45
CoMo
NiMo NiMo/CoMo CoMo/NiMo/CoMo
40
35% total aromatics
35
30
25
SRGO + 30% LCO feed
20
15
SRGO + 15% LCO feed
10
5
SRGO feed
0
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Hydrogen consumption , scfb
Figure 7 Total aromatics content in the liquid products
which seems to indicate a small variation in catalyst activ- ity with time. As expected, this trend was also observed in hydrogen consumption (see Figure 5 ). NiMo catalyst presented the highest HDS capacity dur- ing all the conditions tested, followed by the CoMo/NiMo/ CoMo and NiMo/CoMo systems. As previously observed with hydrogen consumption, the CoMo catalyst showed the lowest activity. This finding suggests CoMo/NiMo/CoMo as an optimal loading scheme with a good balance between catalyst activity and hydrogen consumption. This conclusion draws on the fact that the right catalyst type was used for the right purpose at the right reactor position when the ratio between CoMo/NiMo is fixed (constant catalyst loading cost), 50/50% in this case study. As illustrated in Figure 6 , the CoMo/NiMo/CoMo scheme achieved the ULSD sulphur specification, 8 ppmw, at 30% LCO blending percentage (a practical limit to achieve the minimum cetane number for modern road diesel) with a relatively low SOR temperature (321°C, see Figure 2) and only a marginal increase (5%) in hydrogen consumption (see Figure 5).
In the CoMo/NiMo/CoMo scheme, the low nitrogen zone can be achieved earlier with more NiMo catalyst volume available for HDN reactions, as the first half of the CoMo catalyst is used for easy sulphur treatment near the reactor inlet. Generally, the HDS rate is the highest in the reactor region with low nitrogen levels, so overall RVA increases with the enlarged low nitrogen zone. In the hydrogen constrained units, the last portion of CoMo can be placed near the reactor outlet to optimise hydrogen use further. Moreover, placing CoMo catalysts near the reactor outlet offers better stability for hydrotreating units with a rela- tively low pressure rating. The hydrogen partial pressure drops along the reactor axis, and CoMo works better when the hydrogen partial pressure is below 40 bara (less hydro- gen available for promoting the indirect sulphur removal pathway). Product aromatics and density During hydrotreating, unsaturated hydrocarbon molecules (olefins and aromatics) are usually partially hydrogenated, which affects global hydrogen consumption. Because of
0.800 0.805 0.810 0.815 0.820 0.825 0.830 0.835 0.840 0.845 0.850 0.855 0.870 0.875 0.880 0.865 0.860
CoMo
NiMo NiMo/CoMo CoMo/NiMo/CoMo
0.845 product density
150
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Hydrogen consumption , scfb
Figure 8 Density of the liquid products
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Catalysis 2023
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