Powder Technology 237 (2013) 14–23
Contents lists available at SciVerse ScienceDirect
Powder Technology
journal homepage: www.elsevier.com/locate/powtec
Experimental study on hydrodynamic characteristics of gas–solid pulsed
fluidized bed
Hamed Khosravi Bizhaem, Hassan Basirat Tabrizi ⁎
Amirkabir University of Technology, Mechanical Engineering Department, P.O. Box 15875-4413, Tehran, Iran
a r t i c l e
i n f o
Article history:
Received 22 September 2012
Received in revised form 15 December 2012
Accepted 5 January 2013
Available online 11 January 2013
Keywords:
Pulsed flow
Fluidized bed
Hydrodynamic
Gas–solid flow
a b s t r a c t
The purpose of this study is to investigate the hydrodynamic characteristics of pulsed fluidized bed experimentally. Three different particles, silica with 196 μm mean diameter, alumina with 95 μm mean diameter and
alumina with 10 μm mean diameter were introduced into the fluidized bed. Experiments were performed in
0.11 m ID and 0.5 m height fluidized bed. Pulsed airflow was introduced using square waves at 1 to 10 Hz
pulsation frequency, which were generated using a solenoid valve connected to an electronic circuit. A
high speed camera recorded the bed behavior to measure the bubble size and velocity. Results indicate that
for alumina 95 μm the bed surface oscillation increases with decrease of pulsation frequency. Increasing the pulsation frequency decreases the bed expansion ratio and the bubble's diameter and velocity. In addition, the mean
bed pressure drop becomes closer to the continuous airflow by increasing the pulsation frequency. This study
indicates that the pulsating airflow decreases the minimum fluidization velocity and enhances fluidization of
fine cohesive particles.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Gas–solid fluidized beds have many applications in chemical, oil,
pharmaceutical, biochemical, and power industries. Their widespread
application is due to the suitable mixing characteristics and high surface
contacting between the phases [1]. Problems related to the bubbling
fluidized beds such as gas bypassing and poor fluidization quality of
fine and cohesive particles cause the researchers to focus on it [1–4].
In order to overcome these limitations and to improve their intrinsic
performance, various assisted fluidization techniques have been proposed and tested. To mention a few, magnetic field [5], electric field
[3,6,7], acoustic excitation [2,8,9], mechanical vibration [10,11], and
flow pulsation [4,12–15].
Pulsed fluidization is an operation in which the fluidization velocity
pulsates with time in the form of regular or irregular patterns; e.g., rectangular pulsation, saw tooth wave or any other pattern. Some studies
have shown that pulsed fluidization can improve the fluidization quality because it can eliminate slugs, gas bypassing and channeling, reduce
the bubble size and enhance gas-particle contact in fluidized beds
[12,16,17]. Wang and Rhodes [18] showed numerically that the hardto-fluidize materials could be fluidized when pulsation or vibration is
used. Due to these merits, some research efforts have been devoted to
various aspects associated to pulsed fluidization; e.g., mass oscillation
[19], mass transfer [20], heat transfer [21,13–15,4], nano-powder
fluidization [22], combustion [23–25], numerical models [26,27] and
⁎ Corresponding author.
E-mail address: hbasirat@aut.ac.ir (H. Basirat Tabrizi).
0032-5910/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.powtec.2013.01.001
simulation [28,16–18] and many other industrial application such as
drying [29–35].
In industry, pressure pulses generated by the combustion process
naturally cause the pulsed fluidization and the pressure pulsation is
connected to acoustic effects and combustion inside the bubbles
[36]. Pulsed flow can be generated in many ways. A more usual way
to obtain gas flow pulsation is to use a solenoid valve [4,35] or any
mechanical pulsed flow generator [12,13,30,31,34]. Literature on the
use of this technique is rather scarce. Devahastin and Mujumdar [35]
investigated some hydrodynamic characteristics of pulsed spouted
bed, i.e., spouting mechanism, solid circulation and mixing of particles.
They used solenoid valve to generate flow pulsation at frequencies of
0.2 to 2.0 Hz. Their experimental results revealed that the maximum
spoutable bed height decreases as the pulse frequency increases. Further, they indicated that at a given bed height, there exists an optimum
pulse frequency for the highest particle circulation rate. Li et al. [17]
investigated the bubbling flow in a 2D pulsed fluidized bed using a
developed Eulerian–Eulerian two-fluid modeling approach. Their
results indicate that low frequency of 0.4 Hz produces an unsteady
fluidization; intermediate frequency of 4 Hz causes a resonant fluidization and high frequency of 40 Hz results in a normal fluidization. Zhang
and Koksal [4] studied the effects of flow pulsation on surface-to-bed
heat transfer with a pulsed airflow at 1 to 10 Hz. They found that the
enhancement of heat transfer depends on the particle size, superficial
gas velocity and the frequency of pulsation. Pence and Beasley [14,15]
examined the effect of opposing oscillatory flow on heat transfer from
an immersed cylinder in a bubbling fluidized bed. They found that the
heat transfer characteristics are significantly altered by the opposing
H. Khosravi Bizhaem, H. Basirat Tabrizi / Powder Technology 237 (2013) 14–23
15
Fig. 1. Schematic of experimental setup. (1) compressor, (2) air tank, (3) ball valve, (4) pressure gauge, (5) filter, (6) solenoid valve, (7) electronic circuit, (8) flow meter, (9) distributor,
(10) Plexiglas cylinder, (11) filter, (12) manometer, (13) high speed camera, (14) LED.
oscillatory flow. Further, they observed enhancements in overall and
local heat transfer from submerged cylinder with low pulse frequency.
Ali and Asif [22] used pulsed flow with low frequency (0.05, 0.10 and
0.25 Hz) for fluidization of nano-powders. They observed greater
homogeneity of the bed and reduction in the agglomerate size is
obtained. Moreover, in their study low pulsation frequency was used
while in present study, high frequency pulsed airflow is investigated.
Further, Zhang and Koksal [4] did not comprehensively investigate on
the hydrodynamics of pulsed fluidized bed. Ali and Asif [22] used low
pulsation frequency in their work with only nano-powders.
Therefore, present study investigates various pulsation frequencies
for different diameters and materials of the particles. The main objective
of this study is to investigate the effect of pulsed airflow on the hydrodynamics of gas–solid fluidized bed.
2. Experiment
2.1. Experimental setup
Experiments were performed with a Plexiglas cylinder with height
of 50 cm and diameter of 11 cm. The distributor consists of a perforated
plate under which, there was a homogenized system to prevent the airflow from generating asymmetrical effects inside the freeboard. A wind
filter was installed at the top of the column to prevent fine particles
flying out. The fluidization air was supplied from a compressor. An air
storage tank located after the compressor to prevent fluctuations in
air pressure. Particles were fluidized with air at ambient condition. A
pressure gauge with scale range of 0–10 bar was used to show the
line pressure. The airflow rate was adjusted with a ball valve and measured using a gas flow meter (testo 6441 model).
A two-way normally close solenoid valve was used to generate flow
pulsation. This type of valve opens when energized and closes when
de-energized so that the gas can be intermittently supplied at the
desired frequency. An electronic circuit was designed and fabricated
to control the frequency of the solenoid valve. In this circuit, a microcontroller was used and programmed to generate any desired frequency. The input voltage to the valve was 12 VDC.
Apparatus of the experimental setup and its components are shown
in Fig. 1. The overall pressure drop was measured by using a manometer
at different superficial gas velocity. A digital camera (Fujifilm HS10) was
used to record the flow regimes and bubble formation through the
transparent wall during the experiments. This camera was capable of
recording until 1000 frames per second. Further, different LED in two
directions was used to generate required light intensity for captured
films. The captured films were analyzed frame by frame to get the
best snapshot to study of the flow regimes. Square wave of different
pulse frequencies were used as shown in Fig. 2. Pulsation frequency of
4 Hz means that the valve opens and closes 4 times per second. Thus,
a square wave for 0.125 s duration opened and 0.125 s closed was generated. Different particles from Geldart groups B, A/B and C were used in
the experiments as listed in Table 1. The mean diameter of silica 196 μm
and alumina 95 μm was determined by sieve analysis and for alumina
10 μm mastersizer analysis was used. The particles densities were determined experimentally.
2.2. Repeatability, accuracy and uncertainty
The tests were repeated at least three times to ensure consistency
of the data. It was found that the highest deviation in the data was
about 11% that occurs at low range air velocity. As a typical example,
the error bars for the pressure drop in each air velocity are shown in
Fig. 3.
Further, the accuracy and uncertainty of tests are listed in Table 2.
The uncertainty for bubble rise velocity and fluidization index can be
described by following equations.
Δx x
¼
Δt sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
t
2 2
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
∂U b
∂U b
1
¼
EðU b Þ ¼
dx þ
dt
ðdxÞ2 þ ðU b dt Þ2
t
∂x
∂t
Ub ¼
ð1Þ
Table 1
Particle properties.
Fig. 2. Flow pulsation generated by solenoid valve.
Particle
dp (μm)
ρp (kg/m3)
Group
Ar
Silica
Alumina
Alumina
196
95
10
2550
3860
3570
B
A/B
C
709
122
0.132
16
H. Khosravi Bizhaem, H. Basirat Tabrizi / Powder Technology 237 (2013) 14–23
3.2. Bubble rise velocity
Fig. 3. Repeatability of experiments for continuous flow, alumina particles, dp = 95 μm.
Δp max AΔp max
¼
w
w
sA ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2 2 2
∂ðFIÞ
∂ðFIÞ
∂ðFIÞ
EðFIÞ ¼
dðΔp max Þ þ
dA þ
dw
∂A
∂ðΔp max Þ
∂w
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1
¼
ðΔp max dAÞ2 þ ðAdðΔp max ÞÞ2 þ ðFIdwÞ2
w
FI ¼
ð2Þ
where dA calculated by
A¼
π 2
π
−6 2
d ⇒EðAÞ ¼ 2 d Δd⇒EðAÞ ¼ 17:279 10 m :
4
4
ð3Þ
3. Results and discussions
3.1. Bed expansion
Snapshots of the bed expansion in one pulsation cycle at different
frequencies for alumina 95 μm particles are shown in Fig. 4. The bed
surface oscillates with the pulsed airflow frequency and the amplitude
of the oscillation increases as the pulsation frequency is decreased. At
a high pulsation frequency, when the valve opens, a lower amount of
air enters into the bed compared with low pulsation frequency that
causes shorter bed expansion. Furthermore, at high pulsation frequency
in the off period, the bed has less time to collapse. Zhang and Koksal [4]
reported the same observation for glass beads particles with 160 μm diameter. The more bed expansion was reported due to higher gas velocity in their experiments.
Table 2
Accuracy of measurements and uncertainties.
Measurements or instruments
Accuracy
A.
Flow meter
Manometer
Elevation measurement
Bed diameter
Particles mass
Each frame time step measurement
±3% measured value
±5 Pa
±10−3 m
±10−4 m
±10−4 g
±8.334 × 10−4 s
B.
Parameters
Bubble rise velocity
Fluidization index (FI)
Uncertainty
7.027 × 10−3 m/s
2.414 × 10−3–3.696 × 10−3
In order to investigate the bubble rise velocity, the bubble rise velocity in continuous airflow was calculated experimentally then compared with the existing correlations. In the next step, bubble rise
velocity for pulsed airflow in various pulsation frequencies was
investigated.
Researchers proposed many empirical correlations in order to predict the bubble rise velocity in a bubbling fluidized bed for continuous
airflow. Some of them are shown in Table 3.
In Werther's relation, ψ and α selected according to the experimental results where for Gledart group B particles are ψ = 0.65 and
α = 2 D 0.5.
So the bubble rise velocity is calculated according to the above relations. The calculated bubble rise velocity is compared with the experimental results of the continuous airflow that was obtained from
our image processing. Fig. 5 shows snapshots of the bubble formation
and grows up for alumina 95 μm in continuous flow. The time interval
between each snapshot is 0.05 s. It can be noticed, at the first snapshot from left, two small bubbles “A” and “B” are generated at the bottom of the bed. The bubbles grow up in the second and third
snapshots. In the fourth snapshot, the bubbles coalesce and finally
burst at the bed surface. Hence, the bubble rise velocity is calculated
from this process.
Table 4 indicates the Davidson and Harrison's relation which
shows the best prediction for the bubble rise velocity. Lindborg et
al. [37] also used their correlation in order to evaluate simulation results for 2D fluidized beds. The difference between Werther's relation
and experimental results is about 33%. This difference may be due to
the wrong value of constants ψ and α, so Werther's relation can make
a better prediction by improving their constants.
Snapshots of bubbling regime in the pulsed fluidized bed for alumina 95 μm are shown in Figs. 6–8 for 1 Hz, 4 Hz and 10 Hz pulsation
frequencies, respectively. The time interval of each snapshot is 0.05 s
in 10 Hz pulsation frequency and 0.1 s in 1 Hz and 4 Hz pulsation frequencies. Their average rise velocity of bubble is calculated and is
shown in Table 5. Furthermore, Fig. 8 shows two bubbles “A” and
“B” rise nearly with the same size. In addition, Table 5 indicates that
the bubble average velocity and the bed expansion ratio for alumina
95 μm in given condition decrease as the pulsation frequency increases. Increasing pulsation frequency, causes the shorter on and
off opening time of the valve, so lower amount of air enters to the
bed in each cycle. At 10 Hz pulsation frequency, the bubble average
velocity and bed expansion ratio are close to the continuous airflow.
3.3. Bubble size
Most of the existing correlations in the literature for prediction of
the bubble size in fluidized bed are suitable for Geldart group B particles in continuous airflow. Mori and Wen [38] proposed the bubble
size relation in each elevation as:
−0:3z
dbm −db
¼e D
dbm −db0
2
2:78 U 0 −U mf ; ðcmÞ
db0 ¼
g
hπ i0:4
2
; ðcmÞ:
dbm ¼ 0:65 D U 0 −U mf
4
ð7Þ
Werther proposed another relation for Geldart group B particles in
each elevation as [1]:
h
i1
1:21
db ¼ 0:853 1 þ 0:272 U 0 −U mf 3 ð1 þ 0:684zÞ ; ðcmÞ:
ð8Þ
H. Khosravi Bizhaem, H. Basirat Tabrizi / Powder Technology 237 (2013) 14–23
17
Fig. 4. Snapshots of the pulsed bed behavior at different pulsation frequencies at Q = 10 lit/min, alumina particles, dp = 95 μm, H0 = 15 cm, (a) 1 Hz, (b) 4 Hz, (c) 10 Hz.
For calculating the bubble diameter experimentally, the bubble
area in each snapshot is measured by ImageJ software, and then
used the introduced formula of Kunii and Levenspiel [1]:
db ¼
rffiffiffiffiffiffiffiffi
4Ab
:
π
ð9Þ
Table 3
Relations for predicting the bubble rise velocity [1].
Method
Relation
Davidson and Harrison
U b ¼ U 0 −U mf þ U br ; ðm=sÞ
0:5
U br ¼ 0:711ðgdb Þ ; ðm=sÞ
(4)
Werther
Kunii and Levenspiel
Ub = ψ(U0 − Umf) + αUbr, (m/s)
Ub = 1.6[(U0 − Umf) + 1.13db0.5]D1.35 + Ubr,(m/s)
(5)
(6)
The bubble diameter in each elevation is plotted for experimental
results of the continuous flow in comparison with Mori–Wen and
Werther's relation in Fig. 9. The bubbles increase in size and rise
through the bed due to coalescence with each other and entrained gas
from the emulsion. Li et al. [17] indicated this effect in their simulation
of gas–solid fluidized bed. It is noticed, Werther's method can predict
the bubble diameter more accurately while there is some deviation in
Wen–Mori's relation.
In Fig. 10, the bubble diameter for continuous and pulsed flow is
shown. For 1 Hz pulsation frequency the bubble size, increases very
fast. It is due to the large amount of air enters into the bed in each
on period of pulsed flow. Increasing pulsation frequency, the amount
of entered air in each cycle decreases, so the bubble size decreases
too. At 4 Hz frequency, the bubble grows at the bottom of the bed
and then increases very smoothly through the riser. No change in
the bubble diameter is due to the interruption of airflow in pulsation
18
H. Khosravi Bizhaem, H. Basirat Tabrizi / Powder Technology 237 (2013) 14–23
Fig. 5. Snapshots of bubble formation, continuous flow at Q = 28 lit/min, alumina particles, dp = 95 μm, H0 = 15 cm.
cycle. Near the top of the bed, pressure decreases and the bubble
grows up and bursts.
For 10 Hz pulsation frequency, no change in the bubble diameter
size is noticed. Since the airflow is pulsed with high frequency, the
required air for forming a large bubble is not supplied. The bubble diameter in the middle of the bed has the same size in 10 Hz pulsation
frequency and continuous flow. However, at the bottom of the bed,
10 Hz has the larger bubble because at on period in each cycle, the
amount of airflow forms a bubble whereas in continuous flow bubble
grows up through the entering airflow. For the same reason at the top
of the bed, 10 Hz frequency creates the smaller bubble. Increasing the
pulsation frequency, the bubble diameter decreases because on and
off opening time of the valve decreases, so the amount of air in each
cycle that entered into the bed decreases.
Table 4
Bubble rise velocity, alumina particles. dp = 95 μm, Umf = 1.9 cm/s, U0 = 4.9 cm/s.
Method
Bubble rise
velocity (m/s)
Difference %
Experiment
Davidson and Harrison
Werther
Kunii and Levenspiel
0.4
0.4026
0.2666
0.3898
–
0.65
33.35
2.55
3.4. Bed pressure drop and fluidization index
Fig. 11 shows the bed pressure drop for superficial gas velocity in
continuous flow for silica 196 μm at different initial bed height. The
diagram of bed pressure drop the continuous flow is similar to what
can be noticed in the literature [1,39].
Figs. 12–13 show the mean bed pressure drop at different pulsation frequencies for 15 cm and 10 cm bed height, respectively. Bar
signs, indicate the pressure fluctuations in a certain inlet gas velocity.
The effect of pulsation frequency can be investigated in three regions:
low range frequency such as 1 to 2 Hz, middle range frequency includes
4 Hz and high range frequency such as 8 to 10 Hz. Similar behavior can
be seen in each region. In Fig. 13, the graphs of 2 and 8 Hz have been
omitted for ease of noticing. In the low range frequency, the bed pressure drop has the lowest quantity. While the pulsation frequency increases, the mean bed pressure drop increases. In low range
frequency, due to the long interval between gas entries, the particles
have enough time to settle on the bed and thereby the required air for
fluidization cannot be provided. Therefore, the pressure drop curve is
lower than the continuous flow. Increasing frequency up to 4 Hz, the
time interval between on and off period decreases and the particles
have not enough time to settle, so the fluidization is improved. At
high range frequency, the bed has very fast vibration mode and the
pressure drop nearly equals to the continuous flow. In addition, both
the pulsed and continuous fluidization curve tends to a maximum pressure drop value above the minimum fluidization velocity.
Fig. 6. Snapshots of bubble formation, pulsed flow at Q = 28 lit/min, f = 1 Hz, alumina particles, dp = 95 μm, H0 = 15 cm.
H. Khosravi Bizhaem, H. Basirat Tabrizi / Powder Technology 237 (2013) 14–23
19
Fig. 7. Snapshots of bubble formation, pulsed flow at Q = 28 lit/min, f = 4 Hz, alumina particles, dp = 95 μm, H0 = 15 cm.
Fig. 8. Snapshots of bubble formation, pulsed flow at Q = 28 lit/min, f = 10 Hz, alumina particles, dp = 95 μm, H0 = 15 cm.
At low inlet gas velocity and low pulsation frequency, the pressure
fluctuation is observed but in high pulsation frequency, these fluctuations occur only at high inlet gas velocity. This is related to the
time duration between gas entries. At high pulsation frequency, the
inlet gas amount in each cycle is lower and so at low gas inlet velocity
the pressure fluctuation is not observed.
Bubble coalesces, split and bursting cause the pressure fluctuation.
As shown in Figs. 5–8 in low pulsation frequency, the biggest bubble
is observed. It is noticed that pressure fluctuation in low frequencies
is more than high frequencies. This is due to generation and burst of
big bubble and higher vibration amplitude.
Moreover, the experiments are repeated for 10 cm static bed
height. Fig. 13 illustrates the pressure fluctuation starts by increasing
inlet gas velocity. In 4 Hz pulsation frequency, this fluctuation starts
at higher inlet gas velocity and increases with increasing gas velocity.
At 10 Hz, pulsation frequency the pressure fluctuation is very low and
observes only at high gas velocity. In addition, all of these characteristics are observed for 15 cm static bed height, too.
Fluidization index (FI), which is the ratio of the maximum bed
pressure drop to the pressure exerted by the weight of particles,
Table 5
Bubble average velocity and bed expansion ratio, alumina 95 μm, Q = 28 lit/min, H0 =
15 cm.
Bubble average velocity (cm/s)
Bed expansion ratio
Continuous
f = 1 Hz
f = 4 Hz
f = 10 Hz
40
1.167
70
1.267
50
1.22
45
1.12
Fig. 9. Bubble diameter in fluidized bed, alumina particles; dp = 95 μm, Umf = 1.9 cm/s,
U0 = 4.9 cm/s.
20
H. Khosravi Bizhaem, H. Basirat Tabrizi / Powder Technology 237 (2013) 14–23
Fig. 10. Bubble diameter for continuous and pulsed flow. Alumina particles; dp =
95 μm, Umf = 1.9 cm/s, U0 = 4.9 cm/s.
which is used for pulsed fluid bed dryer by Nitz and Taranto [30,31],
gives:
FI ¼
Δp max
w
=A
ð10Þ
where, this index much closer to one indicates better fluidization
quality. Fluidization index of pulsed fluidized bed for different pulsation frequency and static bed height is shown in Table 6. Two types of
particles and static bed height are used for this purpose. It can be seen
for both alumina 95 μm and silica 196 μm at high pulsation frequency
the fluidization index is closer to one. It is due to the reduction of
channeling and bubbling in the bed. In a higher frequency, the intermittence intervals are shortened. Therefore, there is a less time left
for the bed to settle and the gas–solid contact increases. Consequently, better fluidization quality occurs at higher pulsation frequency
that indicates an improvement on hydrodynamic behavior of the
bed. Nitz and Taranto [30,31] found the same behavior in the fluid dynamic analysis of drying beans in their works. They commented that
better fluidization quality increases the gas–solid contact, so drying
rate would be improved and the final dried beans would have more
uniform mixture. In addition, improvement in fluidization quality
causes a better circulation rate and decrease the time required to
mix particulate mixture. Devahastin and Mujumdar [35] indicated
there is an optimum pulse frequency that allows for the highest particle circulation rate and the speed of mixing depends both on the superficial air velocity and on the pulse frequency. While in this study,
the results reveal that at higher pulsation frequency the best fluidization quality occurs for both silica and alumina particles in 15 cm and
10 cm bed height.
Fig. 12. Bed pressure drop at different pulsation frequencies, H0 = 15 cm, (a) silica particles, dp = 196 μm, (b) alumina particles, dp = 95 μm.
3.5. Minimum pulsed fluidization velocity
The minimum fluidization velocity for continuous and pulsed flow
is compared in Fig. 14. Alumina 95 μm and silica 196 μm particles in
15 cm static bed height are used to determine the minimum fluidization velocity. As marked in Fig. 14, the minimum fluidization velocity
decreases in 10 Hz pulsating flow for both particles. Pulsed airflow
with high frequency vibrates the particles as shown in Fig. 4 that
causes to overcome the interparticle forces. This makes it easier for
particles to fluidize. Reduction at the minimum fluidization velocity
can be observed only in high range frequency.
In Table 7, the minimum fluidization velocity for two types of particles at different static bed height for continuous and 10 Hz pulsation
flow is given. The minimum fluidization velocity in 10 Hz pulsation
frequency is always lower than continuous flow and it can be reduced
to almost one third of the continuous flow. Reduction in the minimum fluidization velocity causes decrease in the required energy
for fluidization of particles, in other words, the bed needs lower
amount of gas velocity and energy in high pulsation frequency to fluidize the particles.
3.6. Fine particles fluidization
Fig. 11. Bed pressure drop for continuous flow, silica particles, dp = 196 μm.
Fine particles widely used in pharmaceutical and paint industries.
The formation of particle agglomeration causes non-homogeneities
and poor mixing of phases in the fluidized bed. However, the pulsed
flow could improve fluidization quality of fine particles.
Fig. 15 illustrates the bed of alumina 10 μm particles (Geldart
group C) at different types of airflow rate vs. time. In Fig. 15(a) for
continuous airflow fine powders form bridges and channels that
leads to a poor contact between the solid and fluid phases of fluidized
H. Khosravi Bizhaem, H. Basirat Tabrizi / Powder Technology 237 (2013) 14–23
Fig. 13. Bed pressure drop at different pulsation frequencies, H0 = 10 cm, (a) silica particles, dp = 196 μm, (b) alumina particles. dp = 95 μm.
bed as reported [22,40]. Channeling regime and particle agglomeration is observed. Furthermore, bubbling regime can be seen in
Fig. 15(b) that shows pulsed fluidized bed at 10 Hz pulsation frequency. High pulsation airflow helps to break agglomerates. This is
mainly due to the motion of particles introduced by the instantaneous
change in the airflow during the valve opening and closing. Ali and
Asif [22] indicated that 0.05, 0.1 and 0.25 Hz flow pulsations reduce
nano-powder agglomeration and lead to a greater homogeneity of
the bed. Low pulsation frequency such as 1 and 2 Hz cannot fluidize
alumina 10 μm particles because required energy was not enough to
overcome particles agglomeration. In 4 Hz frequency, the fluidization
Table 6
Fluidization index.
Particles
dp (μm)
H0 (cm)
f (Hz)
w/A (Pa)
Δp (Pa)
FI
Silica
Silica
Silica
Silica
Silica
Silica
Silica
Silica
Alumina
Alumina
Alumina
Alumina
Alumina
Alumina
Alumina
Alumina
196
196
196
196
196
196
196
196
95
95
95
95
95
95
95
95
15
15
15
15
15
10
10
10
15
15
15
15
15
10
10
10
1
2
4
8
10
1
4
10
1
2
4
8
10
1
4
10
2262
2262
2262
2262
2262
1506
1506
1506
2670
2670
2670
2670
2670
1912
1912
1912
1894
1795
1912
2114
2226
1187
1206
1427
2221
2065
2256
2635
2643
1642
1680
1854
0.84
0.79
0.85
0.93
0.98
0.79
0.80
0.95
0.83
0.77
0.84
0.99
0.99
0.86
0.88
0.97
21
Fig. 14. Minimum fluidization velocity for continuous and pulsed flow, H0 =15 cm;
(a) Silica particles, dp =196 μm, (b) alumina particles, dp =95 μm.
improved slightly and at 10 Hz pulsation frequency, the best fluidization quality of alumina 10 μm is observed.
4. Conclusion
The effect of pulsed flow on the hydrodynamic characteristics of fluidized bed was studied experimentally. A solenoid valve was used for
supplying pulsed airflow to the bed at 1 to 10 Hz frequencies. Results
indicate that the bed surface of alumina 95 μm oscillates with the
pulsed airflow frequency and the amplitude of the oscillation increased
as the pulsation frequency decreased.
Davidson and Harrison's relation predicts the bubble rise velocity accurately and Werther's relation has the best prediction for the bubble
size in the continuous airflow for alumina 95 μm. Increasing pulsation
frequency, the bubble size decreases due to the reduction of air amount
that enters in each cycle for tested particles. In the low range frequencies, the mean bed pressure drop has the lowest quantity. While the
pulsation frequency increases, the bed pressure drop increases until at
highest pulsation frequency and the mean pressure drop approximately
equals to the continuous airflow. These effects were observed for both
silica and alumina particles. Pulsation produces better fluidization
Table 7
Minimum fluidization velocity.
Static bed height
Inlet flow type
Silica,
dp = 196 μm
Alumina,
dp = 95 μm
H0 = 10 cm
Continuous
Pulsed flow, 10 Hz
Continuous
Pulsed flow, 10 Hz
1.7
0.7
2.1
0.7
1.57 cm/s
1.22 cm/s
1.9 cm/s
0.66 cm/s
H0 = 15 cm
cm/s
cm/s
cm/s
cm/s
22
H. Khosravi Bizhaem, H. Basirat Tabrizi / Powder Technology 237 (2013) 14–23
W
Z
weight of particles in the bed, N
bed elevation, m
Greek letter
ρp
particle density, kg/m 3
Acknowledgments
The authors would like to express their appreciation to Mr. M.A.
Ehteram (Ph.D. student) for assistance in the design of the experimental
apparatus.
References
Fig. 15. Fine particles fluidization by pulsed flow, alumina particles dp =10 μm, H0 =15 cm;
(a) continuous flow, (b) pulsed flow, f=10 Hz.
quality with less bed expansion. Pulsed flow at high frequency may
decrease the minimum fluidization velocity by vibrating the particles.
In addition, for fine particles such as alumina 10 μm, channeling regime
and particle agglomeration was observed in the continuous airflow,
while 10 Hz pulsed airflow assisted to break agglomerates and leads
to greater homogeneity of the bed.
Nomenclature
A
cross-sectional area of the bed, m 2
Ab
bubble area, m 2
Ar
Archimedes number
db
bubble diameter, m
dp
mean particle diameter, m
D
bed diameter, m
E
Uncertainty
F
pulsation frequency, Hz
G
gravitational acceleration, m/s 2
H0
static bed height, m
Q
volumetric flow rate, lit/min
U0
superficial gas velocity, m/s
Ub
bubble rise velocity, m/s
Ubr
single bubble rise velocity, m/s
Um
bubble rise velocity, m/s
Δp
bed pressure drop, Pa
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