Hydrocarbon Processing - June 2022 - 21

Special Focus Process Optimization
A. DORSEY, F. YE and A. COTTE, Axens, Rueil-Malmaison,
France; and M. MOLINIER, Axens, Houston, Texas
Xylene-loop scheme for minimized GHG emissions
and PX production cost
Molecular management, energy efficiency
and carbon dioxide (CO2
)
emissions have become major focuses
for petrochemical complexes, including
aromatics plants. The xylene loop accounts
for about two-thirds of the energy
consumption and associated emissions
in a modern reformate-to-paraxylene
(PX) facility. In the xylene loop, xylene
vapor-phase isomerization (VPI) units
carry out two chemical reactions: the
isomerization of para-depleted xylenes
to equilibrium xylenes, and the removal
of ethylbenzene (EB) by chemical conversion.
EB can either be isomerized to
xylenes (the EB-reforming VPI process)
or dealkylated to benzene (the EB-dealkylation
VPI process). With reformate
C8
aromatic cuts containing up to 18%
EB, converting EB to benzene or xylene
has a significant impact on the total PX
vs. benzene production.
With an unprecedented wave of large
steam crackers going onstream in China,
Korea, the U.S. Gulf Coast and other
parts of the world, pygas benzene is expected
to be widely available in these
regions. The abundance of benzene supply,
combined with potential bans on
some benzene derivatives (e.g., bisphenol
A in Europe), may drive operators of
existing and future aromatics complexes
to maximize their PX output while
minimizing benzene.
In xylene liquid-phase isomerization
(LPI), the only reaction taking place is
the isomerization of para-depleted xylenes
to equilibrium xylenes. Except in
specific situations, LPI is normally deployed
in conjunction with VPI to manage
EB removal from the xylene loop.
LPI operates at a significantly lower temperature
than VPI and yields hardly any
C7aromatics
and C9+
aromatics-therefore,
the LPI effluent can be directed to
the xylene column without the C7aromatics
fractionation.
A configuration including LPI with
an EB-reforming VPI process in an aromatics
complex significantly
reduces
overall product losses, energy consumption
and associated greenhouse gas
(GHG) emissions, thereby affording the
lowest PX production cost. In an existing
facility, the addition of an LPI unit
also opens the possibility of revamping
an EB-dealkylation process into an EBreforming
process. Case studies showing
the synergies between LPI and EBreforming
VPI, including the addition of
LPI to an existing facility and grassroots
facility combining both technologies, are
reported here and discussed.
Xylene loop: Two paths for EB removal.
Aromatics producers usually
define the xylene loop as the circuit that
includes the xylene column, the PX separation
process, the xylene isomerization
process (the effluent of which returns
to the xylene column) and the orthoxylene
column, where applicable. In facilities
converting reformate to PX, the
xylene loop accounts for approximately
two-thirds of the capital and operating
expenses. Consequently, opportunities
to reduce PX production costs and
utilities consumption primarily rest with
the xylene loop design and operation.
The C8
aromatic cuts processed in PX
complexes essentially contain three xylene
isomers (orthoxylene, metaxylene
and PX) and EB. Ironically, PX-the
most desired of the xylene isomers-is
also the least abundant at equilibrium.
Therefore, PX is continuously separated
from C8
effluent is re-isomerized to equilibrium
xylenes and recirculated in a large loop
for further PX separation until all xylene
isomers are eventually converted to PX.
The fourth C8
aromatic isomer (EB)
cannot be fractionated out, since its boiling
point is too close to that of xylene
isomers-yet it must be eliminated because
it takes up space in the xylene loop
and reduces the PX production capacity
of any facility.
In VPI units where para-depleted xylenes
are re-isomerized to equilibrium
xylenes, EB is simultaneously removed
via a chemical route. Since two options
exist for EB removal, two processes are
commercially available:
* An EB-reforming VPI
process, where ethylbenzene
is converted to xylene
* An EB-dealkylation VPI
process, where ethylbenzene
is converted to benzene.
While aromatics facilities are usually
erected to produce PX, benzene is an important
byproduct. Naphtha reformate
C8
EB content in C8
cuts may contain up to 18% EB.1,2
aromatic streams vary
widely depending on the source, with
transalkylate C8
as 1% EB,2
and pygas C8
more than 50% EB.1
cuts containing as little
cuts containing
In a typical modaromatic
feed,
ern plant, reformate represents about
half of the xylene-loop C8
while transalkylate represents the other
half, meaning that the feed EB content
is approximately 8%-9%. Therefore,
the decision to convert EB to xylene or
benzene has a significant impact on the
overall feed consumption/PX production
capacity of the plant.
FIG. 1A shows the reaction pathway
aromatic isomers, and, following
this separation step, the para-depleted
for EB conversion to xylene, and FIG. 1B
shows the reaction pathway for EB conHydrocarbon
Processing | JUNE 2022 21
Hydrocarbon Processing - June 2022

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