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Figure 6.22 TAC comparison for the two-column and one-column flowsheets.

6.3.7 Location of Feed Tray

The lower right graph in Figure 6.19 shows that there is an optimal feed tray location. Figure 6.21 shows the effect of feed tray locations on temperature and composition profiles. This optimum location is the top of the reactive section. Above this point, the effect of the rectifying section to keep the reactant in the reactive sections is lessened, and below this point there is an effect similar to reducing the number of reactive trays. Either position will increase the vapor boilup.

6.3.8 Comparison Between These Two Flowsheets

After investigating two possible flowsheets, we will be interested in choosing the appropriate one for a given process. It turns out that the chemical equilibrium constant plays a critical role for selecting the better one, as described in Figure 6.22. The TAC of the one-column flowsheet decreases dramatically into the feasible region. When the chemical equilibrium constant is >0.70, the one-column configuration prevails. On the contrary, if the chemical equilibrium constant is small, the two-column configuration is more economical. When the equilibrium constant is reduced further, past the limit of feasibility, the two-column configuration is the only choice.

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