Hydrocarbon Processing - October 2021 - 24

Plant Safety and Environment
Loss of aromatic product to remove
olefinic contaminants. The bromine
index (BI) is an indirect measure of olefins
content in an aromatic stream.6
For
more than 50 yr, low BI has been achieved
on C6
-C7 aromatics and on heavy reformate
by processing these streams through
clay treaters. Two clay treaters per stream,
operated in series or in parallel, are normally
present. The acid sites of activated
clays catalyze the alkylation of olefins
with aromatic molecules. The heavy compounds
resulting from this alkylation are
then fractionated out, and the overall process
is very efficient for olefins removal.
Furthermore, significant improvements
have been accomplished over the years,
with increasingly active acidified clays
and even zeolite-based catalysts proposed
for clay replacement.7
Despite treater technology advances,
the fundamental olefins alkylation chemistry
remains the same and is illustrated
in FIG. 2. Namely, one aromatic molecule
is consumed per alkylated olefin molecule.
In other words, some of the valuable
aromatic product obtained at high
cost via catalytic reforming is lost for
the purpose of removing olefin contaminants.
Moreover, reforming units in aromatic
mode (i.e. maximizing aromatics
output) usually operate at higher severity,
which subsequently yields higher olefins
content in the product. As producers
increase operating severity to produce
more aromatics, they also consume more
aromatics to remove olefins.
Two main clay treating processes are
present in typical aromatic complexes,
one on the benzene-toluene extract
stream and one on the heavy reformate
stream. BI is usually lower in the benzenetoluene
stream because most olefins in
C6
-C7 aromatics are removed through the
extraction process prior to the separation
of benzene from toluene. This is especially
true when N-formylmorpholine is the solvent
used in the extractive distillation process.
The examples presented later in this
article focus on BI removal from heavy
reformate, meaning C8
+ aromatic streams.
Selective hydrogenation to avoid aromatic
losses. Selective hydrogenation
applied to olefins removal from aromatic
streams is a simple, low-temperature process
operated in an inexpensive vessel using
a hydrogen (H2
reformer H2
) source-typically
-and a catalyst. Depending
FIG. 2. Examples of reaction pathways of olefins alkylation with aromatics in clay treaters.
on the unit location in a complex scheme,
additional hardware may be required.
The reaction pathways involved in selective
hydrogenation are illustrated in FIG. 3.
Two major benefits can be derived from a
molecule management perspective:
1. Reaction pathways do not
consume reformate aromatics;
therefore, valuable product is no
longer destroyed in the process of
removing olefins.
2. When the olefin to be removed
is an alkenyl aromatic, then
hydrogenation produces additional
valuable aromatics (e.g., the
ethylbenzene in FIG. 3 will be
converted to benzene or xylene
in a xylene loop). Based on heavy
reformate compositions provided
by aromatic sites for feasibility
studies, alkenyl aromatics can
account for more than 95% of
the olefins to be removed in
a C8
+ aromatics stream. This
means that the benefit of selective
hydrogenation vs. the clay treating
process is essentially doubled by
alkenyl aromatics conversion to
high-value products.
In addition to the aforementioned aroFIG.
3. Examples of selective hydrogenation reaction pathways for olefins removal.
24 OCTOBER 2021 | HydrocarbonProcessing.com
matics production benefits, selective hydrogenation
brings numerous advantages:

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Hydrocarbon Processing - October 2021

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