Drop-interface Coalescence Research Articles

Mechanistic modelling of separating dispersions in pipes using model-based design of experiments techniques

This work presents a parametric study on a mechanistic model for separating liquid–liquid dispersions in pipes. The model considers drop-settling, drop-interface coalescence and drop-drop coalescence, predicting the evolution of four characteristic layers during separation. Parameter estimation, parametric sensitivity analysis (PSA), and model-based design of experiments (MBDoE) techniques are employed to acquire precise parameter estimates and propose optimal experimental conditions, thereby enhancing the accuracy of existing models. Experimental data from literature using oil-in-water dispersions are used for parameter estimation. PSA reveals regions of high sensitivity of the model outputs to uncertain parameters, which are corresponding to favourable sampling locations. Manipulating the mixture velocity, the dispersed phase fraction, and the layer heights at the inlet influences these sensitive regions. Clustered measurements around highly sensitive regions in the pipe enhance the information content they provide. MBDoE demonstrates that either of the A-, D-, or E-optimal experimental design criteria improves the expected parameter precision.

A mechanistic model for the prediction of flow pattern transitions during separation of liquid-liquid pipe flows

A one-dimensional mechanistic model that predicts the flow pattern transitions during the separation of dispersed liquid-liquid flows in horizontal pipes was developed. The model is able to capture the evolution along the pipe of the four characteristic layers that develop from initially dispersed flows of either oil-in-water or water-in-oil at a range of mixture velocities: a pure water layer at the bottom, a settling (flotation/sedimentation) layer, a dense-packed zone, and a pure oil layer on the top. Coalescence correlations from literature were included in the model to predict the drop growth due to binary drop coalescence and the coalescence rate of drops with their corresponding interface. The model predictions on the evolution of the heights of the different layers were partly compared against available experimental data obtained in a pilot scale two-phase flow facility in a test section of 0.037 m inner diameter using tap water and an oil of density 828 kg m−3 and viscosity 5.5 mPa s as test fluids, and in a 0.1 m inner diameter test section using water and an oil of density 857 kg m−3 and viscosity 13.6 mPa s. It was shown that the evolution of the four characteristic layers depends on the rates of drop settling and drop-interface coalescence. Oil-in-water dispersions separated faster than water-in-oil ones, while dispersions with smaller drop-sizes were more likely to exhibit depletion of the dense-packed zone.

Role of surfactant-induced Marangoni stresses in drop-interface coalescence

We study the effect of surfactants on the dynamics of a drop-interface coalescence using full three-dimensional direct numerical simulations. We employ a hybrid interface-tracking/level-set method, which takes into account Marangoni stresses that arise from surface-tension gradients, interfacial and bulk diffusion and sorption kinetic effects. We validate our predictions against the experimental data of Blanchette and Bigioni (Nat. Phys., vol. 2, issue 4, 2006, pp. 254–257) and perform a parametric study that demonstrates the delicate interplay between the flow fields and those associated with the surfactant bulk and interfacial concentrations. The results of this work unravel the crucial role of the Marangoni stresses in the flow physics of coalescence, with particular attention paid to their influence on the neck reopening dynamics in terms of stagnation-point inhibition, and near-neck vorticity generation. We demonstrate that surfactant-laden cases feature a rigidifying effect on the interface compared with the surfactant-free case, a mechanism that underpins the observed surfactant-induced phenomena.

The secondary drop formation of nanoparticle/surfactant-stabilized water droplets under non-uniform electric fields

Electrocoalescence is an energy-efficient and environmentally-friendly process for breaking water-in-oil emulsions. It has been used extensively in the oil and petroleum industries. In this study, the electrocoalescence process of nanoparticle/surfactant-stabilized water droplets with a planar interface in the presence of non-uniform electric fields was experimentally investigated. The effects of electric field patterns (i.e. uniform and non-uniform fields) and nanoparticles (i.e. concentrations) on the drop-interface electrocoalescence process were systematically examined, analyzed, and discussed. The results showed that the presence of Sodium Dodecyl Sulfate (SDS) significantly increased the volume of secondary droplets and led to the apex disintegration of the main secondary droplet because of the low surface tension. At high SiO2 concentrations, the liquid bridge between the droplet and interface was difficult to form, and the whipping filament regime, sheet breakup regime, and varicose jet breakup regime, were observed, which were determined by the accumulating surface charge. In addition, three types of non-uniform fields induced by different geometrical electrode configurations, i.e. grid electrodes, mesh electrodes, and coupled electrodes, were proposed and the electric field distributions were calculated in COMSOL Multiphysics. With increasing SiO2 concentrations, coupled electrodes produced much smaller detached droplet volumes than those of pairwise mesh and grid electrodes, which was helpful to complete the coalescence of the drop and interface. Furthermore, flat electrodes, which induced a uniform electric field, produced much larger secondary droplet volumes than those of the electrodes generating non-uniform electric fields. In summary, the utilization of coupled electrodes had a positive effect on drop-interface coalescence and increased the overall separation efficiency. The outcome of this work is potentially useful in the design of compact and efficient oil-water electro-dehydration devices.

Emulsion microstructural evolution with the action of environmentally friendly demulsifying bacteria

Demulsifying bacteria, as soft particles, are environmentally friendly agents used to recycle oil and purify water from surfactant-stabilized emulsions. However, details of this demulsification mechanism are unclear. Herein, we investigate the emulsion phase behavior and microstructural evolution in real space and time, as well as the specific role of the bacterium S-XJ-1 in this evolution through multifaceted approaches. Results reveal that new features of emulsion phase behavior with S-XJ-1 exhibited whole zone drop-drop coalescence, which caused emulsion network collapse, followed by drop rearrangement, which led to clear spatial layering of the emulsion. The bacteria adsorb onto the water-oil interface, aggregate into bacterial clusters, and become unevenly distributed at the interfaces in real emulsion systems. The bacterial peculiar behaviors were shown to improve drop-drop and drop-interface coalescence using micropipettes and sessile drop technology. Thus, the distribution and aggregation of bacteria at the interface could contribute to the bridging of neighboring water drops, rupture the original interface films, and improve drop coalescence. This study provides an insight into the role of particle demulsifiers in emulsion microstructural evolution.

The influence and optimisation of electrical parameters for enhanced coalescence under pulsed DC electric field in a cylindrical electrostatic coalescer

A pulsed DC electric field is usually used to enhance drop–drop and drop–interface coalescence in a water-in-oil (W/O) emulsion. The effect and optimisation of electrical parameters are important when designing an electrostatic coalescer. In this work, experiments were performed with a small-scale electrostatic coalescer and image processing technology using water in crude oil emulsions to investigate the influence of electric field intensity, frequency, duty ratio and rise time. The comparison between experimental data and predicted data based on the two-layer capacitor model of a cylindrical electro-coalescer was also obtained to validate whether the two-layer capacitor model is applicable for a concentric cylindrical electrostatic coalescer. Optimum electric field strength exists for different water content emulsions (525.6kVm−1 for water content of 10% and 338.1kVm−1 for 30%), which suggests that it is higher for lower water content emulsion. There are two optimum frequencies in the low and high frequency ranges. For water content of 10%, the optimum frequency is approximately 20Hz, while for 30%, it is 10Hz in the low frequency range. The coalescence effect is also high when the frequency is 2000Hz. The droplet diameter is largest when the duty ratio is 50% for different electric field strengths, frequencies and water contents. The effect of rise time for pulsed DC is negative and the longer the rise time is, the worse the electrostatic coalescence effect is. The optimum frequency and duty ratio are obtained by using a theoretical formula and an empirical formula. The optimum frequency is nearly the same for the low frequency range especially for emulsions with a water content of 10%, but coalescence effects will increase at high frequency, which is different from the theoretical predicted value, indicating that the theory is applicable in the low frequency range for the concentric cylindrical coalescer. The predicted duty ratio is close to 0.5 and is nearly the same as the experimental results. In the electrostatic coalescer, the waveform of pulsed DC will be distorted to a triangle wave at high frequency. The high-frequency pulsed DC electric field is therefore not suitable for use in an electrostatic coalescer with an insulated electrode.

Electro-coalescence of water drops in oils under pulsatile electric fields

Electric fields are commonly used to enhance the coalescence of water drops in oils. However, this process could cause some undesirable phenomena such as secondary droplets formation, reducing the separation efficiency. Here the effect of pulsatile electric fields (PEF) on the secondary droplets formation has been investigated. In the presence of a very low frequency PEF or DC electric field three distinct drop–drop and drop–interface interaction patterns are observed: complete coalescence, partial coalescence and rebound without coalescence. The first is the ideal pattern not leaving any secondary droplets. It has previously been shown that an increase in the electric field strength and/or a decrease in the interfacial tension result in non-ideal patterns in drop–interface coalescence. The application of PEF shifts the coalescence pattern from a non-ideal to an ideal one in both drop–drop and drop–interface coalescences. Three waveform types, i.e. square, half-sinusoidal and sawtooth waves have been applied to the coalescence process. It is shown that the sawtooth waveform is the most effective in reducing the secondary droplets formation in drop–interface coalescence, followed closely by the half-sinusoidal one. The observation of videos sequences suggests that a threshold frequency exists above which a non-ideal pattern switches to an ideal one. For drop–drop coalescence this threshold frequency depends on the PEF amplitude and the size of primary drop pairs, as for bigger primary drop pairs and larger amplitudes of PEF the threshold frequency would be higher. When using pulsatile electric fields higher field strengths can be applied for systems having high water content without causing field breakdown, as compared to the constant DC field. This is useful in optimizing the electro-coalescence process.

Electro-coalescence of an aqueous droplet at an oil–water interface

The coalescence of an aqueous droplet at an oil–water interface under an electric field has been investigated, with a view to quantify conditions that give rise to secondary droplet formation. Two patterns of drop-interface coalescence may occur: complete coalescence and partial coalescence. The former is obviously the desirable pattern for industrial coalescers. However in practice, the process of coalescence could actually produce smaller droplets, which become more difficult to remove, and hence undesirable. This is caused by either necking, due to extensive elongation of the droplet, or reaction to a fast and energetic coalescence and is referred to as partial coalescence. The volume of the droplets formed in this way has been analyzed as a function of the initial droplet size, electric field strength and the distance between the droplet and the interface. The expansion speed of the neck connecting the droplet and interface at the beginning of the pumping process has also been quantified. These results are useful in optimizing the electro-coalescence process.

Electro-hydrodynamic separation of aqueous drops from flowing viscous oil

In a number of industrial processes, gravitation usually plays a significant role in the separation of dispersed particles or drops from another immiscible liquid. This separation, taking place in large tanks, provides low operating cost when the residence time of the liquid–liquid system is long and the density difference between the two phases is large. However, the situation becomes complicated for small liquid drops dispersed in another liquid when the density difference is very small and the carrier liquid is at high velocity. In this paper, using a novel compact electrocoalescer-separator that has been developed recently, an externally applied electric field has been shown to significantly enhance the separation of aqueous drops in a flowing viscous oil, with low concentrations of the dispersed phase. The separation efficiency increases with the electric field strength until a limit, above which, drop deformation and break-up occur. Using pulsed direct current electric fields, an optimum electric field and frequency exist for the enhancement of drop–drop and drop–interface coalescence, thus producing an optimum separation efficiency for the system. The separation efficiency increases with drop size up to a certain diameter, as larger drops have been observed to deform and short-circuit the system. Short-circuiting can be avoided by optimising the electric field. From a force balance on a single sphere, a parameter IP is defined which describes the acceleration of the sphere under an electric field. The parameter IP can predict the behaviour of the system qualitatively up to the limit where drop break-up and electrical short-circuiting occur.

Suggestion of New Correlations for Drop/Interface Coalescence Phenomena in the Absence and Presence of Single Surfactant

After designing and constructing a coalescence cell, drop/interface coalescence phenomenon was studied in the absence and presence of single surfactant.Two surface active agents of sodium dodecyl sulfate and 1-decanol were used. Distilled water was used as dispersed phase. Toluene, n-heptane and aqueous 60% (v/v) of glycerol were selected as continuous phases, separately. It was found that the coalescence time increased with both drop size and falling height. When the chemical system suffered from multi-step (partial) coalescence, number of coalescence steps decreased with either of these variables. Addition of a single ionic or nonionic surfactant made the drop size smaller, and hence caused the onset of partial coalescence. When the surfactant was soluble in the drop phase, it increased the time more effectively. Also, It was found that the viscosity of the continuous phase played an important role in drop-interface coalescence. Based on the experimental results, new correlations were proposed. Then, the results were compared with the other models by application of the existing condition.

The behaviour of a liquid–liquid interface and drop-interface coalescence under the influence of an electric field

The study of drop-interface coalescence provides an insight for the enhancement of the separation rate of a liquid–liquid dispersion in a settler. This is very important as liquid–liquid separation is usually the bottleneck of any industrial process. Moreover, most of the settlers in the chemical industries are huge, leading to high capital and operating costs. The current understanding of drop-interface coalescence is described in the first part of the paper. Experiments reported in this paper show that the measured electric current through water–sunflower oil and water–n-heptane systems, induced by an applied potential difference, increases linearly until a particular value (i.e. the turning point), beyond which the measured current has been observed to increase very rapidly. For various thicknesses of the water layer and the organic phase layer, the electric current, corresponding to the turning points in the voltage–current characteristic curves, is between 3.7 and 18 nA. Furthermore, it is observed that the turning point of the voltage–current characteristic curve for a liquid–liquid system is caused by the formation of a cone at the interface. Above this turning point, the intensification of the local electric field above the tip of the cone is believed to be responsible for the very rapid increase in the measured electric current. Furthermore, the semi-vertical angle of the induced cones are measured to be very close to the value given by Taylor [Proc. Royal Soc. A 280 (1964) 383]. The measurement of the electric current can be used to monitor and control the behaviour of a liquid–liquid interface, thus providing an optimum condition for instantaneous and single-staged drop-interface coalescence. This, consequently, will produce a higher separation rate of the dispersed drops from the continuous phase, thus reducing the residence time of the liquid–liquid dispersion in the settler. As a result, the size of the settler can be reduced, therefore, lowering the capital and operating costs.

Electrostatic and hydrodynamic separation of aqueous drops in a flowing viscous oil

In various industrial processes, sedimentation often plays a significant role in the separation of dispersed particles or drops from another immiscible liquid. This separation, taking place in large tanks, provides low operating cost when the residence time of the liquid–liquid system is long and the density difference between the two phases is large. However, the situation becomes complicated for small liquid drops dispersed in another liquid when the density difference is very small and the carrier liquid is at high velocity. In this paper, using a novel compact electrocoalescer-separator that has been developed recently, an externally applied electric field has been shown to significantly enhance the separation of aqueous drops in a flowing viscous oil, with very low concentrations of the dispersed phase. The separation efficiency increases with the electric field strength until a limit, above which, drop deformation and break-up occur. Using pulsed direct current electric fields, an optimum electric field and frequency exist for the enhancement of drop–drop and drop–interface coalescence, thus producing an optimum separation efficiency for the system. The separation efficiency increases with drop size up to a certain diameter, as larger drops have been observed to deform and short-circuit the system. Short-circuiting can be avoided by optimising the electric field. From a force balance on a single sphere, a parameter I P is defined which describes the acceleration of the sphere under an electric field. The parameter I P can predict the behaviour of the system qualitatively up to the limit where drop break-up and electrical short-circuiting occur.

Electrocoalesce-separators for the separation of aqueous drops from a flowing dielectric viscous liquid

Abstract For small drops dispersed in another liquid with a small density difference, gravitational separation is not effective and other mechanical methods such as centrifugation are expensive. An attractive way to separate the dispersed phase is by the use of electrical phenomena, whereby the differences in conductivity and dielectric properties between the two phases can be exploited to generate significantly high surface forces, where under favourable conditions they can enhance electrocoalescence. In this paper, the design and operation performance of two novel compact electrocoalescer-separators are described. The application of an external electric field has been shown to significantly enhance the separation of aqueous drops in a flowing viscous oil, with a low concentration of the dispersed phase. The separation efficiency increases with the electric field strength until a limit, above which drop deformation and break-up occur. Using pulsed direct current (dc) electric fields, an optimum electric field and frequency is found for the enhancement of drop–drop and drop–interface coalescence, thus producing a maximum separation efficiency for each of the two separators. The separation efficiency increases with inlet drop size up to a certain diameter. Above this diameter, the drops deform and short-circuit the electrodes easily. Short-circuiting can be minimised by optimising the electric field. Larger throughputs of the aqueous and oil phases are obtained with the electrocoalescer-separators that uses combined electrical and centrifugal effects, compared with the electrocoalescer-separator that is based on electrical and gravitational effects. The novel compact electrocoalescer-separators can be easily installed on-line to pipelines for treating water-in-oil dispersions occurring in the crude oil and petroleum industries.

Experimental study of drop-interface coalescence in the presence of polymer stabilisers

An experimental study has been carried out to characterise the performance of polymer stabilisers, partially hydrolysed polyvinyl acetate (PVAc), used in suspension polymerisation processes. The stabilisers are ranked by their ability to stabilise the dispersion characterised by the median coalescence time of a single drop with its homophase at a planar liquid/liquid interface. Results show that the stability of the dispersion relates closely to the molecular properties of the PVAcs. Other conditions being equal, PVAcs with higher molecular weights or lower degrees of hydrolysis can better stabilise a liquid–liquid dispersion. The stability of the dispersion also depends strongly on where the PVAc resides. The presence of a PVAc in the dispersed phase significantly reduces stability. Consistent with results reported in the literature, considerable scatter has been observed on the coalescence times of identical drops under the same conditions. An explanation for the scatter is also proposed in the paper, based on the classical Reynolds model for film thinning.

Effects of Applied Electric Fields on Drop—Interface and Drop—Drop Coalescence*

ABSTRACT In this work, coalescence of a single organic or aqueous drop with its homophase at a horizontal liquid interface was investigated under applied electric fields. The coalescence time was found to decrease for aqueous drops as the applied voltage was increased, regardless of the polarity of the voltage. For organic drops, the coalescence time increased with increasing applied voltage of positive polarity and decreased with increasing applied voltage of negative polarity. Under an electric field, the coalescence time of aqueous drops decreases due to polarization of both the drop and the flat interface. The dependency of organic drop-interface coalescence on the polarity of the electric field may be a result of the negatively charged organic surface in the aqueous phase. Due to the formation of a double layer, organic drops are subjected to an electrostatic force under an electric field, which, depending on the field polarity, can be attractive or repulsive. Pair-drop coalescence of aqueous drops in the organic phase was also studied. Aqueous drop-drop coalescence is facilitated by polarization and drop deformation under applied electric fields. Without applied electric fields, drop deformation increases the drainage time of the liquid film between two approaching drops. Therefore, a decrease in the interfacial tension, which causes drop deformation, accelerates drop-drop coalescence under an electric field and inhibits drop coalescence in the absence of an electric field. *The submitted manuscript has been authored by a contractor of the U.S. Government under contract DE-AC05-00OR22725. Accordingly, the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes.

Separation of liquid-liquid dispersions in a deep-layer gravity settler: Part II. Mathematical modeling of the settler

A mathematical model for the steady-state operation of a deep-layer gravity settler has been developed by using a population balance approach. This model takes into account the size distribution of drops within the dispersion band and uses rate expressions for the description of drop-drop and drop-interface coalescence phenomena. The model was validated with experimental data obtained in a laboratory mixer-settler unit, for a system consisting of 10% Acorga M5640 in Escaid 103–0.25 M Na 2SO 4 aqueous solution. The model successfully predicts the thickness of the dispersion band and the growth of the dispersed phase drops occurring by drop-drop coalescence within the dispersion band.

Effects of Surfactant on Multiple Stepwise Coalescence of Single Drops at Liquid-Liquid Interfaces

The mechanisms of multiple stepwise drop-interface coalescence have been investigated for liquid-liquid systems both with and without surfactant. It was observed that both the coalescence time and the number of coalescence steps depend on the initial drop size and the surfactant concentration. Without surfactant, the drop lifetime decreases as the drop size decreases while, in the presence of surfactant, the drop lifetime increases with decreasing drop size. These observations are explained on the basis of thin liquid film stability and film drainage mechanisms, and it is shown that drop lifetime versus size is governed by the thermodynamic stability of the film rather than film drainage and is dependent on film size and capillary pressure. Film stability is analyzed on the basis of both the concepts of mechanical perturbations and nucleation (hole formation). It is concluded that the experimental data for film lifetime versus film size is in qualitative agreement only with the nucleation mechanism. The mechanism of secondary drop formation from the primary drop and its drainage is interpreted by the Rayleigh capillary instability mechanism, and the effect of interfacial tension on the size of the secondary drop is also analyzed.

Interpretation of continuous settling behavior from batch settling data in a Versatic acid 10-water system

Studies on the prediction of phase disengagement in a continuous settler from batch settling data have been carried out. An equation to predict the variation of dispersion band height with dispersion throughput has been derived. The batch decay curve of the present system showed a typical sigmoidal decay mode, at the first stage of which free sedimentation of drops controlled the overall rate, while at the second stage the drop-drop and drop-interface coalescence dominated. It was found that the initial drop size and dispersed phase hold-up had to be taken into account in the interpretation of dispersion behaviour in a continuous settler. It can be concluded that it is possible to express the dispersion band thickness in terms of parameters proposed in this model and that the dispersed liquid drops collapse through an identical coalescence mechanism in both the batch and the continuous settlers.

Drop-Interface Coalescence Rate in Tertiary Amine Solvent Extraction

Abstract An important requirement for economical application of solvent extraction technology is rapid and efficient phase disengagement. However, much progress toward a clear fundamental understanding of the factors that affect phase disengagement rate will be needed before a logical approach to the problem will be possible. In this paper, we develop the conceptual framework for the study of drop-interface coalescence in collapsing liquid/liquid dispersions and present the details of the experimental setup employed in our initial work. The method for determining the drop-interface coalescence rate requires measurement of the average volume of drops (vf) adjacent to the interface, their number (n) per unit area of interface, and dispersed-phase throughput (Q) per unit area. We have employed recording videomicrography for measurement of vf and n, while Q is found from the changing position of the major interface as the dispersion band collapses (batch mode).

A Study of Oil/Water Separation in Corrugated Plate Separators

Simplified trajectory calculations and small scale experiments suggest that the mechanisms of oil separation in corrugated plates are similar to those of other plate separators and that the radius of curvature is not an important design parameter. Bouyancy force and flow fields control oil drop rise, while drop-interface coalescence should be important when plates are completely oil covered. Results suggest that interdrop coalescence, due to the periodically constricted and expanding flow, is not an important mechanism. Large scale (3–6 gpm) tests show that with light influent oil, corrugated plate separators perform better than simpler baffled vessels. With viscous, low interfacial tension oils, a simpler baffled separator performs better than the corrugated stack separator. This is probably due to viscous oil blockage in the plates and formation of a “foamy” oil-water mixture of high specific gravity.