Secado Neumatico De Particulas Solidas

Drying 2004 – Proceedings of the 14th International Drying Symposium (IDS 2004) São Paulo, Brazil, 22-25 August 2004, vol. A, pp. 366-373

PNEUMATIC DRYING OF SOLID PARTICLES I. Skuratovsky, A Levy and I. Borde The Department of Mechanical Engineering, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva, Israel E-mail: borde@bgumail.bgu.ac.il
Key words: pneumatic drying,two-dimensional model, drying parameters, tube diameter ABSTRACT Pneumatic or flash dryers, one of the most common types of industrial drying units, are characterized by continuous convective heat and mass transfer processes. To improve design procedures for these dryers, reliable models are needed. A twodimensional, two-fluid model was developed for modeling the steady-state flow of particulate materialsthrough a vertical pneumatic dryer. Two-dimensional flow field interpretations provided information about the properties of the continuous and dispersed phases at every point of the flow field. Axial and radial profiles were obtained for the flow variables. The gas phase was treated as a mixture of gas and liquid vapor. The wet particles were composed of a porous solid structure that contained asaturated liquid. The drying model considered a two-stage drying process. In the first drying stage, heat transfer controls evaporation from the saturated outer surface of the particle to the surrounding gas. In the second stage, the particles are assumed to have a wet core and a dry outer crust. The evaporation of the liquid from a particle is assumed to be governed by diffusion through the particlecrust and by convection into the gas medium. As evaporation proceeds, the wet core shrinks as the particle dries. Following the models successful validation the influence of tube diameter on the properties of the continuous and dispersed phase in the axial and radial direction was investigated. INTRODUCTION Drying in general is an energy-intensive process and has important implications on theenvironment since the thermal energy needed for drying is mostly obtained by combustion of fossil fuels, leading to emission of carbon dioxide. Well-designed drying equipment with high thermal efficiencies is becoming very important. The two dimensional two-fluid model was used to describe the steady state, dilute phase

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flow of a wet dispersed phase (solid particles) in a continuous gasphase through pneumatic dryer. The gas phase was assumed to be a mixture of gas and liquid vapor. The wet particle is a porous solid structure, which contains saturated liquid. The drying model considered a two-stage drying process. In the first drying stage, heat transfer controls evaporation from the saturated outer surface of the particle to the surrounding gas. In the second stage, the particlesare assumed to have a wet core and a dry outer crust. It is assumed that the evaporation of the liquid from a particle is governed by diffusion through the particle crust and by convection into the gas medium. As evaporation proceeds, the wet core shrinks as the particle dries. The developed model was solved numerically and validated by simulating the drying process of wet PVC particles in alarge-scale pneumatic dryer and of the drying process of wet sand in a laboratory-scale pneumatic dryer. The predictions of the numerical solutions were compared with the results of independent numerical and experimental investigations. It is well known that many parameters affect the drying process. So, following the models successful validation, the two-dimensional distributions of the flowcharacteristics were examined such as gas velocity, solid moisture content, gas moisture content and gas temperature in the vertical and radial distribution. In the present study the influence of the tube diameter on the two-dimensional distribution of the flow characteristics were investigated. TWO-DIMENSIONAL PNEUMATIC DRYING MODEL Owing to the fact that detailed derivations of the governing equations...