(371f) Al-Si-Cr Coating and Its Inhibition of Coke Formation during Naphtha Thermal Cracking

Thermal cracking of hydrocarbons is the major process for the production of olefins and aromatics. The process is carried out at high temperature in cracking coils which is made of Fe-Cr-Ni alloys. During the process, long whisker-like structure coke that extend from tube wall are formed with the catalytic of Ni and Fe, meanwhile, it tends to catch pyrolytic coke known as radical coke to form complex amorphous coke. The coke adhered to the walls inhibits heat transfer from the burners to the gas mixture, and reduces tube diameter causing a higher pressure drop. Coke must be removed from the coils periodically by decoking, which also reduce the production yield and shorten the coils life. Coating technique to inhibition coking is an effective measure to extend the decoking cycle and the life of the furnace tube. A chromium-aluminum-silicon coating was prepared on the Cr35Ni45 alloy by pack cementation processes. The obtained Al-Si-Cr coating possessed a thickness of about 130  and had a dense multiple layers structure,which consisting of an outer layer composed of Al-Si-rich compounds and an inner layer composed of Cr-Si-rich compounds. Al₂O₃, Cr₃Si and CrSi₃ were found at the surface of coating through XRD pattern. The microstructure of matrix subjected to the coating preparation process had no obvious change. The anti-coking properties were studied by coking experiments using light naphtha feedstock. The coating effectively inhibited coking formation. The SEM pictures revealed that the coke sample at the surface of Al-Si-Cr coating had noticeably different structures with the sample at the no-coating material surface. The later showed filamentous deposits while the former show pyrolytic nature without catalytic filaments. Contrast with no-coating material, the coating had a coke inhibition rate up to 72.5%, and after three times decoking cycles, the coke inhibition rate was still more than 61%.