Hydrocarbon Processing - October 2021 - 23

Special Focus Plant Safety and Environment
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 1
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
zene product specifications, and from
C8
Process background. The recent paraxylene
capacity increase in China is adding
pressure on existing aromatics complexes
to improve overall efficiency to
remain competitive. The very large scale
of the plants coming onstream1,2
means
-C7 aromatics streams to meet ben+
aromatics streams to meet specifications
for paraxylene recovery processes.
Historically, olefins have been removed
from aromatic streams in treaters
using activated clay to promote the
acid alkylation of olefins with aromatic
molecules, thereby producing heavier
compounds further separated by fractionation.
While the process is very
efficient, it inherently consumes valuable
aromatics to remove undesirable
olefins. The higher the olefins content,
the higher the product loss. Given the
current scale of paraxylene facilities,
valuable product losses, even in small
percentages, can have a massive impact
on plant economics.
Selective hydrogenation converts olefins
to paraffins and alkylaromatics via a
reaction pathway that does not consume
aromatics. As a result, aromatics yields
are substantially increased, while the use
of clay, which may account for up to 80%
of the solid waste in a paraxylene complex,
is dramatically reduced. This article
presents key features and performances
for the selective hydrogenation process,
and options for its implementation in
a plant block flow diagram. It also describes
the benefits of selective hydrogenation
addition to an existing facility.
The benefits of selective hydrogenation
integration in a grassroots facility will be
addressed in a separate article.
that even small losses in terms of percentage
can have a huge economic impact on
these new complexes. Optimized heat
integration, reduced utilities consumption,
improved selectivities and reduced
investments are common themes for all
processes at sites currently in operation,
as well as for new aromatics facilities.
Since
the
adsorption
+ aromatprocess
has
grown into the industry workhorse for
paraxylene separation, the removal of
olefins contaminants from C8
ics streams has become a critical step,
because even small olefins concentrations
can adversely impact the recovery
unit's capacity. Furthermore, product
benzene has a very low specification
on acid wash color, which is a measure
of unsaturated impurities;3
removal of olefins from C6
therefore,
aromatics
streams is also a necessary step.
Reformed naphtha (or reformate)
is the primary feed source for aromatic
complexes.4
In the naphtha reforming
process, side dehydrogenation reactions
of paraffins or paraffinic substituents, as
shown in FIG. 1, can take place in addition
to the desired naphthene dehydrogenation
and paraffin dehydrocyclization reactions.
Furthermore, the transition from
high-pressure semi-regenerative reforming
to low-pressure continuous catalytic
reforming has resulted in a substantial
increase of unsaturated species in reformate-derived
streams.5
The concentration
of these contaminants is a function
of reformer feed composition, the severity
of the reforming operation and the
catalyst used in the reforming process,
among other elements.
FIG. 1. Examples of dehydrogenation reactions producing olefins in a catalytic reforming unit.
Hydrocarbon Processing | OCTOBER 2021 23

Hydrocarbon Processing - October 2021

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