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ABSTRACT
Mass spectrometry (MS) was used to study the compositional changes taking place during the hydrocracking process and the effect of composition on viscosity index (VI) of base stocks. The determination of compound types by MS required a separation of the petroleum samples into aromatic and nonaromatic fractions. After the hydrotreating of the feed, an increase in 1–ring naphthenes was observed. The value of an average ring number per mole did not change indicating the absence of ring–opening reactions. After the hydrocracking step, the MS analysis of the nonaromatic fraction showed a significant decrease in 1–6 ring naphthenes. A decrease in average ring number from 0.4 to 0.2 per mole indicated that the ring-opening reactions were taking place. The MS analysis of different hydrocracked base stocks, having a VI in the range from 143 to 148, showed no significant effect of paraffins/isoparaffins, 1-ring naphthenes and the condensed naphthenes on VI. The hydrocracked base stocks, having a VI of 143–144, were found to have an average ring number of 0.3 per mole. The hydrocracked base stocks, having a higher VI of 146–148, were found to have a lower average ring number of 0.2 per mole. The MS results indicated that the severity of hydrocracking and the extension of the ring-opening reactions were better described by an average ring number per mole rather than the contents of different chemistry hydrocarbons.