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Unformatted text preview: t 25oC. This is a different datum than that used to get the given enthalpy of the hot feed.
Pure A: boils at 56.7oC.
Since CP of liquid =134 kJ/kmolK, hL at 56.7oC = 134(56.725)=4248 kJ/kmolK
hV at 56.7oC =4248 + latent heat = 4248 + 29750 = 33998 kJ/kmolK
Pure B: boils at 100oC.
Since CP of liquid =75.3 kJ/kmolK, hL at 100oC = 75.3(10025)=5648 kJ/kmolK
hV at 100oC =5648 + latent heat = 5648 + 42430 = 48078 kJ/kmolK
Equilibrium liquid mixture of 50 mol% A and 50 mol% B has a bubble point at 60oC. Therefore,
hL = 0.5(134)(6025) + 0.5(75.3)(6025) = 3663 kJ/kmolK
Equilibrium vapor mixture of 85 mol% A and 15 mol% B has dew point of 60oC. Therefore,
hV = 0.85[(134)(6025) + 29750] + 0.15[(75.3)(6025) + 42430] = 36034 kJ/kmolK
Calculations for other equilibrium mixtures are done in a similar manner and are summarized in
the following table: Analysis: (continued) Exercise 4.11 (continued) xA
xB hL, kJ/kmol
yA
yB hV, kJ/kmol
T, oC
56.7 1.000 0.000
4248 1.000 0.000
33998
57.1 0.920 0.080
4151 0.944 0.056
34656
60.0 0.500 0.500
3663 0.850 0.150
36034
61.0 0.330 0.670
3408 0.837 0.163
36296
63.0 0.176 0.824
3254 0.805 0.195
36880
71.7 0.068 0.932
3703 0.692 0.308
39069
100.0 0.000 1.000
5648 0.000 1.000
48078
From this table, the hxy plot follows, with tie lines to connect the vaporliquid equilibrium
along the dewpoint and bubblepoint lines. EnthalpyComposition Plot Exercise 4.12
Subject: Vaporizer and condenser heat duties for benzene (A) toluene (B) mixtures, using an
enthalpyconcentration diagram.
Given: P = 1 atm. Vapor pressure data. Saturated liquid and vapor enthalpy data.
Assumptions: Raoult's law.
Find: (a) Construct an hxy plot.
(b) Heat duty for 50 mol% vaporization of a 30 mol% A mixture, starting from liquid
saturation temperature. Heat duty to condense the vapor and subcool it 10oC.
Analysis: (a) First, compute the vapor and liquid equilibrium compositions at 1 atm and
temperatures from 60 to 100oC using Raoult's law with the vapor pressure data. Eq.
(244 ) applies, as well as the sum of the mole fractions in the phases in equilibrium. Thus,
KA = s
yA PA T
=
xA
P s
yB PB T
=
xB
P ,
, yA + y B = 1 KB = xA + x B = 1 (1, 2)
(3, 4) Equations (1) to (4) can be reduced to the following equations,
xA = 1 − KB
KA − KB , y A = KA x A (5, 6) Vapor pressure data in Exercises 4.6 for benzene, and 4.8 for toluene give Antoine equations,
PAs = exp 15.5645 − 2602.34
,
T + 211.271 PBs = exp 17.2741 − 3896.3
T + 255.67 (7, 8) Where vapor pressure is in torr and temperature is in oC. Solving, Eqs. (1) to (8),
T, oC Ps of A, torr Ps of B, torr
KA
KB
xA
yA 80.1
759.9
290.0 0.9998 0.3816 1.000
85.0
880.8
342.7 1.1590 0.4510 0.775
90.0
1019.1
404.4 1.3409 0.5321 0.579
95.0
1173.4
474.9 1.5439 0.6249 0.408
100.0
1345.0
555.2 1.7697 0.7305 0.259
105.0
1535.0
646.2 2.0198 0.8503 0.128
110.5
1766.8
760.1 2.3248 1.0001 0.000
This covers the temperature range of coexistence of vapor and liquid. 1.000
0.899
0.776
0.630
0.459
0.259
0.000 E...
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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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