Hydrocarbon Processing - September 2021 - 76

Process Optimization
flow and control valve opening trends
for the bottom product are shown in the
top section of FIG. 1. Due to the bottom
rundown flow variation, other critical
operating parameters were observed,
such as the variation in the slurry recycle
flow and preheat temperature and
a slight fluctuation in the reactor outlet
temperature. Due to the variation in reactor
temperatures, a slight reduction in
conversion was noticed, resulting in a reduction
in LCO yield.
Implementation of target opening
nonlinear (TON) control. As bottom
level and slurry flow variations were
marginally high in the PID control system,
it was decided to implement TON
control for performance improvement of
bottom level control. TON control is a
correlation-based control, and it works
based on the differences of set level and
actual level. The correlation used for level
control is shown in Eq. 1:
CVOP = (LA
- LS) *M + TOP
(1)
The calculated output to the bottom
rundown flow control valve (CVOP
[Actual bottom level (LA
(LS
) =
)] × M + target output (TOP
) - set level
).
Control configuration can be selected
as:
1. Normal distributed control
system (DCS) mode
2. TON control mode.
This modification can be carried out
in the DCS, with the help of an instrument
engineer. Apart from the selection
of DCS or TON, three fields exist in
the DCS: target output, level set point
and factor M. Target output is the average
opening of the bottom level flow
control valve. The level set point is the
desired bottom level to be maintained.
The actual level indication value will be
taken from the DCS. M is a factor to be
set once as per operating practice and
flow variation with level, which is very
similar to controller gain. It will usually
vary from 1-2.5.
Benefits. After implementing TON
control, the bottom level and flow controller
performances were found to be
highly encouraging. The performance
trend after implementation is shown in
the bottom section of FIG. 1. After implementing
bottom level monitoring with
TON control, the other bottom circuit
flow (e.g., slurry recycle to the reactor,
bottom pumparound and quench flow)
became stable due to a steady slurry
draw rate. After implementing TON
control, LCO dropping to the bottom
was eliminated due to steady slurry rundown
flow. This resulted in a steady reactor
outlet temperature and an improvement
in conversion, with a reduction in
slurry yield of 0.5%-1%.
Based on RFCCU bottom level performance
and experience, a similar TON
control was implemented in the VGO
mild hydrocracking unit (MHCU) bottom
level control. Bottom level control
was in level controller/flow controller
(LC/FC) cascade control, with rundown
flow to the tank. The problems
were level and flow variations in a cyclic
pattern due to the inherent character of
PID control, and the maximization of
hot feed was limited due to the rundown
flow variation. TON control was implemented,
providing a 10%-12% increase
in hot feed to the RFCCU.
Brief description of DCU coke drum
operation. Coke drum changeover
in the DCU is a normal process. After
completion of the coking cycle, the coke
drum switchover takes place. After isolation
of the coke drum, there are two
operational steps before coke cutting:
stripping for the removal of H/C from
the coke drum and cooling of the coke.
During stripping, steam is introduced
from the bottom of the coke drum, followed
by process water for cooling.
Blowdown material from the coke drum
is routed to the blowdown tower, where
tower overhead material is collected in
an overhead receiver vessel after cooling.
The receiver vessel has two separate
compartments with level indication (i.e.,
for hydrocarbon and sour water). Hydrocarbon
is pumped to the blowdown
column for quenching, and sour water
is pumped to the sour-water stripper
(SWS) unit for further treatment. There
are two pumps: one is for hydrocarbon
pumping, and the other is for sour water
pumping. As coke drum switchover is
a batch operation, both sour water and
hydrocarbon pumps will operate (start/
stop) based on minimum and maximum
set level indications.
Analysis of operation. Oil carryover
and sour water were occasionally at elevated
levels due to the pump running at
maximum capacity, along with frequent
starts and stops of the pump. To avoid
frequent starts and stops, one control
valve was provided for the sour water
pumping system, with the objective of
reducing oil carryover and sour water.
Sour water level control was done in LC
control mode with the new control valve.
After installation of the control valve, oil
carryover and sour water were reduced
but could not be eliminated due to the
continuous fluctuation of sour water
flow. Fluctuation in sour water generation
was 15 m3
/hr-400 m3
FIG. 2. DCU blowdown vessel interface level, C/V opening and flow trends before and after
TON control.
76 SEPTEMBER 2021 | HydrocarbonProcessing.com
/hr. The level
and flow fluctuation are shown in the
top section of FIG. 2.
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Hydrocarbon Processing - September 2021

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