(617di) Preparation of MnCr2O4 Spinel Coating for Anti-Coking Use during Light Naphtha Thermal Cracking

Hydrocarbons thermal cracking is the major process for producing olefins and aromatics. Coking is a serious problem for Fe-Cr-Ni alloys used in hydrocarbon thermal cracking environment. Coke layer formed on the inner surface of coils hampers the heat transfer from the coils to the cracking feedstock and results in pressure drop over the coils. Therefore, the coke must be removed from the coils periodically by decoking, leading to the reduction of production yield. Moreover, severe carburization and metal dusting will occur during hydrocarbon thermal cracking. The alloys contains large amounts of Fe and Ni elements responsible for catalytic coke formation, which increases coking rate. It is an efficient method to inhibit coke formation to separate the Fe and Ni elements from hydrocarbons at high temperature. MnCr₂O₄ spinel is inert to hydrocarbons, and it is stable in high temperature carbonaceous environment. Therefore, MnCr₂O₄ spinel is a promising material for anti-coking use. This paper presents a novel two-step way to prepare MnCr₂O₄ spinel coating on HP40 alloy. The first step is Cr-Mn pack cementation process, and the next step is thermal oxidation process. The elemental distribution changes after the first step, and Cr-Mn coating formed on the alloy surface. The coating is rich in Cr, lack in Fe and Ni, and contains proper content of Mn. The thermal oxidation process was conducted in a quartz tube furnace under low oxygen partial pressure created by H₂-H₂O gas mixture. The surface morphology and elemental composition of the as-prepared spinel coating were characterized by SEM, EDX, respectively. The cross sectional morphology and elemental distribution of the coating were also examined by SEM and EDS maps. The phase composition of the coating was detected by XRD and Raman spectra. According to the analyses and tests above, a compact, uniform coating with a favorable thickness was prepared on a selected alloy sample. Coking tests were also adopted to evaluate the coking behavior of the spinel coatings. The coking rate, coke morphology and coke microstructure were characterized by weighing, SEM images and Raman spectra, respectively. The results show that coking rate on spinel coated samples is decreased by about 70%, indicating good anti-coking property of the spinel coating. Filamentous coke is completely inhibited on spinel coated samples, and only granular coke appears on the samples. According to the analyses of Raman spectra, cokes formed under different conditions show considerable difference in microstructure.