Hydrocracking of paraffins is carried out on bifunctional catalysts with a metal function, generally provided by Pt to catalyze the dehydrogenation/hydrogenation and an acid function catalyzing the isomerization and cracking steps. The acid function is often provided by Y-zeolites because of their wider pore structure that minimizes the diffusion resistance for the bulky molecules of heavy feedstocks.

It has been shown in several studies[1-3] on hydroisomerization and hydrocracking of model compounds ranging from C8 to C12 paraffins that on a well-balanced catalyst (i.e. having sufficient metal activity), hydrogenation/dehydrogenation steps are intrinsically much faster than the elementary steps taking place on the acid sites. As a result, the hydrogenation / dehydrogenation reactions attain quasi-equilibrium and the elementary steps on the acid sites are the rate determining steps. Under these conditions, the concentration of a particular olefin intermediate depends only on the concentration of the corresponding saturated paraffin and mono-branched feed isomers are the only primary products, whereas multi-branched and cracked molecules are the secondary products. The concentration of feed isomers is high and secondary cracking is relatively limited. On the other hand, if the catalyst does not have sufficient metal activity to equilibrate the hydrogenation/dehydrogenation steps, the situation becomes considerably more complex. Under these so called ‘non-ideal' hydrocracking conditions, the concentration of a particular olefin intermediate depends on the concentration of all the paraffins present in the reaction mixture[4]. When the rate determining steps shift from purely acidic sites to both the acid and metal sites of the catalyst, multi-branched feed isomers and cracked products become primary products of hydrocracking. It also results in increased cracking selectivities and higher amount of secondary cracking. For a catalyst with given metal to acid activity, high temperatures, low pressures and high hydrogen to hydrocarbon ratios shift the rate determining step from acidic sites to the metal sites of the catalyst in the gas phase hydrocracking[5].

In the work reported here, a generalized mechanistic kinetic model has been developed for the three-phase hydrocracking of heavy paraffins up to C32 in which the rate determining step is assumed to occur on both acid and metal sites of the catalyst. In this model, the frequency factors for the acid site elementary steps are modeled using the single-event concept[6] and the corresponding activation energies using the Evans-Polanyi relationship. The rate coefficients for the dehydrogenation reactions on the metal sites of the catalyst are classified into five different classes depending on the nature (i.e., primary, secondary or tertiary) of the carbon atoms forming the double bond. As a result of this approach, a total of 14 independent parameters are required for the 49636 elementary steps taking place on the acid sites and the 7601 hydrogenation/dehydrogenation reactions taking place on the metal sites of the catalyst in the hydrocracking of C16 paraffin feedstock. These parameters are estimated from the three-phase hydrocracking of n-hexadecane. The model yields a detailed product distribution that is in excellent agreement with the experimental data. A number of reactor simulations have been performed for various operating conditions and hydrocarbons of different chain lengths. The effect of the relative metal to acid activity of the catalyst on the isomerization and cracking selectivities and on the carbon number distribution of the products has been analyzed.

The generalized nature of the model also makes it applicable to the specific case in which the hydrogenation/dehydrogenation steps are at equilibrium, which has been conventionally referred to as ‘ideal' hydrocracking. It follows from the very essence of the model that all the parameters are independent of the chain length of the hydrocarbons. This makes the model applicable to the hydrocracking of mixtures of paraffins and paraffinic waxes.

(1)Baltanas, M. A., Van Raemdonck, K. K., Froment, G. F., Mohedas, S. R. Ind. Eng. Chem. Res. 1989, 28, 899. (2)Martens, G. G.; Marin, G. B.; Martens, J. A.; Jacob, P. A.; V., B. G. J. of Catalysis 2000, 195, 253. (3)Steijns, M.; Froment, G. F. Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 660. (4)Degnan, T. F.; Kennedy, C. R. AIChE J. 1993, 39, 607. (5)Debrabandere, B.; Froment, G. F. Hydrotreatment and Hydrocracking of Oil Fractions; Elsevier Science B. V.: Amsterdam, The Netherlands, 1997; pp 379. (6)Baltanas, M. A., Van Raemdonck, K. K., Froment, G. F., Mohedas, S. R. Ind. Eng. Chem. Res. 1989, 28, 899.