Pressure Membranes

Pressure membranes are membranes that are used to separate materials from a fluid by the application of high pressure on the membrane. Thus, pressure membrane filtration is a high pressure filtration. This contrasts with electrodialysis membranes in which the separation is effected by the impression of electricity across electrodes. Filtration is carried out by impressing electricity, therefore, electrodialysis membrane filtration may be called electrical filtration.

According to Jacangelo (1989), three allied pressure-membrane processes are used: ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO). He states that UF removes particles ranging in sizes from 0.001 to 10 fim, while RO can remove particles ranging in sizes from 0.0001 to 0.001 fim. As far as size removals are concerned, NF stays between UF and RO, being able to remove particles in the size range of the order of 0.001 f m. UF is normally operated in the range of 100 to 500 kPag (kilopascal gage); NF, in the range of 500 to 1,400 kPag; and RO, in the range of 1,400 to 8,300 kPag. Microfiltration (MF) may added to this list. MF retains larger particles than UF and operates at a lesser pressure (70 kPag).

Whereas the nature of membrane retention of particles in UF is molecular screening, the nature of membrane retention in MF is that of molecular-aggregate screening. On the other hand, comparing RO and UF, RO presents a diffusive transport barrier. Diffusive transport refers to the diffusion of solute across the membrane. Due to the nature of its membrane, RO creates a barrier to this diffusion. Figures 8.2 through 8.4 present example installations of reverse osmosis units.

FIGURE 8.2 Bank of modules at the Sanibel-Captiva reverse osmosis plant, Florida.
FIGURE 8.3 Installation modules of various reverse osmosis units. (Courtesy of Specific Equipment Company, Houston, TX.)

Perforated PVC baffle

Perforated PVC baffle

Paper substrate

Cellulose acetate membrane

Grooved phenolic support plate

Plate and frame

Paper substrate

Cellulose acetate membrane iss

Grooved phenolic support plate

Plate and frame

Porous tube .-d^.Membrane

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