Screen filtration

Figure 2.31 Separation of particulates can take place at the membrane surface according to a screen filtration mechanism (a) or in the interior of the membrane by a capture mechanism as in depth filtration (b)...

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. 

Palaemonetes pugio) b) acute (96h) and short-term (7d) fish test (Cyprinodon variegatus) Saline extraction 37 pm and 10 pm mesh screen filtrations followed by decantation... 

Charcoal may also be screened (filtration of contaminants and separation into size classes), crushed into powder, or made into briquettes wherein the charcoal is compressed into compact, high carbon-density masses with or without a binder. Wet charcoal presents a particular combustion hazard because moisture catalyzes oxidation leading to combustion and self-heating. 

During the preparation of medicines, steam, vapour, aerosols, dust and fumes can be released, which may pose a health risk for the operator. It is not always possible to change the process releasing these hazardous substances. As a consequence it can be necessary to protect operators in preparation or quality control areas from exposure to the product or the active substance. This can be done by active ventilation and exhaust and by filtration in order to protect the environment (see also Sect. 26.4.1). The appropriate equipment may be fume cupboards, moveable exhaust ducts, powder exhaust units, (bio)safety cabinets and isolators. Fumes, gas mixtures and volatiles might be absorbed by special filters, but in pharmacy practice only the technique of exhausting and screen filtration is usually used. 

Filtration processes can be divided into screen filtration and depth filtration. These processes are shown in Fig. 30.8. 

Fig. 30.8 Schematic representation of the screen filtration principle (a) and the depth filtration principle. Source Recepteerkunde 2009, KNMP...

---