Effects of organic nitrogen compounds on hydrotreating and hydrocracking reactions

A major issue encountered in hydrotreating and hydrocracking reactions, as in many others fixed bed catalytic processes, is the decrease of catalytic activity with time on stream. The organic nitrogen compounds act as temporary poisons in hydroprocessing catalysts besides being coke precursors. The inhibiting effects of nitrogen compounds present in crude oil have been studied on SRGO hydrotreating reactions and VGO hydrocracking reactions. The results show that selective removal of nitrogen by adsorption using silica and alumina in varying proportions, would not only increase the HDS catalyst activity by more than 60%, but would also reduce hydrogen consumption. This step of nitrogen removal can be installed additionally in the upstream of an existing SRGO HDS reactor to achieve higher desulphurization or can be designed with the grass roots units. The inhibition effects of nitrogen on VGO hydrocracking have been studied at different temperatures and the reaction has been found to be highly non-linear in nature and the conversion rapidly drops and the slope becomes less steep as the nitrogen level increases. At higher reaction temperature, the drop in activity or conversion with feed nitrogen is less than that in lower temperatures due to the higher rate of desorption of nitrogen compounds at elevated temperatures. The drop in conversion with nitrogen compounds present in VGO indicates the presence of organo nitrogen compounds having higher basicity compared to the nitrogen compounds by pyridine doping. The hysteresis exists in adsorption/ desorption of nitrogen compounds and it indicates that desorption is a very slow process. With the increase of nitrogen compounds in the feed, the conversion drops rapidly and it takes long time to reach an equilibrium value. Similarly, with the step increase in reactor temperature, nitrogen desorption takes place at a slow rate and the conversion level comes to an equilibrium value after similar to 8 days. The observed effects of nitrogen inhibition on SRGO hydrotreating and VGO hydrocracking conversion are explained reasonably well by kinetic models. (c) 2005 Elsevier B.V. All rights reserved.