Effect of intermittent ratio on the performance of a closed-loop heat pump drying system with a novel ternary refrigerant and on the drying characteristics of corn kernels

To improve the drying rate (DR) and energy efficiency in the middle and late stages of heat pump drying (HPD), this paper proposes a closed-loop transcritical R744/R290/R32 heat pump intermittent drying system, with corn kernels as the drying material. The effects of different intermittent ratios (0, 1/6, 2/6, 3/6, and 4/6) on specific moisture extraction rate (SMER), drying capacity per unit volume per unit time (DCVT), drying capacity per unit energy consumption (DCEC), drying time, DR, fatty acid, rehydration ratio (Rf) and crack are experimentally investigated. The results indicate that as the intermittent ratio increases, average DR, average SMER, DCVT, DCEC, and Rf all increase, while HPD time shortens and fatty acid decreases initially before increasing. A comprehensive scoring analysis identifies 2/6 as the optimal intermittent ratio. Compared to continuous drying, when the intermittent ratio is 2/6, average DR, average SMER, DCVT, DCEC, and Rf increase by 16.41 %, 42.98 %, 50.01 %, 49.92 %, and 6.78 %, respectively, while both HPD time and total drying time are shortened. The germ shrinks more than the endosperm after drying, indicating moisture content differences among corn kernel components. The research results provide a reference for studying and applying transcritical CO2 mixed refrigerant heat pump intermittent drying technology.