1. 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).
2. 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.
3. 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.
4. Assumption
1) Steady state therefore, dNA/dt = 0
2) Well mixed therefore rA is the same throughout the reactor
∫ 𝑟𝐴 𝑑𝑉 = 𝑟𝐴
𝑣
0
∫ 𝑑
𝑣
0
𝑉 = 𝑟𝐴 𝑉
Rearranging the generation
𝑣 =
𝐹𝐴0 − 𝐹𝐴
−𝑟𝐴
In term if conversion
𝑋 =
𝐹𝐴0 − 𝐹𝐴
𝐹𝐴0
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 𝑡𝑜 𝑝𝑜𝑖𝑛𝑡 𝑖
𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝐴 𝑓𝑒𝑑 𝑡𝑜 𝑓𝑖𝑟𝑠𝑡 𝑟𝑒𝑎𝑐𝑡𝑜𝑟
5. Mol balance on Reactor 1
In – out + generation = 0
FA0 – FA1 + rA1V1 = 0
𝑋1 =
𝐹𝐴0 − 𝐹𝐴1
𝐹𝐴0
FA1 = FA0 – FA0X1
𝑉1 =
𝐹𝐴0 𝑋1
−𝑟𝐴1
Mol balance on Reactor 2
In – out + generation = 0
FA1 – FA2 + rA2V2 = 0
𝑋2 =
𝐹𝐴0 − 𝐹𝐴2
𝐹𝐴0
FA2 = FA0 – FA0X2
𝑉2 =
𝐹𝐴0(𝑋2 − 𝑋1)
−𝑟𝐴2
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
6. 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.
9. Graph result based on data
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
0 10 20 30 40 50 60 70 80 90
CONDUCTIVITY,Q(mS/cm)
TIME (min)
Conductivity change in time for each reactor in
pulse change
Reactor 1 Reactor 2 Reactor 3
10. 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
11. 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
12. 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 63rd 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.
13. 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
