Fischer Tropsch Synthesis: A Study of the Selectivity to Light Hydrocarbons

The effect of co-feeding nitrogen in Fischer–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 P_n+1/O_n+1 versus P_n/O_n 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 O_n, O_n+1 and P_n. 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 (C₅–C₁₉) fraction is particularly enhanced by co-feeding N₂ in FTS.