Advances in Mixing Research: 16th European Conference on Mixing

International audienceMixing operations are frequently encountered in the process industries, covering numerous sectors such as chemicals, pharmaceuticals, foods, water treatment, pulp and paper. Understanding mixing processes is the key to controlling product quality and yield, managing energy and water consumption as well as operating costs, and addressing safety and environmental concerns. Whilst many industrial mixing operations are based on empirical knowledge, the design and operation of more effective and efficient processes and products, which fulfill the stringent demands of today's society now require a much more thorough design approach based on engineering science. The advances in information technology have led to the development of sophisticated experimental and computational tools that enable the collection of vast amounts of data, resolved in both time and space. With such tools, there is now the possibility to better understand physical and chemical phenomena occurring in mixing processes and to make more accurate predictions, which are primordial for effective process and product design. The development and use of advanced experimental tools (e.g., planar laser-induced fluorescence, electrical resistance tomography, positron emission particle tracking) and numerous simulation and modelling techniques (including computational fluid dynamics, dissipative particle dynamics, Lattice Boltzmann modelling, population balances) is now enabling significant scientific advances to be made in the understanding of mixing phenomena and the control of industrial mixing operations. These tools and techniques are currently being applied to complex mixing problems involving multiphase flows, viscous products, nanoparticles, and biologically reactive systems. The outcome is a comprehensive understanding of the multiple physical phenomena occurring at multiple scales that is used to develop modelling and prediction strategies as well as process design rules. With increasing computational resources, and consequently simulation and modelling capabilities, it is expected that the development and use of advanced numerical techniques will continue to intensify, in particular for complex industrial mixing problems

Intensified processes for FAME production from waste cooking oil: a technological review

This article reviews the intensification of fatty acid methyl esters (FAME) production from waste cooking oil (WCO) using innovative process equipment. In particular, it addresses the intensification of WCO feedstock transformation by transesterification, esterification and hydrolysis reactions. It also discusses catalyst choice and product separation. FAME production can be intensified via the use of a number of process equipment types, including as cavitational reactors, oscillatory baffled reactors, microwave reactors, reactive distillation, static mixers and microstructured reactors. Furthermore, continuous flow equipment that integrate both reaction and separation steps appear to be the best means for intensifying FAME production. Heterogeneous catalysts have also shown to provide attractive results in terms of reaction performance in certain equipment, such as microwave reactors and reactive distillation

Current methods for characterising mixing and flow in microchannels

This article reviews existing methods for the characterisation of mixing and flow in microchannels, micromixers and microreactors. In particular, it analyses the current experimental techniques and methods available for characterising mixing and the associated phenomena in single and multiphase flow. The review shows that the majority of the experimental techniques used for characterising mixing and two-phase flow in microchannels employ optical methods, which require optical access to the flow, or off-line measurements. Indeed visual measurements are very important for the fundamental understanding of the physics of these flows and the rapid advances in optical measurement techniques, like confocal scanning laser microscopy and high resolution stereo micro particle image velocimetry, are now making full field data retrieval possible. However, integration of microchannel devices in industrial processes will require on-line measurements for process control that do not necessarily rely on optical techniques. Developments are being made in the areas of non-intrusive sensors, magnetic resonance techniques, ultrasonic spectroscopy and on-line flow through measurement cells. The advances made in these areas will certainly be of increasing interest in the future as microchannels are more frequently employed in continuous flow equipment for industrial applications

A new definition of mixing and segregation: Three dimensions of a key process variable

Although a number of definitions of mixing have been proposed in the literature, no single definition accurately and clearly describes the full range of problems in the field of industrial mixing. An alternate approach is proposed which defines segregation as being composed of three separate dimensions. The first dimension is the intensity of segregation quantified by the normalized concentration variance (CoV); the second dimension is the scale of segregation or clustering; and the last dimension is the exposure or the potential to reduce segregation. The first dimension focuses on the instantaneous concentration variance; the second on the instantaneous length scales in the mixing field; and the third on the driving force for change, i.e. the mixing time scale, or the instantaneous rate of reduction in segregation. With these three dimensions in hand, it is possible to speak more clearly about what is meant by the control of segregation in industrial mixing processes. In this paper, the three dimensions of segregation are presented and defined in the context of previous definitions of mixing, and then applied to a range of industrial mixing problems to test their accuracy and robustness

Modeling Turbulent Flow in Stirred Tanks with CFD: The Influence of the Modeling Approach, Turbulence Model and Numerical Scheme

Single phase turbulent flow in a tank stirred by a down- and an up-pumping pitched blade turbine has been simulated using CFD. The effect of the modeling approach, discretization scheme and turbulence model on mean velocities, turbulent kinetic energy and global quantities, such as the power and circulation numbers, has been investigated. The results have been validated by LDV data. The stationary and time-dependent modeling approaches were found to have little effect on the turbulent flow, however the choice of the numerical scheme was found to be important, especially for the predicted turbulent kinetic energy. A first order method was found to highly underestimate LDV data compared with higher order methods. The type of the turbulence model was limited to the k-e and RNG models due to convergence difficulties encountered with a Reynolds Stress Model (RSM) and there was found to be little effect of these models on the mean flow and turbulent kinetic energy. This latter quantity was found to be largely under predicted in the discharge region of the down-pumping impeller in comparison with LDV data. Better agreement was found for the up-pumping pitched blade turbine. Estimated power numbers were found generally to be in good agreement for the down- and up-pumping data. However, the circulation number tended to be over predicted by about 30% and 40% for the down- and uppumping agitators, respectively

Gas-liquid mass transfer : influence of sparger location

The performance of three sparger diameters (DS = 0.6D, DS = D, DS = 1.6D) in combination with three positions (below, above or level with the impeller) for gas-liquid dispersion and mass transfer were evaluated in the case of the Rushton turbine and the A315 propeller in up- or down-pumping mode. The results show that the best results in terms of gas handling and mass transfer capacities are obtained for all impellers with the sparger placed below it and with a diameter at least equal to the impeller diameter. For the sparger position below the agitator, the kLa values of the Rushton turbine are greater than those of the A315 propeller, whatever the pumping mode. The A315 propeller in up-pumping mode is, however, more economically efficient in terms of mass transfer. In all cases, the up-pumping mode gives better results than the down-pumping one

Effect of microchannel aspect ratio on residence time distributions and the axial dispersion coefficient

The effect of microchannel aspect ratio (channel depth/channel width) on residence time distributions and the axial dispersion coefficient have been investigated for Newtonian and shear thinning non-Newtonian flow using computational fluid dynamics. The results reveal that for a fixed cross sectional area and throughput, there is a narrowing of the residence time distribution as the aspect ratio decreases. This is quantified by an axial dispersion coefficient that increases rapidly for aspect ratios less than 0.3 and then tends towards an asymptote as the aspect ratio goes to 1. The results also show that the axial dispersion coefficient is related linearly to the Reynolds number when either the aspect ratio or the mean fluid velocity is varied. However, the fluid Péclet number is a linear function of the Reynolds number only when the aspect ratio (and therefore hydraulic diameter) is varied. Globally, the results indicate that microchannels should be designed with low aspect ratios (≤ 0.3) for reduced axial dispersion

Alternate operating methods for improving the performance of a continuous stirred tank reactor

The effect of the pumping direction of an axial flow impeller, the feeding rate and the number of feed inlets on the operation of a continuously-fed stirred tank has been studied using CFD. The flow patterns generated by the up-pumping and down-pumping impeller, under both ‘typical’ and ‘intensified’ operating conditions, are compared. The effect of various tank configurations on the performance of the vessel is assessed by analysing the flow and power numbers, as well as the concentration field of a non-reactive tracer. Furthermore, the inlet feed jets are reduced using traditional jet similarity analysis and are compared with that of a typical round jet. The results show that up-pumping impellers improve circulation in the upper part of the tank and reduce shortcircuiting of the feed stream with only a small increase in power consumption. Furthermore, by using multiple feed inlets to increase the total throughput capacity, the amplitude of torque fluctuations is decreased and impeller bypassing is also decreased. The ensemble of conclusions suggest that the throughput capacity and mixing quality of a CSTR can be improved, without problems of short-circuiting, by employing up-pumping impellers coupled with multiple surface feed points

On the combined effects of surface tension force calculation and interface advection on spurious currents within Volume of Fluid and Level Set frameworks

This paper deals with the comparison of Eulerian methods to take into account the capillary contribution in the vicinity of a fluid–fluid interface. Eulerian methods are well- known to produce additional vorticity close to the interface that leads to non-physical spurious currents. Numerical equilibrium between pressure gradient and capillary force for the static bubble test case within a VOF framework has been reached in [35] with the height-function technique [14,35]. However, once the bubble is translated in a uniform flow, spurious currents are maintained by slight errors induced by translation schemes. In this work, two main points are investigated: the ability of Volume of Fluid and Level Set methods to accurately calculate the curvature, and the magnitude of spurious currents due to errors in the calculation of the curvature after advection in both translating and rotating flows. The spurious currents source term is expressed from the vorticity equation and used to discuss and compare the methods. Simulations of gas–liquid Taylor flow at low capillary number show that the flow structure and the bubble velocity can be significantly affected by spurious currents

Characteristics of liquids lugs in gas–liquid Taylor flow in microchannels

The hydrodynamics of liquid slugs in gas–liquid Taylor flow in straight and meandering microchannels have been studied using micro Particle Image Velocimetry. The results confirm a recirculation motion in the liquid slug, which is symmetrical about the center line of the channel for the straight geometry and more complex and three-dimensional in the meandering channel. An attempt has also been made to quantify and characterize this recirculation motion in these short liquid slugs (Ls/w<1.5) by evaluating the recirculation rate, velocity and time. The recirculation velocity was found to increase linearly with the two-phase superficial velocity UTP. The product of the liquid slug residence time and the recirculation rate is independent of UTP under the studied flow conditions. These results suggest that the amount of heat or mass transferred between a given liquid slug and its surroundings is independent of the total flow rate and determined principally by the characteristics of the liquid slug