Rositsa Stoykova, Dessislava Moutafchieva, Dimitrinka Popova, Veselin Iliev

469

Journal of Chemical Technology and Metallurgy, 49, 5, 2014, 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, BulgariaE-mail: rosica2136@abv.bg

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.

Received 10 November 2013Accepted 11 July 2014

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 pro-gress 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 reac-tors 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 limita-tions 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 meas-urements must always be available. Also, such methods do not provide information about the flow in the impel-ler 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 stud-ied 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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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).

RESULTS AND DISCUSSION

The results for the gas hold-up are obtained by simu-lation of the process with the program product ANSYS CFX. The hydrodynamics equations are solved using the

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 hold-up 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 Fig. 1. Work sketches of stirred tank with aeration.

Fig. 2. ANSYS CFX simulated geometrical model.

Fig. 3. Volume mesh model.

Rositsa Stoykova, Dessislava Moutafchieva, Dimitrinka Popova, Veselin Iliev

471

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 break-age of the cavities into tiny bubbles which then escape into the liquid bulk. It is this mechanism of cavity forma-tion 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 sur-rounded 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. 5. Gas hold-up in a horizontal bulk cross-sections along the reactor height (N = 400 rpm, Ug = 4 m s-1).

Fig. 6. Gas velocity distribution plotted by vectors.

Fig. 7. Gas velocity distribution plotted with streamlines.

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CONCLUSIONS

A CFD simulation method is being developed to model gas-liquid dispersion in aerated stirred tank reac--tors. Results to date indicate the correct patterns of gas velocity and gas hold-up distribution throughout the ves-sel. 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

experimental data from an industrial apparatus at oper-ating conditions. This will allow making more precise validation of the simulation results obtained using CFD of stirred tank with aeration.

REFERENCES

1. A. Bakker, Hydrodynamics of stirred gas–liquid dispersions, PhD thesis, Delft University of Tech-nology, 1992.

2. M. Greaves, M. Barigou, Estimation of gas hold-up and impeller power in a stirred vessel reactor”, in “Fluid Mixing III”, Inst. Chem. Eng., Int. Chem. Eng. Symp. Series No.108, 1990, 235 - 255.

3. J.B. Joshi, A.B. Pandit, M.M..Sharma, Mechanically agitated gas-liquid reactors, Chem. Eng. Sci., 37, 1982, 813 - 844.

4. J.B. Joshi, N.K. Nere, C.V. Rane, B.N. Murthy, C.S. Mathpati, A.W. Patwardhan, V.V. Ranade, CFD simulation of stirred tanks: comparison of turbulence models, Part I: Radial flow impellers, Can. J. Chem. Eng., 89, 2011, 23 - 82.

5. G. Lane, P.T.L. Koh, CFD simulation of a Rushton turbine in a baffled tank, Proceedings of the Inter-national Conference on Comp. Fluid Dynamics in Mineral & Metal Processing & Power Generation, CSIRO, Melbourne, 1997, 377 - 385.

6. V.V Ranade, Computational Flow Modelling for Chemical Reactor Engineering, Academic Press, 2002.

7. V.B. Rewatkar, J.B. Joshi, Effect of sparger design on gas dispersion in mechanically agitated gas/liquid reactors, Can. J. Chem. Eng., 71, 1993, 278 - 291.

8. V.B. Rewatkar, A.J. Deshpande, A.B. Pandit, J.B. Joshi, Gas hold-up behavior of mechanically agitated gas–liquid reactors using pitched blade downflow turbines, Can. J. Chem. Eng., 71, 1993, 226 - 237.

9. G. Tabor, A.D. Gosman, R.I. Issa, Numerical simu-lation of the flow in a mixing vessel stirred by a Rushton turbine, in „Fluid Mixing V“, Int. Chem. Eng. Symp. Series No.140, 1996, 25-34.

10. K. Van’t Riet, Review of measuring methods and results in nonviscous gas–liquid mass transfer in stirred vessels, Ind. Eng. Chem. Process Des. Dev., 18, 1979. 357 - 364.

Fig. 8. Influence of gas velocity on gas hold-up along the re-actor 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).