Hydrocarbon Processing - August 2022 - 35

Catalysts
A. KLINE, IGS Environmental, Richmond, Virginia
Case study: SCR catalyst plugging
Hydrogen (H2
to produce H2
posed of H2
) reforming is a process
from a compound comatoms.
A convection pass is
used to heat the process during reforming,
which creates nitrogen oxides (NOx
The NOx
).
-rich flue gas passes through a
selective catalytic reduction (SCR) reactor
before being exhausted into the atmosphere.
Lint-like fibers from the convection
pass and refractory liners create an
obstruction that builds up and fouls the
face of the SCR catalyst (FIG. 1).
The result is decreased NOx
reduction.
The higher pressure drops across
the catalyst and ultimately reduces H2
output. Typically, this buildup will require
an unscheduled outage for manual
cleaning, which can take as long as 5 d at
an estimated cost of $1 MM/d.
Lab study. In a study, samples of ash collected
from the catalyst were sent to a lab
for testing. Microscopy images indicated
hair-like particles as well as granular particles.
These particles combine and create
a dryer lint-like fibrous material.
Testing several online cleaning systems,
such as sonic horns, produced no
discernible results in breaking up the media.
However, the use of air cannon blasts
sufficiently broke apart the mass. Various
styles of screen material and design were
then tested to determine the optimal pitch
and diameter for operation in reformers.
Mitigation method. The exact cause of
the material being caught in the flue gas
is unknown, and it is difficult to pinpoint
and address all locations where the refractory
fiber may break loose. However,
once the material breaks free and accumulates
on the SCR catalyst, the dP will
increase and NOx
removal will decrease.
Bringing the unit offline to vacuum
the material is extremely costly and not
ideal; therefore, units often run at a reduced
output until the next turnaround.
FIG. 1. Typical buildup and fouling of a reformer SCR catalyst face.
Hydrocarbon Processing | AUGUST 2022 35
Case study: Methane reformer in
Texas. Based on these successes, the author's
company installed its screensa
upstream
of the SCR on a methane reformer
in Texas in 2016. The screen is designed
to capture these particles upstream of the
SCR catalyst.
The screen pitch is smaller than
the pitch of the catalyst, as it is critical
to catch all particles too large to pass
through the catalyst. Therefore, the company
selected a screen system designed
to achieve sufficient filtration and flow
optimization. The pressure drop was also
considered during the design phase in
selecting the screen pitch. The goal was
to install a screen to prevent buildup and
minimize pressure drop increase.
The author's company has installed
nearly 100 proprietary screensa
in front
of SCRs since the early 2000s, allowing
companies to maintain better output.
Air cannons. Air cannons are required
because the particles tend to become
trapped on the screen and block the gas
flow, much like fibers in a dryer lint trap.
When this happens, the gas will often
bypass certain catalyst areas and increase
NOx
emissions. Air cannons address this
challenge in two ways:
* Blasting the particles and
breaking them apart so they are
small enough to pass through
the filter and the catalyst.
* The blast is aimed to push the
remaining particles down to the
lower collection area where they
will not impact performance.
A CFD analysis was performed with
and without the screen in place. The results
showed that the screen improved
the uniformity index from 90% to 92%.
The protective screen is also a flow aid-
the better the flow, the better the SCR
performance (FIG. 2).
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