An Investigation of heat and mass transfer characteristics of spin freezing and drying process

Abstract

Freeze drying consists of two stages namely, freezing and drying. During the freezing process, the product is cooled below the eutectic point at atmospheric pressure, and the frozen water content is sublimated and desorbed under vacuum during the drying process. In a conventional freeze dryer, the products are usually filled in a vial or shell and mounted above a shelf heat exchanger. The quality of the freeze-dried product depends on uniformity in the temperature of the product on the vial or shell during the entire process. Even though enormous literature is available for freeze-drying concerning the product and the process, a very few concerning the design aspects of the freeze dryer machine for its effective heat transfer is still very much open for new research. It is needless to mention that holding time results in power consumption and time to hold depends on the type of shelf heat exchanger. Further, the time for a complete freeze-drying cycle depends on the magnitude of heat and mass transfer coefficient. newlineIn the present study, freeze drying of liquid products in a vial and shell that is completely immersed in a low-temperature bath and rotated during the freezing stage is considered. During rotation of liquids, the liquid layers slide on the inner surface of the vial like thin film coating. Such liquid ow pattern on the inner surface is called rimming ow patterns, which depend on the speed of rotation and fill volume. Unlike high speed centrifuging of liquids, in the present study, the vial is rotated at low speed. Since the rimming ow patterns depend on parameters like the aspect ratio of the cylinder, fill volume and speed of rotation, it is proposed that the resulting ice patterns should also depend on the above parameters. Subsequently, spin freezing results in an enhanced ice-vapour surface area for sublimation, which is approximately the inner circumferential surface area of the vial.