Process Technology: An Introduction - Haan A.B. 2015

10 Solid-liquid separation
10.6 Filter media

The filter medium is the heart of any filtration step, and the importance of a careful selection cannot be overstated. Many industrial process difficulties relate to the interaction between impinging particles and the pores of the filter medium. Selection of the filter medium, however, depends largely on experience in small— and large-scale operations. The performance of the filter medium may be characterized by the resistance to filtrate flow, clarity of filtrate, and durability. Resistance to flow depends to a great extend on the porosity or free area of the medium, which itself is dependent, on the material used, and on the type of manufacturing. Clarity of the filtrate depends on the pore size of the filter medium, the particle size, and the ability of the particles to form bridges to cover the pores. The ideal circumstance, where all particles are retained on the surface of the filter, is often not realized. Particle penetration into cloth or membrane pores leads to an increase in resistance of the medium to the flow of filtrate.

A wide variety of filter media are available to the user. Tab. 10.1 gives an overview of some important filter media and their characteristics. For most applications the medium of particular interest will be the one that is readily installed in the filter to be used in the process. Thus woven and nonwoven fabrics, constructed from natural (cotton, silk, wool) or synthetic fibers, are probably the most common industrial filter medium in pressure, vacuum, and centrifugal filters. Wire cloths and meshes, produced by weaving monofilaments of ferrous or nonferrous metals, are also widely used in industrial filtrations. At the small scale filter papers are common. The same materials and also rigid porous media (porous ceramics, sintered metals, woven wires) can be incorporated into cartridge and candle filters. These filter elements are usually constructed in the form of a cylinder. Cartridge filters are either of the depth or surface type, and they are widely used throughout the whole process industry for the clarification of liquids. They are particularly useful at low solid contaminant concentration and particle sizes smaller than 40 µm.

Tab. 10.1: Overview of filter media.

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If the specific cake resistance is too high or the cake is too compressible, the addition of filter aids can improve the filtration rate considerably. Filter aids are rigid, porous, and highly permeable powders that are applied as a precoat, which then acts as a filter medium on a coarse support, or is mixed with the feed suspension as body feed to increase the permeability of the resulting cake. A precoat of filter aids allows filtration of very fine or compressible solids from suspensions of 5 % or lower concentration of solids. In fact we speak here about deep bed filtration. In body feed filtration the filter aid serves as a bodybuilder to obtain high cake porosities in order to maintain high flow rates. Materials suitable as filter aids include diatomaceous earth, expanded perilitic rock, cellulose, nonactivated carbon, ashes, and ground chalk, or mixtures of those materials.