Filter aid selection

Filter aid selection should be based on 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, filter medium, and a liquid phase. Experiments are needed in most applications not only in proper filter aid selection but in defining the method of their administration. Some general guidelines can be applied to such evaluations  

For precoat applications to rotary filters, the relative usefulness of various filter aids requires experimental data on the effects on the following variables  

Tests should be conducted under a constant set of operating conditions such as pressure and temperature. 

Such tests will reveal that the manner in which filter aid is removed depends on its properties and the drum speed. In some cases the filter aid will be removed as flakes, while other tests will show that the remaining portion of the cake will acquire a rough surface. High drum speed and filter aid dispersity are known to enhance the quality of the precoat layer. 

For a low rate of knife advance the resistance of the filter aid decreases in spite of the decrease in cake thickness. This is explained by the fact that for a single rotation of the drum, the particles of the suspension penetrate into the pores of filter at depths greater than 1.5 mm, and blockage occurs. Rotary drum filler systems are characterized not only by rate, but by the ratio of filtrate volume to the weight of filter aid cut. Some generalizations for this system are  

---

Filter aid selection should be based on laboratory tests. The requirements for preliminary evaluation of the selected filter aids may be summarized as follows ... 

Results of test work with this filter, producing cakes of 1 mm thickness using a 3 mm clearance, have been pubUshed (33,34). The cake formed on the medium was generally stable, giving high filtration rates over long periods of time, and the precoat type cake did not blind with time. There was no evidence of any size selectivity of the process the only exception was conventional filter aids which were preferentially picked up by the rotating fluid. This... 

Liquid Dosage Forms. Simple aqueous solutions, symps, elixirs, and tinctures are prepared by dissolution of solutes in the appropriate solvent systems. Adjunct formulation ingredients include certified dyes, flavors, sweeteners, and antimicrobial preservatives. These solutions are filtered under pressure, often using selected filtering aid materials. The products are stored in large tanks, ready for filling into containers. QuaUty control analysis is then performed. 

Use of selected fibrous or granular filter aids (precoating), permitting the high-efficiency collection of submicrometer smokes and gaseous contaminants... 

Develop a list of the issues and parameters to examine when making a selection of the proper filter aid. 

The most commonly used filter medium is woven cloth, but a great variety of other media is also used. The main types are listed in Table 10.2. A comprehensive discussion of the factors to be considered when selecting filter media is given by Purchas (1971) and Mais (1971) see also Purchas and Sutherland (2001). Filter aids are often used to increase the rate of filtration of difficult slurries. They are either applied as a precoat... 

The activity is determined by the Ni surface and its accessibility, which also affects the selectivity of the process. Thus the active phase is usually dispersed on a solid support, which also acts as a filter aid and even as promoter. The... 

Choosing the filter medium to be used in a pressure filter is a very important consideration. The medium should be selected primarily for its ability to retain the contaminant to be separated, and to have an acceptable life in the filter environment. Filter media are manufactured from cotton, synthetic polymers, glass, cellulose, metal, carbon, refractories, and other porous or perforated solids, and sand and other particulate solids. The filter s effectiveness may be enhanced by the use of filter aids. 

Filtration—After an adsorbent has selectively captured the impurities, it must be removed from the oil before it becomes a catalyst for color development or other undesirable reactions. Filtration, the separation method most often used for spent bleaching media removal, is the process of passing a fluid through a permeable filter material to separate particles from the fluid. Examples of the filtration materials used are filter paper, filter cloth, filter screen, and membranes. Filter aid, such as diatomite, perlite, or cellulose, are usually used in conjunction with the permeable filters for surface protection. Traditionally, either plate and frame or pressure-leaf filters have been used for spent bleaching media removal. Currently, self-cleaning, closed filters that operate on an automated cycle are available. 

The manufacturer of filter aids produces filter aid grades in a wide range of particle sizes to meet practically any industrial filtration requirement. The relative flow rates of these grades are determined by a standard filtration test and are shown in Fig. 4. Typical properties of some commercial filter aids are shown in Tables 1-3. Filter Cel, the finest grade, shown in Fig. 4, giving the highest clarity and lowest flow rate, is a natural diatomite, which has been selectively quarried, dried, milled, and air-classified. To make coarser, faster flow rate filter aids, Filter-Cel is calcined and air-classified. These straight... 

Higher drum speeds, however, mean more frequent shaving of the precoat by the blade. To maintain rates, the precoat must be cut to a depth below that which solids have penetrated, and therefore partially blinded, the precoat. A grade of precoat material must be selected, then, to minimize this cake penetration. If this is done, it is possible to take a small enough cut to maintain precoat filter aid economy at high drum speeds and filtration rates. 

When DE is used as an admix to produce a less resistant cake, selection of the type and amounts of filter aids depends on laboratory tests with a constant pressure filter cell. A graph illustrating the effect of filter aid addition on the average flow rate is shown in Figure 22.15 (Tiller 1978). Overdose of Alter aid resulted in a decrease of filtrate rate. 

For a particulate application involving SLS, how to choose the best equipment, to use batch or continuous operation, which filter medium to select, and what is the optimum operating conditions are concerns of engineers. Although theories are available for some SLS operations, solutions for equipment selection, process design, and optimization are still very much dependent on test and experience, and are frequently qualitative or semi-qualitative. In this section, strategy and decision networks for selection of SLS equipment, introduction of filter media, centrifugal pumps for filtration operation, and selection of filter aids by cycle analysis will be discussed. 

Cycle Analysis in Selection oe Filter Aids (Tiller and Li 2003)... 

---