Abstract

Electrostatic coalescence is the most efficient water-in-crude-oil demulsifying technology having been adopted in upstream dehydration and downstream desalting operations. Significant improvements in the capacity and performance of existing crude oil dehydrators/desalters have been achieved through continuous improvement in the mechanism research, the electric field development, and the practice of pre-coalescing step.

The first part of this paper presents the current understanding about the electrocoalescence mechanism of water droplet in crude oil emulsions up to now. Besides analyzing various forces acting on the water droplet at a macro-scale, it also discusses the influence of turbulent flow conditions and electric field voltage/frequency.

The second part of this paper presents systematically some typical technologies in electrostatic demulsification process and online electrostatic pre-coalescers up to now. For the electrostatic dehydrator/desalter, the authors pay more attention to the composite plate electrode, PC-based Load Responsive Controller, improved feed-stream distribution and variable voltage/frequency electric field. The variable voltage/frequency technology is the tendency which makes it possible for the customer to adjust the voltage and frequency simultaneously according to the corresponding physical properties of the inlet crude oil stream, thus the dehydration efficiency will definitely be improved. The typical representatives of compact electrostatic pre-coalescer which have achieved successfully field trial results, such as CEC, IEC, VIEC, highlight the moderate turbulent flow in their corresponding proprietary technologies, rather than the power supply.

Finally, some technological tendencies related with electrostatic coalescence of water-in-oil emulsions have been presented, such as integration with centrifugation, combination with flotation, etc.. Considering that heavy oils are becoming increasingly attractive from the stand point of reserves and economic status including China, the authors introduce the challenge and some successful trials on the electrostatic dehydration of heavy oils. Some suggestions are put forward as concluding remarks.

Introduction

The problem of water-in-crude-oil emulsions accompanies with the dawn of petroleum industry. In most cases, the presence of water droplets in crude oil is highly undesirable as it can lead to several problems, among which the oilfield surface qualification treatment (dehydration) and refinery plant upgrading process (desalting) have been mostly concerned. Therefore, the removal of entrained or emulsified water droplets from continuous oil phase has attracted much attention consistently during the past decades, and it will become more and more challenging with the pressure of limited source and unlimited control of processing cost.

Nowadays, possible separation methods of water-in-oil (W/O) emulsions include gravity or centrifugal settling, thermal treatment, chemical demulsification, pH adjustment, electrostatic demulsification, freeze/thaw method, filtration, membrane separation, microwave radiation and ultrasonic energy etc.. Compared with the other widely used or tried solutions, the electrostatic demulsification is becoming more and more popular, both from environmental and economical points of view. The fundamental principle or ultimate destination of electrostatic demulsification is to take full advantage of the Stokes' law,

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