Filter press operation

Example 4 Cycle time for maximum amount of production from a plate-and-frame filter press. Tests with a plate-and-frame filter press, operated at constant pressure, have shown that the relation between the volume of filtrate delivered and the time in operation can be represented as follows ... 

A slurry is filtered, and the filter cake is washed by use of a plate-and-frame filter press operated at a constant pressure drop of 40 psi throughout the entire run. Experimental tests have been carried out on this equipment, and the results for the slurry mixture used can be expressed as follows for any one pressure drop ... 

The first two columns of Table 14.1 show laboratory data onfiltering calcium silicate with an average particle size of 6.5 gm in a 0.04287-m2 (0.460-ft2) plate-and-frame filter press operating at a pressure... 

The method described above is particularly advantageous for the preparation of dry press mixes however, it is also used in the preparation of plastic mass where the granules have to be remoistened. The main advantages are the continuous process and the savings provided by elimination of the laborious filter-press operation w hich is also demanding in terms of required floor space. The recycling of waste heat for drying is also favourable in this case. 

At the end of the filtration cycle, the dewatered filter cake must be removed from the fabric in preparation for the next cycle. It is important that the cake is effectively discharged at this point since any delays will lead to extended filtration cycle times and therefore reduced process efficiency. This is particularly apt in filter press operations, where manual intervention may be necessary to remove sticky cakes. As a consequence, in addition to longer cycle times, the cost of the operator must also be considered. To some extent this topic may be linked to the cake moisture content because, broadly speaking, wetter cakes will adhere more tenaciously to the cloth. This problem has been partly addressed by the equipment manufacturers with the incorporation of high pressure wash jets and brush cleaning devices, and the filter media producers also continue to pursue the development of fabrics that will facilitate the ultimate goal of perfect, unassisted cake release and hence the achievement of a fully automated operation. 

In similar manner, excessive stretch caused by the mass of filter cakes in filter press operations could result in the port holes in the fabric moving out of alignment with the corresponding holes in the filter plate, thereby restricting the flow of filtrate out of the press. Likewise, in other filtration systems, such as pressure leaf filters, the same stretch could result in the formation of creases and ultimately mechanical damage to the fabric. 

The surfaces against which the fabric will be expected to operate in filter press operations will also be influential in the choice of fabric construction. Whilst the introduction of advanced plastics has considerably reduced the damage that was previously inflicted on fabrics by rough, cast iron surfaces, there remain a large number of applications where the use of cast iron is stiU necessary. 

Pressure filtration differs fi-om vacuum filtration in that the liquid is forced through the filter medium by a positive pressure instead of a vacuum. Among the most widely used in Che chemical process industry (and widely used in Europe for wastewater treatment) is Che filter press. As shown in Figure 3, Che filter press operates by pumping the sludge between plates that are covered with a filter cloth. The liquid seeps throu Che filter cloth leaving the solids bdiind between the plates. Whoi the spaces between the plates are filled, the treatment plant operator separates the plates and removes the solids. 

Whatever method is employed it is necessary to first condition the sludge before dewatering, which, in most cases, is exclusively carried out by addition of a cationic polymer (usually a polyacrylamide) or sometimes a combination of two polymers applied in sequence, or an inorganic pre-conditioner followed by polymer. Some filter press operations still use inorganic conditioning systems, such as ferric chloride and lime, without polymer however, these are now few and far between. 

The typical operating pressure of filter presses is 600 or 700 kPa, although some manufacturers offer presses for 2000 kPa or higher. As the pressure increases during filtration, it forces the plates apart this maybe offset by a pressure compensation faciHty offered with some large mechanized presses. 

The vertical recessed plate automatic press is shown schematically in Figure 15. Unlike the conventional filter press with plates hanging down and linked in a horizontal direction, this filter press has the plates in a horizontal plane placed one upon another. This design offers semicontinuous operation, saving in floor space, and easy cleaning of the cloth, but it allows only the lower face of each chamber to be used for filtration. 

A pressure filter is a plate-and-frame press which operates on an intermittent time cycle. Drier cakes are generally attainable from a filter press. Sludge drying beds are usually used for smaller sludge volumes, which drain and dry rapidly. Thein appHcation is usually restricted to the more arid climates. 

Comparisons are available on the relative performance and costs for dewatering municipal sludges (2). The relative performance of different filters and conditioners on waste sludges is shown in Table 3. The same sludge was treated on two belt-filter presses, two different centrifuges, and rotary vacuum filter (75). In another study, a variable chamber filter press, fixed-volume filter press, continuous belt-filter press, and rotary vacuum filter were compared for performance, capacity, and capital and operating costs (69). 

Other apphcations include dewatering extremely fine (0.1 llm) laterite leach tailings (91). These pelletizing processes should be compared in flocculant consumption and operating and capital costs with belt-filter presses. 

Filter presses are made in plate sizes from 10 by 10 cm (4 by 4 in) to 1.5 by 1.8 m (61 by 71 in). Frame thickness ranges from 0.3 to 20 cm (0.125 to 8 in). Operating pressures up to 689 kPa (100 psig) are common, with some presses designed for 6.9 MPa (1000 psig). Some metal units have cored plates for steam or refrigerant. Maximum pressure for wood or plastic frames is 410 to 480 kPa (60 to 70 psig). 

The filter press has the advantage of simplicity, low capital cost, flexibility, and ability to operate at high pressure in either a cake-filter or a clarifying-filter application. Floor-space and headroom needs per unit of filter area are small, and capacity can be adjusted by adding or removing plates and frames. Filter presses are cleaned easily, and the filter medium is easily replaced. With proper operation a denser, drier cake compared with that of most other filters is obtained. 

Pressure leaf filters are used to separate much the same lands of slurries as are filter presses and are used much more extensively than filter presses for filter-aid filtrations. They should be seriously considered whenever uniformity of production permits long-time operation under essentially constant filtration conditions, when thorough washing with a minimum of hquor is desired, or when vapors or fumes make closed construction desirable. Under such conditions, if the filter medium does not require frequent changing, they may show a considerable advantage in cycle and labor economy over a filter press, which has a lower initial cost, and advantages of economy and flexibility over continuous vacuum filters, which have a higher first cost. 

Plate pr esses. Sometimes called sheet filters, these are assemblies of plates, sheets of filter media, and sometimes screens or frames. Thev are essentially modified filter presses with practically no cakeholding capacity. A press may consist of many plates or of a single filter sheet between two plates, the plates may be rectangular or circular, and the sheets may lie in a horizontal or vertical plane. The operation is similar to that of a filter press, and the flow rates are about the same as for disk filters. The operating pressure usually does not exceed 138 kPa (20 psig). The presses are used most frequently for low-viscosity liqmds, but an ordinaiy filter press with thin frames is commonly used as a clarifier for 100-Pa s (1000-P) rayon-spinning solution. Here the filtration pressure may be 6900 kPa (1000 psig). 

Select a cloth filter for a filter press application in which the water is both alkaline and has a high content of solvents. The operating temperature could reach as high as 200° F on excursions. 

A filter press with membrane plates has the following operational sequence ... 

A filter press with a hydraulic drive has 35 frames with internal dimensions of 1900 X 1200 mm. The unit operates under the following set of conditions ... 

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