Experimental determination of the effective moisture diffusivity and activation energy during convective solar drying of olive pomace waste

The drying of olive pomace waste was performed using a partially indirect solar convective dryer operating in forced convection. This comparative study is focused on the drying kinetics of two types of residue, namely, raw olive pomace and deoiled olive pomace. These products have been spread out in thin layers on perforated circular racks before they be placed in the dryer. The sample thicknesses considered are 0.5, 1.0 and 1.5 cm. Kinetic measurements are carried out for three temperatures (40, 60 and 80 °C) and two drying air flow rates (0.042 and 0.083 m3 s−1). The relative humidity varies between 28% and 65% throughout the drying period. The increase of temperature in the environment reduces significantly the drying time. The characteristic drying curve (CDC) applicable to both types of olive pomace has been established as a polynomial of order 4 in reduced moisture content. Data obtained from dried products were used to determine the effective diffusivity values during the drying period with decreasing curve. In this period, the moisture transfer from the pomace was described by applying the Fick diffusion model. Effective diffusivity varies between 1.6 × 10−8 and 34.7 × 10−8 m2 s−1 with the increase of the medium air drying temperature but also with the sample thickness. The activation energy value is estimated at 29.06 kJ mol−1.