Graeme Lane

Predicting gas–liquid flow in a mechanically stirred tank

Abstract Computational fluid dynamics (CFD) provides a method for investigating the highly complex fluid flow in mechanically stirred tanks. Although there are quite a number of papers in the literature describing CFD methods for modelling stirred tanks, most only consider single-phase flow. However, multiphase mixtures occur very frequently in the process industries, and these are more complex situations for which modelling is not as well developed. This paper reports on progress in developing CFD simulations of gas–liquid mixing in a baffled stirred tank. The model is three-dimensional and the impeller region is explicitly included using a Multiple Frames of Reference method to account for the relative movement between impeller and baffles. Fluid flow is calculated with a turbulent two-fluid model using a finite-volume method. Several alternative treatments of the multiphase equations are possible, including various expressions for drag and dispersion forces, and a number of these have been tested. Variation in bubble size due to coalescence and break-up is also modelled. The CFD simulation method has been used to model a gas-sparged tank equipped with a Rushton turbine, and simulation results are compared with experimental data. Results to date show the correct pattern of gas distribution and the correct trends in local bubble size in the tank. Further work is needed to improve the quantitative agreement with experimental data.

Modelling of the Interaction between Gas and Liquid in Stirred Vessels

Publisher Summary Although existing literature demonstrates substantial progress in developing computational fluid dynamics (CFD) methods for stirred tanks, most studies are limited to single-phase liquid flow. In modeling of multiphase mixtures, there are a range of additional complexities. Further development of CFD modeling is being investigated for gas–liquid contacting in a mixing vessel. This chapter highlights the general method of simulation, and discusses modeling of the gas–liquid interaction. It is shown that predictions of gas distribution and holdup are sensitive to the specification of the drag force. This force is usually determined according to the drag force correlation. There is evidence however that in a forced turbulent flow the drag coefficient on particles or bubbles is increased. Using the correlation, it is found that gas holdup is substantially underpredicted. Alternative methods of calculating drag coefficient in turbulent flow are found to increase the predicted holdup but give an incorrect pattern of gas distribution. A modified correlation based on that of Brucato et al. is found to give improved results, but the generality of the method is uncertain. To improve the accuracy of the CFD model, better knowledge of bubble drag coefficients is needed.

Comparison of CFD Methods for Modelling of Stirred Tanks

Publisher Summary Simulation by computational fluid dynamics (CFD) is becoming an increasingly useful tool in analysis of the flow in mechanically stirred tanks. However, the development of accurate and efficient modeling methods is a continuing process. One significant complication in modeling of baffled stirred tanks is accounting for the motion of the impeller, since there is no single frame of reference for calculation. A number of approaches have been taken to this problem. In some cases an empirical model is provided for the impeller, whereas other methods are capable of predicting the effect of the impeller directly. In the latter category are the Sliding Mesh and Multiple Frames of Reference (MFR) methods. Results are presented for simulation of a standard configuration tank stirred by a Rushton turbine. Using the same geometry and finite volume grid, the fluid flow is simulated using both these methods. The Sliding Mesh and MFR methods are discussed and compared with respect to computation time and accuracy of prediction of mean velocities and turbulence parameters. It is found that the MFR method provides a saving in computation time of about an order of magnitude. Predicted mean velocities using both methods are compared with experimental data, and it is found that both methods provide good agreement with experimental data. Turbulence parameters are also compared with experimental data. It is found that both methods significantly underpredict the values of specific turbulent kinetic energy and rate of dissipation of turbulent energy.

Relation between combustion efficiency and coal rank in fluidized bed combustors

Abstract The relative combustion performance of a range of coals varying in rank from subbituminous to semi-anthracite has been investigated in a technical-scale bubbling fluidized bed combustor. Two coal-rank-dependent combustion efficiency relationships based on coal fixed carbon content and organic carbon content are identified. In the preferred relationship the single-pass combustion efficiency, η sp , d , is shown to vary linearly with the coal organic carbon content, C oc , according to: η sp , d = K oc , d − 1.30 C oc , where K oc , d is a constant related to the particular fluidized bed combustor design and coal feed size. Combustion efficiency data in fluidized beds from other sources is also shown to conform to this type of relationship.

CFD simulation of a solvent extraction pump mixer unit: evaluating Large Eddy Simulation and RANS based models

Mixer-settler equipment is widely used for solvent extraction (SX) operations. The pump mixer is the heart of an SX process. Any improvement in understanding of hydrodynamics and flow instabilities within a SX pump mixer unit would enable effective design of the mixer-settler equipment. In this direction, the present work investigates the predictive performance of the Large Eddy Simulation (LES) model vis-a-vis the PIV experimental results and RANS based model. Comparisons have been made initially for single phase operation of a Mixer unit, and then for the multiphase operation. The ANSYS/CFX modelling package has been used to set-up a transient three-dimensional CFD model using the sliding mesh approach for impeller motion and Eulerian-Eulerian approach for multi-phase flows. The present paper compares the flow patterns predicted by the LES model and compares them to RANS model prediction and PIV data. The prediction of flow structures and turbulence intensities will eventually pave the way for determina...

CFD Modelling of Hydrodynamic Conditions within the Wake of Mixing Impeller Blades

Computational fluid dynamics simulations have been used to simulate the flow field within the impeller swept region of a baffled stirred tank. Rushton turbine and flat paddle impeller configurations are investigated using sliding mesh techniques, for which no experimental input is necessary. The results from the CFD simulations show reasonable agreement with experimental data for the location of important flow structures close to the impeller blades, including the position of flow recirculation and axis of trailing vortex pairs generated within the wake at the rear of each blade, and the angle-resolved periodicity of the flow field between successive impeller blades.