(445g) Spray Drying of Large Particles of Film Forming Materials

We have observed that large spray dried particle product with uniform morphology can be created when using a nozzle atomiser working on the principle of Rayleigh instability of varicose jets. In this paper, we report results from experiments conducted at AkzoNobel Applied Research Group, Wilton, UK, drying dispersions of starch in water in a Niro P-6,3 pilot spray dryer, 1.6m diameter, 7.8m fall height, with co-current gas flow, using both rotary and nozzle atomisers. The starch dispersions used in these experiments form films, rather than porous crusts, as they dry out. This behaviour is key to their industrial use as encapsulation matrices. It was found that, under otherwise identical drying conditions with the nozzle atomiser, by varying the composition of the liquid feed, the maximum mean droplet diameter that could be dried varied from 250µm to 450µm. Our experimental data indicate that for these large droplets containing film-forming polymeric materials, the drying kinetics of the droplets may be rate controlling. This is contrast to the normal expectation of equilibrium limitation in spray drying. A different approach to performance modelling and prediction is therefore required. Ideally, drying kinetic data for a material should be measured at realistic industrial spray dryer operating conditions. An apparatus design is proposed to measure the drying rates of small droplets of aqueous liquids, employing an ultramicrobalance to measure mass loss rate from droplets as small as 500µm diameter, in an air stream which velocity and temperature may be as high as 30m/s and 300°C, with humidity from dry to near saturation. In this paper, we demonstrate that a simple scaling model using distributions of droplet size and residence times can qualitatively explain the experimental observations of increased droplet size and uniform dry particle morphology.