Group Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Title: 8.3.2 - Investigation on Reactive Mass Transfer at Freely Rising Gas Bubbles
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 Material Information
Title: 8.3.2 - Investigation on Reactive Mass Transfer at Freely Rising Gas Bubbles Experimental Methods for Multiphase Flows
Series Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Physical Description: Conference Papers
Creator: Kück, U.D.
Schlüter, M.
Räbiger, N.
Publisher: International Conference on Multiphase Flow (ICMF)
Publication Date: June 4, 2010
 Subjects
Subject: gas/liquid mass transfer
chemical reaction
LIF
PIV
 Notes
Abstract: The impact of local phenomena on mass transfer from free rising single gas bubbles with different bubble diameters, temperatures and ethanol concentrations has been investigated by using the laser induced fluorescence (LIF) and particle image velocimetry (PIV) technique. The influence of these parameters on mass transfer performance will be reported. Furthermore a novel calculation method for mass transfer coefficients has been developed and will be discussed in this paper.
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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Volume ID: VID00202
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Holding Location: University of Florida
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Resource Identifier: 832-Schlueter-ICMF2010.pdf

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




Investigation on Reactive Mass Transfer at Freely Rising Gas Bubbles


Ulf Daniel KUck*, Michael SchlOter* and Norbert Rabiger+


*Hamburg University of Technology, Institute of Multiphase Flows
EiBendorfer StraBe 38, 21073 Hamburg, Germany
E-mail: kueck@ims-tuhh.de

iUniversity of Bremen, Institute of Environmental Process Engineering,
Leobener StraBe, UFT, 28359 Bremen, Germany


Keywords: gas/liquid mass transfer, chemical reaction, LIF, PIV



Abstract

The impact of local phenomena on mass transfer from free rising single gas bubbles with different bubble diameters,
temperatures and ethanol concentrations has been investigated by using the laser induced fluorescence (LIF) and particle
image velocimetry (PIV) technique. The influence of these parameters on mass transfer performance will be reported.
Furthermore a novel calculation method for mass transfer coefficients has been developed and will be discussed in this paper.


Nomenclature

A interfacial area (m2)
C1 constant (-)
D diffusion coefficient (m2s-')
de bubble equivalent diameter (m)
di, distance to the lower surface of the bubble (m)
kL liquid side mass transfer coefficient (ms'1)
ri mass flow rate (gs-')
r radius (m)
Re Reynolds number (-)
Sc Schmidt number (-)
Sh Sherwood number (-)
r Volume flow rate (m3s-')
w local liquid velocity (ms'1)
WF counter flow mean velocity (ms-1)
w.,w, radial velocity components (ms-1)
wy axial velocity component (ms-1)
Greek letters
v kinematic viscosity (m2s-1)
Po dissolved oxygen concentration (gL-1)
p* oxygen saturation mass concentration (gL-1)
p oxygen mass concentration of the fluid (gL 1)
(p angle (0)
Subsripts
mob mobile interface
rel Relative
rig rigid interface


Introduction

In many fields of chemical industry, biotechnology, life
science and environmental process engineering the mass


transfer from a gaseous disperse phase into the circumfluent
liquid phase is of particular importance. Nevertheless
accurate dimensioning of multiphase system reactors is an
extensively undissolved problem, because mass transfer
processes occurring under swarm conditions which cannot
be easily determined by measurement techniques.
Due to insufficient fundamental experimental results, the
design of many types of mass transfer apparatus is still
mainly based on empirical mass transfer correlations which
are developed from integral measurements.
However, the physical effects of hydrodynamics and mass
transfer on bubbles are very complex and are not fully
understood at present. Although it is well known that in
general the mass transfer is unsteady in gas-liquid systems
(Brauer et al., 1971), the dimensionless mass transfer
correlations are usually expressed by the Sherwood number
Sh that only based on the Reynolds number Re and Schmidt
number Sc according to (Brauer, 1981)


Sh=2+C, Rea Sc .


The mass transfer resistance is generally induced by the
liquid phase. Local hydrodynamic effects (Bork et al., 2005)
on the bubble swarm (e.g. mixing in the wake area) are
merely in the form of constants C1 or exponent a, b
empirically considered with the aim of the dimensionless
Sherwood number, which allows the consideration of the
bubble velocity (Re) and the fluid characteristics (Sc v/D).
Due to the pressure of innovations in process intensification
the efficient utilization of the gas phase at low energy costs,
optimal contact times and high mass transfer rates is today
of proper importance. Hence the wakes of dispersed gas
bubbles are notable as local mixing zones which have a
great impact on the meso- and micro-mixing in multiphase









flows. Additionally new perspectives in chemical reaction
engineering can be established. To the extensive
comprehension of the coupled momentum, heat and mass
transfer in multi phase flows detailed knowledge is needed,
which is provided so far only in fragments because of the
complex interdependencies and limited applicability of
measurement techniques. Several studies on gas-liquid mass
transfer at single bubbles using optical measurement
methods have been recently published (Dani et al., 2007;
Mizuta et al., 2008; Vasconcelos et al., 2002).


Experimental Facility

To measure the mass transfer of the migrating
gas-component of a single bubble in the circumfluent fluid
spatiotemporal with high resolution it is necessary to
capture the predominant concentration and velocity fields
simultaneously. In this work a combination of laser-induced
fluorescence (LIF) and particle image velocimetry (PIV) is
used. The light source for both measuring systems is a
Nd:YAG laser (wavelength 532 nm, pulse width 5 ns, New
Wave Research, Inc.). To perform the experimental
investigations, a flow channel with 100 mm diameter and
2.5 m height is in application. In Fig. 1 is the appropriate
flow diagram shown. A syringe pump in combination with
different nozzles is used to generate bubbles with a diameter
range from 0.8 to 2.5 mm. Additionally a variable
counter-current flow can be adjusted with defined flow rate.


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

Fig. 2 shows the principle layout of the measuring method.
The cameras of the LIF- and PIV-measuring technique are
respectively rectangular to the direction of the light sheet
arranged. Reflections of the light source at the bubble
interface are minimized by narrow the light sheet height to
the camera field using a slit shade. Fluorescence marked
tracer particles (Rhodamine B) are utilized to decouple the
frequency range of the light detected by the PIV-camera
from strong interface reflections of the laser wavelength.
Thereby a high resolution double shutter camera (PCO
Sensicam QE) in combination with a low pass filter is
employed. The repetition rate of the simultaneous
measurements is 4 Hz. Synchronisation and evaluation of
the measurements is performed by a measuring system from
Intelligent Laser Applications (ILA GmbH, Jiilich,
Germany).
Laser induced fluorescence (LIF) is used to measure the
oxygen concentration field, therefore the fluorescent dye tris
(1,10-phenan-throlin)-ruthenium(II)-chlorid is dissolved in
the water. The fluorescence intensity of the dye behaves
inversely proportional to the oxygen concentration. The
recording of the fluorescent light is done with a second
camera (PCO Sensicam QE), also in connection with a low
pass filter to prevent the camera chip from direct laser
radiation. Finally are the LIF-measurements evaluated
according to a method described by Bork (Bork, 2006).

Simultaneous PIV-LIF Set-Up


Figure 1: Experimental set-up


Main components of the investigated system are double
distilled water and oxygen. About to assure a high driving
concentration gradient for optimal mass transfer, nitrogen
stripping is utilized to remove dissolved oxygen from the
medium extensively (Po2 < 0.2 mgL-1). Furthermore
different gas separator units should ensure a micro bubble
free flow to avoid measurement errors. By means of a
double tube shell the flow channel is tempered with a
variation of + 1 C.


LIF-Camera


Figure 2: Simultaneus PIV-LIF Set-Up

Further development of the LIF-measurement technique has
been carried out by running the camera exposure not at the
same time with the laser impuls (Fig. 3). Therefore it takes
advantage of the fact that the fluorescence intensity at the
end of the excitation impuls decays exponentially. The
measurement time frame is placed in this decay range to
reduce interface reflection effects. Thus the LIF-exposure
takes place with a short time offset of a few micro-seconds
to the second frame of the PIV-exposures (Fig. 3a).
Furthermore it can be assured that the LIF-measurements
will not be disturbed by the fluorescence light of the tracer
particles (Fig. 3b, c), which lies in a similar wavelength
range but decays more quickly.





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


U


Figure 3: Temporal fluorescence intensity characteristic
during laser excitation

The evaluation of the PIV-measurements is arranged with
the aid of the software VidPIV (ILA GmbH, Jiilich,
Germany). Additionally is the processing of the LIF-images
with 8-bit greyscale bitmap format performed by the
software Camware (PCO). Matlab (MathWorks) is used to
support further evaluation steps e.g. calculating mass
transfer rates.

Results and Discussion

To clarify the local mass transfer from gas bubbles in the
circumfluent fluid the knowledge of the simultaneous
velocity and concentration fields are necessary. A
simultaneous captured velocity and concentration field is
shown in Fig. 4. In this example is the bubble diameter de =
1.7 mm and the counter flow mean velocity wF = 1 cm s- .
The concentration field is described by the colour map and
the velocity field is characterized by vektors.


Concentration
P02 mglL 1 2 3 4 5 6 7

0.005


IL
F;PIV
IF;LIF


Position x I m


Figure 4: Simultaneous captured velocity and
concentration field of a free rising single bubble

With the aid of this new measuring method mass transfer
balances on single bubbles are possible. Therefore a balance
room is arranged around the bubble to determine the local
mass transfer from the gas bubbles into the circumfluent
fluid. The following Fig. 5 indicates a schematically mass
transfer model which is taken as basis. Mass transfer flow
rates induced by diffusion or convection through the upper
end plane (ri) and the lateral area (rh2) are neglected
compared to the flow rate through the lower end plane (ri,)
according to the work of Bork (Bork, 2006). The major
contingent of transferred mass is flowing off the lower end
plane in the wake area by convection. To calculate this mass
flow rate the spatiotemporal velocity and concentration have
to be integrated.


m0
w .L = d
.. ................... ..... .......... r = R = d
....... -.








W~l ~ "~1


ti=


Figure 5: Schematic mass transfer model


IL Laser puls intensity
IF;LIF LIF-Fluorescence intensity
IFPTV PIV-Fluorescence intensity
tlaser pus Laser puls width
delay Delay width
tLFexposure LIF-Exposure width


Velocity
Wre
mis
0.01
-0.18
-0.20
-0.22
-0.25
-0.27
-0.29
-0.32
-0.34


-!PxtosFe t
serpuls de F-exposure


1I-naewthg (U~a


?









The local oxygen mass flow rate m is defined as the
product of the local volume flow rate V and the mass
concentration p02 corresponding to


m =V Po. (2)


While the local mass concentration P02 is determined
with the LIF measurement technique, the local liquid
velocity w is needed to calculate the volume flow rate V .
Assuming that the radial velocity components (w,, w,) are
negligible in comparison to the axial velocity component
Wy the oxygen mass flow rate is defined as follows

m = wy A 0 (3)


By the consideration of local concentration fluctuations the
mass flow rate of the migrating component can be
computed using a surface integral in cylindrical
coordinates balancing the velocity and the mass
concentration fields according to


rm = WY(r,) po(r,yp) r ddr.
r V


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

Furthermore the knowledge of the oxygen mass flow rate
enables the calculation of the liquid side mass transfer
coefficient kL defined from the equation


dm A/ p
mm= = kLA -p)
dt


transformed in

kL =m.f
AVo-p)


In Equation 4 and 5 the interfacial area (bubble surface) is
denoted by A, p* is the oxygen saturation mass
concentration and p is the mass concentration in the fluid.
Due to the restriction that only two-dimensional velocity and
concentration fields can be measured at the same time, a
rotational symmetric wake is obligatory. Thus regarding the
boundary conditions wy (q) = konst. and pg (p) = konst.
equation 4 can be simplified according to equation 7
1
m = 2 w(r) 2 (r)rdr (7)
r=0

Results of the mass transfer coefficient kL calculated with the
above mentioned method are presented in figure 7.


In equation 4 the liquid velocity Wy is given in direction of
the bubble rising path and the concentration P02 in the
regarded plane. In terms of the conservation of mass, the
mass flow rate that is integrated inside the lateral cut of the
wake should be constant along the y-axis of the wake. It is
possible to draw conclusions from these characteristics on
mistakes in measuring and balancing or unfounded
assumptions.

Fig. 6 describes exemplary mass flow rate characteristics.
Strong fluctuations appear at the wake area near to the lower
surface of the bubble with dil < 1 mm which are originated in
the low resolution of the velocity field.

2 50E-06 -
Constant area
2 00E-06
_o 7 Evaluation of the oxygen mass flow =90.
g i rate starting from 0 001 m distance
o ^ to the lower surface ofthe bubble
1 50E-06 4'


1 00E-06
S0two different exemplary bubbles
5 OOE-07 0


0 0001 0002 0003 0004 0005
distance to the lower surface of the bubble dl / m


Figure 6: Oxygen mass flow rate integrated at the lateral
cut of the wake

The constant area of the mass flow rate starts at di = 1 mm.
Hence the evaluation of the mass flow rate takes place by
averaging downwards the y-axis.


1,OE-05
5,001


results water/oxygen (Vasconcelos 2002) results water/oxygen (Sardeing 2006)
O results water/tenside (Sardeing 2006) N results water/oxygen 30 C
results water/oxygen 18 C A results water/ethanol 1 /oxygen
E-04 1,00E-03 1,50E-03 2,00E-03 2,50E-03 3,00E
equivalent bubble diameter d, / m


Figure 7: Comparison of mass transfer coefficients kL

The dashed lines are theoretically calculated and postulated
by Higbie (Higbie, 1935) for a mobile interface (bubble with
inner circulation) and Frossling (Frbssling, 1938) for a rigid
interface (bubble without inner circulation) described in
equation 7 and 8


kL,mob =1,13. -.D21
L~mb ild


0006


2 -1
kLng =c e D3 v 6 (9)

Integrally measured values are given by Sardeing and
Vasconcelos (Sardeing et al., 2006; Vasconcelos et al., 2002)
for the water/oxygen system. Our results are measured in a


E-03









very small time period of few microseconds thus increased
variation originates. Nevertheless regarding averaged values
significant effects are observable.
The trend of the values in the water/oxygen system at 18 C
is comparable to the results of Sardeing et al.. Hence a strong
sensitivity of the mass transfer coefficient to the bubble
diameter can be assumed. Furthermore at a temperature of
30 C increasing values at smaller bubble diameter can be
observed due to stronger mobility of the interface at higher
temperature. In particular, it is interesting to note that the
system water/ethanol does not show the expected decreasing
trend of the results of Sardening et al. for the water/tenside
system in consequence of increasing diffusion resistance.
Comparable effects have been widely evaluated in the
Literature (Painmanakul et al. 2005)
Recapitulatory has to be assumed that the system
water/oxygen at room temperature can be fitted with
Frossling's equation (eq. 9) at small equivalent bubble
diameter (de < 1,25mm). Additionally high equivalent bubble
diameter (de > 1,75mm) can be expressed by Higbie's
equation (eq. 8) and a transition region is observed in
between (1,25mm < de < 1,75mm).

Conclusions

The impact of local phenomena on mass transfer from free
rising single gas bubbles with different bubble diameters,
temperatures and ethanol concentrations has been
investigated by using the laser induced fluorescence (LIF)
and particle image velocimetry (PIV) technique. The
influence of these parameters on mass transfer performance
has been reported. It was becomes clear that simultaneous
measurements of the concentration and velocity field in the
vicinity of a free rising gas bubble are possible. For
axisymmetric flow conditions the instantaneous and local
mass transfer rate as well as the overall mass transfer
coefficient is calculable with measured the data. At free
rising gas bubbles the measurement of the concentration
gradient inside the boundary layer is possible by vt-LIF.
The concentration gradient inside the boundary layer is in
agreement with the analytical solution of the penetration
model. In future quantitative measurements of flow- and
concentrations fields with chemical reaction will be
performed to calculate mass transfer rates and enhancement
factors. With the comparison of experimental results and
numerical simulations of Prof. Bothe (Center of Smart
Interfaces, TU Darmstadt) and the experimental validation of
mass transfer models (e.g. penetration model) under certain
conditions new models will be developed to ensure a more
precise calculation of mass transfer in multiphase flows.
Further more reliable data by combining experimental and
numerical results are expected.


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

Acknowledgements
This work is supported by the German Research Foundation
(DFG), Reactive Mass Transfer on rising Gas Bubbles
(PAK119). The authors gratefully acknowledge this
financial support.
We also thank our cooperation partners, the Institute of
Chemical Engineering, University of Paderborn and the
Institute of Mathematical Modelling and Analysis, Center of
Smart Interfaces, Darmstadt University of Technology


References

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an Gasblasen in Zweiphasenstrbmungen. Dissertation,
Universitat Bremen, Germany, 2006.

Bork, O., Schliiter, M., RAbiger N., The impact of local
phenomena on mass transfer in gas liquid systems,
Canadian Journal of Chemical Engineering, 83, 658-666,
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Brauer, H., Mewes, D., Stoffaustausch einschlieBlich
chemischer Reaktionen, Verlag Sauerlander, Aarau,
Frankfurt am Main, Berlin, Miinchen, Salzburg, 1971.

Brauer, H.: Particle/Fluid Transport Processes. Prog. Chem.
Eng. 19, 61-99, 1981.

Dani, A., Guiraud, P., Cocks, A., Local measurement of
oxygen transfer around a single bubble by planar
laser-induced fluorescence, Chemical Engineering Science,
62, 7245-7252, 2007.

Frossling, N.: Uber die Verdunstung fallender Tropfen,
Geophysik, 52,1/2, 170-216, 1938.

Higbie, R., The rate of absorption of a pure gas into a still
liquid during short periods of exposure, Trans. Am. Inst.
Chem. Eng., 31. 365-377, 1935.

Mizuta, K., Odo, Y, Yoshimitsu, S., Kaji, H., Murakami, M.,
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Eng. of Japan, 41, 7, 553-556, 2008.

Painmanakul, P., Loubiere, K., Hebrard, G,
Mietton-Peuchot, M., Roustan, M., Effect of surfactans on
liquid-side mass transfer coefficients, Chemical Engineering
Science, 60, 6480-6490, 2005.

Sardeing, R., Painmanakul P., Hebrard G: Effect of
surfactants on liquid-side mass transfer coefficients in
gas-liquid systems: A first step to modelling, Chemical
Engineering Science, 61, 6249-6260, 2006.

Vasconcelos, J. M. T., Orvalho S. P, Alves S. S., Gas-Liquid
Mass Transfer to Single Bubbles: Effect of Surface
Contamination, AIChE Journal, Vol.48, No. 6, 1145-1154,
2002.




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