Through-Flow Filtration Applications

Many of the applications for MF derive from the excellent retention these membranes have for microorganisms. Indeed, the retention for bacteria or other organisms is often superior to what may be obtained from tighter UF and RO membranes. We may divide large-scale MF applications according to whether they utilize through-flow filtration (TFF) or cross-flow filtration (CFF). The former are more common. 

Herzig IP, Leclerc DM, LeGoff P (1970) Flow of suspensions through porous media—Application to deep filtration. Ind Eng Chem 62 8-35... 

This is a method of limiting cake growth by pumping the slurry through porous tubes at high velocities, so that the ratio of the axial flow velocity to filtration velocity through the tube walls is of the order of thousands. This is in direct analogy with the now well-established process of ultra-filtration applicable to much finer solids, which itself borders with reverse osmosis on the molecular level. It is therefore appropriate to review briefly the latter and then to follow on with ultra-filtration and, finally, with the relatively recently explored cross-flow filtration in porous tubes. 

Commercialized inorganic membranes exist in three configurations disks or sheets, tubes and multichannels/honeycombs. Usually, flat disks or sheets are limited to small scale industrial, medical and laboratory applications. They are used almost exclusively in flow-through filtration in contrast to cross-flow filtration in tubes and multichannel monoliths. Meanwhile, tubes and monoliths are used for various industrial applications [2, 4]. 

Membrane separation utilizes cross-flow filtration in which feed water flows over the membrane surface, separating the feed water into two streams product water and concentrated water. The driving force for this filtration process is the pressure differential. Reverse osmosis membranes reject ionic species and operate at pressures of 700-4200 kPa (100-600 psig) for brackish water applications. Reverse osmosis is the process of forcing water through a semipermeable membrane against the natural osmotic gradient. When water is... 

The word membrane has stuck to a range of separation media that has expanded enormously from this early form, to embrace solid inflexible inorganic materials, especially ceramics, and an ever-increasing group of polymeric materials, and to applications that now extend through ultrafiltration into the miCTofiltration range. The existence of the membrane as a very effective filtration medium led to the development of the whole field of cross-flow filtration, which also now extends well beyond its reverse osmosis origins. 

Because pore sizes in the cake and filter medium are small, and the liquid velocity through the pores is low, the filtrate flow may be considered laminar hence, Poiseuille s law is applicable. Filtration rate is directly proportional to the difference in pressure and inversely proportional to the fluid viscosity and to the... 

Figures 4-65, 4-66, and 4-67 show several units of the bag. The bags may be of cotton, wool, synthetic fiber, and glass or asbestos with temperature limits on such use as 180°F, 200°F, 275°F, 650°F respectively, except for unusual rnaterials. (See Table 4-12A and B.) These units are used exclusively on dry solid particles in a gas stream, not being suitable for wet or moist applications. The gases pass through the woven filter cloth, depositing the dust on the surface. At intervals the unit is subject to a de-dust-ing action such as mechanical scraping, shaking or back-flow of clean air or gas to remove the dust from the cloth. The dust settles to the lower section of the unit and is removed. The separation efficiency may be 99%-i-, but is dependent upon the system and nature of the particles. For extremely fine particles a precoat of dry dust similar to that used in some wet filtrations may be required before re-establishing the pi ocess gas-dust flow.

The small particles are reported to be very harmful for human health [98]. To remove particulate emissions from diesel engines, diesel particulate filters (DPF) are used. Filter systems can be metallic and ceramic with a large number of parallel channels. In applications to passenger cars, only ceramic filters are used. The channels in the filter are alternatively open and closed. Consequently, the exhaust gas is forced to flow through the porous walls of the honeycomb structure. The solid particles are deposited in the pores. Depending on the porosity of the filter material, these filters can attain filtration efficiencies up to 97%. The soot deposits in the particulate filter induce a steady rise in flow resistance. For this reason, the particulate filter must be regenerated at certain intervals, which can be achieved in the passive or active process [46]. 

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