Filtration depth filters

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. 

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. 

Depth filters are usually preferred for the most common type of microfiltration system, illustrated schematically in Figure 28. In this process design, called "dead-end" or "in-line" filtration, the entire fluid flow is forced through the membrane under pressure. As particulates accumulate on the membrane surface or in its interior, the pressure required to maintain the required flow increases until, at some point, the membrane must be replaced. The useful life of the membrane is proportional to the particulate loading of the feed solution. In-line microfiltration of solutions as a final polishing step prior to use is a typical apphcation (66,67). 

By filtration mechanism. Although the mechanism for separation and accumulation of solids is not clearly understood, hvo models are generally considered and are the basis for the apphcation of theoiy to the filh ation process. When solids are stopped at the surface of a filter medium and pile upon one another to form a cake of increasing thickness, the separation is called cake filtration. When solids are trapped within tne pores or body of the medium, it is termed depth, filter-medium, or clarifying filtration. 

Depressurization of an expanded liquid organic solution (DELOS), 24 17, 18 Depropanizer, 10 614, 615 Deprotection reactions, 15 168-169, 170, 172, 173, 183 Deprotonation, 15 654 alkylborane, 13 660 Depth filters, 15 827, 828 Depth filtration, in protein separation, 12 136... 

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... 

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 the submicron range of particle sizes, there are at least two media types to consider depth and microporous membrane. Typically, depth media is melt-blown fibrous nonwoven type (Fig. 18.13). Membranes are basically cast or expanded microporous films. Depth filters with a wrapped or pleated construction are used for slurry filtration (see Fig. 18.14a and b). Membrane... 

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. 

FIGURE 18.17 LPC for 5.0-pm Planargard CMP5 distribution loop depth filter (10" length) and 1.5-pm multiple layer POU Solaris-01 (SLRl) filter in silica-A slurry, (a) Initial feed to distribution loop filter and filtrate from loop filter after 5-h recirculation and (b) 5-h recirculated slurry feed to the POU filter and filtrate from POU filter. 

Filtration tests for recirculated silica-M slurry from Test 1, using 1 and 3 pm rating depth filters resulted in immediate plugging, whereas tests with 5 and 9 pm nominal filters (Planargard CMP5 and CMP9, respectively) resulted in Ap... 

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. 

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