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Shaping up hydrotreating performance (TIA)

A fine balance of activity and pressure drop has long created a challenge when considering the maximisation of performance for hydroprocessing and hydrocracking units.

James Esteban
Criterion Catalysts & Technologies
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Article Summary
It is especially a critical balance for high profile units in hydrocracking service that receive large margins for product upgrade and also have high incentive for incremental processing capacity. Recent margins have placed a great deal of pressure on refiners to maximise hydrocracking unit throughput up to hydraulic limitations, which in many cases is a limit set by reactor pressure drop. Limitations in reactor pressure drop can be mitigated by many means, but ultimately catalyst selection is the most critical factor in hydrocracker optimisation. Criterion developed the ATX (Advanced Trilobe eXtra) catalyst shape to allow hydrocracking units to reduce pressure drop and improve activity simultaneously. There are several significant advantages of the ATX shape (see Figure 1).

Pressure drop in packed beds is commonly modelled by the Ergun equation which is applicable for single phase flow, but to model two phase flow through packed beds there are modified versions of the Ergun equation. Using the modified Reynolds number the Ergun equation can be simplified as:

This equation illustrates that pressure drop is inversely proportional to both catalyst bed void fraction and the effective particle diameter. Void fraction itself is a function of loading method (dense vs sock)and particle shape. Differing diameter particles of the same extruded shape load at the same void fraction and density since packing efficiency is determined by shape. However, larger particle diameter extrudates of the same shape will result in lower pressure drop. This is due to the effect of liquid hold-up and relative velocity in the bed voids. In addition to particle size, particle shape has a significant impact on pressure drop.

The following two commercial examples demonstrate the effect of the ATX shape applied in hydrocracking units to unlock additional capacity, creating increases in profitability.

Case 1: Canadian refiner
Figure 2 shows the normalised pressure drop from a hydrocracking unit at a Canadian refinery where the previous cycle applied Criterion’s earlier generation Z-3723 TL catalyst and the current cycle is operating with Z-FX20 ATX. The current catalyst load was also dense loaded to maximise reactor performance and yields while still achieving reduced bed pressure drop. The reduction in pressure drop across the reactor has permitted an increase in unit capacity of 20%,

resulting in increased profits, better than $10 million annually. In addition to increases in unit profitability, the catalyst load itself saw a reduced fill cost as a result of lower compacted bulk density.
Case 2: US Gulf Coast refiner
One of the highest capacity hydrocracking units in the world leverages the advantage of ATX shaped catalysts to maximise unit capacity. Over recent operating cycles the following unit has transitioned to Criterion’s ATX catalysts to reduce unit pressure drop. While only ~30% of the overall catalyst load is now ATX shaped cracking catalyst, the unit has been capable of increasing throughput by 35% (see Figure 3).

Figure 4 shows the relative difference between the various shaped extrudates used in hydroprocessing applications in terms of pressure drop per bed height contribution ratio with the typical trilobe setting the scale at 100%. The chart shows that a quadlobe particle with the same outer dimensions as a trilobe particle will result in higher pressure drop across the same bed depth. This is well explained when considering the Ergun equation because the effective diameter of a quadlobe particle is smaller than that of a trilobe particle. Further, the ATX shape offers a significant reduction in pressure drop when compared to standard TL particles.

While the pressure drop advantages of ATX are remarkable in comparison to conventional particle shapes, the greatest advantage afforded by the ATX shape is exceptional activity. Improved reactor pressure drop profiles with improved activity enables performance gains for these hydrocracking units.

James Esteban is a Senior Technical Service Engineer with Criterion Catalysts & Technologies.

This short case study originally appeared in PTQ's Technology In Action feature - Q1 2017 issue.

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