A comparison of crystallisation approaches in spray drying

Simulations of crystallisation behaviour in a spray dryer have been performed using modifications to a model developed by previous workers and applied to lactose produced by a Buchi-290 laboratory scale dryer. Crystal growth behaviour has been modeled using Williams–Landel–Ferry (WLF) kinetics and also using lactose crystallisation kinetics from solution, with the assumption that no further crystal growth occurs when the material temperature is less than or equal to the glass-transition temperature. Explorations using all the approaches have suggested that the air inlet temperature is an important variable affecting the crystallinity of the product. This is confirmed by experimental evidence showing that the apparent degree of crystallinity for lactose is affected by the inlet air temperature over the range from 134 °C (∼55% crystalline) to 210 °C (∼76% crystalline). Comparing the trends predicted by the different approaches with experimental data from the literature suggests that WLF kinetics are most successful in predicting all the data, suggesting that the amorphous to crystalline transition cannot be neglected relative to the initial crystal growth from solution inside the droplets.