Vapor Sidestream Column With Rectifier

If the sidestream is a vapor, a rectifier can be added that removes some of the heavy impurity in the vapor sidestream coming from the main tower. To illustrate this configuration, we take the same DME/MeOH/water system studied in the previous section but remove a vapor sidestream below the feed stage.

10.4.1 Steady-State Design

The feed flowrate is 100 kmol/h, and the feed is fed on stage 11 of a 52-stage column. A vapor sidestream is withdrawn from stage 31 and is fed into a 12-stage rectifier column. The flowsheet is given in Figure 10.20. The distillate D2 from the rectifier is the MeOH product. The bottoms liquid stream from the rectifier is pumped back to stage 32 of the main column.

The rectifier must operate at a pressure lower than that of the main column so that the vapor sidestream can flow "downhill." The main column operates at 11 atm. The pressure

323 K 11 atm

323 K 11 atm

0.02 MeOH 0.98 Water

Figure 10.20 Vapor sidestream with rectifier.

0.02 MeOH 0.98 Water

Figure 10.20 Vapor sidestream with rectifier.

in the rectifier is set at 9 atm to provide some pressure drop over the control valve in the vapor line.

The feed composition is 35 mol% DME, 35 mol% MeOH, and 30 mol% H2O. The distillate from the main column Dj is the DME product and has an impurity specification of 1 mol% MeOH. The bottoms from the main column B1 is the water product and has an impurity specification of 2 mol% MeOH. The distillate from the rectifier D2 is the methanol product and has an impurity specification of 1 mol% water. The DME concentration in this stream is negligible because there is very little DME in the main column below the feed tray. The feed is fed on stage 11 near the top. The vapor sidestream is withdrawn down at stage 31.

The steady-state design of this two-column system with recycle was achieved by a sequential approach. First, the flowrates of the distillates from the two columns were set equal to the molar flowrates of DME and MeOH, respectively, in the feed. The reflux ratio in the main column was set at 2. The flowrate of the vapor sidestream S1 was set at 3 times the MeOH product rate, which gives a reflux ratio in the rectifier of 2. Note that there is only one degree of freedom in the rectifier, so setting the distillate flowrate completely specifies the column.

Then a guess was made of the composition of the recycle stream xB2 j flowing back to the main column (the bottoms of the rectifier B2). The simulation was converged, and the difference between the guessed values of xB2 and the calculated values was observed. New guesses were made until there was little difference. Then the recycle loop was closed using a Tear specification.

The next step was to adjust the various degrees of freedom to attain the desired purities of the three products. Three Design Spec/Vary functions were used in a sequential manner:

1. The impurity of MeOH in the main-column distillate xD1(MeOH) was fixed at 1 mol% MeOH by varying the distillate flowrate D1.

2. The impurity of MeOH in the rectifier distillate xD2(MeOH) was fixed at 1 mol% MeOH by varying the distillate flowrate D2.

3. The impurity of MeOH in the main-column bottoms xB1(MeOH) was fixed at 2 mol% MeOH by varying the flowrate of the vapor sidestream S1.

Finally, the reflux ratio in the main column was reduced to see the effect on the DME impurity in the sidestream. Since DME is much more volatile than MeOH and is fed above the sidestream drawoff tray, the reflux ratio could be reduced to 0.5 without affecting sidestream composition. With a reflux ratio of 0.5, the liquid rate in the top section of the main column is quite small (17 kmol/h) compared to the liquid rates lower in the column (180 kmol/h). Therefore reflux ratios lower than 0.5 were not used.

Figure 10.20 gives design parameters and equipment sizes of this process. The reboiler heat input in the main column is 1.38 MW. It is interesting to compare this with the energy requirements of the sidestream/stripper flowsheet shown in Figure 10.15 (1.20 + 0.022 = 1.22 MW). The two processes produce essentially the same three product streams with the same purities. The energy consumptions of the two flowsheets are quite similar.

The diameter of the main column is 0.524 m, and the diameter of the rectifier is 0.459 m. Thus the size of the rectifier is significantly larger than the stripper (0.12 m) in the alternative flowsheet, indicating higher capital cost for the rectifier process. The inherent reason for this is that the MeOH/water separation (which takes place in the rectifier) is more difficult than the DME/MeOH separation (which takes place in the stripper).

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