Surface filtration

Addition of Inert Filter Aids. FUtet aids ate rigid, porous, and highly permeable powders added to feed suspensions to extend the appheabUity of surface filtration. Very dilute or very fine and slimy suspensions ate too difficult to filter by cake filtration due to fast pressure build-up and medium blinding addition of filter aids can alleviate such problems. Filter aids can be used in either or both of two modes of operation, ie, to form a precoat which then acts as a filter medium on a coarse support material called a septum, or to be mixed with the feed suspension as body feed to increase the permeabihty of the resulting cake. 

In the precoat and body feed mode, filter aids allow appHcation of surface filtration to clarification of Hquids, ie, filtration of very dilute suspensions of less than 0.1% by volume, such as those normally treated by deep bed filters or centrifugal clarifiers. Filter aids are used in this mode with pressure filters. A precoat is first formed by passing a suspension of the filter aid through the filter. This is followed by filtration of the feed Hquid, which may have the filter aid mixed with it as body feed in order to improve the permeabiUty of the resulting cake. The proportion of the filter aid to be added as body feed is of the same order as the amount of contaminant soHds in the feed Hquid this limits the appHcation of such systems to low concentrations. Recovery and regeneration of filter aids from the cakes normally is not practiced except in a few very large installations where it might become economical. 

In cake or surface filtration, there are two primary areas of consideration continuous filtration, in which the resistance of the filter cake (deposited process solids) is veiy large with respec t to that of the filter media and filtrate drainage, and batch pressure filtration, in which the resistance of the filter cake is not veiy Targe with respect to that of the filter media and filtrate drainage. Batch pressure filters are generally fitted with heavy, tight filter cloths plus a layer of precoat and these represent a significant resistance that must be taken into account. Continuous filters, except for precoats, use relatively open cloths that offer little resistance compared to that of the filter cake. 

The concentration of indoor pollutants is a function of removal processes such as dilution, filtration, and destruction. Dilution is a function of the air exchange rate and the ambient air quality. Gases and particulate matter may also be removed from indoor air by deposition on surfaces. Filtration systems are part of many ventilahon systems. As air is circulated by the air-conditioning system it passes through a filter which can remove some of the particulate matter. The removal efficiency depends on particle size. In addition, some reactive gases like NOj and SOj are readily adsorbed on interior surfaces of a building or home. 

Surface-enhanced resonance Raman scattering (SERRS), 21 327-328 advantage of, 21 329 Surface Evolver software, 12 11 Surface excess, 24 135, 136 Surface extended X-ray absorption fine structure (SEXAFS), 19 179 24 72 Surface filtration, 11 322-323 Surface finish(es). See also Electroplating in electrochemical machining, 9 591 fatigue performance and, 13 486-487 Surface finishing agents, 12 33 Surface force apparatus, 1 517 Surface force-pore flow (SFPF) model,... 

The fine structure of the membrane also allows surface filtration to occur. Suspended solid particles are retained on the surface of the membrane and are not allowed to penetrate into or through the supporting felt cloth. In contrast, conventional media depend upon depth filtration in which particles are retained within the medium itself. The results are immediate filtrate clarity without the need to build-up a filter cake, and reduced accumulation of solids within the medium that leads to increasing pressure drop. 

Suspended matter down to bacterial size can best be removed by subsidence in settling ponds and by skimming off floating material such as grease, which may form a paste that coats heat-exchange surfaces. Filtration would, no doubt, do a better job (see later), but on a large scale it is not always cost-effective (besides, filters tend to clog). 

The water filtering direction is lop-to-bottom, and conventional design flux rates are about 2-3 gpm/sq ft Solids suspended in the water are filtered out by the fine sand grains on the top of the bed. These solids may penetrate a few inches into the top of the media, but the process is essentially surface filtration. 

Thus, a large volume of the media participates in solids removal as opposed to just surface filtration in a conventional filler. This is referred to as deep bed" filtration. 

Surface filtration involves the removal of particulate material suspended in a liquid by mechanical sieving. This is achieved by passing the liquid through a thin layer of woven metal meshes, cloth fabrics, or a variety of synthetic materials. 

The clogging effect can be considered as a reduction in the value of the surface filtration constant for practical purposes. Indeed, when clogging takes place, the surface filtration constant can be given by its initial value ko multiplied by a decreasing time function. This assumption is frequently used when the function is obtained from experiments [3.19, 3.20]. In our example, if we do not consider the friction (and heat transfer) we can note that only a concrete mass transfer problem can be associated with the membrane separation process. The first step before starting to build the general mathematical model, concerns the division of the system into different elementary sections. Indeed, we have a model for the filtration device (i.e. the membrane and its envelope), for the pump (P) and for the reservoir of concentrated suspension (RZ) (Fig. 3.7). 

In surface filtration the solids retained are those that do not pass through the smallest cross-section of the capillary flow channels of the filtering layer. Many particles are trapped by adsorption in the labyrinthine three-dimensional sieve of the filter aid. This means that substances can be retained that are smaller than the mesh size of the filter aid. In depth filtration, on the other hand, the solids are trapped in the interior of the layer this is due to the mechanical retention capacity (inertia and size of the particles, sedimentation, diffusion) and to the composition of the juice. 

Surface filtration coarse particles are trapped on the surface because of their size. 

With increasing filtration time, the particles separated from the juice clog up the capillaries in the sheets. In surface filtration the very nature of the operation inevitably means that the surfaces of the sheets get clogged up in the course of filtration. 

In many respects, MF is a simple extension of UF. MF, however, is a genuine filtration process in whidi particulate matter typically 0.1—10 /um is removed from aqueous streams. The membranes are totally non-selective with respect to osmotic-pressuregenerating solutes, and low pre ure operation is the norm. Particle removal is adhieved strictly on the basis of size, and the rules of particle filtration, particularly surface filtration, apply. 

ABSTRACT Panel Bed filter is a new alternative for particle removal. The filter is based on the use of sand or other granulated material as a filtration medium, and is operating in surface filtration mode. The concept was developed in the United States and further developed in Norway. 

Measured pressure build up versus accumulated amount of fly ash is shown in Fig. 5 a). Calculated fly ash accumulation is based on averaged dust load measured gravimetiically during die respective filtration cycle. Observed pressure loss is close to ideal behavior for surface filtration (i.e. inconqiressible cake formation and constant dust concentration). Calculated specific cake resistance is shown in Fig. 5 b). 

Terminal turbidity breakthrough is not generally observed because precoat filtration is dominantly a surface filtration process. 

Screening (Fig. 1C) is a type of surface filtration to separate solids and liquid by screens with openings smaller than particle size. It is usually employed among the first few steps of a SLS process. Vibrations and other mechanism are often applied to avoid blinding of the screens. 

The distinction between cross-flow and dead end (also known as through-flow) filtration can be better understood if we first analyze the mechanism of retention. The efficiency of cross-flow filtration is largely dependent on the ability of the membrane to perform an effective surface filtration, especially where suspended or colloidal particles are involved. Table 2 shows the advantages and versatility of cross-flow filtration in meeting a broad range of filtration objectives, Figure 2 illustrates the differences in separation mechanisms of CFF versus dead end filtration. 

If the preceding precantions are taken, the wine s quality should not be affected as it passes through the filter surface. Filtration, after all, is intended to eliminate turbidity, foreign bodies and impurities that would, in time, form the lees. It would be ridiculous to suggest that these substances make a positive contribution to flavor. 

The effective particle rize distribution can be changed by classifying the suspenrion and feeding the coarse finction to tire filter ahead of the fine fi-action. By doing this the finer material, which is the more difficult to filter is captured by depth and surface filtration mechanisms on the coarse fiaction, vriiich acts as a precoat... 

Winding a string of staple fibres on to a former or core provides a cartridge which can have a hi dirt handling capacity provided the suspended solids are not removed by surface filtration. Figure 6.21 illustrates a yam-wound cartridge. 

The most commonly adopted filtering media is a high-density ceramic characterized by an asymmetric structure, a support material covered with a thin membrane layer containing very small pores. The membrane collects the fine particles and by making it thin the differential pressure of the filter element is kept low. The ideal solution is to have a very thin layer without defects that just covers the support material, so that purely surface filtration takes place. Experimental evidence at industrial scale shows that this ideal solution can be achieved in practice, penetration of particles into the support structure of the filter element being prevented, and that the element can be effectively regenerated by the clean gas back-pulsing procedure described by Cocco et al. [56]. It has also been confirmed at... 

Filtration processes may be classified as either depth filtration or surface filtration. Depth filtration relies on a layer of porous media in which suspended particles in the beer are trapped within the media. Examples in brewing include filter aid filtration, sheet filtration and some forms of filter cartridge. Surface filtration normally refers to membrane technology. A thin layer of membrane has pores throughout the structure. This means that it is possible to achieve very exact filtration, perhaps enabling sterilisation, but typically the quantity of suspended beer particles that may be removed is less than for depth filtration. 

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