Hydrocarbon Processing - February 2022 - 36

Process Optimization
issue can be addressed by changing the
column internals, which may not always
in the extractive distillation heat
exchanger circuit (the higher the
Increasing feed pre-vaporization results in
lowering vapor-liquid traffic in the stripping
section of the extractive distillation column,
which leads to reduced pressure drop and
reboiler duty, thereby debottlenecking the column.
be an economically viable solution. One
of the solutions could be to increase the
feed pre-vaporization. This would help
to reduce the reboiler duty, along with
the vapor-liquid traffic in the bottom section
of the extractive distillation column,
thereby decreasing pressure drop across
the column. The degree of pre-vaporization
is dependent on the percentage of
non-aromatics in the feed. This possibility
was checked by analysis of the extractive
distillation unit preheat circuit, using
a simulation. The extent of feed pre-vaporization
is primarily governed by the:
* Maximum feed temperature
attainable across the feed preheater
feed temperature, the greater
the extent of pre-vaporization)
* Ability of the feed line to sustain
the flow regime with increased
vapor flow
* Acceptable slippage of aromatics
and non-aromatics in the product
streams, as per the operating
plant's specifications.
Vaporizing the feed ensures that light
boiling components go up the feed tray
in the vapor phase. This avoids the light
non-aromatic components entering the
bottom section of the column, which
saves reboiler duty. The reboiler duty
goes down to maintain heat balance inside
the extractive distillation column,
and eventually decreases the vapor flow
through the trays between the feed
tray and the bottom tray. This helps
to reduce the pressure drop in the
stripping section of the column.
Therefore, feed pre-vaporization
might be a practical solution in case
the column is limited by flooding.
However, increasing the feed
vaporization does not always improve
the overall separation efficiency
of a distillation unit. Excessive
feed temperatures can cause a
significant amount of flash of heavy key
components at the distillation column
feed zone. The extent of pre-vaporization
will depend on the allowable specifications
of aromatics slippage in the raffinate
stream of the extractive distillation, as well
as non-aromatics slippage in the extract
stream of the solvent recovery column. In
addition, the capability of the feed line to
endure the increased vapor fraction must
be evaluated. Personnel must look at the
effect of feed pre-vaporization on the operation
of downstream units like the solvent
recovery unit (SRU).
Approach. The following methodology
was used for the case study:
* Perform a simulation of extractive
distillation and solvent recovery
columns using commercial
simulation software and an
appropriate thermodynamic
property model
* Check the possibility of feed
pre-vaporization, with the
help of simulation
FIG. 2. The impact of feed pre-vaporization on the reboiler duty of the extractive distillation
column.
* Perform the extractive distillation
column hydraulic study (pressure
drop studies) by increasing feed prevaporization,
and study its impacts
on reboiler duty and on aromatics
and non-aromatics separation.
Analysis of the extractive distillation
unit preheat circuit. To check the possibility
of feed pre-vaporization and the
maximum reasonable feed temperature
across the feed preheater, it was important
to analyze the extractive distillation
column's preheat circuit to check the adequacy
of all heat exchangers in the circuit.
Depending on the preheater duty margin
and flashing conditions, the authors used
simulation models to determine the feed
FIG. 3. Trends of V-L load below the feed tray with an increase in feed pre-vaporization percent.
36 FEBRUARY 2022 | HydrocarbonProcessing.com
pre-vaporization percent by increasing

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