K

30.51 kmol/h 0.02 MeOH 0.98 Water

Figure 10.15 Liquid sidestream with stripper.

30.51 kmol/h 0.02 MeOH 0.98 Water

Figure 10.15 Liquid sidestream with stripper.

the amount of the heaviest component (water) in the stream that is fed to it. So the main column must be designed so that the water concentration in the liquid at the sidestream drawoff tray is small. In the design shown in Figure 10.15, the concentration of water is only 0.3 mol% at stage 12.

However, the stripper is able to adjust the amount of the lightest component that leaves as its bottoms product B2. Therefore, larger amounts of DME can be present in the liquid at the sidestream tray. This is why the sidestream/stripper configuration can handle larger concentrations of the lightest component in the feed. It is also why this configuration is often economical in systems where the relative volatility between the lightest and intermediate components is not large.

Achieving the steady-state design of this more complex system is not a trivial job. There is a recycle stream and 4 degrees of freedom. The design was successfully achieved by a sequential approach.

1. Initial guesses were made of the flowrates of the distillate, stripper bottoms and sidestream, and the reflux ratio of the main column.

2. A Design Spec/Vary function was set up to achieve a distillate impurity xD1(MeOH) of 1 mol% MeOH by adjusting the distillate D1 flowrate.

3. A second Design Spec/Vary was set up to achieve a main-column bottoms B1 impurity xB1(MeOH) of 0.5 mol% MeOH by adjusting the flowrate of the liquid sidestream Si withdrawn from the main column that is fed to the stripper.

4. A third Design Spec/Vary was set up to achieve a stripper bottoms B2 impurity xB2(DME) of 2 mol% DME by adjusting the ratio of the stripper bottoms to the stripper feed (B2/S1).

The reflux ratio of 5 was held constant during these convergences. This additional degree of freedom could be used to adjust the water concentration in the sidestream fed to the stripper. A reflux ratio of 5 gives 0.3 mol% water in the sidestream, which permits a high-purity methanol product to be produced.

Figure 10.15 gives the steady-state conditions. The energy consumption in the main column is 1.20 MW, while in the stripper it is only 0.0228 MW. The diameter of the main column is 0.635 m, and the diameter of the stripper is 0.12 m.

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