In-situ crystallization of particles in a counter-current spray dryer

- Counter current spray dryer provides higher flexibility for in-situ crystallization.
- In situ crystallization relies on the “wet time” and “residence time” of particle.
- Droplet size and spray rate significantly control crystallization process.
- Thermophysical properties of sugars like solubility play key role in crystallization.

This paper evaluated the potential of counter current spray drying to produce engineered crystalline particles in-situ. One main feature of the counter current spray dryer is in inducing droplet drying history with a progressively increasing drying rate; in contrast to a co-current spray dryer in which the particle is immediately confronted with high driving force of evaporation. In addition, counter current dryer provides higher residence time for particles within the drying chamber. This work explored the manipulation of these unique features of counter current spray drying to control the crystallinity of the particles formed. Sucrose, lactose and mannitol were spray dried as model materials exhibit contrasting crystallization. Counter current spray drying was suitable in producing well defined crystalline lactose particles mainly due to the higher residence time. The produced lactose particles were composed of agglomeration of fine thin lactose crystals. Surprisingly, the counter current spray drying with relatively lower initial evaporation rate resulted in amorphous mannitol particles. Fully crystalline mannitol was produced when the feed spray temperature was elevated leading to rapid supersaturation. This may be an important strategy to control the crystallinity of mannitol particles particularly for pharmaceutical application. Similar feed pre-heating strategy in conjunction with counter current spray drying was used for sucrose drying. Crystalline sucrose was achieved with relatively lower drying temperature. This will be a valuable strategy for producing free flowing sucrose particles specifically for the food industry.

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