Figure 6.8 BTX product purities for benzene and xylene feed composition changes with fixed reflux and stage temperature.

Figure 6.8 gives results for changes in feed composition when the temperature on stage 2, 8, 12, or 23 is held. The upper graphs indicate changes in the mole fraction of benzene in the feed. The xylene mole fraction is fixed at 0.4, so the toluene composition in the feed changes inversely with the benzene composition. For this type of disturbance, holding stage 23 constant produces less variability in both product purities.

The lower graphs in Figure 6.8 show that holding stage 23 temperature for changes in the xylene composition of the feed is also effective. However, lowering the xylene in the feed strongly affects distillate purity.

These results illustrate the problems with the invariant-temperature criteria. Effectiveness is strongly dependent on which components in the feed change.

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