Environmental Engineering Reference
In-Depth Information
11,270 kcal/kg, HDPE has an energy intensity of 76 MJ/kg, or without considering the energy
value of ethylene, 28.7 MJ/kg (Worrell et al., 2000).
LDP has a polymerization energy requirement of 9.3 MJ/kg (Worrell et al., 2000), and the
total energy intensity of LPD is around 87 MJ/kg. Similar polymerization energies have been
reported for polypropylene, polystyrene, and polyvinyl chloride with values of 10.5, 9.36, and
11.6 MJ/kg, respectively (Worrell et al., 2000).
The total energy intensity for manufacturing of polypropylene and polystyrene, consider-
ing the energy content in the feedstock and the process energy, is 93 MJ/kg for polypropylene,
82 for polystyrene obtained by suspension polymerization, and 126 MJ/kg for produced by
bulk polymerization (Bridgewater and Lidgren, 1983).
Polyvinyl chloride (PVC) uses less fossil resources than other polymers because PVC
molecules are composed of 38 percent chlorine. However, production of chlorine is energy-
intensive. It takes about 4380 kWh of electricity and 3.45 GJ or thermal energy to produce 1
tonne of chlorine gas (Worrell et al., 2000). In the end, though, production of PVC is less
energy intensive than other polymers derived from petroleum and natural gas with an energy
intensity of 57 MJ/kg for the suspension polymerization method (European Council of Vinyl
Manufacturers [ECVM], 2008a) and 65.9 MJ/KJ for when the emulsion polymerization
method is used (ECVM, 2008b).
Polylactate
In the United States, PLA is produced from corn starch that is first transformed enzymatically
into dextrose and then fermented into lactic acid. Clearly, any other source of fermentable
sugars can be used to produce the precursor monomer lactic acid. Lactic acid is then polymer-
ized into polylactate by two major routes: direct condensation polymerization and ring-
opening polymerization.
The cradle-to-factory-gate life cycle assessment of PLA using corn as raw material and the
ring-opening polymerization route (applied by Cargill, the major producer of PLA, Fig. 12.1)
indicates that the gross energy requirement to produce 1 kg of polymer is 82.5 MJ/kg:
54.1 MJ/kg comes from fossil fuels.
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28.4 is the energy embodied in the corn feedstock (Vink et al., 2003).
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Wood
The main use of wood in packaging is for the production of wooden pallets. The cumula-
tive energy needed to build a pallet depends on many factors, such as type of wood, size,
and transportation of the materials. Pallets come in different dimensions and weights, but
a typical pallet weights around 25 kg. Boustead and Hancock (1981) estimated that a
25-kg pallet built in the United Kingdom with domestic and imported materials takes
about 668 MJ of energy. When the wood feedstock energy (438 MJ) is incorporated,
then the embedded energy increases to 1106 MJ per pallet (Table 5.1 in Boustead and
Hancock, 1981).
Paper
Energy intensity to produce paper depends on the process, the technology, and the age of the
machinery. New paper mills are more energy efficient than old ones.
Estimates for production of cellulose pulp vary from around 28 MJ/kg (Boustead and
Hancock, 1981) to 44.6 MJ/kg (Bridgewater and Lidgren, 1983). The transformation of pulp
