Hydrogen Requirements for Deep Desulfurization

We ran a series of case studies to calculate the hydrogen requirements for deep desulfurization in a high-pressure gas-oil hydrotreater. 

For the saturation of aromatics in a hydrotreating or hydrocracking unit, equilibrium effects, which favor formation of aromatics, start to overcome kinetic effects above a certain temperature. This causes a temperature-dependent aromatics cross-over effect, which explains the degradation of important middle distillate product properties—including kerosene smoke point and diesel cetane number—at high process temperatures near the end of catalyst cycles. The cross-over temperature is affected by feed quality and hydrogen partial pressure, so it can differ from unit to unit. 

The y-axis shows the mole fraction of total aromatics remaining in product. For this case study, the feed rate, feed type, and catalyst activity were kept constant, which means that deactivation effects were not included. With less-active end-of-run catalysts, the rates of forward (saturation) reactions are inhibited, so the aromatics cross-over effect is amplified. 

we ran the case study using the hydrotreating portion of the Suncor-Samia hydrocracker model with an equal-outlet temperature profile. For 500 wppm sulfur in the product, the makeup H2 flow is 15.2 MMSCFD. For 15 wppm, the makeup flow is 18.6 MMSCFD, more than 40% higher. Higher WART leads to increased conversion, increased HDN, and (up to point) increased saturation of aromatics. This explains why the hydrogen consumption increased so much— and so non-linearly— when the target product sulfur content went from 500 wppm to 15 wppm. 

We can also keep WART constant and achieve the desired level of HDS by allowing changes in LHSV. When this is done, the effect on hydrogen consumption is less severe. But in the real world, decreasing the LHSV is equivalent to lowering feed rate or adding a reactor, both of which are expensive. The WART-based trend shown here is more relevant to an existing unit. 

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