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ABSTRACT Data obtained in batch reactors can be well defined and used to predict performance of larger scale, continuous-flow reactors. The saponification reaction between NaOH and Et(ac) done in a batch reactor at different temperature.The effect of temperature on the reaction rate constant, k in CSTR batch operation was conducted with the addition of back titration process in order to determine the realtionship between the time, the concentration of unreacted sodium hydroxide, CNaOH, the rate law and the reaction rate constant at three temperatures, 26 oC, 40oC and 50oC. It was concluded that the higher the temperature of the reaction, the higher the reaction rate constant by the means of the 1:4 ratio of rise in temperature and the reaction rate.The longer the reaction is allowed to take place, the slower the reaction proceeds.
INTRODUCTION A process in which all the reactants are added together at the beginning of the process and products removed at the termination of the reaction is called a batch process. In this process, all the reagents are added at the commencement and no addition or withdrawal is made while the reaction is progressing . In the majority field of chemical processes, the reactor vessel in which the reaction process take place is the key component of the equipment.The design of the reactors is very important to the success of the production.Batch reactors are used widely in industry at all scales. These commonly provided with agitation and a method of heat transfer (usually by coils or external jacket). This type of reactor is primarily employed for relatively slow reactions of several hours duration, since the downtime for filling and emptying large equipment can be significant. The stirred tank batch reactor is still the most widely used reactor type both in the laboratory and industry. Batch reactors may be preferred for small-scale production of high priced products, particularly if many sequential operations are carried out to obtain high product yields. Batch reactors may also be justified when multiple, low volume products are produced in the same equipment or when continuous flow is difficult, as it is with highly viscous or sticky solids-laden liquids. Because residence time can be more uniform in batch reactors, better yields and higher selectivity may be obtained than with continuous reactors.
OBJECTIVE To measure the rate of saponification reaction between NaOH and ethyl acetate, using a batch reactor as function of temperature and determine the rate law constants.
THEORY Batch reactors are often used because of their convenience mainly in laboratory experimentation. Data obtained in batch reactors can be well defined and used to predict performance of larger scale, continuous-flow reactors
The reaction chosen in this experiment is the saponification of ethyl acetate (EtOAc) with dilute sodium hydroxide (NaOH): NaOH + EtOAc →NaOAc + EtOH This type of reaction is called a saponification because of its importance in themanufacture of soap.The reaction is relatively slow and the changing ester concentration can be followed quite easily by analyzing samples from the reaction mixture every few minutes. The second order rate constant is determined by integrated rate law method: Assuming the reaction to be a simple, irreversible, second order process, then the rate law for a batch process may be written;
is concentration of NaOH and
is concentration of ethyl acetate.
Suppose that the concentrations of the ester and hydroxide are equimolar at the start of the experiment and equal to CA0 mol/dm3, and that at some time t later,both the NaOH and ethyl acetate concentration have fallen to CA. Therefore,
Intergrating the equation give; 2
is concentration of unreacted NaOH,
is initial concentration, k is rate constant
(L/mol.s) and t is time. A plot of 1/CA against t give a straight line with the slope t, k value. In any single homogenous reaction, temperature, composition, and reaction rate are uniquely related. They can be represented graphically in one of three ways as shown in Figure 8 below:
Concentration vs Time
Rate appearence vs Time
Rate appearence vs Concentration
Rate, k vs Temperature
APPARATUS 1. 2. 3. 4. 5. 6.
Stopwatch 100mL Beakers 100mL graduated cylinder Burette and retort stand 250 mL conical flasks Continuous stirrer tank reactor (SOLTEQ model:BP100)
Figure A: Continuous stirrer tank reactor (SOLTEQ model:BP100)
METHODOLOGY Effect of Temperature on Reaction Rate Constant
1. General start-up procedures were performed. (See Appendix A) 2. Prior to the start of the batch reactor experiment, the overflow tube was adjusted to give a working volume of about 2.5 liters. Pump P1 was switch on and 1.25 liters of 0.1M of ethyl acetate from the feed were pumped into the reactor. The stirrer was switched on at a medium speed followed by the heater which the reaction temperature was set at room temperature of 26oC. 3. Consequently, pump P2 was switched on with the valve was set at maximum flowrate where as the 0.1M NaOH pumped into the reactor at the same volume of ethyl acetate, 1.25 liters. As soon as the level of the reactants reached 2.5 liters, pump P2 were switched off and the timer starts immediately at t0. 4. At the same time, 25ml of 0.25 M HCl was prepared in a flask ready for quenching process with the collected sample. 5. After 1 minute of reaction time, 100ml sample was collected by opening the sampling valve and the sample was immediately quenched with the prepared HCl solution. 6. The sample was then titrated with 0.1M NaOH under the hood, to determine the amount of unreacted HCl in the sample. Three drops of phenolphthalein were added prior to the titration. 7. The steps 4 to 6 were repeated for reaction times of 5, 10, 20 and 25 minutes. 8. Subsequently, the steps 1 to 7 were repeated for temperatures of 40 and 50oC. 9. All the switches were switch off after the experiment finished and general shut-down procedures were done, consequently.
RESULT Time (min)
Volume of NaOH titrated (mL)
Temperature: 26ºC Conversion, Concentration of X unreacted NaOH in reactor, CA
Volume of NaOH titrated (mL)
Temperature: 40ºC Conversion, Concentration of X unreacted NaOH in reactor, CA
Volume of NaOH titrated (mL)
Temperature: 50ºC Conversion, Concentration of X unreacted NaOH in reactor, CA
0.015 50 0.01
0 0 -0.005
Figure 1:Concentration of NaOH versus Time at specific temperature
2000 y = 108.8x + 164.0
1800 1600 1400
y = 64.766x - 98.531
y = 13.75x - 16.88
Figure 2: The reciprocal of Concentration of NaOH versus Time at specific temperature
0.008 0.007 0.006
0.001 0 -0.001
Figure 3: The rate of dissapearance of NaOH,-rA versus Time at specific temperature
0.008 0.007 0.006 0.005 26 0.004 -rA
0.002 0.001 0 -0.001
Concentration of NaOH (mol/L)
Figure 4: The rate of dissapearance of NaOH,-rA versus concentration of NaOH at specific temperature
120 100 rate constant,k
y = 3.9401x - 1165.6 80 60 40 20 0 295
Figure 5: The reaction rate constan,k versus Temperature
CALCULATIONS Sample calculation for conversion of NaOH in the reactor, X for temperature = 26°C at 1 minute Unknown quantity: Concentration of NaOH in the reactor
= ? mol/L
Volume of samples
= 100 mL
Concentration of NaOH in the feed vessel
= CNaOH, f
= 0.1 mol/L
Volume of HCl for quenching
= 25 mL
Concentration of HCl in standard solution
= 0.25 mol/L
Volume of titrated of NaOH
= 40 mL
Concentration of NaOH used for titration
= 0.1 mol/L
Calculations: Conc. Of NaOH entering the reactor, CNaOH, 0
Sample calculation of reaction rate constant, k for temperature = 26°C at 1 minute: (
Sample calculation of rate of reaction, -rA for temperature = 26°C at 1 minute : = 13.75(0.0225)2 = 0.0070 Where k value is determined from the slope of the graph of the reciprocal of Concentration of NaOH versus Time at specific temperature.
DISCUSSION Continuous stirred tank reactor (CSTR) is the experiment which to study the effect of temperature on the reaction rate constant, k in standard batch operating process. The aim involves a saponification reaction between sodium hydroxide, NaOH and ethyl acetate which were conducted at three different temperatures, room temperature which was 26oC, 40 oC and 50 oC. According to the figure 1, as the time of reaction increases at the same temperature, the concentration of the unreacted NaOH, CNaOH decreases.Based on the trendline of the graph, at higher temperature, the rate of decreasing of concentration of NaOH is faster. Thus, by means of the back titration calculation, the CNaOH was obtained from the experiment were greatly effects on each temperature. At higher temperature of 40oC and 50 oC, the concentration of NaOH appear 0 because the conversion was completed The rate reaction constant however, was determined by the figure 2 as the slope of the straight line at each corresponding temperature. The, k at 26oC, 40oC and 50oC are 13.75L/mol.minutes, 64.766L/mol.minutes and 108.80L/mol.minutes, respectively. As the rule of thumb, a 10oC in temperature causes the reaction rate constant to double. Though in this case, from that figure, the reaction rate constant is 2.4 times from the calculated 30oC to 40oC however further temperature at 50oC was 1.7 times of the reaction rate constant at 40oC.
The slight error in the experiment was to install the cooling water when the process undergo at 50oC which leads to decrease of 5oC of the desired temperature. While in figure 3, it shows the relationship between the rates of disappearance of NaOH with the time increment. At the start of the experiment of 26oC of temperature, the rate of disappearance of NaOH also called the rate law was 0.007. The rate law decreases as the time increases to 25 minutes of reaction. Comparing to the trendline at different temperature,at higher temperature, the rate of disappearance of NaOH is faster to achieve the maximum conversion. Once the sodium hydroxide entered the reactor with half full of ethyl acetate, the reaction already started thus the rate of disappearance of sodium hydroxide is high at this starting period. At that period, the reactions already occur as time proceeds is not considered could lead to inaccurate data taken. The relation between the rate of disappearance of NaOH with respect to time shown in figure 4. As the concentration of NaOH increase, the rate of disappearance of NaOH increase. Otherwise,the rate of dissapearence of NaOH decrease as there is more product form can reduce the contact between the reactants of NaOH and ethyl acetate. Figure 5 illustrates the relationship between the rate constant and temperature.From that figure, it shown that the reaction rate increase as the temperature increase. This is because the effect of temperature to increase the rate of reaction by lowering the activation energy and increase the effective collision between molecules of reactants. Thus, the reaction become faster.
CONCLUSION In the experiment, the evaluation on the effect of temperature on the reaction rate constant, k in CSTR of batch process where crucial in order to determine the trend of these two properties in chemical reaction engineering. A batch titration process was also conducted to determine the concentration of unreacted sodium hydroxide values thus in addition to verify the values, the conversion also calculated. It was concluded at the higher the temperature of reaction, more reactions were precede at higher reaction rate resulting decreases of CNaOH throughout the time period. Though there was a slight contrast between the result of the experiment and the theory where as in the theory it stated that a 10oC rise in temperature causes a reaction rate
constant to double. However, the values of rate constant from 30 to 40 and 40 to 50 oC are 2.4 and 1.7 respectively are appear double.
RECOMMENDATIONS It is impossible to get the precise result same as the imaginary result without any errors in the experiment but possible to minimize the error that exists in this experiment. There are some recommendations and precautions need to be considered in order to ensure that the result of experiment is accurate. i.
The sample needs to be added with 0.25 M of Hydrochloric Acid quickly in order to stop the reaction.
Make sure the temperature controller was set for desired value before run the pump P2.
In order to get more accurate data, make sure that you not use the same conical flask in order to keep away from contamination of previous sample
Make sure to cool down the reactor when the temperature reaches 45°C to avoid the reactor becomes too hot.
The experiment can be repeated and the value of average reaction time can be taken to make more precise value. This will having a better data than only conducting the experiment only once..
REFERENCES 1. Frogler, H.S., (2006). Elementary of Chemical Reaction Engineering (4th Edition). Prentice Hall. 2. CSTR in series Model: BP 107. Retrieved on 15 November 2011 from http://www.solution.com.my/pdf/BP107%28A4%29.pdf 3. Hafiz Muhammad Zaheer Aslam (2006). Chemical Reaction Engineering Lab. Retrieved on 15 November 2011 from http://www.uet.edu.pk/export/sites/UETWebPortal/faculties/facultiesinfo/chemical/La bs/ChemicalReactionEngineering.pdf 4. Prof. William H. Green (2007). Lecture 5: Continuous Stirred Tank Reactors (CSTRs). Retrieved on 15 October 2011 from http://ocw.mit.edu/courses/chemical-
engineering/10-37-chemical-and-biological-reaction-engineering-spring-2007/lecturenotes/lec05_02212007_g.pdf 5. Smith, J.M., Chemical Engineering Kinetics, McGraw Hill Book Co., Singapore, 1981. 6. Fogler, H.S., Elements of Chemical Reaction Engineering, Prentice Hall, New Jersey, 1992. 7.
Levenspiel, O., Chemical Reaction Engineering, Third ed., Wiley Int. Co., New York, 1999.