Environmental Engineering Reference
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by the pyrolysis temperature. Industrial-scale pyrolysis is carried out in
rotary drum, melt furnace, or fluidized bed reactors.
No commercial scale facilities are operational in North America but
numerous companies have demonstrated technical feasibility of pyrolysis.
9.2.2 Hydrogenation or hydrocracking
Hydrogenation technologies are not that widely used in waste management.
It is usually a two-stage process where plastic waste is first subjected to
mild cracking by low-temperature pyrolysis and the condensable product is
reactedwithhydrogentoobtainpetrochemicalsordieselfuel(JooandGuin,
1997). Typically, the process involves reaction of the plastic degradation
products with hydrogen over a catalyst in an autoclave at moderate
temperatures and pressures (typically 423-673°K and 3-10 MPa hydrogen)
(Panda et al., 2010). Catalysts used include oxide- or zeolite-supported
transition metals that catalyze both reactions. The cracking reaction is
endothermic while the hydrogenation itself is exothermic. Typical feeds
include PE, PET, PS, PVC, and mixed plastics or plastic/tire rubber mixes.
Both pyrolysis and hydrocracking are particularly desirable waste
management approaches from a sustainability standpoint, for several
reasons.
a. They produce only small amounts of waste to be landfilled and result in
minimal air emissions (flue gases and CO 2 ) or water pollution.
b. These yield an energy-rich fuel that is ready for use with little or no
further processing. Pyrolysis yields about a gallon of ready-to-use fuel
per about 7.6 lbs of scrap plastic and the BTU value of the fuel is higher
(by about 15%) than that from incinerating the plastic itself as a solid
fuel (4R Sustainability, Inc., 2011). However, process energy costs
(generally amounting to a few percent of the energy content of waste
(Gonçalves et al., 2008) in producing the fuel is significant.
c. Where monomer recovery in high yield is possible (as with polystyrene
waste), it can, at least in theory, provide a route to converting waste
plastics into virgin resin (Smolders and Baeyens, 2004).
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