Unformatted text preview: oint, xW = zW = 0.5 and xF = zF = 0.5. Then, the only unknown in Eq.
(9) is P. Equation (9) is linear in P and, thus, can be solved directly to give a bubblepoint
pressure of 719 torr. The Kvalues at the bubble point are 1.015 for W and 0.984. The
composition of the vapor bubble is obtained from Eq. (2), which at the bubble point reduces to yi
= xiKi = ziKi . This gives yW = 0.508 and yF = 0.492.
(b) At the dew point, Ψ = V/F =1, and Eq. (1), combined with (4), becomes:
f{xW, xF, P} = zW P
P
− 1 + zF
−1 = 0
s
o
γ W {xW , xF } PW {107 C}
γ F {xW , xF } PFs {107 o C} where because yW = zW = 0.5 and yF = zF = 0.5, xW , and xF =(1  xW) are left as unknowns. The
liquid phase mole fractions are from Eq. (3), xi = zi /Ki . Solving these equations by trial and
error with a spread sheet, starting from P = 760 torr, xW = 0.5 and xF = 0.5, quickly leads to a
dewpoint pressure of 725.2 torr. The Kvalues at the dew point are computed to be KW = 1.006
and KF = 0.977, with xW = 0.497 and xF = 0.512.
(c) To determine the existence of an azeotrope, where yi = xi , a series of bubblepoint
calculations can be made, using the procedure in part (a), starting from say, xW = 0.05 in
increments of 0.05. If, in the range of xW from 0.05 to 0.95, the Kvalue of W switches from
more than 1 to less than 1, then an azeotrope exists in this range. The calculations can then be
refined. The result from a spreadsheet is a maximumboiling azeotrope at 718.2 torr with a
composition of 44.5 mol% W and 55.5 mol% F. Exercise 4.37
Subject: Bubble point, dew point, and equilibrium flash of a ternary mixture.
Given: Mixture of 45 mol% nhexane (1), 25 mol% nheptane, and 30 mol% noctane.
Assumptions: Applicability of SRK method for estimating Kvalues.
Find: (a) Bubblepoint temperature at pressures of 5, 1, and 0.5 atm.
Dewpoint temperature at pressures of 5, 1, and 0.5 atm.
(b) Temperature and phase compositions for flash of V/F = 0.5 at 5, 1, and 0.5 atm.
(c) noctane in the vapor if 90% of the nhexane is vaporized at 1 atm.
Analysis: The following results are obtained with the CHEMCAD simulator.
(a) Bubblepoint and dewpoint temperatures:
Pressure, atm
Bubblepoint T, oF
Dewpoint T, oF 0.5
147
178 1
187
216 5
307
330 (b) 50 mol% vaporization of feed:
Pressure, atm
Temperature, oF
Vapor mole fractions:
nHexane
nHeptane
nOctane
Liquid mole fractions:
nHexane
nHeptane
nOctane
Moles V/mole F
Moles L/mole F 0.5
162 1
201 5
318 0.615
0.231
0.154 0.599
0.233
0.168 0.554
0.239
0.207 0.285
0.269
0.446
0.5
0.5 0.301
0.267
0.432
0.5
0.5 0.346
0.261
0.393
0.5
0.5 (c) By iteration on the isothermal flash calculation, for 90 mol% of nhexane to vapor,
need a temperature of 210.2oF at 1atm. This gives 80.1 mol% vaporization with 65%
vaporization of noctane. Exercise 4.38
Subject: Vaporization of column bottoms in a partial reboiler.
Given: 150 kmol/h of bubblepoint liquid, L1, at 758 kPa, with a molar composition of 10%
propane, 40% nbutane, and 50% npentane leaving bottom sta...
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 Spring '11
 Levicky
 The Land

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