Experimental Quantification of the Lateral Mixing of Binary Solids in Bubbling Fluidized Beds

Abstract

A novel experimental method for the lateral mixing of binary solids in bubbling fluidized beds was developed based on the capacitance probe technique. The evolutions of local mixing ratios in a fluidized bed which can be assumed as one mixing cell were analyzed in detail. The solids mixing within one mixing cell was resolved and the effect of convection and diffusion mechanism on lateral mixing was evaluated individually. The results show that at lower part of the fluidized bed, convection plays a more important role in the mixing process near the wall; meanwhile, diffusion is very important for the mixing around the center line. This is opposite with that at the higher part. A lateral micro dispersion coefficient was proposed to characterize the lateral mixing within the mixing cell and the value is generally between 0.005 and 0.025 m/s. A new mixing index was proposed to evaluate the lateral mixing quality of binary solids. It was found that at the lower part of the fluidized bed, the best mixing is acquired at the half radius, whereas mixing at the center line is the worst. At the higher part, solid mixing is better when increasing the distance from the wall. The influences of gas velocity and static bed on the lateral mixing were also discussed from a microscopic perspective.

Experimental Quantification of the Radial Mixing of Binary Mixtures in Annular Fluidized Beds Using the Capacitance Probe Method

“…The above discussions prove that when the capacitance probe is in emulsion solids, the signal changes linearly with the volume fraction of dense particles at the measurement point, as also claimed in our previous research (Li et al). In other words, when f j changes from 0.0 to 1.0, the signal corresponding to emulsion solids changes from V f to V j .…”

“…It has been proved that the signal of the capacitance probe system and volume fraction of gas or particle follow a linear relationship (Wiesendorf and Werther). In addition, it has been proven in our former research that when the capacitance probe method is used in the fluidized bed dealing with binary mixtures, the volume fraction of dense particles F d and the output signal v follow a linear relationship (Li et al): in which k and b are the calibration constants.…”

“…Instead, we suggest characterizing the mixing state using a new concept, particle mixing ratio. The mixing ratio of dense particles, f j is actually the volumetric fraction of dense particles in only particles neglecting the air (Li et al 27 ). When f j is 0.0, the particles are all light particles, and the output signal corresponding to the emulsion solids is recorded as V f .…”

“…In the mixing process, when emulsion solids flow through the measurement point, the averaged signal is used to calculate f j at the measurement point and the f j is shown as a scatter in Figure 4. The standard deviations (STDs) of f j in each 5 s are calculated to evaluate the fluctuation of f j with time, from which the effect of the convection and diffusion mechanism on the mixing process can be analyzed (Rowe and Partridge, 26 Li et al 27 ).…”

“…The main reason for a lack of deep understanding about the radial particle mixing in annular fluidized beds is the lack of appropriate measurement methods. The advantages and disadvantages of the existing measurement methods for particle mixing in fluidized beds were analyzed in our previous research (Li et al). An appropriate measurement method for the continuous, multipoint measurement of the mixing parameters in annular fluidized beds is still lacking.…”