Figure 8.13 Specifying feedstream.
end of the column, using the reflux ratio specified. One trajectory of liquid compositions from tray to tray moves up the stripping section, and the other moves down the rectifying section. If the two trajectories intersect, the design is feasible.
As Figure 8.18 demonstrates, the compositions that were selected cannot be attained. The problem is clearly shown in the ternary diagram. The bottoms and feed points are in the lower region, but the distillate is in the top region. This separation is infeasible.
The distillate composition must be specified to be in the lower region where the feed and bottoms are located. Figure 8.19 reveals that the distillate point is still in the upper region when the specified distillate composition is increased to 1 mol% TAME. Figure 8.20 gives the results when the specified distillate composition is increased to 7 mol% TAME and the
Overall Component Balance Line Figure 8.18 Ternary diagram.
bottoms composition is changed to 5 mol% methanol. Now the distillate point is just inside the lower region and very near the distillation boundary. With a reflux ratio of 5, the number of stages required is six.
The picture obtained from the ternary diagram clearly shows that a distillate that contains very little TAME cannot be attained with the specified feed composition. Suppose the feed contains less methanol. The feed point moves down in the diagram, which also moves the distillate point down. The distillation boundary is closer to the zero TAME hypotenuse of the triangle. Thus, a distillate with a lower composition of TAME can be attained. In Figure 8.21 the feed composition is changed from 30 to 20 mol% methanol, with an appropriate change in the composition of iC5. The distillate composition is specified to be 2.2 mol% TAME and the bottoms 0.1 mol% iC5 and 1 mol% methanol. A feasible design is obtained, which requires 14.3 stages. Figure 8.22 shows the same diagram with the residue curve added. These are obtained by clicking the third button on the right-hand side of the window, locating the cursor somewhere on the diagram and clicking. A residue curve is drawn through this point.
Keep in mind that the analysis applies to a nonreactive column that is simply separating a ternary mixture. However, the example indicates that adding an excess of methanol in the reactive column may result in the loss of TAME in the overhead. The TAME reactive distillation column is therefore operated in a "pseudoneat" mode. Enough methanol must be added for the reaction plus that required for the C5/methanol azeotropes. The numerical example given in the next section illustrates this point.
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