ABSTRACT

The experiment is carried out to achieve the objectives set beforehand which are,

saponification reaction between Sodium Hydroxide, NaOH and Ethyl Acetate, Et(AC), 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. In order to

achieve these objectives, an experiment using SOLTEQ-QVF Continuous Stirred Tank

Reactor (Model: BP 143), commonly known as CSTR, is finely designed so that these

objectives can be finely met. Apart from CSTR, some common laboratory apparatus for

titration process is also required. In short, the two solutions Sodium Hydroxide, NaOH and

Ethyl Acetate, Et(Ac) were reacted in the CSTR and the product is then analysed by using

titration to determine the reaction. Before we started, we ensured that all the general start up

was carried out properly according to the consultation before with lab assistant, Mr Jamil in

order to avoid any disturbance that may affect our result. After the methodology for general

start up settled, we continued by start up the experiment by open fully V5 and V10 to obtain

adjustable flow rate which are 0.15, 0.20, 0.25 and 0.30. For each flow rate, 100 mL of the

sample are collected at V12 that used in the back titration. Then, the stable conductivity is

recorded into a data for every 5min before collect the samples. In the back titration, the

samples that had been collected are titrated with NaOH for saponification reaction until it

change colour from colourless into light pink. Then, the amount of NaOH titrated were

recorded. From the tabulated data, a few of calculation are carried out and graph had been

plotted.

INTRODUCTION

Based on all industrial sectors, mostly reactor is the main equipment that is used where it

changes from the raw materials into the desire products that are needed. The criteria for the

good reactor is it produced a high production and also economical. There are many types of

reactor depending on the nature of the feed materials and products. The rate of reaction is the

most important thing that we are considered in the reactor because it showed the effectiveness

of the processing of the reactor. A most common ideal reactor type in chemical engineering is

the continuous stirred tank reactor or known as CSTR.

In a continuous stirred tank reactor (CSTR), reactants and products are continuously

added and withdrawn from the reactor. The CSTR is the idealized opposite of the weel-stirred

batch and tubular plug flow reactors. Analysis of selected combination of these reactors types

can be useful in quantitatively evaluating more complex gas-, liquid-, and solid-flow

behaviours.

Furthermore, the continuous stirred tank reactor (CSTR) which is also known as vat-

or back-mix reactor and this kind of model used to estimate the key unit operation variables

when using a continuous agitated-tank reactor to reach a specified output. This reactor can be

used for all fluids, gases and slurries. In a perfectly mixed reactor, thus the output

composition is identical to the composition of the materials inside the reactor which is a

function of residence time and rate of reaction that had been considered in this experiment.

CSTR that used in the experiment, (model: BP 143) is designed for student’s

experiments on chemical reaction in liquid phase which is under adiabatic and isothermal

conditions. CSTR consists of two tanks of solutions and one reactor. The model also consists

of jacketed reaction fitted in the agitated and condenser. The unit comes complete with

vessels for raw materials and products, feed pumps and thermostat that set at 500C. The

reactor is modelled in order to perform the saponification reaction where the reaction is occur

between the sodium hydroxide,NaOH and ethyl acetate, Ac. The saponification process

between this compounds produced sodium acetate in a batch and the continuous stirred tank

reactor evaluate the rate data needed to design a production scale reactor.

OBJECTIVES

The main purposes of conducting this experiment are to carry out saponification reaction

between Sodium Hydroxide, NaOH and Ethyl Acetate, Et(AC) by using a Continuous

Stirred Tank Reactor, CSTR. Secondly, to determine the effect of residence time to the

reaction's extent of conversion. Lastly, to evaluate the reaction rate constant.

THEORY

Reaction Rate

The reaction rate or rate of reaction for a reactant or product in a particular reaction is defined

as how fast or slow a reaction takes place. Consider a typical chemical reaction:

aA + bB → pP + qQ (eq 1)

The lowercase letters (a, b, p, and q) represent stoichiometric coefficients of the species,

while the uppercase letters represent the reactants (A and B) and the products (P and Q). The

reaction rate r for a chemical reaction occurring in a closed system under constant-volume

conditions, is defined as:

Negative notations (-) are used to denotes reactants as seen with the species A and B above.

The rate of reaction can also be related in such a way that :

Conversion

Eq. 1 can be rearranged and adjusted for convenience. A way to do that is to put the

quantities on a 'per mole A basis'. This is done as follows :

As species A has become the basis of calculation, the progress of the reaction can be

quantified by observing how many moles of products are formed for every mole of A

consumed. This parameter is called Conversion. In short, conversion is number of moles of A

that has reacted divided by Number of moles of A originally exists at the beginning of the

experiment. As such :

Assumptions

Steady state, therefore dNA/dt = 0

Well-mixed therefore is the same throughout the reactor.

Rearranging the generation, V = (FAo – FA)/ -rA

In terms of conversion, X = (FAo – FA) / FAo

V = (FAoX) / -rA

A calibration curve is a method used in analytical chemistry to determine the concentration of

an unknown sample solution. It is a graph generated by experimental means, with the

concentration of solution plotted on the x-axis and the observable variable — for example,

the solution’s absorbance — plotted on the y-axis. The curve is constructed by measuring the

concentration and absorbance of several prepared solutions, called calibration standards.

Once the curve has been plotted, the concentration of the unknown solution can be

determined by placing it on the curve based on its absorbance or other observable variable.

Residence Time

The reactor’s residence time is defined as the reactor volume divided by the total feed flow

rates.

Residence time, τ = V CSTR

F0

APPARATUS AND MATERIALS

The unit used in this experiment was Solteq-QVF Continuous Stirred Tank Reactor (Model :

BP 143).

Figure 1: SOLTEQ-QVF Continuous Stirred Tank Reactor (Model: BP 143)

The unit comes complete with a jacketed glass reactor, constant temperature water circulating

unit, vapour condenser, individual reactant feed tanks and pumps, temperature and

conductivity measuring sensors, and of course, data acquisition system. Apart from that, there

were also some laboratory apparatus involved such as :

burette

conical flask

measuring cylinder

ph indicator

beakers

Among the chemicals used are :

0.1 M Sodium Hydroxide, NaOH

0.1 M Ethyl Acetate, Et(Ac)

0.1 M Hydrochloric Acid, HCl

De-ionised water

PROCEDURES:

General start-up Procedures:

1. The following solution were prepared:

i- 40L of sodium hydroxide, NaOH (0.1 M)

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

iii- 1 L of hydrochloric acid, HCl (0.25M) , for quenching.

2. All valves were initially closed.

3. The feed vessels were charged as follows:

i- The charge port caps for vessels B1 and B2 were opened.

ii- The NaOH solution was carefully poured into vessel B1 and Et (Ac) solution

was poured into vessel B2.

iii- The charge port caps for both vessels were closed.

4. The power for control panel was turned on.

5. Sufficient water in thermostat T1 was checked. Refill as necessary.

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

7. Valves V2, V3, V3, V7, V8 and V11 were opened.

8. The unit was ready for experiment.

General shut-down Procedures:

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

flowing.

2. Pumps P1 and pumps P2 were switched off. Stirrer M1 was switched off.

3. The thermostat T1 was switched off. The liquid in the reaction vessel R1 was let to

cool down to room temperature.

4. Cooling water V13 was closed.

5. Valves V2, V3, V7, and V8 were closed. Valves V4, V9 and V12 were opened to

drain any liquid from the unit.

6. The power for control panel was turned off.

Preparation of Calibration Curve for Conversion vs Conductivity:

1. The following solution were prepared:

i- 1 L of sodium hydroxide, NaOH (0.1M)

ii- 1 L of sodium acetate , Et (Ac) (0.1M)

iii- 1 L of deionised water, H2O.

2. The conductivity and NaoH concentration for each valu were determined by mixing

the following solution into 100 mL of deionised water.

i- 0% conversion : 100 mL NaOH

ii- 25% conversion : 75 mL NaOH + 25 mL Et (Ac)

iii- 50% conversion : 50 mL NaOH + 50 mL Et (Ac)

iv- 75% conversion : 23 mL NaOH + 75 mL Et (Ac)

v- 100% conversion : 100 mL Et (Ac)

Back Titration Procedures for Manual Conversion Determination:

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

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

3. A 50 mL sample was obtained from the experiment and immediate the sample was

added to the HCl in the flask to quench the 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 was

neutralized. The amount of NaOH titrated was recorded.

Effect of Residence Time of The Reaction in a CSTR:

1. The general start-up procedures was performed.

2. Pump 1 and pump 2 were switched on and valves V5 and V10 were opened to obtain

the highest possible flow rate into the reactor.

3. The reactor was filled up with both of the solution until it is njust bout to overflow.

4. Valves V5 and V10 were readjusted to give a flow rate of about 0.1 L/min. the flow

rate for both valves must be same. The flow rate were recorded into a data.

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

6. The conductivity value at Q1 was started monitoring until id does not change over

time. This is to ensure that the reactor has reached steady state.

7. The steady state conductivity value was recorded and the concentration of NaOH and

extent of conversion in the reactor was found out from the calibration curve.

8. Sampling valve V12 was opened and 100mL of sample was collected. It directly

proceed with the back titration procedures to manually determine the concentration of

NaOH in the reactor and extent of conversion.

9. The experiments was repeated (steps 5-9) for different residence times by adjusting

the feed flow rate of NaOH and Et(Ac) to about 0.15, 0.20, 0.25, and 0.30 L/min. the

flow rate for both must be same.

RESULTS

Table 1:

NoTemperature

(°C)

Flow rate of

NaOH (mL/min)

Flow rate of

Et(Ac) (mL/min)

Total flow rate of solutions, F0

(mL/min)

1 28.00 0.10 0.10 0.20

2 28.20 0.15 0.15 0.30

3 28.30 0.20 0.20 0.40

4 28.40 0.25 0.25 0.50

5 28.60 0.30 0.30 0.60

Table 2:

No Residence time,

τ (min)

Conductivity

(mS/cm)

Exit concentration of

NaOH, CNaOH (M)

Conversion,

X (%)

Volume of

NaOH

1 50.00 2.01 0.0040 92.0 28.0

2 33.33 1.91 0.0040 92.0 28.0

3 25.00 1.84 0.0036 92.8 28.2

4 20.00 1.75 0.0040 92.0 28.0

5 16.67 1.74 0.0040 92.0 28.0

91.9 92.0 92.1 92.2 92.3 92.4 92.5 92.6 92.7 92.8 92.90.00

10.00

20.00

30.00

40.00

50.00

60.00

Residence time, τ versus Conversion, X

Residence time, τ (min)

Conversion, X (%)

Resid

ence

tim

e, τ

(min

)

Figure 1 shows the effect of residence time to the conversion of Sodium Hydroxide, NaOH.

SAMPLE CALCULATION

1) F0 = 0.1+0.1 = 0.2 L/min

Known quantities:

Volume of sample, Vs = 50 mL

Concentration of NaOH in the feed vessel, CNaOH,f = 0.1 mol/L

Volume of HCL for quenching, VHCl,s = 12 mL

Concentration of HCl in standard solution, CHCls = 0.25 mol/L

Volume of titrated NaOH, V1= 9.8 mL

Concentration of NaOH used for titration, CNaOHs = 0.1 mol/L

i- Concentration of NaOH that entering the reactor, CNaOH0.

CNaOHo = ½ CNaOHf

= ½ (0.1)

= 0.05 mol/L

ii- Volume of unreacted quenching HCl,V2

V2 = (CNaOHs / CHCls) x V1

= (0.1/0.25) x 28

= 11.2 mL

iii- Volume of HCl reacted with NaOH in sample, V3

V3 = VHCls – V2

= 12 – 11.2

= 0.8

iv- Moles of HCl reacted with NaOH in sample, n1

n1 = (CHCls x V3) / 1000

= 0.25 x 0.8/1000

= 0.0002 mol

v- Moles of unreacted NaOH in sample, n2

n2 = n1

= 0.0002 mol

vi- Concentration of unreacted NaOH in the reactor, CNaOH

CNaOH = n2/Vs x 1000

= 0.0002/50 x 1000

= 0.004 mol/L

vii- Conversion of NaOH in the reactor, X

X = (1- CNaOH / CNaOHo) x 100%

= (1 – 0.004/0.05) x 100%

= 92.0%

viii- Residence time, τ

τ = VCSTR / Fo

= 10 / (0.1 + 0.1)

= 50 min

ix- Reaction rate constant, k

k = ( CAo – CA) / τCA2

= ( 0.05 – 0.004) / (50 x 0.0042)

= 57.50 M-1 min -1

x- Rate of reaction, -rA

-rA = kCA2

= 57.50 x 0.0042

= 9.2 x 10 -4 mol/L.min

DISCUSSION

Before the experiment was conducted, there were three objectives that needs to be

achieved, which are to carry out the saponification process between NaOH and Et (Ac) in a

CSTR reactor, to determine the effect of the residence time onto the reaction extent of

conversion and lastly to determine the constant rate of reaction. From the data collected, a

graph of residence time versus conversion was plotted.

For the graph of residence time versus conversion, the residence time increase but the

changes in conversion is very little. In theory, as residence time increase, the conversion

should also increase. However in the experiment, several unwanted factors have affected the

result wanted. For example, the delay in time in which the stopwatch was started affected the

conversion. Apart from that, the control of the valves was not good enough as it was hard to

observe the readings whilst adjusting the valve. Besides that, the titration process wasn’t

conducted at its best as the volume used in the quenching was 12ml instead of 10ml that was

required in the experiment. As there was too much acid used for quenching, the amount of

sodium hydroxide used to change the sample containing 3 drops of phenolphthalein, was

excessive eventhough the colour of the sample was light or pale pink.

For the saponification process, it is a process to make soap. Saponification process is

a continuous reaction which can be conducted using the CSTR. In this experiment, the

reaction of saponification is quenching with hydrochloric acid to stop the reaction. The

reaction rapidly reacts in increasing of experiment. To investigate if the reaction has stopped

or one, back titration is done. There were some errors that were made trying to achieve this

objective. Among them are while taking the reading of the burette, the position of the eyes is

not perpendicular to the scale. So, to improve the reading and get the better results, the

positions of the eyes must be perpendicular to the meniscus and the scale. Secondly, ensure

that rinse all the apparatus before using it as it would be best to avoid any contaminants from

the previous experiment which will affect the accuracy of the experiment.

CONCLUSION

The experiment was conducted with several objectives set beforehand. The first one is to

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

Et(Ac) by using a Continuous Stirred Tank Reactor, CSTR. 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. 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 calculation. Lastly, the main objective of this experiment is to study the

relationship between the residence time and the conversion of the reactant. The experiment

was not fully achieved due to several factors that had affected the result that was obtained.

RECOMMENDATIONS

There are several recommendations that can be applied to reduce the amount of error and

improve the accuracy of the results obtained. Firstly, make sure reactor does not have any

leaks and valve closed and opened as needed, controlled the valve carefully and slowly when

adjusting the flow rate to obtain 0.10 L/min. It is to make sure flow rate will stabilize and the

experiment will run smoothly. Secondly, repeat titrations two or three times because a lot of

error comes from titration, thus it is advisable to take an average reading from 3 titrations.

Thirdly, divide into two teams which is the first team in charge of the CSTR 40 liters

machine while the second team would carry out the back titration procedures so that the

experiment would run more smoothly. Fourth, take conductivity reading when the

conductivity not changes in time because it can change rapidly in short of time. Lastly, make

sure CSTR 40 liters machine is running appropriately, it to prevent harm to the machine and

individual that used the machine.