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.