Hydrocarbon Processing - December 2022 - 16

Catalysts
maldistribution is monitoring and understanding
temperature variations in the
catalyst bed (FIG. 2A). The development
of hotspots is an important indicator that
a reactor is suffering from maldistribution.
Hotspots are problematic, as they
increase the weighted average bed temperature
(WABT) of a catalyst bed. This
results in the EOR temperature being
reached sooner than expected, and the
early termination of the catalyst cycle life.
The pattern (or distribution) of
hotspots provides valuable information
about the probable cause of maldistribution
(FIG. 2B). For example, if the whole
north side of a reactor is hotter than the
south side, this suggests that the north
(hotter) side is experiencing restricted
liquid/gas flow, while the south (colder)
side experiences higher liquid/gas flow,
possibly because of a tilted catalyst bed
A
surface or a tilted internal component,
such as a distribution tray.
What is (really) causing your maldistribution?
The first response of most reactor
operators whose units are suffering
maldistribution is to think that their hardware
is to blame. For example, they may
assume that their distribution trays have
not been installed properly or that they
are faulty. However, more often than not,
the problem does not lie in the hardware,
but somewhere many do not look: in the
quality of the catalyst loading.
Catalyst loading controls four key variables
that are central to the liquid/gas flow
and thermal characteristics of a catalyst
bed: the density, homogeneity, profile of
the catalyst bed and the catalyst integrity.
Get any of these wrong and, no matter
how robust the processes are or how well
B
690˚F
680˚F
675˚F
T1
670±2˚F
Actual ΔT
670±2˚F
T1
T1
660±2˚F
T1
T1
670±2˚F
680±2˚F
T1
670±2˚F
Time 1
T1
T1
670±2˚F
690˚F
680˚F
675˚F
T1
670±2˚F
T1
685±2˚F
T1
680±2˚F
660±2˚F
Time 2
T1
T1
688±2˚F
670±2˚F
the hardware is performing, the reactor
will likely suffer maldistribution and reduced
performance.
Even with industry-leading distribution
trays that offer near-perfect catalyst wetting
(FIG. 3), poor-quality catalyst loading
can reduce the performance advantages offered
by optimized distribution hardware.
Low average catalyst bed density.
The average density of the catalyst bed describes
how tightly individual catalyst particles
are packed together throughout the
entire bed (FIG. 4). If the average density
of the bed is too low, then the total mass of
the loaded catalysts will be lower, resulting
in reduced overall catalyst activity.
Low homogeneity. Homogeneity deT1
680±2˚F
Time
3
T1
Expected ΔT
Decreasing reactor performance
T1
>800˚F
730˚F
710˚F
T1
700±2˚F
T1
690±2˚F
690±2˚F
T1
Days on Stream
T1
680±2˚F
T1
T1
720±2˚F
708±2˚F
>1000˚F
800˚F
745˚F
T1
690±2˚F
T1
740±2˚F
T1
735±2˚F
680±2˚F
Time 4
T1
T1
745±2˚F
735±2˚F
scribes how the localized density of the
catalyst bed varies throughout its volume.
Low homogeneity reflects significant
localized density variations, even if the
average density of the bed meets expectations.
If local density varies, then fluids will
preferentially flow through lower-density
zones, thereby leaving higher-density
zones poorly utilized and prone to the development
of hotspots. Moreover, WHSV
is lower than average in high-density zones,
while it is higher than average (not to say
significantly higher) in low-density zones.
Poor catalyst integrity. Catalyst integT1
730±2˚F
730±2˚F
FIG.
2. Onset of maldistribution shown by increasing and more variable radial ΔTs (A).
Maldistribution may cause localized hotspots to form, resulting in high radial ΔTs (B).
Conventional tray
10%-15% wetting
Bubble-cap tray
20%-30% wetting
Proprietary HD traya
Near-perfect wetting
rity describes the extent to which catalyst
particles remain intact during loading. Broken
catalyst particles form fines and dust
that plug the interstitial space between
particles, and prevent liquid and gas from
freely and homogeneously circulating between
all catalyst particles. Increases in loInert
material
Dry catalyst
Wet catalyst
FIG. 3. Well-designed distribution trays can offer near-perfect catalyst wetting, essential for
optimal liquid/gas distribution.
16 DECEMBER 2022 | HydrocarbonProcessing.com
FIG. 4. Poor-quality catalyst loading results in
poorly aligned catalyst particles (left), larger
void spaces and a lower total mass of the
loaded catalysts. High-quality catalyst loading
(right) results in well-aligned catalyst particles,
smaller void spaces and a higher total mass of
loaded catalysts.
Reactor Radial ΔT
Onset of maldistribution

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Hydrocarbon Processing - December 2022

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