Hydrocarbon Processing - March 2022 - 28

Petrochemical Technology
Bromine index
be regenerated with very high activity recovery
and reused for subsequent cycles.
Assessing small incremental improveBI
outlet
BI inlet
ments can be challenging in a pilot plant
and even more in a commercial environment.
Data are usually scattered with a
significant error bar, because small variations
on large gas chromatography peaks
are difficult to quantify10
and also because
50
100
150
200
250
300
350 400
FIG. 4. BI removal by a commercial
selective olefins hydrogenation unit on
a full reformate stream in the aromatics
block of a CTC complex.
Cumulated valuable aromatics gain through
selective hydrogenation reactor
such data are subject to flowmeter calibration
variations. However, an attempt
to measure valuable aromatic molecules
gain in the plant through a selective olefins
hydrogenation unit, as the result of
the conversion of molecules such as styrene,
methyl styrene and dimethyl styrene
to alkyl substituted aromatics (as
discussed earlier), is shown in FIG. 5. Time
onstream monitoring suggests close to
constant and consistent additional valuable
aromatics generated via olefins selective
hydrogenation throughout the year of
data displayed in FIG. 5.
50
100
150
200
250
Day on stream
FIG. 5. Valuable aromatics gain by alkenyl
aromatics hydrogenation through a selective
olefins hydrogenation unit processing a full
reformate stream.
clay treater cycles can range from a few
weeks up to 12 mos in aromatic plants.8,9
However, in this commercial CTC facility
in service for 3 yr, clay has never been
replaced in either C6
/C7
or C8
+ aromatics
service and the temperature has never
been increased to compensate for loss of
activity, meaning that both treaters are fed
with streams containing extremely low BI
levels. The clay in these treaters is expected
to remain active for a long time.
FIG. 4 shows the stable operation as
a function of time onstream of one of
the selective olefins hydrogenation units
in use at this facility. As can be seen, the
site has decided to target 65%-70% olefins
conversion to achieve (1) extremely
low BI on the C6
-C7 stream following
additional olefins removal in the extraction
unit, (2) extremely low BI on the C8
aromatics stream via selective hydrogenation
conversion combined with dilution
effects explained above, (3) undetectable
aromatic ring loss through the selective
hydrogenation process, and (4) catalyst
cycle exceeding complex turnaround requirements
(≥ 5 yr). The catalyst can then
28 MARCH 2022 | HydrocarbonProcessing.com
300
350 400
Selective olefins hydrogenation benefits
in a CTC complex: Selectivity is
key. The integration of a selective hydrogenation
process on the full reformate of the
aromatics block in a CTC complex offers
massive benefits. For a 4-MMt paraxylene
plant, additional production at constant
feed rate exceeds 110,000 tpy when only
taking into consideration the heavy reformate
stream. This does not include the incremental
benzene/toluene production on
the light reformate stream, nor the incremental
production obtained by avoiding
frequent clay change-outs and associated
start of cycle periods at high xylene losses.
For
the
commercial operation described
in this article, it is estimated that
3 yr of operation without any clay changeout
have already spared the purchase of
about 7 MMkg of clay, as well as associated
solid waste generation and disposal
costs. This represents a massive environmental
footprint reduction for this large
aromatics facility.
Additional benefits include reduced
traffic in the heavy aromatics column and
associated energy consumption, as well as
reduced solvent, energy consumption and
benzene losses in the aromatics extraction
process. Selective olefins hydrogenation
is a must-have process for plants of CTC
scale, and indeed all new CTC facilities
coming onstream or under construction
include a selective olefins hydrogenation
process in their aromatics block flow diagram.
Yet the selection of catalytic technology
and process operating conditions
remains crucial: it is advisable to operate
at slightly lower conversion where the catalyst
nears 100% selectivity to avoid any
loss of aromaticity, which could be fatal
in streams with inherently high aromatics
content. For the commercial operation
described in this article, economic calculations
have shown that 0.2% aromatics
loss through hydrogenation would reduce
by 60% the credits associated with the
selective olefins hydrogenation process,
while 0.5% aromatics loss would erase
these credits almost entirely.
Takeaway. A selective olefins hydrogenation
process with the appropriate
selectivity has become an indispensable
technology in the block flow diagram of
very large aromatics complexes. Given
the extent of aromatics losses and environmental
footprint associated with clay
treaters, selective olefins hydrogenation
should also be considered by producers
operating smaller aromatics facilities.
LITERATURE CITED
Complete literature cited available online at
www.HydrocarbonProcessing.com.
FAUSTINE CLAIRE is an expert in aromatics and
derivatives at Axens' petrochemistry product
line in Rueil-Malmaison, France. She engages in
development projects for reforming and selective
reformate hydrogenation technologies, as well as
for energy efficiency improvement for aromatics
complexes. Ms. Claire has 15 yr of experience in oil
and petrochemicals. She holds an MS degree in
chemistry from the National Graduate School of
Chemistry of Montpellier in France and an MS degree
in chemical engineering from the Royal Institute of
Technology of Stockholm in Sweden.
ARNAUD COTTE heads the aromatics and derivatives
technology group at Axens' petrochemistry product
line in Rueil-Malmaison, France. He has more than 15 yr
of experience in the oil and gas industry. He joined
Axens in 2008 and built almost 10 yr of expertise
in the development and deployment of aromatics
technologies. Mr. Cotte holds an MS degree in process
and chemical engineering from the National Graduate
School of Chemical Industry of Nancy in France and an
MS degree in refining, gas and engineering from the
IFP School of Rueil-Malmaison in France.
MICHEL MOLINIER is a consultant with Axens
North America for aromatics technologies and other
petrochemical processes. He has more than 20 yr
of experience in the petrochemical industry and
30 yr of experience in heterogeneous catalysis.
He has coauthored two book chapters, 30 peerreviewed
articles and 25 U.S. patents. Dr. Molinier
holds an MS degree in physical chemistry from the
University of Bordeaux in France and a PhD in solid
state chemistry from Philipps Universität Marburg
in Germany.
Tons of aromatics
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