Hydrodynamics of Micro- and Ultrafiltration Systems

Permeate flux enhancement by pressure and flow pulsations has been investigated by many authors [36-38]. The effect of flow oscillations in a smooth-walled duct is shown in Fig. 12.13. 

Pulsate flows were applied to mineral microfiltrations membranes during apple juice filtration [36] illustrating the advantage of this method to enhance permeability compared to steady flow regime. With carefully chosen pulsations permeate flux increased up to 45% at 1 Hz pulsation frequency. Moreover well defined pulsations decreased the hydraulic power dissipated in the retentate per unit volume by up to 30%. In an other work on cross-flow filtration of plasma from blood [37] permeate flux increase was also observed when pressure and flow pulsations at 1 Hz are superimposed on the retentate. 

It has been suggested in the literature [34] that filtration devices producing Taylor or Dean vortices can help depolarization of the solute build up on membranes. This seems to be an attractive way because of excellent bulk fluid mixing, high wall shear rates and weakly decoupled cross-flow with transmembrane flux. Unfortunately there are some severe limitations on a technical and economical point of view with such devices. Build up and scale up of these modules are expensive with difficulties in repairing and changing membranes. A good compromise between economic and technical constraints has been described by Charpin et al. [39]. It consists in the preparation of mineral (metal 

12 — TRANSPORT AND FOULING PHENOMENA IN UQUID PHASE SEPARATION 

Rotating disc systems have also been described as efficient devices to overcome flux limitation due to matter deposit on membrane surface during cross-flow filtration [40]. The problem of erosion of a macroscopic particle solid deposit on a rotating disc membrane has been quantified by Aubert et al. [41]. The influence of the transmembrane pressure, the thickness of the initial deposit and the pore size on the critical shear stress have been investigated and described by empirical fits. It results from this study that fouling is more efficiently eliminated at high Ap and large pore size. 

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