Fouling processes coking mechanisms

In a previous work [8], neither parallel or series coking mechanism was found to be satisfactory because the predominant fouling mechanism changes with temperature. Since the dehydrogenation reaction and coking formation takes place on the same type of active sites, a combined parallel and series mechanism is assumed in which ())( can be expressed in terms of process temperature, process time and the concentration of the respective foulents, as -c(t>T)= k(-B YcbCT) PB(t) + kcD Ycd(T) Pd(0 (2)... 

Those deactivation models accounting for both coke and metal sulfides are rather simple. Coke and metals foul residue hydrodesulfurization catalysts simultaneously via different processes, and decrease both intrinsic reaction rate and effective diffusivity. They never uniformly distribute in the commercial reactors. We have examined the activity and diffusivity of the aged and regenerated catalysts which were used at the different conditions as well as during the different periods. This paper describes the effects of vacuum residue conversion, reactor position, and time on-stream on the catalyst deactivation. Two mechanisms of the catalyst deactivation, depending on residue conversion level and reactor position, are also proposed. 

Fouling as a result of chemical reaction is generally associated, although not exclusively, with the processing of crude oil and petroleum fractions. The mechanisms involved include polymerization of components because of the presence of metal ions, and oxygen and coking (break-down reactions) reactions. Chemical... 

The deposits usually accumulate in the downstream separators, heat exchangers, and fractionating towers, and foul the transfer lines, eventually causing unit shutdown. Equipment fouling by the coke-like sediments formed in residual oil hydroconversion or the hydrocracking process can lead to enormous financial burdens in terms of increased costs of operation, maintenance, and shutdown. The refiners often use chemical and mechanical treatments to remove the deposits from the equipment Some refiners use antifoulant chemical additives to the feed to suppress the formation of sediments. 

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