Date of Award
Doctor of Philosophy
Chemical and Biological Engineering
L. K. Doraiswamy
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 chemical effects or mechanical effects, or to both simultaneously. The chemical effects of ultrasound are attributed 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 when the bubble implodes asymmetrically. Research emphasis in this area attempts to discern the mechanisms behind ultrasound's mechanical effects by selecting a model solid-liquid noncatalytic reacting system in which the chemical effects of ultrasound are negligible. A rigorous kinetic modeling approach is used which allows for reaction in both the liquid and solid phases. After an extensive analysis of the experimental data obtained from the system, it is concluded that the reaction occurs on the solid phase, and that the liquid phase reaction is negligible;Using several investigative techniques, the expected effects of ultrasound were observed, such as the degradative effects on particle size 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 an organic reactant through the ionic lattice of the product layer. In addition, ultrasound also induces supersaturation of solid sodium sulfide in the solvent acetonitrile, increasing the solubility by a factor of 1.4 over the equilibrium saturation concentration. This enhanced solubility is attributed to cavitation which creates localized hot spots containing solvent in a supercritical state. The normally sparingly soluble solid is highly soluble in the solvent when it exists as a supercritical fluid. The increased solubility in these localized area has "memory" and is retained, even after the hot-spot dissipates into the bulk liquid. The use of ultrasound to induce supersaturation has significant applications in the areas of chemical kinetics when the reaction occurs in the liquid film or the bulk liquid phase.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Leigh Christine Hagenson
Hagenson, Leigh Christine, "Sonochemical reactions: mass transfer and kinetic studies of a solid-liquid system " (1997). Retrospective Theses and Dissertations. 11801.