Hydrocarbon Processing - February 2022 - 37

Process Optimization
feed temperature. The authors compared
the results for cases with 0%, 0.6%, 3.6%,
5.6%, 7.6% and 18.6% of feed pre-vaporization.
Reboiler duty and specifications
of the end products were checked for each
scenario. In addition, the transition in
flow regimes with vaporization was studied
to inspect the feed line's effectiveness.
Impact of feed pre-vaporization on
reboiler duty. The impact of feed vaporization
to offload reboiler duty in distillation
columns is discussed in literature.2
Likewise, the authors' simulation for the
case study indicated a reduction in reboiler
duty with increasing feed pre-vaporization
(FIG. 2). A further reduction in reboiler
duty from the existing level would depend
on the prospects of pre-vaporization due
to the reboiler margin and the preheating
of feed to the extractive distillation unit.
Impact of feed pre-vaporization on
pressure drop. The increase in feed prevaporization
decreases vapor-liquid (V-L)
traffic in the extractive distillation column,
especially in the stripping section (below
the feed tray). Using simulation software,
stagewise V-L profiles were generated.
The decreasing trends for vapor and liquid
load below the feed tray to the reboiler
for each percent vaporization are shown in
FIG. 3. The V-L load is consistent above the
feed tray. The impact of pre-vaporization
on pressure drops in the stripping and rectification
sections of the column, and on
pressure drops of the entire column, is depicted
in FIG. 4. It is evident that, with increased
feed vaporization, pressure drops
in the column could be reduced further.
Impact of feed pre-vaporization on
aromatics separation. The increase
in feed pre-vaporization might affect
separation efficiency of the extractive
distillation column. When it comes to
extractive distillation separation, for the
raffinate phase, the loss of aromatic hydrocarbon
components must be monitored.
Conversely, in the extract phase,
non-aromatics content is an important
criterion, since it affects final product
quality. The impact of feed preheating
on both parameters is depicted in FIG. 5.
FIG. 4. The impact of feed pre-vaporization on pressure drop of the extractive distillation column.
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Hydrocarbon Processing - February 2022

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