Clean fuel's impact on refinery distillation

The Clean Air Act Amendments of 1990 have prompted U.S, refiners to install facilities to comply with new specifications for gasoline and diesel fuel. Additionally, California refiners have had to comply with the California Air Resources Board (CARE) requirements, which are even stricter than those imposed elsewhere in the United States. This legislation has resulted in the planning, design and construction of new refining facilities along with modifications of existing process units. The new specifications require significant reductions in sulfur, olefins, distillation points, vapor pressure (RVP), aromatics and benzene. Refinery distillation operations have been impacted as a direct result of the new specifications. For example, the reduction of benzene via prefractionation of naphtha reformer feed (to remove precursors cyclohexane and methyl cyclopentane) has been chosen by many refiners as the preferable option. This option involves more than a simple cutpoint shift since product economics drive the volatile C7s (e.g., 2,3-dimethylpentane, 2-methylhexane, 3-methylhexane) into reformer feed (reformate value) versus isomerization feed (fuel value as a result of cracking), requiring more reflux and/or more stages of fractionation. In one installation a 70-tray splitter was justified on this basis. Other similar examples are: for olefin and RVP reduction, C5 olefins fractionated out of FCC gasoline are sent to the alkylation unit: the tail end of hydrocracker heavy naphtha is fractionated out and sent to jet or diesel blending for T90 cutpoint reduction; preparation of a heart cut feed to the reformer or a bottoming column added downstream of the reformer are also options for T90 reduction. In some cases, existing columns required extensive modifications, while in others new equipment was installed; the choice depended on each refiner's specific situation, differences in refinery configurations and individual technology preferences. This paper examines the cumulative impact of the new legislation on various refinery distillation operations. Specific emphasis is placed on examining column configuration options, column modifications and trade-offs involved in product recovery economics.