired, with the 
layers of high density usually of the same but typically less thickness, 
e.g. one to eight inches, than the low density layers. The number of 
layers, e.g. in the nesting or non-nesting, may vary, such as two to 
twelve, e.g. four to eight. The height of the pad may vary, as desired, 
e.g. four to twelve inches or more. 
Another means of obtaining a variegated density in mesh pad mist 
eliminators is to vary the crimp height or thickness of the corrugated 
layers, i.e., the height of the peaks and depth of the valleys or in 
adjacent regions of the mesh pad. The greater the crimp height, the lower 
the density. The mesh pad of the invention would then comprise a pad in 
which the low density regions contain non-nesting mesh having a relatively 
large crimp height, and the high density regions contain nesting or 
non-nesting mesh having a relatively small crimp height or being 
uncorrugated. 
Yet another means of obtaining a variegated density in mesh pad mist 
eliminators is to use parallel knit material or other high density 
material in conjunction with monofilament knit material or other low 
density material in adjacent regions of the mesh pad. Monofilament 
material is made by using a single wire or filament in the knitting 
process while parallel knit material or multifilament material is made by 
using a bundle of several wires or filaments in parallel in the same 
knitting process, e.g. up to twelve filaments, such as four to eight. The 
parallel knit material has a higher density than the monofilament 
material. The mesh pad of the invention would then comprise a pad in which 
the low density regions contain monofilament mesh while the adjacent high 
density regions contain parallel knit, multifilament mesh. 
Other techniques can also be used to vary the density of mesh layers within 
the mesh pad. These techniques include using different materials, e.g. 
plastic and metal, in the high and low density regions, using different 
filament diameters in the high and low density regions, or using a more 
tightly knitted or woven mesh in the high density regions. It is essential 
that the density variation is in a direction perpendicular to the 
direction of gas flow. 
Another configuration for the mist eliminator mesh pads comprises a 
box-type structure wherein an interior, low density mesh is fully or 
partially surrounded by an exterior, high density mesh layer, all of which 
mesh layers represent different embodiments of the concept of a 
mesh-layered density variegation through employing fibrous elements in a 
nesting and non-nesting arrangement. 
A mesh pad with enhanced drainage can be prepared by incorporating a region 
of higher density extending downwardly from the mesh pad into the gas flow 
path so as to permit coalesced liquid from the higher density region of 
the variegated mesh pad to drain from the extending, higher density layer. 
For example, for variegated mesh pads used in vertical flow mist 
elimination, the extensions e.g. one-half to four inches, of the high 
density mesh layers from the bottom of the cylindrical pad, which 
extension is in contact with the higher density layer formed in the pad, 
enhances drainage of coalesced liquid from the higher density region. 
The improved mist eliminator mesh pads of the invention are typically 
employed by placing one or more layers of the mesh pads in a gas-liquid 
contact apparatus and permitting the flow of a mist-containing gas stream 
through the mesh pads thereby providing for coalescence of liquid from the 
gas stream and the draining of coalesced liquid from the mesh pad. 
The invention will be described for the purposes of illustration only in 
connection with certain embodiments; however, it is recognized that 
various changes, modifications, additions and improvements may be made in 
the illustrated embodiment by those persons skilled in the art without 
departing from the spirit and scope of the invention.

DESCRIPTION OF THE EMBODIMENTS 
FIG. 1 shows a gas-liquid contacting system 10 which comprises a gas-liquid 
contact tower 12 having an inlet at the lower portion thereof 14 for the 
introduction of a mist-laden gas stream in which the mist particles are to 
be removed, and an upper portion 16 for the removal of a mist-laden gas 
stream and a lower outlet 18 for the removal of coalesced liquid 20 from 
the gas-liquid tower 12. Across the diameter of the tower is shown a 
variegated density, mist eliminator mesh pad of the invention 22 which 
mesh pad is placed in the gas flow path of the mist-containing gas stream, 
and is a cylindrical, spiral-wound mesh pad 22, spiral-wound to the 
dimensions of the gas-liquid contact tower 12. 
FIGS. 2 and 3 are enlarged views of the variegated mesh pad 22 showing high 
density mesh layers 26, 30 and 34 providing a plurality of spiral-wound, 
higher density regions, e.g. two to six or three as illustrated, and 
layers 24, 28, 32 and 36 providing a plurality of low-density regions, 
e.g. two to six or four as illustrated. The high density regions provide 
for the coalescing and drainage of coalesced liquid 20. As illustrated, 
the high density region may extend slightly downward from the upstream 
face 40 of the mesh pad to promote more rapid drainage from the mesh pad. 
FIG. 4 is a perspective partial sectional view of a horizontal flow, 
variegated density mesh pad 50 with alternate layers of high and low 
density mesh with the lower density layers in regions 52, 54, 56 and 58 of 
greater thickness than the higher density layers in regions 60, 62 and 64, 
the plane of the layers or regions disposed parallel to the gas flow and 
with the coalesced liquid 66 draining from the lower section of the high 
density layers or regions 60, 62 and 64. Optionally, these high density 
layers 60, 62 and 64 may be extended slightly downward from the bottom 
surface of the mesh pad 50 to promote rapid drainage from the extended 
high density regions. This same configuration can be used in vertical flow 
rather than the spiral-wound configuration. In this case, the coalesced 
liquid would drain downward into the rising mist-containing gas stream. 
FIG. 5 is a perspective partial sectional view of a box-type mist 
eliminator mesh pad 80 composed of low, high and medium density knitted or 
woven wire mesh layers, in generally circular form, for enclosing a 
central space and for insertion into a gas-liquid column. The pad 80 
comprises a tubular layer composed of high density mesh layer 82, e.g. 
over 15 pounds per cubic foot, a circular top layer of medium density mesh 
84, e.g. 8 to 12 pounds per cubic foot, and a lower upstream low density 
layer 86, e.g. less than 6 pounds per cubic foot, the layer enclosing an 
empty cylindrical space 88. In mesh pad 80, mist from the mist-containing 
gas stream passes axially upward through the low density coalescing layer 
86 into space 88 (which optionally may contain another layer or be filled 
in whole or part with a mist-coalescing device such as fiber) and upwardly 
through layer 84, while also passing peripherally upward through the 
surrounding high density layer 82 which aids in draining the coalesced 
liquid from the mesh pad 80 about the lower outer periphery of the pad. 
FIG. 6 illustrates two layers of corrugated knitted wire mesh 90 comprising 
non-nesting, angularly disposed, e.g. 10.degree. to 60.degree., e.g. 
30.degree., corrugated knitted mesh layers 92 and 94 arranged in a low 
density, non-nesting relationship to provide the usual low density area or 
region of a mesh pad. 
FIG. 7 illustrates two layers of corrugated knitted wire mesh 96 composed 
of three nesting corrugated mesh layers 98, 100 and 102 to provide a high 
density drainage mesh pad layer, with the adjacent layers nesting along 
the corrugations. 
As illustrated, the variegated mesh pad may vary in thickness and in height 
of the high and low density layers; however, such pad includes two or more 
adjacent regions of different density wherein the direction of density 
variation is generally perpendicular to the direction of gas flow. 
Certain comparative tests were conducted to illustrate the improvement in 
capacity of the low-high density, variegated mesh pads and prior art low 
density mesh pads composed solely of the low density mesh. The variegated 
mesh pad was that illustrated in FIGS. 2 and 3 wherein the density was 
varied as illustrated in FIGS. 6 and 7. Table 1 gives the physical 
characteristics of the two mesh pads tested. Both pads were tested under 
identical conditions in a three-foot diameter, vertical flow, transparent 
tower using a nozzle to spray a water mist into the air stream below the 
mist eliminator. 
The pressure drop and the critical velocity at different gas flow rates and 
liquid loadings for both mesh pads were measured. The results obtained are 
presented in Tables 2 and 3. Enhanced drainage from the higher density 
regions of the variegated mesh pad was observed during the test. This 
accounts for the higher capacity and lower pressure drop of the variegated 
mesh pad as indicated in Tables 2 and 3. 
TABLE 1 
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Physical Characteristics of Mesh Pads Tested 
Variegated Pad 
Conventional Pad 
______________________________________ 
Diameter (ft.) 3 3 
Height (in.) 6 6 
Configuration Spiral Spiral 
Density (lb./ft..sup.3) 
9 (low) 9 
18 (high) 
Variegated Thickness (in.) 
4 (low) Uniform 
1 (high) 
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TABLE 2 
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VARIEGATED MESH PAD TEST RESULTS 
Pressure Drop .DELTA. P (in H.sub.2 O) 
Liquid 
Loading 
Mesh Pad Gas Phase Velocity (ft/sec) 
(gpm/ft.sup.2) 
Design 4 6 8 10 11 
______________________________________ 
0 Variegated 0.069 0.136 
0.218 0.318 0.370 
Conventional 
0.073 0.140 
0.220 0.320 0.370 
0.3 Variegated 0.09 0.170 
0.28 0.47 0.6 
Conventional 
0.09 0.180 
0.32 0.62 0.92 
2.5 Variegated 0.10 0.220 
0.48 1.05 1.65 
Conventional 
0.10 0.240 
0.62 1.70 2.25 
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TABLE 3 
__________________________________________________________________________ 
COMISON: VARIEGATED MESH PAD WITH CONVENTIONAL MESH PAD 
Pressure Drop 
Pressure Drop 
(inches of water) 
(inches of water) 
V.sub.c (ft/sec) @ 
V.sub.c (ft/sec) @ 
at 8 ft/sec and 
at 11 ft/sec and 
0.3 gpm/ft.sup.2 
2.5 gpm/ft.sup.2 
2.5 gpm/ft.sup.2 
2.5 gpm/ft.sup.2 
__________________________________________________________________________ 
Variegated Mesh Pad 
12.49 10.66 0.48 1.65 
Conventional Mesh Pad 
12.09 10.33 0.62 2.25 
% Improvement of 
3.3% 3.2% 23% 26.7% 
Variegated Mesh Pad 
__________________________________________________________________________ 
V.sub.c = critical velocity where droplets get reentrained (i.e., the 
maximum allowable operating velocity)