(131f) Recycling Waste Plastic to Fuels: Detailed Mechanistic Modelling of LLDPE Pyrolysis

Plastic pollution has now turned into an acute environmental problem. Currently, about 400 million metric tons of plastic is generated every year out of which less than 10% is recycled¹. The remaining is dumped in landfills, incinerated (which adds CO₂ into the atmosphere) or ends up as unregulated waste and interferes with the biosphere. Conversion to chemicals (and fuels) by pyrolysis is an effective strategy of recycling waste plastics. Polyethylene is a widely used plastic for packaging applications and forms a significant portion of the waste produced². Linear low-density polyethylene (LLDPE) is a type of polyethylene that is popular for bags, wrap films, etc. owing to its attractive mechanical properties. Studies suggest that thermal pyrolysis of LLDPE yields liquid fractions with diesel-like properties³.

We present a microkinetic model for the thermal pyrolysis of LLDPE. A microkinetic model is a multiscale mechanistic model which predicts reactor level conversions by taking into account the kinetic information of the mechanistic steps involved. Being a system involving a large number of species and reactions, it is computationally challenging to model every mechanistic step of polymer pyrolysis. Therefore, we employ population balance modelling in which species are divided into classes based on their characteristics and the variation of their molecular weight distributions, with the progress in reaction tracked using the moments. The number averaged and weight averaged molecular weights of the species classes and the reaction mixture can be calculated from these moments. The Broadbelt group has used this method of moments extensively for modelling pyrolysis of other polyolefins, including polypropylene, polystyrene, and high density polyethylene (HDPE). ^4–6 In this approach, kinetic parameters are formulated using the Arrhenius relationship, and reaction rates are considered in terms of the moments of species classes, where the classification is done based on features such as unsaturated ends, type of radical (mid versus end), etc. Besides the molecular weight distribution of species classes, specific species of known carbon length can also be tracked to make predictions of product distributions from the model. A semi-batch reactor system is considered. The final model is a differential-algebraic system with differential equations describing the rates of change of moments of species classes and the rates of formation of specific species. Due to the chain length dependency of some of the reactions in pyrolysis, the method of moments has a closure problem which is resolved by providing an algebraic approximation for the third moments. Moreover, the fact that the same reaction could yield different kinds of products necessitates the calculation of likelihoods of their formation using algebraic equations. The Differential Algebraic Equation (DAE) model is written in C and is solved using the solver Double Precision Differential/Algebraic Sensitivity Analysis Code (DDASAC).

LLDPE is made by co-polymerizing ethene and alpha-olefins such as, butene, hexene, octene, etc. As a result, LLDPE contains a large number of short chain branches. The reactions that occur during LLDPE pyrolysis are similar to those during HDPE pyrolysis, but needs several additional equations to solve for, due to the presence of branches. The reaction families considered in the model are chain fission, radical recombination, mid-chain beta scission, radical addition, end-chain beta scission, disproportionation, bimolecular hydrogen abstraction and intramolecular hydrogen shifts, which are the typical free radical reaction types. Mechanistic models based on these reactions have been demonstrated to be able to capture product distribution of HDPE pyrolysis with reasonable accuracy⁶ and will serve as a starting point for LLDPE models as well. The species classes are determined based on the characteristics such as chain types (linear/branched), radical types (end-radical, mid-radical, tertiary mid-radical or dead species) and the types of ends (saturated/unsaturated).

Figure 1 shows the number average molecular weight (M_n) as a function of reaction time predicted by a basic LLDPE model compared to an analogous HDPE model. This basic model tracks only the species classes and not any specific species. Isothermal reaction at 673.15 K is considered. The branch characterization of LLDPE in this model is in terms of number of branches/1000 carbons. The average branch length is considered to be 6. Due to the presence of bonds at branch points involving tertiary carbon atoms that are weaker than bonds involving secondary carbon atoms, LLDPE is expected to decay faster than HDPE. The agreement of the model result with the anticipatory behavior is an indication that the preliminary LLDPE model is reasonable. Further observations about LLDPE pyrolysis can be made after modifying the model to capture more details pertaining to specific species. The quantitative predictions of different fractions of products produced by thermal pyrolysis of LLDPE and their validation with experimental results will be presented.

References

1. United Nations Environment Programme (2021). From Pollution to Solution: A global assessment of marine litter and plastic pollution. Nairobi.

2. UNEP. Single-use Plastics: A Roadmap for Sustainability. 2018. (Rev. ed., pp. vi; 6).

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5. Kruse TM, Wong HW, Broadbelt LJ. Mechanistic modeling of polymer pyrolysis: Polypropylene. Macromolecules. 2003;36:9594–607.

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