Unformatted text preview: ted reflux rate = 24.75 mol/s
Estimated temperatures: Stage 1, 35oC Stage 12, 55oC Stage 2, 40oC Stage 11, 50oC A converged result, which gave just slightly less than the desired 99 mol% purity of ethanol in
the bottoms, was obtained in 4 iterations. The material balance was almost in agreement with
the above table. Although the feed locations could probably be improved, no pinch region is
evident, as shown in the liquidphase composition profile below. Also included on the this page
are the column profiles for temperature, liquid flow, and vapor flow.
Column 1 was sized with Chemcad for sieve trays, with:
24inch tray spacing
12% downcomer area
10% hole area
1/4inch holes
80% of flooding
The resulting column inside diameter was determined to be 14 feet. The condenser duty was
computed to be 19.03 MW, with a reboiler duty of 16.42 MW. For the 5 theoretical plates, the
total pressure drop was computed to be 4.9 kPa. For a final design, the computations should be
repeated taking into account tray pressure drop and tray efficiency. Exercise 11.14 (continued)
Analysis: Column 1 (continued) Exercise 11.14 (continued)
Analysis: (continued)
Column 2:
Using the above material balance table, the specifications for the ShortCut Column model
are 182.41/183.22 = 0.9956 for the recovery of ethanol to the distillate, and 232.16/173.75 =
0.7442 for the recovery to the distillate of benzene. Thus, it is again expected that little
rectification is necessary. The feed to Column 2 is the distillate from Column 1 after being
pumped to a pressure of 120 kPa for entry into Column 2 and heating to the bubble point at that
pressure.
The results are:
Minimum number of equilibrium stages = 5.0
Minimum reflux ratio = very small and probably in error. Therefore, the Gilliland correlation
was not reliable.
The SCDS Column model of the Chemcad program experienced convergence difficulties
and was replaced with the TOWER model, which also experienced difficulty until the following
specifications were made with a damping factor of 0.7.
Pressures as stated above.
Number of stages = 6 (includes the total condenser and the partial reboiler)
Entry for feed = 2 from top (giving in effect a reboiled stripper with some reflux)
Distillate flow rate of ethanol = 182.41 mol/s
Bottoms mole flow rate = 80.61 mol/s
Estimated distillate rate = 414.57 mol/s
Estimated reflux rate = 41 mol/s (corresponding to a small reflux ratio of 0.1)
Estimated temperatures: Stage 1, 67oC Stage 6, 80oC Stage 2, 70oC
A converged result was obtained in 20 iterations, with a small reflux ratio of 0.155 or
64.1 mol/s. The material balance was in agreement with the above table. Although the feed
location could probably be improved, no pinch region is evident, as shown in the liquidphase
composition profile on the next page. Also included on the next page are the column profiles for
temperature, liquid flow, and vapor flow.
Column 2 was sized with Chemcad for sieve trays, with:
24inch tray spacing
12% downcomer area
10% hole area
1...
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 Spring '11
 Levicky
 The Land

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