posted on 2024-01-08, 19:08authored byPramod
P. Kothmire, Vijay M. Naik, Rochish M. Thaokar
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 low-water 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 emulsion
in an electrocoalescer.