Abstract

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

system consists of three agitated, glass reactor vessels in series. The concentration is kept

uniform for each reactor and it is observed that there is a change in concentration as fluids

move from one reactor to the other reactor. This experiment is carried out to determine and

observe the effect of step change input. CSTR is one kind of chemical reactor system with

non-linear dynamics characteristics. The usage of this equipment is to study the reaction

mechanism as well as the dynamics of reactor with various types of inputs. CSTR is widely

used in water treatment and chemical and biological processes. The deionised water are filled

in both tanks with the sodium chloride are diluted in one tank. Then the deionised water from

the second tank will flow through to fill up the three reactors. The flow rate of the deionised

water is set to 159.7 ml/min to prevent from over flow. The readings are taken at the time to

after the conductivity readings showing stable enough. After that, the readings are

continuously taken for every 3 minutes until to the point where the conductivity values for

three reactors are equivalent. Based on the result obtained, the graph has been plotted

between conductivity, Q (mS/cm) against time, t (min).

Aim

To study the effect of step change input to the concentration.

Introduction

In the industrial chemical process, a reactor seems to be the most important equipment in

which raw materials undergo a chemical change to form a desired product. The design and

operation of chemical reactors are essential criteria responsible to the whole success of the

industrial operation. The stirred tank reactor in the form of either single tank, or more often a

series of tanks, particularly suitable for liquid phases reactions and widely used in chemical

industry, i.e pharmaceutical for medium and large scale of production. It can form a unit in a

continuous process, giving consistent product quality, easy to control automatically and low

man power requirement.

The mode of operation of reactors may be batch flow or continuous flow. In a batch flow

reactor, the reactor is charge with reactant, the content are well mixed and left to react and

then the mixture will be discharged. A continuous flow reactor, the feed to reactor and the

discharge from it are continuous. The three types of continuous flow reactor are plug flow

reactor, the dispersed plug flow reactor, and completely mixed or continuously stirred tank

reactors (CSTRs). CSTR consists of a stirred tank that has a feed stream and discharge

stream. Frequently, several CSTRs in series are operating to improve their conversion and

performance (Reynolds and Richards 1996).

Complete mixing in a CSTR reactor produces the tracer concentration throughout the reactor

to be the same as the effluent concentration. In other words, in an ideal CSTR, at any travel

time, the concentration down the reactor is identical to the composition within the CSTR

(Hoboken et al., 2005). It is also important to notice that the mixing degree in a CSTR is an

extremely important factor (Cholette, Blanchet et al. 1960), and it is assumed that the fluid in

the reactor is perfectly mixed in this case, that is, the contents are uniform throughout the

reactor volume. In practice, an ideal mixing would be obtained if the mixing is sufficient and

the liquid is not too viscous. If the mixing is inadequate, there will be a bulk streaming

between the inlet and the outlet, and the composition of the reactor contents will not be

uniform. If the liquid is too viscous, dispersion phenomena will occur and this fact will affect

the mixing extent.

Theory

The continuous flow stirred-tank reactor (CSTR), also known as vat- or backmix reactor, is a

common ideal reactor type in chemical engineering. A CSTR often refers to a model used to

estimate the key unit operation variables when using a continuous[†]agitated-tank reactor to

reach a specified output. The mathematical model works for all fluids: liquids, gases,

and slurries.

The behavior of a CSTR is often approximated or modeled by that of a Continuous Ideally

Stirred-Tank Reactor (CISTR). All calculations performed with CISTRs assume perfect

mixing. In a perfectly mixed reactor, the output composition is identical to composition of the

material inside the reactor, which is a function of residence time and rate of reaction. If the

residence time is 5-10 times the mixing time, this approximation is valid for engineering

purposes. The CISTR model is often used to simplify engineering calculations and can be

used to describe research reactors. In practice it can only be approached, in particular in

industrial size reactors.

Assume:

perfect or ideal mixing, as stated above

Integral mass balance on number of moles Ni of species i in a reactor of volume V. 

General mol balance equation.

Assumption

1) Steady state therefore, dNA/dt = 0

2) Well mixed therefore rA is the same throughout the reactor

∫0

v

r A dV=r A∫0

v

d V =r A V

Rearranging the generation

v=F A0−FA

−r A

In term if conversion

X=F A0−FA

F A 0

Reactors in Series

Given -rA as a function of conversion, , -rA = f(X), one can also design any sequence of

reactors in series provided there are no side streams by defining the overall conversion at any

point.

Xi=moles of A reacted up¿ point i ¿moles of A fed ¿

first r eactor ¿

Mol balance on Reactor 1

In – out + generation = 0

FA0 – FA1 + rA1V1 = 0

X1=F A0−F A1

F A0

FA1 = FA0 – FA0X1

V 1=FA 0 X1

−r A1

Mol balance on Reactor 2

In – out + generation = 0

FA1 – FA2 + rA2V2 = 0

X2=F A0−F A2

F A0

FA2 = FA0 – FA0X2

V 2=F A0

(X ¿¿2−X1)−r A2

¿

Apparatus

1. Distillation water

2. Sodium chloride

3. Continuous reactor in series

4. Stirrer system

5. Feed tanks

6. Waste tank

7. Dead time coil

8. Computerize system

9. Stop watch

Procedure

Experiment 1 : The effect of step change input.

1. The general start up procedure was perfomed by following the instruction of the

manual given at the instrument.

2. Tank 1 and tank 2 was filled up with 20 L feeds deionizer water.

3. 200g of Sodium Chloride was dissolved in tank 1until the salts dissolve entirely and

the solution is homogenous.

4. Three way valve (V3) was set to position 2 so that deionizer water from tank 2 will

flow into reactor 1.

5. Pump 2 was switched on to fill up all three reactors with deionizer water.

6. The flow rate (Fl1) was set to 150 ml/min by adjusting the needles valve (V4). Do not

use too high flow rate to avoid the over flow and make sure no air bubbles trapped in

the piping.

7. The stirrers 1, 2 and 3 were switched on. The deionizer water was continued pumped

for about 10 minute until the conductivity readings for all three reactors were stable at

low values.

8. The values of conductivity were recorded at t0.

9. The pump 2 was switched off after 5 minutes. The valve (V3) was switched to

position 1 and the pump 1 was switched on. The timer was started.

10. The conductivity values for each reactor were recorded every three minutes.

11. Record the conductivity values were continued until reading for reactor 3 closed to

reactor 1.

12. Pump 2 was switched off and the valve (V4) was closed.

13. All liquids in reactors were drained by opening valves V5 and V6.

Result

The effect of step-change input

FT : 159.7 ml/min TT1 : 29.2 oC TT2 : 29.9 oC TT3 : -32768.0 oC

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

0 3.5958 0.2606 0.0199

3 6.4896 1.6928 0.1585

6 6.4562 2.9047 0.7359

9 9.6409 4.8038 1.3953

12 10.1162 5.9755 2.6493

15 11.1444 7.3652 3.5440

18 10.4900 8.3135 4.9323

21 11.8739 9.2744 5.8081

24 12.2028 9.7336 7.0988

27 12.2698 10.0885 7.7742

30 12.3793 10.1459 8.8370

33 12.6829 11.4844 9.4266

36 12.5977 11.1323 10.1873

39 12.6150 11.7495 10.8874

42 12.5411 11.3561 11.2474

45 12.5646 12.6366 11.5306

48 12.5908 11.9001 11.6498

51 12.6415 12.1749 11.9416

54 12.6593 12.0855 12.1671

57 12.6170 11.9264 12.4328

60 12.6079 11.2871 12.4185

63 12.7635 12.1717 12.5523

66 12.6775 12.2884 12.5886

69 12.7378 12.1032 12.6366

72 12.6181 12.2029 12.749

75 12.7205 11.7649 12.7905

78 12.6322 12.6069 12.8175

81 12.6905 12.1663 12.7119

84 12.6945 12.3185 12.6957

Graph result based on data

0 10 20 30 40 50 60 70 80 900

1

2

3

4

5

6

7

8

9

10

11

12

13

14

Conductivity change in time for each reactor in pulse change

Reactor 1 Reactor 2 Reactor 3

TIME (min)

CO

NDU

CTIV

ITY,

Q (m

S/cm

)

Calculation

Vi = FA0 (XAi – XAi-1)/(-rA)i

Where Vi = volume of reactor i

FAi = molal flow rate of A into the first reactor

XAi = fractional conversion of A in the reactor i

XAi+1 = fractional conversion of A in the reactor i-1

For the first order reaction, -rA = kCA1 = kCA0(1-XAi)

v = volumetric flow rate of A = 159.7 ml/min = 0.1597 liter/min

For the first reactor: (V=20 liter)

(-rA)1 = (kCA)1 = kCA1 = kCA0 (1-XA1)

CA0 = FA0/v

i.e. FA0 = vCA0

XAi+1 = XA0 = 0

Therefore,

Tank 1

Vi = FA0 (XAi - XAi-1) / (-rA)i

20 = 0.1597 (XA1 - 0) / (0.158 x (1 – XA1))

XA1 = 0.95

Tank 2

Vi = FA0 (XAi - XAi-1) / (-rA)i

20 = 0.1597 (XA2 – 0.95) / (0.158 x (1 – XA2))

XA2 = 0.997

Tank 3

Vi = FA0 (XAi - XAi-1) / (-rA)i

20 = 0.1597 (XA3 – 0.997) / (0.158 x (1 – XA3))

XA3 = 1

Discussion

In this experiment, we carried out an experimental procedure to determine the effect of step

change input on the concentration of the salt solution used in the experiment which is sodium

chloride, NaCl. The first step in the experiment was filling the reactor tanks with 20L of

deionized water. In the experiment of CSTR in series, there are two main objectives to

observe; effect of step-change input and effect of pulse input. But in this discussion, we are

only focusing on the effect of step-change input. The difference between these two methods

are that step-change input means we are continuously feeding the salt solution NaCl into the

reactor throughout the experiment and through the time the salt solution will fill all three

reactors until the first reactor and third reactor will have an equal value of conductivity. As

for the effect of pulse input, we feed the reactor with 3 minutes worth of salt solution and

then continuing the experiment feeding the reactors with deionized water spreading the salt

solution equally through all three reactors.

The feed is flowed through the reactors at roughly 150 ml/min and the system is running

isothermally with each reactor’s temperature at around 29 0C. In this experiment we took

readings of the conductivity of each reactor every 3 minutes. The experiment ends when the

conductivity of the first reactor and the third reactor are equal and constant for the few last

readings. The first reading of the reactors are as follows; QT1 is 3.5958, QT2 is 0.2606, and

QT3 is 0.0199 mS/min. The results can be observed in the results section of the report. As

observed from the results of the experiment, the conductivity of the mixture increases as time

passes on as more and more salt solution is fed into the reactors. And at the 63 rd minute we

can see that the conductivity of the reactors are starting to slowly get equal and finally after

some time at the 84th minute, the value reads QT1 is 12.6945, QT2 is 12.3185, and QT3 is

12.6957 mS/min.

In a scientific research, there are always unknown variables that could disrupt us from

obtaining the best results possible. During the recording of the data, there were some

problems that occurred to the computer that recorded the data. The computer froze for a few

seconds and thus it did not record accurately every 3 minutes. Because the data was not very

accurate, the plotting of the graph was affected and not very smooth.

Conclusion

As a conclusion, based on the aim of the experiment, we can say that the step-change input

affected the concentration at the reactor. It can be seen from the graph plotted. If we compare

our graph with a theorized graph, the graph is almost the same. But because of the error

during recording of the data, there are some difference compared to the theory and a less

smooth graph was obtained. It is safe to say that based on the results of the experiment, the

experiment was a success as the objective was achieved.

Recommendation

It is in our biggest interest to acquire the best results off of the experimental procedures but

little do we know that most of the time the methodology is always incomplete in a sense that

precautionary steps are rarely given. There is significant amount of external disturbances that

can affect the results of the experiment. To prevent from any inaccuracy, it is advised that the

precautionary steps are to be mentioned. For example in this experiment, make sure that the

reactors are properly cleaned before starting the experiment because we don’t want any salt

residue in the reactors that could affect the readings later on. Just to be on the safe side, the

best way to have a precise outcome is to prepare the proper and complete procedures for the

experiment.

References

1. Elements of Chemical reaction Engineering, Fourth Edition H. Scott Fogler, Pearson

International Edition, 2006 Pearson Education, Inc

2. (2015). Retrieved 1 April 2015, from 2. http://www.solution.com.my/pdf/bp107(a4).pdf

3. (2015). Retrieved 26 March 2015, from http://www.formatex.info/microbiology2/15821594.pdf

4. (2015). Retrieved 1 April 2015, from 3.

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

Appendix

CSTR (s) Solteq model BP107

CSTR (s) front image

CSTR (s) back image