CSTR Final

SCHOOL OF ENGINEERING

CHEMICAL ENGINEERING DEPARTMENT

CHE 451: Transport Phenomena Lab 2

Summer 2013

Experiment # 4

Continuously Stirred Tank Reactor (CSTR)

Submitted to: Dr. Essam

Submission Date: July 6th, 2013

NameID# Moafak Sakka@00041370

Ula Aboutiman@00035366Aliyu Usman@00042426Fatima Rufai@00038850

Javadian farnoosh@00031560

Abstract

Table of Contents

Abstract.......................................................................................................................................................2

Introduction.................................................................................................................................................6

Apparatus..................................................................................................................................................13

Procedure..................................................................................................................................................15

Data Collected...........................................................................................................................................17

Calculations and Results............................................................................................................................19

Run #1: Room temperature & pump speed at 50%...............................................................................20



Run #4: Elevated temperature & pump speed at 30%..........................................................................26

Run #5: Elevated temperature & pump speed at 50%..........................................................................26

Sample Calculation....................................................................................................................................29

Discussion..................................................................................................................................................31

Sources of errors.......................................................................................................................................33

Conclusion.................................................................................................................................................34

References.................................................................................................................................................35

Appendix...................................................................................................................................................36

List of Table

List of Figures

Introduction

Reactors are one of the most vital pieces of equipment in the chemical engineering

industry. These reactors are to get optimum conversion rates by tweaking other variables

such as temperature, contact surface area, stirring speed etc. There are different types of

reactors used in the industry depending on the type of application. Examples of such

reactors are the batch reactor, continuously stirred tank reactor, Plug flow reactor, packed

bed reator etc. Previously, the study of the kinetics and mode of operation of the batch

reactor was carried out but this experiment will be a continuously stirred tank reactor

(CSTR).

Continuously stirred tank reactor (CSTR)

This type of reactor is used for fluid reactants. The reactants are continuously fed

through an inlet and the product leaves the vessel through the outlet. This reactor is

important in the sense that there is homogeneity of the mixture due to the stirrers. It is also

important because heat can easily be added or removed by the use of a coil or jacket. It is

also imperative that we know what the space time is for this type of reactor. The space time

is the time it takes for the fluid to remain in the reactor. This parameter is significant for

the CSTR because it is a continuous process. The schematic below shows what a typical

CSTR looks like.

Figure 1: CSTR Vessel

The saponification of ethyl acetate with sodium hydroxide to give ethanol and a salt was

carried out in the CSTR for this experiment. This experiment is a pseudo-elementary

chemical reaction in theory. The reaction for this process is shown below.

NaOH + CH3COOC2H5 → CH3COO Na + C2H5OH

Firstly, we measure the conductivity of unreacted NaOH because it will be used in the

calculation of conversion. The calculations are based on that of the NaOH because its

conductivity is substantially higher than that of the other reagents involved in this

experiment, hence, making them negligible. The rate law for this reaction is as follows

which show that the reaction is second order:

(1)

The steady state equation for the CSTR is shown below;

(2)

Where:

RA : Rate of reaction

k: Rate constant

CA : Ethyl acetate concentration

CB : sodium hydroxide concentration

FAO= Ethyl acetate inlet flow rate

FA= Ethyl acetate outlet flow rate

VR = Reactor volume filled with liquid

Substituting the rate law into the design equation for the CSTR gives an expression that will

aid in determining the rate constant as shown below.

(3)

Where

CAO = Ethyl acetate inlet concentration

CA = Ethyl acetate outlet concentration

CB = Sodium hydroxide outlet concentration

τ = Space time= VR/vo

vo = Total volumetric flow rate

As previously mentioned, space time is an important factor because it represents the time

the fluid spends in the reactor.

The reaction is monitored by measuring the conductivity of NaOH and using the following

equation.

C - C∞ CA – CA∞

————— = —————— (4) Co - C∞ CAo – CA∞

Where

C = Specific conductivity at time t

Co = Specific conductivity at time t=0

C∞= Specific conductivity at time t=∞

CA= NaOH concentration at time t

CAo = NaOH concentration at time t=0

CA∞ = NaOH concentration at time t= ∞

Knowing that CA∞ → 0 as t → ∞, we determine the conversion using equations below.

C A C - C ∞

————— = ———— = 1 - XA (5) CAo Co - C∞

Where

XA is the fractional conversion of sodium hydroxide.

Equation (5) is manipulated so that we can get the conversion using the conductivity

instead of concentrations.

(7)

Where

κ is the measured specific conductivity at the reactor exit [S·m-1]

κ0 is the measured specific conductivity of the mixture at initial conditions [S·m-1]

κeq is the measured specific conductivity at equilibrium [S·m-1]

With knowledge that the reactant fed in equimolar proportions and the conversion gotten

from equation (7), the equation below can be used to find the rate constant at any

temperature.

XA

————— = CAO K (8) 1 – XA

A plot of plotting XA/[CAO (1-XA)] vs. is generated upon completion of calculations. Thisτ

should depict a straight line that passes through the origin with its slope being the reaction

rate constant. If the plot conforms to the above description, then it is confirmed that the

reaction is indeed a pseudo-elementary reaction.

Also, Arrhenius equation is used to calculate the reaction rate which can be compared with

the other value.

K = A e –Ea/RT (9)

Where:

A =Pre-exponential factor or frequency factor

Ea= Activation energy in J/mole

R =Gas constant = 8.314 J/mole oK

T = Absolute temperature

τ =Space time, second

A plot of ln(k) vs. 1/T gives use the activation energy and the pre-exponential factor. The

slope is used to fing the activation energy while the intercept is used to calculate the pre-

exponential factor.

(10)

Where:

k = specific rate constant

(E/R) = slope of the line

ln (A) = y- intercept of the line

The above equations and relations for the CSTR are all based on the assumption of

steady state. The calculations above were based on the assumption that the CSTR was

operated at steady state where variables do not change with time. Another way of carrying

out these calculations is to eliminate the assuption of steady state. The set overflow at the

beginning of the experiment enables volume (V) determination and knowing the equimolar

flow rates of the reactant Q allows calculation of space time. Since the assumption of steady

state is not applied here, the concentration of NaOH will change with time (decrease) and

also from inlet to outlet. This makes the process to become a transient one at the beginning

until it finally reaches steady state. The concentration of sodium hydroxide at the outlet

(CE) will consequently change until the new equilibrium is reached. On the other hand, the

CE is the same as the concentration of NaOH inside the vessel at a given time. The

equations that are used for the transient part of this process is as follows:

Input = output + consumption + accumulation

d (CE V) Q Co = Q CE + (-r) V + ———— (11) dt

By introducing residence time into the equation, the equation becomes:

dCE

θ ———— + CE – r θ = Co (12) dt

Where:

θ = V/Q = The theoretical residence time of the reactor.

Apparatus

The apparatus used in the continuous stirred tank reactor experiment is G.U.N.T.

Gerrbau Gmbh CH 310.

Figure 2: G.U.N.T. Gerrbau Gmbh CH 310

As seen from figure (2) above, the apparatus consists of a water bath (1) that is used to

keep the reactor (2) at a constant operating temperature. The water bath temperature is

kept constant by recycling the water in the water bath. The water bath temperature is read

on the control panel temperature display (3), with the aid of a temperature sensor that

transmits a signal to the temperature controller (TC) (4), as the reactor’s temperature is

being controlled by an electrical heating coil inside the water bath (5). The reactor

temperature is set on the temperature control panel (6). Switches (7) & (8) are used to

switch on or off the heater and the recycling pump respectively. The feed to the reactor is

supplied from two glass tanks (9). One of the tanks contains sodium hydroxide (NaOH)

solution, while the other contains ethyl acetate solution. The apparatus also has two pumps

(10), which assist in pumping the feed of the reactant solutions from their respective tanks.

Switches (11) & (12) on the control console are used to switch on or off the pumps for the

sodium hydroxide and ethyl acetate solutions respectively. The feed flow to the reactor is

controlled by turning the potentiometers under the flow displays (13) & (14). The

conductivity of the reaction product in the reactor is measured by using the conductivity

sensor (15) with a cell constant 0.09 cm-1 and the conductivity is read on the conductivity

display (16) on the control console. The reactor’s product solution is discharged directly to

the swamp tray (17) and emptied afterwards through a drain to a waste container (18).

Finally, electricity is supplied through an integral cable and turned on or off by turning the

main switch (19)

Procedure

The equipment is operated by the following step-by-step procedure:

1. 5 liters of each of 2.3 weight-% sodium hydroxide and 5 volume % ethyl acetate

solutions should be prepared and filled in the tanks specific for each solution.

2. Measure the conductivity and the temperature of each material at room

temperature.

3. Insert the conductivity-measuring sensor in the measuring opening on the lid and

connect the stirrer.

4. Supply water to the system by connecting the water hose from the heating system to

the connections on the lid placed on the reactor and to to the switch housing,

5. Connect the chemical hoses from the pumps to the tank using the rapid actor

connectors.

6. Adjust the overflow on the tank such that the measuring sensor is immersed about

50 mm in the liquid with the tank filled. Adjust the overflow pipe in preparation for

the measurements of the space time of the reactor.

7. Switch on the main switch

8. Fill the two feed vessels with water and measure the residence time at flow rates of

30%, 40% & 50% using a stopwatch.

9. Fill the two feed vessels with the prepared materials in 1

10. Switch on the chemical pumps and adjust both to the same flow rates (initially

30%).

11. Switch on the stirrer and adjust to a suitable speed.

12. Record the conductivity, reaction temperature and time of the effluent from the

reactor every 10 seconds until a steady reading is obtained.

13. Repeat the procedure for flow rates of 40% and 50%.

14. Same steps (9-12) are applied for elevated temperatures.

15. Stop the NaOH & ethyl acetate flow.

16. Turn off the pumps, empty and rinse the reactor and feed tanks with water

Data Collected

Table 1: Data collected for run #1

Time (sec) Specific Conductivity (mS/cm) Temperature 0C

Space Time 83 0.15 27.4

10 6.65 28.2

20 6.40 28.3

30 6.32 28.4

40 6.24 28.4

50 6.23 28.5

60 6.28 28.5

70 6.23 28.6

80 6.29 28.6

90 6.26 28.7

100 6.29 28.7


Time (sec)Specific Conductivity

(mS/cm)Temperature (oC)

10 6.88 28.9

20 6.62 29

30 6.64 29

40 6.46 29

50 6.44 29.1

60 6.44 29

70 6.38 29.1

80 6.29 29.1

90 6.37 29.1

100 6.32 29.1

Table 3: Data collected for Run #3


Time (sec)Specific

Conductivity (mS/cm)

Temperature (oC)

10 6.13 33.920 5.84 34.230 5.82 34.540 5.64 34.850 5.6 3560 5.57 35.270 5.46 35.380 5.37 35.5

Time (s) Conductivity (mS/cm) Temperature (oC)

Space Time 96 0.03 28.810 6.48 29.120 6.39 29.230 6.30 29.240 6.26 29.250 6.19 29.260 6.17 29.270 6.17 29.280 6.13 29.290 6.13 29.2

100 6.15 29.2

90 5.35 35.7100 5.42 35.7

Calculations and Results

Table 5 : Data Collected before operation

Volume of the CSTR (mL) 1086

Specific conductivity (NaOH) mS/cm 19.1

Cell Constant (1/cm) 0.09

CA0 Ethyl Acetate 0.5502

Table 6: Space Time Data

Pump speed % Space time (sec)τ30 10040 8350 79

Run #1: Room temperature & pump speed at 50%

Table 7 : Calculations for run #1

Time (sec)

Specific Conductivity (mS/cm)

Conductivity (mS)

XA Xa/((1-Xa)*Ca0)Temperature

0C83 0.15 1.6667 0.9968 574.0331 27.4

10 6.65 73.8889 0.6549 3.4494 28.2

20 6.40 71.1111 0.6681 3.6581 28.3

30 6.32 70.2222 0.6723 3.7284 28.4

40 6.24 69.3333 0.6765 3.8005 28.4

50 6.23 69.2222 0.6770 3.8097 28.5

60 6.28 69.7778 0.6744 3.7642 28.5

70 6.23 69.2222 0.6770 3.8097 28.6

80 6.29 69.8889 0.6739 3.7552 28.6

90 6.26 69.5556 0.6754 3.7823 28.7

100 6.29 69.8889 0.6739 3.7552 28.7

0 10 20 30 40 50 60 70 80 90 100 1100.6500

0.6550

0.6600

0.6650

0.6700

0.6750

0.6800

Conversion Vs. Time

Time (Sec)

Xa

Figure 3: Conversion Vs. Time for run #1

0 10 20 30 40 50 60 70 80 90 100 11066.000067.000068.000069.000070.000071.000072.000073.000074.000075.0000

Conductivity Vs. Time

Time (Sec)

Conductivity (mS)

Figure 4: Conductivity Vs. Time for run #1

0 10 20 30 40 50 60 70 80 90 100 11027.9

2828.128.228.328.428.528.628.728.8

Temperature Vs. Time

Time (Sec)

Temperature 0C

Figure 5: Temperature Vs. Time for run #1



Time (sec)


Temperature (oC)

Conductivity (mS)

xa

Xa/((1-Xa)*Ca0)

10 6.88 28.9 76.44444444 0.62712795 3.27100227

20 6.62 29 73.55555556 0.64140582 3.47360809

30 6.64 29 73.77777778 0.64030752 3.45745197

40 6.46 29 71.77777778 0.6501922 3.60650929

50 6.44 29.1 71.55555556 0.6512905 3.62359285

60 6.44 29 71.55555556 0.6512905 3.62359285

70 6.38 29.1 70.88888889 0.65458539 3.67549537

80 6.29 29.1 69.88888889 0.65952773 3.7552327

90 6.37 29.1 70.77777778 0.65513454 3.68424221

100 6.32 29.1 70.22222222 0.65788029 3.72839761

0 20 40 60 80 100 1200.62

0.63

0.64

0.65

0.66

0.67

0.68

Conversion vs Time

Time (sec)

Xa

Figure 6: Conversion vs. Time for run #2

0 20 40 60 80 100 12066

68

70

72

74

76

78

Conductivity vs Time

Time(secs)

C(m

S)

Figure 7: Conductivity vs. time for run #2

0 20 40 60 80 100 12028.75

28.8

28.85

28.9

28.95

29

29.05

29.1

29.15

Temperature vs Time

Time (secs)

Tem

pera

ture

(oC)




Time (s)


Temperature (oC)

Conductivity (mS)

xa xa/Cao(1-xa)

10 6.48 29.1 72.0000 0.6654 3.4294

20 6.39 29.2 71.0000 0.6702 3.5030

30 6.30 29.2 70.0000 0.6749 3.5788

40 6.26 29.2 69.5556 0.6770 3.6131

50 6.19 29.2 68.7778 0.6806 3.6744

60 6.17 29.2 68.5556 0.6817 3.6922

70 6.17 29.2 68.5556 0.6817 3.6922

80 6.13 29.2 68.1111 0.6838 3.7280

90 6.13 29.2 68.1111 0.6838 3.7280

100 6.15 29.2 68.3333 0.6827 3.7100

0 10 20 30 40 50 60 70 80 90 100 1100.66000.66250.66500.66750.67000.67250.67500.67750.68000.68250.68500.68750.6900

Conversion Vs. Time

Time (s)

Xa


0 10 20 30 40 50 60 70 80 90 100 11066.0000

67.0000

68.0000

69.0000

70.0000

71.0000

72.0000

73.0000


Time (s)

Cond

uctiv

ity (m

S)

Figure 10: Conductivity Vs. Time for run #3

0 10 20 30 40 50 60 70 80 90 100 11029.04

29.06

29.08

29.1

29.12

29.14

29.16

29.18

29.2

29.22


Time (s)

Tem

pera

ture

(oC)


Run #4: Elevated temperature & pump speed at 30%

Run #5: Elevated temperature & pump speed at 50%


Time (sec)

Specific Conductivity

(mS/cm)

Temperature (oC)

Conductivity (mS)

Xa xa/Cao(1-xa)

10 6.13 33.9 68.11 0.332 0.8464014120 5.84 34.2 64.89 0.316 0.7881406630 5.82 34.5 64.67 0.315 0.7842164740 5.64 34.8 62.67 0.305 0.7494235550 5.6 35 62.22 0.303 0.7418177960 5.57 35.2 61.89 0.301 0.7361429870 5.46 35.3 60.67 0.295 0.7155489280 5.37 35.5 59.67 0.290 0.6989446790 5.35 35.7 59.44 0.289 0.69528436

100 5.42 35.7 60.22 0.293 0.70814228

0 20 40 60 80 100 1200.25

0.26

0.27

0.28

0.29

0.3

0.31

0.32

0.33

Conversion vs Time

Time (sec)

Xa


0 20 40 60 80 100 12054

56

58

60

62

64

66

68

70

Conductivity vs Time

Time(secs)

C(m

S)

Figure 13: Conductivity vs. time for run #5

0 20 40 60 80 100 12033

33.5

34

34.5

35

35.5

36

Temperature vs Time

Time (secs)

Tem

pera

ture

(oC)


Table 11: Data used to determine the Rate constant at room temperature

Room Temperature Pump speed % Space time (sec)τ Xa/((1-Xa)*Ca0)

30 100 3.71

40 83 3.278

50 79 3.755

75 77 79 81 83 85 87 89 91 93 95 97 99 1013

3.2

3.4

3.6

3.8

4

4.2

4.4

f(x) = 0.0422447004694519 x

Xa/((1-Xa)*Ca0) vs. Space Time

τ (sec)

Xa/(

(1-X

a)*C

a0) (

1/M

)

Figure 15 : Xa/((1-Xa)*Ca0) vs. Space Time

From the Slope the value of K = 0.0422 M-1. Sec-1

Table 12: Data used to determine the Rate constant at elevated temperature

Elevated Temperature

Pump speed % Space time (sec)τ Xa/((1-Xa)*Ca0)

30 100

50 79

Sample Calculation

Run #1:

Cell constant= 0.09 1

cm

Specific conductivity of NaOH = 19.1 mscm

At Time = 20 sec

Conductivity (K) at 20s = 6.4 ms /cm0.09/cm = 71.111 mS

K0= 19.1ms /cm0.091 /cm = 212.222 mS

Kinf = 1 mS

K = conductivity at time 20 sec

Xa= 1−K−Keq

K 0−Keq = 1−71.111−1

212.222−1 = 0.6681

Xa1−Xa = Cao * K* τ

Density of ethyl acetate= 0.897g

ml

Molecular weight of ethyl acetate = 88.11 g

mol

380 ml in 6.65 L of H20

Mass of ethyl acetate= 0.897g

ml * 380 ml =340.86 g

Moles= 340.86 g

88.11g /mol = 3.868 moles

CA0 of ethyl acetate= 3.868 moles

380+6650 ml = 0.0005502 moles

ml = 0.5502moles

L

Slope = K = 0.0442 1

M∗S

Discussion

Conversion Vs. Time

Figure 2 at the beginning of the reaction there is a fast increase in the rate conversion and after

50 second the conversion will fluctuate around 0.677 and 0.673 and it never reached a steady

value because the value of the temperature kept increasing and it wasn’t steady.

Figure 5 shows a curve that is rapidly increasing. This is just as expected because the longer the

reactant spend in the vessel, the more the reactants are in contact with each other and hence an

increase in the degree of conversion.

Figure 12 displays a curve with an unsteady increase of conversion with respect to time at an

elevated temperature of 44.5oC. The unsteady increase is as a result of the system trying to attain

steady state while the reactants try to achieve perfect mixing with respect to time.

.


On Figure 3 the conductivity with time decreased in an exponential fashion, one reason behind

that due to the fact that most of the NaOH reacted with Ethel acetate and the amount of the

hydroxyl ions decreased in the solution.

(Figure 6) rapidly decreasing. This pattern is logical because as more and more NaOH reacts

with ethyl acetate, the quantity of NaOH in the reactor decreases, therefore, the conductivity will

decrease.

As seen from Figure 13,at an elevated temperature 44.5oC, conductivity decreases with time.

This decreasing pattern is due to the fact that the hydroxyl ions in the NaoH react with the ethyl

acetate for the reaction to carry out, thus, leading to a decrease in the hydroxyl ions present in the

reactant solution


Figure 4 shows that the temperature increases with time however it’s not increasing in an

exponential fashion and it did not reached to a steady value but it kept fluctuation. This illustrate

that the 50% speed pump is not a good choice because it didn’t affect the temperature only but tit

affected the conversion only.

Lastly the temperature vs. Time plot (Figure 7) spikes up at the beginning, then it decreases a

little and then it becomes constant. The explanation for these depictions of the graph is that the

increase in the temperature is attributed to the heat released when the reaction occurred, the

decrease might be as a result of heat losses, and the stabilizing of the line is probably because the

heating coil served its function in the end by keeping the temperature constant.

Figure14 shows an almost steady temperature increase with time. The steady temperature increase is due to the fact that the system receives the heat of the reaction being released(exothermic reaction) in a gradual manner and as such, a steady increase in temperature with time.

Sources of errors

Some of limitations that effect the obtaining of data are caused by the size of the reactor,

which in this case the reactor used is only capable of performing small scale experiments that are

under atmospheric pressure.

Another limitation is made by the conductivity meter that has some second delay, which

will affect the real value of conductivity. The overflow level of the reactor’s tank is another error;

this happens when the solution overfills the reactor which causes change in the space time and the

conductivity meter, that will result in a false value. This problem can be solved by enlarging the

overflow hole which use to bends the pipe and closes up and stop the overflow from happening.

The human errors is another limitation, which means that while reading the stop watch and

communicating with the person taking notes some values can be read or written wrongly (parallax

error.)

Also there might be some inaccuracy while preparing the feed solution for the

reactor; this will affect the stoichimetric ratios of the substances used.

If the amount of the solution is not enough in one of the runs, the experiment must be

stopped, so it is better to check the reactants levels before the runs and fill them if needed. But in

case the tank’s run was stopped under this condition it’s is best to do the run all over again to get

the correct values.

The most important errors is that the thermostat also has a default in its design since only a

small part of the coil is merged from the top inside the tank; the solution cannot be heated exactly

to the temperature required. This problem can be solved by using Dr.Essam’s idea and designing

the coil from the bottom of the tank.

Conclusion

References

CHE 451: Chemical Engineering Laboratory II Handout, American University of Sharjah.

Material safety data sheet ethyl acetate msds. (n.d.). Retrieved from

http://www.sciencelab.com/msds.php?msdsId=9927165

Material safety data sheet ethyl alcohol 200 proof msds. (n.d.). Retrieved from


Material safety data sheet sodium acetate anhydrous msds. (n.d.). Retrieved from

http://www.sciencelab.com/msds.php?msdsId=9924952 (Sodium Acetate)

Material safety data sheet sodium hydroxide, pellets, reagent acs msds. (n.d.). Retrieved from




Saponification. (n.d.). Retrieved from http://en.wikipedia.org/wiki/Saponification

Wikipedia [Print Photo]. Retrieved from:

http://upload.wikimedia.org/wikipedia/commons/thumb/b/be/Agitated_vessel.svg/

350px-Agitated_vessel.svg.png

S. Bhaduri & D. Mukesh, Homogeneous Catalysis: Mechanism and Industrial Applications. New York: John Wiley & Sons, 2000. Available: google e-book.

[2] H. Fogler, Elements of Chemical Reaction Engineering. New Jersey: Prentice Hall, 2006.

http://upload.wikimedia.org/wikipedia/commons/thumb/b/be/Agitated_vessel.svg/350px-Agitated_vessel.svg.png

http://upload.wikimedia.org/wikipedia/commons/thumb/b/be/Agitated_vessel.svg/350px-Agitated_vessel.svg.png

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