A potential solution: the MistFix™ mist eliminator
However, there is another solution that works in many cases such as this. It is a patented type of mist eliminator manufactured by Amistco Separation Products, called MistFix™. As seen in Figure 3, a MistFix unit is a hollow cylinder of the same sort of knitted mesh that conventional mesh pads are made of. It is designed to be inserted in a vertical flanged gas exit nozzle, being secured by a base ring that fits between the flanges. Rigidity is provided by a cylindrical frame around which the mesh is wrapped (not shown in the diagram), and the bottom end is closed by a plate. MistFix units are widely applicable for exit nozzles with inside diameter no less than about 6 inches, provided that the length necessary to achieve an efficient gas velocity range is no greater than about 54 inches. By using special high-efficiency, high-density mesh styles, the radial thickness of the mesh can be as low as two inches—sometimes even less.
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FIGURE 3. Mistfix™ insertion mist eliminator
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FIGURE 4. Gas flows through the mesh of a Mistfix unit horizontally, as it does through a vertical flat mesh pad in some knockout drums such as this one.
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A MistFix mist eliminator functions in the same manner as a mesh pad mounted vertically, such as in the knockout drum shown in Figure 4. Gas flows horizontally through the mesh, and captured liquid drains downward. In a Mist-Fix unit as in Figure 3, mist-free gas emerges from the mesh into the central cavity and flows upward into the gas exit pipe.
The MistFix unit is sized to fit closely inside the exit nozzle. Its length depends on the case at hand. The longer the mesh cylinder, the greater the cross-sectional area of the mesh for gas flow, and the lower the gas velocity for a given volumetric throughput. The length of the mesh cylinder is selected so that the velocity through the mesh is within an optimum range for the application.
MistFix units can be advantageous for retrofitting even large vessels that have manways. This method avoids the usual down time and expense for installing a conventional mist eliminator. There is no need to purge and ventilate the vessel, build a scaffold inside, weld supports for the mist eliminator, and recertify the vessel to ASME code.
Vessels with special linings for corrosion resistance but no mist eliminators are especially appropriate candidates for retrofit with MistFix units. If increased throughput causes mist and the need for a mist eliminator, it is difficult to attach a support ring without penetrating the lining. Even slight damage to the lining can lead to corrosion and possible failure of the vessel. A MistFix unit may be the only viable alternative.
Even for new construction, MistFix mist eliminators are preferred over conventional alternatives in some cases. One instance is applications requiring frequent replacement because of fouling or corrosion. Another is those in which entering the vessel for inspection and replacement would be burdensome because down time cannot be tolerated, or the process material is exceptionally hazardous, etc.
Despite the advances that have been made in the technology, design and specification of mesh and vane mist eliminators is still as much art as science. For all but the most experienced users, proper application depends on consultation with the manufacturer’s engineers. This is especially so in the case of MistFix mist eliminators.
This paper provides some background for those discussions with Amistco engineers. Included are some general guidelines for applying MistFix mist eliminators, particularly in retrofits. The information is not intended to enable users to design and specify these devices without assistance.
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