Primary-drying monitoring

Figure 18 Thermograph recorded by a Drying Process Monitor (DPM) during primary drying. (From [18].)...

After freezing the product, most of the solvent is removed by sublimation during the primary drying. In this stage it is necessary to monitor product temperature and the residual water content in such a manner that the control system can both optimize... 

Despite the various drawbacks listed above, thermocouples have been proposed to monitor the primary drying and to detect the endpoint of this stage. When the primary drying ends, an increase in the product temperature is measured at the bottom of the vials due to the loss of thermal contact between the sensor and the ice. Moreover, the product temperature increases as there is no longer an endothermic sublimation process that uses the heat supplied by the heating shelf... 

The various techniques available to monitor the primary drying can be roughly divided into three groups as they can be used to monitor single vials, a group of vials or the whole batch (Barresi et al, 2009a) these techniques will be discussed and compared in the following sections. 

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).

Monitoring the Primary Drying Using Methods Based on the PRT... 

From the previous discussion it becomes evident that various devices should be used together to monitor primary drying. The LyoMonitor (Barresi et al, 2009a) is an example of a system that can manage various devices and collect their measurements to monitor primary drying. Currently, as shown in Fig. 4.14, it includes various sensors, namely... 

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