Hydrocracking of long chain linear paraffins

The hydrocracking reactivity of two model compounds, namely n-C(16)H(34) (n-C16) and n-C(28)H(58) (n-C28), was investigated on a Pt/SiO(2)-Al(2)O(3) catalyst. Conversion and products distribution have been determined under a wide range of operating conditions (i.e. pressure: 20-80 bar; temperature: 270-330 degrees C; weight hourly space velocity: 0.33-1.0 h(-1): H(2)/n-paraffin feeding ratio 0.05-0.15 wt/wt). The latter were changed according to a central composite design. The present paper summarises the results obtained on both the model paraffins, depending on the reaction conditions. A first, simple kinetic elaboration is also presented, based on an ideal PFR model and a first order kinetics. The reaction confirmed to be first order with respect to the n-paraffin. Experimental data showed that for both n-C16 and n-C28 conversion was affected by H(2)/n-paraffin ratio. The change of conversion was explained in terms of vapour liquid equilibrium (VLE), which in turn is affected by the H(2)/n-paraffin ratio, so leading to a different vaporisation degree of reactant. In agreement with the VLE data, the effect of H(2)/n-paraffin on conversion was lower for n-C28. VLE calculations have been carried out to estimate the H(2) partial pressure and degree of vaporisation of the normal paraffin. The reaction order for hydrogen was -1 and -0.5 for n-C16 an n-C28, respectively. However, in the case of n-C16 the data obtained at the lower bound of the pressure range examined displayed an increase of the reaction order. The apparent activation energy was calculated after correction of the contact time taking into account the liquid-vapour equilibrium: similar values have been estimated for n-C16 and n-C28, ca. 32 and 31 kcal/mol, respectively.