Chemical Engineering

Packed and fluidised beds in microfluidic devices offer the potential of enhanced heat and mass transfer capability at a scale where the process can be closely controlled. The knowledge of hydrodynamics of packed and fluidised beds in capillary tubes is essential for the design and optimization of such devices. This study experimentally examines the hydrodynamics of packed and fluidised beds in terms of pressure drop, bed expansion and minimum fluidisation velocity in tube sizes with inner diameters of 0.8, 1.2 and 17.1 mm. Specifically the influence of the wall on the hydrodynamic characteristics of the beds was examined by changing the tube-to-particle diameter ratio. It was found that as the tube diameter reduces the bed voidage sharply increases leading to a reduction in the pressure drop across the bed. Also a pressure drop overshoot was observed at lower tube-to-particle diameter ratios which found to be associated with contact stresses due to wall friction.

The influence of primary particle size distribution (PPSD) on aggregation behaviour and the resulting effect on yield stress of a concentrated colloidal suspension was investigated theoretically. The discrete element model (DEM) combined with the well-known DLVO theory was employed to obtain an insight into the aggregation process of nanoparticles with different PPSDs, where a modified version of the Flatt and Brown model [J. Am. Ceram. Soc. 89 [9] was employed to calculate the corresponding suspension yield stress from the simulation results. Specifically, the aggregate growth and structure in terms of fractal dimension, coordination number and the longest dimension were examined. It was shown that at small PPSD variances, a netlike structure was formed with aggregate branches interconnected in multiple locations, whereas at large variances aggregates with more compact structure and smaller longest dimension were generated. The rate of aggregation and particle assemblage was found to be faster at broader PPSDs, in turn generating aggregates with narrower size distributions and more compact structures. The influence of PPSD on coordination number (CN) was found to be minor while a decrease in PPSD variances led to an increase in both the mass-equivalent size and the longest dimension of aggregates. Further, suspension yield stress decreased as PPSD became broader. The simulation results agreed well with the experimental measurements and the published data.

The success of many industrial processes largely depends on the structural characteristics of aggregates. In intensive aerobic digestion process for wastewater treatment applications, the structural characteristics namely aggregate shape, size and therefore the aggregate surface area strongly influence the transfer of dissolved oxygen from the aeration process to aggregates of harmful contaminants/microorganisms.

Multiphase systems are ubiquitous in industrial applications aimed at the generation of products either by chemical/biological reaction or physical separation based on density, electrical charge or surface properties such as hydrophobicity. The physical processing of these multiphase systems is carried out at all scales of operation and within an endless variety of vessel shapes and ancillary devices. Underpinning each process is a complex interaction between phases involving hydrodynamic, heat and mass transport. These phenomena are in turn governed largely by the nature of the flow, and in particular whether laminar or turbulent conditions prevail. In large-scale industrial processes the flows are almost always turbulent, whilst for microscale operations the flow will be laminar. Each condition provides its own challenge in being able to predict (and optimise) performance in terms of operational stability and efficiency of energy utilisation. Turbulent systems are particularly dif...

A DEM-based model was developed and examined for simulation of aggregation in suspensions of α-alumina nanoparticles. In the model, the random Brownian diffusion and the externally induced dielectrophoresis (DEP) motion were considered as the driving mechanisms for the transport of particles in colloidal suspension. To simulate particle interactions, the non-contact surface force and the contact force were taken into account using the well-known Derjaguin-Landau-Verway-Overbeek (DLVO) theory and the soft-sphere model, respectively. Specifically, the model was used to study the effects of pH, solid volume fraction and external AC electric field on α-alumina aggregate growth which was expressed in terms of coordination number, longest dimension, and fractal dimension. The simulations were carried out over a pH range of 4-10, solid volume fraction of 0.02-0.4, and a variety of AC electric fields. In relatively dilute suspensions, the aggregates predominantly exhibited chainlike structures, whereas at high solid volume fraction, aggregates with complex netlike structures were formed. It was also evident that, in concentrated colloidal suspensions, DEP had a negligible influence on aggregate growth over the examined conditions. The effect of DEP however, was found to be more noticeable on aggregate structure leading to the formation of more compact aggregates with a greater particle number density. The break-up and reattachment of sub-aggregates as well as the rearrangement of nanoparticles in the particle assemblies and subsequent curling of the loose network promoted by a strong AC electric field was deemed to be responsible for this structural transformation. Finally, the DEM-based model was used to predict the size of α-alumina aggregates over a range of pH. The predictions were found to be in good agreement with the published experimental data, particularly around the isoelectric point.

An experimental and numerical study has been conducted to investigate the flow field in the vicinity of a moving solid boundary that passes through a free surface into a liquid phase. Through the use of particle image velocimetry (PIV) techniques, the variation in the liquid velocity field in the vicinity of the three-phase contact line has been quantified for solid

In this paper the hydrodynamics and bubble-particle interactions associated with mineral flotation in mechanically agitated vessels (cells). Firstly, the influence of turbulence intensity on the slip velocity of the bubbles and particles is critically examined, resulting in a correlation that accounts for a reduction in slip velocity. This information is inputted into a computational fluid dynamics (CFD) model that can be used to describe the gas dispersion within the flotation cell. The method explicitly includes the impeller geometry. A simple equation was introduced to allow prediction of cavity formation and the associated reduction in power draw. The model was also able to predict the local bubble size based on a critical number concept. Additionally, the flotation recovery process was incorporated into a CFD model which accounted for probabilities of collision, adhesion and stabilization of the particles at the bubble surface. NOMENCLATURE α 2 volume fraction of gas (-) β =U 0 /U T , C D drag coefficient (-) D bubble diameter (m) K 2 adjustable parameter in equation (1) S adjustable parameter in equation (1) U 0 turbulence fluctuation velocity U T terminal velocity (m/s) μ = L 11 /τ p U T , L 11 integral length scale of the turbulence (m) τ p bubble relaxation time (s)

Although existing literature demonstrates substantial progress in developing CFD methods for stirred tanks, most studies are limited to single-phase liquid flow. In modelling of multiphase mixtures, there are a range of additional complexities. Further development of CFD modelling is being investigated for gas-liquid contacting in a mixing vessel. This paper outlines the general method of simulation, and discusses modelling 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 of Ishii and Zuber [6]. There is evidence however that in a forced turbulent flow the drag coefficient on particles or bubbles is increased. Using the correlation of Ishii and Zuber [6], it is found that gas holdup is substantially underpredicted. Alternative methods of calculating drag coefficient in turbulent flow as proposed by Brucato et al. [7] and Bakker [10] 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. [7] was 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. 1. INTRODUCTION Computational fluid dynamics (CFD) modelling has considerable potential as a tool for design, process improvement and scale-up of stirred vessels. Existing literature has demonstrated substantial progress in developing CFD methods [ 1 ]. However, modelling has usually been limited to single-phase liquid flow, whereas multiphase mixtures are very common in industrial stirred tanks. For multiphase flow, the complexity of modelling increases considerably, and this remains an area for further development. CFD methods are presently being investigated for gas-liquid contacting in a stirred tank. The CFD method is applied to simulate a baffled tank stirred by a Rushton turbine. Progress in this work was reported recently [2]. This paper outlines the general method of simulation and modelling of the forces between gas and liquid. It is shown that the most important force is the drag force, which determines the gas slip velocity, and hence affects the distribution of gas in the vessel and the total gas holdup. There is evidence in the literature to indicate that the effective drag coefficient on bubbles is different in a turbulent liquid compared to that of a bubble rising in a stagnant fluid. In order to improve the accuracy of the CFD modelling method, several expressions for the modified drag coefficient have been tested.