in Wiley InterScience (www.interscience.wiley.com). AIChE J, 52: 185-198, 2006 Keywords: gas-solid fluidization, mixing, hydrodynamics, bubbles, mass-transfer coefficient, zirconium Gas-mixing and bed hydrodynamics associated with the rise of isolated bubbles in gas-fluidized beds of two granular solids are investigated by means of a novel technique. These diagnostics consist of an array of miniaturized zirconia-based oxygen sensors located along the axis of a hot (1123 K) incipiently fluidized bed, in which isolated bubbles of a tracer gas (nitrogen) are injected. Time-resolved oxygen concentrations and relative pressures are recorded at several levels along the bed. It is shown how the response of the sensors array can be related to the concurrent effects of bubble motion, bubble-emulsion phase, mass transfer and interstitial gas flow in the emulsion phase, combined with each other to give rise to the propagation of two perturbations to the basal value of oxygen concentration. Analysis of time-resolved oxygen concentration profiles enables the assessment of: (a) the bubble rise velocity; (b) the gas velocity associated with the interstitial flow; and (c) the interphase bubble-emulsion phase, mass-transfer rate and coefficient. Analysis of the response of sensors located in the splash zone of the fluidization column provides indirect inference of the hydrodynamic patterns associated with bubble bursting at the surface of the bed. © 2005 American Institute of Chemical Engineers oxide sensors IntroductionThe relevance of mass transfer between bubbles and the emulsion phase to the effectiveness of a broad range of fluidized bed, chemical and physical processes, has long been highlighted. Several examples can be provided: (a) char burnout in bubbling fluidized-bed combustors (or in the bottom bed of circulating FBC), where heterogeneous char combustion is fast to the point of making interphase mass transfer the ratecontrolling process; (b) burning of combustible gas in bubbling fluidized beds, where homogeneous reaction is quenched in the emulsion phase, whereas it takes place in the bubbles at a rate that can be controlled by mass transfer; and (c) fast catalytic reactions, like the catalytic combustion of light hydrocarbons. Despite the recognition of the role of interphase mass transfer in a variety of fluidized bed processes, and the extensive literature published on this topic, [1][2][3][4][5][6][7][8][9] there is still a lack of experimental data and phenomenological modeling related to this subject.Mass transfer between a bubble and the emulsion phase is typically described by the following balance on species A Ϫ dN Ab dtwhere N Ab , V b are the number of moles of A in the bubble and the bubble volume K be is the overall bubble-to-emulsion phase mass-transfer coefficient per unit volume of the bubble, and C Ab and C Ae are the concentrations of A in the bubble and in the Correspondence concerning this article should be addressed to P. Salatino at salatino@unina.it. © 2005 American Institute of Chemica...