Removal of nitrogen and sulfur compounds from gas oil by adsorption

Catalytic hydrotreatment is known worldwide as the process where concentrations of species like nitrogen, sulfur, oxygen and various metals are reduced to acceptable levels in the refining process. This process involves high temperatures and pressures, which results in major economic expense to refineries. These process conditions are dependent on the nature of the petroleum feedstock to be processed. Bitumen-derived gas oils from Athabasca oil sands can have high concentrations of organo-sulfur (~ 4 wt.%) and nitrogen (~ 0.4 wt.%) species; thus, posing a major challenge to downstream processing in refineries. Due to the challenge represented by these heteroatoms (S, N) during conventional catalytic hydrotreatment, other methods that involve non-catalytic processes using π-acceptor-immobilized adsorbents have garnered immense interest of researchers in this field due to its low temperature and pressure requirements. Though, mixed metal oxides such as mesoporous alumina with desirable textural properties (surface area, pore diameter and pore volume) have the tendency to be used as adsorbent support for the immobilization of π-acceptor moieties via a linker, their potential has been less explored. In this study, an attempt was made to investigate the mesoporous alumina support-linker-π acceptor, charge-transfer complex (CTC) structure for the adsorption of these heteroatoms present in the oil. This novel alumina-based adsorbent, combined with the non-catalytic adsorption method, was effective to selectively remove the refractory sulfur and nitrogen compounds from model feedstocks under milder conditions (room temperature and atmospheric pressure) as compared to catalytic hydrotreatment. The research work was divided in 3 phases. Phase 1 was focused on the synthesis and characterization of the alumina-based adsorbents and their characterization. Three adsorbents were synthesized using a charge- transfer complex (CTC) moiety consisting of a support, linker and π-acceptor. Alumina was selected to be the supporting materials for its textural properties; three different alumina- based supports were used: mesoporous alumina, titania-substituted alumina and commercial alumina for comparison purposes. These three supports went under reaction to get a linker, ethylenediamine (EDA) attached to them and then a πacceptor, 2,7-Dinitro-9-fluorenone (DNF). The adsorbents and supports were characterized using BET, FTIR, TGA techniques to ascertain their physicochemical properties. The extent of π-acceptor functionalization on supports was characterized using XRD, TGA and XPS techniques. In phase 2, the adsorbents were examined for the desulfurization of a model oil feed with 500 ppm of sulfur. The commercial γ-Al2O3-EDA-DNF adsorbent successfully removed 90.4 wt% of sulfur. The substitution of Ti in the framework of mesoporous Al2O3 did not promote its desulfurization efficiency. The higher desulfurization activity of the commercial γ-Al2O3 based adsorbent than that of others is attributed to its textural properties. Among the three adsorbents screened a commercial γ-Al2O3 CTC adsorbent (Adsorbent C) showed the highest desulfurization in a short time period. Therefore, in Phase 3, the sulfur adsorption isotherms and kinetic were examined. The kinetics of sulfur adsorption followed a pseudo-second-order model with the CTC adsorbents. The regeneration of used adsorbent was studied with three different polar solvents such as chloroform, dichloromethane and carbon tetrachloride. Dichloromethane was found to be the most suitable solvent for extracting a major part of sulfur compounds contained in the pores of the spent adsorbent. Thermodynamic parameters such as Ea, ΔG, ΔH and ΔS provided a better insight into the adsorption