38
December
2014
HYDROCARBON
ENGINEERING
resistance of the body. Ceramic materials, however, offer no
panaceas, and no easy answers.
Ceramics have much lower thermal conductivities than metals,
and their sintered bonds are much less ductile than metallic bonds,
and therefore rapid swings in temperature in dense technical
ceramic grade bodies will be met with brittle fracture. Erosive
particles skip across the harder ceramic surface, not displacing
material, and not scouring the surface. Where they do have an
impact is in the fine cracks that begin to appear after repeated
thermal cycles; the repeated impact between the particles and
cracks begins to expand them. Subsequent thermal shocks expand
them further still. Failure here is sudden and catastrophic to the
ceramic component.
Refractory materials are analogous to alloy steels. They are
comprised of a number of constituents, selected for their
synergistic effects. Unlike alloy steels, refractories require a
distinct binder to cement them together. Refractories typically
have lesser abrasion resistance properties than technical ceramics,
but they are uniquely qualified to survive repeated thermal shocks.
In fact, it is refractory material that lines the reactor, regenerator,
and the transfer lines. These materials are adequate for that use,
but not up to the task for the air grid nozzle, where velocities are
higher, and the effects of erosive wear more pronounced.
The binders used in refractory materials typically have much
better thermal conductivity, and better resistance to thermal
shock than the base materials. Also, by definition, refractory
materials are not fully dense, and it is that microporosity that adds
thousands of pores, which function as crack stoppers, adding to
the ‘toughness’ if not the strength, of the material.
The preferred angle of impact in a ceramic body is the acute
angle, so that the erosive particles rebound off the harder ceramic
particles. As the angle of impact approaches 90˚, erosive particles
can hammer away at the typically softer bond phase, and begin
displacing harder ceramic particles that way.
It is worth noting that the angle of incidence between erosive
particles and the exposed surface of the nozzles is generally acute,
as the flow of air and particles swirls across the tips of the nozzles.
It was not until the late 1980s that a few plants started using
ceramic for the air grid nozzles. They saw the benefit of the
monolithic castable materials used in areas covering the grid and
in the cyclone. Requirements for length of campaigns have been
increasing, along with temperatures and pressures, and it became
imperative to find a material that would be up to the task. Over
the last 20+ years a wide variety of alloys, weld overlays, and
ceramic materials have been tried in grid nozzles, with varying
results. The bulk of the ceramic materials have been fully dense
technical ceramics, selected purely for their abrasion resistance.
Not surprisingly, most of these suffered from early failure due to
thermal shock.
Air grid nozzles feed comparatively cool air into the
regenerator typically operating in excess of 700 ˚C resulting in
significant steady state thermal gradients and associated thermal
stress within the nozzle. Under dynamic operating conditions the
nozzle can see rapid temperature changes which induce significant
thermal gradients within the material. A refractory grade material
would be the more appropriate selection from a thermal shock
point of view, but any conventional refractory grade material
available at the time would have a softer, more vulnerable, binder
and be more prone to abrasive wear.
Beginning a half dozen years ago, Blasch began developing a
unique refractory grade material with a highly proprietary binder
chemistry that, when processed under specific conditions in the
production facility, creates a binder with a very specific chemical
composition yielding excellent thermal shock resistance, and a
very high hardness. This development allowed for the creation of
refractory grade composition that tackles abrasion like a fully
dense technical ceramic. And because it is a refractory, it does
retain the network of microporosity within the body that
contributes to the crack stopping toughness typical of refractory
materials.
This composition, now commercially known as Altron, is
compatible with the forming processes used at Blasch, and it is
possible to cast complex, close tolerance, net shapes repeatedly,
from the thimble sized, up to those weighing hundreds of
pounds, with dimensional accuracies of up to 1% on most
geometries.
One other area of dissimilarity between metals and ceramics
is thermal expansion. Metals expand at a much greater rate than
ceramics and the higher the temperature, the greater the
discrepancy in size at operating temperature. This makes it
impossible to directly affix the ceramic body to the steel
structure at the temperatures seen in the regenerator. It is
necessary to encapsulate or contain the nozzle or a portion of
the nozzle, with or in a steel structure such that the ceramic part
remains in compression even as the steel is expanding away from
it. Further, this should be done in the manufacturer’s plant so that
the plant owner can concentrate on the installation of the parts,
not the assembly. This installation should involve the simple
welding of one metallic part to another.
There are now a number of installations of Altron air grid
nozzles in the field, with excellent results to date, and attention
turns to other applications within the FCCU, perhaps even
including segmented tongue and groove tiles for lining larger
cyclones or even fluidised beds.
Conclusion
The drive toward longer campaigns, combined with higher
temperatures and pressures, ask more of the materials used in
today’s refineries. Knowledge of non-metallic materials and
when best to use them can provide the operator with the tools
they need to put dollars in their pocket; the knowledge that not
all ceramic materials are created equal. Just as with metallics, in
addition to single element sintered bodies, there are a wide
variety of ‘alloys’ available that offer excellent compromises
between incongruent properties. In the FCCU, those properties
are abrasion resistance and resistance to thermal cycling and
shock. A careful review of operating conditions and a good
description of the optimal balance of properties desired, can help
both the supplier and the operator settle on the best material for
the job.
References
1. Chemical Engineering Digital Library,
.
chemeddl.org/services/chemteacher/index.php?option=com_
content&view=article&id=36
2. COMPUTATIONAL MODEL OF DUCTILE EROSION BY SINGLE
PARTICLE IMPACT, Chandrakant Rai, West Virginia University,
Morgantown, WV, 2000
3. COMPUTATIONAL MODEL OF DUCTILE EROSION BY SINGLE
PARTICLE IMPACT, Chandrakant Rai, West Virginia University,
Morgantown, WV, 2000.
