Hydrocarbon Processing - March 2022 - 25

Special Focus Petrochemical Technology
F. CLAIRE and A. COTTE, Axens, Rueil-Malmaison, France;
and M. MOLINIER, Axens, Houston, Texas
Enhance aromatics production with concurrent
reduction of environmental footprint-Part 2
Part 1 of this article (October 2021)
Material balance improvements are
discussed how catalytic reforming processes
produce olefin contaminants in
aromatics streams via paraffin dehydrogenation
side reactions. Operating catalytic
reformers at higher severity results in
higher yields of valuable aromatics, but it
also results in higher olefins content in the
reformate. Olefins must be removed from
C6
-C7 aromatics streams to meet benzene
product specifications, and from C8
+ aromatics
streams to meet specifications for
paraxylene recovery processes.
That article presented key features
and performances for the olefins selective
hydrogenation process, and options for
its implementation in an aromatics plant
block flow diagram. It also described the
benefits of selective hydrogenation addition
to an existing facility. The benefits of
selective hydrogenation integration in a
grassroots facility will be addressed here.
Olefins selective hydrogenation integration
in the aromatics block of
a crude-to-chemicals complex. The
most significant benefits achieved by adding
an olefins selective hydrogenation
process to an existing aromatics complex
have been recently described1
and can be
summarized as:
* End the downgrade of valuable
aromatic molecules to fuel oil in the
process of removing olefins
* Yield additional valuable aromatic
molecules by hydrogenation of
alkenyl aromatics
* Lower the environmental footprint
of aromatic plants by considerably
reducing solid waste production.
Furthermore, process selectivity-
that is, olefins hydrogenation without loss
of aromatic rings-is key to additional
aromatics production.
incremental; they can be very substantial
for smaller existing sites, but they
become of immense proportion when
olefins selective hydrogenation is part
of the aromatics section design in a new
crude-to-chemicals (CTC) complex, due
to the large production capacity of these
facilities. Similarly, the environmental
benefits are massive, as the generation of
millions of kilograms (kg) of clay waste
can be avoided every year.
The olefins selective hydrogenation
unit can be integrated upstream from the
reformate stabilization column for maximum
advantages. In this configuration,
the full reformate stream is processed,
yielding the following benefits:
* The reduced olefins content
in the feed to the extraction unit
results in lower solvent and
energy consumption for the
extraction process
* Aromatics preservation is maximized
on both the C6
and C8
/C7
aromatics stream
+ aromatics stream
* Aromatics net gain is maximized
on the C8
+ aromatics stream
* Diolefins (which may shorten clay
cycles2
both the C6/C7
and C8
+ aromatics stream.
This arrangement takes advantage
of synergies such as desirable feed temperature
and the use of the reformate
stabilization column for removal of unreacted
hydrogen and light ends, without
any need for a flash drum or other
stabilization means. Therefore, the lowcost,
once-through process requires
minimum equipment, namely a vessel
and a heat exchanger. The impact on the
stabilization column design is negligible
because olefins to be hydrogenated rep)
are efficiently removed on
aromatics stream
CCR
stab.
Unstabilized
reformate
Stabilized
reformate
Make-up H2
FIG. 1. Olefins selective hydrogenation unit
inserted between the CCR and the reformate
stabilization column.
resent a small fraction of the stream to
be processed and the technology operates
at low excess hydrogen. The source
of make-up hydrogen is usually the catalytic
reforming unit. The configuration
is depicted in FIG. 1.
Heavy reformate stream: Incremental
aromatic production doubled.
Using
selective hydrogenation
rather
than clay treating for olefins removal [or
bromine index (BI) removal] from aromatic
streams prevents the loss of valuable
aromatics in the benzene/toluene
and heavy reformate streams. However,
as previously reported1
, the olefins to be
removed in a heavy reformate stream are
predominantly alkenyl aromatics. Plant
sample analyses suggest that the prevalent
molecules are methyl styrene, dimethyl
styrene and styrene, accounting
for most of the heavy reformate olefins
content. FIG. 2 illustrates how these molecules,
when hydrogenated via selective
hydrogenation rather than being converted
to heavy aromatics by alkylation with
aromatics of equivalent molecular weight
in clay treaters, contribute to the overall
complex aromatics production. Namely:
Hydrocarbon Processing | MARCH 2022 25
Ofefins sel.
hydrogenation
Hydrocarbon Processing - March 2022

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