Process Considerations

Cross-flow microfiltration is usually carried out in the feed-and-bleed mode, shown in Figure 11. The use of a retentate recycle makes it possible to work at high cross-flow velocities (high Qc) while having low retentate flows (i.e. high volumetric concentration factors Vc = 1 + Qp/Qr). When high concentration factors are desired, several recirculation loops may be placed in series, or in even more complicated schemes, with loops both in parallel and in series (Christmas tree design).

In many cross-flow microfiltration systems and in some dead-end systems, backflushing is applied to remove the fouling layer from the membrane. Back-flushing is achieved by forcing the permeate periodically back through the membranes. Effective back-flushing is obtained by using high counterpressures

Figure 11 Feed-and-bleed operational configuration for cross-flow microfiltration.
Figure 12 Microfiltration flux when filtering a particle suspension, with and without backflush.

(about 0.5 bar) for several seconds every few minutes (Figure 12).

Fouling is reduced by high cross-flow velocities and low transmembrane pressures. High cross-flow velocities cause high-pressure drops along the membrane, which cause the AP to be undesirably high at the entrance of the membrane module. Therefore microfiltration processes have been developed which facilitate a cross-flow both on the feed side and on the permeate side. The pressure drops on both sides are similar in magnitude, guaranteeing a uniform transmembrane pressure. This method of operation has been shown to be effective in many dairy applications.

Other process techniques to reduce fouling are the use of pulsed flow, gas sparging, and electric or acoustic fields, and the use of flow geometries that create secondary flows or vortices resulting in high shear rates (e.g. the use of 'turbulence promoters' or curved channels).

Solar Panel Basics

Solar Panel Basics

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