INTRODUCTION

There two famous types of chemical reactor which are the batch reactor and the flow reactor where the mechanism is completely and utterly different. The reactor is known to be 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 necessities 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. To achieve the objective of this experiment, the Plug Flow Reactor (Model: BP101) is used for 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.

Objective :

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).

THEORY

Flow reactors allows for the direct measurement of reaction rates. At steady state (unlike the batch reactor), the time scales of the analytical technique used and the reaction are decoupled. Additionally, since numerous samples can be acquired at the same conditions, the accuracy of the data dramatically increases. Consider the following problem. In the petrochemical industry, many reactions are oxidations and hydrogenations that are very exothermic. Thus, to control the temperature in an industrial reactor the configuration is typically a bundle of tubes (between 1 and 2 inches in diameter and thousands in number) that are bathed in a heat exchange fluid. The high heat exchange surface area per reactor volume allows the large heat release to be effectively removed. Consider first the tubular reactor. From the material balance (Table 3.5.1), it is clear that in order to solve the mass balance the functional form of the rate expression must be provided because the reactor outlet is the integral result of reaction over the volume of the reactor. However, if only initial reaction rate data were required, then a tubular reactor could be used by noticing that if the differentials are replaced by deltas, then:

Thus, a small tubular reactor that gives differential conversion (i.e., typically below 5 percent) can yield a point value for the reaction rate. In this case, the reaction rate is evaluated at Clo. Actually, the rate could be better calculated with the arithmetic mean of the inlet and outlet concentrations:

One of the types of ideal reactor is the tubular flow reactor operating isothermally at constant pressure and at steady state with a unique residence time. This type of reactor normally consists of a cylindrical pipe of constant cross-section with flow such that the fluid mixture completely fills the tube and the mixture moves as if it were a plug traveling down the length of the tube. Hence the name plug flow reactor (PFR). In a PFR, the fluid properties are uniform over any cross-section normal to the direction of the flow; variations only exist along the length of the reactor. Additionally, it is assumed that no mixing occurs between adjacent fluid volume elements either radially (normal to flow) or axially (direction of flow). That is to say each volume element entering the reactor has the same residence time since it does not exchange mass with its neighbors. Thus, the CSTR and the PFR are the two ideal limits of mixing in that they are completely mixed and not mixed at all, respectively. All real flow reactors will lie somewhere between these two limits.Rate of Reaction and Rate LawRate of reaction can be 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 constantCA-concentration of A speciesCB-concentration of B species-stoichiometric coefficient of A-stoichiometric coefficient of B

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 :

APPARATUS AND 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 for 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

PROCEDURE

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. 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.

RESULT

Reator volume : 4 LConcentration of NaOH in the feed tank : 0.1 MConcentration of Et(Ac) in the feed tank : 0.1 M

NoFlowrate of Naoh (mL/min)Flowrate of Et(Ac) (mL/min)Total flowrate of solutions, V0(mL/min)Outlet conductivity (ms/cm)Amount of NaOH (mL)

13013006016.114.0

22482494975.914.4

32022014035.815.1

41481492975.516.7

5981042024.818.9

652491014.121.6

TABLE 1

Residence Time, , (min)Conversion, X, (%)Reaction Rate Constant,k (L.mol/min)Rate of Reaction, -rA (mol.L/min)

6.655778.05.3273.7496 x 10-3

8.048378.84.6183.1246 x 10-3

9.925680.24.0812.4999 x 10-3

13.468083.43.7301.8749 x 10-3

19.80287.83.6341.2500 x 10-3

39.60493.23.4616.2488 x 10-4

TABLE 2SAMPLE CALCULATION

Residence Time For flow rates of 300 ml/min :Residence Time, Total flow rate, Vo = Flow rate of NaOH + Flow rate of Et(Ac)= 301 mL/min NaOH + 300 mL/min Et(Ac)= 601 mL/min= 0.601 L/minHence,Residence Time, = 6.6556 min placed in Table 2Other residence times were calculated by the same way, and varying the flow rates.2 ConversionFor flow rates of 300 ml/min :Moles of reacted NaOH, n1

n1= Concentration NaOH x Volume of NaOH titrated= 0.1 M x 0.014 L = 0.0014 mole

Moles of unreacted HCl, n2

Moles of unreacted HCl= Moles of reacted NaOH n2= n1 n2= 0.0014 mole

Volume of unreacted HCl, V1

V1= = = 0.0056 L

Volume of HCl reacted, V2V2= Total volume HCl V1 = 0.01 0.0056= 0.0044 L

Moles of reacted HCl, n3

n3= Concentration HCl x V2= 0.25 x 0.0044 = 0.0011 mole

Moles of unreacted NaOH, n4

n4 = n3 = 0.0011 mole

Concentration of unreacted NaOH

CNaOH unreacted = = = 0.022 M

Xunreacted

Xunreacted = = = 0.22XreactedXreacted = 1 - Xunreacted = 1 - 0.22 = 0.78

Conversion for flow rate 301mL/min

0.78 x 100% = 78.0 %placed in Table 2

Hence, at flow rate 301mL/min of NaOH in the reactor, about 78.6% of NaOH is reacted with Et(Ac). Other conversions were calculated by the same way, and varying the flow rates.Reaction Rate Constant, k

For flow rates of 301 ml/min :V0 = Total inlet flow rate = 0.6 L/minVTFR = Volume for reactor = 4 LCAO = inlet concentration of NaOH = 0.1 MX = 0.78 = 5.327 L.mol/minplaced in Table 2Other Reaction Rate Constants were calculated by the same way, and varying the flow rates.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/minplaced in Table 2Other Rate of Reactions were calculated by the same way, and varying the flow rates.

Graph of conversion against residence time

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.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.

CONCLUSION

The experiment was conducted with several objectives to achieve. 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. The conclusion that can be made is the higher the residence time, the higher the conversion rate of the NaOH.

RECOMMENDATION

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. Pumps should never be run dry.

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/reaksi-kinetik.pdf on the 18th October 2013)

APPENDIX1