Estudo da transferência de calor e massa no processo de secagem em leito vibro-fluidizado.

Abstract

Drying processes in fluidized and vibro-fluidized beds are nowadays being widely investigated in the literature, because such beds offer many advantages over other types of dryers. The purpose of this work is to contribute for the research in this area by presenting an extensive investigation of the drying process in fluidized and vibro-fluidized beds. The following topics were investigated in this work: a) fluid dynamic studies in fluidized and vibrofluidized beds were carried out using particles of alumina and silica-gel as inerts. The effects of variables such as the frequency and vibration amplitude (which are combined in a dimensioless vibration number), height of the bed and particle diameters on the fluid dynamic behavior of both beds have been analyzed. Simultaneously, an analysis of the fluid dynamic behavior for these beds was carried out using measurements of standard desviations of the pressure drops in the beds as a function of the superficial gas velocity. The results show that in both beds the dimensioless vibration number has a strong influence on the fluid dynamic behavior of the bed of particles. Besides, the increase in the height of particles in the bed, as well as in the particle diameters affects significantly its characteristic curves; b) equilibrium isotherms for alumina and silica-gel particles were obtained experimentally using the static method. Some equations for equilibrium isotherms available in the literature were compared to experimental data and the best fitted model was obtained using a technique based on non-linear estimation of the parameters. The results show that the equation of Sabbah provided the best fitting for both particles of alumina and silica-gel; c) the drying kinetics for alumina and silica-gel were obtained from experimental data for the drying of a thin layer of particles in a fixed bed. The best model for the drying kinetics came from a semi-empirical equations presented in the literature, using a non-linear estimation of parameters technique. The results show that the equation from Mancini provided the best predictions for experimental data of alumina and silica-gel; d) heat an mass transfer coefficients in the fluidized and vibrofluidized beds were estimated using the methodology proposed by Kmiec (1975). In the range of variables investigated, the results show that the heat and mass transfer coefficients for the vibrofluidized beds have not increased significantly as compared to the values obtained in the fluidized beds; e) the moisture content and temperature profiles of the particles were estimated using the mathematical model proposed by Massarani and Silva Telles (1992) with the heat and mass transfer coefficients obtained experimentally. For the range of conditions investigated, the mathematical model has not provided good predictions for the moisture content profiles, but in the other hand, it provided reasonable good predictions for the temperature profiles of the particles.