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Oxide ceramics handle heat, struggle with corrosion in reverse-flow pyrolysis petrochemical reactors 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. 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 looked at the challenges of selecting hot-zone materials for a reverse-flow pyrolysis reactor 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) easily converts 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 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 lasts between four hours and five days, subjecting the test materials to thousands or tens of thousands of oxidizing-to-reducingatmosphere cycles. Because 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 brick. In the team’s 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. The authors concluded 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 2012 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 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. 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. Keusen- 18 www.ceramics.org | American Ceramic Society Bulletin, Vol. 92, No. 2 (Credit: JACerS; Wiley.) Cross-sectional SEM images of the corroded polycrystalline 8 mol%-YSZ coupon after testing in the reverse–flow pyrolysis reactor for about 70 hours showing the corrosion of polycrystalline bulk material to fine-grained dust. research briefs


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