Membrane Processing

Membrane technology was originally developed in 1960 for production of potable water from seawater and brackish water. The food industry has especially benefited from this technology because it is a gentle and efficient way of fractionating, concentrating, and clarifying components in liquid and gaseous streams. It is based on the use of semipermeable membranes (membranes that are permeable to some components but not to others) to separate molecules primarily on the basis of size and, to a certain extent, on shape and chemical composition. For example, as shown in Figure 1, reverse osmosis (RO) can be used to concentrate the solids in a liquid food, whereas nanofiltration (NF) membranes are designed to separate salts (primarily monovalent ions) from multivalent salts, sugars, and larger compounds. NF can also separate organic compounds by its degree of dissociation. Ultrafiltration (UF) can be used to fractionate components in the feed solution, whereas microfiltration (MF) is used to clarify slurries or remove suspended matter.

In addition, pervaporation (PV) separates liquid mixtures by partial vaporization through a membrane (Fig. 2). It is an enrichment technique similar to distillation, but it uses a membrane that is permeable either to

Figure 1. Definition of pressure-driven membrane processes.

water or selected organic compounds in vapor form. The driving force is maximized by application of low pressure to the permeate side of the membrane (eg, a vacuum), combined with immediate condensation of permeated vapors.

Electrodialysis (ED) is the transport of ions through the membranes as a result of the application of direct electric current (Fig. 2). Only ionic species are transferred directly; thus ED can separate ionic species from nonionic components, so that both concentration and purification are possible. It is used mainly as a desalting technique.

Membrane technology requires less energy than many other dewatering techniques. Whereas open-pan evaporation may need over 600 kW h/ 1000 kg water removed and a five-effect evaporator requires 37-53 kW h/1000 kg, reverse osmosis for desalination requires 5-20 kW h/1000 kg water removed (1,2). In addition, no extremes of temperature are required as with evaporation and freeze concentration, thus preventing damage to heat-sensitive food components.

The technology is very simple. The membrane is assembled in a module, and the feed stream is pumped through the module over the membrane surface in a cross-flow mode (Fig. 3). The pressure gradient across the membrane forces solvent and solute molecules smaller than the pores on the membrane surface through the membrane into the permeate stream, while larger solutes are retained in the retenĂ­ate stream.

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