T.J. Williams

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.

Drug delivery by inhalation of charged particles

Abstract All solid and liquid particles produced naturally or by energetic industrial processes are electrically charged. Although the natural levels of charge are normally insufficient to influence the deposition of inhaled particles in the lung it is possible to increase charge levels so that a significant increase in lung deposition is caused. By careful control of breathing, particle size and charge it is possible to target specific regions of the lung. Predictions of targeted deposition using the Southampton lung model are presented and a brief description of complementary clinical studies is given.

Changes in the Size Distribution of a Water-in-Oil Emulsion Due to Electric Field Induced Coalescence

A knowledge of the droplet size distribution of a water-in-oil emulsion subjected to an electric field provides useful information regarding electrostatic coalescence which can aid the formulation of coalescence models and the design of commercial electrostatic separators. Water droplet size-distribution measurements made using a laser light-scattering technique during the electrostatic resolution of a low-water-content water-in-oil emulsion are reported. A qualitative explanation of the results is presented.

LUNG DEPOSITION OF PARTICLES BY AIRWAY GENERATION IN HEALTHY SUBJECTS: THREE-DIMENSIONAL RADIONUCLIDE IMAGING AND NUMERICAL MODEL PREDICTION

Multi- modality medical imaging enables measurement of the three-dimensional spatial distribution of inhaled, radiolabelled aerosol within the human lung. Using a conceptual model of spatial lung morphology, this data may be transformed to provide information on deposition by airway generation in the conducting airways. This methodology has been used to study intrapul- monary deposition patterns in control subjects for two polydisperse aerosols produced by jet-type nebulisers of mass median aerodynamic diameter (MMAD) 1. 8 and 6.8 pm. Comparison between derived experimental results and those from computer modelling shows reasonable agreement for total body, oropharynx and lung deposition and also for the difference in deposition pattern between the two aerosols. However, experiment suggests significantly less deposition in the central airways than is predicted by modelling. The new methodology has considerable potential in the fields of inhalation therapy and deposition modelling though more detailed validation is still required. 0 1998 Elsevier Science Ltd. All rights reserved

DESTABILISATION OF WATER-IN-OIL EMULSIONS UNDER THE INFLUENCE OF AN A.C. ELECTRIC FIELD : EXPERIMENTAL ASSESSMENT OF PERFORMANCE

Abstract The destabilisation of water-in-oil emulsions is an important industrial process and is often the rate-determining step in the dewatering and desalting of crude oils. In this study the influence of a 50Hz sinusoidal electric field on flowing water-in-oil emulsions is reported. A horizontal rectangular duct was used, with emulsion flowing through at flow rates up to 100 litre/min, giving Reynolds numbers (Re) in the range 1000 to 12,000. Electrocoalescer performance was assessed by measuring water droplet growth, using a specially developed technique with a laser diffraction particle sizer. Under conditions of high water-cut, electrocoalescence can be so efficient that free-water drop-out may occur, even in conditions of turbulent flow. This can lead to the formation of a layer of water coflowing with the depleted emulsion in the duct Study of the electrocoalescer behaviour under these conditions has been carried out by image analysis using a Hamamatsu Image Processor.

Electrostatic discharges between charged insulators and grounded spheres

A commercially available electrostatic field solver enables the accurate calculation of the field distribution existing within the air gap between a charged insulating sheet with a grounded backing and a grounded conducting sphere. These data are input to a computer program which uses a breakdown criterion to determine if the field conditions are sufficient to initiate a discharge between the sphere and the insulator. The program decreases the sphere-to-insulator separation until the breakdown strength of the gap is exceeded. The paper presents results for a range of sphere sizes approaching insulators of different thickness. In addition to this, experimental data on the charge transferred by discharge are discussed in terms of the simulated breakdown field strengths. The reported data should be of interest in some specialist areas of electrostatic technologies such as the manufacture and processing of insulating film and sheet.

Modelling the effect of charge on selective deposition of particles in a diseased lung using aerosol boli.

The technique of using a charged bolus of aerosol to deliver a drug or other agent is advantageous since sites of interest within the lung can be selectively targeted. Ideally, the volume of the bolus should match that of the targeted region allowing the aerosol bolus particles to be confined to the selected area during the pause period after inhalation. Our existing computer model for predicting the deposition of charged aerosol particles has been developed to encompass aerosol boli, some diseased lung morphologies and drug dose administered per breathing cycle. Aerosol deposition in the targeted region is found to be enhanced by increasing particle charge, pause period and particle size. For particles in the size range 1-2.5 microm, aerosol deposition in the region affected by bronchoconstriction does not alter significantly with flow rate variation (range 250-1000 ml s(-1)) for a targeted charged bolus of matched volume. The technique may enable the optimal delivery of therapeutic or other agents to diseased or normal lungs.

Characterisation of electrostatic discharges from insulating surfaces

This paper investigates how the presence of a grounded backing plane influences the electrostatic discharge (ESD) characteristics from a charged polystyrene sheet. High-speed digital sampling techniques are used to capture discharge current waveforms via a specially designed current probe, the resultant scans giving clear indication of how the surface charge distribution contributes to the ESD. Data obtained indicate that the presence of a grounded backing plate close to a charged plastic surface has a significant effect on the resultant discharge from the surface for a given initial surface charge density. The results suggest that the presence of an earthed metal backing plate can mean the difference between an incendiary and a non-incendiary ESD. This has implications for the design of plastic components for use in sensitive flammable atmospheres.

Some characteristics of liquid-to-metal discharges involving a charged “low risk” oil

Abstract Liquids possessing electrical conductivities greater than about 10 pS m −1 may be classified as “low risk” as regards hazardous levels of charge accumulation when loaded into grounded containers. The same liquids, when loaded into insulating containers, may give rise to liquid-to-metal discharges that are incendive either to vapor or to finely-dispersed oil mist in the vapor space. Using both theoretical and experimental approaches some characteristics of these discharges are evaluated in terms of measurable parameters; empirical ignition test data are employed to help establish safety criteria. When identifying “bad actors” in terms of tendency to static ignition it is recommended that both the liquid vapor pressure and the incidence of high negative charging levels be considered. In the present study, a negatively charged test oil produced energetic positive brush discharges accompanied by surface streamers. Positively charged oil yielded negative brush discharges which did not penetrate to the surface or produce surface streamers. The negative brushes carried an order-of-magnitude lower charge and were shown to be non-incendive during empirical ignition tests. Ignition of paraffin vapor by negative oil discharge occurred for surface potentials above 25 kV. High flash-point oil mists were ignited at surface potentials of about 60 kV, this being limited principally by the representative mist droplet diameter and the mist concentration in the discharge region.

The rosin—rammler size distribution for liquid droplet ensembles

Abstract Measurements of the distribution of particle sizes are often fitted to mathematical functions for convenience. The Rosin—Rammler volume distribution function is widely used in this way. Limitations in the use of this function accurately to fit experimental droplet distribution data are discussed. Under certain circumstances a “good” mathematical fit of a function to experimental data can lead to highly inaccurate calculations of such important characterizing parameters as number meart and volume mean diameters. The minimum and maximum droplet size limits chosen either due to resolution limitations of size measurement apparatus or for mathematical convenience can also have a drastic effect upon the accuracy with which such parameters may be determined.