Adsorption process of n-alkanes onto BAX-1100 activated carbon: Theoretical estimation of isosteric heat of adsorption and energy distribution of heterogeneous surfaces

Adsorption of the n-alkane series, C1-C7, on Westvaco BAX-1100 activated carbon were measured over very broad ranges of temperature (293-393 K) and pressure (0.06-4964 Torr), depending on the n-alkane. Different behaviors of experimental adsorption isotherms were observed where various steps were noticed depending on the adsorbate length. A heterogeneous Gauss-monolayer model was developed based on statistical physics approach to evaluate the adsorption energy distribution (AED). Surface heterogeneity was evaluated and confirmed from isosteric heat of adsorption; where the adsorption of longer n-alkanes is energetically more favorable. A new equation was obtained from AED analysis to calculate the isosteric heat of adsorption. A notable difference between the adsorption isosteric heats estimated from AEDs and those obtained from classical Clausius-Clapeyron equation was observed at very low and high capacities. This difference was explained by the fact that, the gas behaves non-ideally at high pressure range. On the other hand, the absolute percentage deviation in adsorbed phase heat capacity was evaluated from the isosteric heat of adsorption predicted from the AED analysis. This percentage deviation increases when increasing the chain length of the alkane.