The behaviour of plastic particles during pyrolysis in bubbling fluidized bed reactors: Incipient agglomeration and axial segregation

Abstract

Plastic-fluidized bed interaction has been investigated by non-invasive X-ray imaging techniques in the temperature range of 500–650 °C and under pyrolysis conditions. Experiments were conducted by feeding a single polypropylene particle using either overbed or underbed feeding methods. The fluidized bed was operated from minimum fluidization conditions to bubbling regime, up to 2Umf. Interestingly, experimental observations show that the understanding of the mixing/segregation behaviour of polypropylene in a hot fluidized bed does not appear to be as straightforward as in the case of more conventional feedstocks, such as biomass. Further analysis and an alternative point of view to investigate the complex behaviour observed for plastic feedstock have been provided. The interaction between the fluidized bed and a single polypropylene particle was modelled by means of an axial segregation model, and a physics-assisted stochastic approach. In the second case, it was possible to simulate the formation of the sand-plastic agglomerate (incipient agglomeration stage) and the evolution of its density over time. The models were then validated by means of axial segregation data obtained via X-ray imaging techniques and by measuring the physical properties of real sand-polypropylene agglomerates produced within the bed operating at high temperatures. Results obtained in this study show that the unpredictable sinking-floating behaviour of the plastic particles is not related to the change in density associated to the formation of the sand-polypropylene agglomerates over time.