Effect of different levels of operating pressure and drying temperature on thin layer modeling and drying costs of mint leaves

Drying medicinal plants such as mint using vacuum dryers is a highly effective method for preserving their bioactive compounds, essential oils, and therapeutic properties. Unlike traditional drying methods, vacuum drying operates at low temperatures and reduces pressure, minimizing heat damage and oxidation. So, the current study aimed to dry mint leaves using a developed automatic vacuum dryer at three different temperatures (40, 50, and 60 °C) and three different operating pressures (atm, − 5 kPa, and − 10 kPa). All mint samples were subjected to drying at normal atmospheric pressure (atm) to establish a reference point for the comparison between the conventional oven system, which utilizes atmospheric pressure drying, and vacuum drying. It is the goal of this comparison study to find out which drying method works best and how changes in pressure affect thin-layer modeling, thermodynamic properties, drying kinetics, and the costs of drying mint leaves. The obtained results indicated that the drying constant increased with decreasing the operating pressure. Furthermore, the highest drying rate was 0.0431 g_water/kg_{dry matter}/min, and it was observed at drying temperature of 60 °C and an operating pressure of − 10 kPa. Additionally, reducing the operating pressure inside the drying room from atm to − 10 kPa decreases the drying time by about 60.6%, 52.63%, and 61.5% at a drying temperatures of 40 °C, 50 °C, and 60 °C, respectively, and also leads to increasing the effective moisture diffusivity (EMD) from 3.34 × 10^–9 to 9.96 × 10^–9 m²/s, while it leads to decreasing the activation energy by about 92.32%. Furthermore, the thin layer modeling showed that the Weibullian (I) model was the best thin layer model to describe the drying process of mint leaves. Among the thermodynamic parameters, it was observed that enthalpy values decrease with increasing drying temperature and decreasing the operating pressure. Additionally, all tests showed negative entropy, suggesting that the chemical adsorption and structural modifications of the adsorbent are responsible for these results. Drying mint leaves at 10 kPa and 60 °C reduced the specific energy consumption used by 49.87% and reduced the investment payback period to 0.079 years (less than 1 month).