Spinning Disc Reactor for Intensified Polymerisation Processes

It is believed that the reactors of the future will operate under continuous as opposed to the traditional batch mode and will be capable of achieving heat and mass transfer rates higher by orders of magnitude than the conventional batch reactors. More importantly the reactors will provide the ideal fluid dynamics environment for optimising product quality, reducing reaction times and enhancing selectivity.

At the Process Intensification and Innovation Centre (PIIC) in Newcastle University, ongoing research is exploiting the opportunity offered by thin highly sheared and unstable films (Figure 1) in their application to polymerisation processes. The thin films are continuously produced on rotating surface of a Spinning Disc Reactor (SDR) under the action of large centrifugal forces which cause the films to be thrown off the disc at very large velocities. Extensive heat and mass transfer studies have shown that convective heat transfer coefficients as high as 14 kW/m²K can be achieved in the SDR giving overall heat transfer coefficients as high as 5–7 kW/m²K. Such enhanced heat transfer characteristics make the SDR an ideal system for performing highly exothermic reactions as good control of reaction exotherms can be achieved even with the use of higher feed concentrations.

One class of highly exothermic reactions which have been studied in the SDR is polymerisation reactions. Conventional polymer reactor technology include large stirred tanks and unagitated towers in batch or continuous mode. These have serious limitations especially at high conversions. Heat removal combined with poor mixing levels in high viscosity/high polymer concentration mixtures result in poor control of reaction temperature. The occurrence of "hot spots" and "temperature peaking" results in a broadening of the molecular weight distribution (MWD) and hence poor polymer product quality. In practice, the operating temperature and hence the rate of polymerisation also have to be restricted to prevent thermal runaways.

Fig. 1 - View of surface waves on a smooth rotating disc	Fig. 2 - Internally cooled Spinning Disc Reactor

Fig. 3 - Free-radical polymerisation of styrene at 90°C initiated by BPO in batch and SDR

Faster rates of thermally initiated free-radical polymerisation of styrene are achieved on a 360 mm diameter grooved disc at 90°C and rotating at 850 rpm than in a conventional batch reactor. Processing times can be reduced in the SDR by as much as 80 to 100 minutes in one disc pass in the high conversion region (Figure 3). Also, good control of molecular weights and molecular weight distribution (MWD) is possible in the SDR as a result of enhanced heat transfer and mixing levels even at high viscosities of the polymerising system. An EPSRC funded project is currently looking at the fundamental kinetics and mechanistic aspects of styrene polymerisation in the SDR, which is being carried out in collaboration with the Centre for Polymer Science at Sheffield University led by Prof. Ebdon.

Fig. 4 - Step-growth polymerisation of maleic anhydride and ethylene glycol at 200°C in batch and SDR

The rate of condensation polymerisations is controlled by the rate of removal of a small by-product molecule, usually water or alcohol from the polymerising system. The diffusion process is slow in a very viscous melt contained in a large batch reactor. Significant reductions in reaction times are achieved in the Spinning Disc Reactor in the low acid value region for the unsaturated polyesterification reaction between maleic anhydride and ethylene glycol at 200°C and disc speed of 1000 rpm.

At high viscosities (or low acid values), it appears that the diffusion control limitations imposed by the bulk viscous reaction mixture in the batch reactor system are easily overcome in the thin film formed on the rotating disc surface enabling the polymerisation to proceed in the SDR at its inherently faster rate.

Fig. 5 - Photopolymerisation of n-butyl acrylate in SDR

Radiation induced polymerisations have very rapid rates of initiation but the major limitation is that the reaction system has to be in the form of a thin film for efficient penetration of the UV radiation. The thin film characteristics offered by the Spinning Disc Reactor make it an ideal reactor for continuous photo-polymerisation processes. For the UV-initiated free-radical polymerisation of n-butyl acrylate, conversions as high as 90%, with average molecular weights Mw in the range 58,000 to 70,000 and polydispersity indices in the range 1.8-2.0, have been achieved at a residence time of less than 3 seconds on a smooth rotating disc of 200 mm diameter. Interestingly, branching effects, which make bulk polymerisation of acrylates difficult in conventional reactors, were absent in the SDR polymer product. The flow characteristics in the SDR clearly suppress any transfer reaction in the polymerising film and thus result in an entirely linear polymer.

Currently, the merits of polymerising styrene in the SDR by photo-initiation and using solid Lewis acid catalysts for cationic polymerisation are being explored in a joint collaboration with the Green Chemistry Group at York University funded by EPSRC under the programme of Collaboration between Chemists and Chemical Engineers.