1) ABSTRACT In chemical engineering, a chemical reactor is used as a container-like to contain chemical reactions in it. A chemical reactor design deals with multiple aspects of chemical engineering and chemical reactions. A plug flow reactor (PFR) being one of them, designed in the area of chemical engineering. The objective of this experiment is to carry out saponification reaction between sodium hydroxide (NaOH) and ethyl acetate (Et(Ac)), to determine the reaction rate constant, and to determine the effect of residence time on the conversion. The experiment is done my first, starting up the general start-up procedure of the equipment. After the general start-up, both NaOH and Et(Ac) entered the reactor and sample was taken at different feed flow rates of 300, 250, 200, 150, 100, and 50. The sample was then immediately taken for titration, following back titration procedure. After all samples have been taken, general shut- down procedure was done on the equipment. After the result was obtained, calculations were made and graph of conversion versus residence time was plotted. The reading of the graph shows that the conversion of NaOH dropped from residence time of 6.6667 minute to 8.0000 minute. After 8.0000, the conversion of NaOH all increases from 67.48%, to 69.48%, 80.80%, 84.00%, and 92.40% for 8.0000 minute, 10.0000 minute, 13.3333 minute, 20.0000 minute, and 40.0000 minute, respectively. The objectives initially set for this experiment was all obtained successfully. 1
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1) ABSTRACT
In chemical engineering, a chemical reactor is used as a container-like to contain chemical reactions in it. A chemical reactor design deals with multiple aspects of chemical engineering and chemical reactions. A plug flow reactor (PFR) being one of them, designed in the area of chemical engineering. The objective of this experiment is to carry out saponification reaction between sodium hydroxide (NaOH) and ethyl acetate (Et(Ac)), to determine the reaction rate constant, and to determine the effect of residence time on the conversion. The experiment is done my first, starting up the general start-up procedure of the equipment. After the general start-up, both NaOH and Et(Ac) entered the reactor and sample was taken at different feed flow rates of 300, 250, 200, 150, 100, and 50. The sample was then immediately taken for titration, following back titration procedure. After all samples have been taken, general shut-down procedure was done on the equipment. After the result was obtained, calculations were made and graph of conversion versus residence time was plotted. The reading of the graph shows that the conversion of NaOH dropped from residence time of 6.6667 minute to 8.0000 minute. After 8.0000, the conversion of NaOH all increases from 67.48%, to 69.48%, 80.80%, 84.00%, and 92.40% for 8.0000 minute, 10.0000 minute, 13.3333 minute, 20.0000 minute, and 40.0000 minute, respectively. The objectives initially set for this experiment was all obtained successfully.
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2) INTRODUCTION
In chemical engineering, a chemical reactor is used as a container-like to contain chemical
reactions in it. A chemical reactor design deals with multiple aspects of chemical
engineering and chemical reactions. The designers will always ensure that the reaction
held in the reactor proceeds with the highest efficiency towards the desired output product,
producing the highest yield of product that required the least amount of money to purchase
and operate (Schmidt and Lanny, 1998).
A plug flow reactor (PFR), the feed enters at one end of a cylindrical tube and the
product stream leaves at the other end. The long tube and the lack of provision for stirring
prevent complete mixing of the fluid in the tube. Hence the properties of the flowing
stream will vary from one point to another, namely in both radial and axial directions.
Figure 2.1 Plug flow reactor
Plug flow conditions means that all the material processed through the reactor must have
the same residence time so that the chemicals exiting the reactor have witnessed the same
reaction conditions of reactive species contact-time aging-temperature history.
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3) OBJECTIVE
To carry out saponification reaction between NaOH and Et(Ac).
To determine the reaction rate constant.
To determine the effect of residence time on the conversion.
4) THEORY
4.1 Rate of Reaction and Rate Law
Simply put, rate of reaction can be roughly defined as the rate of disappearance of
reactants or the rate of formation of products. When a chemical reaction is said to occur, a
reactant (or several) diminishes and a product (or several) produced. This is what
constitutes a chemical reaction. For example:
aA+bB→
cC+dD
A and B represent reactants while C and D represent products. In this reaction, A
and B is being diminished and C and D is being produced. Rate of reaction, concerns it
with how fast the reactants diminish or how fast the product is formed. Rate of reaction of
each species corresponds respectively to their stoichiometric coefficient. As such:
−r A
a=
−r B
b=
rC
c=
r D
d
The negative sign indicates reactants.
A usual equation for rA is:
−r A=k C Aα CB
β
Where:
k - rate constant
CA - concentration of A species
CB - concentration of B species
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α - stoichiometric coefficient of A
β - stoichiometric coefficient of B
4.2 Conversion
Taking species A as the basis, the reaction expression can be divided through the
stoichiometric coefficient of species A, hence the reaction expression can be arranged as
follows:
A+ ba
B+ ca
C+ da
D
Conversion is an improved way of quantifying exactly how far has the reaction
moved, or how many moles of products are formed for every mole of A has consumed.
Conversion XA is the number of moles of A that have reacted per mole of A fed to the
system. As seen below:
X A=moles of A reacted
molesof A fed
4.3 Plug Flow Reactors
This reactor is also known as tubular flow reactor which is usually used in industry
complementary to CSTR. It consists of a cylindrical pipe and is usually operated at steady
state. For analytical purposes, the flow in the system is considered to be highly turbulent
and may be modelled by that of a plug flow. Therefore, there is no radial variation in
concentration along the pipe.
In a plug flow reactor, the feed enters at one end of a cylindrical tube and the
product stream leaves at the other end. The long tube and the lack of provision for stirring
prevent complete mixing of the fluid in the tube. Hence the properties of the flowing
stream will vary from one point to another.
In an ideal tubular flow reactor, which is called plug flow reactor, specific
assumptions are made regarding the extent of mixing:
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1. No mixing in the axial direction
2. Complete mixing in the radial direction
3. A uniform velocity profile across the radius.
Tubular reactors are one type of flow reactors. It has continuous inflow and
outflow of materials. In the tubular reactor, the feed enters at one end of a cylindrical tube
and the product stream leaves at the other end. The long tube and the lack stirring prevent
complete mixing of the fluid in the tube.
4.4 Residence Time Distribution Function
Residence Time Distribution is a characteristic of the mixing that occurs in the
chemical reactor. There is no axial mixing in a plug flow reactor, PFR and this omission
can be seen in the Residence Time Distribution, RTD which is exhibited by this class of
reactors. The continuous stirred tank reactor CSTR is thoroughly mixed and its RTD is
hugely different as compared to the RTD of PFR.
5) APPARATUS AND MATERIAL
5.1 APPARATUS
1. SOLTEQ Plug Flow Reactor (Model: BP101)
2. Burette
3. Conical flask
4. Measuring cylinder
5. pH indicator
6. Beakers
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5.2 MATERIALS
1. 0.1M sodium hydroxide, NaOH
2. 0.1M ethyl acetate, Et(Ac)
3. 0.1M hydrochloric acid, HCl
4. De-ionised water
6) PROCEDURE
6.1 General Start-Up Procedure
1. All valves are closed except valves V4, V8, and V17.
2. The following solutions are prepared:
20 litre of sodium hydroxide, NaOH (0.1M)
20 litre of ethyl acetate, Et(Ac) (0.1M)
1 litre of hydrochloric acid, HCl (0.25M), for quenching
3. Feed tank B1 is filled with NaOH solution while feed tank B2 is filled with Et(Ac)
solution.
4. Water jacket B4 and pre-heater B5 was filled with clean water.
5. The power for the control panel was turned on.
6. Valves V2, V4, V6, V8, V9, and V11 were opened.
7. Both pumps P1 and P2 were switched on. P1 an P2 were adjusted to obtained flow
of approximately 200 mL/min at both flow meters F1-01 and F1-02. Both flow
rates are set the same
8. Both solutions were allowed to flow through reactor R1 and overflowed into the
waste tank B3.
9. Valves V13 and V18 were opened. Pump P3 was switched on for water to circulate
through pre-heater B5. Stirrer motor M1 was switched on and the speed is set to
200 rpm to ensure homogenous water jacket temperature.
10. Unit is ready for experiment.
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6.2 Experimental procedure
1. General start-up procedure is performed as in Section 6.1.
2. Valves V9 and V11 were opened.
3. Both NaOH and Et(Ac) solution is allowed to enter the plug reactor R1 and was
emptied into the waste tank B3.
4. P1 and P2 were adjusted to give a constant flow of 300 mL/min at flow meters FI-
01 and FI-02. Both flow rates are made sure to be the same and the flow rates are
recorded.
5. The inlet (QI-01) and outlet (QI-02) conductivity values were monitored until they
do not change over time. This ensures that the reactor has reached steady state.
6. Both inlet and outlet steady state conductivity values were recorded. The
concentration of NaOH exiting the reactor and extent of conversion from the
calibration curve was found.
7. Sampling valve V15 was opened and a 50 mL sample was collected for back
titration procedure.
8. Experiment was repeated from step 4 to step 7 with different residence time by
reducing the feed flow rates of NaOH and Et(Ac) to 250, 200, 150, 100, and 50
mL/min. Both flow rates are made sure the same.
6.3 Preparation of calibration curve for conversion vs. conductivity
1. The following solutions were prepared:
1 litre of sodium hydroxide, NaOH (0.1M)
1 litre of sodium acetate, Na(Ac) (0.1M)
1 litre of deionised water, H2O
2. The conductivity and NaOH concentration was determined for each conversion
values by mixing the following solutions into 100 mL of deionised water.
0% conversion : 100 mL NaOH
25% conversion : 75 mL NaOH + 25 mL Na(Ac)
50% conversion : 50 mL NaOH + 50 mL Na(Ac)
75% conversion : 25 mL NaOH + 75 mL Na(Ac)
100% conversion : 100 mL Na(Ac)
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6.4 Back titration procedure
1. A burette is filled with 0.1M NaOH solution.
2. 10 mL of 0.25M HCl was measured in a flask.
3. 50 mL of sample was obtained from step 7 of experiment and was immediately
added to the HCl in the flask to quench saponification reaction.
4. A few drops of pH indicator were added into the mixture.
5. The mixture was titrated with NaOH solution from the burette until the mixture is
neutralized. The amount of NaOH used was recorded.
6.5 General shut-down procedure
1. Pumps P1, P2, and P3 were switched off. Valves V2 and V6 were closed.
2. The heaters are switched off.
3. The cooling water circulating the reactor is kept while the stirrer motor is running
to allow the water jacket to cool down to room temperature.
4. Power for the control panel is turned off.
7) RESULT AND CALCULATION
7.2 Result of preparation of calibration curve
Conversion
Solution Mixtures Concentration
of NaOH(M)
Conductivity
(mS/cm)0.1M NaOH
(mL)
0.1M
Na(Ac)
(mL)
H2O (mL)
0% 100 - 100 0.0500 6.39
25% 75 25 100 0.0375 4.24
50% 50 50 100 0.0250 1.397
75% 25 75 100 0.0125 0.615
100% - 100 100 0.0000 0.00191
Table 7.1 Table of preparation of calibration curve
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0 25 50 100 1000
1
2
3
4
5
6
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f(x) = − 1.640118 x + 7.449136R² = 0.922307238406559
Conductivity vs Conversion
Conductivity vs Conversion
Linear (Conductiv-ity vs Con-version)
Conversion (%)
Conductivity (mS/cm)
Figure 7.1 Graph of conductivity vs conversion
7.2 Result of experiment
Flowrate of
NaOH
(mL/min)
Flowrate of
Et(Ac)
(mL/min)
Total inlet
flowrate
(mL/min)
Vo
Outlet Conductivity Volume of
NaOH
titrated (mL)Q1 Q2
300 300 600 7.6 6.7 23.50
250 250 500 8.0 7.0 16.87
200 200 400 8.1 6.8 17.37
150 150 300 7.2 5.7 20.23
100 100 200 6.5 5.0 21.03
50 50 100 6.2 4.2 23.07
Residence time, Conversion, X, Reaction rate constant, k Rate of reaction, -rA
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τ, (min) (%) (L.mol/min) (mol.L/min)
6.6667 94.00 23.5000 8.460 x 10-4
8.0000 67.48 2.5938 2.743 x 10-3
10.0000 69.48 2.2765 2.121 x 10-3
13.3333 80.80 3.1563 1.164 x 10-3
20.0000 84.00 2.6250 6.720 x 10-4
40.0000 92.40 3.0395 1.756 x 10-4
Table 7.2 Effect of residence time on reaction
6.6667 8 10 13.3333 20 4060
65
70
75
80
85
90
95
100
f(x) = 1.51085714285714 x + 76.072R² = 0.0641489876355068
Conversion vs Residence Time
Conversion vs Residence Time
Figure 7.2 Graph of conversion vs residence time
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7.3 Sample of calculation
Vol of sample, Vs : 50 mL
Conc. NaoH in feed vessel, CNaOH,r : 0.1 mol/L
Vol. HCl quenching, VHCl,s : 10 mL
Conc. HCl standard soln, CHCl,s : 0.25 mol/L
Conc. NaOH for titration, CNaOH,s : 0.1 mol/L
(A) Sample: flowrate = 300 mL/min of NaOH
300 mL/min of Et(Ac)
(B) Volume of titrating NaOH = 23.50 mL = 0.02350 L