A CFD MODEL FOR THE PERFORMANCE PREDICTION OF HOLLOW FIBRE HAEMODIALYSIS MODULES

Objectives: The model proposed aims to predict how geometric, transport and operative parameters affect the performances of hollow-fibre membrane modules for haemodialysis, especially solute clearance. Methods: A two-scale approach was used. Preliminarily, dialysate flow and mass transfer around fibre bundles were simulated at Unit Cell level, i.e. in a single periodic unit of the bundle. For a given porosity, both regular lattices (square or hexagonal) and random fibre arrangements were studied. From the predicted friction coefficients and Sherwood numbers, permeability and solute exchange terms were derived to be used in a porous media model of the whole module. Solute concentrations on the blood and dialysate sides were alternatively simulated in an iterative fashion and were coupled by appropriate sink/source terms describing solute transfer. The model predicted 3-D flow fields and solute concentrations both in the blood and in the dialysate, as well as overall performance parameters such as clearance and pressure drop, for given module configuration, solute species and membrane properties. Results and discussions: At unit cell level, the dialysate side Sherwood number depended weakly on the axial velocity, but increased greatly in the presence of cross flow (up to 2.5 times for transversal velocities of ~1/4th the axial velocity). The presence of cross flow affected little the axial permeability and thus the axial flow rate. At module level, several cylindrical configurations sharing the same membrane area but differing in aspect ratio and inlet/outlet arrangement yielded very different pressure drops but similar values of the clearance. Conclusions: For typical commercial membrane properties, most of the resistance to mass transport is located in the membrane and the flow field has only a minor influence. Improving dialysate-side mass transfer coefficients and flow distribution can have beneficial effects on clearance only if membrane performances are also upgraded.