Abstract

In this experiment , the operation of saponificaton of NaOH and Et(Ac) is carry out in

Continuous Stirrer Tank Reactor (CSTR) 40 L. There are three component in this experiment

to be measured which are reaction rate, residence time and conversion of X. This experiment

used model BP 143 as the mechanism to run the reaction. The experiment start to be

measured by 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.After experiment compeleted , the data obtain can be used to plot

graph.The graph is used to see the effect of residence time on conversion of X.The graph

show the residence time is inversely proportional to the conversion which do not follow the

theory due to some error occur in this experiment. . For the reaction rate, the data show the

decreasing over the period of conversion. This also not follow the theory whereby as the

conversion increase the rate of reaction will increase.

Introduction

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

continuously added and withdrawn. In practice, mechanical or hydraulic agitation is required

to achieve uniform composition and temperature, a choice strongly influenced by process

considerations. The CSTR is the idealized opposite of the well-stirred batch and tubular plug-

flow reactors. Analysis of selected combinations of these reactor types can be useful in

quantitatively evaluating more complex gas-, liquid-, and solid-flow behaviors.

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.

The unit used in this experiment, which is SOLTEQ-QVF Continuous Stirred Tank

Reactor (Model: BP 143), The unit conducted a saponification reaction by using ethyl

acetate and sodium hydroxide . 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 saponification process between th2

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

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

Objective

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

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

-To determine the reaction rate constant.

Theory

Rate of Reaction and Rate Law

Rate of reaction is the number of mol A reacting per unit time per unit volume.It also 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.

aA+bB→

cC+dD

where 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 itself 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.

The Equation for rA is :

−r A=k C Aα CB

β

Conversion

Using the equation shown in Equation 1.1 and taking species A as the basis of calculation, the

reaction expression can be divided through by the stoichiometric coefficient of species A, in

order to arrange the reaction expression in the form:

aA + ba

B → pa

P + qa

Q

The expression has now put every quantity on a ‘per mole of A basis’.

A convenient way to quantity how far the reaction has progressed, or how many moles of

products are formed for every mole of A consumed; is to define a parameter called

conversion. The conversion XA is the number of moles of A that have reacted per mole of A

fed to the system.

X A=moles of A reacted

moles o f A fed

To perform a mole balance on any system, the system boundaries must first be specified. The

volume enclosed by these boundaries is referred to as the system volume. We shall perform a

mole balance on species j in a system volume, where species j represents the particular

chemical species of interest, such as water or NaOH.

Continuous Stirred Tank Reactors (CSTR)

CSTR runs at steady state with continuous flow of reactants and products; the feed assumes a

uniform composition throughout the reactor, exit stream has the same composition as in the

tank.

General Mole Balance Equation

Assumptions

1) Steady state therefore

2) Well mixed therefore rA is the same throughout the reactor

Rearranging the generation

In terms of conversion

APPARATUS

Continuous stirred tank reaction (Model: BP143)

0.1M sodium hydroxide, NaOH

0.1M sodium acetate, Et(Ac)

0.25 hydrochloric acid, HCl

De-ionized water,H2O

Burette

Retort stand

Conical flask

pH indicator

Measuring cylinder

PROCEDURE

General Start-up Procedure

1. The following solutions were prepared:

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

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 the Et (Ac)

solution was poured into vessel B2.

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

4. The power for the 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 10 L in the reactor R1.

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

8. The unit was ready for experiment.

General shutdown procedure.

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

flowing.

2. Pumps P1 and pump 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 valve 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 solutions were prepared:

i. 1 L of sodium hydroxide, NaOH (0.10M)

ii. 1 L of sodium acetate, Et (Ac) (0.10M)

iii. 1 L of deionised water, H20

2. The conductivity and NaOH concentration for each value were determined by mixing

the following solutions 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 : 25 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.

Result

Tim

e(mi

n)

Flow

rate of

NaOH

(L/

min)

Flow

rate of

et(ac)

(L/

min)

Total

flow

rate of

soluti

on V0

Q,Con

ductiv

ity(ms

)

NaoH

titrate

d(Ml)

Conver

sion X

Resid

ence

time

Reaction

rate

constant

k

Reaction

rate

5 0.10 0.10 0.2 3.51 24.6 98.4 100 768.75 4.92 x 10

-4

10 0.15 0.15 0.3 3.17 25.6 98.00 66.66

7

734.87 7.35 x 10

-4

15 0.20 0.20 0.4 2.83 26.0 96.00 50.00

0

240 9.6 x 10 -4

20 0.25 0.25 0.5 2,64 25.9 99.64 16.66

7

185.957 8.8 x 10 -4

25 2.30 0.30 0.6 2.52 25.9 99.64 16.66

7

160.99 9.8 x 10 -

4

TABLE 1

19.64 19.64 50 66.667 10094

95

96

97

98

99

100

conversion X VS residence time

conversion X

Residence time

Conv

ersio

n X

Graph 1

Conversion Solution mixture Concentration of

NaoH(M)

Conductivity(ms/

cm)0.1M

NaoH

0.1M

NaoH(Ac)

H20

0 100ml - 100ml 0.0500 10.7

25% 75 ml 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 2

0 20 40 60 80 100 1200

5

10

15

20

25

30

f(x) = 0.21508 x + 1.252R² = 0.895108140667573

conversion vs conversion X

CONVERSION X

CON

DUCT

IVIT

Y

Graph 2

SAMPLE CALCULATION:

F0 =0.1L/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 = 10 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 24.6

= 9.84 mL

iii- Volume of HCl reacted with NaOHin sample, V3

V3 = VHCls – V2

= 10 – 9.84

= 0.16

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

n1 = (CHCls x V3) / 1000

= 0.25 x 0.16/1000

= 0.00004 mol

v- Moles of unreacted NaOH in sample, n2

n2 = n1

= 0.00004 mol

vi- Concentration of unreacted NaOH in the reactor, CNaOH

CNaOH = n2/Vs x 1000

= 0.0004/50 x 1000

= 0.0008 mol/L

vii- Conversion of NaOH in the reactor, X

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

= (1 – 0.0008/0.05) x 100%

= 98.4 %

viii- Residence time, τ

τ = VCSTR / Fo

= 10 / 0.10

i. Reaction rate constant , k

k = (C A0−C A )

τ C A2

= (0.05−0.0008 )100× 0. OOO82

= 768.75 M−1 min−1

ii. Rate of reaction , −r A

−r A = kC A2

= 768.75 ×0.00082

= 4.92 ×10−4 mol / L. min

Discussion

This experiment was conducted to achieve three main adjectives which are to carry

out saponification reaction between NaOH and Et (Ac) in a CSTR, to determine the effect of

the residence time onto the reaction extent of conversion and to determine the reaction rate

constant.

From the data obtain from the experiment, a graph can be plotted to achieve the

objective. The first graph plotted to see the effect of residence time towards reaction extent of

conversion. The graph shows decreasing conversion of X as the residence time increase.

Residence time can be defined as   average amount of time that a particle spends in a

particular system. Its mean that the more time the particle spend in the system the lesser

conversion of X into product. It is suppose the result to be the conversion X directly

proportional to residence time. This is because the more time the particle in the system the

more potential collision can occur among each particle to achieve activation energy. Thus,

more reactant can be converted into product.The reaction rate also will increase as the

conversion increase.

The second graph is plotted for calibration curve which is a method general method

for determining the concentration of a substance in an unknown sample by comparing the

unknown to a set of standard samples of known concentration. Based on the graph,it show

that the conversion is increase as the conductivity increase. This shows that there is a positive

slope obtained from the line which is 0.2151.

Saponification is the process to make a soap. Saponification is a continuous reaction.

In this experiment, the reaction of saponification is quench with hydrochloride acid to stop

the reaction. The reaction rapidly react in increasing of temperature. Back titration is done to

investigate if the reaction is stop.

Conclusion

The objective of this experiment is to determine the effect of residence time on the

reaction extent of conversion.Based on the theory, the result should be conversion and

residence time was directly proportional.But in this experiment, the result slightly different

from the theory due to some error that occur in this experiment. For the reaction rate, the data

show the decreasing over the period of conversion. This not follow the theory whereby as the

conversion increase the rate of reaction will increase.

RECOMMENDATION

-The samples need to immediately quench with hydrochloride acid (HCl) to stop the reaction.

The reaction still occurred as long as no quenching is done. The recommendation is store the

HCl near to the sample so it can immediately quench.

-Immediately stop the titration after the colour turns light pink. The long titration may cause

error to the calculation which the flow rate of NaOH could be more than the initial.

-During titration, the indicator must be put to HCl then followed by sample.

REFERENCES

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

Hall.

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

- Continuous Stirrer Tank Reactor (CSTR) (Retrieved from

http://www.konferenslund.se/p/L16.pdf on 18th October 2013)