Adel O. Sharif

Electrostatic enhancement of coalescence of water droplets in oil: A review of the current understanding

This paper reviews the current understanding of electrocoalescence of water droplets in oil, highlighting particularly the mechanisms proposed for droplet-droplet and droplet-interface coalescence under the influence of an applied electrostatic field, as well as various factors influencing the electrocoalescence phenomenon. Generally, the coalescence behaviour can be described in three stages: droplets approaching each other, the process of film thinning/drainage, and film rupture leading to droplet-droplet coalescence. Other possible mechanisms, such as droplet chain formation, dipole-dipole coalescence, electrophoresis, dielectrophoresis and random collisions, are also presented. Experimental work and mathematical modelling of the coalescence process are both reviewed, including various models, such as molecular dynamic simulation, random collision/coalescence modelling, and linear condensation polymerisation kinetics. The type of electric field, such as alternating, direct and pulsed direct current, plays a significant role, depending on the design and set-up of the system. The concept of an optimum frequency is also discussed here, relating to the electrode design and coating. Other factors, such as the average droplet size and the residence time of the liquid mixture exposed to the electric field, are highlighted relating to coalescence efficiency. The characteristics of the emulsion system itself determine the practicality of employing a high electric field to break the emulsion. Emulsions with high aqueous phase content tend to short-circuit the electrodes and collapse the electric field. Type and concentration of surface-active components have been shown to impart stability and rheological property changes to the interfacial film, thus making the coalescence mechanism more complicated. More investigations, both experimental and by computer simulation, should be carried out to study the electrocoalescence phenomenon and to contribute to the design and operation of new electrocoalescers.

Long-range electrostatic attraction between like-charge spheres in a charged pore

The existence of long-range attractive electrostatic forces between particles of like charge is one of the great current controversies ofcolloid science. The established theory (Derjaguin–Landau–Vervey–Overbeek; DLVO) of colloidal interactions predicts that an isolated pair of like-charged colloidal spheres in an electrolyte should experience a purely repulsive screened electrostatic (coulombic) interaction,. Direct measurements of such interactions have shown quantitative agreement with DLVO theory. Recent experiments, however, provide evidence that the effective interparticle potential can have a long-range attractive component in more concentrated suspensions, and for particles confined by charged glass walls,,. It is apparent that the long-range attraction in concentrated systems is due to multi-body interactions and may have a similar explanation to the attraction observed for otherwise confined colloids. Theoretical explanations have been proposed but remain the subject of controversy,. Here we present a quantitative theoretical explanation of these attractive forces between confined colloidal particles, based on direct solutions of the nonlinear Poisson–Boltzmann equation for two like-charged spheres confined in a cylindrical charged pore. The calculations show that the attraction may be explained by the redistribution of the electric double layers of ions and counterions in solution around the spheres, owing to the presence of the wall; there is thus no need to revise the established concepts underlying theories of colloidal interactions.

Atomic force microscope studies of membranes: force measurement and imaging in electrolyte solutions

Abstract An atomic force microscope has been used to study the electrical double layer interactions between a silicon tip (with an oxidised surface) and two polymeric membranes, one microfiltration (nominally 0.1 μm) and the other ultrfiltration (25 000 MWCO), in aqueous NaCl solutions. Force-distance curves were measured for the two membranes at four ionic strengths. The membranes were also imaged under the same conditions using electrical double layer repulsive forces of differing magnitudes —; “electrical double layer mode” imaging. Image analysis was used to determine surface pore size distributions. The force-distance curves, together with numerically calculated potential profiles at the entrance to a charged pore, allow an explanation and identification of the optimum imaging conditions. The best images were obtained at high ionic strength with the tip close to the membrane surface.

Experimental studies of deformation and break-up of aqueous drops in high electric fields

Abstract High electric fields have been used to separate dispersed aqueous drops from an oil phase in the chemical and oil industries. Nevertheless, very high electrostatic fields can disintegrate aqueous drops which is detrimental to the overall efficiency of the electrocoalescence process. The limit above which an electrostatic field can deform and break-up the drops, rather than coalescing the drops, is of great practical importance. The outcomes of an experimental investigation of drop deformation and break-up in several liquid–liquid systems are presented in this article. The onset of drop instability corresponds to the situation when the ratio of the length of the major axis to that of the minor axis of the drop is about 1.9. This corresponds to an electrostatic field strength of between 350 and 380 kV m−1 for a 1.2-mm diameter aqueous drop, corresponding to a critical electrostatic Weber number of about 0.49. The magnitude of the critical electric field depends on the initial drop size. The investigations were performed using a single aqueous drop in a rectangular Perspex cell. The drop adopted most commonly a prolate shape. However, if the drop was not in the centre of the electric field, then it could take on different shapes altogether. A filament penetrating into the continuous phase could disintegrate and produce a droplet at a time from its tip, and the detached droplets move towards the counter electrode. Movement of a charged drop between a pair of electrodes had also been investigated. To predict the average translational velocity of a charged drop, several aspects need to be considered, such as the non-spherical drop shape, especially as it contacting and leaving the electrodes. Therefore, the mechanism of drop deformation and break-up under high electric field is complex, and warrants more experimental investigations.

A model of the interaction between a charged particle and a pore in a charged membrane surface

Abstract A model of the electrostatic and molecular interactions of a charged colloid particle with a charged membrane surface in an electrolyte solution has been developed. In Derjaguins approximation, the force between a spherical colloid particle and a cylindrical membrane pore (with a rounded inlet) is calculated taking into account both electrostatic and van der Waals interactions. The force and energy are strongly dependent on the zeta-potential of both the particle and the membrane pore, the electrolyte concentration, and geometrical parameters. Conditions are found for which a potential barrier exists at the pore entrance. This barrier prevents a particle from entering the pore and, hence, gives an equilibrium position of the particle above the membrane surface. Therefore, there is a possibility in this case of removing the particle by a tangential flow, preventing pore blocking. The model was verified using a Finite Element Method (FEM) analysis developed earlier for colloidal interactions by two co-authors. It has been found that the accuracies of analytical formulae obtained for the interaction energy and force are within 10 and 20%, respectively, for practical application ranges of physico–chemical and geometrical parameters. Two major advantages of the model proposed compared to FEM calculations are: (1) the possibility of non-centerline calculations (when a particle is not moving along the axis of a membrane pore) without a three-dimensional solution; and (2) speed of calculations using the analytical formulae is much higher. Using a simplified expression for hydrodynamic force, critical values of pressure gradients across the membrane pore have been calculated analytically.

Hydrodynamic and colloidal interactions effects on the rejection of a particle larger than a pore in microfiltration and ultrafiltration membranes

Abstract The application of membrane separation processes, such as microfiltration and ultrafiltration, is one of the most important developments in chemical engineering in recent years. Membrane fouling is the most important problem which restricts application of membrane processes. Recently, it has been demonstrated that colloidal and hydrodynamic interactions govern membrane fouling and they can be manipulated by choice of processing conditions, for example, pH, ionic strength and applied pressure. The paper presents a quantification of both colloidal (electrostatic and van der Waals) and hydrodynamic effects to identify conditions for the operation of such processes with much greater efficiency. In particular, the hydrodynamic and colloidal forces on a charged spherical particle slightly larger than a pore at various distances from a charged cylindrical pore in a charged planar surface have been calculated. In the absence of electrostatic interactions, filtration of such particles can result in a catastrophic loss in flux as they can plug pores highly effectively. The rejection of the particle at a membrane pore is described in terms of a balance between the hydrodynamic force which is driving the particle towards the membrane and the colloidal forces between the charged particle and the charged membrane surface. A Galerkin finite element scheme combined with automatic mesh refinement and error estimation strategy has been used to provide numerical solutions of the non-linear Poisson–Boltzmann equation for electrostatic interactions and of the Navier–Stokes equation for hydrodynamic interactions. The results show that under the conditions covered by the calculations, which correspond to those occurring in practice, the electrostatic interactions can play a crucial role in controlling the approach of such a particle to a pore. The calculations have a number of important consequences for membrane separation processes. Firstly, the quantification of the operating conditions which allow separation without the particles coming into intimate contact with the membrane—potentially non-fouling conditions. Secondly, a demonstration that the manufacture of membranes with a high surface potential would be very beneficial to the efficient operation of such processes.

The effects of electrostatic interactions on the rejection of colloids by membrane pores—visualisation and quantification

The pressure driven membrane processes of microfiltration and ultrafiltration are usually classified in terms of the size of solutes (colloids) separated. However, theoretical calculations have shown that electrostatic double-layer interactions can have a strong influence on rejection at the pores of such membranes. The present paper provides experimental evidence to support these findings. Firstly, atomic force microscopy in conjunction with the colloid probe technique is used to measure directly the repulsive electrostatic force experienced when a single colloidal particle approaches a microfiltration membrane. Scanning of such a membrane with the colloid probe provides a direct visualisation of the membrane surface as would be experienced by a colloidal particle during filtration. Secondly, filtration flux/time data is presented for the case of filtration of particles of size very close to the pore size in an ultrafiltration membrane. For such a case, theoretical calculations allow definition of a critical pressure at which the hydrodynamic force transporting the colloid toward the membrane pore is exactly balanced by the opposing electrostatic force. The experiments show that operating above this pressure results in a rapid loss in filtration flux, but operation below this pressure allows continuing filtration with only a minor decrease in flux, in agreement with the calculations.

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

Abstract 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.

The hydrodynamic and electrostatic interactions on the approach and entry of a charged spherical particle to a charged cylindrical pore in a charged planar surface with implications for membrane separation processes

The application of membrane separation processes, such as microfiltration and ultrafiltration, is one of the most important developments in chemical engineering in recent years. The separation characteristics of such membranes are usually interpreted sterically in terms of the relative size of membrane pores and solutes. However, electrostatic effects are also important, though often neglected. The paper presents a quantification of both electrostatic and hydrodynamic effects to identify conditions for the operation of such proceses with much greater efficiency. In particular, the hydrodynamic and electrostatic forces on a charged spherical particle as a function of distance of approach and entry to a charged cylindrical pore in a charged planar surface have been calculated. A Galerkin finite-element scheme has been used to provide numerical solutions of the nonlinear Poisson-Boltzmann equation for electrostatic interactions and of the Navier-Stokes equation for hydrodynamic interactions, with the Newton sequence technique being used to solve the Poisson-Boltzmann equation. The results show that under the conditions covered by the calculations, which correspond to those occurring in practice, the electrostatic interactions can play a crucial role in controlling the approach and entry of such a particle to a pore. The calculations have several important consequences for membrane separation processes. Firstly, the quantification of the operating conditions which allow separation without the particles coming into intimate contact with the membrane—potentially non-fouling conditions. Secondly, the quantification of the operating conditions allowing fractionation of particles of identical size but differing surface potential. Thirdly, a demonstration that the manufacture of membranes with a high surface potential would be very beneficial to the efficient operation of such processes.

Formation Damage in Iranian Oil Fields

The fine migration and the scale formation into the porous media and the resulting production decline have long been the problem to the petroleum industry. It is also generally accepted that formation due to the particle movement and the scale formation are not thoroughly understood. In contributing to the solution of this problem, an experimentally study of calcium sulphate scale formation and the particle movement in the porous media using of packing bed with 12 different size of the glass and sand bead and the 8 core plug that gathered from the Siri oilfields. The purpose was to study the different physical and mechanical aspects of the processes leading to the formation damage caused by the movement and the entrapment of the suspended particles and the scale formation. The permeability is the key parameter among several others that control the reservoir performance. The experiments are based on the results of the permeability reduction. The interception of the permeability reduction by the interaction between the operational parameter is very complex. Therefore, several of these factors such as the temperature, the concentration, the fluid dynamic and the type of porous media are considered. The experimental results are analyzed and a new model which can predicts particle movement and the scaling tendency of the common oilfield water deposits in the water disposal wells, the water flooding systems, and in surface equipment’s and the facilities is developed. The developed of the model is based on the experimental data and the empirical correlation, which perfectly mach the Iranian oilfields condition. This model has been applied to the investigate of the potential of the scale precipitation in the Iranian oilfields, either in the onshore or the offshore fields, where the water injection is being performed for the desalting units water disposal purpose or as the method of secondary recovery or the reservoir pressure maintenance.