An experimental study on drying kinetics of some herbal leaves

In this study, thin-layer drying characteristics of some herbal leaves, mainly nettle and mint leaves, are investigated experimentally in a convective drier. Effects of the drying air parameters including temperature, velocity and relative humidity on the total drying time are determined. Initially, sorption isotherms of the dried leaves are determined for different temperatures and equilibrium relative humidity (e.r.h.). Experiments are conducted for air temperatures at 35, 45 and 55 °C, velocities at 0.2, 0.4 and 0.6 m/s and, relative humidity values at 40%, 55% and 70%. In the ranges that covered, the values of the moisture diffusivity Deff are obtained between 1.744 × 10−9 and 4.992 × 10−9 m2/s for nettle leaves and 1.975 × 10−9 and 6.172 × 10−9 m2/s for mint leaves from the Fick’s diffusion model. Using Deff, the value of Ea is determined assuming the Arrhenius-type temperature relationship, which varies from 79.873 to 109.003 kJ/mol for nettle leaves and 66.873 to 71.987 kJ/mol for mint leaves.