Hydrocarbon Processing - August 2022 - 42

Catalysts
FIG. 11. Two-stage hydrocracker startup plots.
ing in a hydrocracking unit, the circulating
startup oil (diesel) will be completely
desulfurized and switching to VGO will
result in a significant and rapid temperature
rise in the hydrotreating catalyst
beds. VGO can be cut in over a period
of 3 hr-5 hr at constant catalyst bed inlet
temperatures (determined by the catalyst
vendor) and in incremental tranches to
control the transition.
During this
transition there should
be no hydrocracking conversion activity.
The hydrocracking catalyst will be
sensitive to the high organic N-slip from
the hydrotreating catalyst until target hydrotreating
temperatures and hydrodenitrogenation
(HDN) activity are attained.
The organic N-slip will temporarily passivate
the hydrocracking catalyst, reducing
its initial activity. The aim, once the liquid
feed has been switched to 100% VGO, is
to increase the hydrotreating catalyst temperatures
quickly, within the constraints
set by the catalyst vendor, to establish the
design N-slip to the hydrocracking catalyst
while keeping the hydrocracking catalyst
temperatures as low as possible.
When design conditions have been
reached at the outlet of the hydrotreating
catalyst, the hydrocracking catalyst
temperatures can be increased carefully.
A descending temperature profile should
be adopted in the hydrocracking catalyst
beds (each catalyst bed inlet temperature
lower than the previous bed inlet). Once
conversion has been established, the feed
rate can be increased to 70%-75% of design
and the fractionation section lined
42 AUGUST 2022 | HydrocarbonProcessing.com
out. Over time, during this period, the
organic N will be stripped from the hydrocracking
catalyst and its activity will
slowly increase. This additional activity
will allow a higher conversion, or a higher
feed rate at constant conversion.
Two-stage hydrocracking unit. For
a two-stage hydrocracking unit, two approaches
to the operation of the second
stage are available once VGO is introduced.
The second stage of a two-stage
unit normally operates in an almost Nfree
environment except for a small organic
N-slip from the first stage (< 5 ppmw)
and a quantity of ammonia in the recycle
gas (higher if the unit does not have a recycle
gas scrubber).
As mentioned above, the organic
N-slip from the first stage will be high
until the hydrotreating catalyst reaches its
target temperature and HDN activity. If
the hydrocracking catalyst can tolerate the
high initial organic N-slip, the approach
outlined above can be followed. However,
if it is deemed inappropriate to subject
the second-stage catalyst to high levels of
organic N, then the feed can be removed
and the catalyst temperatures reduced to
< 260°C, until such time as the organic
N-slip from the first stage is within specification
(normally 12 hr-24 hr). When the
second-stage feed (recycle oil) is nitrogenfree,
it can be reintroduced and the second
stage conversion established, as above.
FIG. 11 provides an example of the predicted
behavior of the main process variables
of a two-stage hydrocracker for a
typical startup procedure (liquid-phase
sulfiding), as described above.
The catalyst vendor will suggest temperature
limits on the operation of the
hydrotreating catalyst as well as the conversion
levels for the first few weeks of operation.
During this initial period of operation,
it is beneficial to limit the severity
of operation (no difficult refractory feeds
or full conversion) until the catalyst has
stabilized. These short-term limitations,
for a few weeks, can add several months
to the catalyst cycle.
NOTES
a
Petroval's S-Load system
ACKNOWLEDGEMENT
The authors want to thank Trevor Penny from CR
International for the images and helpful comments.
SHAUN DYKE is an experienced chemical engineer
who has worked in the refining and petrochemical
industries for more than 40 yr in numerous technical,
managerial, governance and consulting roles around
the world. He lives in and works from New Zealand
and writes in his spare time. Mr. Dyke earned a
BSc degree (First Class Honors) in chemistry from
Massey University, New Zealand. The author can be
reached at shaun.dyke@petroquantum.co.nz.
NATTAPONG PONGBOOT is an experienced chemical
engineer with extensive experience in refining and
petrochemical technologies, specializing in hydroprocessing
technology. Before partnering with Global
R&D, he held various technical roles with Honeywell
UOP, PTT Global Chemical and SCG Chemicals, with
broad exposure to process and equipment design,
refinery operation, plant troubleshooting and process
optimization. Additionally, he is also a technical
instructor for www.chemengedu.com. Mr. Pongboot
obtained B.Eng. and M.Eng. degrees in chemical
engineering from Kasetsart University. The author
can be reached at nat.pongboot@gmail.com or
nattapong.globalrd@gmail.com.
http://www.chemengedu.com http://www.HydrocarbonProcessing.com
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