ABSTRACT From this experiment, our objectives are to carry out the saponification reaction between NaOH and Et(Ac) in plug flow reactor, to determined the reaction rate constant and the rate of reaction of the saponification process. First of all, the equipment is set up before started the experiment. After collecting the data, the value of reaction rate constant and rate of reaction is calculated. The reaction rate constant we get for 0.10 L/min flowrate is 60.00 M-1s-1, for the 0.15 L/min reaction rate constant is 34.28 M-1s-1, for the 0.20 L/min reaction rate constant is 31.94 M-1s-1, for the 0.25 L/min reaction rate constant is 29.44 M-1s-1, and for the 0.30 L/min reaction rate constant is 27.00 M-1s-1. Besides that, we are also able to determine the rate of reaction for this process. Then, a graph of conversion factor against residence time is plotted. From the graph we can see that the conversion factor is directly proportional to the residence time. As the residence time increases, the conversion factor also increases.
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INTRODUCTION A common type of reactor is the mixing, or stirred reactor .The basic components of this device will include a mixer or agitator mounted to a tank. One of the stirred reactors is Continuous Stirred Tank Reactors (CSTR). The flow stirred tank reactor in series is a common reactor type in environmental applications. The principle characteristic is that the reactor is assumed to be instantaneously and perfectly mixed. Majority of industrial chemical processes, a CSTR reactor is the equipment in which raw materials undergo a chemical change to form desired product. The types of process this equipment is the continuous stirred tank reactor which is this reactor is almost always operated at steady state. The CSTR reactor used are most commonly used in industrial processing, primarily in homogeneous liquid-phase flow reactions, where constant agitation is required. They may be used by themselves, in series, or in a battery. It is referred to as the continuously stirred tank reactor (CSTR). It is normally operated at steady state and is assumed to be perfectly mixed. The characteristic of this equipment is run at steady state with continuous flow of reactants and products, the feed assumes a uniform composition throughout the reactor, and exit stream has the same composition as in the tank.
Figure 1 Cross-sectional diagram of Continuous stirred-tank reactor
The CSTR can run as single reactor and also in series. The CSTR reactor is connected in series so that the exit stream of one reactor is the feed stream for another reactor. There are three reactor vessels 2
connected in series by piping, each containing a propeller agitator driven by a variable speed electric motor and the unit based on the simplest classic case of well mixed, multi-staged process operation. The solution in each reactor is well stirred and the concentration can be measured. These three reactors are to compare the measured responses of the vessel concentrations to deliberate change at the inlet with a theoretical prediction.
Figure 2 Single Continuous Stirred Tank Reactors (CSTRs)
The piping arrangement has been designed to include a dead time coil in the system. Feed liquid to the first vessel is drawn from of the two sump tanks by a pump, via a flow meter and control valve. The trace material concentration in each sump tank is made to be different. At a selected instant, a sudden change from one feed to the other is made: either for continuous period is known as the step function, or for a short interval is known as impulse function, and the concentration or conductivity changer with time in each vessel is measured.
Figure 3 Continuously Stirrer Tank Reactor (CSTR) in series.
The advantages of CSTR are easily maintained, good temperature control, cheap to construct, reactor has large heat capacity and interior of reactor is easily accessed. Meanwhile, the disadvantages of using CSTR are lowest conversion per unit volume and also by-passing and channeling possible with poor agitation
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OBJECTIVE The objectives of this experiment is to carry out a saponification reaction between NaOH and Et(Ac) in a CSTR. Another objective is to determine the reaction rate constant and lastly to determine the effect of residence time onto the reaction extant of conversion.
THEORY Saponification is a process which esters in fat are hydrolyzed by sodium or potassium hydroxide (NaOH or KOH) to produce a carboxylate anion which can act as surfactant,i.e. soap. The equation below shows the saponification process between sodium hydroxide and ethyl acetate (irreversible reaction) to produce sodium acetate and by-product ethanol.
NaOH
→
C2H5O2CCH3
+
Ethyl Acetate
Sodium Hydroxide
CH3CO2Na +
H3CCH2OH
Sodium Acetate
Ethanol
A. Preparation of Calibration Curve for Conversion vs. Conductivity
The reaction to be studied is the saponification reaction of ethyl acetate Et(Ac) and sodium hydroxide (NaOH). Since this is a second order reaction, the rate of reaction depends on both concentrations of Et (Ac) and NaOH. However, for analysis purposes, the reaction will be carried out using equimolar feeds of Et (Ac) and NaOH solutions with the same initial concentrations. This ensures that both concentrations are similar throughout the reaction.
NaOH + Et (Ac)
Na(Ac) + EtOH
From this experiment, it is required to calibrate the conductivity measurements of conversion values for the reaction between 0.1 M ethyl acetate and 0.1 M sodium hydroxide.
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B. Back Titration for Manual Conversion Determination
It is advisable to carry out manual conversion determination on experiment samples to verify the conductivity measurement values. It is based on the principle of quenching the sample with excess acid to stop any further reactions, then back titrating with a base to determine the amount of unreacted acid.
NaOH + HCl
NaCl + H2O
The back titration calculations as follow;
Conc. of NaOH entering the reactor, (CNaOH,o)
Volume of unreacted quenching HCl, (V2)
Vol. of HCl reacted with NaOH in sample, (V3)
Moles of HCl reacted with NaOH in sample, (n1) ⁄ Moles of unreacted NaOH in sample, (n2)
Conc. of unreacted NaOH in the reactor, (CNaOH) ⁄ Conversion of NaOH in the reactor, X (
)
C. Reaction rate constant
K = (CAO- CA) ƬCA2
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APPARATUS AND EQUIPMENT
Continuous stirred tank reactor 40 L equipment
40 liter of 0.1 M Sodium Hydroxide, NaOH
40 liter of 0.1 M Ethyl Acetate, Et(Ac)
1 liter of 0.25M Hydrochloric Acid, HCl for quenching
Beakers
Titration equipment
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PROCEDURES 1. All valves are ensured to initially close. 2. The following solutions are prepared: a) 40 litre of sodium hydroxide, NaOH (0.1M) b) 40 litre of ethyl acetate, Et(Ac) (0.1M) c) 1 litre of hydrochloric acid, HCl (0.25M) for quenching 3. The charged pot caps for vessels B1 and B2 are opened. 4. The feed tank B1 is filled with the NaOH solution and feed tank B2 with the Et(Ac) solution 5. The charged port caps for both vessels are closed. 6. The power of control panel is turned on. 7. The thermostat T1 tank are checked that there is sufficient water and refilled as necessary. 8. The cooling water valves V13 are opened and let the cooling water flow through the condenser W1. 9. Adjusted the overflow tube to gave a working volume of 10 L in the reactor R1 10. Valves V2, V3, V7, V8 and V11 are opened 11. Both pumps P1 and P2 are switched on simultaneously and valves V5 and V10 are opened to obtain the highest possible flow rate into the reactor. 12. Let the reactor filled up with both the solution until it is just about to overflow. 13. The valves V5 and V10 are readjust to give a flow rate of about 0.1 L/min. make sure that both flow rates are the same and the flow rate are recorded. 14. Switch on the stirrer M1 and set the speed to about 200 rpm. 15. Started monitoring the conductivity value at Q1-401 until it does not change over time. This is to ensure that the reactor has reached steady state. 16. The steady state conductivity values are recorded and find the concentration of NaOH in the reactor and extent of conversion from the calibration curve. 17. A 50 mL sample are collected from sampling valve V 12 and carry out a titration to determine the concentration of NaOH in the reactor and extant of conversion. 18. Steps 14 - 18 are repeated for different residence times by adjusting the feed flow rates of NaOH and Et(Ac) to about 0.15, 0.20, 0.25 and 0.30 L/min. Both flow rates are make sure to be the same.
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RESULTS
Flowrate of NaOH, (L/min) Flowrate of Et(Ac), (L/min) Conductivity, (µS/cm) Volume of NaoH titrated, V1 (mL) Residence time, τ (min) Volume of unreacted quenching HCl, V2 (mL) Volume of HCl reacted with NaOH , V3 (mL) Conversion of NaOH in the reactor, X (%) Rate Constant,k (M-1s-1) Rate of reaction, -rA (M/s)
Conversion, % 0 25 50 75 100
0.10
0.15
0.20
0.25
0.30
0.10
0.15
0.20
0.25
0.30
2.94
2.86
2.83
2.81
2.81
24.0
23.4
23.1
22.8
22.5
200
133.33
100.00
80.00
66.67
9.6
9.36
9.24
9.12
9.0
0.4
0.64
0.76
0.88
1.0
96
93.6
92.4
91.2
91.0
60
34.28
31.99
29.44
27.00
3.51 x 10-4 4.62 x 10-4 5.70 x 10-4 2.40 x 10-4 Table 1 Effect of residence time tubular flow reactor
0.1 M NaOH, mL 100 75 50 25 100
Solution mixture 0.1 M Et(Ac), mL
H2O, mL
0 100 25 100 50 100 75 100 100 100 Table 2 Preparation of calibration curve
6.75 x 10-4
Conductivity, mS/cm 6.73 4.55 2.75 1.016 0.0311
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Figure 4 Calibration curve (Conversion vs. Conductivity)
97
Conversion, %
96 95 94 93 92 91 90 0
50
100
150
200
250
Residence Time, min Figure 5 Graph of conversion against residence time
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CALCULATION
NaOH + HCl
NaCl + H2O
Sample calculations for flowrate = 0.10 L/min Volume of sample,Vs = 50 mL Concentration of NaOH in the feed vessel, CNaOH,f = 0.1 M Volume of HCl for quenching, VHCl,s = 10 mL Concentration of HCl in standard solution, CHCl,s = 0.25 mol/L Volume of NaOH titrated, V1 = 24.0 mol/L Concentration of NaOH used for titration, CNaOH,s = 0.1 mol/L Concentration of NaOH entering the reactor, CNaOH,0 = (1/2)(0.1) = 0.05 mol/L Volume of unreacted quenching HCl, V2 = (CNaOH,s /CHCl,s) x V1 = (0.1/0.25) x 24.0 = 9.6 mL Volume of HCl reacted with NaOH in sample, V3 = VHCl,s - V2 = 10 – 9.6 = 0.4 mL Moles of HCl reacted with NaOH in sample, n1 = (CHCl,s x V3)/1000 = (0.25 x 0.4) / 1000 = 0.0001 mol Moles of unreacted NaOH in sample, n2 = n1 = 0.0001 mol Concentration of unreacted NaOH in the reactor, CNaOH = n2/ Vs x 1000 =
x 1000 = 0.002
Conversion of NaOH in the reactor, X = (
) x 100%
=(
) x 100% = 96 %
Residence time, τ = VCSTR/F0 = 40 L/ (0.10 + 0.10) L/min = 200 min
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Rate constant,
=
= 60 M-1s-1
Rate of reaction, -rA = kCA2 = 60(0.0022) = 2.4 x 10-4 M/s
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DISCUSSION The experiment is conduct to determine the reaction rate constant as well as to determine the effect of the residence time on the conversion in a Tubular Flow Reactor. The CSTR model is used to predict the behavior of chemical reactors, so that key reactor variables, such as the dimensions of the reactor, can be estimated.
The experiment is conducted with setting up the flow rate of both NaOH and Et(Ac) into 0.10 L/min, followed by 0.15 L/min, 0.20 L/min, 0.25 L/min and 0.30 L/min at each run of experiment. From the experiment, when the flow rates of the reactant in the reactor become slower, the residence time tends to increase. Residence time is known as the removal time which is the average amount of time that a particle spends in a particular system. This measurement varies directly with the amount of substance that is present in the system.
When using the residence time equation, it is significant to made a variety of assumptions. It is assumed that chemical degradation does not occur in the system in question and that particles do not attach to surfaces that would hinder their flow. If chemical degradation were to occur in a system, the substance that originally entered the system may react with other existing compounds in the system, causing the residence time to be significantly shorter since the substance would be chemically broken down and effectively be removed from the system before it was able to naturally flow out of the system.
Therefore, when the residence time increases, it indicates that more molecules of reactants are reacted with each other. Thus, the conversion of reactant into product is increased. From the result, it shows that when the flow rates was set into 0.30 L/min for both reactants which is the highest flow rate in this experiment; the residence time of that reactants in the CSTR is the shortest which is 66.67 min and give out the result for conversion of 91.0%.
Oppositely, when the flow rate for both reactants was set into 0.10 L/min which is the lowest flow rate, the residence time of those reactants in the CSTR is the longest which is 200 min and the highest conversion of the reactants is 96.0 %. While for flow rates of 0.15 L/min, 0.20 L/min and 0.25 L/min; the residence time are 133.33 min, 100.00 min and 80.00 min respectively and the conversion of reactants are 93.6 %, 92.4 % and 91.2 % respectively.
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CONCLUSION The purpose of this experiment is to determine the reaction rate constant and as well as the effect of the residence time on the conversion of sodium hydroxide. Continuous stirred tank reactor is used in order to achieve the objectives of this experiment. After completing this lab experiment, all the purposes are met and the results are collected. From the results, it shows that as for each flow rates decrease from 0.10 L/min to 0.30 L/min, the conversion of sodium hydroxide decrease from 96.0 % to 91.2 %. The graph of conversion of sodium hydroxide versus residence time is plotted. It is a directly proportional. As the conversion increase, the residence time increase as well.
As all the purposes of this experiment is achieved, this experiment is considered as a successful.
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RECOMMENDATION 1. The device needs to be well maintenance in order to avoid it from malfunctioning during the experiment period like the one we are having in our session.
2. To get a better result, only one person is needed to take care of the opening and closing of the valve and other person take care of the pump. This is because some valve needed to be opened or closed simultaneously.
3. Make sure the tank is filled with the correct solution and to the correct amount. Different substance reacts differently and lack of substance can damage the apparatus.
4. Make sure general start-up procedure is done first in order to check the machine functionality.
5. Sodium hydroxide is corrosive to flesh and it can cause blindness. To prevent this from happen, eye protection should be wear at all the time.
6. The burette should be rinsed with sodium hydroxide after rinsed using the distilled water.
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REFERENCES Fogler, Scott H. Elements of Chemical Reaction Engineering. 4th ed. Englewood Cliffs, NJ:PrenticeHall, 2006. Thomas, Charles E. Process Technology Equipment and Systems. 3rd ed. Clifton Park, Cangage
NY:Delmar
Learning, 2011
Smith, J.M., Chemical Engineering Kinetics, McGraw Hill Int., 1981. Levenspiel, O., Chemical Reaction Engineering, 3rd ed.,John Wiley and Sons, New York, 1999.
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APPENDICES
16
