Kinetic modelling of hydrocracking of low-density polyethylene in a batch reactor

Hydrocracking offers potential for the selective recovery of useful chemical fractions from polyolefin waste at relatively moderate reaction conditions with the possibility of heteroatom- and contaminant-tolerance. This study develops a kinetic model for low density polyethylene (LDPE) hydrocracking over a bifunctional zeolite namely, 1%Pt-Beta using a lumping model that describes the kinetics in a batch process. In developing the kinetic model, mass transfer limitations and vapour-liquid equilibrium were taken into consideration. Kinetic parameters were estimated from experimental results obtained at a hydrogen pressure of 20 bar and different reaction temperatures (250-300 °C) as well as different batch reaction times (0-40 min). Kinetic parameters, mass transfer coefficients and effectiveness factors were determined using a nonlinear regression model of the experimental results via MATLAB software. The physical properties of the product streams as well as vapour-liquid equilibrium data of the system were estimated using the flash unit in Aspen HYSYS software. The product stream was dominated by the naphtha fraction, decreasing with longer batch times. The results of the model indicate mild gas-liquid mass transfer limitation and unavoidable diffusion limitations of the macro-molecules of molten LDPE and heavy liquid through the catalyst pores, especially at high reaction temperature.