CHEMICAL ENGINEERING LABORATORY II

1.0 Title of Experiment: Continuous Stirred Tank Reactor (CSTR)

2.0 Objectives of Experiment

The objectives for this experiment were to observe and control the operation a

continuous-stirred tank reactor and to determine the effects of flow rate on conversion

rate in a continuous-stirred tank reactor.

3.0 Introduction

Continuous Stirred Tank Reactor (CSTR) is used to conduct the whole experiment. The

CSTR is an easily constructed, versatile and cheap reactor, which allows simple catalyst

charging and replacement. This reactor permits straightforward control over temperature

and pH of the reaction and the supply or removal of gases due to its well-mixed nature.

CSTRs tend to be larger in size as the need for the efficiently mixed.

There are some basic assumptions which can be made. For example, this reactor

runs at steady state, i.e. all the time derivations go to zero. Besides, none of the variables

are function of position, i.e. all of the spatial derivatives go to zero. The conditions that

exist at the exit are the same as those everywhere in the reactor. dNA/dt term is zero since

steady state us assumed. –rA is set to be the rate term and the equation can now be solved

for the volume to yield

V CSTR=FA 0−F A

−r A

where,

V CSTR volume of the reactor

F A0 inlet molar flow rate

F A outlet molar flow rate

−r A rate of reaction

4.0 Materials and Equipment

Beaker: 2L ×2

Measuring Cylinder: 100 mL ×1

Volumetric Flask 1 L ×1

Glass rod

Stopwatch

15 L of 2.3 % sodium hydroxide (NaOH) solution

15 L of 5 % ethyl acetate (Et(Ac)) solution

500 mL of 0.5 M sodium acetate, Na(Ac)

1 L of deionised water, H2O

A - Main Power

Switch

B - Conductivity and

Temperature

Meters

C - Hot Water Pump

D - Sump Tank

E - Hot Water Tank

F - NaOH Feed Tank

G - Et(Ac) Feed Tank

H - Reactor Vessel

I - Dosing Pumps

J - Tank Drain Valves

K - Hot Water Valves

L – Pump Bypass

Valve

Figure 4.1: Continuous Stirred Tank Reactor

5.0 Results and Calculations

Table 5.1: Results for Experiment 1

Conversion (%) Conductivity (mS)

0 7.81

25 6.44

50 5.06

75 3.97

100 2.84

Table 5.2: Results for Experiment 2(a) Speed of Dosing Pumps: 15%

Time

Measured

(min)

Reaction

Temperature

(°C)

Conductivity (mS/cm)Conversion of Reactants

(%)

1 2 1 2

2 38.1 22.0 1.2 83.49 99.84

4 39.7 22.0 1.2 83.49 99.84

6 40.8 22.1 1.2 83.41 99.84

8 41.6 22.1 1.2 83.41 99.84

10 42.1 22.1 1.2 83.41 99.84

Table 5.3: Results for Experiment 2(b) Speed of Dosing Pumps: 30%

Time

Measured

(min)

Reaction

Temperature

(°C)

Conductivity (mS/cm)Conversion of Reactants

(%)

1 2 1 2

2 42.7 24.7 1.2 81.37 99.84

4 42.9 26.1 1.3 80.27 99.76

6 43.1 27.1 1.4 79.48 99.69

8 43.3 28.3 1.4 78.54 99.69

10 43.5 28.6 1.4 78.30 99.69

Table 5.4: Results for Experiment 2(c) Speed of Dosing Pumps: 50%

Time

Measured

(min)

Reaction

Temperature

(°C)

Conductivity (mS/cm)Conversion of

Reactants (%)

1 2 1 2

2 44.1 26.6 1.9 79.87 99.29

4 44.4 24.8 1.7 81.29 99.45

6 44.5 24.1 1.6 81.84 99.53

8 44.5 24.1 1.7 81.84 99.45

10 44.4 24.0 1.7 81.92 99.45

12 44.1 24.2 1.7 81.76 99.45

Calculations for Conversion of Reactants (%)

X=[1− (k−k e)(ko−ke) ]×100%

¿ [1−(22−1)

(128.2−1) ]×100 %

¿83.49 %

where,

X extent of conversion

k measured value for conductivity (mS/cm)

k O initial conductivity for 2.3% sodium hydroxide solution (128.2 mS/cm)

k e conductivity of the end product (1 mS/cm for a 5% sodium acetate solution)

Table 5.5: Table of Conductivity versus Conversion

Dosing Pump (%) Conductivity (mS/cm) Conversion (%)

C1 C2 X1 X2

15 22.1 1.2 83.41 99.84

30 28.6 1.4 78.3 99.69

50 24.2 1.7 81.76 99.45

10 15 20 25 30 35 40 45 50 5515

17

19

21

23

25

27

29

31

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

Conductivity(mS) Vs. Dosing Pump (%)

C1C2

Dosing Pump (%)

Cond

uctiv

ity (m

S/cm

)

C2C1

Figure 5.1: Graph of Conductivity versus Dosing Pump

10 15 20 25 30 35 40 45 50 5575767778798081828384

99.2

99.3

99.4

99.5

99.6

99.7

99.8

99.9

Conversion (%) Vs. Dosing Pump (%)

X1X2

Dosing Pump (%)

Conv

ersio

n (%

)

X1 X2

Figure 5.2: Graph of Conversion versus Dosing Pump

6.0 Discussion

Based on Figure 5.1, Series C1 represents the conductivity for mixture at the inlet of

CSTR while C2 represents the conductivity for mixture at the outlet of the CSTR.

Increase of dosing pump, increases the conductivity of the mixture solution. This is

because increasing the dosing pumps increases the flow rate of raw reactants into the

CSTR, and hence the residence time of reactants in the reactor is lesser. The reactant

does not have sufficient time to react and transform into product before being overflow

to the tank drain where the conductivity of mixture is measured. So, the amount of

NaOH remained in the fluid coming out from the CSTR is higher. Therefore, the

conductivity is high due to the large amount of hydroxide ion (from NaOH) remaining in

the outlet stream.

By referring to Figure 5.2, Series X1 represents the conversion for mixture at the

inlet of CSTR while X2 represents the conversion for mixture at the outlet of the CSTR.

Increase of dosing pump, increases the conductivity of the mixture solution. The

percentage conversion of the saponification reaction is dependent on the conductivity

measured from the product. Therefore, from Figure 5.2 the percentage conversion is

high at low percentage dosing pump. As the percentage dosing pump or flow rate

increases, the resulting percentage conversion of NaOH decreases. This is due to less

residence time of reactants in the CSTR, which does not give ample time for the

saponification reaction to take place. Hence, lower percentage conversion of NaOH

results. In short, the conversion of NaOH is inversely proportional to the percentage of

dosing pump. The results proved that this theory applies on this experiment.

The residence time of a chemical reactor is the average amount of time a particle

spends inside the reactor, with the general formula of

τ=VQ

where,

τ residence time,

V volume of fluid in reactor, m3

Q volumetric flow rate, m3/min

To calculate the residence time in a CSTR, first the volume of the CSTR has to be

determined. Since the volume of CSTR in this experiment is set to be constant (the

amount of fluid is maintained at a per-determined level by a level adjustor in the CSTR),

so we can assume that the volume of fluid is around 20% of the volume of reactor. To

get the volumetric flow rate of fluid into the reactor, multiply the dosing pump

percentage to the total flow rate as the dosing peristaltic pumps are fitted with speed

control to adjust the feeding rate. Therefore, for this experiment, the higher the dosing

pump, the higher the volumetric flow rate and hence lower the residence time (as the

volume of fluid, V is constant).

By referring to the equation below,

V=F AO X A

(−r A )

where,

V volume of reactor

F A0 initial feed rate of A

X A extent of conversion of A

r A rate of reaction

we know that as the volume of a CSTR increases, the conversion of reactant A increases

as well. To be more precise, with higher volume of reactant fluid in the reactor and fixed

volumetric flow rate, the residence time of the reactant in the reactor increases.

Therefore, the reactant has more time to react in the reactor and hence the conversion

will be higher.

Temperature in the reactor affects the conversion and the conversion rate. By

heating the mixture, the kinetic energy of the reactant’s molecules in a reactor increases.

This promotes the movement of the molecules and more collisions between the

molecules happen in the reactor. When two chemicals react, their molecules have to

collide with each other with sufficient energy for the reaction to take place. The collision

theory explains that two molecules will only react if they have enough energy. By

heating the mixture, you will raise the energy levels of the molecules involved in the

reaction. According to kinetic theory, molecules move faster and increase the frequency

of collision under higher temperature. Hence, the conversion of a reaction will be higher

in a reactor with higher temperature.

The reaction between nitrogen gas and hydrogen gas to produce ammonia gas is

exothermic, releasing 92.4kJ/mol of energy at 298K (25oC).

N2 (g )+3 H 2 ( g ) ´heat , pressure , catalyst 2NH 3 (g )

∆ H=−92.4 kJ /mol

where,

N2 nitrogen gas

H 2 hydrogen gas

NH 3 ammonia gas

∆ H change of enthalpy in the system

Le Chatelier’s principle states that:

‘If a chemical system at equilibrium experiences a change in concentration, temperature,

volume or partial pressure, then the equilibrium shifts to counteract the imposed change

and a new equilibrium is established’.

To increase the conversion:

Removing the product constantly to decrease the concentration of product. This

helps to shift the equilibrium to the side with fewer moles of component (product

side). Changes in the initial concentrations of the substances only affect the

amount of product produced but not the conversion.

Decreasing the temperature causes the equilibrium position to move to the right

resulting in a higher yield of ammonia since the reaction is exothermic (releases

heat). Le Chatelier’s Principle states that the system will react to remove the

added heat, thus the reaction must proceed in the reverse direction, converting

the products back to the reactants. Reducing the temperature means the system

will be adjusted to minimise the effect of the change, that is, it will produce more

heat since energy is a product of the reaction, and will therefore produce more

ammonia gas as well.

Reducing the volume or increase in pressure. By increasing the pressure, the

distance between molecules decreases, the frequency of collision will be higher.

So, more products will be formed and consequently the conversion will be

higher.

It is important to note that adding catalyst will not affect the conversion of the reaction.

It only speeds up the rate for the reaction to reach equilibrium. Apart from that, the rate

of the reaction at lower temperatures is extremely slow, so a higher temperature must be

used to speed up the reaction which results in a lower yield of ammonia.

Theoretically, after increasing the dosing pump, the product conductivity has to

be higher due to more NaOH molecules remained in the solution. However, the

conductivity does not change much. This is because the conductivity meter is measuring

the fluid with the previous set of condition (lower dosing pump percentage) since the

solution in the CSTR is mixture of product from current dosing pump and previous

dosing pump. So, the conductivity shown does not represent the actual case.

Recommendation for future work:

The solution in the CSTR has to be drained completely before changing dosing

pump.

Prepare enough solution to ensure constant feed concentration

Develop more accurate rotameter calibration for CSTR

Research conductivity probe calibration more carefully to determine actual

effects of all components

Precaution steps:

Always wear gloves when filling up the feed tank with chemicals and taking out

the waste from the sump tank.

Hold the apparatus tightly so prevent them from slipping and fell down.

7.0 Conclusion

In a CSTR, the fluid reagents are introduced into a tank reactor equipped with

an impeller while the reactor effluent is removed. By increasing the flow rate into the

CSTR, the residence time of reactant in the CSTR decreases, hence the conversion of

ethyl acetate and sodium hydroxide decreases as well since the reactants do not have

sufficient time to transform into the product which are ethanol and sodium acetate.

8.0 References

Anonymous. (n.d.). Chapter 2 Flowing Reactors: Continuous Stirred Tank Reactors CSTRs. Retrieved February 19, 2011, from http://unix.eng.ua.edu/~checlass//che354/Che354Site/Library/Modules/Chapter1/ C!CSTR.pdf

Chaplin, Martin. (20 December, 2004). Continuous flow stirred tank reactors. Retrieved February 19, 2011, from LONDON SOUTH BANK UNIVERSITY website: http://www.lsbu.ac.uk/biology/enztech/cstr.html

Coulter (2009). Factors That Affect Chemical Equilibrium. Retrieved February 18, 2011 from http://www.mrcoulter.com/LECTURES/08_equilibrium3.pdf

David N. Blauch (2009). Chemical Equilibria – Le Chatelier’s Principle. Retrieved February 19, 2011 from http://www.chm.davidson.edu/vce/equilibria/temperature.html

Purchon N. (2006). Rates of reaction. Retrieved February 18, 2011 from http://www.purchon.com/chemistry/rates.htm#temperature

Wikimedia Foundation Inc. (2011). Chemical Kinetics. Retrieved February 18, 2011 from http://en.wikipedia.org/wiki/Chemical_kinetics