Comparison of the effects of ultrasound and mechanical agitation on a reacting solid-liquid system

Ultrasound has been shown to have desirable effects on both homogeneous and heterogeneous reactions, such as increasing the conversion, enhancing the selectivity, and improving the yield. Enhancements due to ultrasound may be attributed to its chemical or mechanical effects, or to both simultaneously. The chemical effects of ultrasound are due to the implosion of microbubbles, generating free-radicals with a great propensity for reaction. Mechanical effects are caused by shock waves formed during symmetric cavitation, or by microjets formed during asymmetric cavitation. Research emphasis in this area has largely been restricted to the chemical effects of ultrasound and physical descriptions of cavitation. The present study is among a very few to attempt a chemical 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 techniques, 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 ultrasound enhances the intrinsic mass transfer coefficient as well as the effective diffusivity of the organic reactant through the ionic lattice of the product layer. Discerning the effects of ultrasound on mass transfer parameters, as done in this paper, is an important step towards understanding the effects of ultrasound and determining its applications in the chemical industry. (C) 1997 Elsevier Science Ltd.