(25b) 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