Composition and polymerisation products influence on the viscosity of coal tar wash oil

Abstract Wash oil is a fraction obtained by the distillation of coal tar and is primarily used for the absorption of light oil from coke oven gas. During operation, the oil undergoes polymerization and loses some components, necessitating the removal of the used oil and its replacement with fresh wash oil. The rheological properties of the studied oils were determined using a Brookfield DV2T rotational controlled-shear rate rheometer. Gas chromatographic analysis, gas chromatography/mass spectrometry, and IR spectroscopy were employed to evaluate the oil’s characteristics. By adding individual components to the wash oil, it was found that these components could be categorized into three groups: viscosity enhancers, viscosity reducers, and non-polar oil substances with medium molecular weight that have no significant effect on viscosity. The addition of relatively polar components with greater molecular weight and a planar, rigid structure leads to an increase in viscosity. Conversely, the incorporation of naphthalene and its methyl homologues, which have lower molecular weights, reduces viscosity by disrupting intermolecular interactions; their lower heats of crystallization also inhibit the formation of structured order in the liquid, further contributing to the reduction in viscosity. The most substantial increase in oil viscosity was observed with the addition of indene-coumarone resins, attributed to their high molecular weight and polymer structure.