(617cz) Fischer Tropsch Synthesis: A Study of the Selectivity to Light Hydrocarbons

Muleja, A., University of South Africa
Yao, Y., University of South Africa
Hildebrandt, D., University of South Africa
Glasser, D., University of South Africa
The effect of co-feeding nitrogen in Fischerâ??Tropsch synthesis (FTS) in a fixed bed reactor over a 10%Co/TiO2 catalyst was investigated. The partial pressure of CO and H2 in the feed to the reactor was kept constant, while the partial pressure of N2 was switched between a low and high value. We plotted the data in the standard formats typically used, but also in two other ways: Yao plot is a plot of Pn+1/On+1 versus Pn/On where for us n is a carbon number from n=2 to 5; Lu plot is a plot of the relationship between normalized mole fractions of On, On+1 and Pn. in ternary plot.

A comparison of the results obtained indicates that there was almost no effect on the overall reaction rate, but that there was a significant, positive effect on the product selectivity. A lower selectivity to the undesired light hydrocarbons was observed when co-feeding N2. From Yao plot, we find that the data lies on two straight lines, one for n=2 and the other for n>2. Ternary plots show that the reactor operation tends to a stable point. Such results cannot be explained by classic kinetics alone but reaction equilibrium among some species, vapour-liquid equilibrium (VLE), boiling, or a combination of these factors may be contributing to the observed behaviour in FT. The laboratory data indicates that the selectivity to the (C5â??C19) fraction is particularly enhanced by co-feeding N2 in FTS.