Kinetics of sulfur removal in high shear mixing-assisted oxidative-adsorptive desulfurization of diesel

Abstract

Abstract Current global environmental regulations require deep desulfurization of prevailing high-sulfur crude oil reserves to limit toxic sulfur oxide emissions during fuel combustion. In this study, deep desulfurization of diesel was carried out in two sequential batch steps: (1) oxidation of refractory sulfur compounds in diesel into sulfones and (2) adsorption of the polar sulfur-species in the resulting organic phase onto commercially available adsorbents. Oxidative desulfurization of diesel was performed in a glass vessel equipped with a high shear mixer set at 12,000 rpm. Phosphotungstic acid, tetraoctylammonium bromide and hydrogen peroxide were used as catalyst, phase transfer agent and oxidant, respectively. Adsorptive desulfurization, on the other hand, was carried out using powdered alumina, granular alumina, powdered activated carbon (PAC), and granular activated carbon (GAC) adsorbents. Characterization results showed that powdered alumina and PAC appear to be aggregates of small crystalline structures, with PAC having larger surface area of 846 m2 g−1 compared to powdered alumina at 129 m2 g−1. Micropores were detected in PAC, while the porosity of powdered alumina was attributed to the presence of mesopores. Sulfur removal by the four types of adsorbents conformed to the pseudo-second order model, implying that chemisorption was the rate-limiting step. The computed adsorption capacities from the kinetic model at 3.47, 1.09, 3 and 1.09 mg g−1 were in agreement with the experimental adsorption capacities at equilibrium of 3.36, 0.98, 3 and 1 mg g−1 for powdered alumina, granular alumina, PAC and GAC, respectively. The 2-line Weber-Morris plots of the four adsorbents indicated the effects of boundary layer diffusion and intraparticle diffusion in sulfur removal. The values of kid1 and kid2, as well as I1 and I2, implied that boundary layer diffusion proceeded at a faster rate than the rate-determining step which was intraparticle diffusion. Higher intraparticle diffusion coefficient values were observed for powdered alumina due to its larger particle size and, consequently, smaller surface area where the sulfur compounds tend to be more readily adsorbed.