Hydrocarbon Processing - January 2021 - 43

Process Optimization
wash water flow and maintain a 1st-stage
interface level. Initially, APC became
very popular with the desalter operators.
As process water flow was being adjusted with the change of crude blend and
change of slop oil processing tank and
flow, the idea of wash water manipulation
became a discussion point as process
water flow was reduced or increased due
to changes in the water content in crude
oil or slop oil. However, with the performance of the desalter interface level with
wash water flow control in APC mode,
the brine flow remains absolutely steady,
with a slight variation in wash water flow.
Implementation of target opening non-linear (TON) control. Avoid

operating parameter adjustment by operators, such as the manipulation of level
control valve opening locking based on
brine flow estimation, or by APC for
wash water flow adjustment. Both options were working better than normal
but were not accepted by operation crew
members. A new idea was identified and
subsequently implemented for interface
level control: TON control, which works
based on level differences (set level and
actual level) with a correlation:2
The calculated output to the
interface level control valve =
(Actual level - set level) ×
M + target output.

The configuration of control logic
can be selected as: 1) for normal DCS
mode, or 2) for TON control mode.
This modification can be carried out in
the DCS with the help of an instrumentation engineer. Apart from the selection
of DCS or TON, three fields exist in the
DCS: target output, level setpoint and
factor M. The target output of the interface level control is basically the control
valve opening at which the estimated
brine flow is achieved. The level setpoint
is the desired interface level to be maintained-the actual level indication value
will be taken from the DCS. M is a factor
to be set once as a system requirement;
usually it will vary from 1-2.5.
Performance and benefit achieved.
The performance trend after the implementation of TON control, the interface
level and flow trend are shown in FIG. 3. It
can be seen during 8 hr of operation that

FIG. 4. Desalter 1st- and 2nd-stage interface level, valve opening and brine flow trend with
wash water flow during blend changeover with TON control with abnormal water contentment
along with crude oil.

the interface level, control valve opening and brine flow are steady. With this
steady brine flow, the following benefits
were achieved:
* No water carryover along with
the crude, and no water surge
during blend changeover
* Steady brine flow, resulting
in reductions in oil carryover
along with the brine
* With little to no interface level
control valve fluctuation, crude
flow ex desalter remained steady
* Steady desalter current and
no desalter transformer tripping
due to high current, even
during blend change.
Crude unit blend/crude changeover
is a common process and is done every
2 d-3 d. During each blend changeover, the desalter gets disturbed due to
changes in API, BS&W and free water
content (bottom portion crude tank).
It is observed during blend changeover
that brine flow varies abnormally, resulting in water carryover along with crude
during low brine flow; oil carry will be
high during high brine flow. In the case
of TON control, the brine flow variation
will be very low and post desalter crude
flow and brine flow will remains almost
steady, resulting in no disturbance during blend changeover. Only minor variations in the interface level, control valve
opening and brine flow were noticed
during the blend changeover period.
TON control also works well during abnormal water content in crude oil/crude

oil tank, as shown in FIG. 4. Here, only
one action was taken by the operator:
the reduction of process water, as brine
flow was increasing at a faster rate due to
very high water content in the crude oil.
Takeaway. With the challenges presented by a market for low-cost, high-TAN,
high-sulfur crude, the desalter must operate with maximum efficiency. TON
control is a viable solution for efficient
interface level control and improved
desalter performance. The cost for this
modification is minimal and it can be
performed easily within the DCS. This
modification has been implemented in
three sets of two-stage desalters of Nayara
Energy Ltd. at Vadinar, Gujarat, with
varying crude flow capacities of 350 m3/
hr-1,500 m3/hr, and all are working exceptionally well on a continuous basis.
LITERATURE CITED
1 Mandal, K. K., " New level control techniques, "
Hydrocarbon Processing, October 2004.
2 Mandal, K., " Improve desalter control, " Hydrocarbon
Processing, February 2005.
KISHORE KUMAR MANDAL is
a Technical Consultant for Nayara
Energy Ltd., in Vadinar, India, after
retirement from Indian Oil Corp.
as General Manager (TS) of the
Panipat Refinery, Panipat, India.
He has worked on different refinery
units, including atmospheric vacuum distillation,
FCC, DHDT and hydrocracker units. Mr. Mandal has
carried out assignments in commissioning,
operations, technical services and projects. He has
worked for Indian Oil Corp. for 36 yr. Mr. Mandal
obtained his B.Tech (honors) degree in chemical
engineering from IIT, Kharagpur, India.
Hydrocarbon Processing | JANUARY 2021 43

Hydrocarbon Processing - January 2021

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