Experimental and Theoretical Investigation of Moisture Dynamics in Intermittent Drying of Rough Rice

Intermittent drying is a satisfactory technique for drying of heat sensitive grains especially rough rice. This method consists of two separate stages applied intermittently: drying and tempering. In this work, intermittent drying of rough rice is investigated both theoretically and experimentally. In order to characterize this process; a multi-scale model consisting of macroscopic and microscopic submodels has been developed. Macroscopic model calculates the moisture changes of the rough rice bed assuming a lumped model for grains, while the microscopic submodel determines the rate of moisture gradient removal during the resting period. In latter submodel, moisture is assumed to diffuse through the grains governing by Fick’s law. Further, a set of experiments were designed and carried out in a lab-scale fluidized bed dryer to estimate the model parameters as well as to evaluate the effects of different parameters such as temperature, air velocity, and tempering time on the drying rate. The model estimates show good agreement with experimental data. Model's results reveal that thermal equilibrium is rapidly obtained within the first two minutes of grain-hot air exposure. Air velocity shows to have no significant effect on drying rate when fluidized condition is prevailed. In addition, a significant kink occurs in drying rate diagrams which implies that drying rate is improved as a result of moisture gradient removal by applying tempering period.

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