Secagem da fibra do pseudocaule da bananeira: avaliação de modelos cinéticos, difusividade efetiva, propriedades termodinâmicas e caracterização estrutural

Abstract

The drying process of banana pseudo-stem fibers was studied, experimentally, in a vertical fixed-bed convective dryer (at 60, 75, and 90 °C). Nine mathematical models (theoretical, semi-theoretical and empirical) were used to analyze the drying behavior and effective diffusivity, activation energy and thermodynamic properties were calculated. Dry fibers were evaluated also by thermogravimetric, spectroscopic, and morphological analyses. Diffusion approximation with adjusted coefficient of determination, r2adj ≥ 0.978, reduced chi-square χ2 ≤ 2.21×10-4 and root mean square error RMSE ≤ 0.0140 was the model that better fitted experimental data. The effective diffusion coefficients (𝐷𝑒) increased with temperature, reaching the order of 10-7 m2 s-1. Its dependence on temperature was described by the Arrhenius Equation and its activation energy was 47.61 kJ mol-1. When temperature increases, enthalpy decreases contrarily to entropy and Gibbs free energy, indicating that drying is an endergonic and non-spontaneous process. Characteristic absorption bands of lignocellulosic components were identified (1423, 1327, 1160 cm-1) and the degradation of this material only occurred at temperatures above 190 °C, as confirmed by thermogravimetric analyses. Morphological changes were observed in dry fibers. These mainly occur at 90 °C and led to an irreversible damage of sample structure. Alterations are caused by the tensile strength generated because of temperature and humidity gradient, produced during the drying process.