Manometric Temperature Measurement (MTM) and SMART Freeze-Drying

MTM is an analytical tool to monitor primary drying using the pressure increase in the freeze-dryer chamber during a short test period. An isolation valve between chamber and condenser is closed rapidly in regular intervals (Figure 1), mostly between 30-120 minutes, for a short time, mostly 25 s. The pressure increase in the chamber is recorded with the capacitance manometer, and fitted using a complex equation (Figure 2). The fit function allows the calculation of the vapour pressure of ice and thereby the product temperature at the sublimation interface as well as product resistance and other derivative parameters.

The derivative parameters calculated from the fitting curve are generally correct until 2/3 of the ice have sublimed. Pressure of ice at the sublimation front can be used as an endpoint indicator of primary drying. Product temperatures are more representative for center vials with less radiation effects, improvement of results is possible by using thermal shields.

The results of each MTM measurement can be used for process optimization, i.e. adjustment of product temperature to a target temperature slightly below the collapse or eutectic temperature by changes of shelf temperature and chamber pressure. Changes in process parameters such as increasing product resistance can be taken into account and compensated. This way, the development of an optimized freeze-drying cycle is possible within one run. This principle is marketed in form of the SMART^TM Freeze Dryer by FTS Systems / SP Industries.

Figure 1: MTM apparatus schematic

Figure 2: MTM curve fit

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