Hydrocarbon Processing - December 2022 - 15

Special Focus Catalysts
E. MAAS, Shell Catalysts & Technologies, Houston, Texas;
J. SIGAUD, Shell Catalysts & Technologies, Amsterdam,
the Netherlands; and G. POUSSIN and T. CALDAS,
CREALYST-Oil, Paris, France
Solving maldistribution: Catalyst loading
and why it matters
Are your reactor yields lower than
forecast? Does your fixed-bed catalytic
reactor suffer from hotspots? Are your
cycle lengths shorter than planned? If you
answered " yes " to any of these, then your
reactor could be suffering from liquid/gas
maldistribution-one of the most common
causes of poor reactor performance.
The impact of maldistribution can be
severe and long-lasting, yet its root cause is
often misunderstood. Although faulty or
poorly installed reactor internals are often
to blame, maldistribution can be frequently
traced back to poor catalyst loading.
The authors' companies collaborated
to share insights into the impacts of maldistribution
and the critical role that catalyst
loading plays in reactor performance, as
well as the important steps that plant personnel
can take to ensure optimal performance
of expensive catalyst and hardware.
Maldistribution: How is it affecting
production? Maldistribution is described
as the preferential flow and the uneven
distribution of liquids and/or gases
within a catalyst bed, which can have the
following consequences:
* Off-spec products due to
the suboptimal use of the
loaded catalyst volume
* Shorter-than-expected cycle length
due to end-of-run (EOR) conditions
reached sooner than expected
* Hotspot formations and the
resulting threat to the mechanical
integrity of the reactor.
Performance impacts: Poor use of
catalysts. Ideally, the weight hourly space
velocity (WHSV) should be uniform
within the entire catalyst bed volume;
however, if a reactor is suffering from
FIG. 1. The presence of hotspots (A) that can reduce distillate (diesel) yield. Localized hotspots
(B) can lead to over-cracking and the conversion of desired products (e.g., distillates) into
undesirable light ends.
Hydrocarbon Processing | DECEMBER 2022 15
maldistribution, then portions of expensive
catalyst are being underused (with a
lower-than-average WHSV), resulting in
lower-than-expected yields and off-spec
products. Consequently, the reactor must
run under more severe conditions to meet
the targeted product properties or yields.
Time impacts: Shorter run/cycle
times and increased downtime. In
the most severe cases, maldistribution
can lead to the early termination of the
reactor cycle. This is caused by the development
of hotspots-sections of the
catalyst bed where the liquid/gas flow is
impeded, causing high temperatures-
that result in EOR temperatures being
reached sooner than scheduled. Should
a cycle be cut short, valuable run time is
replaced with the costly and time-consuming
process of replacing exhausted
catalysts. In the most extreme cases, the
hotspot temperature can be high enough
to threaten the mechanical integrity of
the reactor and cause severe safety incidents
[e.g., reactor rupture, fire, explosion
or hydrogen sulfide (H2
S) release].
For example, imagine that plant personnel
have targeted a 1,000-d unit cycle
length, but the development of maldistribution
forces the cycle to terminate after
only 730 d, thereby reducing cycle run
time by 27%. Stopping the unit earlier than
scheduled and taking up to 3 wk to load a
fresh batch of catalyst can impact the bottom
line by as much as $18 MM (the cycle
scenario assumes a planned yield quota of
$500,000/d, a total planned cycle revenue
of $500 MM, and replacement catalyst
and downtime costs of $18 MM).
Cost impacts: Lost revenue and
poor return on investment. Maldistribution
results in lower yields, shorter
cycles and more downtime, which, together,
have a significant impact on unit
economics by increasing the total cost of
ownership (TCO) of each reactor (FIG. 1).
Moreover, each cycle requires an investment
in catalyst, hardware, feed and labor,
which is calculated based on the forecast
yield. If the yield is down and downtime
is up, then return on investment (ROI)
will be lower. The impacts of maldistribution
are unpredictable, and unpredictability
is difficult to budget.
Spotting maldistribution: It is all in
the temperature. The key to identifying

Hydrocarbon Processing - December 2022

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