(122g) Options for Reducing Benzene in the Refinery Gasoline Pool

Authors: 
Bundick, B., Mustang Engineers and Constructors, LP


US refiners are in the process of planning and executing capital projects to comply with the new MSAT (Mobile Source Air Toxics) II regulations that become effective January 1, 2011. These new rules will restrict the annual average benzene level in the gasoline sold in U.S. except California to 0.62 vol%. California has similar restrictions on gasoline benzene content. Of the various refinery streams that are blended into gasoline 70 ? 85% of the benzene is contributed by reformate from catalytic reforming and 10-25% by FCC gasoline. Most MSAT II compliance strategies focus on reducing benzene in reformate. The benzene content in reformate can be changed by either removing compounds in the reformer feed that form benzene in the reforming reaction or by removing benzene from reformate by hydrotreating or solvent extraction. Removal of benzene from FCC gasoline is less straight forward. The relationship of feed properties and reaction process conditions to the production of various compounds in a FCC unit is complex and thus does not present a straight forward solution for benzene control. Further, FCC gasoline contains olefins and heavier aromatics that are the major contributors to the octane of this stream. Any hydroprocessing route focused on benzene reduction would also saturate a significant portion of these compounds.

There are several options for reducing the benzene content in gasoline:

? Reduce benzene precursors in catalytic reformer feed via fractionation.

? Saturate benzene contained in light straight run and/or light hydrocrackate.

? Install a reformate splitter to produce a benzene rich stream followed by hydroprocessing to remove benzene.

? Remove benzene from reformate with solvent extraction.

? Purchase benzene credits from other refineries. The maximum average benzene content must still be below 1.3 vol%.

The magnitude of the benzene reduction achievable with the above options is presented in a refinery case study. An optimization study to establish the design parameters for a new naphtha fractionator is also presented. This study considers the sensitivity of the column design to reflux ratio and light and heavy naphtha qualities. The tower size is then optimized based on capital and operating costs.

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