Secagem em leito fixo de materiais granulares com diferentes esfericidades
Altino, Heitor Otacílio Nogueira
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The drying process of particulate biological materials in fixed bed is influenced by several factors associated with the particles and the bed packing. The main objective of this research was to evaluate the drying of particulate materials with different internal and external characteristics and to investigate how these characteristics influence the bed fluid dynamics, axial temperature profiles and thin layer drying kinetics. It is intended to revisit the analysis of grain drying with experimental data obtained in a standardized way and incorporating on-line acquisition techniques for temperature and humidity measurements. The selected materials were: soy, green lentil, barley, oats and spheres of alumina. The physicochemical properties of the particles as well as the bulk and fluid dynamics characteristics of the packed-beds were measured in different moisture contents of the solid phase. The thickness of the thin layer, as well as the heat transfer in the bed, was analyzed by evaluation of the axial temperature profiles along the bed. The drying kinetics of the materials was studied in a thin layer fixed bed under constant operating conditions. The results evidenced that the characteristics of the beds are strongly dependents of properties like size, shape and surface roughness of the particles, resulting in different bed packing characteristics, such as bulk density, porosity, permeability and tortuosity. The hydraulic radius and permeability of the biological materials presented the following decreasing order of magnitude: soy, lentil, barley and oats. On the other hand, the tortuosity exhibited an inverse order. The inertial resistance factor showed similar values among the materials, with the exception of lentils. The pressure drop was higher in the beds with lower permeability and higher inertial resistance factor. The axial temperature profiles revealed that packed materials with initial moisture of 0.24 g.g-1 d.b. had different heating rates, according to the descending order: soy, lentil, barley, oats and alumina. The effective diffusivities were estimated based on the fit of a diffusive model to the experimental data, assuming a condition of thermodynamic equilibrium at the exit of the bed. The model predicted with good accuracy the kinetic curves for all materials.