Ultrafiltration hydrodynamic resistance

Wijmans J.G., Nakao S., Van Den Berg J.W.A., Troelstra F.R. F.R., Smolders C.A. (1985), Hydrodynamic resistance of concentration polarization boundary layers in ultrafiltration. Journal of Membrane Science, 22, 117-135. 

Ultrafiltration is a membrane process whose nature lies between nanofiltration and microfiltration. The pore sizes of the membranes used range from 0.05 um (on the microfiltration side) to 1 am (on the nanofiltration side). Ultrafiltration is typically used to retain macromolecules and colloids from a solution, the lower limit being solutes with molecular weights of a few thousand Daltons. Ultrafiltration and microfiltration membranes can both be considered as porous membranes where rejection is determined mainly by the size and shape of the solutes relative to the pore size in the membrane and where the transport of solvent is directly proportional to the applied pressure. Such convective solvent flow through a porous membrane can be described by the Kozeny-Carman equation (see eq. VI - 27) for example. In fact both microfiltration and ultrafiltration involve similar membrane processes based on the same separation principle. However, an important difference is that ultrafiltration membranes have an asymmetric structure with a much denser toplayer (smaller pore size and lower surface porosity) and consequently a much higher hydrodynamic resistance. 

Tubular Modules. Tubular modules are generally limited to ultrafiltration appHcations, for which the benefit of resistance to membrane fouling because of good fluid hydrodynamics overcomes the problem of their high capital cost. Typically, the tubes consist of a porous paper or fiber glass support with the membrane formed on the inside of the tubes, as shown in Figure 24. 

There are two main issues in an ultrafiltration process productivity and selectivity. Productivity is quantified in terms of the permeate flux, this being defined as the permeation rate per unit membrane surface area. Factors that affect permeate flux are solute type, solute concentration, membrane type, solution pH, solution ionic strength, apphed pressure (also called the transmembrane pressure), and the hydrodynamic conditions on the feed side. The volumetric permeate flux, which is the volume of permeate collected per unit time per unit membrane area is given by the following generalized equation, based on a resistance model ... 

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