Due to the progressive restrictive environmental specifications that impose the production of ultra low sulfur diesel (under 10 ppm from 2009) and introduce new emphasis on the petroleum refining process, the worldwide refineries have to optimize the hydrodesulfurization (HDS) process conditions as a function of feed properties, like flow rate, temperature and pressure. Most of hundreds of sulfur compounds present in gasoil can be easily desulfurized under typical hydrotreating operating conditions but the removal of refractory alkyldibenzothiophenes results much more difficult due to their steric configuration. Moreover their conversion decreases because of the presence of nitrogen compounds that compete against the refractory sulfur compounds on the catalyst surface and strongly inhibit their hydrodesulfurization. In this work the kinetic behavior of nitrogen compounds has been investigated, considering three lumps with a different cross-effect on the hydrodesulfurization of sulfur compounds. The resulting model is able to satisfactorily predict the total sulfur content and the distribution of the refractory compounds in the hydrotreated products.

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