Vanadium catalyst activity after deposition

Fig. 42. Catalyst activity after vanadium deposition (Takeuchi et al., 1985).

Takeuchi et al. (1985) tested the catalytic activity of deposited Ni and V by use of a catalytic metal-free alumina base. These interesting results are shown in Fig. 42. After accumulation of 10wt.% vanadium on the catalyst, the alumina base, with little initial activity, has essentially the same activity for HDM and asphaltene cracking as the catalytic metal-... 

The uniform layer model of deposition has been implicit since Sato [3] and Newson [11]. This model leads to difficulties for a support surface of about 200 square meters per gram. A 20 weight % coke would occupy about 400 square meters, and 20 weight % vanadium pentasulfide would occupy 200 square meters. After a few months of operations, there would be 5 to 6 monolayer equivalent of deposits on the surface, so that the original cobalt-molybdenum surface would be completely covered. The remaining catalyst activity must be attributed to the activities of nickel and vanadium, which is perhaps ten times lower for the HDS reaction. 

For regeneration to be technically viable, it must be able to remove deposited vanadium and nickel quantitatively as well as the carbonaceous coke which was co-deposited. The catalyti-cally active metals should remain unaffected in amount, chemistry, and state of dispersion. The alumina support should remain intact, with the surface area, pore-size distribution and crush strength after treatment comparable to that of the original. To be economically viable, the process should be accomplished in a minimum of steps at nearly ambient temperatures and preferably in aqueous solution. The ultimate proof of any such scheme is for the catalytic activity of the regenerated catalyst to be equal to that of a fresh one. 

A number of refinery processes require the use of a fixed-bed catalyst These processes include catalytic reforming, hydrodesulfurization, hydrotreating, hydro-cracking, and others. These catalysts become inactive in six months to three years and are eventually replaced in the reactors with fresh catalyst during a unit shutdown. Many of these catalysts contain valuable metals which can be recovered economically. Some of these metals, such as platinum and palladium, represent the active catalytic component other metals such as nickel and vanadium are contaminants in the feed which are deposited on the catalyst during use. After valuable metals are recovered (a service usually performed by the outside companies), the residuals are expected to be disposed of as solid waste. 

Vanadium is present in crudes mainly in the +4 state (58). In fact, up to 50% of the total vanadium in crude oil can be found as V02+ in organometallic compounds such as porphyrins and naphthenates (59-63). During the cracking reaction in a FCCU, these compounds deposit V (probably in the form of VO+2 cations) on the catalyst surface. Then, after steam-stripping and catalyst regeneration, formation of V+5 surface phases occur. The effects of vanadium on FCC properties are more severe than any of the other metals present in petroleum feedstocks. In fact, vanadium causes an irreversible loss of cracking activity which is the result of a decrease in crystallinity, pore volume and surface area of the catalyst, Figure 5. 

The decrease in activity of heterogeneous Wacker catalysts in the oxidation of 1-butene is caused by two processes. The catalyst, based on PdS04 deposited on a vanadium oxide redox layer on a high surface area support material, is reduced under reaction conditions, which leads to an initial drop in activity. When the steady-state activity is reached a further deactivation is observed which is caused by sintering of the vanadium oxide layer. This sintering is very pronounced for 7-alumina-supported catalysts. In titania (anatase)-supported catalysts deactivation is less due to the fact that the vanadium oxide layer is stabilized by the titania support. After the initial decrease, the activity remains stable for more than 700 h. 

The causes of activity decay of HDS catalysts are deposits of carbonaceous materials and heavy metals such as nickel and vanadium. Aggregation or crystal growth of active molybdenum component has been observed as well. Regeneration usually consists of i) removal of heavy metals by washing the catalysts with sulfuric acid after removal of oil, ii) burn-off of coke by air at about 770K, and iii) resulfidation at about 670K. Steps i) and ii) can be interchanged. It has been reported that complete removal of heavy metals is not necessary to restore the activity since they usually only block the entrance of catalyst pores. ... 

---