Unformatted text preview: s vapor. Reflux
drum temperature is 100oF, and composition of total distillate in mole fractions is 0.10 C2, 0.20
C3, and 0.70 nC4. Reflux pressure is not given, but Kvalues at 100oF and 200 psia are given.
Assumptions: Kvalues are inversely proportional to pressure.
Find: Pressure in the reflux drum.
Analysis: As shown in Exercise 4.24, the compositions of net equilibrium vapor and liquid are
independent of recycle or, in this case, reflux. Therefore, the flash equations can be applied
using the total distillate composition as the feed composition. Therefore, Ψ =V/F = 0.10. The Kvalues
are given by: KC2 = 2.7 200
200
200
, KC3 = 0.95
, KnC 4 = 0.34
P
P
P Substituting these equations into Eq. (3), Table 4.4,
200
200
200
01 1 − 0.95
.
01 1 − 0.34
.
P
P
P
f { P} =
+
+
=0
200
200
200
1 + 01 2.7
.
− 1 1 + 0.1 0.95
− 1 1 + 0.1 0.34
−1
P
P
P
01 1 − 2.7
. (1) Eq. (1) is a nonlinear equation that can be solved by various means. Using a spreadsheet, in a
manner similar to that used to solve Exercise 4.24, we obtain P = 126 psia. Exercise 4.28
Subject: Comparison of flash calculations using three different Kvalue correlations.
Given: A stream at 7.2oC and 2,620 kPa with the overall composition given below.
Find: Phase conditions
Analysis: Using the CHEMCAD process simulator, the following results are obtained using the
SoaveRedlichKwong (SRK), PengRobinson (PR), and BenedictWebbRubinStarling
(BWRS) correlations:
Kvalues:
Component
N2
C1
C2
C3
nC4
nC5
nC6 SRK
17.5
5.73
1.07
0.327
0.098
0.036
0.0096 PR
17.6
5.71
1.12
0.337
0.102
0.032
0.0110 BWRS
16.3
6.00
0.99
0.292
0.087
0.029
0.0071 The Kvalues for the SRK and PR correlations are in reasonably good agreement, deviating from
each other by less than 15 %. Except for C1, the BWRS correlation predicts lower values, with
the biggest deviation for nC6.
Product compositions: Componen
t
N2
C1
C2
C3
nC4
nC5
nC6
Total: SRK:
f, kmol/h υ,
kmol/h
1.0
0.70
124.0
52.92
87.6
11.13
161.6
6.81
176.2
2.31
58.5
0.25
33.7
0.05
642.60
74.17 PR:
l, kmol/h υ,
kmol/h
0.30
0.69
71.08
52.06
76.46
10.45
154.79
6.42
173.89
2.16
58.25
0.26
33.65
0.04
568.43
72.08 BWR:
l, kmol/h υ,
kmol/h
0.31
0.67
71.94
53.04
77.15
9.63
155.18
5.67
174.04
1.88
58.24
0.21
33.66
0.03
570.52
71.13 l, kmol/h
0.33
70.96
77.97
155.93
174.32
58.29
33.67
571.47 All three correlations predict about the same V/F ratio, which ranges from 0.1107 to 0.1154. Exercise 4.29
Subject: Equilibrium flash calculations at different temperatures and pressures
Given: Mixture of 100 kmol of 60 mol% benzene (A), 25 mol% toluene (B), and 15 mol% oxylene (C). Sources of vapor pressure data.
Assumptions: Ideal solutions using vapor pressure with Raoult's law.
Find: Amounts and compositions of vapor and liquid products at:
(a) 100oC and 1 atm.
(b) 100oC and 2 atm.
(c) 105oC and 0.1 atm.
(d) 150oC and 1 atm.
Analysis: Instead of Figure 2.4 for the vapor pressures of benzene and toluene and three vaporpressure data points for oxylene, use the builtin vapor pressure data in the CHEMCAD pr...
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 Spring '11
 Levicky
 The Land

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