Distillate Hydrotreating to Ultra-low-sulfur Diesel – the Impact of Aromatics

Abstract

Distillate hydrotreating has become a key concern in the refining industry owing to ultra-low-sulfur diesel legislation. Rate-limiting parameters that had previously been overlooked, such as aromatic inhibition, may need to be considered for the production of ultra-low-sulfur diesel (<10 weight parts per million wppm sulfur). As hydrodesulfurization (HDS) and aromatic saturation (HDA) reactions occur in parallel in a shared environment, they compete for the necessary resources, namely, hydrogen and catalyst sites. A series of pilot plant and numerical simulation experiments were completed in order to explore how the competition for hydrogen and catalyst sites impacts the rate of HDS in the presence of poly-cyclic aromatics. A diagnostic pilot plant test showed that external mass transfer was not rate-limiting. Intra-particle mass and energy transport were investigated by calculating the concentration and temperature gradients in a catalyst pellet. The effectiveness factors for HDS and HDA decreased in the presence of increased aromatics concentrations. However, the magnitude of the changes was insufficient to conclude that the competition for hydrogen was the sole mechanism needed to explain the suppression of HDS activity. Furthermore, the pilot plant data indicated that the presence of poly-aromatics has a more significant impact on the slower HDS reactions (sterically hindered dibenzothiophenes). As a result, it was concluded that aromatics inhibition (competitive adsorption on catalyst sites) impacted the rate of HDS most significantly. Although the consequences were significant, the impact of aromatic inhibition was minor relative to the analogous nitrogen inhibition.