Filter press

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Filter press

A filter press is a tool used in separation processes, specifically in solid/liquid separation using the principle of pressure drive, provided by a slurry pump. The filter press is used in fixed-volume and batch operations, which means that the operation must be stopped to discharge the filter cake before the next batch can be started. The major components of a filter press are the skeleton and the filter pack. The skeleton holds the filter pack together while pressure is being developed inside the filtration chamber. The chamber however can only hold a specific volume of solids.

Concept behind filter press technology

Generally, the slurry needed to be dewatered is injected into the center of the press and each chamber filled. The filling time should be as quick as possible in order to avoid cake formation in the first chamber before the last chamber is filled. 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 cloths 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.

History

The first form of filter press was invented in the United Kingdom 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. 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. A detailed historical review, dating back to when the Shang Dynasty used presses to extract tea from camellia the leaves and oil from the hips in 1600 BC, was compiled by K. McGrew.

Types of filter presses

There are three main basic types of filter presses: plate and frame filter presses, recessed plate and frame filter presses and automatic filter presses.

Plate and frame filter press

A plate and frame filter press is the most fundamental design, and many now refer it as a "membrane filter plate". This type of filter press consists of many plates and frames assembled alternately with the supports of a pair of rails. The presence of a centrifuge pump ensures the remaining suspended solids do not settle in the system, and its main function is to deliver the suspension into each of the separating chambers in the plate and frame filter. For each of the individual separating chambers, there is one hollow filter frame separated from two filter plates by filter cloths. The introduced slurry flows through a port in each individual frame, and the filter cakes are accumulated in each hollow frame. As the filter cake becomes thicker, the filter resistance increases as well. So when the separating chamber is full, the filtration process is stopped as the optimum pressure difference is reached. The filtrate that passes through filter cloth is collected through collection pipes and stored in the filter tank. Filter cake (suspended solid) 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 then fall off from those plates and are discharged 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. A scraper can also be used, by moving from one chamber to another and scraping the cake off the cloth. At the end of each run, the cloths are cleaned using wash liquid and are ready to start the next cycle.

Automatic filter press

An automatic filter press has the same concept as the manual filter and frame filter, except that the whole process is fully automated. It consists of larger plate and frame filter presses with mechanical "plate shifters". The function of the plate shifter is to move the plates and allow rapid discharge of the filter cakes accumulated in between the plates. It also contains a diaphragm compressor in the filter plates which aids in optimizing the operating condition by further drying the filter cakes. An animation showing this principal of operation can be seen here.

Recessed plate filter press

A recessed plate filter press is made up of polypropylene squares at about 2 to 4 feet across with a concave depression and a hole in the center of each. Two plates join together to form a chamber to pressurize the slurry and squeeze the filtrate out through the filter cloth lining in the chamber. It is capable of holding 12 to 80 plates adjacent to each other, depending on the required capacity. When the filter press is closed, a series of chambers is formed. The differences with the plate and frame filter are that the plates are joined together in such a way that the cake forms in the recess on each plate, meaning that the cake thickness is restricted to 32mm unless extra frames are used as spacers. However, there are disadvantages to this method, such as longer cloth changing time, inability to accommodate filter papers, and the possibility of forming uneven cake.

Applications

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. According to G.Prat, the "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 the 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.

Other industrial uses for automatic membrane filter presses include municipal waste sludge dewatering, ready mix concrete water recovery, metal concentrate recovery, and large-scale fly ash pond dewatering.

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. According to M.Isselhardt, "appearance can affect the value of maple syrup and customer's perception of quality". This makes the raw syrup filtration process extremely crucial in achieving desired product with high quality and appealing form, which again suggested how highly appreciated filter press methods are in industry.

Main process characteristics

Table 1 Classification of filter press.

Specifications	Parameters	Units
Scale	1-100	m3/h
Operation	Batch, Continuous
Objectives	Clarification, Solids Recovery
Slurry settling characteristics
Rate	<0.1 to >5	cm/s
Clarification	Poor
Proportion of sludge	< 2% vol to > 20% vol
Slurry filtering characteristics	Slow at the rate of cm/h, up to 10 h

Feed

Typical range of particle size and feed concentration are 1-100 μm and 1-30% by weight. The slurry feed concentration generally has large amount of ultrafine particles. The percentage of solids concentration in the slurry feed is normally more than 10% by weight.

Operation

Limitation in pressure resistance of the filter cake is up to a maximum of 800 kPa. Flow rate of filtrate is controlled by feed pump. In filter press methodology, positive pressure filtration is used instead of vacuum filtration with high-energy consumption.

Efficiency

Plate and frame filter press produce up to 99% of solids recovery, and the moisture left in the cake commonly ranges from 15% to 20%. During cake washing, 90% of the filtrate can be removed by the wash liquid.

Assessment of important characteristics

Here are some typical filter press calculation used for handling operation applied in waste water treatment:

Solids loading rate

S=​^{(B x 8.34 lb/gal x s)}⁄_A
Where,

S is the solid loadinjgs rate in ​lb h⁄ft2.<r />
B is biosolids in ​gal⁄h
s is the % solids/ 100.
A is the plate area in ft2.

Net filter yield

$NFY=\frac{S\times P}{TCT}$

Where:

• NFY is the net filter yield in kg/h/m².
• S is the solids loadings rate in kg/h/m².
• P is the period in h.
• TCT is the total cycle time in h.

(S × P) gives the filter run time.

Flow rate of filtrate

$u=\frac{1}{A}\frac{dV}{dt}=\frac{\Delta P}{\mu\times(R_c+R_f)}$

Where:

• u is flow rate of filtrate through cloth and cake (m/s),
• dV/dt is volumetric filtration rate (m³/s),
• R_c is the resistance of the filter cake (m-1),
• R_f is the initial resistance of the filter (resistance of an initial layer of cake, filter cloths, plate and channel) (m-1),
• μ is the viscosity of the filtrate (N·s/m²),
• ΔP is the applied pressure difference (N/m²) one side to another side of the filter medium,
• A is the filtration area (m²).

Those are the most important factors that affect the rate of filtration. When filtrate pass through the filter plate, deposition of solids are formed and increases the cake thickness, which also increase Rc while Rf is assumed to be constant. The flow resistance from cake and filter medium can be studied by calculating the flow rate of filtration through them.

If the flow rate is constant, the relationship between pressure and time can be obtained. The filtration must be operated by increasing pressure difference to cope with the increase in flow resistance resulting from pore clogging. The filtration rate is mainly affected by viscosity of the filtrate as well as resistance of the filter plate and cake.

Optimum time cycle

High filtration rate can be obtained from producing thin cake. However, a conventional filter press is a batch system and the process must be stopped to discharge the filter cake and reassemble the press, which is time consuming. Practically, maximum filtration rate is obtained when the filtration time is greater than the time taken to discharge the cake and reassemble the press to allow for cloth's resistance. Properties of the filter cake affect the filtration rate, and it is desirable for the particle's size to be as large as possible to prevent pore blockage by using a coagulant. From experimental work, flow rate of liquid through the filter medium is proportional to the pressure difference. As the cake layer forms, pressure applies to the system increases and the flow rate of filtrate decreases. If the solid is desired, the purity of the solid can be increased by cake washing and air drying. Sample of filter cake can be taken from different locations and weighed to determine the moisture content by using overall material balance.

Possible heuristics to be used during design of the process

The selecting of filter press type depends on the value of liquid phase or the solid phase. If extracting liquid phase is desired, then filter press is among the most appropriate methods to be used.

Materials

Nowadays, filter plates are made from polymers or steel coated with polymer. They give good drainage surface for filter cloths. The plate sizes are ranged from 10 by 10 cm to 2.4 by 2.4 m and 0.3 to 20 cm for the frame thickness.

Filter medium

Typical cloth areas can range from 1 m² or less on laboratory scale to 1000 m² in a production environment, even though plates can provide filter areas up to 2000 m². Normally, plate and frame filter press can form up to 50 mm of cake thickness, however, it can be push up to 200 mm for extreme cases. Recessed plate press can form up to 32 mm of cake thickness.

In the early days of press use in the municipal waste biosolids treatment industry, issues with cake sticking to the cloth was problematic and many treatment plants adopted less effective centrifuge or belt filter press technologies. Since then, there have been great enhancements in fabric quality and manufacturing technology that have made this issue obsolete. Unlike the USA, automatic membrane filter technology is the most common method to dewater municipal waste biosolids in Asia. Moisture is typically 10-15% lower and less polymer is required—which saves on trucking and overall disposal cost.

Operating condition

The operating pressure is commonly up to 7 bars for metal. The improvement of the technology makes it possible to remove large amount of moisture at 16 bar of pressure and operate at 30 bars. However, the pressure is 4-5 bars for wood or plastic frames. If the concentration of solids in the feed tank increase until the solid particles are attached to each other. It is possible to install moving blades in the filter press to reduce resistance to flow of liquid through the slurry. For the process prior to cake discharge, air blowing is used for cakes that have permeability of 10⁻¹¹ to 10⁻¹⁵ m².

Pre-treatment

Pre-treatment of the slurries before filtration is required if the solid suspension has settled down. Coagulation as pre-treatment can improve the performance of filter press because it increases the porosity of the filter cake leading to faster filtration. Varying the temperature, concentration and pH can control the size of the flocs. Moreover, if the filter cake is impermeable and difficult for the flow of filtrate, filter aid chemical can be added to the pre-treatment process to increase the porosity of the cake, reduce the cake resistance and obtain thicker cake. However, filter aids need to be able to remove from the filter cake either by physical or chemical treatment. A common filter aid is Kieselguhr, which give 0.85 voidage.

In terms of cake handling, batch filter press requires large discharge tray size in order to contain large amount of cake and the system is more expensive compared to continuous filter press with the same output.

Washing

There are two possible methods of washing that are being employed, the "simple washing" and the "thorough washing". For simple washing, the wash liquor flows through the same channel as the slurry with high velocity, causing erosion of the cakes near the point of entry. Thus the channels formed are constantly enlarged and therefore uneven cleaning is normally obtained. A better technique is by thorough washing in which the wash liquor is introduced through a different channel behind the filter cloth called washing plates. It flows through the whole thickness of the cakes in opposite direction first and then with the same direction as the filtrate. The wash liquor is normally discharged through the same channel as the filtrate. After washing, the cakes can be easily removed by supplying compressed air to remove the excess liquid.

Waste

Nowadays filter presses are widely used in many industries, they would also produce different types of wastes. Harmful wastes such as toxic chemical from dye industries, as well as pathogen from waste stream might accumulate in the waste cakes; hence the requirement for treating those wastes would be different. Therefore, before discharge waste stream into the environment, application of post-treatment would be an important disinfection stage. It is to prevent health risks to the local population and the workers that are dealing with the waste (filter cakes) as well as preventing negative impacts to our ecosystem. Since filter press would produce large amount of waste, if it was to be disposed by land reclamation, it is recommended to dispose to the areas that are drastically altered like mining areas where development and fixation of vegetation are not possible. Another method is by incineration, which would destroy the organic pollutants and decrease the mass of the waste. It is usually done in a closed device by using a controlled flame.

Advantages and disadvantages compared to other competitive methods

Many debates have been discussed about whether or not filter presses are sufficient to compete with modern equipment currently as well as in the future, since filter presses were among one of the oldest machine-driven dewatering devices. Efficiency improvements are possible in many applications where modern filter presses have the best characteristics for the job, however, despite the fact that many mechanical improvements have been made, filter presses still remain to operate on the same concept as when first invented. A lack of progress in efficiency improvement as well as a lack of research on conquering associated issues surrounding filter presses have suggested a possibility of performance inadequacy. At the same time, many other types of filter could do the same or better job as press filters. In certain cases, it is crucial to compare characteristics and performances.

Batch filter press versus a continuous vacuum belt filter

Filter presses offer a wide range of application, one of its main propositions is the ability to provide a large filter area in a relatively small footmark. Surface area available is one of the most important dimensions in any filtering process, since it maximises filter flow rate and capacity. A standard size filter press offers a filter area of 216 m², whereas a standard belt filter only offers approximately 15 m².

High-solids slurries: continuous pressure operation

Filter presses are commonly used to dewater high-solids slurries in metal processing plants, one of the press filter technology that could deliver the job is the Rotary Pressure Filter method, which provides continuous production in a single unit, where filtration is directed via pressure. However, in cases where solids concentration in high-solids slurries is too high (50%+), it is better to handle these slurries using vacuum filtration, such as a continuous Indexing Vacuum Belt Filter, since high concentration of solids in slurries will increase pressure and if pressure is too high, the equipment might be damaged and/or less efficient operation.

Current development

In the future, market demands for modern filtration industry are going to become finer and higher degree in separation, and particularly on the purpose of material recycling, energy saving, and green technology. In order to meet increasing demands for higher degree of dewatering from difficult-to-filter material, super-high pressure filters are required. Therefore, the trend in increasing the pressure for the automatic filter press will keep on developing in the future.

The conventional filter press mechanisms usually use mechanical compression and air to de-liquoring; however, the efficiency of producing low-moisture cake is limited. An alternative method has been introduced by using steam instead of air for cake dewatering. Steam dewatering technique can be a competitive method since it offers product of low-moisture cake.