The reactor effluent is fed into a 35-stage reactive distillation column (C1) on stage 28. Catalyst is present on stages 7-23. The reactor effluent is fed five trays below the reactive zone. A methanol stream is fed at the bottom of the reactive zone (stage 23). The flowrate of the methanol fed to the reactive column is 235 kmol/h.
Figure 9.8 gives composition and temperature profiles in the reactive column C1. The reflux ratio is 4, which gives a bottoms purity of 99.2 mol% TAME and a distillate impurity of 0.1 ppm TAME. Reboiler heat input and condenser heat removal are 38.2 and 39 MW, respectively. The operating pressure is 4 bar. The column diameter is 5.5 m. The overall conversion of 2M1B and 2M2B in the C5 fresh feed is 92.4%. Table 9.1 gives stream information for the prereactor and column C1.
The distillate Dj has a methanol composition (28 mol% methanol) that is near the azeo-trope at 4 bar. It is fed at a rate of 1122 kmol/h to stage 6 of a 12-stage extraction column. Water is fed on the top tray at a rate of 1050 kmol/h and a temperature of 322 K, which is achieved by using a cooler (heat removal 1.24 MW). The column is a simple stripper with no reflux. The column operates at 2.5 atm so that cooling water can be used in the condenser (reflux drum temperature is 326 K). Reboiler heat input is 5.96 MW. The overhead vapor is condensed and constitutes the C5 product stream.
This column is designed by specifying a very small loss of methanol in the overhead vapor (0.01% of methanol fed to the column) and finding the minimum flowrate of extraction water that achieves this specification. Using more than 10 trays or using reflux did not affect the recovery of methanol.
The bottoms is essentially a binary methanol/water mixture (23.5 mol% methanol), which is fed to a 32-stage column operating at atmospheric pressure. The number of trays in the second column is optimized by determining the total annual cost of the column over a range of tray numbers. Reboiler heat input and condenser heat removal are 8.89 and 9.53 MW, respectively. The column diameter is 2.24 m.
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