Hydrocarbon Processing - November 2022 - 35

Process Optimization
carryover with LPG. The LPG rundown
flow control was not working smoothly in
auto cascade mode, so some was
kept in manual control mode for
less flow variation.
sulting in amine carryover with LPG
(upper half of FIG. 3).
stabilizer overhead in the DHDT unit for
a reduction in wild naphtha and unstabiAnalysis
of feed to the stabilizer.
The incoming stream
flow was set for 24 hr of operation,
and the product outgoing
stream flow was critically analyzed
to identify the problems and improve
the stabilizer performance feed.
Approximately 80%-85% of the CDU
Improving the operation of stabilizers increases profit margins
by reducing product quality giveaways. TON control is an
ideal tool for achieving maximum improvement in stabilizer
performance. TON control is also beneficial for any level of
control for steady flow to the downstream section.
stabilizer feed was from CDU overhead
and unestablished naphtha, and about
15%-20% of total feed was from other
units like wild naphtha from DHDS,
DHDT and VGO MHC stripper. Wild
naphtha flow from DHDS, DHDT and
unstabilized naphtha flow from VGO
were analyzed (upper half of FIG. 1).
Primarily, the CDU, overhead naphtha
(80%-85%) and compressed unsaturated
gas were routed to the re-contacting
column from the main CDU fractionator
and the subsequent column bottom
was routed to the stabilizer as feed. The
feed flow from the CDU overhead and
the re-contacting column was critically
analyzed, and a wide flow variation was
noticed for the CDU overhead liquid
flow (upper half of FIG. 2).
Wild and unstabilized naphtha from
DHDT, DHDS and VGO MHC flow
variation was continuous, causing trouble
in stabilizer operation. Feed coming to
the re-contracting column from the CDU
overhead was a wide flow variation with
continuous disturbances.
In the crude unit, blend changeover
is a common requirement and is done
every 2 d-3 d. It is also observed that
higher levels of disturbance occur during
this blend changeover. The disturbance
from re-contacting the column to the
stabilizer was less than the CDU overhead
feed flow.
Analysis of product flow from the
stabilizer. The rundown product from
the stabilizer bottom and overhead vessel
with the downstream LPG, an amine
extraction column and LPG KOD level
were analyzed critically with the flow.
The stabilizer bottom level and flow
were stable with a marginal variation.
However, the overhead vessel LPG
level and flow variation were wide, reFIG.
3. The stabilizer overhead vessel product flow, level and control valve opening trends.
Hydrocarbon Processing | NOVEMBER 2022 35
Implementation of target opening
nonlinear (TON) control. The feed
and product flow fluctuations were marginally
higher in the normal distributed
control system (DCS), as stated above in
the flow trends. It was decided to implement
TON control for performance
improvements of selected level control
where flow deviation was on the extreme
side, such as the stripper overhead in the
DHDS and VGO MHC units and the
lized flow variation. TON control was
also considered for the CDU overhead
flow to the re-contacting column level
and stabilizer overhead level control to
reduce flow variation.
TON control is a correlation-based
control that works based on the level difference,
set level and actual level. The correlation
used for level control is (Eq. 1):
CVOP
= (LA
- LS) × M + TOP
(1)
FIG. 2. The level and feed flow from the CDU overhead trends.
Hydrocarbon Processing - November 2022

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