Due to increased political pressure to improve the air quality, tighter and tighter specifications on transport fuels are being introduced. The sulfur specification for diesel in the EU is 10 wt. ppm from 2009, and a similar specification has been introduced in Japan. The sulfur specification for diesel in the US has been 15 wt. ppm since 2006.
Within this decade it is expected that a significant part of the world will also move towards an ultra low sulfur diesel (ULSD) specification of 50 ppm (however expected to e as low as 10 wt. ppm S in the large cities). As a consequence of these tight specifications, the need for hydrotreating capacity grows.
Removal of organic sulfur is carried out in hydrotreating units, where diesel oil and hydrogen flow concurrently down through a bed of porous catalyst particles (usually CoMo or NiMo on an alumina support). Several reactions take place in a hydrotreater:
The saturation of aromatics (HDA) is important, since several properties of the diesel fuel, e.g. the cetane number and the emission characteristics, depend on the content of aromatic compounds.
Saturation of fused poly-aromatic rings is known to be fast at typical hydrotreating conditions, and the conversion is therefore often limited by thermodynamic equilibrium, whereas the saturation of mono-aromatics is much slower. The catalytic reactions take place inside the liquid-filled pores of the catalyst particles and therefore require the diffusion of hydrogen from the gas phase through a liquid film and into the particle before a reaction can take place. Thus, the conversion may be limited by mass transport of the reacting species depending on the process conditions.
Using naphthalene hydrogenation as a model reaction, the project develops a model to describe the hydrodearomatization reactions taking place in a trickle-bed reactor, accounting for the intrinsic kinetics, thermodynamic equilibrium constraints, vapor-liquid equilibrium and mass transfer/diffusion limitations.
Rasmus Risum Boesen, DTU, Department of Chemical and Biochemical Engineering
Master thesis in the R&D Division (Refinery Process)