Important Factors

Several factors must be taken into account when considering a membrane process for a particular application: membrane material, membrane properties, module design, engineering factors, fouling and cleaning, and process design (1). Membranes have been made from more than 150 different polymers or inorganic materials, although fewer than 30 have achieved widespread commercial use. The majority of materials have been used for MF membranes and much fewer have worked successfully for RO, PV, and ED. Cellulose acetate and derivatives are still widely used for pressure-driven processes, despite their limitations of pH (generally usable only at pH 2-8) and temperature (not higher than 30°C). Thin-film composite membranes, containing a polyamide separating barrier on a polysulfone or polyethylene supporting layer, generally give better performance for RO and NF applications. Polysulfone, polyethersulfone, polyvinylidine fluoride, and polyacrylonitrile membranes are most common in UF and MF. MF membranes are also available in inorganic materials (alumina, zirconia/carbon composites, carbon/carbon composites, and stainless steel). Inorganic membranes have considerably widened the range of membrane applications, particularly in waste treatment, recovery and reuse of chemicals, and biotechnology applications where high temperature, acid and alkali stability, steam steril-izability, and cleanability are important.

Pore size is the most important property of a membrane. In reality, there is a distribution of pore sizes, which makes it difficult to get sharp or clean separations of molecules with membrane technology. Other factors affecting

Figure 2. Schematic of electrodialysis and perva-poration.

Depleted feed stream

Product

Electrodialysis a

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