Experimental Study and Numerical Simulation of Green Tea Drying Kinetics using a Heat Pump System Integrated with Auxiliary Electric Heating

Abstract

Controlling the moisture removal process in Thai Nguyen green tea drying is a significant challenge due to the complex capillary-porous structure of the leaves and the high thermal sensitivity of bioactive compounds. This study proposes an advanced hybrid drying solution that combines heat pump technology (HPD) with auxiliary electric heating to optimise drying kinetics and energy efficiency. In addition to the experimental investigation of three multi-stage temperature-control modes, a mathematical model based on 1D moisture-diffusion partial differential equations (PDEs) was developed. This model was solved using the implicit finite difference method (FDM) in MATLAB to accurately describe the moisture concentration gradient from the core to the material surface in real time. The experimental and simulation results show high compatibility (RMSE < 0.05). The data indicate that the stepped drying mode (35–40–45°C) is the optimal strategy, reducing the drying time by 27.2% compared to the conventional constant-temperature heat pump method. This strategy ensures uniform moisture distribution and effectively prevents surface "case hardening." Among the six thin-layer kinetic models evaluated, the Midilli & Kucuk model exhibited superior performance, with a coefficient of determination (R2) of 0.9981 and an RMSE of 0.00179. The novelty of this research lies in establishing a "thermal safety zone" through a multi-stage heating mechanism, which maintains a maximum drying rate of 0.035 g/min during the critical falling-rate period. The findings confirm that PDE-based numerical simulation is a fundamental tool for optimising hybrid drying systems, enabling accurate prediction of the drying endpoint and establishing temperature profiles to preserve sensitive bioactive substances in industrial applications.