Effect of the Fuel Equivalence Ratio on the Mechanisms of Thiophene Oxidation in Water Vapor at Increased Density of the Reagents

The article presents the results of a study of thiophene oxidation in high-density C 4 H 4 S/O 2 mixtures (ρ Thi = 0.12 and 0.15 mol/dm 3 , ρ O2 = 0.74-1.26 mol/dm 3 ), diluted with water vapor and argon (dilution level x D = 35-65% mol), at uniform heating (1 K/min) of a stainless-steel tubular reactor up to 823 K. It is established that the temperature of thiophene oxidation onset weakly depends on the nature of the diluent and the oxygen content in the reaction mixture. From the time dependences of the reaction mixtures on temperature and pressure, it follows that the oxidation of thiophene in the water vapor and argon media proceeds according to the mechanisms of homogeneous and heterogeneous reactions. Upon oxidation of thiophene in the stoichiometric mixtures in argon with a small amount of water vapor, as well as in the lean mixtures in water vapor, the contribution of reactions on the surface of the Pt-Rh/Pt thermocouple, inserted into the center of the reaction volume, is increased. Upon oxidation of thiophene in water vapor in the fuel-enriched and stoichiometric mixtures, reactions on the oxidized surface of the reactor wall (primarily iron oxides) prevail. Increasing the density of water vapor both reduces the contribution of heterogeneous reactions on the reactor wall and prevents complete carbon burnout. It is shown that the neutralization of sulfuric acid, resulting from the oxidation of thiophene, with calcium carbonate reduces the corrosion of stainless steel. The X-ray diffraction analysis revealed the presence of ferrochromite, iron and chromium oxides, iron, nickel, and chromium sulfates in the corrosion products.