Calculation of combined diffusive and convective mass transfer.

The clearance of a dialyzer is calculated under the most general conditions, allowing not only for a mixed diffusive and convective mass transfer, but also for a variation along the membrane of the local ultrafiltration, the membrane permeability and the sieving coefficient. The study is then carried on for the case in which these are all constant, to reach a relatively simple expression for the influence of a low ultrafiltration rate on the clearance. In this study, the permeabilities of the boundary layers on both sides are treated as included in the (equivalent) membrane. In an appendix, the stacking of membranes is studied, giving a general law for the calculation of overall permeabilities of a stack of individual membranes, regarded as one (equivalent) membrane (such as a physical membrane with two boundary layers). Permeability data for boundary layers are quoted from earlier works. In other appendices, the variation of the local ultrafiltration along the dialysis path is studied, as well as its effect on the effective permeability of the membrane.