PTQ Q3 2022 Issue

A (naphthalene)

COKE

Dicoronene

C (pyrene)

B (phenanthrene)

F (coronene)

Diovalene

J (ovalene)

Figure 3 Sequential growth of polyaromatics. 5 Modified to include the dimerisation of coronene and ovalene via the Scholl reaction 6

HPNA formation and accumulation At temperatures beyond crossover, polyaromatics grow to form HPNA. HPNA also are known as PNA or PAH (polyaromatic hydrocarbons). They are condensed hydro- carbons containing several fused aromatic rings. 4 Light PNAs, with two to six rings, are found in straight-run VGO. Larger HPNAs with more than six rings are found in heavy cracked stocks such as heavy coker gas oil (HCGO) or heavy FCC cycle oil (HCO). They are called the ‘red death’ due to their colour and deleterious impact on operations: they can foul equipment and/or deactivate catalysts. Figure 3 illustrates ways in which HPNAs might grow and accumulate in high-conversion recycle hydrocrack- ers. In one mechanism, growth proceeds via the addition of 2-carbon and 4-carbon units. 5 Another route involves dimerisation via the Scholl reaction. 6 HPNA build-up depends on operating conditions, primarily temperature, pressure, and the bleed rate of UCO. As previously stated, at high enough concentrations, they can foul equipment and/ or deactivate catalysts. They must be frequently measured

and carefully controlled. The conventional way to reduce HPNAs is to bleed away some UCO, which is expensive because it is equivalent to decreasing conversion to higher- value products. At a constant UCO bleed rate, HPNAs increase from start-of-run (SOR) to end-of-run (EOR). Table 2 illustrates the change in PNA and HPNA from SOR to EOR for a two- stage unit. 5 Due primarily to higher temperatures at EOR, there was a 2x to 4x increase in polyaromatics with three- plus rings. There was also a shift in the relative amounts of three-ring aromatics versus four- and five-plus ring aromatics. Figure 4 is based on the data in Table 2 . It presents a plot of EOR/SOR concentration ratios for different ring sizes, illustrating that, in general, concentration growth is great- est for larger ring families. HPNA control: precursor removal Opening saturated rings eliminates the reversibility of aromatic saturation. Therefore, removing precursors with

Composition of UCO at SOR to EOR for two-stage hydrocracking (measured by HPLC)

0.0 1.0 0.5 1.5 2.0 2.5 3.0 4.0 3.5 4.5

Concentration in UCO, wt%

Ratio

Fraction (HPLC)

SOR 57.8 27.5

EOR 53.0

EOR/SOR

Saturates

0.92 1.03 1.10 2.39 3.90 2.84 1.11

1-ring aromatics 2-ring aromatics 3-ring aromatics 4-ring aromatics 5-ring aromatics +

28.46 14.35 2.862 0.729 0.380 46.78

13.04 1.198 0.187 0.134 42.05

Saturates

1-ring 2-ring

3-ring

4-ring

5-ring

Total aromatics

Fraction (HPLC)

Figure 4 EOR to SOR ratio increase in aromatic build-up

Table 2

PTQ Q3 2022