Enhancing flexibility in two-stage hydrocrackers
Shifting markets and refined product priorities compels refiners to find the flexibility for shifting from middle distillate to naphtha for petrochemical feedstock.
Devansh Dhar, Pronit Lahiri and Paul Ronald Robinson
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A Mediterranean refinery recently installed Topsoe catalysts in a two-stage hydrocracker. Taking an unconventional approach and using a zeolite-based catalyst in the second stage provided product flexibility while maintaining high selectivity in diesel mode. The required temperatures in the second stage are lower than for amorphous-based catalysts. This one solution solved three challenges: it satisfied the need for naphtha flexibility, it relieved limitations due to tight heat integration, and it tempered the build-up of heavy polynuclear aromatics (HPNA), thereby leading to significant operational benefits in the hydrocracking unit.
The hydrocracking unit (HCU) comprises three reactors: Reactor 1 contains guard and pretreat catalysts, Reactor 2 contains the zeolite-based first-stage hydrocracking catalyst, and Reactor 3 contains the zeolite-based second-stage hydrocracking catalyst. In addition, post-treatment catalyst is loaded in the hot high-pressure separator presented in the block flow diagram (see Figure 1).
The operating pressure exceeds 150 bar, and the target conversion level is 99.5 wt%. The primary aim is to maximise middle distillates while processing heavy vacuum gas oil (HVGO), coker gas oil, and other heavy streams. In addition, the refiner desires flexibility to increase naphtha yields in response to changes in market demand. The product slate from this unit comprises LPG, light naphtha, heavy naphtha, kerosene, diesel, and unconverted oil (UCO).
Thanks to its Mediterranean location, the refinery has ready access to many crudes. In a given year, more than 25 different crudes are processed. Consequently, the feed to the 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 aromatics, polyaromatics, and other unsaturated compounds, as well as high microcarbon residue (MCR). The nitrogen content 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 facilitate HPNA formation.
HCU operating challenges
Primary HCU operating challenges include:
1 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.
2 HPNA formation: Another constraint is HPNA formation.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 properties such as density, cetane index, smoke point, and UCO colour.
3 Flexible product slate: For this refinery, the ever-fluctuating 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 formation and accumulation. HPNA formation depends on the concentrations of precursors in the feed and operating conditions. Precursors include polyring compounds: polyaromatics, polynaphthenes, and naphthenoaromatics. Important process conditions include temperature, pressure, 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 catalysts, which allow operation at lower temperatures.
Aromatics crossover effect: saturation vs condensation
Aromatics saturation is essential to hydroprocessing, especially deep HDS, deep HDN, and hydrocracking. In gas oil, VGO, and cracked stocks, the most difficult-to-remove sulphur and nitrogen compounds are those in which the heteroatoms 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 subsequent hydrogenolysis.2 Moreover, the catalytic hydrocracking 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+ aromatics remaining after hydrotreating as a function of temperature 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, saturation 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.
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 hydrocarbons 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 hydrocrackers. 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.
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