Experimental study on CO2 closed-cycle heat pump drying for lemon assisted by ultrasonic pretreatment

Abstract

Drying lemon can effectively prevent them from rotting and spoiling. Closed-cycle heat pump (HP) offers advantages such as environmental sustainability and energy efficiency. However it has low energy utilization in the middle and late stages of the system's drying process. Therefore, this paper proposed the application of ultrasonic pretreatment (UP) in the CO₂ closed-cycle HP system for lemon drying. The effects of different UP power assisted HP drying for lemon on the system performance and drying quality were experimentally investigated. The results showed that UP process increased the porosity of lemon slices by 49.94 % (at a power of 150 W), shortened the drying time by 11.5 %–30.7 %. The average drying rate (DR), average specific moisture extraction rate (SMER), energy consumption per unit of drying capacity (ECDC), and moisture drying volume treated per unit time (MDVT) of the HP drying system were increased by 13.02 %–44.38 %, 8.39 %–33.00 %, 8.60 %–31.18 %, and 13.37 %–44.70 %, respectively. After drying, the rehydration ratio (R_f), luminance (L∗), and vitamin C content (V_c) of the lemon slices increased by 12.8 %–26.7 %, 3.99 %–14.94 %, and 3.29 %–14.48 %, respectively. As the UP power increased from 0 to 200 W, the average DR, average SMER, ECDC, MDVT, rehydration ratio, luminance, and V_c all increased first and then decreased. When the UP power was 150 W, the maximum values for average DR, average SMER, ECDC, MDVT, rehydration ratio and luminance were achieved, which were 0.732 kg/(kg·h), 0.935 kg/(kW·h), 1.22 kg/(kW·h), 3.14 kg/(m³·h), 4.84 and 46.63, respectively. The optimum drying kinetics model, based on experimental data, was fitted to the Page model. Compared to other methods of lemon drying, the use of UP assisted HP drying was more energy-efficient and carbon-reducing, resulting in a reduction of the payback period for the HP drying system to 0.86 years. The findings of this research can provide reference for improving the energy efficiency of closed-cycle HP drying for lemon and enhancing drying processes for other fruits and vegetables.