Structural Evolution of Coal Tar Wash Oils in Industrial Benzene Recovery: An Integrated Rheological–Molecular Approach

Coal tar wash oil degradation during industrial benzene recovery alters both the absorbent’s performance and the liquid-phase rheology. This study introduces an integrated rheological and quantum-chemical framework to evaluate the non-ideal flow behavior of light and heavy wash oils (fresh and operational) diluted with 0–30 vol.% benzene. Compositional changes were monitored via GC–MS, while viscosity non-ideality was quantified using relative viscosity deviations (η%) and characteristic empirical parameters (φ1max). Quantum-chemical calculations revealed stronger interactions between benzene and the polycyclic aromatic and heterocyclic compounds enriched in heavy operating oil, consistent with a lower (φ1max) for heavy oil (0.23) compared to light oil (0.34–0.37). This multi-method approach provides a sensitive, phenomenological diagnostic probe for monitoring absorbent degradation, supporting operational decisions regarding wash oil regeneration or replacement in industrial scrubbing units.