Optimisation of solute transport in dialysers using a three-dimensional finite volume model

Dialyser manufacturers only provide limited information about mass removal under well-defined flow and solute conditions in commercially available dialysers for hemodialysis. This computational study aimed at assessing the solute transport efficiency in a dialyser for different geometries (fiber lengths and diameters). A three-dimensional finite volume model of a single fiber in a high flux polysulphone dialyser (Fresenius F60) was developed. Different equations describe blood and dialysate flow (Navier-Stokes), radial filtration flow (Darcy) and solute transport (convection-diffusion). Fluid and membrane properties were derived from in vitro and in vivo tests as well as from literature data. Urea (MW60) was used as marker to simulate small molecule removal, while middle molecule transport was modelled using vitamin B12 (MW1355) and inulin (MW5200). Keeping the fluid velocity in a single fiber constant, fiber diameter and length were changed in a wide range for evaluation of solute removal efficiency. Clearances were found enhanced by 13% (urea), 50% (vitamin B12) and 89% (inulin) for a fiber twice as long as a standard one and by 5.5% (vitamin B12) and 21% (inulin) for a fiber diameter of 150 mum instead of 200 mum. The impact of fiber dimensions was more pronounced for the middle molecules compared to urea.