Advances in Carbon-Based Nanocomposites for Deep Adsorptive Desulfurization

Introduction

Removal of sulfur compounds (desulfurization) is a vital unit operation in petroleum refining since the combustion products of sulfur compounds is the main reason for acid rain and environmental pollution. In addition, sulfur is also a catalyst poison during industrial processes. Compounds with sulfur are removed catalytically at high temperature and pressure. Desulfurization is gaining a lot of attention and efforts have been channeled towards investigating several methods that are effective and economically viable. The attention is warranted by stricter environmental regulations on the amount of sulfur that should be present in transportation fuels (Song, 2003; Yang et al., 2004). Sulfur limit (mass-percent) of 0.015, 0.035, and 0.2 for gasoline, diesel fuel, and light fuel oil, respectively has been set. A new and more stringent limit of 0.003-0.005 mass-percent (30-50 ppm) is imminent for transportation fuels in Europe and United States of America (Song, 2003; State Announcer, 2001; Babich and Monlijn, 2003). It’s worthy to note also that; desulfurization processes have found applications in converting used tyres and shale oil to fuel oils. Using calcium oxide in binding up sulfur oxide in emissions has been achieved for stationary applications in the desulfurization of non transportation fuels but the use of the harmful compounds still a challenge (Svobodal et al., 1994). Therefore, the need to innovate effective technologies for desulfurization processes is very paramount.

Numerous methods of desulfurization processes have been investigated. The need to achieve a lower level of sulfur in fuel oils has also called for different innovative ways of achieving deep desulfurization where synergy of methods yielding better results (Agarwal and Sharma, 2010; Sundaraman et al., 2009). Hydro-desulfurization (HDS) is a popular process, but there is wide variation in the reactivity of sulfur-containing heterocyclic compounds. Alkyl-substituted derivatives of dibenzothiophene like 4-methyldibenzothiophene, 4, 6-dimethyldibenzothiophene e.t.c from fuel oil have been reported to be relatively unreactive towards hydrotreating (Gate and Tropsoe, 1997). In order to achieve deep-desulfurization and take care of attending challenges of hydro-desulfurization which include high hydrogen consumption, energy (heat) cost, catalyst volume and many other methods are combined with HDS for better results (Rana et al., 2007). In recent years, efforts are being directed to other methods, including; adsorptive desulfurization, oxidative desulfurization where different catalysts are used (Kumar et al., 2012) extractive desulfurization involving ionic liquids, photochemical activation, bio-adsorption, bio-desulfurization, ultrasonic-desulfurization, microwave desulfurization and electrochemical approach (Bhatia and Sharma, 2006; Lam et al., 2012).

Adsorptive desulfurization where adsorbents are used to remove sulfur-containing compounds in fuel oils is holding the future promise of ultra-clean sulfur-free fuel oils. Adsorption processes can be performed at ambient temperature and pressure, thus saving a lot of energy as compared to other methods of desulfurization (Seredych et al., 2009). Therefore, intensive research is on-going to produce new adsorbents with great emphasis on good selectivity, high capacity for adsorption and regeneration of the adsorbents with special attention to the mechanism of adsorption (Yang et al., 2007).