Secagem de sólidos porosos granulares
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In the present work, an experimental study about heat and mass transfer phenomena was carried out during drying of porous media when subjected to an upward flow of heated air. The cylindrical porous media consisted of non-consolidated particles of activated alumina of several diameters. The particulate system was initially characterized based on the achievement of its main physical, thermal and structural properties. The drying experiments consisted of obtaining sorption isotherms at different operational conditions and in determining moisture and temperature data versus time. The temperature profiles were obtained by thermocouples inserted in the porous body and the moisture content data by the gravimetrical method. The concept of time in series analysis was applied to determine the stationarity of the experimental data. The drying of the porous media was studied theoretically based on the context of the pseudo-homogeneous formulation, which led to the estimation of transport parameters of interest considering two situations: the presence of the external resistance to mass transfer and the interfacial equilibrium condition. The estimated transport coefficients were analyzed as constant or variable throughout the drying process. The diffusive mechanisms of mass transfer were estimated in order to identify the limiting one in drying. According to the results obtained, the vapor flux is controlled by liquid superficial diffusion into the particle. However, it was found that the effective moisture diffusivity is a harmonic mean of all the diffusive mechanisms, being the all type of moisture diffusivity of both particles and the porous medium important parameters in drying. The results showed that the drying of the porous medium studied is controlled by external resistance of heat and mass transfer. The pseudo-homogeneous formulation presented restrictions for the situation in which the interfacial equilibrium is admitted in the drying problem.