VARIEGATED DENSITY MESH PAD FOR MIST REMOVAL AND METHOD OF PREPARING SAME

Abstract: 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.

Description: 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 0.10 to 0.38 mm (4 to 15 mil) diameter stainless steel wire and are arranged from about 7.6 to 61 cms (3 to 24") in thickness, having a density ranging from about 64 to 240 kg/m³ (4 to 15 lbs/ft³) and range in diameter from about 0.3 to 9.1 m (1 to 30 ft) 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 1 to 5 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 localised, separate regions of flow interruption and interception, therefore creating preferential drainage foci. (See for example, US-A-4 022 593, upon which are based the prior art portions of claims 1 and 9.) In some limited cases, it has been the past practice to employ variable high and low density mesh pads in a vapour 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 vapour 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. The invention relates to apparatus using mist eliminating mesh pads and to the method of using such mesh pads in gas-liquid contact, such as mist eliminator apparatus. Particularly, the invention relates to the use of a variegated density mist eliminator mesh pad having increased capacity and low pressure drop. The present invention, as defined in claims 1 and 11 used an improved mesh pad which 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 localised regions of higher density the provision of which enhances drainage within the mesh pad itself with the direction of variegated or variation of density being 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 characterised 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 characterised within the mesh pad by defined adjacent areas of high and low density, with the high, localised 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 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 an 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 in 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 the use of mist eliminator mesh pads in accordance with 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 64 to 240 kg/m³ (four to fifteen pounds per cubic foot) for the low density, to 128 to 481 kg/m³ (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 in 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. 10.2 to 30.5 cms (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 one 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 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 one 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. 1.27 to 10.2 cms (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, enhance, drainage of coalesced liquid from the higher density region. The improved mist eliminator mesh pads are typically employed by placing one or more layers of the mesh pads in a gas-liquid contact apparatus and permitting the flow or 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. Brief Description of the Drawings Fig. 1 is on illustrative, sectional view of a gas-liquid contact tower containing a mist eliminator mesh pad and embodying the invention. Fig. 2 is a top plan view of the mist eliminator mesh pad of Fig. 1. Fig. 3 is an enlarged perspective partial sectional view of the mist eliminator mesh pad of Fig. 1. Fig. 4 is a sectional view of a variegated density mesh pad in which the layers of mesh lie in adjacent planes. Fig. 5 is a perspective partial sectional view of a box mist eliminator mesh pad. Fig. 6 is an enlarged fragmentary view of two non-nesting, corrugated knitted mesh layers of a low-density region of Fig. 2; and Fig. 7 is on enlarged fragmentary view of two testing, corrugated knitted mesh layers of a high density region of Fig. 2. 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 outlet 16 for the removal of a 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 regions 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 240 kgs/m³ (15 pounds per cubic foot), a circular top layer of medium density mesh 84, e.g. 128 to 192 kg/m³ (8 to 12 pounds per cubic foot), and a lower upstream low density layer 86, e.g. less than 96 kg/m³ (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 much as fiber) and upwardly through layer 84, while also passing peripherally upward through the surrounding high density layor 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° to 60°, e.g. 30°, 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 a layer 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 91 cm (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. Physical Characteristics of Mesh Pads Tested Variegated Pad Conventional Pad Diameter cms (ft) 91 (3) 91 (3) Height cms (in) 15 (6) 15 (6) Configuration Spiral Spiral Density kg/m³ (lb/ft³) 144 (9) (low) 144 (9) 288 (18) (high) Variegated Thickness cm (in) 10.2 (4) (low) Uniform 2.5 (1) (high)

Claims: Mist-eliminator apparatus (12) comprising a open-mesh pad (22) comprising fibrous elements to permit the flow of a mist-containing gas stream through said pad for the coalescing of the liquid mist from the gas stream, characterised in that the pad (22) comprises a plurality of layers providing adjacent regions (24-36 or 52-64) of high and low density mesh, relative one to the other, the direction of density variation being generally perpendicular to the direction of gas flow whereby the direction of flow through the pad is parallel to the direction of extent of said layers. Apparatus according to claim 1, wherein the pad comprises alternate regions of high and low density mesh material with each high density region comprising a plurality of nested layers (98-102). Apparatus according to claim 1, wherein the low density regions are comprised by corrugated mesh material (92,94). Apparatus according to claim 3, wherein the high density regions are comprised by corrugated mesh material (98-102) disposed in a nesting relation and the low density regions are comprised by corrugated mesh material (92,94) disposed in a non-nesting relation. Apparatus according to claim 1 or 2, wherein the low density region comprises monofilament fibrous elements and the high density region comprises multifilament fibrous elements. Apparatus according to claim 1 or 2, wherein one or more of the low density regions comprises non-nesting layers of high crimp height and one or more of the high density regions comprises nesting layers of low crimp height. Apparatus according to any one of claims 1 to 4, wherein the fibrous elements comprise stainless steel and the low density regions have a density of from 64 to 240 kg/m³ (4 to 15 lbs/ft³), and the high density regions have a density of from 128 to 481 kg/m³ (8 to 30 lbs/ft³). Apparatus according to any preceding claim, wherein the layers (52-64) of different density regions of the pad (50) are disposed generally vertically and parallel to the gas flow. Apparatus according to any one of claims 1 to 7, wherein the pad comprises an upright cylinder composed of axially extending cylindrical layers of the high and low density regions. A method according to any one of claims 1 to 7, wherein the pad comprises one layer which has been rolled with another layer to form a cylindrical, variegated density mesh pad. Apparatus according to any preceding claim, wherein the lower edges of the high density regions (26,30,34) project downwardly from the lower surface of the pad beyond the lower edges of the low density regions (24,28,32,36). A method of eliminating mist from a mist-containing gas stream wherein the stream is passed through an open-mesh pad (22) of fibrous material with separated liquid (20) being collected from the lower surface of the pad, characterised in that the pad comprises alternating regions (24-36) comprised by layers of high and low fibrous mesh density relative to each other, extending parallel to the direction of fluid flow through the pad with the direction of the variation of density of said regions being generally perpendicular to the direction of gas flow. A method according to claim 12, wherein the lower edges of the higher density regions project below the lower edges of the lower density regions with the collected liquid dropping from the projecting edges.

Inventors: HANSEN, OLE V., MCNULTY, KENNETH J., MONAT, JAMIE P., OZOLINS, KARLIS P.
Applicants: KOCH ENGINEERING COMPANY, INC.
Application year: 1988
Main classification: B01D46
IPC codes: B01D46