Unformatted text preview: t data, using the FUG results, with N = 2 x Nmin.
A guess of R is made:
Pressures as stated above.
Number of stages = 12 (includes the total condenser and the partial reboiler)
Feed stage for recycle, D2 = 4 from the top
Entry for feed, F = 6 from top
Reflux ratio = 0.5
Bottoms mole flow rate = 55.1 mol/s
Estimated distillate rate = 275.76 mol/s
Estimated reflux rate = 138 mol/s
Estimated temperatures: Stage 1, 35oC Stage 12, 40oC Stage 2, 36oC Stage 11, 39oC Exercise 11.13 (continued)
Analysis: Column 1 (continued) A converged result, which gave a 99.8 mol% purity of ethanol in the bottoms, was obtained in 10
iterations. A purity of only 99 mol% was needed. This indicated that the reflux could be
reduced. Accordingly, using the profiles from the first rigorous run, a second run was made with
a column top specification of D = 173.75 mol/s in place of the reflux specification. A converged
result, with the desired 99 mol% ethanol purity in B1, was obtained at a reflux ratio of 0.279 in 24
iterations. The material balance was in agreement with the above table. Although the feed
locations could probably be improved, no pinch region is evident, as shown in the liquid-phase
composition profile on the next page. Also included on the next page are the column profiles for
temperature, liquid flow, and vapor flow.
Column 1 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 12 feet. The condenser duty was
computed to be 12.91 MW, with a reboiler duty of 11.58 MW. For the 10 theoretical plates, the
total pressure drop was computed to be 8.2 kPa. For a final design, the computations should be
repeated taking into account tray pressure drop and tray efficiency.
Using the above material balance table, the specifications for the ShortCut Column model
are 101.60/102.05 = 0.9956 for the recovery of ethanol to the distillate, and 129.30/173.75 =
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
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 = 101.60 mol/s
Bottoms mole flow rate = 44.9 mol/s
Estimated distillate rate = 230.90 mo...
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