Concentration polarization hydraulic boundary layer

An effect not considered in the above models is the added resistance, caused by fouling, to solute back-diffusion from the boundary layer. Fouling thus increases concentration polarization effects and raises the osmotic pressure of the feed adjacent to the membrane surface, so reducing the driving force for permeation. This factor was explored experimentally by Sheppard and Thomas (31) by covering reverse osmosis membranes with uniform, permeable plastic films. These authors also developed a predictive model to correlate their results. Carter et al. (32) have studied the concentration polarization caused by the build-up of rust fouling layers on reverse osmosis membranes but assumed (and confirmed by experiment) that the rust layer had negligible hydraulic resistance. 

Reverse osmosis is a cross-flow process and, as in any dynamic hydraulic process, the fluid adjacent to the membrane moves slower than the main stream. While the main stream flow may be turbulent, the layer next to the membrane surface is laminar. This thin, laminar flow film is called the boundary layer. When water permeates through the membrane, nearly all of the salt remains behind in the boundary layer next to the membrane. The salt must then diffuse across the boundary layer and back into the bulk stream. This results in a boundary layer with a salt concentration which is more concentrated than the bulk stream. The effect has been termed concentration polarization, and it is defined by the following equation ... 

To avoid concentration polarization, an improved mass transfer should be realized in the feed compartment. Determining parameters are feed flow velocity (modified through the hydraulic diameter of the feed cell or the pump characteristics), solute diffusion (changed via the feed temperature), feed viscosity (idem), shape and dimensions of the module (introduction of turbulence promoters, use of pulsating flows to break the boundary layer, increased Reynolds numbers,...). 

In a membrane filtration process, the retentate in suspension may build up a high concentration adjacent to the membrane surface forming a dyamic boundary layer (a gel layer). This concentration gradient becomes a driving force to pull the retentate from the boundary layer back to the bulk flow. This phenomenon is referred to as concentration polarization. The accumulation of retentate at the membrane surface will result in a hydraulic resistance that may reduce the permeability of the membrane. This phenomenon is called fouling. Membrane fouling is a common phenomenon observed in the operation of any membrane filtration process, which leads to a reduction in permeate flux and selectivity. 

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