A simple plug-flow model was developed to study the deactivation mechanisms of hydroprocessing catalysts in atmospheric residue desulfurization (ARDS) units. The three stages of catalyst deactivationsat the start of the run, middle of the run, and end of the runsare considered. The catalyst deactivation is mainly due to metal and coke deposition. The model parameters considered are the unit temperature, fluid velocity, reaction rate constant, catalyst combination, catalyst bed length, and feed concentration of sulfur, metals, and asphaltenes. The simulated results compared very well with our laboratory data for a long catalyst life test operated under constanttemperature mode. The model is further applied to a parametric study that examines the effects of space velocity, unit temperature, and maximum metal capacity on the performance of catalyst systems. A constant-sulfur-mode simulation is also given.

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A hydroprocessing multicatalyst deactivation and reactor performance model was originally developed to predict the life and performance of atmospheric residue hydroprocessing catalysts in a pilot-plant long-life test. The model has been subsequently applied to predict catalyst and reactor behavior in an accelerated life test using Boscan crude oil as the feedstock. Simulations from the original model did not compare well with accelerated test run data. Because Boscan crude oil has very high metal and asphaltene contents, mass balance terms that emphasized noncatalytic hydrothermal reactions in the model were needed, especially when catalysts were severely deactivated. Excellent comparisons were obtained after including hydrothermal reaction terms to refine the model. The model was then used to simulate the Boscan crude oil hydroprocessing in constantsulfur-mode operations, and it was found that it would be more beneficial to operate the atmospheric residue desulfurization unit at a lower initial temperature.

Due to environmental constraints, sulfur content of diesel fuel has been restricted to very low levels (500 ppm maximum) in many countries. As a result, a greater emphasis has been placed in recent years on the development of catalysts and processes for deep desulfurization of diesel blending streams to produce low sulfur diesel fuel. In the present work we have compared the performance of a conventional Co-Mo catalyst with that of high metal loading Co-Mo and Ni-Mo catalysts in deep desulfurization of Kuwait atmospheric gas oil. The tests were carried out in a fixed bed reactor unit using 75 ml of catalyst under the conditions: P=32 bar; LHSV = 4h-'; H,Ioil ratio = 100 mVml; temperature range = 330 -390°C. HDS activity of the high molybdenum Co-Mo catalyst was superior to that of the conventional Co-Mo hydrotreating catalyst. High metal loading Co-MolA 1,0, catalyst also showed a substantially higher HDS activity than the Ni-Mo/AJ,O, catalyst containing a similar high metal loading. The unreacted sulfur compounds remaining in the product after high severity hydrotreating were identified as dibenzothiophenes with alkyl substituents next to the sulfur atom. The desulfurization of such low reactive alkyl dibenzothiophenes was found to occur at a substantially lower temperature over the high metal loading Co-Mo catalyst compared with the conventional Co-Mo catalyst. The results have been explained on the basis of the stacking and dispersion of MoS, slabs as well as in terms of the nature of the sulfur vacancies in the MoS, layers in these catalyst systems 1135

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