The effect of adsorption on mass transfer in fluidized bed catalytic reactors

Tracer gas residence time distributions (RTD) in a laboratory scale fluidized bed system have been measured for pulses of three different tracer gases (methane, ethane and propane) at different temperatures in the range 323 to 435 K. The fluidized solid was a commercial zeolite based FCC catalyst (CBZ-2), and measurements were carried out in a superficial air velocity range of 0.01 to 0.04 m/s. The data were interpreted with two-phase dense phase dispersion models for adsorptive tracers, available in the literature. In addition, modified models were considered by assuming a stationary dense phase and neglecting axial dispersion in this phase. Mean residence time, μ1, and the variance of the residence time, σ2, of RTD data were calculated for each experimental run. Applying the moment technique in the Laplace domain, the differential equations for all models considered were analytically solved. Mass transfer coefficients obtained from dynamic experiments were compared with the values estimated from the relations available in the literature. It was found that methods considering convective flux alone between the bubble and emulsion phases give closer values to the experimental ones than the methods also including the diffusive flux. Les distributions de temps de sejour d'un traceur gazeux dans un systeme a lit fluidise a l'echelle de laboratoire ont ete mesurees pour des pulsations de trois traceurs (methane, ethane et propane) a des temperatures comprises entre 323 et 435 K. Le solide fluidise est un catalyseur FCC commercial base sur une zeolite (CBZ-2), et des mesures ont ete prises dans une gamme de vitesses d'air superficielles de 0, 01 a 0, 04 m/s. Les donnees ont ete interpreters a l'aide de modeles de dispersion de phase dense biphasiques publies pour les traceurs adsorbants. En outre, des modeles modifies ont ete pris en compte en supposant une phase dense stationnaire et en negligeant la dispersion axiale dans cette phase. Le temps de sejour moyen μ1., et la variance du temps de sejour σ2 des donnees de DTS ont ete calcules pour chaque essai experimental. Les equations differentielles de tous les modeles consideres ont ete resolues analytiquement en appliquant la methode du moment dans le domaine de Laplace. Des coefficients de transfert de matiere provenant d'experiences dynamiques ont ete compares aux valeurs estimees a partir de relations disponibles dans la litterature scientifiques. On a trouve que les methodes ne retenant que le flux de convection entre la phase des bulles et la phase d'emulsion donnent des valeurs plus proches des valeurs experimentales que les methodes qui tiennent egalement compte du flux de diffusion.