The public concern on environmental issues and fuel specifications are causing an increase in the number and stringency of legislative actions. This is because the sulfur is converted to SOx, a major air pollutant, during the combustion process of fuel in auto-mobiles. In oil refineries, sulfur poisons the catalyst in pipeline, pumping and refining equipment, and in auto-mobile engines it leads to the premature failure of combustion engines and the poisoning of the catalytic converters. Due to these environmental impacts, sulfur specifications were proposed with the goal of reducing the sulfur content in transportation fuel particularly diesel, petrol and light fuels. For instance, the sulfur specification in many countries is 150ppm while some countries have much tighter specifications. The new regulations in many countries have brought down the sulfur level to 50ppm while the Euro 5 specification has adopted a sulfur content of 10ppm. In South Africa, the government is planning to implement a fuel standard of 10ppm by 2017.
Different methods have been proposed in the last decades for the removal of sulfur in petroleum streams i.e. the hydrodesulfurization (HDS) method. Much effort has been dedicated to exploring other desulfurization processes such as oxidation, extraction, precipitation, biodesulfurization and adsorption (Campos-Martin et al., 2010; Zhang et al., 2004; Monticello, 2000; Ma et al., 2005). However, adsorption has received enormous interest as a promising technique. This is due to low operating temperatures and pressures, its capability of removing refractory sulfur compounds and the adjustable pore structure as well as the surface functional groups of the adsorbents. But there are still opportunities to develop highly selective adsorbents which can be regenerated.
The reaction of sulfur compounds with imidation agents (chloramines T, PI) had been reported (Mann & Pope, 1922). However, few studies have been done on the use of imidation agents in the desulfurization process (Shiraishi et al., 2001; Shiraishi et al., 2002; Shiraishi et al., 2003; Fadhel, 2010) and it was found that at high sulfur concentration (12354 ppm), ultra-deep desulfurization could not be achieved by using both chloramine T (Sodium N-Chloro-p-toluene sulfonamide) and sodium N-chloro-polystyrene sulfonamide (PI) (Fadhel, 2010). The main objective of the current study is to synthesize PI and to improve its adsorption capacity. This was done by synthesizing the PI with a relatively high surface area and using carbon nanotubes (CNTs) as a catalyst. Its applicability in desulfurization was explored by varying the temperature, stirring speed, contact time and the amount of an adsorbent.
A Polymer-supported imidation agent (PI) was synthesized according to the procedure given by Fadhel (2010). The surface area and the porosity of the synthesized PI were characterized using a Micromeritics Tristar-Surface area and Porosity analyzer. The synthesized PI was found to have a BET surface area of 0.5333 m2/g, total pore volume of 0.003690 cm3/g and an average pore size of 25.80 nm. The functional groups present in the synthesized PI and its intermediates were identified using a Brüker Tensor 27 Fourier Transform Infrared spectrometry. The IR study confirmed the presence of the sulfur-containing group at a wavelength range of 1400 to 850 cm-1; N-H group at 1600 cm-1; N-Cl group at 822 cm-1. The external surface morphology and topography of the three products was obtained using the JSM 840 SEM. The obtained SEM images showed that the synthesized PI with its intermediate products has irregular shape and a rough surface structure. It was also observed that the structure became less porous after the sulfonamidation reaction. The presence of the sulfonic groups, amine group and other elements was confirmed using an Oxford X-act energy dispersion spectroscopy (EDS) detector. Batch desulfurization tests were conducted using the synthesized PI and carbon nanotubes as a catalyst. The tests were done on the commercial diesel fuel which was obtained from the inlet stream to the HDS reactor with a sulfur content of 5200 mg/kg. This was supplied from Natref refinery in South Africa. The experiment was conducted following a procedure given by Shiraishi et al. (2003) and the results show a promising trend.
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