Assessment of gas‐fluidized beds mixing and hydrodynamics by zirconia sensors

Abstract

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...

Gas interchange between bubble and emulsion phases in a 2D fluidized bed as revealed by two-fluid model simulations

“…Their simulation showed similar results to the model by Davidson and Harrison [2]. More recently, Solim ene et al [6] employed a novel technique based on zirconia oxygen sensors to study the nitrogen mixing in a bubble injected in an air fluidized bed, and Pavlin et al [9] measured the gas interchange in a three dimensional (3D) fluidized bed by means of nuclear mag netic resonance. Though these new experimental methods can ad vance the understanding of the gas transport in a bed, they are still unable to provide a full characterization of the gas interchange be tween the emulsion phase and the bubbles.…”

Fluidized Bed Combustion of Liquid Biofuels: Application of Integrated Diagnostics for Micro-explosions Characterization

“…An electromotive force, V, is generated when the two sensor tip surfaces are exposed to gases at different oxygen concentrations. The signals of the oxygen solid-state sensors have been elaborated according to Solimene et al 19 in light of a Nernst-like equation that relates the electromotive force, V, to the oxygen molar fraction in the investigated gas…”

Electrochemical Sensors: Fundamentals, Key Materials, and Applications

“…While sensors used in automotive combustion processes [383][384][385][386] have in the past received the most attention, combustion gas sensors are also important for improving efficiency and NiCr 600ºC [29,328] NiCr 600ºC [275,309] NiCr 700ºC [320] NiCr 700ºC [309] CdCr 500ºC [313] CdCr 500ºC [324,325] CdCr 600ºC [313] ZnFe 600ºC [328][329][330] ZnFe 700ºC [ reducing emissions in other processes, such as glass processing [387,388], liquid petroleum gas (LPG) boilers [389], and even wood burning [390]. The sensors developed for these applications can also be used in other areas, where information on gas compositions is important, such as controlling the atmosphere during sintering [391] or characterizing the gas flow and distribution in a fluidized-bed reactor [392].…”