Filter aids capacity

In case 4, the increasing pressure compresses the cake to such as extent that it actually squeezes off the flow so that as the pressure increases the flow rate decreases. This situation can be compensated for by adding a filter aid to the slurry. This is a rigid dispersed solid that forms an incompressible cake (diatomaceous earth, sand, etc.). This provides rigidity to the cake and enhances its permeability, thus increasing the filter capacity (it may seem like a paradox that adding more solids to the slurry feed actually increases the filter performance, but it works ). 

In surface filtration the solids retained are those that do not pass through the smallest cross-section of the capillary flow channels of the filtering layer. Many particles are trapped by adsorption in the labyrinthine three-dimensional sieve of the filter aid. This means that substances can be retained that are smaller than the mesh size of the filter aid. In depth filtration, on the other hand, the solids are trapped in the interior of the layer this is due to the mechanical retention capacity (inertia and size of the particles, sedimentation, diffusion) and to the composition of the juice. 

A major alternative to direct flow membrane filtration is depth filtration, in which particles are removed throughout the filtration matrix rather than just at the membrane surface, by various mechanisms such as size exclusion, electrostatic, and hydrophobic interactions. Depth filters are typically composed of a bed of cellulose or polypropylene fibers together with an inorganic filter aid such as diatomaceous earth and a binder to form a filter sheet. The filter aid imparts the matrix very high surface areas and plays an important role in increasing both retention and the capacity. Depth filters can also have an electrostatic charge usually associated with the binder polymer. 

Should cake bridging occur between leaves in pressure filters, unequal pressures on the leaves can cause severe warping. The amount of filter aid that can be added without bridging can be calculated from the cake capacity of the filter (area times cake thickness. 

Tubular filters (Fig. 10) offer low cost construction and high hydraulic capacities. They are made with both rigid and flexible tubes. Celite is used on the rigid tube filters in the usual combination of precoat and body feed. With flexible tube filters, instead of body feed, an extra heavy precoat [25-30 lb (11.4-13.6 kg) filter aid per 100 ft (9.29 m ) filter area] is used. After filter pressure has reached a maximum, the precoat is bumped from the tubes, re-slurried, then re-deposited. This sequence is repeated until pressure is no longer reduced significantly, at which time the precoat is discarded and a new one applied. 

Bulk orders of filter aids are shipped in 40-ton to 50-ton pressure differential cars. The filter aid is generally conveyed from the car to a storage tank of from 75 to 100 tons capacity, using a conventional air conveying pressure system. From the storage tank it may be conveyed to any location within several hundred feet. 

Belt presses have the advantage of large capacity and automatic operation. The initial capital cost is also low. They were originally developed in the pulp and paper industry. Any slurry with fibers will do well in a belt press, and high-fiber material can be added to the slurry as a filter aid for belt press operation. Today, in addition to pulp dewatering, the belt press is widely used in wastewater sludge dewatering. 

The horizontal plate filter is a pressure filter with a number of horizontal circular drainage plates in a stack in a cylindrical shell. A filter cloth or paper is placed on each plate. Filter aid may be applied if necessary. Filtration is continued until the cake capacity of the unit is reached or the filtration rate becomes too slow owing to cake resistance. Pressure drop across the filter medium is generally designed for SO lb,/in.. Cake may be washed or air-blown prior to manual removal. This type of filter is flexible and easily cleaned or sterilized, but it has a high labor requirement. 

Considerations in design of the body feed system are quite similar to those above related to the precoat. The major difference is in the rate of utilization. Body feed is added continuously at low rates. Centrifugal pumps are not appropriate in most systems piston or diaphragm pumps are used instead. These should have variable capacity, which can be obtained by varying stroke length or speed. Filter aids also can be educted into the brine. 

Pressure leaf filters are generally employed where fine suspended impurities are to be removed. This operation is made easier by addition of suitable filter aids, e.g. Hyflo-Super-Cel, a diatomaceous inert powder which makes the filter cake porous, and filtration can be continued for longer time till the cake holding capacity is properly utilised. The following need to be considered for the procurement of this type of filter ... 

Feed Slurry Temperature Temperature can be both an aid and a limitation. As temperature of the feed slurry is increased, the viscosity of the hquid phase is decreased, causing an increase in filtration rate and a decrease in cake moisture content. The limit to the benefits of increased temperature occurs when the vapor pressure of the hquid phase starts to materially reduce the allowable vacuum. If the hquid phase is permitted to flash within the filter internals, various undesired resiilts may ensue disruption in cake formation adjacent to the medium, scale deposit on the filter internals, a sharp rise in pressure drop within the filter drainage passages due to increased vapor flow, or decreased vacuum pump capacity. In most cases, the vacuum system should be designed so that the liquid phase does not boil. 

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