Liquid-liquid Dispersions Research Articles

Drop Distribution Determination in a Liquid-Liquid Dispersion by Image Processing

This paper presents the implementation of an algorithm for automatic identification of drops with different sizes in monochromatic digitized frames of a liquid-liquid chemical process. These image frames were obtained at our Laboratory, using a nonintrusive process, with a digital video camera, a microscope, and an illumination setup from a dispersion of toluene in water within a transparent mixing vessel. In this implementation, we propose a two-phase approach, using a Hough transform that automatically identifies drops in images of the chemical process. This work is a promising starting point for the possibility of performing an automatic drop classification with good results. Our algorithm for the analysis and interpretation of digitized images will be used for the calculation of particle size and shape distributions for modelling liquid-liquid systems.

EXPERIMENTAL INVESTIGATION OF PHASE INVERSION OF LIQUID-LIQUID SYSTEMS IN A SPRAY EXTRACTION COLUMN

An experimental study of the phase inversion behavior of liquid-liquid dispersion has been conducted in a spray extraction column for systems of toluene / water, n-hexane/water, CCl4/water, toluene /water + glycerol (25 % wt), toluene + CCl4 (25 % wt) / water and toluene / acetic acid (5 % wt)/water. The effects of physical properties, mass transfer and column geometry on phase inversion have been investigated. The results show that the dispersed phase hold up sufficient for phase inversion increases in o/w dispersion and decreases in w/o dispersion by increasing interfacial tension. Also, it was found that by increasing the viscosity of aqueous phase, dispersed phase hold up decreases at phase inversion point in both o/w and w/o dispersions. The tendency to phase inversion increases in o/w dispersion with an increase in density difference of two phases. It was observed that dispersed phase hold up at phase inversion point decreases in the presence of mass transfer, when the direction of mass transfer is from dispersed phase to continuous phase. The results show that dispersed phase holdup increases with drop size at phase inversion point and column diameter has an important effect on phase inversion because of wall effect.

Flow patterns for liquid-liquid dispersions in force fields of various intensities

Dependence of the velocity of drops on their size is studied for the case in which the field intensity is varied in a wide range, forming a particular flow pattern for the drop-liquid dispersion, which predicts the velocity of the drop relative to the medium and is characterized by the prominent basic lines related to different flow (settling) modes. The processes of drop settling in the fields of gravity and centrifugal forces are two particular cases of the proposed generalization for predicting the efficiency of separating the liquid-liquid system. For polydisperse systems, the calculation of the average velocity takes into account the boundaries of settling modes.

Roles of Interfacial Properties on the Stability of Emulsified Bitumen Droplets

Because of the deformable nature of emulsion droplets, it is imperative to consider the inter-droplet pressure in assessing the stability of a liquid-liquid dispersion. In a novel methodology, the pressure generated between droplets in axial compression is estimated through considerations of equilibrium shape mechanics. The applied compressive pressure is compared to the disjoining pressure, induced by the surface charges, resisting droplet-droplet interaction. The stability of bitumen droplets, emulsified in aqueous media, is assessed from surface mechanical and electrostatic perspectives. In support of the analysis, the tensions at, and zeta potentials near, the bitumen droplet surfaces are measured. The predicted trends of stability against coalescence are confirmed by novel droplet interaction experiments: Individual emulsified bitumen droplets, manipulated by suction micropipettes, are compressed against one another along a mutual axis at up to 100 nN force. The force is measured via the deflection of a sensitive microcantilever. Because of the statistical nature of the experimental observations, a probability of coalescence is quantified. The water chemistry dramatically influences bitumen droplet coalescence, which is enhanced in acidic environments and at high ionic strength.

On Evolution of Drop-Size Distributions in Turbulent Pipe Flow Revisited

This paper corrects an error in the work of Nere and Ramkrishna [Ind. Eng. Chem. Res. 2005, 44, 1187] on solving population balance equations for the evolution of drop-size distributions in fully developed turbulent flow of a liquid-liquid dispersion in a circular pipe. We rely on the previously noted Nere and Ramkrishna's work for the formulation of the problem. An alternative linear operator formulation is developed here to solve the population balance equation, compared with detailed numerical solution obtained by computational fluid dynamics (CFD) and is determined to be considerably more efficient.

Investigation of Phase Inversion of Liquid-Liquid Dispersions in Agitated Vessels

In this paper we report results from on-going theoretical and experimental studies carried out jointly at Imperial College London and University College London. Laser-induced fluorescence (LIF) is used to investigate liquid-liquid phase inversion experimentally and to observe in detail phenomena that accompany the inversion process, such as secondary dispersions and drop coalescence and breakup. Theoretically, a two-region model together with a criterion based on a dynamic balance between drop coalescence and breakup is employed to predict phase inversion. The concept of a radial distribution function for hard spheres was also utilized in order to better model the interaction of drops at high dispersed phase holdup. The modeling work is capable of predicting the existence of ambivalent ranges which are in good agreement with experimental observations.

Electrophoresis of Carreau-Drop Dispersions with a Charge-Regulated Surface

The electrophoresis of a liquid-liquid dispersion, where the dispersed phase comprises drops of a shearing-thinning Carreau fluid with a charge-regulated surface and the dispersion medium is an aqueous electrolyte solution, is analyzed theoretically under the conditions of low surface potential, uniform weak applied electric field, and arbitrary double layer thickness. This is the first attempt for the description of the electrophoretic behavior of a dispersion containing non-Newtonian drops with a charge-regulated surface. We show that, in general, the more significant the shear-thinning nature of the drop fluid, the lower the concentration of drops, the lower the pH of bulk solution, or the higher the concentration of dissociable functional groups on drop surface, the larger the mobility. Also, the mobility of a drop may exhibit a local maximum as the thickness of the electric double layer surrounding it varies.

Turbulent Mixing of Two Immiscible Fluids

Abstract The emulsification process in a static mixer HEV (high-efficiency vortex) in turbulent flow is investigated. This new type of mixer generates coherent large-scale structures, enhancing momentum transfer in the bulk flow and hence providing favorable conditions for phase dispersion. We present a study of the single-phase flow that details the flow structure, based on LDV measurements, giving access on the scales of turbulence. In addition, we discuss the liquid-liquid dispersion of oil in water obtained at the exit of the mixer/emulsifier. The generation of the dispersion is characterized by the Sauter diameter and described via a size-distribution function. We are interested in a local turbulence analysis, particularly the spatial structure of the turbulence and the turbulence spectra, which give information about the turbulent dissipation rate. Finally, we discuss the emulsifier efficiency and compare the HEV performance with existing devices.

On a new model for continuous coalescence and breakage processes with diffusion

We study a new model for the evolution of a liquid-liquid dispersion. The droplets of the dispersed phase are supposed to move due to diffusion and to undergo coalescence and breakage. The main feature of the model is the inclusion of a maximal droplet size. This requires a consistent mechanism opposing the increase of droplets due to coalescence. The resulting system of uncountably many coupled reaction-diffusion equations is interpreted as a vector-valued Cauchy problem. We prove existence and uniqueness of nonnegative and mass-preserving solutions. Furthermore, we give sufficient conditions for global existence.

Scalable Synthesis of a New Class of Polymer Microrods by a Liquid–Liquid Dispersion Technique

An efficient scalable process for forming polymer microrods, based on a liquid–liquid dispersion technique, is reported. Rod-like particles with diameters of 0.5–3 μm and lengths of tens of micrometers form via three concurrent processes: emulsification of polymer solution, shear-driven deformation of the drops into cylinders, and solidification into rod-like particles (see Figure). The microrods are readily oriented by electric fields.

Sedimentation and Coalescence Profiles in Liquid‐Liquid Batch Settling Experiments

AbstractA simple mathematical method for predicting the phase separation profiles of a batch liquid‐liquid dispersion is developed based on the empirical description of drops resting at a liquid‐liquid plane interface. The inflexion time, at which the sedimentation zone vanishes, can be obtained by analysis of the coalescence profile. Therefore, the sedimentation profile can be predicted and the size of drop and interfacial area can be estimated. The experiments were carried out with an O/W‐type dispersion of kerosene‐water system in a 154 mm I.D. mixing vessel. The experimental results agree well with the model prediction. Other experimental data in the literature were also adopted for testing the model.

Dynamic Interfacial Tension of Aqueous Solutions of PVAAs and Its Role in Liquid-Liquid Dispersion Stabilization

In liquid-liquid dispersion systems, the dynamic change of the interfacial properties between the two immiscible liquids plays an important role in both the emulsification process and emulsion stabilization. In this paper, experimentally measured dynamic interfacial tensions of 1-chlorobutane in the aqueous solutions of various random copolymers of polyvinyl acetate and polyvinyl alcohol (PVAA) are presented. Theoretical analyses on these results suggest that the adsorption of the polymer molecules is controlled neither by the bulk diffusion process nor the activation energy barrier for the adsorption but the conformation of polymer molecules. Based on the concept of critical concentration of condensation for polymer adsorption, as well as the observation that the rate at which the dynamic interfacial tension changes does not correlate to the PVAA’s ability to stabilize a single drop, it is postulated that the main stabilization mechanism for the PVAAs is by steric hindrance, not the Gibbs–Marangoni effect offered by the small molecule surfactants.

Utilization of population balances in simulation of liquid-liquid systems in mixed tanks

A simulation model has been developed to model drop populations in a mixed tank. A multiblock mixed tank model has been used with the drop population balance equations developed in the literature. The drop breakage and coalescence functions used in the population balance model take into account the local turbulent energy dissipation values. The drop breakage and coalescence function parameters are fitted against drop size measurements from dense liquid-liquid dispersions, which were assumed fully turbulent. Since the local turbulence and flow values of a mixed tank are used in the present model, the fundamental breakage and coalescence phenomena can be taken into closer examination. Furthermore, the present model is capable of predicting inhomogeneities occurring in a mixed tank. It is also considered as an improved tool for process scale-up, compared to the simple vessel-averaged population balance approach, or use of correlations of dimensionless numbers only. The present model can use two sources of data for fitting parameters in the drop rate functions. One is the transient data of the measured drop size distribution as the impeller speed is changed. The other is the time-averaged data measured at different locations of the mixed tank. Different flow regions can be chosen from direct measurements or from the CFD simulations in a straightforward manner. CFD flow simulation results can be used when no experimentally obtained flow conditions are available. This is especially useful for nonstandard vessels, such as reactors containing cooling coils.

Characterization of Liquid-Liquid Dispersion in Batch and Continuous Toroiedal Minimixer.

The present work deals with the use of a torus mixer for the formation of liquid-liquid dispersion. The objective is to determine the performance of the torus reactor to form the primary emulsion of the microencapsulation process of an active product by a solvent evaporation technique. The encapsulated product is dissolved in an aqueous phase, which is emulsified into a solution of the polymer in methylene chloride to form a water/oil emulsion. The methylene chloride is gradually removed from the droplets by evaporation. Hard microcapsules are obtained when the solvent is completely removed. The formation of the water/oil primary emulsions is the key step to control the size distribution of the microcapsules.The characteristics of the dispersed phase of the water-in-oil emulsion produced in a torus mixer in batch and continuous conditions is determined by an optical method based on the use of the microscope connected to a CCD camera. The flow inside the torus mixer is directly linked to the rotation speed of the impeller. The effect of the rotation speed of the impeller and the dispersed phase volume fraction of the liquid-liquid dispersion is analyzed. The experimental data are expressed by a correlation giving the mean droplet diameter in function of the Weber number and dispersed phase concentration. The influence of the superimposed throughput, for the continuous process, on the size distribution of the dispersed phase is also taken into account.

Preparation of Biodegradable Capsules Composed of Paper Fibers by Utilizing Liquid-Liquid Dispersion

Biodegradable and controlled-release capsules composed of waste paper fibers as matrix and limonene droplets as core materials were prepared by utilizing the liquid-liquid dispersion. Encapsulation was performed due to coalescence between a droplets containing binder and fibers and those dissolving a gelling agent. In this experiment, methylcellulose as a binder and tannic acid as a gelling agent were used respectively. Paper fibers were treated hydrophobic prior to the encapsulation operation in order to improve waterproofing of capsules. It was investigated whether capsules could be prepared by this encapsulation process or not. Furthermore, it was investigated how the fiber concentration affected the morphology and release characteristics of capsules. As a result, it was found that capsules were able to be prepared due to coalescence between two kinds of droplets and the capsule morphology was extremely changed with the fiber concentration. Moreover, it was found that the change in morphology considerably affected the release characteristics. Namely, in the case of the lower fiber concentration, monocore-type capsule containing limonene droplets were formed. However, as the fiber concentration increased, multicore-type capsules were formed.

Drop Breakup in Liquid-Liquid Dispersions at an Orifice Plate Observed in a Large-Scale Model

Fine emulsions, i.e., a dispersion of fine liquid droplets in another liquid, can be obtained by forced flow of a coarse predispersion through small orifices. Direct observations of droplets at the original scale is nearly impossible. A promising solution is a large scale-up, considering the most important scale factors. The experiments presented in this paper were conducted on a model 250 times the original size, while maintaining strong hydromechanical similarity. The breakup of single oil droplets in water was documented with a highspeed camera.

Salt Effects on Single Aqueous Drops Falling through an Immiscible Organic Liquid

Drop settling (or rising) is crucial to the breaking of liquid-liquid dispersions. Most of the empirical correlations for terminal velocity of drops are formulated based on experimental data for systems of dispersed organic phase and continuous aqueous phase using pure and nearly pure liquids. Because the presence of the various third components in one or both of the phases mayaffect the settling of the drops, the terminal velocity and the onset of oscillation of aqueous drops containing various concentrations of electrolytes falling through n-dodecane were studied in this work. The applicability of existing empirical correlations to these reversed systems of continuous organic phase and dispersed aqueous phase was also examined. Dissolved electrolytes have the effect of causing the terminal velocities and their peak velocities at the onset of drop oscillation to become faster and the peak diameters to become smaller when their concentrations are high enough to cause sufficient changes of physical properties. Existing correlations for drop terminal velocity in pure systems are shown to be adequate in the presence of electrolytes. For high interfacial tension systems, the Hu-Kinter correlation predicts the terminal velocity of organic drops in water and aqueous drops in organics equally well, independent of the presence of electrolytes. The criteria of Hu-Kintner can be used to estimate the peak velocities and peak diameters at the onset of oscillation within a deviation of 3.7%.

The Dynamics of Two-Phase Liquid Dispersions: Necessity of a New Approach

This paper is a short overview of the problem of modelling the dynamics of liquid-liquid dispersions produced by a rotating device in a closed vessel under the assumption of spatial homogeneity. The main scope is to criticize the traditional approach, based on the definition of suitable coalescence and breakage operators, pointing out the necessity of modelling a third evolution mechanism, that we called volume scattering, having basically the same importance. This theory was presented at the Seminario Matematico e Fisico di Milano.

Dynamics Of Dispersions With MultipleBreakage And Volume Scattering

In some previous papers we have developed a model for the dynamics of liquid-liquid dispersions including some new features, among which the socalled volume scattering phenomenon, as a third mechanism of evolution, besides coalescence and breakage. So far we confined to binary breakage. The present paper analyses the more general case in which more than two droplets may be produced as the result of breakage or volume scattering. The main point is the introduction of a probability function for each breakage mode: from this point of view the paper completes our approach to the description of the dynamics of droplets dispersions which are spatially homogeneous.

The Influence of Macroscopic Elongational Flow on Dispersion Processes in Agitated Tanks

The influence of elongational and shear gradients in the macroscopic flow field in agitated tanks on dispersion processes is investigated. Measurements of droplet size distribution for a liquid-liquid dispersion process using phase-Doppler anemometry (PDA) reveal that axial-flow impellers, such as the 24°-pitched-blade turbine and propeller, produce smaller droplets than the Rushton turbine at the same average specific power and energy input. These results stand in contradiction to the usual assumption that only the maximum turbulent shear stress determines the breakup process and the Rushton turbine is well known to produce higher turbulent shear stresses. Particle image velocimetry (PIV) measurements of the macroscopic flow field indicate that the 24°-pitched-blade turbine and propeller produce larger areas with higher elongational gradients. Therefore, the proposed consideration of particle breakup due to macroscopic elongational flow in addition to turbulent stresses improves the understanding of dispersion processes in agitated tanks.