Flow characteristics of vortex based cavitation devices

Vortex based hydrodynamic cavitation (HC) devices offer various advantages over conventional linear flow devices, such as early inception, low erosion risk, and higher cavitational yield. Despite several promising applications, the key underlying flow characteristics are not yet adequately understood. This article presents results of a computational investigation into cavitation in vortex devices. Multiphase computational fluid dynamics results are presented and compared with experimental data on pressure drop over a range of flow rates. The results highlight the unique hydrodynamic characteristics of this type of device in relation to conventional linear flow reactors; cavitation inception occurs in the liquid bulk away from sold surfaces, and rapid pressure recovery rates are achieved. The models were used to simulate detailed time–pressure histories for individual vapor cavities, including turbulent fluctuations. The developed approach, models and results will provide a sound and useful basis for comprehensive multiscale modeling of vortex-based devices for HC.     

Modeling hydrodynamic cavitation in venturi: influence of venturi configuration on inception and extent of cavitation

Hydrodynamic cavitation (HC) is useful for intensifying a wide variety of industrial applications including biofuel production, emulsion preparation, and wastewater treatment. Venturi is one of the most widely used devices for HC. Despite the wide spread use, the role and interactions among various design and operating parameters on generated cavitation is not yet adequately understood. This article presents results of computational investigation into the cavitation characteristics of different venturi designs over a range of operating conditions. Influence of the key geometric parameters such as the length of venturi throat and diffuser angle on the inception and extent of cavitation is discussed quantitatively. Formulation and solution of multiphase computational fluid dynamics (CFD) models are presented. Appropriate turbulence and cavitation models are selected and solved using a commercial CFD code. Care was taken to eliminate the influence of numerical parameters like mesh density, discretization scheme, and convergence criteria. The computational model was validated by comparing simulated results with three published data sets. The simulated results in terms of velocity and pressure gradients, vapor volume fractions and turbulence quantities, and so on, are critically analyzed and discussed. Diffuser angle was found to have a significant influence on cavitation inception and evolution. The length of the venturi throat has relatively less impact on cavitation inception and evolution compared to the diffuser angle. The models and simulated flow field were used to simulate detailed time–pressure histories for individual vapor cavities, including turbulent fluctuations. This in turn can be used to simulate cavity collapse and overall performance of HC device as a reactor. The presented results offer useful guidance to the designer of HC devices, identifying key operating and design parameters that can be manipulated to achieve the desired level of cavitational activity. The presented approach and results also offer a useful means to compare and to evaluate different designs of cavitation devices and operating parameters. © 2018 American Institute of Chemical Engineers AIChE J, 65: 421–433, 2019     

Modeling of an industrial scale hydrodynamic cavitation multiphase reactor for Prileschajew epoxidation

The hydrodynamic cavitation multiphase reactor (HCMR) is emerging as a promising alternative for the intensification of liquid–liquid heterogeneous reactions, but research on HCMR modeling is lacking. In this article, an HCMR model was developed using Prileschajew epoxidation as the model system. First, based on experimental measurements of oil/water two-phase flow downstream of hydrodynamic cavitation devices, semiempirical correlations were proposed to describe the droplet size and droplet size distribution (DSD) as functions of flow conditions and geometry parameters. Then, with boundary conditions calculated by the DSD correlation, a droplet dynamics simulation in a reaction tank was performed by computational fluid dynamics coupled with population balance model to obtain the two-phase interfacial area. Finally, the acquired reactor model was substituted into an overall kinetic model, to simulate the epoxidation reaction in HCMR. Model predictions were verified by experimental results measured on an industrial scale HCMR.     

Specific features of power characteristics of in-line rotor–stator mixers

Methodthat can be used to predict agitation power of in-line rotor–stator mixers are considered in this paper. Results of experimental investigations of power draw are presented and interpreted by using proposed phenomenological models and CFD predictions for the Silverson rotor–stator 150/250 MS device and its modifications. Results of theoretical analysis lead to extended version of previously used correlation for the agitation power. Based on new correlation and the multifractal model of turbulence the problems of energetic efficiency and scale-up of drop breakage in the rotor–stator devices are presented and discussed.     

Operating Regimes and Hydrodynamics of a Rotating‐Disc Contactor

The operating regimes for a pilot‐scale rotating‐disc contactor (RDC) were investigated by a computational fluid dynamics/population balance model (CFD‐PBM) simulation. The model successfully predicted the critical rotor speed, which divided the entire operating range into two regions. In the low rotor speed region, the input energy was insufficient to break droplets, resulting in an almost constant droplet diameter. Therefore, the increasing revolution slightly affected the interfacial area, while the axial mixing became severe. In contrast, the interfacial area increased significantly in the high rotor speed region because of the increased breakage rate. Moreover, the axial mixing extent increased slightly because the dispersed‐phase accumulation enhanced the advection effect. The results indicate that the CFD‐PBM approach can be applied to engineering practice for extractors.     

A CFD‐PBM‐PMLM integrated model for the gas–solid flow fields in fluidized bed polymerization reactors

Although the use of computational fluid dynamics (CFD) model coupled with population balance (CFD‐PBM) is becoming a common approach for simulating gas–solid flows in polydisperse fluidized bed polymerization reactors, a number of issues still remain. One major issue is the absence of modeling the growth of a single polymeric particle. In this work a polymeric multilayer model (PMLM) was applied to describe the growth of a single particle under the intraparticle transfer limitations. The PMLM was solved together with a PBM (i.e. PBM‐PMLM) to predict the dynamic evolution of particle size distribution (PSD). In addition, a CFD model based on the Eulerian‐Eulerian two‐fluid model, coupled with PBM‐PMLM (CFD‐PBM‐PMLM), has been implemented to describe the gas–solid flow field in fluidized bed polymerization reactors. The CFD‐PBM‐PMLM model has been validated by comparing simulation results with some classical experimental data. Five cases including fluid dynamics coupled purely continuous PSD, pure particle growth, pure particle aggregation, pure particle breakage, and flow dynamics coupled with all the above factors were carried out to examine the model. The results showed that the CFD‐PBM‐PMLM model describes well the behavior of the gas–solid flow fields in polydisperse fluidized bed polymerization reactors. The results also showed that the intraparticle mass transfer limitation is an important factor in affecting the reactor flow fields. © 2011 American Institute of Chemical Engineers AIChE J, 58: 1717–1732, 2012     

Azimuthally oscillating membrane emulsification for controlled droplet production

A novel membrane emulsification (ME) system is reported consisting of a tubular metal membrane, periodically azimuthally (tangentially) oscillated with frequencies up to 50 Hz and 7 mm displacement in a gently cross flowing continuous phase. A computational fluid dynamics (CFD) analysis showed consistent axial shear at the membrane surface, which became negligible at distances from the membrane surface greater than 0.5 mm. For comparison, CFD analysis of a fully rotating ME system showed local vortices in the continuous phase leading to a variable shear along the axis of the membrane. Using an azimuthally oscillating membrane, oil‐in‐water emulsions were experimentally produced with a median diameter of 20–120 μm, and a coefficient of variation of droplet size of 8%. The drop size was correlated with shear stress at the membrane surface using a force balance. In a single pass of continuous phase, it was possible to achieve high dispersed phase concentrations of 40% v/v. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3607–3615, 2015     

Flow Characteristics in a Honeycomb Structure to Design Nanobubble Generating Apparatus

A nanobubble generator with honeycomb structures producing a large amount of water including large nanobubble density in a short time is described. The nanobubble-generating performance is investigated for large and small apparatus having different honeycomb cell dimensions by applying computational fluid dynamics (CFD) coupled with a population balance model (PBM). The CFD simulation shows that a significant pressure drop and shear stress occur in the bubbly flow in the honeycomb cell. The numerical model is based on the Eulerian multiphase model and the PBM is used to calculate the bubble size distribution. The obtained CFD-PBM results are compared with the experimental results for large and small apparatus. Bubble size distributions in the honeycomb structure under different inlet absolute pressure can be predicted by the PBM. The maximum shear stress is determined as the main controlling factor for nanobubble generation.     

Low-Field High-Resolution PFG-NMR to Predict the Size Distribution of Inner Droplets in Double Emulsions

In double emulsions, the inner and outer droplet size distribution (DSD) determines the quality of the double emulsion and is therefore essential to be measured. Low-field high-resolution pulsed-field gradient nuclear magnetic resonance is used to measure the inner DSD in water-in-oil-in-water double emulsions. The Gaussian phase distribution approach is employed with a mixture of two normal distributions to predict bimodal inner droplets. This approach allows the prediction of the swelling of inner droplet during storage of double emulsions, and thus to validate a phenomenological population balance model estimating inner droplet swelling. Only a fraction of the inner droplets are found to swell during storage, due to differences in the Laplace pressure, thus leading to the formation of a bimodal size distribution of the inner droplets. Practical Applications: This methodology is useful to predict the evolution of double emulsions during storage, in a wide range of applications, such as food and pharmaceutical products.     

Modeling Emulsification in Static Mixers: Equilibrium Correlations versus Population Balance Equations

Two different modeling approaches are adopted to model turbulent breakage during continuous emulsification in static mixers. First, a correlation is developed to predict the droplet mean diameter. Second, a population balance equation (PBE) is applied to track the droplet size distribution (DSD) using two different breakage kernels. The performances of the two approaches are assessed against a large number of experimental data. The correlation is fast to develop and is found to be capable of predicting the mean diameter with an acceptable accuracy while the PBE‐based model gives an excellent prediction of the entire DSD.     

Experimental Investigation and CFD Modeling of Hydrodynamic Parameters in a Pulsed Packed Column

In this work, a combination of computational fluid dynamics (CFD) and droplet population-balance model (DPBM) in the framework of Fluent was applied to simulate the drop-size distributions and flow fields in a pilot-plant liquid–liquid extraction pulsed packed column. The three-dimensional unsteady-state liquid–liquid flow was modeled using the Eulerian two-fluid equations in conjunction with the realizable k – ε turbulence model. The classes method (CM) was chosen for solving population-balance equations. Two models for breakage and coalescence, the models of Luo and Garthe, were used in the CFD code. The model was validated by comparing the simulated drop-size distributions and holdup with experimental measurements. After the validation of the model, the effects of the operating conditions (feed rates and pulsation) on the dispersed phase holdup and drop-size distributions were studied. The results of linked CFD-DPBM model and experiments revealed that the dispersed phase holdup was increased when the organic and aqueous flow rates increased and when the intensity of pulse was increased, the holdup increased. Increasing the dispersed and continuous feed rates caused the Sauter mean diameter of the drops decreased and when the intensity of pulse was increased, because of high droplets break up rate, the Sauter mean diameter decreased. Results of linked CFD-DPBM model show that the CFD-DPBM tool is able to predict hydrodynamic parameters in a pulsed packed column.     

A Novel Low‐Pressure Device for Production of Nanoemulsions

A novel device is applied to produce emulsions of methyl methacrylate in water with a controllable size in the range of 30–100 nm. The process is based on the reciprocating flow of the material through an abrupt contraction which generates a strong elongational flow. This results in highly efficient dispersive mixing even at moderate pressures, thus reducing viscous dissipation and improving temperature control. The original design of the device also allows easy feeding and sampling, easy adjusting of the total volume of the emulsion, and processing volatile components owing to the liquid‐ and gas‐tightness of the device. The influence of process parameters (like pressure drop and mixing time) and composition of the system (volume ratio of the dispersed and continuous phases, surfactant and hydrophobic agent weight percentages) on the droplet size and stability of the emulsions is investigated and discussed.     

湍流分散体系中液滴破碎频率模型的黏性修正

在分析研究分散相黏度对液滴变形和破碎影响的基础上,提出了一个改进的液滴破碎频率模型并拓展了液滴破碎判据标准.同时通过Monte Carlo模拟的随机方法,得到了湍流搅拌槽中液-液分散体系的液滴直径分布和Sauter平均直径d₃₂.通过与文献中关于d₃₂的实验结果比较发现,该模型预测的Sauter平均直径更接近实验值,对于黏性分散相改进的液滴破碎频率模型要优于Coulaloglou和Tavlarides提出的模型.计算结果表明对于黏性分散相液滴,其黏度限制了液滴变形,使得液滴破碎频率被大大减少, 液滴直径明显增加,液滴直径分布向右偏移.     

Influence of Different Silica Nanoparticles on Drop Size Distributions in Agitated Liquid‐Liquid Systems

The impact of different silica nanoparticles on rheology, interfacial tension and drop size distributions in liquid‐liquid systems is determined experimentally. The particles vary in wettability and specific surface area. In contrast to commonly used high‐energy devices for Pickering emulsion preparation, low energy input by stirring allows to quantify drop breakage and coalescence in steady state and dynamic conditions. The experiments can provide essential information for drop size model development in nanoparticle‐stabilized emulsions.     

Analysis of energy dissipation in stirred suspension polymerisation reactors using computational fluid dynamics

This work evaluates the spatial distribution of normalised rates of droplet breakage and droplet coalescence in liquid–liquid dispersions maintained in agitated tanks at operation conditions normally used to perform suspension polymerisation reactions. Particularly, simulations are performed with multiphase computational fluid dynamics (CFD) models to represent the flow field in liquid–liquid styrene suspension polymerisation reactors for the first time. CFD tools are used first to compute the spatial distribution of the turbulent energy dissipation rates (ε) inside the reaction vessel; afterwards, normalised rates of droplet breakage and particle coalescence are computed as functions of ε. Surprisingly, multiphase simulations showed that the rates of energy dissipation can be very high near the free vortex surfaces, which has been completely neglected in previous works. The obtained results indicate the existence of extremely large energy dissipation gradients inside the vessel, so that particle breakage occurs primarily in very small regions that surround the impeller and the free vortex surface, while particle coalescence takes place in the liquid bulk. As a consequence, particle breakage should be regarded as an independent source term or a boundary phenomenon. Based on the obtained results, it can be very difficult to justify the use of isotropic assumptions to formulate particle population balances in similar systems, even when multiple compartment models are used to describe the fluid dynamic behaviour of the agitated vessel. © 2011 Canadian Society for Chemical Engineering     

On the steady‐state drop size distribution in stirred vessels. Part I: Effect of dispersed phase viscosity

Previous studies on emulsification have used the maximum drop size (d_max) or Sauter mean diameter ( ) to investigate the effect of viscosity on the drop size distribution (DSD), however, these parameters fall short for highly polydispersed emulsions. In this investigation (Part I), the steady‐state DSD of dilute emulsions is studied using of silicon oils with viscosities varying across six orders of magnitude at different stirring speeds. Different emulsification regimes were identified; our modeling and analysis is centered on the intermediate viscosity range where interfacial cohesive stresses can be considered negligible and drop size increases with viscosity. The bimodal frequency distributions by volume were well described using two log‐normal density functions. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3293–3302, 2018     

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     

The effect of hydrodynamic parameters on the production of Pickering emulsions in a baffled stirred tank

Pickering emulsions are potential industrial scale alternatives to surfactant-based emulsions. The stability of Pickering emulsions depends on the physicochemical nature of the liquid–particle interface and the hydrodynamic conditions of the production process. This article investigates the effect of hydrodynamic conditions on the drop size of concentrated Pickering emulsions in baffled stirred tanks. Oil in water emulsions composed of silicon oil, water, and hydrophilic glass beads as stabilizing particles were produced. Two impellers were used at different sizes: Rushton turbine (RT) and pitched blade turbine. The effects of power per mass, Reynolds number, tip speed, and Weber number on the droplet sizes were studied. The energy dissipated around the impeller and the size of the impeller high shear zone were found to be critical to the emulsion droplet sizes. The breakup and droplet-particle contact mechanism of the RT was found to be more favorable for the production of the Pickering emulsions.     

A unified theoretical model for breakup of bubbles and droplets in turbulent flows

Pressure has a significant effect on bubble breakup, and bubbles and droplets have very different breakup behaviors. This work aimed to propose a unified breakup model for both bubbles and droplets including the effect of pressure. A mechanism analysis was made on the internal flow through the bubble/droplet neck in the breakup process, and a mathematical model was obtained based on the Young–Laplace and Bernoulli equations. The internal flow behavior strongly depended on the pressure or gas density, and based on this mechanism, a unified breakup model was proposed for both bubbles and droplets. For the first time, this unified breakup model gave good predictions of both the effect of pressure or gas density on the bubble breakup rate and the different daughter size distributions of bubbles and droplets. The effect of the mother bubble/droplet diameter, turbulent energy dissipation rate and surface tension on the breakup rate, and daughter bubble/droplet size distribution was discussed. This bubble breakup model can be further used in a population balance model (PBM) to study the effect of pressure on the bubble size distribution and in a computational fluid dynamics‐population balance model (CFD‐PBM) coupled model to study the hydrodynamic behaviors of a bubble column at elevated pressures. © 2014 American Institute of Chemical Engineers AIChE J, 61: 1391–1403, 2015