Analyse quantitative détaillée des distillats moyens par couplage CG/SM. Application à l'étude des schémas réactionnels du procédé d'hydrotraitement Quantitative Analysis of Middle Distillats by Gc/Ms Coupling. Application to Hydrotreatment Process Mechanisms

Abstract

L'analyse détaillée des distillats moyens est une étape indispensable à la compréhension des mécanismes réactionnels et à la cinétique de certains procédés de raffinage comme l'hydrotraitement. Une nouvelle méthode associant, d'une part un couplage chromatographie en phase gazeuse/spectrométrie de masse (CG/SM) et, d'autre part une analyse quantitative par famille chimique par spectrométrie de masse a été développée. La chromatographie en phase gazeuse, réalisée sur une colonne apolaire, effectue la distillation des composés présents dans le gazole et la spectrométrie de masse quantifie les familles chimiques par intervalle de nombre d'atomes de carbone ou de point d'ébullition. Elle permet d'accéder ainsi à la répartition par nombre d'atomes de carbone de chaque famille chimique (alcanes, cycloalcanes, hydrocarbures aromatiques à un ou plusieurs noyaux, hydrocarbures aromatiques soufrés). Cette méthode a été validée et appliquée à une charge et à une recette d'hydrotraitement. A detailed analysis of middle distillates is essential for understanding the reaction mechanism and for studying the kinetics of refining processes such hydrotreatment. In fact, when we see the complexity of saturated and aromatic hydrocarbon mixtures appearing in gas oil, we realize that it's necessary to have a very detailed analysis of those cuts to understand the mechanisms involved in refining processes and to be able to describe their kinetics. Each gas oil has a very different composition and therefore a specific reactivity. That is why we have tried to develop predictive kinetic models to avoid experimenting in pilot plants, which is very expensive. But, even if all the compounds of a gasoline (PI-200°C) have now been identified and quantified, using gas chromatography (1), such is not the case for heavier cuts. Only an overall characterization can be made, by chemical family. The techniques employed are, for example, HPLC (3,4) or UV spectrometry (5). For a few years now, methods using mass spectrometry have been developed and also standardized for some of them. These methods quantify chemical families in middle distillates and determine the detailed composition of saturated hydrocarbons (alkanes, cycloalkanes with one or more rings), aromatic hydrocarbons (monoaromatics = CnH indice(2n-6), CnH indice(2n-8), CnH indice(2n-10); diaromatics = CnH indice(2n-12), CnH indice(2n-14), CnH indice(2n-16); triaromatics and polyaromatics), and sulfur-containing aromatic hydrocarbons. But all those methods give us only overall compositions. That is why a method using GC/MS (electronic impact at 70 eV) has been developed. Compounds are separated according to boiling point by gas chromatography in an apolar column. For each carbon number range (or boiling range), a quantitative group types analysis method is applied and enables us to determine the breakdown by chemical families (Fig. 1 and 2). Hence, it is possible to obtain concentration profiles for each family in terms of the number of carbon atoms in molecules. This analytical protocol is easy to develop and could be used with all types of spectrometer (quadripolar or magnetic). This analysis has been used for the study of hydrotreatment reactions and, more precisely, the reactivity of middle distillate aromatic hydrocarbons. To illustrate this approach, the hydrogenation of a catalytic cracking middle distillate (LCO) has been followed by this method. It was then possible to see the evolution of the different family concentrations in terms of contact time and to study precisely the hydrogenation of aromatic hydrocarbons. The distribution curves (weight%), foreach family quantified, in terms of the number of carbon atoms, were then plotted. The postulated reaction mechanisms were verified by checking the evolution of these curves. On the whole, the following succession of reactions can be proposed:Triaromatics - Diaromatics - Monoaromatics - SaturatedSulfur-containing aromatics - MonoaromaticsThis kind of analysis enabled us to develop a kinetic model of hydrodesulfurization, hydrodenitrogenation and hydrodearomatization reactions (17), which can be used to predict performances of LCO gas oil for fixed operating conditions.