Separation And Molecular Characterization Of Heavy Petroleum Fractions By Medium Pressure Liquid Chromatography And Mass Spectrometry

With the crude oil resources dwindling and their quality deteriorating,efficiently utilization heavy oil has become an important field of research in modern oil refining.Analytical technology to study the molecular composition of heavy oil and knowledge of the refinery reaction mechanism in molecular level is especially in great need.Component separation is an important method to evaluate the chemical composition of heavy oil,and it is also an important pretreatment method for molecular composition analysis.In this paper,a separation method of heavy oil was developed based on medium pressure liquid chromatography,and the molecular composition of nitrogen-containing compounds in the separated components was analyzed by ion mobility time-of-flight mass spectrometry,and the transformation law of nitrogen compounds in the process of hydrodenitrogenation was studied.A multi-fraction separation method of heavy oil was developed based on medium pressure liquid chromatography.Using alumina chromatographic column and gradient elution with n-heptane,toluene and ethanol,the VGO was separated into seven fractions including saturates,light aromatics,medium aromatics,heavy aromatics,light resin,medium resin and heavy resin.The separation conditions,repeatability and applicability of this method were investigated.The hydrocarbon composition of saturates and three aromatics was analyzed by gas chromatography-mass spectrometry(GC-MS),and the structure of each fraction was studied by ~1H nuclear magnetic resonance spectrometry(~1H-NMR)using improved Brown-Ladner method(B-L method).The content of sulfur and nitrogen elements in each fraction was analyzed,and the distribution of nitrogen compounds in each fraction was analyzed by time-of-flight mass spectrometry with ion mobility resolution(IMS-TOF-MS).The multi-fraction separation method realizes further separation and enrichment of different classes of aromatic compounds.About 70%of the light aromatics fraction is mono-aromatics,50%of the medium aromatics is bi-aromatics,and 70%of the heavy aromatics is poly-aromatics with 3 or more benzene rings.The method utilizes automated medium-pressure liquid chromatography for gradient elution,achieving higher separation efficiency,more general sample applicability and better repeatability than traditional fraction separation methods.In addition,a method to characterize basic and neutral nitrogen compounds in VGO by IMS-TOF MS was developed,and the method was applied on traditional four fractions(SARA).The basic nitrogen compounds are mostly concentrated in the aromatics,while the non-basic nitrogen compounds such as benzocarbazole and dibenzocarbazole are concentrated in the resin.The DBE of basic nitrogen compounds in resin is lower than that of aromatics,and the DBE of non-basic nitrogen compounds in resin is higher than that of aromatics.After the multi-fraction separation,the basic nitrogen compounds are mainly concentrated in the heavy aromatics,of which the carbon number gradually increases and the DBE decreases from the light aromatics to the medium resin;Benzocarbazole and dibenzocarbazole compounds in neutral nitrogen are enriched in heavy resin,with the carbon number gradually decreases and DBE increase from light aromatics to medium resin.The basic and neutral nitrogen in Daqing vacuum gas oil(VGO)and its hytrotreated products are characterized by electrospray ionization IMS-TOF MS and the conversion mechanism of nitrogen compound in the process of hydrodenitrogenation was discussed.The results of element analysis showed that the content of neutral nitrogen compound was higher than that of basic nitrogen compound before hydrotreating,which is reversed after mild denitrogenation,and gradually became even after further denitrogenation.The results of mass spectrometry analysis demonstrated that in untreated VGO,benzocarbazole and dibenzocarbazole classes were dominant in the neutral nitrogen compounds,and pyridine classes were dominant in the basic nitrogen compounds.After mild hydrodenitrogenation,The content of neutral nitrogen compounds decreased and cycloalkylamines were observed,which was hardly detected after deep hydrodenitrogenation,This result indicated that the neutral nitrogen compounds were firstly saturated to form cycloalkylamines,then removed by further hydrodenitrogenation.For both neutral and basic nitrogen compounds,it was observed that nitrogen compounds with higher DBE and shorter/less alkyl side chains are easier to remove by hydrotreating.