2244 E. Oakridge Dr.
Layton, Utah 84040
December 8, 2005
Dr. Terry A. Ring
Department of Chemical Engineering
University of Utah
Salt Lake City, Utah 84112
Dear Dr. Ring:
During the period from November 15 to December 7, 2005, the members of
Group D characterized the ideality of the Continuously Stirred Tank Reactor (CSTR) at
30 rpm (donated by Chevron, located in MEB 3520f). The ideality of the CSTR was
characterized by performing a residence time distribution test and comparing the
measured mean residence time to the theoretical mean residence time. The CSTR was
found to be non-ideal at 30 rpm. However, the ideality of the CSTR improved as the
mixing speed increased, due to the promotion of turbulence. Additionally, the conversion
of the saponification of ethyl acetate reaction was investigated. Theoretically, 17.5%
conversion was expected if the reaction was performed just in a CSTR. Coupling a Plug
Flow Reactor (PFR) in front of the CSTR was predicted to increase the conversion to
18.6%. A maximum conversion of 20 ± 4.18% was obtained when the mixing speed was
60 rpm. However, the conversion was more consistent when the mixing speed was
greater than 200 rpm. This is again due to the promotion of turbulence in the system
which causes better mixing. A brief summary of the procedure and results follows:
The major pieces of equipment that were used in this project were a jacketed 1.22
± 0.03 L CSTR coupled to a PFR which was filled with glass beads and had a void
volume of 0.08 ± 0.001 L (see Figure 1, attached). In the first set of experiments, the
ideality of the CSTR was characterized using impulses of sodium hydroxide. The
hydroxide concentration was monitored using a pH 2500 Series meter (with an
uncertainty of ± 0.05 pH units) that was recorded by Opto 22 with a sampling interval of
1 second. The mixing speed in the CSTR was controlled by an ADI 1012 Applikon mixer
controller. In the second set of experiments, the PFR was connected to the CSTR in series
and the saponification reaction was performed at 20oC as measured by Omega type T
thermocouples. The temperature was maintained at 20oC by a VWR scientific water bath.
The reactant and product flow rates were maintained using Masterflex pumps (Cole
Parmer console drive model 7521-40, range 6-600 rpm). Finally, the flow rates were
measured by Dwyer flow meters (range: 0-20 gal/hr H2O).
The following procedure dictates the steps used to determine the ideality of the
CSTR and the conversion of the reaction performed in the coupled PFR and CSTR:
Determining the ideality of CSTR:
1 – A water bath at 20oC was turned on and allowed to fill the jackets of both the
PFR and CSTR. The system was also rinsed thoroughly with water at a flow rate of 8
gal/hr. A solution of NaOH was prepared. (The concentration of NaOH was 0.1 M in the
1
experiments with mixing speeds of 200-1000 rpm, and significantly lower in the
experiments with mixing speeds of 15-100 rpm. The exact initial NaOH concentration for
the lower mixing speed experiments was unknown, but not necessary because the
concentration was measured.)
2 – The speed for the CSTR mixer was set by using the Opto display time R3.1e
computer program. (Experiments were conducted with mixer speeds between 15 and
1000 rpm). Impulses of NaOH (approximately 40 mL) were inserted into the bottom of
the CSTR.
3 – The pH in the CSTR was monitored and recorded using Opto 22 while the
system was allowed reach the initial steady state pH value.
4 – Steps 2 and 3 were repeated with different mixing speeds to obtain pH
measurements for each mixing speed.
Measuring the conversion of reaction with coupled CSTR and PFR:
1 – A water bath was turned on and the system was rinsed as in step 1 from
“Determining the ideality of the CSTR.” The water was then emptied from the CSTR.
2 – A flow rate of 0.395 ± 0.008 L/min of each reactant was introduced into PFR.
The reaction mixture was directed through the coupled PFR/CSTR series.
3 – The reaction proceeded for 10 minutes (approximately 5 times the measured
mean residence time), which allowed the reaction to reach steady state. Then a sample of
the product was collected from the CSTR outlet.
4 – The reaction was quenched with 0.1 M HCl and then titrated with 0.1 M
NaOH until phenolphthalein turned purple (pH = 7). The amount of NaOH needed to
titrate the mixture was used to calculate the concentration of the product.
The ideality of the CSTR was characterized using mean residence time
distribution tests. The mean residence time was calculated using Equations (1)1 and (2)2,
while the variance was calculated using Equation (3)2,
C (t )  C (t  0)
(1)
E (t )  t
end
 C (t )  C (t  0)dt
0
tm 
t end
 tE(t )dt  
(2)
0
tend
 2   (t  t m ) E (t )dt
(3)
0
where E(t) is the residence-time distribution function, C(t) is the concentration of NaOH
(mol/L), tM and τ are both representations of the mean residence time (sec), t is the time
(sec), σ2 is the variance, and σ is the standard deviation of the residence time distribution.
The ideal residence time was calculated using Equation (4),
Chow, B., B. Wan, S. Philyaw, K.
. A. Ring “Resident Time Distributions for 1.3
L Stirred Tank Reactor.” Accessed online, December 8, 2005.
http://www.che.utah.edu/~ring/CrystallizationWeb/Residence%20Time%20Distributions%20in%20a%20S
tirred%20Tank-2.doc
2
Fogler, H. S. Elements of Chemical and Reaction Engineering. 3rd edition. Apprentice Hall: New Jersey,
2002. p. 821-836.
1
2
V
(4)
v
where τideal is the ideal residence time (sec), V is the volume of the CSTR (L) and υ is the
volumetric flow rate (L/sec).
 ideal 
The ideal residence time was calculated to be 185 ± 9.0 seconds. The NaOH
concentration profiles (calculated from the pH measurements) from the experiments with
a mixing speed range of 200 to 1000 rpm are shown in Figure 2, attached. These profiles
were obtained by averaging the raw data to decrease the error associated with random
fluctuations in pH readings. (The mixing speed range of 15-100 rpm is not shown
because the hydroxide concentration for those experiments was substantially smaller.
Therefore, the corresponding profiles could not be adequately visualized on the same
plot.) From these concentration profiles, the mean residence time was calculated. The
mean residence time for mixing speeds between 15 and 1000 rpm are shown in Figure 3,
attached, and compared to the theoretical mean residence time. The measured mean
residence time became more consistent after a mixing speed of 200 rpm, but was always
significantly lower than the ideal mean residence time. Additionally, the ideal standard
deviation should be equal to the mean residence time2. As shown in Table 1, attached, the
ratio of the mean residence time to the standard deviation became relatively stable and
close to 1 as the mixing speed increased. Unfortunately, the uncertainty in these
measurements was difficult to calculate and may not be exactly accurate. Nevertheless,
the uncertainty was large due to variations in flow rates as well as fluctuation in the
volume of the CSTR. Therefore, the CSTR was not ideal at low mixing speeds. This
could be due to dead zones, insufficient mixing and short-circuiting. However, the CSTR
became more ideal as the mixing speed increased, as shown by the ratio of the mean
residence time to the standard deviation.
The conversion of the saponification of ethyl acetate was investigated using a
PFR in series with a CSTR. The kinetics of this reaction and the reaction rate were
obtained from the literature.3 Our client was previously using a CSTR for this reaction
and wanted to know if adding a PFR in series or in parallel would increase the conversion
of reactants to products. Using a series configuration, the conversion was predicted to
increase. The order of reactors in this configuration was investigated to find which
reactor should be first to produce the most product. In order to maintain the integrity of
the glass beads in the PFR, the reactant concentrations had to be low (0.1 ± 0.005 M). At
this concentration, using a PFR first was predicted to yield a conversion of 18.56%, while
using a CSTR first was predicted to yield a conversion of 18.55%. Even though the
difference was small, the PFR was placed first in the series of reactors to produce the
highest conversion.
The saponification reaction was performed with several mixing speeds in the
CSTR. The best conversion was found at a mixing speed of 60 rpm. However, a
conversion above 12% was consistently found with a mixing speed above 200 rpm. The
measured conversions and their uncertainties are shown in Table 2, attached. These
Hovorka, R. B., and H. B. Kendall. “Tubular Reactor at Low Flow Rates.” Chemical Engineering
Progress. 56(8):58-62. 1960.
3
3
results indicate that a higher conversion will be more consistently obtained if a higher
mixing speed in the CSTR is employed.
Two other experiments were also performed. The saponification reaction was
performed with the CSTR (mixing speed of 300 rpm) placed first in the reactor
configuration and the reaction conversion was the same as when the experiment was
performed with the PFR first. This indicates that the predictions were accurate.
Additionally, the mixing speed where the transition to turbulence occurred was found by
changing the mixing speed and visualizing when eddies began to form. This mixing
speed was 200 rpm. The transition to turbulence explains why the ideality of the CSTR
increased when the mixing speed was above 200 rpm because eddies generate better
mixing. The transition to turbulence also explains why the conversion of the
saponification reaction is more consistent above 200 rpm.
The CSTR and PFR in MEB 3520f were characterized to the best of our ability in
these experiments. However, the characterization could be improved if more accurate
methods of maintaining the flow rate and the volume of liquid in the CSTR at constant
values were employed. Additionally, the ideality of the CSTR could be improved if
baffles were included and a larger impellor was used. The saponification reaction was
only performed at one temperature. Therefore, it is suggested that the reaction be
performed at multiple temperatures to identify the optimal temperature for this reaction
and configuration of reactors. Finally, the reaction conversion could be improved if the
void volume of the PFR was increased.
Sincerely,
Trina Sudweeks
4
PFR
CSTR
Figure 1. Schematic of CSTR/PFR apparatus. The reactants (ethyl acetate and sodium
hydroxide) were introduced into the PFR via a “tee” fitting to induce mixing. The
mixture was then sent to a CSTR to increase the conversion of the reaction. The products
(ethanol and sodium acetate) were removed from the top of the CSTR. The CSTR and
PFR were both jacketed and a water bath was used to maintain the reaction temperature
at 20oC.
5
0.0014
Concentration OH-
0.0012
0.001
200 rpm
300 rpm
400 rpm
500 rpm
600 rpm
700 rpm
800 rpm
900 rpm
1000 rpm
0.0008
0.0006
0.0004
0.0002
0
0
100
200
300
400
500
600
700
800
Time (s)
Figure 2. Hydroxide concentration profiles used to determine the ideality of the CSTR.
Each concentration profile had a peak at 40 seconds. These profiles were obtained by
averaging the raw data to decrease the error associated with random fluctuations in pH
readings. The uncertainty in time was ± 1.0 second. (The mixing speed range of 15-100
rpm is not shown because the hydroxide concentration for those experiments was
substantially smaller. Therefore, the corresponding profiles would not be adequately
visualized on this plot.)
6
mean residence time (s)
250
200
150
Experimental
Mean Residence
Time
100
Ideal Mean
Residence Time
50
0
0
200
400
600
800
1000
1200
RPM
Figure 3. Comparison of expected residence time and actual residence time. At mixing
speeds greater than 200 rpm, the mean residence time remained consistent. This indicated
that the CSTR was not ideal because the actual residence time was not within the range of
the ideal residence time. Discrepancies come from short circuiting, insufficient mixing
and dead zones. The uncertainty in mixing speed was ± 2.5 rpm.
Table 1. Mean residence times (τ) and the standard deviations (σ) were
calculated for a mixing range of 15 to 1000 rpm. The ideal mean residence time
was 185 ± 9.0 seconds. The ideal standard deviation was also 185 ± 9.0
seconds. Ideally, the standard deviation and mean residence time should be
equal (σ/τ = 1). The measured ratio was more ideal at higher mixing speeds.
RPM
TAU (SEC)
SIGMA (SEC)
SIGMA/TAU
15
30
45
60
100
200
300
400
500
600
700
800
900
1000
144.4
110.1
136.4
109.0
106.8
109.0
101.4
105.6
103.9
110.7
103.2
101.4
98.7
101.5
102.9
86.6
110.2
72.4
70.1
97.7
93.0
100.9
106.0
112.7
94.9
95.4
94.4
90.8
0.71
0.79
0.81
0.66
0.66
0.90
0.92
0.95
1.02
1.02
0.92
0.94
0.96
0.89
UNCERTAINTY
IN SIGMA/TAU
0.22
0.57
0.32
0.41
0.43
0.44
0.58
0.43
0.43
0.36
0.47
0.45
0.48
0.51
7
Table 2. Conversion of saponification reaction
at several mixing speeds. The largest conversion
was obtained with a mixing speed of 60 rpm.
However, the conversion was consistently above
10% when the mixing speed was greater than
200 rpm.
RPM
CONVERSION
30
60
100
200
300
400
500
600
700
1000
14%
20%
10%
10%
16%
12%
16%
14%
16%
18%
±
±
±
±
±
±
±
±
±
±
CONVERSION
ERROR
4.19%
4.18%
4.19%
4.19%
4.19%
4.19%
4.19%
4.19%
4.19%
4.18%
8