Modeling and Optimization of a Cracked Naphtha Post-Treater

FCC cracked naphtha is a valuable component of the gasoline pool that, when untreated, is characterized by high olefin and sulfur content. The limitation on sulfur of commercial gasoline imposed by tier 3, has pushed refiners to adapt existing technologies that remove sulfur in cracked naphtha to meet new and more stringent specifications. The simulation software ProMax® was used to model the desulfurization of a cracked naphtha to meet tier 3 specifications while minimizing octane loss caused by olefin saturation. The full boiling range naphtha is sent to a post-treater complex comprising a naphtha splitter, a selective hydrogenation reactor to remove diolefins, a caustic treatment unit to remove light mercaptans, and a hydrotreating unit to selectively remove sulfur while avoiding substantial olefin saturation.

The selective hydrogenation and hydrotreating reactors are modeled as plug flow reactors, with detailed reaction networks that include hydrogenation of olefins/dehydrogenation of paraffins, mercaptan, thiophene and benzothiophenes species desulfurization, and the recombination of olefins with H₂S to form mercaptan species. The reactions in oxidizer reactor of the caustic treatment unit are considered to reach equilibrium between the reacting mercaptans and disulfide products.

Different scenarios were simulated while varying the cut point in the naphtha splitter, solvent flowrate and caustic concentration in the caustic treatment unit, and reactor temperature, hydrogen to hydrocarbon ratio in the hydrotreater unit to study their effect on sulfur removal, hydrogen consumption, and research octane number of the product. Optimum conditions were selected for a maximum octane barrel of final product that meets a cracked naphtha sulfur specification between 10-30 ppm.