Hydrocarbon Processing - October 2021 - 26

Plant Safety and Environment
Adsorption units have tight
benzene specifications,
and each clay start-of-cycle
constitutes a potential
exposure to off-specification
feed for the adsorption unit.
e. Selective hydrogenation
upstream from an extraction
process reduces extraction
solvent consumption, as
paraffins are easier to separate
from aromatics than olefins.
FIG. 4. Cumulated solids consumption in an aromatics complex.
1. Solid waste reduction: Clay treaters
produce up to 80% of an aromatics
complex's total solid waste. The
massive environmental footprint
of clay waste is illustrated in FIG. 4,
showing solids consumption as
a function of time in aromatics
facilities. With selective
hydrogenation, clay is essentially
eliminated, meaning not only
a huge reduction in solid waste
generation but also significant
savings in disposal costs.
2. Operating efficiency: Clay treaters
are designed for a target cycle
length at the start of aromatics
production. With repeated
complex debottlenecks via
increased feed processing capacity
and/or increased reformer severity,
it is not unusual for treater cycle
length to rapidly decline, forcing
operators to unload/reload every
3 mos or even more frequently,
with the following consequences:
a. Clay start-of-cycle unavoidably
produces high xylene losses,8
for a period of 1 wk-2 wk,
due to high initial clay activity.
Xylenes are lost through
transalkylation reactions
involving two C8
aromatics
that produce a toluene and a
C9
aromatic. At sites replacing
clay every 3 mos, this means
1 wk-2 wk every 12 wk at high
xylene losses, with a severe
economic impact:
i. In complexes operating a
transalkylation process,
the impact is essentially
26 OCTOBER 2021 | HydrocarbonProcessing.com
energetic (fractionation,
recirculation) because
toluene and C9
f. Selective hydrogenation
reduces traffic in the heavy
aromatics column bottoms,
yielding energy savings.
aromatics
will be recombined in the
transalkylation unit to
produce xylenes.
ii. In complexes that do not
operate a transalkylation
process, lost xylenes are
irreversibly downgraded
since toluene and C9
aromatics are exported to
the gasoline pool.
b. Frequent clay change-outs
constitute a significant
operating (recurrent unloading
and reloading events with
potential safety concerns) and
logistics (waste handling and
disposal) burden, which is
eliminated when clay cycles are
exponentially extended.
c. Short clay cycles resulting
from complex debottlenecks
make it difficult to maintain
the low feed BI specification
for the adsorption unit. The
consequence is the potential
permanent loss of a fraction of
site recovery capacity during
high feed BI excursions if
clay cannot be replaced fast
enough. With an upstream
selective hydrogenation unit,
this threat no longer exists.
d. The transalkylation reactions
catalyzed by initial high clay
activity, as mentioned above,
produce benzene when the
C8
aromatic involved in the
reaction is ethylbenzene.
g. Pressure drop excursion events
can occur in clay treaters and
potentially disrupt production
when heavy reformate dienes
content is high, because
dienes oligomerize over acidic
catalysts,9
leading to rapid
coke deposition. Selective
hydrogenation is very efficient
at converting dienes, thereby
eliminating this risk.
Selective hydrogenation process
in the aromatics complex. The typical
location of the two main clay treating
processes in a modern aromatics complex
is shown in yellow in FIG. 5. As previously
explained, the benzene/toluene extract
stream is clay treated to meet benzene
product specifications, while the heavy
reformate stream is clay treated to achieve
separation process specifications on the
xylene column overhead stream. In both
cases, the heavy compounds resulting
from olefins alkylation with aromatics are
fractionated out via the toluene column
bottoms in the case of the extract stream
and via the xylene column bottoms in the
case of the heavy reformate stream.
Using green dots, FIG. 5 shows potential
locations for a selective hydrogenation
process in a modern aromatics complex:
1. Reformer effluent upstream
from the reformate splitter.
In this configuration, the selective
hydrogenation process converts
olefins for both C6
and C8
-C7
aromatics
+ aromatics streams.
This arrangement results in
the maximum conservation of
valuable aromatic compounds and
reduces energy consumption in
the extraction unit, as well as in
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