Two-stage hydrothermal liquefaction converts ABS plastic into low-nitrogen aromatic oils

A new study published in Engineering introduces a two-stage sequential hydrothermal liquefaction (HTL) process that efficiently recycles acrylonitrile–butadiene–styrene (ABS) plastic into energy-dense oils with low nitrogen content, offering an improved chemical recycling route for this widely used polymer. Researchers from Aarhus University designed the method to address nitrogen-related drawbacks in conventional single-stage HTL and pyrolysis of ABS, which often produce oils with high nitrogen levels requiring costly upgrading.

In the first subcritical hydrothermal stage, ABS is treated at 250–350 °C with potassium hydroxide (KOH) as a homogeneous alkaline catalyst. Under optimized conditions of 325 °C, 20 min of reaction time, and 0.30 g of KOH per gram of ABS, the process achieves a denitrogenation efficiency of 95%. The nitrile groups in ABS hydrolyze to form a potassium–ABS salt complex that transfers to the aqueous phase, while nitrogen is released as ammonia and removed by evaporation. This step yields two solid fractions, solid product 1 (SP1) and solid product 2 (SP2), which serve as feedstock for the second stage.

The denitrogenated solids undergo supercritical hydrothermal liquefaction at 375–425 °C and 250 bar, converting into low-nitrogen oils with high heating values reaching up to 41.6 MJ/kg. These oils are mainly composed of mono‑, di‑, and polycyclic aromatic compounds, with boiling ranges similar to kerosene and vacuum gas oil. Gaseous products from the supercritical stage consist mainly of C₂–C₅ hydrocarbons derived from the butadiene component of ABS.

Compared with single-stage supercritical HTL of ABS, the two-stage approach significantly reduces nitrogen in the final oil, as most nitrogen is separated in the first subcritical step rather than remaining in the liquid product. The co‑liquefaction of SP1 and SP2 without HCl neutralization delivers an oil yield of 66.3 wt% based on the initial ABS mass, with an energy recovery of 63.8% from the oil product. The process avoids the need for pre‑drying ABS or denitrogenated solids, taking advantage of their hygroscopic nature under supercritical water conditions.

The findings show that sequential denitrogenation and liquefaction provides a feasible way to produce high‑quality aromatic oils from waste ABS, supporting circular economy goals for plastic waste management. The resulting low‑nitrogen oils can potentially be used directly as fuel or co‑processed in petroleum refineries without extensive hydrotreatment, unlike oils from single‑stage thermochemical processes.

The paper “Sequential Denitrogenation and Liquefaction of Acrylonitrile–Butadiene–Styrene via Two-Stage Hydrothermal Liquefaction Using Homogeneous Catalysts,” is authored by Subramanian Harisankar, Juliano Souza dos Passos, Sofie Klara Gissel Skibsted, Esben Damgaard, Patrick Biller. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.12.037. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.