Thermal analysis and freeze drying microscopes have helped improve the understanding of the critical temperature — the temperature at which the product may collapse or melt-back — of the product being freeze dried. This knowledge provides the information required to produce a robust and efficient freeze drying cycle.

Classic freeze drying control is open loop, where the shelf temperature and chamber pressure are controlled based on a predetermined profile. It is assumed that the temperature of the product stays below its critical temperature, and the result is a reproducible — but very conservative and long — freezedrying cycle.

Closed loop control of the shelf temperature is required to both prevent collapse and minimise the length of the freezedrying cycle. The latest control systems use critical temperature information to dynamically control shelf temperature, which both protects against collapse and meltback whilst optimising the freezedrying cycle.

Methods that use an average measurement of the product in the chamber, such as calculated via pressure rise testing, adjust the temperature of the shelves a few times throughout the first half of the primary drying process. This process is limited to the first half of primary drying and only provides a conservative protocol. However, it is not optimised and does not take into account variations inside the chamber.

The latest control systems take into account both average and specific measurements to ensure there is no melt back and constantly control the shelf temperature throughout the entire cycle to produce a userselectable conservative or aggressive protocol.

In the future, more advanced closed loop control systems will be available that offer improved process control. Today, most applicable measuring instruments, such as tunable diode laser absorption spectroscopy, near infrared and mass spectrometry, are expensive and provide only marginal process improvement, which means they are not economically feasible for process control. As instrumentation and techniques advance, they will be incorporated into real-time process control systems.

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