Mesh pads

In drying towers of sulfur-burning plants, mesh pads or inertial impaction-type mist eliminators are usually adequate. High efficiency mist eliminators are usually used in drying towers of spent acid or metallurgical plants. 

Entrainment Due to Gas Bubbling/Jetting through a Liquid Entrainment generally hmits the capacity of distiUation trays and is commonly a concern in vaporizers and evaporators. Fortunately, it is readily controllable bv simple inertial entrainment capture devices such as wire mesh pads in gravity separators. 

Lm. The coarseness results from the relatively low power dissipation per mass on distillation trays. This means that it is relatively easy to remove by a device such as a wire mesh pad. Over 50 percent is typically captured by the underside of the next higher tray or by a turn in the piping leaving an evaporator. Conversely, though small on a mass basis, the smaller drops are extremely numerous. On a number basis, more than one-half of the drops in the lower curve are under 5 [Lm. These can sei ve as nuclei for fog condensation in downstream equipment. 

As indicated previously, mesh efficiency drops rapidly as particles decrease in size Below 5 [Im. An alternative is to use two mesh pads in series. The first mesh is made of fine wires and is operated beyond the... 

Figure 4-23. Typical Installation of wound mesh pads In vertical vessel. Courtesy of Metal Textile Corp., Bulletin ME 9-58.

V (separator) = Separator vapor velocity evaluated for the gas or vapor at flotving conditions, ft/sec V = Vapor velocity entering unit, lbs, per minute per square foot of inlet pipe cross section Va = Maximum allowable vapor velocity across inlet face of mesh calculated by relation, ft/sec Van Actual operating superficial gas velocity, ft/sec or ft/min, for tvire mesh pad Vu = Design vapor velocity (or selected design value), ft/sec... 

V ,ax = Calculated maximum allowable superficial gas velocity, ft/sec, or ft/min tvire mesh pad Vs = Superficial gas velocity, ft/sec Vsa = Separator vapor velocity evaluated for air-water system, ft/sec... 

Entrainment removal of 99% is attained with mesh pads of 4-12 in. thicknesses 6 in. thickness is popular. 

The separation of gas-liquid (or vapor-liquid) mixtures can be enhanced by installing a mesh pad at the top of the disengagement zone to coalesce the smaller droplets to larger ones. If this is done, then the KT in Equation 8.3 is normally specified to be 0.11 m s-1, although this can take lower values down to 0.06 m-s-1 for vacuum systems8. 

An empty vessel may be employed, but horizontal baffles can be used to reduce turbulence and assist the coalescence through preferential wetting of the solid surface by the disperse phase. More elaborate methods to assist the coalescence include the use of mesh pads in the vessel or the use of an electric field to promote coalescence. Chemical additives can also be used to promote coalescence. 

A mixture of vapor and liquid ammonia is to be separated in a cylindrical vessel mounted vertically with a mesh pad to assist separation. The flowrate of vapor is 0.3 m3-s. The density of the liquid is 648 kg-m"3 and that of the vapor 2.71 kg m-3. Assuming KT = 0.11 m s-1, estimate the diameter of vessel required. 

Coalescence. Coalescence by gravity in simple settling devices can often be used to separate immiscible liquid-liquid mixtures. The coalescence can be enhanced by the use of mesh pads and centrifugal forces, as discussed in Chapter 8. 

A second type of impingement separation device is a knitted wire mesh pad. The primary mechanism of separation in the knitted wire mesh is impingement. It also utilizes centrifugal and gravitational force in the collection of small liquid particles. 

Aimes all droplets greater than 10 microns in diameter were removed by the knitted wire mesh pads within the capacity limitations. However, a few droplets as large as 20 microns in diameter were allowed to pass through the unit. This is not a serious limitation since it would lake 90,000 of these droplets in each standard cubic foot of gas passing... 

Impingement type mist extractor exemplified by knitted wire mesh pads are widely used in oilfield liquid and gas separation equipment. Their liquid and gas handling capacities are good with high liquid droplet removal efficiencies with small gas pressure drop. 

Paraffin. Separator operation can be adversely affected by paraffin accumulation. Coalescing plates in the liquid section and mesh pad mist extractors in the gas section are particularly prone to plugging by paraffin. If paraffin is an actual or potential problem, use of plate type or centrifugal mist extractors should be considered. Manways, handholes and nozzles should be provided to allow steam, solvent or other types of cleaning of separator internals. 

The mechanism of vapor-phase separation is illustrated in Fig. 1. Both stages—coalescence and disengagement followed by gravity settling—must be considered in sizing the drum and selecting the wire-mesh pad. 

As in the case of Dpg, b i function of physical properties and the geometric characteristics. S. of the wire-mesh pad. A more closely woven and thicker pad produces larger droplets, and hence a higher Kg, which makes possible a faster vapor velocity, V(H and a smaller drum diameter. However, this is limited by the interfacial tension between the mesh material and the liquid (i.e., the wettability of the material), lnlcrfacial tension is another property that is difficult to determine. 

A wire-mesh pad in a horizontal drum greatly enhances separation and significantly reduces drum size. For lower pressure drop across the pad. the requirement for a high-performance pad can be relaxed by designing so the vapor space removes larger droplets, reducing the load on the pad. 

The design-base particle diameter to be separated in the vapor space should not exceed the pad-disengage-mt-nt droplet diameter. 1). To be on the safe side, a maximum droplet diameter should be limited to 400 pm. In horizontal drum design, the mesh pad should be regarded as a polisher for removing small droplets not separated in the vapor space—i.e., as a secondary mechanism in vapor-liquid separation... 

To further improve the steam quality, the experienced design engineer places a dome, as shown in Fig. 15.4, on top of the kettle. The dome will have a sloped demister mesh pad. A demister acts to coalesce small droplets of water into larger droplets. The larger droplets easily settle out of the flowing steam. The demister is sloped, to promote liquid drainage from the surface of the pad. 

Kettle reboilers consist of a bundle of tubes in an oversize shell. Submergence of the tubes is assured by an overflow weir, typically 5-15 cm higher than the topmost tubes. An open tube bundle is preferred, with pitch to diameter ratios in the range of 1.5-2. Temperature in the kettle is substantially uniform. Residence time is high so that kettles are not favored for thermally sensitive materials. The large shell diameters make kettles uneconomic for high pressure operation. Deentraining mesh pads often are incorporated. Tube bundles installed directly in the tower bottom are inexpensive but the amount of surface that can be installed is limited. 

Figure 18.4. Typical installations of mesh pads in equipment (Metal Textile Corp, Bulletin ME-7-, from Ludwig, 1977, Vol. I, p. 159).

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