Hydrocarbon Processing - June 2021 - 52

Catalysts
duced in the alkylator. Increasing the
EB production rate will directionally
lead to high PEB production, which increases
the weight hourly space velocity
(WHSV) of the transalkylator. To maintain
constant conversion to reduce PEB
recycle, the reactor temperature must be
increased. However, high-temperature
operation results in higher heavies yield
and fast catalyst deactivation.
The other mitigation steps include increasing
the B/E ratio in the alkylator operation.
As shown in FIG. 2, the selectivity
to PEB decreases with the increase of B/E.
However, high B/E operation increases
benzene recycle and results in high energy
consumption. In such a case when the operation
flexibility in the alkylation section
is limited, the transalkylator becomes a
bottleneck in the EB process unit. Therefore,
a higher-activity transalkylation
catalyst is required to convert more PEB
1
2
3
4
Proprietary
transalkylation
catalystc
FIG. 4. Catalyst screening for a higher-activity transalkylation catalyst.
Next-generation catalystd
DEB conversion
Previous catalystc
TEB conversion
75
65
55
45
35
FIG. 5. Next-generation transalkyation catalystd
(2013 cycle) at a PEB WHSV of 1.2 h-1
.
52 JUNE 2021 | HydrocarbonProcessing.com
(2017 cycle) vs. the previous catalystc
Next-generation catalystd
TEB conversion
Next-generation catalystd
temperature
Previous catalystc
DEB conversion
Previous catalystc
temperature
205
200
195
190
185
Cat 1
Cat 2
Cat 3
Cat 4
Cat 5
Cat 6
Cat 7
Cat 8
Cat 9
Cat 10
at increased EB production rates, while
minimizing residue production.
Overview of Tianjin Dagu Chemical
Co.'s EB alkylation operation
and constraints. Tianjin Dagu Chemical
Co.'s (TDCC's) EB alkylationa
unit
was first commissioned in January 2010
with a production rate of 500,000 metric
tpy of styrene monomer. The unit was
originally designed to use a proprietary
catalystb
in the reactor guard bed (RGB)
in the transand
six-bed alkylator, along with a proprietary
transalkyation catalystc
alkylator. FIG. 3 shows the process flow
scheme of the proprietary EB alkylation
process at TDCC's facility.
During the booming period of the styrene
market, TDCC's primary objective
was to increase the plant's capacity. TDCC
and the co-authors' companies' technical
teams collaborated for months to evaluate
the proprietary EB alkylation performance
and identify the bottleneck in the
transalkylation section at increased capacity.
It was found that the unit was unable
to maintain the designed PEB conversion
at design temperature and WHSV when
higher PEB was made in the alkylator;
therefore, the operation needed to increase
the reactor temperature, which inevitably
led to high heavies yield and catalyst aging.
Plant operation optimization was attempted
to achieve a high EB production
rate while minimizing residue purge.
While the results showed improvement,
they did not meet TDCC's operational expectations
(i.e., high production rate and
low residue make at low OPEX). Measures
were then taken that included increasing
the alkylation section's B/E ratio. The increased
B/E ratio did reduce PEB yield,
but resulted in high OPEX due to high
benzene recirculation. As a second option,
TDCC reluctantly shut down the unit
temporarily to load additional proprietary
transalkylation catalystc
in the remaining
limited space of the reactor. Unfortunately,
the additional catalyst was still unable to
bring adequate activity for the required
PEB conversion. Eventually, there were essentially
two remaining options:
1. Invest in a new larger reactor to
load more catalyst
2. Deploy a higher-activity
transalkylation catalyst.
Option 1 was not economical in
terms of capital and schedule. To support
TDCC's objective to increase EB
production, and to enable increased plant
capacity and address the transalkylation
section bottleneck, the technical teams
started an accelerated catalyst development
program to identify a higher-activity
transalkylation catalyst for potential
deployment at TDCC.
High-activity transalkylation catalyst
development: Pilot plant experience.
With an extensive catalog of active materials,
the technical teams quickly identified
10 catalysts with varying zeolite types,
compositions and treatment methods to
address TDCC's constraints. These materials
were screened in a lab-scale batch
reactor with commercially representative
feed specifications and conditions. A
normalized catalyst activity plot from this
testing (FIG. 4) illustrates that six out of the
10 catalysts tested showed two times the
activity vs. the proprietary transalkyation
Conversion, %
Normalized catalyst activity
Temperature, °C
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