New catalysts for low and medium pressure hydrotreating

A catalyst platform provides higher performance in middle distillate hydrotreating applications with limited hydrogen, difficult feedstocks and severe operating conditions.

Albemarle Catalysts; Utrecht University

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Article Summary

In recent years, new and improved hydrotreating catalysts have been introduced to the market primarily to serve operations with high pressure and hydrogen availability. On the other hand, catalyst innovation for operations limited by hydrogen, such as low and medium pressure middle distillates-hydrotreating (MD-HT), has been slower. The challenge is to be able to deliver both premium activity and stability in applications with limited hydrogen, difficult feedstocks and severe operating conditions in general.

To improve stability for hydrogen constrained MD-HT operations, suppliers followed a defensive approach and sought to develop moderately active catalyst systems with low selectivity for nitrogen removal. The drawback in adopting such catalysts is that, while benefiting from their stability in operation, refiners are still not able to maximise activity and thereby extract full operating potential and profit from their critical units.

Albemarle responded to the need for low and medium pressure, higher performance catalysts first by introducing KF 780, a highly active and versatile CoMo grade developed for FCC pretreat (FCC-PT) as well as middle distillate hydrotreating applications. KF 780 delivers enhanced metals efficiency for HDS and HDN activity and higher robustness in operation.

KF 780 was introduced to the market at the beginning of 2015, first of all for FCC-PT applications. Its main operational targets in FCC-PT are to maintain a low sulphur product level via improved HDS and to reduce nitrogen and aromatics levels for better yields and operations in FCC units. After proving itself in this demanding service, it was rolled out to the ultra low sulphur diesel (ULSD) segment.

The acceptance of KF 780 by refiners for both applications has been positive, with more than 12000 tonnes delivered and 130 operations served worldwide since 2015. An overview of the combined FCC-PT and ULSD applications worldwide is shown in Figure 1.

Research into alternative approaches to the design of hydrotreating catalysts has led Albemarle to introduce a new generation of catalysts, Pulsar. This is a breakthrough technology that effectively controls the morphology and dispersion of the metal active phase.

The first grade of this new class is KF 787 Pulsar which delivers high activity without compromising stability, even in operations with challenging feedstock and constrained by low hydrogen availability.

Challenges in low and medium pressure middle distillates hydrotreating
A premium ULSD catalyst for low and medium pressure hydrotreating applications that processes difficult feedstock requires a perfectly balanced combination of high activity, stability and robustness against operational upsets.

Performance in these challenging operations is constrained by both kinetics and thermodynamics.

Figure 2 shows a simplified reaction pathway for the hydrodesulphurisation (HDS) reaction and the response of its different routes to operating hydrogen pressure under thermodynamically favourable conditions.

The HDS reaction consists of two routes. The direct desulphurisation route (DDS) is a single step reaction in which sulphur is converted via direct hydrogenation to H2S. DDS is typically the fastest HDS pathway at very low pressure, especially for easy sulphur removal, and is the one that requires the lowest hydrogen consumption. Hydrogen-assisted HDS (HYD-HDS) occurs in parallel with DDS and is a more complex reaction, requiring hydrogenation (of at least one aromatic ring) as a first step and DDS as a final step. Therefore, DDS is necessary also in the HYD-HDS route, in particular at low and medium pressure (ppH2 <35-40 bar) where hydrogenation reactions are slow and DDS is necessary to shift the equilibrium of the first hydrogenation step. While at very low pressures pure DDS is the dominant reaction, at low to medium pressure (25 bar <ppH2 <40 bar) the DDS and the HYD-HDS routes are both potentially important. The more refractory the sulphur species are to convert, the more important will be the contribution of the HYD-HDS route.

Note that the hydrodenitrogenation (HDN) reaction also proceeds via a hydrogenation step, hence its response to hydrogen pressure and thermodynamics is similar to that of the HYD-HDS route.

For the HYD-HDS and HDN routes to be effective in a hydrotreating reactor, though, additional conditions are required:
• Sufficiently high hydrogen coverage, to preserve enough hydrogen partial pressure at the bottom of the reactor
• Sufficiently high ppH2/temperature ratio to avoid thermodynamic limitation of the hydrogenation step
• Limited inhibition effects by refractory feed nitrogen, especially basic nitrogen, which adsorbs on the catalyst’s hydrogenation sites and inhibits HYD-assisted reactions

Based on these considerations, it is possible to identify three typical operating regimes, or regions, for a hydrotreating reactor. These regions are illustrated in Figure 3. The effectiveness of the different reaction routes varies by region:
•  In the green operating region, which is characterised by a low temperature/hydrogen pressure ratio, all three reactions, DDS, HDN and HYD-HDS, are effective, with DDS being dominant for HDS at low pressure, and HDN and HYD-HDS becoming increasingly more important at higher pressure
• In the intermediate region (depicted in yellow), the rates of HDN, HYD-HDS and HDA start to slow down because of limitation of the hydrogenation steps by thermodynamics
• In the red region, the one with the highest temperature/hydrogen pressure ratio, all hydrogenation-assisted reaction routes are severely hindered. In this zone, the rate of removal of sulphur and nitrogen is significantly lower, and HDS has to proceed almost exclusively via the DDS route.

Low and medium pressure hydrotreaters processing difficult feedstock often operate, totally or partially, in the intermediate or in the red region already at the beginning of their cycles. This is particularly true for units with very low ppH2, low hydrogen coverage and/or high space velocity, which are all conditions that lead to higher operating temperature.

In the red operating region, not only are HDS and HDN reaction rates slower, but other phenomena are also favoured and these can negatively affect the performance of a catalyst. Depending on a catalyst’s properties, dehydrogenation and condensation reactions of (nitrogen-containing) polyaromatics can occur, leading to the formation of coke that can block the catalyst active sites. In addition, high temperature can cause metals migration from the active metal slabs into larger agglomerates with significantly lower activity.

When designing a premium catalyst for low and medium pressure for upgrading difficult feedstock to high value diesel, all these aspects must be taken into account. The optimal catalyst would be the one that can maximise DDS activity without compromising the potential of the hydrogenating reactions for HDS and HDN, and that can still provide high robustness and full operating stability.

Developing such a catalyst has been the focus of Albemarle Catalyst Research over the last five years and has required a fundamentally new approach to catalyst design.

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