(137c) From Plastic Waste to Ethylene: Steam Cracking of Supercritically Produced Naphtha

A promising route for producing ethylene from wastes and renewable fractions is the combination of supercritical conversion with a traditional steam cracker. The Mura Hydro-PRT technology is a hydrothermal process to recycle a broader scope of plastic waste, including post-consumer, contaminated, multi-layer flexible films and rigid. Therefore, post-consumer wastes that would otherwise be incinerated or landfilled can be addressed. To begin the process, the plastic waste is shredded and pre-sorted (non-plastic contaminants such as stones, glass, and metal must be removed to protect downstream machinery alongside non-target plastics). The resulting plastic waste is combined with supercritical water and heated to reaction conditions. The water breaks down the carbon-carbon bonds in the plastics into molecules and donates hydrogen to form new, stable, shorter-chain hydrocarbons, resulting in increased final product stability. This conversion process takes approximately 25 minutes. Flash distillation separates the product fractions for use in the petrochemical industry, producing high yields due to water use in the conversion, which doesn’t produce char. The industry-ready liquid hydrocarbon products produced following flash distillation include circular liquid hydrocarbons for manufacturing circular plastics. This heavy residual oil can be an additive in producing asphalts or bitumen and a light vapour product. All products are REACH registered. Warwick Manufacturing Group (WMG) at the University of Warwick published an independent, academic Life Cycle Assessment in early 2023, which concluded that the first Hydro-PRT site in Teesside would save 1.86 tonnes CO₂ eq. per tonne of plastic processed, when compared to incineration, a comparable end-of-life treatment for plastic waste.[1] This equates to over 40,000 tonnes CO₂ eq. annually for the first manufacturing line in Wilton, Teesside. In addition, Mura’s Hydro-PRT was shown to have a 50% lower Global Warming Potential (GWP) than pyrolysis technologies studied[2], and the recycled hydrocarbon feedstocks produced are at a lower GWP than fossil. These figures are expected to improve with increased renewable energy supplied to the site.

The current contribution will discuss details about the Mura Hydro-PRT Process for producing naphtha, such as process technology, production capacity, and economics. It will also report on recent progress on the Plant, which started in 2024.

On the other hand, the contribution will discuss the results of an extensive steam cracking pilot plant campaign performed with naphtha from the Hydro-PRT technology. This study was conducted on the pilot plant[3] of the Laboratorium voor Chemische Techniek (LCT) in Ghent University and involved in the detailed characterization of the naphtha using GC×GC TOF-MS, AED and GC×GC FID,[4] important for addressing the quality of the produced naphtha (detailed PIONA, sulfur, nitrogen, chlorine and oxygenates content, etc.). Cracking of the naphtha leads to high light olefin yields and low amounts of aromatics. For a residence time of 0.4 s, a Coil Outlet Pressure (COP) of 1.7 bar, a dilution of 0.45 kg steam per kg hydrocarbons and Coil Outlet Temperature (COT) of 850°C, the naphtha gives an ethylene yield of more than 27 wt% and a propylene yield of 17.5 wt%. The absence of a significant aromatic fraction in the feed results in low C5+ yields, low amounts of pyrolysis gasoline and lower amounts of pyrolysis fuel oil than classical pyrolysis oils. The influence of the process conditions on the product yields is as expected.⁴ more methane, ethylene, 1,3-cyclopentadiene, benzene and toluene are formed at higher severities. The propylene yield is highest at a COT of 820°C. Higher dilutions result in higher light olefin yields and lower amounts of C5+ products for identical process-gas-temperature profiles. Comparison with the results of several oil-derived naphthas shows that the naphtha can be considered a desirable feedstock for a steam cracker. The latter is confirmed by coking runs [COT = 850°C; δ = 0.45 kg/kg; COP = 1.7 bar] with and without DMDS addition and comparison with coking data of reference feeds. The naphtha has a low coking tendency, and long-run lengths can be expected. The results obtained in this study are scaled up to industrial furnaces (Lummus SRT, Millisecond, TechnipEnergies USC) using COILSIM1D[5, 6], resulting in product yields and run lengths for a typical range of industrial operating conditions.

[1] M. C. Ozoemena and S. R. Coles, "Hydrothermal Treatment of Waste Plastics: An Environmental Impact Study," Journal of Polymers and the Environment, vol. 31, no. 7, pp. 3120-3130, Jul 2023, doi: 10.1007/s10924-023-02792-3.

[2] E. Commission et al., Environmental and economic assessment of plastic waste recycling : a comparison of mechanical, physical, chemical recycling and energy recovery of plastic waste. Publications Office of the European Union, 2023.

[3] K. M. Van Geem, S. P. Pyl, M.-F. Reyniers, J. Vercammen, J. Beens, and G. B. Marin, "On-line analysis of complex hydrocarbon mixtures using comprehensive two-dimensional gas chromatography," Journal of Chromatography A, vol. 1217, no. 43, pp. 6623-6633, 2010/10/22/ 2010, doi: 10.1016/j.chroma.2010.04.006.

[4] K. M. Van Geem, "Plastic waste recycling is gaining momentum," Science, vol. 381, no. 6658, pp. 607-608, Aug 2023, doi: 10.1126/science.adj2807.

[5] P. P. Plehiers, S. H. Symoens, I. Amghizar, G. B. Marin, C. V. Stevens, and K. M. Van Geem, "Artificial Intelligence in Steam Cracking Modeling: A Deep Learning Algorithm for Detailed Effluent Prediction," Engineering, vol. 5, no. 6, pp. 1027-1040, Dec 2019, doi: 10.1016/j.eng.2019.02.013.

[6] K. M. Van Geem, M. F. Reyniers, and G. B. Marin, "Taking optimal advantage of feedstock flexibility with COILSIM1D," in AIChE Spring Meeting: Ethylene producers conference, 2008/04/15/ 2008.