Catalysis 2023 Issue

Improved diesel hydrotreating catalyst loading scheme A European refiner used an independent catalyst testing approach to confirm their existing hydrotreating unit’s ability to cope with different LCO blending targets

Tiago Vilela and Nattapong Pongboot Avantium

R efineries process blends of straight-run gasoil (SRGO) and light cycle oil (LCO). LCO is convention- ally processed in hydrotreaters along with straight- run middle distillates to upgrade its economic value. From a product quality perspective, LCO has a relatively lower cetane number (poorer ignition performance in diesel engine) compared to straight-run middle distillates derived from the crude distillation unit. The aromatics content (low cetane components, mainly 2-ring aromatics) of LCO from FCC units can be as high as 85 wt% in a high severity FCC operation (such as high-octane gasoline or propylene mode). Generally, the cetane number of LCOs ranges from 15-25 compared to 40-60 for straight-run diesel. It is also impor- tant to note that the cetane number is directly proportional to the total aromatics content.1 As such, the amount of LCO permitted in the diesel blending pool is often limited by this combustion property, forcing refiners to dispose the remaining LCO to the low-value fuel oil blending pool as the viscosity adjuster. To make matters worse, disposing of LCO as fuel oil is becoming more and more constrained by declining demand for heavy fuel oil as the world is moving towards zero-carbon emissions. In addition to high aromatics content, a significant por- tion of organic sulphur (normally 0.2-2.5 wt%) is in the form of alkyl dibenzothiophenes (DBT), while organic nitrogen (typically 100-750 ppmw) is mostly constituted of non-basic organic nitrogen compounds (such as 5-ring membered carbazoles). These organic sulphur and nitrogen components are known to be refractive, posing challenges to ultra-low sulphur diesel (ULSD) operation. In most cases, LCO processing requires more severe hydrotreating (higher temperature) at the start-of-run (SOR) to meet the same product sulphur target (<10 ppmw for ULSD), thus limit- ing the cycle length. It should be noted that cycle length can also be limited by diesel ASTM colour specifications. It is common for the product colour to deteriorate over time from declined hydrogenation activity. In general, feeding LCO along with straight-run middle distillates requires higher hydrogen consumption due to hydrogenation of unsaturated hydrocarbon compounds. From a ULSD perspective, a preferred reaction pathway is saturating the first aromatic ring of alkyl DBT (for bet- ter sulphur accessibility of metal active sites) prior to the

sulphur removal by hydrogenolysis, thus adding to the total hydrogen consumption (see Figure 1 ). Removal of organic nitrogen compounds, an essential step before converting alkyl DBT, also contributes to the additional hydrogen con- sumption by increasing the total nitrogen content. Special grading requirements In addition to aromatics, the fact that LCO also contains a certain level of olefinic compounds (typically indicated by Bromine number) is a challenge in terms of grading bed design. Catalyst activity must initially be low enough and gradually increase over the reactor length to prevent rapid heat release, local hydrogen starvation, bed fouling from polymerisation, and coke deactivation. This addi- tional special grading requirement can limit the volume of higher-activity hydrotreating catalysts (which is particularly important when processing LCO) when the reactor volume is fixed, such as existing reactors. To accommodate a high portion of LCO in hydrotreaters, the hydrogen intake capacity must be large enough with adequate reactor volume, hydrogen partial pressure, and hydrogen circulation rate to ensure an acceptable catalyst deactivation rate. With higher temperature rises in the cat- alytic bed, a higher quenching rate will also be required to maintain catalyst bed thermal stability, thus adding to the

3 H

Saturation

4,6-dimethyldibenzothiophene

4,6-dimethyl- hexahydrodibenzothiophene

2 H (hydrogenolysis)

2 H

+ HS

Figure 1 Two sulphur removal pathways: 1) single-step hydrogenolysis (direct) 2) Pre-aromatic ring saturation followed by hydrogenolysis (indirect). The reaction rate of the second pathway is faster for sterically hindered sulphur compounds like alkyl DBT and can further be promoted by using nickel-molybdenum (NiMo) hydrotreating catalysts

Catalysis 2023