Energy Transfer During Freeze-Drying in Dual-Chamber Cartridges

Freeze-drying essentially requires knowledge about the heat and mass transfer characteristics to assure product quality. Whereas this understanding has been created for freeze-drying in vials, only limited information is available for state-of-the-art multiple compartment container systems such as dual-chamber cartridges (DCCs). Therefore, the aim of this study was to investigate the heat transfer characteristics of this novel container format. Sublimation tests were carried out using pure water at 60, 100, 150, and 200 mTorr chamber pressure at a shelf temperature of 0°C. Custom-made aluminum blocks were used as holder systems. Two heat transfer coefficients could be identified: the coefficient characterizing heat transfer between shelf and block, KAl, and between block and cartridge, KDCC. KAl was dependent on all three modes of heat transfer: contact conduction, gas conduction, and radiation. For KDCC, contact conduction was negligible. Radiation strongly influenced the overall energy transfer as it is the major mode of heat transfer for KDCC and contributes up to 44% to KAl. A third coefficient, Ktot, was defined as an overall heat transfer coefficient. This knowledge about heat transfer enables a purposeful development and control of optimized lyophilization processes for this novel container system.