Drop Size Distribution in a Standard Twin‐Impeller Batch Mixer at High Dispersed‐Phase Volume Fraction

The preparation of concentrated aqueous silicone oil emulsions has been investigated with particular attention to the effect of the dispersed‐phase volume fraction ? from 0.01 to 0.5 for a wide range of oil viscosities (50 to 1000 cSt). Oil was added on the top surface of a 6‐L vessel. Drop size distribution and Sauter mean diameter, d₃₂, measurements were carried out over 24 h mixing time. Emulsification was found to be relatively sensitive to the oil phase viscosity, μ_d, for the same ? yielding a narrower drop size distribution for low oil viscosity (50 cSt) and a wider drop size distribution for the highly viscous oil (1000 cSt). For the same ?, increasing μ_d resulted in increasing d₃₂. The equilibrium d₃₂ was found to be well correlated to the viscosity number by for ? = 0.5. For the same oil viscosity, d₃₂ was found to increase with increasing ?. A multiregression of d₃₂ with both ? and V_i for various silicone oil viscosity grades was successfully correlated by with a regression coefficient (R²) of 0.975. This shows a very weak dependence of the equilibrium d₃₂ on ?.     

Behavior of Elongated Liquid Jets in Viscous Liquid‐Liquid‐Systems

The stability of jets in elongational flow is exploited to obtain thin threads before breakup. Fine drops can be generated in suitable geometries with comparably large ducts. The examination deals with the stability of liquid threads simultaneously extended with the continuous phase in convergent flow. Breakup limits and regimes are discussed.     

A Probabilistic‐Statistical Model of Change of Particle Size Distribution in Settlers and Tanks

A probabilistic and statistical model of changes in the particle size distribution of the dispersed phase of low‐concentrated suspensions has been developed. The applicability of the method is theoretically substantiated. It is demonstrated that changes in the particle size distribution, in particular in the field of gravity, can be described by the Fokker‐Planck‐Kolmogorov equation.     

Solid Dissolution and Liquid Mixing in Turbulent Stirred Tanks

The dissolution of salt particles in turbulent stirred vessels is investigated. A novel method for achieving the simultaneous identification of the liquid mixing time and the dissolution time of salt particles from the conductivity data collected by electrical resistance tomography is presented. The characteristic time of the liquid mixing during the dissolution is found to be either uninfluential or significant on the dissolution itself, depending on the salt particle diameter and the vessel size. The adoption of a combination of literature correlations, which are proved to adequately estimate the two times, is suggested as a simple tool for identifying the minimum particle size for optimizing the dissolution operation.     

Rotor‐Stator and Disc Systems for Emulsification Processes

Emulsions now find a wide range of applications in industry and daily life. In the pharmaceutical industry lipophilic active ingredients as well as many nutritional products such as vitamins are often formulated in the dispersed phase of oil‐in‐water emulsions. Emulsions can be produced with different mechanical emulsification techniques. In the following review, the process of rotor‐stator systems and disc systems are compared to other popular mechanical emulsification systems. On the basis of experimental results from the authors' laboratory, a discontinuous gear‐rim dispersing system, discontinuous disc system, and a continuous high pressure system are compared with regard to their attainable mean droplet diameter and drop size distribution in an oil‐in‐water emulsion. It can be shown that dissolver discs with a very simple geometry attain very small mean droplet diameters and a very narrow droplet size distribution, comparable to the emulsions obtained with established rotor‐stator systems such as gear‐rim dispersers.     

Micromixing of Solid‐liquid Systems in a Stirred Tank with Double Impellers

The effect of solid particles on micromixing has been studied using the competitive iodide/iodate reaction system in stirred, multi‐impeller, solid‐liquid systems. The influences of particle size, impeller speed, solid holdup, feed position, and energy input have been investigated. The change of the segregation index with the power input was more distinguishable only for the 450–600 μm particles as compared with the large ones, at the same solid holdups. Also, for the small ones, cloud formation was observed at a particle concentration of 12.1 wt %. However, the influence of larger particles of 1–1.25 mm on micromixing was negligible, though both energy input and solid loading were increased. Besides, the optimal feed position was identified, and multiple feeds were also explored.     

CFD Prediction of the Critical Agitation Speed for Complete Dispersion in Liquid‐Liquid Stirred Reactors

A computational fluid dynamics (CFD) model is adopted to simulate the turbulent immiscible liquid‐liquid flow in a stirred vessel based on a two‐fluid model with a k‐ϵ‐A_P turbulence model. An improved inner‐outer iterative procedure is adopted to deal with the impeller rotation in a fully baffled stirred tank. Different drag formulations are examined, and the effect of the droplet size on both the dispersed phase holdup distribution and the velocity field is analyzed. Two different numerical criteria are tested for determining the critical impeller speed for complete dispersion. The simulated critical impeller speeds are generally in good agreement with the correlations in the literature when the fixed droplet size is properly selected. This demonstrates that the modeling approach and the numerical criteria proposed in this work are promising for predicting the dispersion characteristics in liquid‐liquid stirred tanks.     

New Predictive Correlations for the Drop Size in a Rotating Disc Contactor Liquid‐Liquid Extraction Column

Sauter mean drop sizes (d₃₂) generated from a hole distributor in liquid extraction RDC columns were studied under various conditions. Experiments were designed to generate data required to determine the main variables that control the drop sizes in RDCs. Two precise correlations were proposed for predicting d₃₂ in a RDC extraction column. The first was based on operating variables, hole‐distributor diameter, disc speed, column geometry, and system physical properties. The second one considered the same variables, except the column geometry. This model can be used for design purposes. The two correlations are the first of their type to consider the distributor hole inlet diameter in a RDC column. This diameter has been neglected by previous investigators. The maximum standard deviation for all data is 0.75 %, with a maximum absolute error of 6.8 %.     

Flow Field Analysis of Stirred Liquid‐Liquid Systems in Slim Reactors

Previous studies have shown the great potential, but also the great challenges, in handling slim reactors often used for polymerization reactions. Experiments and simulations were carried out in reactors with aspect‐to‐diameter ratios of up to 5, to test and to evaluate the mixing and dispersion efficiency for liquid‐liquid systems of single‐ and multiple‐stage impellers. Therefore, power consumption, mixing time and minimum dispersion speed were determined for five different stirrer types under turbulent conditions. It was found that the dimensionless mixing time is highly sensitive to the configuration of the impellers, with almost no dependency on the turbulent power number. Another focus was the analysis of the effect of the baffles. The influence of the baffle length in slim reactors on the mixing time and the macroscopic flow field was determined.     

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.     

Phase Inversion and Associated Phenomena in Oil‐Water Vertical Pipeline Flow

Phase inversion and its associated phenomena are experimentally investigated in co‐current upward and downward oil‐water flow in a vertical stainless steel test section (38 mm I.D.). Oil (ρ_o=828 kg/m³, µ_o=5.5 mPa s) and tap water are used as test fluids. Two inversion routes (w/o to o/w and o/w to w/o) are followed in experiments where either the mixture velocity is kept constant and the dispersed phase fraction is increased (type I experiments), or the continuous phase flow rate is kept constant and that of the dispersed phase is increased (type II experiments). By monitoring phase continuity at the pipe centre and at the wall it was found that phase inversion does not happen simultaneously at all locations in the pipe cross‐section. In type I experiments, the velocity ratios (U_o/U_w) where complete inversion appeared acquired the same constant value in both flow directions, although the phase inversion points, based on input phase fractions, were different. In contrast to previous results in horizontal flows, frictional pressure gradient was found to be minimum at the phase inversion point. The interfacial energies of the two dispersions before and after phase inversion, calculated from the measured drop sizes, were found to be different in contrast to the previously suggested criterion of equal energies for the appearance of the phenomenon. In type II experiments the phase inversion point was found to depend on mixture velocity for low and medium velocities but not for high ones. In all cases studied an ambivalent region, commonly reported for inversion in stirred vessels, was not observed.     

A Hydration Shell‐Based Thermodynamic Model for Aqueous Two‐Phase Systems

In this work a Flory‐Huggins model modified to account for some unique features of Aqueous Two‐Phase Systems is presented. The model takes into account observed solvation between water and polymer molecules through the incorporation of a hydration shell to express the number of water molecules bonded to each polymer molecule. The parameters of the modified equation were determined using experimental data of ATPS containing poly (ethylene glycol) and dextran. The results revealed remarkable improvement in the correlation ability of the model. A general expression that defines the number of water molecules in the hydration shell was also obtained.     

Droplet Size in Two‐Phase Liquid Dispersed Pipeline Flows

Few experimental data exists on drop size distribution during dispersed liquid‐liquid pipeline flows. In the majority of cases dilute dispersions have been used and the results have mainly been compared with models for drop breakup. A review of this work shows that the Rosin‐Rammler distribution represents satisfactorily the existing experimental data. However, the commonly used Hinze model (Hinze, 1955) often underpredicts the experimentally found maximum drop sizes. Later models, many of which are developments of the Hinze one, are also unable to predict the resulting maximum drop size for a wide range of experimental conditions. A more comprehensive database is needed for the further development and refinement of theoretical models.     

Dispersion Characteristics of Gas‐Liquid Contactors Agitated by Single and Double‐Stage Rushton Turbines

The study of the loading/complete dispersion transition is of great importance especially in processes with enhanced mixing requirements. In the present work, new data and correlations concerning the dispersion characteristics in gas‐liquid contactors agitated by single and dual Rushton turbine systems are reported. The maximum amount of gas which can be completely dispersed, in the presence of gross, well defined recirculation patterns of gas at a given stirrer speed is predicted. Under these conditions, an increase of flow number with increasing Froude number could always be estimated. With decreasing impeller diameter, d, the same gas amount could be dispersed at higher stirrer speeds. At impeller spacing ΔH = 2 d, for d equal to 0.06 and 0.08 m, and ΔH = 1.54 d, for d = 0.10 m, the complete dispersion conditions of the dual impeller systems were slightly better than the corresponding conditions of the single impeller systems.     

Radial Liquid Dispersion and Bubble Distribution in Three‐Phase Circulating Fluidized Beds

The liquid dispersion and bubble distribution in the radial direction have been investigated in the riser of a three‐phase circulating fluidized bed whose diameter is 0.102m and 3.5m in height. Effects of gas and liquid velocities and solid circulation rate have been determined. It has been found that the radial distribution of bubbles is related closely to the liquid dispersion in the radial direction. The size and rising velocity of bubbles tend to increase as the radial position approaches to the center of the riser. The bubble size increases with increasing U_G, but it decreases with increasing U_L or G_S in all radial positions. The radial dispersion coefficient of the liquid phase increases with increasing U_G or G_S, however, it tends to decrease with increasing U_L. The value of Dr has been well correlated in terms of dimensionless groups based on the isotropic turbulence model.