Figure 5.25 Flowsheet with recycle/D2 loop closed.

Figure 5.26 Ternary diagram.

condenser for the high-pressure (high-temperature) column can be used as the reboiler in the low-pressure (low-temperature) column.

5.3.1 Flowsheet

Figure 5.27 gives the conceptual flowsheet. The specific system used as an example is methanol/water. Product specifications are 99.9 mol% methanol in the distillate streams (there is one from each column) and 99.9 mol% water in the two bottoms streams. The fresh feed is 1 kmol/s with a composition of 60 mol% methanol and 40 mol% water. The feed is split between the two columns so that the system operates "neat," where the condenser heat removal in the high-pressure column is exactly equal to the reboiler heat input in the low-pressure column. Each column has 32 stages and is fed on the stage that minimizes reboiler heat input.

To achieve the required temperature differential driving force in the condenser/reboi-ler, the pressures in the two columns must be appropriately selected. The low-pressure column C1 operates at a pressure of 0.6 atm (vacuum conditions, 456 mmHg) that gives a reflux drum temperature of 326 K so that cooling water can be used. The pressure drop per tray is assumed to be 0.0068 atm (0.1 psi). The base temperature of C1 is 367 K.

A reasonable differential temperature driving force is about 20 K. If the AT is too small, the heat transfer area of becomes quite large. The pressure in the second column is adjusted to give a reflux drum temperature of 367 + 20 = 387 K. The pressure in C2 is 5 atm. The base temperature in C2 at this pressure is 428 K, which will determine the pressure of the steam used in this reboiler.

0.2033 kmol/s 0.999 H2O

Methanol D

0.2950 kmol/s 0.999 MeOH

0.2033 kmol/s 0.999 H2O

Methanol D

0.2950 kmol/s 0.999 MeOH

Figure 5.27 Heat-integrated columns.

5.3.2 Converging for Neat Operation

Initially the total feed is split equally between the two columns. This is achieved in the Splitter labeled "T1" on the flowsheet shown in Figure 5.28. Two Design Spec/Vary functions are set up in each column to adjust distillate flow and reflux ratio to attain the 99.9 mol% product purities of all four streams. The optimum feed tray location is determined by finding the feed stage that minimizes reboiler heat input. In column C1, it is stage 19; in column C2, it is stage 18.

Under these conditions, the resulting reboiler heat input in the low-pressure column C1 is 17.91 MW. The resulting condenser heat removal in the high-pressure column C2 is 18.62 MW. These are very close, but if the system is to be operated "neat" (with no auxiliary reboilers or condensers), these heat duties must match exactly.

One way to do this is to manually adjust the feed split in "T1" until QR1 is equal to QC2. This can be automated by going to Flowsheeting Options on the Data Browser window and selecting Design Spec. This is similar to the Design Spec/Vary in the column blocks, but now variables from any block can be used. Figure 5.29a shows the window that opens after New and OK are clicked. On the first page tab Define we enter two variables QR1 and QC2, and click on Edit to define what they are. Figure 5.29b shows how the QR1 is defined.

On the Spec page tab (Fig. 5.29c) a parameter "DELTAQ" is specified with the desired value Target and Tolerance. Clicking the Vary page tab opens the window shown in Figure 5.30. The "T1" block is selected and the variable is Flow/Frac. The ID1 is set at "2" since the flowrate of the stream "2" leaving the splitter is the first variable and is the one specified. The final item is to define DELTAQ on the Fortran page tab (Fig. 5.31). Remember that the convention in Aspen Plus is that heat addition is a positive number and heat removal is a negative number. Therefore we want the sum of QR1 and QC2 (in watts) to be small. Running the program yields a feed split with 0.5086 kmol/s fed to the low-pressure column C1 and 0.4914 fed to the high-pressure column C2. The heat duty in the condenser/reboiler is 18.10 MW as shown in Figure 5.32, which is obtained by selecting Results under the DS-1 design spec. The final flowsheet conditions are given in Figure 5.27.

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