(A) In the control system finally chosen, the toluene impurity content in the distillate product is controlled by the reflux ratio. (B) The five alternative sidestream tray positions and their controls, which regulate the benzene and xylene impurities in the sidestream drawoff, are shown in this blowup.

FIGURE 7.7 D-scheme
FIGURE 7.8 L-scheme

intermediate boiler, and (2) to remove essentially all of the high boiler out of the bottom, together with the remainder of the intermediate boiler. The two product streams from the prefractionator are fed to two different trays of the sidestream drawoff column. The intermediate product is then withdrawn from a tray or trays located between the two feed trays. Thus the heaviest and lightest components are detoured around the section of the column where the sidestream is withdrawn, and "pinches" are avoided.

Figure 7.7 has been designated by Doukas and Luyben as the "L" scheme. Sidestream drawoff location is used to control the concentration of the lightest component in the sidestream product. The "D" scheme of Figure 7.8 provides manipulation of the distillate product flow from the prefractionator to control sidestream composition.

Both of these schemes were shown to provide effective control for modest changes in feed composition. Since the D scheme is probably easier to implement, it is recommended for most systems.

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