Thermodynamics and Kinetics of Hydrodesulfurization HDS

The kinetics of HDS have been studied intensively. Here, we only examine some basic principles. For details see Kabe, Ishihara, and Qian (1999), Satterfield, Modell, and Mayer (1975), McCarthy and Schrader (1987), Delmon, Froment, and Grange (1999), Givens and Venuto (1970), Gates, Katzer, and Schuit (1979), Daly (1978), 

For CoMo-catalysts direct HDS via biphenyl is preferred as the reaction of DBT to form biphenyl is by about three orders of magnitudes faster than hydrogenation to 1,2,3,4-tetrahydrodibenzothiophene and 1,2,3,4,10,11 -hexahydrodibenzothiophene, respectively. For NiMo-catalysts the ratio of direct to indirect route of HDS is about two (Girgis and Gates, 1991). 

A simplified intrinsic rate expression for HDS (in the liquid phase such as for diesel oil and residues, see Section 6.8.3) is for the example of DBT  

Although HDS of S-compounds is a first-order reaction, the decrease in total sulfur content is better approximated by a second-order rate with respect to sulfur [Eq. (4.10.26)]  

The reason for this, at first sight unexpected, second-order behavior is the stepwise conversion of S-compounds, which differ strongly in reactivity. As a consequence, the rate of HDS (with respect to total S) decreases 

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