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Generals Estimates.

Figure 8,28 Composition estimates.

1. The first maintains the composition of TAME in the bottoms at 99.14 mol% by varying the bottoms flowrate.

2. The second maintains the composition of TAME in the distillate at 1 x 10 7 mole fraction by varying the reflux ratio.

Figure 8.29 shows that the two Operating Specifications are bottoms rate and reflux ratio. Figure 8.30 illustrates setting up one of the Design Spec functions using the three page tabs (Specifications, Components, and Feed/Product Streams). Figure 8.31 shows the selection of the reflux ratio as the Vary.

Effect of Pressure. The base case pressure is 4 bar. Table 8.3 gives results for TAME production, reactant losses, and energy consumption over a very narrow range of pressures. These results clearly show the extremely high sensitivity to pressure. A small reduction in pressure from 4 to 3.8 bar produces a very significant reduction (27%) in energy consumption (36.82 to 26.94 MW). The amount of TAME produced increases by 3% from (234.2)(0.9914) = 232.2 kmol/h in the base case to (242.2)(0.9914) = 240.1 kmol/h when the column pressure is 3.85 bar. The increase in TAME comes from the decrease in the losses of reactants in the distillate. The total of the two reactants losses drops from 17.53 to 9.71 kmol/h.

Further decreases in pressure below 3.8 bar begin to increase energy consumption and reactant losses in the distillate. Increasing pressure from the base case produces very rapid increases in energy consumption and reactant losses, and TAME production drops sharply. Figure 8.32 provides the temperature profiles at two different pressures, and Figure 8.33 shows composition profiles. Note the large changes in the position of the methanol and reactant profiles over the small range of pressures.

Figure 8.29 Manipulated variables B and RR.

The TAME reactive column is very sensitive to pressure. This example illustrates that reactive distillation columns are much more affected by pressure than are conventional columns.

Effect of Number of Reactive Trays. In the base case, stages 7-23 contain catalyst, so the reactive zone has 17 trays. Reactor effluent is fed on stage 28 and methanol on stage 23 (the lowest reactive stage). There are 6 rectifying trays and 11 stripping trays.

Table 8.4 gives results over a range of values of the number of reactive trays. The numbers of stripping and rectifying trays are held constant, as are the pressure and the design specifications on the concentration of TAME in the bottoms and distillate.

Adding more reactive trays produces a small decrease in energy consumption, and reducing the number from the base case of 17 results in increases in energy consumption. If only 10 reactive stages are used, energy consumption increases quite significantly.

Note that the effects of reactive stages on the losses of the two reactants are different. The 1M2B reactant losses decrease as fewer reactive trays are used until NRX drops to 10.

Figure 8.30 Design spec on TAME in distillate.
Figure 8.30 (Continued)

However, the 1M1B reactant losses increase as NRX is reduced over the whole range of tray numbers.

These results show that the TAME reactive distillation column is fairly insensitive to the number of reactive stages.

TABLE 8.3 Effect of Pressure

Pressure (bar)

Reboiler Heat Input (MW)

Bottoms (kmol/h)

1M2B Losses in Distillate (kmol/h)

1M1B Losses in Distillate (kmol/h)

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