Overlapping end points of ice sublimation and desorption drying as detected by temperature probes and Pirani gauge pressure measurements

In general, as the ice mass retracts to leave behind the dry layer, the resistance to vapour flow increases and reduces the ice sublimation rate, however, the increase in the surface area for desorption, leads to an increase in the desorption rate; which is then facilitated further by a generalised warming of the dry layer as the temperature of the vial increases, from the slowing rate of ice sublimation (and hence slowing heat consumption to drive the sublimation process). The overall result is that the impact of drying on the temperature of the product shifts from being controlled by ice sublimation to being controlled by water desorption. Similarly, in terms of the concentration of vapour in the drying chamber; it is at first saturated by the large volumes of water generated by ice sublimation. As this process subsides it then gives way to an opportunity for the concentrations of vapour in the chamber being influenced by the water evolving from water desorption, such that vapour in the chamber being defined by the equilibrium of these gases with the water still residing in the solids fraction. This equilibrium and therefore the amount of residual water contained within at the end of the primary drying stage is then a function of the temperature of the solid.This phenomenon of one process giving way to another is illustrated in this powerpoint presentation with an imaginary construct of a temperature profile (recorded by a thermocouple) at the end of the sublimation phase, being influenced by the overlap of two imaginary thermal profiles, which are associated with the end of ice sublimation leading into the increase in water desorption, which ultimately itself decreases such that the temperature stabilises at some asymptotic value.In both cases of temperature measured or predicted in the vial, and the vapour detected in the drying chamber, the trajectories of the end point defining characteristics for sublimation and desorption are in the same direction; either showing a positive gradient that gets less steep (in the case of TC probes) or a negative gradient which gets less steep (in the case of the Pirani curve).