Blocking filter media

Constant-pressure drop filtration can result in saturation or blockage of the filter medium. The network of pores within the filter medium can become blocked because of one or a combination of the following situations ... 

The transition from pore-blocked filtration to more favorable cake filtration can therefore be achieved with a suspension of low settling particles by initially feeding it to the filter medium at a low rate for a time period sufficient to allow surface accumulation. This is essentially the practice that is performed with filter aids. 

The blocking of the pores of the filter medium by particles is a complex phenomenon, partly because of the complicated nature of the surface structure of the usual types of filter media, and partly because the lines of movement of the particles are not well defined. At the start of filtration, the manner in which the cake forms will lie between two extremes — the penetration of the pores by particles and the shielding of the entry to the pores by the particles forming bridges. Heertjes(11) considered a number of idealised cases in which suspensions of specified pore size distributions were filtered on a cloth with a regular pore distribution. First, it was assumed that an individual particle was capable on its own of blocking a single pore, then, as filtration proceeded, successive pores would be blocked, so that the apparent value of the specific resistance of the filter cake would depend on the amount of solids deposited. 

These are housings of metal or plastic containing one or more replaceable and renewable cartridges which contain the active filter element, usually based on a polymeric filter medium or in some cases, sintered stainless steel. They are useful as polishing filters where the level of solids to be removed is relatively low, to prevent the filter from blocking up. 

Clarifying filters remove small amounts of solids or liquid droplets from either liquids or gases. The particles are trapped inside the filter medium or on its surfaces. Clarification differs from screening in that pores in the filter medium are larger—sometimes much larger—than the particles to be removed. The particles are caught by surface forces and immobilized on the surfaces or within the flow channels, where they reduce the effective diameter of the channels but usually do not block them completely. 

In filtration the solid particles are removed from the slurry by forcing the fluid through a filter medium, which blocks the passage of the solid particles and allows the filtrate to pass through. In settling and sedimentation the particles are separated from the fluid by gravitational forces acting on the particles. 

Deep-bed filtration (or depth filtration) is known by various terms like blocking filtration, surface filtration, and clarification. Deep-bed filtration (Rajagopalan and Tien, 1979 Stamatakis and Tien, 1993 Tien and Payatakes, 1979) is normally preferred in treating large quantities of liquids containing low solid concentration (less than 500mg/liter) with particles size less than 30 pm. In this operation, the particles to be removed are often substantially smaller than the pores of the filter medium and will penetrate a considerable depth... 

It has been stated that a filter medium is a porous (or at the very least semi-permeable) barrier placed across the flow of a suspension to hold back some or all of the suspended material. If this barrier were to be very thin compared with the diameter of the smallest particle to be filtered (and perforated with even sized holes), then all the filtration would take place on the upstream surface of the medium. Any particle smaller than the pore diameter would be swept through the pores, and any particle larger than that (assuming the particles to be rigid) would remain on the upstream surface. Some of the larger particles, however, would be of a size to settle into the individual pores and block them. The medium surface would gradually fill with pores blocked in this way, until the fluid flow reduced to below an acceptable level. At this point filtration would be stopped and the medium surface would be brushed or scraped clean (although many automatic filters have their surface continuously brushed or scraped). 

Most real media are, of course, not infinitely thin, but have a finite thickness in the direction of fluid flow, while most pores through such material vary in diameter along the fluid path. A second mechanism, termed depth straining, then applies when a particle moves through a pore until it meets a point where the pore is too small, and the particle is held entirely because of its size. The pore then is blocked, and remains so until the filter medium becomes too clogged in this way for it to have any further use. At this point it must be discarded, or, preferably, blown free of the trapped solids, by a reverse flow of fluid. 

The applications and efficiencies of dust collecting filters are given in more detail in Section 6, where it will be seen that filter panels can achieve very high filtration efficiencies. Such high efficiency filters are normally installed in series with a prefilter, using coarser filter medium, to remove larger particles from the inlet air, which would block the fine filter too quickly. 

Figure 7.2 Optical block diagram of the wide field Kerr-gated microscope. Note the position of the sample (S), the sequence of three matched Cassegrain objectives (COi, CO2 and CO3), polarizers (Pi and P2), the Kerr medium (K) and blocking filters (F). A prism spectrometer (PR) can be inserted into the path of the gated light allowing monitoring of the collective spectral dynamics of objects within the field of view.

Filters for mists and droplets have more open area than those used for dry parhcles. If a filter is made of many fine, closely spaced fibers, it will become wet due to the collected liquid. Such wethng will lead to mathng of the fibers, retenhon of more liquid, and eventual blocking of the fiter. Therefore, instead of fine, closely spaced fibers, the usual wet filtrahon system is composed of either knitted wire or wire mesh packed into a pad. A looser filtrahon medium results in a filter with a lower pressure drop than that of the filters used for dry parhculates. The reported pressure drop across wire mesh mist eliminators is 1-2 cm of water at face velocihes of 5 m sec T The essenhal collechon mechanisms employed for filtrahon of droplets and mists are inertial impachon and, to a lesser extent, direct intercephon. 

In the manufacture of baker s yeast, the stock strain is inoculated into a medium that containing molasses and com steep liquor. The pH of the medium is adjusted to be slightly acidic at pH 4-5. The acidic pH may retard the bacterial growth. The inoculated medium is aerated during the incubation period. At the end, the cells are harvested by centrifuging out the fermentation broth, and they are recovered by filter press. A small amount of vegetable oil is added to act as plasticiser, and then the cell mass is moulded into blocks. The process is shown in Figure 1.2. 

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