M.P. Schwarz

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.

Applicability of the standard k-ϵ turbulence model to gas-stirred baths

Abstract Recently, Mazumdar and Guthrie found that they were unable to simulate the flow fields in a model steel processing ladle using the standard k -ϵ turbulence model. They made adjustments to the empirical constants in order to obtain agreement between the predicted and experimental velocities. In this paper a more sophisticated two-phase model that solves for void fraction and both phase velocities is used. This method in conjunction with the standard k -ϵ model yields flow fields in good agreement with the data published by Mazumdar and Guthrie.

CDF simulation of bubble-particle collisions in mineral flotation cells

The efficiency of the flotation process depends highly on the initial contact between the air bubble and the mineral particle. To enhance this contact, flotation cells are designed to achieve good mixing between the suspending solids and the dispersing air. CFD simulation of flotation cells provides an opportunity to analyse the influence of variations in design features and operating conditions on the performance of flotation cells. A laboratory flotation cell designed by CSIRO Minerals and a cylindrical tank fitted with a Rushton turbine used as a flotation machine have been modelled. The impeller and cell geometries have been set up using multi-blocking and sliding mesh techniques. Complex two-phase flow fields within the cells are predicted. Three-dimensional profiles of the turbulence dissipation rates and volumetric fraction of the air are also obtained. These form the basis for determining the number of bubble-particle collisions per unit time and unit volume. These profiles are important for locating positions within flotation cells where the initial contacts between bubbles and particles are made. Collision rates in the CSIRO flotation cell and the stirred tank have been compared. As flotation machines, the CSIRO cell is superior because of the higher maximum collision rate in comparison to the stirred tank.

Simulation of gas injection into liquid melts

Abstract Gas injection is used in several metal smelting and refining operations to increase mixing in the melt and as a carrier for particle addition. Some of the fluid dynamical phenomena that occur in liquid baths when injected with gas are discussed. Mathematical models for simulating three of these phenomena are described: a two-fluid model for simulating bath circulation is compared with data obtained in air-water experiments; a technique that allows the swelling of the bath surface to be simulated is illustrated; and a mathematical model for the prediction of wave motion in gas-agitated baths is summarized.

A numerical model for predicting bubble formation in a 3D fluidized bed

Abstract Fluidized bed systems have the potential to be widely used in the power generation, mineral processing and chemical industries. One factor limiting their increased use is the lack of adequate design techniques for scaling such systems. A model has been developed for simulating gas–solid fluidized bed plant. The model uses a multiphase Eulerian–Eulerian technique to predict the transient behaviour of fluidized bed systems. The commercial CFD code CFX is used as the computational framework for solving the discretized equations. To overcome the problem of accurate geometrical representation experienced in previous models a body fitted grid system is employed. The model is used to predict isothermal flow in a three-dimensional bubbling fluidized bed. Predictions of the three-dimensional model show bubble formation with gas bubbles or voids preferentially moving along the centre of the bed. Predicted behaviour is qualitatively consistent with experimental observations.

CFD modelling of thickeners at Worsley Alumina Pty Ltd

Abstract Computational fluid dynamics (CFD) modelling of process unit operations is a tool that is being used increasingly by mineral processing industries to reduce operating and capital costs and increase throughputs. Worsley Alumina first became involved with CFD modelling through support of the AMIRA Thickener Technology Project 266A in 1994, and subsequent extension projects in which CSIRO has been developing validated CFD models of thickener operation. The CSIRO Division of Minerals has been involved with CFD modelling since 1984 mainly on high temperature furnace applications and has been developing multi-phase thickener models since 1992. The benefits of obtaining a better understanding of flow patterns in thickeners using this modelling method became obvious and projects commenced in the third quarter of 1995 to utilise the CSIRO expertise. Projects have been ongoing almost continuously since that time. The CFD modelling was verified using tracers to measure actual flow patterns in a settler. Once verification had been achieved the CFD model was used to test innovative changes in design aimed at achieving higher throughputs and improved operation. These innovative changes when implemented on the full plant gave results similar to the CFD model predictions and resulted in improved process stabilisation, reduced chemical costs and very large savings in capital requirements for our major expansion that has just been completed. There are a number of assumptions made in the CFD model and these are discussed in detail in the paper together with details of individual CFD modelling projects and cost benefits achieved from completed projects.

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.

CFD modelling of waste heat recovery boiler

Abstract A model has been developed of a waste heat recovery boiler utilising typical plant off-gas consisting of both gaseous and particulate combustibles. The model allows the calculation of temperatures of gas and particles within the boiler and hence the likelihood of deposition onto the boiler walls. The model was applied to a typical waste heat boiler geometry, and a typical off-gas composition including a mixture of combustibles (char) and non-combustible particulates. Mixing in the burner region, char burnout and char particle temperature were analysed using the model. Combustion stability was also studied using a simple Eddy break-up model which accounts for combustion kinetics and the results compared with a Mixed-is-burnt model.