Hydrocarbon Processing - August 2022 - 37

Catalysts
S. DYKE, PetroQuantum, Northland, New Zealand; and
N. PONGBOOT, Global R&D, Samut Prakan, Thailand
Hydroprocessing catalyst reload and
restart best practices-Part 2
Hydroprocessing (hydrotreating and
hydrocracking) units are high-pressure,
high-temperature units that have multiple
reactors, multiple beds per reactor
and specialized metallurgy. Catalysts in
these units are replaced on a 2 yr-5 yr
cycle, depending on feed quality, unit design,
catalyst selection, operational constraints
and performance.
One of the key drivers for a catalyst
change-out turnaround on these highmargin
units is minimizing the time that
the unit is offline-the cost of downtime
is high, particularly when a turnaround is
extended.
Managing a catalyst change-out turnaround
is vastly different than normal
refinery maintenance activities: the
complexity is higher, the risks and consequences
of unplanned events are much
greater, and the required resources are
significant but limited. The experience
of the key personnel involved is a significant
factor during this activity; however,
decades of experience are disappearing
from refineries due to economic pressures
and the age profile of operations/
engineering/maintenance personnel.
The responsibilities of refinery process
engineers change every 2 yr, so
building and promoting experience and
expertise in-house for hydroprocessing
catalyst reloads is difficult. In the past,
the catalyst vendor often provided much
of the necessary technical expertise, specifically
for the catalyst loading and unit
restart activities. However, the impact of
the COVID-19 pandemic and ongoing
restrictions means that catalyst vendors
are generally unable to provide this level
of onsite support.
This situation leads to the question of
how to best manage the complexities and
risks associated with a catalyst changeout
and restart for refinery hydroprocessing
units, as well as hydroprocessing units
for renewable fuels (e.g., vegetable oils
and fatty acids).
This article (Part 2 of 2) will discuss
catalyst
the
activities
associated with
loading and the restart. Part 1 of this article,
published in the July issue, discussed
the activities associated with turnaround
planning and shutdown, catalyst unloading
and reactor inspection. The two articles
cover the entire process across the
full catalyst cycle and many of the best
practices used to manage and mitigate
the underlying risks for these units.
Catalyst loading. Catalyst is manufactured
in the metal-oxide form, but it can
also be provided in a presulfided or presulfurized
form. Note: See the Startup
section for an explanation of presulfided
and presulfurized catalyst. The two latter
forms are more expensive; however, they
provide the advantage of reducing the
time and complexity of the startup. These
catalysts are loaded in an inert (nitrogen)
atmosphere-which adds time and complexity
to the catalyst loading-although
they can be loaded in air, depending on
temperature and catalyst storage conditions.
An economic evaluation must be
made to determine the benefits and risks
of using presulfided or presulfurized catalysts
over the conventional metal-oxide
catalysts in each case.
Catalyst loading is performed using
one of two techniques: sock loading or
dense loading (shown in FIGS. 5 and 6,
respectively). Sock loading is simpler and
faster but carries a higher risk of non-uniform
loading, leading to a higher probability
of flow maldistribution during
operation and a shorter run length.
A dense-loading machine spreads the
catalyst evenly over the full cross-sectional
area of the reactor, normally using shaped
ports on a specially designed, variablespeed
rotor so that it falls like a uniform
rain of catalyst particles. This results in
greater alignment of catalyst particles and
provides a harder and more homogenous
catalyst bed, more evenly distributed flow
and a higher catalyst density. The result
is 15%-25% higher density for extrudate
catalysts and, therefore, a longer cycle, as
the run length is proportional to the mass
of catalyst loaded, in most cases. Consequently,
dense loading is maximized unless
pressure drop limitations are reached.
The first bed in a hydrotreating reactor
is normally sock loaded, as it usually
includes bulk physical filtering materials
and/or a graded bed (layers of successively
smaller catalyst) to limit pressure
drop issues resulting from fine scale and
other contaminants in the feed. The
demetalization catalysts are also normally
sock loaded. The hydrotreating catalyst
in the first bed can be sock or dense
loaded, or a combination, depending on
pressure drop constraints. The bottom
portion of the bed will be dense loaded
if a combined sock-/dense-loading
approach is proposed.
Numerous different proprietary denseloading
machines are used, most of which
can provide excellent dense-loading results.
It is important to ensure that the
technician performing the dense loading
is experienced with the machine being
used, as the quality of loading will have a
direct impact on refining margins for the
unit over its subsequent catalyst cycle. Using
the lowest cost option for dense loading
will likely prove very costly.
Stab-in thermocouple assemblies are
withdrawn for dense loading of the catalyst
and replaced as the catalyst layer apHydrocarbon
Processing | AUGUST 2022 37
Hydrocarbon Processing - August 2022

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