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25Cr-35Ni base alloys are the most frequently used materials for steam cracking reactors. The influence of cyclic aging, reactor temperature, and adding sulfur containing compounds before or during cracking on the rate of coke deposition on a classical 25Cr-35Ni alloy is evaluated using a jet stirred reactor equipped with an electrobalance. As expected, the initial and asymptotic coking rate increased with increasing reactor temperature. Scanning electron microscopy coupled with energy dispersive X-ray (SEM-EDX) analysis indicated that more Ni and Fe is present on the surface at higher cracking temperatures. Presulfidation led to increased coke deposition and decreased CO yields compared to the reference. When a sulfur containing compound was added continuously, coke deposition increased significantly but carbon oxide formation was suppressed. A pronounced amount of coke was measured in the reactor, followed by suppressed generated amounts of carbon oxides downstream. When combined with the continuous addition of sulfur containing compounds, presulfidation has little effect. Depending on the conditions, the effect of aging of the material is different: during the reference run and when only presulfidation was applied, coking rates increased as the material aged. When sulfur containing compounds were added continuously, with our without presulfidation, coking rates decreased as the material aged. This can be related with increased amounts of MnCr2O4 and Cr2O3 observed by SEM and EDX analysis. [ABSTRACT FROM AUTHOR]

The coking tendency under steam cracking conditions of CoatAlloy, a newly developed multilayered Al barrier coating deposited on a commercial 25/35 Cr-Ni base alloy and aimed at reducing the coke formation under hydrocarbon atmosphere at >1100 K temperatures was investigated. It was benchmarked to the uncoated commercial 25/35 Cr-Ni base alloy with a known low coking tendency in ethane steam cracking in a pilot plant. The influence of process conditions, such as coil outlet temperature, presulfidation, continuous sulfur addition and aging was evaluated. The applied coating resulted in a reduced coking tendency as well as reduced yields of both CO and CO2 compared to the uncoated coil. The surface of both tested reactor materials was studied by means of SEM and EDX analysis. Further scale up was assessed by simulations of an industrial ethane cracker. All the findings show that the CoatAlloy barrier coating is capable of reducing coke formation and maintains its anticoking activity over multiple cracking-decoking cycles. [ABSTRACT FROM AUTHOR]