Feedstock Recycling of Polymer by Hydrocracking

Abstract

With more stringent environmental regulation and taxation, rising landfill costs and the drive towards a circular economy, there is an increasing need to redirect polymer waste from landfill/energy recovery towards the enhanced recovery of chemicals and monomers. The most widespread approach to chemical recycling is pyrolysis (or cracking). However, a more effective option is that of hydrocracking, which offers the potential for the selective recovery of useful chemical fractions but is also tolerant of the presence of heteroatoms. The starting point for this study was to convert pure polymers representing around 70% of the total global production, namely, polyethylene (LDPE, HDPE), polypropylene (PP) and polystyrene (PS), into hydrocarbon gas and liquid, mostly in naphtha (C5-C12). Reactions were carried out using a laboratory batch system at a relatively moderate temperature (≤330 oC), pressure (20 barg) and in the presence of zeolite catalysts of Beta and USY (typically loaded with Pt or Ni). The study was extended to test post- consumer polymers to simulate different types of waste streams, for example, solvent bottles (LDPE) and centrifuge tubes (PP) as well as consumer products by milk bottles (HDPE) and water drinking cups (PS). Typically, total conversion of pure polymers was achieved, and more importantly, a high proportion of the post-consumer polymers and blends tested were converted into gas (C3-C4) and naphtha fractions with a high proportion of branched isomers, which are valuable products in the petrochemical industry. A kinetic study using a lumping model that describes the hydrocracking of LDPE was carried out. In developing kinetic model, mass transfer was taken into consideration and the results of the modelling project were compared with similar studies found in the literature but over thermal or catalytic cracking to prove the lower energy requirements of the hydrocracking.