Freeze during primary drying stage

Before concentrating on process parameters (Section III), we will briefly outline the behavior of the product during the three separate but interdependent stages of freeze-drying freezing, sublimation (primary drying), and desorption (secondary drying). 

The structural stability of a material relates to its ability to go through the freeze drying process without change in size, porous structure, and shape. The maximum allowable temperature in the frozen layer is determined by both structural stability and product stability (e.g., product bioactivity) factors that is, the maximum value of the tanperature in the frozen layer during the primary drying stage must be such that the drying process is conducted without loss of product property (e.g., bioactivity) and structural stability. 

Fig. 4.6 Example of results obtained during a freeze-drying cycle using the special balance with the embedded wireless temperature measurement in order to monitor the primary drying stage of a mannitol-dextran solution (6-14% by weight) in a pilot-scale freeze-dryer. The freezing stage was run at 223 K for about 5 h, while the main drying was carried out setting the fluid temperature at 293 l< and the chamber pressure at 10 Pa (Nv = 98 on tray, dv= 14.2 X 10 m, Lp = 7.2 x 10 m).

Fig. 5.1 Sketch of a freeze-drying process during the primary drying stage.

---