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Principle, Construction, Working, Uses, Merits and Demerits of Plate and Frame Filter

A surface filtration mechanism is used in this filter. As the slurry flows through the filter medium, it enters the frame under pressure.

Plate and Frame Filter

Principle

A surface filtration mechanism is used in this filter. As the slurry flows through the filter medium, it enters the frame under pressure. Upon entering the outlet, the filtrate collects on the plates. It is possible to process large volumes of sludge simultaneously without or with washing by using multiple frames and plates to increase surface area.

Construction

As shown in the figure below, the plate and frame filter press is constructed. Plates and frames make up the filter press.


This filter consists of the following components.
  • It has an inlet (eye) for slurry and maintains the slurry reservoir.
  • Filter mediums are designed to provide solid retention.
  • A plate with a section is used to support the filter medium, receive the filtrate and to disperse it (outlet).
  • Placing the plates and frames of a filter press together.
Aluminum alloy is usually used for plates and frames. These are sometimes also coated with an anticorrosive chemical and steam sterilized if necessary. Within the frame there is an open space where a slurry reservoir and an inlet are maintained for filtration. The filter cloth and outlet are supported by a plate with a textured or grooved surface. Between plate and frame, the filter fabric is installed. It is possible to choose between frames of different thicknesses. The frame with the optimum thickness is selected. In filtration, cake thickness is a major determinant of selection. In the figure above, there is a plate, a filter medium, a frame, a filter medium, and a plate clamped into a structure. The large surface area is achieved by using many plates and frames. They are operated parallel to each other. By connecting the inlet of the slurry and the filtrate outlet of the frames and plates, channels can be created for the slurry to enter and the filtrate to exit. The inlet channel is only formed in some types, while the outlet valves on each plate are individually controlled.

Working

Filtration and washing of the cake (if appropriate) are the two steps of frame and plate process operation. Below is a diagram illustrating the plate and frame press in action? Slurry enters the frame from the feed channel as it passes through the filter medium and onto the plate. The solids remain in the frame as a filter cake. The cake is usually half the thickness of the frame because of the filtration that takes place on each side of the frame. Therefore, two filter cakes form in the frame, which eventually meet in the center. Solid content in a slurry and its resistance should be considered when determining filter cake thickness. Upon draining between the projections on the plate's surface, the filtrate is released from the outlet. The cake resistance increases over time and the filtering rate decreases. Stopping the process at a certain point instead of continuing at a low flow rate is a better idea. Filtration is completed by emptying and restarting the press.

Washing

A plate and frame press cannot be used to wash filter cakes, which is generally necessary. There is a halt in the flow of the picture due to the cake built up at the center of the frame. The same channels used for filteration of the filtrate should not be used for the water washing of cake. It would be best to modify the frame and plate press. The modified version includes a second channel for washing, see Figure below (d). The wash water channel is connected to the plate's surface in half.


Filtration involves the following steps:
  • Plates are filled with slurry and filtration is performed as usual until cake is filled in the frames.
  • Washing plates are fitted with closed outlets so that filter cake can be washed.
  • The washing channel is pumped with wash water. On the surface of the washing plates, water enters through the inlets.
  • On the surface of the washing plates, water enters through the inlets. Through the filter cloth, the water washes the cake, flowing down the surface of the plate to enter the plate.
A final outlet on that plate allows the washed water to escape.

Because washing plates have special contours, the wash-water can flow over all surfaces evenly, making a smooth resistance between cake and water, and thus, washing the whole cake thoroughly. It is only when the filters are filled with filter cake that the water wash is effective. Unless the cake fills the frame completely, washing will result in less effective washing if the cake breaks on the washing plate side of the frame. The frames must be completely filled with cake for the washing to be effective. In addition, this simplifies the process of emptying the cake frames and washing it properly.

Special Provision

At the outlet of each plate is a glass tube for observing contamination. This enables the quality of the filtrate to be checked. It is directed to an outlet channel through a control valve. It can be seen that each plate filters the filtrate. Broken cloths can be isolated and filtration can resume with one plate fewer if the faulty plate is isolated. Filter sheets made up of asbestos and cellulose, capable of retaining bacteria, are available for sterilized filtration with sterile filter presses and filter mediums. In most cases, the assembled unit is sterilized with steam. These examples include separating precipitated antitoxins from insulin liquors, removing precipitated proteins from insulin liquors, and separating the cell broth from the fermentation medium. For the filtration of viscous liquids, the press can be equipped with heating and cooling coils.

Uses

  • Among the many applications of filter presses, there is dewatering of slurries and purification. A filter press is widely used for dewatering ultrafines and reclaiming filtrates at the same time.
  • There are many industries that utilize solid-liquid separation, including alcohol, chemistry, metallurgy, pharmaceuticals, light industry, coal mining, food, textiles, environmental protection, and energy sources.

Merits

  • With multiple safety devices, this device is convenient to use, easy to maintain, and safe to use. Various materials may be used, including cast iron (to handle common substances), bronze (for smaller units), stainless steel (to avoid contamination), hard rubber or plastic (to avoid metal) and wood (for lightness).
  • In a relatively small footprint, it offers a large filtering surface.
  • It is a versatile device whose capacity can be adjusted based on the thickness and the number of frames used. By using 60 chambers, the surface can be increased significantly.
  • Strongly constructed presses use pressure differences of 2000 kilopascals to operate.
  • It is very efficient to wash cakes.
  • Due to its lack of moving parts, it requires little maintenance, and the filter cloths can be easily replaced.
  • By placing all the joints on the outside, the plate can be isolated from leaks and the filter can be prevented from being contaminated.
  • Dry slabs of cake are produced.

Demerits

  • There is a considerable amount of downtime with a batch filter, which is not productive.
  • It is an expensive filter press. It is a time-consuming and labor-intensive process that results in a high-cost method.
  • In order to remove cake effectively, the frames must be full otherwise the washing is inefficient and difficult.
  • Solids in slurries less than 5% are removed via the filter press. As a result of the high cost, this filter press is used for expensive materials such as removing precipitated proteins from insulin liquors and for collecting precipitated antitoxins.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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