Desulfurization and Denitrogenation of Bitumen-Derived Gas Oils using Functionalized Polymers

Recent developments in the petroleum refinery processes are focused on the production of ultra-low sulfur fuels due to the stringent environmental regulations. Heavy crude oil, also known as the synthetic crude derived from the oil sands is emerging as one of the sources to meet the present and future energy demands. However, heavy oil contains high concentration of heterocyclic sulfur and nitrogen compounds. A major challenge in achieving deep hydrodesulfurization with the conventional hydrotreating technology is the inhibition and deactivation of the catalyst caused by heterocyclic nitrogen compounds. Amongst the sulfur compounds present in the petroleum feed, benzothiophene and its derivatives are unsusceptible to conventional hydrotreating catalyst leading to impediment in achieving deep hydrodesulfurization. There is a critical need for the improvement in hydrotreating technologies to reduce sulfur and nitrogen levels in the fuels derived from unconventional resources. Therefore, a pre-treatment process using functionalized polymers has been developed for the removal of nitrogen and sulfur compounds from gas oil via charge transfer complex mechanism. Functionalized polymer comprises of polymer support, linker and π-acceptor; these parts were modified to enhance nitrogen and sulfur removal. The research work was divided into five phases. The first phase was focused on studying the effects of different electron withdrawing π-acceptors on the nitrogen and sulfur removal. Three π-acceptors with 2, 3 and 4 electron withdrawing nitro groups attached on the fluorenone moiety were synthesized; viz., 2,7-dinitro-9-fluorenone (DNF); 2,4,7-trinitro-9-fluorenone (TriNF) and 2,4,5,7-tetranitro-9-fluorenone (TENF), respectively. Porous polymer support [copolymer of glycidyl methacrylate and ethylene glycol dimethacrylate, poly (GMA-co-EGDMA)] was functionalized with the π-acceptors via hydroxylamine linker. The optimized polymers successfully removed 14.4 wt.% of total nitrogen and 1.4 wt.% of total sulfur from light gas oil at room temperature. It was found that the removal of these impurities is due to the formation of charge transfer complexes between π-acceptor functionalized polymers and heterocyclic nitrogen and sulfur species present in gas oil. In the second phase, the effects of changing the linker length on nitrogen and sulfur removal was studied. The linkers were varied from a two-carbon (diaminoethane, DAE (2)), a three-carbon (diaminopropane, DAP(3)) to a four-carbon (diaminobutane, DAB(4)) containing compounds while the functionalized polymer consisted of identical polymer support, poly (GMA-co-EGDMA) and π-acceptor (TENF). 19 wt.% removal of nitrogen compounds from light gas oil was observed using diaminopropane (DAP(3)) substituted polymers in a batch reactor at ambient temperature with polymer to oil loading ratio of 1:4 wt./wt. In the third phase, polymers with high internal phase emulsion (polyHIPEs) were synthesized to study the effects of polymer support on the removal of nitrogen and sulfur compounds from bitumen-derived gas oil. Four different types of polymer supports were prepared by varying the amount of monomers (unsaturated polyester resin, glycidyl methacrylate and divinylbenzene) and the type of porogen (toluene or tetrahydrofuran). All four polymer supports were functionalized with identical π-acceptor (TENF) and tested in a batch reactor at ambient temperature. 14.6 wt.% of total nitrogen was removed while no significant sulfur removal was observed. Reusability studies were performed by regenerating the used polymers with toluene. Regenerated polymer was capable of removing 8.2 wt.% of total nitrogen in second contact with light gas oil. A fundamental insight into the role of π-acceptors forming charge transfer complexes with heterocyclic nitrogen and sulfur compounds was obtained in the fourth phase of this work. Adsorption isotherms were studied using model nitrogen and sulfur compounds (quinoline, 9-ethylcarbazole and dibenzothiophene) in a batch reactor at three sets of temperature (298 K, 313 K and 328 K). Calculation of the thermodynamic parameters disclosed the exothermic and spontaneous nature of the adsorption of quinoline, 9-ethylcarbazole and dibenzothiophene over functionalized polymer. The polymeric adsorbent was found selective towards the removal of nitrogen compounds and the adsorption capacity followed the order: quinoline > dibenzothiophene > 9-ethylcarbazole. In the last phase, hydrotreatment of polymer treated heavy gas oil was studied in a trickle bed reactor at industrial conditions. The combination of adsorption and catalysis has resulted in 94.3% hydrodesulfurization (HDS) and 63.3% hydrodenitrogenation (HDN) as compared to 93.1% HDS and 60.5% HDN activities during hydrotreatment of heavy gas oil using conventional NiMo/γ-Al2O3 catalyst