CSTR 40L.docx

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 SOLTEQQVF Continuous Stirred Tank Reactor (Model: BP 143), commonly known as CSTR, 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 CSTR and the product is then analyse 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 increases almost linearly with the increasing of residence time. Further details can be obtained in the results and discussion sections.

2.0 Introduction In the majority of industrial chemical processes, the reactor is the key equipment in which raw materials undergo desired change to form desired products. The design and operation of chemical reactors is essential to the whole success of an industrial process. Reactors can take various form depending on the nature of the feed materials and the products. Understanding the behaviour of reactors is necessary to properly control and handle a raction system. Basically, there are two groups of reactors, batch reactors and continuous flow reactors. The unit used in this experiment, which is SOLTEQ-QVF Continuous Stirred Tank Reactor (Model: BP 143), With this unit, a saponification reaction was conducted by using ethyl acetate and sodium hydroxide among others.

3.0 Aims/Objectives The main purposes of conducting this experiment are :  

to carry out saponification reaction between Sodium Hydroxide, NaOH and Ethyl Acetate, Et(AC) by using a Continuous Stirred Tank Reactor, CSTR to determine the effect of residence time to the reaction's extent of conversion



to evaluate the reaction rate constant

4.0 Theory 4.1 - Reaction Rate The reaction rate or rate of reaction for a reactant or product in a particular reaction is defined as how fast or slow a reaction takes place. Consider a typical chemical reaction: aA + bB → pP + qQ

(eq 1)

The lowercase letters (a, b, p, and q) represent stoichiometric coefficients of the species, while the uppercase letters represent the reactants (A and B) and the products (P and Q). The reaction rate r for a chemical reaction occurring in a closed system under constant-volume conditions, is defined as:

Negative notations (-) are used to denotes reactants as seen with the species A and B above. The rate of reaction can also be related in such a way that :

4.2 - Conversion Eq. 1 can be rearranged and adjusted for convenience. A way to do that is to put the quantities on a 'per mole A basis'. This is done as follows :

As species A has become the basis of calculation, the progress of the reaction can be quantified by observing how many moles of products are formed for every mole of A consumed. This parameter is called Conversion. In short, conversion is number of moles of A that has reacted divided by Number of moles of A originally exists at the beginning of the experiment. As such :

4.3- Continuous Stirred Tank Reactors CSTR runs at steady state with continuous flow of reactants and products; the feed assumes a uniform composition throughout the reactor, exit stream has the same composition as in the tank.

figure 1 When the CSTR is operated at steady state, the rate of reaction with respect to species A is :

The familiar form knwon as the design equation for a CSTR is obtained,

(eq 2)

Normally, conversion increases with time as reactants spend in the reactor.For continuous flow system, this time usually increases with increasing reactor volume. So, the conversion, X is a function of the reactor volume. If FA0 is the molar flowrate of species A fed to a system operated at steady state, the molar flowrate at which species A is reacting whithin the entire system will be [FA0 X]. The molar flowrate of A to the system minus the reaction of A within the system equals the molar flowrate of A leaving the system, FA0. This is shown, mathematically : (eq 3) The entering molar flowrate FA0 is just the product of entering concentration CA0 and the entering volumetric flowrate v0,

Combining equaton 2 and 3, yields the design equation with a conversion term for CSTR,

Since the exit composition from the reactor is identical to the composition inside the reactor, the rate of reaction is evaluated at the exit condition.

5.0 Apparatus/Materials

The unit used in this experiment was Solteq-QVF Continuous Stirred Tank Reactor (Model : BP 143).

SOLTEQ-QVF Continuous Stirred Tank Reactor (Model: BP 143)

The unit comes complete with a jacketed glass reactor, constant temperature water circulating

unit, vapour condenser, individual reactant feed tanks and pumps, temperature and conductivity measuring sensors, and of course, data acquisition system. 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/Procedure 1. Both pumps on the unit were open to the fullest. This is to obtain the highest possible flowrate. 2. The reactor was filled up until both the solution is about to overflow. 3. Valves V5 and V10 were fine tuned so that the flowrate of both solution, NaOH and Et(AC), is 0.1 L/min. 4. The stirrer M1 was switched on to the speed of 200 rpm. 5. The conductivity value was monitored and, after it has been constant, recorded. 6. 50 mL of sample was collected, mixed with 10 mL HCl, and titrated with NaOH. 7. Step 3 to 6 were repeated with both solutions having the flowrates of 0.15 L/min, 0.20 L/min, 0.25 L/min, and 0.30 L/min.

7.0 Results Table 1 Flowrate of NaOH, (L/min)

0.10

0.15

0.20

0.25

0.30

Flowrate of Et(Ac), (L/min)

0.10

0.15

0.20

0.25

0.30

Volume of NaoH titrated, V1 (mL)

24.2

23.5

22.9

22.6

22.1

50

33.33

25

20

16.67

9.68

9.4

9.16

9.04

8.84

0.6

0.84

0.96

1.16

Residence time, τ (min) Volume of unreacted quenching HCl, V2 (mL) Volume of HCl reacted with NaOH , V3 (mL)

0.32

Conversion of NaOH in the reactor, X (%)

96.8

94

91.6

90.4

88.4

378.125

156.666

103.855

98.09

78.8347

9.68 x 10-4

0.00141

0.00183

0.0023

0.00265

Rate Constant ,k (M-1s-1) Rate of reaction, -rA (M/s)

Conversion of NaOH vs Residence Time 98 97 96 95 94 Conversion of NaOH, %

93 92

Y-Values

91

Linear (Y-Values)

90 89 88 87 0

10

20

30

40

50

60

Residence Time, min Figure 1

Figure 1 shows the effect of residence time to the conversion of Sodium Hydroxide, NaOH. The relationship can be seen as almost linear. Hence from the graph, it can be safely said that the amount of conversion of NaOH increases linearly with the residence time

8.0 Sample of Calculation

When the flowrate of both solution is 0.1 L/min (Column 1 of Table 1), the known quantities are :

Volume of sample,Vs

50 mL

Concentration of NaOH in the feed vessel, CNaOH,f

0.1 M

Volume of HCl for quenching, VHCl,s

10 mL

Concentration of HCl in standard solution, CHCl,s

0.25 mol/L

Volume of NaOH titrated, V1

24.2 mol/L

Concentration of NaOH used for titration, CNaOH,s

0.1 mol/L

And the calculated values are : 1. Concentration of NaOH entering the reactor, CNaOH,0

CNaOH,0

= (1/2) CNaOH,s = (1/2) 0.1 = 0.05 mol/L

2. Volume of unreacted quenching HCl, V2 V2

=

(CNaOH,s /CHCl,s) x V1

= ( 0.1/0.25) x 24.2 = 9.68 mL

placed in table 1

3. Volume of HCl reacted with NaOH in sample, V3

V3

=

VHCl,s - V2

=

10 – 9.68

=

0.32 mL

placed in table 1

4. Moles of HCl reacted with NaOH in sample, n1 n1

=

(CHCl,s x V3)/1000

= (0.25 x 0.32) / 1000 = 0.00008 mol Moles of unreacted NaOH in sample, n2 = n1 = 0.00008 mol

5. Concentration of unreacted NaOH in the reactor, CNaOH CNaOH

=

n2/ Vs x 1000

=

x 1000

= 0.0016 6. Conversion of NaOH in the reactor, X X

=

(

) x 100%

=

(

) x 100%

= 96.8 %

placed in table 1

7. Residence time, τ τ

=

VCSTR/F0

=

10 L/ (0.10 + 0.10) L/min

=

50 min

placed in table 1

8. Rate constant, k

=

= 378.125 M-1s-1

placed in table 1

9. Rate of reaction, -rA -rA = kCA2 = 378.125 (0.00162) = 9.68 x 10-4 M/s

placed in table 1

The calculation of these nine values were repeated for other flowrates and tabulated in Table 1.

9.0 Discussion Continuous Stirred Tank Reactor(CSTR) also called Mixed Flow Reactor(MFR) is, as the name suggests, a type of reactor where the contents are uniform throughout the reactor due to its well-stirred nature. It is a real reactor designed to almost match the performance of an ideal reactor. To do that, it continuously stir the contents of the reactor, that is the reactants, in a certain amount of time so that the products are form uniformly throughout the system. Thus, when a certain amount of sample is obtained from the reactor, it can be assumed that the composition of products in the sample is similar to the composition of product in the reactor.

Now that it is known that CSTR thoroughly mix the contents of the reactor for a certain period of time, one can ask a further questions, does the 'certain period of time' have an effect on the reaction? Does varying the amount of time for the reactants to react actually improve or worsen the efficiency of the reaction? These are the major questions tried to be answered by conducting this experiment. Hence the objective : to determine the effect of residence time to the reaction's extent of conversion.

For the purpose of achieving that particular target, the experiment is designed so that two reactants which are Sodium Hydroxide, NaOH and Ethyl Acetate, Et(Ac) react with each other in the saponification process. The reactor used is CSTR since the property that is to be varied is the residence time. To get different residence times, the flowrate of both of the reactants from their respective feed tanks, are varied. The relationship between the two parameters is descibed as :

where VCSTR refers to the volume of the reactor (in this case 10 L) and F0 is the flowrate of the feed that can be manipulated to get different residence time, . And that is exactly what was done. The flowrate of the feed, in the experiment, was varied to be 0.2 L/min, 0.3 L/min, 0.4 L/min, 0.5 L/min, and 0.6 L/min which can be seen in columns 1-5 in Table 1 in the Result section. With that, the residence times are determined to be 50 min, 33.33 min, 25 min, 20 min and 16.67 min respectively. These values can also be seen in Table 1 (row 4). Now that the residence times are successfully varied, here comes the big question, how does this varying residence times actually affect the reaction conversion? To answer that, a relationship between residence time and the conversion of the reactant (in this case NaOH) has to be formed. The values of residence times are known, as explained before, and the values of conversion, X of NaOh can be determined by : X

=

(

) x 100%

where

is the initial concentraion of NaOH, pre-determined to be 0.05 mol/L and

is the concentration of NaOH in the sample of product obtained by the process of titration. With all the values of conversion, X is now determined for each respective residence times, the relationship between these two parameters is graphed in Figure 1 in the Result section. From that particular figure, it can be seen that the relationship of these two parameters is almost linear. This is only logical as one can always hazard a guess that when two reactants is left to react with each other for a longer time, they will react more as compared to when a shorter time is allowed. By this experiment, that guess (or hypothesis) is confirmed.

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 Continuous Stirred Tank Reactor, CSTR, 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. The reaction rates determined to be 9.68 x 10-4 M/s, 0.00141 M/s, 0.00183 M/s, 0.0023 M/s and 0.00265 M/s respectively. Lastly, the main objective of this experiment is to study the relationship beteween the residence time and the conversion of the reactant. This relationship was successfully studied and graphed in Figure 1 of the Result section. It shows that the relationship of these two parameters is almost linear.

11.0 Recommendation     

It is better to prepare the Hydrochloric acid first, so that when the sample is collected, it can be quickly quenched. This will avoid further reaction of the sample after it is collected. Titration should be immediately stopped when the indicator turned pink. The indicator should be mixed with the acid first, then the sample. When the sample is being collected, the first few mililiters should be thrown away, for it is the remaining of the previous sample trapped in the pipe. Pumps should never be run dry.

12.0 References

 

Sullivan, J.A (1997). Fluid Power : Theory and Application. Levenspiel, O (1999). Chemical Reaction Engineering.

 

McCabe. (2005). Unit Operations of Chemical Engineering. McGuire, J.T. (1990). Pumps for Chemical Processing.



Fogler. H.S (2005). Elements of Chemical Reaction Engineering.

13.0 Appendix