Unformatted text preview: 8oC Stage 2, 65oC
Stage 45, 75oC
A converged result was obtained in 6 iterations, with a computed reflux ratio of 14.9, compared
to the above 15.5 estimate by the Underwood equations. However, as shown in the composition
profile on the next page, a pinch region (where no composition changes occur) of approximately
7 stages is evident just above the feed stage. Accordingly, the calculations with the SCDS
Column model were repeated by removing 7 stages above the feed, resulting in the following
Number of stages = 39 (includes the total condenser and the partial reboiler)
Feed stage = 25
A converged result was obtained in 5 iterations, with a computed reflux ratio of 15.0, just slightly
greater. The composition profile for this run also appears on the next page. No pinch is evident. Analysis (continued) Exercise 11.9 (continued) Analysis (continued) Exercise 11.9 (continued) The ordinary distillation column for separating methanol from ethanol was sized with Chemcad
for sieve trays, with:
24-inch tray spacing
12% downcomer area
10% hole area
80% of flooding
The resulting column inside diameter was determined to be 6.5 feet, which gives 60.0% of
flooding at the top tray and 72.4% of flooding at the bottom.
The tray pressure drop averages 0.091 psi/tray, for a total of 3.4 psi or 0.23 atm for the 37 trays.
Thus, the assumption of negligible tray pressure drop is not satisfactory. For a final design, the
computations should be repeated taking into account tray pressure drop and tray efficiency.
The final material balance for the ordinary distillation column was as follows:
68.76 Because the flow rate of ethanol in the bottoms is only 68.69 mol/s and 70.00 is needed
for the extractive distillation column, the makeup ethanol rate is 70.00 - 68.69 = 1.31 mol/s.
Comparison with Example 11.3:
In Example 11.3, water is used as the solvent. It is much more effective than the ethanol
solvent used in this exercise, as shown by the following comparison:
Solvent flow rate, mol/s
Solvent makeup rate, mol/s
Equilibrium stages in extractive distillation column
Equilibrium stages in ordinary distillation column
Diameter of extractive distillation column, feet
Diameter of ordinary distillation column, feet Water solvent
2.5 Ethanol solvent
6.5 Exercise 11.10
Subject: Separation of acetone from methanol by extractive distillation with methyl-ethylketone (MEK)
Given: Bubble-point feed of 30 mol/s acetone (A) and 10 mol/s methanol (M) at 1 atm. MEK
(K) as the solvent. Results of Example 11.3 with a solvent of water.
Assumptions: Because the feed is close to the azeotropic composition of 22 mol% methanol,
use a two-column system, with the first column being extractive distillation and the second
ordinary distillation to recover the solvent. UNIFAC for K-values. Negligible tray pressure
drop. 100% tray efficiency.
Find: Suitable column designs to obtain an acetone product of at least 95 mol%, a methanol
View Full Document
- Spring '11
- The Land