Unformatted text preview: or solving Eq. (1) is to use a spreadsheet to make a plot of f{Ψ} vs. Ψ in
increments of 0.1 from 0.0 to 1.0. Then, use smaller increments in Ψ in the vicinity of f{Ψ}= 0
to obtain the solution. The results are shown in the two figures on the next page. The converged
solution is
Ψ = 0.8697. Therefore, V = 0.8697(4,600) = 4,000.6 lbmol/h. The composition of the
equilibrium vapor from Eq. (2) is as follows: Component
Hydrogen
Methane
Benzene
Toluene zi
0.4348
0.4348
0.1087
0.0217 yi
0.4990
0.4925
0.0078
0.0007 xi
0.0062
0.0493
0.7820
0.1625 Exercise 4.24 (continued)
Analysis: (a) (continued) Exercise 4.24 (continued)
Analysis:
(b) When the flash conditions of temperature and pressure are fixed, the compositions of the
equilibrium vapor and liquid are independent of any recycle of equilibrium liquid or vapor. To
prove this, draw a material balance envelope around the system in Fig. 4.38 as shown below.
Now, the flash equations are the same as in Table 4.3, except for the energy balance, Eq. (6).
But, that equation is only solved after all of the other equations are solved. Thus, the results for
the compositions of the net vapor and liquid products are the same as when there is no recycle. Exercise 4.25
Subject: Partial condensation of a gas mixture at 120oF and 300 psia.
Given: Gas at 392oF and 315 psia, with a composition in kmol/h of 72.53 N2, 7.98 H2, 0.13
benzene, and 150 cyclohexane. The gas is cooled and partial condensed to 120oF and 300 psia,
followed by phase separation.
Find: Equilibrium vapor and liquid flow rates and compositions.
Analysis: The flash calculations are made conveniently with a process simulator, using an
appropriate Kvalue correlation. The following results were obtained with CHEMCAD, using
the ChaoSeader, GraysonStreed (CSGS) method for Kvalues.
Component
Hydrogen
Nitrogen
Benzene
Cyclohexane CSGS Ki
79.7
7.54
0.024
0.022 fi , kmol/h
72.53
7.98
0.13
150.00 υi , kmol/h
70.82
6.36
0.0016
1.67 li , kmol/h
1.71
1.62
0.1284
148.33 Exercise 4.26
Subject: Rapid determination of phase condition without making a flash calculation.
Given: A hydrocarbon mixture at 200oF and 200 psia, with a composition in lbmol/h of 125 C3,
200 nC4, and 175 nC5, and Kvalues at these conditions.
Find: Phase(s) present without making a flash condition.
Analysis: From Eq. (412), have a subcooled liquid if C
i =1 From Eq. (413), have a superheated vapor if zi Ki < 1 .
C i =1 Component
C3
nC 4
nC5
Total: fi
125
200
175
500 zi
0.25
0.40
0.35
1.00 Ki
2.056
0.925
0.520 zi
< 1.
Ki zi K i
0.514
0.370
0.182
1.066 > 1 zi/Ki
0.122
0.432
0.673
1.227 > 1 Therefore stream is partially vaporized. Both vapor and liquid phases present. Exercise 4.27
Subject: Determination of refluxdrum pressure for a specified temperature and total distillate
(vapor and liquid phases) composition
Given: Overhead partial condensing system of a distillation column that produces vapor
distillate, liquid distillate, and liquid reflux. Of the total distillate, 10 mol% i...
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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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