Page 241 - Separation process engineering

CMO and constant relative volatilities. The column has a total condenser and a partial reboiler.
L/D = 6. As a first guess, assume that n-propanol does not distribute (it all exits in the bottoms).
Relative volatilities are: methanol = 3.58, i-propanol = 1.86, and n-propanol = 1.0.

a. Find D, B, x i,dist , and x i,bot .

b. Step off stages and find the optimum feed location and the total number of stages.
H2. [VBA required] Either write your own program or use the program in Appendix A of Chapter 5
to solve the following problem. A feed of 100 mol/h of a saturated liquid that is 25 mol% A =
benzene, 35 mol% B = toluene, and 40 mol% C = cumene is fed on the optimum feed plate to a
distillation column that has a total condenser and a partial reboiler. Fractional recoveries of B
(toluene) in the distillate of 0.9 and of C in the bottoms of 0.97 are desired. The relative
volatilities are α AB = 2.25, α = 1.0, and α = 0.21. Use an external reflux ratio of L/D = 0.3.
BB
CB
Find the optimum feed stage, the total number of stages, the fractional recovery of A (benzene) in
the distillate, D and B. After solving the problem, try “What if?” simulations to explore the effects
of changing the feed concentrations, the fractional recoveries, L/D, and the relative volatility α .
CB
H3. [VBA required] Distillation programs such as the ternary stage-by-stage program in Appendix A
of Chapter 5 can be used to find the minimum external reflux ratio by setting an arbitrarily high
feed location and an arbitrarily high maximum number of stages and then varying L/D until the
lowest L/D at which the separation can be achieved is found. Arbitrarily set the feed stage to 100
(in the spreadsheet) and set the maximum number of stages to 200 (in the VBA program). For the
separation of benzene (A), toluene (B), and cumene (C) with a 0.99 fractional recovery of B in the
distillate and a 0.999 fractional recovery of C in the bottoms, find (L/D) min . The feed and alpha

values are the same as in Problem 5.H2. Check your answer by trying a feed stage of 125 with a
maximum number of stages of 250.
H4. [VBA required] Although it is usually not possible to make an accurate guess of the fractional
recovery of a sandwich component, a stage-by-stage calculation often still converges, since the
fractional recovery of the sandwich component is finite for both distillate and bottoms. For a
ternary system, it often does not matter at which end of the column one starts the calculation. In the

process of doing the stage-by-stage calculation, one also obtains reasonably accurate values for D
and B.
a. The easiest way to run a sandwich component is probably to run a standard program (e.g.,
Appendix A of Chapter 5) as if A = LNK, B =LK, and C = HK. Since the fractional recovery of
A in the distillate and of C in the bottoms are specified but the program wants B and C to be
specified, guess the fractional recovery of B in the distillate. Run the program and check if the
calculated value of the fractional recovery of A is equal to the specified value. If not try,
another value for fractional recovery of B.

b. Solve the following problem. A feed of 100 mol/h of a saturated liquid that is 25 mol% A, 35
mol% B, and 40 mol% C is fed on the optimum feed plate to a distillation column that has a
total condenser and a partial reboiler. Fractional recoveries of A in the distillate of 0.94 and of
C in the bottoms of 0.995 are desired. The relative volatilities are α AB = 1.4, α = 1.0, and
BB
α = 0.7. Use an external reflux ratio of L/D = 5.0. Find the optimum feed stage, the total
CB
number of stages, the fractional recovery of B in the distillate, distillate and bottoms flow rates.
c. Plot the profiles for both liquid and vapor mole fractions of the sandwich component.

d. Compositions of trace components will build up in the column if they are sandwich
components. Repeat the sandwich component analysis, but with z = 0.38, z = 0.02, and z =
C
B
A