Toluene Destruction in the Claus Process by Sulfur Dioxide: A Reaction Kinetics Study

The presence of aromatics such as benzene, toluene, and xylene (BTX) as contaminants in the H 2 S gas stream entering Claus sulfur recovery units has a detrimental effect on catalytic reactors, where BTX forms soot particles and clogs and deactivates the catalysts. BTX oxidation, before they enter catalyst beds, can solve this problem. A theoretical investigation is presented on toluene oxidation by SO 2 . Density functional theory is used to study toluene radical (benzyl, o -methylphenyl, m -methylphenyl, and p -methylphenyl)–SO 2 interactions. The mechanism begins with SO 2 addition on the radical through one of the O atoms rather than the S atom. This exothermic reaction involves energy barriers of 4.8–6.1 kJ/mol for different toluene radicals. Thereafter, O–S bond scission takes place to release SO. The reaction rate constants are evaluated to facilitate process simulations. Among four toluene radicals, the resonantly stabilized benzyl radical exhibited lowest SO 2 addition rate. A remarkable similarity between toluene oxidation by O 2 and by SO 2 is observed.