Hydrocarbon Processing - July 2021 - 74

Digitalization
debutanizer column. The aim of this recycle is
to optimize column separation. Proper recycle flowrate
is crucial for debutanizer operation.
* Debutanizer column feed rate. It was observed that
the debutanizer column performance was fluctuating
at a low debutanizer feed rate. Therefore, a column
tray analysis was performed, and the weeping condition
in all trays was obtained.
* Debutanizer column bottom temperature.
The temperature at the bottom of the debutanizer
was also one of the parameters affecting column
performance. For the debutanizer top section load,
the bottom temperature and reboiler duty needed
to be optimized.
Simulation of the coker debutanizer and stripper columns.
A commercial simulation program was used to achieve
accurate simulation models on the stripper and debutanizer
sections of the DCU. The configuration of the simulations
covered rigorous column, condensers (heat exchangers and air
coolers), reboilers, drums, pumps, valves and pipe segments.
The first step in simulation modeling of the unit was to
carry out several selections and identifications. To estimate
the properties of hydrocarbons, water and steam, appropriate
property packages should be used. In this study, the PengRobinson
equation of state (EOS) and ASME steam fluid
packages were used. A unit process flow diagram was the basis
for the simulation flowsheet.
The debottlenecking study via simulation models had the
following primary equipment: a stripper column, a debutanizer
column, condensers (heat exchangers and air coolers), reboilers
and overhead receiver drums. The primary goal of the
study was to investigate and provide solutions to a bottleneck
problem in the debutanizer column section. Product back
blend was used in all simulation case studies at the beginning
of the detailed analysis. The base model of the stripper and
Air cooler
Debutanizer
overhead drum
debutanizer section of the unit was completed (at steady-state
conditions, according to normal operational conditions) and
then used for the bottleneck analysis.
Simulation model validation. Simulation models were validated
according to the unit's design data or field data before
they were used in the study. The steps used during this validation
are summarized here:
1. Generation of the model, using field data: Plant
data (such as distillation, flowrates and operational
data) and equipment process data sheets were used
to configure the model. The debutanizer and stripper
column models were completed according to steadystate
operational conditions via two different data sets.
Using different data sets, feed conditions were varied
to obtain the most validated simulation models and
to witness the effects of operational parameters on
process dynamics, such as bottleneck conditions.
2. Validation of the model results: Based on predefined
validation limits, the simulation results were validated
according to plant data. Critical parameters for
validating the simulation model include:
* Product flowrates (LPG, light naphtha and
heavy naphtha)
* Product specifications for light naphtha, heavy
naphtha (ASTM D86 T5-T95) and LPG (C5
* Condensers (heat exchangers and air coolers)
and reboiler outlet temperatures.
3. Acceptance of the deviations from actual data:
When the model was compared with plant data,
the results were acceptable if the difference was
±5%. These criteria were based on possible
measurement errors and general refinery practice.
LPG
Stripper
HP
HP
Gasoil
Naphtha
Debottleneck analysis. Before evaluating alternative solutions
for a bottleneck analysis, the unit base model was created
according to the 8-hr average values of a day when product
and charge analyses were made, and production was stable.
Because bottleneck problems occurred at low flowrates, the
debottleneck study was performed via turndown simulation
models. From the simulation model results, the authors observed
the following:
* The simulation results showed weeping in all trays
of the column at turndown conditions.
* The lower feed was raised to higher temperatures
in the reboilers of the debutanizer column, with vapors
remaining nearly unchanged at the top (given the
weeping conditions).
* If the column began to weep, then there was no
equilibrium in the trays, and the vapor from the
feed (including the vapor produced by the reboiler)
bypassed the trays on its way up the column.
This typically causes higher pressure and very low
tray efficiency.
FIG. 1. Overview of the debutanizer and stripper sections of the DCU.
74 JULY 2021 | HydrocarbonProcessing.com
* If the reboiler duty was kept the same, then more
light components could reach the top of the column,
thus increasing pressure and tripping the PSV.
limit)
* Temperature and pressure profiles of the stripper
and debutanizer columns
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