Valorization of refinery sludges by hydrothermal carbonization towards circular economy

Despite continuous efforts by the petroleum industry to address societal needs, the effective management of potentially hazardous wastes, such as refinery sludge, remains a significant challenge. Refinery sludge is a stable emulsion comprising water, petroleum hydrocarbons (e.g., oils, fats, organic compounds), solids, and metals. Hydrothermal carbonization (HTC) is a promising and environmentally friendly approach for valorizing oily sludge from refineries, given its adaptability and high efficiency in processing wet or high-moisture-content organic wastes. This study aims to systematically evaluate the effects of three key factors-initial sludge composition (through the mixing of three distinct real oily sludge streams from refinery), HTC temperature, and reaction time-on the properties of the resulting hydrochar, aqueous phase and oily phase. This scope was reached through a full factorial design of experiments and a deepened characterization of the as-received petrochemical samples and the separated HTC products. The study resulted in predictive models for 32 response variables associated with the HTC process using real refinery sludge mixtures. These variables include the HTC product distribution, hydrochar yield, higher heating value (HHV) of produced hydrochar, and the characteristics of the process water, among others. The developed models provide a reliable framework for optimizing HTC outputs and forecasting quality variations in response to changes in the operating conditions studied.