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A filter press separates solids from various types of waste fluids and by-products created in a variety of industrial applications&#;typically in food, chemical, mining, edible oils, pigments and dyes and pharmaceutical industries.
The fluids processed through a filter press are often viscous sludge or slurries. They can be abrasive and chemically aggressive in nature.
Once processed, the trapped solids, referred to as a &#;cake," remain in the filter press. The cake can be reused in some processes or sold off for further processing.
In many applications, the major advantage of the filter press process is that it minimizes the disposal costs tied to wet slurry. Because disposal costs are commonly based on the resulting cake weight, filter press operators strive to eliminate as much moisture as possible.
Fluid is pumped through a filter press that contains a series of filters to trap particles. As fluid passes through the press, the flow gets more and more constricted. Meanwhile, solids collect on the surface of the filters.
Because the filter continues to trap solids over time, the fluid pressure must be gradually increased to maintain adequate filtration flow rates.
As a result, the pump processing the fluid must be stopped periodically so the filters can be cleaned.
Because filter press applications often involve corrosive or chemically aggressive fluids, safety and reliability are of the upmost importance.
The abrasive nature of certain fluids can lead to downtime; so, it is important to select the right materials to ensure all system components are suitable for the application.
Sometimes working with familiar pumps does not yield the best results. Single-stage centrifugal pumps are one of the most widely used pumps in the industry. They are often used in filter press applications but can be prone to leaks and can be energy intensive. Their efficiency is dependent on the flow speed, which is variable given how backpressure arises as the filter cake forms.
Progressive cavity pumps can handle highly viscous fluids such as sludge. But they need costly maintenance if they are run dry and can wear prematurely when processing abrasive applications. As a positive displacement pump, progressive cavity pumps need pressure relief valves to prevent excess pressure buildup, which can cause damage and safety issues if not handled properly. Or, they need to be installed with additional instrumentation and expensive control systems to prevent over-pressurized conditions.
Air operated double diaphragm (AODD) pumps may be able to avoid filter press application concerns. Some of the benefits of AODD pumps include:
Additionally, filter presses often use air to control the hydraulic systems required to open and close the press, to press more filtrate from filters (through specialized pneumatic filters bladders) and for the blowdown sequence (needed to flush remaining filtrate from the filter press).
Using an AODD pump can eliminate the need to run electricity to the press, which facilitates operation, minimizes installation costs and increases safety.
AODD pumps can be ordered with components that have a high degree of chemical compatibility. Materials such as stainless steel, polypropylene and conductive polyvinylidene fluoride (CPVDF) are intended to be used in the most aggressive chemical transfer applications.
For more information, please visit Filter Press Belts.
AODD pumps can be outfitted with diaphragms, balls and seats for abrasive applications, or with long-life polytetrafluoroethylene (PTFE) diaphragms.
A 2:1 diaphragm pump can be used in parallel with conventional AODDs to achieve higher pressures (up to 200 pounds per square inch [psi]). This pump pairing will extend filter press run times and further dewater filter cakes.
When selecting an AODD pump, look for reliable and efficient air motor technology. This can cut downtime and reduce energy costs.
Consider an AODD pump with an integrated cycle counting sensor that can be wired into a plant control alarm. This will alert personnel when the pump has stalled, allowing for minimal filter press downtime.
Diaphragm pumps also can be equipped with integrated sensors that alert maintenance personnel of the need to perform a diaphragm change should it rupture, and to stop the pump from running as needed.
Fluid power products can further maximize efficiency.
Filters and regulators (FRLs) are typically installed in the airline before connecting the diaphragm pump. These FRLs ensure that the diaphragm pump is operating at optimal pressure while protecting it from airline contaminants and moisture.
Point of use filtration is critical. Moisture and contaminants are absorbed in the air supply as it travels from the main compressor filter to where the equipment operates.
Author: Peter Schüten, Almatec Product Manager, PSG
In many industrial processes that feature the handling of liquids, the substances that are being transferred may occur in an aqueous or &#;muddy&#; state, with the liquid portion needing to be removed. This liquid-removal stage may take place during the production process &#; for example, in the filtration of edible oils or yeast solutions &#; but it more frequently occurs at the end of a process like wastewater treatment when the waste sludge that has formed must be withdrawn.
Since the disposal of such sludges is calculated based on weight and volume, their thickening and drying is particularly lucrative for the processor. During the thickening and drying process, the sludges are treated using chemicals and/or physical processes so that the waste sludge forms into flake-like solids. After adjusting the pH level to neutral or alkaline via the use of milk of lime, the subsequent dewatering process separates the water for disposal, with the remaining volume of sludge significantly reduced.
The simplest technology for this process involves collection and thickening of the sludge through the use of gravity. However, it is significantly more effective to use technical drying methods via processes such as centrifuges and evaporators, or more commonly, chamber filter presses (Fig. 1).
The operational principle of the chamber filter press relies on the use of a number of plastic frames that are pressed together under high pressure. Inside the frames are hollow chambers &#; from which the press gets its name &#; that are surrounded by filter cloth. When the sludge is fed into the chambers using pressure, a &#;filter cake&#; forms inside the chambers and the filtrate flows through the filter cloths into drainage channels.
When all the chambers are completely filled, the sludge feed stops. The press is then opened and the solid filter cakes can be removed. After closing, the press is ready for a new pressing process.
To fill these presses, filter material and pressure is required. The pressure &#; usually between 8 and 15 bar (116 to 218 psig) at its peak &#; should be even in order not to destroy the sludge flocs during feeding. The flocs should also have enough free space in the feed area. In addition to the constantly increasing counter-pressure that occurs until the end of the pressing, a further constraint is the eventuality that an empty-running sludge tank can lead to dry running of the pump being used to generate pressure.
To build pressure, different systems of displacement pumps are frequently used, including piston diaphragm pumps, which are applied most frequently for large presses. In these large and costly units, one or two diaphragms are hydraulically actuated and feed the sludge into the press through a series of valves. These pumps require large overhead costs even for small plants, such as an air-pressure vessel to equalize the feed rate and a maximum pressure monitor or bypass. Eccentric screw pumps are also used, either as self-regulating pumps (with motors that are electronically controlled via a frequency converter) or as cyclical systems (where an air-pressure vessel is &#;charged&#; by the pump). This valve-free procedure is advantageous when processing large sludge quantities and when long fibers prevent the use of valves. However, there are operational constraints that need to be considered in small- and medium-sized plants due to their sensitivity to abrasion and dry running. The space required to use this system is also considerable.
The list of filter-press pumps also includes hose-diaphragm piston pumps, which function in a similar way to piston diaphragm pumps but with crimped hoses rather than diaphragms. Piston pumps typically generate strong pulsation and require constant lubrication. Both of these pumps are characterized by their simple electrical operation, with fairly high installation and maintenance costs.
By comparison, it is much easier to use an air-operated double-diaphragm (AODD) pump; they are resistant to dry running, virtually maintenance-free, self-priming, self-regulating and highly compact. Without the need of an operator or electronic system, the counter-pressure of the chamber filter press regulates the feed rate automatically. The feed rate decreases continuously as counter-pressure increases simultaneously to the degree of filling. This effect can be used to detect when the chamber filter press is full. When this point is reached, the pump virtually stops &#; zero feed rate &#; or only occasionally makes a delivery stroke. In addition, the use of compressed air as drive power to move the diaphragms results in a highly efficient, regular and gentle cyclic drive that allows the medium to be fed smoothly.
A standard AODD pump is typically limited to the pressure of the supplied air, which is often insufficient to fill the press. For this reason, it is often necessary to increase the pressure, for which there are three very different technical solutions:
Using compressed air to power a pump is very effective. The pump operates with minimal dead space, i.e. the space inside the pump that must be filled with air without contributing to the actual pumping process. As a result, the pump always has sufficient power reserves to handle large volumes of wastewater.
With the introduction of a new generation of high-pressure AODD pumps, there is now an additional variant that combines the highly robust housing of the pressure-converting pump with an air section where no conversion takes place (Fig. 4). This version is therefore suitable for all applications under heavy load conditions, ranging from low-feed pressures to high-pressure applications of up to 15 bar (218 psig). If the user operates a pump at such high air pressure &#; whether it is from an external booster or directly from the compressor &#; he or she can do so secure in the knowledge that the pump is structurally designed for such pressure ranges and does not need to be held together by external reinforcements.
These new pumps are equipped with specially developed heavy-duty diaphragms with an integrated metal core for long service life and the ability to handle heavy loads. The vulcanized core of the diaphragm supports extremely thick layers of elastomer. To transfer the suction forces, the core is also reinforced with a special textile that is barely flexible in any direction.
In addition, these pumps can be combined with the optional use of a sensor that responds to the movements of the diaphragm and allows the cycle to be easily monitored. Accordingly, the slow stroke frequency that accompanies a full press rarely triggers a signal. If a Programmable Logic Controller (PLC) is used to program a time window within which a stroke should take place, the operator will know that the chamber press is full when no PLC-generated signals occur during the time window.
The compressed air can be switched off and a signal can be set for the operator to empty the press. This method functions purely by physical means and is independent of sensitive pressure gauges and contaminating sensors in the wastewater current.
In conclusion, when selecting pumps for filter press operations, AODD pumps are the clear solution, incorporating a number of operational advantages. Conventional positive displacement pumps with electric drive and control elements do not have these properties that are specific to the design of AODD pumps, including dry-run capability, good controllability and a gasket-less mechanical design, to mention but a few.
About the Author:
Peter Schüten is a Product Manager for Almatec Maschinenbau GmbH, Kamp-Lintfort, Germany. He can be reached at or +49 961-0. Almatec® is one of the world&#;s leading manufacturers of air operated double-diaphragm (AODD) pumps, owns numerous patents and has one of the largest product ranges in the field of pneumatic diaphragm pumps. Almatec is part of PSG®, a Dover company. Headquartered in Oakbrook Terrace, IL, USA, PSG is comprised of several of the world&#;s leading pump brands, including Abaque®, Almatec®, Blackmer®, Ebsray®, Finder, Griswold&#;, EnviroGear®, Mouvex®, Neptune&#;, Quattroflow&#;, RedScrew&#; and Wilden®. For more information on Almatec or PSG, please go to www.almatec.de or www.psgdover.com.
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