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Determination of a reasonable estimate for the reflux ratio was difficult. A rough estimate was
made with the Class I Underwood equation (920), using Feed 1, with x = ethanol mole fraction 13
.
R = 13Rmin =
. xD
1 − xD
− α EtOH,H 2 O
x F1
1 − x F1
α EtOH,H 2 O − 1 13
.
= 0.925
1 − 0.925
− 105
.
0.88
1 − 0.88
1.05 − 1 = 10.3 Since the relative volatility in the section between the overhead and the Feed 1 entry is close to
one, the reflux ratio is sensitive to relative volatility. Accordingly, a conservative reflux ratio
estimate of 15 was used.
The specifications for running Chemcad with the SCDS model were as follows:
Feed 1 at the bubble point at 120 kPa. Feed 2 at the bubble point at 120 kPa. Thus, both feeds
would become partially vaporized upon entry into Column 1 operating under vacuum.
Column ∆P = 15 kPa
Top pressure = 13 kPa
Condenser ∆P = 2 kPa
No. of stages = 42 (includes total condenser and partial reboiler)
1st feed stage = 23
2nd feed stage = 35
Distillate flow rate = 78.82 mol/s (from above material balance)
Molar flow rate of ethanol in bottoms = 0.00007 mol/s
Estimated distillate rate = 78.82 mol/s (specified value)
Estimated reflux rate = 15D = 15(78.82) = 1182 mol/s
Estimated temperatures: Stage 1, 35oC Stage 12, 60oC Stage 2, 40oC Stage 11, 55oC Exercise 11.15 (continued)
Analysis: Column 1 (continued) A converged result, was obtained in 5 iterations, with a resulting reflux ratio of 13.47. The
material balance was in agreement with the above material balance table for the system.
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 next page are the column
profiles for temperature, liquid flow, and vapor flow, as well as the stage compositions at the
bottom of the column, which show rapid changes, making the high bottoms purity easy to attain.
Column 1 was sized with Chemcad for valve trays, with a 30inch tray spacing.
The resulting column inside diameter was determined to be 24.5 feet, which is large of the high
reflux ratio and vacuum conditions. The percent of flooding varied from 86% at the top to 38%
at the bottom. The condenser duty was computed to be 48.11 MW, with a reboiler duty of 47.42
MW. For the 40 theoretical plates, the total pressure drop was computed to be 32 kPa. For a
final design, the computations should be repeated taking into account tray pressure drop and tray
efficiency. Exercise 11.15 (continued)
Analysis: Column 1 (continued) Exercise 11.15 (continued)
Analysis: Column 1 (continued) Exercise 11.15 (continued)
Analysis: Column 1 (continued) Exercise 11.15 (continued) Analysis: (continued)
Column 2:
Using the TPXY plot feature of Chemcad, it was found that the relative volatility, α, for
water with respect to ethanol, at 300 kPa, is close to one, but does not vary widely over the
composition range, from 1.027 at 88 mol% ethanol at the distillate to 1.149 at 99.8 mol% etha...
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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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