Unformatted text preview: fer coefficients and interfacial area.
Assumptions: Assume the liquid phase is perfectly mixed and the vapor phase is in plug flow
on each tray. Operation is at about 80% of flooding.
Find: Separation achieved. The number of equilibrium stages to achieve the same separation.
Analysis: The Chemsep method is applied with the Extractive Distillation option, a total
condenser, a partial reboiler, and 42 stages (counting the condenser and the reboiler). Thus, the
feeds are sent to stages 6 and 36 from the top. To estimate Kvalues, the ideal gas law is
assumed with DIPPR vapor pressure, and excess enthalpy for the liquid phase.
For physical properties necessary to size the column diameter and compute masstransfer
coefficients and interfacial area, the following options were chosen:
Rackett equation for liquid density.
Wilke and DIPPR for vapor viscosity.
Molar averaging of DIPPR pure component liquid viscosity, vapor thermal conductivity, liquid
thermal conductivity, and surface tension. Binary liquid diffusivities from WilkeChang
method.
Neglect pressure drop.
Automatic initialization.
Newton's method, but maximum change of liquid or vapor flow rate of only 10% and maximum
temperature change of only 2oC.
Problem converged in 9 iterations in 8 seconds with a Pentium 90. The separation achieved is:
Component
Flow, kmol/h:
Acetone
Methanol
Water
Total:
Mole fraction:
Acetone
Methanol
Water 2 Feeds Distillate Bottoms 50.0
150.0
50.0
250.0 47.59
0.20
2.20
49.99 2.41
149.80
47.80
200.01 0.2000
0.6000
0.2000 0.9519
0.0040
0.0441 0.0120
0.7490
0.2390 Exercise 12.11 (continued) Analysis: (continued)
The backcalculated Murphree vapor tray efficiencies are computed by Chemsep to be:
From about 62.6 to 78.5% for acetone
From about 60.4 to 84.1% for methanol
A wide range of 20 to 172% for water
On the basis of these values, assume an overall average tray efficiency of 75%. Therefore, for an
equilibrium stage calculation, take 0.75(40) = 30 equilibrium stages or 32 total stages (including
the condenser and reboiler). Try feeding the water to stage 5 from the top and the main feed to
stage 1+0.75(35) = 27. However, a separation closer to the one for the ratebased result above
was achieved by sending the main feed to stage 23. The result is as follows with convergence in
7 iterations in 2 seconds with a Pentium 90: Component
Flow, kmol/h:
Acetone
Methanol
Water
Total:
Mole fraction:
Acetone
Methanol
Water 2 Feeds Distillate Bottoms 50.0
150.0
50.0
250.0 47.20
0.22
2.58
50.00 2.80
149.80
47.42
200.02 0.2000
0.6000
0.2000 0.9439
0.0045
0.0516 0.0140
0.7489
0.2371 Exercise 12.12
Subject: Separation of methanol (M) from a mixture containing isopropanol (P) and water (W)
by distillation in a packed column with a dumped packing, using a ratebased method.
Given: A bubblepoint feed of 100 kmol/h of M, 50 kmol/h of P, and 100 kmol/h of W at 1
atm. Column contains 40 ft of 2inch stainless steel Pall rings in two sections (one above the feed
and one below the feed). Feed entry is 25...
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 Spring '11
 Levicky
 The Land

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