Catalytic cracking of n-dodecane over HZSM-5 zeolite under supercritical conditions: Experiments and kinetics

The catalytic cracking of n-dodecane over HZSM-5 zeolite catalyst was investigated at 400–450 °C under supercritical and subcritical pressures (0.1–4.0 MPa). The results show that both the activity of the catalyst and its stabilization towards deactivation decrease with increasing pressure, and the catalyst maintains substantially higher activity when feed rate exceeds 4.00 ml/min under supercritical conditions. A first-order Langmuir kinetic model with a novel decay function is developed for the supercritical catalytic cracking of hydrocarbon incorporating supercritical extraction effect on catalyst stability, which is satisfactory to describe the kinetic behaviors of catalytic cracking of supercritical n-dodecane. According to the estimated reaction rate and adsorption constant of n-dodecane on HZSM-5 at different temperature, the activation energy of 125.4 kJ/mol and adsorption heat 109.5 kJ/mol were calculated. An index of CRSE is proposed to define contribution ratio of supercritical extraction to the activity of the HZSM-5 catalyst in the developed kinetics model, and it is found that the CRSE increases with increasing hydrocarbon feed rates and decreasing catalytic activities, and reaches maximum value when the coke formation rate equals to the coke removal rate by supercritical hydrocarbon.