Drying characteristics of orthodox broken type tea

Abstract

Drying characteristic of Orthodox broken type tea was examined using a laboratory-scale fluid bed tea dryer. Drying experiments were carried out at loading of 29 kg/m2 and moisture content of tea was reduced from 106% to 7% on dry basis. Hot air temperature was varied in the range of 108 - 127 °C as applicable for industrial type tea dryers. Semi-theoretical thin-layer drying models of Lewis, Henderson and Pabis, Logarithmic, Page, Modified Page and Two-term exponential model were tested. Page model gave better predictions than other models, and satisfactorily described drying characteristics of Orthodox broken type tea. Results suggested that internal mass transfer resistance has fully controlled the drying process after 4 minutes of drying. The effective diffusivity of water in tea during latter stage of drying was found to be 3.796 x 10-11 (m/s) and 5.062 x 10-11 (m/s) at hot air temperatures in the range of 108–121 °C and 124–127°C respectively. An empirical model was proposed to describe the variation of moisture content with tea-bed temperature and the predicted values were in close agreement with the measured data. The bulk density of the tea bed was found to increase with time and that might have reduced the channeling effect during latter stages of drying.