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Kinetic engine drives catalyst development

Development of a NiMo catalyst for medium-to-high and high pressure middle distillates hydrotreating.

Albemarle Corporation
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Article Summary
Hydroprocessing is an important refinery process for upgrading low value feed streams to higher value intermediates and on-specification fuels, including ultra-low sulphur diesel (ULSD). Albemarle’s Stax kinetic model continues creating commercial value by helping refiners optimise hydrotreater loadings, and has become a fundamental tool with Albemarle’s research and development for the design of new hydrotreating catalysts.

Ketjenfine (KF) 880 Stars for medium-to-high and high pressure middle distillates hydrotreating is the first catalyst designed using the Stax kinetic engine. This NiMo catalyst offers superior hydrogenation activity for demanding operations. The benefits include higher feedstock upgrading, increased throughput and/or cycle length, improved product properties and higher volume swell. Together with the STAX technology, KF 880 brings significant advantages for the most demanding middle distillates hydrotreating operations.

ULSD middle distillates hydrotreating: reactor zones and catalyst selection

The chemical reactions and related kinetics in a hydrotreater change with the nature of the feedstock, the operating conditions, the degree of conversion and the time on-stream. This means that applying a single catalyst load is rarely the optimal solution. Stax technology enables the refiner and catalyst manufacturer to design the right loading scheme for each situation by taking into account process objectives, economics and constraints.

In its Stax engine, Albemarle identifies three main reactor zones for middle distillates hydrotreating operations (see Figure 1). In zone 1, the primary reactions are hydrogenation of olefins and the conversion of easy sulphur via the direct hydrodesulphurisation (HDS) route, also called direct desulphurisation (DDS). At the same time, polynuclear aromatics (PNA) and nitrogen species start being converted via hydrogenation.

In zone 1, the main risks to operation are condensation reactions of PNA species and coke deposition. Consequently, the preferred choice is a catalyst that does not have too high an affinity for the adsorption of coke precursors.

As the feedstock moves down the reactor, the sulphur removal rate decreases because more refractory species need to be converted. DDS of non-sterically hindered (di-) benzothiophenes proceeds well into zone 2, but sulphur removal by hydrogenation becomes increasingly important. Hydrogenation (HYD) becomes the main HDS reaction route for refractory sulphur at higher pressure and, in general, when hydrogenation reactions are not thermodynamically limited.

Residual organic nitrogen inhibits the HYD route. The faster the organic nitrogen removal, the more effective is the removal of refractory sulphur. At the same time di- and di+-aromatics are further saturated to monoaromatics. Most of the HDS and polyaromatics saturation to monoaromatics takes place in zones 1 and 2. Conversely, the saturation of monoaromatics to naphthenes in zones 1 and 2 typically does not occur as the inhibition effect by nitrogen and PNAs is still too high.

In low and most medium 
pressure ULSD hydrotreaters, Zone 2 extends to the reactor bottom. At higher pressure or when easier feedstock is processed, on the other hand, the hydrodenitrogenation (HDN) reaction rate can be high enough to remove the organic nitrogen fully, thus creating zone 3. At that point, all hydrogenation reactions are boosted, including that of sterically hindered sulphur. The hydrogenation of residual di- and monoaromatics is also boosted, which can be used for additional volume swell.

Catalyst selection for zone 2 is the most critical for both the operation’s activity and stability. If the pressure is low or if thermodynamic limitation occurs for hydrogenation reactions, a catalyst with a high DDS activity is preferable. Conversely, where conditions are suitable for hydrogenation, catalysts with a high HYD activity are preferred, as they enable much faster removal of organic nitrogen and, with it, of refractory sulphur.

For zone 3, catalyst selection strongly depends on operating targets and constraints. The absence of organic nitrogen boosts the activity of all types of catalyst, but mostly of highly hydrogenating ones. An additional consideration is that zone 3 may be replaced by zone 2 during the cycle as hydrogenation activity decreases with catalyst aging. This must be taken into account when designing the catalyst load. Where hydrogen availability and consumption are not limitations, a catalyst with high HYD activity would be the preferred solution. Conversely, where hydrogen consumption is a constraint, or for stability considerations, a catalyst with a lower HYD activity would be the best choice.

The right catalyst system is always the one that best balances the operation’s objectives (desulphurisation, cetane uplift, volume swell, and so on) with its limitations (ppH2/hydrogen availability, operating temperature and cycle length) over the entire cycle.

Stax reactor zones: learning from testing
Tests run by Albemarle highlight the effects of specific catalyst functionalities on performance in the various reactor zones as a function of ppH2. The HDS and HDN results for KF 848 (Type II Stars NiMo catalyst) and KF 757 (Type II Stars CoMo catalyst) are shown in Figures 2 and 3. KF 848 and KF 757 catalysts are used as references to illustrate the behaviour of Type II NiMo catalysts (high HYD activity) and Type II CoMo catalysts (high DDS activity), respectively.

The figures depict the evolution of sulphur and nitrogen as SRGO (total sulphur = 1.2 wt%; refractory sulphur ∼1200 ppmwt) is being converted along the reactor at 30 and 60 bar inlet ppH2. The catalyst systems were run in parallel in the same conditions and in varying amounts to simulate conversion at the different heights in a hydrotreater. The reference reactor length of 1.0 (dimensionless) considered here is the length needed by the KF 757 system to reach 8 wtppm sulphur product at 30 bar.

By comparing the results, it is possible to understand the main differences in behaviour between a NiMo and a CoMo Type II catalyst.

When operating at moderate pressure (30 bar, see Figure 2), the main route for sulphur removal for most of the reactor is DDS. Because of this, a CoMo catalyst provides a higher sulphur conversion than a NiMo catalyst, despite the higher nitrogen slip. In this case, this holds well into zone 2, down more than half of the reactor.

A NiMo catalyst can remove nitrogen faster than a CoMo catalyst, which can boost sulphur removal via the HYD route when approaching the reactor’s bottom. In this case, the NiMo catalyst even reaches a nitrogen-free zone (zone 3) in view of the easy feedstock utilised. However, the advantage of the overall process remains limited mainly because the ppH2 is low, so that at the end the CoMo and the NiMo catalysts reach basically the same HDS performance for ULSD production.

The case illustrated is boundary for the use of either a CoMo or a NiMo catalyst. With more difficult feedstocks, at low pressure the nitrogen level remains higher down to the bottom of the reactor. Also, for a NiMo catalyst, zone 3 cannot be reached. As a result, the HYD reaction route is significantly less effective and a CoMo catalyst typically delivers an overall better performance for ULSD. Also, in terms of stability a CoMo catalyst is preferable at low pressure.
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