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.