Transcript Ideality of a CSTR - Department of Chemical Engineering

Ideality of a CSTR
Jordan H. Nelson
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Brief Overview
Introduction – General CSTR Information
Three Questions
Experimental Conclusions
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Schematic of the CSTR
Item
Description
1
Mixing Point
2
Mixing Point
3
Mixing Point
4
Mixing Points
5
Water Bath Inlet and Outlet
6
Four Wall Mounted Baffles
7
Mixer Drive
8
Marine Type Impeller
9
CSTR Vessel
10
Water Bath Vessel
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3 Questions

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
?
Where is the best mixing in the CSTR?
What is τmean and how does it compare to
τideal?
What configuration of PFR-CSTR will
produce the greatest conversion?
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Where is the Best Mixing?

Impeller selection

Food Dye Test

Dead Zones

Impeller Speed
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Flow Patterns of different impellers
Rushton Impeller
Marine Impeller
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τMean vs τIdeal ?
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τMean – Measured mean residence time
The amount of time a molecule spends in
the reactor
τIdeal – Ideal residence time is calculated
from the following equation
 ideal 
V
o
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Experiment

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
Fill reactor with low concentration salt
(baseline)
Spike reactor at most ideal mixing
Create spike concentration at least one
order of magnitude larger than baseline
Measure change in conductivity over time
Run experiment at different impeller
speeds
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Yikes!
Plot of Concentration vs Time with Error
35
30 RPM
15 RPM
Concentration NaCl(g/mL)
30
25
20
15
10
0
100
200
300
400
500
600
700
Time(s)
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800
Measured Concentration over time
in the CSTR.
26
Concentration NaCl(g/mL)
25
24
30 RPM
15 RPM
23
22
21
20
0
200
400
600
800
Time(s)
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RTD Function E(t)

Measured concentrations are used to
create the residence time distribution
function
E (t ) 
C (t )  C (t  0)
tend
[
C
(
t
)

C
(
t

0
)]
dt

0
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Plot of an ideal residence time distribution
function
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Residence time distributions
0.0023
0.0021
0.0019
E(t)
0.0017
0.0015
0.0013
0.0011
Ideal E(t)
0.0009
E(t) Conductivity 15 RPM
E(t) Conductivity 30 RPM
0.0007
0.0005
0
20
40
60
80
100
120
140
160
180
200
Time(s)
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Mean Residence Time

Using E(t) the following equations produce
the mean residence time
t mean 
t end
tE
(
t
)
dt


mean

0
t end
   (t  t m ) E (t )dt
2
0
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Comparison of Residence Times
RPM
Mean
Residence
Time
Standard
Deviation
Sigma
Sigma/
Tau
15
357.57
11.58
206.87
0.58
30
358.14
11.58
206.35
0.58
466.97
5.90
Ideal
CSTR
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Loss of Data


Over an hour of data was lost from Opto 22
Calculation of Reynolds number over 4000
2
(Turbulent)



ND

Equation applies to a baffled CSTR
RPM speed of 300 obtained full turbulence
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CSTR-PFR Configurations ?
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Schematic of arrangements
Levenspiel Plot
Conduct saponification reaction in the
reactor at different RPM’s
Use Equimolar flow rates and
concentrations of reactants
Quench reaction with a HCl and titrate
with NaOH
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Series Reactor with CSTR Before
PFR.
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Series Reactor with PFR Before
CSTR.
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Et  Ac  NaOH  NaAc  Et  OH
Levenspiel Plot for NaOh+EtOAc
8
Levenspiel Plot for
NaOh+EtOAc
-1/ra
6
4
2
0
0
0.1
0.2
0.3
0.4
0.5
0.6
Conversion
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CSTR-PFR Configurations ?





Schematic of arrangements
Levenspiel Plot
Conduct saponification reaction in the
reactor at different RPM’s
Use Equimolar flow rates and
concentrations of reactants
Quench reaction with a HCl and titrate
with NaOH
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Measured Conversion for PFR-CSTR
Configuration
Speed
(RPM)
Conversion
(%)
Conversion
Error (%)
30
19.7
+/-
4.30
60
21.7
+/-
3.91
200
21.2
+/-
4.00
400
24.3
+/-
3.48
875
24.7
+/-
3.41
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Measured Conversion for CSTR-PFR
Configuration
Speed
(RPM)
Conversion
(%)
Conversion
Error (%)
30
21.5
+/-
3.94
60
21.2
+/-
4.00
200
21.4
+/-
3.97
400
20.9
+/-
4.06
875
21.5
+/-
3.94
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3 Questions



?
Where is the best mixing in the CSTR?
What is τmean and how does it compare to
τideal?
What configuration of PFR-CSTR will
produce the greatest conversion?
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Conclusions




Better mixing for a Rushton impeller is
below the impeller
The reactor is far from ideal at low
impeller speeds
The PFR-CSTR arrangement provided
better conversions
Run the PFR-CSTR reactor at RPM’s of
higher than 300
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Opportunities



Run the experiment again to obtain the
lost residence time values
Run the saponification reaction at higher
temperatures
Exit sampling stream should be at the
bottom of the reactor
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Acknowledgements

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
Taryn Herrera
Robert Bohman
Michael Vanderhooft
Dr. Francis V. Hanson
Dr. Misha Skliar
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REFERENCES
De Nevers, Noel, Fluid Mechanics, McGraw Hill, New York
N.Y. (2005)
Fogler, H. Scott, Elements of Chemical Reaction
Engineering, Prentice Hall, Upper Saddle River, N.J.
(1999)
Havorka, R.B., and Kendall H.B. “Tubular Reactor at Low
Flow Rates.” Chemical Engineering Progress, Vol. 56. No.
8 (1960).
Ring, Terry A, Choi, Byung S., Wan, Bin., Phyliw, Susan.,
and Dhanasekharan, Kumar. “Residence Time
Distributions in a Stirred Tank-Comparison of CFD
Predictions with Experiments.” Industrial and Engineering
Chemistry. (2003).
Ring, Terry A, Choi, Byung S., Wan, Bin., Phyliw, Susan.,
and Dhanasekharan, Kumar. “Predicting Residence Time
Distribution using Fluent” Fluent Magazine. (2003).
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What to expect from your CSTR.
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Question?
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Design Equations
b
 ra  k * Cao * (1  X ) * Cbo ( b  X )
a
 ra  k * Cao (1  X )
2
2
FA0 X
VCSTR 
kC Ao (1  X )
2
VPFR  
X
0
2
dX
kC A0 (1  X )
2
2
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Design Equations

 
2
0
(t   )

2
t

* e dt
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