A critical study on the adsorption of heterocyclic sulfur and nitrogen compounds by activated carbon: Equilibrium, kinetics and thermodynamics

Adsorption of heterocyclic sulfur and nitrogen compounds by activated carbon was studied using model diesel fuels, light cycle oils and shale oil. It is observed that the carbon favours adsorption of cyclic nitrogen compounds. This work also investigates the equilibrium, kinetics and thermodynamics of adsorption of nitrogen and sulfur compounds from diesel fractions by activated carbon. Quinoline, indole, and carbazole are typical N compounds while dibenzothiophene and 4,6-dimethyldibenzothiophene are the representatives of refractory S compounds in diesel fractions, were selected as the model compounds. The total N adsorbed by the activated carbon is more than the S compound. Comparing the three nitrogen compounds, quinoline shows a greater removal rate than indole and carbazole. Adsorptive removal kinetics for N/S compounds was monitored by a GC-FID (gas chromatograph coupled with flame ionization detector) technique and was found to follow pseudo second-order kinetics. The external diffusion is not a controlling step in the adsorption process. The isotherm indicates that activated carbon presents a highly heterogeneous surface in the adsorption of DBT, quinoline and indole, while a homogeneous surface is observed in the adsorption of carbazole. Negative adsorption free energy suggests that the adsorption process is favourable and spontaneous for all S/N compounds.