On the use of a micro freeze-dryer for the investigation of the primary drying stage of a freeze-drying process

This paper deals with the use of a small-scale freeze-dryer, where very few vials are loaded (e.g. 19, each 10 mL, or 7, each 20 mL), for freeze-drying cycle investigation. The system has a metallic ring surrounding the batch of vials, in contact with the external ones, and its temperature is manipulated independently from that of the shelf on the basis of the temperature of the product measured by thermocouples in some vials of the batch. The experimental study was carried out using two sucrose solutions (5% and 10% w/w), aiming to verify the homogeneity of the batch. Both product temperature and the weight loss after 6 hours from the onset of the primary drying stage were selected as key parameters. Experiments were carried out according to a 2N design of experiments, with two values of chamber pressure (60 and 90 mTorr) and two values of shelf temperature (-20 and 0°C). Satisfactory results may be obtained by selecting a ring temperature 5°C lower than that of the monitored samples in case of both products investigated. Besides, the system appears to be useful for the estimation of the coefficient of heat transfer to the product (Kv) and of the resistance of the dried cake to vapour flux (Rp), thus enabling the use of mathematical modelling for process design and optimization.