Velocities filtration

Pressure filters can treat feeds with concentrations up to and in excess of 10% sohds by weight and having large proportions of difficult-to-handle fine particles. Typically, slurries in which the sohd particles contain 10% greater than 10 ]lni may require pressure filtration, but increasing the proportion greater than 10 ]lni may make vacuum filtration possible. The range of typical filtration velocities in pressure filters is from 0.025 to 5 m/h and dry sohds rates from 25 to 250 kg nY/h. The use of pressure filters may also in some cases, such as in filtration of coal flotation concentrates, eliminate the need for flocculation. 

Cross-Flow Filtration in Porous Pipes. Another way of limiting cake growth is to pump the slurry through porous pipes at high velocities of the order of thousands of times the filtration velocity through the walls of the pipes. This is ia direct analogy with the now weU-estabHshed process of ultrafiltration which itself borders on reverse osmosis at the molecular level. The three processes are closely related yet different ia many respects. 

V/ Filtration velocity (superficial gas velocity tbrougb filter) m/min ft/min... 

Panel filters may use either viscous or dry filter media. Viscous filters are so called because the filter medium is coated with a tacky liquid of high viscosity (e.g., mineral oil and adhesives) to retain the dust. The filter pad consists of an assembly of coarse fibers (now usually metal, glass, or plastic). Because the fibers are coarse and the media are highlv porous, resistance to air flow is low and high filtration velocities can be used. 

Dry filters are usually deeper than viscous filters. The dry filter media use finer fibers and have much smaller pores than the viscous media and need not rely on an oil coating to retain collected dust. Because of their greater resistance to air flow, dry filters must use lower filtration velocities to avoid excessive pressure drops. Hence, dry media must have larger surface areas and are usually pleated or arranged in the form of pockets (Fig. 17-64), generally sheets of cellulose pulp, cotton, felt, or spun glass. 

Typical new equipment design efficiencies are between 99 and 99.9%. Older existing equipment have a range of actual operating efficiencies of 95 to 99.9%. Several factors determine fabric filter collection efficiency. These include gas filtration velocity, particle characteristics, fabric characteristics, and cleaning mechanism. In general, collection efficiency increases with increasing filtration velocity and particle size. 

The hydraulic performances required of the sand with slow filters are inferior to those for rapid filters. In the case of slow filters, one can use fine sand, since the average filtration velocity that is usually necessary lies in the range 2 to 5 m/day. 

For filter design and performance prediction it is necessary to predict the rate of filtration (velocity or volumetric flowrate) as a function of pressure drop, and the properties of the fluid and particulate bed. This can be achieved using the modified Darcy equation developed in Chapter 3. 

Reverse-pulse filters are typically operated at higher filtration velocities (i.e., air-to-cloth ratios) than shaker or reverse-flow designs. Filtration velocities may range from 3 to 15 feet per minute in reverse-pulse applications, depending on the dust being collected. However, the most the commonly used range is 4 to 5 feet per minute. 

Collection Efficiency The inherent collection efficiency of fabric filters is usually so high that, for practical purposes, the precise level has not commonly been the subject of much concern. Furthermore, for collection of a given dust, the efficiency is usually fixed by the choices of filter fabric, filtration velocity, method of cleaning, and... 

Filtration velocity (cm/s) Pore size 7 ( rm) Porosity (%) Wall thickness ws (mm)... 

Figure 16 shows the normalized mass distribution inside the filter wall vs. the normalized wall thickness as a function of the utilized capacity of the filter wall, for the low porous and the high porous material (small wall thickness) at a filtration velocity of 4cm/s. The line of the highest utilized capacity gives the state of loading inside the filter wall when the transition from the deep-bed to cake filtration has occurred and there is no more mass entering inside the filter wall. This final state of the mass distribution along the filter wall thickness was calculated for all the cases listed in Table I and the results are shown in Fig. 17. 

Figure 17 shows that a more uniform mass distribution can be achieved at higher filtration velocities, both for the low and the high porous material. Finally, Fig. 18 shows the utilized capacity of the filter wall for all the cases in study, computed with a gas temperature of 280°C and a primary and aggregate particle size of 20 and 90 nm, respectively, as a function of the Peclet number. It is seen (Fig. 18) that the more porous materials with a smaller wall thickness can attain a better usage of the capacity of the filter wall, when the Peclet number increases. 

Fig. 16. Normalized mass distribution inside the filter wall vs. the normalized wall thickness as a function of the utilized capacity of the filter wall. Low porous material (top) and high porous material (bottom), for a filtration velocity of 4cm/s and a wall thickness of 0.305 mm.

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