(74a) Experimental Study of Intrinsic Kinetics for the Pyrolysis of Polypropylene in a Microreactor Using Detailed Compositional and Principal Component Analysis

The U.S. produced 35.67 million tons of plastic waste in 2018, of which 3.09 million tons were recycled—while 26.97 million tons of plastic waste were landfilled, corresponding to 75% of all plastic waste produced in the U.S. in 2018. Polypropylene (PP) is widely used polyolefin due to its chemical and temperature resistance. The U.S. produced 8.15 million tons of PP waste in 2018, of which only 0.6% was recycled. Pyrolysis is a promising thermochemical recycling technique defined as the chemical transformation of a polymer by heating to a moderate temperature (400 – 600˚C) under a non-reactive atmosphere in the absence or presence of catalysts. However, the successful implementation of pyrolysis on a commercial scale relies on having a mechanistic understanding to effectively manage changes in the feedstock characteristics without compromising the product quality and to design catalysts and processes that maximize the production of high-value added chemicals, fuels, and materials. In addition, detailed knowledge of the composition of the pyrolysis oils is fundamental to upgrading the pyrolysis oil in processes such as steam cracking for the production of monomers and other chemicals.

Studying the speciation data under experimentally controlled conditions that allow intrinsic kinetics is fundamental to developing (micro-)kinetic models for process development and optimization. In this work, we systematically studied the primary decomposition of PP under isothermal kinetically controlled conditions using a tandem microreactor coupled to a comprehensive two-dimensional gas chromatography with a time-of-flight mass spectrometer and flame ionization detector (Py-GC×GC-FID/TOF-MS). GC×GC is a powerful technique with superior resolution compared to traditional one-dimensional gas chromatography (GC). GC×GC allows a comprehensive identification and quantification of the pyrolysis products. Box-Behnken design of experiments (BBD) was used to evaluate the pyrolysis variables (i.e., pyrolysis temperature, sample size, helium flow rate, and particle size) along principal component analysis (PCA) to evaluate statistical differences between the product distribution of the experiments conducted. In addition, BBD border conditions and conditions beyond BBD were evaluated to identify the changes that extreme conditions cause to product distribution under enhanced heat and mass transport effects.

The analysis of 600 pyrolysis products identified and quantified in the pyrolysis of PP for the condition combinations evaluated in BBD resulted in statistical differences at temperatures of 460, 530, and 600 ^oC, with C₃ to C₃₅ yields ranging from 39.88 2.08 to 66.61 3.26 wt.%. However, no statistical differences were found in the pyrolysis product distribution within the ranges of sample weight (50 – 150 µg), particle size (53 – 125, 125 – 300, and >300 µm), and carrier gas flow rate (100 – 300 ml min^-1) studied in the BBD experimental space. The analysis of the border conditions showed that a combination of the lowest carrier gas flow rate (100 ml min^-1) and the highest sample weight (150 µg) resulted in statistically different product distribution at the studied pyrolysis temperatures compared to the combination suggested by BBD. Conditions beyond the BBD, including a sample weight of 800 µg and a carrier gas flow rate of 30 ml min^-1, resulted in statistically different product distributions compared to the BBD. Notably, new products such as polycyclic aromatic hydrocarbons (PAHs) were identified. The findings from this work clearly show the importance of selecting appropriate pyrolysis parameters to obtain kinetically relevant experimental data, which in turn serves as a valuable guide for the development and validation of (micro-) kinetic models.