Hydrocarbon Processing - January 2021 - 41

Process
Optimization
K. K. MANDAL, Nayara Energy Ltd.,
Vadinar, Gujarat, India

Search for advanced technique of desalter control-
TON control for interface level
While it may seem like a simple vessel,
the desalter is a critical section in a crude
distillation unit. Its purpose is to remove
salt and impurities from the crude oil.
Improving desalter performance will
increase reliability of the downstream
equipment, as well as secondary units.
Most crude available in today's market is
heavy and tough in nature, so the removal of salt and impurities from crude oil,
the reduction of oil carryover and brine
pose significant challenges.
Several articles are available that
offer valuable suggestions and best
practices about desalter operations, as
well as operational variables, including
chemical treatment/dosing. However,
very little literature is available on interface level control, how it can help to improve desalter performance with respect
to oil carryover along with brine, and
the maximization of wash water flow
for the reduction of salt. Advance process control (APC) systems are available
for almost all process units and working
well for quality prediction and process
parameter correction.1 This is not very
effective for desalter interface level control, as well as crude blend change (i.e.,
crude switch over).
The desalter is a closed vessel operating with pressurized condition at a
temperature of 140°C-145°C (284°F-
293°F) and an operating pressure of
10 kg/cm2-15 kg/cm2. Crude enters
the desalter vessel after preheating and
mixing with wash water (approximately
4%-7%). This water-mixed crude oil is
settled and gravity separated in the vessel with a residence time of about 15
min-20 min. One mixing valve is provided after the wash water injection to

improve the mixing of the water with
crude oil, and a 0.25 kg/cm2-1 kg/cm2
differential pressure is maintained. An
electric field is used to encourage water
separation inside the desalter vessel. Interface level indication for monitoring
and control was provided through an
interface level measurement instrument
and a control valve. The desalter can be
a single-stage or two-stage type-some
refineries are also installing three-stage
desalters. Details of single-stage desalter
arrangement are given in FIG. 1.
Analysis on desalter operation. For
desalter performance analysis, interface
level indications, control valve openings, brine flow and wash water flow
trends were considered for 1st and 2nd
stage for 8 hr of operation. Typical level
trends of a desalter interface level, control valve opening and flow indication
trend are shown for better understanding (FIG. 2). As seen from the trend, the
interface level and flow are both fluctuating steadily for both stages with steady
wash water flow. Interface level fluctua-

tion is seen more in the 1st stage compared to the 2nd stage.
Interface level control and interface
control are discussed here to understand
water balance/material balance of the
desalter system. Water balance is basically water input along with crude oil and
can be estimated based on basic sediment and water (BS&W) and wash water, and water output will be water content along with post desalter crude oil
and brine flow. Theoretically, some increase in brine flow is seen as extra water
content in crude oil will come out along
with the brine. The desired operational
requirement is that brine flow should remain steady; in turn, crude flow from the
ex-desalter will remain steady.
Desalter interface level control arrangement. The desalter interface level

control valve operates based on the interface level indication in LC control
mode [normal distributed control system (DCS)/proportional integral derivative (PID) control]. This control system operates on input and feedback, so
Crude oil ex-desalter

Clear crude oil

Float for level
measurement

Oil and water
mix (emulsion)
Crude oil desalter
Wash water injection

Clear water
interface
Mixing
valve

Brine
Brine level
controller

FIG. 1. Typical single-stage desalter configuration with mixing valve and water injection.
Hydrocarbon Processing | JANUARY 2021 41

Hydrocarbon Processing - January 2021

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