The hydrogenation of pyrolysis gasoline (PyGas) over nickel and palladium catalysts

摘要

Pyrolysis gasoline (PyGas) is a by-product of high temperature naphtha cracking during ethylene and propylene production. It is a high octane number mixture which contains aromatics, olefins and paraffins ranging from C5s to C12s. PyGas has high potential for use as a gasoline blending mixture and/or as a source of aromatics. Currently, PyGas is generally used as a gasoline blending mixture due to its high octane number, but global production of PyGas is very high and further increases are anticipated in the future due to higher demands for ethylene and propylene. However, current strict fuel regulations for aromatic content make PyGas utilisation as a blending mixture more difficult, therefore a useful avenue for PyGas consumption is desired. Catalytic hydrogenation of PyGas is an important industrial and academic research area for the stabilisation, upgrading and utilisation of PyGas. Limited work has been performed on the hydrogenation of PyGas and an incomplete picture of the process has been obtained. The composition of PyGas is very complex and therefore most of the studies have been carried out with single compounds or a mixture of a few model compounds for simplicity and generality. However, the single model compound cannot be representive of the entire PyGas hydrogenation process. Furthermore, the behaviour of these compounds is generally different in mixtures than as individual compounds. Hence, the hydrogenation of a PyGas, which contained styrene, toluene, 1-octene, cyclopentene, heptane, decane and 1,3-pentadiene/1-pentene, was investigated over alumina supported nickel and palladium catalysts. This is a comprehensive model for the broader groups of hydrocarbons present in PyGas. The aim of the work was to investigate the effect of reaction parameters such as reaction temperature, total reaction pressure, hydrogen partial pressure and WHSV of PyGas on the hydrogenation of PyGas. Different strategies were proposed for PyGas consumption to obtain; (i) a high octane number gasoline blend, (ii) an aromatic source mixture and (iii) hydrogenation of surplus aromatics present in PyGas. These desired products were achieved during PyGas hydrogenation over both the nickel and palladium catalysts by using different reaction conditions. However, the palladium catalyst was found to be preferable for the selective hydrogenation of reactive species such as styrene, diolefins and for the isomerisation of olefins, without the hydrogenation of aromatics during PyGas hydrogenation at mild reaction temperatures. However, surplus aromatics hydrogenation can be achieved over the palladium catalyst with an increase in reaction temperature. Conversely, a greater amount of aromatics saturation was observed during PyGas hydrogenation over the nickel catalyst. Therefore the nickel catalyst was found to be preferable when aromatic ring saturation is desired during PyGas hydrogenation. The selective hydrogenation of PyGas without aromatics saturation was achieved over a nickel catalyst when using a low hydrogen partial pressure. The kinetics of PyGas hydrogenation were also investigated for a better understanding of the process. The apparent orders for hydrogenation /isomerisation of the PyGas components were investigated by using an empirical rate equation. During PyGas hydrogenation, first order (1.1 to 1.6) kinetics were observed for the hydrogenation of olefins to their respective paraffins over both the nickel and palladium catalysts with an increase in hydrogen partial pressure, which became zero or negative order kinetics when sufficient amounts of hydrogen were available on the surface of the catalyst. Negative order kinetics were observed for olefin isomerisation to internal olefin with an increase in hydrogen partial pressure. Meanwhile the hydrogenation of styrene to ethylbenzene followed zero order kinetics with respect to hydrogen over both catalysts due to the strong adsorption of styrene onto the catalyst. Third order kinetics were observed for aromatics hydrogenation over both catalysts. On the other hand, the hydrogenation of olefins to paraffins followed zero to negative order (0 to -0.7) kinetics, whilst positive order (1.6 to 3.6) kinetics were observed for isomerisation of olefins to internal olefins with respect to PyGas. Moreover, positive order (0.7 to 1.0) kinetics were observed for styrene hydrogenation to ethylbenzene with respect to PyGas. The hydrogenation of aromatics followed negative orders kinetics with respect to PyGas over both catalysts due to competitive hydrogenation of the olefinic and aromatic components. Coke deposition is believed to be the main reason for catalyst deactivation during the PyGas hydrogenation reaction. The amount and nature of the coke deposited was investigated by in-situ temperature programmed oxidation (TPO). The increase in reaction temperature not only increased the amount of coke deposition but also produced more condensed hydrogen deficient type coke. Conversely, the carbon laydown decreased with an increase in hydrogen partial pressure. Larger amounts of coke deposition took place over the nickel catalyst when compared to the coke deposited over the palladium catalyst under identical reaction conditions. Moreover, a soft type coke (with lower C/H ratio) was deposited over the palladium catalyst, while a comparatively hard type coke (with higher C/H ratio) was deposited over the nickel catalyst. Both the nickel and palladium catalysts used during PyGas hydrogenation were effectively regenerated by in-situ TPO with no significant loss to their catalytic properties.