|M.Sc Thesis||Department of Chemical Engineering|
|Supervisors:||Prof. Emeritus Nir Avinoam|
|Prof. Grader Gideon|
Most of the chemical processes that are being conducted in industry would not be available without proper usage of catalysts. Many of the catalysts that are being used in industrial processes can be classified as heterogeneous catalysts.
One of the factors that limit the catalyst performance is a concentration gradient between the bulk and the interior or the solid catalyst. This gradient occurs when the mass transport into the catalyst is slower then the reaction rate, thus creating a shortage of reactant in the internal pores. Such are most cases when the reaction is in the liquid phase.
Much effort has been directed in an attempt to improve this mass transport problem to improve the effectiveness of the catalysts. It has been found that by inducing an external field (e.g. increasing the flow rate) an interior convection can be achieved. This convection combined with the ordinary diffusion increases the mass transport in the catalysts, thus reducing the concentration gradient.
The aim of this work was to examine whether the usage of an external ultrasonic field can create internal convection, thereby influencing the internal mass transfer.
In a series of tests that were conducted on the depolymerization of Paraldehyde - - to Acetaldehyde - - on NiSO4 site supported on alumina it was found that the ultrasonic field does not enhance the performance of the catalysts, but instead causes the reaction to come to a complete halt as soon as the ultrasonic field is switched on.
After the catalysts were examined by SEM and X-ray, it was found that the catalytic pallet was physically broken and that places that were initially coated with the active agent were removed. Moreover, the X-ray analysis showed that the coating agent of the pellet was converted from NiSO4 (the active initial agent), into NiO, which is inactive.
It was concluded that the shock wave induced by the collapse of the ultrasonic cavitation bubbles causes the physical breakdown of the support, and cleans it from the active agent. Furthermore, the high temperature that develops locally during the cavitation process causes the transition from NiSO4 to NiO.