1.0ABSTRACT

This experiment is to study the saponification reaction between sodium hydroxide and ethyl acetate

in PFR. It is also conducted to determine the relationship of residence time and conversion and how

they affect the reaction rate. In this experiment, equimolar of sodium hydroxide and ethyl acetate

are used. This experiment is carried out for six different flow rates (600mL/min, 500mL/min,

400mL/min, 300mL/min, 200mL/min and 100mL/min). For every flow rates, the conductivities are

recorded and samples are taken and titrated with sodium hydroxide along with 10mL of

hydrochloric acid. The results obtained is then compared with the calbration curve plotted. It is

found that the residence time influent the conversion in such manner that as the residence time

increases increases, the conversion increases and the rate of reaction decreases.

2.0INTRODUCTION

Reactor is one of the important unit operation in chemical industry where various reaction took

place in reactor. In industries, a reactor is design to produce the desired product in low cost but at

high yield. There are are many types of reactors used in today's industries and the usage of the

reactors depends on many criterion and application.

There are three important chemical reactors used, they are batch reactor, continuous flow

reactor(CSTR) and plug flow reactor(PFR). In CSTR, the reaction is a complete mixing reaction

where there is a presence of strirrer or mixer. In PFR, the reaction is also complete mixing reaction

and the shape of the reactor is cylindrical.

In this experiment, it concern the usage of PFR. PFRs are used to model the chemical

transformation of compounds as they are transported in systems resembling "pipes". PFR is the

steady state of tubular reactor (TFR), where it operates at ideal condition. The reactants enter the

cylindrical reactor at one end and exit at another end. In PFR, there are few assumptions made.

They are:

1. no mixing in the axial direction, which is in the direction of the flow

2. complete mixing in the radial direction

3. a uniform velocity profile across the radius

According to Gilmour and Samson, the validity of the assumptions will depend on the geometry of

the reactor and the flow conditions. Deviation which are frequent but not always important are of

two kind:

1. mixing in longitudinal direction due to vortices and turbulence

2. incomplete mixing in radial direction n laminar flow condition

The advantages of using PFR is that it has a high volumetric unit conversion, able to operate for

long periods of time without maintenance, and the heat transfer rate can be optimized by using

more, thinner tubes or fewer, thicker tubes in parallel. Disadvantages of plug flow reactors are that

temperatures are hard to control and can result in undesirable temperature gradients. PFR

maintenance is also more expensive than CSTR maintenance[1].

Plug flow reactors are used for large-scale reactions, fast reactions, homogeneous or heterogeneous

reactions, continuous production and high-temperature reactions[1].

3.0AIM

1. To carry out a saponification reaction between NaOH and Et(Ac) in a PFR

2. To determine the reaction constant

3. To determine the effect of residence time on the conversion in a PFR

4.0THEORY

For different types of reactor, the reaction kinetic for each reactor is different. Therefore, the

equation for each of the reactor is differrent. For PFR, the design equation is

dFA = rA

dV

The residence time distribution is the characteristic of the mixing that occurs in the chemical

reactor. Residence time on the other hand ,is the time for which the reaction can takes place.

Residence time is given by,

τ = VPFR

Vo

The rate of reaction indicates that the rate of disappearance of reactant or the rate of formation of

reactant. For this reaction,

NaOH + CH3COOC2H5 produce CH3COONa + C2H3 OH

the rate of reaction can be represented by,

-rA = k[CA][CB]

where, k is the rate constant, [CA] is the concentration of sodium hydroxide and [CB] is the

concentration of ethyl acetate.

Since the concentration of sodium hydroxide and ethyl acetate are equimolar, the equation can be

reduce to

-rA = k[CA]2

-rA = kCAo (1-X)2

-rA = FAo dX = voCAo dX dV dV

VTFR = vo X kCAo(1-X)

5.0 APPARATUS/MATERIAL

Solteq tubular flow reactor

NaoH(0.1M)

Et(Ac) ().1M)

HCl(0.25M)

phenophthalene

6.0PROCEDURE

Calibration Curve:

1.The following solution are prepared:

a) 0% conversion: 100 ml NaOH

b) 25% conversion: 75ml NaOH + 25mL Na(Ac)

c)50% conversion: 50ml NaOH + 50mL Na(Ac)

d)75% conversion: 25ml NaOH + 75mL Na(Ac)

e) 100% cnversion: 100mL Na(Ac)

2.calibration curve of conductivity vs conversion is plotted.

General Start-up:

1. All valves are ensured to be initially closed except valve V7.

2. The following solutions are prepared:

a.20 liter of sodium hydroxide, NaOH (0.1M)

b.20 liter of ethyl acetate, Et(Ac) (0.1M)

c.1 liter of hydrochloric acid, HCl (0.25M)

3. The feed tank B1 is filled with NaOH solution and tank B2 is filled with Et(Ac) solution.

4. The water jacket B4 and pre-heater B5 are filled with clean water.

5. The power for control panel is turned on.

6. Valves V2, V6, V9, V11 were opened.

7. Both pumps P1 and P2 are switched on. Flow controller at P1 and P2 are adjusted to obtaian

flow of approximately 300 ml/min at both flow meters Fl-01 and Fl-02. Both flowrates are

ensured to be the same.

8. Both solutions are allowed to flow through the reactor R1 and overflow into the waste tank

B3.

9. Valves V13 and V8 are opened. Pump P3 is switched on to circulate the water through pre-

heater B5. The stirrer motor M1 is switched on and the speed is set about 200 ppm to ensure

homogeneous water jacket temperature.

Effect of Residence Time on the Reaction in a PFR:

1. Valves V9 and V11 are opened.

2. Both NaOH and Et(Ac) solutions are allowed to enter the tubular reactor R1 and

empty into the waste tank B3.

3. Pump P1 and P2 flow controller are adjusted to give a constant flow rate of about

300ml/min at flow meter Fl-01 and Fl-02. Both flow rates are ensured to be the same. The

flow rate are recorded.

4. The inlet (Ql-01) and outlet (Ql-02) conductivity values are monitored until they do

not change over time. This is to ensure that the reactor has reached steady state.

5. Both inlet and outlet steady staete conductivity values are recorded. The

concentration of NaOH exiting the reactor and extent of conversion from calibration curve

are found.

6. Sampling valve V15 is opened and 50 ml sample is collected. Back titration is

carried out to determine the concentration of NaOH in the reactor and extend of conversion.

7. The experiment is repeated for steps 3-6 for different residence times by increasing

the feed flow rates of NaOH and Et(Ac) to about 250, 200, 150, 100, 50 ml/min. Both flow

rates are ensured to be the same.

7.0RESULTS

Table 1: Data obtained from the experiment

No Flow rate

of NaOH

(mL/min)

Flow rate

of Et(Ac)

(mL/min)

Total

flow rate

of

solutions,

Vo

(mL/min)

Residenc

e time

(min)

Outlet

conductiv

ity

(mS/cm)

Inlet

conductivit

y (mS/cm)

Conversion,

X

Reaction

rate

constant

(L/

mol.min)

Rate of

reaction

(mol/L.min)

Volume of

NaOH

(cm3)

1 300 300 600 3.3333 5.7 7.9 83.28 7.4713 0.02089 23.4

2 250 250 500 4.0000 5.2 7.8 88.84 9.9507 0.01239 23.7

3 200 200 400 5.000 5.1 7.3 89.96 8.9602 0.00903 23.8

4 150 150 300 66.6667 4.6 7.2 95.52 15.9911 0.00321 27

5 100 100 200 10.0000 4.5 7.0 96.64 14.3810 0.00162 25.5

6 50 50 100 20.0000 3.9 5.7 100 - - 25.4

Table 2: Data obtained from calibration curve

Conversion(%) Conductivity(mS/cm)

0 10.70

25 12.77

50 8.65

75 6.59

100 4.75

8.0SAMPLE CALCULATION

1.From the graph: f(x) = -11.13x + 146.72

where, f(x) is conversion value and x is conductivity value.

A) Calculation of conversion(%)

From the data obtained, for total flow rate = 600 mL/min, the outlet conductivity is 5.7

hence,

f(x) = -12.4x + 157.79

f(x) = -12.4(5.7) + 157.79

= 87.11%

For flow rate of 50 mL/min with the outlet conductivity of 3.9,

f(x) = -12.4x + 157.79

f(x) = -12.4(3.9) + 157.79

= 109.43%

Since, conversion value cannot exceed 100%, the conversion for flow rate of 50 mL/min is 100%.

B) Calculation of residence time,τ

τ = VPFR

Vo where, VPFR is volume of the reactor and Vo is flow rate

For total flow rate = 600 mL/min,

τ = VPFR

Vo τ = 2L (300 x 10-3)L/min = 6.6667 min

CH3–COO–CH2 –CH3 +NaOH→CH3 –COONa+CH3 –CH2 –OH

C) Calculation for reaction constant

Since this reaction is a second order reaction and CAO = CBO, the rate law is,

-rA = kCACB = kCA2 = kCAO(1-X)2

-rA = FAO dX = VoCao dX dV dV

VPFR = Vo (X) kCAO(1-X)

k = Vo (X) VPFRCAO(1-X)

For total flow rate = 600 mL/min, conversion = 87.11%,

k = Vo (X) VPFRCAO(1-X)

k = (600 x 10-3)L/min(0.8711)

(2)L(0.05)M(1 – 0.8711)

= 40.5477 L mol-1min-1

D) Calculation for rate of reaction

-rA = kCACB = kCA2 = kCAO(1-X)2

-rA = kCAO(1-X)2

For total flow rate = 600 mL/min, conversion = 87.11%,

-rA = (40.5477)(0.05)(1 – 0.8711)2

= 0.2613

Table 3: Results from calculation using calibration curve

Conversion (%) Residence Time(min)

Reaction constant, k(L mol-1min-1)

Rate of reaction

(mol/L.min)

87.11 3.3333 40.5477 0.2613

93.31 4.0000 69.7384 0.2333

94.55 5.0000 69.3945 0.1891

100 6.6667 - -

100 10.0000 - -

100 20.0000 - -

2. By using data obtained from titration

A) Calculation of conversion

Concentration = (VHCl)(CHCl) – (VnaOH)(CNaOH) volume of reaction mixture

For total flow rate = 600 mL/min,

Concentration = (10)(0.25) – (23.4)(0.1)

50= 0.0032 M

Conversion, X = 1 – CA CAO

= 1- 0.0024

0.05

= 0.936

B) Calculation of residence time,τ

τ = VPFR

Vo where, VPFR is volume of the reactor and Vo is flow rate

For total flow rate = 600 mL/min,

τ = VPFR

Vo τ = 2L (600 x 10-3)L/min = 6.6667 min

CH3–COO–CH2 –CH3 +NaOH→CH3 –COONa+CH3 –CH2 –OH

C) Calculation for reaction constant

Since this reaction is a second order reaction and CAO = CBO, the rate law is,

-rA = kCACB = kCA2 = kCAO(1-X)2

-rA = FAO dX = VoCao dX dV dV

VPFR = Vo (X) kCAO(1-X)

k = Vo (X) VPFRCAO(1-X)

For total flow rate = 600 mL/min, conversion = 93.6%,

k = Vo (X) VPFRCAO(1-X)

k = (600 x 10-3)L/min(0.936) (2)L(0.05)M(1 – 0.936)

= 90.75 L mol-1min-1

D) Calculation for rate of reaction

-rA = kCACB = kCA2 = kCAO(1-X)2

-rA = kCAO(1-X)2

For total flow rate = 600 mL/min, conversion = 93.6%,

-rA = (87.75)(0.05)(1 – 0.936)2

= 0.01797 mol/L.min

Table 4: Results obtained through calculation using titration data

Conversion (%) Residence Time(min)

Reaction constant, k(L mol-1min-1)

Rate of reaction

(mol/L.min)

93.6 3.3333 87.75 0.01797

94.8 4.0000 91.15 0.01232

95.2 5.0000 73.33 8.4476 x 10-3

100 6.6667 - -

100 10.0000 - -

100 20.0000 - -

9.0DISCUSSION

The experiment is about saponification reaction between sodium hydroxide NaOH and ethyl

acetate(Et(Ac)) in a plug flow reactor (PFR). This reation is a saponification reaction where

saponification process is the reaction between a strong base which in this case is sodium hydroxide and

a natural fat which, for this experiment is ethyl acetate. The reaction is a second order reaction where,

-rA = k[CA][CB]

and for this case, the concentration of sodium hydroxide fed to the reactor is equal to the

concentration of ethyl acetate fed to the reactor. Since the equimolar concentration of reactant is fed

to the reactor,

[CA] = [CB]

Thus,

-rA = k[CA]2

This experiment is conducted in order to understand the relationship of conductivity and conversion

and how this may affect the reaction. A calibration cuve of conversion vs conductivity is plotted to

determine the unknown conversion of reactant at a given flow rate.

Conductivity or salinity is a measure of the ability of water to conduct electricity, which provides a

measure of what is dissolved in water. A higher conductivity value indicates that there higher content of

chemical dissolved in the water. Conductivity is the inverse of resistivity. Resisitivity is the measure of

the resisting power of a specified material to the flow an electric current.

From the calibration curve obtained, it is seen that as the conversion increases, the conductivity

decreases. This is because, the charge particles in material or substance aid in conduction of electric

current in that particular substance. Therefore, higher concentration will results in higher

conductivity. It should be noted that there are substances that do not follow this behaviour such as

concentrated sulphuric acid. Hence, when conversion is high, it means that the concentration of

sodium hydroxide is low, therefore it is only reasonable if the conductivity is low as well.

From the data obtained it can be seen that as the flow rate increases, the conductivity increases. This

means that when the flow rate is high, the conversion of sodium hydroxide is low which means that

there is high concentration of unreacted sodium hydroxide left in the reactor. In addition, as the total

flow rate increases, the residence time decreases. Therefore, residence time increases with the

increasing of conversion of sodium hydroxide.

Residence time refers to the average length of time a molecule spends in a container, where material

flow is concern. Therefore, basically residence time indicates the time taken or given for the

concentration to change or for the reactant to react. That is the reason as the residence time

increases, the conversion increases. When the residence time is longer, there are more time for the

reaction to take place, hence causing the conversion to be higher. The theory of residence time is

applied to conserved quantities of mass, momentum, energy and charge.

In determining the unknown conversion value of sodium hydroxide, the conductivity value from the

experiment is used along with the calibration curve plotted. The results is recheck with the data

obtained from the titration of sodium hydroxide and hydrochloric acid.

There a slight difference in conversion values obtained by both method. By using calibration curve,

the conversion values obtained is less that the conversion values gained from titration of sodium

hydroxide. However, both results shows the same pattern in which as the residence time increases,

the conversion increases. When conversion vs residence time graphs are plotted for both, the pattern

of the graph is the same. The graph shows that, as the conversion inreases, the residences time

increases even after the conversion reaches 100% for total flow rate of 300mL/min, 200mL/min and

100mL/min.

For both methods of calculating the conversion that occur during the reaction at each flow rate, the

data shows that as the residence time increases, the rate of reaction decreases. The rate of reaction

can be expressed as,

-rA = dCA

dt

From this expression, it can be concluded that the rate of reaction is the change of concentration

over time.

Let,

dCA = CA – CAo

dt = residence time

and,

CA = CAo (1 – X)

Here, it can be seen that, as conversion increases, concentration decreases. When concentration

decreases, the change of concentration become small. It was stated earlier that as residence time

increases, conversion increases. This means that as the residence time increases, the concentration

will decrease, hence it will cause the change of concentration to decrease as well along with the

increase of residence time. Therefore, when small value of dCA is divided with large value of time,

it is only significant if the rate of reaction dcreases when residence time increases.

This can be supported by the collision theory, where it states that the rate of reaction will increase

proportional to the number of effective collision per second between the reactant molecules. If the

concentration increases, the greater the frequency of collision and the greater it will be for the frequency

of effective collisions and the consequently the greater will the rate of reaction be.

10.0CONCLUSION

From the data gained, it can be concluded that as the residence time has the influence on the conversio

of the reactant as well as the rate of reaction. The longer the residence time, the higher will the

conversion be and the lower the rate of reaction as the concentration of reactant is decreasing. The

reaction rate constant varies with the values of conversion and the total flow rate.

11.0RECOMMENDATIONS

In order to increase the yield of the reaction, plug flow reactor can be combined with membrane

seperator. The products are selectively pulled out of the reactor as they are made so that the equilibrium

in the reactor itself continues to shift towards making more product.

The conversion of the reactor can be increases by adding another reactor to it such as PFR or CSTR.

Since the rate of reaction is low when the resindence time is high, the rate of reaction can be fasten

by adding catalyst to the reaction. Catalyst will make the reaction process faster.

REFERENCES

September 11, 2011, http://en.wikipedia.org/wiki/Plug_flow_reactor_model#PFR_modeling

November 16, 2010,

Gilmour D., Samson P., Chapter 10: Reaction Kinetic, http://docs.google.com/viewer?

a=v&q=cache:359IYQ1TuVcJ:www.che.boun.edu.tr/courses/che302/Chapter

%252010.pdf+reaction+constant+vs+flow+rate&hl=en&gl=my&pid=bl&srcid=ADGEESh_PXZg

kAmRlwKpk-X9bKdqYSzbyxbvLGRMsxVPk-

gNuC9cXnMTlzbZ6heSJ1wW8MXx5iGMdPSI4MeFeNFnxngVtd4LrZz7saix8TJ5FutMNA8YSI

ScjSBj-CODDBAKB0kYXd-C&sig=AHIEtbRBEdaLLBKTJ9q0EGF0LNCESov-ZA

APPENDICES

Figure 1: Graph conversion vs conductivity from calibration curve

Figure 2: Graph conversion vs residence time through calculation from calibration curve

Figure 3: Graph conversion vs residence time through calculation obtained from titration data