Hydrocarbon Processing - June 2021 - 51

Catalysts
G. LIU, Tianjin Dagu Chemical Co., Ltd.,
Tianjin, China; R. SUNDARARAMAN,
ExxonMobil Chemical Co., Spring, Texas; and J. CAO,
ExxonMobil Catalysts and Licensing, Spring, Texas
Ethylbenzene plant debottleneck
with a high-activity transalkylation catalyst
Modern commercial ethylbenzene
(EB) plants use liquid-phase alkylation
processes with zeolite catalysts to achieve
high product yields. The desired primary
reaction is the alkylation of benzene with
ethylene. However, successive undesired
reactions also occur in the alkylator, producing
polyalkylated compounds such as
diethylbenzene (DEB) and triethylbenzene
(TEB). To maximize EB yield, these
polyethylbenzenes (PEBs) are separated
in the fractionation section and recycled
into a transalkylation reactor in which the
ethyl groups are transferred onto the benzene
ring to produce additional ethylbenzene.
FIG. 1 shows the schematic of the alkylation
reactions and the transalkylation
reactions in EB production.
At the heart of the technology is the
shape-selective MWW family of zeolite
catalysts that enables EB production in
high purity and yields. The MCM family
of alkylation zeolite catalysts revolutionized
EB production. Tailored to the molecular
diffusion rate of the different reactants,
the MWW family zeolite pocket
structure is designed to achieve high selectivity
to EB. This high-activity catalyst
enables the process to be operated in liquid
phase at low temperatures, which dramatically
lowers capital expenditures and
operational expenditures (OPEX). This
proprietary EB alkylation processa
was
first commercialized at the Chiba Styrene
Monomer Co., Chiba, Japan, in 1995.1-3
The MCM family of zeolite catalysts is
more EB or monoalkylate selective than
large-pore zeolites, including zeolites beta
and Y.3
This enables the process to use
low feed ratios of benzene to ethylene.
The lower benzene-to-ethylene (B/E)
Transalkylator
RGB
Alkylator
ratio reduces the benzene circulation rate,
which, in turn, improves operational efficiency
and reduces the throughput to the
benzene recovery column.4
The monoalkylating
selectivity to EB is measured by
the once-through production rate of PEB,
which decreases with the increase of the
B/E ratio (FIG. 2).
The transalkylation reactor in the
proprietary EB alkylation process is designed
to match the alkylation reactor
performance. Because of the high monoselectivity
of the alkylation catalyst, the
production rate of PEB in the alkylator
Benzene
EB
DEB
FIG. 1. EB production via alkylation and
transalkylation reactions.
Lights column
Lights
H2O
Benzene (wet)
Ethylene
Benzene (dry)
Benzene,
EB, PEB
Benzene
column
Benzene to reactors
Benzene column
FIG. 3. Process flow scheme of the proprietary EB alkylation processa
PEB, heavies
PEB to transalkylation
EB column
PEB column
at TDCC's facility.
Hydrocarbon Processing | JUNE 2021 51
EB product
TEB
Alkylator B/E ratio
FIG. 2. Impact of B/E ratio on catalyst
selectivity.
is small. Therefore, the size of a transalkylator
in the EB alkylation process tends
to be small. At a constant EB production
rate, the amount of activity of transalkylation
catalyst required to maximize
the PEB conversion, while minimizing
the low-value heavy residue purge, is
determined by the amount of PEB proResidue
PEB/EB
selectivity, wt%
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