An Overview of New PAT Freeze-Drying Methods Based on Shelf Temperature Inlet/Outlet Difference or Chamber/Condenser Pressure Difference: Theory and Practical Use

Background/Objectives: Recently, new methods of monitoring sublimation flow during freeze-drying operations have been proposed. They are based either on measuring the difference between the temperature of the heat transfer liquid at the inlet and outlet of the shelves (ΔT) or the difference between the chamber pressure and the condenser pressure (ΔP). In this article, we briefly explain the two methods and review their main applications. Methods: Multiple pilot or commercial-scale freeze dryers were used. The inlet and outlet shelf temperature or the capacitance pressures of the chamber and condenser were measured. Results: ΔT and ΔP methods can be implemented in most recent freeze dryers to monitor the sublimation flow. Both methods provide very consistent results and are also comparable to Tunable Diode Laser Absorption System (TDLAS) measurements. The methods can be used for different purposes: calculating the heat transfer coefficient (Kv) distribution from the mass flow curve and estimating the average product temperature and the product temperature range. Furthermore, these methods can be used as a measure of success for transferring the process from the lab to the industrial scale, or from one plant to another, or demonstrating the shelf-to-shelf homogeneity. Finally, the ΔT method is able to detect the ice nucleation during the freezing step. Conclusions: The ΔT and ΔP methods are bringing a new, easy-to-implement, cost-effective, and versatile tool to the freeze-drying study toolbox.