Hydrocarbon Processing - November 2022 - 34

Process
Optimization
K. K. MANDAL, Nayara Energy Ltd., Vadinar, India
Implement advanced level control techniques to
improve crude distillation unit stabilizer performance
The performance of refinery operations
depends on the stable operation
of the plant. The primary crude distillation
unit (CDU) generates a lighter
component from its overhead and processes
it into a stabilizer to separate liquified
petroleum gas (LPG) and naphtha
components. Therefore, any operational
improvement leads to improved refinery
margins by improving the product quality
and optimizing the reboiler heat duty.
The capacity of a particular CDU
was 18 MMtpy, and it processes different
types of crude with sulfur levels of
1 wt%-3 wt%. The production of LPG
and naphtha varies depending on the
type of crude processed. The overhead
vapor from the CDU is cooled and routed
to the overhead receiver vessel. However,
the condensed liquid from the overhead
vessel is partly routed to the column top
as a reflux to maintain the temperature,
and the balance liquid is sent to the recontacting
column to feed the stabilizer.
The overhead gases (e.g., uncondensed
material from the overhead vessel
top) were routed through a two-stage
wet gas compressor, where the gases were
pressurized up to 9.0 kg/cm2
-9.5 kg/cm2
;
after cooling, the overhead gases were
routed to the re-contacting column. Some
parts of saturated gases from other units,
such as vacuum gasoil (VGO) and diesel
hydrotreating (DHDT) units, were also
routed to the re-contacting column along
with the compressed gas. Finally, the liquid
hydrocarbon from the re-contacting
vessel bottom was routed to a stabilizer
for the separation of naphtha and LPG.
The main feed to the stabilizer was from
the CDU overhead via the re-contacting
column based on the vessel level control
in level flow cascade mode.
The stabilizer consists of a column, an
overhead vessel and a reboiler at the bottom
to maintain the stabilizer's bottom
S) before sending it to the
downstream unit to remove mercaptan.
The stabilizer bottom product stabilized
naphtha, which is then routed to
the naphtha hydrotreater (NHT) unit
for further processing. A level indication
was provided with a control valve in the
rundown line to maintain a steady level in
the overhead vessel and column bottom.
Unstabilized naphtha generated from
diesel hydrodesulfurization (DHDS),
DHDT and VGO mild hydrocracking
(MHC) was also directed to the stabilizer
feed pump suction to get processed
in the CDU stabilizer.
Typically in refineries, advanced process
control (APC) is implemented for
stabilizer operation, product yield and
quality optimization; however, due to
continuous disturbance in its operation,
APC was not implemented in this stabilizer.
Another reason for not implementing
APC is because unit operations were
more than the design capacity, resulting
in fewer margins in operating parameters
such as reflux flows, limitations in gas
compressors and cooling systems.
FIG. 1. The feed flow trend from DHDS, DHDT and VGO MHC units.
34 NOVEMBER 2022 | HydrocarbonProcessing.com
Problems. The CDU stabilizer performance
was unsatisfactory and was
operating with a significant amount of
disruption, such as a disturbance in LPG
quality (mainly LPG weathering), amine
carryover with LPG and hydrocarbon
carryover with an amine. An additional
knock-out drum (KOD) was provided
after amine treatment to arrest the amine
temperature. Parts of the cooled liquid
were routed to the column top as a reflux
to maintain the top temperature. The balanced
cooled product (e.g., LPG from the
stabilizer-overhead vessel) was routed to
an amine column for the removal of hydrogen
sulfide (H2
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