Dual-site Langmuir modeling of the liquid phase adsorption of linear and branched paraffins onto a PVDC carbon molecular sieve

The liquid phase adsorption equilibrium and kinetics of three paraffins onto a carbon molecular sieve at a temperature of 30 °C were obtained, namely n-heptane (nC7), 2-methylheptane (2MC7), and 2,5-dimethylhexane (25DMC6), as well as a mixture of these three components in a non-adsorbing 2,2,4-trimethylpentane (224TMC5) solvent. The adsorption capacities of the individual hydrocarbons ranged from 1.6 to 2.3 mmol/gads. The total adsorption capacity of the molecular sieve in the tri-component mixture was 2.0–2.1 mmol/gads, where the individual adsorption capacity of the nC7, the 2MC7 and the 25DMC6 was approximately 0.9, 0.7 and 0.4 mmol/gads, respectively, with small variations depending on the initial concentration of the hydrocarbons. The combined results were modeled as a dual-site Langmuir kinetic model where two consecutive adsorption processes were considered, namely the non-selective adsorption of molecules in the meso-supermicropores, followed by the movement of these adsorbed molecules into even smaller pores through the barriers of the micropore mouths. The results obtained in this study showed that the selective part of the adsorption process was crucial for achieving the observed kinetic separation, and the model could be used to satisfactorily describe the observed adsorption.