Endpoint estimation using a measurement sensor for freeze-drying process control

The drying stage in a freeze-drying process plays a significant role in determining product quality. Determination of the endpoint of the drying stage is a tedious process. Thus, a measurement device is designed, developed, and tested to determine the endpoint of the drying process in this work. The experiments were conducted during freeze-drying of skimmed milk using a pre-calibrated velocity sensor connected with an Arduino unit. The developed sensor is placed between the product chamber and condensing unit. The flow rate of the water vapour sublimated and desorbed from the product is measured using the sensor. The measured parameters determine the amount of product sublimated and desorbed, water content removed, and mass transfer coefficient. The drying process's endpoint is determined based on the amount of water vapour sublimated and desorbed during the drying process. The initial stage of the drying process exhibited a falling rate period, where unbound water molecules on the surface started to sublimate faster, resulting in higher velocity. The ice layers formed in the condensing unit are drained after the experiments and compared with the values determined from the sensor during the drying process. The average deviation was reported to be minimal (4.4%). Thus, the developed measurement device can be implemented to determine the endpoint during a freeze-drying process with better accuracy and sensitivity. Further, the developed sensor is economical and can be easily installed without significant modifications in the freeze-drying unit.