Hydrocarbon Processing - December 2022 - 38

Catalysts
sults (i.e., the winner on paper is also the
winner in pilot plant testing). In other
words, selecting catalyst Package C over
Package E is simply choosing a very good
catalyst package over an almost-as-good
catalyst package. In truth, selecting catalyst
Package E over Package C for alternative
reasons is still not a bad choice in
terms of economics.
In most cases, catalyst vendors must
guarantee key performance parameters
(e.g., product yield or hydrogen consumption).
As such, it would be risky for them
to over-exaggerate their proposal just to
win the contract. When the catalyst vendors
know that the catalyst evaluation is
pilot plant-based, it provides more incentive
to be more realistic with the yield estimates
in their proposals.
For instance, if the middle distillate
yield based on the original estimation is
70 wt% for a middle distillate selective
application, it can be almost certain that
no catalyst vendor would aggressively
put 75 wt% in the catalyst proposal-it
would obviously fail during the pilot
plant testing or performance test run,
and random excuses will be insufficient.
In the authors' experience, the middle
distillate yield gap between " the best "
and " the worst " hydrocracking catalyst in
the market could be greater than 5 wt%.
Therefore, the hydrocracking catalyst
price tends to play a minor role in the hydrocracking
catalyst evaluation.
For these reasons, the risks associated
with hydrocracking catalyst selection are
not necessarily as high as perceived. At a
minimum, this prevents the selection of
poor hydrocracking catalysts. Nonetheless,
pilot plant testing is still the best method
to evaluate hydrocracking catalysts, as
the gap between the best and second-best
hydrocracking catalyst can be significant.
More importantly, it is also possible that
the catalyst ranking might change if the
catalyst performances are close (i.e., catalyst
Packages C and E in FIG. 2).
Higher-activity
pretreating catalysts
improve the middle distillate
selectivity of hydrocrackers. In principle,
an overall conversion is a result of
both pretreating (hydrotreating) and
hydrocracking reactions. The primary focus
of the pretreating section is to lower
the organic nitrogen concentration to
an acceptable level (typically < 50 ppm
for modern designs) to prevent nitrogen
inhibition effects in the hydrocracking
section. In parallel, the pretreating reactor
also removes other contaminants like
metals, sulfur, oxygen and halides, as well
as saturates olefins and aromatics (FIG. 5).
These hydrotreating reactions reduce
the boiling point of the original hydrocarbon
molecules, also known as pretreating
conversion (FIG. 6). The gross pretreating
conversion can be up to 30% for modern
pretreating catalysts with a relatively low
nitrogen slip target (e.g., < 10 ppm). In
other words, the hydrocracking conversion
could be as low as 69% for a fixed
overall conversion of 99%.
Targeting a higher pretreating conversion
improves product selectivity, especially
middle distillates, as pretreating
catalysts are typically much more middle
distillate selective than hydrocracking catalysts.
A higher-activity pretreating catalyst
saturates more aromatic compounds
(increased hydrogen consumption), thus
higher pretreating conversion and better
overall middle distillate selectivity,
assuming the same nitrogen slip target.
With a lower pretreating temperature, the
extent of thermal cracking (which results
in poor liquid yields) will also be reduced.
Additionally, higher-activity pretreating
catalysts will permit refiners to operate
at the lower nitrogen slip level, allowing
the addition of more middle distillate
selective hydrocracking catalysts (lower
zeolitic content).
Unfortunately, this best practice is
not always the case. The authors once
tried adding a small layer of ultra-high
activity trimetallic unsupported catalyst
(Ni-Mo-W) into a pretreating reactor to
improve the overall middle distillate yield,
as there was excess hydrogen available
then. According to the catalyst supplier,
this modification should enhance the overall
middle distillate yield by another 0.5
vol%. As this unsupported catalyst had just
been recently launched to the market and
had few references, the authors requested
the catalyst supplier to conduct pilot plant
testing to confirm the benefit claims.
In contrast with the already explained
conversion balance principle, the pilot
plant testing campaign revealed that the
trimetallic unsupported catalyst increased
light ends production, thus reducing the
overall middle distillate yield by 0.6 vol%.
Although it also lowered kerosene and diesel
density, the density improvement was
nonetheless insufficient to compensate for
the middle distillate weight yield loss. The
catalyst vendor offered no clear explanation
for this phenomenon-perhaps the
vendor did not know, or it did not want to
reveal flaws in its newly launched catalyst.
One hypothesis for this phenomenon
FIG. 5. Reaction classes in a typical hydrocracking process.
FIG. 6. A demonstration of how heteroatom
removals and hydrocarbon saturations reduce
hydrocarbon boiling points. In this case, a
di-benzothiophene (DBT) molecule is hydrotreated
and ends up as bi-cyclohexane (BCH).
38 DECEMBER 2022 | HydrocarbonProcessing.com
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