The mechanical device which is specially used in solid/liquid separation using the principle of pressure driven which is provided by a slurry pump is called as plate and frame filter press.
Plate and frame filter press is a fixed volume and batch operation which simply means that the operation must be stopped to discharge the filter cake before the next batch can be started. Its specific volume of solids can be holded by the filter press.
The first form of filter press was invented in British in 1853, used in obtaining seed oil through the use of pressure cells. However, there were many disadvantages associated with them, such as high labour requirement and discontinuous process.
The kicked off the major developments in filter press technology started in the middle of 20th century.
In 1959, K. Kurita and S. Suwa succeeded in developing the world's first automatic horizontal-type filter press to improve the cake removal efficiency, and moisture absorption.
Nine years later, Kurita Company began developing flexible diaphragms to decrease moisture in filter cakes. The device enables optimisation of the automatic filtration cycle, cake compression, cake discharge and filter-cloth washing leading to the increment in opportunities for various industrial applications.
Filter presses are sometimes called "Plate-and-Frame Filters." This refers to the style of filter element that was most prominent in the mid 1800's until the late 1960's.
While this specialized style of the filter press is still manufactured today, the current manufactured units are more accurately referred to as "Recessed Chamber Filter Press" and/or "Diaphragm (Membrane) Filter Press".
According to G.Prat "filter press is proven to be the most effective and reliable technique to meet today's requirement". One of the examples is Pilot scale plate filter press, which is specialized in dewatering coal slurries.
In 2013 the Society for Mining, Metallurgy and Exploration published an article highlighting this specific application. It was mentioned that the use of filter press is very beneficial to plant operations since it offers dewatering ultraclean coal as product, as well as improving quality of water removed to be available for equipment cleaning.
Many specialized applications are associated with different types of filter press that are currently used in various industries. Plate filter press is extensively used in sugaring operations such as the production of maple syrup in Canada, since it offers very high efficiency and reliability.
Old frame and filter press
- Plate and frame filter press is the most fundamental design among all the other filter presses and majority nowadays refer it as the membrane filter plate.
- Many plates and frames are present in the plate and filter press and presence of a centrifuge pump
- The separating chambers consist of one hollow filter frame separated from two filter plates by filter clothes.
- The introduced slurry flows through a port in each individual frame, and the filter cakes are accumulated in each the hollow frame.
- As the filter cake becoming thicker, the filter resistance increases as well. So when the separating chamber is full, the filtration process is stopped as the optimum pressure difference had reached.
- The filtrate that passes through filter cloth are collected through collection pipes and stored in the filter tank. Whereas filter cakes (suspended solids) accumulation occurs at the hollow plate frame, then being separated at the filter plates by pulling the plate and frame filter press apart. The cakes would just fall off from those plates and being discharge to the final collection point .
- Cake discharge can be done in many ways.
- For example: Shaking the plates while they are being opened or shaking the cloths. Scraper can also be used, by moving from one chamber to another and scrape the cake off the cloth. At the end of each run, the cloth are cleaned using wash liquid and ready to start the next cycle.
CONCEPT BEHIND PLATE AND FRAME FILTER PRESS TECHNOLOGY
- Generally the slurry needed to be dewatered is injected into the center of the press and fill up each chamber.
- The filling time should be as quick as possible in order to avoid cake formation in the first chamber before filling up the last chamber.
- While the chambers are being filled up the pressure inside the system will increase due to the formation of thick sludge.
- Then the liquid is filtered out through the filter clothes by adding streams of compressed air or water.
- The use of pressurized water require more time to pass into the chamber compared to pressurized air, however this method is much more cost efficient.
- The mechanism is surface filtration.
- The slurry enters the frame by pressure and flows through the filter medium.
- The filtrate is collected on the plates and sent to the outlets.
- A number of frames and plates are used so that the surface area increases and consequently large volumes of slurry can be processed simultaneously with or without washing.
- The construction of an plate and frame filter press is made of two types of units, plate and frames.
- These are usually made of aluminium alloy.
- Sometimes these are also lacquered for protection against corresive chemicals and made suitable for steam sterilisation.
- Frame contains an open space inside where in the slurry reservour is maintained for the filtration and an inlet to receive the slurry.
- It indicated by the two dots in the descripition. The plate has a studded or grooved surface to support the filter cloth and an outlet.
- It ondicated by the one dot in the descripition. The filter medium usually cloths is interposed between the plate and frame.
- Frames of different thicknesses are available.
- It is selected based on the thicknes of the cake formed during the filtration.
- Optimum thickness of the frame should be chosen. Plate, filter medium, frame and plate are arranged in the sequence and clamped to a supporting structure.
- It is normally described by dots as 188.8.131.52.1 so on.
- A number of plates and frames are employed so that the filtration area is as large as necessary. In other words a number of filtration units are operated in parallel.
- Channels for the slurry inlet and filtrate outlet can be arranged by fitting eyes to the plates and frames, these join together to form a channel.
- In some types only one inlet channels is formed, while each plate is having individual outlets controlled by valves.
Frame, filter medium, plate along with section and assembly of plate and frame filter press
The pressure behind the slurry typically 100 psi, but up to 900 psi [7 to 60 bar] is provided by a feed pump sometimes a positive displacement or centrifugal pump. With a gravity drain on the filtrate side of the press, a pressure differential between the feed pressure and the gravity discharge is created across the media and the filter cake solids as they build in thickness.
It is the existence of this pressure differential, not just the feed pump pressure, that causes the filtering action to occur. Solids within the slurry will flow to the area of cake development with the lowest pressure differential, resulting in a filter cake that builds uniformly over the drain-field on either side of the chamber walls.
This fill cycle continues until the filter cakes forming on the chamber walls bridge at the center, completely filling the press with solids. It is at this point that the filtration process is complete.
Once this is achieved, the hydraulic closure of the press is retracted, the individual filter elements are separated and the filter cakes are discharged, usually by gravity, to an appropriate receptacle.
The working operation of the filter frame and plate process can be described in two steps namely
- Filtration operation
- Washing oeration
- The working of the plate and frame press is explained as that the slurry enters the frame which are marked by two dots from the feed channels and passes through the filter medium on to the surface of the plates which are marked by one dot.
- The solids form a filter cake and remains in the frame. The thickness of the cake is half of the frame because on each side of the frame filtration occur.
- Thus the two filter cakes are formed which meet eventally in the centre of the frame. In general there will be an optimum thickness of the filter cake for any slurry depending on the solids content in the slurry and the resistance of the filter cake.
- The filtrate drains between the projections on the surface of the plates and the ecae from the outlets. As the filtration proceeds the resistance of the cake increases and the filtration rate decreases.
- At a certain point it is preferable to stop the process rather than continuing at very low flow rates. The press is emptied and the cycle is restarted.
- If it is necessary to wash the filter cake the ordinary plate and frame is unsatisfactoy. Two cakes are built up in the frame meeting eventually in the middle.
- This means that flow is brought virtually to a stand still. Hence water wash using the ame channels of the filtrate is very inefficient, if not impossible.
- A modification of the plate and frame press is used. For the purpose an addditional channel is included. These wash plates are identified by three dots.
- In half the wash plate there is a connection from the wash water channel to the surface of the plate.
- The sequence of the arrangement of plates and frame canbe represented by the dots as 184.108.40.206.220.127.116.11.18.104.22.168.1 so on. Such an arrangement is shown for the operation of the filtration and water washing respectively.
The procedures for the washing the press are as shownin the following steps
- Filtartion proceeds in the ordinary way until the frames are filled with cake.
- To wash the filter cake the outlets of the washing plates are closed.
- Wash water is pumped into the washing channel. The water enters through the inlets on to the surface of the washing plates.
- Water passes through the filter cloth and enters frame which contains the cake. Then the water washes the cake, passes through the filter cloth and enters the plate down the surface.
- Finally washed water escapes through the outlets of that plate.
- Thus with the help of th special washing plates it is possible for the wash water to flow over the entire surface of washing plate so that the flow resistance of the cake is equal to all points. Hence the entire cake is washed with equal efficiency.
- It should be noted that water wash is efficient only if the frames are full with the filter cake.
- If the solids do not fill the frame completely the wash water causes the cake to break that is on the washing plate side of the frame, then the washing will be less effective.
- Hence it is essential to allow the frames become completely filled with the cakes.
- This helps not only in emptying the frames but also helps in washing the cake correctly.
Plate and frame filter press, principle of operation
Principle of filtration operation using the three types of plate and frame
Principle of cake washing using three types of plate and frame
END PRODUCT COLLECTION
- Filter sheets composed of asbestos and cellulose are capable of retaining bacteria, so that the sterile filtrate can be obtained provided that the whole filter press and filter medium have been previously sterilized. Usually the steam is passed through the assembled unit for the sterilization.
- Example includes collection of precipitated antitoxins, removal pf the precipitated protiens from the insulin liquors and the removal of cell broth from the fermentation medium.
- Heating and cooling coils are incorporated in the press so as to make it suitable for the filtartion of viscous liquids.
- Filter presses are used in a huge variety of different applications, from dewatering of mineral mining slurries to blood plasma purification.
- At the same time, filter press technology is widely established for ultrafine coal dewatering as well as filtrate recovery in coal preparation plants.
- The detailed applications in food industry, mining industry, pharmaceutical industry, chemical industry, waste water treatment etc. are as follows:
- Pharmaceutical industry :
Enzymes, amino acids, antibiotics, pharmaceutical intermediates, bulk drugs, medicine, blood products, antibiotics (chlortetracycline, erythromycin, spiramycin, Jinggangmeisu, Midecamycin, tetracycline, berberine, oxytetracycline), calcium phytate, Chinese inositol, growth derived sand, organic phosphorus, glucoamylase.
- Chemical industry:
Dyes, pesticides, silicic acid, glycerin, white carbon, busy, sodium carbonate, additives, basic chemicals, chemical filler, pigment, white alumina, manganese, caustic soda, soda ash, alkali salts mud, saponin, graphite, bleaching powder, Synaptic powder phosphor, sodium hydrosulfite, potassium chlorate, potassium sulfate, ferrous sulfate, ferric hydroxide, water purification agent ( aluminum sulfate, chloride, basic aluminum chloride ) and so on,filter press is the ideal filtration equipment in these applications ;
- Food industry :
Wine filter press, yeast, fruit juice filter press, edible oil, vegetable oil, soy sauce, sugar mills, rice wine, white wine, fruit juices, soft drinks, beer, yeast, citric acid, vegetable protein, plant density sweetener, glucose, stevioside, maltose, starch, rice flour, corn syrup, gum, carrageenan, monosodium glutamate, spices, butter, oral liquid, soy, seaweed.
- Metallurgical industry :
Gold, platinum, uranium, cobalt, nickel, zinc, manganese, copper, iron sands filter press.
- Non-metallic minerals :
Tailings, coal, acid clay, bentonite, kaolin, ceramics, cement, clay, industrial sand.
- Environmental Engineering :
Chemical waste water, wastewater filter press, mining wastewater filter press, domestic wastewater, waste water, waste steel, waste removal, leather waste water, ports silt, mud, salt mud wastewater, carbide slag, gypsum, waste acid recovery, gold clarification of liquids mine tailings, biological wastewater, wastewater, sewage smelting, printing and dyeing wastewater, sewage brewing, pharmaceutical waste water, sewage and other environmental particulates, new equipment purification process.
- Refining :
White oil, sesame oil, light oil, glycerin, mechanical oil, vegetable oil.
- Clay industry :
Kaolin, bentonite, activated clay, clay, electronic ceramics clay.
- The automotive industry :
- Construction of the filter press is simple and variety of materials can be used.
- Cast iron for the handling common substance
- Bronze for smaller units
- Stain less steel is usd there by contamination can be avoided.
- Hard rubber or plastics where metal must be avoided.
- Wood for the lightnes though it must be kepy wet.
- It is provided a large filtering area in the relatively small floor space. It is versatile, the capacity being variable according to the thickness of the frames and the number used. Surface area can be increased by employing chambers up to 60.
- The sturdyconstruction permits the use of the considerable pressure difference. About 2000 kilo pascals can be normally used.
- Efficient washing of the cake is possible.
- Operation and the maintenance is straight forward because there are no moving parts, filter cloths are easily renewable. Since all joints are external, a plate can be disconnected if any leaks are visible. Thus the contamination of the filtrate can be avoided.
- It produces dry cake in the form of slab.
- It is a batch filter so there is a good deal of down time which is non productive,
- The filter pres is an expensive filter. The emptying time, the labor involved and the wear and tear of the cloth resulting in high cost.
- Operation is critical as the frames should be full other wise the washing is inefficient and the cake is difficult to remove.
- The filter press is used for the slurries containing less than 5% solids. So high costs make its imperative that the filter press is used for expensive materials. Examples include in the collection of precipated antitoxins and removal of the precipated protiens from the insulin liquors.
- The permit inspection of the quality of the frame.
- The broken cloths, the faulty plate can be isolated and filtration can be continued with the plate less.
- C.v.s subrahmanyam et al., pharmaceutical engineering principles and practices-filtration, 2001, page no:260-266