MANAGING PLASTIC WASTE - Plastics and Environmental Sustainability - page 394

less oil per unit mass, and PVC degradation produces corrosive HCl that has
to be removed during the process. Though it has never been demonstrated,
bio-based plastics such as poly(lactic acid) (PLA) should also be amenable
to conventional pyrolysis.
A carbonaceous char residue waste (2-13%), often contaminated with
catalyst residue and inorganic fillers, is formed in pyrolysis. This may be
either used as solid fuel, activated C, or disposed of in a landfill.
Mechanism of pyrolytic degradation is complex involving a large set of
reactions, even for a single class of plastic. Generic degradation reactions
such as chain scission, H-transfer, unzipping, disproportionation, and
combination occur in the process. Typical mix of products from mixed
plastic waste streams with different pyrolysis conditions are shown in Table
9.3 .

Table 9.3 Yield of Products from Pyrolysis of Mixed Plastic
Waste a at 440°C (dehydrochlorination Step was at 300°C for 30
Min)

Source: Based on data from Lopez-Urionabarrenechea et al. (2012).
Pyrolysis method Liquid Gas Residue
Uncatalyzed pyrolysis 79.3 ± 1.9 17.7 ± 1.9 3.0 ± 0.3
Catalyzed pyrolysis (zeolite ZSM-5) 56.9 ± 3.0 40.4 ± 3.0 3.2 ± 0.2
Uncatalyzed followed by catalyzed 69.0 ± 0.1 29.0 ± 0.1 2.00 ± 0.0
Dehydrochlorination + catalytic 56.8 ± 2.0 41.2 ± 2.0 2.00 ± 0.2

a PE = 40%, PP = 35%, PS = 18%, PET = 4, and PVC = 3.

The rate of loss in weight of the plastic can be described by first-order
kinetics (Marcilla et al., 2003), allowing an activation energy for the process
to be estimated and is as follows:

where, m is the mass of solid plastic left at time t , n is the order of reaction,
A is the preexponential factor, and E is the activation energy. For PE, for
instance, E ~ 250 kJ/mol and the reaction rate is very significantly affected