Group Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Title: 11.2.1 - Bed Height and Material Density Effects on Minimum Fluidization Velocity in a Cylindrical Fluidized Bed
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 Material Information
Title: 11.2.1 - Bed Height and Material Density Effects on Minimum Fluidization Velocity in a Cylindrical Fluidized Bed Fluidized and Circulating Fluidized Beds
Series Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Physical Description: Conference Papers
Creator: Escudero, D.
Heindel, T.J.
Publisher: International Conference on Multiphase Flow (ICMF)
Publication Date: June 4, 2010
 Subjects
Subject: bed height
biomass
fluidized bed
minimum fluidization velocity
 Notes
Abstract: Minimum fluidization velocity is one of the most important parameters when characterizing the hydrodynamics of a fluidized bed. Experimental studies on the effects of bed height and material density on the minimum fluidization velocity were carried out using a 10.2 cm diameter cylindrical fluidized bed. Three different Geldart type-B particles were tested: glass beads, ground walnut shell, and ground corncob, with material densities of 2600 kg/m3, 1300 kg/m3, and 1000 kg/m3, respectively. The particle size was selected to be the same for all three materials and corresponds to 500 - 600 μm. In this study, five different bed height-to-diameter ratios were used: H/D = 0.5, 1, 1.5, 2 and 3. Minimum fluidization velocity was determined for each H/D ratio using pressure drop measurements. Results show that minimum fluidization velocity for the three tested materials was insensitive to bed height. However, as the material density increased, the minimum fluidization velocity increased.
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
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Bibliographic ID: UF00102023
Volume ID: VID00277
Source Institution: University of Florida
Holding Location: University of Florida
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Resource Identifier: 1121-Escudero-ICMF2010.pdf

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Paper No 1674


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


Bed Height and Material Density Effects on Minimum
Fluidization Velocity in a Cylindrical Fluidized Bed

David Escudero and Theodore J. Heindel

Iowa State University, Department of Mechanical Engineering
Ames, Iowa, 50011, USA
drescude@tiastate.edu, theindel@tiastate.edu



Keywords: Bed height, biomass, fluidized bed, minimum fluidization velocity




Abstract

Minimum fluidization velocity is one of the most important parameters when characterizing the hydrodynamics of a
fluidized bed. Experimental studies on the effects of bed height and material density on the minimum fluidization velocity
were carried out using a 10.2 cm diameter cylindrical fluidized bed. Three different Geldart type-B particles were tested:
glass beads, ground walnut shell, and ground corncob, with material densities of 2600 kg/m3, 1300 kg/m3, and 1000 kg/m3,
respectively. The particle size was selected to be the same for all three materials and corresponds to 500 600 ntm. In this
study, five different bed height-to-diameter ratios were used: H/D = 0.5, 1, 1.5, 2 and 3. Minimum fluidization velocity was
determined for each H/D ratio using pressure drop measurements. Results show that minimum fluidization velocity for the
three tested materials was insensitive to bed height. However, as the material density increased, the minimum fluidization
velocity increased.


Introduction

Fluidized bed hydrodynamic behavior is very complex
and must be understood to improve fluidized bed
operations. One of the most important parameters to
characterize fluidized bed conditions is the minimum
fluidization velocity (Umf) (Ramos Caicedo et al., 2002),
which quantifies the drag force needed to attain solid
suspension in the gas phase. The minimum fluidization
velocity also constitutes a reference for evaluating
fluidization intensity when the bed is operated at higher
gas velocities (Zhong et al., 2008). In general, Umf is a
function of particle properties/geometry, fluid properties,
and bed geometry.

Fluidizing biomass particles is challenging due to their
irregular size, shape, and density. Understanding the
influence of these particular characteristics on the fluidized
bed hydrodynamics is important. Zhong et al. (2008)
studied the effects of particle size, density, and shape on
the minimum fluidization velocity using wood chips, mung
beans, millet, corn stalks, and cotton stalks. In this study,
they used a rectangular fluidized bed with a cross section
of 0.4 x 0.4 m and air as the fluidizing gas. They
determined that for long, thin biomass types, the minimum
fluidization velocity increased with increasing
length-to-diameter (L/dpt) ratio. Their experiments showed
their biomass did not fluidized when L/dpt,>20, indicating
that the biomass size and shape affected its fluidization.

Sau et al. (2007) determined the minimum fluidization
velocity for a gas-solid system in a tapered fluidized bed


and studied the effects that bed geometry, specifically the
tapered angle, had on the minimum fluidization velocity.
They used three different angles (4.61, 7.47, and 9.52) to
observe the effect on minimum fluidization velocity.
Results showed that as the tapered angle increased, Umf
increased, which implied a dependence of the minimum
fluidization velocity to the geometry of the fluidized bed.
Moreover, Hilal et al. (2001) analyzed the effects of bed
diameter, gas distributor, and inserts on minimum
fluidization velocity. It was shown that both the bed
diameter and the type and geometry of the distributor
affected Umf. For example, Umf increased with an increase
in the number of holes in the distributor plate.
Furthermore, with an increase in the bed diameter there
was a decrease in Umf. Finally, the insertion of tubes along
the fluidized bed reduced the effective cross sectional area
for fluidization, which produced a high interstitial gas
velocity causing a decrease in Umf.

The influence of bed height on minimum fluidization
velocity has been studied using different types of fluidized
beds. Zhong et al. (2006) completed minimum fluidization
experiments in spouted fluidized beds. In a spouted
fluidized bed, the bed chamber is tapered like a funnel,
which creates different hydrodynamics, and the
fluidization air is typically injected through a single
orifice. Zhong et al. (2006) used a two dimensional
spouted fluidized bed with dimensions 300 mm x 30 mm
and a height of 2000 mm, and fluidized a variety of
Geldart Type-D particles mungg beans, polystyrene,
millet). Filling the bed with these materials to different
heights (300-550 mm), they determined the minimum






Paper No 1674


spouting fluidization velocity, defined as the minimum
superficial gas velocity at which the spout initiates in the
central region and the surrounding annulus is fluidized;
this is analogous to minimum fluidization velocity in a
bubbling fluidized bed. They concluded that the static bed
height for a spouted bed influenced the minimum spouting
fluidization velocity; increasing the bed height increased
the spouting velocity.

Sau et al. (2007) used a gas-solid conical tapered
fluidized bed to find the minimum fluidization velocity and
the pressure drop across the bed. The dimensions of the
fluidized bed at the bottom were 48, 42, and 50 mm, the
top of the bed measured 132, 174, and 212 mm, and the
column heights were 520, 504 and 483 mm, respectively.
They concluded that bed height for this type of bed did not
have a significant effect on the minimum fluidization
velocity, i.e., Umfwas independent of bed height for this
type of conical tapered fluidized bed.

Ramos et al. (2002) studied the minimum fluidization
velocity for gas-solid 2D fluidized beds. They used a
rectangular bed (1 x 0.2 x 0.012 m) filled with glass beads
of three different diameters (160-250, 250-400, and
490-700 ntm) and various bed heights (2, 4, 8, 16, 20, 40,
and 60 cm). Their results revealed that as the static bed
height increased, Umf increased.

Gunn and Hilal (1997) studied gas-solid fluidized beds
using glass beads with beds that had 89 and 290 mm ID.
The glass bead diameters were 100 and 500 uim. They used
four different bed heights (20, 30, 40, 50 cm). The results
for minimum fluidization velocity showed that for all the
material and experimental conditions used in that study,
there was no significant change in the minimum
fluidization velocity when the bed height was increased.
Therefore, Umf was independent of bed height.

Cranfield and Geldart (1974) studied the fluidization
characteristics of large particles (1000-2000 ntm) of
alkalized alumina in a fluidized bed with a cross sectional
area of 61x61 cm at different bed heights (5, 10, 15, 20,
25, and 30 cm). They showed that for 3D beds, the
minimum fluidization velocity remained constant no
matter the bed height used in the experiments

The goal of this paper is to determined the effects
caused by varying the bed height and material density on
the minimum fluidization velocity in a 3D cylindrical
fluidized bed.

Nomenclature
dpt Particle diameter (mm)
D Fluidized bed internal diameter (cm)
H Bed height (cm)
H/D Height-to-diameter ratio
L Particle length (mm)
L/dpt Length-to-diameter ratio
Umf Minimum fluidization velocity (cm/s)

Subscripts
mf Minimum fluidization
pt Particle


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

Experimental Facility

The reactor used in these experiments is a cold flow
fluidized bed reactor. The cylindrical fluidized bed was
fabricated with 10.2 cm internal diameter (ID) acrylic with
a 0.64 cm wall thickness. As shown in Figure 1, the reactor
consists of three main chambers: the top chamber or
freeboard region, the bed chamber, and the plenum.
Fluidization occurs in the bed chamber which is 30.5 cm
tall and 10.2 cm ID. Square flanges (16.5x16.5 cm)
connect each section. An aeration plate is located
immediately below the bed chamber; it is fabricated from a
1.27 cm thick acrylic plate with 62, 1 mm diameter holes
spaced approximately 1.27 cm apart in a circular grid for a
total open area of 0.62%. To avoid material blocking the
aeration holes, a 45 mesh screen with openings of 0.04 cm
is attached to the plate using silicone adhesive.


Bed chamber


Bed Material


& screen


Plenum


S I I I I Air inlet plate
Pressure tap Air inlet
Figure 1: Fluidized bed reactor (not to scale). The static
bed height is identified by H.

Compressed air from the laboratory's building air
supply is used as the fluidizing gas for this research. The
pressure at which the compressed air is delivered inside the
laboratory is 620 kPa (90 psi). However, since the flow
rates used for fluidization vary depending of the specific
conditions of each experiment, an air flow control board
with four independent air lines is used to deliver the
required air to the fluidized bed

The fluidized bed air flow is regulated by a manual
stainless steel pressure regulator and attached filter, with a
pressure range of 0-862 kPa (0-125 psi) and maximum
inlet pressure of 2.07 MPa (300 psi). The regulated air
flows through two different mass flow meters: a 0-1000
Lpm stainless steel Aalborg GFM771 flow meter, which is
used for high gas flow applications, and a 0-200 Lpm
Aalborg GFM571 flow meter, used in lower gas flow


-I-






Paper No 1674


applications. This allows for better measurement resolution.
The flow through the respective mass flow meter is
controlled through ball valves.

Pressure is measured with a Dwyer 0-34.5 kPa (0-5
psig), 4-20 mA output pressure transducer located in the
bottom of the plenum. The signals obtained from the
pressure transducer and mass flow meters are connected to
a computer controlled data acquisition system.

LabView 8.5 acquisition software records real-time
pressure and flow rate measurements over a user-specified
period, and then the average pressure and flow rate are
calculated and recorded. Average measurements are
necessary due to the highly variable pressure signal caused
by the bubbling fluidized bed. In this study, data collection
occurs at a rate of 1000 Hz for a time interval of 5 seconds;
average pressure, and average flow rate are subsequently
written to a data file.

For this study, three different materials were selected
using the four criteria specified by Franka (2008). These
four criteria were (i) fluidization behavior, (ii) size range,
(iii) density, and (iv) aspect ratio. The fluidization behavior
refers to how easily the particles can be fluidized. To
compare the fluidization characteristics, the chosen
particles must fall within the same fluidization category.
The particle size between the compared materials follows
that of Franka (2008) and corresponds to 500-600 pm. This
size range is chosen because of its availability and low cost.
Three different materials were selected to cover a range of
particle densities. Finally, the aspect ratio desired for the
particles should be on the order of 1 to allow comparisons
between the particles. Additionally, particles that have a
uniform shape provide a better quality of fluidization. Thus,
the three materials used in this study are glass beads,
ground corncob, and ground walnut shell.

The bed bulk density was determined knowing the
material mass and the static bed volume. Bed material was
slowly added until the desired static bed height was
determined, which corresponded to H/D = 0.5, 1, 1.5, 2, or
3. Before the bed height was measured the bed was
fluidized and then allowed to collapse to avoid any packing
effects due to the filling process The material mass was
then measured and the given bed bulk density was
calculated. Table 1 summarizes the characteristics of the
various fluidized beds used in this study. Note that the bed
bulk density generally decreases slightly as the bed height
increases because the amount of air entrainment increases
as the bed is filled.


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

Table 1: Bed Material Characteristics.
Glass Beads
Bed Mass Bulk density
H/D (g) (kg/3)
0.5 670 161070
1 1320 159070
1.5 1860 149070
2 2560 154070
3 3610 144070
Diameter (fm) 500-600
Particle Density (kg/m3) 2600
Ground Corncob
Bed Mass Bulk density
H/D (g) (kg/m3)
0.5 170 41020
1 340 41020
1.5 490 39020
2 675 41020
3 1110 44020
Diameter (fm) 500-600
Particle Density (kg/m3) 800-1200
Ground Walnut Shell
Bed Mass Bulk density
H/D (g) (kg/3)
0.5 295 71050
1 500 60050
1.5 760 61050
2 1000 60050
3 1470 59050
Diameter (gm) 500-600
Particle Density (kg/m3) 1200-1400

To avoid electrostatic effects that may build up during
fluidization, the fluidization air is passed through a
humidifier before entering the fluidized bed inlet. Several
trials in the laboratory have shown that using this simple
solution completely eliminated electrostatic effects.

The minimum fluidization velocity is defined as the
minimum superficial gas velocity where particle
fluidization is achieved. Minimum fluidization velocity is
determining using the following pressure measurement
procedure. First, the reactor is filled with the desired
material to a specified height. Air at Ug= 40.8 cm/s is
passed through the bed for about an hour to allow the
material to absorb any moisture from the humidified air;
this process is repeated each time the material is replaced.
After this conditioning period, the pressure and flow rate
are acquired using the DAQ system. Data are collected at
1000 Hz over a 5 second interval, averaged over this
period, and then output to an Excel file. Next, the air flow
rate is decreased by 1 cm/s by closing the pressure
regulator. After waiting 60 seconds, a period such that the
bed is in a quasi steady state, the pressure and flow rate are
again averaged over a 5 second interval. This process is
repeated until the flow rate reaches Ug= 0 cm/s; at this
point the test is completed. For statistical purposes, each
test for the specified material and bed height is repeated 5
times.

After all the bed material data are collected, the same
procedure is repeated in an empty reactor. This is done to
quantify the pressure drop through the aeration plate and
plenum. The empty reactor pressure data are then
subtracted from the fluidized bed data at the respective






Paper No 1674


superficial gas velocity. Since the flow rates between the
empty reactor and fluidized bed tests do not match exactly,
a linear interpolation method is employed to calculate the
empty bed pressures corresponding to the fluidized bed
flow rates. Finally, the bed pressure drop is plotted as a
function of superficial gas velocity and the minimum
fluidization velocity is defined as the point in which the
pressure drop across the bed remains constant. Figure 2
shows a sample plot obtained for glass beads where the
static bed height corresponds to H/D =1.


1800

1600

S1400

r.1200
0
e-
1000

goo

P-1 600
cJ
M 400


0 10 20 30
U, (cm/s)


40 50


Figure 2: Sample minimum fluidization plot for glass
beads with H/D = 1.

Results and Discussion

Bed pressure drop increased when the H/D ratio
increased, this effect is related to the bulk density and mass
of the material. Hence, the bed pressure drop for glass
beads is larger than for ground corncob and ground walnut
shell. Figure 3 shows the bed pressure drop as a function of
gas velocity for the glass beads at different H/D ratio.

4500
0.5H/D
4000 EIH-D o
1 ,D
-' 3500 1 A1.5HD 0

3000 *2 HD 0
03HRD 0 f
2500 o

2000 0



f 1000
,o.^^.---------,


500 A

0
1oo o.Z


0 10 20 30
U (cm/s)


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

increased. Figure 4 shows that the minimum fluidization
velocity for the three materials is approximately constant.
Note the error bars represent 1 standard deviation from the
average of the five tests. Hence, it can be concluded that
there is not a correlation between bed height and minimum
fluidization velocity for this cylindrical fluidized bed.


30


25


20


S15


10


5


0


Figure 4: Minimum fluidization velocity as a function of
height-to-diameter ratio (H/D).

A force balance between the gravity and pressure
force was obtained for each material to emphasize the
minimum fluidization velocity. As it is shown in Figures 5,
6, and 7, the knee of the graphs (Umf) is approximately
independent of bed height for the three materials, and is
located on the y axis near 1. However, the values of the
bed pressure force over the bed weight that surround the
knee of the graph are not perfectly 1 due to wall frictional
forces.


1.2


1


0.8


S0.6


0.4


S02

o


0 10 20 30
Ug (cm/s)


40 50


40 50 Figure 5: Bed pressure force/bed weight as a function of
superficial gas velocity for glass beads.


Figure 3: Bed pressure drop as a function of superficial
gas velocity for glass beads.
Minimum fluidization velocity, on the other hand, did
not show considerably changes when the H/D ratio


0 Glass Beads

0 Ground Corncob

A Ground Walnut
Shell






Paper No 1674


12





S0-8

I-
o 0.6


04
I



0


0 10 20 30
U, (cm/s)


40 50


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

higher superficial gas velocity is required to overcome the
bed weight. Consequently, a larger pressure drop is
produced with high density materials, increasing Umf.


1800

1600

; 1400

a1200
I,
S1000

g800

P 600

m 400


Figure 6: Bed pressure force/bed weight as a function of.
superficial gas velocity for ground corncob.


0 10 20 30
Ug (cmls)


40 50


Figure 7: Bed pressure force/bed weight as a function of.
superficial gas velocity for ground walnut shell.


At higher superficial gas velocities, the ratio between
bed pressure drop and bed weight shows a slight decrease,
which is attributed to the frcitional forces on the walls of
the fluidized bed. However, it is interesting that for ground
corncob with H/D = 1 (Figure 6) the behavior is inverse to
that of the other materials. This behavior can be attributed
to the low density of the material, and at higher superficial
gas velocities, some of the material is elutriated,causing
less material to be present inside the bed chamber reducing
the bed weight. Subsequesnt experiments were completed
with a screen over the top chamber to minimuze this effect.

The minimum fluidization velocity is influenced by
changes in density, as shown in Figure 8. In this figure,
denser material (glass beads) exhibit a larger pressure drop
than less dense materials (ground walnut shell, and ground
corncob). Since the volume of each material is constant,
high density materials have more mass than low density
materials. Therefore, in order to fluidized the material a


0 10 20 30
U, (cm/s)


40 50


Figure 8: Effect of material density on bed pressure drop
at H/D=1.

The fluidization force balance depends on material
density. A denser material requires more bed pressure force
to equalized the gravity force of the bed in order to achieve
fluidization. Figure 9 shows the effect of material density
on the fluidization force balance. The knee, indicating the
minimum fluidization velocity, occurs approximately at a
force balance equal to 1. This figure clearly shows Umf
increases with increasing material density.


1


^ 0.8

c.s
" 0.6




PI
: 0.4



0


0 10 20 30
U, (cm/s)


40 50


Figure 9: Effect of material density on fluidization force
balance for H/D= 1.

Conclusions

Minimum fluidization velocity was determined for
three different materials (glass beads, ground corncob and
ground walnut shell) over a range of bed heights. As
discussed in the literature, the bed height affects the
minimum fluidization velocity for only specialized
fluidized beds. For example for tapered beds, the minimum


Glass Beads

Ground Walnut shell

SAGround Comcob



, ," ,^">


h-as


U Glass Beads


* GroundWalnut Shell


A Ground Comcob






Paper No 1674 7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

fluidization velocity is independent of bed height. Whereas
for spouted fluidized beds or 2D bubbling fluidized beds,
the minimum fluidization velocity is influenced by
material bulk height. For a 3D bubbling fluidized bed,
studies show that the minimum fluidization velocity is
independent of the bed height; the results obtained in this
research using a 3D fluidized bed corroborate the data
presented in the literature.

The density difference between the three materials
used in this study influenced the minimum fluidization
velocity. A denser material required a higher superficial gas
velocity to start fluidization. Therefore, the minimum
fluidization velocity increased when the density of the
material increased.

Acknowledgements

This work is supported through a Fulbright
Scholarship and Iowa State University.

References

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fluidisation." Chemical Engineering Science,
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Crowe, C. T. (2006). Multiphase Flow Handbook. Boca
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Franka, N. P. (2008). "Visualizing fluidized beds with
X-rays". M.S. Thesis, Department of Mechanical
Engineering, Iowa State University. Ames, IA.
Gunn, D. J. and Hilal, N. (1997). "The expansion of
gas-fluidised beds in bubbling fluidisation."
Chemical Engineering Science, 52(16):
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Hilal, N., Ghannam, M. T. and Anabtawi, M. Z. (2001).
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minimum fluidization velocity." Chemical
Engineering and Technology, 24(2): 161-165.
Ramos Caicedo, G, Garcia Ruiz, M., Prieto Marqu6s, J.,
and Guardiola Soler, J. (2002). "Minimum
fluidization velocities for gas-solid 2D beds."
Chemical Engineering and Processing, 41(9):
761-764.
Sau, D. C., Mohanty, S. and Biswal, K. C. (2007).
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beds." Chemical Engineering Journal, 132(1-3):
151-157.
Zhong, W., Chen, X. and Zhang, M. (2006).
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spouting/spout-fluidizing velocity." Chemical
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