1.0 INTRODUCTION

A tubular reactor is a vessel through which flow is continuous, usually at steady state, and configured so that conversion of the chemicals and other dependent variables are functions of position within the reactor rather than of time. Flow in tubular reactors can be laminar, as with viscous fluids in small-diameter tubes, and greatly deviate from ideal plug-flow behavior, or turbulent, as with gases. There are tubular flow reactors applications which are:

Large-scale reactions Fast reactions Homogeneous or heterogeneous reactions Continuous production High-temperature reactions

In an ideal plug flow reactor, a pulse of tracer injected at the inlet would not undergo any dispersion as it passed through the reactor and would appear as a pulse at the outlet. The degree of dispersion that occurs in a real reactor can be assessed by following the concentration of tracer versus time at the exit. This procedure is called the stimulus-response technique.

High temperature reactions Residence Time Distribution (RTD) analysis is a very efficient diagnosis tool that can be used to inspect the malfunction of chemical reactors. Residence time distributions are measured by introducing a non-reactive tracer into the system at the inlet. The concentration of the tracer is changed according to a known function and the response is found by measuring the concentration of the tracer at the outlet. The selected tracer should not modify the physical characteristics of the fluid and the introduction of the tracer should not modify the hydrodynamic conditions. In general, the change in tracer concentration will either be a pulse or a step.

The residence time distribution of a real reactor deviated from that of an ideal reactor, depending on the hydrodynamics within the vessel. A non-zero variance indicates that there is some dispersion along the path of the fluid, which may be attributed to turbulence, a non-uniform velocity profile, or diffusion. If the mean of the E (t) curve arrives earlier than the expected time τ  it indicates that there is stagnant fluid within the vessel. If the residence time distribution curve shows more than one main peak it may indicate channeling, parallel paths to the exit, or strong internal circulation.

2.0 OBJECTIVES

1. To study the effects of residence time on a reaction by using a Plug Flow Reactor. 2. To determine the reaction rate constant by saponification reaction between Sodium

Hydroxide, NaOH and Ethyl Acetate, Et(Ac).

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3.0 THEORY

Rate of Reaction and Rate Law

Simply put, rate of reaction can be roughly defined as the rate of disappearance of reactants or the rate of formation of products. When a chemical reaction is said to occur, a reactant diminishes and a product produced. This is what constitutes a chemical reaction. For example:

aA + bB→cC+dD

Where A and B represent reactants, while C and D represent products. In this reaction, A and B is being diminished and C and D is being produced. Rate of reaction, concerns itself with how fast the reactants diminish or how fast the product is formed. Rate of reaction of each species corresponds respectively to their stoichiometric coefficient. As such:

−r A

a=

−r B

b=rCc

=rD

d

The negative sign indicates reactants.

A usual equation for rA is:

−r A=k C AαCB

β

Wherek - Reaction rate constantCA - concentration of A speciesCB - concentration of B speciesα- stoichiometric coefficient of Aβ- stoichiometric coefficient of B

Conversion

Taking species A as the basis, the reaction expression can be divided through the stoichiometric coefficient of species A, hence the reaction expression can be arranged as follows:

A+ baB+ c

aC+ d

aD

Conversion is an improved way of quantifying exactly how far has the reaction moved, or how many moles of products are formed for every mole of A has consumed. Conversion XA is the number of moles of A that have reacted per mole of A fed to the system. As seen below:

X A=moles of A reactedmolesof A fed

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Plug Flow Reactors

This reactor is also known as tubular flow reactor, which is usually used in industry complementary to CSTR. It consists of a cylindrical pipe and is usually operated at steady state. For analytical purposes, the flow in the system is considered to be highly turbulent and may be modeled by that of a plug flow. Therefore, there is no radial variation in concentration along the pipe.

In a plug flow reactor, the feed enters at one end of a cylindrical tube and the product stream leaves at the other end. The long tube and the lack of provision for stirring prevent complete mixing of the fluid in the tube. Hence the properties of the flowing stream will vary from one point to another.

In an ideal tubular flow reactor, which is called plug flow reactor, specific assumptions are made regarding the extent of mixing:

1. No mixing in the axial direction2. Complete mixing in the radial direction3. A uniform velocity profile across the radius.

Tubular reactors are one type of flow reactors. It has continuous inflow and outflow of materials. In the tubular reactor, the feed enters at one end of a cylindrical tube and the product stream leaves at the other end. The long tube and the lack stirring prevent complete mixing of the fluid in the tube.

Residence Time Distribution Function

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.

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4.0 MATERIAL AND APPARATUS

Plug Flow Reactor (Model: BP101) is used as it has been properly designed for students' experiment on chemical reactions in liquid phase under isothermal and adiabatic conditions. Included in the unit is a jacketed plug flow reactor; individual reactant feed tanks and pumps, temperature sensors and conductivity measuring sensor. Apart from that, there were also some laboratory apparatus involved such as :

burette conical flask  measuring cylinder  ph indicator  Beakers

Among the chemicals used are : 0.1 M Sodium Hydroxide, NaOH 0.1 M Ethyl Acetate, Et(Ac) 0.1 M Hydrochloric Acid, HCl De-ionised water

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5.0 PROCEDURES

General Startup Procedures 1. 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 about200 rpm to ensure homogeneous water jacket temperature.

Experiment Procedures 1. 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.

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Back Titration Procedures

1. 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.

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6.0 RESULTS

Calibration Curve

Conversion Solution Mixture Concentration of NaOH (M)

Conductivity (ms/cm)0.1 M NaOH 0.1 M Et(Ac) Water

0 % 100 mL - 100 mL 0.0500 7.66025 % 75 mL 25 mL 100 mL 0.0375 5.40050 % 50 mL 50 mL 100 mL 0.0250 2.90075 % 25 mL 75 mL 100 mL 0.0125 1.300100 % - 100 mL 100 mL 0.0000 0.157

Flowrate of NaOH and Et(Ac) (mL/min)

Conductivity Volume of NaOH (mL)Q1 Q2

300 8.2 6.9 10.2250 8.5 7.0 8.5200 8.7 7.1 8.7250 7.7 6.0 9.5100 7.3 5.4 12.050 6.2 4.7 11.5

Residence Time, τ (min)

Conversion, X (%) Reaction rate constant, k

(L.mol/min)

Rate of Reaction, -rA

(mol.L/min)

6.67 70.4 3.567 3.125 x 10-3

8.00 70.2 2.945 2.615 x 10-3

10.00 70.0 2.333 2.0997 x 10-3

13.33 70.0 1.750 1.575 x 10-3

20.00 70.2 1.178 1.046 x 10-3

40.00 70.2 0.589 5.230 x 10-4

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5 10 15 20 25 30 35 40 4569.7

69.8

69.9

70

70.1

70.2

70.3

70.4

70.5

Residence Time, (min)

Conv

ersio

n, X

(%)

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7.0 SAMPLE CALCULATION

Residence time

τ = reactor volume , liter(V )total flow rate ,L /min (V 0)

Total flow rate, V0 = flow rate of NaOH + flow rate of Et(Ac)= 300 ml/min NaOH + 300 ml/min of Et(Ac)= 600 ml/min= 0.6 L/min

∴ τ= 4 L0.6 L/min

=6.6667min

Conversion

Moles of NaOH reacted, n1

n1 = concentration of NaOH x volume of NaOH titrated = 0.1M x 0.0102 L= 0.00102 mol

Moles of unreacted HCl, n2

Moles of unreacted of HCl = moles of reacted NaOH n1 = n2 = 0.00102 mol

Volume of unreacted HCl, V1

V 1=n2

concentration HCl quenchV 1=

0.00102mol0.25mol/L

V 1=0.00408 L

Volume of HCl reacted, V2

V2 = total volume of HCl – V1

= 0.01 L – 0.00408 L= 0.00592 L

Moles of reacted HCl, n3

n3 = concentration HCl x V2

= 0.25 mol/L x 0.00592 L= 0.00148 mol

Moles of unreacted NaOH, n4n4 = n3 = 0.00148 mol

Concentration of unreacted NaOH

CNaOH unreacted=n4

volume sampleCNaOH unreacted=

0.00148mol0.05 L

CNaOH unreacted=0.0296 M

Xunreacted

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Xunreacted=concentration of NaOH unreacted

concentration of NaOHXunreacted=

0.0296mol /L0.1mol /L

Xunreacted=0.296

Xreacted

Xreacted = 1 - Xunreacted

= 1 - 0.296= 0.704

Conversion for flow rate 300 ml/min, 0.704 x 100% = 70.4%

Reaction Rate Constant, k

k=V 0

V TFRCA 0( X1−X )

Flow rate = 300 ml/minV0 = total inlet flow rate = 0.6 L/minVTFR = volume for reactor = 4 LCA0 = inlet concentration of NaOH = 0.1MX = 0.704

k= 0.6 L /min

4 L( 0.1molL )(

0.7041−0.704 )

k=3.567L

min.mol

Rate of Reaction, -rA

−r A=k (CA 0)2(1−X )2−r A=

3.567 Lmin

.mol(0.1mol /L)2(1−0.704)2

−r A=0.003125molmin

.L

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8.0 DISCUSSION

Plug Flow Reactor (PFR) is a type of reactor that consists of a cylindrical pipe and is usually operated at steady state. In a plug flow reactor, the feed enters at one end of a cylindrical tube and the product stream leaves at the other end. The long tube and the lack of provision for stirring prevent complete mixing of the fluid in the tube. Hence the properties of the flowing stream will vary from one point to another. The fluid in PFR is considered to be thin, unmixed layer of volume segments or 'plugs', hence the name.

As seen in figure 2, the solution in the tube is treated as a series of layers of volume segments that are unmixed with the segment before and after it. Like a series of plugs, stacked together in a pipe. In this particular experiment, the solutions used are NaOH and Et(Ac). These two solutions react together in the PFR to complete saponification reaction. The main objective of this particular experiment is to study the effect of residence time on the performance of this reactor, the PFR. To do that, of course, residence times have to be manipulated throughout the experiment and the effects of each one is studied. Residence time, in this particular experiment, is varied by the means of changing the flow rates of the feed solutions. This is shown by the formula:

Residence time

τ = reactor volume , liter(V )total flow rate ,L /min (V 0)

After, the experiment is conducted, raw data consisting inlet flow rates, conductivity value and volume of NaOH used in the titration process are tabulated in the Result Section. From the raw data obtained, a series of calculations were made, as seen in the Sample of Calculation section, and the values of residence times, conversion of the reactions, reaction rate constants and rate of reactions were determined. These values are tabulated in the Result section.

As the data of residence time and conversion is plotted into a graph. The reason for plotting a graph consisting these two parameters is so that the effects of residence time can be studied. Conversion is a property that shows how much of the reaction has taken place. Hence, by comparing this property with the residence time parameter, one can analyze the effects of increasing residence time to the reaction itself. By analyzing the graph and tabulated data, it can be clearly seen that the conversion of the reaction remains fairly constant with the increasing residence time.

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Therefore, one can postulate that residence time is not a factor for reaction conversion, as far as plug flow reactors are concerned. One can also postulate that the reason for this phenomenon is that the PFR lacks a good mixing process. Since the PFR is designed not to stir the solution vigorously to maximize mixing process, the conversion of the reaction by using PFR is fairly low.

The experiment also aims to evaluate the reaction rate constants and rate of reaction values of the reaction. Both of these properties have been determined in the result section.

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9.0 CONCLUSION

The experiment was conducted with several objectives in mind. The first one is to carry out a saponification process between Sodium Hydroxide, NaOH and Ethyl Acetate, Et (Ac). By using a Plug Flow Reactor, PFR, these two substances were flowed into the reactor, mixed and let to react for a certain period of time. By doing that, saponification process was completed. The experiment also targets to determine the reaction rate of this particular reaction. This was also done by calculating the reaction rate as seen in the Sample Calculation section. Lastly, the main objective of this experiment is to study the relationship between the residence time and the conversion of the reactants. This relationship was successfully studied and graphed in the Result section.

10.0 RECOMMENDATIONS

1. It is better to time the sample well so that time-wasting in taking samples can be reduced or, if possible, avoided.

2. All valves should be properly placed before the experiment started.3. Flow rates should be constantly monitored so that it remains constant throughout the

reaction, as needed.4. Titration should be immediately stopped when the indicator turned pink.5. Pumps should never be run dry.

11.0 REFERENCES

1. Fogler, H.S (2006). Elements of Chemical Reaction Engineering (3rd Edition). PrenticeHall.

2. Levenspiel, O. (1999). Chemical Reaction Engineering (3rd Edition). John Wiley.3. The Plug Flow (Retrieved from http://www.konferenslund.se/p/L16.pdf on 18th

October   2013 )4. Reaction Kinetics (Retrieved from http://smk3ae.files.wordpress.com/2007/10/reaksi-

kinetik.pdf on the 18th October 2013)

12.0 APPENDICES

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