Near Infrared Monitoring of Low Conjugated Diolefins Content in Hydrotreated FCC Gasoline Streams

Abstract : Fluid catalytic cracking (FCC) gasolines contain some undesirable compounds like the conjugated diolefins which are highly reactive to polymerization and plug the downstream refining processes. These compounds also affect the commercial gasolines stability. The Selective Hydrogenation Units (SHU) transform the conjugated diolefins into olefins. The conjugated diolefins content is indirectly measured as the "Maleic Anhydrid Value" (MAV), or as the "Diene value" (DV). These methods are based on the Diels-Alder reaction between conjugated diolefins and maleic anhydrid. These analyses are very time consuming (5 to 7 hours per sample) when daily analyses are required to survey the performances of the SHU process. Since Near InfraRed spectroscopy (NIR) is a faster and more repeatable Recent Advances in the Analysis of Catalysts and Petroleum Products Avancées récentes en analyse des catalyseurs et des produits pétroliers technique this work tested the PLS NIR modeling of the conjugated diolefins content according to the MAV method. The diolefins content in these samples is nevertheless very low (< 1 wt%) compared to the other hydrocarbons present in these gasolines, in particular compared to olefins (20–60wt%) which may interfere with the NIR diolefins wavenumbers absorption. In the SHU process, for a given feed, the chemical differences between the reactor inlet and outlet are mainly the diolefins content. Hence, instead of directly correlating the MAV content, we modeled the MAV variation (Delta_MAV) as a function of spectra subtraction between effluents. The NIR model successfully fitted the experimental data from the calibration database. External validation results based on monitoring analyses of SHU pilot plant effluents that confirm the predictive ability of the NIR model are also presented.