From the shape of stationary profiles of temperatures and moisture for the gas and solid phases, two distinct zones clearly appears in a counter-current cascading rotary dryer of sugar; a first zone in which the drying takes place, a second zone in which the dried sugar is reheated by contact with the entering hot air. The fit of experimental data of temperature and humidities was used to test a mathematical model for the drying zone and to find optimal values of heat and mass transfer coefficients. Additional investigations at laboratory scale have provided information on the kinetics of drying of sugar crystals falling in a curtain as in the airborne phase of a rotary dryer. Sorption isotherms show that a strong re-moistening or deliquescence of sugar occurs at about 80% relative humidity. Obviously such a value must not be reached in an industrial dryer.
Disposal of agricultural and agro-food industry by-products has become an increasing environmental problem. Drying is one of the possible conditioning processes that can be applied to these by-products before using them as cattle feed or soil conditioner. In this paper, a dynamic model to simulate the dehydration process of wastes of vegetable from a wholesale market in a rotary dryer is proposed. The dryer was divided into 10 sections and mass and energy balances were established in each of them. The results have been validated in a semi-industrial dryer. The model predicts air and product moisture and temperature depending on working conditions of the rotary dryer. Inlet air temperature has been shown to be the variable that has the greatest effect, on both outlet moisture content of the product and on outlet air temperature.