Quantitative extraction of nitrogen compounds in oils: Atomic balance and molecular composition

The aim of this study was to get an atomic mass balance on nitrogen compounds in petroleum fractions such as crude oils, distillation cuts, or rock extracts. Two methods have been developed for the separation of basic compounds: a liquid-solid extraction on a commercial acid-bound silica and a new technique by liquid-liquid extraction. Both methods allowed for a selective and quantitative recovery of the basic compounds. For neutral nitrogen species, a method is proposed based on deprotonation by a strong base and liquid-liquid extraction. The efficiency of the three methods was checked on appropriate standards. These techniques were then applied to a crude oil from the Middle East and its distillation cut 375-550 degrees C. All the basic and neutral fractions were submitted to elemental analyses in order to get the total nitrogen balance to be compared to the initial amount present in the crude oil. Results show that a large part of nitrogen is contained in the residue of the oil distillation above 550 degrees C. The recovery of nitrogen as isolated compounds reached 93 wt % in the distillation cut 375-550 degrees C. However, only 41 wt % of the nitrogen content was recovered as basic and neutral species from the crude oil. This must mean that nitrogen structures in the crude oil are found largely as high molecular weight polyfunctional compounds. The major part of the nitrogen compounds isolated from the petroleum samples was found in the neutral fraction. However, the proportion between basic and neutral nitrogen compounds was higher in the crude oil than in the distillation cut, 375-550 degrees C, meaning that bases were preferentially found in the high molecular weight fraction of the crude oil.