Let us consider a numerical case in which there are several design objectives:
1. The concentration of the methyl acetate in the distillate should be fairly close to the azeotropic composition because this stream is sent for further processing. It may be sent to another distillation column operating at a higher pressure that shifts
the azeotropic composition enough to permit the production of methyl acetate out the bottom and the high-pressure azeotrope out the top of this second column. In other plants the stream may be fed to a reactor where it reacts with water to form acetic acid and methanol. In any event, we want as high a concentration of methyl acetate as possible in the distillate.
2. The concentration of water in the distillate should be 0.1 mol%.
3. The concentration of methyl acetate in the bottoms should be 0.1 mol%.
The feed flowrate is 0.1 kmol/s, and the design feed composition is 30 mol% methyl acetate (MeAc), 50 mol% methanol (MeOH), and 20 mol% water. Column pressure is initially set at 1.1 atm, but we will check the reflux drum temperature to make sure that cooling water can be used in the condenser.
For a preliminary design, a column with 32 stages is selected. We will vary the feed tray location to minimize reboiler heat input. Initially stage 16 in the middle of the column is specified. Figure 5.11 shows the flowsheet with valves and pumps installed.
Because of the methyl acetate azeotrope, the distillate cannot be richer in methyl acetate than 66.4 mol%. To guess an initial distillate flowrate, let us assume that the distillate contains all the methyl acetate and that its composition is 62 mol% MeAc. Therefore the distillate flowrate is calculated from a methyl acetate balance:
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