QUANTITATIVE-ANALYSIS OF MIDDLE DISTILLAT S BY GC MS COUPLING - APPLICATION TO HYDROTREATMENT PROCESS MECHANISMS/

A detailed analysis of middle distillates is essential for understandi ng the reaction mechanism and for studying the kinetics of refining pr ocesses such hydrotreatment. In fact, when we see the complexity of sa turated and aromatic hydrocarbon mixtures appearing in gas oil, we rea lize that it's necessary to have a very detailed analysis of those cut s to understand the mechanisms involved in refining processes and to b e able to describe their kinetics. Each gas oil has a very different c omposition and therefore a specific reactivity. That is why we have tr ied to develop predictive kinetic models to avoid experimenting in pil ot plants, which is very expensive. But, even if all the compounds of a gasoline (PI-200 degrees 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 deve loped and also standardized for some of them. These methods quantify c hemical families in middle distillates and determine the detailed comp osition of saturated hydrocarbons (alkanes, cycloalkanes with one or m ore rings), aromatic hydrocarbons (monoaromatics = CnH2n-6, CnH2n-8, C nH2n-10; diaromatics = CnH2n-12, CnH-2(n-14), CnH2n-16; triaromatics a nd polyaromatics), and sulfur-containing aromatic hydrocarbons. But al l those methods give us only overall compositions. That is why a metho d using GC/MS (electronic impact at 70 eV) has been developed. Compoun ds are separated according to boiling point by gas chromatography in a n apolar column. For each carbon number range (or boiling range), a qu antitative group types analysis method is applied and enables us to de termine the breakdown by chemical families (Fig. 1 and 2). Hence, it i s possible to obtain concentration profiles for each family in terms o f the number of carbon atoms in molecules. This analytical protocol is easy to develop and could be used with all types of spectrometer (qua dripolar or magnetic). This analysis has been used for the study of hy drotreatment reactions and, more precisely, the reactivity of middle d istillate aromatic hydrocarbons. To illustrate this approach, the hydr ogenation of a catalytic cracking middle distillate (LCO) has been fol lowed 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 distributio n curves (weight %), for each family quantified, in terms of the numbe r of carbon atoms, were then plotted. The postulated reaction mechanis ms were verified by checking the evolution of these curves. On the who le, the following succession of reactions can be proposed: Triaromatic s --> Diaromatics --> Monoaromatics --> Saturated Sulfur-containing ar omatics --> Monoaromatics This kind of analysis enabled us to develop a kinetic model of hydrodesulfurization, hydrodenitrogenation and hydr odearomatization reactions (17), which can be used to predict performa nces of LCO gas oil for fixed operating conditions.