Single event kinetic modeling of the hydrocracking of paraffins

A mechanistic kinetic model for the hydrocracking of paraffins based on the single-event kinetics approach has been studied. Several elements of the model have been improved and the parameters of the model have been estimated from experimental data on n-hexadecane hydrocracking.

A detailed reaction network of elementary steps has been generated based on the carbenium ion chemistry using the Boolean relation matrices. A total of 49,636 elementary steps are involved in the hydrocracking of n-hexadecane. The rate coefficients of these elementary steps are expressed in terms of a limited number of single event rate coefficients. By virtue of the single event concept, the single event rate coefficients of a given type of elementary steps are independent of the structure of reactant and product. Given their fundamental nature they are also independent of the feedstock composition and the reactor configuration. There is no lumping of components involved in the generation of the reaction network. Partial lumping is introduced only at a later stage of the model development and the lumping is strictly based on the criterion that the individual components in any lump will be in thermodynamic equilibrium. This definition of lumping requires a total of 49 pure components/lumps in the kinetic model for the hydrocracking of n-hexadecane. The "global" rate of reaction of a lump to another lump is expressed using lumping coefficients which account for the transformation of all the components of one lump into the components of another lump through to a given type of elementary steps. The rate expressions thus formulated are inserted into a one-dimensional, three-phase plug flow reactor model. Experimental data have been collected for the hydrocracking of n-hexadecane. The model parameters are estimated by constrained optimization using sequential quadratic programming by minimizing the sum of squares of residuals between experimental and model predicted product profiles. The optimized parameters are finally used for the reactor simulation to study the effect of different process variables on the conversion and product distribution of n-hexadecane hydrocracking. The model is also used to predict the product distribution for the hydrocracking of a heavy paraffinic mixture consisting of C9 to C33 normal paraffins.