Experimental determination of dynamic pseudo-equilibrium moisture content: A practical limit for the drying process

Simulation and rigorous design of industrial dryers combine a large number of models, which feed three fundamental balances: (1) mass; (2) energy; and (3) quantity of movement of the material through the dryer. Many of these models represent physical phenomena affecting the three balances at the same time, which makes these calculations extremely complex, hence, accurate models are essential. The hypothesis that the kinetic stage of drying of any material culminates in the thermodynamic moisture equilibrium between solid and drying gas has been in effect for many years. However, recent findings show that there is a transition stage between the kinetic stage and the thermodynamic equilibrium, which, experimentally, looks like an equilibrium. The beginning of this transition stage or dynamic pseudo-equilibrium stage would mark the end of the drying kinetics models, which has been named as the dynamic pseudo-equilibrium moisture contents (X(dpe)). The non-observance of this phenomenon presupposes a model limited in its prediction capacity, especially in the last stages of drying and even more so at low drying temperatures. As a consequence, sizes of industrial dryers could be underestimated during the simulation and rigorous design process, or underestimate drying times, in batch dryers. On the other hand, the optimal conditions may never be found, during the optimization of existing industrial drying processes. The objective of this work is to present the procedure to determine X(dpe), during the experimental determination of drying curves of any material. Likewise, to propose the practical moisture ratio, which uses X(dpe), instead of the equilibrium moisture, to be used in the modeling of the drying kinetics. • The drying process is divided into three stages: kinetic, transition, and equilibrium. • The dynamic pseudo-equilibrium moisture content divides the kinetic and the transition stages. • The practical moisture ratio should be used in rigorous industrial dryer design calculations.