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Mar-2015

Four-stage hydrocracking pretreatment

A four-zone approach to hydrocracking pretreatment catalysis aims for greater flexibility in activity, stability, hydrogen consumption and pressure drop

Stefano Melis
Albemarle

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Article Summary

In the current market that rewards distillate production, many refiners are looking closely at the performance of their hydrocrackers. To improve hydrocracking margins, refineries explore solutions ranging from increasing feed severity to improving yield structure and product quality. Multiple operational constraints, in particular cycle length and hydrogen availability, often determine the feasibility of each approach. In most cases, significant improvements require highly reliable and tailored catalytic solutions.

Albemarle’s portfolio for hydrocracking pretreatment has eight different catalysts (each available in two different sizes) meeting refiners’ needs in terms of activity, stability, hydrogen consumption and pressure drop (see Table 1). These catalysts can be sequenced in many ways according to Albemarle’s Stax technology (optimal catalyst system design technology) to generate solutions for specific requirements.

Stax in hydrocracking pretreatment
In a traditional approach to catalyst system design, pretreatment has two parts (see Figure 1): the top of the reactor is filled with grading/demetallisation catalyst and the rest of the reactor is filled with a single active catalyst.

With such an approach during the design phase, once the catalyst is selected and the operational target is set (typically, nitrogen slip) there is relatively little flexibility left, apart for some minor adjustments that can be achieved with quenching strategy. This two-part approach to pretreatment has consequences:
a) The top part of the active catalyst will be exposed to much heavier feed than the bottom part and could be susceptible to fast deactivation. To prevent this deactivation, the low-activity grading zone could be expanded but this sacrifices activity.

b) Product sulphur and aromatics cannot be independently controlled but are determined by the given nitrogen slip. For example, if the hydrogen consumption becomes too high, the operating temperature of the pretreatment at that point must be reduced, so the product nitrogen slip increases, thereby causing an imbalance in the conversion section. Alternatively, an active catalyst with lower activity can be selected but this will result in worse yields and a shorter cycle length. When the product sulphur specification is not met, this imbalance becomes even more problematic. In such a case, the nitrogen slip has to be reduced. This has positive effects on the conversion section but is detrimental to hydrogen consumption and cycle length.

Albemarle’s approach is markedly different. The basic principle is that each reactor zone is exposed to an oil product of a different quality, so the reactions taking place in each zone are very different. Hydro-cracking pretreatment generally has four zones that, depending on the situation, could vary in length and location (see Figure 2) or disappear.

Zoning
The first zone (Zone a) only exists for extremely heavy feeds, for example with an asphaltene content exceeding 500 ppm or an end boiling point higher than 650°C. For Zone a, dedicated catalysts with specific pore size distribution and texture composition are necessary. Indeed, even demetallisation catalysts quickly coke when the feedstock is so heavy, which results in metals and coke precursors slipping to the main catalyst and consequent rapid deactivation. For such conditions, Albemarle recommends KFR 22, which combines hydrodemetallisation activity and efficient removal of asphaltenes and Conradson carbon residue.

Although by Zone b the feed is partially cleaned up, stability is still the main problem, so a robust catalyst is required. In this zone, multi-ring polyaromatics (four rings or more) are strongly adsorbed onto the active sites and converted to coke during the cycle. Similarly to Zone a, a catalyst with a specific pore size distribution and texture composition is required, but now it needs good activity for HDS and HDN.

For this zone, we propose four different catalysts, depending on duty and unit limiting factors. In most cases, KF 860 Stars is applied, which delivers excellent HDS, HDN and robustness. Alternatively, KF 851 and KF 861 Stars are options if the unit is unconstrained by product nitrogen or product sulphur. Finally, KF 905 Stars can be applied to units where the nitrogen target is relatively easy while the product sulphur is limiting, which happens in some 
mid- to low-pressure mild hydrocrackers.

Zone c is perhaps the zone that reflects the most typical perception of hydrocrackers. Here, the only critical performance characteristic is HDN activity, so catalysts are selected on activity requirements weighted to cost. For this zone, we recommend, in order of increasing activity, KF 851, KF 861 Stars, KF 848 Stars and KF 868 Stars.

Zone d, the final zone, is where product nitrogen is typically below 100 ppm. Here, conversion of the most difficult nitrogen molecules occurs. In addition, as the product nitrogen content is low, extensive hydrogenation of monoaromatics may occur with a corresponding increase in hydrogen consumption. The catalyst selection for this zone therefore depends on unit objectives and hydrogen availability. Typically, a high-activity HDN catalyst like KF 848 Stars or KF 868 Stars is selected.

When extreme performance is desired, we recommend our highest activity catalyst, Nebula 20, for Zone d. This has twice the hydrogenation activity of other conventional catalysts. Such activity can be exploited in two ways: by reducing product nitrogen to very low levels, thereby facilitating the operation of the cracking catalyst and improving the yields; or by operating at the same product nitrogen slip as conventional systems but at a lower temperature (longer cycle length) or with more difficult feedstock, for example, larger intakes of distressed feedstocks.

When the unit is heavily constrained by hydrogen consumption, we recommend using a declining activity profile. Very active catalyst, such as KF 868 Stars, is then placed in Zone c to provide most of the required HDN activity without penalties in hydrogen consumption, as here product nitrogen is relatively high and hydrogenation of monoaromatics is not extensive. Less active catalyst, for example, KF 851 or KF 861 Stars, is then placed in Zone d to provide the additional HDN activity necessary to reach the product nitrogen slip target with minimal hydrogen consumption, as mono-
aromatics saturation is mitigated because the hydrogenation activity of these catalysts is insufficient to facilitate this reaction.

In action
Stax technology has been already applied in numerous hydrocracking units worldwide. One example is a hydrocracking unit operating at 150 bar and processing a feedstock with extremely heavy tails. For this unit, a loading with KFR 22 (Zone a), KF 860 Stars (Zone b) and KF 868 Stars (Zones c and d) was selected to replace a competitor’s catalyst. The result was a 30% improvement in cycle length despite operating at lower product nitrogen slip, with the latter factor guaranteeing an appreciable improvement in distillates yield.

A second example is a hydrocracking unit running at 110 bar. In this case, the feedstock characteristics were such that Zone a was absent. The loading consisted of a combination of KF 860 Stars (Zone b), KF 868 Stars (Zone c) and Nebula 20 (Zone d). In comparison with the previous cycle, it was possible to process an additional 5–10% of a low value, heavy distressed feedstock while maintaining the same cycle length, which improved the refiner’s margin.


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