Plug Flow Reactor

1.0 Abstract The conduction of this experiment is based on a few targets, namely to carry out saponification reaction between Sodium Hydroxide, NaOH and Ethyl Acetate, Et(AC), to determine the effect of residence time to the reaction's extent of conversion and lastly to evaluate the reaction rate constant of this particular saponification reaction. To achieve these targets, an experiment is finely designed so much so that these targets can be finely met. Such experiment involves using a unit called SOLTEQ Plug Flow Reactor (Model: BP 101), commonly known as PFR, as well as some common laboratory apparatus for titration process. To put it simply, the two solutions Sodium Hydroxide, NaOH and Ethyl Acetate, Et(Ac) were reacted in the PFR and the product is then analysed by the method of titration to determine how well did the reaction go. Hence, the experiment was conducted and the results shows that the amount of conversion of Sodium Hydroxide, NaOH is almost unchanged as residence time increases. Further details can be obtained in the results and discussion sections.

2.0 Introduction

Type of chemical reactors remains a highly discussed subject in chemical process industries worldwide.The reactor is of course, the place where chemical reactions take place. Hence it is arguably the single most important part of any chemical process design. The design of a reactor must be finely tuned so that its mechanisms suit the necesseties of the process that is to be carried. Depends on the nature of the materials in both the feed and of course the products, the reactors may take a wide range of forms. This is why full comprehension of a reactor of a particular design as well as its working mechanisms is very much vital to actually conduct a particular chemical process. In this experiment, the 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. By using this particular unit, students will be capable to conduct the typical saponification reaction between ethyl acetate and sodium hydroxide among the others reaction.

3.0 Objectives This experiment is conducted to study the effects of residence time on a reaction by using a Plug Flow Reactor. Also, reaction rate constant is also to be determined by saponification reaction between Sodium Hydroxide, NaOH and Ethyl Acetate, Et(Ac). 4.0 Theory 4.1 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(or several) diminishes and a product(or several) produced. This is what constitutes a chemical reaction. For example : → 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 :

The negative sign indicates reactants.

A usual equation for rA is :

where k

-

rate constant

CA

-

concentration of A species

CB

-

concentration of B species

α

-

stoichiometric coefficient of A

β

-

stoichiometric coefficient of B

4.2 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 :

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 :

4.3 Plug Flow Reactors This reactor also known as tubular flow reactor whic 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 direction 2. complete mixing in the radial direction 3. 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.

4.4 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.

5.0 Apparatus/Materials The unit used in this experiment is SOLTEQ Plug Flow Reactor (Model: BP101)

SOLTEQ Plug Flow Reactor (Model: BP101)

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

6.0 Methodology/Procedures 6.1 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 about 200 rpm to ensure homogeneous water jacket temperature.

6.2 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. 6.3 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.

7.0 Results Reactor Volume.

: 4L

Concentration of NaOH in the reactor, CNaOH

: 0.1M (2L)

Concentration of NaOH in the feed vessel, CNaOH,f

: 0.1M (2L)

Concentration of HCl quench, CHCl,s

: 0.25 M (0.01L)

Volume of sample, Vs

: 0.05L

Table 1 Residence Outlet Conductivity

Flow Rate of

Flow Rate of

NaOH

Et(Ac)

Time,

(ml/min)

(ml/min)

(min)

300

300

250

Volume of

Q1

Q2

NaOH

5

11.0

9.7

0.3

250

5

9.5

8.4

0.2

200

200

5

8.4

7.4

0.1

150

150

5

7.5

6.4

0.1

100

100

5

6.7

5.5

0.2

50

50

5

5.7

4.7

0.2

Table 2 Residence Time,

Conversion, X,

Reaction Rate Constant,k

Rate of Reaction, -rA

τ, (min)

(%)

(L.mol/min)

(mol.L/min)

6.6667

50.6

1.5365

3.7496 x 10-3

8.0000

50.4

1.2701

3.1246 x 10-3

10.0000

50.2

1.0080

2.4999 x 10-3

13.3333

50.2

0.7560

1.8749 x 10-3

20.0000

50.4

0.5081

1.2500 x 10-3

40.0000

50.4

0.2540

6.2488 x 10-4

8.0 Sample of Calculations 8.1 Residence Time For flow rates of 300 ml/min : ( )

Residence Time, 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/min

Hence, Residence Time,

= 6.6667 min

placed in Table 2

Other residence times were calculated by the same way, and varying the flow rates.

8.2 Conversion For flow rates of 300 ml/min : Moles of reacted NaOH, n1 n1

= Concentration NaOH x Volume of NaOH titrated = 0.1 M x 0.0003 L = 0.00003 mole

Moles of unreacted HCl, n2 Moles of unreacted HCl

= Moles of reacted NaOH n2

= n1

n2

= 0.00003 mole

Volume of unreacted HCl, V1

V1

= = = 0.00012 L

Volume of HCl reacted, V2 V2

= Total volume HCl – V1 = 0.01 – 0.00012 = 0.00988 L

Moles of reacted HCl, n3 n3

= Concentration HCl x V2 = 0.25 x 0.00988 = 0.00247 mole

Moles of unreacted NaOH, n4 n4

= n3 = 0.00247 mole

Concentration of unreacted NaOH

CNaOH unreacted

= = = 0.0494 M

Xunreacted

Xunreacted = = = 0.494 Xreacted Xreacted = 1 - Xunreacted = 1 - 0.494 = 0.506

Conversion for flow rate 300mL/min

0.506 x 100% = 50.6 %

placed in Table 2

Hence, at flow rate 300mL/min of NaOH in the reactor, about 50.6% of NaOH is reacted with Et(Ac). Other conversions were calculated by the same way, and varying the flow rates. 8.3 Reaction Rate Constant,k

(

)

For flow rates of 300 ml/min : V0

= Total inlet flow rate = 0.6 L/min

VTFR

= Volume for reactor =4L

CAO

= inlet concentration of NaOH = 0.1 M

X

= 0.506 ( )(

)

(

) = 1.5365L.mol/min

placed in Table 2

Other Reaction Rate Constants were calculated by the same way, and varying the flow rates.

8.4 Rate of Reaction, -rA -rA = k (CA0)2 (1-X)2 For flow rates of 300 ml/min : -rA

= 1.5365 (0.1)2 (1-0.506)2 = 3.7496 x 10-3 mol.L/min

placed in Table 2

Other Rate of Reactions were calculated by the same way, and varying the flow rates.

Conversion vs Residence Time 60 58 56 54 52 Conversion, % 50 48 46 44 42 40 0

5

10

15

20

25

Residence Time, min

Figure 1

30

35

40

45

9.0 Discussions 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.

Figure 2 : courtesy of wikipedia.com 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 reacts 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,

( ) (

)

From the equation above, it can bes seen that residence time is a function of total flow rates of the feed. Hence, by varying the flow rate of the feed solutions, several residence times can be obtained and the effects of each one, studied.

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 Table 1 of 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 Table 2 of the Result section. As the data of residence time and conversion from table 2 is plotted into a graph, the graph is shown in figure 2. 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 analyse the effects of increasing residence time to the reaction itself. By analysing figure 2, it can be clearly seen that the conversion of the reaction remains fairly constant with the increasing residence time. 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 maximise 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.

10.0 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 beteween the residence time and the conversion of the reactants. This relationship was successfully studied and graphed in Figure 2 of the Result section.

11.0 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.

12.0 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. The Plug Flow (Retrieved from http://www.konferenslund.se/p/L16.pdf on 18th October 2013) 5. Reaction Kinetics (Retrieved from http://smk3ae.files.wordpress.com/2007/10/reaksikinetik.pdf on the 18th October 2013)

13.0 Appendix