Hydrocarbon Processing - September 2021 - 26

Refining Technology
superior hydrogenation functionality
were chosen to maximize the volume
swell and HPNA conversion. A pilot
plant (PP) test was performed with
TÜPRAŞ feed and operating conditions
to verify representations.
The process unitk
was shut down in
FIG. 8. Plot showing impact of HPNAs on unit conversion.
living with HPNA woes for the next cycle.
The authors' company continued to work
closely with TÜPRAŞ to develop a comprehensive
HPNA mitigation solution
based on its future needs. The primary
objective was to consistently push the unit
operating severity by processing HS feed
blends with high EP (> 600°C) at 110%
capacity at ≥ 98 wt% conversion. TÜPRAŞ
also wanted to extend the catalyst length
from 3 yr to 4 yr with > 80 wt% distillate
yields without any HPNA buildup.
Equipment solution: Different HPNA
options, such as an activated carbon bed
chamber, split-shell fractionator and
SDA on HCGO stream, were reviewed
with TÜPRAŞ. In 2016, the split-shell
fractionator design and carbon beds for
HPNA absorption were fully commercialized
solutions. The standalone HPNA
stripper design, though commercially
unproven, met TÜPRAŞ' needs and processing
objectives. The risk in trying this
option was low, as it was based on the
split-shell fractionator working principle
and offered the same separation efficiency
but without any major modifications
to the existing equipment.
The carbon beds option was
also
considered but rejected due to plot constraints
and the associated recurring
costs of adsorbent replacement. A standalone
HPNA stripper was found to be
the most economically attractive option,
requiring no additional unit downtime to
include the new equipment with the unit
flow scheme. With the installation of an
HPNA stripper, TÜPRAŞ could increase
the unit conversion to 99.5 wt%, while
26 SEPTEMBER 2021 | HydrocarbonProcessing.com
processing high-end point (> 600°C)
HVGO/HCGO feed. Project work commenced
in January 2016 and the basic engineering
design package was delivered
within a couple of months.
Catalytic solution. Maximizing the
middle distillate yields and flexibility to
process opportunity feedstocks without
any HPNA buildup was a key requirement
to improve refinery margins. TÜPRAŞ
had good commercial success with the
typical low-activity and high-selectivity
catalyst system designed specifically for
a second-stage environment. However,
catalytic activity was simply not enough
to guarantee a low HPNA make with constant
fluctuations in feed quality throughout
the cycle. The knowledge gained
from the unit operation, constraints and
catalysts' shortcomings were leveraged to
develop a tailored catalytic solution specifically
designed, keeping the following
TÜPRAŞ needs in mind:
* Ability to operate the unit at 110%
design feed rate, while meeting
desired cycle length of 48 mos
* Flexibility to intermittently process
heavy feed blends
* Superior hydrogenation for HPNA
conversion in Stage 2 environment
* Flexibility to shift the conversion
between the two stages with minimal
impact on middle distillate yields
* Ability to operate the unit at 99.5
wt% conversion with the HPNA
stripper online
* Improved product quality
throughout the catalyst lifecycle.
High-activity treating catalyste,g
with
February for a scheduled turnaround and
catalyst changeout, and was reloaded with
the selected catalyst system and started up
in May 2019. While all the tie-in connections
for the HPNA side stripper were taken
during the unit turnaround, the HPNA
stripper could only be commissioned in
September 2020 due to delays in mechanical
completion.
The new catalyst system has now been
online for more than 18 mos and has successfully
met or exceeded all key processing
objectives. Unlike the last cycle, the
unit is consistently running at 110% design
capacity at or above design conversion,
with significantly reduced and steady
HPNA make. High second-stage SOR activity
and improved hydrogenation has allowed
TÜPRAŞ to regularly process heavier
feed. It is also now possible to swing the
conversion between the two stages and
balance the catalyst deactivation rates, especially
while processing difficult feeds,
without impacting MD selectivity.
The first plot on the left in FIG. 9 shows
high distillate yields, better
consistent
than the previous catalyst system. The plot
on the right in FIG. 9 shows the difference
between the WABT of two catalyst systems.
An SOR temperature of the catalyste
was and continues to be about 30°C lower
than the previous cycle. Improved activity
has not only helped to reduce HPNA
make, but offers the flexibility to regularly
process heavier feeds without having to
compromise on conversion.
Low SOR temperatures and higher
bed exotherms in the second stage also
reduced the fired heater duty by about
10%. The resultant spare margin provides
further flexibility to increase the feed rate
to second stage and therefore improve the
middle distillate selectivity by reducing
the conversion per pass, especially while
processing difficult feed stocks.
Good and consistent catalyst performance
prompted TÜPRAŞ to push the
operating severity further both by increasing
the feed end point and by increasing
the HCGO in the feed blend. In the last
cycle, processing such heavy feeds was unimaginable
without having to reduce the
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