Coke formation decoking effect

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. 

The effect of the decoking operation on coke formation in subsequent cracking runs was also studied and the results shown pictorially in Figure 10. On an uncoated Incoloy 800 tube, the high coke yield remained approximately constant throughout four successive steam cracking/air decoking cycles. This tube was subsequently coated with silica to produce an immediate xlO... 

Some deactivation processes are reversible. Deactivation by physical adsorption occurs whenever there is a gas-phase impurity that is below its critical point. It can be reversed by eliminating the impurity from the feed stream. This form of deactivation is better modeled using a site-competition model that includes the impurities—e.g., any of Equations (10.18)-(10.21)— rather than using the effectiveness factor. Water may be included in the reaction mixture so that the water-gas shift reaction will minimize the formation of coke. Off-line decoking can be... 

C/OKE FORMATION IS AN UNDESIRABLE SIDE EFFECT of many chemical operations because it leads to costly decoking steps, reduced efficiency of operation, and increased rates of metal corrosion and erosion. There is, therefore, a tremendous incentive to minimize or at least better control coke deposition in numerous commercial units. 

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