Rositsa
Stoykova,
Dessislava
Moutafchieva,
Dimitrinka
Iliev
Journal
of Chemical
Technology
and Metallurgy,
49,Popova,
5, 2014,Veselin
469-472
GAS HOLD-UP PREDICTION IN GAS – LIQUID STIRRED TANK REACTOR
USING CFD SIMULATION
Rositsa Stoykova, Dessislava Moutafchieva, Dimitrinka Popova, Veselin Iliev
University of Chemical Technology and Metallurgy,
8 Kl. Ohridski, 1756 Sofia, Bulgaria
Received 10 November 2013
Accepted 11 July 2014
E-mail: [email protected]
ABSTRACT
This paper presents results from CFD simulation of gas–liquid mixing in an aerated stirred tank reactor. The ANSYS
CFX simulation program method has been used to predict the distribution of the overall gas hold-up in gas-sparged tank
with 1000 l work volume, equipped with two six-blade pitched turbines and a ring sparger of 0.41 m diameter. Fluid flow
is calculated with a turbulent k – ε two fluid model using a finite volume method.
The CFD simulation results indicate that the method used is suitable for the prediction of the change of overall gas
hold-up with increasing both of the impeller speed and the gas flow rate.
Keywords: ANSYS, CFD, gas hold-up, gas-liquid dispersion, simulation, stirred tank reactors.
INTRODUCTION
Computational fluid dynamics (CFD) is becoming
an increasingly useful tool in the analysis of the highly
complex fluid flow in mechanically stirred tanks. There
are a number of papers published to date which present
simulation methods for stirred tanks [1, 5, 9]. However,
most simulations reported in the literature deal with just
single-phase liquid flow, whereas applications in the
process industries often involve gas–liquid, solid–liquid,
and hence modelling methods need to be extended to
deal with multiphase flows. This paper describes progress in developing a simulation method for gas-liquid
contacting in stirred tanks with aeration. These types
of reactors are widely used in industries, such as the
petrochemical, paper and pulp, pharmaceutical, fine
chemicals, food industries, etc. Mixing and dispersion
of gas in liquid in aerated gas-liquid stirred tank reactors are still regarded among the most difficult topics
to tackle because of the complexities in terms of flow
regimes and multiphase operations [6]. So far a number
of simulations of gas–liquid dispersion in stirred tanks
have been presented in the literature, and although some
degree of success is reported, several significant limitations are apparent.
The limitation common to all published methods is
that the impeller is not directly simulated, but is instead
modelled, for example using experimentally determined
impeller boundary conditions, in which case valid measurements must always be available. Also, such methods
do not provide information about the flow in the impeller region. In order to better design these reactors, it is
very important to understand the mixing and dispersion
of gas in the liquid, especially the distribution of gas
hold-up, because the latest plays a very important role
in determining the effects both of mass and heat transfer.
Therefore, gas hold-up is one of the most widely studied parameters in the literature on stirred tank reactors.
Greaves and Barigou, Rewatkar et al. [2, 7, 8] briefly
summarized the work done by various investigators on
gas hold-up in stirred tank reactors. The correlations
presented by different workers can be divided into two
main categories: correlations using approach based on
dimensionless groups i.e. Froude number, flow number,
D/T ratio, etc.; correlations using the Kolmogoroff’s
theory approach of power dissipated as the basis.
The overall gas hold-up, power demand and overall
gas-liquid mass transfer coefficients are very strong
functions of the local fluid dynamics of the gas and liquid
phases in the stirred tank.
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Journal of Chemical Technology and Metallurgy, 49, 5, 2014
EXPERIMENTAL
The geometry of the stirred tank simulated in this
work is created with the program Design Modeler on the
basis of design sketches of pilot plant scale stirred tank
reactor with aeration (Fig. 1). It consists of a cylindrical
tank with two standard six-blade pitched turbines each
of 0.3 m diameter.
The total work volume of the reactor is 1000 l with
D/T ratio = 0.3. The height of the reactor is 1.68 m and
the diameter is 1 m. The reactor is filled with water at
room temperature.
The impellers are situated one over another and both
of them are stirring in same direction with speed from
100 to 600 rpm. The aeration is accomplished by air from
a sparger with 100 holes each 2 mm in diameter (Fig. 2).
The created model consists of 207124 tetrahedra
(Fig. 3).
Fig. 2. ANSYS CFX simulated geometrical model.
RESULTS AND DISCUSSION
The results for the gas hold-up are obtained by simulation of the process with the program product ANSYS
CFX. The hydrodynamics equations are solved using the
Fig. 3. Volume mesh model.
Fig. 1. Work sketches of stirred tank with aeration.
470
finite volume element method and algebraic turbulence
models: k - ε model for the liquid phase and dispersion
model for the gas phase.
The overall gas hold-ups εg for the studied conditions
are seen to be very low with the maximum overall holdup being slightly more than 2.0 % (Figs. 4 and 5). The
gas hold-up in aerated stirred tank depends on impeller
speed and gas flow rate, and respectively on gas velocity
(Fig. 6 and Fig. 7). The experimental data (Fig. 8 and
Fig. 9) show that the gas hold-up increases with increase
in impeller speed and with increase in gas flow rate. At
constant gas flow rates a bypass zone is observed for
small agitator speeds where the gas bubbles flow through
the tank without recirculation. With further increase
Rositsa Stoykova, Dessislava Moutafchieva, Dimitrinka Popova, Veselin Iliev
in impeller speed the gas hold-up reaches a maximum
and then there is no further increase in the hold-up with
increase in impeller speed.
With increasing impeller speed the bubbles become
much more distorted, their cylindrical form follows the
vortex structure and their roughened surfaces reflect the
increased turbulence. The turbulence also causes breakage of the cavities into tiny bubbles which then escape
into the liquid bulk. It is this mechanism of cavity formation and its subsequent breakage and escape into the bulk
liquid which is responsible for the good dispersion of gas
into the fluid bulk. At higher impeller speeds there is gas
phase recirculation in the tank. Larger bubbles entering
the impeller region are rapidly extended into a roll form
and broken up. The frequency with which bubbles are
broken up by the cavity are far higher than breakup of
larger bubbles just rising up with the impeller discharge
stream. With increasing gas flow rates the diameter of the
circulating core tends to increase, but there is a natural
limit to this when the liquid film between the blade and
the gas filled vortex breaks down.
At low impeller speeds the tips of the blades are surrounded by liquid and the power number remains high
because of the presence of two low pressure vortices at
the back of each blade.
At low gas sparging rates (i.e. less than 2 m s-1) the
overall gas hold-ups at different impeller speeds are
pretty close to each other. It is beyond the gas sparging
rate of 2 m s-1 that with increase in impeller speeds the
gas hold-ups get higher. Even for this case the average
hold-ups for impeller speeds between 100 - 300 rpm
are very close to each other. It is only at 400 rpm that a
distinct increase in the gas hold-up can be seen for the
higher gas sparging rates. This clearly shows that the
CFD simulation can capture the right trends for variation
of overall hold-ups.
Fig. 4. Gas hold-up in a vertical bulk cross-section (N =
400 rpm, Ug = 4 m s-1).
Fig. 6. Gas velocity distribution plotted by vectors.
Fig. 5. Gas hold-up in a horizontal bulk cross-sections along
the reactor height (N = 400 rpm, Ug = 4 m s-1).
Fig. 7. Gas velocity distribution plotted with streamlines.
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Journal of Chemical Technology and Metallurgy, 49, 5, 2014
experimental data from an industrial apparatus at operating conditions. This will allow making more precise
validation of the simulation results obtained using CFD
of stirred tank with aeration.
REFERENCES
Fig. 8. Influence of gas velocity on gas hold-up along the reactor height when the impeller speed is constant (400 rpm).
Fig. 9. Influence of impeller speed on gas hold-up along the
reactor height when the gas velocity is constant (4 m s-1).
CONCLUSIONS
A CFD simulation method is being developed to
model gas-liquid
dispersion in aerated stirred tank reactors. Results to date indicate the correct patterns of gas
velocity and gas hold-up distribution throughout the vessel. A number of different researchers report the overall
gas hold-up measurements and found that as the impeller
speed was increased the overall gas hold-up increased
[3, 4]. The overall hold-up also increased when the gas
flow rate was increased. The simulation results show the
same trend as in [10].
Development of the investigations is continuing
in order to provide better quantitative agreement with
experimental measurements of gas hold-up by further
development of the various sub-models in the simulation
method. Further work is needed in order to collect more
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