The final sidestream column configuration studied in this chapter is the case where there is a very small amount of the intermediate component in the feed. The lightest component is the distillate product. The heaviest component is the bottoms product. The intermediate component builds up in the middle of the column, forming a composition "bubble." The objective is to purge off a very small stream from the column at the location where the intermediate composition is fairly large so that there is only a small loss of valuable products. Thus a high-purity sidestream is not required.
The feed flowrate is 100 kmol/h and its composition is 49 mol% DME, 1 mol% MeOH, and 50 mol% water. The feed is fed on stage 16 of a 31-stage column. A small liquid sidestream is withdrawn from stage 12 at a flowrate of 1.21 kmol/h with a composition of 17.4 mol% DME, 81.7 mol% methanol, and 0.9 mol% water. Thus the purity of the sidestream is low, but this is not a problem because only small amounts of valuable products (the DME and the water) are lost in this small stream.
The purities of the distillate and bottoms products are very high (99.99 mol%). If the specifications for the two product purities were low (e.g., 1 mol% MeOH), the MeOH
0.0001 MeOH 0.9999 Water
Figure 10.29 Purge sidestream.
0.0001 MeOH 0.9999 Water
in the feed could simply be removed in the distillate and bottoms. The column operates at 11 atm with a reflux ratio of 0.5 and a reboiler heat input of 0.513 MW. Figure 10.29 gives the flowsheet of the process with design parameters.
The steady-state simulation is converged by first setting the reflux ratio equal to 0.5. Then two Design Spec/Vary functions were used. The first function adjusted distillate flowrate to achieve 0.01 mol% MeOH in the distillate; the second one adjusted sidestream flowrate to achieve 0.01 mol% MeOH in the bottoms.
The purity of the sidestream and the shape of the composition profiles depend on the reflux ratio used. Figure 10.30 gives the composition profiles with a reflux ratio of 0.5. Note that there are two peaks in the methanol composition profile. The sidestream is withdrawn at stage 12, where the larger peak occurs. Figure 10.31 shows how the methanol composition profile changes with reflux ratio. The higher the reflux ratio, the higher the purity of the sidestream. The reflux ratio of 0.5 is selected because it gives a sidestream that is reasonable pure.
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