Ultrasonic stimulation on enhancement of air gap membrane distillation

An ultrasonic irradiation technique was applied to an air gap membrane dist illation (AGMD) system to enhance the permeability for the membrane distill ation of various aqueous solutions. In this study, an ultrasonic stimulatio n of resonance frequency of 20 kHz and irradiation power up to 90 W was app lied to a flat-plate AGMD system of 1 mu m PTFE membrane with a membrane di stillation temperature difference up to 55 degrees C. The feed solutions te sted include tap water and various sodium chloride solutions. Permeate mass flow rate, heating and cooling temperatures, irradiation power and acousti c intensity in feed solutions are directly measured. It was found that the permeate flow rate with the ultrasonic stimulation increases up to 25% than that without ultrasonic irradiation The permeate how rate also increases w ith an increase of irradiation power. Experimental investigation of basic m echanisms pertaining to the ultrasonic enhancement of AGMD was also attempt ed via both intermittent ultrasonic irradiation and study of the effect of ultrasonic irradiation on surface temperature of membrane. The results sugg est that, for a continuous ultrasonic stimulation, the domination mechanism s are microstreaming and cavitation. The significant reduction of temperatu re polarization is validated by both temperature measurements and ultrasoni c intensity measurements. (C) 1999 Elsevier Science B.V. All rights reserve d.