Modelling of the separation of normal paraffins from kerosene fractions by a simulated moving bed process

Linear paraffins are widely used in the manufacturing of industrial and domestic detergents. Some adsorbents selectively separate these linear hydrocarbons by adsorption from petroleum feedstocks. LTA molecular sieves (5A zeolite) adsorb linear paraffins while excluding the rest of the components of kerosene (branched hydrocarbons and aromatics). Equilibrium and kinetic parameters are available in the literature for light paraffins in the vapour phase, however, there is scarce information concerning high molecular weight paraffins in liquid phase, especially at the operating conditions of commercial processes. In a previous work, the equilibrium and kinetics of high molecular weight n-paraffins C5, C10, C14 and C18 were studied for the adsorption in liquid phase on 5A zeolite. The aim of this work is to study the equilibrium and kinetics of n-paraffins C12 and C16, as well as mixtures of n-paraffins C10, C12 and C14 in the same conditions. n-pentane has been included in the study as it is mainly used as desorbent in the cyclic simulated moving bed (SMB) commercial process. Pure component isotherms were obtained, as well as a multicomponent isotherm. By comparing them, it was observed that selectivities are significantly lower in mixtures (for example, selectivity towards C14 with respect to C12 is lowered from 2.84 for pure paraffins to 1.05 for mixtures). A theoretical model has been developed to describe the column adsorption dynamics of the studied systems. The model has been included in an SMB simulation program (SMBSIM), and the model prediction has been validated by comparison with the separation performance data reported for a commercial SMB unit that separates normal paraffins from a hydrotreated kerosene fraction The model predicts the separation of linear paraffins with 99.2% purity and 96.3% recovery (5% error obtained for n-paraffin concentration in the extract and non-adsorptives in the raffinate).