Graduate Thesis Or Dissertation
 

Desulfurization of thiophene and dibenzothiophene with hydrogen peroxide in a photochemical microreactor

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  • Sulfur content of fuels and desulfurization of fuels is becoming an important environmental restriction and regulatory issue. In this study new technology for the desulfurization of fuels is considered. Micro scale reactors are introduced for desulfurization of thiophene and dibenzothiophene. Thiophene and dibenzothiophene are mixed with hexane at concentration, C₀ of 300 ppm. The mixture is introduced into the microreactor concurrently with hydrogen peroxide in equal volumetric flow rates. The two reactant steams form two thin layers inside the microreactor. The microreactor is 2.2 cm long, and 1.05 cm wide. Thin spacers, 100 μm and 50 μm, are used to establish the thickness of the microchannels. The microreactor is housed between stainless steel plates which have rectangular windows to facilitate UV irradiation of the reaction mixture. UV light helps the desulfurization reaction to proceed. The overall desulfurization process for both thiophene and dibenzothiophene is a pseudo first-order reaction. The thiophene and dobenzothiophene reacted with hydroxyl radicals to form their respective sulfoxides and sulfones. The experiments are conducted at room temperature, and at steady state conditions. The ranges of the operating conditions are: the residence time, 0.15 to 9.59 (mm.), the spacer thickness (half-thickness of the reactor), 50 and 100 (μm), and the distance between the microreactor and the UV light source, 1.5 to 6.0 (cm). The thiophenes desulfurization study includes, comparison between model and experimental results, comparison between thiophene and dibenzothiophene reactivity, the effect of the spacer thickness, the effect of the UV light source distance, and comparison between results obtained by introducing the microreactor and to those obtained from batch process done by other researchers. A mathematical model reflecting geometry and flow conditions inside the microreactor is developed to predict the conversion of thiophene and dibezothiophene. The model includes axial convection and diffusion, lateral diffusion, and pseudo first order reaction kinetics at the interface between the two reactant streams. The model is solved numerically using FEMLAB (Finite Element Method Laboratory) software package. The experimental data are fitted to the model equations and the pseudo first order reaction rate constant (k) is evaluated. It is found that the reaction rate constant for thiophene is 1.31*10⁻⁷ [m/s] for spacer thickness of 100 μm, and 1.33*10⁻⁷ rn/s for spacer thickness of 50 μm. The reaction rate for the desulfurization of dibenzothiophene is 1.47*10⁻⁷ (m/s). It is found that the mathematical model explained the experimental data very well. Therefore, the model developed in this study may be used to design desulfurization processes and to predict the concentration profile in the micro reactor. In general, the microreactor was much more efficient in desulfurization of thiophene and dibenzothiophene when compared to other batch processes. The microreactor was capable of achieving 98.20% conversion at 8.11 minutes mean residence time for thiophene desulfurizationthus, thus reducing the thiophene concentration from 300 ppm to less than 6 ppm.
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