Single droplet drying at high temperatures

Particle formation in spray drying is a complex phenomenon; understanding and predicting the particle structures formed requires a deep understanding of the link between drying kinetics, morphology evolution and key material properties. This is further complicated when drying at high temperatures above the boiling point of the droplet due to boiling driven effects, including internal bubble expansion which can influence morphology evolution and thus results in diverse morphologies. This work investigates the effect of boiling on the drying rate, morphology evolution and the structure of final dried particles. The novelty in the methodology of this work involved the development a single droplet drying apparatus where the mass can be measured directly using a microbalance with a high resolution (six digits to a gram) combined with the measurements of the droplet size and temperature. The single droplet drying apparatus was validated by drying of pure water droplets. This apparatus is then utilised to measure drying kinetics and monitor the morphology evolution of droplets of aqueous solutions of sucrose, sodium silicate and hydroxypropyl methylcellulose (HPMC), as well as HPMC-acetone-water solutions. To study the effect of boiling on the morphology evolution, drying kinetics and final structures, single droplet drying experiments are conducted at air temperatures in the range of 60-180 ̊C and across a range of initial solute concentrations.
The systematic study on the three different systems (sucrose, silicate and HPMC) enhanced the scope of the work in this thesis and allowed for a comprehensive investigation of drying above the boiling point. The novel elements of this work include the consideration of a film-forming material, HPMC, which interestingly upon drying results in the formation of a skin layer and a phase change which dominates the drying behaviour and suppresses the effects of boiling and bubble nucleation. The properties of the skin formed are altered when drying HPMC-acetone-water system. The use of water/organic solvent mixture to manipulate the skin of the particle and the final morphology at a high range of temperatures is studied in this work for the first time to the best of our knowledge.
The effect of boiling on the morphology evolution and drying rate are also reported and related to the evolution of rheological and mechanical properties. Drying of sucrose solution droplets results in inflation/deflation cycles, which enhances the evaporation rate. Sucrose droplets exhibit liquid-like behaviour during boiling and thus the droplets collapse at the end of the inflation/deflation cycles. On the other hand, the drying of aqueous solutions of sodium silicate results in inflation and less degree of collapse. As a result, “puffed” dried particles are produced. The differences in behaviour above boiling are further investigated by analysis of drying kinetics such as droplet diameter, droplet temperature and evaporation rate. The structures obtained from the single droplet drying experiments are also compared with laboratory spray drying in the case of drying of aqueous solutions of HPMC or silicate.