CFD analysis of turbulence non-homogeneity in mixing vessels: A two-compartment model

A two-compartment model has been developed for calculating the droplet/particle size distribution in suspension polymerization reactors by taking into account the large spatial variations of the turbulent kinetic energy and its dissipation rate in the vessel. The two-compartment model comprised two mixing zones, namely an impeller zone of high local energy dissipation rates and a circulation zone of low kinetic energy. Computational fluid dynamics (CFD) was employed for generating the spatial distribution of energy dissipation rates within an unbaffled mixing vessel agitated by a flat two-blade impeller. A general methodology was developed for extracting, from the results of the CFD simulations, the volume ratio of the impeller over the circulation zone, the ratio of the average turbulent dissipation rates in the two zones, and the exchange flow rate between the two compartments. The effect of agitation rate, continuous phase viscosity, impeller diameter, and mixing vessel scale on the two-compartment model parameters was elucidated. The two-compartment model was then applied to a non-homogeneous liquid-liquid dispersion process to calculate the time evolution of the droplet size distribution in the mixing vessel. An excellent agreement was obtained between theoretical and experimental results on droplet size distributions obtained from a laboratory-scale reactor operated over a wide range of experimental conditions.     

Computational fluid dynamic modelling of flow patterns in an oscillatory baffled column

Numerical simulations to date within the context of oscillatory flow in a baffled column have been limited to flows in a symmetrical regime, i.e. eddies are generated symmetrical to the central line of the column where the oscillatory Reynolds numbers are below 400. In this paper, 3-D computational fluid dynamic (CFD) simulation of flow patterns of oscillatory flow in a baffled column has, for the first time, been carried out and the results extended to all regimes of oscillatory Reynolds numbers covering from symmetric to asymmetric flows. The flow patterns simulated have also been validated by both direct flow visualisation and by digital particle image velocimetry measurements. The success of such CFD simulations opens doors for many potential studies, from optimisation of geometry for plug flow to suspension of particles, and from droplet breakage and coalescence to mass/heat transfer of particles.     

Droplet breakage and coalescence in liquid–liquid dispersions: Comparison of different kernels with EQMOM and QMOM

Droplet coalescence and breakage in turbulent liquid–liquid dispersions is simulated by using computational fluid dynamics (CFD) and population balance modeling. The multifractal (MF) formalism that takes into account internal intermittency was here used for the first time to describe breakage and coalescence in a surfactant‐free dispersion. The log‐normal Extended Quadrature Method of Moments (EQMOM) was for the first time coupled with a CFD multiphase solver. To assess the accuracy of the model, predictions are compared with experiments and other models (i.e., Coulalogou and Tavlarides kernels and Quadrature Method of Moments [QMOM]). EQMOM and QMOM resulted in similar predictions, but EQMOM provides a continuous reconstruction of the droplet‐size distribution. Transient predictions obtained with the MF kernels result in a better agreement with the experiments. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2293–2311, 2017     

Interfacial phenomena and droplet size of particle stabilized emulsions in oscillatory shear

Production of particle stabilized oil in water emulsions has been investigated both theoretically and experimentally under oscillatory shear conditions using different stabilizing particles (SPs). The investigation included analysis of the interaction between particles interfacial stability and droplets breakage and coalescence. For hydrophobic SPs, droplets maintained their sizes as determined by torque balance (TB) without significant breakage or coalescence. For the more hydrophilic SPs, larger droplets formed that broke by eddies in the inertial subrange. At higher fluid shear stresses, loss of the SPs occurred during droplet formation leading to near bare droplet surface and coalescence to much larger sizes with subsequent fragmentation by capillary instabilities. The final droplet size in both cases was very different from TB model predictions. A modeling approach is proposed that combined both TB and droplet breakage and coalescence mechanisms. Comparison between the experimental results and the models predictions showed satisfactory agreement. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2902–2911, 2016     

CFD simulations and particle image velocimetry measurements in an industrial scale rotating disc contactor

Fluid dynamics of the single‐phase and two‐phase flow in a segment of a rotating disc contactor (RDC) liquid–liquid extraction column with 450 mm inner diameter were studied by performing computational fluid dynamics (CFD) simulations and particle image velocimetry (PIV) measurements. The fluid dynamics were investigated to test the predictivity of CFD at industrial scale. Different turbulence models in conjunction with the Eulerian approach were applied in the single‐phase and two‐phase simulations. The turbulent flow characteristics were analyzed by PIV measurements to validate the CFD simulations. An iso‐optical system composed of CaCl₂/water–butylacetate allows for the two‐phase PIV measurements. Local turbulent energy dissipation was derived from velocity gradients in PIV data. In this connection, the influence of the PIV spatial resolution on the measured energy dissipation was also analyzed, and different fit functions were tested to scale the measured energy dissipation. Simulated velocity fields as well as the energy dissipation were compared with the experimental PIV data. The results from the simulations and experiments are in good agreement. The work shows that CFD can predict hydrodynamic characteristics even at bigger scales but is still subject to some minor restrictions. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Convective Drying of Agitated Silica Gel and Beech Wood Particle Beds—Experiments and Transient DEM-CFD Simulations

With coupled discrete element (DEM)–computational fluid dynamics (CFD) simulations, drying processes can be simultaneously described on the system scale while resolving detailed subprocesses on the particle scale. In this contribution, DEM-CFD simulations are used to analyze the transient heat and mass transfer in mechanically agitated particle beds during drying. Results are compared to convective batch-drying experiments with silica gel and beech wood spheres and mixing effects are studied in detail. A good agreement with the measurements of both single-particle and particle bed drying is achieved by resolving heat and moisture transport three-dimensionally inside each particle.     

The Influence of Process Parameters on the Properties of PLGA‐Microparticles Produced by the Emulsion Extraction Method

Controlled release poly(lactic‐co‐glycolic acid) microparticles for use as active pharmaceutical ingredient carriers were prepared by the emulsion extraction method. Particle formation experiments were carried out in a stirred vessel. The local flow conditions in these experiments, that is, local shear rates and dissipation rates, and the extraction rate of the organic solvent were examined by a computational fluid dynamics (CFD) simulation. The local flow conditions in the stirred tank reactor have a significant influence on the final properties, specific surface area, skeletal density, organic solvent content, and size of the microparticles. We determined nondimensional correlations for predicting these particle properties as functions of the process parameters as, for example, the stirrer speed, emulsion injection point, and oil droplet size in the initial emulsion. The results demonstrate that CFD simulations offer insight into the particle formation process for different batch sizes and provide a basis for scale‐up and optimization of the process. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1868–1881, 2013     

悬浮聚合法制备磁性微球的粒度分布特性

本文研究了含有微细铁黑颗粒的混合单体悬浮聚合产物的粒度分布特性。分析了分散剂、超声预分散和无机铁黑颗粒对形成粒度多峰分布的影响。结果表明,分散剂是体系中形成小颗粒的主要因素;超声波的预分散作用使悬浮体系的液滴破裂以“腐蚀破碎（erosive breakage)”为主;无机铁黑颗粒由于其表面亲水性,倾向于分布在油性单体液表面,不仅有利于悬浮液滴的“磨蚀破碎”,同时也对分散液滴具有良好的稳定作用。上述因素的共同作用使得聚合产物的粒度呈三峰分布。     

Application of CFD and breakage modelling for predicting the size reduction of protein precipitates during transport

Particle breakage due to fluid flow through various geometries can have a major influence on the performance of particle/fluid transport and separation processes. Whey protein precipitate dispersions were used as a case study to investigate the effect of flow intensity and exposure time on the breakage of precipitates during transport. Computational fluid dynamic (CFD) simulations were performed to evaluate the turbulent energy dissipation rate (?) and associated exposure time along various flow geometries. A breakage model, incorporating the CFD output and experimentally determined parameter values, was found to provide a satisfactory capability for predicting the breakage of the protein precipitate particles. The breakage modelling approach was then applied to particles formed under different agitation intensities during the precipitation process. The formation history of the precipitates had a significant effect on their structure and strength and hence different breakage rates were observed. The precipitate dispersions were propelled through a number of different geometries such as bends, tees and elbows. The shape of the flow geometry was found to have an important effect on particle size reduction. This predictive particle breakage modelling approach was then applied to larger-scale flow geometries with cross-sectional area of 150 times greater than the experimental.     

In situ mass-suspension polymerisation

In situ mass-suspension polymerisation of MMA was carried out in a single reactor. The mass polymerisation was carried out in a gently agitated monomer layer of a two stratified layers of monomer and water in the reactor. The degree of conversion at which mass polymerisation changed to suspension polymerisation, by increasing the rate of agitation, was altered systematically. The polymer content of the monomer/polymer solution, formed during the mass polymerisation stage, significantly affected the evolution of the particle size distribution. Mass-suspension polymerisation was found to be more vulnerable to drop coalescence and process failure than conventional suspension polymerisation. The results indicate the importance of the transition stage in a typical suspension polymerisation during which the rate of polymerisation is very low and the adsorption of stabiliser on the surface of drops is completed. The polymer beads from the mass-suspension polymerisation had a very broad size distribution with a large contribution from satellite particles.     

CFD based extraction column design——Chances and challenges

This paper shows that one-dimensional (1-D) [and three-dimensional (3-D) computational fluid dynamics (CFD)] simulations can replace the state-of-the-art usage of pseudo-homogeneous dispersion or back mixing models. This is based on standardized lab-scale cel experiments for the determination of droplet rise, breakage, coalescence and mass transfer parameters in addition to a limited number of additional mini-plant experiments with original fluids. Alternatively, the hydrodynamic parameters can also be derived using more sophisticated 3-D CFD simulations. Computational 1-D modeling served as a basis to replace pilot-plant experiments in any column geometry. The combination of 3-D CFD simulations with droplet population balance models (DPBM) increased the accuracy of the hydrodynamic simulations and gave information about the local droplet size. The high computational costs can be reduced by open source CFD codes when using a flexible mesh generation. First combined simulations using a three way coupled CFD/DPBM/mass-transfer solver pave the way for a safer design of industrial-sized columns, where no correlations are available.     

Dynamic evolution of the particle size distribution in suspension polymerization reactors: A comparative study on Monte Carlo and sectional grid methods

In the present study, an efficient Monte Carlo (MC) algorithm and a fixed pivot technique (FPT) are described for the prediction of the dynamic evolution of the droplet/particle size distribution (DSD/PSD) in both non‐reactive liquid–liquid dispersions and reactive liquid(solid)–liquid suspension polymerization systems. Semi‐empirical and phenomenological expressions are employed to describe the breakage and coalescence rates of dispersed monomer droplets/particles, in terms of the type and concentration of suspending agent, quality of agitation, and evolution of the physical, thermodynamic and transport properties of the polymerization system. Moreover, the validity of the numerical calculations is first examined via a direct comparison of simulation results obtained by both numerical methods with experimental data on average particle diameter and droplet/particle size distributions for both non‐reactive liquid–liquid dispersions and the free‐radical suspension polymerization of methyl methacrylate (MMA). Additional comparisons between the MC and the FP numerical methods are carried out under different polymerization conditions. The simulation results reveal that both numerical methods are capable of predicting the mean and the distributed particulate properties of both non‐reactive and reactive suspension processes.     

MODELING OF TURBULENT, NEUTRALLY BUOYANT DROPLET SUSPENSIONS IN LIQUIDS

The mixing of neutrally buoyant, immiscible droplets in suspension in a turbulent liquid is being studied. In a statistically homogeneous field, it is anticipated that the droplets will affect the turbulent eddies, and that the turbulence will cause the droplets to break-up and coalesce. A cascade model is constructed by extension of the Desnyansky and Novikov equation, accounting for the wavenumber dependence of the fluctuating energy, for the intermittency factor of the turbulence and for the droplet population. In the absence of breakage and coalescence, interactions between eddies and droplets are assumed to be of collision type, so that the exchange of energy and the modifications to the eddy and droplet populations can be described. The resulting equations are solved for a fixed droplet population, showing the effect of droplet size on the turbulent energy spectrum. Continuation of the work is discussed, including droplet breakage and coalescence, as well as the introduction of non-homogeneous distributions.     

Development of a multi-scale simulation method for design of novel multiphase reactors

In this paper a multi-scale simulation method for modelling dispersions in a novel multiphase reactor is presented. This novel reactor is a continuous reactor which consists of repeated identical small mixing elements. The reactor is excellent for studying the effect of turbulence on drop size distributions since turbulence is continuously produced and dissipated along the reactor. Furthermore the energy dissipation within each element is very homogeneous. In addition it allows optical access at all positions along the reactor.Simulations were performed for a wide range of turbulence intensities for different dispersed phase hold-up. Each simulation was validated with measurements of the size distribution along the reactor. Good quantitative agreement was obtained at low hold-up in terms of prediction of the breakage rates and prediction of the size distributions. At higher hold-up the model gave reasonable predictions at low turbulence intensity however too large drops were predicted at high turbulence intensity. This can be a result of turbulence modulation and shows that reliable turbulence models for multiphase flows are necessary in this simulation method. The results show that physical models describing breakup and coalescence combined with CFD provide a good tool for efficient development and optimisation of novel multiphase reactors.     

Modeling fluid behavior and droplet interactions during liquid-liquid separation in hydrocyclones

A mechanical separation process in a hydrocyclone is described in which disperse water droplets are separated from a continuous diesel fuel phase. This separation process is influenced by droplet-droplet interaction effects like droplet breakup and coalescence resulting in a change of droplet size distribution. A simulation model is developed coupling the numerical solution of the flow field in the hydrocyclone based on computational fluid dynamics with population balances. The droplet size distribution is discretized and each discrete droplet size fraction is assumed to be an individual phase within a multiphase-mixture model. The droplet breakup and coalescence rates are defined as mass transfer rates between the discrete phases by the aid of user-defined functions. All model equations are solved with the CFD software package FLUENT™. The investigations show the impact of the cyclone geometry on the coupled population and separation dynamics. Cyclone separators with an optimized geometry show less steep velocity gradients increasing the coalescence rates and improving the separation efficiency. The calculated droplet size distributions at the cyclone overflow and at the underflow show good accordance with experimental data. The basic modeling approach can be extended and adapted to other disperse multiphase flow systems.     

Drop mixing in suspension polymerisation

In suspension polymerisation, monomer is suspended as liquid droplets in a continuous water phase by means of strong agitation and the presence of a suspending agent. As the suspension polymerisation proceeds, the viscosity of a monomer-polymer droplet increases with conversion. Hence, the physical behaviour of the droplet changes during the process. When new dispersible material is added to the existing suspension drops, the new material and existing drops can remain segregated for significant amounts of time. The aim of this project was to study the behaviour of drop mixing when new material is added to the existing suspension polymerisation. This study concentrated on the effect of the dispersed phase viscosity on drop mixing. The results show that viscosity affects drop size and that may then affect the rate of coalescence between drops. A critical drop size exists which determines the coalescence efficiency effect. Above the critical drop size, mixing rate increases as the drop viscosity decreases. While below the critical drop size, drop size of the dispersion determines the coalescence rate; as the drop size increases, coalescence rate also increases. The investigation of the effect of suspending agent shows that Tween 20 is more efficient in stabilising and protecting the drops, based on a weight basis, than PVA as the coalescence rate is lower with Tween 20.     

CFD simulation of impeller shape effect on quality of mixing in two-phase gas–liquid agitated vessel

Mixing efficiency in two-phase gas–liquid agitated vessel is one of the important challenges in the industrial processes. Computational fluid dynamics technique (CFD) was used to investigate the effect of four different pitched blade impellers, including 15°, 30°, 45° and 60°, on the mixing quality of gas–liquid agitated vessel. The multiphase flow behavior was modeled by Eulerian–Eulerian multiphase approach, and RNG k − ε was used to model the turbulence. The CFD results showed that a strong global vortex plays the main role on the mixing quality of the gas phase in the vessel. Based on the standard deviation criterion, it was observed that the axial distribution of the gas phase in the 30° impeller is about 55% better than the others. In addition, the results showed that the 30° impeller has a uniform radial distribution over the other impellers and the maximum gas phase holdup in the vessel. Investigation of the power consumption of the impellers showed that the 30° impeller has the highest power consumption among the other pitched blade impellers. Also, examine the effect of same power condition for pitched blade impellers showed that the 30° impeller has the best mixing quality in this condition.     

Fluid-fluid interactions and hydrodynamics in agitated dispersions: A simulation model

A model which accounts for complex fluid-fluid interactions and hydrodynamic effects in a fully baffled turbulently agitated dispersed phase system for batch, semi-batch, or continuous operation is developed. Both micromixing and macromixing effects on particle size distributions are taken into account. Coalescence and breakage functions are developed and examples are given of the normalized number and/or volume size distributions for various vessel regions. Calculated distributions are compared with experimental data to estimate values of coalescence and breakage parameters. Effects of coalescence, breakage, and system parameters on transient and steady state distributions are determined.