ABSTRACT

The objective of this experiment is to carry out the saponification reaction between NaOH

and Et(Ac) in plug flow reactor, to determine the reaction rate constant and the rate of

reaction of the saponification process. Besides that to determine the effect of residence time

to the reaction's extent of conversion and lastly to evaluate the reaction rate constant of this

particular saponification reaction. To achieve these targets, an experiment is conducted using

a unit called SOLTEQ Plug Flow Reactor (model: BP101), commonly known as PFR, as well

as some common laboratory apparatus for titration process. In this experiment, reaction

between two solutions NaOH and Et(Ac) were reacted in the PFR. Then, the product is then

analysed by the method of titration to determine how well did the reaction go. After

collecting the data, the value of reaction rate constant and rate of reaction is calculated. Then,

a graph of conversion against residence time is plotted. Supposedly, we can say that the

conversion factor is inversely proportional to the residence time. Where, when the residence

time increases, the conversion factor also decreases. However, the experiment conducted may

consist with some error that the graph conversion versus residence time obtained are not

identically with the theory stated. Although some of the result obtain from the experiment is

incorrect due to some errors, but the overall objectives of this experiment is successfully

conducted.

1

TABLE OF CONTENT

PAGE

ABSTRACT 1

TABLE OF CONTENT 2

INTRODUCTION 3

OBJECTIVE 4

THEORY 5

APPARATUS 8

PROCEDURE 9

RESULTS 11

SAMPLE OF CALCULATION 13

DISCUSSION 17

CONCLUSION 19

RECOMMENDATION 19

REFERENCES 20

APPENDIXES 20

2

INTRODUCTION

Reactor is one of the most important parts in industrial sector. Reactor is equipment

that changes the raw material to the product that we want. A good reactor will give a high

production and economical. The design of a reactor must be finely tuned so that its

mechanisms suit the necessities of the process that is to be carried depends on the nature of

the materials in both the feed and of course the products, the reactors may take a wide range

of forms. Thus, it is important to fully comprehend the reactor of a certain reactor as well as

its process mechanism before conduct the experiment.

There are three types of commonly used continuous flow reactors in industrial that

are continuous stirred tank reactor (CSTR), plug flow reactor (PFR) and packed bed reactor

(PBR). In this experiment, the plug flow reactor provided in laboratories is used as it has been

properly designed for students’ experiment on chemical reactions in liquid phase under

isothermal and adiabatic conditions. Included in the unit is a jacketed plug flow reactor;

individual reactant feed tanks and pumps, temperature sensors and conductivity measuring

sensor. By using this particular unit, students will be capable to conduct the typical

saponification reaction between ethyl acetate and sodium hydroxide among the others

reaction.

A plug flow reactor is a pipe-shaped tank where a chemical reaction takes place with

walls coated with a catalyst and an inlet flow of pure reactant. It consists of a cylindrical pipe

and is normally operated at steady state, as is the CSTR A simple illustration for what a

typical plug flow reactor is:-

Figure 1: A simple schematic diagram for plug flow reactor.

There are various different types of reactors due to the numerous different factors that

can control the formation of product during the reaction. Plug flow reactors are an idealized

scenario where there is no mixing involved in the reactor. It is the opposite of the continuous-

stirred tank reactor (CSTR), where the reaction mixture is perfectly mixed. It is impossible to

3

have no mixing at all during a reaction, but the amount of mixing in the reactor can be

minimized. There are several advantages to minimizing the amount of mixing so that the

reactor closely resembles a Plug Flow Reactor. The plug flow reactor has an inlet flow

composed of the reactants. The reactant flows into the reactor and is then converted into the

product by a certain chemical reaction. The product flows out of the reactor through the outlet

flow.

Before the reactants are continually flow inside the Plug flow reactor, there are must

have a specific assumptions are made about the extent of mixing. The validity of the

assumptions will depend on the geometry of the reactor and the flow conditions-:

1) Complete mixing in the radial direction.

2) No mixing in the axial direction, i.e., the direction of flow.

3) A uniform velocity profile across the radius.

4) Mixing in longitudinal direction due to vortices and turbulence.

5) Incomplete mixing in radial direction in laminar flow conditions.

Basically, it is impossible to proceed to 100% completion in chemical reactions. This

is because due to the rate of reaction decreases when per cent completion gradually increases

until the point where the system achieve dynamic equilibrium (no net reaction occurs)

(Fogler, 2006). In fact, the equilibrium point mostly is less than 100% complete. Thus,

distillation is used as a separation process, in order to separate any remaining reagents or by

products from the desired product. Sometimes the reagents may be reused at the beginning of

the process as a recycle back, such as in the Haber process.

OBJECTIVES

The objective of this experiment is:

1. To find the calibration curve of conversion versus conductivity.

2. To carry out a saponification reaction between sodium hydroxide, NaOH and ethyl

acetate, Et(Ac).

3. To determine the reaction rate constant.

4. To determine the effect of residence time on the conversion in a plug flow reactor.

4

THEORY

In this experiment, we used Sodium Hydroxide and Ethyl Acetate to produce Sodium

Acetate and Ethyl Alcohol. The feed of reactor enters at one end of a cylindrical tube and the

product stream leaves at the other end. The function plug flow reactor which is long tube and

the lack of provision for stirring prevent complete mixing of the fluid in the tube. 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 will diminishes

and a product will produced. That product is Sodium Acetate and Ethyl Alcohol. For

example:

aA+Bb cC+Dd General Equation

Based on example, A and B shown that the reactants. Meanwhile C and D represent

products. In this reaction, A and B is being diminished and C and D is will be 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. For instance:

−r Aa

=−r Bb

=rCc

=rDd

The negative sign indicates reactants.

For the rate of reaction for reactant A, -rA :

−r A=k C Aα CB

β

Where: k - Rate constant.

CA - Concentration of reactant A.

CB - Concentration of reactant B.

α - Stoichiometric coefficient of A

β - Stoichiometric coefficient of B

5

Conversion, X

Consider the general equation as before, we choose species A as the basis of

calculation or as limiting reactant, hence the reaction expression can be arranged as follows:

A+ baB+ c

aC+ d

aD

The conversion of species A in a reaction is equal to the number of moles of A reacted per

mole A feed.

X A=moles of A reactedmoles of A feed

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.

Mass Balance in PFR

In a plug flow reactor, reactants are fed to the reactor at the inlet and the products are

removed from the reactor at the outlet. The reaction takes places within the reactors as the

reacting mixtures moves through the pipe. In an ideal plug reactor, the reacting mixture is

assumed to move as a plug and its properties are assumed to be uniformly distributed across

the cross section of the reactor.

Where,

FA: molar flow rate of A in moles per time.

FAo: molar flow rate of A at the inlet in moles per time.

6

Differential volume, dV

FAf : molar flow rate of A at the exit in moles per time.

Vo: volumetric flow rate at the inlet in volume per time.

Vf: volumetric flow rate at the exit in volume per time.

Design equation for reactant A in the PFR is obtained by writing the mass balance for

reactant A over a differential volume of the reacting mixture dV as follows:

Mass of A entering the volume dV per unit time = Mass of A leaving the volume dV per

unit time + Mass of A accumulated within the volume dV per unit time + Mass of A

disappearing by the reaction within the volume dV per unit time.

At steady state, no accumulation takes place. Therefore, at steady state, the above equation

reduces to:

FAMA = (FA + dFA)MA + (-rA)MA dV (Eq. 1)

Where,

FA: number of moles of A per unit time entering the differential volume dV

(FA+dFA): number of moles of A per unit time leaving the deferential volume dV

MA: molar mass of A

-rA: molar rate at which A is disappearing because of the reaction.

Removing MA from (Eq. 1) and rearranging it, we get the design equation for reactant A in

an ideal PFR operated at steady-state as follows:

d F A

dV=r A (Eq. 2)

Working out in terms of the concentration of A, CA:

Concentration CA in an ideal PFR is defined as follows:

C A=F A

v(Eq. 3)

Equation 3 is substitute into equation 2:

7

dC A v

dV=r A (Eq. 4)

In PFR the volumetric flow rate is constant, so the differentiations of equation 4 yield to:

V PFR=∫C Af

CAo

v(−r¿¿ A)

dC A¿ (Eq. 5)

Where CAo and CAf are the respective concentrations of A at the entrance and at the exit of the

reactor respectively, and (-rA) should be expressed as a function of CA.

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.

APPARATUS AND MATERIALS

The apparatus in this experiment is:-

1. Conical flask

2. Measuring cylinder

3. Beakers

4. Burette

5. Retort stand

6. Stop watch

7. Plug Flow Reactor (Model: BP101)

Among the chemicals used are:

1. 0.1M Sodium Hydroxide, NaOH.

2. 0.1M Ethyl Acetate, Et(Ac).

3. 0.1M Hydrochloric Acid, HCl.

4. De-ionized water.

8

5. Phenolphthalein

PROCEDURE

General Start-Up Procedure

1. All the valves are ensured closed except V4, V8 and V17.

2. The following solutions are prepared:

a. 20 liter of NaOH (0.1M)

b. 20 liter of Et(Ac) (0.1M)

c. 1 liter of HCL (0.25M) for quenching

3. Feed tank B1 was filled with NaOH while feed tank B2 was filled with the Et(Ac).

4. The water jacket B4 was filled with water 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 and P2 were adjusted to obtained flow

rate approximately 300mL/min at both flow meters Fl-01 and Fl-02. Both flow rates

must be equal.

8. Both solutions then were allowed to flow through the reactor R1 and overflow into

waste tank B3.

9. Valves V13 and V18 was opened. Pump P3 then was switched on in order to circulate

the water through pre-heater B5. The stirrer motor M1 was switched on and set up to

speed about200 rpm to ensure homogeneous water jacket temperature.

Experiment Procedure

1. The general starts up procedures were performed.

2. Valves V9 and V11 were opened.

3. Both the NaOH and Et(Ac) solutions were allowed to enter the plug reactor R1 and

empty into the waste tank B3.

4. P1 and P2 were adjusted to give a constant flow rate of about 300 ml/min at flow

meters FI-01 and FI-02. Both flow rates were ensured same. The flow rates were

recorded.

9

5. The inlet (QI-01) and outlet (QI-02) were started to monitor the conductivity values

until they do not change over time. This is to ensure 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.

7. Sampling was opened from valve V15 and 50ml of sample was collected. A back

titration procedure was carried out manually to determine the concentration of NaOH

in the reactor and extent of conversion.

8. The experiment was repeated from step 4 to 7 for different residence times by

reducing the feed flow rates of NaOH and Et(Ac) to about 250,200,150,100 and 50

ml/min. Both flow rates were made sure to be equal.

Back Titration Procedure

1. The burette was filled up with 0.1 M NaOH solution.

2. 10 mL of 0.25 M HCl was poured in a flask.

3. 50 mL samples that were collected from the experiment at every controlled flow rate

(300,250, 200, 150, 100 and 50 mL/min) were added into the 10mL HCl to quench

the saponification reaction.

4. 3 drops of phenolphthalein were dropped into the mixture of sample and HCl.

5. The mixture then was titrated with NaOH until it turns light pink.

6. The amount of NaOH titrated was recorded.

General Shut-down Procedure

1. Both pumps P1, P2 and P3 were switched off. The valves V2 and V6 were closed.

2. The heaters were switched off.

3. The cooling water was keep to circulating through the reactor while the stirrer motor

is running to allow the water jacket to cool down to room temperature.

4. If the equipment was not going to be used for long period of time, drain all liquid

from the unit by opening valves V1 to V19. Rinse the feed tanks with clean water.

5. The power for the control panel was turned off.

10

RESULT

Constant temperature at 30°C

Table 1: Calibration

Conversion,

%

Solution mixture

Concentration Conductivity0.1M NaOH,

mL

0.1M Et(Ac),

mL

H2O, mL

0 100 - 100 0.0500 7.66

25 75 25 100 0.0375 5.45

50 50 50 100 0.0250 2.95

75 25 75 100 0.0125 1.35

100 - 100 100 0.0000 0.15

0 20 40 60 80 100 1200

1

2

3

4

5

6

7

8

9

Conductivity vs Conversion

Conversion, %

Cond

uctiv

ity, m

S/cm

Experiment 3: Effect of residence time on the reaction.

Reactor Volume : 4L

Concentration of NaOH in the reactor, CNaOH : 0.1M (2L)

Concentration of NaOH in the feed vessel, CNaOH,f : 0.1M (2L)

11

Concentration of HCl quench, CHCl,s : 0.25 M (0.01L)

Volume of sample, Vs : 0.05L

Flowrate of

NaOH,

(ml/min)

Flowrate of

Et(Ac), (ml/min) Outlet conductivity, (mS/cm)

Volume of NaOH in

titration, mL

Q1 Q2

300 300 6.00 4.90 24.0

250 250 6.30 5.30 23.8

200 200 6.60 5.20 21.5

150 150 5.70 4.70 21.3

100 100 5.10 4.00 23.0

50 50 4.40 3.20 23.6

Table 2: Result

Residence Time, τ

(min)

Conversion, X (%) Reaction Rate

Constant, k

(L.mol/min)

Rate of Reaction, -rA

(mol.L/min)

6.6667 98 73.50 0.2940 x 10-3

8.0000 97.6 50.83 0.2928 x 10-3

10.0000 93 13.28 0.6507 x 10-3

13.3333 92.6 9.39 0.5142 x 10-3

20.0000 96 12.00 0.192 x 10-3

40.0000 97.2 8.67 0.0680 x 10-3

Table 3: Result of calculation.

12

5 10 15 20 25 30 35 40 4589

90

91

92

93

94

95

96

97

98

99

CONVERSION VS RESIDENCE TIME

Residence Time, min

Conv

ersio

n (%

)

SAMPLE OF CALCULATION

Residence Time, τ

For flow rate of 200 ml/min:

Residence time , τ=V , Reactor volume (L)

vo , Total flow rate(Lmin

)

Total flow rate, vo = Flow rate of NaOH + Flow rate of Et(Ac)

= 200 mL/min NaOH + 200 mL/min Et(Ac)

= 400 mL/min

= 0.4 L/min.

Thus,

Residence time , τ= 4 L0.4 L/min

= 10.000 min

Other residence times were calculated by the same way, and varying the flow rates.

13

Conversion

For flow rate of 200 mL/min:

Moles of reacted NaOH, n1:

n1= Concentration NaOH x Volume of NaOH titration

= 0.1M x 0.0215 L

= 0.00215 mole

Moles of unreacted HCl, n2

Moles of unreacted HCl = Moles of reacted NaOH

n2 = n1

n2 = 0.00215 mole

Volume of unreacted HCl, V1

V 1=n2

concentrationof HCl quench

= 0.00215

0.25

= 0.0086 L

Volume of reacted HCl, V2

V2 = Total volume of HCl – V1

= 0.01 – 0.0086

= 0.0014 L

Moles of reacted HCl, n3

n3 = Concentration of HCl x V2

= 0.25 x 0.0014

= 0.00035 mole

14

Moles of unreacted NaOH, n4

n4 = n3

= 0.00035 mole

Concentration of unreacted NaOH

CNaOH, unreacted = n4

volume sample

= 0.00035

0.05

= 0.007 M

Xunreacted

Xunreacted = concentration of NaOH unreacted

concentration NaOH

= 0.007

0.1

= 0.07

Xreacted

Xreacted = 1 - Xunreacted

= 1 – 0.07

= 0.93

Conversion for flow rate 200mL/min

0.93 x 100% = 93%

Thus, at flow rate 200mL/min of NaOH in the reactor, about 93% of NaOH is reacted with

Et(Ac). Other conversions were calculated by the same way, and varying the flow rates.

Reaction rate constant, k

15

k=vo

V PFRCAo( X1−X )

For flow rate of 200mL/min:

Vo = Total inlet flow rate = 0.4 L/min

VPFR = Volume for reactor = 4 L

CAo = inlet concentration of NaOH = 0.1 M

X = 0.93

Hence, k= 0.44 (0.1) ( 0.93

1−0.93 ) = 13.28L.mol/min

Other reaction rate constants were calculated by the same way, and varying the flow rates.

Rate of reaction, -rA

−r A=k (C Ao )2 (1−X )2

For flow rate of 200ml/min:

−r A=13.28 (0.1 )2 (1−0.93 )2

= 0.6507 x 10-3mol.L/min

Other rate of reactions were calculated by the same way, and varying the flow rates.

16

DISCUSSION

The main objective of this particular experiment is to study the effect of residence

time on the performance of PFR reactor. Plug Flow Reactor (PFR) is a type of reactor that

consists of a cylindrical pipe and is usually operated at steady state. The feed enter at one end

of a cylindrical tube and leaves product from the end of cylindrical tube. 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. The fluid in PFR is

considered to be thin, unmixed layer of volume segments or 'plugs', hence the name is PFR.

At the end of the experiment, we are able to determine the reaction rate constant by

using formula and to determine the effect of residence time on the conversion in the plug

flow reactor. The experiment is started by running up the equipment in order to start the

saponification process. There are two method where to done the experiment saponification

process which is variation in temperature or variation in contact time. In this experiment we

will let the flow rate of both solutions as the varying components because the flow rate of

both solutions is controlled by the temperature of the reactor. At the end of the experiment,

the saponification process is successfully done.

After, the experiment is conducted, the data consisting inlet flow rates, conductivity

value and volume of NaOH used in the titration process are tabulated in Table 1 and Table 2

of the Result Section. A series of calculation on one of the flow rate were made based on the

data tabulated that can see in Sample of Calculation section. After that, the values of

residence times, conversion of the reactions, reaction rate constants and rate of reactions were

determined. These values are tabulated in Table 3 of the Result section.

The reaction rate constant we get for flow rate of 300 ml/min is 73.50 L/mol.min, for

flow rate of 250 mL/min is 50.83 L/mol.min , for flow rate of 200 mL/min is 13.28

L/mol.min, for flow rate of 150 mL/min is 9.39 L/mol.min , for flow rate of 100 mL/min is

12.00 L/mol.min and for flow rate of last which is for the 50 mL/min reaction rate constant is

8.67 L/mol.min. Although some of the error may exist during experiment conducted, from

the reaction rate constant we can determine that the value of reaction rate constant is

17

decreased as the flow rate is decrease. Thus, these shows that the reaction rate constant is

depend to the flow rate flow in the plug flow reactor.

The rate of reaction also can be determined after we had done find the reaction rate

constant. The rate of reaction we get for 300ml/min flow rate is 0.2940 x 10 -3 mol/L.min, for

the 250mL/min the rate of reaction is 0.2928 x 10-3 mol/L.min, for the 200mL/min is 0.6507

x10-3 mol/L.min, for the 150mL/min is 0.5142 x 10-3 mol/L.min, for the 100 mL/min is 0.192

x 10-3 mol/L.min and for the 50mL/min the rate of reaction is 0.0680 x 10-3 mol/L.min. After

all value of rate of reactions has been calculated, a graph of conversion factor against

residence time is plotted. From the graph that had been plotted, we can say that the

conversion factor is inversely proportional to the residence time at certain point then a small

changes an increase of graph conversion to the residence time .

Supposedly, the result of conversion factor is inversely proportional to the residence

time. This is maybe due to the error occurred during conducted the experiment. Thus, when

the residence time is increases, the conversion factor also decreases. Graph conversion versus

residence time showed that the conversion of sodium hydroxide increased with increasing

residence time. Residence time was defined as the length of time the fluid would stay in the

reactor. The longer the reactants would stay in the reactor, more products would be formed.

18

CONCLUSION

The experiment was conducted with several objectives in mind. The first one is to

carry out a saponification process between Sodium Hydroxide, NaOH and Ethyl Acetate,

Et(Ac). By using PFR, these two substances were flowed into the reactor, mixed and let to

react for a certain period of time. By doing that, saponification process was completed. There

are many scenarios that must be considered when deciding on which type of reactor to use for

a certain process. A plug flow reactor is one of many types of reactors. It is most useful when

the reaction is not allowed to reach equilibrium, and the reaction is kinetically limited by the

reaction rate. The experiment also targets to determine the reaction rate of this particular

reaction. This was also done by calculating the reaction rate as seen in the sample of

calculation section. Lastly, the main objective of this experiment is to study the relationship

between the residence time and the conversion of the reactants. The graph had been plotted

based on the result data after calculation and it allow to study the relationship between

residence time and conversion. Supposedly, we can say that the conversion factor is inversely

proportional to the residence time. Where, when the residence time increases, the conversion

factor also decreases. However, the experiment conducted may consist with some error that

the graph conversion versus residence time obtained are not identically with the theory stated.

RECOMMENDATION

There are several recommendations that can be taken in order to get more accurate result that

are:

1. Flow rates should be constantly monitored so that it remains constant throughout the

reaction, as needed.

2. To obtain more accurate results, run several trials on tubular flow reactor so we can

take the average value from each different molar rate.

3. During titration, students should be more alert and carefully because the volume of

NaOH that will convert the solution to light pink colour are the most important. Thus,

the excess of drop of NaOH will give effect on the result in the calculations.

19

4. Titration should be immediately stopped when the indicator turned light pink.

REFERENCES

Laboratory Manual Plug Flow Reactor.

Fogler, H.S (2006). Elements of Chemical Reaction Engineering (3rd Edition). PrenticeHall.

The Plug Flow Reactor (Retrieved from http://www.cs.montana.edu/.html on the 20th April

2015).

Fundamentals of Chemical Reactor Theory (Retrieved from,

http://www.seas.ucla.edu/stenstro/Reactor.pdf on the 20th April 2015).

APPENDIXES

20