Bulk, filter media

The cleaning action of the pulse is so effective that the dust layer may be completely removed From the surface of the fabric. Consequently, the fabric itself must sei ve as the principal filter medium for at least a substantial part of the filtration cycle. Woven fabrics are unsuitable for such service, and felts of various types must be used. The bulk of the dust is still removed in a surface layer, but the felt ensures that an adequate collection efficiency is maintained until the dust layer has formed. 

Filter aids should have low bulk density to minimize settling and aid good distribution on a filter-medium surface that may not be horizontal. They should also be porous and capable of forming a porous cake to minimize flow resistance, and they must be chemically inert to the filtrate. These characteristics are all found in the two most popular commercial filter aids diatomaceous silica (also called diatomite, or diatomaceous earth), which is an almost pure silica prepared from deposits of diatom skeletons and expanded perhte, particles of puffed lava that are principally aluminum alkali siheate. Cellulosic fibers (ground wood pulp) are sometimes used when siliceous materials cannot be used but are much more compressible. The use of other less effective aids (e.g., carbon and gypsum) may be justified in special cases. Sometimes a combination or carbon and diatomaceous silica permits adsorption in addition to filter-aid performance. Various other materials, such as salt, fine sand, starch, and precipitated calcium carbonate, are employed in specific industries where they represent either waste material or inexpensive alternatives to conventional filter aids. 

To continuously separate FT wax products from ultrafine iron catalyst particles in an SBCR employed for FTS, a modified cross-flow filtration technique can be developed using the cross-flow filter element placed in a down-comer slurry recirculation line of the SBCR. Counter to the traditional cross-flow filtration technique described earlier, this system would use a bulk slurry flow rate below the critical velocity, thereby forcing a filter cake of solids to form between the filter media and the bulk slurry flow, as depicted in Figure 15.2b. In this mode, multiple layers of catalyst particles that deposit upon the filter medium would act as a prefilter layer.10 Both the inertial and filter cake mechanisms can be effective however, the latter can be unstable if the filter cake depth is allowed to grow indefinitely. In the context of the SBCR operation, the filter cake could potentially occlude the slurry recirculation flow path if allowed to grow uncontrollably. 

The cylindrical capillary model predicts that the size of the largest pore present in a membrane filter medium is inversely proportional to the pressure at which bulk flow of a test gas is not present. 

As pressure is increased above the bubble point pressure, pores of decreasing size have the liquid forced out, and this allows additional bulk flow of the test gas. By measuring and comparing the bulk gas flow rates of both a wetted and a dry filter medium at the same pressure, the percentage of the bulk gas... 

Another integrity test referred to as the pressure hold test makes use of the fact that below the transition pressure no bulk flow of the test gas takes place. Therefore, in a pressure hold test, once a filter system is in place and the filter medium wetted, pressure is applied to the system and then shut off and sealed. If there are no leaks in the system or holes in the membrane larger than the corresponding test pressure used, the pressure should remain constant. If the pressure drops, there is a leak somewhere in the system that should be corrected. The pressure hold test is popular in testing filter assemblies and systems in... 

This is the anaerobic equivalent of the trickling filter. It contains support media (rock, gravel, plastic) with a void space of 50% or more. The bulk of anaerobic bacteria grow attached to the filter medium, but some form floes that become trapped inside this medium. Wastewater upflow through the reactor helps retain suspended solids in the column. This system achieves a modest BOD removal, but a higher removal of solids. Approximately 20% of the BOD is converted to CH4. 

Proper selection of the filter medium is more of an art than a science. The filter cutoff must be chosen to capture the smallest particles of interest. Other factors that must be considered are the type of filter (bulk or surface), the required flow rate, and the size of the membrane. These parameters are not independent and the best choice will usually involve trade-offs. Finally, the material from which the filter is made must be considered. It must be selected for compatibility with the intended postfiltration processing. Glass-fiber filters, for example, often have very high blanks for common ions such as chloride and sodium. 

Filter aids should have low bulk density to minimize settling and aid good distribution on a filter-medium surface that may not be horizontal. They should also be porous and capable of forming a porous cake to minimize flow resistance, and they must be chemically inert to the filtrate. These characteristics are all found in the two most popular... 

The transfer process is illustrated in Figure 2-12b. The bulk of the precipitate is moved from beaker to filter by directed streams of wash liquid. As in decantation and washing, a. stirring rod provides direction for the flow of material to the filtering medium. 

Definition Hard jet-black coal with high carbon content and calorific value Properties Bulk dens. 0.80-0.93 kg/l Uses Filler for NR, SR, adhesives domestic fuel filter medium in water treatment Trade Name Synonyms Carb-O-Fil [Shamokin Filler]... 

---