(20b) Hydroconversion of Fischer-Tropsch Waxes Over Platinum Loaded Catalyst: Assessment of Product Distribution

Since the 1990s the European Union has established emission standards to set limits for vehicles exhaust gasses, in terms of NOx, CO, HC and particulate emissions. Since 1992, when the Euro 1 standard for Light Duty Vehicles came into force, the limits have become more stringent and also fuel regulations have been introduced. The current fuel regulations require a minimum diesel cetane number of 51 and a maximum sulfur content for gasoline and diesel of 50 ppm (2005), while "sulfur free" fuels (≤ 10 ppm) must be available since 2009. Also aromatics and olefins, that are usually added to gasoline to improve the octane number, must be substituted by paraffins and naphthenes. Natural gas conversion to liquid fuels (GTL) via Fischer-Tropsch technology provides middle distillates characterized by very low concentrations of sulfur and nitrogen and, due to their paraffinic nature, high cetane numbers (>75) but poor cold flow properties, thus they are not conforming to the standard specifications for fuels. The current cobalt catalyzed low temperature slurry FT technologies are aimed to the synthesis of heavy products (waxes), which are subsequently hydrocracked to produce high quality isomerized middle distillates. Besides the increase of MD yields, the hydrocracking reaction leads also to the formation of isoparaffins which have a deep influence on both cold flow properties and cetane number. Kerosene and gasoil specifications require a freezing point of -47°C and a cold filter plug point of -5°Â -20 °C respectively: isomerization reaction holds a fundamental role in FT wax upgrading because it allows to lower the melting point of paraffinic compounds with a consequent improvement of the cold flow properties of middle distillates.This solution was extensively analyzed in the last decades by many oil companies, like Sasol, BP, Shell, Chevron, Exxon. During the last years, Eni has widely studied the chemistry and kinetics of FT products hydrocracking.

The hydrocracking of a C10-C70 Fischer-Tropsch derived wax on a Pt/amorphous SiO2-Al2O3 catalyst was investigated on a lab trickle bed reactor. In particular the C4-C22 fraction composition was described in details, distinguishing among monobranched and multi-branched isomers. Furthermore in the monobranched lump the distribution among methyl- ethyl- and propyl- isomers was determined.

Experimental results show that during hydrocracking longer aliphatic chains are hydrocracked preferentially leading to an increase of middle distillate cut. Furthermore, the conversion was also characterized by a low naphtha selectivity and scarce formation of C1-C4 fraction. The latter data together with the evolution of Kerosene/Gasoil ratio indicate an "ideal hydrocraking" mechanism, where consecutive reactions of the primary products are negligible. The isomer content of reaction products shows a strong increase with conversion reaching values of 80% and 60% for the C15-C22 and C10-C14 fractions respectively. A detailed analysis of the C22- fraction in the product mixture evidenced that isomers are mainly made up of monobranched paraffins, while multibranched are more abundant at the highest conversion; this behavior is consistent with the accepted hydroconversion pathway of n-parafins.