Modelling of the separation of long-chain normal paraffins from kerosene in a simulated moving bed process: effect of the desorbent

Linear paraffins can be selectively separated from the rest of components of kerosene (branched hydrocarbons, aromatics and naphthenes) by means of liquid phase adsorption on 5A zeolite using the technology of simulated moving bed (SMB). In previous works, the kinetic and equilibrium parameters required for modelling and design of the SMB unit were obtained for pure n-paraffins and n-paraffin mixtures. However, the simulation of the SMB process indicated the presence of n-C5, used as a desorbent, in the separation zone, especially after the feed mixture is introduced. This finding motivated this work, in which n-paraffin mixtures (n-C10, n-C12, n-C14) including n-C5 were studied to address its influence in the process. The kinetic and equilibrium parameters for these mixtures were obtained and included in the model for the simulation of an SMB unit. While mixtures without n-C5 preferentially adsorbed shorter n-paraffins, it was found that including n-C5 in the mixtures reverses the selectivity of the adsorbent. In this case, longer n-paraffins are preferentially adsorbed, matching the trend observed for pure n-paraffins. In addition, n-C5 significantly increases the mobility of n-paraffins, as indicated in their higher mass transfer coefficients. The model was validated by comparing the predicted performance with the reported separation achieved by a commercial SMB unit that separates n-paraffins from hydrotreated kerosene fractions. The predicted separation performance is very similar to that achieved in our previous works, slightly improving the purity (99.6%) of the extract as a trade for a small loss in recovery (95.4%).