Vial lyophilization: calculations on rate limitation during primary drying.

Abstract

The aim of this study was to assess the rate limiting factors in the sublimation phase of freeze drying and to propose a simple model on the basis of these rate limitations. A programmable freeze dryer was used. The load consisted of vials of varying size and various contents. To increase heat transfer, conductive paste was applied while the resistance toward mass transport was varied by using different restrictive capillaries. It was found that heat transfer limits the rate of sublimation. Presence of the commonly used excipient mannitol did not have a consequence on the rate of sublimation. The same applied for the restrictions towards mass transport. It was found that there exists not only a barrier against heat transport under the vial, but also between the glass wall and the frozen solution. From the results, a set of equations is proposed that enables to predict optimum sublimation conditions. For the pharmaceutical technologist this can serve as a simple and useful tool to derive a suitable freeze drying program.