Desalter Interface Control using BMA-System#30 pages

BMA-System#3 Technical Application Sheet
DESALTER INTERFACE CONTROL
It is all too easy to forget that the primary function of the desalting
system is the removal of inorganic chlorides and other water-soluble
compounds from crude oil.
One need not be a corrosion specialist to realize that the acids that
form from these compounds can do tremendous, long-term damage in
the downstream processes of the refinery (as the inspection of crude
tower overhead condensers can prove). However, the desalting
process has become the focus of attention in areas that are no longer
limited to the simple removal of salts and water.
Most critical of these recent areas of concern has been the degree to
which the desalting system contributes to the load of volatile organic
contaminants (VOCs) at the wastewater system. In fact, the condition
of the brine from a modern desalter is frequently under greater scrutiny
than the condition of the desalted crude.
There can be no doubt that the operation of the desalting system is an
exercise in compromise. A constant balance must be maintained
between mixing intensity, wash water quality, chemical demulsifier feed
and control of other parameters that can provide optimal salt removal
without forming an emulsion so tenacious that it compromises the
system’s dehydration capabilities. Adding to this balancing act the new
legislative demands placed on effluent water quality present the
operator with a difficult challenge.

Optimizing the desalting process is a matter of optimizing the individual components and maximizing the efficiency of the
electrostatic dehydration stage .By “pushing” the electrostatic process, one seeks to obtain the greatest amount of
electrical work possible. The work performed near the grids can provide the dual benefits of enhanced salt removal
(secondary mixing) and optimal coalescence.
The question therefore becomes how to maximize the electrical work of the grid.
In the vessel, the structural parameters such as vessel size, grid elevations and feed discharge points are all fixed. The
most critical remaining variables then become interface condition and position. In fact, optimal interface control has been
proven to have significant impact on both the oil and water quality resulting from the dehydration process. Yet, in spite of
the obvious need for such control, the traditional methods of control have operated on a fundamentally flawed
assumption: Level.
The very term “level control” indicates the presumption that the interface between oil and water in the desalter exists at a
single point (such as that observed between gasoline and water).
Any review of the internal conditions in the desalter vessel via the try-lines or swing-arm will dispel this notion. There is no
level, rather the interface consists of a transition zone from oil to water in a continuous change of volume percent.
Understanding the true nature of the interface leads to the conclusion that efficient control comes from controlling these
water/ oil percentages and not an imaginary level.

TASMZ90710BR1_BMASYS3.3 DESALTER.DOC

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