Cstr in Seriess

NO. SECTIONS PAGE

1 Abstract/summary 2

2 Introduction 2-3

3 Aims/objectives 4

4 Theory 4-7

5 Apparatus 8

6 Experimental procedure 9

7 Results 10-11

8 Sample of calculations 11

9 Discussions 11-12

10 Conclusions 12

11 Recommendations 13

12 References 13=14

13 Appendices 14

1.0) ABSTRACT

1

This experiment involves a continuous stirred tank reactor (CSTR) in series.

CSTR is one in which the contents are stirred so uniformly that it is assumed that no variation

or concentration gradients exist within the vessel. The objective was to determine the

concentration response to a step change and pulse input and also to determine the effect of

residence time on the response curve. Firstly, the deionised water are filled in the both two

tanks with the sodium chloride were diluted in the tank one. The deionised water from the

tank two will flow through to fill up the three reactors. The flow rate of the deionised water is

set to 150 ml/min to prevent from over flow. The only readings were taken at time to after we

get the readings of the conductivity are stable which the readings of the conductivity are quite

similar from one to another. Furthermore, readings are continuously taken every 3 minutes

until to the point that the conductivity values for the reactors are closed to each other. The

concentration of the three reactors almost becomes constant which is at the 75 minutes. The

graph of the conductivity versus time was plotted. We can determine the effect of the step

change and pulse input to the concentration from the graph plotted. The graph shows that the

concentration in the reactor 1 is higher at the initial compared to the reactors 2 and reactors 3.

2.0) INTRODUCTION

Reactors used for carrying out chemical or physical reactions can be characterized as

ideal or non ideal, according to the nature of the hydraulic and mixing conditions. In contrast

with non ideal reactor, ideal reactors are assumed to have uniform mixing and hydraulic

conditions, depending on the specific reactor configurations. Common reactor configurations

include (1) plug flow reactors (PFRs), (2) completely mixed batch reactors (CMBRs) and (3)

completely mixed flow reactors (CMFRs). In addition, a CMFR may also be referred as a

complete-mix reactor (CMR), continuous stirred tank reactor (CSTR), constant flow stirred

tank reactor (CFSTR), or backmix rector. (Crittenden and Montgomery Watson Harza ,

2005).

2

In a continuous stirred tank reactor(CSTR), reactants and products are continuously

added and withdrawn. In practice, mechanical or hydraulic agitation is required to achieve

uniform composition and temperature, a choice strongly influenced by process

considerations. The CSTR is the idealized opposite of the well-stirred batch and tubular plug-

flow reactors. Analysis of selected combinations of these reactor types can be useful in

quantitatively evaluating more complex gas-, liquid-, and solid-flow behaviors. (Laboratory

of Environmental and Science Engineering, n.d)

The continuous stirred tank reactor or back mix reactor is a very common processing

unit in chemical and polymer industry. Its names suggest, it is a reactor in which the contents

are well stirred and uniform throughout. The CSTR is normally run at steady state, and is

usually operated so as a to be quite well mixed. The CSTR is generally modeled as having no

spatial variations in concentrations, temperature, or reaction rate throughout the vessel. Since

the temperature and concentration are identical everywhere within the reaction vessel, they

are the same at the exits point as they are elsewhere in the tank. (Chemical Process

Dynamics, n.d)

Figure 1 : Continuous stirred tank reactor

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3.0) OBJECTIVES

1. To determine the effect of step changes and pulse input to the concentration.

2. To determine the effect residence time on the response curve.

4.0) THEORY

Continuous flow reactors have three types which are continuous stirred tank reactor

(CSTR), plug flow reactor (PFR) and packed-bed reactor (PBR). For this experiment, the

continuous stirred tank reactor (CSTR) is used. Continuous stirred tank reactor (CSTR) is

also known as vat or backmix reactor. It is normally operated at steady state and it assumed to

be perfectly mixed (Fogler, Nov 2010). There are three types of kind of phases present in the

CSTR which are liquid phase reaction, gas-liquid phase reaction and solid-liquid phase

reaction. CSTR is used primarily for liquid phase reaction. The CSTR are used when the

agitation is required and when there are series configurations for different concentration

streams. The CSTR have a few advantages and disadvantages. The advantages of CSTR are

good temperature control, continuous operation, easily adapts to two phase runs and easy to

clean. The disadvantages of CSTR are lowest conversion per unit volume and by-passing and

channeling possible with poor agitation.

The general mole balance equation

Figure 4.1 (from http://www.umich.edu/~elements/5e/asyLearn/bits/cstr/index.htm)

4

F A0−F A+∫0

V

r A dV=dN A

dt Equation 4.1

The assumptions for this equation are:

a) Steady state. Therefore dN A

dt=0

b) Well mixed. Therefore rA is the same throughout the reactor.

∫0

V

r A dV=r A∫0

V

dV=r A V Equation 4.2

Rearranging the generation

V=F AO−F A

−r A Equation 4.3

In term of conversion

X=F AO−F A

F AO Equation 4.4

V=F AO X

−r A Equation 4.5

Reactor sizing

Given that rA is a function of conversion, rA=f(X), the one that can size of any type of reactor.

The volume of the CSTR can be represented as the shaded area in the Levenspiel Plot as

shown below.


5

Reactors in series

Given that rA is a function of conversion, rA=f(X), the one that can also design any sequence

of reactors in series provided. There are no side streams by defining the overall conversion at

any point.

X i=molesof A reacted up¿ point i ¿molesof A fed t o first reactor

Equation 4.6


Mole balance Reactor 1

In – out + generation = 0

FAO – FA1 + rA1V1 = 0 Equation 4.7

X1=F AO−F A1

F AO

Equation 4.8

FA1 = FAO – FAOX1 Equation 4.9

V 1=FAO X 1

−r A1

Equation 4.10



FA1 – FA2 + rA2V2 = 0 Equation 4.11

6

X2=F AO−F A2

F AO

Equation 4.12


V 2=∫X 1

X 2 F AO

−r A 2

dx Equation 4.14



FA2 – FA3 + rA3V3 = 0 Equation 4.15

X3=F AO−F A3

F AO

Equation 4.16


V 3=F AO ( X3−X 2)

−r A3

Equation 4.18

Below is the Levenspiel Plot for Reactor in series


7

5.0) APPARATUS

8

Reactor 1

Reactor 2

Reactor 3

Pump 1

Pump 2

6.0) PROCEDURES

Experiment 2: The Effect of Pulse Input

1. Feed tanks (tank1 and tank 2) are filled up 20L with deionised water.

2. 300g og Sodium chloride (salt) is dissolved in tank 1. The salts are ensured dissolved

entirely and the solution is homogeneous.

3. The V3 is set to position 2 and Pump 2 is switch on. So that deionised water from

tank 2 will flow in to reactor 1 then fill up all three reactors with deionised water.

4. The flow rate (FT1) is set to 150mL/min by adjusting V4. Do not use too high flow

rate to avoid overflow. Ensure that no air bubbles are trapped in the pipings.

5. Stirrers 1, 2 and 3 are switch on.

6. The deionised water is continuing pump for about 10 minutes until the conductivity

readings for all three reactors are stable at low values.

7. The conductivity values at t0 are recorded.

8. Pump 2 is switch off after 5 minutes.

9. V3 is set to position 1 and Pump 1 is switch on. The timer is started for 5 minutes.

10. Pump 1 is switch off after 5 minutes.

11. V3 is set to position 2 and Pump 2 is switch on.

12. The conductivity values for each reactor are recorded for every 3 minutes.

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Feed tank 1

Feed tank 2

13. The conductivity values are continuing recorded until the reading is close to the

starting value recorded.

14. The pump 2 is switch off. V4 is closed.

15. All liquids in reactors are drained by opening V5 and V6.

7.0) RESULTS

FT: 74.1ml/min TT1: 28.2oC TT2: 29.4oC TT3: 32768oC

Time (min) QT1 (ms/cm) QT2 (ms/cm) QT3 (ms/cm)

0 6.45 2.21 1.508

3 5.5 3.18 1.924

6 3.89 3.67 2.16

9 2.89 3.49 2.66

12 2.01 3.12 2.88

15 1.52 2.72 2.97

18 1.076 2.38 2.92

21 0.837 2.07 2.73

24 0.6046 1.648 2.56

27 0.4846 1.406 2.25

30 0.3524 1.088 2.03

33 0.289 0.9062 1.703

36 0.2563 0.6622 1.52

10

39 0.2002 0.5689 1.241

42 0.187 0.4465 1.082

45 0.1556 0.3823 0.8826

48 0.1359 0.2986 0.6789

51 0.1326 0.2561 0.5671

54 0.1236 0.2292 0.4427

57 0.1217 0.1963 0.3767

60 0.1172 0.1708 0.3534

63 0.1159 0.158 0.2929

66 0.1161 0.1487 0.2861

69 0.1124 0.1364 0.2183

72 0.1117 0.1231 0.1871

75 0.1109 0.1209 0.1867

0 10 20 30 40 50 60 70 800

1

2

3

4

5

6

7

QT1 (ms/cm)QT2 (ms/cm)QT3 (ms/cm)

Time vs QT1, QT2 and QT3

Graph 7.1: Time versus QT1, QT2 and QT3

9.0) DISCUSSIONS

This experiment is conducted to determine the concentration response to a step

change. Based on the data collected in Table 7.1, graph of the conductivity versus time is

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plotted in Graph 7.1 for QT1, QT2 and QT3 respectively. Step change is a sudden change in a

process variable. For this experiment our variable that been change is the input. Reactor

feedstock is suddenly switched from one supply to another, causing sudden changes in feed

concentration, flow and excectra.

The graph shows that the concentration in the reactor 1 is higher at the initial

compared to the reactors 2 and reactors 3. This can be explained as the diluted sodium

chloride enters the reactor 1 first and then reactor 2 bypass with the deionised water

containing from the deionised water flow into the reactors. The diluted sodium chloride flow

bypass through reactor 1 to reactor 3 because of the deionised water still containing in the

reactors as it is not fully removed in the third reactor.

Besides that, the concentration of the three reactors almost become constant which is

at the 75 minutes after the valve is switched to position 2 as the time increased. For QT1, the

concentration at minute 75th is 0.1109, while for QT2 is 0.1209 and QT3 is 0.1867.

Moreover, the flow rate of the inlet that is not stable at 150mL/min causes the concentration

at reactor 1 that is the inlet concentration of sodium chloride diluted are not constantly

increased as referring to the graph.

Unfortunately there are some problems occurred to the computer that recorded the

data during the experiment. The computer is stuck and thus the data has to be recorded

manually by taking the reading for every 3 minutes. Therefore the result is also affected and

the graph is not so smooth because of the data recorded is not accurate.

10.0) CONCLUSIONS

As the conclusion, we can say that a step change in input affected the concentration at the

reactor. It can be seen from the graph plotted to the theory that the graph is almost the same. But

because of the error during the data recorded, there are some different of the graph for all reactors as it

does not smooth compared to the theory. From the results, sometimes the time recorded is more than

3 minutes. So, it will affect the readings. From the theory, we should get the nearly value of

conductivity for the reactor 1 and 3. Therefore from the experiment conducted at 75 minutes, we got

QT 1 = 0.1109 mS/cm, QT 2 = 0.1209 mS/cm and QT 3 = 0.1867 mS/cm. For the step change it will

increase the concentration until it reaches a constant value and for pulse input it will first increase then

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decrease until it reaches a constant value. The feed of the systems effect the concentration in the

reactor, if the feed contain a concentration then the concentration in the tank will increase and if the

feed only contain deionised water then the concentration will decrease. So, we can conclude that the

step change in input concentration to the concentration of solute in stirrer reactor in series is

proportional to the time.

RECOMENDATION

After we have finished this experiment, we find that are several factors in this experiment that can

be fixed to make sure that the experiment runs better. This is some of my recommendation for this

experiment:

1. When we are doing the experiment the program that used to record the data was not

function. This cause us a high error in reading the data. My recommendation is to make sure

better maintainers of the apparatus.

2. Make sure that there are no air bubbles in the piping.

3. Check the tank 1 and 2 before start the experiment to make sure that it full with

deionised water and sodium chloride to make sure that our experiment run properly.

4. Make sure that the reactor and turbine are cleaned properly. Flush the system with

deionised water until no trances of salt are detected.

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12.0) REFERENCES

1) Cengel, Y. A. & Cimbala, J. M. (2006). Fluid Mechanics Fundamental and Applications.

(2nd ed.). New York: MC Graw-Hill Education

2) Crittenden, J. and Montgomery Watson Harza (Firm) (2005). Water treatment principles

and design. Hoboken, N.J., J. Wiley. Retrieved from:

http://www.egr.msu.edu/~hashsham/courses/ene806/docs/CSTR%20in%20Series%202.pdf

3) Laboratory of Environmental and Science Engineering. (n.d) .Bioreactors for metal

bearing wastewater treatment: Continuous Flow Tank Reactor. Retrieved from:

http://www.metal.ntua.gr/~pkousi/e-learning/bioreactors/page_06.htm

4) Chemical Process Dynamics. (n.d) .Continuous Stirred Tank Reactor. Retrieved from:

http://iitkgp.vlab.co.in/?sub=35&brch=107&sim=1175&cnt=1

5)Green, W. D. and Perry, H. D. (2007). Perry’s Chemical Engineers’ Handbook,8th edition.

U.K: Amazon

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http://iitkgp.vlab.co.in/?sub=35&brch=107&sim=1175&cnt=1

http://www.metal.ntua.gr/~pkousi/e-learning/bioreactors/page_06.htm

http://www.egr.msu.edu/~hashsham/courses/ene806/docs/CSTR%20in%20Series%202.pdf

6) Continuous Stirred Tank Reactors (CSTRs). (n.d.). Retrieved May 3, 2014, from

http://www.umich.edu/~elements/5e/asyLearn/bits/cstr/index.htm

7) Fogler, H. (2010). Continuous-Flow Reactors. In Essentials of Chemical Reaction

Engineering: Mole Balances (p. 4). Prentice Hall.

13.0) APPENDIX

Figure 4.5 : Continuous stirred tank reactor in series

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http://www.umich.edu/~elements/5e/asyLearn/bits/cstr/index.htm

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Cstr in Seriess