Roberto Zennaro

Wax composition transients during Fischer–Tropsch synthesis

Abstract The temporal evolution of the composition of the products collected from a slurry reactor during Fischer–Tropsch synthesis over a Co/Al 2 O 3 catalyst is studied, in order to rationalize the time required for a complete renewal of the liquid phase used to initially dilute the catalyst. The transients are well described by a simple equation with no assumption on the phase repartition of the hydrocarbons in the reactor, which also provides an estimate of the time constant of the system. The mathematical model can be used to predict the time necessary to reach steady state of the composition of the reactor outlet and hence collect reliable data for product selectivity in a slurry reactor. The change in the composition at the outlet of a fixed bed reactor after step changes of a co-fed liquid olefin is also approximately analyzed: it is shown that the transient is much faster for a fixed bed reactor than for a slurry reactor.

Reactivity of paraffins, olefins and alcohols during Fischer-Tropsch synthesis on a Co/Al2O3 catalyst

Mechanistic aspects of the Fischer Tropsch synthesis are here investigated by chemical enrichment experiments of olefin, paraffin and alcohol molecules. The results indicate that paraffins and alcohols are terminal products, whereas olefins are hydrogenated, hydroformilated and partecipate in the chain growth.

On the performance of a Co-based catalyst supported on modified γ-Al2O3 during Fischer–Tropsch synthesis in the presence of co-fed water

The effect of water on the Fischer–Tropsch performance of a supported cobalt catalyst has been studied in a fixed bed reactor by running co-feeding experiments for more than 1000 h under industrially relevant process conditions. The adopted γ-Al2O3 support (Dp = 300 μm) has been modified with superficial cobalt aluminates to reduce the risk of formation of these species during the reaction as a consequence of the strong interaction between the active phase and the support. In spite of this precaution, the addition of water results in the slow deactivation of the catalyst, possibly caused by the oxidation of the reduced cobalt metal centers. The effect of water on the product distribution is more complex: in this case, water has both irreversible effects, which become stronger upon increasing the concentration of co-fed water, and reversible effects, which are clearly visible even with small concentrations of co-fed water. The two effects lead, in the presence of co-fed water, to decreased CH4 and alcohol selectivities and increased C25+ hydrocarbon, olefin and CO2 selectivities. These results can be explained by assuming that water suppresses hydrogenation reactions.