Adsorption Behavior of a Mixture of C13 Isomers onto a Fixed-Bed of H-ZSM-5

Multicomponent liquid-phase adsorption of C-13 paraffins (mainly monomethyldodecanes and tridecane) onto a fixed bed of H-ZSM-5 (Si/Al = 200) has been studied (428 K and 21 barg), obtaining complete breakthrough curves for all the solutes, in order to ensure that equilibrium between the liquid feed and the adsorbent was achieved. The concentration of adsorbates in the feed mixture has been varied by diluting a concentrated mixture of C-13 paraffins with a nonadsorbing solvent (2,2,4-trimethylpentane) in different proportions. The relationship between the branching position of the isomer and its adsorption affinity has been studied. The following affinity order is obtained (estimated as adsorbed concentration/liquid concentration): 2-MC12 approximate to 3-MC12 < 4-MC12 < n-C-13 < 5-MC12 < 6-MC12. This order can be attributed to the difference in length of the lateral chains of each isomer, considering that a CH3-CH-CH2 group is placed the pore intersections of H-ZSM-5 for the branched isomers, and a CH2-H-2 group for n-C-13. For the more symmetric isomers (n-C-13, 5-MC12, 6-MC12), the affinity decreases continuously as the concentration of the feed mixture is increased, whereas it passes through a maximum for the asymmetric ones (2-MC12, 3-MC12, 4-MC12), because the asymmetric isomers adsorb weakly if the concentration of the symmetric ones is not very high, but they are displaced from the adsorbed phase as the concentration of the feed mixture is increased. The practical implication of this result is that, if the recovery of both symmetric and asymmetric C-13 isomers from kerosene with H-ZSM-5 is desired for the production of tensoactives, it is convenient to use a feed concentration where the displacement of asymmetric isomers is minimal.