Monitoring methods based on the measure of the sublimation flow: Pressure Rise Test and other direct technologies.

In this chapter, monitoring methods based on the measurement of the sublimation flow are presented. A direct measurement is possible in the drying chamber using one of the pressure rise (PRT) or pressure decrease (PDT) tests methods. All the methods allow us to monitor the residual amount of ice in the product, integrating the sublimation flux in time. Besides, these methods allow estimating the batch average product temperature, the cake resistance to mass transfer and the heat transfer coefficient: these parameters are required for optimizing the process in-line, using a model-based control system, or to develop a design-space approach. The main features of the three most widely used methods are described in detail: manometric temperature measurement (MTM), pressure rise analysis (PRA), and dynamic parameters estimation (DPE), focusing on the most significant differences and on the influence of some important assumptions (i.e. the effect of vial wall, and of radiation). The most recent developments of DPE approach, to improve robustness and performance are also presented. As an alternative, the sublimation flow can be measured directly in the duct, by measuring the gas velocity and temperature, by the TDLAS technology, or the pressure drop, as with the valveless monitoring system (VMS), which is also suitable for water-organic solvents mixtures. Finally, the applicability of these technologies to estimation of end of primary drying, to monitoring residual moisture in secondary drying, and for process control are discussed.