Unformatted text preview: ly, a tray efficiency of 80% was used to obtain an initial
estimate of the number of actual trays and feed tray location. These values were then varied to
meet the mol% water specification in the distillate. The result was 25 trays plus a condenser and
reboiler, with the feed to tray 21 from the top. 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 89.45
3.64
0.01
93.10 3.66
1.36
44.34
49.36 0.9607
0.0392
0.0001 0.0743
0.0274
0.8983 These results for the ratebased case compare well with the equilibriumstage case. They indicate
an overall tray efficiency of approximately 22/25 = 0.88 or 88%.
The computed Murphree vapor tray efficiencies were as follows:
Methyl alcohol:
78.2 to 88.5%
Ethyl alcohol:
44.0 to 127.8%, a very wide range, with most values between
44 and 128%
Water:
82.4 to 90.4%
Computing time was 2 s with 3 iterations for the equilibriumstage calculation.
Computing time was 5 s with 5 iterations for the ratebased method of calculation. Exercise 12.19
Subject: Separation of methanol (M) from a mixture with ethanol (E) and water (W) by
distillation in a packed bed, using a ratebased method.
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. 2inch metal Pallring packing.
Assumptions: Operation at about 75% of flooding.
Find: Height of packing above and below the feed entry.
Analysis: The ChemSep method was applied, using the ideal gas law, Antoine vapor pressure,
and excess enthalpy with UNIFAC. Default properties were selected with the WilkeChang
method for liquid diffusivities. In section above the feed, 80 segments (called stages) were used.
In the section below the feed, 20 segments were used. A column bottoms pressure of 1.3 atm
was assumed, but the actual pressure drop was estimated using the BilletSchultes method of
1992. Masstransfer coefficients and interfacial area were estimated with the BilletSchultes
method of 1992. 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. To determine an initial estimate of the packed
heights above and below the feed, an HETP of 0.6 m was assumed giving 12.8 m above the feed
and 2.4 m below. These heights were then varied to meet the mol% water specification in the
distillate. The final result, which was achieved in 14 s with 8 iterations, was 23 m above the feed
and 4 m below. The split was:
Component
Feed Distillate Bottoms
Flow, kmol/h:
Methanol
93.11
89.28
3.84
Ethanol
5.00
3.82
1.1...
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 Spring '11
 Levicky
 The Land

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