Ice Sublimation in Vial Lyophilization

Ice sublimation in vial lyophilization was studied experimentally and theoretically. A theoretical dynamic and two-dimensional axisymmetric model that comprised both the vial and its content was developed. The model included the curvature of the bottom of the vial, the temperature of the ice surface as a function of the chamber pressure, and physical models for the heat of sublimation, thermal conductivity of gases at low pressure, thermal conduction, and thermal radiation. The coefficients in the model were based on physical constants, except for vials on the periphery of the shelf, where an overall heat transfer coefficient derived from experimental data was used to model the transport of heat from the walls of the freeze dryer. The study demonstrated that the sublimation was, apart from the shelf temperature and the chamber pressure, also influenced by the curvature of the bottom of the vial, the position on the shelf, and the atmosphere in the freeze dryer.