Simulation of the UHF convective drying process of alfalfa herbage in a vertical dryer

Abstract

A physics-based mathematical model of UHF convective drying of chopped alfalfa herbage in a vertical dryer under reduced pressure (60–80 kPa) was developed and validated. The model forecasts moisture removal kinetics, temperature fields, and carotene losses under combined microwave heating and vacuum conditions. Unlike single-component approaches, it accounts for morphological heterogeneity by separately describing leaf (65%) and stem (35%) fractions with distinct heat and mass transfer kinetics and dielectric properties. Leaves and stems were represented as flat plates and cylinders enabling quantification of asynchronous drying and redistribution of absorbed microwave energy. A kinetic equation for thermal carotene degradation dependent on leaf temperature and moisture content was incorporated to assess product quality. The temperature difference between fractions reaches 1.9–5.6°C, while the radial gradient within the chamber is limited to 2.5–3.5°C. The maximum material temperature is 89°C with leaf temperature not exceeding 87.1–91.2°C which constrains carotene degradation. The final moisture content ranges from 20.5% to 29.6% with a leaf–stem difference of 2.7–7.9%. Residence time varies from 220 to 580 s depending on screw rotation speed (5–12 rpm). The optimal range of 7–10 rpm ensures leaf moisture of 20.1–21.8%, stem moisture of 23.8–25.9%, and carotene losses below 3.9–5.1%. Model validation demonstrated high accuracy with a coefficient of determination of 0.987, relative moisture prediction error below 1.7%, and carotene loss error within 0.5%. The results provide a basis for engineering design of efficient microwave–convective drying systems.