Primary drying model
Mass transfer during the sublimation of ice from a frozen solution, leaves behind a dry layer as the ice sublimation interface recedes down the vial. The skin and the dry layer resistances to vapour flow are dominant over those smaller resistances from the vent in the stopper and through the duct to the condenser, and hence it is these resistances that effectively control the flux of water vapour and hence the sublimation rate.
The restrictions to heat transfer from the shelf to the vial, that result from the poor thermal contact between the base of the vial and the shelf results in energy being drawn from the frozen layer and hence the sublimation interface temperature drops below that of the shelf. In the process recipe this then allows for the shelf temperature to be set higher than the critical temperature of the product.
The system reaches a state of thermal equilibrium when the mass transfer (dm/dt) equals the heat transfer to the vial (dq/dt), a condition known as heat and mass balance.