In vitro study of combined convection- diffusion mass transfer in hemodialysers

Clearance measurements have been obtained for urea, Vit B12 and myoglobin by simultaneous hemodialysis and ultrafiltration (UF) techniques, using various commercially available hemodialysers. The experimental results confirm that the overall clearance is less than the clearance of each process occurring separately due to interaction between convection and diffusion. Adequate urea clearance (100 ml/min) can be obtained either by using high ultrafiltration (60 ml/min) with low dialysate flow rate (70 ml/min) or moderate ultrafiltration (less than 30 ml/min) with higher dialysate flow rate (120 ml/min). The comparison of experimental clearance with the predictions of theoretical model (1) for urea are found to be in good agreement for dialysate flow rates of more than 300 ml/min and for smaller dialysate flow rates the predicted clearances are higher than measured ones. An approximate correlation for the overall clearance is CL = CLD + QF/2 where CLD is the dialytic clearance and QF the UF flow rate.