Mixing Characteristics of Irregular Binaries in a Promoted Gas-Solid Fluidized Bed: A Mathematical Model

The vertical as well as horizontal dispersion for the gas–solid fluidization has been analyzed in the present work. The fluidization and solids mixing characteristics of large irregular particles (Geldart-BD type) for both homogeneous and heterogeneous materials (size-variant and density-variant respectively) have been investigated in a 15 × 100 cm2 cylindrical column. A theoretical model for concentration of jetsam particles has been developed as a function of height of any layer of particles (measured from the grid) by considering the counterflow of solids and circulation model together with the dispersion model for the vertical and the horizontal displacement of the particles in the fluidized bed. The mixing index at any position in the bed has thereby been expressed as a function of the concentration of the jetsam. The values of mixing index calculated from the above model as well as the experimentally measured ones for both the homogeneous and the heterogeneous systems have been compared with each other. Attempt has also been made to develop correlations for the mixing index using various system parameters based on the dimensional analysis approach for the unpromoted and the promoted beds. The mixing index values thus obtained through the dimensional analysis approach have been compared with those obtained from the developed theoretical model. The values of the mixing index calculated for promoted beds have also been compared with the corresponding values for the unpromoted beds. Segregation effect is found to be stronger for the heterogeneous systems than the homogeneous ones. The density difference is the major factor affecting the segregation. On a analyse dans ce travail la dispersion verticale et horizontale en fluidisation gaz-solides. Les caracteristiques de la fluidisation et du melange de solides pour des particules irregulieres larges (de type Geldart-BD) dans le cas de materiaux homogenes et heterogenes (variant en taille et en masse volumique respectivement), ont ete etudiees dans une colonne cylindrique de 15 × 100 cm2. On a developpe un modele theorique pour la concentration des particules rejetees en fonction de la hauteur pour toutes les couches de particules (mesuree a partir de la grille) en considerant le contre-courant des solides et le modele de circulation conjointement au modele de dispersion pour le deplacement vertical et horizontal des particules dans le lit fluidise. L'indice de melange a une position donnee dans le lit est ainsi exprime en fonction de la concentration des rejets. Les valeurs de l'indice de melange calculees a partir du modele ci-dessus ainsi que les valeurs mesurees experimentalement pour les systemes homogenes et heterogenes ont ete comparees entre elles. On a egalement tente d'etablir des correlations pour l'indice de melange a l'aide de differents parametres de systemes en s'appuyant sur une methode d'analyse dimensionnelle pour les lits non actives et les lits actives. Les valeurs d'indice de melange ainsi obtenues par la methode d'analyse dimensionnelle ont ete comparees a celles obtenues avec le modele theorique mis au point. Les valeurs de l'indice de melange calculees pour les lits actives ont egalement ete comparees aux valeurs correspondantes pour les lits non actives. On a trouve que l'effet de segregation etait plus important pour les systemes heterogenes que pour les systemes homogenes. La difference de masse volumique est le principal facteur influant sur la segregation.