(155g) Predicting Spray Dried Product Properties from Single Droplet Drying Experiments

Spray drying is a powerful particle formation technique through which particles with a range of characteristics can be formed in a single process step. Despite the simplicity of the process the scale-up of spray dried products is challenging due to the complexity of the many simultaneous, multi-scale, mechanisms involved in the process. The development process for a spray drying operation can therefore be time consuming and expensive due to the requirement for numerous, large-scale experiments. This is particularly the case in the pharmaceutical industry where raw materials can be expensive. Single droplet drying is a small-scale technique that has the potential to improve the development process by reducing this requirement. This work uses three different single droplet drying techniques (suspended droplet dryer, electodynamic balance and acoustic levitator) to investigate the drying behaviour of aqueous solutions of choline bitartrate, which was chosen as a surrogate organic molecule. The same drying behaviour was observed across all the techniques, even for this potentially complex crystallising system, and even though the initial droplet size varies by up to an order of magnitude. The mechanisms for particle formation and crystallisation were explored using single droplet drying models, to help interpret behaviours seen, and as a guide for optimising particle properties. The choline bitartrate was also spray dried on a small-scale dryer and the powders characterised. X-ray micro-tomography revealed interesting particle structures both on the small droplet scale and in the spray dried product. Overall the characterisation demonstrated that particle morphology was consistent between the scales and the single droplet tests gave valuable mechanistic understanding.