Page 272 - Separation process engineering

Using the K-value results in Aspen Plus, calculate and report the relative volatility of propane-butane
(Kpropane/Kbutane) in both reboiler and condenser at pressures of 0.25, 1.0, 4.0 and 16.0 atmospheres
for the runs in Ia. At the condenser, what is the trend of relative volatility of propane-butane as the
pressure is raised? Is the general trend similar in the reboiler? Do the trend results agree with the
predictions from the DePriester charts?

V. Pressure effects—Azeotropes. Switch the system to isopropanol and water. Use NRTL or NRTL-2 as
the VLE package. We want to look at the analysis at different pressures. To do this, you need to set up
a column (dimensions and so forth are arbitrary). Run the simulation so that Aspen Plus will let you
use Analysis. Look at the T-y,x and y,x diagrams at p = 1.0 atm, p = 10.0 atm, and p = 0.1 atm. Notice
how the concentration of the azeotrope shifts. (In the Binary Analysis Results Table the azeotrope
occurs when K = 1.000. Record the azeotrope mole fractions). This shift may be large enough to
i-p
develop a process to separate azeotropic mixtures (see Chapter 8).
VI. Tray Efficiencies. Switch to the ethanol-water system using NRTL as the VLE package. Make the
feed 60 mole% water and 40 mol% ethanol, 100 kmol/h, saturated liquid at 1 atm. Column has a total
condenser and a partial reboiler. Set N = 15, Nfeed = 10, on stage. D = 44, L/D = 2. Column pressure
is constant at 1.0 atm.

a. Run the system. Report the liquid compositions on stages 1 and 15 (distillate and bottoms). This run
is equivalent to efficiencies of 1.0.
b. Go to the Data Browser and click on the block for your distillation column. Click on Efficiency.
Click on Murphree efficiency and Specify stage efficiencies. Then click on tab for vapor-liquid. For

stage 1, specify an efficiency of 1 (although for a total condenser this does not matter). Click on
Enter. Then for stage 2 specify a Murphree vapor efficiency of 0.6. Click on Enter. Continue for
stages 3 to 14, specifing a Murphree vapor efficiency of 0.6. Click on Enter. For stage 15 (partial
reboiler) specify efficiency of 1.0. Click on the Next button and run the simulation. Compare liquid
compositions on stages 1 and 15 with the run in part VI.a. Explain the effect of the lower efficiency
(the effect will be larger if there is no pinch point).
Note: To save time, the runs in Lab 5 were not at the optimum feed plate. Operating at the optimum feed
plate should not change any of the general trends. Practice finding the optimum feed plate for at least
one of these problems. A general procedure to do this is: For your initial value of N, find the optimum
feed location by trial and error. Then, reduce or increase the total number of contacts N to just reach the

desired specifications. While you do this, choose the feed stage by noting that the ratio N feed /N is
approximately constant. Once you have an N that just gives the desired recoveries, redo the
optimization of the feed location (your value should be reasonably close). This practice will be very
helpful for Lab 6, which requires a lab report.

Lab 6. Use RADFRAC with an appropriate VLE package for this assignment. Use the direct sequence in
Figure 11-9A. Do the overall mass balances for both columns to determine both distillate values before
lab.
We are separating 1000 kmol/h of a feed containing propane, n-butane, and n-pentane. The feed
pressure is 4.0 atm. This feed is 22.4 mol% propane, 44.7 mol% n-butane, and the remainder n-

pentane. In the overall process we plan to recover 99.6% of the propane in the propane product, 99%
of the n-butane in the n-butane product, and 99.7% of the n-pentane in the n-pentane product. In column
1 recover 99.5% of the n-butane in the bottoms product. For purposes of your initial mass balances,
assume: 1) There is no n-pentane in the propane product stream, and 2) There is no propane in the n-
pentane product stream. Check these guesses after you have run the simulations. Both columns operate
at 4.0 atm. Operate each column at 1.15 L/D minimum. Use the optimum feed stage for each column.