Hydrocarbon Processing - August 2022 - 40

Catalysts
the hydrocracker catalyst activity has increased
in recent years (with the inclusion
of zeolitic base material to hydrocracking
catalysts), the heat of adsorption exotherms
have increased. In addition, if the
startup has been delayed and the catalyst
subjected to an extended period (several
days) of recycle gas-only circulation without
any liquid feed, the wetting exotherm
can be very high. In gas-phase sulfiding,
the catalyst will be extremely dry by the
end of the sulfiding step and pre-wetting
exotherms can be extreme and must be
proactively controlled with quench.
Therefore, the wetting exotherm can be
significant, > 40°C for hydrotreating catalyst
and > 100°C for hydrocracking catalyst,
hence the low oil cut-in temperature.
An additional factor that must be understood
is that the wetting exotherm is extremely
fast in its increase and subsequent
decrease-much faster than the onset of
a reactor catalyst temperature runaway
during normal operation. If the reactor(s)
has
temperature rate-of-change alarm/
trip functionality, a high probability exists
of it being activated during catalyst prewetting.
This very severe exotherm must
be controlled proactively. If the operator
waits until the exotherm starts in a particular
catalyst bed, it is too late to control it
with quench. Therefore, it is imperative to
start the quench flow a few minutes before
the liquid reaches the top of the respective
reactor/bed. For example, in a multi-bed
reactor, as the exotherm reaches the first
thermocouples in the bottom of the first
bed, the quench between the first and
second beds should be opened (to at least
50% of the quench valve capacity) and the
quench to the current bed decreased and
closed. This process should be repeated
for each bed (FIG. 10).
After pre-wetting and catalyst flushing
are complete, the catalyst temperatures can
be increased for low-temperature catalyst
sulfiding (normally 230°C). At this time,
the H2
above 90 vol%, ideally, and there should be
no wash water or amine circulation.
Catalyst conditioning. This consists of
sulfiding of the active metal sites on hydrotreating
and hydrocracking catalysts
and is the most important and most sensitive
step in the startup procedure. Most
hydrotreating and hydrocracking catalysts
have metals-nickel, molybdenum, cobalt
and tungsten are most prevalent-
40 AUGUST 2022 | HydrocarbonProcessing.com
purity in the recycle gas should be
that are in the oxide state when manufactured
and must be converted to the
sulfide state to make them effective and
stable for hydrotreating and hydrocracking
reactions. The sulfiding reactions include
the following (Eqs. 1-3):
3NiO + H2
+ 2H2S j
Ni3S2 + 3H2O + heat
MoO3 + H2 + 2H2S j
MoS2 + 3H2O + heat
WO3 + H2 + 2H2S j
WS2 + 3H2O + heat
(1)
(2)
(3)
If the catalyst has been presulfided
or pre-sulfurized, the startup process is
much simpler, using a procedure provided
by the vendor. For presulfided catalysts,
the above reactions have been completed
using a carefully controlled ex-situ procedure
following manufacture; therefore,
the active metals are already in their sulfided
form. Startup comprises heating in
a controlled manner in a H2
Presulfurized catalyst have a sulfurrich
compound (normally a polysulfide)
bound to the catalyst that decomposes at
a known temperature and provides sulfur
at the active metal sites during the startup.
This eliminates the need to inject a sulfur
compound,
such as dimethyl disulfide
(DMDS), into the reactor circuit. For presulfurized
catalyst, the startup procedure is
like that for catalyst, where the active metal
sites are in the oxide form but simpler
and so a little quicker. The startup of presulfurized
catalyst is vulnerable where the
startup is interrupted shortly after the decomposition
of the sulfur-rich compound.
If the reactor circuit must be depressured
before the high-temperature soak, a loss of
sulfur may require replacement by DMDS
injected into the high-pressure circuit.
In-situ sulfiding can be done in a gasphase
procedure or in a procedure that
uses liquid feed to the unit, normally on
full recycle mode. To enable an efficient
catalyst sulfiding step, a sulfur compound
is injected into the high-pressure
circuit, either directly or with the liquid
feed, to provide the sulfur for the sulfiding
reactions. The most common sulfiding
compounds are dimethyl disulfide
or DMDS (CH3
(C4
SSCH3) and Sulfrzol 54
H9 C4
DMDS: CH3
2CH4
S4 H9), shown in Eqs. 4 and 5:
SSCH3
+ 3H2
+ 2H2
Sulfrzol 54: C4
8CH4
+ 4H2
S
H9 C4
S
S4
H9
+ 11H2
j
(4)
j
(5)
DMDS is preferred due to its high-sulfur
content, low decomposition temperature
and low vapor pressure.
Gas-phase sulfiding has been replaced
by liquid-phase sulfiding, especially since
the development of Type-II hydrotreating
catalysts. As mentioned above, the highactivity
Type-II hydrotreating catalysts
have a temperature limit of 140°C-150°C
prior to being fully wetted. To allow gasphase
sulfiding, a protective compound
must be applied to the catalyst before
delivery and loading to allow the catalyst
to be fully sulfided at temperatures up to
315°C-320°C before liquid feed is cut in.
At 200°C, the injection of the sulfidenvironment.
ing
agent can be started. Increasing the
injection rate should be done carefully
and in a step-wise manner to avoid excessive
exotherms from the decomposition
of the sulfiding agent and the initial sulfur
laydown. The injection rate can be kept at
the maximum (set by the catalyst vender)
until H2
S breakthrough is achieved, unless
excessive catalyst exotherms are experienced,
at which time the injection rate
must be reduced until the exotherms return
to within acceptable limits.
Regular sampling and composition
testing of the recycle gas, low-pressure
separator and fractionator off-gasses, and
liquid streams should commence with
the start of catalyst sulfiding to monitor
the progress and allow the calculation of
a sulfur balance. Shortly after sulfiding is
begun, water will begin to accumulate in
the cold high-pressure separator and will
continue until sulfiding is complete.
Low-temperature sulfiding conditions
should be maintained until H2
S breakthrough
is achieved and at least 70% of
the theoretical sulfur up-take has been
injected in the form of the sulfiding agent.
When these criteria have been met, the
catalyst temperatures can be increased for
high-temperature sulfiding.
Through the catalyst sulfiding process,
purity decreases as H2
recycle gas H2
is
consumed by the decomposition of the
sulfiding agent and the only H2
make-up
is to compensate for solution losses. If the
H2
the recycle gas and should be minimized.
As the H2
ane (CH4
purity drops to 70%-75%, the highpressure
bleed should be opened to purge
some recycle gas and allow a higher H2
make-up. This will result in H2
S loss from
purity drops and the meth)
content increases through the
sulfiding process, the density of the recycle
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