Profiting with FCC feedstock diversity
Gasoline sulphur-reduction catalysts and additives can provide additional options and flexibility while maximising refinery profitability
Natalie Petti, Larry Hunt and George Yaluris, Grace Davison
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The FCC unit can process a variety of feeds, from severely hydrotreated gas oil (VGO) to resid (Figure 1). It can be operated to maximise gasoline or distillate production, or it can be operated at high severity to maximise petrochemical feed production such as propylene. Some refiners have endeavoured to comply with clean fuels regulations while maximising profitability by processing cheaper, heavier and more sour crudes. This can be achieved by combining the proper process units either upstream or downstream from the FCC unit with the right catalytic solution. Units such as hydrotreaters, hydrocrackers, cokers and visbreakers are common components of strategies used to upgrade low-value hydrocarbon streams before incorporating a portion of these streams in the feed to the FCC unit. Diesel or gasoline treatment units are also employed after FCC processing to further improve product quality and allow blending into the gasoline or diesel pool without compromising compliance with clean fuel requirements.
The changing needs of refiners present new challenges for FCC catalyst manufacturers. Many refiners increasingly require FCC catalysts capable of handling diverse feed slates. Even if the feed composition to the FCC is relatively constant, the varieties of crude and refinery processing schemes have resulted in a huge range of needs for which the catalyst vendor must make optimum formulations. In addition, many refiners are finding that despite the installation of new process units to reduce sulphur in gasoline and distillate products, the overall profitability of their operation can be enhanced by using FCC catalysts and additives that reduce gasoline sulphur. Such catalysts can improve economics, for example, by reducing the severity of operation of hydrotreating units upstream or downstream of the FCC unit and decreasing gasoline octane loss.
Catalyst technology platform
As refiner needs have become more complex and diverse, the importance of FCC catalyst technology to adapt and serve these needs has increased dramatically. Grace Davison’s proprietary alumina-sol technology platform has been designed to allow maximum possible flexibility for formulating FCC catalysts. As shown in Figure 2, alumina-sol catalysts can be formulated to deliver the desired activity, coke selectivity and bottoms cracking activity to suit the needs of FCC units. Additional functionalities can be incorporated to further optimise catalyst performance. For example, while the alumina-sol technology is inherently coke selective, further improvement in coke selectivity as well as in bottoms cracking and activity retention has been achieved by incorporating special matrices to provide V and Ni passivation. The recently introduced proprietary Tunable Reactive Matrix (TRM) technology has further allowed for the extended application of alumina-sol catalysts to cover applications for units processing any type of feed.1
Through a combination of materials and processing technology advancements, TRM technology provides control of the matrix acidity and porosity, so that the catalyst matrix has properties specifically calibrated to the requirements of the feed to be processed. Alumina-sol catalysts with TRM technology have been used successfully to process resid in the presence of high levels of contaminant metals with step-out coke selectivity and bottoms cracking activity; to maximise bottoms cracking in units processing hydrotreated feeds without a coke penalty; and to minimise coke and maximise bottoms cracking for units processing gas oil feedstocks.
In addition, alumina-based catalysts can provide a measure of resistance to iron (Fe) poisoning, because alumina resists the formation of low-temperature melting phases, which destroy the surface pore structure of catalytic particles and block access of the hydrocarbon molecules to the interior structure.2,4 Grace Davison catalysts made with alumina-sol technology are especially resistant to Fe poisoning. The alumina from the alumina-sol process is the most effective form of alumina for Fe tolerance because it is dispersed throughout the catalytic particle, providing the pore structure required for diffusion of the heavy hydrocarbon molecules inside the particle for cracking. It also has active sites for bottoms cracking activity, and its pore structure is not susceptible to closing by Fe contamination. This technology has been incorporated into premium resid cracking technologies, providing exceptional Fe tolerance.
Processing resid feeds
As the availability of easy-to-process feeds continues to dwindle, efforts to improve profitability have resulted in increased resid processing. This is especially true for refiners having recently constructed FCC units, since new FCC units are often built to handle the high levels of coke made by cracking resid. Regardless of the FCC unit configuration, when processing resid feeds several considerations are important in selecting the best cracking catalyst:
• Amount of high boiling point feed components
• Amount of contaminant metals (for example, Ni, V, Fe, Na, Ca and Mg) on equilibrium catalyst (e-cat)
• Unit constraints and economics (for example, air blower and slurry handling limits, bottoms upgrading requirements).
As has been reported elsewhere, there are three types of bottoms cracking mechanism: Type I involves vapourisation and pre-cracking of the feed; Type II is dealkylation of alkyl-aromatics primarily catalysed by the zeolite component of the catalyst; Type III involves the cracking and conversion of naphtheno-aromatics.5 All types of mechanism are operative during the processing of any type of feed. However, because a large amount of the feed is too heavy to vapourise since the regenerated catalyst cools down from the vapourisation of the lighter and easier-to-vapourise feed components when processing resid the ability of the catalyst to conduct Type I bottoms cracking acquires added importance. Heavy feed components prevalent in resid feeds are more likely to be multicore naphtheno-aromatics, chal-lenging the catalyst to upgrade them into valuable products while minimising their natural tendency to make coke.
In addition to containing heavy hydrocarbon components, resid feeds tend to contain high levels of contaminant metals, most often Ni and V, but also other metals having a deleterious effect on catalyst performance such as Na, Fe and Ca. These contaminants affect catalyst performance in two ways. First, they deactivate the catalyst, reducing conversion, and thus decreasing the yields of valuable products and increasing bottoms make. The mechanism of deactivation depends on the metal, but it is often the result of zeolite destruction (V, Na, Ca) or destruction of the pore structure of the exterior surface of the particle (Fe, Na, Ca). In addition, many contaminant metals (for example, Ni, V, and Fe) are active as dehydrogenation catalysts, increasing H2 and dry gas, as well as coke make. Thus, to be able to process resid within the limits and constraints of the unit, a resid catalyst must be formulated to be resistant to poisoning by contaminant metals and it must be able to passivate metals causing dehydrogenation reactions, both of which are detrimental to the FCC unit’s operation and economics.
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