CHE 425 (Fall 2010)
__________________
LAST NAME, FIRST
Optional set 1 (Show all your work)
Note: Your answers must be correct to 3 significant figures and have the appropriate units.
I. A mixture contains 55 mole % isobutane and 45 mole % isopentane is at 30 psia. The K values
for these compounds can be obtained from
ln K = A/T2 + B + C ln P where T is in oR and P is in psia
Compound
A
B
Isobutane
-1,166,846
7.72668
Isopentane
-1,481,583
7.58071
C
-.92213
-.93159
1) The vapor mole fraction of isobutane at the bubble point of 527.6oR is
___________
2) The liquid mole fraction of isobutane at the dew point of 549.7oR is
___________
3) The mixture is flash at 536.0 oR, 30 psia where V/F = 0.4,
then the mole fraction of isobutane (iC4) in the liquid phase is
__________
II. A liquid containing 50 mol % benzene and 50 mol % toluene is subjected to a flash
vaporization process at a pressure of 760 mmHg where the temperature inside the flash drum is
o
o
95oC. At 95oC, Pbenzene
= 1176.8 mmHg, Ptoluene
= 476.87 mmHg
(4) The mole fraction of benzene in the exit liquid is
____________
III. A homogeneous reaction A  P takes place inside a porous cylindrical catalyst with radius
R. The two ends of the catalyst particle are impermeable to A so diffusion is only in the r
direction. The length of the catalyst particle is L = 1 cm. The effective diffusivity, De, of A
within the catalyst is 1.210-7 m2/s. The concentration of A within the catalyst is given by

 r 
CA = CAo 0.20  sinh   , where CAo = 0.10 kmol/m3 and R = 1.00 mm
 R 

(5) The molar flux of A at r = R/2 is
__________
(6) The consumption rate of A within the catalyst is
__________
IV. An ethanol-water separation is achieved in a distillation column at 1 atm. Feed flow rate is
5,000 lb/hr. The distillate contains 90 wt% ethanol, and the bottoms contain 5 wt% ethanol.
Assume theoretical trays, a total condenser, a partial reboiler, and saturated reflux.
7) The minimum number of equilibrium stages (including the reboiler is)
________
8) If the reflux ratio R is 1.8, the enthalpy H’ of the upper operating point D’ in Btu/lb is _____
9) If the feed contains 30 wt% ethanol and H’ = 1,200 Btu/lb, the condenser duty is
___________
10) If the feed is saturated liquid with 30 wt% ethanol and the enthalpy of W’ =  100 Btu/lb,
then the enthalpy of the upper operating point D’ in Btu/lb is
__________
11) If the feed is saturated vapor at 190oF, then the minimum reflux ratio is
__________
12) If the feed is saturated liquid, the enthalpy H’ of the upper operating point D’ is 1100 Btu/lb,
and the enthalpy of W’ is  100 Btu/lb then temperature of the feed in oF is
__________
V. A saturated vapor mixture of benzene and toluene containing 40 mole percent benzene is to
be separated into an overhead product containing 98 mole percent benzene and a bottom product
containing 2 mole percent benzene. Benzene and toluene mixture may be considered as ideal.
Use optimum feed plate.
ln Psat = A  B/(T + C) P where T is in K and Psat is in kPa
Compound
benzene
toluene
A
14.1603
14.2515
B
2948.78
3242.38
13) If the feed is at 420 K, the feed pressure is
C
 44.5633
 47.1806
__________
14) Consider an equilibrium tray at 415 K and the liquid mole fraction of benzene leaving the
tray is 0.3, the vapor mole fraction of benzene leaving the tray is
____________
15) If the reflux ratio is 3.0 and the liquid mole fraction of benzene leaving an equilibrium tray is
0.3, the vapor mole fraction of benzene entering the tray is
____________
16) If the reflux ratio is 3.0 and the vapor mole fraction of benzene entering an equilibrium tray
is 0.35, the liquid mole fraction of benzene leaving the tray is
VI. A feed with q = 0.4, consisting of 100 mol/s phenol and cresols, is separated in a plate
column with a total condenser and a partial reboiler. Assume CMO with a reflux ratio of 3. The
following compositions and relative volatilities are given
Compound
1) Phenol
2) o-cresol
3) m-cresol
Feed, mol%
40
25
35
Distillate, mol%
94.2
4.5
1.3
Bottoms, mol%
5.2
17) The mole fraction of o-cresol leaving the column in the bottoms is
Relative volatility
1.30
1.00
0.70
____________
18) The mole fraction of phenol in the liquid stream leaving the top (first) tray is ____________
19) The mole fraction of o-cresol in the liquid stream leaving the top tray is
____________
20) The liquid flow rate in the stripping section of the column is
____________
21) The vapor flow rate in the stripping section of the column is
____________
The mole fraction of phenol and o-cresol in the liquid stream leaving the second tray are 0.8945
and 0.0680, respectively.
22) The mole fraction of phenol in the vapor stream entering the second tray is
____________
23) The mole fraction of o-cresol in the vapor stream entering the second tray is
____________
VII. 10,000 lb/hr of saturated liquid at 1 atmosphere is fed into an adiabatic flash drum operating
at 0.1 atmosphere. The feed stream is a binary mixture of 29 wt% ethanol and 71 wt% water.
24) The temperature in the flash drum is
____________
25) The mole fraction of ethanol in the vapor phase is
____________
VIII. (26) The degree of freedom for an equilibrium stage with feed and heat transfer for 10
components is
____________
IX. 1000 kg/hr of a saturated liquid with 30 wt% ethanol in water is fed into a distillation
column. We desire a distillate composition xD = 0.90 mass fraction ethanol and a bottoms
composition xB = 0.02 mass fraction ethanol. Reflux is return as a saturated liquid with a reflux
ration of 1.5. The column is adiabatic with a partial reboiler and a total condenser. The enthalpies
of ethanol-water system are given below
h(kJ/kg) = 114x2  279x + 393 = enthalpy of saturated liquid mixture
H(kJ/kg) =  1614y + 2676
= enthalpy of saturated vapor mixture
x, y = mass fraction of ethanol in the liquid and vapor mixture, respectively
27) The condenser duty in kJ/hr, QC, is
____________
28) If QC = 2.0×105 kJ/hr, determine the reboiler duty, QR, in kJhhr
____________
X. Consider an air cooler where the flow rate of water is large compared to the air, and the
effectiveness, , of the cooler is known to depend on the flow rate of air according to the relation,
0.3
 ~  mair  . The air inlet temperature is 25oC.
29) The water inlet temperature is 100oC. If the air flow rate increases 50%, the percentage
increase in the heat removal rate is
__________
30) At a given air flow rate, the outlet air temperature is 50oC. When the air flow rate increase
50%, the heat transfer rate increase 20%, the new air outlet temperature is
__________
XI. Component A in a water stream is to be stripped out using an air stream in a counter current
staged stripper. Inlet air is pure with a flow rate of 600 lbmol/hr. Inlet liquid stream has 25 mol
% A and a flow rate of pure water of 500 lbmol/hr. The desired outlet mole fraction of A is x =
0.05. Assume that water is nonvolatile and air is insoluble. Equilibrium data can be represented
by y = 1.3x for x < 0.5 .
31) At a location in the stripper where mole ratio X = 0.12 then
Y = __________
32) At a location in the stripper where mole ratio Y = 0.12 then
X = __________
XII. A cross flow heat exchanger consists of 120 tubes, arranged in four rows with two passes.
Process water in the tube at 100oC, 20 kg/s is to be cooled to 80oC by cross-flow of air at 30oC, 1
atm. The tube has an inner diameter of 20 mm, outer diameter of 24 mm, and length of 12.0 m.
The mass flow rate of air is 40 kg/s. The following data are given: water = 995 kg/m3, air = 1.16
kg/m3, cp,water = 4178 J/(kgK), and cp,air = 1007 J/(kgK).
33) The total inside area for heat transfer is
___________
34) The velocity of water is
___________