Selectivity of the Filter

In a mixture, there should be three substances with different particle sizes. We address these kinds of particles with (1), (2), and (3). The mixture is suspended in a liquid. The substances (1), (2), and (3) are present in the amounts wi,W2, W3. They have the transmissions for the filter of Ti, T2, T. We presume that we know the numerical values of these transmissions. But we do not know the mass amounts of the respective particles. 

Using the transmissions we can calculate the amount that is accumulating as filter cake, if we know the initial feed, Wi, W2, W3. After filtration the filter cake will be found  

If we would evaporate the liquid, from the suspension before filtering, then the total amount of the initial feed present would be recovered  

The index F represents the filtrate, the index K represents the filter cake, and the index T represents the total amount of the initial feed of the suspended particles to be filtered, without the liquid. 

Obviously, also with a mixture of more than three substances with known transmission such a calculation can be performed. However, we must assume that the individual substances do not interact somehow. Thus, 

---

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 ... 

Inlet filter. In air compressors filter selection is an important factor in preventing fouling. Most high-efficiency air filters have a triple-stage filtration system. Also these filters often have rain shades to prevent water from entering the filters. Site conditions play a very important part in the selection of the filters. 

The most important consideration in the selection of the filter is the compatibility of the hlter materials of constmction with the solvent. The solvents used in pharmaceutical processes can be very aggressive. They include acetone, methanol, ethanol, isopropyl alcohol (IPA), acetonitrile, dimethyl acetamide, dimethyl formamide (DMF), ethyl acetate, tetrahydrofuran (THF), methyl isobutyl ketone (MIBK), and methyl ethyl ketone (MEK). Filters with PTFE membranes and polypropylene supports are used in most applications. 

Proper selection of the filter medium is more of an art than a science. The filter cutoff must be chosen to capture the smallest particles of interest. Other factors that must be considered are the type of filter (bulk or surface), the required flow rate, and the size of the membrane. These parameters are not independent and the best choice will usually involve trade-offs. Finally, the material from which the filter is made must be considered. It must be selected for compatibility with the intended postfiltration processing. Glass-fiber filters, for example, often have very high blanks for common ions such as chloride and sodium. 

The alternative method to turbidimetric detection used for measuring solubility in early discovery is to quantify the aqueous supernatant directly via UV absorbance [13, 20, 21]. Typically, DMSO stock solution is added to aqueous buffer such that the final DMSO composition is kept to a minimum (5% or less) and the resulting precipitate is removed by filtration. A UV plate reader is then used to determine the aqueous solubility by comparing the filtrate absorbance against that of a calibration solution prepared in an identical solvent. It is important to match the sample and calibration solutions to prevent solvochromic effects. Care must also be taken in the selection of the filter plate since nonspecific binding of compound can occur with some filter materials leading to erroneously low solubility values [22], Like nephelometry, the plate-based UV detection approach is amenable to automation. 

After preliminary selection of the filter aid type, the optimum amount of addition to a suspension and the preferred rate of filtration must be established. Final selection should be based on comparison of the suspension separation efficiency for different filter aids and filter aid grades. 

The most important factors influencing the selection of the filter type include particle size distribution in the solids to be separated, the specific gravity of these solids and the solids content of the slurry to be filtered. 

The IMC design framework can yield Dahlin s controller and the VogehEdgar controller for appropriate choices of F z) and G+( ). It can also readily be applied to higher-order systems, where Direct Synthesis is not as reliable. For details on treatment of process model zeroes and selection of the filter, see Zafiriou and Morari (1985). 

Chromium speciation is very often performed in workplace air and a careful selection of the filter medium used for collecting dusts is of paramount importance. Cellulosic filters and glass fibre filters containing binders are ordinarily unsuitable, as these filter types can lead to significant reduction of Cr(Vt). Filter materials acceptable for sampling of airborne Cr(VI) include PVC, PVF, PTFE or quartz fibre filters. ... 

---