Experiments show that the minimum fluidization velocity of particles increases as the diameter of the fluidization column is reduced, or if the height of the bed is increased. These trends are shown to be due to the influence of the wall. A new, semicorrelated model is proposed, which incorporates Janssen's wall effects in the calculation of the minimum fluidization velocity. The wall friction opposes not only the bed weight but also the drag force acting on the particles during fluidization. The enhanced wall friction leads to an increase in the minimum fluidization velocity. The model predictions compare favorably to existing correlations and experimental data.

A numerical study of a dilute turbulent gas‐particle flow with inelastic collisions and turbulence modulation in an Eulerian framework is described. A new interpretation is provided for the interaction/coupling terms, based on a fluctuating energy transfer mechanism. This interpretation provides for a new robust closure model for the interaction terms with the ability to predict the turbulence dampening as well as the turbulence enhancement phenomenon. Further, the model developed herein is investigated along with a variety of other published closure models used for the interaction/coupling terms, particle drag, and solid stress. The models are evaluated against several sets of benchmark experiments for fully‐developed, turbulent gas‐solid flow in a vertical pipe. © 2011 American Institute of Chemical Engineers AIChE J, 2012

Experimental investigations show that fluidized binary mixtures exhibit varied pressure drop profiles and segregation patterns, depending on the level of disparity due to size and/or density differences. In this study, different mixture types are mapped on a graph of density versus size ratios. It is found that the ratio of the minimum fluidization velocities of individual components can be used to categorize these mixtures. A simple correlation is developed to compute the ratio of the minimum fluidization velocities based on the density and size ratios. Categorizing the binary mixtures in this manner gives a qualitative understanding of how the different mixtures behave on fluidization. V V C 2010 American Institute of Chemical Engineers AIChE J, 57: 1446AIChE J, 57: -1458AIChE J, 57: , 2011 Keywords: fluidization, segregation, mixing, binary mixture IntroductionFluidized beds are popular in industry. For example, drying of pharmaceutical powders 1 and catalytic cracking of petroleum and coal combustion for generating electricity 2 are commonly performed in fluidized beds. Fluidization operations are based on contact between a fluid stream and a mixture of solid materials, which varies for each process. A common observable phenomenon associated with multisolid beds is that they may segregate when subjected to fluidization. Segregation is primarily due to particle size and/or density differences.The segregation tendency of a powder mixture influences the overall process efficiency and, hence, segregation via fluidization is one of the key areas of fluidization research. Much experimental data exists in the literature concerning fluidized bed segregation, yet the current understanding of the mechanisms controlling multisolid fluidization segregation is very poor, even for two-component particle mixtures. In fact, for a simple binary mixture fluidized by a gas, there are a variety of pressure drop profiles that have been observed, and the different pressure drop profiles lead to different segregation patterns. For example, Formisani et al. 3 presented several binary mixtures having large density and/ or size ratios that fluidize at two different velocity points, while Joseph et al. 4 has reported mixtures that fluidize at a single velocity point, just like a monocomponent powder. Marzocchella et al.5 studied an extremely disparate mixture that mixes well at low velocities, just beyond the point of fluidization, but segregates at larger velocities. Complicating matters further, the phenomenon of layer inversion has also been observed in fluidized mixtures containing smaller, denser particles and coarser, less dense particles. 6This study is the first attempt to catagorize the various gas fluidized binary mixture types reported by various authors. In addition, a corelation between the minimum fluidization Correspondence concerning this article should be addressed to A. Rao at akhilrao@hotmail.com. is proposed that can be used to distinguish between these mixture types, the observed pressure drop profiles, and segr...