Effect of air recirculation and heat pump on mass transfer and energy parameters in drying of kiwifruit slices

The simultaneous transfer of mass and heat in the drying process has turned it into a complicated process with respect to mass transfer and moisture removal. A hot-air dryer equipped with an auxiliary heat pump and air recirculation system was used to dry kiwifruit slices at three different temperatures (45, 55 and 65 °C) to determine mass transfer and activation energy using two different models, namely Dincer-Dost and Crank's models. When the heat pump was on, compared with 45 °C and air recirculation rate (0%) at higher temperature (65 °C) and higher air recirculation rate (100%) the drying rate constant increased from 1.113 × 10−4 s−1 to 2.357 × 10−4 s−1and the effective moisture diffusion coefficient increased from 1.94 × 10−9 m2/s to 7.12 × 10−9 m2/s. When the heat pump was off, both parameters decreased with increasing recirculation (from 0 to 100%) and increased with rising the temperature (from 45 to 65 °C). When the heat pump was turn off and on, at 65 °C and 100% recirculation the change in the range of activation energy, convective mass transfer coefficient, specific energy consumption, drying efficiency and specific moisture extraction rate (SMER) were 14.04–20.39 kJ/mol, 4.12–8.55 × 10−7 m/s, 1.08–1.49 kWh/kg, 9.84–12.15% and 0.11–0.15 kg/kWh, respectively.