Page 264 - Separation process engineering

operating at 5.0 atm. The column has a total condenser and a partial reboiler. The feed flow rate is
1000 kmol/h. The feed is a saturated liquid. Feed is 8.0 mol% ethane, 33.0 mol% propane, 49.0
mol% n-butane and 10.0 mol% n-pentane. The column has 4 equilibrium stages plus the partial
reboiler, which is an equilibrium contact. Feed is on 2nd stage below total condenser. Reflux
ratio L /D = 2.5. Distillate flow rate D = 410 kmoles/hr. Develop the mass balance and
0
equilibrium matrix (Eq. 6-13) with numerical values for each element (A, B, C, and D). Do this
j
j
j
j
for your first guess: T = bubble-point temperature of feed (use same temperature for all stages); K
j
values are from DePriester chart; L and V are CMO values. Do the matrix for propane only.
D3. Do the matrix for n-butane for Problem 6.D2.
D4. A distillation column is separating 100 kmol/h of a saturated liquid feed that is 30 mol%

methanol, 25 mol% ethanol, 35 mol% n-propanol, and 10 mol% n-butanol at a pressure of 1.0
atm. The column has a total condenser and a partial reboiler. We want a 98.6% recovery of i-
propanol in the distillate and 99.2% recovery of n-propanol in the bottoms (but realize that this
first trial will not provide this amount of separation). Operation is with L/D = 5, D = 60, N = 4,
and N feed = 3 (#1 = total condenser and #4 = partial reboiler), set up the mass balance matrix Eq.
(6-13) for the first trial for n-butanol, and then solve. This is a hand calculation.

a. Use CMO to estimate liquid and vapor flow rates in the column for the first trial. Report these
flow rates.
b. For a first guess of K values, assume the K values in the column are constant and equal to those
found in a bubble-point calculation for the feed. The K values are (y/x) where y and x np
np
np
np
are from the bubble-point calculation with constant alpha, Eq. (5-30). The other K = α K .
np
i
i
c. Calculate all the A, B, C, and D values (but for n-butanol only) and write the complete matrix.
d. Solve the n-butanol matrix using the Thomas algorithm and find the n-butanol flow rates l
j,n-
leaving each stage.
butanol
e. The T implicitly used in the calculation to find the K values is the bubble-point temperature of
the feed. To determine T, first calculate the K value for n-propanol as (y/x) n-propanol where y and
x are found from the constant relative volatility solution for the bubble-point. Then use Raoult’s
law to find the T that gives this K value. Report this T.

Do not do additional trials.
System properties: If we choose n-propanol as the reference, the relative volatilities are methanol
= 3.58, ethanol = 2.17, n-propanol = 1.0, and n-butanol = 0.412. These relative volatilities can be
assumed to be constant. The K value for n-propanol can be estimated from Raoult’s law. The

vapor pressure data for n-propanol from Perry’s is:

F. Problems Requiring Other Resources

F1.* A distillation column with two stages plus a partial reboiler and a partial condenser is separating
benzene, toluene, and xylene. Feed rate is 100 kmol/h, and feed is a saturated vapor introduced on
the bottom stage of the column. Feed compositions (mole fractions) are z = 0.35, z = 0.40, z =
B
X
T
0.25. Reflux is a saturated liquid and p = 16 psia. A distillate flow rate of D = 30 kmol/h is
desired. Assume K = VP/p. Do not assume constant relative volatility, but do assume CMO. Use
i
i
the matrix approach to solve mass balances and the bubble-point method for temperature