Page 192 - Separation process principles 2

Exercises 157

4.30 Prove that the vapor leaving an equilibrium flash is at its
dew point and that the liquid leaving an equilibrium flash is at its
bubble point.
4.31 The following mixture is introduced into a distillation col-
umn as saturated liquid at 1.72 MPa. Calculate the bubble-point v
Reboiler
temperature using the K-values of Figure 2.8.
Compound kmolh QR
Figure 4.41 Conditions for Exercise 4.38.
Ethane 1.5
Propane 10.0
n-Butane 18.5
4.37 For a mixture consisting of 45 mol% n-hexane, 25 mol%
n-Pentane 17.5
n-heptane, and 30 mol% n-octane at 1 atm, use a simulation
n-Hexane 3.5
computer program to:
4.32 An equimolar solution of benzene and toluene is totally (a) Find the bubble- and dew-point temperatures.
evaporated at a constant temperature of 90°C. What are the pres- (b) Find the flash temperature, and the compositions and relative
sures at the beginning and end of the vaporization process? Assume amounts of the liquid and vapor products if the mixture is subjected
an ideal solution and use the vapor pressure curves of Figure 2.4. to a flash distillation at 1 atm so that 50 mol% of the feed is
vaporized.
4.33 The following equations are given by Sebastiani and
Lacquaniti [Chem. Eng. Sci., 22, 1155 (1967)l for the liquid-phase (c) Find how much of the octane is taken off as vapor if 90% of the
activity coefficients of the water (W)-acetic acid (A) system. hexane is taken off as vapor.
Repeat parts (a) and (b) at 5 atm and 0.5 atm.
log ny = X;[A + B(4xw - 1) + C(xw - xA)(6xw - I)]
4.38 In Figure 4.41, 150 krnoVh of a saturated liquid, L1, at
log y~ = X&[A + B(4xw - 3) + C(xw - xA)(6xw - 5)]
758 kPa, of molar composition, propane lo%, n-butane 40%, and
n-pentane 50%, enters the reboiler from stage 1. What are the com-
positions and amounts of VB and B? What is QR, the reboiler duty?
Use a simulation computer program to find the answers.
4.39 (a) Find the bubble-point temperature of the following mix-
ture at 50 psia, using K-values from Figure 2.8 or Figure 2.9.

Find the dew point and bubble point of a mixture of composition Component Zi
xw = 0.5, XA = 0.5 at 1 atm. Flash the mixture at a temperature
Methane 0.005
halfway between the dew point and the bubble point. Ethane 0.595
4.34 Find the bubble-point and dew-point temperatures of a n-Butane 0.400
mixture of 0.4 mole fraction toluene (1) and 0.6 mole fraction
n-butanol (2) at 101.3 Ha. The K-values can be calculated from (b) Find the temperature that results in 25% vaporization at
(2-72), the modified Raoult's law, using vapor-pressure data, and yl this pressure. Determine the corresponding liquid and vapor
and from the van Lax equation of Table 2.9 withAI2 = 0.855 and compositions.
AZI = 1.306. If the same mixture is flashed at a temperature midway 4.40 As shown in Figure 4.42, a hydrocarbon mixture is heated
between the bubble point and dew point, and 101.3 kPa, what frac- and expanded before entering a distillation column. Calculate,
tion is vaporized, and what are the compositions of the two phases? using a simulation computer program, the mole percent vapor
4.35 (a) For a liquid solution having a molar composition of phase and vapor and liquid phase mole fractions at each of the three
ethyl acetate (A) of 80% and ethyl alcohol (E) of 20%, calculate the locations indicated by a pressure specification.
bubble-point temperature at 101.3 kPa and the composition of the
corresponding vapor using (2-72) with vapor pressure data and the
van Laar equation of Table 2.9 with AAE = 0.855, AEA = 0.753. 100 Ibmollh 260 OF,
(b) Find the dew point of the mixture. 150 OF, 260 psia 250 psia h A 100 psia *
(c) Does the mixture form an azeotrope? If so, predict the temper-
ature and composition.
4.36 Abinary solution at 107OC contains 50 mol% water (W) and
50 mol% formic acid (F). Using (2-72) with vapor pressure data
and the van Laar equation of Table 2.9 with AWF = -0.2935 and Mole
Component fraction
AFW = -0.2757, compute:
C2 0.03
(a) The bubble-point pressure. c3 0.20
(b) The dew-point pressure. nC4 0.37
nC5 0.35
Also determine whether the mixture forms a maximum- or nC6 -
0.05
minimum-boiling azeotrope. If so, predict the azeotropic pressure 1 .oo
at 107°C and the azeotropic composition. Figure 4.42 Conditions for Exercise 4.40.