Page 270 - Separation process engineering

scroll down to Reinitialize. Click on OK to messages) and run again.
c. Try an intermediate cut where butane exits from the top, hexane from the bottom, and pentane
distributes between the top and bottoms. (Change D or B.) Do with saturated liquid feed. Compare

Q and Q in runs b (V/F = 0) and c. Explain.
C
R
d. Continue item b with V/F = 0. Find the total number of stages and the optimum feed location if we
want butane mole fraction in the bottoms to be just less than 1.0 E –3 and pentane mole fraction in the
distillate to be just less than 1.0 E –3. This calculation is trial and error with a simulation program
(commercial simulators have a “design” option that will do this trial-and-error process for you, but it
should not be used until you have a firm grasp of distillation). In the “Results Summary” Browser
accurate values for the distillate and bottoms compositions can be found in the section titled
Compositions. Use the menu bar to switch to “Liquid.” Use the other menu bar for mole units. Since
Aspen Plus calls the partial condenser #1, the vapor composition leaving stage 1 is the distillate, and
the last liquid mole fraction is the bottoms. Is it more difficult to meet the C or the C requirement?
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e. Look at your condenser and reboiler temperatures (continuing item d). The condenser temperature is
low enough that refrigeration is needed. This is expensive. To prevent this, raise the column pressure
until the condenser temperature is high enough that cooling water can be used for condensation. (The
appropriate value depends on the plant location. Use a cooling water temperature appropriate for
your location and add 5°C for ΔT in the heat exchanger.) Changing the pressure changes the VLE.
Check to see if you still have the desired separation. If not, find the new values for optimum feed and
total number of stages to obtain the desired mole fractions. Note: Be sure to also raise your feed
pressure so that it is equal to the column pressure. Why are more stages required at the higher P?

f. Try different values for L/D or boilup ratio. See how this affects the separation. Try using different
operating specifications in RADFRAC.
g. Now, try a different feed temperature. For example, try a feed at 30°C. Look at how reboiler and

condenser heat loads change and compare to run 1e. Try changing the feed composition. (Remember
to change D or B to satisfy mass balances.)
h. Try a different feed flow rate (but same concentrations and fraction vapor) at conditions that you
optimized previously. The number of stages, optimum feed location and separation achieved should
not change. The heat requirements will be different, as will outlet flow rates. Compare Q /(Feed
R
rate) for the two runs. What does this say about the design of distillation for different flow rates?
i. Change the column configuration and have a liquid distillate product or two feeds. This will require
redrawing your flowsheet. Compare results to 1e, but remember distillate is a liquid.

j. On the menu bar, click on Tools, then Analysis, then Property and then on Residue. Then click on Go.
This gives a residue curve for your chemical system. A residue curve shows the path that a batch
distillation will follow for any starting condition. The absence of nodes and azeotropes for this
nearly ideal chemical system shows that the designer can obtain either the lightest or the heaviest
species as pure products for any feed concentration. This is not true when there are azeotropes. (This
topic is covered in detail in Chapter 8.)

II. (Optional) Try a more complicated chemical system such as methanol-ethanol-water at 1 atm (use
NRTL-2 for VLE). Look at the residue curve map. Expand the size of the map and look for the ethanol-
water azeotrope (where one of the curves intersects the side of the triangle running from ethanol to
water). Try a distillation with this system.
Feel free to further explore Aspen Plus on your own.

Lab 5. In this lab we continue to use RADFRAC to explore distillation in more detail and to learn more