Dispersion coefficient of coffee berries in vibrated bed dryer

This paper interprets the experimentally measured residence time distribution of coffee berries on a vibrated tray dryer with recycle by means of the dispersion coefficients and the Peclet number. Drying was carried out on a vibrated tray dryer operating with recycle consisting of four parts: a vertical drying tunnel, vibration system, warm air supply to the drying tunnel and recycle system of coffee berries. Using the stimulus–response method the flow behavior of the coffee berries was examined. The dispersion coefficients were calculated by the Taylor Dispersion Model and Free Dispersion Model. The differences in prediction of the dispersion coefficient between the two models were appreciable, but the more reliable values for the dispersion coefficients Ez were those obtained by the Free Dispersion Model. The dispersion coefficient (Free Dispersion) ranged from 2.32 × 10−4 to 76.81 × 10−4 m2/s. The average Peclet number, Pe was approximately equal to 6.5, despite greatest variation of the Ez. Therefore, flow velocity variation of coffee berries was the same magnitude of the dispersion coefficient variation.