Surface and depth filtration

The basis of the particular mechanical separator that is called the filter is the placing across the fluid flow of a barrier, the filter medium. This acts like a porous screen, allowing those arriving particles - which are below a certain size - to pass through the openings that give the medium its porosity, together with the carrier fluid. Those 

The first major point to note is that any particle, in the absence of electrical charges on fibre or particle, once hronght close enough to the fibre, will be attracted to the fibre, until contact is made, and then the particle will stay put. The attractive forces are quite weak (known as van der Waal s forces), but are sufficiently strong to hold the particle on the fibre surface once it is there, independent of the way in which the particle arrived. The particle should be very close to the fibre for this process to occur, but once it has been trapped, then the trapped particle acts like an extension of the fibre, and can then, in its turn, trap other particles. 

It follows that, if the fluid flow is such that the particle is brought into contact with the fibre, it will be caught and held by the fibre - it has been filtered. Another point to remember is that the flow inside the medium is close to, if not actually, laminar, so that the fluid flows in smooth streamlines around obstacles in its path, as 

This ability to trap particles smaller than the apparent aperture size is a very important characteristic of filter media, and shows that media need to be tested. 

It has been stated that a filter medium is a porous (or at the very least semi-permeable) barrier placed across the flow of a suspension to hold back some or all of the suspended material. If this barrier were to be very thin compared with the diameter of the smallest particle to be filtered (and perforated with even sized holes), then all the filtration would take place on the upstream surface of the medium. Any particle smaller than the pore diameter would be swept through the pores, and any particle larger than that (assuming the particles to be rigid) would remain on the upstream surface. Some of the larger particles, however, would be of a size to settle into the individual pores and block them. The medium surface would gradually fill with pores blocked in this way, until the fluid flow reduced to below an acceptable level. At this point filtration would be stopped and the medium surface would be brushed or scraped clean (although many automatic filters have their surface continuously brushed or scraped). 

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Addition of another powdered solid, termed a filter aid, to the system can produce a significant improvemaat to a filtration operation. The filter aid may be used in two ways, either separately or in conjunction. The first of these methods is to precoat the filter medium with a layer of fiher ail cake. Precoat aids must filter quickfy without bleeding or penetrating throu the cloth and must give a uniform thickness with a reproducible filtering surfiice. The susp ntion is then fihered onto the precoat by surface and depth filtration mechanisms. Thus surface properties are inq>ortant in the choice of aid. 

This system is designed for fine slimy sohds that would not be retained on a filter cloth. The capture mechanism is a combination of surface and depth filtration so some feed suspension particles will penetrate into the precoat layer. It is a matter of ejqperhnentation to find the optimum depth of precoat cut which will preserve a high filtration rate with a long overall cycle time. 

On the basis of particle collection characteristics, filtration can be classified into cake filtration and depth filtration. In cake filtration, particles are deposited on the front surface of the collecting filter, as shown in Fig. 7.13(a). Filtration of this type is achieved mainly as an effect of screening. This is by far the most common type employed in the chemical and process industries. In depth filtration, particles flow through the filter and are collected... 

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. 

Figure 6.3.8. Principles of surface filtration and depth filtration for removing particles from a fluid.

These mechanisms - surface straining and depth filtration - are the clarification processes that are the prime topic of this book. This is because most clarification applications involve low solids concentration. 

Dust collection mechanisms include aU of the entrapment processes described in Section 1C, with the effect of electrostatic forces also being very important in many systems. Filtration of dusts is achieved by depth filtration - for which the filter pad is very effective - and by the combination of surface and cake filtration that the pleated sheet media do so well. The two materials have vied with one another over the years as to which can achieve the highest degree of separation, a history of success that is largely determined by the development of new materials of one kind or the other. 

Cross-flow filtration can be classed as one of a number of thin layer filter systans. It is one of the three main practical filtration processes (the others being cake filtration and depth filtration, with surface straining being a less-used fourth process). 

Other types of coalescing filters include combination units that use three basic operations of liquid-gas surface and depth coalescing, vapour phase adsorption and final particle filtration to remove free entrained water, water-oil emulsions, free oil, oil vapours, dirt particles and some types of entrained organic liquids and vapours. 

Filtration. In filtration, suspended solid particles in a liquid or gas are removed by passing the mixture through a porous medium that retains the particles and passes the fluid. The solid can be retained on the surface of the filter medium, which is cake, filtration, or captured within the filter medium, which is depth filtration. The filter medium can be arranged in many ways. 

Several types of aggregate-bed filters are available which provide in-depth filtration. Both gravel and particle-bed filters have been developed for removal of dry particulates but have not been used extensively. Filters have also been developed using a porous ceramic or porous metal filter surface. 

Another common problem with sand filters is clogging on the surface of the sand media, resulting in short runs and high head loss. This often is due to the finest grains of sand rising to the top of the media bed. If the problem persists, the easiest solution may be to replace the top few inches of sand with a slightly coarser grain so that some depth filtration occurs. 

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. 

In the second type of filtration, depth or deep-bed filtration, the particles penetrate into the pores of the filter medium, where impacts between the particles and the surface of the medium are largely responsible for their removal and retention. This configuration is commonly used for the removal of fine particles from very dilute suspensions, where the recovery of the particles is not of primary importance. Typical examples here include air and water filtration. The filter bed gradually becomes clogged with particles, and its resistance to flow eventually reaches an unacceptably high level. For continued operation, it is therefore necessary to remove the accumulated solids, and it is important that this can be readily achieved. For this reason, the filter commonly consists of a bed of particulate solids, such as sand, which can be cleaned by back-flushing, often accompanied by... 

The mechanism of particle capture by depth filtration is more complex than for screen filtration. Simple capture of particles by sieving at pore constructions in the interior of the membrane occurs, but adsorption of particles on the interior surface of the membrane is usually at least as important. Figure 2.34 shows four mechanisms that contribute to particle capture in depth membrane filters. The most obvious mechanism, simple sieving and capture of particles at constrictions in the membrane, is often a minor contributor to the total separation. The three other mechanisms, which capture particles by adsorption, are inertial capture, Brownian diffusion and electrostatic adsorption [53,54], In all cases, particles smaller than the diameter of the pore are captured by adsorption onto the internal surface of the membrane. 

Separation takes place in microfiltration primarily between solids and liquids, and many established applications are simply extensions of conventional filtration into a lower particle size range. (See Section I.A.) A homogeneous porous membrane used as a conventional depth filter traps particles on its surface and inside the tortuous pores. The membrane can become clogged... 

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. 

Graded-density submicron depth filters (Fig. 18.13) are suitable for broader PSD slurry global distribution loop filtration. These filters with large surface area and low face velocity are suitable for high flow POU and POD filtration applications, are typically disposable in nature, and may have nominal ratings of 0.2, 0.3, 0.5, and 1.0 pm. 

A major alternative to direct flow membrane filtration is depth filtration, in which particles are removed throughout the filtration matrix rather than just at the membrane surface, by various mechanisms such as size exclusion, electrostatic, and hydrophobic interactions. Depth filters are typically composed of a bed of cellulose or polypropylene fibers together with an inorganic filter aid such as diatomaceous earth and a binder to form a filter sheet. The filter aid imparts the matrix very high surface areas and plays an important role in increasing both retention and the capacity. Depth filters can also have an electrostatic charge usually associated with the binder polymer. 

There are two main types of barriers depth and screen. A depth filter retains particles both on its surface and within its matrix. Figure 37-6, p. 433, shows a depth filter made of fiberglass bonded with acrylic resin to prevent pieces from getting into the filtrate. Depth filters have a random matrix of fibers. The pore structure is irregular so they retain a variety of particle sizes. These filters are given a nominal rating, and they will retain 98% of all particles larger than that size. 

These other mechanisms of retaining particles within the depth of membranes may be mechanical or physicochemical. Mechanical means of entrapment apply equally to liquid and gas filtration they include inenial impaction to the walls or surfaces of the pores and lodgement in crevices and dead ends." Theoretical models and other illustrations of membrane filters often portray pores as being cylindrical in shape and as passing directly from the top surface of the membrane to the bottom surface by the shortest siraightesl route. In fact pores are rarely cylindrical, they do not have smooth walls, and their passage may be extremely convoluted even through very thin (typically 0.015 cm) depths of membrane. Physicochemical interactions with the filler medium can be very... 

Cartridge-type hydrophobic membrane Filtration has largely replaced depth filtration as a means of sterilizing gases. Collection of panicles from a gas stream by membrane Filtration is, as with liquid filtration, a function of both sieving and other means of retention. Adsorption and electrostatic attraction are far more important to retention of particles in gas Filtration than in liquid Filtration. Because there are more mechanisms and interactions between pore surfaces and particles, removal of particles is more easily accomplished from gas streams than from liquids. Gases are quite satisfactorily sterilized using 0.45 pm pore size rated membranes. 

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