The effect of high frequency ultrasound on diffusion boundary layer resistance in ion-exchange membrane transport

Boundary layer diffusion resistance in ion-exchange membrane increases the total system resistance and reduces the power output from electro dialysis process. In this research, the influence of ultrasound wave on diffusion boundary layer and electrochemical membrane properties for a cationic and an anionic exchange membrane is investigated. High frequency (1.7 MHz) piezoelectric transducers were used to induce ultrasound waves in the medium. Mixing was accomplished using a magnetic stirrer, ultrasonic and a combination of ultrasound and magnetic stirrer. The results showed that the membrane potential, transport number and selectivity were decreased with increase in electrolyte concentration in the anionic membrane. In cationic membrane, all mentioned properties were initially increased and then decreased with increase in the solution concentration. The SEM characterization of the membranes indicated that the cationic membrane has a smoother surface that led to its higher selectivity and membrane resistance compared with those of the anionic membrane. Moreover. In order to explain the observed experimental results, the CFD modeling technique has been used to illustrate flow pattern and acoustic streams propagation in the cell and shows when only the ultrasound transducer activated the established velocity near the membrane surface is twice that of stirrer activated case. (C) 2011 Elsevier B.V. All rights reserved.