Materials challenges in cyclic carburizing and oxidizing environments for petrochemical applications

Steam cracking is a petrochemical process that cleaves a broad range of hydrocarbon feed molecules into a variety of light olefinic products, including the highly desirable ethylene. Over the course of a cracking operation using a feed mixture of a saturated hydrocarbon and steam around 900-1000 degrees C in tubular alloy coils located in fired heaters, coke inevitably forms on the inside surfaces of the furnace tubes and must be burned off and/or spalled off periodically using steam or a steam-air mixture. Furnace tube materials are predominantly based on chromia-forming alloys; such alloys can degrade by carburization and oxide-carbide conversion in such a mixed carburizing-oxidizing environment. These hurdles have been largely overcome by using an alumina-forming material that provides superior corrosion and coking resistance. Cracking hydrocarbons at much higher temperatures results in high selectivity to acetylene, which can be converted into many petrochemical products including ethylene. The desired hydropyrolysis reaction from hydrocarbons to acetylene can be realized in a reverse-flow reactor operating above 1500 degrees C in a scaleable manner. The reactor elements include ceramic components that are placed in the hottest regions of the reactor, and must withstand temperatures in the range of 1500-2000 degrees C. Moreover, the materials in the hot zone are exposed alternately to a regeneration (heat addition) step that is mildly oxidizing and a pyrolysis (cracking) step that is strongly reducing with a correspondingly high carbon activity. This paper addresses the thermodynamic stability of selected ceramic materials based on alumina, zirconia, and yttria for such an application. Results from laboratory tests involving the exposure of these ceramic materials to simulated process conditions followed by their microstructural characterization are compared with expectations from thermodynamic predictions.