Unformatted text preview: ification for acetone purity, and a high acetone recovery were met. The following
Chemcad SCDS results, for the listed conditions, satisfy the product specifications, but other
combinations of stages, reflux, and solvent rate could also give satisfactory results:
Number of stages = 57 (includes total condenser and partial reboiler)
Solvent enters stage 12 from the top
Feed enters stage 36 from the top
Reflux ratio = 5
Bottoms flow rate = 78.774 mol/s
Solvent flow rate = 70 mol/s
Initial guesses:
Distillate rate = 108 kmol/h
Reflux rate = 540 kmol/h
o
Temperatures: Stage 1, 55 C
Stage 57, 75oC
Stage 2, 60oC
Stage 56, 70oC
Convergence was achieved in 34 iterations with the following material balance:
o Temperature, C
Flow rates, mol/s:
Acetone
Methanol
Ethanol
Total: Solvent
60 Feed
55.2 Distillate
56.3 Bottoms
75.4 0
0
70
70 30
10
0
40 29.87
0.11
1.24
31.22 0.13
9.89
68.76
78.78 The liquid phase composition profile is shown on the next page. The extractive distillation
column 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 7 feet, which gives 79.5% of flooding
at the top tray and 72.2% of flooding at the bottom.
The tray pressure drop averages 0.094 psi/tray, for a total of 5.2 psi or 0.35 atm for the 55 trays.
Thus, the assumption of negligible tray pressure drop is not satisfactory. For a final design, the
computations should be repeated taking into account tray pressure drop and tray efficiency. Analysis (continued) Exercise 11.9 (continued) Exercise 11.9 (continued)
Analysis (continued)
Ordinary Distillation Column: This column separates the bottoms from the extractive
distillation column to obtain 98 mol% methanol and 99.9 mol% ethanol. A starting point for this
separation is obtained by using the ShortCut Column (FUG) model of Chemcad with an
operating reflux ratio of 1.3 times the minimum ratio. The feed is assumed to be at the bubble
point. To achieve the purity specifications, a preliminary material balance gives:
mol/s:
Component
Feed Distillate Bottoms
Acetone
0.13
0.13
0.00
Methanol
9.89
9.82
0.07
Ethanol
68.76
0.07
68.69
Total:
78.78
10.02
68.76
The specifications for the ShortCut Column model are 0.982/0.989 = 0.993 for the LK
(methanol) and 0.07/68.76 = 0.001 for the HK (ethanol). The results are:
Minimum number of equilibrium stages = 24.6
Minimum reflux ratio = 11.9
Reflux ratio = 1.3(11.9) = 15.5
Number of equilibrium stages = 44.4
Feed stage location (Kirkbride equation) = 31 from the top
The SCDS Column model of the Chemcad program was then used to make a rigorous calculation
based on the following input data:
Pressure constant at 1 atm
Number of stages = 46 (includes the total condenser and the partial reboiler)
Feed stage = 32
Methanol purity in the distillate = 98 mol%
Ethanol purity in the bottoms = 99.9 mol%
Estimated distillate rate = 36.1 kmol/h (10.03 mol/s)
Estimated reflux rate = 560 kmol/h (155.6 mol/s or a reflux ratio of 15.5)
Estimated temperatures: Stage 1, 64oC Stage 46, 7...
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 Spring '11
 Levicky
 The Land

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