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:
0.3113 Distillate Bottoms 89.45
0.8983 These results for the rate-based case compare well with the equilibrium-stage case. They indicate
an overall tray efficiency of approximately 22/25 = 0.88 or 88%.
The computed Murphree vapor tray efficiencies were as follows:
78.2 to 88.5%
44.0 to 127.8%, a very wide range, with most values between
44 and 128%
82.4 to 90.4%
Computing time was 2 s with 3 iterations for the equilibrium-stage calculation.
Computing time was 5 s with 5 iterations for the rate-based 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 rate-based 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. 2-inch metal Pall-ring 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 Wilke-Chang
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 Billet-Schultes method of
1992. Mass-transfer coefficients and interfacial area were estimated with the Billet-Schultes
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:
Feed Distillate Bottoms
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