Equipment filtration

Filtration equipment is commercially available in a wide range. Proper selection must be based on detailed information of the slurry to be handled, cake properties, anticipated capacities and process operating conditions. One may then select the preferred operational mode (batch, semibatch or continuous), and choose a particular system on the above considerations and eomomic constraints. 

Either continuous or batch filters can be employed in cake filtration. In filter-medium filtration, however, ere particulates are retained within the fiamework of the filter medium, batch systems are the most commcm. Batch filters may be operated in any filtration regime, whereas continuous filters are most often operated under constant pressure. 

In an attempt to organize the almost overwhelming number of different types of filtration equipment, two classification schemes have evolved for continuous operations. The first scheme is based on operating pressure differentials and is provided in Table 1. The second scheme is based on the relative difference between gravity force and filtrate motion. Three orientations are possible forces acting in 

Hydrostatic pcessuce of the suspension layer to be separated Usually no more than 5 

Because the influence of gravity is so important to most filtration operafimis, the second classification basis is used in this chapter. The operating principles and important features of filtration equipment are described in diis chapter with the intent of providing the reader a background in the versatility and selection opticms available. 

The types and styles of filtration equipment are legion, but generally fall into one of the four categories viz, gravity, vacuum, centrifuge and pressure. 

In centrifuge filtration, the textile filter element is usually a tighter fabric and manufactured in the form of a suitably shaped bag, which fits inside 

Compared with the previously mentioned systems, pressure filters consume by far the greatest volume of fabric, being employed in a host of industries including mining applications, food processing, chemical process industries and effluent treatment. The most extensively used type of pressure filter is the filter press (of which there are several variations), though other pressure filters are also widely found with titles such as candle filter tower belt press , pressure leaf filter tube press , multi-roll press and pressure 

The most common types of filters used in industrial processing are pressure filters such filter presses, leaf filters, and cartridge filters, as well as vacuum filters like the rotary drum vacuum filter, and the rotary vacuum disc filter. Most filtration units produce a cake, which at long cycles may become reasonably dry at the expense of declining the flow rafes. Keeping consfanf 

FP Filter plate F Frame M Medium WP Wash plate 

Description and arrangements of filter press components (a) detail of plates and frames, (b) arrangement for the filtration cycle, and (c) arrangement for the washing cycle. 

Schematic diagram of a plate-and-frame filter press assembled and mounted. 

Pressure-vessel leaf filters (a) vertical-tank, vertical-leaf filter, and (b) horizontal-leaf filter. 

Filtration is a very much more complex process than sedimentation since the large degree of design control that one has also, of course, allows a far greater number of parameters to be varied. For a tme evaluation it will, therefore, be necessary to have access to a reasonably well equipped filtration laboratory. For the purpose of this chapter, however, we will review purely the shortlisting and basic evaluation and assume that only the most simple laboratory equipment is available. 

A prerequisite for any filtration tests is the Buchner funnel with filter flask and the filter leaf. In addition a good vacuum source is required. This should preferably be better than the simple water tap system. 

To establish what effect greater pressure differentials have one will have to rely on proper test equipment and, of course, the same would apply to compression filtration. 

We stress that, before starting any filtration tests, the following basic principles must be adhered to  

Use only the material which will be used under plant conditions. 

The most important factors in filter selection are the specific resistance of the filter cake, the quantity to be filtered, and the solids concentration. For free-filtering materials, a rotary vacuum filter is generally the most satisfactory since it has a very high capacity for its size and does not require any significant manual attention. If the cake has to be washed, the rotary drum is to be preferred to the rotary leaf. If a high degree of washing is required, however, it is usually desirable to repulp the filter cake and to filter a second time. 

A compressibility-permeability test cell has been developed by Ruth(7) and Grace(8) for testing the behaviour of slurries under various conditions of filtration. A useful guide to the selection of a filter type based on slurry characteristics is given in Volume 6. 

Bed filters provide an example of the application of the principles of deep bed filtration in which the particles penetrate into the interstices of the filter bed where they are trapped following impingement on the surfaces of the material of the bed. 

For the purification of water supplies and for waste water treatment where the solid content is about 10 g/m3 or less, as noted by Cleasby(23) granular bed filters have largely replaced the former very slow sand filters. The beds are formed from granular material of grain size 0.6-1.2 mm in beds 0.6-1.8 m deep. The very fine particles of solids are removed by mechanical action although the particles finally adhere as a result of surface electric forces or adsorption, as Ives 24 points out. This operation has been analysed by Iwasaki 25) who proposes the following equation  

If uc is the superficial flowrate of the slurry, then the rate of flow of solids through the filter at depth Z is ucC per unit area. Thus the rate of accumulation of solids in a distance dZ = —uc(9C/dl) dZ. If o is the volume of solids deposited per unit volume of filter at a depth Z, the rate of accumulation may also be expressed as (do/dt) dZ. 

The ability of the gas stream to permeate the filter is also affected by the shrinking and elongation of the fabric. Elongation of the fibers results in increased pore space, and conversely shrinkage decreases the pore volume. Fabrics of good dimensional qualities are essential to fabric life and efficiency. 

Fabric filters can be more costly to operate and maintain than electrostatic precipitators, cyclones, and scrubbers however, fabric filters are more practicable for filtration of specific dusts. For example fabric systems are the typical control method for toxic dusts from insecticide manufacturing processes, salt fumes from heat treating, metallic fumes from metallurgical processes, and other applications. Any other control method may not be as efficient, nor economically feasible for such applications. 

Clearly the largest application for filtration is air pollution control. On a worldwide basis the annual sales revenues for all types of pollution control equipment is around 150 billion dollars, with air pollution control devices being a substantial part of tliese equipment sales. The following is a summary of Web sites recommended to be consulted for specific vendor information on air filtration devices. A brief description of each site is provided. Many of these sites will link the reader to other sites containing additional information on product information. 

Recommended Web Sites on Product Information for fabric filters and related devices  

Particle Size Small Tiny Invisible Submicron 

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Benefits of Prethickening. The feed soHds concentration has a profound effect on the performance of any cake filtration equipment. It affects the capacity and the cake resistance, as weU as the penetration of the soHds into the cloth which influences filtrate clarity and medium resistance. Thicker feeds lead to improved performance of most filters through higher capacity and lower cake resistance. 

Filtration. In many mineral processing operations, filtration follows thickening and it is used primarily to produce a soHd product that is very low in moisture. Filtration equipment can be either continuous or batch type and either constant pressure (vacuum) or constant rate. In the constant pressure type, filtration rate decreases gradually as the cake builds up, whereas in the constant rate type the pressure is increased gradually to maintain a certain filtration rate as the cake resistance builds. The size of the device is specified by the required filter surface area. 

Water plays a primary role in corrosion of the metal walls of tanks and pipes (17), and increases the tendency for high speed pumps to produce wear particles and to exhibit shortened life. Formation of corrosion products can be controlled by addition of corrosion inhibitors, a mandatory additive in military fuels. However, corrosion inhibitors may also degrade other fuel properties and adversely affect ground filtration equipment. Thus they are not generally acceptable in commercial fuels where rigorous attention is given to clean and dry fuels upon aircraft fueling. 

Lime-Sulfuric. Recovery of citric acid by calcium salt precipitation is shown in Figure 3. Although the chemistry is straightforward, the engineering principles, separation techniques, and unit operations employed result in a complex commercial process. The fermentation broth, which has been separated from the insoluble biomass, is treated with a calcium hydroxide (lime) slurry to precipitate calcium citrate. After sufficient reaction time, the calcium citrate slurry is filtered and the filter cake washed free of soluble impurities. The clean calcium citrate cake is reslurried and acidified with sulfuric acid, converting the calcium citrate to soluble citric acid and insoluble calcium sulfate. Both the calcium citrate and calcium sulfate reactions are generally performed in agitated reaction vessels made of 316 stainless steel and filtered on commercially available filtration equipment. 

Step 4 Define the System Boundaries. This depends on the nature of the unit process and individual unit operations. For example, some processes involve only mass flowthrough. An example is filtration. This unit operation involves only the physical separation of materials (e.g., particulates from air). Hence, we view the filtration equipment as a simple box on the process flow sheet, with one flow input (contaminated air) and two flow outputs (clean air and captured dust). This is an example of a system where no chemical reaction is involved. In contrast, if a chemical reaction is involved, then we must take into consideration the kinetics of the reaction, the stoichiometry of the reaction, and the by-products produced. An example is the combustion of coal in a boiler. On a process flow sheet, coal, water, and energy are the inputs to the box (the furnace), and the outputs are steam, ash, NOj, SOj, and CO2. 

Croat and Associates Suppliers of filtration equipment and filter media including membranes, reverse osmosis and desalination systems. http //croalassociates.com... 

Filtration is a fundamental unit operation that, within the context of this volume, separates suspended particle matter from water. Although industrial applications of this operation vary significantly, all filtration equipment operate by passing the solution or suspension through a porous membrane or medium, upon which the solid particles are retained on the medium s surface or within the pores of the medium, while the fluid, referred to as the filtrate, passes through. 

When specifying filtration equipment for an intended application one must first account for the parameters goverrung the application and then select the filtration equipment best suited for the job. There are two important parameters that must be considered, namely the method to be used for forcing liquid through the medium, and the material that will constitute the filter medium. 

Final filtration equipment selection should be based on the ability to meet all objectives of the application within economic constraints. 

Filtration may also serve as the preparatory step for the operation following it. The latter stages may be drying or incineration of solids, concentration or direct use of the filtrate. Filtration equipment must be selected on the basis of their ability to deliver the best feed material to the next step. Dry, thin, porous, flaky cakes are best suited for drying where grinding operations are not employed. In such cases, the cake will not ball up, and quick drying can be achieved. A clear, concentrated filtrate often aids downstream treatment, whereby the filter can be operated to increase the efficiency of the downstream equipment without affecting its own efficiency. 

Cheremisinoff, N. P. and P. N. Cheremisinoff, Filtration Equipment for Wastewater Treatment, Prentice Hall Publishers, Englewood Cliffs, New Jersey, 1993. 

The third category, cake filters, although well developed in many wastewater treatment applications, are the least developed of the filtration equipment use by the Biotech Industry. In the organic synthesis laboratory sometimes very simple equipment like a funnel and filter paper is used to accomplish this operation. Some other operations used for this filtration step in the lab are more sophisticated, but many are very labor intensive and limit the capacity of the overall production process itself. As a result, there is a need for optimization of the cake filtration equipment used in biotechnology. Cake filtration equipment is available in batch and continuous modes. Following are several examples of cake filtration units ... 

Cake Filtration Equipment for Biotechnology. There are three general categories of filters used. .. http //members.aol.com/jmk7/feb97.htm... 

The principal factors to be considered when selecting filtration equipment are ... 

Three basic types of air filter are used viscous, dry and continuous. Viscous and dry units are similar in construction, but the filter medium of the viscous type is coated with a viscous material, such as a mineral oil, to retain the dust. The filters are made up from standard, preformed, sections, supported on a frame in a filter housing. The sections are removed periodically for cleaning or replacement. Various designs of continuous filtration equipment are also available, employing either viscous or dry filter elements, but in which the filter is cleaned continuously. A comprehensive description of air-filtration equipment is given by Strauss (1975). 

Wakeman, R. and Tarleton, S. (1998) Filtration Equipment Selection, Modelling and Process Simulation (Elsevier). 

All production processes, such as ampoule washing and sterilization, solution filtration, equipment set-up and operation, sorting, and freeze-drier cleaning and operation, should be covered in detail in a procedure manual to ensure that all operations are understood as well as carried out properly and uniformly. Cleaning, sterilization, sterile filtration, filling, and aseptic processing operations must be validated. 

Figure 1.4 shows a typical curve demonstrating the dependence of concentrations of copper, lead, and cadmium in the filtrate on the volume of seawater sampled. Metal levels become constant after 1-1.51 of sample have been filtered, and it can be concluded that at this point, contamination of the sample by the filtration equipment is negligible. 

Centrifuges, 22 288. See also Hydrocyclones basket, 11 389-390, 391 filtration equipment, 5 542-548 fixed-bed, 11 389-391 materials of construction and operational stress, 5 522-524 moving-bed, 11 391-392 noise, 5 524-525... 

The synthetic route may be exemplified by the preparation of a tetra-chloroisoindolinone pigment. A mixture of one mole of 1,4-diaminobenzene in o-dichlorobenzene with a solution of two moles of 3,3,4,5,6,7-hexachloroisoindoline-1-one in o-dichlorobenzene is heated to 160 to 170°C for 3 hours. Closed filtration equipment is used to filter the hot product, which is then washed with o-dichlorobenzene and alcohol, dried, and milled. The resulting product is a reddish yellow pigment with the structure 45 ... 

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