Context

Global plastic production reached almost 370 million metric tons (MMT) in 2020. Most of this plastic ends up in landfills as waste at the end of its useful life with only a small fraction of the disposed plastics being recycled. Plastic upcycling to valuable chemicals could ensure a circular economy for plastics and reduce their environmental burden. In our study, we designed a process to convert high-density polyethylene (HDPE) to a high-quality liquid (HQL) product, which has tribological properties similar to premium lubricant oils such as Group III and PAO.¹

Method

Based on experimental results for plastic hydrogenolysis using a novel catalyst technology that we developed, a large scale plant that would upgrade 250 MT/day of post-consumer HDPE to a high-quality lubricant-like product was designed. The process was modeled with Aspen Plus v12. Two different scenarios, a low-yield (LY) and a high-yield (HY) case, were studied. To compare the different scenarios, the minimum selling price (MSP) was determined (i.e., the production cost that would lead to a zero net present value). Life cycle assessment (LCA) was performed to evaluate the onsite and upstream emissions of the conversion process. Emissions were calculated using Argonne’s GREET (Greenhouse Gases Regulated Emissions and Energy Use) LCA tool. The results were compared to the fossil fuel-based production process for conventional Group III and PAO lubricants.

Results and Conclusions

A techno-economic analysis (TEA) shows a breakeven production cost of $1.85 for the HY case and $6.03 for the LY case per gallon of HQL based on the operating conditions (Figure 1). For comparison, the average selling price for lubricant was $10.9/gal in 2020. The lowest production cost was obtained for the HY case with a 90% HQL yield. The HY case was characterized by a shorter reaction time and lower amount of catalyst usage compared to the LY case. At a MSP of $1.85/gal, the HDPE conversion has a greater margin than sorting and separately selling the plastic feedstock, converting it to liquid fuels via gasification and syngas processes or generating a synthetic crude via pyrolysis. Our conceptual design exhibited a lower cradle-to-gate CO₂ emissions compared to the conventional processes for producing Group III and PAO lubricants from fossil fuels. For the HY scenario, the onsite and upstream emissions were 0.33 kgCO₂/kgHQL. For comparison, emissions for conventional Group III lubricants is about 1 kgCO₂/kg_Lub, and for PAO is 1.65 kgCO₂/kg_Lub.^2,3 Thus, lubricants derived from plastic waste can reduce CO₂ emissions by 67% compare to Group III and 80% compared to PAO.

References

1 R. A. Hackleret al., ChemSusChem, DOI:10.1002/cssc.202100912.

2 P. Sun et al., Environ. Sci. Technol., 2019, 53, 6556–6569.

3 Wang, Michael et al. Greenhouse gases, Regulated Emissions, and Energy use in Technologies Model ® (2021 Excel), Argonne National Laboratory (ANL), Argonne, IL (United States), 2021.