Page 110 - Separation process engineering

with VP in mm Hg and T in °C, the constants for n-pentane are: A = 6.853, B = 1064.8, C =
233.01. n-hexane constants are: A = 6.876, B = 1171.17, C = 224.41 (Dean, 1985)
a. Predict the vapor pressure at 0°C for pure n-pentane

b. Predict the boiling point of pure n-pentane at 3 atmosphere pressure.

c. Predict the boiling pressure if pure n-pentane is boiling at 0°C.
d. At a pressure of 500 mm Hg and temperature of 30°C, predict the K values for n-pentane and n-
hexane using Raoult’s law.

e. For a binary mixture with two phases at equilibrium, F = C – P + 2 = 2 – 2 + 2 = 2 degrees of
freedom. If T = 30°C and p = 500 mm Hg, determine the mole fractions in the liquid and vapor
phases of an equilibrium mixture of n-pentane and n-hexane.
f. One mole of a mixture that is 75 mol% n-pentane and 25 mol% n-hexane is placed in a closed
chamber. The pressure is adjusted to 500 mm Hg and the temperature to 30°C. The vapor and

liquid mole fraction were found in part e. How many moles of liquid and moles of vapor are
there at equilibrium?
g. If 1 mol/min of a mixture that is 75 mol% n-pentane and 25 mol% n-hexane is fed continuously
to an equilibrium flash chamber operating at 30°C and 500 mm Hg, what are the flow rates of
the liquid and vapor products?

D28. Repeat Example 2-4, but with F = 3000 lbmol/h, and use a horizontal flash drum with h/D = 4.
D29. Design a horizontal flash drum to separate 15,000 kg/h of a feed with the following mass
fractions: methane 0.21, propane 0.39, n-butane 0.24, i-butane 0.11, and n-pentane 0.05. The feed
is at 0°C and a pressure just high enough that it is all liquid. The drum pressure is 4.0 atm and the
drum is adiabatic (heat duty = 0).

a. Find the minimum feed pressure (to within a whole number of atmospheres) that keeps the feed
a liquid. Report this pressure and the VLE correlation used, and use this as the feed pressure for
the flash calculation.

b. Find L, V (in kg/h and in kmol/h), y, x (in mole fractions), and T drum (in °C).
i
i
c. If the ratio of drum length to diameter, L/D = 4, find the length and diameter (in meters) of the
horizontal drum.
Note: The easiest way to solve this problem is to use Aspen Plus for part a (trial and error) and once
you have solved part a, obtain the solution for part b. Although Aspen Plus does not do the drum
sizing, it does calculate the physical properties needed for drum sizing. Obtain these and do the
drum sizing with a hand calculation.

D30. Data for the equilibrium of water and n-butanol at 1.0 atmosphere is given in Table 8-2. Plot y
w
versus x . A feed of 100 kmol/h that is 20 mol% water is fed to flash chamber A in the figure that
w
follows. The vapor from flash chamber A is 40 mol% water. This vapor is then partially
condensed and fed to flash chamber B. In flash chamber B, 40% of the vapor from chamber A is
condensed.
a. What are the flow rates L and V and what is the value of x ?
A
A
A
b. What are the flow rates L and V and what are the values of x and y ?
B
B
B
B
c. Suppose we changed the operation of chamber B so that enough vapor was condensed to give a
liquid mole fraction of x = 0.20 mole fraction water. If x = 0.20, what are y , L , and V ?
B
B
B
B
B
We could now easily recycle this liquid to the feed (also 20% water) and produce only V B,new