Intrinsic Millisecond Kinetics of Polyethylene Pyrolysis via Pulse-Heated Analysis of Solid Reactions

Abstract

Continued demand for polyolefins can be met by recycling plastic materials back to their constituent monomers, ethylene and propylene, via thermal cracking in a pyrolysis reactor. During pyrolysis, saturated polyolefin chains break carbon−carbon and carbon−hydrogen bonds, yielding a distribution of alkanes, alkenes, aromatic chemicals, light gases, and solid char residues at temperatures varying from 400 to 800 °C. To design a pyrolysis reactor that optimizes the chemistry for a maximum yield of light olefins, a detailed description of the chemical mechanisms and associated kinetics is required. To that end, the reaction kinetics of isothermal films of low-density polyethylene (LDPE) have been measured by the method of "pulse-heated analysis of solid reactions", or PHASR, which allows for quantification of intrinsic kinetics via isothermal reaction-controlled experimental conditions. The evolution of LDPE films from 20 ms to 2.0 s for five temperatures (550, 575, 600, 625, and 650 °C) was characterized by measurement of the yield of chromatography-detectable compounds (<C20) in addition to the total yield of volatile products. The kinetics of volatile product evolution were interpreted via a lumped kinetic model with an activation energy of 225 ± 16 kJ mol −1 , compared with existing kinetic models of polyethylene pyrolysis, and validated from first principles.

Chloride and Hydride Transfer as Keys to Catalytic Upcycling of Polyethylene into Liquid Alkanes

“…Despite the conversion rates of LDPE varying with different Al halides, plotting the logarithmic correlation between the initial conversion rates of LDPE and n ‐C 16 H 34 as a model reactant against the inverse temperature (Figure S4) showed a consistent activation energy range of 40–44 kJ/mol. These values are similar to the activation energies for the hydride transfer reaction between two isobutane molecules (55 kJ/mol), [37] but significantly lower than the reported activation energy barrier for polyethylene depolymerization, which spans a range of 160–310 kJ/mol [38,39] . This observation suggests that the underlying mechanism and rate‐limiting steps are consistent across the examined aluminum halides.…”

Fast Pyrolysis Product Analysis of Commercial Polyethylenes

“…Relevant observations are also seen during the thermolysis of a thin LDPE film at 550–650 °C, supporting the view of surface decomposition . Temperatures below 650 °C lead to residues with char, whereas pyrolysis above 650 °C only gives gases.…”

“…Successive β-scission in midchain radicals will lead to primary alkyl radicals and terminal olefins. Considerations based on these processes lead to the prediction of a mixture of dienes, alkanes, ethylene, and 1-olefins as “simple pathway” products (in a ratio of 1:1: x :2). , The formation of products may also be influenced by mass transport of volatile products and hence be dependent on the dynamics of the polymer (melt), i.e., processes that will scale with densities and viscosities. ,, Further reactions (e.g., 1, x -hydrogen shifts in primary radicals) of the initial products may eventually lead to cyclizations and subsequently the formation of hydrogen, aromatics, and finally coke. ,, …”

Chloride and Hydride Transfer as Keys to Catalytic Upcycling of Polyethylene into Liquid Alkanes

“…These values are similar to the activation energies for the hydride transfer reaction between two isobutane molecules (55 kJ/mol), [37] but significantly lower than the reported activation energy barrier for polyethylene depolymerization, which spans a range of 160-310 kJ/ mol. [38,39] This observation suggests that the underlying mechanism and rate-limiting steps are consistent across the examined aluminum halides. The congruence of the activation energy with that of hydride transfer processes led us to hypothesize that the hydride transfer represented the pivotal step in this reaction sequence.…”