ENHANCEMENT OF STRAUSS' SECONDARY FLOW MIXING IN THIN FILM COMMERCIAL REACTORS

Experiments involving both viscous mixing and flow visualization in complex geometries simulating thin film commercial reactors were performed. Highly complex recirculation flows of elastic polymers were reproduced and studied on a TV screen by injecting small volumes of dye (representing process catalyst) into polyvinyl acetate melt, on a blade simulator developed by us recently. High resolution motion pictures taken at 30 frames per second demonstrate that secondary flows, as predicted by Strauss' equations, fail to correct the tendency of the fluid-fluid interface to remain parallel to the velocity streamlines. This problem is further aggravated by the presence of stagnant zones at the center of the vortices. This situation often led to reaction excursions in poorly mixed regions. Specially designed notches were developed to enhance the mixing process, and these increased the interfacial area for mass transfer several fold and also reduced viscous heat dissipation at the gap between the stationary blade and the moving plate. Commercial scale tests with the new blade design corroborate the laboratory findings.

A simplified apparatus has been developed to simulate the secondary flow behavior that occurs in thin film commercial reactors. The extension of Strauss' mathematical model to the case of viscoelastic fluid enabled reactor performance improvement through high speed cinematographic studies using the simulator. The rate of recirculation in the secondary flow was shown to be proportional to the speed of the impeller; however, the center of the secondary flows was noted to be a zone of poor mixing. This was corrected through a special impeller design that destroys stagnation zones in the reactor.