Hydrocarbon Processing - September 2021 - 77

Process Optimization
Implementation of TON control. The
system was working better after the installation
of the control valve. However,
oil carryover continued. To improve
operations, TON control was implemented
for sour water interface level
control. This simple modification was
configured in the DCS, and the system's
performance is now highly encouraging
and satisfactory.
Benefits. After implementing TON
control, a drastic reduction in oil carryover,
along with sour water, was noticed.
An additional advantage of TON
control is that flow increases based on
the level difference-hence, additional
separation volume for higher flow. A reduction
in sour water flow variation resulted
in less level variation in the SWS
surge vessel level. The blowdown receiver
sour water level, control valve opening
and flow trend (24-hr operation) are
provided in the bottom section of FIG. 2.
Brief description of the amine regeneration
unit (ARU). The ARU
plays a critical role to maintain product
quality, especially for the removal of
H2
S from products like LPG and FG.
The ARU supplies regenerated lean
amine to the DCU, the FCCU and the
CDU/VDU amine scrubber. Similarly,
rich amine is sent to the ARU from the
amine scrubber (i.e., DCU LPG and
FG, FCCU FG, CDU LPG and FG, and
VDU offgas). Rich amine accumulates
in a surge vessel for the removal of hydrocarbons,
if any. It is then routed to
a stripper for the removal of H2
S. The
stripper bottom's temperature is maintained
by the steam reboiler. Overhead
acid gas from the regenerator (H2
S with
water vapor and other gases) is cooled
through an air and water cooler, and
then routed to the overhead vessel. Acid
gas from the overhead vessel is routed
to the sulfur recovery unit (SRU). A
differential pressure indication across
the column is provided to understand
amine carryover, along with the gas and
performance of the amine stripper.
Analysis of operation. The system
operates in a steady manner; however,
sudden increases of differential pressure
across the column can sometimes be observed.
This is an indication of amine
carryover, along with acid gas, resulting
in a disturbance in the SRU. On analyFIG.
3. ARU overhead vessel level, flow, control valve opening and reboiler steam flow trends
before and after TON control.
sis of the ARU overhead vessel level,
flow and opening trends, a fluctuation
was noticed with the reboiler steam flow
control valve. The overhead level, flow
and control valve opening trends are
shown in the top section of FIG. 3. Based
on these observations, TON control logic
was implemented. The performance
after implementation of TON control is
shown in the bottom section of FIG. 3.
Benefits. Steady overhead level control
resulted in a steady acid gas flow
to the SRU and the elimination of abnormal
and/or sudden increases in differential
pressure across the column.
The column temperature and pressure
are steady due to steady reboiler steam
flow and top reflux flow, which improves
amine regeneration.
Approximately 60% of the rich amine
comes from the DCU FG scrubber and
LPG scrubber, and the gas generation
rate fluctuates by 12%-15% during coke
drum changeover. It was observed that
the incoming rich amine flow to the ARU
was varying to some extent and that the
combined flush vessel provided for FG
and LPG scrubber level control was not
working properly. TON control was implemented
for the FG scrubber and flush
vessel level control. Both are working satisfactorily
in auto control mode, and rich
amine flow variation was reduced.
Based on one ARU's performance,
two more ARUs in the refinery-used
for supplying amine to hydroprocessing
units and to catalytic reforming units
(CRUs) and SRUs-have implemented
TON control for overhead vessel level
control.
Takeaway. Refinery units like the RFCCU
and the VGO MHCU are crucial
for optimization of refinery performance
and maximization of profitability. Stable
operations of these units will help to increase
refining margins. Additionally,
stable operations of the sour water vessel
level will reduce oil carryover and
sour water, which will improve SWS unit
performance. Improved performance
of the ARU will increase H2
from rich amine and H2
S removal
S recovery from
FG, LPG and recycled gas will increase.
TON control has been implemented in
the RFCCU and VGO MHCU bottom
levels; three ARU overhead levels; the
DCU FG amine scrubber and flash vessel
levels; and the DCU blowdown overhead
sour water level control at Nayara
Energy Ltd.'s refinery in Vadinar, Gujarat,
India. All are working exceptionally
well on a continuous basis.
KISHORE KUMAR MANDAL is a
Technical Consultant for Nayara
Energy Ltd. Prior to Nayara Energy
Ltd., he worked for Indian Oil Corp.
for 36 yr, retiring as General
Manager of Indian Oil Corp.'s
Panipat Refinery. Mr. Mandal has
worked on different refinery units, including the
atmospheric vacuum distillation unit, the FCCU, the
diesel hydrotreater (DHDT) unit, and the
hydrocracking unit. He has also carried out
assignments in commissioning, operations, and
technical services and projects. Mr. Mandal earned his
BTech degree in chemical engineering from the Indian
Institute of Technology (IIT) in Kharagpur.
Hydrocarbon Processing | SEPTEMBER 2021 77
Hydrocarbon Processing - September 2021

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