Thin-layer modelling of black tea drying process

Abstract

An experimental dryer was developed for determining the kinetics of black tea drying. Drying characteristics of tea were examined using heated ambient air for the temperature range 80–120°C and air flow velocity range 0.25–0.65 m/s. The data of sample weight, dry- and wet-bulb temperatures and air velocity of the drying air were recorded continuously during each test. The drying data were then fitted to the different semi-theoretical models such as Lewis, Page, modified Page, two-term and Henderson and Pabis models, based on the ratios of the difference between the initial and final moisture contents and the equilibrium moisture content. The Lewis model gave better predictions than other models, and satisfactorily described the thin-layer drying characteristics of black tea particles. The effective diffusivity varied from 1.14×10 −11 to 2.98×10 −11 m 2/s over the temperature range. The temperature dependence of the diffusivity coefficient was described by the Arrhenius-type relationship. The activation energy for moisture diffusion was found to be 406.02 kJ/mol. Temperature and air velocity dependence on drying constant was described by the Arrhenius-type and Power-type relationships. The coefficients of determination were above 0.996 for both relationships. The Arrhenius-type model was used to predict the acceptable moisture ratios at the experimental drying conditions and to understand better the influence of drying variables on drying rate constant. The results illustrate that in spite of high initial moisture content, the drying of tea particles takes place only in the falling rate period. This single-layer drying equation can be used for the simulation of deep-bed drying of black tea.