CSTR LAB REPORT

ABSTRACT The experiment is to study the effect of temperature and reaction time and to determine the relationship between reaction rate with conversion, reactor volume and feed rate. The reaction started when Ethyl Acetate and Naoh were mixed with equal volume by using continuous stirred tank reactor at 30°C. After 1 minute the first 50mL sample of solution is collected and thus undergo the titration process with 0.1M NaoH. The sample is taken again for minutes 5, 10, 15, 20, and 25 consecutively. The amount of Naoh been used in that titration has been recorded in the result. The same procedures were repeated for different temperature which are 40°C. All the result has been recorded, thus calculation is made and the graphs were plotted based on the results. By the graph we can determine the rate of the reaction which is 2.1359L/mol.min and 1.0987L/mol.min respectively for 30°C and 40°C.

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INTRODUCTION Chemical kinetics and reactor design are important in producing almost all industrial chemicals. The selection of a reaction system that operates in the safest and most efficient way is the key to the success or failure of a chemical plant. Reactor is the 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 design or to choose a reactor knows the effectiveness of the reactor itself. There are many reactors depending on the nature of the feed materials and products. One of the most important we need to know in the various chemical reaction is the rate of reaction. The reaction occurred in a reactor is exothermic or endothermic. A reactor is generally assembled with a jacket or coil in order to maintain the reaction temperature in the reactor. If heat is evolved due to exothermic reaction, a coolant stream is required to pass through the jacket or coil to remove the extra heat. On the other hand, if endothermic reaction occurs in the system, the flow of heating medium is passing through jacket or coil for maintain the reaction temperature. A reactor operates at a constants temperature, then that is called as the isothermal reactor. If any exothermic or endothermic reactions are involved in the reactor, the temperature of the reactions mixture varies with time and we need to develop the energy balance equation for this non-isothermal reactor. In adiabatic reactor, no interchange of heat occurs between the system and surroundings. Thus no heating and cooling medium are required. A chemical reactor is a vessel where reactions are carried out purposely to produce products from reactants by means of one or more chemical reactions. A chemical reactor may be characterized by the mode operation according to the flow condition. In this experiment, the Continuous Stirred Tank Reactor has been used to conduct a chemical process. OBJECTIVE To determine the effect of temperature on reaction rate constant, k for batch reaction and determine the activation energy of saponification.

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THEORY The continuous stirred tank reactor or back mix reactor is a very common processing unit in chemical and polymer industry. Its names suggest, it is a reactor in which the contents are well stirred and uniform throughout. The CSTR is normally run at steady state, and usually quite well mixed. The CSTR is generally modeled as having no spatial variations in concentrations, temperature, or reaction rate throughout the vessel. Since the temperature and concentration are identical everywhere within the reaction vessel, they are the same at the exits point as they are elsewhere in the tank.

Figure 1 : The Continuous Stirred-Tank Reactor Assumptions in CSTR: 1) Operate at steady state therefore,

(Steady state is a situation, in which all the state variables remain constant despite parallel processes trying to change them.) 2) Well mixed therefore rA is the same throughout the reactor,

3) Rearranging the generation 3

4) In terms of conversion

Reactor Sizing Given –rA as a function of conversion, -rA = f(X), one can size any type of reactor. It can be done by constructing a Levenspiel Plot which FAo / -r A or 1 / -rA as a function of X. For FAo / -r A vs. X, the volume of a CSTR can be represented as the shaded area in the Levenspiel Plots shown below:

Figure 2: The plot of CSTR volume

Effect of temperature on rate of reaction 4

Increasing the temperature can cause the rate of reaction also increase. An increase of 10 oC typically doubles the rate of a reaction. In a chemical reaction, at low temperatures the molecules collide with each other, but bounce apart. If, however, molecules collide at high temperatures, bonds may be broken and new are molecules formed. Collision theory states that three conditions must be met for a reaction to occur: 

Molecules must collide with one another



Molecules must have the right orientation



Molecules must have sufficient energy

Every reaction has an energy barrier. The fact that a reaction increases with increasing temperature suggests that only molecules with sufficient energy are able to react. The energy barrier or minimum energy a molecule must possess to overcome this barrier is called activation energy (Ea) which is can be shown in Arrhenius Law Equation: k = A * e-Ea/RT 

k



Ea = The activation energy



R = The gas constant



T = Temperature in Kelvin



A = Frequency factor constant or also known as pre-exponential factor or Arrhenius

= The rate constant

factor. APPARATUS AND MATERIALS 1. Continuous-stirred Tank Reactor (Model: BP100) 2. Conical flask 3. 50 mL burette 4. 100 mL measuring cylinder 5. 0.25 M Hydrochloric Acid 6. 0.1 M Sodium Hydroxide 7. 0.1 M Ethyl Acetate 8. Phenolphthalein 5

Motor stirrer

CSTR

Drain valve

V2

V1

6

Tank 1

Pump 1

V3

V4 Pump 2

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PROCEDURE General Start-up Procedure 1. The following solutions were prepared: a) 20 L of sodium hydroxide, NaOH (0.1 M). b) 20 L of ethyl acetate, Et(Ac) (0.1 M). c) 1 L of hydrochloric acid, HCl for quenching (0.25 M). 2. All valves were initially closed. 3. Feed tanks were charged: a) Charge port caps were opened for tanks T1 and T2. b) NaOH solution was carefully poured into vessel T2 and Et(Ac) solution into vessel T1. c) The charge port caps were closed for both tanks. 4. The power was turned on for the control panel. 5. The heater was not switched on until it was fully submerged in the liquid. Liquid level was maintained above the heater to avoid damage to the heater. 6. The stirrer assembly was secured properly to avoid damage to the mechanical seal. 7. All tubings were inspected periodically for leakage and worm out. Leakage might cause damage to equipment by corrosive reactants. Experimental Procedures 1. Pump P1 was switched on to pump 1.25 L of 0.05 M Ethyl Acetate, Et(Ac), from the feed tank into reactor. Pump P1 was switched on. 2. Pump P2 was switched on to pump 1.25 L of 0.05 M Sodium Hydroxide, NaOH, into the reactor. Pump P2 was stopped when 2.5 L of total volume was reached. 8

3. The stirrer and the heater were switched on and temperature was set to be . The timer was started. 4. After 1 minute of reaction, 50 mL of sample was collected from the reactor. The sample was titrated with sodium hydroxide, NaOH. 5. Step 4 was repeated for reaction times of 5, 10, 15, 20, and 25 minutes. 6. The experiment was repeated for temperature of . Titration Procedures 1. 10 mL of 0.25 M hydrochloric acid, HCl, was prepared in a conical flask. 2. 50 mL sample collected was added to the conical flask to quench the saponification reaction. 3. 3 drops of phenolphthalein were added to the conical flask as indicator. 4. The mixture was then titrated with 0.1 M NaOH until it turned light pink (neutralized). 5. The volume of NaOH used was recorded.

General Shut-down Procedures 1. The cooling water valve from main pipe was kept open to allow the cooling water to continue flowing. 2. Both pumps P1 and P2 were switched off. The stirrer and heater were switched off to let the equipment to cool down to room temperature. 3. Cooling water valve from main pipe was closed. 4. The power for control panel was turned off. 5. After each experiments, the reactor tubings were cleaned properly as NaOH and Et(Ac) are corrosive and could damage the tubings. RESULTS For temperature 30oC Table 1 : The volume of NaOH solution titrated at 30oC. Time (min)

Volume of NaOH

CA (mol/L)

used for titration 9

1/CA (L/mol)

(mL) 16 17.8 18.1 19.0 19.7 20.5

1 5 10 15 20 25

0.018 0.014 0.014 0.012 0.011 0.009

55.556 69.444 72.464 83.333 94.340 111.111

Figure 1: graph of 1/CA vs time Value of k based on the slope = 2.1359 L/mol.min and the order of reaction is 2nd order. For temperature 40oC Table 2 : The volume of NaOH solution titrated at 40oC. Time (min)

Volume of NaOH

CA (mol/L)

1/CA (L/mol)

used for titration 1

(mL) 17.5

0.0150 10

66.667

5 10 15 20 25

18.0 18.7 19.0 19.4 19.6

0.0140 0.0126 0.0120 0.0112 0.0108

71.429 79.365 83.333 89.286 92.593

Figure 2: graph of 1/CA vs time Value of k based on the slope = 1.0987 L/mol.min and the order of reaction is 2nd order. Table 3 : Calculated values of k, -rA, ln k and 1/T Temperature, T

Specific

Rate of

(K)

reaction rate

reaction, -rA

constant, k

(mol/min.L)

(L/mol.min) 2.1359 1.0987

0.0007 0.0002

303 313

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ln k (L/mol.min)

1/T (K-1)

0.767 0.094

0.0033 0.0032

Figure 3 : Graph of ln k vs 1/T SAMPLE OF CALCULATIONS To calculate CA at 30OC at 1 min Volume of unreacted quenching HCl, V1 : V1 = (CNaOH / CHCl) x /Volume of titrated NaOH = ( 0.1 mol L-1 / 0.25 mol L-1 ) x 16 mL = 6.4 mL Volume of HCl reacted, V2 : V2 = VHCl - V1 = 10 mL - 6.4 mL = 3.6 mL Moles of reacted HCl, n1 : 12

n1 = CHCl x V2 = 0.25 mol/L x 3.6 mL x 1 L / 1000 mL = 0.0009 mol Moles of unreacted NaOH in sample, n2 : N 2 = n1 = 0.0009 mol Concentration of unreacted NaOH, CNaOH : CNaOH unreacted = n2 / volume sample = 0.0009 mol/ 0.05 L = 0.018 mol/ L 1/CA = 1/0.018 L/mol = 55.556 To find specific reaction rate constant, k From the slope of the graph.

To calculate the rate of reaction, -rA -rA = kCA2 = (2.1359 L/mol.min) x (0.018 mol/L)2 = 0.0007 mol/L.min To calculate the activation energy from the reaction Ea From the graph ln k vs 1/T, the equation for the best fit of the data is y = 6730x - 21.442 ln k = 6730 (1/T) - 21.442 and the slope of the line given is - Ea/R = 6730K Ea = -6730 K (8.314 J/mol.K) = -55953.22 J/mol 13

Activation energy from Arrhenius equation ln (k2/k1) = E/R (1/T1-1/T2) ln (1.0987 /2.1359)/ (1/303 - 1/313) = E/(8.314) E = -52415.79 J/mol DISCUSSION NaOH + Et(Ac)

→Na(Ac) + EtOH

The experiment was carried out by using special NaOH and Et(Ac). Order of the reaction is referring to the powers of the concentration which are raised in the kinetic law. Moreover, rate of reaction for NaOH or any species as reactant could be found from the graph plotted using data. Firstly, the order should be guess between zero, first or second order. The rate law equation of -rA can be modified into straight line equation y= mx+c. From the result and calculations, the data was fitted to second order reaction. Hence the rate law for both experiments is -rA= kCA2.The slopes of the graphs which represent the specific reaction rate constant, k could also be obtained. All the calculated values as shown in table 3. For the first temperature of 30oC, the value of k is 2.1359 L/mol/min whereas 1.0987 L/mol.min for temperature 40oC. Furthermore, the value of -rA that we calculated also decreasing as temperature increasing which is 0.0007 mol/L/min at 30oC and 0.0002 mol/L.min at 40oC. Based on the equation, the value of rate constant should be increasing as the temperature increasing. The errors in result is probably due to reading error at time 5 to 10 minutes at 30 oC due to miscalculation or error that occurred during titration process. During titration, the volume of NaOH needed to neutralize should be taken as soon as the sample turns into pink color. Result could be affected if volume taken is slightly late than it should be. Arrhenius Equation has been use to give the temperature behavior of the most reaction rate constant within experimental accuracy over temperature range. A graph (Figure 3) was plotted to find the value of -E/R. From the graph, the slope shown a straight line. The slope indicated value of -E/R. The value of ln k2 is higher that ln k1 which cause the positive value of slope. Based on the equation obtained from the graph ln k vs 1/T , y = 6730x - 21.442, the value of activation energy, E could also be calculated. Activation energy is important for the molecules to use energy to complete the reaction. The value of activation energy obtained from this 14

experiment is -55.953 kJ/mol. Another way could also be used to calculate activation energy, by By using from Arrhenius Equation which is ln (k 2/k1) = E/R (1/T1 - 1/T2). The E value calculated is -52.415 kJ/mol. Both amount of activation energy are not too large for a reaction. The larger the activation energy, the more temperature sensitive in the rate of reaction. Thus this shows that the temperature is not too sensitive in the rate of reaction. CONCLUSION After all experiment has been done, first conclusion that can be made from this experiment was this reaction was elementary and it is 2nd order. We concluded this by the graph 1/Ca versus t(time) that has been plotted in Figure 1, 2 and 3. We get straight line graph that has a positive slope value. Our second conclusions is the value of k is dependent on temperature and the rate constant will only constant for a constant temperature. When the temperatures increase the value of reaction rate also increase. This satisfied with the Arhenius’s Equation. We also conclude that activation energy is constant for reactions that have a same concentration but different temperatures. This has been proven by the equation ln (k2/k1)= E/R (1/T1-1/T2) and the graph that we have plotted and we get almost the same values for both experiments. For overall experiment, it can be considered as succeed because the effect of temperature and reaction can be determined. This is because, the k values obtained were in positive values. RECOMMENDATION 1

This experiment should be done in 4 different temperatures in order to get more accurate results. Yet ours just in 3 different temperatures due to the maintenance problem occurred during the experiment.

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The apparatus should be clean thoroughly by using deionized water to avoid contamination or defect in titration process.

3

The color of light pink that obtained in titration must be same for entire experiment to avoid any variation in result.

REFERENCE 1) Stenstrom, M. K. (2003). Fundamentals of Chemical Reactor Theory. Los Angeles: Civil and environmental Engineering Department. 2) Fogler, H. (2010). Continuous-Flow Reactors. In Essentials of Chemical Reaction Engineering: Mole Balances (p. 4). Prentice Hall. 15

3) Aliff, (2006). Continuous Stirred Tank Reactor. Retrieved May 13, 2015 from http://www.scribd.com/doc/36549783/Continuous-Stirred-Tank-Reactor-Cstr-2#scribd 4) Reaction kinetic studies in a mixed flow reactor. Retrieved May 13, 2015 from http://solve.nitk.ac.in/dmdocuments/theory_Mixed%20Flow%20Reactor.pdf 5) Zarif, M. (2010). Continuous Stirred Tank Reactor. Retrieved May 13, 2015 from http://www.scribd.com/doc/181148523/CONTINUOUS-STIRRED-TANK-REACTORLAB-REPORT#scribd APPENDIX

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