Fundamental bubbling characteristics in a rotating fluidized bed: Modeling and measurement of bubble size
Modeling and measurement of bubble size in a rotating fluidized bed (RFB) were conducted. The model for predicting the bubble growth in RFB has been proposed based on the following concepts: (i) local centrifugal acceleration and excess gas velocity are considered as the parameters in the radial direction, and (ii) bubble volume flow locally changes depending on the radial distance. Bubbling behaviors were also experimentally observed by means of a high-sped video camera. Bubble sizes were measured using an image analysis technique, and the measured bubble sizes were compared with the estimated values by our proposed model and the available one for a conventional fluidized bed. The estimated bubble sizes by our proposed model showed good agreement with the measured ones, while the predicted ones by the model for conventional fluidized bed underestimated. The validity of our proposed concepts for modeling of bubble growth in RFB was confirmed.
On partial fluidization in rotating fluidized beds
In a rotating fluidized bed, unlike in a conventional fluidized bed, the granules are fluidized layer by layer from the (inner) free surface outward at increasing radius as the gas velocity is increased. This is a very significant and interesting phenomenon and is extremely important in the design of these fluidized beds. The phenomenon was first suggested in a theoretical analysis and recently verified experimentally in the authors' laboratory. However, in the first paper, the equations presented are too cumbersome and the influence of bed thickness is not clearly stated. In this note the authors present simplified equations, based on that paper, for the pressure drop and the minimum fluidizing velocities in a rotating fluidized bed. Experimental data are also shown and compared with the theoretical model, and the effect of bed thickness is shown. Furthermore, an explanation for the observation of a maximum in the pressure drop vs. velocity curve instead of the plateau derived by Chen is proposed.
Enhanced fluidization of nanoparticles in an oscillating magnetic field
Some experimental observations on the fluidization characteristics of nanoparticles in the form of agglomerates with magnetic assistance are presented. The nanoagglomerates consist of Degussa Aerosil® R974 fumed silica, with a primary particle size of 12 nm. An oscillating AC magnetic field is used to excite large (mm size) permanent magnetic particles mixed in with the nanoparticle agglomerates, and the fluidization behavior of the nanoagglomerates, including the fluidization regime, the minimum fluidization velocity, the bed pressure drop, and the bed expansion are investigated. It is shown that, with the aid of an oscillating magnetic field at low frequencies, the bed of nanoparticle agglomerates can be smoothly fluidized, and the minimum fluidization velocity is significantly reduced. In addition, channeling or slugging of the bed disappears and the bed expands uniformly without bubbles, and with negligible elutriation. The bed expansion and the minimum fluidization velocity depend on the mass ratio of magnetic particles to nanoparticles, and the intensity and frequency of the oscillating magnetic field. © 2005 American Institute of Chemical Engineers AIChE J, 2005