Feb-2008

Combining ULSD objectives with FCC pretreatment

Catalysts for ULSD and FCC pretreatment combine higher activity, better stability and lower hydrogenation consumption.

Lars Skyum, Haldor Topsøe

Article Summary

Although a hydrotreating catalyst contains only a few active ingredients, the nature of the active phase is difficult to elucidate and has been the subject of numerous characterisation studies using a multitude of different techniques. It is not the purpose of this article to go into details on the characterisation techniques that have been employed. The following discussion is instead focused on two important functionalities: the catalyst’s ability to either use the direct desulphurisation route or to use the hydrogenation route for desulphurisation.

In the mid-1980s, Topsøe researchers demonstrated that the sites for direct desulphurisation are placed on the edges of CoMoS slabs, and that it is possible to have two different CoMoS structures, where the hitherto unknown structure, which was labelled as Type II, had significantly higher activity than the Type I structure. Fifteen years later, the researchers were involved in fundamental work that led to an understanding of the sites for hydrogenation, and it was shown that hydrogenation takes place on the top of the CoMoS slabs. The reaction sites are situated on very electron-dense “S” atoms close to the edges of the CoMoS slab, and the researchers chose to name them brim sites (Figure 1).

In the beginning of the new millennium, researchers developed a new catalyst-preparation technology, which is known as the BRIM technology. To date, over 27 000 tonnes of catalysts based on BRIM technology have been put into service in more than 165 hydrotreating units. With this technology, it is possible to enhance and better control the number and the activity of the direct reaction sites and brim sites. Further discussion will focus on why it is advantageous to have either high activity for the direct route or high activity for the hydrogenation route. First, the kinetics in ULSD will be discussed, followed by the kinetics of FCC and hydrocracking pretreatment.

ULSD kinetics
In the production of ULSD, it is relatively easy to remove 95–98% of the sulphur, but the remaining sulphur consisting of sterically hindered di-benzothiophenes (DBTs) has a very low reactivity. As shown in Figure 2, the DBTs can be removed either via the direct route or via the hydrogenation route, where one of the benzo-rings is hydrogenated before the sulphur is extracted in the second step.

When it comes to sterically hindered DBTs, the hydrogenation route is faster than the direct route, but the hydrogenation route is inhibited by nitrogen compounds (and in particular basic nitrogen compounds), and for the catalyst to use this faster route these inhibiting species must first be removed in the upper part of the catalyst bed. The key to understanding ULSD kinetics lies in this fact. The optimal catalyst system depends on whether or not it is possible in the hydrotreater in question to remove the inhibitors so the hydrogenation route becomes active.
The best catalyst for removing the inhibiting nitrogen species is one with high hydrogenation activity, since they are removed exclusively via the hydrogenation route. What this means is that the best catalyst system for a hydrotreater that can make use of the fast hydrogenation route consists of a catalyst with a high hydrogenation activity in the upper part of the catalyst bed (with the purpose of removing the inhibitors), and a catalyst with a high hydrogenation activity in the bottom part of the catalyst bed (with the purpose of removing sterically hindered DBTs via the hydrogenation route). It is thus clear that if it is possible to use the hydrogenation route in ULSD service, the best HDS catalyst is the one that has the highest hydrogenation activity. Within the BRIM family of catalysts, the TK-575 BRIM NiMo catalyst has been developed for these applications.

But for many applications, it is not possible to remove the inhibitors. This is the case for hydrotreaters that are either operated at low-to-medium pressures or process feedstocks containing a relatively high content of inhibiting species, such as cracked stocks, or both. The significance of the pressure is related to the fact that the first step in the hydrogenation route is a reversible reaction, and the equilibrium is strongly dependent on the hydrogen pressure (Figure 2). The best catalyst system for these hydrotreaters has maximum activity for the direct desulphurisation, and for such applications Topsøe uses the TK-576 BRIM CoMo catalyst.

It is therefore understood that as long as HDS is the primary objective there is no justification for a stacked-bed solution of CoMo and NiMo catalyst. Either the catalyst system should have maximum hydrogenation activity (NiMo) or maximum activity for the direct route (CoMo). With the help of the BRIM technology, the activity for either of these two routes can be enhanced compared to previous generations of hydrotreating catalysts.

FCC and hydrocracking pretreatment
The BRIM technology has been used to develop better FCC pretreatment catalysts. As was the case with ULSD, the improvements stem from reaction kinetics studies. The function of the FCC pretreater is partly to remove aromatics and nitrogen compounds, thereby improving the performance in the FCC unit, but it is increasingly HDS that is the main objective of the pretreater, in order to obtain low sulphur levels in the FCC gasoline. Contrary to ULSD production, it is rare that the hydrogenation route predominates in FCC pretreatment, partly because the conversion is lower and the sterically hindered DBTs do not have to be desulphurised, but also because the VGO feed has a much higher concentration of nitrogen compared to diesel feed. The best HDS catalyst in FCC pretreatment is therefore one with high activity for direct desulphurisation, and for this purpose the TK-558 BRIM CoMo catalyst has been developed. If the hydrogen pressure in the FCC pretreater is, for example, higher than 80–90 bar (1150–1270 psi), the equilibria for aromatics and nitrogen removal are shifted favourably, and some refiners wish to take advantage of this and use a catalyst that has higher hydrogenation activity and a better capability to remove nitrogen than a CoMo catalyst. The pressure is, on the other hand, rarely high enough for the HDS hydrogenation route to become really active, which means the catalyst in FCC pretreatment service must also have a good activity for the direct route in order to comply with the sulphur specification. For this purpose, the TK-559 BRIM NiMo catalyst offers a good combination of direct and brim sites.