Hydrocarbon Processing - March 2022 - 26

Petrochemical Technology
* Methyl styrene is converted to
methyl ethylbenzene via selective
hydrogenation. Then, in the
transalkylation process, methyl
ethylbenzene is dealkylated to
produce a toluene molecule. Finally,
still in the transalkylation process,
toluene is transmethylated either
with another toluene to yield a
benzene and a xylene molecule, or
with a trimethylbenzene to yield
two xylene molecules (FIG. 2A).
* Dimethyl styrene is converted to
dimethyl ethylbenzene via selective
hydrogenation. Then, in the
transalkylation process, dimethyl
ethylbenzene is dealkylated to
produce a xylene molecule (FIG. 2B).
* Styrene is converted to ethylbenzene
via selective hydrogenation. If the
complex operates an ethylbenzenedealkylation
isomerization,
ethylbenzene is converted to
benzene, while if the complex
operates an ethylbenzene-reforming
isomerization, ethylbenzene is
converted to xylene (FIG. 2C).
Consequently, with olefins selective
hydrogenation on the heavy reformate
stream, not only is aromatics consumption
for olefins removal avoided, but olefin-substituted
aromatics are converted to
additional valuable aromatics.
Process conditions, catalyst, selectivity
and impact on C6
/C7
and C8
+
aromatic streams. Since typical olefins
content in reformate aromatic streams is
in the order of 1%-2% by weight3
In the
, the hydrogenation
process is only useful if the
olefins can be hydrogenated selectively,
i.e., without simultaneous hydrogenation
of aromatic compounds.3,4
literature, palladium catalysts have been
reported to be more selective than nickel
catalysts.4,5
In pyrolysis gasoline hydrogenation,
palladium catalysts are usually
preferred for the selective hydrogenation
of styrene and diolefins, while nickel catalysts
are preferred when some amount of
aromatic saturation is targeted.5
Further,
conditions such as mild temperature and
low excess hydrogen favor olefins hydrogenation
while minimizing aromatic
saturation, and also result in slower coke
deposition, allowing longer cycles and
softer coke (lower C/H ratio) formation
that renders catalyst regeneration easier.
Diolefins selective hydrogenation, as
well as the selective hydrogenation of
the olefinic function of olefin-substituted
aromatics, are achievable at milder
conditions than the selective hydrogenation
of aliphatic olefins, because the
former molecules adsorb more strongly
on catalysts' surface than the latter.5,6
An
olefins selective hydrogenation process
positioned upstream from the reformate
splitter must operate at mild conditions
to avoid undesirable aromaticity loss.
This drives BI removal efficiency differences
on the C6
/C7
and C8
+ aromatics
streams, but these differences work to
the advantage of the user due to the aromatics
complex configuration:
* On the heavy reformate stream
(C8
+ aromatics), where BI mostly
results from the presence of
alkenyl aromatics, the BI removal
achieved by selective hydrogenation
processing the full reformate stream
is very high. Moreover, this stream
is diluted downstream of the clay
treater by the isomerate recycle
C8
aromatics stream containing
very low BI, and-where a
transalkylation unit is part of the
process flow scheme-diluted by
the transalkylate C8
+ aromatics
stream, which also contains very
low BI. Finally, any heavy olefinic
species is removed from the xylene
loop via the xylene splitter bottoms.
All these conditions ensure that
the BI feed specification to the
paraxylene recovery unit is achieved
for extremely long periods. Hence,
the clay treater becomes a guard
bed present for infrequent upsets
or unusual feed contaminants;
under normal conditions, the clay
treater is expected to operate
for many years without any need
for clay change-out.
FIG. 2. Reaction pathways for the production of additional valuable aromatics produced by
olefins selective hydrogenation.
26 MARCH 2022 | HydrocarbonProcessing.com
* On the light reformate stream,
BI mostly results from the
presence of aliphatic aromatics.
BI removal achieved by selective
hydrogenation processing the full
reformate stream is substantially
lower than on heavy reformate.
However, light reformate is then
processed in the extraction unit
where additional BI is removed.
The combined BI reduction
by selective hydrogenation and
aromatics extraction process yields
a benzene/toluene extract stream
with extremely low BI. Here as
well, the clay treater becomes a
guard bed essentially present for
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