An evaluation of the coking resistance of a high performance oxide dispersion strengthened(ODS) alloy tube

JGC Corporation and Special Metals Corporation have developed a new ethylene cracking tube made of oxide dispersion strengthened (ODS) TNCOLOY(TM) alloy MA956 with extremely good resistance to coking and carburization, excellent creep strength and high melting point. Laboratory and long term coking-decoking cycle naphtha cracking bench scale tests have been performed and compared with conventional 25Cr-35Ni alloys. Coke deposition rate of MA956 tubes is about half of conventional alloy tubes and no carburization was observed even for high severity operation conditions (Coil Outlet Temperature : 950 degreesC) during which the conventional tubes developed about 0.3mm depth of carburization. Analysis of the coke formed during the naphtha cracking bench scale tests revealed very low levels of metallic compounds in the deposits on the MA956 tubes compared with conventional 25Cr-35Ni alloys. The presence of such metallic compounds normally accelerates the catalytic coking mechanism. The application of MA956 cracking tube will allow extended decoking intervals, longer lives of tube and flexible operation for cracking furnaces. The manufacture of MA956 as 12 metre long tubes is available now and the installation of an ODS cracking tube in a commercial furnace to evaluate its performance is scheduled to take place shortly.