Evaluating the environmental impact of crude glycerol purification derived from biodiesel production: A comparative life cycle assessment study

In recent decades, surplus crude glycerol has been generated in large amounts as a waste product of biodiesel production, leading to bottlenecks in the supply chain of the biodiesel industry. This waste glycerol represents an important potential renewable feedstock and platform chemical; however, its purification is often needed for further processing. Advancements towards glycerol purification are being made using sustainable purification techniques aimed at improving the biodiesel industry's environmental footprint. Many studies focussing on various techniques to purify glycerol can be found in the literature; however, very few studies to evaluate the environmental impacts of the purification processes have been reported. This paper provides a critical investigation on the cradle-to-gate life cycle assessment (LCA) of three different processes for purifying crude glycerol, namely, physicochemical treatment and membrane purification (PMP) processes, vacuum distillation purification (VDP) processes and ion exchange purification (IEP) processes having a functional unit (FU) of 1000 kg of purified glycerol. These purification processes were modelled using Aspen plus software v12.1 in combination with Super Pro Designer v13. CCaLC2 (Carbon Calculations over the Life Cycle of Industrial Activities) was used to measure the environmental impacts associated with each process. By following the ISO 14044:2006 methodology and utilising the CCaLC2 tool, seven different types of potential environmental impacts have been investigated, which include carbon footprint, water footprint, acidification, eutrophication, ozone layer depletion, photochemical smog and human toxicity. Sensitivity analysis of the LCA was carried out using the response surface method (RSM) to determine the most effective parameter within the LCA. The total carbon footprint of the PMP, VDP and IEP processes are 3466.82, 1745.72 and 2239.71 kg CO2 eq. FU−1 respectively. The LCA study determined that waste generated as a result of crude glycerol impurities from the three processes had one of the highest environmental impacts on the overall process. For the PMP and IEP processes, the raw materials used in the physicochemical treatment also contribute significantly to the carbon footprint and other environmental impacts. Lastly, aspects concerning the environmental impacts from the PMP glycerol purification process have been addressed by analysing the raw materials from different sources accompanied by altered waste disposal methods (i.e. the incineration of generated wastes as opposed to landfilling) in an attempt to reduce the overall environmental impacts. For the PMP process, which has the highest carbon footprint, usage of differently sourced raw materials and altered waste disposal treatments resulted in 39% reduction in total carbon footprint and 54% reduction in the total ozone layer depletion. Sensitivity analysis of the LCA shows that the glycerol content within the crude glycerol was the most significant parameter.