| Incoloy 800, HK-40, HP-40, CORET, and aluminized Incoloy 800 alloys often used in ethylene furnaces were pretreated with {dollar}rm Hsb2/Hsb2O,{dollar} CO, CO/H{dollar}sb2{dollar}O, CO{dollar}sb2,{dollar} air, Ar, N{dollar}sb2,{dollar} He, and H{dollar}sb2.{dollar} For pretreatments at 650 to 1000{dollar}spcirc{dollar}C from 0.25-24 hours, the surfaces for the first three alloys became enriched at depths up to 3.0 microns with chromium, manganese, and often with titanium, aluminum, and silicon. The chromium and manganese concentrations sometimes increased to 60-80% and 20-35% respectively, and the iron and nickel concentrations were then reduced to {dollar}{dollar}50%. These surface changes depend on the temperature, time of pretreatment, and oxidation potential of the pretreating gases.; Coking experiments were performed with pretreated alloys at 850{dollar}spcirc{dollar}C using ethane/steam and toluene/nitrogen mixtures with volumetric ratios of 3:1 and 0.02:1 respectively. For ethane/steam coking experiments, more coke was formed but the adherence of coke was generally less as compared to toluene/nitrogen coking experiments. Alloys pretreated with {dollar}rm Hsb2/Hsb2O{dollar} or CO generally resulted in lower rates of coke formation and poorer adherence between coke and the metal surface. Decoking experiments were performed for 0.5-6 hours at 850-950{dollar}spcirc{dollar}C using either {dollar}rm Hsb2/Hsb2O, Nsb2/Hsb2O{dollar} mixtures, or air. For the first known time, a relationship has been found between the method of decoking and the rate of coking during pyrolysis. {dollar}rm Nsb2/Hsb2O{dollar} and especially air cause rapid decoking but then result in rapid coke formation and strong adherence of coke to metal surface, as compared to {dollar}rm Hsb2/Hsb2O.{dollar}; Spalling information was also obtained. Spalled surfaces are often rich in iron, nickel, and silicon. Such surfaces promote coke formation and strong adherence between the coke and metal. Recommendations are given on how to improve ethylene plant operation. |
