Finding The Optimum Feed Tray And Minimum Conditions

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Now that the pressure has been determined and the product specifications attained, we need to go back and find the "optimum" feed tray. In addition, the minimum reflux ratio and the minimum number of trays can be determined. These will be useful for heuristic optimization, which is discussed in detail in Chapter 4.

3.7.1 Optimum Feed Tray

In most distillation columns, the major operating cost is reboiler energy consumption. Of course, if refrigeration were used in the condenser, this heat removal expense would also be quite large. For our propane/isobutane example, the pressure was deliberately set so that cooling water could be used in the condenser. Therefore, reboiler heat input is the quantity that should be minimized.

The simulation is run using different feed stages. The purities of both products are held constant. The feed stage that minimizes reboiler heat input is the optimum. Table 3.1 gives the results of these calculations. Feeding on stage 14 gives the minimum energy consumption.

3.7.2 Minimum Reflux Ratio

The simulation can be used to find the minimum reflux ratio by increasing the number of stages until there is no further reduction in the reflux ratio. Product purities are held

TABLE 3.1 Effect of Feed Stage on Reboiler Heat Input

Feed Stage

Reboiler Heat

Condenser Heat

Reflux

Number

Input (MW)

Removal (MW)

Ratio

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