Adsorptive desulfurization of diesel using activated sewage sludge: kinetics, equilibrium and thermodynamics studies

Abstract

Combustion of fossil fuels gives rise to sulfur oxides, which are harmful to the environment. Adsorptive desulfurization (ADS) of diesel was conducted using sewage sludge activated with H₂O₂ as the oxidizing agent. A full 2² central composite response surface design was employed to determine optimum conditions for the production of activated sewage sludge (ASS). The adsorbent (ASS) was characterized using SEM, EDX and FTIR and the results of the analysis showed that it has the capacity to desulfurize diesel significantly. The ASS was subsequently used to conduct batch ADS of diesel with a view to investigate the kinetics, equilibrium and thermodynamics of the process. The optimum conditions established for the production of ASS using H₂O₂ as the oxidizing agent were: temperature 400?°C and holding time 60?min. The Elovich model gave the best fit to the kinetic data of the ADS of diesel using ASS, while the equilibrium study showed that the Freundlich isotherm fitted the data at 35?°C better than Temkin and Langmuir isotherms. The positive values of the free energy and enthalpy changes revealed that the process was non-spontaneous and endothermic, respectively, while the negative entropy change is evidence of decrease in randomness of the adsorbed species. 33% desulfurization was achieved in 100?min during ADS of diesel showing that the adsorbent developed by activating SS with H₂O₂ was very good and effective. Thus, ASS can be used to gain more insight into kinetics, equilibrium and thermodynamics of the ADS of middle-distillate petroleum fractions.