Distillation Section

The purpose of distillation section (see Figure 3-4) is to remove H2S and light ends from the first-stage reactor effluent and fractionate the remaining effluent into naphtha, kerosene, and diesel cuts. The bottom stream is either fed to the second stage of the hydrocracker, recycled to extinction with the fresh feed, or withdrawn as product.

Hydrocarbon liquid flows to H2S stripper V-ll from stripper preflash drum V-10. The preflash drum removes some of the light ends and H2S from the low-pressure separator oil before it is stripped and fractionated. The stripper column contains packed sections below the feed plate and two sieve trays above the feed inlet. Stripping is achieved with steam, lH0

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Figure 3-4. Distillate hydrocracker (distillation section).

which removes H2S and light ends. The stripper overhead vapor is partially condensed in air cooler E-07 and a trim water cooler then flashed in reflux drum V-12. The sour gas from the reflux drum is sent to a low-pressure amine contactor. The condensed hydrocarbon liquid is refluxed back to the stripper. H2S stripper bottoms are sent to product fractionator V-13 after heat exchange with diesel in E-16, circulating reflux in E-18, and fractionator bottoms in E-18. The fractionator feed is then brought to column temperature by heating in the feed heater H-02. After heating, the partially vaporized fractionator feed is introduced into the flash zone of product fractionator V-12. In the flash zone, the vapor and liquid separate. The vapor passes up through the rectifying section, containing approximately 27 trays.

Heat is removed from the fractionator column in the overhead condenser and in a circulating reflux heat removal system. Vapor leaving the top tray of the column is condensed in overhead condensers. The condensed overhead vapor is separated into hydrocarbon and water phases. Part of the hydrocarbon is recovered as overhead product, and the rest is sent back to the column as reflux to ensure good separation.

The portion of the column below the flash zone contains five trays. Superheated steam is injected below the bottom tray. As the steam passes up through the stripping section, it strips light components from the residual liquid from the flash zone.

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