(154e) Mixing and Segregation of Biomass Particles In a Bubbling Bed

Fluidized beds are being widely considered for converting
solid biomass into liquid fuels, syngas, and chemical
products.     While fluidized beds have
been extensively studied for many applications, the detailed hydrodynamics and
mixing patterns in systems with disparate types of bed particles is still not
fully understood.  A number of biomass
gasification and pyrolysis processes now being considered employ bubbling beds
of fine sand into which larger biomass particles are fed.  At steady state, the biomass particles
usually constitute only a small fraction of the bed so that the dynamics
involve a few coarser, lighter particles circulating in a finer, denser
medium.  This disparity in particle sizes
and densities creates the potential for segregation, especially at low
fluidizing velocities.  The extent of
segregation can be extremely important, because it determines the intimacy and
duration of contacting between released gases and bed material.  In turn, the degree of gas-particle
contacting can greatly affect product yield and composition, especially when
the bed material has catalytic properties. 
We report results from visual observation of bubbling laboratory beds
combined with magnetic particle tracking to measure detailed mixing and
segregation patterns for simulated biomass particles.  Our ultimate goal is to develop correlations
and statistical models which can be used to predict biomass mixing and
segregation patterns in bubbling bed reactors.

Experimental
Materials and Methods

We use our previously reported (1) magnetic particle
tracking method in a 55 mm three-dimensional bed fluidized with air at ambient
conditions.  Safe, inexpensive magnetic
tracers and detectors are used, thus avoiding the issues associated with past
particle-tracking methods.  In this
study, we used neodymium magnets embedded in wooden spherical and cylindrical
particles and externally positioned magnetic field detectors to continuously
locate the position of a tracer particle. 
The method takes advantage of the tendency of the tracer particles to
align their magnetic axes with the earth's magnetic field like a compass
needle.  Proper positioning of the probes
can take advantage of this tendency, which simplifies the data analysis.  We also collected high-definition slow-motion
videos of the bed surface to observe top-segregated tracer motion.

Both beds of approximately 200 micron glass beads and 100 to
200 micron sand were studied.  Single
biomass tracer particles were dropped into the beds and their positions
determined over a 5-minute run time. 
Tracers had diameters of 3 to 5 mm and densities from 0.55 to 1.2
g/cc.  Tracer positions were determined
using the algorithm we previously reported on. 
Statistical analysis on the time series position data was used to
characterize the tracer behavior. 
Fluidization velocity varied from 1.1 to 6 times
minimum fluidization.  Slumped bed depth
to diameter ratio was held constant at 1.

Results and
Discussion

Frequency distributions of the vertical position for various
conditions were determined.  In some
cases these show segregation at the top or bottom and under other conditions a
uniform distribution across the bed.  We
found that these distributions could be reasonably well fitted with the
two-parameter Weibull distribution function and that
the two parameters correlated with tracer density and superficial
velocity.  Vertical velocity
distributions were also determined.  In
addition, typical three-dimensional trajectories showed typical bubble-inducted
solids circulation at low velocities, top and bottom segregation behaviors, and
mixing at higher velocities.  It was
found that a tracer with a density of 0.89 g/cc
continued to circulate (appeared to be neutrally buoyant) at even very low
fluidization velocities (1.1 times minimum fluidization velocity) while heavier
tracers segregated to the bottom and lighter ones to the top at low
velocities.  At high velocities, all
tracers in the 0.55 to 1.2 g/cc density range mixed and circulated.  Higher-density tracers, however, did not always
circulate, even at the highest velocities studied.  The implications for biomass processing is
that appropriate processing conditions will likely depend on the type of
biomass, degree of dryness, and whether it is processed or raw.

1. Patterson et al., Ind. Eng. Chem. Res. 2010, 49, 5037-5043.

Abstract submitted for
presentation at the 2011 Annual A.I.Ch.E. meeting in Minneapolis, MN