Enhancement of mass transfer in spacer-filled channels under laminar regime by pulsatile flow

Improving mass transfer in membrane modules is always desirable, but not easily achieved under laminar regime. Pulsatile flow induced by a solenoid valve was investigated as a technique to improve mass transfer in membrane modules with spacer-filled channels. Mass-transfer coefficients were measured by the limiting current technique in open and spacer-filled narrow rectangular channels, both for steady-state flow and pulsatile flow, at Reynolds numbers ranging from 10 to 50 (based on the channel height). The test cell had a narrow rectangular channel (170mm×15mm×2mm) and 8 consecutive nickel electrodes (11.2mm long and 10mm wide) fitted in the top wall, with and without spacer inside the flow channel. The pulsatile flow followed a step function with three different frequencies (1, 10 and 50Hz) and two different duty cycles (fraction of time during which the valve is closed in each pulse) of 25% and 50%. The results show that pulsatile flow generated with solenoid valves with frequencies between 10Hz and 50Hz can increase up to 50% the mass-transfer coefficient in membranes modules with spacer-filled channels, operating in laminar flow. The effect is particularly intense near the channel outlet, close to the solenoid valve, but vanishes for large distances from the channel outlet. •Step pulsatile flow in laminar regime is investigated for open and spacer-filled rectangular channels.•Pulsatile flow generated with solenoid valves with frequencies of 10Hz and 50Hz can increase wall mass transfer by 50%.•Mass-transfer coefficient increases with increasing pulse frequency and Reynolds number.•The enhancement effects are intense near the channel outlet, close to the solenoid valve.•The mass-transfer enhancement effect fades away from the pulsatile source.