Metal dusting is a dramatic and catastrophic high-temperature corrosion mechanism. As the term implies, the corrosion process converts a structural alloy into a pile of dust. Not good.
Metal dusting occurs in carbon-rich atmospheres, so it is a serious risk at chemical and petrochemical plants that process hydrocarbons. A New Jersey-based team looks at the challenges of selecting hot zone materials for a reverse-flow pyrolysis reactor team in two newly published papers. The team is a collaboration between researchers at ExxonMobil Corporate Strategic Research and Princeton University.
At present, steam cracker technology produces light olefin “feed molecules” for ethylene production at process temperatures in the 900-1,000°C range, but the efficiency is low. For example, a steam cracker using a naphtha feed is only 30 percent selective for ethylene at 900˚C. However, reverse-flow reactors, which operate at very high temperatures in the 1,700-2,000°C range, are 70 percent selective for acetylene. Acetylene (C2H2) is easily converted to ethylene (C2H4) by standard hydrogenation processes, hence, the interest in replacing steam cracker technology with reverse-reactor pyrolysis technology, if the materials issues can be solved.
The environment in a pyrolysis reverse-flow reactor is hellish with high temperatures, temperature fluctuations, and gaseous atmospheres that cycle between oxidizing and reducing. The reactor is consists of two packed bed heat exchangers placed back-to-back to sandwich a hot zone in the middle of a bed of solids. In operation, fuel and air first flow through the bed and combust in the middle to create a hot zone that is maintained at 1,500-2,000°C. Next, hydrocarbons flow in from the opposite direction and they are pyrolyzed, or cracked, with steam that gathers heat from the hot zone. The temperature fluctuates 100-500°C every several seconds with each cycle. In addition, the hot zone environment goes from a mildly oxidizing atmosphere during the combustion phase to a strongly reducing atmosphere during the pyrolysis phase. A typical lab-scale reverse-reactor run last between 4 hours and 5 days, subjecting the test materials to thousand or tens of thousand of oxidizing-to-reducing atmosphere cycles.
Noting that the melting point of most high-temperature structural alloys is less than 1,600°C, the authors looked at candidate oxide material that could withstand the extreme environment. Components in the hot zone that can be made of ceramic include honeycomb monoliths, mixer plates, and refractory bricks.
In their first paper published in the International Journal of Applied Ceramic Technology, they report on four oxides: alumina, magnesia-partially stabilized zirconia, yttria-stabilized zirconia, and yttria. They found the alumina “had insufficient temperature capability” to handle the hot zone. Magnesia evaporated out of the zirconia at high temperatures, thus negating the stabilization. YSZ materials suffered from a dusting-like degradation, and the authors conclude that yttria is the most promising candidate material.
That outcome might be interesting enough by itself, but they followed up with a deeper investigation of the degradation of YSZ in a paper that was published in the December issue of the Journal of the American Ceramic Society. They call the phenomenon “ceramic dusting” because the YSZ tended to crumble similar to the way metal dusting happens.
To isolate the mechanisms, they studied a porous YSZ (similar to service component morphologies), a fully dense YSZ (to eliminate porosity effects), and a single crystal YSZ (to eliminate grain boundary effects). All three YSZ morphologies were affected: the carbon found its way in through the surface, grain boundaries, or lattice.
The abstract explains that both the combustion and pyrolysis reactions set the stage for a harmful reaction to happen in the next cycle. In a diabolical synergy, the pyrolysis reaction carburizes the YSZ by carbon diffusion through porosity, grain boundaries, and the lattice, and creates a porous, nonprotective carbide layer. Carbon builds up in the pores during pyrolysis and during the oxidizing combustion step, graphite and (oxy)carbide are reoxidized. The corrosion progresses “by a repetition of oxide-carbide interconversion, carbon precipitation, and reoxidation steps.” As the carbon and reaction products build up, they push the grains apart, eventually loosening them enough so that they shed as dust.
By the way, ACerS’ Refractory Ceramics Division and its St. Louis Section are tackling similar issues in March at the groups’ annual refractories symposium. The theme is “Refractory Challenges in the Chemical and Petro-Chemical Industries.”
For more information on ceramic dusting see these papers:
“Materials Challenges in Reverse-Flow Pyrolysis Reactors for Petrochemical Applications,” by C.M. Chun, S. Desai, F. Hershkowitz, P.F. Keusenkothen, G.D. Mohr, and T.A. Ramanarayanan, International Journal of Applied Ceramic Technology, doi:10.1111/j.1744-7402.2012.02848.x.
“Ceramic Dusting Corrosion of Yttria-Stabilized Zirconia in Ultra High Temperature Reverse-Flow Pyrolysis Reactors,” by C.M. Chun, S. Desai, F. Hershkowitz, T.A. Ramanarayanan, Journal of the American Ceramic Society, doi:10.1111/jace.12035.