Granular filtration particles

While the development of codeposition theories was essentially dormant, the understanding of the kinetics of particle deposition from suspensions was rapidly evolving. The omnipresence of the interaction of particles with surfaces [70] and the importance to deep-bed granular filtration, deposition of paints, fouling of coolant circuits, chemical reactors and membranes, led to careful theoretical and experimental investigations of the mechanism of deposition. The theory is most advanced in the area of filtration and a number of comprehensive reviews exist [71-73]. It is striking that of... 

The collection efficiency of a fiber is defined as the ratio of the number of particles striking the fiber to the number which would strike it if the streamlines were not diverted [Dorman, 1966]. For particles with dp > 1 pm, the particles are collected primarily by inertial impaction and interception. Collection by diffusion is only important for submicron particles. In the following, we only discuss the methodology of collection efficiency by interception. Details about the filtration of fine and ultrafine powders or granular filtration in general can be found in Davies (1973), Fuchs (1964), Dickey (1961), Matteson and Orr (1987), and Tien (1989). 

Most granular filters are based on the principle of deep bed filtration (particle penetration deep into the filtration medium) and the purification degree has been relatively poor for particle sizes in the area 0.1-5jim. 

An important means by which small particles in suspension are separated from solutions is through capture by collectors, which may be larger particles, or granular, porous, or fibrous media. An example of such collection is filtration. The separated solids may be collected as a cake on the surface of the filter medium (much like ultrafiltration), and this is termed cake filtration. Alternatively, the solids may be retained within the pores of the medium, and this is termed depth filtration. It is important to recognize that particle collection in a porous medium is not simply a matter of straining that is, the capture is not purely steric, since, in filtration, particles are captured that are much smaller than pores of the medium. The capture of small suspended particles from fluids in laminar flow by a collector is a consequence of the simultaneous action of fluid mechanical forces and forces between the particle and collector, such as van der Waals or electrostatic forces. It is the combined forces, at least close to the collector, that govern the particle trajectories and determine whether a particle will be transported to and retained at the surface of a collector that is fixed in the flow (Spielman 1977). 

In Section 7.2.2, we consider fixed-bed granular filtration for clarifying dilute suspensions of particles from a liquid. We will identify the similarities between this process and the fixed-bed adsorption/desorption processes studied in Section 7.1.1 both processes employ a low velocity bulk liquid flow through a bed of fixed particles and solute molecules or particles are removed from the liquid by forces acting perpendicular to the fixed particles. Both processes achieve high levels of purification. From a black-box type of perspective, granular filtration may also look very similar to the dead-end filtration studied in Section 6.3.3.1, but there are significant differences. 

Very detailed models of granular filtration have been developed using such idealizations of the porous medium (Tien, 1989). A simple aspect of this filtration is that, as the liquid flows down the porous medium, particles are transported perpendicular to the flow by electrostatic force. Brownian difiusion, various surface interaction forces and interception. Gravitational force is also present its... 

Figure 7.2.10. Fixed-bed granular filtration (a) various mechanisms of particle capture by a spherical collector granule (after Davis (2001)) (b) granular packed-bed schematic for filtration.

Filtration operations are capable of handling suspensions of varying characteristics ranging from granular, incompressible, free-filtering materials to slime-like compositions, as well as finely divided colloidal suspensions in which the cakes are incompressible. These latter materials tend to contaminate or foul the filter medium. The interaction between the particles in suspension and the filter medium determines to a large extent the specific mechanisms responsible for filtration. 

Granular media filtration is used for treating aqueous waste streams. The filter media consists of a bed of granular particles (typically sand or sand with anthracite or coal). The anthracite has adsorptive characteristics and hence can be beneficial in removing some biological and chemical contaminants in the wastewater. This material may also be substituted for activated charcoal. 

Sand filters vary in sophistication. A simple filter will remove most particles down to 5 pm. Multi-media filters which use sand and anthracite, and possibly a third medium, in discrete layers, can yield very efficient filtration down to 2 pm. Granular activated carbon can be used instead of sand to add some measure of organic removal to the filtration process. The quality produced by any filter depends largely on the efficiency of the backwash. Sand filters in some form provide a satisfactory solution for the majority of water-filtration problems. 

In some cases the interaction between the particles and the surrounding fluid is of little significance, although at other times this can have a dominating effect on the behaviour of the system. Thus, in filtration or the flow of fluids through beds of granular particles, the characterisation of the porous mass as a whole is the principal feature, and the resistance to flow is dominated by the size and shape of the free space between the particles. In such situations, the particles are in physical contact with adjoining particles and there is... 

Both media filters and cartridge filters can be used in a pretreatment process. Granular media filters involve the filtration of large particles through different layers of fine particles, usually coal, pumice, sand or garnet (Bonnelye et al. 2004). Cartridge filters act as the final filtration step before the water passes through the membranes, and remove fine particles as small as 1 pm. 

Advanced treatment plants employ either granular filters or membrane filters. The former is exemplified by activated carbon, whereas membrane filtration has been developed only in recent memory. Besides the general principle of excluding contaminants based on size, these advanced filtration systems also have a charge that enables them to exclude particles, with the removal of anionic compounds being higher than that of nonionic ones. Both systems come at a premium. 

Modem coal combustion employs two principal techniques combustion in a fluidized bed or pulverization, followed by combustion of fine particles suspended in moving air. Figure 1 shows a schematic of pulverized coal combustion, a process much used in steam-raising plants. Each process produces a characteristic residue fluidized bed combustion gives rise mainly to a clinker-like or granular product, whereas pulverization, followed by combustion, produces mainly a much finer, micrometre-sized ash residue. Pulverization also yields a coarser fraction, the so called bottom ash , which is periodically removed without difficulty. However, the finer fly ash has to be recovered by filtration and electrostatic precipitation. Commercially, fly ash has... 

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