Cleaning of lubricating products from machinery parts using subcritical water

Lubricating products composed of hydrocarbon mixtures modified with various thickeners and other additives have been widely used in a large number of machine elements. Cleaning of used lubricants is critical for ensuring the proper machine operations. Conventional cleaning processes, however, involve very toxic, hazardous, and/or persistent organic or harsh aqueous solvents. In this study, an alternative cleaning method was developed using subcritical water. Lubricating oily materials were effectively removed by both liquid and vapor subcritical water extractions at a relatively low operating temperature (∼150°C). The cleaning efficiency was mainly dependent on the physical properties of lubricants. The operating temperature also had a significant impact on the cleaning efficiency since the solubility of subcritical water for the base oil was greatly enhanced due to the reduction of its dielectric constant (€ ∼43 at an operating temperature of 150°C). Upon the results of this study the subcritical water cleaning system demonstrated a potential as an alternative of conventional technologies for the clean-up of lubricants from machinery parts.