Characterisation of oil sludges from different sources before treatment: High-field nuclear magnetic resonance (NMR) in the determination of oil and water content

Abstract

Oil sludges are wastes from the oil industry containing complex mixtures of oil hydrocarbons, water, sediments, and heavy metals. These wastes are of main importance for the petroleum industry since it is estimated that more than a billion tons have been stored worldwide. There are several types of oil sludges, but the oil tank bottom sludges are the most studied. Therefore, it is necessary to analyse a variety of oil sludges to establish a detail characterisation, with fast and reliable methods before selecting a suitable treatment. Five sludges were analysed in this study: an oil drilling, oil refinery, oil-water separator, and two waste engine oil sludges. This is a rare report detailing the use of high-field (500 MHz) nuclear magnetic resonance (NMR) to determine oil and water contents in oil sludges, which are more commonly analysed by low-field NMR (below 100 MHz). The proposed NMR procedure was validated by the analysis of oil-water calibration standards (experimental errors < 15%). There was a good agreement among the data obtained from the 1D proton spectra and the Carr-Purcell-Meiboom-Gill (CPMG) T2 decays (percentage differences < 5%). The T2 decays data showed that all sludges had only two components attributed to oil and water, and their relative ratios were determined. The sediment content was determined by the oven-drying method, and the oil hydrocarbon fractions and trace elements were assessed. The oil sludges presented different ranges in the oil, water, sediment and heavy metals contents. The higher presence of C10-C18 aliphatic fractions in the oil from the sludges was an indicator of the potential to be reused as diesel fuel. The sludges had potentially toxic elements (PTEs) values under the limit of the landfilling standards established by the European Union, except for the WSS sludge. Ca and Fe had the highest concentrations in all sludges, which are characteristic of these wastes. The techniques used in this study can be an alternative for a rapid characterisation of the oil sludges, so their most appropriate treatment can be established.