36
December
2014
HYDROCARBON
ENGINEERING
containing catalytic cracking catalyst, which circulates
continuously from one vessel to the other through a set of
transfer lines. Those vessels are referred to as the reactor and
regenerator. The desired reaction occurs in the reactor, and over
time the surface of the catalyst particles is coated with coke, a
condensed hydrocarbon. Coke reduces the activity of the
catalyst and makes it necessary to continually remove
deactivated, or spent, catalyst to the regenerator, where the
coke is burned off in what is a very exothermic process. Once
the catalyst is cleaned, or regenerated, it is returned to the
reactor. This loop constitutes a continuous process that must
run every day, year after year, to yield maximum benefit.
FCC catalysts are typically microporous aluminosilicate
materials, and in a fluidised state, they can be quite abrasive.
Add to that high temperature and pressure, the presence of
steam, and a host of metallic and nonmetallic impurities, and
what one is faced with is an extremely difficult environment.
The impact of abrasive wear
The fluidised bed in the regenerator is maintained through the
use of compressed air, fed through a large number of nozzles
situated in one of a number of proprietary configurations. The
key element to performance in the regenerator is consistency,
and a uniform fluidisation must be maintained for maximum
regenerating efficiency. The size and shape of the orifices of the
air grid nozzles are critical to maintaining this uniformity. Any
abrasion to the nozzle has the potential to significantly impact
the geometry of the individual nozzle, and with enough damage,
eventually the overall performance of the regenerator.
When a grid nozzle wears, the shape of the flow path
through the nozzle changes, resulting in an alteration in the air
flow rate of the nozzle and, potentially, the flow pattern, as the
air flows into the bed. Distribute these changes in varying
magnitudes across the complete air grid and the resultant
variability in fluidisation and combustion can result in serious
performance issues. Catalyst regeneration rates can suffer and
reaction chemistry can change driving up catalyst deactivation
rates. Temperature distribution within the bed can change
from optimum design conditions resulting in unexpected
consequences such as HCN generation rates high enough to
carry through to flue gas discharge. The value of abrasive
wear resistance in FCCU air grid nozzles as it impacts process
consistency is significant.
Abrasion and metals
There are a number of different engineering strategies for
dealing with abrasive wear, but with regard to material
selection the generally accepted rule is the harder, the
better.
This premise can be a little tricky when it comes to metals
where in many cases resistance to abrasion is more heavily
influenced by specific constituents in alloyed compositions,
exhibiting higher individual hardness compared to the overall
average macrohardness of the composition. This characteristic
peak microhardness forms a basis for understanding abrasion
resistance in metals. Regardless, whether pure metal or alloy,
metallic compounds, as a class, are less hard than ceramic, and
because they deform under tensile stress (due to the elastic
nature of metallic bonds
1
) they are considered to be ductile
bodies; they deform, rather than fracture. Because they are
relatively soft as well as ductile, metallic material can be
displaced by erosive particles as they move across the surface.
2
This scouring effect continues for the length of the contact
between the erosive particle and the metal compound. Ergo,
the more acute the angle of impact, the longer the contact
path, and the greater the abrasive wear. The greater the angle,
the less total surface area is exposed to the effect of the
erosive particle and the less displacement occurs.
Consequently, metals wear faster at acute angles of impact
than at angles closer to 90˚. Further, metals begin to soften at
temperatures much lower than ceramic, and any excursions
can fatally undermine the material properties.
Abrasion and ceramics
Ceramics simply are harder, and this helps explain why more
often than not ceramic materials exhibit significant
advantages in wear applications. Ceramic materials,
surprisingly to some, share an important similarity to the
metallic compounds described above. They can be made
from a single ceramic constituent, or they may be ‘alloyed’ if
you will; made of multiple ceramic compounds. These
ceramic compounds do differ in one key way from metallic
alloys, the bond phase.
There are a number of grades of ceramic materials,
including structural, technical, refractory, and white ware.
Technical ceramics include tiles used in the Space
Shuttle program, gas burner nozzles, ballistic protection,
biomedical implants, jet engine turbine blades and missile
nose cones. Refractory ceramics include such things as kiln
linings, steel and glass making crucibles, and various
monolithic, or unformed, compositions used in the
petrochemical industry.
When one thinks of hard, abrasion resistant ceramic
materials, one is generally thinking of a technical ceramic,
with a fine grained, fully dense, sintered body. This is
analogous to the pure metallic composition described above.
The hardness of the single constituent dictates the abrasion
Figure 1.
Erosion resistance of ductile versus
brittle materials.
3
off the harder ceramic particles. As the angle of impact approaches 90 degrees, 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.
Figure 1. Erosion Resistance of Ductile vs. Brittle Materials
3
It wasn’t until the late 1980’s 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 resistanc . N t surprisingly, most of these suffered from early failure du to thermal
shock.
3
COMPUTATIONAL MODEL OF DUCTILE EROSION BY SINGLE PARTICLE IMPACT, Chandrakant Rai, West Virginia
University, Morgant wn, WV, 2000
Erosion in g/g ( x 10 ) [Ductile]
Ductile Material
Brittle Material
Angle of Attack in degrees
Erosion in g/g ( x 10 ) [Brittle]
