Group Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Title: 14.5.2 - Effect of Temperature on Onset Velocity of Turbulent Regime in Gas Fluidized Beds
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 Material Information
Title: 14.5.2 - Effect of Temperature on Onset Velocity of Turbulent Regime in Gas Fluidized Beds Fluidized and Circulating Fluidized Beds
Series Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Physical Description: Conference Papers
Creator: Choi, J.-H.
Ryu, H.-J.
Yi, C.-K.
Publisher: International Conference on Multiphase Flow (ICMF)
Publication Date: June 4, 2010
 Subjects
Subject: model
temperature effect
transition velocity
onset velocity
turbulent fluidized bed
 Notes
Abstract: This study proposed a model to predict the temperature effect on the onset velocity of turbulent fluidization (uc) of Geldart’s type A particles. It was found that void splitting could occur in the whole bed when the initial bubble size at the distributor (dbo) was greater than the maximum stable bubble size (dbmax) or the equilibrium bubble size (dbeq). The proposed model was successful to fit the trend of temperature effect on the uc. When the bubble growth was limited to the dbmax, the uc increased with an increase of temperature. However, when the dbeq was smaller than dbmax, the uc decreased with an increase of temperature because the minimum fluidizing velocity and thus the dbeq decreased. The uc increased initially and decreased later when the dbeq/dbmax starting with the ratio > 1 decreased to the ratio < 1 as temperature increased. The present model predicted the trend properly that any other existing correlations could not reflect.
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: VID00356
Source Institution: University of Florida
Holding Location: University of Florida
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Resource Identifier: 1452-Choi-ICMF2010.pdf

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


Effect of Temperature on Onset Velocity of Turbulent Regime in Gas Fluidized Beds


Jeong-Hoo Choi, Ho-Jung Ryu and Chang-Keun Yi

Konkuk University, Department of Chemical Engineering
Seoul, 143-701, Korea
choijhoo~konkuk.ac.kr
*Korea Institute of Energy Research
Daejon, 305-343, Korea


Keywords: model, temperature effect, transition velocity, onset velocity, turbulent fluidized bed




Abstract

This study proposed a model to predict the temperature effect on the onset velocity of turbulent fluidization (ue) of Geldart's
type A particles. It was found that void splitting could occur in the whole bed when the initial bubble size at the distributor
(dbo) WaS greater than the maximum stable bubble size (dbmax) Or the equilibrium bubble size (dbeq). The proposed model was
successful to fit the trend of temperature effect on the u,. When the bubble growth was limited to the dbmax, the u, increased
with an increase of temperature. However, when the dbey WaS Smaller than dbmax, the u, decreased with an increase of
temperature because the minimum fluidizing velocity and thus the dbey decreased. The u, increased initially and decreased later
when the dbeq/dbmax Starting with the ratio > 1 decreased to the ratio < 1 as temperature increased. The present model predicted
the trend properly that any other existing correlations could not reflect.


Introduction

The turbulent fluidization regime is commonly considered
to lie between bubbling fluidization and the fast fluidization
regime. The turbulent regime is known to result from
disappearance of large voids by predominance of void
splitting over void coalescence. Turbulent fluidization is
commonly utilized in industrial fluidized-bed reactors due
to vigorous gas-solids contacting, favorable bed-to-surface
heat transfer, high solids hold-ups, and limited axial mixing
of gas (Bi et al., 2000).

Although extensive results have been reported to quantify
the onset velocity of turbulent fluidization (ue), the effect of
temperature on it is not clear yet for Geldart's type A
particles (Chehbouni et al., 1995). The u, increased with
temperature for Geldart's type A and B particles in the study
of Cai et al. (1989, 1992). However, they measured the
static pressure in the middle of bed height that was improper
to reflect the property of the whole bed. Chehbouni et al.
(1995) reported that the u, decreased with an increase of
temperature in the bed of FCC particles (Geldart's type A:
78 pLm, 1450 kg/m ). These results were related to the effect
of temperature on bubble size which was significantly
different for fine and large particle diameters. Moreover, no
published correlation could predict adequately those
experimental data. The u, in the bed of sand particles
(Geldart's type B: 250 pLm, 2560 kg/m ) increased with an
increase of temperature. According to the study of Peeler et
al. (1999) as temperature increased, the u, decreased after an
initial increase in the bed of sand particles (Geldart's type
B: 130 pLm, 4400 kg/m ) using N, as fluidizing gas and


decreased in the bed of alumina particles (Geldart's type A:
70 pLm, 2800 kg/m ) using N2 or He as fluidizing gas. None
of the reported correlations for estimating u, satisfactorily
predicts this behavior.

This study is to propose a model for temperature effect on
transition velocity to turbulent fluidization (ue) of Geldart's
type A particles. Two cases of void splitting were
considered for the fluidized bed of Geldart's group A and
AB particles. One is that the equilibrium bubble diameter is
smaller than the maximum stable bubble diameter. Then the
u, decreased with an increase of temperature. Another is that
the equilibrium bubble diameter is greater than the
maximum stable bubble diameter. Then the u, increased
with temperature.


Nomenclature

Ar Archimedes number (-)
dbey equilibrium bubble diameter (m)
dbmax m8Ximum stable bubble diameter (m)
dbo initial bubble diameter formed at the distributor
(m)
d, particle diameter (m)
dt column diameter (m)
f, splitting frequency of a single bubble (1/s)
g gravitational acceleration, 9.8 (ms1i'
k bubble flow fraction of excess gas velocity
(u-umf) (-)
Re, particle Reynolds number at the onset condition
of turbulent fluidization (-)





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


The Reme can be given by the correlation of Wen & Yu
(1966):
Reme = [(33.7)' + 0.0408 Ar]0 33.7 (5)

Then, the onset condition of turbulent fluidization is in
another form:
Re, = [(33.7)' + 0.0408 Ar]" 33.7 + 0.5985 ArO 5 (6)

When the bubble growth is limited to the maximum stable
bubble size, the onset velocity of turbulent fluidization u,
increases with an increase of temperature as can be seen in
Equation (4a).

The equilibrium bubble size can be given by following
correlations of Choi et al. (1998):
6.792k(u -umnf )
dbe (7)


Paper No


Reme particle Reynolds number at minimum
fluidization (-)
u gas velocity (m/s)
u, onset velocity of turbulent fluidization (m/s)
ume minimum fluidizing velocity (m/s)

Greek letters
me bed voidage at minimum fluidization condition

CL gas viscosity (Pa-s)
p, gas density (kg/m3)
p, solid density (kg/m3)


Model

The void size in bubbling or slugging bed is limited to the
maximum stable bubble size according to the study of
Harrison et al. (1961). The bubble size in the bed of
Geldart's type A particles is controlled additionally by
equilibrium bubble size because bubble splitting occurs
(Choi et al., 1998). Therefore, bubble breakup could occur
in the whole bed when the initial bubble size at the
distributor is greater than the smaller one of the maximum
stable bubble size and the equilibrium bubble size.

At the usual gas velocity of turbulent bed including the bed
of Chehbouni et al. (1995) and Peeler et al. (1999) which
were considered in this study, the bubble diameter formed
initially at the distributor is bigger than the pitch of the
distributor nozzle so given by the following correlation for
the porous plate distributor (Kunii & Levenspiel, 1991:
Miwa et al., 1972).
3.685(u-uinf )2
dbo= (1)

Harrison et al. (1961) proposed a following relationship for
the maximum stable bubble size:


0.620
u -(u /uinf) Uinf
k =
inuf
-4 u 0.454 g
fs = 6.47 x 104


The k is bubble flow fraction of excess gas velocity and the
f,* splitting frequency of a single bubble. The equilibrium
bubble size is considered now that the bubble splitting is
important in the bed of Geldart's type A particles. When
Equation (7) is applied to the initial bubble size (Equation
(1)) formed at the distributor as a criterion to start the
bubble breakup throughout the bed, the onset velocity of
turbulent fluidization is
( Uc )2.454 -2( Uc )1.454 -2849( Uc )+2849( Uc )0.62+ Uc )0.454
"nt "nt "nt "nt "nt


PP
PP -P PpP-g nmf
1.32
p, 1- nf


dbmax
dp


Since


The solution is

uc= 217.5


Pp-Pg -mf

l-mf


,st gas


is nearly unity in mo


Therefore, if the bubble growth is limited to the equilibrium
bubble size, the u, decreases with an increase of temperature
because the minimum fluidizing velocity and thus the
equilibrium bubble size decrease.
Combining Equations (3) and (7) to (9) gives


fluidized beds, the simplified form gives
PP P
dbmax = 1.32 dp (3)

When this relationship is applied to the initial bubble size
(Equation (1)) formed at the distributor as a criterion to
begin bubble breakup throughout the bed, the onset velocity
of turbulent fluidization u, is


U 0.62


beq enf unf
d ru
b max (-93 I m


dP (pp-'p )g )0.5
uc = un{ + 0.5985( )


d u~gdpunf Pg dpp (Pp



Re, = Reme + 0.5985 ArO


(4a)





(4b)
(4c)


First of all, the bubble size in the bed of Geldart's type A, B
and D particles is limited to the maximum stable bubble size.
The bubble size in the bed of Geldart's type A particles is
controlled additionally by the equilibrium bubble size
because the bubble splitting is important. The bubble





































































102 103 104 106 1

Ar [-]


Figure 1: dbeq/dbmax VeTSus Ar at u,.

Figure 2 shows comparison of the present model with data


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

measured by Chehbouni et al. (1995). The maximum stable
bubble diameter is greater than the equilibrium bubble
diameter at temperature > 1040C. So the u, was determined
by Equation 4(a) for temperature < 1040C and Equation (11)
for temperature > 1040C. The present model could predict
the temperature effect on the u, reasonably well that
decreased with an increase of temperature. However, the
equilibrium diameter seemed to be somewhat
underestimated so that the present model underestimated the


Paper No


splitting frequency in the bed of Geldart's type B and D
particles is so low that the equilibrium bubble size is greater
than the stable bubble size and thus no real limitation to the
bubble growth. Therefore, bubble breakup occurs in the
entire bed if the initial bubble size formed at the distributor
is greater than the smaller one of the maximum stable
bubble size and the equilibrium bubble size.


Results and Discussion

Figure 1 shows the ratio of the equilibrium bubble diameter
to the maximum stable bubble diameter dbeq/dbmax aS 8
function of Ar and u/ume which is estimated on the
experimental condition of Chehbouni et al. (1995) and
Peeler et al. (1999). The data were plotted at the u=ue. The
dbeq/dbmax for the bed of sand and FCC particles was smaller
or greater than unity, whereas for the bed of alumina
particles smaller than unity. Therefore, this result confirms
that bubble breakup in the bed could be caused by the initial
bubble size formed at the distributor greater than the
maximum stable bubble size or the equilibrium bubble size.
The present discussion did not consider the data of
Chehbouni et al. (1995) obtained from sand beds because
the dbeq/dbmax > 1 fOr those beds. The bubble splitting
frequency in the bed of Geldart's type B and D particles is
so low that the equilibrium bubble size is greater than the
stable bubble size and thus no real limitation to the bubble
growth.


9 0~~ uc measured C.
0.8 -1 uc calculated
1.2
\ --- dbeq/dbmax
\ 1.0 '
? 0.6 -\O O
E 0.8

0.4 0.6
0.2 -.. .
C ebuiet al. (1995): .
00FCC 0.078 mm, Air 0.
0 100 200 300 400 500
Temperature [oC]

Figure 2: Effect of temperature on u, and dbeq/dbmax
(Chehbouni et al., 1995: FCC 0.078 mm, air).

Figure 4 shows comparison of the present model with the
data of Peeler et al. (1999), which were measured in sand
beds using N, gas as fluidizing gas. The u, initially
increased but decreased later as temperature increased. The
dbeq/dbmax WaS predicted greater than unity at temperature <
7250C so the u, was determined by Equation (4a). However,
the dbeq/dbmax WaS predicted smaller than unity at
temperature > 7250C so Equation (11) determined the u,.
The present model could predict the measured trend well. It
was the trend that any other existing correlations could not
follow reasonably (Peeler et al., 1999).


102



101



S100



-0 10-1


1111 ""'"I 11 "' 1 1111 I 1 I "I




V


*


/ B


-

O"' "" "" "" ""


Peeler et al. (1999):
Sand 0.130 mm, N2 O O O




\ O u, measured
O \ u, calculated
\ _ dbeq/dbmax


10-2


-3
- 2

- 1


E 1.5

1.0 -

0.5

0.0 -


10-1 100 101


O ChehbounI etal (1995) FCC, 78pm, 1450 kg/m3
n Chehbounl et al (1995) sand, 250 pm, 2650 kg/m3, dt 0 2 m
SChehbounl et al (1995) sand, 250 pm, 2650 kg/m3, dt 0 082 m


1000 1200


sand/Nltrogen, 130 pm, 4400 kg/m3
alumina/Nltrogen, 70 pm, 2800 kg/m3
alumina/Hellum, 70 pm, 2800 kg/m3


Peeler et al (1999)
Peeler et al (1999)
Peeler et al (1999)


0 200 400 600 800
Temperature [oC]


Figure 3: Effect of temperature on u, and dbeq/dbmax (Peeler
et al., 1999; sand 0.130 mm, N2).
























I


d the u, This study proposed a model to predict the temperature
00C and effect on the u, of Geldart's type A particles. The present
97 m/s) study found that bubble breakup in the whole bed could be
caused by the dbo > dbmax Or dbeq. The present model was
successful to fit the trend of temperature effect on the u,.
When the bubble growth was limited to the dbmax ch u
increased with an increase of temperature. However, when
S0.4 dbeq < dbmax, the u, decreases with an increase of temperature
because the minimum fluidizing velocity and thus the
- 0.3 equilibrium bubble size decreases. The u, increases initially
and decreases later when the dbeq/dbmax Starting with a ratio >
-0.2 L1 1 decreases to a ratio < 1 as temperature increases. The
present model predicts the trend properly that any other
.1' existing correlations cannot reflect. The present model
-o~lu~seemed to underestimate the u, because the dbeq WaS
underestimated. Further study must be needed to improve
0.0the model accuracy in the future.


Figure 5 compares the present model with the data (
et al. (1999), measured in the alumina bed using H
fluidizing gas. The dbeq/dbmax WaS predicted small
unity within the tested temperature range. The u, d
with an increase of temperature. The present
predicted the trend properly. But it underestimate
considerably at temperatures 150C, 4200C and 63
could not follow the unlikely big decrease (0.
between 6300C and 8200C.



2.5


2.0 -
O
1.5 -\



\- o
0.5 "- -O
Peeler et al. (1999):
Alumina 0.070 mm, He
0.0


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

Figure 6 shows the relationship between u, and ume when
dbeq < dbmax. Equation (11) was in reasonable agreement with
the data of Chehbouni et al. (1995) and Peeler et al. (1999)
except some data of the alumina-He system at temperatures
150C, 4200C, and 6300C. The present knowledge hardly
seemed to explain the reason of the disagreement. The
present model just seemed to underestimate the uc because
the equilibrium diameter was underestimated. It is evident
that further study must be needed to improve the model
accuracy.


Paper No


Figure 4 compares the present model with the data of Peeler
et al. (1999), measured in the alumina bed using N2 gaS as
fluidizing gas. The dbeq/dbmax WaS predicted greater than
unity at temperature < 2380C so the u, was determined by
Equation (4a). However, the dbeq/dbmax WaS predicted smaller
than unity at temperature > 2380C so Equation (11)
determined the u,. So the u, looked just decreasing with an
increase of temperature. The present model could predict the
measured trend well at the same condition.


1.4

1.2

1.0 -

5i0.8





0.2

0.0 -


O O u, measured
\ I us calculated


o o




Peeler et al. (1999):
Alumina 0.070 mm, N2


V






O O FCC 0078 mm, air
9 Sand 0130 mm, N2
O Alumna 0070 mm, N2
O Alumina 0 070 mm, He
uc=217 5umf


- 1.5 .-
1 ~2.0


-s 1.0
o-


0 200 400 600 800 1000
Temperature [oC]


1200


0.000 0.002 0.004


0.006 0.008 0.010 0.012 0.014


Figure 4: Effect of temperature on u, and dbeq/dbmax (Peeler
et al., 1999; alumina 0.070 mm, N2 -


umt [m/s]


Figure 6: u, versus umf.


Conclusions


of Peeler
[e gas as
ller than
ecreased
t model


0 200 400 600
Temperature [oC]
O uc measured
-uc calculated


800 1000


Figure 5: Effect of temperature on u, and dbeq/dbmax (Peeler
et al., 1999; sand 0.130 mm, N2 *


Acknowledgements

The authors are grateful for the financial support of the
Energy R&D program of Korean Government.


References

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state-of-the-art review of gas-solid turbulent fluidization.






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

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Cai, P., Chen, S. P., Jin, Y., Yu, Z. Q. & Wang, Z. W. Effect
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Kunii, D. & Levenspiel, O. Fluidization Engineering. 2"d ed.,
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