Hydrocarbon Processing - September 2021 - 24

Refining Technology
set up an in-house capability to perform
quick analysis and make necessary adjustments
to prevent HPNA buildup. Another
change was to replace the ASTM D1160
distillation test method with SIMDIST
D7169 to better control the feed tail end
and minimize residue entrainment.
Simultaneously, process unitk
feed and
FIG. 6. Actual vs. design feed properties.
UCO samples were shipped to the authors'
company's laboratories to develop a better
understanding of the feed recalcitrant species
using advanced characterization techniques.
Detailed compositional analysis of
these hydrocracker streams was then used
to estimate the difficulty of feedstock processability
using the authors' company's
correlations to both develop catalytic solution
for the next cycle and to optimize the
unit operation for the existing cycle.
FIG. 7 shows how advanced characterFIG.
7. HPNA break-up for different feed types.
Reducing the feed end point and conversion
helped improve the situation but
was obviously not the most optimum
solution. Processing heavier feeds was
essential to sustain the unit operation at
110% of design, but not at the cost of unit
reliability and cycle length. The company
performed several yield estimates at different
conversion levels and feed rates for
the revised feed definition. Each estimate
included details on catalyst deactivation
rates, products yield slate and UCO
purge rate to develop a risk vs. profit metrics
to help TÜPRAŞ make an informed
decision. Eventually, operating the unitk
at 110% design capacity at a slightly lower
conversion (~95 wt%) was selected as the
most cost-effective short-term solution.
FIG. 6 shows the key differences between
the design and actual feed properties at
high capacity.
The next step was to devise a robust
feed monitoring system to effectively
analyze and limit the contaminants in the
feed blend and UCO. Tracking these refractory
and unusual species in thermally
24 SEPTEMBER 2021 | HydrocarbonProcessing.com
cracked feedstocks like HCGO is often a
challenge, and bulk properties analysis is
simply not enough. Over the years, the authors'
company has developed a number
of analytical techniques for obtaining a detailed
molecular compositional analysis of
extremely complex heavy streams. These
analyses identify feedstock compounds
that are known to be precursors of catalyst
coke and of 11+ ring HPNAs.
The two most frequently used and reliable
analyses are high-performance liquid
chromatography (HPLC) and fluorescence
spectroscopy (FS). The HPLC test
method harvests and quantifies the refractory
species-especially pure ovalenes
and coronenes-in both feeds and UCO,
while FS reports HPNA types as a function
of different wavelengths at which peak
fluorescence intensity occurs. The higher
the wavelength, the greater the number of
rings. FS is usually the preferred test method
for tracking HPNAs with 11+ rings.
The authors' company provided the
necessary training, lab equipment specifications
and test methods to help TÜPRAŞ
ization methods like high-resolution mass
spectroscopy (HRMS) can be utilized to
optimize unit operations by identifying
the feed contaminants at a molecular level
and reduce the potential for rapid catalyst
deactivation. The four bars indicate the
PNA/HPNA breakup by ring numbers.
Red dots are the distillation end point of
each of the feed blend components. The
relative concentration of 5 and 7+ ring
compounds or PNA precursors is much
higher in HCGO, even though its end
point is lower than the straight-run VGO.
An increased proportion of these precursors
in the feed eventually results in
HPNA buildup in the unit at high conversion.
These analyses were consistent with
the authors' company's recommendations
to either control the HCGO end point
to operate the unit at design conversion
or reduce the unit conversion to address
HPNA buildup issues. It also serves as a
reminder that feed bulk properties like
distillation, API, etc., are insufficient to
predict the HPNA precursors, particularly
in thermally cracked feedstocks. Advanced
feed characterization and improved inhouse
feed and UCO monitoring helped
improved the unit reliability and achieve
the guaranteed cycle. FIG. 8 shows the impact
of HPNA buildup on unit conversion.
Addressing HPNA woes. The first
catalyst cycle clearly exposed the vulnerabilities
of high-conversion units without
any active HPNA management solution to
changes in the feed slate. Costs associated
with reduction in MD yields and shorter
cycle length were simply too high to justify
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