Optimal Drying Conditions of Grains

The problem we are concerned with is to dry as quick as possible a grain from an initial known moisture to a desired value, with a minimal energy cost and with respect to quality constraints for the product. It is evident that more we spend energy and money, shorter is the drying time, and that more one accepts a long drying time, more we save energy. We are looking for the optimal solution of this dilemma, taking into account the cost of energy and an equivalent cost of time. For this purpose a mathematical model of a dryer has been written as the discrete sequence of several cross-flow elementary dryers. The control variables are the characteristics of drying airs entering each of these steps as well as the global drying time. Their optimal values are computed by using dynamic programming. This optimisation includes the renewing of some used airs if they keep some interesting drying capacities. Numerous abacus and simulation curves show the main results of this study, and applications to optimal design and automatic control of dryers are discussed at last.