Equilibrium Constants for Isomerization of n-Paraffins

A correlation between lumped isomerization equilibrium constants (Keq) and the number of carbon atoms in paraffinic chains has been developed. The aim of the work is to check the meaningfulness from a chemicophysical point of view of previously estimated values of Keq in order to improve the performances of a simulation model for the isomerization/hydrocracking of Fischer-Tropsch (F-T) waxes, a mixture made up of normal paraffins covering a wide range of molecular weights. Owing to the lack of experimental data for paraffins with more than ten carbon atoms, a procedure has been developed to determine equilibrium constants extrapolating the thermodynamical data of low carbon number paraffins. Since in our case the hydrocracking simulation model considers lumped classes of isomers (i.e., monobranched and multibranched), the equilibrium data do not take into account single isomerization reactions but those from a n-paraffin to the lump of its monobranched isomers and from the lump of monobranched isomers to the lump of multibranched ones. The coherence of the estimated constants has been verified by comparison with the little data available in the literature on lumped equilibrium constants. The analysis of equilibrium constants at different temperatures has shown that isomerization becomes endothermic from a number of carbon atoms equal to about 13-14 onward; this result is also supported by enthalpy data.