Reaction temperature, heat of combustion, and main chemicals produced by hydrothermal oxidation of polypropylene

Plastic pollution is a global emergency. Since the 1950s around 4900 Mt of plastic waste has been lost to the environment, and chances to find it in the open ocean have increased fivefold since the 1990s. Clean technologies are needed to convert plastic waste into less environmentally harmful substances. There are advanced technologies reported in the literature for plastic waste treatment, but hydrothermal oxidation (HO) is generally missing. HO can be regarded as an intermediate technology between incineration and waste conversion. It is clean because it minimizes CO2 and toxic emissions in comparison with incineration, generate enough heat to self-maintain the process, and retains low molecular weight organic compounds that could be further processed into bioplastics, biomethane or added value chemicals. This study reports our initial investigations towards developing clean HO technology for conversion of plastic waste into water soluble organic compounds. Two different HO reactors were used: a fluidized sand bed system for batch oxidation with short residence time, and a reaction calorimeter for measuring oxidation rates as function of the temperature. The experiments were designed to characterize heat and mass balances, which are necessary for optimizing the process. A main distinction from published literature is that the proposed HO technology aims at retaining as much organic compounds in the water as possible, contrary to traditional HO technologies aiming at producing clean effluents from waste. It was concluded that acetic acid appears to be a refractory intermediate which limits the oxidation rate, thus high temperatures and residence times are required to produce clean effluents. On the contrary, using lower temperatures and residence times will positively increase the production of organic compounds in the proposed technology, as well as reduce capital and operation costs. The next step of the research is to determine the lowest temperature and residence time that produce the highest yield of organic compounds.