Development of zeolite-based adsorbents for deep desulfurization of liquefied petroleum gas

Abstract

Air pollution has become one of the major problems facing humanity. A significant part of air pollution is originated from transportation sector. Organic sulfur compounds present in transportation fuels are converted to sulfur oxides (SOx) during combustion, which cause smog, global warming, and acid rain. Also, stringent sulfur regulations imposed on fuels have been mandated worlwide. Thus, the urgent need for deep desulfurization methods, such as adsorption, is required. Understanding the factors that affect the sulfur removal performance of adsorbents is important for both adsorbent and fuel industries. Most commercial desulfurization adsorbents are derived from zeolites due to their relatively high performance and low cost. Desulfurization adsorbents are expected to have the properties like high sulfur adsorption capacity, selectivity, thermal stability, regenerability and economical advantage. The effectiveness of zeolites to adsorb organic sulfur compounds in the fuels has been strongly influenced by the structural, textural and chemical properties of the adsorbent. On the other hand, the type of sulfur compound plays an important role in the overall capacity since different adsorption mechanisms are effective for each compound. Therefore, zeolites used in desulfurization should be developed to be suitable on a target-based seperation. The sulfur adsorption capacity and selectivity of zeolites are continuing to be explored in depth as new materials and methods are developed. According to the investigations on adsorptive desulfurization from fuels, sulfur removal performance of zeolites can be enhanced by ion-exchange method which increases the number of active sites for adsorption. A fundamental understanding of ion-exchange process and its effects on capacity is needed to design high performance desulfurization adsorbents. Considering the difficulties associated with industrial applications, studies should be evaluated by using real fuels to model adsorption system and optimize the parameters. In this thesis, the adsorptive removal of dimethyl disulfide (DMDS) and thiophene (TP) from real liquefied petroleum gas (LPG) over zeolites was studied. NaX and NaY type zeolites were modified using Cu2+, Zn2+ and Cu2+-Zn2+ ions by liquid phase ion-exchange (LPIE) method in order to improve their adsorption capacity and selectivity. The adsorbents were characterized to investigate their surface area, crystal structure, surface acidity, ion-exchange rate, and adsorption mechanism. The desulfurization performances of zeolites were tested and compared in both static and dynamic adsorption conditions. Then, the best performing adsorbent, Cu-Y, was investigated in detail to understand the effects of calcination temperature, concentration of Cu(NO3)2 solution, initial sulfur concentration, competitive adsorption between sulfur compounds and LPG flow rate. Cu-Y zeolite was characterized for its chemical composition, crystal structure, auto-thermal reduction of copper species, surface acidity, adsorption mechanism, textural properties and surface morphology to reveal performance-structure relationships. Furthermore, the equilibrium isotherms and kinetics for both DMDS and TP adsorption over zeolites in real LPG were also investigated for the first time in the literature. Results indicate that ion-exchange process enhances the DMDS and TP removal performance of the zeolites in the order of Cu > CuZn > Zn. Among the all prepared zeolites, Cu-Y ion-exchanged with 7 wt.% Cu2+ and calcined at 550 ℃ displays the highest sulfur removal capacity in both static and dynamic tests. It is clear that the calcination temperature has great influence on adsorption capacity since the auto-reduction of Cu2+ ions into Cu+ improves mainly the Lewis acid sites of the zeolites that enhance the sulfur adsorption. Mechanistic investigation shows that DMDS is attached to Cu-Y via direct sulfur-metal (S-M) interaction while both direct S-M interaction and π-complexation contributed in TP adsorption onto zeolite. According to the characterization results, textural, structural and chemical properties directly affect the adsorption performance. The adsorption of both DMDS and TP on the adsorbents appears to fit Langmuir isotherm and the pseudo-second-order kinetic models. Furthermore, Cu-Y has good thermal stability and can be reused efficiently up to 3 cycles that makes it highly beneficial and economical for deep desulfurization of LPG in industrial applications.