Page 271 - Separation process engineering

about the capabilities of Aspen Plus. For parts I, II, III, and IV use the following feed: 100 kmol/h at
20°C. Mole fractions of components are: ethane 0.091, ethylene 0.034, propane, 0.322, propylene 0.064,
n-butane 0.413, n-pentane 0.057, n-hexane 0.019. Use Peng-Robinson for VLE. Pressure of feed should be
0.1 atm above that of column. The column has a total condenser and a partial reboiler. There is a single
feed, a liquid distillate and a liquid bottoms. Draw the flowsheet and input all data. Then save the file.
For part I record the mole fractions of butane in distillate and propane in bottoms. Record the
temperatures, and the K values for butane and propane in reboiler and condenser.

I. Pressure effects—temperatures.
a. Suppose we want to split between the C3 and the C4 components. Set D = 51.1 kmoles/hr (Why is

this value selected?), p = 1 atm., N = 30 (includes reboiler and condenser), Nfeed = 15 (on stage),
L/D = 2. Report distillate and bottoms purities and condenser and reboiler temperatures.
b. Repeat run Ia but at p = 10 atm. Same report as part a.
c. Since cooling water is much cheaper than refrigeration, we want to operate with the condenser at a

temperature that is high enough that cooling water can be used. Assume the minimum temperature is
30°C (25°C cooling water plus 5°C approach in heat exchanger). Have we satisfied this for either
run? If not, what pressure is necessary? Try 20 atmospheres and go down.
Reduction in pressure is used if the reboiler temperature would be too high or if there is excessive
thermal degradation.

II. Pressure effects—Changing split. Suppose we want to split between the C2 and the C3 compounds
(ethane and ethylene are in distillate and everything else in bottoms). Change the value for distillate
flow rate in Aspen Plus to achieve this. Operate at pressure of 15 atmospheres. Use the same settings
as previously. Run Aspen Plus. Compare the temperature in the condenser to the temperature in run I.c.
(also at 15 atm). What can you conclude? Report distillate and bottoms mole fractions and
temperatures.

III. Pressure effects—column diameter. Aspen Plus will size the column diameter and set-up the tray
dimensions. Click on Data (toolbar) and then Blocks. In the Data Browser open up the block that is
your distillation column (click on the box with the + in it). Then click on the file “Tray Sizing.” Click
on New (or Edit if you have previously set this up) and OK for section 1. Section 1 can be the entire
system—stages 2 to 29 (remember that Aspen Plus calls the condenser 1 and the reboiler 30). You
want 1 pass (the default) and for tray type use the menu to specify “sieve.” For tray spacing 2 feet (or
the default) is OK. Use the default value for hole area/tray area. Then click on the tab for Design.
Fractional approach to flooding should be in range 0.75 to 0.85—use 0.75. For minimum downcomer
area, the default value of 0.1 is good for larger columns. Use 0.15 here. The default values for foaming
and overdesign (1) are both fine. Use the menu to select the “Fair” method for flooding design (this is

the procedure used in Chapter 10).
Now run the design (for Ia, D = 51.1) at pressures of 0.25, 1.0, 4.0 and 16.0 atm with constant feed
pressure 16.1 atm. If convergence is a problem in the Column Block Configuration tab, change
Convergence from Standard to Petroleum/Wide-boiling. Look at the column diameter and other new
information now included in the results. What is the effect of increasing column pressure on column
diameter? This result is explained in Chapter 10. Note: Do Part IV at the same time as Part III. Report
the largest column diameter for each pressure.

Save the file. Feel free to explore the other possible alternatives in the Data Browser when the
distillation block is open. If you have time, look at Convergence and Tray Rating. Downcomer backup
(under profiles in tray rating) is useful. We will look at Efficiency in section VI below.

IV. Pressure effects—VLE changes and changes in separation.