Unravelling the relation between processed crude oils and the composition of spent caustic effluents as well as the respective economic impact

Spent caustic discharges are responsible for increasing oil and grease (O&G) matter in refineries wastewater, leading to increasing treatment costs due to low water quality and environmental constraints associated with high O&G concentration discharges. As a way to settle and optimize treatment technologies for such complex effluents, more insight regarding the effluents impact and deeper characterization is necessary. The present study intends to assess the possibility of a relationship between the processed crude oils with the polar O&G concentration in naphthenic spent caustic as well as in the final wastewater; Sines refinery was considered as case-study. Also, in order to get insights about the nature of the polar O&G compounds, their structures and their prevalence in the effluent treatment system was carried out through detailed analytical characterization studies. Proton nuclear magnetic resonance (¹H NMR), Fourier transform infrared spectroscopy (FT-IR) and gas chromatography-mass spectrometry (GC-MS) were chosen. It was found that, for the Sines refinery, spent caustic discharges may increase the refinery effluent management cost up to 3 €/ton of processed crude oil, every time a high kerosene cut acid crude oil is processed. It was also found that the typical spent caustic O&G effluents are composed by organic contaminants with low molecular weight (MW), with aromatic and polar arrangements, like phenolic groups and naphthenic acids. This outcome is crucial for subsequently establishing the best technologies able to deal with such complex effluents.