Kinetics of alfalfa drying: Simultaneous modelling of moisture content and temperature

• The kinetics of the dehydration of alfalfa stem, leaf and whole plant was studied.
• The influence of bed thickness, air mass rate and air temperature was evaluated.
• A kinetic model of globalised parameters was proposed to simulate the dehydration.
• The model simulated moisture and temperature inside the substrates simultaneously.
• The model accounted for 70–95% of the variance in experimental results.

The kinetics of the dehydration of alfalfa stems, leaves and the whole alfalfa plant (Medicago sativa L) was studied experimentally at lab-scale using a fixed-bed dryer with warm air at 328, 333, 338 and 343 K. Following the evaluation of the influence of bed thickness, air mass rate and air temperature on the kinetics of drying, a kinetic model of globalised parameters is proposed. The kinetic model was designed to simulate the exchange of moisture between alfalfa and the surrounding air and the variation of temperature inside the substrates with time simultaneously. In a first step, the kinetic model was applied to stems and leaves separately. The model reproduced 94.4% and 70.1% of the moisture experimental results obtained for stems and leaves, respectively, within an error band of 15%. Moreover, 95% of the experimental results regarding the variation of temperature inside stems with time were simulated. The kinetic model was then applied to the whole alfalfa plant considering its content of stems (60% wet weight) and leaves (40% wet weight) using the same kinetic parameters and variables fixed for their single modelling. The model reproduced 82.2% of the moisture experimental results obtained for the drying of the whole alfalfa plant. This kinetic model could be a useful tool for the design of a drying device based on scientific evidence.