Filter media—general

Just as there are vast numbers of different types of filters, the same holds true for filter media, on which the filters are so dependent. 1 will present as many types of media as feasible, and will concentrate on those that are most commonly used. 

An important question on filter media is to ask how fine a degree of filtration it wiU give. There are three groups of considerations which are raised when attempting to define the degree of fineness of filtration that can be achieved with a given medium. The first is the structure of the medium the second is the mechanism by which filtration occurs, which is dependent upon the medium and the fluid-particle system to be handled and the third group is the method of measurement used. 

Obvious uniformity is found in simple form with plain-weave metal cloth of light gauge wire. As the gauge of the wire becomes heavier and the weave is changed to a twilled or Dutch-type weave, we have a more elaborate medium that is generally used for filtration. The nature of the holes is more complex and more difficult to recognize with the unaided eye. Woven fabrics become more complicated due to the flexible nature of yams, and therefore it is more difficult to try to define the size of the hole in a woven fabric. The same is tme for media with random stmcture, such as felts, paper, fibrous and porous material. 

The main types of mechanism by which filtration occurs are surface and depth. These are also referred to as classifications of filter media. Other classifications are rigid versus flexible, loose versus integral, and permanent versus disposable media. 

An example of siuface filtration would be a straining process with wire cloth, wherein all of the solids larger than the media openings are deposited 

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An additional benefit of prethickening is reduction in cake resistance. If the feed concentration is low, there is a general tendency of particles to pack together more tightly, thus leading to higher specific resistances. If, however, many particles approach the filter medium at the same time, they may bridge over the pores this reduces penetration into the cloth or the cake underneath and more permeable cakes are thus formed. 

By filtration mechanism. Although the mechanism for separation and accumulation of solids is not clearly understood, hvo models are generally considered and are the basis for the apphcation of theoiy to the filh ation process. When solids are stopped at the surface of a filter medium and pile upon one another to form a cake of increasing thickness, the separation is called cake filtration. When solids are trapped within tne pores or body of the medium, it is termed depth, filter-medium, or clarifying filtration. 

The second method of exploitation occurs when the electric field is of a polarity such that the charged-particle migration occurs away from the filter medium. The contribution to the net-particle velocity of the elec trqphoreticaUy induced flow away from the filter medium is generally orders of magnitude less than the contribution to the net-... 

Note that filter aid selection must be based on planned laboratory tests. Guidelines for selection may only be applied in the broadest sense, since there is almost an infinite number of combinations of filter media, filter aids, and suspensions that will produce varying degrees of separation. The hydrodynamics of any filtration process are highly complex filtration is essentially a multiphase system in which interaction takes place between solids from the suspension, filter aid, and filter medium, and a liquid phase. Experiments are mandatory in most operations not only in proper filter aid selection but in defining the method of application. Some general guidelines can be applied to such studies the filter aid must have the minimum hydraulic resistance and provide the desired rate of separation an insufficient amount of filter aid leads to a reduction in filtrate quality — excess amounts result in losses is filtration rate and it is necessary to account for the method of application and characteristics of filter aids. 

Proper selection of the filter media is often the most important consideration for assuring efficient suspension separation. A good filter medium should have the following general characteristics ... 

Filtration experiments are typically conducted in pilot scale equipment and generally tests are conducted either at constant pressure or constant rate to determine axo, as well as s and Rf, for a given sludge and filter medium. Such tests provide empirical information that will enable the time required tor the pressure drop to reach the desired level for a specified set of operating conditions to be determined. In the initial stages of filtration, the filter medium has no cake. Furthermore, Ap is not zero, but has a value that is a function of the resistance of the medium for a given flowrate. This initial condition can be stated as ... 

Normally the filter strueture consists of a stack of plates attached to a hollow shaft which are mounted inside a pressure vessel with eaeh plate eovered with a suitable filter medium. The slurry is fed under pressure into the vessel and the eake, which is retained by the filter medium, forms on the top of eaeh plate whilst the filtrate passes through the hollow shaft further to the proeess. Filter sizes may vary but generally the maximum is 60 m area and designed for a 6 bar operating pressure. Each circular plate in the staek is eonstructed with radial ribs that are welded to the bottom and support a horizontal eoarse mesh screen whieh is eovered with a fmer woven metal screen or filter cloth to retain the cake. The bottom of the plate slopes towards the hollow eentral shaft whieh lets the filtrate flow freely through circumferential holes and further down the shaft to the filtrate outlet. The elearanee between the plates is maintained by speeial spacers... 

The choice of the filter medium is often the most important consideration to ensure efficient operation of a filter. Its function is generally to act as a support for the filter cake, while the initial layers of cake provide the actual filter. The filter medium should be selected primarily on the basis of its ability to retain solids without binding. It should be mechanically strong and corrosion resistant, and should offer as little resistance as possible to the flow of the filtrate. The media are made from widely different materials such as cotton, wool, linen, nylon, jute, silk, glass fiber, porous carbon, metals, rayon and other synthetics, and miscellaneous materials like porous rubber. Cotton fabrics are most commonly used because they are available in a wide variety of weaves, and are cheap. 

Results from constant differential pressure filtration tests have been analyzed according to traditional filtration science techniques with some modifications to account for the cross-flow filter arrangement.11 Resistivity of the filter medium may vary over time due to the infiltration of the ultrafine catalyst particles within the media matrix. Flow resistance through the filter cake can be measured and correlated to changes in the activation procedure and to the chemical and physical properties of the catalyst particles. The clean medium permeability must be determined before the slurries are filtered. The general filtration equation or the Darcy equation for the clean medium is defined as... 

The function of the filter medium is generally to act as a support for the filter cake, and the initial layers of cake provide the true filter. The filter medium should be mechanically... 

If the resistance of the filter medium is neglected, t = BiV2 and the time during which filtration is carried out is exactly equal to the time the press is out of service. In practice, in order to obtain the maximum overall rate of filtration, the filtration time must always be somewhat greater in order to allow for the resistance of the cloth, represented by the term B2V. In general, the lower the specific resistance of the cake, the greater will be the economic thickness of the frame. 

A wide variety of filters are available for the cell recovery. There are generally two major types of filters pressure and vacuum filters. The detailed descriptions of those filter units can be found in Chemical Engineers Handbook (Perry and Chilton, 1973). The two types of filters most used for cell recovery are the filter press and rotary drum filters. A filter press is often employed for the small-scale separation of bacteria and fungi from broths. For large-scale filtration, rotary drum filters are usually used. A common filter medium is the cloth filter made of canvas, wool, synthetic fabrics, metal, or glass fiber. 

As a filtration process proceeds, generally under an applied driving force of pressure, solids are removed by and begin to accumulate on the filter medium. The liquid portion continues to move through the filter medium and out of the filter system. The separated liquid is referred to as the filtrate. The amount of pressure applied to accomplish the filtration depends on the filtration resistance. Filtration resistance is a result of the frictional drag on the filtrate as it passes through the filter medium and the accumulated solids. In equation form,... 

The filter medium may be precoated by placing the precoat solution in the immersion tank before being replaced by the process slurry. Generally precoated filters must operate on a semicontinuous basis because after a period of time the precoat layer will deteriorate and must be replaced. Filters without a precoat can be operated continuously. 

Solid particles are generally removed from the solution by filtration. The fluid is passed through a filtering medium, namely, a bed of fine particles, cloth, mesh, or sintered plate. The fluid flow is achieved either by forcing the mixture under pressure or by applying a vacuum on the opposite side. 

Temperature Requirements of a Process. The temperature of filtration may affect the viscosity of the fluid, the corrosion rate of the housing, and filter medium compatibHity. Elevated temperatures tend to accelerate corrosion and may weaken the gaskets and seals of filter housings. In general, the viscosity of fluids decreases with increasing temperature. EHtration of highly viscous fluids maybe conducted at elevated temperature (8). For example, pharmaceutical products containing oil or a Hpid emulsion as a dmg carrier may require filtration at elevated temperature to enhance filterability characteristics. 

Horizontal basket or moving bed solvent extractors are preferred over vertical tower extractors in processing safflower cake. The fibrous nature of safflower provides a natural channel through which the solvent can move, and the bed acts as a natural filter medium. Tower extractors generally have problems extracting... 

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