Improving FCC catalyst performance
Higher volumes of heavier feeds can be processed with additives designed to enhance base catalyst effectiveness. Deactivation and dehydrogenation problems are avoided, while catalyst selectivity and stability are preserved
Michael K Maholland
Intercat (Johnson Matthey)
Viewed : 7059
Fresh catalyst begins to undergo significant deactivation immediately upon its addition to the FCC unit. Causes of this deactivation are several-fold. First, the Y-zeolite undergoes dealumination as a result of the high temperature and steam partial pressure in the regenerator. As the zeolite continues to dealuminate under these severe hydrothermal conditions, the selectivity changes, the activity decreases and, eventually, the crystalline structure collapses. The collapse of the zeolite crystal leads to a loss of surface area as well as catalyst activity. Another cause of fresh catalyst deactivation is the deposition of contaminant metals from the feed onto the catalyst. The most significant of these metals in terms of their impact on conversion are vanadium and sodium.
Vanadium, which is deposited on the catalyst from the metalloporphyrins in the feed, is oxidised and hydrolysed in the FCCU regenerator, forming vanadic acid. Vanadic acid is capable of destroying the Y-zeolite of the catalyst by hydrolysis of its SiO2-Al2O3 framework. The impact of vanadium is exacerbated in the presence of sodium. Vanadium in the V5+ oxidation state (as can occur in the regenerator) is highly mobile and can migrate within the catalyst particle and from particle to particle. Thus, vanadium destruction is not limited to the initial particle onto which the metal adsorbs. In addition, vanadium, along with nickel, is active as a dehydrogenation catalyst.
While the dehydrogenation activity of vanadium is observed to be about one-fourth of that of nickel, it is not passivated by the standard passivating agents, antimony or bismuth. The effects of this catalytic dehydrogenation activity are increased yields of coke, dry gas and hydrogen, along with a decreased yield of gasoline. The higher yields of coke and dry gas can bring the FCC unit to constraints on the air blower, main fractionator and wet gas compressor, and thereby limit the feed rate.
The problems associated with contaminant metals, particularly vanadium, are increased when processing residual feedstocks. Refiners who are equipped to take advantage of lower-cost heavy crudes and process the heavy residual fractions through the FCCU are typically at a significant economic advantage over the light crude oil refiner. The residual feeds such as atmospheric or vacuum tower bottoms are characterised by high Conradson carbon numbers (greater than 1 wt%), a large percentage of high boiling point components (typically those boiling greater than 565°C) and high levels of the contaminant metals V and Ni.
Catalyst selection and management
While it is economically attractive to process residual feeds in the FCCU, the feeds themselves pose special problems for the refiner in terms of catalyst selection and management. Selecting an appropriate catalyst for the feed being processed can have a large effect on profitability. Experience has shown that the task of upgrading these residual feeds is often too great for a single-component catalyst formulation to handle effectively. The necessary functions of metals trapping, passivation, selective bottoms upgrading and high matrix accessibility, along with incorporating high zeolitic activity and stability functions, cannot be adequately and economically manufactured into a single attrition-resistant catalyst particle.
The difficulties posed by processing residual feeds are ideally handled by a multi-component catalyst system. This system involves one component, a catalytic additive, containing the metals trapping, metals passivation and selective bottoms upgrading functions, and a second, base catalyst component, containing additional selective matrix activity and the zeolitic function to upgrade the intermediates generated by the additive particles to the final desired and highly valued products. By decoupling the catalytic functionalities into separate particles, the overall catalyst composition and resulting yield slate can be optimised.
Managing high metals feeds
The most common method of managing the effects of high metals feeds, and thus the FCC unit activity in a residual feed operation, is by adjusting fresh catalyst additions based on the level of contaminant metals on the equilibrium catalyst. This pre-selected metals level is often determined from past experience and is set so as to avoid exceeding constraints on the air blower or wet gas compressor. Fresh catalyst additions are increased when feed metals begin to increase, and vice versa. However, when responding to higher metals feeds, adding more fresh catalyst alone may not be an effective catalyst management strategy.
In an analysis of equilibrium catalysts from over 150 FCC units, Guglietta, et al1, showed that catalyst type and unit operating conditions have a greater impact on performance than the catalyst addition rate. They discovered that despite an almost three-fold variation, the catalyst addition rate had little effect on unit profitability. Without the appropriate catalytic formulation and functionalities, increasing the addition rate of a catalyst may have little or no effect on reducing the impact of contaminant metals, on maintaining conversion or on improving the net product value.
Another approach to catalyst management commonly practiced in high metals, residual feed operations is to use a purchased, low metals equilibrium catalyst to supplement the fresh catalyst make-up. The primary function of the purchased equilibrium catalyst is to act as a flush catalyst, limiting the metals level attained on the base catalyst. Problems with equilibrium catalyst usage are that the availability and consistency of good-quality equilibrium catalyst is typically limited. In addition, low metals equilibrium catalysts usually do not incorporate catalyst technologies designed for high metals, heavy feed applications. Purchased equilibrium catalysts often have poor metals tolerance and can contribute to poor selectivity and activity maintenance in a resid operation.
The incorporation of an additive particle with metals trapping functionality, added to the FCCU separate from the base catalyst, has none of the drawbacks of the previous two catalyst management techniques. This trapping technology works by capturing the volatile and mobile metal contaminants, primarily vanadium, to form a stable and catalytically inactive compound.
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