ABSTRACT 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. In this experiment, sodium hydroxide and ethyl acetate react in tubular flow reactor. Both of the reactants fed to the reactor at equimolar flowrate for a certain time. The reaction is carried out at different volumetric flowrate. The conductivity value of outlet stream is measured to determine the conversion achieve at different retention time.
The retention time is highest for the lowest flowrate.
The result shows that the conversion is increases as the residence time increases. INTRODUCTION Reactor is one of the most important parts in industrial sector. Reactor is equipment that changes the raw material to the product that we want. A good reactor will give a high production and economical. One of criteria to choose or to design a good reactor is to know the effectiveness of the reactor itself.
There a many types of reactor depending on the nature of the feed materials and products. One of the most important we need to know in the various chemical reaction was the rate of the reaction.
By studying the saponification reaction of ethyl acetate and sodium hydroxide to form sodium acetate in a batch and in a continuous stirred tank reactor, we can evaluate the rate data needed to design a production scale reactor. A stirred tank reactor (STR) may be operated either as a batch reactor or as a steady state flow reactor (CSTR). The key or main feature of this reactor is that mixing is complete so that properties such as temperature and concentration of the reaction mixture are uniform in all parts of the vessel. Material balance of a general chemical reaction described below.
The conservation principle requires that the mass of species A in an element of reactor volume dV obeys the following statement: (Rate of A into volume element) – (rate of A out of volume element) + (rate of A produced within volume element) = (rate of A accumulated within vol. element) THEORY General Mole Balance Equation Assumptions 1) Steady state therefore 2) Well mixed therefore rA is the same throughout the reactor Rearranging the generation In terms of conversion The reaction to be studied is the saponification reaction of ethyl acetate Et(Ac) and sodium hydroxyde NaOH.
Since this is a second order of reaction, the rate of reaction depends on both of these reactants. The reaction will be acrried out using equimolar feeds of both the reactants with same initial concentrations. The raction equation is; NaOH + Et (Ac) >Na(Ac) + EtOH or A + B > C + D For a second order equimolar reaction with the same initial concentration (CAO = CBo), the rate law is; -ra=kCACB=kCA2 -rA= VCSTRFA0X Thus, the volume of the reactor is ; VCSTR = FA0XkCA2= F0(CAo-CA)kCA2 For equimolar feed rate, the reaction constant is ; k=(CAo-CA)? CA2
The residence time of a chemical reactor or vessel is a description of the time that different fluid elements spend inside the reactor is given by ; Residence time, ? =VCSTRF0 OBJECTIVE 1. To carry out a saponification reaction between sodium hydroxide,NaOH and ethyl acetate Et(Ac) in a CSTR 2. To determine the reaction rate constant of sodium hydroxide,NaOH and ethyl acetate Et(Ac) 3. To determine the effect of residence time on the conversion in a CSTR. APPARATUS 1. A laboratory scale of Continuous Stirred Tank Reactor 2. A conductivity meter 3. 50 mL Beakers 4. 250 mL Conical Flasks 5.
A burette 6. A retort stand. 7. Sodium Hydroxide 8. Ethyl acetate 9. Hydrochloric acid 10. Phenolphtalein 11. Deionized water PROCEDURE a) Preparation of Calibration Curve 1. 1 Liter of sodium hydroxyde,NaOH (0. 1M) and 1 Liter of sodium acetate,Na(Ac) (0. 1M) was prepared. 2. The conductivity and NaOH concentration for each conversion values were determined by mixing the following solution into 100mL of deionised water; a) 0% conversion : 100mL NaOH b) 25% conversion : 75mL NaOH + 25mL Na(Ac) c) 50% conversion : 50mL NaOH + 50mL Na(Ac) d) 75% conversion : 25mL NaOH + 75mL Na(Ac) ) 100%conversion : 100 mL Na(Ac) 3. The value of conductivity for each conversion was recorded. 4. The callibration curve of conductivity versus conversion plotted. The slope and y-axis intercept was determined. b) Back Titration Procedures for manual Conversion Determination 1. A burette was filled up with 0. 1M NaOH solution. 2. A 10 mL of 0. 25M HCl was measured in a flask. 3. A 50 mL sample from the experiment obtained and immediately was added to the HCl in the flask to quench the saponification reaction. 4. A few drops of phenolphtalein were fadded into the mixture. . The mixture was titrated with NaOH solution from the burette until the mixture was neutralized. The amount of NaOH titrated recorded. c) Effect of Residence Time on The Reaction in a CSTR 1. The general start-up procedures were performed. 2. Both pumps P1 and P2 simultaneously were switched on and the valves V5 and V10 were opened to obtain the highest possible flow rate into reactor. 3. Both the NaOH and Et(Ac) solutions were allowed to enter the reactor until it is just about to overflow. 4. The valves V5 and V10 were readjusted to give a flowrate about 0. 1L/min. 5.
The stirrer M1 were switched on and the speed were setted to about 200 rpm. 6. The conductivity value at (QI-401) were started monitoring until the did not change over the time to ensure that the reactor had reached steady state. 7. The sampling valve V12 was opened and a 50mL sample collected. A back titration procedure was carried out to manually 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 time by increasing the feed flow rates of NaOH and Et (Ac) to about 0. 15, 0. 0, 0. 25 and 0. 30 ml/min. Make sure that both was the same. RESULTS a)Determination of conductivity value for conversion of sodium hydroxide,NaOH. Solution| Conversion, X| Conductivity (ms)| | | 100mL solution| 100mL solution + deionized water| 100 mL NaOH| 0| 7. 60| 7. 01| 75 mL NaOH + 25 mL Na(Ac)| 0. 25| 4. 83| 4. 27| 50 mL NaOH + 50 mL Na(Ac)| 0. 5| 2. 23| 2. 02| 25 mL NaOH + 75 mL Na(Ac)| 0. 75| 4. 69| 4. 24| 100mL Na(Ac)| 1| 5. 65| 38. 8| b)Manual conversion determination of NaOH by Back Titration Method. Sample (ml/min)| Amount of NaOH (mL)| Conversion,x| 0. 10| 12. | 0. 512| 0. 15| 13. 0| 0. 520| 0. 20| 13. 5| 0. 540| 0. 25| 14. 0| 0. 560| 0. 30| 14. 5| 0. 580| c) Effect of Residence Time on the Reaction in CSTR. Flowrate set value(L/min)| Conductivity (ms)| 0. 10| 7. 08| 0. 15| 6. 79| 0. 20| 6. 73| 0. 25| 6. 70| 0. 30| 6. 67| Flowrate set value(L/min)| Residence Time (min)| Conversion,X| Reaction rate constant,k| Rate law,-rA| 0. 10| 400| 0. 512| 0. 1075| 4. 47×10-5| 0. 15| 266. 67| 0. 520| 0. 1693| 9. 75×10-5| 0. 20| 200| 0. 540| 0. 2552| 1. 35×10-4| 0. 25| 160| 0. 560| 0. 3616| 1. 75×10-4| 0. 30| 133. 33| 0. 580| 0. 4932| 2. 75×10-4| d) Graph of calibration curve of conductivity versus conversion. Conversion, X Figure 1 e) Graph of conversion versus residence time. Residence Time Figure 2 CALCULATION Sample calculation: A) Back titration 1. Known quantities Volume of sample, VS = 50 ml Concentration of NaOH in the feed vessel, CNaOH,f = 0. 1 mol/L Volume of HCI for quenching, VHCi,S = 10 ml Concentration of HCl in standard solution, CHCi,s = 0. 25 mol/L Volume of titrated NaOH, V1 = 16. 50 ml Concentration of NaOH used for titration, CNaOH, S = 0. 1 mol/L 2. Calculations
Concentration of NaOH entering the reactor, CNaOH,0 = (CNaOH,f)/2 = 0. 1/2 = 0. 05 mol/L Volume of unreacted quenching HCI, V2 = CNaOH, SCHCi,s X V1 = 0. 10. 25 x 0. 0128 = 0. 00512 L Vol. of HCI reacted with NaOH in sample, V3 = VHCi,S – V2 = 0. 01 – 0. 00512 = 0. 00488 L Moles of HCl reacted with NaOH in sample, n1 = CHCi,s x V3 = 0. 25 X 0. 00488 = 0. 00122mol Moles of unreacted NaOH in sample, n2 = n1 = 0. 00122 mol Conc. Of unreacted NaOH in the reactor, CNaOH = n2 / (VS / 1000) 0. 00122 / (50/1000) = 0. 0244 mol/L Conversion of NaOH in the reactor, X = 1- CNaOHCNaOH,0 = 1-0. 02440. 05 = 0. 512 B) Effect of residence time ?=VCSTRF0 = 40L0. 1L/min = 400 min Reaction Rate constant,k=(CAo-CA)? CA2 = [(0. 05-0. 0244)mol/L] / [400min x (0. 0244mol/L)2] = 0. 1075 L/mol. min 1) Rate law, -ra=kCA2 = (0. 1075) (0. 0244)2= 0. 000064 DISCUSSION The calibration curve is plotted to determine the conversion of the reaction of between sodium hydroxide and ethyl acetate at certain value of conductivity.
It was obtained that the conductivity of the sodium hydroxide solutions varied linearly with concentration of sodium hydroxide. The conductiivity is decrease when the molar concentration of NaOH decrease. The mixture of different of moles of both the reactants give different value of conductivity. As they mix, sodium hydroxide is consumed and sodium acetate is produced. Because ethyl acetate and ethanol are not electrically conductive, conductivity measurements can be used to measure the concentration of unreacted NaOH that remains solution that relate to conversion.
The reaction is carried out at different volumetric flowrate that relate to the residence time. It was found that the conversion is increase as the volumetric flowrate increase. By defination, the residence time is time that the fluid element spend within reactor. Therefore, fluid entering the reactor at time t will exit the reactor at time t + ? , where ? is the residence time of the reactor. At low flowrate,the velocity of fluid moving inside the reactor is low means the reactant spend more time within the reactor. As the residence time increase due to increase of flowrate, the conversion achieve also increase.
When the reactant flow slowly, they are possible to contact and react to each other to form the product. This is very important for the reactants to have complete mixing in the radial direction and a uniform velocity profile across the radius. The reaction between equimolar of NaOH and Et(Ac) is the second order, the rate of reaction is L/mol. min. Relate to the reaction rate constant for this order of reaction, when the ‘k’ value is increase means more volume of NaOH require to convert just a mole of NaOH in 1 second. In other words, it mean less mole of NaOH converted for a big volume of NaOH solution.
For this experiment,the reaction rate constant is decrease as the volumetric flowrate is increase. It is means that the reaction rate increase when the rate constant increase. There is more moles of NaOH converted for a less volume of NaOH solution require. This is very important key design to have a high conversion for large scale production. Other than determine the conversion based on the conductivity value measured by electronic device on the reactors, the conversion also determined manually by back titration procedure. From the calculation, it was obtained that the value of conversion is decrease when the volumetric flowrate decrease.
Relate to the theory and comparison to measured by the device, the conversion should be increased. This may happen because of some mistakes when collecting, mixing and measuring the sample from the outlet stream of reactor, NaOH to be use as titrant and hydrochloric acid for quenching. For example, measured 50mL sample with a large beaker that less accurate compared to measured cylinder. The next mistake is measure 10 ml of HCl with 100ml of measured cylinder instead of 25ml measured cylinder that more accurate with small scale. The inaccurate measuring the 0. 5M HCl is very significant because it may affect more the volume of 0. 1M NaOH to neutralized it and affect the calculation of determination the conversion value. The most mistakes is allow the collected sample in the beaker for a few minutes before quenching with HCl. The reaction could proceeds in the beaker after the CSTR. CONCLUSION The saponification reaction between NaOH and Et(ac) done in a CSTR at different conversion due to different volumetric flowrate. As the flowrate decreases,the reaction rate constant,k for the second order of reaction is decreases.
The conversion achieve within the reactors is proportional to the residence time. The experiment was successful because the entire objectives have been achieved and related to the theory of study. RECOMMENDATIONS 1. Use the apparatus with appropriate size and scale to the amount to be measured. 2. Hydrochloric acid for quenching should be prepared early and added to the samples as soon as possible so that the reaction between NaOH and Et(Ac) cannot proceed because all the unreacted NaOH Neutralized by HCl. 3.
The samples that already mix with HCL should be titrated as soon as possible. 4. This experiment could be done with more variables such as temperature,type of flow stream and so on. REFERENCES 1. Fogler, H. S. , ‘Elements of Chemical Reaction Engineering’, 2nd edition, Prentice Hall, 1992, New Jersey. 2. Gilbert F. Froment and Kenneth B. Bischoff. , ‘Chemical Reactor Analysis and Design’John Wiley & Sons, 2nd Edition, 1990. 3. Levenspiel O. Chemical Reaction Engineering. John Wiley & Sons, NewYork, third edition, 1999.
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