Mass Transfer in Hemodialyzers cf

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 

Many membrane materials have been developed and are used for hemodialyzers. Today, these include regenerated cellulose, cellulose acetate, polyacrylonitrile, poly(methylmethacrylate), vinyl alcohol-ethylene copolymer, polysulfone, polyamide, and others. 

The relative magnitudes of the three terms on the right-hand side of Equation 15.25 vary with the diffusing substance, the flow conditions of both fluids, and especially with the membrane material and thickness. With the hollow-fiber-type hemodialyzers that are widely used today, membrane resistance usually takes a substantial fraction of the total resistance, and the fraction increases with increasing molecular weight of the diffusing component. 

One widely used performance index of hemodialyzers is that of clearance, defined similarly to that of the human kidney. The clearance of a hemodialyzer is the conceptual volume of blood (cm inin ) from which a uremic substance is completely removed by hemodialysis. Let Qg (cm min ) be the blood flow rate through the dialyzer, Qjj fern min ) the dialysate flow rate, and Cg and Cj3 (mgem ) the concentrations of a uremic substance in the blood and the dialysate, respectively, with the subscripts i and o indicating values at the inlet and outlet, respectively. The rate of transfer of the substance in the dialyzer w (mgmin ) is then given as 

Another index of hemodialyzer performance is the dialysance, (cm min ), defined as 

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