Filter medium selection

All filters require a filter medium to retain solids, whether the filter is for cake filtration or for filter-medium or depth filtration. Specification of a medium is based on retention of some minimum parficle size at good removal efficiency and on acceptable hfe of the medium in the environment of the filter. The selection of the type of filter medium is often the most important decision in success of the operation. For cake filtration, medium selection involves an optimization of the following factors ... 

Filter-medium selection embraces many types of construction fabrics of woven fibers, felts, and nonwoven fibers, porous or sintered solids, polymer membranes, or particulate solids in the form of a permeable bed. Media of all types are available in a wide choice of materials. 

The difficulty becomes accentuated by several other requirements that cannot be achieved through the selection of a single filter medium. Therefore, selection is often reduced to determining the most reasonable compromise between different, mutually contradictory requirements as applied to the filter medium at a specified set of filtration conditions. Because of this, some problems should be solved before final medium selection. For example, should attempts be made to increase filtration rate or filtrate purity Is cost or medium life more important In some cases a relatively more expensive filter medium, such as a synthetic cloth, is only suitable... 

One can to outline a general approach for medium selection along with a test sequence applicable to a large group of filter media of the same type. There are three methods of filter media tests laboratory- or bench-scale pilot-unit, and plant tests. The laboratory-scale test is especially rapid and economical, but the results obtained are often not entirely reliable and should only be considered preliminary. Pilot-unit tests provide results that approach plant data. The most reliable results are often obtained from plant trials. 

Proper filter medium selection is based on understanding these mechanisms and analyzing the impact each has on the filtration process. 

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... 

Select filter medium from those with suitable chemical resistance... 

For sample preparation before chromatography select filter size according to the bead size of the chromatographic medium. 

Barlow G. Fabric Filter Medium Selection for Optimum Results. In FUtech Conference. Croydon Uplands Press 1981. p. 345. 

Restriction of the first fines by poor drainage will increase the probability of retention by the medium, with concomitant bfinding. On the other hand, the modem dinqiled filter plate used in conjunction wife properly selected filter media has reduced the fiequoicy in fire use of backing cloths. An exan le of fee latter is described below in fee section dealing wife industrial information. An altemative to backing cloth is the use of reinforced filter febric in fee flmrge and stay-boss areas of the filter. See Section 4.4.11 below. 

There are a variety of ultraviolet detection devices. They convert radiation arriving at a sensing device or medium to some form of display. Conversion methods include electrical (photovoltaic cells and phototubes), thermal (thermopile), and chemical (photographic plates). For most devices, selective filters determine what wavelengths arrive at the sensor. 

A simplification of the COSY H-n.m.r. spectra of oligosaccharides by use of a 2-ditnensional version of 3-dimensional HOHAHA COSY has been described. A new and economical (in spectrometer time) technique for assigning oligosaccharide n.m.r. specua, which employs selective excitation combined with a DANTE pulse train and chemical shift selective filtering (CSSF), was found suitable for problems of medium complexity, where each spin network contains at least one separate multiplet. It has been applied to hepta-O-acetyl-a-cellobiosyl ande as an example. ... 

Table 6.8 shows some adsorbents used to prepare the stationary phase in the chromatographic separation of carotenoids by TLC. The choice between them depends on the solvent or mixture of solvents to be used as eluent phase. The adsorbent layer is placed on the glass plate (normally 20 X 20 cm) as a slurry, with a thickness that is variable but small (0.2-0.7 mm). The adsorbent is allowed to air-dry and is activated in the oven at 110°C. The pigment extract is applied to the base of the plate, and the plate is put into a tank containing the eluent. Development is usually carried out upwards, and when complete, the band or bands of interest are selected, scraped off, and eluted from the silica with either diethyl ether (in the case of polar carotenoids) or acetone or ethanol (if the polarity is medium), and filtered to remove the sihca. 

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... 

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