Oct-2017

Use of non-abrasive ceramic materials in the FCCU

Fluid catalytic cracking (FCC) technology has been a part of the petroleum industry since it was introduced in the US in 1942 to help feed the voracious appetite of the first truly mechanised war effort.

Tim Connors, Ted Collins and Jeffrey Bolebruch
Blasch Precision Ceramics

Article Summary

It remains one of the most important conversion processes used in petroleum refineries today. This process is widely used to convert the high molecular weight hydrocarbon fractions of petroleum crude oils to more valuable gasoline, olefins, and other products. The FCC process vaporises and breaks the molecules of the long chain hydrocarbons into much shorter molecules by contacting the feedstock, at high temperature and moderate pressure, with a catalyst.

The FCC process occurs in a portion of the refinery referred to as the fluid catalytic cracking unit (FCCU), which contains, among various other ancillary pieces of equipment, a pair of fluidised bed vessels 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 micro-porous 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 macro-hardness of the composition. This characteristic peak micro-hardness 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 bonds1) 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 surface2. 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 degrees. 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 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.