PTQ Q3 2022 Issue

properties such as density, cetane index, smoke point, and UCO colour.  Flexible product slate: For this refinery, the ever-fluc - tuating markets for fuels and petrochemicals require high product flexibility. This, in turn, demands high product flexibility from the hydrocracking unit. Flexibility depends on catalyst choice and process conditions. A conventional amorphous-based hydrocracking catalyst in the second stage of HCU might be inherently more selective to middle distillates, but it won’t be able to enhance heavy naphtha production when desired. Moreover, given the previously described limitations, it might not be able to achieve the desired conversion during the latter part of the cycle. Fundamentals of HPNA formation Due to its significant impact on recycle hydrocracking, it is worthwhile here to present a discussion on HPNA for - mation and accumulation. HPNA formation depends on the concentrations of precursors in the feed and operat - ing conditions. Precursors include polyring compounds: polyaromatics, polynaphthenes, and naphthenoaromatics. Important process conditions include temperature, pres - sure, LHSV, and UCO bleed rate. Increasing UCO bleed rate decreases conversion, which is expensive. Another option to reduce HPNA formation is to employ higher activity cat - alysts, which allow operation at lower temperatures. Aromatics crossover effect: saturation vs condensation Aromatics saturation is essential to hydroprocessing, espe - cially deep HDS, deep HDN, and hydrocracking. In gas oil, VGO, and cracked stocks, the most difficult-to-remove sul - phur and nitrogen compounds are those in which the het - eroatoms are present in ring compounds, such as hindered dibenzothiophenes and carbazoles. In deep HDS and deep HDN, saturation of one or more rings must precede subse - quent hydrogenolysis. 2 Moreover, the catalytic hydrocrack - ing of polynuclear aromatics also requires saturation prior to ring opening because aromatic rings do not crack. 3 Unfortunately, right in the middle of the usual operating temperature range for hydroprocessing, aromatics become more difficult to saturate. As temperatures increase through this ‘crossover region’, kinetics become less important, and thermodynamics start to dominate. At high enough temperatures, saturation becomes essentially impossible. Figure 2 illustrates this phenomenon. It shows di+ aro - matics remaining after hydrotreating as a function of tem - perature for different space velocities (LHSV) at constant pressure. Note that higher LHSV means lower residence time and, hence, a lower extent of reaction. Also note that the crossover temperature depends on pressure, an aspect not shown in the bespoke Figure 2 . As shown on the left of Figure 2 , below crossover, satu - ration depends on LHSV (residence time). The extent of saturation is higher for lower LHSV (i.e., higher residence time). In this region, kinetics determines reaction rates. To the right, above crossover, the extent of reaction depends less on LHSV. Thermodynamics dominate. In this region, using a catalyst with higher saturation activity makes less and less difference.

Comparison of properties of typical HVGO and HCGO

HVGO 0.920

Heavy coker GO

Sp.Gr.

0.961

S, wt%

1.34 1100

2.0

N, wppm CRR, wt%

4100

0.5

1.2

SimDist 70/95/FBP

473/556/586

476/547/590

Total aromatics

34 16 18

63 17 46

Mono

Di+

Table 1

HCU fluctuates widely in composition. For a complex unit like the HCU, this means frequent adjustment of process variables to meet changing processing objectives and a need for robust operational guidelines. Roughly 20% of the feed to HCU consists of heavy coker gas oils from a delayed coking unit (DCU). DCU streams typically contain high concentrations of aromat - ics, polyaromatics, and other unsaturated compounds, as well as high microcarbon residue (MCR). The nitrogen con - tent also tends to be high, and DCU nitrogen tends to be harder to remove than the nitrogen from straight-run VGO. These parameters require careful consideration in pretreat catalyst selection. Table 1 compares the properties of a representative HVGO with a representative DCU gas oil. Note the differences in nitrogen, total aromatics, and di+ aromatics. Higher concentrations of polyaromatics facili - tate HPNA formation. HCU operating challenges Primary HCU operating challenges include:  Impact of first-stage issues: The unit is limited by tight heat integration. This impacts the first stage and can limit its conversion, putting extra pressure on the second stage.  HPNA formation: Another constraint is HPNA forma - tion. 1 HPNA molecules form and accumulate in recycle hydrocrackers. They can precipitate in heat exchangers and other cold surfaces. They also deactivate catalysts, which has an adverse effect on yields and distillate product

90 100

LHSV = 4 Equilibrium

LHSV = 0.5 LHSV = 1 LHSV = 2

0 20 10 30 40 50 60 70 80

310 320

330

340

350 360

370 380 390

400

Temperature, ˚C

Figure 2 Di+ aromatics remaining after hydrotreating vs temperature for different LHSV (1/h).

PTQ Q3 2022