ABSTRACT

This is the experiment of continuous stirred tank reactor 40 L. The reaction to be studied

is the saponification reaction of ethyl acetate Et(Ac) and sodium hydroxide NaOH in the

continuous stirred tank reactor. The calibration curve of conversion vs conductivity is also

perform in this experiment. The objectives of this experiment are to perform saponification

reaction between NaOH and Et(Ac), to determine the reaction rate constant and to study the

effect of residence time on the conversion. This experiment is done by varying the residence time

by manipulating the flow rate of the system. The flow rate of both feeds that is introduced,

Et(Ac) and NaOH is the same and varied from 0.1 L/min to 0.3 mL/min. From the result, the

percentage of conversion of NaOH is increasing with the increase of residence time. When the

residence is 200 mins, the conversion is 98.4%. Then as the total flow rate provided for the

system is increased, the residence time is decreasing and that makes the conversion of NaOH to

decrease as well. When the residence time is 66.6667 mins, the conversion is increasing to

96.4%. The lower flow rate gives a higher residence time and helps increasing the conversion of

the reaction in the CSTR.

1

INTRODUCTION

Chemical kinetics and reactor design are at the heart of producing almost all

industrial chemicals. The selection of a reaction system that operates in the safest and most

efficient manner can be the key to the success or failure of a chemical plant. The reaction

occurred in a reactor is exothermic or endothermic. A chemical reactor is a vessel where

reactions are carried out purposefully to produce products from reactants by means of one or

more chemical reactions. A chemical reactor may be characterized by the mode operation

according to the flow condition.

Continuous stirred tank reactors (CSTR) are the most basic of the continuous

reactors used in chemical processes. The continuous stirred tank reactor or back mix reactor is a

very common processing unit in chemical and polymer industry. Its names suggest, it is a reactor

in which the contents are well stirred and uniform throughout. The CSTR is normally run at

steady state, and is usually operated so as a to be quite well mixed. The CSTR is generally

modeled as having no spatial variations in concentrations, temperature, or reaction rate

throughout the vessel. Since the temperature and concentration are identical everywhere within

the reaction vessel, they are the same at the exits point as they are elsewhere in the tank.

Continuous stirred‐tank reactors (CSTRs) are open systems, where material is free

to enter or exit the system that operate on a steady‐state basis, where the conditions in the reactor

don't change with time. Reactants are continuously introduced into the reactor, while products

are continuously removed. CSTRs are very well mixed, so the contents have relatively uniform

properties such as temperature, density, etc. throughout. Also, conditions in the reactor's exit

stream are the same as those inside the tank. Continuous stirred‐tank reactors are most

commonly used in industrial processing, primarily in homogeneous liquid‐phase flow reactions,

where constant agitation is required. They may be used by themselves, in series, or in a parallel.

The Continuous Stirred Tank Reactor (Model: BP 143) unit is suitable for student

experiments on continuous chemical reactions. The unit consists of a jacketed reaction fitted in

the agitator and condenser. The unit comes complete with vessels for raw materials and product,

feed pumps, and thermostat. . Students will conduct continuous saponification reaction of ethyl

2

acetate and sodium hydroxide. A conductivity measurement is provided and students will relate

the conductivity value to extent of reaction. Students will study the effects of residence time and

reaction temperature on the reaction rate constant.

OBJECTIVE:

To carry out a saponification reaction between NaOH and Et(Ac) in a CSTR

To determine the effect of residence time onto the reaction extent of conversion

To determine the reaction rate constant

3

THEORY

The behavior of a CSTR is often approximated or modeled by that of a Continuous Ideally

StirredTank 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 to 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,

Integral mass balance on number of moles Ni of species i in a reactor of volume V;

where Fio is the molar flow rate inlet of species i, Fi the molar flow rate outlet, and stoichiometric

coefficient. The reaction rate, r, is generally dependent on the reactant concentration and the rate

constant (k). The rate constant can be determined by using a known empirical reaction rates that

is adjusted for temperature using the Arrhenius temperature dependence. Generally, as the

temperature increases so does the rate at which the reaction occurs. Residence time, is the

average amount of time a discrete quantity of reagent spends inside the tank.

Assume:

constant density (valid for most liquids; valid for gases only if there is no net change in

the number of moles or drastic temperature change)

isothermal conditions, or constant temperature (k is constant)

steady state

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single, irreversible reaction (νA = 1)

first order reaction (r = kCA)

A → products

NA = CA V (where CA is the concentration of species A, V is the volume of the reactor, NA is the

number of moles of species A)

The values of the variables, outlet concentration and residence time, in the equation are major

design criteria. To model systems that do not obey the assumptions of constant temperature and a

single reaction, additional dependent variables must be considered. If the system is considered to

be in unsteady state, a differential equation or a system of coupled differential equations must be

solved. CSTR's are known to be one of the systems which exhibit complex behavior such as

steady state multiplicity, limit cycles and chaos.

Conversion:

One of the reactants is choosed as the basis of calculation and the other species involved is

related in the reaction to this basis. Consider the general equation

aA + bB → cC + dD

We will choose A as our basis of calculation.

A+ ba

B → caC+ d

aD

The basis of calculation is most always the limiting reactant. The conversion of species A in a

reaction is equal to the number of moles of A reacted per mole of A fed.

5

APPARATUS/MATERIALS

tubular flow reactor

deionized water

sodium hydroxide

ethyl acetate

6

PROCEDURE/METHODOLOGY

General Start-Up Procedures

1. The following solutions are prepared:

a) 40 L of sodium hydroxide, NaOH (0.1M)

b) 40 L of ethyl acetate, Et(Ac) (0.1M)

c) 1 L of hydrochloric acid, HCL (0.25M), for quenching

2. All valves are ensured to be initially closed.

3. The feed vessels are charged as follows:

a) The charge port caps are opened for vessels B1 and B2.

b) The NaOH solution is carefully poured into vessel B1 and the Et(Ac) solution into vessel B2.

c) The charge port caps for both vessels are closed.

4. The power for the control panel is turned on.

5. Water in the thermostat T1 tank is check whether it is sufficient. Refill as necessary.

6. Cooling water valve V13 is opened and the cooling water is let to flow through the condenser

W1. (Only for Experiment 2)

7. The overflow tube is adjusted to give a working volume of 10 L in the reactor R1.

8. Valves V2, V3, V7, V8 and V11 are opened.

9. The unit is now ready for experiment.

7

General Shut-Down Procedures

1. The cooling water valve V13 is kept open to allow the cooling water to continue flowing.

2. Both pumps P1 and P2 are switched off. Stirrer M1 is switched off.

3. The thermostat T1 is switched off. The liquid in the reaction vessel R1 is let to cool down to

room temperature.

4. Cooling water valve V13 is closed.

5. Valves V2, V3, V7 and V8 is closed. Valves V4, V9 and V12 are opened to drain any liquid

from the unit.

6. The power for the control panel is turned off.

A. Preparation of Calibration Curve for Conversion vs. Conductivity

The reaction to be studied is the saponification reaction of ethyl acetate Et(Ac) and sodium

hydroxide NaOH. Since this is a second order reaction, the rate of reaction depends on both

concentrations of Et(Ac) and NaOH. However, for analysis purposes, the reaction will be carried

out using equimolar feeds of Et(Ac) and NaOH solutions with the same initial concentrations.

This ensures that both concentrations are similar throughout the reaction.

NaOH + Et(Ac) Na(Ac) + EtOH

The following procedures will calibrate the conductivity measurements of conversion values for

the reaction between 0.1 M ethyl acetate and 0.1 M sodium hydroxide:

Procedures:

1. The following solutions is prepared:

a) 1 liter of sodium hydroxide, NaOH (0.1 M)

b) 1 liter of sodium acetate, Na(Ac) (0.1 M)

8

c) 1 liter of deionised water, H2O

2. The conductivity and NaOH concentration for each conversion values are determined by

mixing the following solutions into 100 ml of deionised water:

a) 0% conversion : 100 ml NaOH

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

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

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

e) 100% conversion : 100 ml Na(Ac)

B. Back Titration Procedures for Manual Conversion Determination

It is advisable to carry out manual conversion determination on experiment samples to verify the

conductivity measurement values. The following procedures will explain the method to carry out

back titration on the samples. It is based on the principle of quenching the sample with excess

acid to stop any further reactions, then back titrating with a base to determine the amount of

unreacted acid.

Procedures:

1. A burette is filled up with 0.1 M NaOH solution.

2. 10 ml of 0.25 M HCl is measured in a flask.

3. A 50 ml sample from the experiment is obtained and immediately added to the HCl in the

flask to quench the saponification reaction.

4. A few drops of pH indicator is added into the mixture.

5. The mixture is titrated with NaOH solution from the burette until the mixture is neutralized.

The amount of NaOH titrated is recorded.

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Experiment procedure:

1. The general start-up procedures is performed.

2. Both pumps P1 and P2 is switched on simultaneously and open valves V5 and V10 to obtain

the highest possible flow rate into the reactor.

3. The reactor is filled up with both of the solution until it is just about to overflow.

4. The valves V5 and V10 are readjusted to give a flow rate of about 0.1 L/min. Both flow

rates are made sure to be the same. The flow rate is recorded.

5. The stirrer M1 is switched on and the speed is set to about 200 rpm.

6. The conductivity value at QI-401 is monitored until it does not change over time. This is to

ensure that the reactor has reached steady state.

7. The steady state conductivity value is recorded and the concentration of NaOH in the reactor

and extent of conversion is found from the calibration curve.

8. Sampling valve V12 is opened and a 50 mL sample is collected. A back titration procedure to

is determined manually determine the concentration of NaOH in the reactor and extent of

conversion.

9. The experiment (steps 5 to 9) is repeated for different residence times by adjusting the feed

flow rates of NaOH and Et(Ac) to about 0.15, 0.20, 0.25 and 0.30 L/min. Both flow rates are

made sure to be the same.

10

RESULTS

Table of the preparation of calibration curve.

Conversion Solution Mixtures Concentration

of NaOH (M)

Conductivity

(mS/ cm)0.1M

NaOH

0.1M

Na(Ac)

H2O

0% 100mL - 100mL 0.0500 10.7

25% 75mL 25mL 100mL 0.0375 9.7

50% 50mL 50mL 100mL 0.0250 7.5

75% 25mL 75mL 100mL 0.0125 5.6

100% - 100mL 100mL 0.0000 4.0

Table for experiments 1

Reactor volume = 40 L

Concentration of NaOH in feed vessel = 0.1 M

Concentration of Et (Ac) in feed vessel = 0.1 M

Flow

rate of

NaOH

(L/

min)

FT 01

Flow

rate of

Et(Ac)

(L/

min)

FT 02

Total

flow

rate of

solutio

ns, V0

(L /

min)

Residen

ce

time, t

(min)

Cond

uctivit

y, Q

(mS/c

m)

Exit

Concent

ration of

NaOH

Convers

ion,

X (%)

Reaction

rate

constant,

k

Reaction

rate

Vol of

NaOH

Titrate

d

(L)

0.10 0.10 0.2 200 3.51 0.0008 98.4 384.375 2.46 x 10-4 0.0246

0.15 0.15 0.3 133.3333 3.17 0.0010 98 367.5 3.68 x 10-4 0.0255

0.20 0.20 0.4 100 2.83 0.0020 96 120 4.8 x 10-4 0.0260

0.25 0.25 0.5 80 2.64 0.0018 96.4 185.957 6.03 x 10-4 0.0259

0.30 0.30 0.6 66.6667 2.52 0.0018 96.4 223.148 7.23 x 10-4 0.0259

11

SAMPLE CALCULATIONS

Residence Time:

Calculation for flow rates of 0.1 L/min:

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

= 0.1 L/min NaOH + 0.1 L/min Et(Ac)

= 0.2 L/min

Residence Time, τ=

Reactor volume ( L ) ,V

Total flow rate ( Lmin ) , v0

= 40 L0.2L/min = 200 mins

Initial concentration of NaOH entering the reactor:

= Concentrationof NaOH∈feed

2

= 0.12

= 0.05 M

Conversion:

Calculation for flow rates of 0.1 L/min :

Moles of reacted NaOH, n1

n1 = Concentration NaOH x Volume of NaOH titrated

= 0.1 M x 0.024.6 L

12

= 0.00246 mole

Moles of unreacted HCl, n2 = Moles of reacted NaOH, n1

n2 = 0.00246 mole

Volume of unreacted HCl, V1

V1 = n2

concentrationHCl quench

= 0.00246

0.25

= 0.00984 L

Volume of HCl reacted, V2

V2 = Total volume HCl – V1

= 0.01 – 0.00984

= 0.00016 L

Moles of reacted HCl, n3

n3 = Concentration HCl x V2

= 0.25 x 0.00016

= 0.00004 mole

Moles of unreacted NaOH, n4

n4 = n3

= 0.00004 mole

Concentration of unreacted NaOH

CNaOH unreacted = n4

volume sample

=0.00004

0.05

= 0.0008 M

13

Xunreacted

Xunreacted = Concentrationof NaOH unreacted

concentrationNaOH

=0.0008

0.05

= 0.016

Xreacted

Xreacted = 1 - Xunreacted

= 1 - 0.016

= 0.984

Conversion for flow rate 0.1 L/min,

0.984 x 100% = 98.4 %

Reaction rate constant, k

k = ( CAo – CA) / τCA2

= ( 0.05 – 0.0008) / (200 x 0.00082)

= 384.375 M-1 min -1

Rate of reaction, -rA

-rA = kCA2

= 24.44 x 0.0062

= 8.79 x 10 -4 mol/L.min

14

DISCUSSION

0 10 20 30 40 50 60 70 80 90 1000

2

4

6

8

10

12

f(x) = − 0.07 x + 11R² = 0.989819004524887

Conversion vs Conductivity

Conversion (%)

Cond

uctiv

ity (m

S/cm

)

For the preparation of calibration curve for conversion vs conductivity, the NaOH

solution and Na(Ac) is mixed together in order to give a value of conversion from the 0% which

means a pure NaOH solution to the 100% conversion which means a pure Na(Ac) solution

produced. The conductivity of the solution is then obtained by using a conductivity meter. The

curve obtained is shown in the figure above. The higher the conversion of NaOH, the lower the

conductivity value obtained based on the result. The best fit lined constructed from the result

gives the line of y = -0.07x + 11. Thus, the slope obtained is -0.07 while the y-intercept is at 11

mS/cm.

The CSTR is normally run at steady state, and is usually operated so as a to be quite well

mixed. The conductivity value in the CSTR is assumed as the same anywhere in the reactor.

From the experiment, the conductivity of the reactant decreases as the conversion decreases. The

98.4% conversion gives a conductivity value of 3.51 mS/cmw. The conversion which is 96.4%

gives the lowest conductivity value of 2.52 mS/cm. The decrease in the conductivity value of the

solution is because of the decrease in the ionic activity of a solution in term of its capacity to

transmit current. As the electrical current is transported by the ions in solution the conductivity

increases as the concentration of ions increases. Thus, the higher conversion of the NaOH gives a

15

lower ionic activity for the preparation of the calibration curve result. However, the lower

conversion of the NaOH gives a lower ionic activity from the experiment conducted. The ionic

activity is higher in the higher conversion of NaOH. This is possibly because of the agitation in

CSTR which making the ionic activity still going on in the solution even after the conversion.

60 80 100 120 140 160 180 20080828486889092949698

100

Conversion vs Residence Time

Residence Time (min)

Conv

ersio

n (%

)

For the experiment of investigating the effect of conversion on residence time, the reaction to be

studied is the saponification reaction of ethyl acetate Et(Ac) and sodium hydroxide NaOH. The

reaction is carried out using equimolar feeds of Et(Ac) and NaOH solutions with the same initial

concentrations. This ensures that both concentrations are similar throughout the reaction. The

introduced flow rate of both feeds also the same throughout the experiment. The flow rate is then

varied from 0.1 L/min to 3.0 L/min for both feeds in order to give variation in the residence time.

The residence time is determined from the equation:

Residence Time, τ=

Reactor volume ( L ) ,V

Total flow rate ( Lmin ) , v0

The results obtained from the experiment are shown in the graph above. From the graph,

we can see that the percentage of conversion of NaOH is increasing with the increase of

residence time. When the residence is 200 mins, the conversion is 98.4% that is the highest

16

conversion of NaOH from the experiment. Then as the total flow rate provided for the system is

increased, the residence time is decreased and that makes the conversion of NaOH to decrease as

well. When the residence time is 66.6667 mins, the conversion is decreasing to 96.4%.

The CSTR is normally run at steady state, and is usually operated so as a to be quite well

mixed. From the result, when flowrate is low, it helps the reaction more in the CSTR as

conversion value is higher as shown in the experiment. This is because the reactants have a lot of

time to make contact with each other before exiting the outlet when the flow rate is low. Thus,

the lower flowrate helps the conversion to increase as the contact between the reactant. As for

the reaction rate of the reaction, -rA from the result it is increasing as the residence time is higher

while the rate constant, k is decreasing.

17

CONCLUSION

As a conclusion, the objectives for this experiment are achieved. This is because we get

to perform saponification reaction between NaOH and Et(Ac) by using the continuous stirred

tank reactor 40 liter unit and then determine the reaction rate constant for each reaction which is

different from each flow rate introduced to the system. Besides that we also get to study the

effect of residence time on the conversion which is the lower flow rate gives a higher residence

time and increasing the conversion in the reaction.

RECOMMENDATION

1. It is recommended that the same experiment is conducted by using other type of reactor

in order to know which reactor will give a better process for the reaction and their

characteristics can be compared.

2. It is recommended that the flowrate of the feed NaOH and Et(Ac) to be varied and not

the same with each other to know how will it affects the reaction process in the

continuous stirred tank reactor.

3. It is also recommended to varied the concentration of the feed for example like how

would the reaction process go if the feed introduced into the system is not equimolar.

18

REFERENCES

1. Amrita Virtual Lab Collaborative Platform. Continous Stirred Tank Reactor. Retrieved

on 6th April from http://iitkgp.vlab.co.in/?sub=35&brch=107&sim=1175&cnt=1

2. Encyclopedia Of Chemical Engineering Equipment. Retrieved on 6th April 2015 from

http://encyclopedia.che.engin.umich.edu/Pages/Reactors/CSTR/CSTR.html

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

Hall.

4. Levenspiel, O. (1999). Chemical Reaction Engineering (3rd Edition). John Wiley.

5. Wikipedia. Continuous Stirred Tank Reactor. Retrieved on 6th April from

http://en.wikipedia.org/wiki/Continuous_stirredtank_reactor

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APPENDIX

20