cstr design for rtd applications

1. CSTR Ideality
November 29, 2006
By Taryn Herrera

2. Importance of CSTR Ideality
 Economics
 Design of Reactors
 Higher Conversion of Reactants

3. CSTR Ideality
 The Experiment
 The Results
 Characterization of Ideality
 Residence Time Distribution Function
 Causes of CSTR Non-Ideality
 Conclusions and Questions

4. The Experiment
 Characterize the ideality of client’s bench
CSTR reactor
 Operate currently at 30 rpm
 Determine most ideal conditions and make
recommendations

5. The Experiment (cont.)
 Injection Impulse Step Test
 Sodium Chloride is the Tracer
 Concentration Measurements by
Conductivity
 Different Mixer Speeds

6. Figure 1 Schematic Diagram for 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
The CSTR Apparatus

7. Characterization of Ideality
 Ideal Residence Time vs. Mean Residence
Time
 Residence Time is how long material stays
in reactor
 Comparison is done numerically and
graphically

8. Ideal Residence Time
o
ideal
V

 

9. Residence Time Distribution
 

end
t
mean
mean dt
t
tE
t
0
)
( 
 

end
t
m dt
t
E
t
t
0
2
)
(
)
(

 



 end
t
dt
t
C
t
C
t
C
t
C
t
E
0
)]
0
(
)
(
[
)
0
(
)
(
)
(

10. Residence Time Distribution (cont.)
 E(t) or RTD, measures fraction of material
in reactor between two times
 tm, mean residence time
 Sigma or variance, measures the
distribution’s spread

11. Residence Time Distribution (cont.)
Figure 2 Graph from Fluent Magazine showing RTD functions for
different methods (Ring, 2004)

12. The Results
Concentration of Sodium Chloride versus Time
10
15
20
25
30
35
0 100 200 300 400 500 600 700 800
Time(s)
Concentration
NaCl(g/L)
30 RPM
15 RPM

13. The Results (cont.)
Residence Time Distributions
0.0005
0.0007
0.0009
0.0011
0.0013
0.0015
0.0017
0.0019
0.0021
0.0023
0 20 40 60 80 100 120 140 160 180 200
Time(s)
E(t)
Ideal E(t)
E(t) Conductivity 15 RPM
E(t) Conductivity 30 RPM

14. The Results (cont.)
Mean Residence Time
0
100
200
300
400
500
0 5 10 15 20 25 30 35
RPM
Mean
Residence
Time(s)
Ideal Mean Residence Time
Experimental Residence Times

15. The Results (cont.)
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
Ideal CSTR 466.97 5.90

16. Causes of CSTR Non-Ideality
 Poor Mixing
 Dead Zones in CSTR
 Short Circuit

17. CSTR Non-Ideality
Figure 3 Impulse Injection for CSTR

18. CSTR Non-Ideality
Rushton Impeller Marine Impeller

19. CSTR Non-Ideality

2
ND

 5
3
0
D
N
P
r
PowerNumbe


 Turbulent Mixing
 Done by Baffles and Mixing Speed
 Promotes Better Mixing of Reactants
 Dead Zones Minimized
 Reynolds Number of 4000

20. Review
 Characterization of Ideality
 Residence Time Distribution Function
 Mean Residence Time vs. Ideal Residence
Time
 CSTR Non-Ideality

21. Conclusions
 Use Marine Type Impeller
 Promote Turbulence in CSTR
 Choose Wisely the Mixing Points and
Sampling Points
 Use Fluent
 Observe other RTD data

22. Questions?