Hydrocarbon Processing - June 2022 - 22

Process Optimization
version to benzene. The former reaction
is an equilibrium reaction under thermodynamic
control-hence, conversion
per pass is limited. The latter is an irreversible
reaction under kinetic control;
in this case, conversion per pass can be
adjusted with parameters such as temperature,
residence time and hydrogen
partial pressure.2
State-of-the-art,
optimized
EB-reforming
and EB-dealkylation VPI processes
have made substantial progress in
minimizing product loss per pass. However,
due to the thermodynamically vs.
kinetically controlled reaction pathways,
the achieved EB conversion per pass is
nearly twice as high with EB-dealkylation
VPI than with EB-reforming VPI,
resulting in higher energy consumption
at equivalent EB conversion for the latA
H2
-
H2
ter process. Conversely, at equivalent
PX production, feed consumption will
be substantially higher with EB-dealkylation
VPI, since EB is converted to
benzene instead of xylene. Furthermore,
with EB-dealkylation VPI, two out of
eight carbons in the EB molecule are
converted to light gas, which significantly
affects overall plant economics when
C8
aromatic feeds are EB rich.
LPI: Low investment for low product
loss and low energy consumption.
LPI has been previously discussed in
literature.3,4
This simple and inexpensive
process affords lower energy consumption
vs. VPI processes-not only
because of the absence of feed vaporization
and associated hardware, but also
because of the lack of side reactions,
meaning little to no byproduct generation
and a considerably simplified effluent
fractionation scheme. Marginal side
reactions also mean near undetectable
product loss per pass, which is another
significant advantage compared to VPI
processes. Since the LPI process does
not remove EB, it is usually deployed in
conjunction with a VPI process to prevent
EB accumulation in the xylene loop.
The higher the percentage of PX separation
raffinate that is processed through
LPI vs. VPI, the higher the energy savings
and product loss minimization.
Conversely, the higher the percentage of
PX separation raffinate that is processed
through LPI vs. VPI, the lower the overall
EB removal. Therefore, the split between
LPI-processed and VPI-processed
raffinate is the result of a xylene-loop
optimization effort.
The two commercial case studies
detailed in this article demonstrate that
Ethylbenzene
B
H2
+
Ethylbenzene
Benzene
C2H4
Ethylene
Benzene
+
C2H6
Ethane
FIG. 1. Reaction pathways for EB conversion to xylene or benzene: (A) EB reforming and
(B) EB dealkylation.
Raffinate
Reformer
H2
Benz/tol
extraction
R
S
Separation
XC
and
XS
Separation
XC
and
XS
Ox
C
Liquid phase
isomerization
Orthoxylene
H
A
R
Heavies
FIG. 2. The addition of two LPI units in an existing facility operating two xylene loops in parallel.
22 JUNE 2022 | HydrocarbonProcessing.com
Liquid phase
isomerization
Paraxylene
EB reforming
isomerization
Benzene
Benz/tol
fractionation
Transalkylation
Paraxylene
EB reforming
isomerization
Naphthene intermediates
Xylene
PX production can be maximized via
EB-reforming VPI, while both energy
consumption and product loss are minimized
by the addition of an LPI process.
In other words, a scheme comprising
EB-reforming VPI and LPI processes effectively
leads to the lowest PX production
cost and the lowest GHG emissions
for a xylene loop.
Case Study 1: LPI process addition
to an existing xylene loop operating
an EB-reforming VPI process.
The flow scheme of this commercial facility
is depicted in FIG. 2. The plant has
opted for EB-reforming VPI to maximize
PX production. A continuous catalytic
reforming process and a transalkylation
process are feeding two xylene loops operated
in parallel and comprising a fractionation
section including a xylene column
and a xylene splitter, an adsorption
unit and an EB-reforming VPI unit. The
two xylene splitter bottom streams feed
an orthoxylene column. The proposed
LPI process addition includes two units
(shown in green)-namely, one per each
xylene loop.
The added LPI heating duty-and
marginally increased loop traffic because
of the slightly higher EB content-are
more than compensated for by the reduction
in:
* VPI traffic and associated duty
* VPI effluent fractionation duty
* The xylene column duty.

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Hydrocarbon Processing - June 2022

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