Interplay of Residence Time Distribution and Electric Chaining Tendency due to Distributor Design Determine Electrocoalescer Performance

Abstract

An electrocoalescer is a piece of equipment wherein an electric field is applied across a water-in-oil emulsion to enhance the separation of its constituent phases. The equipment is employed in all petroleum refineries, almost without exception, to produce dehydrated, desalted crude oil, which is safe and suitable for further processing in the downstream equipment. The performance of an electrocoalescer critically depends upon the electrohydrodynamics and hydrodynamics in the equipment vessel, which acts at two levels. First the droplet−droplet interaction, fundamental to the microphysics of separation of a water-in-oil emulsion, is affected by electrohydrodynamics of coalescence and noncoalescence. Second, at a more macroscopic level, the electrohydrodynamics and hydrodynamics in the equipment vessel jointly influence the local water volume fraction of the emulsion and the probability of formation/disruption of water droplet chains across electrodes, which if formed, can result in electrical short-circuiting in the electrocoalescer. In this work, we present some avenues to intervene in the electrohydrodynamics and hydrodynamics at the second level to improve the performance of a continuously operated electrocoalescer. The study was carried out in a 6 L capacity custom-built laboratory-scale electrocoalescer of a design similar to that of industrial electrocoalescers. The apparatus comprised a horizontal axis cylindrical vessel, two horizontal-plane electrode grids mounted one above the other, and a single horizontal pipe with multiple orifices along its length, acting as a feed distributor. The flow characteristics of this bigrid electrocoalescer were studied in terms of residence time distribution (RTD) curves by conducting tracer experiments and by carrying out particle tracking simulations using commercial softwares COMSOL as well as ANSYS-FLUENT. Alternative physical flow models comprising combinations of stirred tanks and tubular elements were proposed and analytical expressions were derived to best fit the experimental and simulation data. The findings were compared with results obtained by carrying out electric dehydration of a model water-in-oil emulsion using the same experimental setup. We demonstrate that the vertical position of the emulsion distributor with respect to the two-electrode grids and the particular direction in which the emulsion ejects into the vessel by virtue of the orientation of the orifices, significantly affect the dehydration performance. For lowwater cut emulsions, it was found beneficial to keep the distributor between the two electrodes. On the other hand, for high-water cut emulsion, it was better to place the distributor between the lower electrode and the emulsion−water interface at the bottom. The work demonstrates how the presence of a complex interplay between RTD and water droplet chaining tendency dictates the optimal position of the feed distributor and the best possible orientation of the orifice on the same, for achieving effective dehydration of water-in-oil emulsi...