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Unformatted text preview: n Eqs. (1) and (2) are solved simultaneously, the results are Rmin = 1.058 and Nmin = 2.668.
Using the latter value in Eq. (1327),
.
01004
x DC =
= 0.0244
2 .668
2 .668
0.3989 2
+ 0.5007 15
.
+ 01004(1) 2.668
.
From Eq. (1325),
0.0244 2.668
0.0244
x DA = 0.3989
2
= 0.6164 and x DB = 0.5007
152.668 = 0.3592
.
0.1004
01004
. Exercise 13.25 (continued) Analysis:
The calculations above for the first time increment are repeated for subsequent time
increments, using a spread sheet, up to 1 hour. The results are as follows. Exercise 13.26
Subject: Calculation of batch rectification of a ternary mixture by a method that accounts for
holdup.
Given: Charge of 100 lbmol of 35 mol% nhexane (C6), 35 mol% nheptane (C7), and 30
mol% noctane (C8). Batch rectification at 1 atm. Column with the equivalent of 10 theoretical
stages, plus a partial reboiler and a total condenser. Constant reflux ratio, R = 5 and vapor boilup
rate, V = 50 lbmol/h. Total reflux condition is established by time zero. Ideal thermodynamics
(Raoult's law Kvalues). Holdup of condenserreflux drum = 1 lbmol. Three operation steps:
Step 1. Stop when accumulated distillate purity drops to 95 mol% C6.
Step 2. Empty the C6rich cut and resume rectification until instantaneous distillate composition
drops reaches 80 mol% C7.
Step 3. Empty the C6rich cut and resume rectification until accumulated distillate composition
reaches 4 mol% C8.
Assumptions: No pressure drop.
Find: Compositions and amounts of each of the 4 products for:
Column 1: Plate column with a total liquid holdup of 8 lbmol.
Column 2: Packed column with a total liquid holdup of 2 lbmol.
Effect of holdup for the two columns.
Analysis: Use the Batch distillation program of Chemcad to make the calculations. The
flowsheet below includes a batch column (1), a time switch (2), and 3 tanks (3, 4, and 5). Exercise 13.26 (continued)
Analysis (continued):
To determine the effect of holdup, make a run with no holdup in the column or the
condenserreflux drum. For all three runs, the initial charge is first brought to the bubble point
temperature, which is computed to be 193.2oF at 1 atm.
Case 1: Column and condenserreflux drup with no holdup:
The input data for Chemcad are as follows, where distillate rate = D = V/(1+R) = 50/6 = 8.333
lbmol/h.
Batch column:
No. of stages = 12 (10 in column + reboiler + condenser)
No. of operation steps = 3
Total condenser
Condenser pressure = 1 atm
Holdup units in moles (lbmoles in this case)
Condenser holdup = 0
Stage holdup = 0
Method of calculation = Insideout
Operation parameters:
Step
1
2
3
Startup option
Total reflux
Column content
Column content
Tank no.
3
4
5
First specification
Reflux ratio = 5
Reflux ratio = 5
Reflux ratio = 5
Second specification
Distillate rate = 8.333 Distillate rate = 8.333 Distillate rate = 8.333
lbmol/h
lbmol/h
lbmol/h
Step size, hr
0.02
0.02
0.02
Record frequency
3
3
3
Stop criterion
Mole fraction of C6 in Mole fraction of C7 in Mole fraction of C8 in
accumulator = 0.95 inst...
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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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