Fall 2011
ChE 471
Exam 3
(closed book and closed notes, three crib sheets allowed)
1.
A first order irreversible reaction A  products (– RA = 2.5 CA (mol/L min)) is
conducted in a flow reactor at constant temperature. The reactor volume is V = 200 L
and it processes 300 L/min of the feed at CAo = 1 (mol/L).
a)
What conversion would we obtain if this reactor behaved as a:
i) CSTR, ii) PFR?
You perform an impulse tracer experiment on your actual reactor and obtain the following
tracer response to an instantaneous injection of mt = 6000 mg of tracer
C
20
mg

 

 L 
t(min)
1
b)
c)
d)
e)
2.
Run 1
Run 2
2
Is the mass balance preserved for the tracer experiment and if so, what is the E–
curve?
Is all the volume of the system accessible to flow?
Model the observed E-curve with an appropriate number of stirred tanks in series and
predict conversion based on that model.
How can you predict most accurately the conversion that will be achieved in your
reactor with the E-curve obtained from the above tracer experiment and what is that
conversion?
A first order irreversible catalytic reaction A → P is carried out at isothermal
conditions, in absence of all external transport effects, in a stirred basket reactor on
two different size catalyst pellets. The pellets are spherical. The catalyst activity and
pore structure of the pellets are identical. Therefore the kinetic rate constant and
effective diffusivity are identical in pellets of both sizes. The temperature, pressure
and bulk reactant concentration are identical in both runs. The following data are
obtained.
Pellet Diameter
(cm)
1.0
0.1
1
Observed Rate x 105
(mol/cm3cat.s)
3.0
15.0
Estimate the Thiele modulus  and effectiveness factor  for each particle. Are
internal diffusion effects pronounced in each pellet size?
What pellet diameter is needed to completely eliminate internal diffusion resistance at
the temperature of these experiments (i.e.   1).
a)
b)
3.
What can you tell about the controlling resistance (film, diffusion, kinetic) and activation
energy for a 1-st order catalytic reaction based on the data below obtained in a small packed
bed reactor with large gas recycle. Therecycle rate is always sufficiently high to achieve
CSTR behavior. Changes in recycle rate above the high value affect and further improve
the external mass transfer coefficient. Isothermal conditions prevail in the reactor during
each run and particle temperature equals the exit gas temperature. Constant density can be
assumed. (No need to worry about units for the data below, they are consistent. Note that
temperature is reported in degrees centigrade).
Run
1
2
3
4
Volumetric
Flow Rate
5
2
8
9
CA feed
CA exit
5
4
2
3
1
1
1
1
Mass of
Catalyst
10
6
4
9
Pellet
Diameter
1
2
2
2
Recycle
Rate
high
highest
highest
high
Temp
(oC)
320
320
360
360
a) What is the dominant resistance?
b) What is the activation energy? Report it in (cal/mol). Hence R= 1.987 (cal/mol K)
Document and explain your answers.
4.
A secretary spilled coffee all over the flow chart for your process line in which A
isomerizes to B on a solid catalyst in packed bed adiabatic reactors. However, the
conversion-temperature diagram has been saved and is shown below. Your boss asked you
to reconstruct the process flow chart from that diagram.
Show:
a)
The molar flow rates of all streams. (If recycle is involved calculate all recycle
ratios).
b)
The location and heat duty of all heat exchangers, and indicate whether they add or
remove heat and report the rate of heat removal or addition in calories per hour.
Data: Molar feed rate Fo = 1,000 (mol/h), Cp = 300 (cal/mol K)= ρCP/CA0;
H RA  26,000(cal / mol) (Hence, the slope of adiabatic lines ab and cd is Cp/(-ΔHRA)
To avoid the ambiguity in reading the attached graph, the coordinates (T, xA) of the points
needed for your calculations are given here: b(350, 0.4); e(365, 0.4); f(305, 0.4).
In addition note that conversion at point a is 0.2; at point c is 0.5; and at point d is 0.8.
2
1.0
d
0.8
0.6
xA
c
0.4
b
f
0.2
e
a
g
350
400
To = 300K
3
T(K)