the effect of coking on kinetics of hds reaction under steady and transient states

a study was made of the coking of a commercial fresh sulfide ni-mo/al2o3 catalyst in a fixed-bed reactor. the catalyst was coked using different coke precursors in the gas oil under accelerated conditions at temperatures of 400 to 450°c to yield different deactivated catalysts containing 2-20 wt% c. two cases were studied; crushed catalyst without diffusional resistance and extruded pellets with diffusional resistance. physical properties and catalytic activities of the coked catalysts were measured using the thiophene sulfur removal in the gas oil.it is concluded that coking occurs by selective deactivation on hydrotreating catalyst and the experimental results of the catalyst activity under different operating conditions, obey a power law as a function of the coke cantent. in the pellet catalyst showed a lower rate of coking and deactivation in comparison to the catalyst without diffusional resistance. it is inferred that high level of coke content ( higher than 12 wt% c) affects the tortusity factor of the catalyst,considerably.in the study of transient deactivation, initial activity of the catalyst  was derived by a time variable function, then this equation was used in  dynamic model of hydrodesulfurization reaction in a packed bed reactor to determine  the activity change of the catalyst in the reactor during actual operational conditions.