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Unformatted text preview: se above for the equilibriumstage calculations.
However, the % absorption of propane is now 39.2%. Five passes were needed to handle the
high liquid flow rate.
The backcalculated Murphree vapor tray efficiencies were mainly in the following ranges:
Methane:
54 to 80%
Ethane:
55 to 75%
Propane:
46 to 77%
nButane:
65 to 77%
nPentane:
74 to 80%
nDodecane: 73 to 78%
(c) The tray efficiencies were higher than anticipated, probably due to somewhat higher
temperatures at the elevated pressure, which reduces the liquid viscosity. The ratebased method
has the advantage of predicting the efficiency. However, in this exercise, as pointed out above,
the near 40% absorption of propane is independent of the number of equilibrium stages above
three. A column diameter of 13 m is probably too large. Two or more columns in parallel
should be considered. Exercise 12.18
Subject: Separation of methanol (M) from a mixture with ethanol (E) and water (W) by
distillation, using equilibriumstage and ratebased methods with a trayed column.
Given: A feed at 1.3 atm and 316 K of 142.46 kmol/h of 65.36 mol% M, 3.51 mol% E, and
31.13 mol% W. Column is equipped with a total condenser (1.1 atm in and 1.0 atm out) and a
partial reboiler. Reflux ratio = 1.2 and distillate rate = 93.10 kmol/h (almost equal to the M feed
rate). Distillate is to have a mole fraction of W less than or equal to 0.0001. UNIFAC method
for activity coefficients. Sievetray efficiency of 85%.
Find: (a) Number of equilibrium stages, optimal feed stage location, and the split of all
components.
(b) Actual number of sieve trays, the split of all components, and the Murphree vapor
tray efficiencies. Comparison and discussion of results.
Analysis: The ChemSep method was applied to both parts, using the ideal gas law, Antoine
vapor pressure, and excess enthalpy with UNIFAC. A column bottoms pressure of 1.3 atm was
assumed. The calculations used automatic initialization with Newton's method and defaults for
step limits on flows, temperature, and composition. The bottoms flow rate was specified as
142.46  93.10 = 49.36 kmol/h.
(a) For the equilibriumstage case, the total number of stages and the optimal feed stage
were varied, starting with 42 stages (including the condenser and reboiler) and a feed stage of 21
from the condenser, until the water mol% specification in the distillate was met. A satisfactory
result was achieved for 24 stages (including the condenser and reboiler) with the feed to stage 20
from the condenser. The splits were as follows: Component
Feed
Flow, kmol/h:
Methanol
93.11
Ethanol
5.00
Water
44.35
Total:
142.46
Mole fraction:
Methanol
0.6536
Ethanol
0.0351
Water
0.3113 Distillate Bottoms 89.49
3.60
0.01
93.10 3.62
1.40
44.34
49.36 0.9612
0.0387
0.000087 0.0734
0.0283
0.8983 Exercise 12.18 (continued)
Analysis: (continued)
(b) For the ratebased case, a sievetray column was used, with default properties and the
WilkeChang method for liquid diffusivities. The ChanFair method was used to estimate masstransfer coefficients and interfacial area. Plug flow was used for the vapor and the mixed flow
option was applied to the liquid. Initial...
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This document was uploaded on 02/24/2014 for the course CBE 2124 at NYU Poly.
 Spring '11
 Levicky
 The Land

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