Optimization of the drying of Moringa oleifera leaves by determination of thermophysical parameters

Abstract

Moringa oleifera leaves are increasingly used as a dietary supplement. The present study deals with the estimation of its thermophysical properties. It is based on the Clausius-Clayperon and Gibbs-Helmholtz equations. This study required knowledge of the sorption isotherms obtained by the static gravimetric method in the relative humidity range of 5.4 to 97% and the temperatures of 30, 35, 45 and 55 °C. The BET models (1938), Smith (1947), Henderson (1952), Iglesias (1981) and GAB (1985) are used for the smoothing of the experimental points. The isotherms obtained are of type II and characterized by a sigmoid shape. The theoretical curves of GAB and Smith show a better similarity with the experimental curves. The monolayer moisture content (Xm) is 8.54/7.62% at 30 °C; 7.0/5.52% at 35 °C; 6.74/ 5.41% at 45 °C and 5.45/4.65% at 55°C respectively for the GAB and BET models. The net isosteric heat (qst) and desorption differential entropy (ΔS) tend towards zero for very high moisture contents. The activation energy of desorption of the Moringa oleifera leaves in the temperature range tested is -13.1010 kJ/mol. Isokinetic temperature (Tβ) and the average harmonic temperature (Thm) are respectively 314.172 K and 314.459 K while the free energy (ΔGβ) is 308.18 J/mole.

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Keywords: Desorption isotherm, Modelization, Isosteric heat, Differential Entropy, Activation energy