cstr 40l

ABSTRACT In chemical processes, reactor are very important for conversion of the at as a vessel to change the reactant into or product, this made out as key component of the equipment in chemical processes.The design of the reactors is very important to the success of the production. In this experiment, sodium hydroxide and ethyl acetate react in continuous stirred tank reactor.Both of the reactants feed to the reactor at equimolar flowrate for a certain time.The reaction is carried out at different volumetric flowrate.The conductivity value of outlet stream is measured to determine the conversion achieve at different retention time. The retention time is highest for the lowest flowrate. The result shows that the conversion is increases as the residence time increases.

INTRODUCTION In a continuous-flow stirred-tank reactor (CSTR), reactants and products are continuously added and withdrawn. In practice, mechanical or hydraulic agitation is required to achieve uniform composition and temperature, a choice strongly influenced by process considerations. The CSTR is the idealized opposite of the well-stirred batch and tubular plug-flow reactors. Analysis of selected combinations of these reactor types can be useful in quantitatively evaluating more complex gas-, liquid-, and solid-flow behaviors. Because the compositions of mixtures leaving a CSTR are those within the reactor, the reaction driving forces, usually the reactant concentrations, are necessarily low. Therefore, except for reaction orders zero- and negative, a CSTR requires the largest volume of the reactor types to obtain desired conversions. However, the low driving force makes possible better control of rapid exothermic and endothermic reactions. When high conversions of reactants are needed, several CSTRs in series can be used. Equally good results can be obtained by dividing a single vessel into compartments while minimizing back-mixing and short-circuiting. The larger the number of CSTR stages, the closer the performance approaches that of a tubular plug-flow reactor.

Continuous-flow stirred-tank reactors in series are simpler and easier to design for isothermal operation than are tubular reactors. Reactions with narrow operating temperature ranges or those requiring close control of reactant concentrations for optimum selectivity benefit from series arrangements. If severe heat-transfer requirements are imposed, heating or cooling zones can be incorporated within or external to the CSTR. For example, impellers or centrally mounted draft tubes circulate liquid upward, then downward through vertical heat-exchanger tubes. In a similar fashion, reactor contents can be recycled through external heat exchangers. By studying the saponification reaction of ethyl acetate and sodium hydroxide to form sodium acetate in a batch and in a continuous stirred tank reactor, we can evaluate the rate data needed to design a production scale reactor. A stirred tank reactor (STR) may be operated either as a batch reactor or as a steady state flow reactor (CSTR). The key or main feature of this reactor is that mixing is complete so that properties such as temperature and concentration of the reaction mixture are uniform in all parts of the vessel. Material balance of a general chemical reaction described below.The conservation principle requires that the mass of species A in an element of reactor volume dV obeys the following statement: (Rate of A into volume element) - (rate of A out of volume element) + (rate of A produced within volume element) = (rate of A accumulated within vol. element)

THEORY A mathematical model to predict ideal transient concentration in a CSTR is developed by using principles of a simple material balance. From the material balance, the ideal residence time distribution is derived. In order to create the experimental model, a negative step input method is utilized. This process is used instead of the positive step method due to the difficulty of keeping an initial tracer concentration in the feed stream.

General Mole Balance Equation

Assumptions

1) Steady state therefore 2) Well mixed therefore rA is the same throughout the reactor

Rearranging the generation

In terms of conversion

The reaction to be studied is the saponification reaction of ethyl acetate Et(Ac) and sodium hydroxyde NaOH.Since this is a second order of reaction, the rate of reaction depends on both of these reactants.The reaction will be acrried out using equimolar feeds of both the reactants with same initial concentrations.The raction equation is; NaOH + Et (Ac) →Na(Ac) + EtOH or A

+

B

→ C + D

For a second order equimolar reaction with the same initial concentration (CAO = CBo), the rate law is;

Thus, the volume of the reactor is ;

For equimolar feed rate, the reaction constant is ;

The residence time of a chemical reactor or vessel is a description of the time that different fluid elements spend inside the reactor is given by ;

This equation gives the concentration of species i in the outlet stream at any time t. The residence-time distribution function, E(t), is given:

By substituting this equation

into above equation and solving, we obtain the following expression which describes the amount of time a tracer spends in the reactor:

The ideal cumulative concentration distribution, F(t), is also practical when evaluating the residence time distribution, providing the percent of material that has a RTD of time t or less

By definition, E(t) = -dF(t)/dt for a negative step input. Therefore, by differentiating Eq. 6, we obtain the residence-time distribution function for a non-ideal CSTR.

OBJECTIVE 1. To carry out a saponification reaction between sodium hydroxide,NaOH and ethyl acetate Et(Ac) in a CSTR 2. To determine the reaction rate constant of sodium hydroxide,NaOH and ethyl acetate Et(Ac) 3. To determine the effect of residence time on the conversion in a CSTR.

APPARATUS 1. A laboratory scale of Continuous Stirred Tank Reactor 40litre 2. A conductivity meter 3. 50 mL Beakers 4. 250 mL Conical Flasks 5. A burette 6.

A retort stand.

7. Sodium Hydroxide 8. Ethyl acetate 9. Hydrochloric acid 10. Phenolphtalein 11. Deionized water

PROCEDURE a) Preparation of Calibration Curve 1. 1 Liter of sodium hydroxyde,NaOH (0.1M) and 1 Liter of sodium acetate,Na(Ac) (0.1M) was prepared. 2. The conductivity and NaOH concentration for each conversion values were determined by mixing the following solution into 100mL of deionised water; a) 0% conversion

: 100mL NaOH

b) 25% conversion

: 75mL NaOH + 25mL Na(Ac)

c) 50% conversion

: 50mL NaOH + 50mL Na(Ac)

d) 75% conversion

: 25mL NaOH + 75mL Na(Ac)

e) 100%conversion : 100 mL Na(Ac) 3. The value of conductivity for each conversion was recorded. 4. The callibration curve of conductivity versus conversion plotted.The slope and y-axis intercept was determined.

b) Back Titration Procedures for manual Conversion Determination 1. A burette was filled up with 0.1M NaOH solution. 2. A 10 mL of 0.25M HCl was measured in a flask.

3. A 50 mL sample from the experiment obtained and immediately was added to the HCl in the flask to quench the saponification reaction. 4. A few drops of phenolphtalein were fadded into the mixture. 5. The mixture was titrated with NaOH solution from the burette until the mixture was neutralized.The amount of NaOH titrated recorded. c) Effect of Residence Time on The Reaction in a CSTR 1. The general start-up procedures were performed. 2. Both pumps P1 and P2 simultaneously were switched on and the valves V5 and V10 were opened to obtain the highest possible flow rate into reactor. 3. Both the NaOH and Et(Ac) solutions were allowed to enter the reactor until it is just about to overflow. 4. The valves V5 and V10 were readjusted to give a flowrate about 0.1L/min. 5. The stirrer M1 were switched on and the speed were setted to about 200 rpm. 6. The conductivity value at (QI-401) were started monitoring until the did not change over the time to ensure that the reactor had reached steady state. 7. The sampling valve V12 was opened and a 50mL sample collected. A back titration procedure was carried out to manually 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 time by increasing the feed flow rates of NaOH and Et (Ac) to about 0.15, 0.20, 0.25 and 0.30 ml/min. Make sure that both was the same.

RESULTS a) Preparation of Calibration Curve Conversion (%)

Volume deionised water (ml)

Volume NaOH (ml)

Volume NaCACl (ml)

Conductivity, Q (ms)

0

100

100

-

6.73

25

100

75

25

4.55

50

100

50

50

2.75

75

100

25

75

1.016

100

100

-

-

0.0311

b)Manual conversion determination of NaOH by Back Titration Method. Flowrate set value (L/min) 0.10 0.15 0.20 0.25 0.30

Amount of NaOH (mL)

Conversion,x

24.0 23.4 23.1 22.8 22.5

0.960 0.936 0.924 0.912 0.900

c) Effect of Residence Time on the Reaction in CSTR. Flowrate set value (L/min) 0.10 0.15 0.20 0.25 0.30

Conductivity (µs/cm) 2.94 2.86 2.83 2.81 2.81

Flowrate set value (L/min)

Residence Time (min)

Conversion,X

Reaction rate constant,k

Rate law,-rA

0.10

400

0.960

0.31

2.4x10-7

0.15

266.67

0.936

0.2892

1.84x10-7

0.20

200

0.924

0.3239

1.87x10-7

0.25

160

0.912

0.3488

2.7x10-7

0.30

133.33

0.900

0.4092

2.77x10-8

d) Graph of calibration curve of conductivity versus conversion.

Figure 1

e) Graph of conversion versus residence time.

Figure 2

Residence Time

DISCUSSION The calibration curve is plotted to determine the conversion of the reaction of between sodium hydroxide (NaOH) and ethyl acetate (Et(Ac)) at certain value of conductivity. It obtain that the conductivity of the sodium hydroxide solutions varied linearly with concentration of sodium hydroxide. The conductiivity is decrease when the molar concentration of NaOH decreases. Both the reactants give different value of conductivity when mixture of different of moles was used. When they mix, sodium hydroxide is use as a reactant while sodium acetate is produced as a product. Because ethyl acetate and ethanol are not electric conductor, the conductivity of the mixture measurements can be used to measure the concentration of unreacted NaOH that remains solution that relate to conversion. Volumetric flow rate is related to the residence time therefore is experiment is conduct in varies flow rate. It was found that the conversion is increase as the volumetric flow rate decrease. Residence time is time that the fluid elements spend within reactor. The longer the time of the reactant spend in the more conversion of the reactant therefore the concentration of the reactant will decrease and the concentration of product will increase. Fluid entering the reactor at time t

will exit the reactor at time t + τ, where τ is the residence time of the reactor. At low flow rate, the velocity of fluid moving inside the reactor is low means the reactant spend more time within the reactor. The reaction between equimolar of NaOH and Et(Ac) is the second order, the rate of reaction is L/mol.s. Relate to the reaction rate constant for this order of reaction, when the ‘k’ value is increase means more volume of NaOH require to convert just a mole of NaOH in 1 second. In other words, it mean less mole of NaOH converted for a big volume of NaOH solution. For this experiment,the reaction rate constant is decrease as the volumetric flowrate is increase. It is means that the reaction rate increase when the rate constant decrease.There is more moles of NaOH converted for a less volume of NaOH solution require.This is very important key design to have a high conversion for large scale production. Other than determine the conversion based on the conductivity value measured by electronic device on the reactors, the conversion also determined manually by back titration procedure. From the calculation, it was obtained that the value of conversion is increase when the volumetric flow rate decrease. This proved the theory form the calibration curve from the first experiment. Therefore the experiment is success.

CONCLUSION From the experiment, reaction between NaOH and Et(ac) done in a continous stired tank reactor at different conversion due to different volumetric flow rate. As the flow rate decreases, the reaction rate constant, k for the second order of reaction is decreases. The conversion achieve within the reactors is proportional to the residence time. The experiment was successful because the entire objectives have been achieved and related to the theory of study. The continous stired tank reactor is idle for produce higher conversion with small amount concentration of reactant in output stream.

RECOMMENDATIONS

1. Use the apparatus with appropriate size and scale to the amount to be measured. 2. Make sure all the error is avoided to get more accurate result. 3. Hydrochloric acid for quenching should be prepared early and added to the samples as soon as possible so that the reaction between NaOH and Et(Ac) cannot proceed because all the unreacted NaOH Neutralized by HCl. 4. The samples that already mix with HCL should be titrated as soon as possible. 5. This experiment could be done with more variables such as temperature,type of flow stream and so on.

REFERENCES 1. Fogler, H. S., ‘Elements of Chemical Reaction Engineering’, 2nd edition, Prentice Hall, 1992, New Jersey. 2. Gilbert F.Froment and Kenneth B.Bischoff., ‘Chemical Reactor Analysis and Design’John Wiley & Sons, 2nd Edition, 1990. 3. ‘Continuous Stirred Tank Reactor model’, http://en.wikipedia.org/wiki/plug-flow-reactormodel, accessed in October 2011. 4. Levenspiel O. Chemical Reaction Engineering. John Wiley & Sons, NewYork, third edition, 1999. 5. http://www.scribd.com/doc/74373588/cstr

UNIVERSITI TEKNOLOGI MARA FACULTY OF CHEMICAL ENGINEERING CHEMICAL ENGINEERING LABORATORY III (CHE574) STUDENTS’ NAME GROUP EXPERIMENT DATE PERFORMED SUBMIT TO No. 1 2 3 4 5 6 7 8 9 10 11 12 13

: : : : :

MUHAMAD AIMAN BIN HASSAN (2012266886) EH2203D CONTINUOUS STIRRED TANK REACTOR 19th APRIL 2013 MDM. NORASMAH MOHAMMED MANSHOR

Criteria Abstract / Summary Introduction Aims Theory Apparatus Methodology / Procedure Results Calculations Discussion Conclusion Recommendations Reference Appendixes TOTAL MARKS Remarks:

Allocated Marks (%) 5 5 5 5 5 10 10 10 20 10 5 5 5 100

Checked by:

Rechecked by:

__________________

__________________

Date:

Date:

APPENDIX CALCULATION

Marks

Sample calculation: A) Back titration 1. Known quantities

Volume of sample, VS

= 50 ml

Concentration of NaOH in the feed vessel, CNaOH,f

= 0.1 mol/L

Volume of HCI for quenching, VHCi,S

= 10 ml

Concentration of HCl in standard solution, CHCi,s

= 0.25 mol/L

Volume of titrated NaOH, V1

= 24.0 ml

Concentration of NaOH used for titration, CNaOH, S

= 0.1 mol/L

2. Calculations Concentration of NaOH entering the reactor, CNaOH,0

= (CNaOH,f)/2 = 0.1/2 = 0.05 mol/L

Volume of unreacted quenching HCI, V2

=

=

x 0.0240

= 0.00960 L

Vol. of HCI reacted with NaOH in sample, V3

= VHCi,S – V2 = 0.01 – 0.00960 = 0.0004L

Moles of HCl reacted with NaOH in sample, n1

= CHCi,s x V3 = 0.25 X 0.0004 = 0.0001mol

Moles of unreacted NaOH in sample, n2

= n1 = 0.0001 mol

Conc. Of unreacted NaOH in the reactor, CNaOH

= n2 / (VS / 1000) = 0.0001 / (50/1000) = 0.002 mol/L

Conversion of NaOH in the reactor, X

=

=

= 0.96 B) Effect of residence time

=

= 400 min

Reaction Rate constant,

= [(0.05-0.002)mol/L] / [400min x (0.002mol/L)2] = 0.31 L/mol.min 1) Rate law,

= (0.06) (0.002)2= 2.4 Exp-7