Evaluation of immersion-contact type heat transfer for continuous pharmaceutical spin freeze-drying process

Spin freeze-drying process is a recently proposed method to achieve continuous processing of unit doses in freeze-drying application. An experimental study is carried out to investigate the heat transfer and drying characteristics of a spin-frozen liquid product. To enhance heat transfer, unlike freeze drying performed in shelves, the shell is immersed into a temperature-controlled bath. The influence of different ramping condition of bath temperature on drying rate, time and variation of moisture content in spinfrozen products (deionized water and skimmed milk) are analyzed and reported in this paper. Among different ramping conditions, ramping of bath temperature to 10 degrees C resulted in higher drying rate (1.8 kg hr(-1) m(-2)) and lesser drying time (44%). The overall heat transfer coefficient (K-v) between the bath and product is evaluated for the above condition in both the test liquids. This direct contact type heat transfer helps in reducing air gap resistance and radiation effect, which primarily helps in minimizing thermal resistances. The effect of dry layer resistance is minimal in the spin-frozen product due to lower product thickness and larger ice-vapor surface area.