Hydrocarbon Processing - August 2022 - 41

Catalysts
gas increases significantly and the load on
the recycle gas compressor driver also increases.
For an electric motor-driven recycle
gas compressor, the load on the motor
(amps) should be monitored and the H2
purity increased, if necessary-at the expense
of H2
pressor does not trip on motor overload.
After H2
S loss-to ensure that the comS
breakthrough has occurred
and before catalyst temperatures are increased,
the injection rate of the sulfiding
agent should be decreased by about 30%-
50% to keep the H2
gas in the 1 vol%-1.5 vol% range. If the H2
content goes too high, the H2
S content in the recycle
S
S losses resulting
from solution losses become excessive.
As the catalyst temperatures increase,
the rate of sulfur laydown on the catalyst
increases again and the injection rate of the
sulfiding agent can be increased. The hightemperature
sulfiding soak for hydrotreating
catalyst is normally 315°C-320°C and
290°C for the hydrocracking catalyst.
When increasing catalyst temperatures
from the low-temperature sulfiding step in
the transition to high-temperature sulfiding,
an additional precaution is required
for highly active hydrocracking catalysts
(higher zeolyte content). An injection of a
nitrogen (N)-based modifier stream (ammonia
or high-N liquid feed) is required
before the catalyst reaches 260° to avoid
the impact of excessive cracking of the circulating
liquid and coking of the fresh, hyperactive
hydrocracking catalyst. Production
of significant quantities of liquefied
petroleum gas (LPG) is a sign that hydrocracking
reactions are occurring: the temperature
increase should be stopped and
the injection rate of the nitrogen-based
inhibitor increased.
The high-temperature soak period
(normally 4 hr-8 hr) commences after the
catalyst temperatures reach the designated
high-temperature sulfiding temperatures.
The H2
S content in the recycle gas should
be controlled in the 1 vol%-2 vol% range.
When this period is complete and when
catalyst bed temperatures have stabilized
(no exotherm), no more water is being
produced and the injection rate of the sulfiding
agent has reduced to the level of the
H2
S losses via solution losses (recycle gas
H2S content stable or increasing slowly),
the catalyst sulfiding step is deemed to be
complete. The sulfur balance should also
confirm the completion of sulfiding.
A catalyst sulfiding process that is
smooth and relatively fast normally creates
the best results. Time wasted between
the various startup steps results
in a higher probability of unwanted outcomes,
such as running out of sulfiding
agent or having the catalyst at elevated
temperatures (> 200°C) for too long (hr)
before injecting the sulfiding agent.
After completing gas-phase sulfiding,
the catalyst temperatures are reduced for
the cut in of liquid feed (see the discussion
above on catalyst pre-wetting). If liquid
feed has already been established, normally
on circulation, the catalyst temperatures
should be reduced for the switch to
normal feed quality: straight run vacuum
gasoil (VGO) or diesel for a hydrotreating
unit, or VGO for a hydrocracker. Normally,
cracked or high endpoint feedstocks
are not permitted for 3 d-7 d to allow the
catalyst to stabilize without excessive coke
laydown on the fresh catalyst.
After completing liquid-phase sulfidON
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Hydrocarbon Processing
Hydrocarbon Processing | AUGUST 2022 41
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