Permeability from suspension flow

Figure 15. Permeability damage due to flow of suspensions in linear and radial cores. (Reproduced with permission from reference 17. Copyright 1991.)...

Precoat filtration can be incorporated within a wide range of pressure filters including leaf, multi-element and plate and frame types (see Section 1.4.2). Up to 700 g m of precoat is typically filtered onto the filter medium prior to introduction of the feed suspension. The feed, which may also contain a significant addition of filter aid to improve cake permeability, is filtered until the filtrate flow rate is sufficiently low to warrant cake discharge in the normal way. It is not economical to recover the feed solids from the precoat, and it follows that washing of the solids is not practised. Moreover, the filter aid tends to abrade the pumps used to promote the filtration. Precoat pressure filtration is most often used for the removal of finer particles from dilute suspension where other potential processes would require too high an investment. 

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. 

One commercially available ultrafiltration membrane is claimed to have a permeability of 0.62 m /m day under a pressure difference of 3.4 atm. This membrane initially rejects 96% of a 3 wt% suspension of partially hydrolyzed starch (molecular weight 17,000). However, if 4.2 cm of membrane separates 65 cm of a starch solution from the same volume of pure water, the volumetric flow is zero, and the osmotic difference is 85% of the original value in one week. Assuming the temperature is 25 °C, find the permeability ip, the solute permeabilty m, and the reflection coefficient [Pg.547]

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