Monitoring the behaviour and fate of nickel and vanadium complexes during vacuum residue hydrotreatment and fraction separation

The size distribution of nickel and vanadium complexes was monitored by Size Exclusion Chromatography separation (SEC) and Inductively Coupled Plasma Mass Spectrometry (ICP/MS) detection, at different steps of a vacuum residue fractionation and hydrotreatment. Sample preparation includes asphaltene/maltene separation, and asphaltene fractionation by cross-flow ultrafiltration. The asphaltene fractionation gives asphaltene fractions enriched in low and high molecular weight compounds. The impact of asphalteneʹs size on hydrotreatment reactivity was investigated. Finally, a vacuum residue was reconstituted in order to monitor and control the fate of nickel and vanadium complexes through hydrotreatment. Results show that metals in the asphaltene fraction are associated with molecules that present a complex continuum of polydispersed compounds, with a majority of metal complexes found at 15 kDa polystyrene equivalent. Nickel is generally present in higher molecular weight compounds than vanadium. The size of the metal compounds impacts significantly their reactivity during hydrotreatment; metals present in high molecular weight compounds are the most refractory to conversion. Only the light metal complexes were easily converted during hydrotreatment.