Comparative analysis of drying kinetics, thermodynamic properties, and mathematical modeling of pomegranate peel (Punica granatum L.) in a hybrid solar dryer and an oven dryer

Abstract

Drying pomegranate peels, a by-product of juice extraction, offers an effective means to preserve bioactive compounds while reducing waste. This study investigates the influence of drying temperature and layer thickness on the drying kinetics, thermodynamic properties, and modeling of pomegranate peels using a hybrid solar dryer (HSD) and a conventional oven dryer (OD). Fresh peels with an average initial MC of 325.53% (dry basis) were dried under three temperatures (50, 60, and 70 °C) and three-layer thicknesses (1, 2, and 3 cm). The HSD included a temperature and humidity control unit and an auxiliary electric heater to ensure stable conditions. Weight loss during drying was recorded at regular intervals to track moisture content changes until equilibrium MC (EMC) was reached. Experimental data were used to calculate effective moisture diffusivity (EMD) using Fick's second law and activation energy through Arrhenius-type relationships. Thermodynamic parameters-Gibbs free energy, enthalpy, and entropy-were also determined. Additionally, twelve thin-layer drying models were fitted to the data using nonlinear regression, with performance evaluated using R², RMSE, and χ². Results showed that final MCs ranged from 2.15 to 2.80% (OD) and 2.92-3.01% (HSD). EMD increased with temperature and thickness, reaching up to 12.17 × 10⁻⁹ m²/s (OD) and 11.66 × 10⁻⁹ m²/s (HSD) at 70 °C for 3 cm layers. Activation energy varied with thickness, ranging from 25.76 to 43.55 kJ/mol (OD) and 25.82 to 41.09 kJ/mol (HSD). Among all models, the Modified Midilli II model best described the drying behavior. Thermodynamic analysis indicated that Gibbs free energy increased with temperature, while enthalpy and entropy decreased, reflecting improved energy efficiency at higher drying temperatures. The results demonstrate that optimized drying conditions can enhance the preservation and quality of pomegranate peels, promoting their use as functional ingredients in the agri-food industry.

Keywords: Effective moisture diffusivity; Food by-product; Fruit drying; Renewable energy; Thin layer modeling.

Fig. 1

Step-by-step drying process flow diagram using the HSD.

Fig. 2

Main components of the HSD. Whereas a. real photo of the HSD, b. double air circulation passes inside the flat plate solar collector.

Fig. 3

Internal components and control units of the HSD. Whereas, a. internal components of the drying room, and b. control units for temperature, humidity and electrical heater.

Fig. 4

MR on dry basis of pomegranate peels at different drying temperatures and layers thicknesses, (a) OD, and (b) HSD.

Fig. 5

Relation between LnMR and drying time of pomegranate peels at different drying temperatures and layers thicknesses, (a) OD, and (b) HSD.

Fig. 6

EMD for OD and HSD under different drying air temperatures and different layers thicknesses.

Fig. 7

LnD_eff with invers absolute temperature of pomegranate peels at different drying temperatures and layers thicknesses, (a) OD, and (b) HSD.

Fig. 8

Activation energy of pomegranate peels for OD and HSD under different drying air temperatures and different layers thicknesses.

Fig. 9

Observed and predicted MR for OD at the best mathematical model (Modified Midilli II).

Fig. 10

Observed and predicted MR for HSD at the best mathematical model (Modified Midilli II).