(44d) Impact of Vaporization On Catalyst Deactivation: Part I - Low Pressure ULSD Production

To comply with the recent implementation of Ultra Low Sulfur Diesel (ULSD, Product S < 10 wppm) requirements in North America and Europe, refiners have opted to either revamp existing diesel hydrotreaters, build grass-roots units, or modify unit feed properties. Many older, lower pressure (500 ? 850 psig) units were revamped by adding reactor volume in order to increase residence time and facilitate the conversion of the refractory feed sulfur molecules. Grass-roots ULSD units tend to be designed for much higher operating pressure (1100+ psig). In the past, typical operating condition requirements for hydrotreating diesel streams yielded satisfactory run lengths and basic bulk properties were sufficient to predict the performance of the catalyst. Commercial run lengths are shorter for ULSD production because the operating conditions required for ULSD are more severe. It has been found in commercial units that some combinations of feed properties and operating conditions lead to unexpected accelerated deactivation rates. These rates cannot be predicted simply by the traditional monitoring parameters and feed bulk properties used in the refining industry and are not typically due to catalyst poisons. The conditions and mechanism for the accelerated deactivation rates have been identified by observation and evaluation of different commercial operations. Operations where co-processing of coker naphtha, heavy cat naphtha and kerosene at low operating pressure are especially prone to accelerated deactivation even though the desulfurization of the sulfur components in these lighter streams requires less energy. In ULSD operations accelerated deactivation rates are attributed to excessive vaporization of the hydrocarbon feed at high temperatures (end-of-cycle) leading to operation outside of the trickle bed regime and reduced hydrogen partial pressure. Operating outside the trickle bed regime adversely affects physical parameters such as catalyst wetting and liquid distribution. High extents of vaporization (> 80 wt%) result in reduced hydrogen partial pressure and long residence times for heavy aromatic molecules. These factors lead to an operating environment conducive for coke formation and catalyst deactivation. The effects of vaporization due to chemical properties of its feed components and physical phenomena in the reactor at high operating temperature of a specific commercial low pressure (~ 500 psig) ULSD unit causing the acceleration of catalyst deactivation at end-of-cycle are discussed.