Steam Cracking of Polyolefins in Fluidized Bed Systems: Influences of the Bed Materials on the Hydrogen Transfer Reactions

Licentiate thesis, 2022

Plastic materials are crucial to the supply of everyday necessities, such as clothes, packaging, and building materials. Most plastic materials are disposed of after a short period of use due to their low cost of production. The linear use of plastics has resulted in high levels of plastic waste materials. In addition, the production of plastics is largely dependent upon fossil resources. A paradigm shift in relation to the production and management of plastic waste is necessary for the development of a sustainable plastic usage.
Plastic waste is challenging to recycle, given its inherent heterogeneous nature. In this context, recycling using thermochemical processes is a promising approach towards fossil-free production of new plastics of original quality from the abundantly available plastic waste. For this application, steam cracking in a dual fluidized bed (DFB) reactor is a suitable process owing to its flexibility. This work focuses on strengthening the position of the DFB steam cracking technology in the recycling of plastic waste. Experimental studies were conducted on steam cracking of polyethylene, a polyolefin, and rapeseed oil, a polyolefin-like feedstock, in a laboratory reactor under operating conditions similar to that of a DFB system. The suitability of such feedstocks for the DFB steam cracking process is discussed with respect to the distribution of the obtained products. The influence of the bed material on the steam cracking reactions of polyolefins is also examined.
The results presented in this thesis demonstrate that steam cracking in a DFB system is a suitable process for the recycling of polyolefin and polyolefin-type feedstocks. Steam cracking of such feedstocks yields a product distribution that is similar to that obtained from thermal cracking of petroleum naphtha. Experiments investigating the influences of the bed material on the steam cracking reactions reveal that the activity of the bed material towards the hydrogen transfer (C-H bond scission) reactions is a critical parameter for the process.
An increase in the catalytic activity of the bed material towards the hydrogen transfer reactions results in either dehydrogenation or hydrogenation of the polyolefin feedstock. Dehydrogenation is associated with the oxidizing nature of the bed material, and results in the formation of compounds with H/C ratios <2. In contrast, hydrogenation promoted by the bed material leads to the enhanced formation of compounds with H/C ≥2. Hydrogenation was observed in the presence of a reduced bed material and was associated with the water dissociation and hydrogen transfer capabilities of the bed material. Extensive experimental results supporting these findings are discussed throughout the thesis.