Influence of drying condition on the drying constants and activation energy for robusta coffee using heat pump drying

Heat pump drying (HPD) is a method of drying agricultural products which has great potential due to its high efficiency and energy savings. Coffee is a heat sensitive product, HPD was chosen as an alternative to existing drying methods for coffee processing. Understanding of the HPD process is important to know for optimizing the coffee drying process with HPD especially in drying characteristics. The purpose of this research is to determine the number for the drying rate constant and the energy of activation in coffee drying. The material used in this study was wet parchment Robusta coffee beans (Coffea canephora). The research was carried out experimentally for 5 h at 700 lpm air flow with varying drying air conditions by adjusting the drying temperature and specific humidity. The drying temperatures used were 60, 65, 70, 75, and 80 °C. The drying temperature is limited to 80 °C based on SNI 7467-2008. Variations in specific humidity are determined based on temperature settings in the refrigeration system of 10, 15, 20 °C and without a refrigeration system. The maximum drying rate constant was 10.69×10-3 s-1, achieved at a temperature of drying variation of 80 °C while a specific humidity of 6.16 g H2O/kg dry air, while the minimum activation energy value was 23.43 kJ/mol at the same specific humidity. The lowest drying rate constant value was 5.79×10-3 s-1, achieved at a temperature of drying variation of 60 °C and a specific humidity of 17.24 g H2O/kg dry air, while the greatest activation energy level was 26.18 kJ/mol at the same specific humidity.