Elutriation and Species Segregation Characteristics of Polydisperse Mixtures of Group B Particles In a Dilute CFB Riser

Elutriation and Species Segregation Characteristics of Polydisperse Mixtures of Group B particles in a dilute CFB Riser

Jia Wei Chew, Drew M. Parker, Christine M. Hrenya^*

Department of Chemical and Biological Engineering

University of Colorado at Boulder

Boulder, CO 80309

Elutriation has long been recognized as an issue in gas-solid fluidized beds, with a largely empirical understanding focused on binary mixtures.  In this work, experiments in a gas-solids circulating fluidized bed (CFB) have been carried out using two categories of polydispersity: binary mixtures with varying compositions and continuous particle size distributions (PSDs) with varying widths. Measurements include total and species elutriation flux, and local mass flux and species segregation. Qualitative differences between the binary mixtures and continuous PSDs were observed.  First, while an increase of the total elutriation flux is linked to an increase in the mass % of fine particles (with terminal velocity, U_t, less than the superficial gas velocity, U_s) for the binary mixtures, the trends are opposite for the continuous PSDs, the latter of which is counter to the plethora of composition-based elutriation correlations. Second, while the elutriation flux of coarse particles (U_t > U_s) increases non-monotonically with mass % of fines for binary mixtures, that of continuous PSDs is monotonic. Both of these qualitative differences can be explained by the increasing disparity of size differences with an increase in the distribution width for the continuous PSDs, while the size disparity remains constant in binary mixtures of varying compositions.  Third, while the mass % of coarse particles decreases non-monotonically with riser height for binary mixtures near the wall, the decrease is monotonic for continuous PSDs. These observations underscore the inherent differences between binary mixtures and continuous size distributions, and point to the need for independent experiments and model validation for each type of polydispersity.

^*Corresponding author: hrenya@colorado.edu