Fischer-Tropsch synthesis over Al2O3- supported cobalt catalysts

The low temperature Fischer-Tropsch synthesis (FTS) is a CO hydrogenation process applied to produce synthetic long chain hydrocarbons and oxygenates that can be marketed as high added value fuels (gasoil, kerosene) or chemicals (base oils, normal paraffins, naphta). Several aspects have been addressed in this work. A model for the prediction of the yields of vapor and liquid hydrocarbon species formed during the low-temperature Fischer-Tropsch synthesis over a representative cobalt-based catalyst at industrially relevant process conditions has been developed in this work. At the typical process conditions more than 95 mol.% of the C1-C30 hydrocarbons are in the vapor phase the most abundant product in the liquid phase formed in the reactor is the heptacosane, whose liquid fraction represents more than 78% of the total amount of C27 formed in the reactor. A kinetic model able to describe the distribution of FTS products, not only in terms of hydrocarbon products, but also of oxygenates, including carbon dioxide has been developed. Given that the Co-based FTS does not produce many oxygenated species, it has been decided to group the alcohols with more than five carbon atoms into a single pseudo component. The developed model accurately predicts the reactants conversions and the hydrocarbon products distribution, distinguishing between n-paraffins and -olefins. In particular this model is able to describe the linear hydrocarbon product distribution typical of the polymerization reactions like the FTS. Furthermore, the same model is able to properly describe the distribution of oxygenated products and the carbon dioxide selectivity. Mass transfer limitations may affect the net rate of reactants conversion and the product distribution. This work addresses indeed the effect of internal diffusive limitations on the reactivity of Co/Al2O3 catalysts for the FTS. In order to study the effects of diffusive limitations tests were carried out on two catalytic systems having the same composition but different particle size, and operating at different temperatures. Both catalyst systems were tested at different temperatures, so as to be able to estimate the activation energy of the reaction as a function of the catalyst particle size. The effects observed pointed out that, upon increasing the temperature the CO conversion increases, the selectivity to heavy products (especially C25+) decreases, while the light species (CH4 in particular) increases, the olefin content in the products is reduced the CO2 selectivity increases. In order to prevent the onset of mass transfer limitations, it is also possible to choose a catalytic system of egg-shell type, constituted by particles in which the catalytically active phase is arranged on a thin layer which constitutes the outer part of the granule. Adoption of these catalyst systems enables a compromise between pressure drop and diffusive limitations. With the developed method eggshell catalysts based on cobalt and supported on alumina. An activity test is carried out to demonstrate the ability of the catalyst to be active at the FTS.