ABSTRACT

The objective of this experiment is to determine the rate constant and rate of reaction and also to

study the effect of residence time on conversion for the reaction between sodium hydroxide,

NaOH and ethyl acetate Et(Ac). To achieve the objectives, an experiment is conducted. The

experiment involves using a unit called SOLTEQ Plug Flow Reactor (Model: BP 101),

commonly known as PFR, as well as some common laboratory apparatus for titration process.

The output conductivity is recorded at different flow rates. Based on the result, as the flow rate

decrease, the conversion, the rate constant and the rate of reaction will increase. The graph of

conversion against residence time has been plotted. From the graph, the conversion is increasing

when the residence time is increasing too. The last part of the experiment is to determine manual

conversion by using back-titration method. From the result, as the feed flow rates are increasing,

less volume of NaOH is needed to titrate the sample.

INTRODUCTION

Plug flow reactor also known as tubular reactor is the type of reactor that is commonly used in

industries. The reactor is usually at steady state, continuous flow, and configured so that

conversion of the chemicals and other dependent variables are functions of position within the

reactor rather than of time. This type of reactor consists of hollow pipe or tube through which

reactants flow. The reactants are continually consumed as they flow down the length of the

reactor. For ideal tubular reactor, the mixtures are assumed to be complete mixing perpendicular

to the direction of flow and there is no back-mixing in the reactor.

During the operation, the reactants are continuously fed into the reactor. As plug flow

down the reactor, the reaction will take place. This would result in an axial concentration

gradient whereby change in concentration over a distances from left to right but not radial

direction. The products and unreacted reactants will flow out of the reactor continuously. When

isothermal operation is possible, the temperature will also vary with the axial direction.

The reactor can be applied in either gas or liquid phase systems and most common industrial uses

like gasoline production, oil cracking and synthesis of ammonia from its elements.

OBJECTIVES

The objectives of the experiment are are to carry out a saponification reaction between NaOH

and Et(Ac), to determine the reaction rate constant and to determine the effect of residence on

the conversion.

THEORY

Plug flow reactor consists of a water jacket, variable speed stirrer, inlet and outlet ports for the

feed and product stream, sampling, conductivity measurements and temperature measurements

and control. A cooling coil and immersion heater are provided inside the vessel to provide

constant reaction temperature. The desired reaction temperature is achieved by controlling the

heating using a digital temperature controller located on the front panel. This unit also equipped

with two non-corroding feed storage vessels, chemically resistance pump and flow meters. It

have a coil of long tubing wound around the cylinder inside the vessel. It is designed to ensure a

good radial mixing while minimizing longitudinal dispersion.

In industry, reactors are commonly used to mixing the reaction to produce the product.

One of the famous reactors widely used is tubular reactors and another type of reactor is

continuous stirred tank reactor. Continuous stirred tank reactor (CSTR) had continuous inlet and

outlet flow of materials. Inside the tubular reactor, the feed or the stream enters at one end of a

cylindrical tube and the product stream leaves at the other end. The long tube and the lack of

provision for stirring will prevent complete mixing of the fluid in the tube. Hence the properties

of the flowing stream will differ from one point to another, namely in both radial and axial

directions. Ideal tubular reactor is known as a plug flow reactor (PFR). PFRs are frequently

referred as piston flow reactors. This is because as the plug flows through a tubular reactor, the

fluid is perfectly mixed in the radial direction but not in the axial direction.

Mass Balance

For a time element t and a volume element V, the mass balance for species i is given by the

following equation:

QA

CA

v t- Q

A C

A

v+v t - r

AVt = 0 (Equation 1)

where QA : molar feed rate of reactant A to the reactor, mol/sec

CA

: concentration of reactant A

rA

: rate of disappearance of reactant A, mol/ltsec

The conversion, X, is defined as:

X = (initial concentration - final concentration) / (initial concentration)

Since the system is at steady state, the accumulation term in Equation 1 is zero. Equation 2 can

be written as:

-QA

CA

- rAV = 0 (Equation 2)

Dividing by V and taking limit as V 0

dCA/dV = -r

A/Q

A (Equation 3)

This is the relationship between concentration and size of reactor for the plug flow reactor. Here

rate is a variable, but varies with longitudinal position (volume in the reactor, rather than with

time). Integrating:

-dV/ QA

= dCA/r

A (Equation 4)

At the entrance: V = 0

CA

= CAo

At the exit: V = VR

(total reactor volume)

CA

= CA (exit conversion)

=

0

APPARATUS AND MATERIALS

Tubular flow reactor (Model: BP101)

500 mL beakers

50 mL burette

Retort stand

0.1M NaOH solution

0.25M HCl

Phenolphthalein

0.1M ethyl acetate

Deionized water

Figure:1 SOLTEQ Plug Flow Reactor (Model: BP101)

PROCEDURE

Effect of Residence Time on the Reaction

1. The general start-up procedure is performed before preceding the experiment.

2. Valves V9 and V11 is opened.

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

empty into waste tank B3.

4. P1 and P2 are adjusted to give a constant flow rate of about 300 mL/min at flow meters

Fl-01 and Fl-02. Both flow rates need to ensure are same. The flow rates are recorded.

5. The inlet (Ql-01) and outlet (Ql-02) conductivity values is monitored 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 are recorded. The concentration of

NaOH exiting the reactor and extent of conversion is determined from the calibration

curve.

7. Optional: Sampling valve V15 is opened and 50 mL of the sample is collected. Back

titration procedure is carried to manually determine the concentration of NaOH in the

reactor and extent of conversion.

8. Steps 4 to 7 is repeated 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 need to be

ensure are same.

9. A graph of conversion against residence times is plotted.

RESULTS

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)

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

Volume of sample, Vs : 0.05L

Flow

Rate of

NaOH

(ml/min)

Flow

Rate of

Et(Ac)

(ml/min)

Total folw

rate of

solution,

Vo(mL/min)

Residence

Time,

(min)

Outlet

conductivity

(mS/cm)

Titrated

NaOH

Volume

(mL)

Conversion

X, (%)

Reaction

Rate

Constant,k

(L.mol/min)

Rate of

Reaction, -rA

(mol.L/min)

300 300 600 6.67 6.4 16.5 66 5.8235 1.68 x 10-3

250 250 500 8.0 6.4 18 72 6.4253 1.26 x 10-3

200 200 400 10.0 5.8 18.9 75.6 6.1968 9.07 x 10-4

150 150 300 13.33 5.1 20.8 83.2 7.428 5.24 x 10-4

100 100 200 20.0 4.6 20.15 86 6.143 3.01 x 10-4

50 50 100 40.0 4.2 22.3 89.2 4.1297 1.2 x 10-4

Table 1: Result

Figure 2: The graph of conversion against residence time

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30 35 40 45

CO

NV

ERSI

ON

, X

(%

)

RESIDENCE TIME, t (time)

CONVERSION VS RESIDENCE TIME

SAMPLE OF CALCULATIONS

Residence time calculations

For flow rates of 300 ml/min:

Residence Time, = ()

(/)

Total flow rate, V0 = Flow rate of NaOH + Flow rate of Et(Ac)

= 300 mL/min NaOH + 300 mL/min Et(Ac)

= 600 mL/min

= 0.6 L/min

Hence,

Residence Time, = ()

(/)

= 4 ()

0.6 (/)

= 6.67 min

Conversion calculations

For flow rate of 300 mL/min:

Concentration of NaOH entering the reactor, CNaOH,0 = ,

2

= 0.1

2

= 0.05 mol/L

Volume of unreacted HCl, V1

V1 = ,

,

= 0.1 /

0.25 / 16.5

= 6.6 mL

Volume of HCl reacted, V2

V2 = VHCl,s V1

= 10 mL 6.6 mL

= 3.4 mL

Moles of reacted HCl, n1

n1 = CHCl,s x V2

= 0.25 mol/L x 0.0034 L

= 0.00085 mol

Moles of unreacted NaOH in sample, n2

n2 = n1

= 0.00085 mol

Concentration of unreacted NaOH, CNaOH

CNaOH unreacted = 2

= 0.00085

0.05

= 0.017 mol/L

Conversion of NaOH in the reactor, X = ( 1 -

,0) x 100%

= (1 0.017

0.05) x 100%

= 66 %

Reaction Rate Constant, k

= 0

(

1 )

V0 = Total inlet flow rate

= 0.6 L/min

VTFR = Volume for reactor

= 4 L

CAO = inlet concentration of NaOH

= 0.05 M

X = 0.66

= 0.6

(4)(0.05)(

0.66

10.66)

= 5.8235 L/mol.min

Rate of Reaction, -rA

-rA = k (CA0)2(1-X)

2

For flow rates of 300 ml/min :

-rA = 5.824 (0.05)2

(1-0.66)2

= 1.68 x 10-3

mol/L.min

DISCUSSION

The aim of this experiment is to determine the rate constant and rate of reaction and also to study

the effect of residence time on the conversion in the saponification reaction between sodium

hydroxide, NaOH and ethyl acetate, Et(Ac). This experiment the residence times have to be

manipulated, and the effects of each one is studied. Residence time, in this particular experiment,

is varied by the means of changing the flow rates of the feed solutions. This is shown by the

formula :

Residence Time, = (),

(

),0

From the equation above, it can be seen that residence time is a function of total flow

rates of the feed. Hence, by varying the flow rate of the feed solutions, several residence times

can be obtained and the effects of each one, studied.

From the raw data obtained, a series of calculations were made, as seen in the Sample of

Calculation section, and the values of residence times, conversion of the reactions, reaction rate

constants and rate of reactions were determined. These values are tabulated in Table 1 of the

Result section.

As the data of residence time and conversion from table 1 is plotted into a graph, the

graph is shown in figure 2. The reason for plotting a graph consisting these two parameters is so

that the effects of residence time can be studied. Conversion is a property that shows how much

of the reaction has taken place. Hence, by comparing this property with the residence time

parameter, one can analyse the effects of increasing residence time to the reaction itself.

By analysing figure 2, it can be clearly seen that the conversion of the reaction remains

fairly constant with the increasing residence time. Therefore, one can postulate that residence

time is not a factor for reaction conversion, as far as plug flow reactors are concerned. One can

also postulate that the reason for this phenomenon is that the PFR lacks a good mixing process.

Since the PFR is designed not to stir the solution vigorously to maximise mixing process, the

conversion of the reaction by using PFR is fairly low.

Then titration method is conducted. Phenolphthalein is used as indicator to determine 50

mL of sample for different flow rates are neutralized. From the titration, it can be observed that

the volume of NaOH is increasing as the flow rates of the feed are decreasing. The experiment

also aims to evaluate the reaction rate constants and rate of reaction values of the reaction. Both

of these properties have been determined in the result section.

CONCLUSION

From the experiment, it can be concluded that experiment is success too. Based on the

experiment result, it can be concluded that as the flow rates of the feed is increasing, the

conversion, rate constant and the rate of reaction is decreasing. Besides that, the conversion

increases as the residence time increase

The conductivity and conversion are related to each other. As the conversion increasing, the

value of conductivity is decreasing. This is because, the conductivity is decreasing when the

concentration of its base is decreasing.