Figure 5.11 Methyl acetate flowsheet.
A guess for the reflux ratio of 2 is made, and the simulation is run. The resulting product compositions are 2.475 x 10"3 mol% H2O and 61.98 mol% MeAc in the distillate and 3.356 x 10"7 mol% MeAc in the bottoms. To drive the bottoms methyl acetate composition to the specified 0.1 mol%, we set up a Design Spec/Vary manipulating distillate flowrate and click the N button. The column does not converge after the default 25 iterations. Clicking Convergence under the C1 block lets us change the number of iterations to 50 and try again. The column still does not converge.
The problem is that we have been using the standard method of convergence. This must be changed to either Azeotropic or Strongly nonideal liquid to achieve convergence in this nonideal system. This is done on the Configuration page tab under Setup, as shown in Figure 5.12.
It is sometimes necessary to switch back and forth between Azeotropic and Strongly nonideal liquid to achieve convergence. It is also sometimes necessary to temporarily Hide one or both of the Design Spec/Vary functions to achieve convergence. This is done by right-clicking on the individual Design Spec and selecting Hide. The same procedure is followed for the corresponding Vary. After the simulation has converged without the Design Spec and Vary active, they can be reactivated by right-clicking Design Spec or Vary and selecting Reveal.
The new distillate flowrate to achieve the bottoms methyl acetate of 0.1 mol% is 0.04424 kmol/s. The water composition of the distillate is now 0.565 mol% instead of the desired 0.1 mol%. A second Design Spec/Vary is set up manipulating the reflux ratio. The simulation is run, and the required reflux ratio is 4.833.
Now the effect of the feed tray location is explored to find the optimum where reboiler heat input is at a minimum. However, we also need to keep track of what is happening to the methyl acetate concentration in the distillate. Figure 5.13 gives results for varying the feed stage number. Unlike the ideal system explored in Chapter 3, energy consumption and product compositions are very sensitive to feed tray location. The feed stage that minimizes reboiler heat input is stage 27, which is very near the bottom of the column. However, the methyl acetate composition in the distillate drops drastically for feed stages greater than stage 26. Therefore a design tradeoff must be made between the cost of energy in this column and the cost of feeding a lower-purity methyl acetate stream
Figure 5.13 Effect of feed stage; 32 total stages.
to the downstream unit. This is a good example of the typical multiunit tradeoffs that occur in complex chemical plants.
This also illustrates that the simple heuristics cannot always be applied in complex nonideal systems. The basic complexity in this example is that we have three design specifications instead of the normal two. Therefore another degree of freedom is required, which is the feed tray location.
The next step is to see the effect of using different numbers of total stages. Figure 5.14 gives results for four different total stages: 17, 22, 27, and 32. The breakpoints where the distillate methyl acetate purity drops precipitously in the four cases are stages 12, 17, 21, and 26, respectively. At these four points, the reboiler heat inputs are 6.611, 3.318, 3.199, and 4.452 MW, respectively. So adding more stages reduces energy consumption up to about 27 stages and then it actually increases. This indicates that either the 22-stage design or the 27-stage design is the economic optimum.
Evaluating the total annual cost of these four designs shows that the 27-stage design gives the lowest TAC. Table 5.1 gives details of the design and economic parameters.
It is interesting to check the rigorous simulation developed above with the ternary analysis using the graphical column design method in DISTIL. Figure 5.15a gives the Spec Entry page tab with the feed composition and thermal condition (q = 1) given. The reflux ratio is specified to be 1, which would correspond to that found in the optimum 27-stage design. Figure 5.15b gives the bottoms specification (0.001 mf MeAc). Figure 5.15c gives the two specifications made for the distillate (0.001 mf water and 0.63 mf MeAc). Note that the latter is close to the azeotropic composition.
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