Pyrolysis of polyolefins for increasing the yield of monomers’ recovery

Pyrolysis of plastic waste is an alternative way of plastic recovery and could be a potential solution for the increasing stream of solid waste. The objective of this work was to increase the yield the gaseous olefins (monomers) as feedstock for polymerization process and to test the applicability of a commercial Ziegler–Natta (Z–N): TiCl4/MgCl2 for cracking a mixture of polyolefins consisted of 46% wt. of low density polyethylene (LDPE), 30% wt. of high density polyethylene (HDPE) and 24% wt. of polypropylene (PP). Two sets of experiments have been carried out at 500 and 650 °C via catalytic pyrolysis (1% of Z–N catalyst) and at 650 and 730 °C via only-thermal pyrolysis. These experiments have been conducted in a lab-scale, fluidized quartz-bed reactor of a capacity of 1–3 kg/h at Hamburg University. The results revealed a strong influence of temperature and presence of catalyst on the product distribution. The ratios of gas/liquid/solid mass fractions via thermal pyrolysis were: 36.9/48.4/15.7% wt. and 42.4/44.7/13.9% wt. at 650 and 730 °C while via catalytic pyrolysis were: 6.5/89.0/4.5% wt. and 54.3/41.9/3.8% wt. at 500 and 650 °C, respectively. At 650 °C the monomer generation increased by 55% up to 23.6% wt. of total pyrolysis products distribution while the catalyst was added. Obtained yields of olefins were compared with the naphtha steam cracking process and other potentially attractive processes for feedstock generation. The concept of closed cycle material flow for polyolefins has been discussed, showing the potential benefits of feedstock recycling in a plastic waste management.

► Thermal and catalytic pyrolysis of mixed polyolefins in fluidized bed has been studied.
► We tested applicability of a commercial Ziegler–Natta catalyst (Z–N: TiCl4/MgCl2).
► The catalyst has a strong influence on product distribution, increasing gas fraction.
► At 650 °C the monomer generation increased by 55% when the catalyst was used.
► We showed the concept of treatment of mixed polyolefins without a need of separation.