Feb-2008

Maximising ULSD with cracked feedstocks

Balancing hydrogen consumption can give flexibility when processing cracked feedstocks

Charles Olsen and Brian Watkins, Grace Davison
Woodrow K Shiflett, Advanced Refining Technologies

Article Summary

It has been documented for some time that the desulphurisation of dibenzothiophene (DBT) and substituted DBTs occurs through two reaction pathways: the direct sulphur abstraction route and the hydrogenation abstraction route. The former involves the adsorption of the 
molecule on the catalyst surface via the sulphur atom, followed by a C-S bond scission. This 
path is favoured over cobalt-molybdenum (CoMo)-based hydrotreating catalysts. The second pathway involves the saturation of one aromatic ring of the dibenzothiophene species, followed by the extraction of the sulphur atom. Nickel-molybdenum (NiMo) catalysts have a higher selectivity for desulphurisation via this route.

It is also expedient and efficient to model ULSD kinetic schemes by lumping the various sulphur species into facile sulphur and difficult sulphur. The so-called facile, or easy, sulphur is made up of compounds that are readily desulphurised via direct abstraction and boil below about 365°C, while the difficult, or hard, sulphur is made up of compounds that are more readily removed via hydrogenation followed by abstraction. These compounds include 4,6 dimethyl-DBT and other di- and tri-substituted DBTs. The relative amounts of easy and hard sulphur in a feed are a critical property to consider, since the concentration of each can vary significantly from feed to feed, depending on the crude source, boiling range and the prior thermal or catalytic treatment of the feedstock.

The SmART catalyst system was first introduced in 2001 in anticipation of the stringent new demands required for ULSD.1,2 This catalyst system concept is based on a staged catalyst approach designed to exploit both reaction pathways for desulphurisation. The system utilises a high-activity CoMo catalyst such as ART CDXi for the efficient removal of sulphur via the direct abstraction route, and a high-activity NiMo catalyst such as ART NDXi to effectively remove the multi-substituted DBTs via the hydrogenation route.

A number of factors need to be considered when designing the optimum SmART system for a given application. These have been reviewed in detail previously and include such things as feed endpoint (amount of difficult sulphur), feed nitrogen and H2 availability.3,5

H2 selectivity
The ability to balance HDS activity and H2 consumption to meet individual refiners’ requirements is shown in Figure 1. As NiMo catalyst is added to the SmART system, there is a large increase in HDS activity relative to the all-CoMo reference. A maximum in HDS activity is eventually reached. The position and magnitude of this maximum varies with feed and operating conditions, especially the H2 partial pressure. Figure 1 also includes the relative H2 consumption and, again, as a percentage of the NiMo component increases the H2 consumption relative to the base CoMo system also increases. Notice, however, that in this case the relationship between H2 consumption and the fraction of NiMo catalyst is non-linear. In the region where the system shows the highest activity, the hydrogen consumption is only slightly greater than that for the all-CoMo system, and well below that for the all-NiMo catalyst. Again, the nature of this relationship varies with feed and operating conditions, with a strong correlation to hydrogen availability.

The balancing of hydrogen consumption and HDS activity is due to the different kinetics and reaction pathways for sulphur removal and aromatic hydrogenation. The previously discussed two reaction pathways for sulphur removal, with the main point being the hydrogenation pathway, are critical for ULSD production. The multi-substituted hard sulphur DBTs are polynuclear aromatic (PNA) species containing two aromatic rings, one of which must be hydrogenated for the efficient removal of the sulphur atom. Thus, the catalyst system needs good hydrogenation activity and selectivity in order to minimise hydrogen usage.

To more fully understand how this works, it is useful to review PNA hydrogenation in general. To begin with, the hydrogenation of aromatics is reversible, and equilibrium conversion is less than 100% under practical hydrotreating conditions. Equilibrium conversion decreases with increasing temperature. Therefore, increasing the temperature to get higher hydrogenation rates may ultimately result in lower conversion. These reactions are also exothermic, which can has an impact on the conversion in adiabatic systems.

The hydrogenation of a number of poly-aromatic species such as naphthalene and biphenyl have been studied by investigators, and the work has led to the reaction networks presented in Figure 2.6 In the case of naphthalene, the reaction begins with the hydrogenation of one of the aromatic rings to form tetralin, a mono-ring aromatic. The next reaction is the hydrogenation of the remaining aromatic ring to produce decalin, the fully saturated species. As indicated in Figure 2, the reactions proceed sequentially with the rate of hydrogenation of the final aromatic ring (tetralin) at least an order of magnitude lower than the saturation of the first aromatic ring (naphthalene). Interestingly, the rate of tetralin hydrogenation is similar to that observed for benzene hydrogenation. A variety of substituted naphthalenes have also been shown to follow a similar reaction network, with the rate of hydrogenation of the first aromatic ring approximately equal to that observed for naphthalene.

The hydrogenation of biphenyl proceeds similarly in a stepwise fashion, with the rate of hydrogenation of the first aromatic ring about an order of magnitude faster than that of the 
mono-ring compound. An important difference is that the rate of the first hydrogenation reaction in naphthalene is approximately an order of magnitude faster than the rate of hydrogenation of the first ring in biphenyl. This indicates that there is a significant difference between the hydrogenation of an aromatic with two fused rings compared to a two-ring aromatic where the rings are not fused. This is an important difference when considering hard sulphur removal, since these species can be expected to behave more like a biphenyl aromatic species.

The challenge when designing a SmART system is to provide enough hydrogenation activity to efficiently saturate the first ring on the two-ring aromatic (biphenyl type, sulphur containing) molecule, but not so much as to catalyse the final hydrogenation step in the previously discussed reaction pathway.