Deep desulfurization limitations

As we have shown previously, obtaining both good cold operation characteristics and sufficient cetane numbers constitutes the principal objective for the refiner in the formulation of diesel fuel. To this is added the need for deep desulfurization and, perhaps in the future, limitations placed on the chemical nature of the components themselves, e.g., aromatics content. 

Besides the thermodynamic limitations of HDN, the reactivity of N-compounds is strongly influenced by their molecular structure, which interferes with the absorption of the N-atom on the active site [30]. Basic N-compounds are also the strongest inhibitors of hydrogenation sites and consequently of all reactions that may follow this pathway (e.g., HDS). Therefore, removing N-inhibitors is an essential strategy to achieve deep desulfurization of diesel fuels. 

Hence, for deep desulfurization, a two-phase reactor (oil externally pre-saturated with H2 and solid catalyst) could be an alternative to the trickle bed. The H2-recycle is then redundant, and scale-up problems do not occur. In addition, the two-phase technology utilizes the maximum intrinsic chemical reaction rate as pore diffusion does not play a role in the slow desulfurization of refractory compounds left in predesulfurized feedstocks. For a trickle bed this rate is an upper limit, which caimot, or only hardly, be reached with regard to improper gas-liquid distribution and/or wetting of the catalyst. A laboratory-scale tricHe-bed and two-phase reactor with pre-saturation are compared in Figure 6.8.10 for a model oil. 

Every year many new catalysts become available for a wide variety of applications. Because it is impossible to discuss all these catalysts, we will limit the discussions to a summary of new grades introduced in the last five years and review only the most important developments for the various application segments, being resid upgrading, hydrocracking, FCC pretreatment, middistillate desulfurization and deep hydrogenation. Table 6 summarizes new catalysts introduced in the last 5 years and still being sold in 1995 [35]. Each year 15-20 new catalysts are introduced in the market. Akzo Nobel, Chevron and Criterion appeared to be the most active companies. 

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