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Studies on gas holdup in a bubble column using porous spargers with additives.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2008; 3: 417–424
Published online in Wiley InterScience
(www.interscience.wiley.com) DOI:10.1002/apj.137
Research Article
Studies on gas holdup in a bubble column
using porous spargers with additives
Abhishek Jha, B. Raj Mohan, S. Chakraborty and B. C. Meikap*
Department of Chemical Engineering, Indian Institute of Technology, Kharagpur 721302, India
Received 20 March 2008; Revised 14 April 2008; Accepted 15 April 2008
ABSTRACT: High gas holdup and uniform fine bubble generation is very essential in froth floatation cell of mineralbased process industries. In this work, to generate uniform fine bubbles a bubble column with porous sparger have
been used. The bubble column was characterized to study the effect of gas flow rate and concentration of additive
solution on the performance of bubble column with porous spargers. Various types of spargers namely poly par sparger,
refractory brick sparger and ceramic sparger were used to produce air bubbles in the liquid phase water and the gas
holdup is estimated. The variation of gas holdup with respect to gas flow rate for different spargers and for different
conditions is presented in this paper. It has been found that the gas holdup with addition of surfactants was in the range
of 25–40%. The behavior of gas holdup is consistent with the existing theory and shows improvement over previous
results.  2008 Curtin University of Technology and John Wiley & Sons, Ltd.
KEYWORDS: bubble column; fine bubble; filter sparger; gas holdup; porous sparger; polypar sparger; surfactant
INTRODUCTION
Bubble columns are of great importance in chemical and
biochemical process industries where gas–liquid operations like distillation, fractionation, humidification,
gas–liquid reactions, etc. are carried out and reported by
Meikap et al .[1] They are widely used because of their
simplicity in construction and operation, low operating
costs and high energy efficiency. In these processes, a
gas holdup is an important design parameter, because
greater gas holdup implies greater interfacial area available for mass transfer. Moreover considering the applications in the mineral beneficiation, greater gas hold
up indicates greater bubble population which means a
higher probability of the particles getting attached to
the bubbles that leads to greater amount of mineral
being separated from its ore by means of this froth
flotation method. The bubble size distribution and gas
holdup in gas–liquid dispersions largely depend upon
the column geometry, operating conditions, physicochemical properties of the two phases and type of
gas sparger and reported by Mouza et al .,[2] Colella
et al .,[3] Tao,[4] Zahradnik et al .,[5] Polli et al .,[6] Ruzicka, et al .,[7] Ruzicka et al . and Parthsarthy.[8,9]
The two main approaches proposed to analyze the
gas holdup and bubble size are the computational fluid
*Correspondence to: B. C. Meikap, Department of Chemical Engineering, Indian Institute of Technology, Kharagpur 721302, India.
E-mail: bcmeikap@che.iitkgp.ernet.in;
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
dynamics model and the classical chemical engineering
approach (flow regimes, global and local gas holdup
etc.). The aim of this work is to improve the experimental knowledge of bubble behavior in bubble columns.[10]
Bubble behavior has a direct bearing on hydrodynamics, mass transfer and reactor performance. The design
and scale-up of bubble column has been based so far
on empirical methods.[11] It is generally accepted that
two regimes can be distinguished based on the gas
flow rate: the homogenous (bubbly flow regime) and
the heterogeneous (churn turbulent flow) regime. The
homogeneous bubbly flow regime encountered at low
gas velocities and characterized by a narrow bubble size
distribution and radially uniform gas holdup; and the
heterogeneous (churn turbulent flow) regime observed
by Kazakis et al .[12] at higher gas velocities and characterized by the appearance of large bubbles, formed
by coalescence of the small bubbles and bearing a
higher rise velocity hence leading to relatively lower
gas holdup values depending on the type of the gas distributor and the properties of the liquid phase.[11] Both
regimes can be obtained in the same equipment by varying the gas input flow rate.
EXPERIMENTAL SETUP AND TECHNIQUE
The experimental setup consists of a perspex column
of height 1.45 m and the diameter is 0.116 m with a
porous sparger fitted at the bottom. For experimental
418
A. JHA ET AL.
Asia-Pacific Journal of Chemical Engineering
purpose three different types of spargers are used.
These are polypar sparger (acrylic fiber), refractory
bricks material and ceramic with porosity 23, 20 and
16% respectively. The first one is made up of a
polymer named as polypar with a height of 0.15 m
and diameter of 0.04 m, the rest two are 0.13 m height
and 0.04 m diameter and 0.06 m height and 0.12 m
diameter respectively. A Compressor has been used to
suck the atmospheric air, which is sent to the porous
sparger for producing bubbles in the bubble column
containing plain water. A rotameter of range: 0–50
l/min is fitted along the air line to the sparger from
the compressor to regulate the air flow rate. A paper
scale was attached to the column. The experimental set
up is presented in Fig. 1.
Water and dilute soap solutions are used for gas
holdup experimentation in the bubble column. The
soap solution is prepared by using 0.5 g of surfactant
(detergent) in 1 l of tap water and diluted to various
concentrations as per the requirement. The physical
properties of different surfactant solution presented in
Table 1. Type of surfactant employed was short-chain
alkyl naphthalene sulphonate type anionic surfactant.
The bubble column was first filled with 5 l of plain
water and the initial level of the water was noted. The
gas (air) flow rate to the first sparger (polypar sparger)
was varied from 15 to 50 l/min and final level of the
water in the bubble column was noted for each gas flow
rate at steady state. The gas holdup for each gas flow
rate was estimated by subtracting the initial level of
the water to from the final level of water in the bubble
column. In a similar way the gas holdup for 6, 7 and 8
l of plain water in the bubble column was estimated.
The surface tension of the plain water is reduced by
adding the prepared surfactant solution (detergent) in
different volumes 30, 60, 90, 120 and 150 ml to the
bubble column and made up 5 l using plain water.
The gas holdup for different gas flow rates at different
dilutions of the surfactant solution in the bubble column
was estimated. Exactly 36, 72, 108, 144, 180 ml of the
surfactant solutions are added to the bubble column
and made up to 6 l for each set of experiments.
The gas holdup for different gas flow rate for each
of the above-mentioned dilutions of the surfactant
was estimated. Further 42, 84,126,168 and 210 ml of
surfactant solution was added to the bubble column and
the total volume was made up to 7 l. The gas holdup
for each flow rate was for the above-mentioned dilutions
was noted. The above procedure both with pure water
and surfactant solution was repeated for different type
of sparger namely poly par sprger, refractory brick
sparger and ceramic sparger. A photographic view of
the above sparger are shown in Fig. 2. Experiments
were conducted for other type sparger of higher radius
following the same procedure.
Figure 1. Schematic diagram of the experimental set up. This figure
is available in colour online at www.apjChemEng.com.
Table 1. Physical properties of surfactant solution.
Surfactant
dose (Vol(%))
Density (g/m3 )
Viscosity Pa s (kgm/s)
Surface tension × 103 , N/m
0.5
1
1.5
2
2.5
3
3.5
4
999
0.2028
53
998
0.2067
48
996
0.2171
42
995
0.2247
40
993
0.2286
38
990
0.2324
36
988
0.2356
34
985
0.2432
33
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2008; 3: 417–424
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
STUDIES ON GAS HOLDUP IN A BUBBLE COLUMN
Photographs of (a) Ceramic sparger (b) Refractory brick
sparger (c) Polypar sparger. This figure is available in colour online at
www.apjChemEng.com.
Figure 2.
RESULTS AND DISCUSSION
Effect of superficial gas velocity on the gas
holdup with water without surfactant
The effect of gas velocity on the gas holdup in
bubble column, fitted with polypar sparger, containing
measured volume water in column is presented in
Fig. 3. It gives the clear understanding of the variation
of gas holdup with respect to the change in the gas flow
rate. Figure 3 reveals that the holdup increases almost
linearly with the increase in the gas velocity from 0.02
to 0.08 m/s. A maximum holdup of 42% is attained for
the maximum initial height of 0.76 m of water. As the
volume of the water in the bubble column is increased,
the holdup is also found to increase. The rate of increase
in the gas holdup is observed to be steady upto 0.07 m/s
gas velocity, after which the rate decreases. This trend is
found common in all the initial levels of the liquid in the
bubble column. This may be because of turbulent flow
development resulting in churn turbulent regime where
there is comparatively less increase in gas holdup. With
8 l of water in the bubble column, the holdup is found
to increase very much and linearly when compared to
other volumes. Moreover there is no fall in the rate of
increase in the hold up also. Thus, the turbulent regime
may not be developed in greater liquid volume for the
given gas velocity of 0.08 m/s. In greater liquid volume,
greater energy is required for the development of the
turbulent flow regime.
Effect of gas flow rate on gas holdup with
addition of surfactant solution in the bubble
column
Figure 4 shows the effect superficial gas velocity on gas
holdup for a bubble volume for a particular surfactant
dose. It is very clear from this figure that at 3%
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
Figure 3. Effect of superficial gas velocity on gas holdup
at different initial height for poly par sparger. This figure is
available in colour online at www.apjChemEng.com.
of surfactant dose, the gas holdup is more when the
bubble volume increased from 5 to 7 l. Experiments
conducted with addition of the prepared surfactant
solutions making it to the 5 l volume (5l) of the bubble
column and polypar sparger as the sparger are presented
Asia-Pac. J. Chem. Eng. 2008; 3: 417–424
DOI: 10.1002/apj
419
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A. JHA ET AL.
Asia-Pacific Journal of Chemical Engineering
Figure 4.
Effect of superficial gas velocity on gas
holdup at different bubble column volume for poly
par sparger. This figure is available in colour online at
www.apjChemEng.com.
Figure 5. Effect of superficial gas velocity on gas holdup
at different surfactant concentrations for refractory brick
sparger. This figure is available in colour online at
www.apjChemEng.com.
in Figs 4 and 5 for refractory sparger. When an input of
30 ml of the prepared solution to the bubble column and
made up to 5 l, the gas holdup increased by an average
of 4–6% for all gas flow rates when compared to plain
water of 5 l. Thus, the addition of surfactant reduces
the surface tension of the water and promotes more
number of bubbles getting formed. Further increase in
the concentration of the surfactant by the addition of 60,
90, 120 and 160 ml and made up to 5 l, the gas holdup
increases by 6% per 30 ml of addition of solution.
This increase in gas holdup due to increased bubble
formation by coalescence. It was observed that at higher
volume of solution, first the foam forms which fills the
entire column but afterwards it settles down. Figure 5,
reveals that for decreasing gas flow rate gas holdup is
not the same as with the increasing volume. It was also
observed that the holdup increase rate reduces when the
gas flow rate goes above 0.07 m/s. This may be due to
turbulent flow development resulting in churn turbulent
regime where there is comparatively less increase in
gas holdup. The gas holdup increases more rapidly for
the initial sets of surfactant solutions than with the
subsequent dosage of solution. This may be due to the
easy breakage of the bubbles caused by low surface
tension and thus giving way for the gas to pass through
the column quickly.
Experiments carried out using prepared solution in
6 l of the water and surfactant solution and the
results are presented in Fig. 6. For the 36 ml of the
prepared solution being made up to 6 l in the bubble,
the increase in the gas flow rate increases the gas
holdup steadily from 25 to 43%. When 72 ml of
surfactant solution is made up to 6 l in the bubble
column, the gas holdup showed a 24% difference
from the previous holdup values. And further for
every increase in the concentration of the surfactant
by addition 36 ml of the surfactant solution, the gas
holdup increases by 25%. From the Fig. 6, it is clear
that for the same concentration of the surfactant solution
the increase in gas holdup remains independent of the
initial volume of liquid in the bubble column for this
polypar sparger. A slight higher value of gas holdup is
observed for decreasing gas flow rates that may be due
inertia.
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2008; 3: 417–424
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
Effect of superficial gas velocity on gas
holdup at different surfactant concentrations for ceramic
sparger. This figure is available in colour online at
www.apjChemEng.com.
Figure 6.
Figure 7 represents the effect of gas flow rate on
the gas holdup for ceramic sparger for 3% by volume
of surfactant dose. The gas holdup was found to
increase by 4% with increase of volume of liquid in
the column. This effect remains almost same at higher
flow rates resulting in a difference of 3% with 7 l
solution. The gas holdup tends to shows nonlinear
behavior for 5, 6 and 7 l of water volume in the
bubble column. This may be due to transition regime
between bubbly flow regimes and churn turbulent flow
regime. A highest of 45% of gas holdup is achieved
in the bubble column for the given maximum gas
velocity from 0.02 to 0.08 m/s with 3 vol% of surfactant
solution.
Effect of gas velocity on gas holdup (with
water only) for refectory sparger
To study the effect of gas flow rate on the gas
holdup in bubble column with respect to different
types of spargers experiments are conducted on the
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
STUDIES ON GAS HOLDUP IN A BUBBLE COLUMN
Effect of gas flow rate on gas holdup in a
bubble column at different liquid volumes for ceramic
sparger. This figure is available in colour online at
www.apjChemEng.com.
Figure 7.
bubble column for the same volume and concentration.
Figure 8 reveals the effect of gas rate on gas holdup
for different volumes of water using refractory sparger
as the diffuser in the bubble column. The gas holdup
increases with increase in the liquid (water) volume for
the given gas velocity (0.02 to 0.08 m/s). There was a
marked difference in the gas holdup for each volume
of the liquid for this filter type sparger. A maximum of
27, 29, 31 and 33.5% of gas holdup was observed for
5, 6, 7 and 8 l of water volume respectively at 0.08 m/s
of gas rate. This may be due to increased number of
bubbles being present with larger residence time in
the water column (greater volume). For 8 l volume
of bubble column, the increase in gas holdup is pretty
rapid when compared to the polypar sparger where as
the gas holdup is less than that in the case of polypar
sparger. Moreover the increase in the gas holdup seems
to be steady, which is different from the polypar sparger
holdup trend hence the entire range of operation is in
bubbly flow regime which is characterized by uniform
bubble size.
Asia-Pac. J. Chem. Eng. 2008; 3: 417–424
DOI: 10.1002/apj
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A. JHA ET AL.
Asia-Pacific Journal of Chemical Engineering
Figure 8.
Effect of superficial gas velocity on gas
holdup at different initial height for refractory brick
sparger. This figure is available in colour online at
www.apjChemEng.com.
Figure 9.
Effect of gas velocity on gas holdup at
different liquid volume of bubble column at 3
vol% concentrations of surfactants
Fig. 10 is for 5 and 7 l of the volume, the solution in the
bubble column respectively, the gas holdup increases
with increase gas velocity for all types of sparger.
Results indicate that the poly par sparger and ceramic
sparger gives very good holdup than that of refractory
sparger. The relationship between the gas holdup and
gas flow rate are almost linear at low holdups. This
represents that the entire region of operation was in
the bubbly regime. The gas holdup was least in this
sparger as the gas holdup with pure water is least and
also the increase in gas holdup upon addition of solution
is less. The increase of gas holdup was very close to
linear values compared to all other spargers. The gas
holdup increased by 10%. It can be concluded that
with increase of concentration of surfactant solution gas
holdup increases. This effect remains the same at higher
flow rates resulting in a difference of 7–10% increase
in solution. The gas holdup was least in this sparger
as the gas holdup with pure water is also least and
the increase in gas holdup upon addition of solution
is also less. The liquid level was most stable and there
is almost no difference in increasing and decreasing gas
The results on the effect of gas holdup for 5 and 7 l
of volume of water at 3% concentrations of surfactant
in the bubble column with refractory brick sparger are
presented in Fig. 9. It can be observed from Fig. 9 that
the gas holdup increased by 0.75% for a lower gas
velocity. As the gas velocity increases from 0.02 to
0.08 m/s, the gas holdup increases from 26 to 31%.
Effect of gas velocity on gas holdup for
various type of sparger and different liquid
volumes of bubble column at 3 vol%
concentrations of surfactants
Figure 10 shows a typical comparison of various type of
sparger performance and their gas holdup values under
different superficial gas velocity. The observations of
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
Effect of gas flow rate on gas holdup in a
bubble column at different liquid volumes for refractory
brick sparger. This figure is available in colour online at
www.apjChemEng.com.
Asia-Pac. J. Chem. Eng. 2008; 3: 417–424
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
STUDIES ON GAS HOLDUP IN A BUBBLE COLUMN
Figure 11. Comparison of the experimental gas holdup
and predicted values reported in the literature. This figure is
available in colour online at www.apjChemEng.com.
Figure 10. Effect of gas flow rate on gas holdup at
different volumes of water in the bubble column for
various sparger. This figure is available in colour online
at www.apjChemEng.com.
dimensionless numbers Froude (Fr), Archimedes(Ar)
and Eotvos (Eo) numbers defined as follows.
Fr =
flow rates. Moreover the increase of gas holdup is also
constant with increase of the solution. The behavior of
this sparger can be predicted most accurately because
of its linearity in relationship between gas flow rate and
gas holdup for different concentrations of surfactant.
Comparison of gas holdup experimentally
found out and reported by various researchers
To compare data obtained in the present study an
attempt has been made by for better fit of the data. The
gas holdup correlation as reported by Kazakis et al .[12]
given in Eqn (1) have been used to compare the gas
holdup.
εG = 0.2 Fr 0.8 Ar 0.2 Eo 1.6
ds
dc
0.9 dp
ds
0.03 2/5
(1)
where, dS is diameter of sparger dP is the diameter of
pore and dC is dimeter of column and following are
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
Ar =
Eo =
U 2 GS
dc g
dC3 ρL2 g
µ2L
dC2 ρL g
σL
(2)
(3)
(4)
Where ρL , µL , σL are the density, viscosity and
surface tension of the liquid respectively.
The experimentally observed values have been compared with the values reported in the literature[2,12 – 14]
and presented in Fig. 11. It has been found that most of
the experimental values at lower superficial gas velocity
are in good agreement with that reported in the literature
cited values within ±20%.
CONCLUSION
The gas holdup increased almost linearly with gas
flow rate for all three spargers investigated. The gas
holdup in pure water was maximum for polypar sparger
Asia-Pac. J. Chem. Eng. 2008; 3: 417–424
DOI: 10.1002/apj
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A. JHA ET AL.
Asia-Pacific Journal of Chemical Engineering
and minimum with refractory type sparger with greater
radius. The gas holdup in case of additive solution is
comparable for polypar sparger and ceramic sparger
with smaller radius. This is because the rate of increase
of gas holdup upon increase of concentration is higher
with filter sparger. The rate of increase of gas holdup
for a fixed concentration of solution is independent of
the level of water in the column and dependent upon
type of sparger. The stability of liquid level varies more
in case of Polypar sparger and less in refractory sparger
making it more stable in predicting gas holdsup. The gas
holdup increased more linearly with filter type sparger
with greater radius but at the cost of lowest gas holdup.
Thus it can be said for polypar types of sparger that for
superficial gas velocity of 5–6.67 cm/s the turbulent
regime occurs. A comparison of the present experimental data has been compared with the correlations and
data available in the literature and found in good agreement at lower superficial gas velocities.
[2] A.A. Mouza, G.K. Dalakoglou, S.V. Paras. Chem. Eng. Sci.,
2005; 60, 1465–1475.
[3] D. Colella, D.D. Vinci, R. Bagatin, E.A.M. Bakr. Chem. Eng.
Sci., 1999; 54, 4767–4777.
[4] D. Tao. Sep. Sci. Technol., 2004; 39(4), 741–760.
[5] J. Zahradnik, M. Fialova, M. Ruzicka, J. Drahos, F. Kastanek,
N.H. Thomas. Chem. Eng. Sci., 1997; 52, 3811–3826.
[6] M. Polli, M. Di Stanislao, R. Bagatin, E. AbuBakr, M. Masi.
Chem. Eng. Sci., 2002; 57, 197–205.
[7] M.C. Ruzicka, J. Drahos, P.C. Mena, J.A. Teixeira. Chem.
Eng. J., 2003; 96, 15–22.
[8] M.C. Ruzicka, J. Zahranik, J. Drahos, N.H. Thomas. Chem.
Eng. Sci., 2001; 56, 4609–4626.
[9] R. Parthsarthy, N. Ahmed. J. Chem. Eng. Jpn., 1996; 29,
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[10] G. Vatai, M.N. Tekic. Chem. Eng. Sci., 1987; 42(1), 166–169.
[11] B.C. Meikap, G. Kundu, M.N. Biswas. Can. J. Chem. Eng.,
2002; 80, 306–312.
[12] N.A. Kazakis, I.D. Papadopoulos, A.A. Mouza. Chem. Eng.
Sci., 2007; 62(12), 3092–3103.
[13] M. Kaji, T. Sawai, K. Mori, M. Lguchi. Behaviours of Bubble
formation from a bottom porous Nozzle Bath. In Proceedings
of the Fifth ExHFT , Thessaloniki, 2001, 1503–1508.
[14] E. Camarasa, C. Vial, S. Poncin, G. Wild, N. Midoux,
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REFERENCES
[1] B.C. Meikap, G. Kundu, M.N. Biswas. AIChE J., 2002; 48(8),
2074–2083.
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2008; 3: 417–424
DOI: 10.1002/apj
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