Membranes And Membrane Modules

UF and RO membranes are usually asymmetric, which means the pores change in size from one surface of the membrane to the other. They also differ from microporous membranes, which have pores of uniform size throughout the membrane. The two techniques have been developed to manufacture asymmetric membranes: one utilizes the phase-inversion process (6) and the other forms a composite structure by depositing a very thin homogeneous solid polymer film on a microporous membrane (7). The phaseinversion process consists of three steps: (J) a polymer is dissolved in an appropriate solvent to form a solution containing 10 to 30 wt % polymer; (2) the solution is cast into a film about 100 to 500 jum thickness; and (3) the film is quenched in a nonsolvent, which for most polymers is typically water or an aqueous solution (3,8,9). Variations of the phase-inversion process such as wet, dry, or melt spinning processes have been reviewed (5). More than 90 different homopolymers, copolymers, and blends have been investigated for the manufacture of the phase-inversion membranes (10). Cellulose acetate, polysulfone, and polyamide membranes are the most common types used commercially.

The methods of forming composite membranes have been reviewed (5,11). These methods can be briefly summarized as follows: (1) casting of the barrier layer separately, which is followed by lamination to the support film; (2) dip-coating of the support film in a polymer solution and drying, or in a reactive monomer or prepolymer solution followed by curing with heat or radiation; (3) gasphase deposition of the barrier layer from a glow-discharge plasma; and (4) interfacial polymerization of reactive monomers on the surface of the support film.

Mineral or ceramic membranes are another class of membranes that are lately receiving increased attention. They are formed by the deposition of inorganic solutes, such as zirconium oxide, onto reusable porous carbon tubes or porous metal tubes. Because they are made of inorganic materials, they have little or none of the disadvantages associated with polymeric membranes. They possess a high degree of resistance to chemical and abrasion degradation and tolerate a wide range of pH and temperature (5).

For the successful application of UF and RO, the design of the membrane module and the layout of the system in which the module is installed are as important as the selection of the proper membranes (9). There are four important membrane module designs used in both UF and RO: tubular, plate-and-frame, spiral-wound, and capillary or hollow-fiber.

The tubular membrane module is shown in Figure 1 and consists of a membrane cast on the inside of a porous fi-

Outer tube Permeate Porous tube

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Figure 1. A tubular membrane module.

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