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IIT Jodhpur

Department of Chemical Engineering

World University root node

Stability and Destabilization of Water-in-Crude Oil Emulsion

Drop-interface interaction under an electric field is relevant in commercial desalters wherein water droplets suspended in oil coalesce under an electric field, move down under gravity, and eventually coalesce with the water pool at the bottom of the desalter. In this work, we report our observation that the transition from coalescence to partial coalescence can be described by a critical electrocapillary number and is independent of the Ohnesorge number. On the other hand, the partial coalescence to noncoalescence transition depends upon both the electrocapillary number and the Ohnesorge number. The bridge during partial coalescence exhibits an electrocapillary-number-independent growth and collapse dynamics, although the transition time for growth to collapse depends upon the electrocapillary number (CaE). Lastly, contrary to previous studies, our results indicate that the secondary droplet size varies as CaE3/2 unlike the CaE1/2 reported in the literature. Copyright © 2020 American Chemical Society.

Langmuir

Coalescence, Partial Coalescence, and Noncoalescence of an Aqueous Drop at an Oil-Water Interface under an Electric Field

The coalescence of water drops, dispersed in oil, is critical to the destabilization of a water-in-oil emulsion under an electric field. In this work, we determine the conditions for coalescence or non-coalescence of two aqueous water drops that are suspended in an insulating oil and subjected to a uniform electric field. Specifically, we investigate the effect of the conductivity of the droplet phase (σ), the type of oils (silicone oil and castor oil), the effect of waveform AC and DC and frequency, and the separation between the droplets on the electrocoalescence/non-coalescence of two suspended aqueous drops. The main highlights are, (i) we provide expressions for critical electrocapillary number (Cac) vs. σ for silicone oil (Cac = 0.010 × σ−0.20) that shows cone-cone and castor oil (Cac = 0.022 × σ−0.15) that shows cone-dimple mode of contact. (ii) Two closely spaced drops can coalesce at electrocapillary numbers much higher than the critical electrocapillary number for widely spaced drops due to a smaller cone angle at the point of contact. These results should be important in the design of industrial electrocoalescers. © 2019

Chemical Engineering Research and Design

An experimental study on the effect of conductivity, frequency and droplets separation on the coalescence of two aqueous drops under an electric field

The effect of an electric field on the coalescence of two water droplets suspended in an insulating oil (castor oil) in the non-coalescence regime is investigated. Unlike the immediate breakup of the bridge, as reported in earlier studies, e.g. Ristenpart et al. (Nature, vol. 461 (7262), 2009, pp. 377-380), the non-coalescence observed in our experiments indicate that at strong fields the droplets exhibit a tendency to coalesce, the intervening bridge thickens whereafter the bridge dramatically begins to thin, initiating non-coalescence. Numerical simulations using the boundary integral method are able to explain the physical mechanism of thickening of this bridge followed by thinning and non-coalescence. The underlying reason is the competing meridional and azimuthal curvatures which affect the pressure inside the bridge to become either positive or negative under the effect of electric field induced Maxwell stresses. Velocity and pressure profiles confirm this hypothesis and we are able to predict this behaviour of transitory coalescence followed by non-coalescence. © 2019 Cambridge University Press.

Journal of Fluid Mechanics

Breakup and non-coalescence mechanism of aqueous droplets suspended in castor oil under electric field

Electrocoalescence is an industrially relevant process, especially in oil exploration and petroleum refining industries, and is the working principle of the process equipment, an electrocoalescer. In this work, electrocoalescence is modelled using dipolar electrostatic forces which bring the droplets together but are resisted by Stokesian drag forces. We demonstrate that hydrodynamic interactions are important to model electrocoalescence. Similarly, a multi box methodology is necessary for getting improved statistics for drop size distribution, since any coagulation/coalescence process, typically results in the loss of droplets. Moreover, the process of chaining can be modelled by assuming a “wait-time” on contact of the interacting drops, and n-mer formation can be predicted. Our results show a reasonable agreement between model predictions and experimental data. The simulations also show that an increase in volume fraction, average droplet size, and polydispersity enhance the dynamics of the coalescence process. © 2018

Computers and Chemical Engineering

Modelling and particle based simulation of electro-coalescence of a water-in-oil emulsion

The electrocoalescence of aqueous drops in an insulating oil is the fundamental process in the dehydration of emulsions of water in insulating oils, such as crude oils. The great difference in the conductivity of the aqueous droplets and the oil used as the suspending medium suggests that the conductivity of the oil medium should not affect the coalescence process. In this work, contrary to this expectation, we show a very strong dependence of the conductivity of the insulating oil on the mode of droplet coalescence. Experiments are reported using silicone oils, vegetable oils, and hydrocarbons, in the anchored drop setup that also facilitates the use of low viscosity oils. Our study indicates that the droplet undergoes cone-cone mode of interaction at low conductivities of the oil medium. On the other hand, when the conductivity of the oil is increased, the cone-dimple mode of coalescence is observed when the viscosity of the oil phase is higher, while a cone-spray-dimple mode is observed when oils of low viscosity are used. © 2019 Elsevier B.V.

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Modes of coalescence of aqueous anchored drops in insulating oils under an electric field

The effect of an electric field on the coalescence of two water drops suspended in an insulating oil is investigated. We report four new results. (i) The cone angle for the non-coalescence of drops can be significantly smaller (as small as ) than the value of reported by Bird et al. (Phys. Rev. Lett., vol. 103 (16), 2009, 164502). (ii) A surprising observation of the dependence of the mode of coalescence/non-coalescence on the type of insulating oil is seen. A cone-cone mode for silicone oil is observed as against cone-dimple mode for castor oil. (iii) The critical capillary number for non-coalescence decreases with increase in the conductivity of the droplet phase. (iv) Systematic experiments prove that the apparent bridge during non-coalescence is indeed transitory and not permanent, as reported elsewhere. Theoretical calculations using analytical theory and the boundary integral method explain the formation of the cone-dimple mode as well as the transitory bridge length. The numerical calculation and thereby the physical mechanism to explain the occurrence of very small non-coalescence angles as well as the dependence of the phenomenon on the conductivity of the insulating oil and the water droplets remain unexplained. © 2018 Cambridge University Press.

Electrocoalescence of a pair of conducting drops in an insulating oil

The efficacy of electrocoalescence is critically dependent upon the interfacial tension of the crude–water interface. This study demonstrates the effect of interfacial tension on the electrocoalescence efficiency in crudes with high acidity. The interfacial tension is estimated using spinning drop tensiometer (SDT) and electrocoalescence experiments are performed at an electric field = 1.15 kVrms/cm at a frequency of 50 Hz. It is observed that separation of water from the crude is hindered at high pH for two very different reasons depending upon the source of alkalinity. Calcium hydroxide induced alkalinity leads to more rigid interface, resulting in delayed electrocoalescence. On the other hand, sodium hydroxide based alkalinity leads to ultra-low tension of crude–water interface, thereby causing oil-in-water emulsion. Increase in the pH also leads to poor quality of brine resolution, in case of sodium hydroxide based alkalinity (pH = 10) we get unresolved turbid emulsion. © 2018 Elsevier B.V.

Interrelationship between electrocoalescence and interfacial tension in a high acidity crude: Effect of pH and nature of alkalinity

Photocatalytic degradation of nitrobenzene (NB) and an azo dye (namely azophloxine) was carried out using zinc oxide (ZnO) nanoparticles which were prepared by electrochemical method using zinc electrodes in aqueous oxalic acid solution. Taguchi methodology was applied for studying the effects of four experimental parameters namely catalyst dosage (g/1). time of reaction (h), pH and H2O2 dose. Degradation efficiency of NB was found to increase with an increase in H2O2 concentration and decrease in catalyst dosage and pH. Maximum NB degradation efficiency of 98% was observed at catalyst dosage=l g/1, pH=3, H2O2 to C6H5NO2 ratio of 7 mol/mol after 5 h of treatment. Similarly, maximum dye degradation efficiency of 58% and color removal efficiency of 78% was observed at catalyst dosage=l.25 g/1, pH=4, H2O2 concentration of 8 mmol/1 in 5 h. Overall, prepared ZnO nanoparticles performed better for NB degradation in comparison to azo dye degradation. © 2016 Scientific Publishers. All rights reserved.

Journal of Scientific and Industrial Research

Photocatalytic degradation of nitrobenzene and azo dye using zinc oxide nanoparticles prepared by electrochemical method

In this study, zinc oxide nanoparticles were synthesized using electrochemical method. Zinc was used as electrode whereas oxalic acid in aqueous solution was used as an electrolyte. A L9 (34) Taguchi optimization methodology was used to find out the individual and interactive effect of all four independent experimental parameters namely pH (pHo): 5-8, oxalic acid concentration (m): 0.05-0.15 M, conductivity (k): 20-30 (mS/cm) and operating voltage (Vo): 5-8 V. These experimental parameters were optimized so as to maximize the productivity (g) and correspondingly find out specific energy consumption (kW h/kg) and specific electrode consumption (kg/kg). At the optimum condition of pHo = 5, m = 0.05 M, k = 30 (mS/cm) and Vo = 8 V, values of productivity, SENC and SELC were found to be 1.03 g, 3.79 kW h/kg and 1.76 kg/kg, respectively. Nanoparticles synthesized at optimum conditions have been further characterized by scanning electron microscopy, X-ray diffraction and UV-Visible diffuse reflectance spectroscopy techniques so as to confirm its ZnO nature. © 2015 Elsevier B.V. All rights reserved.

Journal of Alloys and Compounds

Zinc oxide nanoparticles synthesis by electrochemical method: Optimization of parameters for maximization of productivity and characterization

Herein, copper nanoparticles were synthesized using electrochemical method at pH 5, 6.5, 9.5 and 12.5 (coded as Cu5, Cu6.5, Cu9.5 and Cu12.5, respectively). Copper was used as electrode whereas 0.15 M oxalic acid in aqueous solution was used as an electrolyte. Effect of pH of the electrolyte solution on the morphological, structural and textural properties of prepared copper nonoparticles was studied. Prepared nanoparticles were characterized by X-ray crystallography, Field emission scanning electron microscope, transmission electron microscopy, thermogravimetric analysis, differential thermal analysis and textural analysis. The morphology and sizes of the nanoparticles prepared varied with the initial pH of the solution. Sizes of synthesized Cu particles were found to be in the range of 20 nm to 7 μm. All the particles were mesoporous in nature. Cu5 was found to contain 67% copper hydroxide and 33% copper oxalate whereas Cu6.5, Cu9.5 and Cu12.5 essentially consisted of copper hydroxide. © (2015) Trans Tech Publications, Switzerland.