Feb-2003

Ultra-deep desulphurisation of gas oils

In order to choose the right combination of catalyst and technology for the production of diesel with 10ppm sulphur or less, it is essential to have a thorough understanding of the reaction kinetics in diesel hydrotreating from SOR to EOR

Lars Skyum, Haldor Topsoe

Article Summary

With increased hydrotreating severity, a new set of operational limitations determines unit performance. To identify these new limitations, it is important to under-stand the reaction kinetics in diesel hydrotreating. For moderate to deep desulphurisation of gas oils (typically less than 90–99 per cent HDS), sulphur is removed using the direct route where the sulphur-containing molecule is absorbed into a catalyst site and the sulphur is then extracted.

When ultra-deep desulphurisation is required (typically above 95–99 per cent HDS), it is necessary to remove the most refractive sulphur compounds from the diesel. Many studies have shown that the most refractive sulphur compounds in diesel streams are alkyl-substituted dibenzothiophenes (DBT), where the substituents are in positions 4 and 6 (Figure 1). Due to an “undesirable” configuration of the 4,6 alkyl-substituted DBT, the sulphur atom cannot easily gain access to the active site on the catalyst. For this reason, compounds such as 4,6 alkyl-substituted DBT are known as “sterically hindered” sulphur compounds.

It is possible to desulphurise 4,6 alkyl-substituted DBT using the direct route, but the reaction rate is very slow. Under certain operating conditions, it is instead possible to desulphurise the sterically hindered sulphur compounds using the hydrogenation route (Figure 2). Here, one ring of the DBT is hydrogenated before the sulphur atom is extracted. This route is slower than the direct route for the non-sterically hindered alkyl-substituted DBTs, but faster for sterically hindered DBTs. Therefore, if feasible, it is better to operate the HDS unit in a such a way that allows for the removal of the sterically hindered sulphur compounds using the hydrogenation route.

Both of the reaction routes are sensitive to inhibitors. The direct route is inhibited by the presence of H2S and by nitrogen components, whereas the hydrogenation route is inhibited by certain basic nitrogen compounds that are present in the feed.

Reaction pathway parameters
As mentioned, it is better to operate the HDS reactor in an operating regime where the catalyst can use the fast hydrogenation route to remove the sterically hindered DBTs. A refiner choosing a catalyst system for ultra-deep HDS must first investigate (together with the catalyst supplier) whether or not the unit operates in a regime where it is thermodynamically possible to use the hydrogenation route.

Generally, the hydrogenation route is predominant when no or only few inhibitors are present in the reactor environment. The key to defining whether or not the hydrogenation route will predominate is therefore to determine if the inhibiting specific nitrogen compounds are present in a large concentration in the feed and/or if they can be removed in the top of the catalyst bed so that the hydrogenation route can be used in the bottom part of the bed.

To predict the catalyst cycle length, it is also important to define the operating window in which the hydrogenation route can be used. For example, it is not unusual for the hydrogenation route to predominate at SOR conditions, but as the catalyst deactivates during the cycle and the reactor temperature is increased it becomes no longer feasible to remove the sterically hindered sulphur compounds using this route. The refiner registers this as a relatively high rate of deactivation in the catalyst, but in reality it is a change in the kinetics from the hydrogenation route to the direct route.

To acquire a better understanding of when the hydrogenation route can be used, it is necessary to focus on some of the parameters that influence the presence of the inhibitors right down through the catalyst bed. These parameters include:
- Feedstock properties
- Temperature
- Hydrogen pressure and hydrogen availability.

Feedstock properties
In general, the concentration of basic nitrogen inhibitors increases with the final boiling point of the feed. Feeds containing cracked material such as coker gas oil have a higher concentration of inhibitors than straight-run fractions. Topsoe has developed an analytical method whereby it is possible to measure the concentration of the specific nitrogen compounds in diesel. However, since most refiners do not have access to the equipment that is required for such analyses, generally available information such as the amount of cracked feedstock, the boiling range, the density and the total nitrogen content can be used to correlate the inhibiting effect of the feed.

Temperature
Some of the initial steps in the removal of the feed inhibitors and the sterically hindered sulphur compounds are equilibrium limited, which means the rate of removal diminishes at high reactor temperatures. Since the reactor temperature throughout the run is raised to counteract catalyst deactivation, it is often observed that a level of high initial activity in a catalyst system drops significantly as the temperature increases. To predict a reliable cycle length, it is important to be aware of this temperature effect, as the best catalyst system at SOR conditions may not be the most stable system giving the longest cycle.

A catalyst with a high level of hydrogenation activity (for example, NiMoP, CoMoP or CoMoSi catalysts) can (in a pilot plant test) exhibit higher levels of HDS activity than a catalyst with a lower level of hydrogenation activity, such as a CoMo type that specifically uses the direct route. However, when the temperature is increased, the direct route may become the predominant reaction pathway, and it is now the catalyst with a high level of activity for the direct desulphurisation that has the highest level of activity (Figure 3).

H2 pressure and availability
A high level of hydrogen pressure favours the removal of the inhibitors and the sterically hindered sulphur compounds via the hydrogenation route. This is because the equilibrium curve is moved in a more favourable direction when the hydrogen pressure is increased.