Hydrocarbon Processing - July 2022 - 71

Catalysts
Do not assume the catalyst loading diagrams provided by the
catalyst vendor are free of errors. Challenge any details that do
not appear to be correct, based on your understanding. For example,
if the vendor proposes to use 6-mm ceramic balls to directly
support 1.7-mm extrudate hydrotreating catalyst it should
be challenged, especially if the hydrotreating catalyst is to be
sock loaded. In this case, 3-mm support material should be used
under 1.7-mm extrudate catalysts (a maximum size factor of 3:1
is normally applied) to avoid catalyst migration.
A minimum 150-mm layer of large-diameter ceramic balls
should be used above the outlet collector, and minimum 75-mm
layers of finer ceramic balls are deemed appropriate to support
the catalyst. Layers of less than 75 mm have an increased chance
of allowing migration due to variations in the layer depth due to
poor loading practices. Ceramic hold-down material is not used
above 3-mm extrudate catalyst beds (i.e., more catalyst can be
loaded). For smaller catalyst, the hold-down material is still used
to guard against the potential for reverse flow.
Shutdown and startup procedures should be reviewed in detail
prior to the turnaround to build in changes in catalyst conditioning
requirements, lessons learned from the last shutdown
and startup, newly acquired best practices from other locations,
and to consider specific requirements or opportunities for the
coming turnaround.
The turnaround planning and execution phases should be a
cooperative effort between engineering/maintenance and operations,
with an integrated plan and schedule (starting with
feed out) all the way to product rundown on grade to storage.
This integrated plan should define when each piece of equipment
will be handed over from operations to engineering/maintenance
in a gas-free condition and when it is expected back for
startup preparations.
Prior to the turnaround, a detailed discussion with the catalyst
vendor and the catalyst handling contractor should finalize
the detailed plans and contingencies for catalyst unloading and
loading activities.
Throughout the catalyst change-out turnaround, the highest
probability of delays and errors occur during handovers and
transitions, such as the handover of equipment from operations
to engineering/maintenance after shutting down the unit and
gas-freeing individual systems and equipment, the handover between
engineering/maintenance to the catalyst contractor, and
when inspectors are requested to perform equipment inspection.
Safe work permit renewals for confined space entry, for example,
occur at the start of every shift (2-3 times per day), so
any delays in this process can add a considerable amount of time
to the entire turnaround. All delays on each interface can add
hours and sometimes days to the turnaround-by managing
these interfaces and handovers efficiently, considerable time can
be potentially saved during the turnaround.
Shutdown, catalyst unloading and reactor inspection. As
discussed here, the process of shutting the unit down is one of
the highest risk periods for hydroprocessing units. During this
non-steady-state operation, conditions (temperature, pressure,
composition, etc.) are fluctuating significantly and, in some
steps, very close to engineering limits. An incident on shutdown
can impact the scope of the turnaround (e.g., a heat exchanger
leak or a heater tube failure).
In the case of a hydrocracker unit, the whole complex is likely
to be shutting down at the same time, including the vacuum
distillation unit, hydrogen plant and sour gas treating units.
Particularly for a hydrocracker unit, ensure that the gasoil flush
and hot hydrogen strip are performed thoroughly to remove as
much heavy hydrocarbon from the catalyst as possible. If these
steps are cut short, the probability of a delayed entry into the reactor
will be higher due to lingering hydrocarbon on the catalyst
[high hydrocarbon vapor (LEL) in the reactor]. Do not exceed
maximum cooling rates of 28°C/hr for heavy wall equipment,
especially the reactors, as it can lead to hydrogen stress cracking
and overlay disbonding.
Cooling of the reactor and catalyst system at lower temperatures
is the rate-controlling step that determines when the reactor
can be opened. Ambient temperature is a key factor and
some locations have experience with accelerated cooling using
liquid nitrogen injection (as shown in FIG. 2); however, the metallurgy,
piping design and equipment at and downstream of the
injection point must be rated for the lower temperature operation.
The composition of the recycle gas changes significantly
during the accelerated cooling operation (nitrogen injection),
and the impact on the recycle gas compressor must be evaluated
prior to implementing this practice.
For most units, maintaining full hydrogen pressure for as
long as possible down to the minimum pressurization pressure
(MPT), as well as other cooling strategies, will allow for an acceptable
cooling rate. Most refiners use the nominal MPT (to
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Hydrocarbon Processing | JULY 2022 71
Hydrocarbon Processing - July 2022

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