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1. Types of Mist Eliminators
2. Fundamental Considerations
3. Sizing for Gas Velocity
4. Mesh vs. Vanes - or both
5. Applying Mist Eliminators
6. Curves
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TYPES OF MIST ELIMINATORS
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There are three general types of mist eliminators — mesh, vane and fiber bed — and AMISTCO makes all three. Each is suited for a different class of applications, either alone or combined with another type.
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Figure 2: Flattened tube of knitted 0.011 inch wire with diagonal crimps
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Figure 3: Co-Knit mesh, including multifilament glass fibers knitted along with the wire
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Mesh — pads and insertion type
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The most widely applicable type of mist eliminator is made of metal or plastic wire with typical diameter of 0.006 to 0.011 inch, loosely knitted in a form resembling a cylindrical net. This tube is flattened to form a two-layer strip typically 12 inches wide, which is then crimped in a diagonal pattern with ridges as shown in Figure 2. When these strips are laid together, the ridges slant in alternate directions, forming an open structure through which gas flows freely. Such mesh can efficiently capture mist droplets as small as 5 microns (micrometers). For eliminating droplets down to 1 micron in diameter, multi-filament yarns of various plastics or glass are knitted into the mesh. The result is called a composite or co-knit mesh (Figure 3).
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Mesh Pads
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In the most familiar application of knitted mesh, the crimped strips are stacked to form a pad with typical thickness of four or six inches. (See Figure 4.) Rigidity is provided by a frame—usually metal—consisting of a grid on each side and rods passing through the mesh. Pads larger than about three feet across are fabricated in sections narrow enough to pass through a manway for assembly inside a vessel. Mesh pads can be made in almost any shape, but most are round (as in Figure 4) or rectangular.
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Figure 4: Typical mesh-type mist eliminator pad
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MistFix™ Insertion Mist Eliminator
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In an exclusive Amistco innovation (U. S. Patent Number 5,985,004), knitted wire mesh is wrapped into a cylindrical core with a flange at one end. (See Figure 5.) MistFix mist eliminators extend vertically into a vessel from the vapor exit nozzle. As replacements or substitutes for pads, they greatly speed turnaround time, avoid entry of hazardous vessels, and eliminate the need for vessel cutting where there is no access port. They are ideally suited for existing vessels that do not have mist eliminators.
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Figure 5: MistFix™ insertion mist eliminator
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Vane Packs
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Also known as chevron or plate type, vane mist eliminators consist of closely spaced corrugated plates that force mist-laden gas to follow serpentine paths. These devices are generally not efficient for mist droplets smaller than about 20 microns, but they are sturdier than mesh pads and impose less pressure drop. Vane arrays can be mounted horizontally or vertically. They are preferred in applications involving high vapor velocities, low available pressure drop, viscous or foaming liquids, lodging or caking of solids, slugs of liquid, or violent upsets. Like mesh pads, vane units are usually round or rectangular. They are sometimes used in combination with mesh pads for optimum performance in special situations. Amistco’s
standard vanes (front cover) are available in metal or plastics and have various blade spacings and profiles. For special requirements, Amistco also supplies curved vanes such as the non-metallic variety shown in Figure 6.
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Figure 6: Mist eliminator with curved non-metallic vanes
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Double-Pocket Vanes
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Amistco’s high-performance double-pocket vanes (Figure 7) can operate at higher capacity and higher efficiency than conventional vanes. The design features liquid pockets that prevent re-entrainment of the separated liquid droplets. This helps increase the capacity up to twice that of conventional vanes. The higher gas velocities also help in obtaining 100% removal of 8-micron droplets.
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Figure 7: Top edge of Amistco double-pocket vane unit
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Fiber Candles and Panels
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Fiber mist eliminators can capture mist droplets so small (below 1 micron) that they appear as smoke or nearly invisible haze. These units employ fine fibers—typically cellulose, glass, or plastic—packed into a mat with thickness of a few inches. Fiber mist eliminators are mostly used in cylindrical form called candles (Figure 8) but are also available in flat panels. Amistco designs and manufactures these devices under an exclusive Western Hemisphere license from Begg Cousland. For additional details, see the “Amistco Candle Filters” brochure.
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Figure 8: Typical fiber mist eliminator candle
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