A parametric CFD study of hollow fiber membrane modules for hemodialysis

Hemodialysis is a membrane-based process in which solute transport from the patient’s blood to a rinsing solution (dialysate) occurs by diffusion and ultrafiltration. Devices used in hemodialysis are cylindrical modules filled with hollow-fiber membranes which allow the removal of toxic substances and metabolic wastes from the blood, but inhibit the passage of proteins and cells to the dialysate. A predictive porous-media model of hemodialysis was developed and validated against experimental data. Unlike previous literature models, it requires only basic membrane properties (hydraulic and diffusive permeabilities and reflection coefficients) instead of relying on empirically adjusted global mass transfer coefficients. The necessary porous-media characteristics, notably Darcy permeabilities and shell-side mass transfer coefficients, were obtained by combining theoretical results, CFD predictions for regular fiber arrays and experimental data for commercial modules. A parametric analysis was conducted to assess the influence of different physical quantities and operating conditions. Simulation results for different solutes showed that clearance is affected, in decreasing order of significance, by the membrane’s diffusive permeability (kM), the dialysate flow rate (QD) and the ultrafiltration flow rate (QUF). Doubling kM yields an enhancement in clearance of ~7% for urea and ~20% for B12 vitamin, while halving kM yields a decrease of ~14% for urea and ~32% for B12 vitamin. Changes in the flow rates affect clearance to a lesser extent: a 50% increase of QD yields a clearance increase of ~4−5%, while a ±10 mL/min variation of QUF with respect to the reference value of 10 mL/min leads to a clearance change of ±1.3% for urea and ±3.2% for B12 vitamin. The oncotic pressure in blood has almost no influence. Therefore, possible performance improvements, at least in terms of clearance, strictly rely on the development of a novel generation of membranes characterized by a significantly higher solute diffusive permeability.