Reactive precipitation of TiO 2 in a spinning disc reactor (SDR) has been performed. Physical parameters such as rotational speed, disc surface texture, and operating parameters such as flowrate, ratio of water to precursor and location of feed introduction points have been studied in terms of their effects on TiO 2 particle size, particle size distribution (PSD) and particle yield. Smaller particles of less than 1 nm mean diameter with narrower PSDs are generally formed at higher yields at higher disc speeds, higher flowrates and on grooved disc surfaces, all of which provide the best hydrodynamic conditions for intense micromixing and near ideal plug flow regime in the fluid film travelling across the disc surface. Similar observations are made for particle characteristics at higher water/TTIP ratios which are attributed to the increased rate of the hydrolysis reaction favouring nucleation over growth. The introduction of the TTIP feed stream into the water stream away from the centre of the disc is also conducive to the generation of smaller and more uniformly sized particles due to the greater energy dissipation for improved micromixing at these locations. Comparisons with reactive-precipitation of TiO 2 in a conventional stirred tank reactor (STR) also demonstrate that the SDR performs better in terms of much improved particle characteristics and higher TiO 2 yields per unit processing time. This is attributed to more uniform and intense mixing conditions in the smaller volume, continuous SDR than in the STR.

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.

The bulk free-radical polymerization of styrene was carried out on a spinning disc reactor (SDR) with prepolymer feeds previously prepared in a stirred batch reactor at 70 C. The SDR significantly enhanced the reaction rate when the conversion of the prepolymer feed was about 50%. There was an optimal disc rotation speed, that is, 1500 rpm, at which the conversion enhancement was maximized. Increasing the disc rotation speed up to 1500 rpm increased the conversion, but a further increase beyond this speed caused the conversion enhancement to decrease. This behavior could be attributed to the dual effect of the disc speed (controlling both the residence time and the effective radial mixing of the reaction media) on the disc. On the other hand, the multipass disc feeding mode was used as an approach to increase the residence time and evolution of the conversion of the reacting mixture in the SDR. Surprisingly, a maximum conversion change of about 40% was achieved for a prepolymer feed of 50% after three passes across the rotating disc. Furthermore, the effect of multipass disc feeding on the molecular characteristics of the SDR products was investigated. In comparison with the results obtained at a feed conversion of 30%, the weightaverage molecular weight, the z-average molecular weight, and the polydispersity index of the three-pass SDR products prepared with a 50% feed conversion at a rotation speed of 1500 rpm were lower than those of the one-pass products. This behavior could be attributed to probable chain scission of long polymer chains as a result of the high shear and elongation forces that arose on the disc surface.

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