Operating Parameters

Separation of solutes by UF membranes occurs by a sieving mechanism. The transport of fluids through the pores is modelled as laminar flow through channels, with flux directly proportional to applied transmembrane pressure. However, it has frequently been observed that under certain operating conditions, flux becomes independent of pressure as shown in Figure 4. This is owing to 'concentration polarization' which is shown in Figure 5. Molecules or particles that are partially or completely retained by the membrane accumulate on the surface of the membrane during ultrafiltration. This build-up of solids will cause a concentration gradient within the boundary layer, resulting in back-transport of solute into the bulk stream owing to diffusion. Eventually a steady state is reached where the two phenomena balance each other. Solute concentration reaches a maximum at the 'gel concentration'. This con-

Figure 4 Effect of operating conditions on flux of an ultrafiltration system. (Adapted from Cheryan (1998) with permission from Technomic.)

Figure 5 Concentration polarization in ultrafiltration.

Figure 4 Effect of operating conditions on flux of an ultrafiltration system. (Adapted from Cheryan (1998) with permission from Technomic.)

Feed flow

Figure 5 Concentration polarization in ultrafiltration.

solidated gel layer is the reason that pressure independence in Figure 4 is observed. Flux is no longer controlled by pressure but by the mass-transfer characteristics of the system which in turn depends on the diffusion coefficient of the rejected molecules in the boundary layer, turbulence in the flow channel, viscosity and density of the fluid stream. Higher temperatures lead to higher flux because of its favorable effect on diffusivity and viscosity. In the pressure-independent region, flux decreases in a semi-logarithmic manner with bulk feed concentration and increases with higher turbulence (usually achieved by higher flow rates through the module).

Solar Panel Basics

Solar Panel Basics

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