Transient heating and cooling of cocurrent three‐phase fluidized beds

Abstract

The thermal transient behaviour of three-phase fluidized beds have been investigated for a liquid viscosity ranging from 35 to 75 mPas. For the operating conditions used in this study, a 6 mm glass particle bed was found to have a thermal response similar to that of a fixed bed. The transient responses, which were not significantly affected by gas sparging, were, however, faster for heating than for cooling. This result has been analyzed from a model assuming liquid plug flow through stationary particles using combined free and forced convection correlations for heat transfer around the particles. Different correlations are then proposed to predict the contribution of natural convection to the liquid-to-particle heat transfer in heating and cooling modes. The effect of gas sparging was found to strongly affect the 2.0 mm particle bed responses but only moderately the 3.9 mm bed responses. These responses were analyzed using axial dispersion models for the liquid and solid phases. For the 3.9 mm particle bed, the axial dispersion coefficient of the solids, &, was found to be of the same order of magnitude as that of the liquid coefficient, E, . However, the value of E,, for the 2 mm particle bed was found to be five times that of EzL,.On a etudie le comportement thermique transitoire d'un lit fluidise triphasique mettant en oeuvre un liquide de visco-sit6 variant entre 35 et 75 mPa a s . Dans ce travail, les lits de particules de verre de 6 mm, ont montr6 un comportement similaire a celui d'un lit fixe. De plus on a observe que les reponses du lit n'etaient pas affectees par le bullage du gaz et qu'elles etaient plus rapides en chauffage qu'en refroidissement. On a interpret6 ces differences en introduisant des effets mixtes de convection naturelle et de convection forcke autour des particules. Chacune de ces contributions au transfert de chaleur a ete quantifiee. Pour les experiences conduites respectivement avec des particules de 3.9 et 2.0 mm, le bullage du gaz avait des effets respectivement modkres et importants. Les rkponses thermiques ont kt6 analysies en dkcrivant les melanges des phases liquide et solide par les modkles de dispersion axiale. Pour les lits de particules de 3.9 mm, les valeurs du coefficient de dispersion axiale du solide (ED) obtenues par regression, sont du mCme ordre de grandeur que celles du coefficient de dispersion du liquide, EzL . Pour les particules de 2.0 mm, les valeurs de Ezs son1 approximativement 5 fois celies de Ezl,.Keywords: three-phahe fluidization, liquid-to-particle heat transfer, solids axial mixing.hree-phase fluidized beds are used in a variety of