ABSTRACT

This is the experiment of plug flow reactor (PFR). The reaction to be studied is the saponification reaction of ethyl acetate Et(Ac) and sodium hydroxide NaOH. The calibration curve of conversion vs conductivity is also perform in this experiment. The objectives of this experiment are to perform saponification reaction between NaOH and Et(Ac), to determine the reaction rate constant and to study the effect of residence time on the conversion. This experiment is done by varying the residence time by manipulating the flow rate of the system. The flow rate of both feeds, Et(Ac) and NaOH is the same and varied from 300mL/min to 50 mL/min. From the result, the percentage of conversion of NaOH is increasing with the increase of residence time. When the total flowrate of the residence is 6.6667 mins, the conversion is 89.4%. Then as the total flow rate provided for the system is decreased, the residence time is increasing and that makes the conversion of NaOH to increase as well. When the total flowrate is 40 mins, the conversion is increasing to 96.8%. The lower flow rate gives a higher residence time and helps increasing the conversion of the reaction.

INTRODUCTION

Plug Flow Reactor (PFR) is the the third general type of reactor. PFRs are used to model the chemical transformation of compounds as they are transported in systems resembling "pipes". The "pipe" can represent a variety of engineered or natural conduits through which liquids or gases flow. The system may be either contained or open. An ideal plug flow reactor has a fixed residence time. A real plug flow reactor has a residence time distribution that is a narrow pulse around the mean residence time distribution. A typical plug flow reactor could be a tube packed with some solid material. Sometimes the tube will be a tube in a shell and tube heat exchanger.Plug flow reactors are used for the applications of large scale reactions, fast reactions, homogeneous or heterogeneous reactions, continuous production and high temperature reactions. The advantages of Plug Flow Reactors are they have a high volumetric unit conversion, run for long periods of time without maintenance, and the heat transfer rate can be optimized by using more, thinner tubes or fewer, thicker tubes in parallel. Disadvantages of plug flow reactors are that temperatures are hard to control and can result in undesirable temperature gradients. PFR maintenance is also more expensive than CSTR maintenance. The Pug Flow Reactor (Model : BP 101) has been designed for student demonstration on reactor ad reaction behavior. This mobile bench type of unit consists of mainly a reactor (PFR) made of flexible tubing, hot water jacket with a variable speed stirrer, a heating system, reactant feeding systems and instruments for the measurement and control of reaction temperature and reactants flow rates. Students will conduct continuous saponification reaction of ethyl acetate and sodium hydroxide. A conductivity measurement is provided and students will relate the conductivity value to extent of reaction. Students will study the effects of residence time and reaction temperature on the reaction rate constant. Students will also conduct RTD studies on the reactor.

OBJECTIVES

To perform saponification reaction between NaOH and Et(Ac)To determine the reaction rate constantTo study the effect of residence time on the conversion

THEORY

In the simplest case of a PFR model, several key assumptions must be made in order to simplify the problem. Note that not all of these assumptions are needed. However the removal of these assumptions does increase the complexity of the problem. The PFR model can be used to model multiple reactions as well as reactions involving changing temperatures, pressures and densities of the flow. Although these complications are ignored in what follows, they are often relevant to industrial processes.Assumptions:a) plug flowb) steady statec) constant density (reasonable for some liquids but a 20% error for polymerizations valid for gases only if there is no pressure drop, no net change in the number of moles, nor any large temperature change)d) single reaction occurring in the bulk of the fluid (homogeneously).A material balance on the differential volume of a fluid element, or plug, on species i of axial length dx between x and x + dx gives:[accumulation] = [in] [out] + [generation] [consumption]

Accumulation is 0 under steady state therefore, the above mass balance can be rewritten as follows:Fi(x) Fi(x+dx) + Atdxvir = 0where:x is the reactor tube axial position, mdx the differential thickness of fluid plugthe index i refers to the species iFi(x) is the molar flow rate of species i at the position x, mol/sD is the tube diameter, mAt is the tube transverse cross sectional area, m2 is the stoichiometric coefficient, dimensionlessr is the volumetric source/sink term (the reaction rate), mol/m3s.The flow linear velocity, u (m/s) and the concentration of species i, Ci (mol/m3) can be introduced as:

On application of the above to Equation 1, the mass balance on i becomes:

When like terms are cancelled and the limit dx 0 is applied to Equation 2 the mass balance on species i becomes:

The temperature dependence of the reaction rate, r, can be estimated using the Arrhenius equation. Generally, as the temperature increases so does the rate at which the reaction occurs. Residence time, is the average amount of time a discrete quantity of reagent spends inside the tank.

Assume:a) Isothermal conditions, or constant temperature (k is constant)b) Single irreversible reaction (A = 1)c) First order reaction (r = k CA)After integration of Equation 3 using the above assumptions, solving for CA(x) we get an explicit equation for the concentration of species A as a function of position:

where CA0 is the concentration of species A at the inlet to the reactor, appearing from the integration boundary condition.ConversionOne of the reactants is choosed as the basis of calculation and the other species involved is related in the reaction to this basis. Consider the general equation aA + bB cC + dDWe will choose A as our basis of calculation.A+ B The basis of calculation is most always the limiting reactant. The conversion of species A in a reaction is equal to the number of moles of A reacted per mole of A fed.Design equation for Plug Flow Reactor: = rAIn terms of conversion FAo = -rA V = FAo The volume of PFR can be represented as shaded area under the curve.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.

APPARATUS/MATERIALS

Apparatus : burette conical flask measuring cylinder ph indicator beakersAmong the chemicals used are : 0.1 M Sodium Hydroxide, NaOH 0.1 M Ethyl Acetate, Et(Ac) 0.1 M Hydrochloric Acid, HCl 0.25 M Hydrochloric Acid, HCl De-ionised water

METHODOLOGY/PROCEDURES

A. Preparation of Calibration Curve for Conversion vs. ConductivityThe 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) + EtOHThe following procedures will calibrate the conductivity measurements of conversion values for the reaction between 0.1 M ethyl acetate and 0.1 M sodium hydroxide:

Procedures:1. The following solutions is prepared:a) 1 liter of sodium hydroxide, NaOH (0.1 M)b) 1 liter of sodium acetate, Na(Ac) (0.1 M)c) 1 liter of deionised water, H2O2. The conductivity and NaOH concentration for each conversion values are determined by mixing the following solutions into 100 ml of deionised water:a) 0% conversion : 100 ml NaOHb) 25% conversion : 75 ml NaOH + 25 ml Na(Ac)c) 50% conversion : 50 ml NaOH + 50 ml Na(Ac)d) 75% conversion : 25 ml NaOH + 75 ml Na(Ac)e) 100% conversion : 100 ml Na(Ac)

B. Back Titration Procedures for Manual Conversion DeterminationIt is advisable to carry out manual conversion determination on experiment samples to verify theconductivity measurement values. The following procedures will explain the method to carry out back titration on the samples. 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.

Procedures:1. A burette is filled up with 0.1 M NaOH solution.2. 10 ml of 0.25 M HCl is measured in a flask.3. A 50 ml sample from the experiment is obtained and immediately added to the HCl in the flask to quench the saponification reaction.4. A few drops of pH indicator is added into the mixture.5. The mixture is titrated with NaOH solution from the burette until the mixture is neutralized. The amount of NaOH titrated is recorded.

General Startup Procedures1. 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.Experiment Procedures1. 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.

Back Titration Procedures1. 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.

RESULT

Reactor Volume: 4LConcentration of NaOH in feed tank: 0.1M (20L)Concentration of Et(Ac) in feed tank: 0.1M (20L)

Flow rate of NaOH(mL/min)

FT 01Flow rate of Et(Ac)(mL/min)

FT 02 Total flow rate of solutions, V0(mL / min)Residencetime, t(min)Outlet conductivity(mS/min)

Conversion,X (%)Reaction Rate Constant(L/mol.min)Rate of Reaction(mol/L.min)

Vol of NaOH Titrated(mL)

Q101Q202

3003006006.666711.29.889.42.10852.3691 x 10-419.7

25025050089.68.389.82.20102.567 x 10-419.9

200200400108.37.292.83.22221.6704 x 10-421.4

15015030013.33337.56.393.63.65631.4976 x 10-421.8

100100200206.85.693.63.65631.4976 x 10-421.8

5050100405.85.896.87.56257.744 x 10-523.4

ConversionSolution MixturesConcentration of NaOH (M)Conductivity(mS/ cm)

0.1M NaOH0.1M Na(Ac)H2O

0%100mL-100mL0.050010.7

25%75mL25mL100mL0.03759.7

50%50mL50mL100mL0.02507.5

75%25mL75mL100mL0.01255.6

100%-100mL100mL0.00004.0

SAMPLE CALCULATIONS

Residence Time:

Calculation for flow rates of 300 ml/min:Total flow rate, Vo = Flow rate of NaOH + Flow rate of Et(Ac) = 300 mL/min NaOH + 300 mL/min Et(Ac) = 600 mL/min= 0.6 L/minResidence Time, = 6.6667 minsConversion:Calculation for flow rates of 300 ml/min :Moles of reacted NaOH, n1n1= Concentration NaOH x Volume of NaOH titrated= 0.1 M x 0.0197 L = 0.00197 mole

Moles of unreacted HCl, n2= Moles of reacted NaOH, n1 n2= 0.00197 mole

Volume of unreacted HCl, V1V1= = = 0.00788 L

Volume of HCl reacted, V2 V2= Total volume HCl V1 = 0.01 0.00788= 0.00212 L

Moles of reacted HCl, n3n3= Concentration HCl x V2= 0.25 x 0.00212 = 0.00053 mole

Moles of unreacted NaOH, n4 n4 = n3 = 0.00053 mole

Concentration of unreacted NaOHCNaOH unreacted = = = 0.0106 M

XunreactedXunreacted = = = 0.106Xreacted Xreacted = 1 - Xunreacted = 1 - 0.106 = 0.894

Conversion for flow rate 300 mL/min,0.894 x 100% = 89.4 %

Reaction Rate Constant, k

For flow rates of 300 ml/min :V0 = Total inlet flow rate = 0.6 L/minVTFR = Volume for reactor = 4 LCAO = inlet concentration of NaOH = 0.1 MX = 0.894 = 2.1085 L.mol/min

Rate of Reaction, -rA

-rA = k (CA0)2 (1-X)2For flow rates of 300 ml/min :-rA = 2.1085 (0.1)2 (1-0.894)2 = 2.3691 x 10-4 mol.L/min

DISCUSSION

For the preparation of calibration curve for conversion vs conductivity, the NaOH solution and Na(Ac) is mixed to give a value of conversion from 0% which means a pure NaOH solution and also 100% conversion which means a pure Na(Ac) solution produced. The curve obtained is shown in the figure above. From the graph, for 0% conversion, the conductivity value is 10.7 mS/cm. The 100% conversion gives the conductivity value of 4.0 mS/cm. The higher the conversion of NaOH, the lower the conductivity value obtained based on the result. The best fit lined constructed from the result gives the line of y = -0.07x + 11. Thus, the slope obtained is -0.07 while the y-intercept is 11 mS/cm. The decrease in the conductivity value of the solution is because of the decrease in the ionic activity of a solution in term of its capacity to transmit current. As the electrical current is transported by the ions in solution, the conductivity increases as the concentration of ions increases. Thus, the higher conversion of the NaOH gives a lower ionic activity for the preparation of the calibration curve result. From the experiment, the conductivity of the reactant also decreases as the conversion increases. The 89.4% conversion gives a conductivity value of 11.2mS/cm for the NaOH and 9.8mS/cm for the Et(Ac). When the conversion increases the conductivity value is decreasing. The highest conversion which is 96.8% from the flowrate of 50 mL/min for each feeds gives the lowest conductivity value of 5.8 mS/cm.

For the experiment of investigating the effect of conversion on residence time, the reaction to be studied is the saponification reaction of ethyl acetate Et(Ac) and sodium hydroxide NaOH. The reaction is 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. The flowrate of both feeds are also the same throughout the experiment. The flowrates is then varied from 300mL/min to 50 mL/min in order to gives variation in the residence time. The residence time is determined from the equation:Residence Time, The results obtained from the experiment is shown in the graph above. From the graph, we can see that the percentage of conversion of NaOH is increasing with the increase of residence time. When the total flowrate of the residence is 6.6667 mins, the conversion is 89.4%. Then as the total flow rate provided for the system is decreased, the residence time is increasing and that makes the conversion of NaOH to increase as well. When the total flowrate is 40 mins, the conversion is increasing to 96.8%. The design of a plug flow reactor is usually cylindrical and long. Feed enters the reactor at the inlet and continuously flow and exit at the outlet without the help of agitation or stirrer. Thus, the composition in the reactor will be different from one point to another also. The conductivity in unit helps in order to know the reaction in the reactor stability of the system. From the result, when flowrate is high, the reactants only have a little time to make contact with each other before exiting the outlet. Thus, the lower flowrate helps the conversion to increase as the contact between the reactants is longer before exiting the outlet. As the plug flow reactor has a very low mixing process, thus lowering the flowrate of the or might as well said as increasing the residence time, helps the reaction process to takes place better to gives higher conversion value. As for the reaction rate of the reaction, -rA from the result it is decreasing as the residence time is higher while the rate constant, k is increasing.

CONCLUSION

As a conclusion, the objectives for this experiment is achieved. This is because we get to perform saponification reaction between NaOH and Et(Ac) by using the plug flow reactor unit and then determine the reaction rate constant for each reaction which is different from each flowrate introduced to the system. Besides that we also get to study the effect of residence time on the conversion which is the lower flowrate gives a higher residence time and increasing the conversion in the reaction. This is because the design of the plug flow reactor itself which having a poor mixing process. Thus higher residence time helps increasing conversion in the reaction process. From this experiment, it is also discovered that when the conversion value is high, the conductivity value is decreasing.

RECOMMENDATION

1. It is recommended that the same experiment is conducted by using other type of reactor in order to know which reactor will give a better process for the reaction and their characteristics can be compared.2. It is recommended that the flowrate of the feed to be varied and not constant to know how will it affects the reaction process in the plug flow reactor. 3. It is also recommended to varied the concentration of the feed for example like how would the reaction process go if the feed introduced into the system is not equimolar.

REFERENCES

1. Fogler, H.S (2006). Elements of Chemical Reaction Engineering (3rd Edition). Prentice Hall.2. Levenspiel, O. (1999). Chemical Reaction Engineering (3rd Edition). John Wiley.3. Laboratory Manual Tubular Flow Reactor.4. Reactor Theory and Practice. Plug Flow Reactor (PFR). Retrieved on 27th March from http://www.cs.montana.edu/webworks/projects/stevesbook/contents/chapters/chapter008/section002/blue/page004.html5. Wikipedia. Plug Flow Reactor Model. Retrieved on 27th March from http://en.wikipedia.org/wiki/Plug_flow_reactor_model

APPENDIX

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