1.0
ABSTRACT
The purpose of conducting this experiment were to carry out the 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 to evaluate the reaction rate constant of this saponification reaction. SOLTEQ Plug Flow Reactor (Model: BP101) unit was used to run this experiment or usually called as PFR, and also some common laboratory apparatus for titration process. The NaOH and Et(Ac) solution were mix and reacted in the PFR and the output product was analysed by using titration method. The results shows that the conversion of NaOH solution did shows a large difference at each residence time, because the conversion should nearly constant at each residence time. This can happen due to some errors that occurred.
2.0
INTRODUCTION
Type of chemical reactors remains a highly discussed subject in chemical process industries worldwide.The reactor is of course, the place where chemical reactions take place. Hence it is arguably the single most important part of any chemical process design. The design of a reactor must be finely tuned so that its mechanisms suit the necesseties of the process that is to be carried. Depends on the nature of the materials in both the feed and of course the products, the reactors may take a wide range of forms. This is why full comprehension of a reactor of a particular design as well as its working mechanisms is very much vital to actually conduct a particular chemical process. In this experiment, the Plug Flow Reactor (Model: BP101) is used as it has been properly designed for students' experiment on chemical reactions in liquid phase under isothermal and adiabatic conditions. Included in the unit is a jacketed plug flow reactor; individual reactant feed tanks and pumps, temperature sensors and conductivity measuring sensor. By using this particular unit, students will be capable to conduct the typical saponification reaction between ethyl acetate and sodium hydroxide among the others reaction.
3.0
OBJECTIVES
The objectives of this experiment were to carry out the saponification reaction between Sodium Hydroxide ( NaOH) And Ethyl Acetate (Et(Ac)), to determine the effect of residence time to the extent of reaction of conversion and to evaluate the reaction rate constant of this saponification reaction.
4.0
THEORY
4.1 Rate of Reaction and Rate Law Simply put, rate of reaction can be roughly 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. For example : 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 −r B r C r D = = = a b c d The negative sign indicates reactants. A usual equation for rA is : −r A=k C αA C βB where k
-
rate constant
CA
-
concentration of A species
CB
-
concentration of B species
α
-
stoichiometric coefficient of A
β
-
stoichiometric coefficient of B
4.2 Conversion Taking species A as the basis, the reaction expression can be divided through the stoichiometric coefficient of species A, hence the reaction expression can be arranged as follows : b c d A + B+ C + D a a a Conversion is an improved way of quantifying exactly how far has the reaction moved, or how many moles of products are formed for every mole of A has consumed. Conversion XA is the number of moles of A that have reacted per mole of A fed to the system. As seen below : X A=
moles of A reacted moles of A fed
4.3Plug Flow Reactor A Plug Flow Reactor (PFR) consistd in a long, straight pipe in which the reactive fluid transits at steady state ( no accumulation). The main assumption of this models are that the fluid is completely mixed in any cross-section at any point, but it exprience no axial mixing, i.e contiguous cross sections cannot exchange mass with each other.
In a plug flow reactor, the feed 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 prevent complete
mixing of the fluid in the tube. Hence the properties of the flowing stream will vary from one point to another.
In an ideal tubular flow reactor, which is called plug flow reactor, specific assumptions are made regarding the extent of mixing: 1. no mixing in the axial direction 2. complete mixing in the radial direction 3. a uniform velocity profile across the radius. Tubular reactors are one type of flow reactors. It has continuous inflow and outflow of materials. In the tubular reactor, the feed enters at one end of a cylindrical tube and the product stream leaves at the other end. The long tube and the lack stirring prevent complete mixing of the fluid in the tube.
4.4Residence time distribution function Residence Time Distribution is a characteristic of the mixing that occurs in the chemical reactor. There is no axial mixing in a plug flow reactor, PFR and this omission can be seen in the Residence Time Distribution, RTD which is exhibited by this class of reactors. The continuous stirred tank reactor CSTR is thoroughly mixed and its RTD is hugely different as compared to the RTD of PFR.
5.0
APPARATUS & MATERIALS
SOLTEQ Plug Flow Reactor ( Model : BP101) was used in this experiment.
LAB APPARATUS :
burette
conical flask
measuring cylinder
ph indicator
beakers
CHEMICAL USED :
0.1 M Sodium Hydroxide, NaOH
0.1 M Ethyl Acetate, Et(Ac)
6.0
0.1 M Hydrochloric Acid, HCl
De-ionised water
METHODOLOGY / PROCEDURE
6.1General Start-up Procedures 1. All the valves are ensured closed except V4, V8 and V17. 2. The following solutions are prepared: 20 liter of NaOH (0.1M) 20 liter of Et(Ac) (0.1M) 1 liter of HCL (0.25M) for quenching 3. Feed tank B1 was filled with NaOH while feed tank B2 was filled with the Et(Ac). 4. The water jacket B4 was filled with water and pre-heater B5 was filled with clean water. 5. The power for the control panel was turned on. 6. Valves V2, V4, V6, V8, V9 and V11 were opened. 7. Both pumps P1 and P2 were switched on. P1 and P2 were adjusted to obtained flow rate approximately 300mL/min at both flow meters Fl-01 and Fl-02. Both flow rates were made sure to be equal. 8. Both solutions then were allowed to flow through the reactor R1 and overflow into waste tank B3. 9. Valves V13 and V18 was opened. Pump P3 then was switched on in order to circulate the water through pre-heater B5. The stirrer motor M1 was switched on and set up to speed about 200 rpm to ensure homogeneous water jacket temperature.
6.2Experiment Procedures 1. The general starts up procedures were performed. 2. Valves V9 and V11 were opened.
3. Both the NaOH and Et(Ac) solutions were allowed to enter the plug reactor R1 and empty into the waste tank B3. 4. P1 and P2 were adjusted to give a constant flow rate of about 300 ml/min at flow meters FI-01 and FI-02. Both flow rates were ensured same. The flow rates were recorded. 5. The inlet (QI-01) and outlet (QI-02) were started to monitor the conductivity values 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 were recorded. The concentration of NaOH exiting the reactor and extent of conversion from the calibration curve. 7. Optional. Sampling was opened from valve V15 and 50ml of sample was collected. A back titration procedure was carried out manually to determine the concentration of NaOH in the reactor and extent of conversion. 8. The experiment was repeated from step 4 to 7 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 were made sure to be equal. 6.3Back Titration Procedures 1. The burette was filled up with 0.1 M NaOH solution. 2. 10 mL of 0.25 M HCl was poured in a flask. 3. 50 mL samples that were collected from the experiment at every controlled flow rate (300, 250, 200, 150, 100 and 50 mL/min) were added into the 10mL HCl to quench the saponification reaction. 4. 3 drops of phenolphthalein were dropped into the mixture of sample and HCl. 5. The mixture then was titrated with NaOH until it turns light pink. 6. The amount of NaOH titrated was recorded.
7.0 Sample of Calculations 7.1 Residence Time For flow rates of 300 ml/min :
Residence Time,
Reactor volume ( L ) ,V L Total flow rate , v0 min
Total flow rate, Vo
= Flow rate of NaOH + Flow rate of Et(Ac)
τ=
( )
= 300 mL/min NaOH + 300 mL/min Et(Ac) = 600 mL/min = 0.6 L/min So, Residence Time,
τ=
4L 0.6 L/ min
= 6.6667 min
placed in Table 2
Other residence times were calculated by the same way, and varying the flow rates.
7.2 Conversion For flow rates of 300 ml/min : Moles of reacted NaOH, n1 n1 = Concentration NaOH x Volume of NaOH titrated = 0.1 M x 0.0161 L = 0.00161 mole Moles of unreacted HCl, n2 Moles of unreacted HCl
= Moles of reacted NaOH
n2 = n1 n2 = 0.00161 mole Volume of unreacted HCl, V1 V1
=
n2 concentration HCl quenc h
=
0.00161 0.25
= 0.00644 L Volume of HCl reacted, V2 V2
= Total volume HCl – V1 = 0.01 – 0.00644 = 0.00356 L
Moles of reacted HCl, n3 n3
= Concentration HCl x V2 = 0.25 x 0.00356 = 0.00089 mole
Moles of unreacted NaOH, n4 n4
= n3 = 0.00089 mole
Concentration of unreacted NaOH CNaOH unreacted
=
n4 volume sample
=
0.00089 0.05
= 0.0178 M
Xunreacted Xunreacted =
Concentration of NaOH unreacted concentration NaOH
0.0178 0.1
=
= 0.178 Xreacted Xreacted
= 1 - Xunreacted = 1 - 0.178 = 0.822
Conversion for flow rate 300mL/min 0.822 x 100% = 82.2 %
placed in Table 2
Hence, at flow rate 300mL/min of NaOH in the reactor, about 50.6% of NaOH is reacted with Et(Ac). Other conversions were calculated by the same way, and varying the flow rates. 7.3 Reaction Rate Constant,k
k=
v0 X V TFR C AO 1−X
(
)
For flow rates of 300 ml/min : V0
= Total inlet flow rate = 0.6 L/min
VTFR
= Volume for reactor =4L
CAO
= inlet concentration of NaOH = 0.1 M
X
= 0.822 ( X- reacted) k=
0.6 0.822 1−0.822 (4)(0.1)
(
)
= 6.9269L.mol/min
placed in
Table 2 Other Reaction Rate Constants were calculated by the same way, and varying the flow rates. 7.4 Rate of Reaction, -rA
-rA = k (CA0)2 (1-X)2 For flow rates of 300 ml/min : -rA
= 6.9269 (0.1)2 (1-0.822)2 = 2.19472 x 10-3 mol.L/min
placed in Table 2
Other Rate of Reactions were calculated by the same way, and varying the flow rates.
8.0
RESULT
Reactor volume
:4L
Concentration of NaOH in feed tank
: 0.1 M
Concentration of Et(Ac) in feed tank
: 0.1 M
No.
Flow rate
Flow rate
Total
Residence
Outlet
Inlet
of NaOH
of Et(Ac)
flowrate
time, t
conductivit
conductivit
(mL/min)
(mL/min)
of
(ms/cm)
y (ms/cm)
y (ms/cm)
5 5 5 5 5 5
6.7 6.7 6.8 5.5 5.0 4.0
8.4 8.7 9.0 7.6 7.1 6.7
solutions. V0 1 2 3 4 5 6
300 250 200 150 100 50
300 250 200 150 100 50
(mL/min) 600 500 400 300 200 100 Table 1
Residence Time,
Conversion, X,
Reaction Rate
Rate of Reaction, -rA
τ, (min) 6.6667 8.0000 10.0000 13.3333 20.0000 40.0000
(%) 82.2 85.0 87.4 89.8 85.8 92.2
Constant,k (L.mol/min) 6.92690 7.08330 6.93665 6.60294 3.021126 2.955130
(mol.L/min) 0.00219472 0.00159375 0.00110126 0.00068696 0.00060918 0.00017979
Table 2
Conversion vs Residence Time
conversion, %
94 92 90 88 86 84 82 80 78 76
Residence Time,
Figure 1
9.0
DISCUSSION
Plug Flow Reactor (PFR) consists of cylindrical pipe that operate at steady state. In this type of reactor, the feed enters at one end of cylindrical tube and the product will leaves at the other end. The properties of the flowing stream will vary from one point to another. In this experiment, the type of solution that used were NaOH and Et(Ac). These two solutions will react in the PFR to undergo saponification reaction. In order to achieve the objective of this experiment, the residence time have to be manipulated throughout the experiment, and thus the effect of each one is studied. It can be seen that residence time is a function of total flow rates of the feed. The flow rates of the feed solution have been varied by 300, 250, 200, 150, 100 and 50 mL/min respectively. In table 1, the raw data that consisting inlet flow rates, conductivity value and residence time was tabulated. From this graph, the effect of residence time can be studied. Conversion is a property that shows the percentage of the reaction has taken place. From figure 2, it can be seen that the conversion values shows fairly increasing in value and it have to be almost
constant. Since the PFR is designed not to stir the solution vigorously to maximise mixing process, the conversion by using PFR is also fairly slow.
10.0
CONCLUSION
The objective of this experiment which were to carry out the saponification process between NaOH and Et(Ac) and the relationship was graphed in figure (1) was conducted in this experiment. The results that were obtained was not really satisfied the theory of PFR as the increasing of residence time, suppose the value of conversion of reaction should be almost constant. From this experiment, the value rate of reaction this particular reaction were also being done by doing some calculation as shown in sample calculation section.
11.0
RECOMMENDATIONS
1)
The titration should be stop as soon as the Et(Ac) solution turned pink pale colour.
2)
The flow rates of the feed should be constantly monitored so that it remain constant throughout the reaction.
3)
All valves should be properly placed before the experiment started.
4)
Pump should never run dry.
12.0
REFERENCES
1)
Perry, R.H., and D. Green, Perry’s Chemical Engineer’s Handbook, 6 th Edition, McGraw-hill, 1987.
2)
Smith, J.M, Chemical Engineer Kinetics, 3rd edition, McGraw-Hill, 1981.
3)
The Plug Flow (Retrieved from http://www.konferenslund.se/p/L16.pdf on 18th October 2013)
4)
Reaction Kinetics (Retrieved from http://smk3ae.files.wordpress.com/2007/10/reaksikinetik.pdf on the 18th October 2013)
