Study of Esterification Reactions in a Batch Reactor:
Modeling the Industrial Synthesis of Benzoic Acid and Biodiesel
Chida Balaji
chidabalaji@gmail.com
Brett Levine
levine.brett@gmail.com
Shirin Poustchi
spoustchi@msn.com
Abstract
This experiment examines two esterification reactions: the de-esterification of
ethyl benzoate into benzoic acid and the transesterification of palm oil into biodiesel.
Through the de-esterification of ethyl benzoate, we mimicked the processes and
experimental designs that are involved in the production of API’s (active pharmaceutical
ingredients). The transesterification of palm oil allowed us to observe the production of
biodiesel on a small scale. In a 1L batch reactor at 40oC and 0.25 ethanol mole fraction,
the rate constant was experimentally found to be 0.47 M-1s-1. This value is important in
that it tells us the rate (speed) of the reaction under the given conditions and compares
favorably with the literature value of 0.51 M-1s-1 [2]. For the biodiesel reaction, we
successfully produced biodiesel in a 1L batch reactor with a percent yield of 23% at
60oC.
Introduction
The demand for pharmaceuticals
is increasing; consequently, the need for
efficient production designs is vital to
the success of the pharmaceutical
industry. By studying the synthesis of
APIs, chemical engineers are striving to
discover new ways to optimize the
efficiency of these valuable reactions.
Although API synthesis covers a
multitude of chemical reaction types, our
research focused on one specific reaction
type: esterification.
As with pharmaceuticals, the
petroleum industry is constantly
searching for new ways to increase their
productivity.
Additionally, these
companies are actively pursuing viable
and renewable alternative energy sources
as a result of the decreasing fossil fuel
reserves, which include wind power,
solar power, and the focus of our second
experiment: biodiesel.
Esterification reactions involve
either adding (transesterification) or
removing (de-esterification) an ester
group to/from a molecule. Esters are a
type of molecule formed from an organic
acid and an alcohol and have the general
structure of R-CO-OR’. Ester molecules
exist in a variety of forms, ranging from
naturally occurring esters such as
vegetable oils to commercially prepared
products such as biodiesel.
Commercially, esters are very
prevalent and a valuable resource to
many
industries,
especially
the
pharmaceutical industry. Many APIs, or
active pharmaceutical ingredients, are
formed in esterification reactions (ex.
Aspirin). Without these quintessential
ingredients,
the
medical
and
pharmaceutical industry would not be
where it is today. Furthermore, it is of
great importance to continue to study
and understand how these esterification
reactions work so engineers and other
professionals can continue to produce
products that will further benefit
mankind.
Commercially, these reactions
take place in very large batch reactors.
These specially designed vessels are
often tailored to the reaction taking
place, and provide a closed system that
can often easily be controlled by a
computer. These reactors also have the
ability to control temperature, reactant
concentrations, and many other things
such as pressure and pH depending on
the specific reactor involved. [1]
Carefully analyzing past experiments
enables chemical engineers to make
changes to the reactor conditions that
would make the reactions more efficient
and effective.
Figure 1: The Batch Reactor
Above is a picture of the 1L batch reactor used in
both experiments. The outer layer of the reactor
is a water jacket with a dedicated temperature
probe that constantly monitors the reactor’s
temperature.
This paper analyzes two different
esterification reactions. The first, the deesterification of ethyl benzoate into
benzoic acid, serves as a model for the
synthesis of APIs in batch reactors. The
second, the transesterification of palm
oil into biodiesel, offers a source of
renewable and clean alternative energy.
The purpose of both experiments is to
simply study esterification reactions,
both
transesterification
and
deesterification, and how to conduct both
experiments in a way that models their
industrial production.
There were two forms of objectives set
to accomplish the goals of this project:
the first being quantitative and the
second
being
qualitative.
The
quantitative objectives of this research
were:
• Calculate the rate constant value,
k, (ethyl benzoate reaction)
• Calculate the % yield (biodiesel
reaction)
The qualitative objectives were:
• Carry
out
a
model
deesterification reaction using ethyl
benzoate
• Carry
out
a
model
transesterification reaction using
palm oil (to produce biodiesel)
• Analyze the above small-scale
reactions to serve as a model for
industrial production
The rate constant, k, describes how
quickly the reaction proceeds (measured
by the change in concentration of the
reactants with respect to time). The
percent yield is a measure of how much
product was produced in relation to the
predicted yield (from stoichiometry).
Although both reactions were
quantitatively analyzed, our study of
these reactions was mostly qualitative.
The quantitative measures simply serve
to gauge our accuracy in comparison to
others who have completed similar
experiments. This paper does not strive
to find ways to maximize the efficiency of
these experiments; it simply strives to
model the nature of esterification
reactions.
Background
De-esterification of Ethyl Benzoate
The first reaction involved the deesterification of ethyl benzoate to
benzoic acid. De-esterification reactions
typically involve hydrolysis, where
water cleaves a molecule (ethyl
benzoate) into its respective alcohol
(ethanol) and acid (benzoic acid).
Typically (and as with our experiment),
these reactions take place with a basic
catalyst (NaOH).
amount of desired product in the
smallest amount of time.
The calculations for the biodiesel
percent yield are relatively simple when
compared to those for the ethyl benzoate
reaction.
The differential rate law for this
reaction is:
R = k[EB]2
Figure 2: Ethyl Benzoate Reaction [1]
The product of this reaction,
benzoic acid, has several important uses
in consumer products. Benzoic acid is
primarily used as a food/drink
preservative and has been shown to
inhibit the reproduction of mold and
yeast molecules.
As previously mentioned, this
reaction serves as a model for the
synthesis of APIs in a batch reactor
similar to the one we used. In the
chemical engineering world, measures
such as yield (how much of the product
is created compared to the theoretical
yield) and speed are of great importance.
Pharmaceutical companies want to be
able to manufacture the APIs they need
for their medicine; however, they also
want to make the reactions as efficient as
possible.
The speed and yield are
influenced by a variety of conditions,
mainly mole fraction (the relative
composition of the reactant mixture) and
temperature for the two reactions studied
in this experiment. Most esterification
reactions are reversible, which means
that often the reactions are not complete
(the actual yield is less than the
theoretical yield).
This is where
chemical engineers use their knowledge
and experience to optimize the reactions
such that they yield that maximum
(Where EB represents Ethyl Benzoate)
Equation 1: Differential Rate Law
By taking the integral of both sides of
this equation, we obtain the integrated
rate law, which is what we used in the
experiment:
[EB] = [EB]0
1+[EB]0kt
Equation 2: Integrated Rate Law
We also know that [EB] at time t is
equal to the concentration of EB at t=0
minus the concentration of the acid used
to quench it:
[BA] = [EB]0 – [EB]
Equation 3 (where BA represents Benzoic Acid)
By substitution, we get:
[EB]0 – [HCl] = [EB]0
1+[EB]0kt
Equation 4
Solving for k:
k = [BA]
[EB][EB]0t
Equation 5
This equation can be further simplified
by:
k=
[BA]
([EB]0 – [BA])[EB]0t
Equation 6
The above equations, most importantly
the integrated rate law, allow us to
calculate a value of ‘k’.
Note that the [EB] = [OH]c in the
original mixture. Also note that this
reaction is considered irreversible.
Transesterification of Palm Oil
While the de-esterification of
ethyl benzoate isn’t especially practical,
the second experiment certainly is. As
the world’s reliance on fossil fuels
increase and the supply of these energy
sources deplete, there is an increasing
necessity for an alternative and
renewable energy source. Biodiesel, a
blanket term for a combustible and
energy rich hydrocarbon chain, is
produced by the esterification of palm
oil (from palm trees) in the presence of
methanol and a catalyst.
Figure 3: Biodiesel Reaction [6]
The end result of the reaction is an
immiscible mixture of biodiesel and
other waste products including excess
methanol and glycerin. These parts can
be effectively separated to leave high
purity biodiesel.
Worldwide, biodiesel interest is
increasing due to the looming oil crisis.
The results of this study offer valuable
insight into the production of biodiesel
and introduce it as a valuable and
renewable alternative energy source.
Method
Both
reactions,
the
deesterification and transesterification,
took place in a 1L glass batch reactor.
The reactor was surrounded by a water
jacket, which allowed us to carefully
regulate the temperature throughout the
progression of both reactions.
The
reactor also had a temperature probe that
recorded the temperature inside the
reactor (a separate probe existed for the
jacket), down to a tenth of a centigrade.
Additionally, the reactor contained an
inert stirrer on the bottom side which
spun at a rate ranging from 0-500
revolutions per minute (rpm). With any
reaction, a well-stirred reactor is needed
to properly evaluate the rate constant.
For the ethyl benzoate reaction,
we calculated the rate constant first by
continuously extracting samples from
the reactor. The composition of the
samples was then analyzed using
filtration.
Based
on
previous
experiments conducted by others on this
same topic, it was determined that this
reaction was second order with respect
to ethyl benzoate [2].
To experimentally determine the
rate constant, samplings of the reactor
mixture (which contained a mixture of
ethyl benzoate, sodium hydroxide,
ethanol, and benzoic acid) were taken at
certain intervals of time. At specified
times, we took a small ~10 mL sample
of the reactant mixture from the
sampling tray. Immediately after, we
measured exactly 5.00 mL using a
micropipette and added that to 5.00 mL
of cold Hydrochloric acid.
This
important step is referred to as
quenching, which means we used the
HCl to effectively stop the reaction in
the sample that we took. As soon as the
HCl was added, the time was recorded.
After agitating the sample
solution in a vortex, it was taken over to
the titrator. The titrator used 0.10N
sodium hydroxide to titrate the benzoic
acid in the sample solution that was
formed in the reaction.
batch reactor such that they could be
heated to the desired temperature before
adding the final ingredient: palm oil.
Note that NaOH was added in pellet
form instead of solution form purposely
to prevent any water from entering the
reactor. When water is present, deesterification takes place via hydrolysis
(and forms soap), which is exactly what
we do not want to happen [1]. Once this
temperature was reached, the palm oil
was added and the reaction began.
The following day, we returned
(with the assumption that the reaction
was finished), and turned off the stirrer.
The hydrophilic and denser glycerol
migrated to the bottom of the reactor
while the less dense biodiesel rose to the
top. Using a peristaltic pump, we slowly
extracted the biodiesel layer into an
Erlenmeyer
flask.
Figure 4: The Titrator
Solver, an Excel application,
compared the actual concentrations at
each respective time to the theoretical
concentrations as proposed by the
second order rate law. Solver used the
non-linear least squares regression test in
order to minimize the sum of the squares
of the errors to calculate ‘k’ [5].
The experiment with the biodiesel
involved simply determining the percent
yield, which compared the actual amount
of biodiesel formed with respect to the
amount dictated by stoichiometry [1].
Ideally, we would have liked to study the
kinetics of the biodiesel reaction;
however, by using palm oil it is
extremely impractical and difficult.
Palm oil is a conglomerate of about five
different hydrocarbons, which makes it
nearly impossible to write a rate law
(which is needed to calculate ‘k’).
First, methanol and dry sodium
hydroxide (NaOH) were added to the
Figure 5: Separation
(Formation of Biodiesel)
of
Layers
Next, we used a vacuum filtration
system to filter out some of the glycerol
waste products that collected in the
interphase of the mixture and thus were
extracted into our biodiesel mixture.
The product of this filtration was then
taken to the evaporator, which further
purified the biodiesel by evaporating any
methanol, glycerol, or other volatile
products that were in the biodiesel
solution. From here, the volume of the
biodiesel was taken and converted to a
mass using the density of biodiesel and
thus the yield was calculated.
The
processes we
used in our
experiments
are ones that
are utilized in
a much larger
scale
by
industries.
The
batch
reactors
easily
allowed
us to
Figure 6: Evaporator
replicate the
esterification
reactions
that
are
performed by large-scale industries. [1]
One of our objectives was to analyze the
reaction mechanisms for the deesterification of ethyl benzoate to
benzoic acid. This was accomplished by
evaluating the second-order rate
constant, k, through analysis of the deesterification reaction.
The rate
constant, k, is affected by temperature,
concentration of ethyl benzoate, and the
ethanol mole fraction. Through the
analysis of the reaction mechanisms of
the formation of benzoic acid we gained
information that we can apply to the
production of different APIs.
Results
De-esterification of Ethyl Benzoate
The concentrations of the ethyl benzoate
experiment are shown below:
Time(min)
0.00
5.38
16.23
31.97
44.90
60.18
81.15
100.57
[BA]
0
0.029
0.059
0.052
0.056
0.075
0.072
0.074
[EB]
0.1
0.071
0.041
0.048
0.044
0.025
0.028
0.026
Predicted
0.1
0.080
0.057
0.040
0.032
0.026
0.021
0.018
Figure 7: Ethyl Benzoate Data
Solver minimized the difference of the
squares between the actual and
theoretical values to calculate a k value
of 0.47 M-1s-1 . Again, ‘k’ describes how
fast the concentration of the ethyl
benzoate decreases with respect to time.
Figure 8: EB concentration vs. time
The above graph plots the concentration
of the ethyl benzoate with respect to
time. The solid line shows the expected
concentration versus time according to
the second order integrated rate law.
Transesterification of Palm Oil
For the biodiesel reaction, the
main indicator was the percent yield.
According to stoichiometry, 440 grams
of biodiesel should have been produced
given the initial concentrations and
volumes. In reality, we produced 92.8
grams of biodiesel (115 mL) which
yields a density of 0.81 g/mL. The
actual density of biodiesel ranges from
0.86-0.90 g/mL.
92.8g biodiesel 1 mL =115 mL biodiesel
0.81 g
%Yield=Experimental=92.8 g = 21.1%
Predicted 440 g
The above calculations show the
determination of mass of the biodiesel
produced and its percent yield. Note that
the density of biodiesel depends on the
exact composition of the triglycerides
used, which can not be determined
definitively (the composition of the palm
oil is only given in percentage ranges of
the triglycerides).
Discussion/Conclusions
For the de-esterification reaction, we
obtained a rate constant of k=0.47 M-1s-1
at 40°C and 0.25 ethanol mole fraction.
The rate constant was determined
through analysis of the decreasing
concentration of ethyl benzoate as the
reaction progressed. The data that we
gathered for the concentration of ethyl
benzoate was slightly skewed from the
predicted values because of problems
with the experimental apparatus.
Specifically, the micropipette, which is
supposed to measure exact volumes, had
a slight crack that was discovered upon
completion of the experiment. This
minor fault prevented the necessary
precision that we needed and thus is
certainly a source of error.
The ‘k’ value that we calculated
corroborates with the literature value
(0.51 M-1s-1) at the same conditions and
ethanol mole fraction [2]. This
experiment provides a means for us to
find ways by which to make the
production of APIs more efficient. By
calculating the rate constant at different
temperatures and ethanol mole fractions
we can obtain the fastest means to carry
out the experiment. Faster processes &
better experimental designs in the
synthesis of APIs can help make the cost
of essential drugs cheaper. Thus this
research is applicable to pharmaceutical
companies that strive to make cheaper
drugs. The production of benzoic acid
gave us an insight into the efficient
synthesis of APIs as well as the
mechanisms of a de-esterification
reaction. This area of research and work
is vital to the progress of pharmaceutical
drugs and their effect on society.
Biodiesel was the chief focus of
the second stage of our research.
Biodiesel can be produced through many
ways but the method we researched was
the transesterification of palm oil.
Through this experiment we tried to see
if biodiesel could be produced in a
productive
way
using
the
transesterification of vegetable oils. We
used 500 grams of palm oil and mixed it
with 450 mL of methanol and added 8.5
grams of NaOH pellets.
Although the reaction was
inefficient, it is the most practical
method of producing biodiesel for
commercial use [3]. The ingredients for
biodiesel production are relatively
cheap, and thus biodiesel can become a
viable alternative to more expensive
gasoline. Biodiesel has a lot of potential
to become the primary fuel source of not
just the nation but also the world [3].
Additionally, biodiesel has a less of an
environmental impact than conventional
fuel sources. Biodiesel’s applications
are growing from just vehicular use to
domestic and industrial use. Research
into cheap and efficient synthesis of
biodiesel is pivotal to the resolution of
the world’s energy crisis [3].
On the contrary, our research
with the biodiesel helped us identify the
flaws of biodiesel and biodiesel
production. Primarily, biodiesel has a
relatively short shelf life and poor cold
flow properties. Although the majority
of it is liquid, biodiesel forms small solid
clumps at room temperatures, which
diminishes its ability to flow. Currently,
research is being performed by chemical
engineers to optimize the flow rate of
biodiesel by mixing it with different
solvents [4].
Related Work
Phillip Moseley and Mustafa Ohag
published a paper in 1997 dealing
directly with the thermodynamic
functions of the alkaline hydrolysis of
ethyl benzoate into benzoic acid.
Moseley and Ohag performed the same
experiment (the de-esterification of ethyl
benzoate) we did over 70 times, with
ethanol mole fractions ranging from 0.10.9 and temperatures from 5-45
centigrade at 5 degrees intervals. A
chart of their results is included below:
Figure 9: Rate constant values for
Moseley’s experiment [2]
This graph shows the rate constants, k,
throughout different ethanol mole
fractions (x-axis) and temperatures. As
a general trend, the rate constant
decreases with increasing ethanol mole
fraction and increases with increasing
temperature. Additionally, varying the
ethanol mole fraction has a more
pronounced effect at lower temperatures
(it is almost negligible at relatively high
temperatures).
Due to time constraints, we only
performed one experiment with the ethyl
benzoate (0.25 mole fraction, 40oC). We
gauged our accuracy by comparing our
experimental rate constant to the one
Moseley and Ohag calculated for the
same temperature and mole fraction.
For the biodiesel production
experiment, we referred to a previous
paper published by Invensys Foxboro, an
engineering firm
specializing in
commercial automation in terms of
production and manufacturing systems.
The paper outlines the different
ways that biodiesel is manufactured,
providing the following useful chart
which
details
a
generic
transesterification reaction to produce
biodiesel:
Chart 2: Biodiesel production flowchart
[3]
Note that biodiesel production is a
relatively cyclic process in that many of
the
byproducts
can
be
reprocessed/recycled back into the
reaction. Consequently, this is one of
the main reasons that biodiesel is a
feasible alternative energy source.
Future Work
Being that our research project covered
such a current issue (especially with the
biodiesel reaction), there exists myriad
possibilities for future work.
The
world’s oil reserves will only continue to
deplete, thus exacerbating the already
prevalent oil crisis. Additionally, the
world’s reliance on oil products has
severe environmental implications.
We could implement Moseley’s
design for his ethyl benzoate experiment
into our biodiesel experiment by using
different initial mole fractions and
temperatures to measure the percent
yield. This experiment would show
under which conditions the production
of biodiesel is most efficient.
As
with
any
experiment,
repetition breeds more accuracy and
precision. Because of time constraints,
we were only able to run the ethyl
benzoate and biodiesel reaction once
each.
In the future, it would be
beneficial to run each experiment again
at the same conditions and furthermore
at different conditions.
These
experiments would add validity to the
experiment and offer a wider scope of
analysis on both reactions.
Acknowledgements
We would like to thank the following
people without whom this experiment
would not have been able to take place.
First, the New Jersey Governor’s School
Board of Overseers for allowing the
Governor’s School of Engineering and
Technology to take place. Second, we
would like to thank Rutgers University
and Dean Don Brown. Next, we would
like to thank Blase Ur, Program
Coordinator, for not only arranging this
research project, but also for planning
this entire program, which has truly been
an invaluable experience. Additionally,
we thank Dr. Henrik Pedersen, Chemical
Engineering Department Chair, for his
everyday guidance and breadth of
knowledge as our primary project
advisor. We also thank Patrick Nwaoko,
our counselor advisor for his assistance
throughout this entire experience. Most
importantly, we’d like to thank the
following program sponsors who
sustained this Governor’s School
program in a financially difficult year:
Prudential, Morgan Stanley, Rutgers
University, the John and Margaret Post
Foundation, and John and Laura
Overdeck. We finally thank the entire
Governor’s School staff for their
friendship and guidance regarding not
only this research project but also the
program as a whole.
References
[1] – Pedersen, Henrik. The Batch Reactor.
[2] – Moseley, Phillip, and Mustafa Ohag. "Thermodynamic functions of activation of
the alkaline hydrolysis of ethyl benzoate and of ethyl p-nitrobenzoate in
ethanol–water mixtures of various compositions at different temperatures."
(1997).
[3] – "Guide to Instrumentation for Biodiesel Fuel Production." Invensys Foxboro
[4] – Levine, Brett. "Batch Reactor." E-mail to Michael Boczon. 15 July 2008.
[5] – Harris, Daniel C. "Nonlinear Least-Squares Curve Fitting with Microsoft Excel
Solver." Computer Bulletin Board. 15 July 2008
<http://jchemed.chem.wisc.edu/Journal/issues/1998/jan/abs119.html>.
[6] - http://en.wikipedia.org/wiki/Image:Generic_Biodiesel_Reaction1.gif