Careful attention to even the smallest details will help to prevent operational problems in filtration processes
Filtration operations are widely used to achieve effective solid-liquid separations in the chemical process industries (CPI). Industrial filtration installations can involve a variety of components, including paper sheets, wire mesh, filter cloth and so on. If not properly specified and installed, even the smallest elements within a filtration system can lead to serious operational problems. This article outlines some of the typical problems encountered in industrial filtration processes, and how they can be rectified.
Filter paper problems
In filtration applications, loss of clarity in product streams occurs when the solids in the feed liquor bypass filter media and end up in the filtrate, contaminating the clean filtered liquor. The contamination resulting from the bypassed solids will cause problems, and if not corrected by a trap filter or recycling, it will allow unwanted solids into downstream equipment. To handle such contamination, the liquor must be filtered again either through a recycling step or by a second filter that ends up acting as the trap filter. This not only causes delays in the process, but also added expense if a trap filter must be installed. Often, loss of clarity occurs when the solids bypass filter-paper media through gaps resulting from improper placement of the filter paper. Filter paper sheets should be correctly placed on the filter plates to ensure that the filter paper is providing an effective seal in between the filter plates. Figure 1 illustrates the placement of the filter paper on the filter plate, and also some problematic situations that should be avoided.
Additional problems can arise when the filter paper becomes folded. Operators should pay close attention to ensure that the paper is placed flat on the filter plate. Furthermore, operational problems occur when the filter paper is crumpled, also causing contaminant bypass and loss of clarity.
To avoid issues, operators should also be aware that filter paper may swell up when the paper becomes wet with the liquid to be filtered. As it swells, the filter paper, if already compressed by the filter-plate compression rings, may start forming wrinkles. Once the filter cake starts forming within the filter paper’s wrinkles, it will cause cracks in the filter cake and loss of clarity. This problem may be avoided by prewetting the filter paper as it is placed on the filter plate, thus allowing the filter paper to swell up before it is compressed in place by the filter-plate compression rings. The prewetting of the filter paper may be done with either the liquid to be filtered or a compatible liquid, such as water.
Sometimes, if air gets trapped in the filter plate during the initial filling of the filter, it will start to push up on the filter paper, causing bubbles to form on the wet filter paper. Depending on the strength of the filter paper, the wet bubble may burst, causing a tear on the paper and consequently, loss of clarity. Conversely, the bubble in the wet filter paper may flatten as the pressure builds up during filtration. This phenomenon causes the filter cake formed on the bubbled paper to flatten and the filter cake to crack, allowing impurities to pass through the cracks, again leading to loss of clarity. This problem may be avoided by slowly filling the filter, allowing the air inside the filter plate to properly vent off through the air vent during filling.
Issues with tearing
Tears on the filter cloth bags can be another source of operational issues. Sometimes, operators may be a bit careless and drag the cloth-covered filter leaf on the floor, causing the cloth on the corners or edges to tear. Care must be taken when removing the filter leaves from shipping crates or from storage racks to avoid the filter leaf being dragged or dropped on the floor or other hard surfaces. These tears will cause loss of clarity. Small tears or holes may be patched up with silicone sealant, or another sealant that is compatible with the process liquid.
The wire mesh covering filter leaves can also be vulnerable to tearing (Figure 2). Again, due to carelessness, the wire mesh on the filter leaves may develop small pinholes or tears. Due to the abrasiveness of the filter aids, a small pinhole (Figure 3) may wear into a larger hole that also causes loss of clarity. Small pinholes may be patched with a compatible sealant. Larger tears may be patched by soldering the tear or by placing a small piece of wire mesh over the hole to secure it in place. Care must be taken not to use too much soldering because the excess solder material may build up under the wire mesh, which will cause bumps on the mesh. These bumps will result in additional problems, especially if the filter cake has to be scraped off of the wire mesh. This will cause even more tears. Filter leaves with tears on the wire mesh should be re-covered with new wire mesh or eventually replaced with new filter leaves.
Solids buildup and leaf damage
Once a filter is in operation, an excessive amount of filter cake can be produced during filtration and the filter leaves can become buried under such a large volume (Figure 4). In order to discharge the filter cake, the filter must be opened and the filter operator must remove the excess filter cake with a shovel to be able to reach the filter leaves. Figure 5 shows an opened filter with an overload of filter cake. When the filter leaves are buried by an excessive solids load, it prevents the self-cleaning feature of the filter to not function as designed. The excess volume of solids will prevent the internal spray system of the filter from properly washing the solids off of the filter internals. In the case of a filter with dry cake discharge, it will prevent the filter cake mechanism from working as designed, even possibly damaging the filter internals if the excessive solids volume has caused bridging. Bridging is a phenomenon where the filter cakes are touching and may not be properly discharged from the filter leaves. This problem forces the operators to open the filter to dig out the filter leaves from the solids, which may eventually cause damage to the filter leaves.
Other issues that can lead to filter leaves becoming overwhelmed by plugged solids include: improper precoat to protect the wire mesh; not correctly discharging the filter cake; and improper cleaning of the filter.
Plugging causes the filter leaves to have spots that are blinded by solids buildup inside the filter leaves. If the filter continues to be used without properly cleaning the filter leaves, the filter cake will build only in areas that are not plugged up, as shown in Figure 6.
Operating with filter leaves that are in poor condition leads to many filtration problems, including ineffective filter cake release when cleaning the filter, loss of clarity and the overall deterioration of the filtration process. Figure 7 shows filter leaves that are very dirty and have damaged wire mesh.
Design, testing and validation
When a filter does not perform as expected, it is an indication to go back and look into the laboratory testing that was done to determine the filter application. For example, preliminary filter testing was done using a particular limestone from Texas to produce calcium chloride by reacting the limestone with hydrochloric acid. Once the plant was built and the filter was sized based on the laboratory test results, the plant decided to buy the limestone from a different source that had different components and impurities. When used in the plant, this mismatched feedstock caused problems with the operation of the filter, including the presence of additional solids in the reaction step and shorter filter cycles. An additional prefiltration treatment was introduced in order to minimize the load on the filter.
Sometimes, filter installations are built without any provisions for what is actually required to operate the filter properly. Such is the case of an installation for two filters with dry cake discharge. Once the filters were started up, it was found that there was not enough compressed-air supply to dry the filter cake. The automatic sequence of the filters had to be reprogrammed to allow a step to build up the air pressure in the filter to provide sufficient air volume to blow through the filter cake to dry the cake so that it could be released from the filter leaves. The filter-cake drying step was reprogrammed to let the air pressure build up with the filter outlet valve partially closed for some air to blow through the filter cake at 10 psi(g). This would prevent the filter cake from falling off the filter leaves and also allow air to build up inside the filter tank to a sufficient volume up to 50 psi(g) to quickly blow through the filter cake down to 10 psi(g). This step was repeated up to three times prior to discharging the filter cake with an air vibrator.
When selecting the auxiliary equipment or other components around the filter, care must be taken that the correct devices and materials of construction are selected to avoid possible malfunction and equipment performance problems. Although not directly related to the operation of the filters, ancillary components can impact filtration performance. For instance, a caustic plant experienced some serious operating problems when the air operators on the caustic filters’ automatic process valves were supplied with aluminum mounts. The engineers that selected the automatic valves installed on the filters did not take into consideration that the caustic environment corroded the aluminum mounts, causing them to fail at the startup of the plant. The valves had to be operated manually until the valve operators were changed. When selecting equipment and mounting hardware in a caustic or corrosive environment, care and consideration must be given to select equipment and materials that will perform well in the corrosive environment. ■
Edited by Mary Page Bailey
Jose M. Sentmanat is the president of Liquid Filtration Specialist, LLC (P.O. Box 1064, Conroe, TX 77305-1064; Website; www.filterconsultant.com; Email: email@example.com; Phone: 936-756-5362). Sentmanat has over 50 years of experience in application engineering, sizing and selection of filtration equipment for a multitude of process-related industries in the food and beverage, pharmaceutical, consumer, chemical and petrochemical sectors. Sentmanat has authored several publications, references and operations manuals, and has given numerous seminars and short courses worldwide.
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