Drying kinetics, energy efficiency, and moisture diffusion characteristics of African walnut (Tetracarpidium conophorum) during cabinet and oven drying

Abstract

This study investigated the drying kinetics, energy consumption, and mathematical modelling of African (Nigerian) walnut ( Tetracarpidium conophorum ) using cabinet and oven drying systems at temperatures of 50, 65, and 80 °C, with and without shells. Drying experiments were carried out to determine the removal of moisture, effective diffusivity of moisture, and specific energy consumption. The curves of moisture ratio were analyzed by means of six thin-layer drying equations, and the performance of the equations was tested by means of the coefficient of determination ( R 2 ), root mean square error (RMSE), and sum of squared errors (SSE). Analysis of the results indicates that the drying time decreased significantly with an increase in the drying temperature, with the shortest drying time being obtained using the oven drying method at 80 °C. The value of the effective diffusivity coefficient increased with an increase in the temperature from 6.23 × 10 −8 m 2 s -1 to 4.72 × 10 −7 m 2 s -1 , which indicates that the rate of diffusion of the moisture inside the material increases with an increase in the thermal gradient. The activation energy required for moisture diffusion ranged from 4.79 kJ/mol to 24.61 kJ/mol depending on the method of drying and condition of the sample material. The Midilli & Kucuk model was found to be the best fit among the models used to describe the experimentally obtained results for the different drying conditions. The energy analysis indicated an improvement in the energy efficiency of the drying process with an increase in the drying temperature, owing to the reduced drying time and SEC, with the highest energy efficiency for oven drying at 80 °C, where the SEC was found to be 0.11 kWh/kg of water removed. In a nutshell, the results show that oven drying at 80 °C is the best compromise between drying rate and energy efficiency. This study is significant because it can help optimize the drying process and enhance the industrial utilization of the underutilized oil-rich nut.