Radial Variation of Local Gas Holdup in a Bubble Column... Exchanging Internals Using Four-Point Optical Probe

Radial Variation of Local Gas Holdup in a Bubble Column Reactor with Heat
Exchanging Internals Using Four-Point Optical Probe
Sai Abhishek Palaparty, National Institute of Technology,Warangal, Warangal, India; Missouri S&T, Rolla, MO,
Moses Kagumba, Chemical Engineering, Missuori S&T, Rolla, MO, Muthanna H. Al Dahhan, Chemical and
Biological Engineering, Missouri University of Science and Technology, Rolla, MO
Slurry bubble column reactors have been demonstrated to be the reactors of choice for the clean
utilization and conversion of syngas and commercialization of the Fischer-Tropsch (F-T) synthesis due
to their advantages over other multiphase flow reactors. The local spatial variation of the gas holdup is
one of the key parameters of the gas holdup since it gives rise to pressure variations radially and axially
leading to varied strengths in the large scale and small scale liquid re-circulations which are important
aspects for both mass and heat transfer in bubble and slurry bubble columns. The radial variations of
the local gas holdup have been studied in the past mainly in bubble columns without the heat
exchanging internals. Therefore, the focus of this study is to investigate how the local gas holdup varies
radially in bubble column equipped with dense mimicked heat exchanging internals. The experimental
work was carried out in 0.14 m diameter Plexiglas column using a non-reactive system of air as the gas
phase and filtered tap-water as the liquid phase at ambient conditions of temperature and pressure. The
superficial gas velocities were varied from 0.08 to 0.45 m/s covering both the transition flow regime
and the churn-turbulent flow regime. The internals used were vertical Plexiglas rods of 0.013 m in
diameter occupying 25 % of the column cross-sectional area. The four points fiber optical probe
technique was used with a suitable algorithm to measure the local gas holdup at different radial
locations. The distance of the probe from the column center was varied for every 0.0015 m. The
experimental results obtained suggest that there is irregular variation of the local gas hold up in
presence of dense internals which is a contrast to the regular variation observed in a bubble column
without internals. The irregular variation is likely to affect the performance of the bubble column
reactor. Detailed results and findings will be discussed in the presentation.
most of the reported studies in literature do not
investigate the variation of local gas holdup in a
understanding of radial variation of the local
gas hold up and how it affects the performance
of the reactor is lacking in literature.
Among the previous studies, Schweiter
et al.1 , investigated radial variation of the local
gas holdup without the presence of internals in
a 50mm ID column with a non-reactive system
of Heptane and nitrogen. The study was carried
out for a superficial gas velocity range of
0.01m/s to 0.26m/s. This study suggests a
regular variation in the local gas holdup profile.
It also suggested a parabolic profile for the
local gas holdup.
Bubble Column Reactor is of wide
application in the industry as it is the reactor of
choice in Methanol Synthesis, waste water
treatment, oxidation and Fisher-Tropsch(FT)
synthesis. Many of these applications involve
highly exothermic reactions which involves
heat removal by large number of heatexchanging internals to maintain the desired
reaction temperature.
Re-circulating eddies are responsible for
high heat transfer rates and mass transfer rates
in these reactors. The strength of these eddies
depends upon the pressure change causing
them, which in turn depends on the local gas
hold up at a particular radial distance. However,
Chen et al.3 investigated the effect of internals
on gas holdup, liquid recirculation and
turbulent parameters using gamma ray
computed tomography(CT) and computed
tracking(CARPT)techniques. In this study Airwater and air-Drake oil(Drake oil 10) systems
were used in a Plexiglas 0.45m diameter bubble
column. The internals geometry comprised of
two concentric circular bundles of 0.025 m
O.D. aluminum tubes each containing eight
tubes equally spaced, covering 5% of the cross
sectional area of the column. Chen et al.
concluded that the internals did not have a
significant effect on the liquid recirculation
velocity whereas the gas hold increased by
about 10% at the center of the column and less
around the wall region.
Forret et al.4 used a basic tracer
technique in a 1m diameter column with
internals. The study showed a decrease in liquid
fluctuation velocity and an increase in large
scale liquid recirculation in presence of
Youssef11 et al. investigated the effect
of internals on gas hold up and bubble
properties in a 0.23m O.D. column using an airwater system with gas velocities up to 0.2m/s.
The study was conducted using dense internals
( occupied 22 percent of the surface area) and
less dense internals (occupied about 5 percent
of the surface area).Four Point optical probe
technique developed by Xue et al5,7-9. was used
in determination of results. They found that that
local gas holdup increases in the presence of
dense internals but the increase is less
significant in case less dense internals were
No study in Literature shows the radial
variation of the local gas holdup and its effects
on the performance of the reactor in presence of
internals. Therefore, This study focuses on the
radial variation of the local gas hold up(in
presence of internals) and presents new data.
In this study, work was carried out in a
0.14m inner diameter and 1.83m high Plexiglas
column. Compressed Air and filtered tap water
were used as gas and liquid phase respectively
at ambient temperature and pressure.
Compressed Air entered the column from the
bottom through a perforated plate gas
distributor. The heat exchanging internals
having 0.013m external diameter made of
Plexiglas were used. The internals occupied
about 25 percent of the column surface area.
There were 30 internals in total and were
arranged as shown in Figure 2. Throughout the
study, the dynamic bed height was constant and
maintained just above 0.167m (z/D=11.9) from
the gas distributor. The gas distributor had 121
holes of 1.32mm Diameter arranged in a
triangular pitch with a total free area of 1.09%.
A four-point probe developed by Xue et
was used to measure local gas holdup
within the fully developed flow region at a
height of 0.71 m(z/D=5.1) from the gas
The optical probe in its initial stages was first
developed by Frijlink6 at Delft University and
then validated and further developed by Xue et.
al.5,7-9 This probe, with its developed algorithm,
gives direct values for the local gas holdup and
other bubble parameters.
The experiments were carried out over a
superficial gas velocities ranging 0.08m/s to
0.45m/s covering the intermediate and the
churn-turbulent flow regime. Dense Internals
having OD 0.013m were used which mimicked
internals in Fisher-Tropsch Synthesis covering
25% of the surface area of the column.
The local gas holdup measurements
were taken for the normalized radius(r/R)
values ranging from 0 to 0.9 at an interval of
r/R equal to 0.02 or r equal to 0.0015m.
Four Point Optical Probe
Vertical Plexiglas
0.013m OD
Data Acquisition
System (DAQ)
Figure 2. Internals arrangement,
Triangular Pitch ~ 0.73cm
Figure 1: Schematic Diagram of Bubble
Column Setup.
irregularity tends to increase with increase with
superficial gas velocity.
Figure 3 shows irregular variations of
local gas holdup in presence of the internals
which in contrast to the regular variation
obtained in system without internals by
Schweiter et al1. When dense internals are
present, bubble rise velocity decreases which
was determined by Youssef et al.11 leading to a
longer bubble residence time thus high gas
holdup. With a change(say 0.05) in normalized
radius, the holdup decreases and then again
increases. The decrease in holdup is because of
proximity to one of the internal's wall resulting
in wall effects. These irregular changes in the
local holdup give rise to irregular axial and
radial pressure differences that result in
irregular strengths of small scale and large scale
liquid recirculation. The local maximum occurs
at r/R equal to approximately 0.65-0.7.This
maximum has relatively higher value as
compared to holdup values around its
neighborhood. This point is the inversion point
which is in range as suggested by Rados10 for a
system without internals. At this point the
bubble velocity is theoretically zero giving rise
to a high value of local gas holdup. The data
also suggests that as the wall
approaches(r/R>0.7), the irregularity becomes
insignificant as the holdup seems to decrease
regularly. This observation may be due to the
fact that at locations past the inversion point
most of bubbles flow from top to bottom and
only few bubbles rise from bottom to top. Also,
the probe is always placed downwards. It is
also evident from the figure that this
Figure 3a: Local Holdup vs Dimensionless
radius at Superficial gas velocity of 45cm/s
Figure 3b: Local Holdup vs Dimensionless
radius at Superficial gas velocity of 20cm/s
r = distance of probe from the centre of the
R= internal radius of the bubble column
r/R= dimensionless or normalized radius
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Mudde, R. F. Bubble
Figure 3c: Local Holdup vs Dimensionless
radius at Superficial gas velocity of 8cm/s
Radial variation of Local gas holdup
for a system with internals is irregular. The
system shows regular variation (decrease) in
local gas holdup after the inversion point has
reached. It is also evident that this irregularity
tends to increase with increase with superficial
gas velocity.
It is very important to note that these
results are explicit for this particular column of
0.14m I.D.,0.0167m O.D. internals and airwater system. These results throw light into
complexity involved in understanding bubble
reactors operating with dense internals and
need to model these results. There is need to
study more about the effects of the irregular
variation in local gas holdup on large scale and
small scale liquid recirculation thus, on the
performance of the reactor.
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Mudde, R., F. Fourpoint
optical probe for measurement of bubble
dynamics: Validation of the
technique. Flow Measure. Instrument. 2008, 19
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10. Rados, N. Slurry bubble column
hydrodynamics, D.Sc. Thesis,
Washington University, Saint Louis, Missouri,
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Bubble Properties of a Bubble
Ahmed A. Youssef and Muthanna H. AlDahhan, Ind. Eng. Chem. Res. 2009, 48, 8007–
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On the Bubble Dynamics in a Slurry Bubble
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