Online conductivity measurement of residence time distribution of thin film flow in the spinning disc reactor

Abstract

Spinning disc reactors provide effective continuous flow mixing in highly sheared liquid films flowing under the action of high centrifugal forces generated as a result of the disc rotation. Although the flow is theoretically characterised to be laminar on the basis of the low Reynolds numbers, the film surface is normally covered with numerous ripples which can change the film hydrodynamics and velocity distribution and thereby affect its residence time distribution (RTD) on the rotating disc. The aim of this study is to determine the experimental conditions for which near plug flow behaviour prevails on the spinning disc. Our findings indicate that the higher the disc speed and liquid flow rate and the lower the viscosity of the processing liquid, the closer conditions within the liquid film on the disc approximate to plug flow behaviour. These effects are attributed to the greater degree of turbulence induced in the liquid film as a result of an increased intensity of surface waves, promoting transverse mixing across the film thickness and thus a more uniform velocity profile at any given radial position. The centrifugal force directing the liquid radially to the disc periphery may also play a part in reducing radial dispersion. The texture of the disc surface is also an influential parameter in achieving plug flow on the rotating disc. With flow on a grooved disc, the number of tanks-in-series increased quite significantly under identical operating conditions as those for a smooth disc, confirming that radial dispersion is lowered. One of the explanations for this effect is that the constantly changing topography enabled by the series of concentric grooves across the disc surface offers the opportunity for repeated film detachment and re-attachment which cause induced recirculation or vortices within the film, giving better transverse mixing and eliminating velocity gradients.