Sustainability within the polyester value chain
Polyethylene terephthalate (PET) is used to make textile fibres, bottles and packaging films. The global production in 2013 was 65 Mt, growing at 5-7% per year over the last decade. PET is manufactured by the continuous polymerisation of ethylene glycol and terephthalic acid, both of which are produced from fossil fuels. This Dissertation examines the environmental impact of manufacturing PET using process modelling and life cycle assessment. The work focused on ways of reducing the environmental impact of the polymer manufacture by using biomass instead of conventional fossil fuels, either as a raw material for producing ethylene glycol or terephthalic acid, or as a fuel to supply process heating or electricity. The environmental impacts of producing a PET bottle using ethylene glycol derived from two types of biomass, sugarcane and willow, were investigated and compared with conventional production. For sugarcane, the sugars were fermented to bioethanol, then dehydrated to ethylene. By using sugarcane, it was found that the global warming potential (GWP) and non-renewable resource use could be reduced by 28% and 16% respectively. Ethanol, and hence ethylene, can also be produced from willow, a lignocellulosic biomass, which could also potentially reduce nonrenewable resource use by 16%. However, for sugarcane there was a significant increase in other environmental impacts, e.g. acidification and eutrophication potential; these increases were smaller when using willow. From supply chain analysis, the transport of finished and intermediate products only made a minor contribution to the environmental impacts. The principal raw material for terephthalic acid is p-xylene, conventionally made from naphtha. It is feasible, however, to manufacture p-xylene by the catalytic conversion of sugars extracted from biomass sources. A PET bottle made using p-xylene derived from willow could reduce the GWP and non-renewable energy use by 32% and 2%, respectively, or 87% and 26% using sugarcane. Again, the disadvantage of using biomass was that all other environmental impact categories were increased over materials derived from petrochemicals. Biomass can also be used for generating process heat or electricity. It was found that the best possible use of biomass within the PET value chain would be combustion to supply process heat, followed closely by burning to generate electricity. In fact, only where ethylene is produced via the fermentation of sugars from hydrolysed willow, and for one measure, GWP, was producing a chemical from biomass more sustainable than combustion for process heating. This conclusion is sensitive to the energy sources from which heat and grid electricity are otherwise produced and might therefore alter as future conventional energy sources change. Finally, the possible savings in GWP and energy use by recycling PET bottles were evaluated for both closed-loop and open-loop systems. Open-loop recycling gave better savings for GWP and energy use when compared with closed-loop recycling. The transport associated with the international trade of baled bottles, largely imported by China, has a minimal effect on the possible savings by recycling. This work has established that there is scope for improving the sustainability of the polyester industry; however trade-offs need to be carefully considered on a case by case basis.
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