Group Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Title: P3.37 - Mixing observations in a two component fluidized bed
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Permanent Link: http://ufdc.ufl.edu/UF00102023/00527
 Material Information
Title: P3.37 - Mixing observations in a two component fluidized bed Fluidized and Circulating Fluidized Beds
Series Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Physical Description: Conference Papers
Creator: Keller, N.K.
Fox, R.O.
Heindel, T.J.
Publisher: International Conference on Multiphase Flow (ICMF)
Publication Date: June 4, 2010
 Subjects
Subject: biomass processing
fluidized bed
hydrodynamics
mixing
segregation
 Notes
Abstract: Fluidized bed technology is useful for combustion, pyrolysis, and gasification of solid fuels such as biomass, which is important to industry because biomass is a potential alternative to petroleum-based fuels. There is usually a notable difference in the fluidization behavior between the solid fuel particle and the fluidized bed media (e.g., refractory sand) due to contrasting size, shape, and particle density; these differences can lead to poor solid-fuel distribution and diminished performance. The hydrodynamics in a fluidized bed drive gas-solid contact, and thus, have a significant influence on fluidized bed performance. Although fluidized bed hydrodynamics are key parameters in their operation, they are still poorly understood, particularly when the solid fuel component, like biomass, is significantly different from the fluidized bed media. This study provides a summary of the visual observations when a model biomass material is mixed with inert fluidized bed material in a 9.5 cm diameter cold-flow fluidized bed. The model biomass is composed of ground corncob in three different size ranges (200-300 μm, 500-600 μm, and 800-1000 μm), while the inert bed material is 500-600 μm glass beads. The biomass comprises 25% of the static bed volume. The ground corncob/glass bead mixtures are fluidized at several flow velocities while particle segregation is observed and documented. It is shown that segregation can be significantly enhanced by reducing particle electrostatic effects simply by humidifying the fluidizing gas stream.
General Note: The International Conference on Multiphase Flow (ICMF) first was held in Tsukuba, Japan in 1991 and the second ICMF took place in Kyoto, Japan in 1995. During this conference, it was decided to establish an International Governing Board which oversees the major aspects of the conference and makes decisions about future conference locations. Due to the great importance of the field, it was furthermore decided to hold the conference every three years successively in Asia including Australia, Europe including Africa, Russia and the Near East and America. Hence, ICMF 1998 was held in Lyon, France, ICMF 2001 in New Orleans, USA, ICMF 2004 in Yokohama, Japan, and ICMF 2007 in Leipzig, Germany. ICMF-2010 is devoted to all aspects of Multiphase Flow. Researchers from all over the world gathered in order to introduce their recent advances in the field and thereby promote the exchange of new ideas, results and techniques. The conference is a key event in Multiphase Flow and supports the advancement of science in this very important field. The major research topics relevant for the conference are as follows: Bio-Fluid Dynamics; Boiling; Bubbly Flows; Cavitation; Colloidal and Suspension Dynamics; Collision, Agglomeration and Breakup; Computational Techniques for Multiphase Flows; Droplet Flows; Environmental and Geophysical Flows; Experimental Methods for Multiphase Flows; Fluidized and Circulating Fluidized Beds; Fluid Structure Interactions; Granular Media; Industrial Applications; Instabilities; Interfacial Flows; Micro and Nano-Scale Multiphase Flows; Microgravity in Two-Phase Flow; Multiphase Flows with Heat and Mass Transfer; Non-Newtonian Multiphase Flows; Particle-Laden Flows; Particle, Bubble and Drop Dynamics; Reactive Multiphase Flows
 Record Information
Bibliographic ID: UF00102023
Volume ID: VID00527
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: P337-Keller-ICMF2010.pdf

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7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010


Paper No.: 1673


Mixing observations in a two component fluidized bed


Norman K. Keller Rodney O. Foxt, and Theodore J. Heindel


*Department of Mechanical Engineering, Iowa State University, Ames, IA 5001 1-2161, USA

Department of Chemical and Biological Engineering, Iowa State University, Ames, IA, USA 50011

nkeller~iastate.edu, rofox~iastate.edu, and theindel~iastate.edu

Keywords: biomass processing, fluidized bed, hydrodynamics, mixing, segregation


Abstract

Fluidized bed technology is useful for combustion, pyrolysis, and gasification of solid fuels such as biomass, which is important to
industry because biomass is a potential alternative to petroleum-based fuels. There is usually a notable difference in the fluidization
behavior between the solid fuel particle and the fluidized bed media (e.g., refractory sand) due to contrasting size, shape, and particle
density: these differences can lead to poor solid-fuel distribution and diminished performance. The lwdrodynamics in a fluidized bed
drive gas-solid contact, and thus, have a significant influence on fluidized bed performance. Although fluidized bed hydrodynamics
are key parameters in their operation, they are still poorly understood, particularly when the solid fuel component, like biomass, is
significantly different from the fluidized bed media.

This study provides a summary of the visual observations when a model biomass material is mixed with inert fluidized bed material in
a 9.5 cm diameter cold-flow fluidized bed. The model biomass is composed of ground corncob in three different size ranges (200-300
pLm, 500-600 pLm, and 800-1000 pLm), while the inert bed material is 500-600 pLm glass beads. The biomass comprises 25% of the
static bed volume. The ground comncob/glass bead mixtures are fluidized at several flow velocities while particle segregation is
observed and documented. It is shown that segregation can be significantly enhanced by reducing particle electrostatic effects simply
by humidifying the fluidizing gas stream.


Introduction

With expected shortages in fossil fuel supplies and
environmental changes, biomass as an energy source has
gained a lot of interest during the past decade due to its
potential to greatly reduce greenhouse gas emissions and to
serve as a permanent, renewable energy source. Many studies
have been carried out on the potentials of renewable energy as
well as on the technologies to most efficiently convert biomass
into useful energy forms (Balat 2006, Balat, Acici & Ersoy
2006, Brown 2003, Demirbas 2001, Kaygusuz 2001, Kelly-
Yong, Lee, Mohamed & Bhatia 2007, Searcy, Flynn, Ghafoori
& Kumar 2007). Among these technologies, thermochemical
processing of biomass appears to be one of the most
promising. Most likely, thermochemical processing will utilize
either pyrolysis or gasification in a fluidized bed reactor
because of its efficient mixing properties, low pressure drop
and high heat and mass transfer rates. The process efficiency
is determined by the mixing and segregation behavior of the


particles in the bed. However, since biomass particles have
different plwsical properties, e.g. size, shape, density, than the
bed material, e.g. refractory sand, an investigation on the
mixing and segregation behavior in a fluidized bed is
necessary to efficiently utilize it for energy production.

Mixing and segregation of different kinds of particles in two-
and three- dimensional fluidized beds has been researched
over the past three decades and cover aspects such as
mixing/segregation mechanisms and patterns as well as other
important factors of fluidized bed operation (Das, Meikap &
Saha 2008, Nienow, Rowe & Chiba 1978, Rice & Brainovich
1986, Rowe & Nienow 1975, Rowe & Nienow 1976, Rowe &
Yacono 1976, Shen, Xiao, Niklasson & Johnsson 2007, Van
Ommen & Mudde 2008, Wirsum, Fett, Iwanowa & Lukjanow
2001, Yang 2006, Zhang, Jin & Zhong 2009, Zhang, Jin &
Zhong 2008). The lwdrodynamic behavior of binary fluidized
beds is strongly influenced by the difference in plwsical
properties of the respective particles, especially size and



































Gas enters the plenum through the inlet and pressure
measurements are used to identify the minimum fluidization
velocities for this research. Pressure is recorded with a Dwyer
0-34.5 kPa pressure transducer located in the plenum wall.
The transducer has a maximum error of 10.25% of the full
scale reading (86 Pa). The compressed air supply from the
laboratory serves as the fluidizing gas for the beds. The air
stream is controlled through a series of ball valves, pressure
regulators, and four flow meters as outlined by Franka (Franka
2008). Flow meter error is less than 2% of the full scale
reading. The pressure transducer and flow meters are
interfaced to a computer-based data acquisition system.
Average pressure and gas flow rates are recorded to determine
the minimum fluidization velocity as well as the superficial
gas velocities of interest.

Material selection
The fluidization hydrodynamics of various bed compositions
are examined in this study. The properties of the two particle
types used in this study, glass beads and ground corncob, are
summarized in Figure 1.

Table 1: Properties of bed material.
Particle properties Glass beads Corncob
diameter [pm] 500-600 200-300
500-600
800-1000
particle density [g/cm3] 2.6 0.8-1.2


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010


Paper No.: 1673


density. Nienow et al. (Nienow, Rowe & Chiba 1978) pointed
out the relation between the superficial gas velocity and the
mixing/segregation behavior in a coal gasifier. According to
this study, segregation will occur at a lower superficial gas
velocity while mixing is solely due to rising bubbles. The
effects of relative humidity on the electrostatic charge of the
particles in the fluidized bed has been studied as well
(Guardiola, Rojo & Ramos 1996).

The objective of the current study is to examine the mixing
and segregation behavior of biomass particles in a fluidized
bed composed of a model biomass (ground corncob) and bed
material (glass beads) by means of visual observation.
Emphasis has been laid on the influence of superficial gas
velocity and humidity of the fludizing air stream. It will be
shown that the humidity of the fluidizing gas stream plays a
significant role in the mixing and segregation behavior of
biomass in fluidized beds.

Nomenclature
D Fluidized bed internal diameter (cm)
H Bed height (cm)
U, Superficial gas velocity (cm/s)
Ume Minimum fluidization velocity (cm/s)



Experimental Procedures

Fluidized bed reactor
As illustrated in Figure 1, the cold flow fluidized bed used in
this study is made from a 9.5 cm inner diameter acrylic tube.
The total height of the bed chamber and riser is 40 cm. Air
enters the column through the inlet at the bottom and is
distributed by a tube with 16, 0.6 cm diameter holes that serve
to gradually expand the air. Leaving the plenum chamber, the
air passes through a distributer plate and screen. The
distributer plate is comprised of 100, 0.1 cm diameter holes,
each spaced 0.4 cm apart on a square grid. To prevent small
particles from dropping through the distributer plate or
clogging it, a 45 mesh screen with openings of 0.04 cm is
attached to the plate. The top of the reactor is open to the
atmosphere.


40 cm


bed chamber
plenum chamber
air inlet
inlet air distributor
plenum pressure taps
distributor plate & screen
bed pressure taps


9.5 cm




o


o



Fiur : xprmeta ludie od





























Figure 3: Close-up of the 500-600 pLm ground corncob.

Minimum fluidization and experimental conditions
The minimum fluidization velocity, Ume, is one of the most
important fundamental parameters related to fluidization
hydrodynamics and is used to normalize flow conditions in
this study. Ume is experimentally determined for glass beads
(15.3 cm/s) using the procedure outlined elsewhere (Franka
2008); for this test, the bed was filled to a height of 1 diameter
with the 500-600 Cpm glass beads. On this basis, three different
superficial gas velocities have been applied to each bed, as
summarized in Table 2.

Table 2: Superficial gas velocity of experiments, all
referenced to glass bead values.
Min. fluidization velocity of glass beads
Umf,GB [Cm/S] 15.3
Superficial gas velocity applied [cm/s]
1 xUmf,GB 15.3
2 xUmf,GB 30.6
3 xUmf,GB 45.9


Experiments have been conducted with varying ground
corncob particle size. Each bed has a total static height of 9.5
cm, which corresponds to one diameter bed height (H/D =
1.0). A 25% volume fraction of corncob, with the remainder
glass beads, has been used for all experiments. Initially, the
ground corncob is located on top of the glass beads, as
illustrated in Figure 4. Experiments have also been conducted
in which the glass beads are on top of the ground corncob, as
well as with an initially well-mixed system, to investigate the
effect of initial bed material distribution.


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010


Paper No.: 1673


Glass beads have been chosen as the inert bed material
because they have similar properties to that of sand (which is
usually found in a fluidized bed reactor), but are better
characterized and uniform. Figure 2a shows a high resolution
photograph of the glass bead particles in the 500-600 pLm size
range. Most glass bead particles are spherical, smooth and
solid.

The biomass is simulated with ground corncob in varying
particle size ranges. Figure 2b shows a high resolution
photograph of the ground corncob particles in the 500-600 pLm
size range. Note that the ground corncob particles appear to be
"chuck-like", "plate-like", and "stalk-like" in shape even
though they may be modeled as spherical (Deza, Battaglia &
Heindel 2008, Deza, Battaglia & Heindel 2007, Deza,
Battaglia & Heindel 2008). Close-up pictures of the ground
corncob (Figure 3) also show large caverns and porous
structures. These particle characteristics could impact particle
mixing and segregation.


(8)


(b)


Figure 2: 500-600 pLm (a) glass beads and (b) ground corncob.







Paper No.: 1673


500-500 u
-- ground comcob

soo-so Cn
- glass beads


Aeration plate
Plamum
etrnsducer


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

also covered with a screen to prevent particle elutriation at the
higher superficial gas velocities. As the superficial gas
velocity was increased, the mixing rate increased. Figure 5
shows a time sequence of pictures from the initial condition to
a long time in which the conditions do not change (identified
as "Equilibrium"). The applied superficial gas velocity was U,
= 2Ume = 30.6 cm/s. For each image the gas flow rate was
abruptly stopped by closing a ball valve and the bed was
allowed to settle before the bed conditions were recorded. The
bed was then refluidized by slowly opening the ball valve
(over a period of 2-3 seconds). The time period corresponds to
the total fluidization time since the initial bed conditions (time
equals 0 seconds in Figure 5).


Figure 4: Sample initial condition with 500-600 pLm glass
beads (bottom material) and ground corncob (top material).

In addition to varying the ground corncob particle size and
superficial gas velocity, experiments were conducted with
humidified air as well as with as-supplied compressed air (low
humidity). It was assumed that the humidified air was nearly
saturated and the as-supplied air was nearly dry, which, as
shown below, has an impact on electrostatic build-up.

Results

Observations of the mixing and segregation results are
recorded by means of still and video images. Multiple
visualization tests at each condition have been completed and
the results presented below are based on repeatable
observations; hence, only selected still images are presented
that represent typical results. Several operating parameters
may influence bed segregation and mixing, including
superficial gas velocity, biomass particle size and density,
relative humidity of the gas stream, volume ratio of biomass to
bed material, and initial bed conditions. All but biomass
density changes have been tested in this study and only those
that showed a significant effect are discussed below.

Effect of superficial gas velocity
Several tests have been performed to investigate the effect of
superficial gas velocity on the mixing and segregation
behavior of biomass. It has been found that higher gas
velocities lead to better particle mixing. These tests were
completed with 500-600 pLm glass beads as bed material and
either 200-300 pLm, 500-600 pLm or 800-1000 pLm ground
corncob as model biomass. Initially, ground corncob
comprised a height of 1/4D, where D is the fluidized bed
diameter, and was located on top of 3/4D glass beads (Figure
4). The fluidizing gas was humidified. The reactor top was


20 sec


time
Figure 5: Mixing and segregation with U, = 2Ume = 30.6 cm/s.

In general, approximately 10 to 20% of the ground corncob
particles mix with the glass beads while the rest of the ground
corncob remains segregated on top of the bed. This is
demonstrated by the upward shift of the segregation line
between phases. The equilibrium condition changes depending
on the superficial gas velocity. Lowering the superficial gas
velocity enhances segregation, resulting in a condition that is
closer to the initial condition. Increasing the superficial gas
velocity, on the other hand, enhances particle mixing. For
example, Figure 6 shows a time sequence for the same bed
composition as in Figure 5 but with U, = 3Ume = 45.9 cm/s. As
shown, increasing U, increased the particle mixing and this
effect was noted with all corncob particle sizes used in this
study.


5 min


time

Figure 6: Mixing and segregation with U, = 3Ume = 45.9 cm/s.


5 mln Equilibrium~


Equilibrium





























Almost na mixingof the bed material
0 30sec 5 mire 40 min
time
Figure 8: Particle mixing and segregation using humidified
fluidization gas.

Effects of particle size of the biomass
As shown above, humidification promotes particle
segregation. Therefore, a series of experiments were
completed in which the 25% by volume ground corncob was
initially well-mixed with the glass beads. The bed was then
fluidized at U, = 2Ume = 30.6 cm/s and images were recorded
at selected time intervals. In general, fluidizing the well-mixed
particle bed with humidified air promoted ground corncob
segregation. Smaller ground corncob particles typically
segregated sooner than larger ground corncob particles. These
results are illustrated in Figures 9-11, where 500-600 pLm glass
beads and the respective ground corncob particles were
initially well mixed. Also, the amount of biomass being
segregated at equilibrium varied according to the biomass
particle size. This is indicated by the segregation line between
the phases at the equilibrium condition; a rough estimate (in
percentage) of how much biomass is segregated out is also
provided in the figures.


time
Figure 7: Particle mixing and segregation using unhumidified
(dry) fluidization gas.

In contrast, Figure 8 shows an experiment with similar bed
composition but using a humidified gas stream, and the
ground corncob initially on top. Note that the change of colors
in the pictures is due to improved camera settings (i.e.
autoflash) during the experiment. Only a few particles stick to
the reactor walls for this condition. Even more interesting,
only a small amount of mixing is observed and the bed
remains segregated. Hence, a clear demarcation separates the
two material phases. This trend was observed for all particle
sizes and superficial gas velocities considered in this study.
When the ground corncob was initially on the bottom, the bed


.. LI_ I ll g~~l
time
Figure 9: Particle segregation using 200-300 pLm ground
corncob.


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010


completely turned over in about 10 seconds and then remained
segregated.
Almost no particles sticking ta the walls ofthe
reactordueto electrostatic charge buildup


Paper No.: 1673


Effect of humidilled gas stream
The effect of fluidization gas humidity was determined by
performing experiments with and without humidifying the
fluidization gas. These experiments were conducted with Ug=
2Ume = 30.6 cm/s and 500-600 pLm ground corncob particles.
The results show that a fluidization gas with a high humidity
significantly lowers electrostatic charge buildup and promotes
segregation. In contrast, fluidizing with low humidity or dry
air yields a well mixed bed, but it also has a high electrostatic
charge. Figure 7 shows the results from an experiment with
1/4D of 500-600 pLm corncob initially on the bottom and 3/4D
500-600 pLm glass beads on top when (dry) compressed air is
used to fluidize the bed. With time, more and more particles
stick to the walls of the reactor due to electrostatic charge
buildup. Also the bed materials appear to be well mixed. From
the initial condition, the bed completely tumns over during the
first 10 seconds of fluidizing, so that the ground corncob floats
on top of the glass beads. It then becomes mixed in the bed, as
shown in Figure 7.

Particlesstickingtothe walls of the reactor
due to electrotstaic charse bul dup


Miningofthe bed material
30sec 5 min


120 mire


20 sec


1 mln Equillbriur







Paper No.: 1673


Figure 9 shows that with very small biomass particles (200-
300 pLm), almost all the biomass is segregated out. More than
half of the material accumulates on top of the glass beads af ter
only 20 seconds. It is estimated that only about 10% of the
biomass is mixed with the bed material at equilibrium (~ 5
minutes).


20sec


time
Particle segregation using


Figure 10:
corncob.


500-600 pLm ground


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010


Figures 9-11 show that the less dense material, i.e. biomass,
segregates from the glass beads and floats on top of the bed.
Even the largest biomass particles, which have a mass larger
than the individual glass beads, rise to the surface when the
fluidizing gas is humidified.

Conclusions

Mixing and segregation were observed in a two-component
fluidized bed. Mixing was enhanced at higher superficial gas
velocities. Humidifying the gas stream showed a significant
effect on the electrostatic charge buildup as well as on the
mixing of the biomass with the bed material. High humidity in
the gas stream lowered the electrostatic charge buildup of the
particles and promoted segregation of the biomass. Finally, the
biomass particle size influenced the amount of biomass being
segregated as well as the time for segregation. Smaller
biomass particles segregated faster than larger ones. Smaller
biomass particles also tended to segregate more evenly and
leave fewer particles mixed with the bed material.

Acknowledgements

Support for portions of the work described in this paper from
ConocoPhillips Company is acknowledged.

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With 500-600 pLm biomass particles, the biomass does not
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-50%




0 20sec Imin Equillbrium
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1 mln Eqruilibrium







7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010


Paper No.: 1673


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