Page 210 - Separation process engineering

results since they have no meaning). If Aspen Plus has a critical error, such as the column drying up,
reinitialize before you do another run. The systems for this lab were chosen so that they should
converge.

When you do runs, record L/D, feed rate, feed condition (fraction vaporized or temperature), the feed
stage location and the number of stages (you set these values), and the results: distillate and bottoms
compositions, the temperatures of distillate and bottoms, the maximum and minimum flow rates of liquid
and vapor, and the heat loads in condenser and reboiler. Do not print out the entire Aspen Plus report.
Binary Distillation. Separate 1,2-dichloroethane from 1,1,2-trichloroethane. Feed is 100 kmol/h of 60%

dichloroethane. Operation is at 1.0 atm. Column has a total condenser and a kettle type reboiler. Peng-
Robinson is a reasonable VLE package. Be sure to select the correct components from the AspenPlus
menu. Note that 1,1,1-trichloroethane should not be used since it has very different properties.
a. Plot the x-y diagram from Aspen Plus and compare to an approximate plot with a constant a relative
volatility of 2.24 (an over-simplification, but close). Note: Input the data for Step 1b first.

b. The feed is 100% vapor at 1.0 atm. (Set the vapor fraction in the feed to 1.0.) We want a distillate
product that is 92 mol% dichloroethane and a bottoms that is 8 mol% dichloroethane. Since Aspen
Plus is a simulator program, it will not allow you to specify these concentrations directly. Thus, you
need to try different columns (change number of stages and feed location) to find some that work. Use
L/D = 2.0. Set Distillate flow rate that will satisfy the mass balance. (Find D and B from mass
balances. Do this calculation accurately or you may never get the solution you want.) Pick a
reasonable number of stages and a reasonable feed stage (Try N = 21 and feed at 9 [vapor] to start).
Simulate the column and check the distillate and bottoms mole fractions. This is easiest to do with the
liquid composition profiles for stage 1 and N. They should be purer than you want. Also, check the K
values and calculate the relative volatility at the top, feed stage, and bottom of the column to see how
much it varies.

c. Continue part 1b: Find the optimum feed stage by trying different feed stages to see which one gives
the greatest separation (highest dichloroethane mole fraction in distillate and lowest in bottoms).
Then decrease the total number of stages (using optimum feed stage each time) until you have the
lowest total number of stages that still satisfies the specifications for distillate and bottoms. Note that
since the ratio (optimum feed stage)/(total number) is approximately constant you probably need to
check only three or four feed stages for each new value of the total number of stages. If your group
works together and have different members check different runs, this should not take too long.

d. Triple the pressure, and repeat steps a and b. Determine the effect of higher pressure on the relative
volatility.

e. Return to p = 1 atm and your answer for part c. Now increase the temperature of the feed instead of
specifying a vapor fraction of 1. What happens?
f. Return to the simulation in part c. Determine the boilup ratio ( /B). Now, run the simulation
reboiler
with this boilup ratio instead of specifying Distillate rate. Then start decreasing the boilup ratio in the
simulation. What happens?

g. Return to the simulation in part e. Instead of setting fraction vaporized in the feed, set the feed
temperature. (Specify L/D = 2 and boilup ratio, N and feed location.) Raise the feed temperature and
see what happens.

h. Return to part b, but aim for a distillate product that is 99% dichloro and a bottoms that is 2%
dichloro. Do the mass balances (accurately) and determine settings for Distillate flow rate. Try to
find the N and feed location that just achieves this separation with an L/D = 2. Note that this