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Apr-2005

Improved residuum cracking

The detrimental effect of nickel and vanadium in forming contaminant coke and hydrogen can be reduced via appropriate passivation technology

Mingting Xu and Rostam J Madon, BASF Catalysts
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Article Summary
Vanadium (V) by itself has little impact on zeolite stability; its role is to facilitate the formation of NaOH. Surface Nad+OHd- species, formed by the reaction of available sodium and steam, attack the Si-O bonds of the Y-zeolite, leading to its destruction. This understanding has been incorporated into Engelhard’s proprietary Flex-Tec resid-cracking catalysts.

A fresh FCC catalyst is a relatively attrition-resistant microsphere with an average particle size of 75µm. It consists of 20–45% Y- or USY-zeolite with exchanged rare earth concentrations in the range of 0–5 wt% and either an active or non-active amorphous matrix bound together with a silica or alumina binder. A fresh FCC catalyst typically contains 0.2–0.5 wt% Na2O – some as Na cations on the Y-zeolite and the remainder tied up in the clay or alumina matrix. Cationic sodium reduces the number of acid sites in Y-zeolite, resulting in lower cracking activity. Sodium, if available, is known to be highly destructive for Y-zeolite in the hydrothermal environment of the regenerator.

Passivation technology
During resid cracking, vanadium and nickel from the feed deposit onto the catalyst. An equilibrium FCC catalyst often contains more than 5000ppm nickel and vanadium. Vanadium is known not only for destroying the zeolite structure under the hydrothermal conditions of the regenerator, but also for producing coke and dry gas when in contact with the feed. While nickel has little effect on the hydrothermal stability of zeolite, it is more active than vanadium for making contaminant coke and hydrogen.

In order to develop a superior catalyst for resid oil cracking, the detrimental effect of nickel and vanadium in forming contaminant coke and hydrogen needs to be reduced via appropriate passivation technology. In addition, the destructive effect of sodium and vanadium on the zeolite structure must be minimised to preserve activity. Therefore, to improve the hydrothermal stability of Y-zeolite, issues affecting stability have to be considered. In addition, a comprehension of the mechanism of destruction from sodium and vanadium is necessary.

This understanding of vanadium’s role in the destruction of Y-zeolite is incorporated in the design of Engelhard’s proprietary Flex-Tec resid FCC catalysts1,2, which combine the advantages of Engelhard’s proprietary distributed matrix structure (DMS) architecture with metal passivation matrix technology.3 Flex-Tec thus comprises a matrix pore structure optimised for the facile transport of heavy feed molecules and the resulting products. Also included are technologies for the passivation of nickel and vanadium, as well as for maintaining high stability in resid FCC operations.

The role of sodium (Na) in zeolite destruction is best understood by focusing on the Na+ exchanged on Y-zeolite. Let us start with a USY-zeolite with different levels of sodium made by exchanging a 4.1 wt% Na2O USY with ammonium and sodium nitrate solutions. Figure 1 (plot a) shows that under dry calcination at 1500°F, there is actually a slight increase in zeolite stability. The small initial decrease in stability, especially at lower sodium levels, is due to the presence of adsorbed water, which causes dealumination and consequent Y destruction. In contrast to thermal stability, Figure 1 (plot b) shows that under hydrothermal conditions the USY surface area decreases rapidly with increasing sodium. This cannot be explained by Y framework dealumination, since the steamed unit cell size (ucs) in Figure 1 (plot c), an indication of the extent of dealumination, is larger at higher sodium loading. In fact, Na+ inhibits dealumination even though more zeolite is destroyed, indicating that a second pathway other than dealumination is responsible for the destruction of zeolite by sodium under hydrothermal conditions.

To determine the pathways via which sodium destroys zeolite, model investigations were carried out on a pure USY with a high SiO2/Al2O3 ratio of 325. Details of this study are described elsewhere.4 USY-zeolite, which is used in FCC catalysts and dealuminates readily when steamed, has framework SiO2/Al2O3 molar ratios from 8–12. In this case, due to the high SiO2/Al2O3 ratio of the model USY, steam dealumination is not a factor. Hence, this material exhibits high hydrothermal stability with no surface area loss when steamed at 1500°F in 100% steam. However, when sodium is added to the USY via incipient wetness using NaHCO3 or NaOH solution, drying at 180°F is sufficient to destroy around 25% zeolite at only 0.19% Na2O loading. The low retention of the zeolite surface area in the presence of NaHCO3 (sodium bicarbonate or sodium hydrogen carbonate) and NaOH is identical. Why does this happen? Sodium bicarbonate, a strong base-weak acid salt, hydrolyses in water to form NaOH. In water, NaOH ionises readily. Destruction of Y-zeolite is related to the facility by which NaOH is formed and made available. The destructive component is the basic anion OH-.

This study in liquid water was carried out just to show the strong destructive nature of NaOH. Under FCC conditions, where the catalyst is exposed to high-temperature steam, such facile formation of Na+ and OH- ions is not possible. However, during FCC regeneration, the exchanged cationic sodium on the Y-zeolite will react with steam to form Nad+...OHd- on the catalyst surface. This is sufficient to destroy Y-zeolite.

Destruction of Y-zeolite by sodium in the presence of steam does not take place via dealumination, but by a second pathway that involves the formation of NaOH and the destruction of the silicon-oxygen bond by NaOH.

Vanadium species, deposited on FCC catalysts from porphyrin complexes in the crude oil during cracking, help destroy Y-zeolite in the FCC regenerator. Wormsbecher et al have suggested that vanadic acid, formed in high-temperature steam under oxidising conditions, promotes the acid-catalysed hydrolysis of framework alumina (Al) and consequently results in the destruction of zeolite.5 Research by Pine has indicated that there is a synergy between sodium and vanadium that results in the destruction of zeolite under hydrothermal conditions.6 Pine’s conclusions resulted from his study with USY samples having low UCS between 24.24 and 24.28Å. Engelhard’s research has shown that this synergy between sodium and vanadium in zeolite destruction is applicable at all UCS values.4 In Figure 2, a standard USY with two levels of Na2O (0.03 
and 1.19 wt%) is used as exchanged sodium cations. The higher-level Na2O is similar to that present in typical FCC catalysts on a zeolite basis.
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