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

(C) Volume of quenching HCl unreacted

with NaOH in sample=

CNaOH ,std

C HCl, std x (B)

= 0.1 mol/ L

0.25 mol/ L x 0.02350L

= 9.4 x 10-3 L

(D) Volume of HCl reacted with NaOH in

sample

= VHCl – (C)

= 10 mL – 9.4 mL

= 0.6 mL

= 0.6 x 10-4 L

(E) Mole of HCl reacted with NaOH in

sample

= CHCl,std x (D)

= 0.25 mol/L x (0.6 x 10-4) L

= 1.5 x 10-5 mol

(F) Mole of NaOH unreacted in sample = (E)

= 1.5 x 10-5 mol

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(G) Concentration of NaOH unreacted with

Ethyl Acetate, CA

= (E)¿¿

= 1.5× 10−5 mol0.05 L

= 3 x 10-3 mol/L

(H) Steady state fraction conversion of

NaOH (XA)= 1−

C A

C AO

= 1−3×10−3mol / L0.05mol / L

= 0.94

(I) Concentration of NaOH reacted with

Ethyl Acetate

= CNaOH,O – (G)

= 0.05 mol/L – (3 x 10-3) mol/L

= 0.047 mol/L

(J) Mole of NaOH reacted with Ethyl

Acetate in sample

= (I) x Vs

= 0.047 mol/L x 0.05 L

= 2.35 x 10-3 mol

(K) Concentration of Ethyl Acetate reacted

with NaOH=

(J )V s

= 2.35× 10−3 mol0.05 L

= 0.047 mol/L

(L) Concentration of Ethyl Acetate

unreacted, CB

= CEA,O – (K)

= 0.1 mol/L – 0.047 mol/L

= 0.053 mol/L

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(M) Residence time, τ=

V CSTR

F0

= 4.0 L

0.6 L/min

= 6.6667 min

For reaction rate constant number 1,

V0 = Total inlet flow rate = 600 mL/min = 0.6 L/min

VTFR = Volume for reactor = 4 L

CAO = Inlet concentration of NaOH = 0.1 M

(N) Reaction rate constant, k=

V 0

V TFR CA 0

( X1−X

)

= 0.6 L/min

(4 L)(0.1 M )( 0.94

1−0.94)

= 23.5 M.L/min

(O) Rate of reaction, -rA = k(CA0)2 (1 – X)2

= 23.5 (0.1)2 (1 – 0.94)2

= 0.0141 M.L/min

The calculation steps above were repeated for all data and the graph was plotted.

8) DISCUSSION

In this experiment, two substances have been used in the plug flow reactor which is

sodium hydroxide, NaOH, and ethyl acetate, Et(Ac). The residence time and conversion of

NaOH are calculated and tabulated in table 7.2. For the residence time, τ , of 6.6667 min,

the conversion of NaOH is 94%. At the 8th minute of residence time, the conversion of

NaOH dropped to 67.48%. As for the 10.0000 minute of residence time, the conversion of

NaOH increased for 2% from 67.48% at 8.0000 minute to 69.48% at 10 th minute. As the

residence time increases to 13.3333 min, conversion of NaOH has a sudden increase to

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80.80%. The conversion of NaOH was further increased to 84% at the residence time of

20.0000 minute. Lastly, at 40.0000 min of residence time, the final conversion of NaOH

obtained was at 92.40%. All these data was the plotted into graph in figure 7.2 of

conversion versus residence time.

While in titration procedure, when both flowrates are at 300 mL/min, the volume of

NaOH titrated is 23.50 mL. When the flowrates are at 250 mL/min, the volume of NaOH

titrated dropped to 16.87 and later at 200, 150, 100, and 50 mL/min, the volume of NaOH

titrated increases to 17.37, 20.23, 21.03, and 23.07 mL respectively. This follows

accordingly to the theory that whereas concentration is higher at lower flow rate, more

volume of NaOH is used in titration to neutralize the sample.

From the graph 7.2 plotted, initially, it shows a drop of NaOH conversion from

6.6667 minute of residence time to 8.000 minute of residence time. This is being due to

error while performing the experiment. The sample might be exposed to long before

titration is performed on them. As for the rest of the graph chart, the conversion of NaOH

gradually increases with residence time. The data mainly shows that the longer the

residence time, the percentage of conversion increases which abides the expected result

where the longer time left in the reactor, more reaction occurs between NaOH and Et(Ac).

Figure 8.1 Rate of reaction vs time

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The rate of reaction for this reactor shows that it decreases over time. There was a

stable supply of heat, making temperature a constant variable, and no unnecessary heat is

applied onto the system. This makes the particle speed slower as time increases, losing

energy. In the end, the reaction becomes slower until it reaches equilibrium where there

are no more changes in the rate of reaction as shown in figure 8.1 above.

9) CONCLUSION

The objective of this experiment is successfully achieved. Saponification reaction between

NaOH and Et(Ac) was obtained in the reactor. Through the data obtained and certain

calculation, both reaction rate constant and effect of residence time on the conversion was

also known. Furthermore, a graph was also plotted in light of more understanding of the

relation between these two data.

10) RECOMMENDATION

All valves should be properly placed before starting the experiment.

Titration should be done as soon as the sample is taken, to avoid any

contaminations into the sample.

Temperature should also be constantly monitored so that it remains the same

throughout the experiment.

Perform general start-up and shut-down procedure so that the equipment is in the

best shape.

11) REFERENCE

Schmidt, Lanny D., The Engineering of Chemical Reactions. New York: Oxford

University Press, 1998.

Plug Flow Reactor Static Mixer. (n.d.). Retrieved April 19, 2015, from

http://www.stamixco-usa.com/plug-flow-reactors

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12) APPENDIX

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