PTQ Q4 2026 Issue

Sulphur compound

Structure (Example)

Most reactive (Easiest to remove)

Increasing refractori n ess (Decreasing reactivity)

Most refractory (Most dicult to remove)

Benzothiophene (BT)

Dibenzothiophene (DBT)

S S

4,6-DMDBT (4,6 -d imethyldibenzothiophene)

Nitrogen compound

Structure (Example)

Most reactive (Easiest to remove)

Increasing refractori n ess (Decreasing reactivity)

Most refractory (Most dicult to remove)

Indole

Quinoline

N H

1,8- d imethylcarbazole

Figure 1 Relative reactivity of sulphur and nitrogen compounds in diesel boiling range molecules

service do not always provide the strongest solution for mixed diesel feeds, such as LCO-containing feeds, or units operating at limited hydrogen partial pressure conditions. Not all nitrogen compounds influence hydrotreating to the same extent. Basic nitrogen compounds can strongly inhibit active sites. However, non-basic polyaromatic nitrogen spe- cies such as carbazole are particularly problematic in diesel hydrotreating because of their structure, concentration, and close association with cracked feed components. Carbazole molecules combine aromatic stability with non-basic pyrrolic nitrogen functionality, making them among the most challenging nitrogen species encountered in diesel hydrotreating. Unlike basic nitrogen compounds, which primarily inhibit catalyst activity through acid-base interactions, carbazoles influence catalyst performance through competitive adsorption on the hydrogenation func- tionality of the active phase. Their fused polyaromatic structure exhibits a strong affin - ity for catalytically active edge sites, allowing these mole- cules to remain adsorbed for relatively long residence times and reducing the availability of hydrogenation sites required for deep sulphur removal. As diesel sulphur concentration decreases through- out the reactor, the HDS mechanism gradually shifts from predominantly direct desulphurisation toward hydrogena- tion-assisted reaction pathways. At this stage, competition between refractory nitrogen compounds and the remaining refractory sulphur species becomes increasingly important. Carbazoles therefore reduce catalyst effectiveness not by permanently deactivating the catalyst, but by temporarily occupying catalytically important sites that are simultane- ously required for hydrogenation of highly refractory sulphur molecules such as alkyl-dibenzothiophenes and particularly 4,6-dimethyldibenzothiophene (4,6-DMDBT).

The consequence is a progressive reduction in the avail- ability of hydrogenation functionality precisely when it becomes most critical for achieving ULSD specifications. As the catalyst approaches end-of-run (EOR) conditions, this competitive adsorption becomes even more significant because the remaining sulphur compounds are themselves increasingly refractory and rely heavily on hydrogena- tion-assisted conversion pathways. Consequently, even relatively moderate carbazole concentrations can have a disproportionate impact on final product sulphur by shifting the balance between parallel HDS pathways. From a catalyst design perspective, efficient carbazole conversion therefore requires more than simply increas- ing intrinsic hydrogenation activity. The catalyst must also maximise the accessibility and utilisation of the active phase throughout the catalyst particle. A highly dispersed active- metal structure exposes a larger population of catalytically accessible edge sites, allowing HDN and HDS reactions to proceed simultaneously while minimising competitive inhi- bition. This relationship explains why improved carbazole removal is frequently accompanied by improved conversion of the most refractory sulphur species under nitrogen-con- strained ULSD operating conditions. A carbazole is therefore not simply another nitrogen com- pound, because it can inhibit the hydrogenation path and reduce the catalyst’s ability to remove refractory sulphur compounds. Its presence significantly increases the sever - ity required to achieve sulphur targets, reduces tolerance to feed variability, and can accelerate carbon laydown if oper- ating the reactor beyond the thermodynamic equilibrium and subsequently increase the deactivation rate of the cata- lyst system. In practical terms, the unit may require a higher weighted average bed temperature (WABT) earlier in the cycle, resulting in a shorter cycle length, greater sensitivity

PTQ Q4 2026