Upcycling HDPE plastic into value-added products via γ-alumina catalyzed steam cracking in a fountain confined spouted bed

Abstract

This paper proposes an alternative approach for plastic wastes valorization towards light olefins in a novel reactor design, which avoids the use of high temperatures (800–900 °C) and zeolite catalysts, as is the case in thermal and catalytic cracking, respectively. Thus, the steam cracking of HDPE was studied in a fountain confined spouted bed reactor (FCSBR) using γ-alumina as in-bed catalyst due to its moderate activity for hydrocarbons cracking reactions and relatively low cost. This reactor design enables operation with continuous plastic feed without defluidization problems and preventing elutriation of fine catalyst particles. The cracking temperature was optimized between 650 and 750 °C to maximize the production of valuable chemicals, such as light olefins and BTX. The results showed that high temperatures enhance the breaking down of long chain hydrocarbons into shorter compounds, narrowing the product distribution. The product stream was composed of mainly C 2 -C 4 olefins, with a maximum yield of 40.3 wt% at 700 °C. Propylene was the major olefinic compound at 650 °C (15.9 wt%), but the distribution shifted towards ethylene (24.5 wt%) at 750 °C, at the expense of decreasing the content of C 3 -C 4 paraffins and olefins. Apart from typical cracking products, CO and CO 2 were also obtained, yielding 4.9 and 15.9 wt% at 750 °C. In addition to promoting cracking and steam reforming reactions, high temperatures also favoured other secondary reactions. Thus, Diels-Alder cyclization and condensation reactions were observed, leading to a BTX yield of 16.7 wt% at 750 °C. Considering the combined yield of all high-value chemicals, 700 °C turned out to be the optimum temperature, enabling to recover 53.5 wt% of high added value products. Upcycling plastic waste into valuable chemicals like olefins and BTX is a promising and sustainable solution to address plastic pollution crisis, promote natural resource preservation and progress towards circular economy. • The fountain confined spouted bed is suitable for upcycling plastics via steam cracking. • The product stream was composed of mainly C 2 -C 4 olefins, peaking at 700 °C. • Lighter olefins are enhanced by increasing temperature. • Ethylene production (24.5 wt%) peaked at 750 °C. • High temperatures yielded 16.7 wt% BTX.

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