Optimization of the energy consumption, drying kinetics and evolution of thermo-physical properties of drying of forage grass for haymaking

Abstract

The study aims to evaluate and optimise the process parameters for effectively drying two forage plants indigenous to Africa to make hay. The goal is to generate data using experimental, theoretical and empirical steps for process model development that can be applied in forage dryer design. The study also considers the analysis of the following: the energy consumption, drying rate, evolution of thermal conductivity, thermal diffusivity, specific heat capacity and heat and mass transfer coefficient of the drying Andropogon tectorum and Pennisetum purpureum grass. Drying of the grasses was done using sun, oven and microwave drying techniques. The optimum specific energy consumption for oven drying was 8.38 kWh/kg at a drying time of 5 h at a temperature of 54.8 °C for Pennisetum purpureum grass. Similarly, for the same method, the specific energy consumption of 27.4 kWh/kg and drying time of 70 °C was attained for Andropogon tectorum. Additionally, for the microwave drying technique, the specific energy consumption was 0.04kWh/kg at drying time of 0.15 h exist for Pennisetum purpureum grass while the specific energy consumption and drying time for Andropogon tectorum grass were not greater than 0.15 h and 0.072 kW/kg respectively. The Effective moisture diffusivity, the specific heat capacity, thermal conductivity, thermal diffusivity, heat transfer coefficient and mass transfer coefficient were determined for both minimum and maximum range. Converesly, the Logarithmic model best described the experimental data for drying both grasses. However, all the thin layer models performed well with R² and RMSE values of 0.5606—0.999, 0.0002—0.512. The drying constants (k) were high due to high drying rates for the leaves.