چكيده به لاتين
The purpose of the present dissertation was investigation of electrochemical desulfurization of model fuels. Firstly, electrochemical elimination of inorganic sulfide species from an aqueous solution was studied. For this purpose, a platinum electrode and cyclic voltammetry method was utilized. In these experiments, different operating parameters including concentration of sulfidic species, concentration of supporting electrolyte, temperature, extent of mixing, and potential scan rate were investigated. The obtained results showed that, increasing temperature leads to enhancement of anodic oxidation rate. Extent of mixing affect sulfide oxidation periodically. As potential scan rate is directly related to residence time, increasing potential scan rate leads to progress of sulfide oxidation. Based on the gathered results, appropriate values of supporting electrolyte concentration, temperature, and extent of mixing are 0.05 M, 40-60 °C and 200 rpm, respectively. Results revealed that anodic oxidation method has excellent potential for electrochemical elimination of inorganic species and especially sulfidic ones from domestic and industrial wastewaters.
Afterward, electrochemical desulfurization of organic compounds (i.e., thiophene and benzothiophene) using platinum electrode was investigated. Oxidation of thiophene, reduction of thiophene and reduction of benzothiophene were studied. For each of these processes, the following steps were performed:
Investigating the effect of electrochemical adsorption of sulfur-containing compound from an aqueous solution using chronoamperometry method and finding the appropriate adsorption and oxidation/reduction potentials.
Investigating the adsorption and oxidation/reduction processes of the sulfur-containing compound from an aqueous solution using square wave potentiometry method and finding the appropriate frequency of square wave potential.
Electrochemical desulfurization of model fuels in the appropriate conditions that found before.
Analysis of feed and products (using gas chromatography, ion chromatography, total sulfur analyzer, and Fourier transform infrared spectroscopy) to determine desulfurization efficiency and mechanism of reactions.
In the oxidative desulfurization of thiophene, appropriate values of adsorption potential, oxidation potential, and frequency of square wave potential were 0.2 V, 1.1 V, and 50 Hz, respectively. In these experiments, aqueous solutions of sulfuric acid and perchloric acid were used as the supporting electrolyte. Results revealed that sulfur content of the sample was reduced from 466 ppm in feed to 56 ppm in product, which is equal to a desulfurization efficiency of 88%.
In the reductive desulfurization of thiophene, appropriate values of adsorption potential, reduction potential, and frequency of square wave potential were -0.54 V, -0.95 V, and 1 Hz, respectively. In these experiments, aqueous solutions of potassium hydroxide was used as the supporting electrolyte. Results revealed that sulfur content of the sample was reduced from 294 ppm in feed to 55 ppm in product, which is equal to a desulfurization efficiency of 81%.
As the results shows, desulfurization efficiency of thiophene oxidation is slightly higher than thiophene reduction. Nevertheless, in the electrooxidative desulfurization, oxidized sulfur-containing compound must be separated from the fuel via an extraction or adsorption process, however, in the electroreductive desulfurization, sulfur is converted to hydrogen sulfide gas, which can be easily separated by a gas-liquid separation process. As the two obtained desulfurization efficiency are almost equal, electroreductive process is more economically feasible.
In the reductive desulfurization of benzothiophene, appropriate values of adsorption potential, reduction potential, and frequency of square wave potential were 0.30 V, -0.15 V, and 10 Hz, respectively. In these experiments, aqueous solutions of sulfuric acid was used as the supporting electrolyte. Results revealed that sulfur content of the sample was reduced from 344 ppm in feed to 199 ppm in product, which is equal to a desulfurization efficiency of 42%.
As the results shows, desulfurization efficiency of benzothiophene is lower than thiophene. This is due to larger size of benzothiophene molecules respect to thiophene molecules, which results in higher spatial restriction of sulfur atom in benzothiophene respect to thiophene toward adsorption and reaction on the electrode surface.
