Mass transfer filtration

Heat and mass transfer Filtration Distillation Mixing Separation Fluidization Sedimentation Reaction Polymerization Drying Forming Ventilation Emission control Incineration Combustion Materials processing... 

The porous medium, namely the P.S.Z., possesses a certain filtration capacity to filter out the solute. This corresponds to the rate of forward mass transfer across the P.S.Z. in zone refining. 

In the situation where the effect of filtration - that is, water movement across the membrane due to the difference in hydrostatic pressure and/or osmolarity - can be neglected, the overall resistance for mass transfer in hemodialyzers with flat membranes is given as... 

The effectiveness factor is very low, indicating that intraparticle mass transfer resistance is very significant. The gas-liquid mass transfer resistance is also important, as expected. On the other hand, the liquid-solid mass transfer resistance is negligible. As a result, the rate of reaction in the slurry reactor is about 50 times higher than that in the trickle-bed. Therefore, in cases of such high rates of reaction, the slurry reactor is a better choice, although the gas-liquid mass transfer and the filtration of the catalyst may be a problem. 

Mechanical forces can disturb the elaborate structure of the enzyme molecules to such a degree that de-activation can occur. The forces associated with flowing fluids, liquid films and interfaces can all cause de-activation. The rate of denaturation is a function both of intensity and of exposure time to the flow regime. Some enzymes show an ability to recover from such treatment. It should be noted that other enzymes are sensitive to shear stress and not to shear rate. This characteristic mechanical fragility of enzymes may impose limits on the fluid forces which can be tolerated in enzyme reactors. This applies when stirring is used to increase mass transfer rates of substrate, or in membrane filtration systems where increasing flux through a membrane can be accompanied by increased fluid shear at the surface of the membrane and within membrane pores. Another mechanical force, surface... 

Membrane reactors - no mass transfer limitations - pyrogen-free products - scale-up simple - pre-filtration required - no polymeric products possible - no product precipitation possible... 

Mass-transport limitations are common to all processes involving mass transfer at interfaces, and membranes are not an exception. This problem can be extremely important both for situations where the transport of solvent through the membrane is faster and preferential when compared with the transport of solute(s) - which happens with membrane filtration processes such as microfiltration and ultrafiltration - as well as with processes where the flux of solute(s) is preferential, as happens in organophilic pervaporation. In the first case, the concentration of solute builds up near the membrane interface, while in the second case a depletion of solute occurs. In both situations the performance of the system is affected negatively (1) solute accumulation leads, ultimately, to a loss of selectivity for solute rejection, promotes conditions for membrane fouling and local increase of osmotic pressure difference, which impacts on solvent flux (2) solute depletion at the membrane surface diminishes the driving force for solute transport, which impacts on solute flux and, ultimately, on the overall process selectivity towards the transport of that specific solute. 

The following processes can be described as selective therapeutic plasmapheresis. In a first step, blood is withdrawn from the patient and separated by crossflow filtration in a hollow-fiber membrane cartridge water and some plasma solutes are transferred through a semipermeable membrane under a convection process. The transmembrane pressure applied from blood to filtrate compartment ensures flow and mass transfers. Then, the filtrate perfuses the adsorption columns where toxins are retained and is finally mixed with blood cells and other plasma components before returning to the patient (Figure 18.11). 

A comparison of the duration of the impregnation by the traditional method and by the procedure here proposed is favourable to the latter. This is related to mass transfer easier in a supercritical medium than in liquid phase because of higher diffiisivity of the species. The advantage of the supercritical method is all the more marked since further steps of filtration and elimination of the solvent are necessary in the classical technique. 

---