Limiting conditions, permeate flow

The preceding chapter shows that, for the limiting conditions of countercurrent flow with no reject produced, the less-permeable component hypothetically could concentrate in the far end of the reject side of the cell. Similarly, with no net permeate produced and compression from the permeate side to the reject side, it is theoretically conceivable for the more-permeable component to concentrate in the far end of the permeate side of the cell. 

In the case of dense membranes, where only hydrogen can permeate (permselectivity for H2 is infinite), the permeation rate is generally much lower than the reaction rate (especially when a fixed bed is added to the membrane). Experimental conditions and/or a reactor design which diminishes this gap will have positive effects on the yield. An increase of the sweep gas flow rate (increase of the driving force for H2 permeation) leads to an increase in conversion and, if low reactant flow rates are used (to limit the H2 production), conversions of up to 100% can be predicted [55]. These models of dense membrane reactors explain why large membrane surfaces are needed and why research is directed towards decreasing the thickness of Pd membranes (subsection 9.3.2.2.A.a). 

Boundary layer formulation. Many membrane processes are operated in cross-flow mode, in which the pressurised process feed is circulated at high velocity parallel to the surface of the membrane, thus limiting the accumulation of solutes (or particles) on the membrane surface to a layer which is thin compared to the height of the filtration module [2]. The decline in permeate flux due to the hydraulic resistance of this concentrated layer can thus be limited. A boundary layer formulation of the convective diffusion equation can give predictions for concentration polarisation in cross-flow filtration and, therefore, predict the flux for different operating conditions. Interparticle force calculations are used in two ways in this approach. Firstly, they allow the direct calculation of the osmotic pressure at the membrane. This removes the need for difficult and extensive experi-... 

PS telechelics formed from AIBN show an end group functionality of approximatedly /=2. Moad et al. demonstrated by NMR that primary radical termination plays an important role in the formation of functional PS oligomers. The elegant gel permeation chromatography (GPC) analysis of the oligomers by Heitz and co-workers demonstrated the advantages of a flow reactor to prepare telechelics. It appears that a limited conversion of monomer (70-80%) under dead end conditions is essential to successfully prepare PS telechelics from azo initiators. ... 

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