^Calculations begin

Dck: CI Model: RADFRAC

Convergence iterations:

OL HL IL Err/Tol

Block: P12

Block: V12

Block: Pll

Block: Vll

S 0.88S16E-01

Model: PUMP Model: VALVE Model: PUMP Model: VALVE

->Generating block results . . . Block: Pll Model: PUMP

Block: P12 Model: PUMP

->Siinulation calculations completed .

lAII blocks have been executed

Figure 3.41 (a) Control panel, (b) new stream results.
Figure 3.42 (a) Setting up second design spec; (b) selecting Propane; (c) selecting Bottoms.

been changed to produce the desired product purities. The stream results show that the flowrate of D1 is 0.4021 kmol/s. To ascertain the reflux ratio, click on Results Summary under the C1 block. The window shown in Figure 3.45 opens, on which the conditions at the top of the column are given. The reflux ratio is 3.095.

The other important pieces of information in the window are the condenser heat removal [- 22.29 MW (megawatts)] and the reflux drum temperature (317.06 K) at the 14 atm pressure that we specified. If you recall, we guessed that a pressure of 14 atm would give us a reflux drum temperature of ~325 K so cooling water could be used in the

Figure 3.43 Set up second Vary.

condenser. To attain the desired 325 K, the pressure should be increased a little. If we rerun the simulation with a pressure of 16.8 atm, the reflux drum temperature will be 325.06 K.

Of course, at this new pressure the required reflux ratio changes. It increases from 3.095 to 3.511 (see Fig. 3.46). This shows the adverse effect of pressure on relative volatilities that occurs in most hydrocarbon systems. The column should be operated at as low a pressure as possible to save energy.

To find the conditions at the base of the column, we use the dropdown menu that is next to View on the Results Summary window and select Reboiler/Column base. Figure 3.47 shows the information obtained. The most important piece of information is the reboiler heat input 27.409 MW. The base temperature is 366.11 K. This will dictate the pressure

Figure 3.44 Stream results.
Figure 3.45 Results for top of column at 14 atm pressure.

of the steam used in the reboiler. A reasonable differential temperature is 40 K, which corresponds to a saturated steam pressure of ~3 atm at 406 K. So, if steam is available in the plant at ~6 atm, it can be used to supply the heat required in the reboiler, assuming a 3 atm pressure drop over the steam control valve.

The stream conditions at the 16.8 atm column pressure are shown in Figure 3.48. The column temperature and composition profiles can be obtained by selecting Profiles in the C1 block. The window that opens is shown in Figure 3.49. There are several page tabs. The first TPFQ (temperature, pressure, flow, heat) gives the temperature and pressure on each stage. Selecting the second page tab Compositions opens the window shown in Figure 3.50, in which Liquid has been selected from the dropdown menu in the View box.

Figure 3.46 Results for top of column at 16.8 atm pressure.
Figure 3.47 Results for base of column at 16.8 atm pressure.

Using the Plot Wizard program makes generating plots of these profiles quite easy. Click on Plot at the top toolbar of the Aspen Plus simulation window. Then click Plot Wizard. This opens the window shown in Figure 3.51a. Clicking Next opens the window shown in Figure 3.51b. Clicking on the upper left picture labeled Temp produces the temperature profile plot given in Figure 3.51c. Clicking on the picture labeled Comp and then clicking Next opens the window on which you can select what components to plot and what phase (liquid or vapor compositions). Figure 3.52a shows the selections, and Figure 3.52b gives the composition profile.

Block CI (RadFrac) Stream Results - Data Browser

Stream Results

^lilif 3

22J aN Nil

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