Formation during hydroprocessing

Hereafter we focus on a detailed understanding and model description of coke formation on catalysts in a trickle-bed reactor during hydroprocessing of VGO under the severe conditions mentioned above. Firstly, we will address the nature of the coke deposits in relation to that of the catalyst. A distinction between catalytic and thermal coke is made, based on information obtained from analytical techniques as well as from re-testing of the spent catalysts. Secondly, the extent of coke formation is dealt with on the basis of both experimental and modelling work. In this part the impact of vapour liquid equilibria is shown to be of prime importance. 

Slower coke formation on the carbon-supported catalysts than that on the y-Al203-supported catalysts has usually been attributed to the neutral surface of the carbon support. This should diminish the interaction of the support with N-bases that are one of the contributors to coke formation. However, the decreased coke formation on carbon-supported catalysts compared with the y-Al203 catalysts was also observed during hydroprocessing of heavy feeds such as In this case, coke deposition was dominated by fouling... 

A comprehensive study on coke deposition in trickle-bed reactors during severe hydroprocessing of vacuum gas oil has been carried out. On the basis of results obtained with different catalysts on the one hand, and analytical and catalytic characterisation of the coke deposits on the other, it is argued that coke is formed via two parallel routes, viz. (i) thermal condensation reactions of aromatic moieties and (ii) catalytic dehydrogenation reactions. The catalyst composition has a large impact on the amount of catalytic coke whilst physical effects (vapour-liquid equilibria, VLE) predominate in determining the extent of thermal coke formation. The effect of VLE is related to the concentration of heavy coke precursors in the liquid phase under conditions which promote oil evaporation such as elevated temperatures. A quantitative model which describes inter alinea the distinct maximum of coke deposited as a function of temperature is presented. 

C-Me-C, Me-C and C-Me-S may be involved in hydrogen activation and transfer. On contact with H2, these entities shall be converted to Me-CH-Me, HC-Me-CH, Me-CH and HC-Me-SH because of a higher strength of the C-H bonds compared with Me-H bond, of course, under certain conditions, formation of the Me-H structures and their involvement in catalysis could not be ruled out. This only indicates a significant complexity during the initial stages of hydroprocessing reactions over carbon-supported catalysts. 

Deactivation of heavy oil hydroprocessing catalysts is driven by two factors coking and metals buildup. It is well documented that coke formation is responsible for the rapid initial activity decay, which occurs during the first hours of operation (generally the first 100 h) and then apparently reaches equilibrium, whereas metals are accumulated during the whole cycle in a linear fashion (Furimsky and Massoth, 1999 Sie, 2001). The contribution of these two processes to the global catalyst deactivation rate can be expressed as follows ... 

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