Part 9: ISOMERIZATION

Overview

Dana K. Sullivan

UOP LLC
Des Plaines, Illinois

The introduction of reformulated gasoline with mandated limits on benzene content has caused many refiners to take steps to reduce the benzene in their gasoline products. The major source of benzene in most refineries is the catalytic reformer. Reformate typically contributes 50 to 75 percent of the benzene in the gasoline pool.

The two basic approaches to benzene reduction involve prefractionation of the benzene and benzene precursors in a naphtha splitter before reforming, postfractionation in a reformate splitter of the benzene after it is formed, or a combination of the two (Fig. 9.1.1). The benzene-rich stream must then be treated to eliminate the benzene by using extraction, alkylation, isomerization, or saturation (Figs. 9.1.2 and 9.1.3).

Figure 9.1.1: Fractionation for benzene reduction.

Figure 9.1.2: Prefractionation options.

Figure 9.1.3: Postfractionation options.

If the refiner has an available benzene market, the benzene-rich stream can be sent to an extraction unit to produce petrochemical-grade benzene. Alkylation of the benzene may also be an attractive option if propylene is available, as in a fluid catalytic cracking (FCC) refinery. An isomerization unit saturates the benzene and also increases the octane of the stream by isomerizing the paraffins to a higher-octane mixture. Saturation in a stand-alone unit is a simple, low-cost option.

PROCESS DISCUSSION

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