Formation of Thiophene Derivatives and Destruction Kinetic Regularities during the Cracking Process of High-Sulfur Vacuum Gasoil Oxidation Products

The results of a study of the composition of cracking products of high-sulfur vacuum gasoil from the Novokuibyshevsk Refinery under various conditions are presented. The quantitative content of sulfur in the liquid cracking products of vacuum gasoil, as well as in the liquid cracking products of oxidized gasoil and its non-polar and polar parts after separation by liquid-adsorption chromatography, is established. It is shown that in the process of thermal cracking of high-sulfur vacuum gasoil, sulfur is practically not removed from the composition of the liquid products. Derivatives of thiophene and benzo- and dibenzothiophene are intensively formed and accumulated, which worsens the quality of the obtained distillate fractions. Pre-oxidation reduces the thermal stability of the high-molecular-weight sulfur-containing components that are stable during conventional heat treatment. Pre-oxidation significantly slows down the accumulation of low-molecular-weight aromatic sulfur compounds in the liquid cracking products. As a result, sulfur-containing components both undergo condensation reactions into coke and destruct with the formation of gaseous sulfur compounds (hydrogen sulfide and lower mercaptans), which contributes to an increase in the degree of sulfur removal. Chromatographic analysis of the liquid cracking products of oxidized vacuum gasoil shows that under such conditions, no degradation processes of newly formed thiophene derivatives are observed. Separate heat treatment of non-polar and polar products of vacuum gasoil oxidation allows minimizing the occurrence of gas formation reactions by eliminating side interactions between the components. It is shown that the formation and accumulation of aromatic sulfur compounds (thiophene derivatives) in the obtained distillates occurs during the cracking of non-polar products, while the sulfur content in the composition of the liquid products practically does not decrease. Cracking of polar products leads to intense gas and coke formation, the sulfur content in the composition of liquid products decreases threefold, and thiophene and benzothiophene homologues predominate in the products.