Separation of mono- and dibranched hydrocarbons on silicalite

Breakthrough curves for mixtures of C5 and C6 hydrocarbons with different degrees of branching were obtained experimentally on a silicalite molecular sieve by fixed-bed experiments. The kinetic separation of di- from monobranched hydrocarbons was feasible on this type of zeolite: dimethyl molecules enter silicalite crystals very slowly, whereas monomethyl molecules are quickly adsorbed. Experimental results were compared to a theoretical isothermal model, considering the variation of diffusivity with concentration according to the Maxwell - Stefan theory. The parameters of the model (adsorption equilibria and diffusivities) were determined from single-component breakthrough curves. Experimental curves were generally well represented by the model, but the variation of diffusivity with concentration, as predicted by the Maxwell - Stefan theory, did not significantly improve the model prediction of experimental breakthrough curves by using a mean value of diffusivity.