COMPARISON OF THE EFFECTS OF ULTRASOUND AND MECHANICAL AGITATION ON AREACTING SOLID-LIQUID SYSTEM

Ultrasound has been shown to have desirable effects on both homogeneou s and heterogeneous reactions, such as increasing the conversion, enha ncing the selectivity, and improving the yield. Enhancements due to ul trasound may be attributed to its chemical or mechanical effects, or t o both simultaneously. The chemical effects of ultrasound are due to t he implosion of microbubbles, generating free-radicals with a great pr opensity for reaction. Mechanical effects are caused by shock waves fo rmed during symmetric cavitation, or by microjets formed during asymme tric cavitation. Research emphasis in this area has largely been restr icted to the chemical effects of ultrasound and physical descriptions of cavitation. The present study is among a very few to attempt a chem ical engineering analysis of the problem. More specifically, it seeks to discern the mechanisms behind the mechanical effects by selecting a model solid-liquid noncatalytic reacting system in which the chemical effects of ultrasound are negligible. Using several investigative tec hniques, the expected effects of ultrasound are observed, such as the degradation of the solid reactant, leading to increased surface area. More importantly, some novel findings of the effects of ultrasound on mass transfer parameters are reported. Results clearly show that ultra sound enhances the intrinsic mass transfer coefficient as well as the effective diffusivity of the organic reactant through the ionic lattic e of the product layer. Discerning the effects of ultrasound on mass t ransfer parameters, as done in this paper, is an important step toward s understanding the effects of ultrasound and determining its applicat ions in the chemical industry. (C) 1997 Elsevier Science Ltd.