Effects of Drying Temperature and Specific Humidity on Drying Rate Constant and Activation Energy of Robusta Coffee

Drying characteristics are largely determined by the value of the drying rate constant and the activation energy of the material. Heat pump drying is a mechanical dryer with several advantages such as being independent of weather, low energy consumption, reduced quality loss, flexible system settings, and suitability for drying heat-sensitive products. Heat pump drying has the potential as an alternative to existing drying methods for coffee processing due to its advantages. Unfortunately, few studies are related to heat pump drying for coffee dryers. Therefore, it is necessary to investigate the drying rate constants and activation energy of coffee drying. The wet parchment Robusta coffee is dried via forced convection by a heat pump drying at different drying temperatures and specific humidity. Drying was carried out at various heater temperatures of 60, 65, 70, 75, and 80 °C for 5 h, with an airflow rate of 400 lpm. Specific humidity varies based on the refrigeration system's settings or the evaporator output temperature of 10, 15, and 20 °C. Drying is also carried out without a refrigeration system. There are 4 variations of specific humidity. Variations in the value of specific humidity were obtained from the calculation of the average exit temperature of the evaporator and relative humidity during the drying period. At a variation of drying temperature of 80 °C and specific humidity of 0.006158 kg H2O/kg dry air, the highest drying rate constant was 0.008053 s^-1, and the lowest activation energy value was 38.62 kJ/mol at the same specific humidity. For comparison, with variations in drying temperature of 60 °C and specific humidity of 0.017243 kg H2O/kg, the lowest drying rate constant was 0.002831 s^-1, and the highest activation energy value was 45.93 kJ/mol at the same specific humidity.