Kinetic and thermodynamic studies on pyrolysis of waste HDPE polymers

Abstract

Pyrolysis is a promising technology for converting waste plastic into high-value hydrocarbons, which can help to protect the environment and improve the waste management industry. Available methods for finding kinetic parameters and heat of solid state reactions are not compatible with the complexity of pyrolysis reactions, and do not give reliable parameters to design an industrial pyrolysis reactor. This work developed two new techniques to determine kinetic parameters and heat of solid state reactions with high-certainty. The proposed kinetic study method is a differential isoconversional technique that finds activation energy and pre-exponential factor at different extents of reaction using isothermal Thermogravimetric Analysis (TGA) datasets. Employing this method, kinetic parameters of pyrolysis of high-density polyethylene (HDPE) were determined at different reaction conversions. The obtained apparent activation energy values were obtained in the range of 270 to 290 kJ/mol. The developed method for finding heat of reaction employs Differential Scanning Calorimetry (DSC) technique at constant temperatures. This method involves a new procedure to find heat loss from the DSC instrument as function of temperature and sample weight. The method was employed to find heat of cracking of HDPE at constant temperature of 400, 410, 420 and 430 °C, and the average heat of reaction was determined to be 1375±233 kJ/kg. Based on available recommendations in the literature for using ZSM-5 catalysts in polymer pyrolysis, catalytic cracking of high-density polyethylene was studied using three ZSM-5 catalysts with different Si/Al ratios of 25, 38 and 80. Using a TGA instrument and altering the variables such as temperature and the catalyst to HDPE ratio, catalytic activity of the catalysts was investigated, and proper operating conditions were estimated. ZSM-5 catalysts with Si/Al ratios of 25 and 38 at constant temperatures of 330, 340, 350, 360 and 370 °C, and cat/HDPE ratio of 15 % showed considerably high catalytic activity in cracking of HDPE (especially the ZSM-5 with Si/Al ratio of 25). Using the developed kinetic study method, kinetic parameters of catalytic cracking of HDPE were determined at the aforementioned conditions, and apparent activation energy values were dropped dramatically to the range of 20 to 90 kJ/mol. In addition, catalytic activity, deactivation behavior, regenerability and reuse of the ZSM-5 catalyst with Si/Al ratio of 25 in consecutive cracking tests were also investigated. When activity of the used catalyst dropped to 20% of its initial value, a catalyst regeneration at 480 °C for 5 h was conducted; however, due to dealumination reactions occurred in the regeneration step, the initial catalytic activity could never be recovered. After catalyst regeneration, the regenerated catalyst was used in the same cracking tests. With 10 regeneration cycles, the ZSM-5 was used in 54 cracking tests. The effect of calcination temperature on the activity of ZSM-5 in cracking of high-density polyethylene was then explored. Calcination at 600 and 700 °C reduced acidity and activity of the ZSM-5 mainly due to catalyst dealumination. On the contrary, no drop in activity of the 500 °C-calcined catalyst was detected. Overall, the findings of this study can be employed to design an industrial reactor for pyrolysis of waste polymers. Additionally, the methods developed in this study for obtaining kinetic parameters and heat of pyrolysis can be used in any other solid state reactions.