Process intensification : spinning disc reactor for the polymerisation of styrene

Abstract

This investigation is concerned with the assessment of the performance of a novel spinning disc reactor (SDR) for the polymerisation of chemically initiated freeradical polymerisation of styrene. The application of high acceleration fields such as those created on the surface of the grooved rotating disc to the polymerising system is aimed at intensifying the polymerisation rate and producing a better quality polymer product. As part of the experimental programme, four separate sets of experimental runs were conducted on a 360 mm diameter grooved rotating disc at a fixed temperature of 88-90°C to explore the effects of disc rotational speed and prepolymer feed conversion/viscosity on the extent of monomer conversion and molecular weight properties (M., MW and MWD) of the product from the SDR. The performance data of the SDR was compared with conventional batch polymerisation data. Both the disc rotational speed and prepolymer feed conversion/viscosity variables were found to have a profound influence on the performance of the SDR. A steady increase in conversion, rate of polymerisation and hence time saving in one pass in the SDR were observed with a rise in the prepolymer feed conversion and rotational speed until, for the latter, an optimal speed of rotation which gave the highest rate of polymerisation was reached. The results have been explained in relation to the effect of disc speed and prepolymer feed viscosity on mean film thickness, mean residence time and film surface instabilities. Furthermore, the SDR product is seen to have generally improved characteristics in terms of narrower molecular weight distribution when compared to polymer prepared in the batch at the same conversion. The large enhancement of the rate of styrene polymerisation in the SDR was discussed in terms of a possible improvement in the BPO initiator efficiency f and non-stationary state polymerisation conditions likely to be prevalent on the rotating disc. The general improvement in SDR product quality was ascribed to the combined effects of a reduced diffusion path length and an intense mixing mechanism within the thin film. A separate experimental study exploring the effects of micromixing efficiency on the conversion and molecular weight properties of styrene polymerisation in the batch was also undertaken. The opposing effects of enhanced micromixing in batch and continuous polymerisation systems were contrasted in a theoretical manner. A theoretical case study highlighting the energy efficiency of the SDR was also carried out. Savings in energy of more than 70% was calculated for a semi-batch process using an industrially adapted spinning disc reactor in comparison to a purely batch process. Finally, a two-stage continuous industrial process for free-radical polymerisation has been proposed consisting of an enhanced tubular reactor in the first stage followed by a parallel arrangement of several rotating disc surfaces. Improvements in intrinsic safety and minimised risks of polymer degradation and thermal runaways are the expected potential benefits. Keywords: Process Intensification, Thin Film, Spinning Disc Reactor, Free Radical Polymerisation, Polystyrene