Sulfided surfaces, coke formation

How does the composition of the metal surface change during pyrolysis Surface reactions that have been identified on Incoloy 800 surfaces include oxidation, reduction, sulfidation, desulfidation, and coke formation (14). Do increased concentrations of nickel and chromium ever occur in the surface in view of the fact that iron is incorporated into the coke Tsai and Albright (14) found increased iron concentrations on inner surfaces of tubes used for pyrolyses. 

Although more information is needed to determine details concerning factors that favor inactive coke formation, relatively high levels of surface sulfides probably promote formation of such coke. On the other hand, metal oxides on the surface likely favor production of active coke. Sulfiding the reactor tube immediately upon completion of the decoking step would form metal sulfides. An aluminized surface, such as provided by the alonized Incoloy 800 reactor, also has been found to be an effective way to prevent the production of active coke. Quite possibly, the initial type of coke formed on the just-cleaned tube would have an important effect on the length of time a reactor tube could be used in a commercial plant before decoking would be required. 

For all metal reactors, the yields of ethylene and total coke were often found to vary significantly with the past history or pretreatment of the reactor. Pretreatments used were with either oxygen, steam, or hydrogen sulfide. Total coke is defined here as the sum of the CO, C02, and net coke it is postulated that CO and C02 were formed by oxidation of part of the coke which formed and that net coke is the amount of coke left on the reactor walls at the end of the run. The results for runs M01 and D44 are one example of the large differences in yields that can be obtained in reactors of the same material of construction (see Table I). Run M01 was made after a stainless steel 304 reactor had been treated with hydrogen sulfide. Hydrogen sulfide results in the formation of metal sulfides (8) and acts in most cases to passivate the surface so that coke formation is minimized. D44 was made using a new stainless steel 304 reactor, whose surface became very active when it was treated with steam. 

In the present Investigation, considerable Information has been obtained to clarify the role of the surface reactions that occur during pyrolysis. Reactions Investigated Include the formation and destruction of metal oxides and metal sulfides. Information has also been obtained relative to coking and decoking. 

---