cstr final
TRANSCRIPT
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