Hydrocarbon Processing - July 2022 - 72

Catalysts
FIG. 2. Accelerated cooling using liquid nitrogen injection.
Once the reactor(s) are cooled and blinded from the rest
of the high-pressure circuit, the inlet elbow is removed and
nitrogen purge connections are made to the reactor(s), the
catalyst handling contractor can begin catalyst unloading.
Strict control of reactor isolation (blinds), nitrogen connections
and the inert/air atmosphere inside the reactor(s) must
be maintained throughout the full process of unloading spent
catalyst and reloading fresh catalyst. Fatalities have occurred
during catalyst unloading, mostly due to nitrogen asphyxiation
or to accidents in the inert atmosphere of the reactor. A
detailed safety and rescue plan should be produced and agreed
upon before work commences.
The environment inside the reactor is non-life supporting
(almost 100% nitrogen) and risks associated with the potential
presence of hydrocarbons and pyrophoric metal sulfides must
be managed, as well. If oxygen ingress cannot be controlled,
the carbon on the catalyst will oxidize to form carbon monoxide
CO, carbon dioxide (CO2
) and potentially nickel carbonyl
(an extremely poisonous gas), along with generating excess
heat. These risks must be actively mitigated, including continuous
monitoring of the reactor(s) for temperature, oxygen
and other contaminants mentioned above.
Once the top distribution tray is opened and any physical
FIG. 3. Vacuum unloading.
prevent the risk of brittle fracture or temper embrittlement)
for their reactor system, which is determined by the bulk metal
composition of the reactor base metal. It is worthwhile to request
a specific MPT to be calculated by the reactor fabricator
(J-factor calculation) for the reactor(s), based on impact
testing (Charpy-V impact testing) of the base metal test blocks
and welded coupons from the actual retained material used in
the reactor fabrication. Most reactors have an actual MPT well
below the nominal MPT of 66°C for post-1990 metallurgy
(2¼ Cr - 1 Mo-0.25 V and 3 Cr - 1 Mo-0.25 V) (i.e., closer to
ambient temperatures). Note: Naphtha hydrotreating (NHT)
and kerosene hydrotreating (KHT) reactors (1¼ Cr - ½ Mo)
do not have an MPT constraint above ambient temperatures.
72 JULY 2022 | HydrocarbonProcessing.com
filter material is removed by the inert entry team, the graded
bed material and any demetalization catalyst layers are normally
removed by vacuum (FIG. 3). Sampling of the spent catalyst
in these top layers, including the first hydrotreating bed, is important
to confirm the level of feed contaminants (V, Ni, As,
Si, Na, etc.) during the previous cycle. The top hydrotreating
catalyst bed in a hydrocracker unit is often very hard and difficult
to remove; this is caused by a very hard crust or agglomerated
catalyst. It is common for this bed to require the use of
jack hammers to break it up before the catalyst can be removed.
The other beds in the reactor system are normally gravitydumped
from dump nozzles at the base of each catalyst bed
(FIG. 4). Only 65%-75% of the catalyst will flow from the bed
under gravity. The inert entry team then enters the reactor
and " chases " the remaining catalyst and ceramic balls from
the bed manually, although robots are being developed for
some of this work. The manway sections are then removed
from the catalyst support grid and the quench zone trays to
access the next catalyst bed, where the process is repeated until
the reactor is empty.
During catalyst removal in the bottom of the reactor, it is
possible that catalyst and small-diameter ceramic balls fall
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