PLATE AND FRAME FILTER PRESS LISSA HANNAH B. ETOR Department of Chemical Engineering, College of Engineering and Architecture Cebu Institute of Technology - University N. Bacalso Ave. Cebu City,6000 Philippines
_________________________________________ Abstract Filter press operates under pressure using the process of cake filtration. This involves sending a slurry through the press, which is equipped with filter cloths. The cloths pick up the particles in the slurry and allow the solvent (water) to pass through. As more slurry moves through the press, the cake builds up and assists the filtration process. The rate at which the slurry moves through the press depends on the viscosity of the liquid, the thickness and resistance of the cake, and the pressure drop across each plate. A positive displacement pump permits the plate-and-frame filter to be fed with a suspension at either a uniform rate or, by adjusting a bypass valve, with a constant pressure. As filtration proceeds, layers of solids build up in the frame while the filtrate passes through the press. However, as more and more solid builds up in the frame, filtration becomes slower and slower. When a sufficient thickness of solids or cake has built up in the frames, filtration is stopped, the press is dismantled and the cake is removed from the frames. The resistance of the cake increases with time as the cake builds up in the frame (i.e. it becomes harder to force the filtrate through the cake, as the cake becomes thicker). At any time, this resistance to the flow of filtrate is proportional to the volume of filtrate that has passed through the cake.
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1. Introduction 1.1 Objectives To determine how concentration is related to the weight and volume of cake obtained. To determine how concentration affects the length of time a certain volume of filtrate is obtained. To plot the volume of filtrate versus time per volume of filtrate. 1.2 Theory Plate and frame filter press is the most fundamental design among all the other filter presses, and majority nowadays refer it as "membrane filter plate". This type of filter press consists of many plates and frames assembled alternately with the supports of a pair of rails. Presence of a centrifuge pump would ensure 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 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.
_________________________________________ 2. Materials and Methods 2.1 Materials and Equipment Plate & Frame Filter Press Equipment water (H2O) calcium carbonate (CaCO3) 2.2 Sketch of the Set-Up
2.3 Procedure About 18 liters of slurry was prepared containing 10% by weight of CaCO3 & 90% by weight water. The feed tank was filled with predetermined amount of slurry. The removable head was tightened to lock. The necessary valves (pressure and control valves) were opened. The agitator and the pump were switch on simultaneously. The switch of both the agitator and the pump were turn off when the slurry from the feed tank was already consumed. The procedure was repeated using 20% by weight CaCO3.
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3. Results
_____________________________________ 4. Discussion This experiment consists of a plate & frame filter to separate slurry of calcium carbonate from water at determined feed pressure of; 0.55 kgf/cm2 using 10% CaCO3 & 0.60 kgf/cm2 using 20% CaCO3. The performance of the filter is characterized by analyzing the filtrate volume versus time per volume of filtrate (see graph above) to determine the specific cake resistance and the degree of compressibility. Based on the graph obtained, the pressure affects the rate of filtration for which an increased pressure would maximize the rate of filtration & produce a final filter cake with a low water content (see red graph). The cloth resistance is generally negligible compared to the cake resistance since the latter increases as the cake gets thicker. Also, decreasing the number of plate/frame would decrease its efficiency as more slurry will produce as filtrate instead of pure water liquid.
_______________________________________ 5. Conclusions As the thickness of the cake increases, resistance to flow of filtrate increases ; rate of filtration gradually decreases. If the rate is maintained to be constant then pressure difference driving force (-∆P) will increase. Therefore, a batch filter is operated either at constant pressure or at constant rate. When the separating chamber is full, the filtration process is stopped at the optimum pressure difference had reached.
_______________________________________ References: 1. e.g. Microsoft Corporation (January 2011). What is a Filter Press?. [ONLINE] Available at: http://www.evoqua.com/en/products/sludge_biosolids_processing/filter_press/Pages/d ewatering_systems_generic_what_is_a_filter_press.aspx. [Last Accessed March 21, 2014]. 2. (1999). ANDRITZ Filter press, side-bar and overhead design. [ONLINE] Available at: http://www.andritz.com/products-and-services/pf-detail.htm?productid=5439. [Last Accessed March 19, 2014] 3. Barry A. Perlmutter, President & Managing Director (2000). A REVIEW OF FILTER PRESS BASICS AND ISSUES VERSUS ALTERNATIVE BATCH OR CONTINUOUS REPLACEMENT TECHNOLOGIES . [ONLINE] Available at: http://www.bhsfiltration.com/documents/A_Review_of_Filter_Press_Basics_Versus_Alternative_Re place%20Final.pdf. [Last Accessed January 13, 2015].
