ABSTRACTOn the 30th or March 2015, an experiment on Continuous
Stirred Tank Reactor was carried out in a group. The model used for
the simulation of this experiment is MODEL: BP 143 which is the 40L
Continuous Stirred Tank Reactor. The purpose of conducting this
experiment was to determine the effect of residence time of the
reaction in a Continuous Stirred Tank Reactor. To summarize the
whole experiment, it was started likewise in all experiments, which
is the general start-up procedure. After the general start-up
procedure was carried out and the equipment was set to a desired
setting, the solution of tank 1 and tank 2 were allowed to flow in
the tank with the highest flow rate as possible. Next, as soon as
the reactor has been filled up, the flow rate was adjusted to 0.1
L/min at both pumps. The stirrer was turned on and equipment was
left for a few minutes to ensure a steady state condition. Then,
the initial conductivity value was recorded and 50mL sample was
collected to be titrated to determine the concentration of sodium
hydroxide in the reactor and the extent of conversion. The
experiment was then repeated by adjusting the flow rates of sodium
hydroxide and ethyl acetate to 0.15, 0.20, 0.25 and 0.30 L/min.
Results that were obtain was calculated and tabulated further.
INTRODUCTIONContinuous Stirred Tank Reactor is a type reactor
that is commonly used in the industrial chemical processes. It is
also known as CSTR, vat or back mix reactor. Basically, in this
reactor, the reactants and products are continuously feed in and
withdrawn. Thus, hydraulic agitation is needed to achieve uniform
composition and temperature, which is the alternative that strongly
influenced by process considerations.CSTR is used mostly for liquid
phase reactions. As for the operations, it is usually operated at
steady state and the reaction is assumed to be perfectly mixed. As
a result, there is no dependence in time or position dependence of
the temperature, the concentration, also the reaction rate in the
reactor.Due to the compositions of mixtures leaving the reactor,
the reaction driving forces, normally the reactant concentrations,
is necessarily low. Thus, excluding the reaction for zero order and
negatives, CSTR requires big amount of volume of the reactor types
to get desired conversions.In addition, in CSTR every of the
variables are the same at every points. This is due to the
concentration and temperature in the exit streams is similar
everywhere within the reactions vessel. This will lead to the
temperature and concentration in the exit stream is modeled as
being similar as those in the reactor.It is important to understand
the behavior of on how the reactors function to know the correct
way in handling and controlling the reaction system. There are two
main groups of reactors, batch reactors and continuous flow
reactors. In CSTR, to obtain good result, divide a single vessel
into compartments while minimizing back mixing and short
circuiting. The bigger number of CSTR stages, the closer the
performance approaches that of a tubular plug flow reactor. (H.S.
Fogler, 2006)In this experiment, the model used is CSTR (Model:
BP143) which has been designed suitable for student to run the
experiment on chemical reactions in liquid phase under both
conditions; isothermal and adiabatic. The unit is completely came
with jacketed glass reactor, constant temperature water circulating
unit, vapor condenser; individual reactant feed tanks and pumps,
temperature sensors, conductivity measuring sensor and data
acquisition system.OBJECTIVEThere are a few objectives for this
experiment: To carry out saponification reaction between sodium
hydroxide and ethyl hydroxide, To determine the effect of residence
time onto the extent of conversion, To determine the reaction rate
constant.THEORYRate of Reaction and Rate LawRate at which the given
chemical reaction proceeds can be articulate either as the rate of
disappearance of the reactants or the rate of formation of
products.The rate equation, for an example the rate law, for rj is
an algebraic equation that is solely a function of the properties
of the reacting materials and reaction conditions (eg. Temperature,
pressure or type of catalyst if it does exist at any point in the
system). The rate equation is then classified as independent of the
type of reactor in which the reaction is carried out.For an
instant,
Where in the equation, A and B are the reactants, C and D are
the products. Meanwhile, a,b,c and d are the stoichiometric
coefficients for the respective species.Similarly, the rate of
reaction can also be represented by the rate of disappearance of
another species, such as rB instead of -rA. Also, the rate of
formation of a product, such as rC or rD In an equation, it can be
written as;
The rate law is essentially an algebraic equation involving
concentration. It may be linear function of concentration or maybe
some other types of algebraic equations. One of the common
equations is;
ConversionTaking species A as the basis, the reaction expression
can be divided through the stoichiometric coefficient of species A,
in order to arrange the reaction expression as the following :
Conversion is a better way to quantify how much the reaction has
moved, or how many moles of products are formed for every mole of A
has consumed. Conversion XA is the number of moles of A that have
reacted per mole of A fed to the system.
Continuous Stirred Tank ReactorsCSTR is sort of reactors that
commonly used in industrial processing which required a stirred
tank operated continuously. It runs normally at steady state and
usually operated so as to be quite well mixed. In CSTR there a few
kinds of phases presents; liquid phase, qas-liquid reactions, and
solid-liquid reactions. The usages are when agitation is required
and for series configurations for different concentration
streams.The advantages of CSTR are it runs for continuous
operation, has good temperature control, it easily adapts to two
phase runs, a good control, simplicity of construction, low
operating cost and it is easy to clean. How there are few
disadvantages too. The contras are it has lowest conversion per
unit volume and its by-passing and channeling possible with poor
agitation.When general mole balance is applied to species A in a
CSTR operated at steady state, in which there are no spatial
variations in the rate of reaction, and the equation based on that
is;
The familiar form known as the design equation for a CSTR
is,
Usually, the conversion will increase with time the reactants
spend in the reactor. As for continuous flow systems, time usually
increases with growing reactor volume. As a result, the conversion
X is a function of reactor volume V.Let FA0 is the molar flow rate
of species A fed to a system operated at steady state, the molar
flow rate at which species A is reacting within the entire system
will be [ FA0 X ]. The molar feed rate of A to the system subtract
rate of reaction of A within the system equals the molar flow rate
of A leaving the system, FA0. Mathematically,
Molar flow rate which enter, FA0 is just the product of the
entering concentration CA0 and the entering volumetric flow rate
v0,
Combining both of the previous equation,
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.
APPARATUS
Model: BP 143 40L Continuous Stirred Tank Reactor (CSTR)Solvent
used: 1. 0.1 M sodium hydroxide, NaOH2. 0.1 M ethyl acetate, Et
(Ac)3. 0.25 M hydrochloric acid, HCl3. De-ionized water, H2O4.
Phenolphthalein
PROCEDURESGeneral Start-Up Procedures1. The following solutions
were prepared;a) 40 L of sodium hydroxide, NaOH (0.1M)b) 40 L of
ethyl acetate, Et(Ac) (0.1M)c) 1 L of hydrochloric acid, HCl
(0.25M), for quenching.2. All the valves are ensured initially
closed.3. The feed vessels were charged as follows;a) The charge
port caps were open for vessels B1 and B2.b) The NaOH was carefully
poured into vessel B1 and the Et(Ac) solution into vessel B2.c) The
charge port caps were closed for both caps.4. The power was turned
on for the control panel.5. The thermostat T1 tank was check for
sufficient water. Refill if necessary.6. The overflow tube was
adjusted to give a working volume of 10L in the Reactor R1.7.
Valves V2, V3, V7, V8 and V11 were opened.8. The unit is then ready
for the experiment.General Shut-Down Procedures1. The cooling water
valve V13 was kept open to allow the cooling water to continue
flowing.2. Both pumps P1 and P2 were switched off. Stirrer M1 was
switched off.3. Thermostat T1 was switched off. The liquid in the
reaction vessel R1 was let to cool down to room temperature.4.
Cooling water valve 13 was closed.5. Valves V2, V3, V7 and V8 were
closed. Valves V4, V9 and V12 were opened to drain any liquid from
the unit.6. The power for the control panel was turned off.
Experiment Procedures1. The general start up procedures was
performed.2. Both pumps P1 and P2 were switched on simultaneously
and valves V5 and V10 were opened to obtain the highest possible
flow rate into the reactor.3. The reactor was let to fill up both
solutions until it is just about to overflow.4. The valves V5 and
V10 were readjusted to give a flow rate of about 0.1 L/min. Both
flow rates were ensured the same. The flow rate was recorded.5. The
stirrer M1 was switched on and the speed was set to about 200
rpm.6. The conductivity at QI-401 was start monitored until it does
not change over time. This was to ensure the reactor has reached
its steady state.7. The steady state conductivity value was
recorded and the concentration of NaOH was found in the reactor and
extent of conversion from the calibration curve.8. Sampling valve
12 was opened and 50mL sample was collected. A titration was
carried out back to manually determine the concentration of NaOH in
the reactor and extent of conversion.9. The experiment was repeated
from step 6 to 9 for different residence times by adjusting the
feed flow rates of NaOH and Et(Ac) to about 0.15, 0.20, 0.25 and
0.30 L/min. The both flow rates were ensured the same.
RESULTSFlowrate of NaOH, (L/min)0.100.150.200.250.30
Flowrate of Et(Ac), (L/min)0.100.150.200.250.30
Volume of NaoH titrated, V1 (mL) 30.3630.3834.0038.7344.77
Residence time, (min)200133.33100.0080.0066.67
Volume of unreacted quenching HCl, V2
(mL)12.14412.51213.615.49217.908
Volume of HCl reacted with NaOH , V3
(mL)-2.122-2.512-3.60-5.492-7.908
Conversion of NaOH in the reactor, X
(%)121.44123.32136154.92179.08
RateConstant,k(M-1s-1)2.64193.40162.09881.28410.8590
Rate of reaction, -rA (M/s)2.42 x 10-4
3.53 x 10-46.80 x 10-49.68 x 10-41.34 x 10-4
Table 1: Table of result
Flowrate of NaOH, (L/min)0.100.150.200.250.30
Flowrate of Et(Ac), (L/min)0.100.150.200.250.30
Volume of NaoH titrated, V1 (mL)2724221917
Residence time, (min)200133.33100.0080.0066.67
Volume of unreacted quenching HCl, V2 (mL)10.89.68.87.66.8
Volume of HCl reacted with NaOH , V3 (mL)-0.80.41.22.43.4
Conversion of NaOH in the reactor, X (%)10896887668
RateConstant,k(M-1s-1)16.87590.012.223.391.992
Rate of reaction, -rA
(M/s)2.7x10-43.6x10-44.4x10-44.7x10-45.1x10-4
Table 2: Table of result obtained from another group as
reference
SAMPLE CALCULATIONSCalculation for flow rate = 0.10 L/minVolume
of sample,Vs= 50 mLConcentration of NaOH in the feed vessel,
CNaOH,f = 0.1 MVolume of HCl for quenching, VHCl,s = 10
mLConcentration of HCl in standard solution, CHCl,s = 0.25
mol/LVolume of NaOH titrated, V1 = 30.36 mol/LConcentration of NaOH
used for titration, CNaOH,s = 0.1 mol/LConcentration of NaOH
entering the reactor, CNaOH,0 = (1/2)(0.1) = 0.05 mol/L
Volume of unreacted quenching HCl, V2 = (CNaOH,s /CHCl,s) x V1 =
(0.1/0.25) x 30.36 = 12.144 mLVolume of HCl reacted with NaOH in
sample, V3 = VHCl,s - V2 = 10 12.144 = -2.144 mLMoles of HCl
reacted with NaOH in sample, n1 = (CHCl,s x V3)/1000 = (0.25 x
-2.144) / 1000 = -0.000536 molMoles of unreacted NaOH in sample, n2
= n1 = -0.000536 molConcentration of unreacted NaOH in the reactor,
CNaOH = n2/ Vs x 1000 = x 1000 = -0.01072Conversion of NaOH in the
reactor, X = x 100% = x 100% = 121.44 %Residence time, = VCSTR/F0 =
40 L/ (0.10 + 0.10) L/min = 200 min
Rate constant, = = 2.6419 M-1s-1Rate of reaction, -rA = kCA2 =
2.6419 (-0.01072)2 = 3.036 x 10-4 M/s
Calculation for flow rate = 0.15 L/minVolume of sample,Vs= 50
mLConcentration of NaOH in the feed vessel, CNaOH,f = 0.1 MVolume
of HCl for quenching, VHCl,s = 10 mLConcentration of HCl in
standard solution, CHCl,s = 0.25 mol/LVolume of NaOH titrated, V1 =
30.83 mol/LConcentration of NaOH used for titration, CNaOH,s = 0.1
mol/LConcentration of NaOH entering the reactor, CNaOH,0 =
(1/2)(0.1) = 0.05 mol/LVolume of unreacted quenching HCl, V2 =
(CNaOH,s /CHCl,s) x V1 = (0.1/0.25) x 30.83 = 12.332 mLVolume of
HCl reacted with NaOH in sample, V3 = VHCl,s - V2 = 10 12.332 =
-2.332 mLMoles of HCl reacted with NaOH in sample, n1 = (CHCl,s x
V3)/1000 = (0.25 x -2.332) / 1000 = -0.000583 molMoles of unreacted
NaOH in sample, n2 = n1 = -0.000583 mol
Concentration of unreacted NaOH in the reactor, CNaOH = n2/ Vs x
1000 = x 1000 = -0.01166Conversion of NaOH in the reactor, X = x
100% = x 100% = 123.32 %Residence time, = VCSTR/F0 = 40 L/ (0.15 +
0.15) L/min = 133.33 minRate constant, = = 3.4016 M-1s-1Rate of
reaction, -rA = kCA2 = 3.4016 (-0.01166)2 = 4.6247 x 10-4 M/s
DISCUSIONRecalling the objectives of this experiment, which are
to carry out the reaction between sodium chloride and ethyl acetate
in a Continuous Stirred Reactor, to determine the effect of
residence time onto the reaction extent of conversion and to
determine the reaction rate constant of the reaction, were yet to
be achieved. In this experiment, five different flow rates of
sodium hydroxide and ethyl acetate were manipulated; ranging from
0.1 L/min to 0.30 L/min with 0.05 L/min in difference respectively.
The volume of sample was constant, 50mL, collected from the CSTR to
be titrated. From that, the volume of NaOH titrated were collected
and recorded.Flow rate (L/min)0.10.150.20.250.30
Vol. of NaOH titrated, V130.6330.8334.0038.7344.77
Table 3: Table of volume of NaOH titratedBy referring to the
result table above, to the volume of sodium hydroxide used titrate
the samples can be seen. The result obtained was then tabulated
further by using some equations in order to enable the conversion,
rate constant and rate of reaction to be calculated. From the
tabulated data, which can be seen below, the volume of hydrochloric
acid that reacted with sodium hydroxide in the sample, v3,
calculated is to be negative in value which is not acceptable
because the volume should not exceed the volume of hydrochloric
acid added to the sample, vHCl,s which is 10 mL. The maximum volume
that can be used in order to achieve an accurate result should be
less than 27 ml. Flow rate (L/min)0.10.150.20.250.30
Vol. of HCl reacted with NaOH in sample,
V3-2.122-2.512-3.60-5.492-7.908
Table 4: Table of volume of HCl reacted with NaOH in sample.
The following objectives will be further discussed:1. To carry
out a saponification reaction between sodium hydroxide and ethyl
acetate in a continuous stirred tank reactor.Saponification is a
continuous reaction process used to produce soap. In order to stop
the reaction, hydrochloric acid is used for quenching to stop the
reaction after the sample was taken. The reaction is highly
dependent of temperature. In the mixture of the solution containing
sodium hydroxide and ethyl acetate, it is said to undergo a
decrement in its conductivity with time. The reason for this
occurrence is because hydroxyl ion, which is a highly conductive
ion, is replaced with a poor conductive acetate ion during the
reaction. Thus, that explains the decreasing in the conductivity
value in this experiment.CH3COOC2H5 + Na+ + OH- CH3COO- + Na+ +
C2H5OH2. To determine the effect of residence time onto the
reaction extent of conversion.Residence time is the time taken for
a substance to remain in the system. It depends highly on the
conversion, x, of the reaction.
Figure 2: Graph of conversion against residence time from the
other group as reference.
Since the data obtain from the experiment cannot be used, a
reference data will be used to further discuss this part of the
objective. Judging from the graph plotted, it can be seen clearly
that both of the parameters do depend on each other. The highest
conversion, which is 96%, took about 133.33 minutes to be achieved.
As for the lowest conversion, 68%, only took about 66.67 minutes.
Thus, it can be concluded that, the longer the residence time, the
higher the conversion of the reaction.3. To determine the reaction
rate constant.The reaction rate constant, k, varies with the flow
rates used for the pumps. From the unit obtained from the tabulated
result, the reaction falls on the second order reaction. A graph of
rate constant was plotted to see the relationship between both
variables.
Figure 3: Graph of rate constant against total volume flow
obtained from the result.
Figure 4: Graph of rate constant against total volume flow from
reference result.Two graphs of rate constant against total volume
flow rate was plotted in order to compare the value of the rate
constant obtained. From both graphs, it can be clearly seen that
there is an unusual peak at the 200 L/min flow rates. It might be
caused by some interference during the reaction. Neglecting point
at the 200 L/min, the pattern of the rate constant seems to be
decreasing as the flow rates increases. It can be verified by the
equation used to calculate the reaction rate constant
below:Residence time, = VCSTR/F0 Rate constant, From the equations
above, residence time, total volume of flow rates and the reaction
rate constant do have a relationship against each other.
Hypothetically, the greater the residence time, the lower the flow
rates, the greater the rate constant.
The result of this experiment might be more accurate if
precaution were to be taken in counter. There are a few reasons
that contribute to this failure to obtain a more accurate result.
Firstly, during the titration, 3 samples were taken to be titrated.
Therefore, the average reading is taken in order to obtain a more
accurate result. Unfortunately, this experiment requires the
samples to be titrated as fast as possible. As a result, there is a
huge range volume of hydrochloric acid used to titrate the first
sample and the third sample so when the average volume was
calculated, the value obtained was abruptly high. Secondly, due to
the large consumption of sodium hydroxide, the burette needs to be
filled constantly after each sample. Thus, this will surely take
some time. As mentioned earlier, this experiment requires the
sample to be titrated immediately. This explains why the volume of
sodium hydroxide used increases. In other words, the longer the
sample is left before it is titrated, the greater the volume of
sodium hydroxide used for titration, the lower the accuracy of the
data obtained. Lastly, due to limited amount of fume chamber, two
groups need to share one fume chamber. If there are more fume
chambers, the titration might be able to be done faster without the
needs to wait for first sample to be completed. However, a
reference result from the other group was used to be discussed in
this part of the report.
CONCLUSIONAs a conclusion, this experiment is a failure as the
value obtained deviate from the limit. Even so, the objectives are
able to be achieved but not completely accurate. As the residence
time increases, the conversion increases, the reaction rate
constant increases with it too and so does the total volume flow
rates.
RECOMMENDATION1. The timing to get the sample must be correct.
Reduce the extension of time. Try to minimize the time wasted in
taking the samples.2. Do not directly get the first flow out of the
samples. Blow it first then only take the samples to be titrated.3.
Check for any leaking in the pipes of the instrument. In case there
is, do report to the lab technician.4. Control the speed from time
to time, as well the flow rate, do not let it increase or drop
rapidly for a long time of period.5. For every samples taken, let
the system run for at least 5 minutes for stability then only read
and recorded the data needed.6. For the calculation, if possible
standardize the decimal points. Take at least 4 decimal points to
get the more precise valuesREFERENCES1. Fogler, H.S (2006).
Elements of Chemical Reaction Engineering (3rd Edition). Prentice
Hall.2. Levenspiel, O. (1999). Chemical Reaction Engineering (3rd
Edition). John Wiley.3. Continuous Stirred Tank Reactor, University
of Minnesota Duluth, Department of Chemical Engineering. (Retrieved
from http://www.d.umn.edu/~dlong/excstr.pdf on the 1st October
2013)4. W.H. Green (2007). Continuous Stirred Tank Reactor CSTRs.
(Retrieved from
http://ocw.mit.edu/courses/chemical-engineering/10-37-chemical-and-biological-reaction-engineering-spring-2007/lecture-notes/lec05_02212007_g.pdf
on the 1st October 2013)