The Deactivation Mechanism Of FCC Catalyst By Vanadium And Nickel Contamination

The fluid catalytic cracking?FCC?is a secondary refining process used primarily for conversion of high-boiling,high-molecular weight hydrocarbon fractions of crude oil to more valuable products.In this work,the various modes of the deactivation of FCC catalyst by vanadium and nickel were investigated and the results are discussed.Findings obtained are believed will close the existing gaps of knowledge on the mechanisms of deactivation of FCC catalyst by vanadium and nickel.The catalytic behaviors of the individual metals,nickel and vanadium,are demonstrated.The catalytic implications of vanadium include activity decay,non-selectivity to gasoline and selectivity to heavier hydrocarbon cut,whereas nickel effect is mild on the catalyst activity but greatly influences selectivity.At high vanadium loadings,important catalytic properties such as crystalline structure collapse,acid sites destruction and surface area degradation occur.Loss of acid sites correlates with activity loss.Coke yield exhibits a similar trend,decreasing with acid and surface area losses.Conversely,the coke formation characteristic of nickel is due to nickel dehydrogenation activity,and an increase in nickel concentration in the catalyst raises coke yield.The interactive effects of nickel and vanadium co-depositing on FCC catalysts were studied by extensive characterization of catalyst samples after treatment under simulated FCC conditions.Overall,the results revealed that vanadium,above a certain level in the catalyst under hydrothermal condition,is highly detrimental to the physio-chemical-structural properties and activity of the catalyst.Conversely,nickel hardly affects the catalyst properties;however,its co-presence in the catalyst reduces the destructive effects of vanadium.Nickel plays an important role as a vanadium scavenger,forming a refractory vanadium-nickel species by absorbing a part of the vanadium that migrates to destroy the zeolite structure,hence reducing the destructive actions of vanadium.Nickel is found to exist mainly as NiAl₂O₄ on both the surface and the bulk of the catalyst,with some traces of NiO and hydroxylated nickel.The formation of a large amount of NiAl₂O₄on alumina support by nickel indicates its preferential location in the alumina component of the FCC catalyst.Vanadium exists largely as bulk V₂O₅ with very low proportion of AlVO₄ solid solution.The coke on the modelled E-cats which consists of graphitic carbon layers with dominantly aromatic carbon species,distributes within the micro-and macro-pore spaces.In unsteamed and metal doped catalysts,coke is located at the entrance of micropores,blocking access to active sites,while in metal doped and steam-aged catalysts,coke is distributed within the micropores and macropores,majorly in the macropores owning to the initial occupation of the micropores by metals.Since micropores of FCC catalysts are seriously implicated by vanadium,a study design to understand the effects vanadium on the micropores of Y zeolite revealed interesting results.In the presence of steam,and in addition to the fundamental destructive effects such as acid sites destruction and dealumination,vanadium caused excessive evolution of non-intracrystalline mesopores and structural damage to USY zeolite.The evolved mesopore size averaged about25.0 nm at 0.5 wt%vanadium loading.A mechanism of mesopore formation based on accelerated dealumination has been proposed.Vanadium immobilization experiments conducted to mitigate vanadium migration into the framework clearly showed vanadium is mobile at reaction conditions.The interaction of vanadium with the passivator limits and decreases mobility,and acidity of vanadium into channels of zeolite capable of causing huge structure breakdown and acid sites destruction.Based on these interesting results,the last part of the work studied the behavior of a mesostructured USY on the metals activities.Surprisingly,the mesoporous formulated catalyst is more stable to metals deactivation than that of the conventional microporous zeolite.This improved catalytic behavior could be attributed not only to the enhanced diffusion and faster desorption of the adsorbed oil molecules within the pores,but also to the increased accessibility to the undestroyed acidic sites in the pores due to well interconnected pore structure and the large mesopore size that developed by mesostructuring.Compared with the parent USY sample,the gasoline yields for the mesoporous samples:USY-5 and USY-10 are higher by 2.99 and3.68 wt%,while the diesel yields are higher by 6.34 and 4.71 wt%,respectively,indicating improved liquid yield.This research work has revealed important characteristics of the deactivation of FCC catalyst by vanadium and nickel.Vanadium is mobile at FCC unit reaction conditions,reaching the zeolite framework where it destroys the active sites of the catalyst by inter/intra-particle migration mechanism.Unlike vanadium,nickel implicates selectivity of the products,favoring coke and gases production.Nickel is located mainly in the matrix of the FCC catalyst,while vanadium prefers zeolite.With an effective vanadium passivator,the harmful effects on the catalyst could be reduced.A new catalyst system comprising mesoporous zeolites could be optimized to control excessive vanadium poison.