Saponification reaction system: A detailed mass transfer coefficient determination

Article abstract of DOI:10.17344/acsi.2014.1110

The saponification of an aromatic ester with an aqueous sodium hydroxide was studied within a heterogeneous reaction medium in order to determine the overall kinetics of the selected system. The extended thermo-kinetic model was developed compared to the previously used simple one. The reaction rate within a heterogeneous liquid-liquid system incorporates a chemical kinetics term as well as mass transfer between both phases. Chemical rate constant was obtained from experiments within a homogeneous medium, whilst the mass-transfer coefficient was determined separately. The measured thermal profiles were then the bases for determining the overall reaction-rate. This study presents the development of an extended kinetic model for considering mass transfer regarding the saponification of ethyl benzoate with sodium hydroxide within a heterogeneous reaction medium. The time-dependences are presented for the mass transfer coefficient and the interfacial areas at different heterogeneous stages and temperatures. The results indicated an important role of reliable kinetic model, as significant difference in k_La product was obtained with extended and simple approach.

Full text of DOI:10.17344/acsi.2014.1110

DOI: 10.17344/acsi.2014.1110
Acta Chim. Slov. 2015, 62, 237–241
237
Short communication
Saponification Reaction System:
a Detailed Mass Transfer Coefficient Determination
Darja Pe~ar* and Andreja Gor{ek
Faculty of Chemistry and Chemical Engineering, University of Maribor,
Smetanova 17, SI-2000 Maribor, Slovenia
* Corresponding author: E-mail: darja.pecar@um.si
Tel.: +38622294442
Received: 09-10-2014
Abstract
The saponification of an aromatic ester with an aqueous sodium hydroxide was studied within a heterogeneous reaction
medium in order to determine the overall kinetics of the selected system. The extended thermo-kinetic model was deve-
loped compared to the previously used simple one. The reaction rate within a heterogeneous liquid-liquid system incor-
porates a chemical kinetics term as well as mass transfer between both phases. Chemical rate constant was obtained
from experiments within a homogeneous medium, whilst the mass-transfer coefficient was determined separately. The
measured thermal profiles were then the bases for determining the overall reaction-rate.
This study presents the development of an extended kinetic model for considering mass transfer regarding the saponifi-
cation of ethyl benzoate with sodium hydroxide within a heterogeneous reaction medium. The time-dependences are
presented for the mass transfer coefficient and the interfacial areas at different heterogeneous stages and temperatures.
The results indicated an important role of reliable kinetic model, as significant difference in kLa product was obtained
with extended and simple approach.
Keywords: Kinetics; mass transfer; saponification; heterogeneous system
equipment. Those properties that affect the reaction rate
are the densities and viscosities during each phase, inter-
1. Introduction
Industrial applications such as reactions, extrac-
tions, and emulsifications are often conducted within he-
terogeneous systems. Proper knowledge about kinetics
and mass transfer between phases is essential, especially
when any changes in operating conditions (temperatures,
compositions, stirring speed) and equipment need to be
taken into account.
It should be noted that special concern has to be fo-
cused on safety analyses regarding those runaway reac-
tions that involve two immiscible liquid-liquid phases,
one being mainly organic and the other aqueous. The im-
portant heterogeneous reactions where chemical reaction
and mass transfer occur simultaneously are sulfonations,
saponifications, nitrations, hydrochlorations, esterifica-
tions, hydrolyses, and oxidations.1,2
facial surface properties, diffusion coefficients of the rea-
gents and products, and the reaction rate constants. From
amongst the mechanical features of the equipment, the
geometry of the reactor, type and diameter of the stirrer,
flow rates of each phase, and stirrer speed are important.3
The influence of the afore-mentioned parameters has to be
determined experimentally. Due to the complexity of he-
terogeneous liquid-liquid reactions’ modeling, a lot of ef-
fort should be invested in experimental, theoretical, and
computational work.
Mass-transfer is often a rate-limiting process, thus
much of the research is oriented towards its intensifica-
tion. Because mass transfer efficiency also depends on the
process equipment mini or micro-reactors are very often
used, which can be especially competitive for fast and
exothermal reactions.4–6
In heterogeneous systems the overall reaction rate
and conversion depend, not only on the chemical and
physical properties of the studied system, but also on the
mechanical and geometrical characteristics of the used
After saponification reactions, the products become
species with surface-active properties having influence on
interfacial properties, and alcohols that usually act as co-
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Acta Chim. Slov. 2015, 62, 237–241
238
solvents changing the solubility properties of the reagents.
transfer of sodium hydroxide to the surface of ethyl ben-
zoate droplets, followed by reaction on the surface. The
chemical kinetic term for this second order reaction is:
Altogether they can greatly modify the mass-transfer and
affect the overall reaction rate.7
An extended kinetic model for sodium benzoate
synthesis within heterogeneous media was developed du-
ring this work. Although some saponification reactions
have been reviewed by other researchers,7–9 no data about
our selected system has yet been published. During this
study we turned our attention towards extending a previ-
ously-simplified thermo-kinetic model valid for heteroge-
neous liquid-liquid reaction systems, where several as-
sumptions were used for mass transfer estimation.10 In
this work the mass transfer coefficient, interfacial area,
and the kLa product were estimated precisely using expe-
rimentally determined thermal power profiles.
(1)
where cA is the concentration of sodium hydroxide on the
surface of ethylbenzoate droplets, t time and k the reaction
rate constant.
The rate expression considering mass transfer
among two liquid phases is:
(2)
where kLa is the mass transfer coefficient multiplied by
the interfacial area, and cAL the concentration of sodium
hydroxide in the bulk liquid.
By the assumption of two steps in the series relating
to the overall rate of reaction (-rAK = –rAM) and after rear-
rangements the final form of the rate equation becomes:
2. Experimental
2. 1. Experimental Procedure
The experimental procedure, equipment, and mate-
rials used are described in our previous study.10 Only a
brief description will be given here. All chemicals, ethyl
benzoate, ethanol, sodium benzoate and sodium hydro-
xide are commercially available and were used as recei-
ved without further purification. The experiments were
conducted within a RC1 reaction calorimeter (Mettler
Toledo), which measures the heat flow that corresponds
to the temperature difference between the temperature of
the reactor’s content and the temperature of the reactor’s
jacket. The reactor (V = 2 L) was equipped with an anc-
hor glass stirrer, a glass AP01 temperature sensor, and a
25 W glass calibration heater. RC1 is a double walled
glass-made automatic laboratory batch reactor. Heat
flow was further used for calculating the heat during the
reaction. The individual experiment was started by char-
ging the reactor with ethyl benzoate, ethanol, and water.
The reaction was triggered by the addition of sodium
hydroxide solution. The reaction was performed at fixed
rotational frequency of the stirrer, N = 80 min–1, and
reaction media temperatures of ϑ = (22, 26, 30, 34, and
38) °C. Ethanol was added to the ethyl benzoate as a co-
solvent. For studying different stages of heterogeneity
different ethanol mole fractions (xEtOH = 0.1, 0.2, 0.3)
were applied. The experiments were performed at equi-
molar initial concentrations of ethyl benzoate and so-
dium hydroxide, c0 = (1.17, 1.14, 1.11) mol/L depending
on ethanol mole fraction.
(3)
where kL is the mass transfer coefficient, and a the interfa-
cial area.
The interfacial area within a heterogeneous liquid-
liquid media is crucial for determining the kinetic parame-
ters when considering mass transfer between phases, and
is important for any further designing and developing of a
process. From amongst several parameters affecting the
interfacial area, the composition of the reaction mixture,
stirrer type, and stirrer speed are the more important ones.
The interfacial area per continuous phase volume unit was
calculated using a Sprow’s type correlation as:8
(4)
where D is the stirrer diameter, α the dimensionless para-
meter (it was set to 0.2, numerical values vary between
0.04 and 0.48), We the Weber number, and εd the volume
ratio between the dispersed and continuous phases:
(5)
The volume of the dispersed phase Vd, was calcula-
ted from:
2. 2. Kinetic Model
The kinetic model was presented in detail in our pre-
vious study,10 only the major points are highlighted here.
The overall rate expression regarding the base-hydrolysis
of ethyl benzoate with sodium hydroxide in a heterogene-
ous medium incorporates two rate steps in series – mass
(6)
where VB0 is the initial volume of ethyl benzoate, nB0 the
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Acta Chim. Slov. 2015, 62, 237–241
239
initial mole number of ethyl benzoate, and nB the mole
number of ethyl benzoate.
When calculating the volume of the continuous
phase Vc, all the instantly present components (i) of the
continuous phase were taken into account:
sity of the continuous phase was calculated for each hete-
rogeneous stage, and was assumed to be constant during
the reaction (Table 1).
The surface tensions for both phases were measured
using a Krüss tension-meter (Table 2).
(7)
Table 2: Surface tensions of the dispersed and continuous phases at
the temperature, ϑ = 30 °C. Concentration dependence was not per-
ceived.
Weber’s number can be obtained from:
(8)
phase
γ/mN m–1
34.8
29.6
dispersed
continuous
where ρc is the density of the continuous phase, N the stir-
rer speed, and σ the interfacial tension.
Four solutions of different concentrations were pre-
pared, according to reaction progress for determining the
surface tension of the continuous phase. There was no
clear concentration dependence therefore the average va-
lue was taken. The surface tension of the dispersed phase
corresponded to pure ethyl benzoate.
Good and Girifalco’s model was used for the calcu-
lation of interfacial tension:
(9)
where γc is the surface tension in the continuous phase, γd
the surface tension of the dispersed phase, and ϕ the inte-
raction parameter, calculated as:
For calculating the overall reaction rate the interfa-
cial area versus time has to be known. The mass transfer
coefficients, kL, during the reaction were calculated using
Matlab software (eq. (3)). This values were compared on-
ly with that of similar reaction system4 (alkaline hydroly-
sis of methyl benzoate) and are the same order of magni-
tude (5.10–6 m s–1). The simulation results at a temperatu-
re of ϑ = 30 °C, and the ethanol mole fraction xEtOH = 0.3,
are presented in Figure 1. The reaction rate constants, k,
were previously determined from the data regarding the
homogeneous mixture.10
(10)
where Vmd is the molar volume of the dispersed phase and
Vmc the molar volume of the continuous phase.
3. Results and Discussion
The calculation procedure for obtaining the reaction
rates from calorimetric data has been explained in our pre-
vious work.10 Mass transfer has to be avoided in order to
study sodium benzoate’s effect on chemical kinetics. Et-
hanol was added to the reaction media for reaching the
homogeneity.
The concentration of sodium hydroxide within the
bulk liquid, as obtained from the calorimetric data, was
used for surface area determination. Throughout the reac-
tion’s progress the volume of the dispersed phase regar-
ding ethyl benzoate decreased whilst the volume of the
continuous phase increased. Both volumes were calcula-
ted by considering their concentration profiles. The den-
Figure 1. Relationship between interfacial area, mass transfer coef-
ficient and kLa product at the temperature, ϑ = 30 °C and ethanol
mole fraction, xEtOH = 0.3. Rotational frequency of the stirrer was
constant for all experiments, N = 80 s–1.
Table 1: Densities of the continuous phase for different heteroge-
For our reaction system we presumed the reaction
is taking place on the surface of the ethyl benzoate dro-
plets and it starts when sodium hydroxide diffuses to the
surface. Figure 1 shows that during the progress of the
reaction, the droplets of ethyl benzoate slowly disappear
and consequently the interfacial area decreases. The
neous stages. Constant densities were assumed during the reaction.
xEtOH
ρc /kg dm–3
0.1
0.2
0.3
0.96
0.93
0.90
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Acta Chim. Slov. 2015, 62, 237–241
mass transfer coefficients increase whilst the kLa pro-
240
Using the simple kinetic model, the kLa product was
determined without any time-dependence. For the purpose
of comparison we calculated the average values of kLa
products for the extended model (Table 3). They were cal-
culated considering kLa products from complete time ran-
ge at a certain temperature and heterogeneity stage.
ducts decrease. Temperature and concentration depen-
dence of the mass transfer coefficients, is presented in
Figure 2.
Table 3: Comparisons between the kLa products, at different hete-
rogeneity stages, calculated for both the simple (S)10 and extended
(EM) kinetic models at the temperature, ϑ = 30 °C.
xEtOH
kLa (S)/10–4 s–1
kLa (EM)/10–4 s–1
0.1
0.2
0.3
2.9
9.4
19.4
1.8
3.6
4.4
It is evident from Table 3, that the values obtained
from the extended model are in general lower compared to
the ones obtained from the simple one. This difference
was expected because many assumptions were used in the
simple model. Nevertheless, it can be seen that the values
for the kLa product increase with any increasing mole
fraction of ethanol when the reactive mixture becomes
more homogeneous.
Finally, knowing mass transfer coefficient and che-
mical kinetic term, the overall reaction rate can be calcu-
lated according to Eq. (3). The comparison of experimen-
tal and calculated values of reaction rate is presented in
Figure 4.
Figure 2. Surface plot of mass transfer coefficients, kL, as a func-
tion of temperature and sodium hydroxide concentration in the bulk
liquid, at mole fraction of ethanol, xEtOH = 0.1.
It is evident from the surface plot (Figure 2) that a
larger increase occurred in the values for mass transfer
coefficient at the end of the reaction, where the mass-
transfer was no longer within the rate-limiting process.
The mass transfer coefficients also increased with increa-
sing temperature.
The dependence of the mass transfer coefficients on
the ethanol mole fraction, is presented in Figure 3.
Figure 4. Experimental and calculated reaction rates, -rA, as a
function of time, at the temperature, ϑ = 30 °C and mole fraction of
ethanol, xEtOH = 0.1.
Figure 3. Surface plot of mass transfer coefficients, kL, as a func-
tion of sodium hydroxide concentration in the bulk liquid and mole
fraction of ethanol, at the temperature, ϑ = 30 °C.
Figure 3 shows that when approaching a more ho-
mogeneous mixture at the end of the reaction, the values
of the mass transfer coefficients increased steeply, as they
also increased with increasing initial mole fractions of et-
hanol.
As we can see in Figure 4 the agreement between
experimental and calculated values of reaction rate is
good. Only at the beginning of the reaction the discrepan-
cies among both data sets are larger, which is due to the
significant changes of the reaction rate.
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Acta Chim. Slov. 2015, 62, 237–241
241
ρc = density of continuous phase (kg m–3)
4. Conclusions
σ
ϕ
A
B
c
= interfacial tension (mN m–1)
= interaction parameter
= component A
The saponification of ethyl benzoate with sodium
hydroxide in heterogeneous liquid-liquid reaction mixture
was used to test an extended thermo-kinetic model, by ta-
king into account mass transfer between both phases.
The results show that during the reaction in the hete-
rogeneous liquid-liquid mixture the interfacial area de-
creases whilst the mass transfer coefficient increases. The
product of the mass transfer coefficient and interfacial
area, kLa, decreases during the reaction because any de-
crease in the interfacial area prevails over any increase in
the mass transfer coefficient.
= component B
= continuous phase
= dispersed phase
d
EtOH = ethanol
i
m
0
= component i
= molar
= initial (at time t = 0)
The obtained results lead us to the conclusion that
for any prediction by a reliable kinetic model it is obliga-
tory to evaluate the rate steps, the chemical kinetics, and
the mass transfer precisely, using minimum assumptions.
This is extremely important for industrial applications and
safety studies.
6. References
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Process Intensif. 2000, 39, 521–537.
http://dx.doi.org/10.1016/S0255-2701(00)00099-4
2. B. A. A.Van Woezik, K. R.Westerterp, Chem. Eng. Process.
Process Intensif. 2002, 41, 59–77.
http://dx.doi.org/10.1016/S0255-2701(01)00106-4
3. J. M. Zaldívar, E.Molga, M. A. Alós, H. Hernández, K. R.
Westerterp, Chem. Eng. Process. Process Intensif. 1996, 35,
91–105.
5. Nomenclature
a
= interfacial area (m2 m–3)
cAL = concentration of sodium hydroxide in the bulk li-
http://dx.doi.org/10.1016/0255-2701(95)04119-2
4. G. Dummann, U. Quittmann, L. Gröschel, D. W. Agar, O.
Wörz, K. Morgenschweis, Catal. Today 2003, 79–80, 433–
439.
http://dx.doi.org/10.1016/S0920-5861(03)00056-7
5. Y. Su, Y. Zhao, G. Chen, Q. Yuan, Chem. Eng. Sci. 2010, 65,
3947–3956.
http://dx.doi.org/10.1016/j.ces.2010.03.034
6. R. S. Abiev, I. V. Lavretsov, Chem. Eng. Sci. 2012, 74, 59– 68.
http://dx.doi.org/10.1016/j.ces.2012.02.024
7. N. Mouhab, L. Balland, I. Ben Talouba, J.-M. Cosmao,
Chem. Eng. Process. Process Intensif. 2008, 47, 363–369.
http://dx.doi.org/10.1016/j.cep.2007.01.016
8. N. Mouhab, L. Balland, I. Ben Talouba, J.-M. Cosmao,
Chem. Eng. Sci. 2007, 62, 1456–1465.
quid (mol L–1)
D
k
= stirrer diameter (m)
= reaction rate constant (L mol–1 s–1)
kL = mass transfer coefficient (m s–1)
kLa = kLa product (s–1)
nA = number of moles which corresponds to the compo-
nent A (mol)
N
= rotational frequency of the stirrer (s–1)
rA = overall reaction rate (mol L–1 s–1)
t
V
= time (s)
= volume (m–3)
We = Weber number
x
= mole fraction
α
γ
= dimensionless parameter
= surface tension (mN m–1)
http://dx.doi.org/10.1016/j.ces.2006.11.041
9. X.-C. Zeng,Y. Chen,Y.-Q. Zhang, X.-N. Chen, Thermochim.
Acta 1998, 317, 151–156.
εd = volume ratio between dispersed and continuous
phase
ϑ
= temperature (°C)
http://dx.doi.org/10.1016/S0040-6031(98)00376-1
10. D. Pe~ar, A. Gor{ek, Chem. Eng. Technol. 2011, 34, 2033–
2036.
http://dx.doi.org/10.1002/ceat.201100186
Povzetek
Umiljenje aromatskega estra v vodni raztopini natrijevega hidroksida smo preu~evali v heterogenem reakcijskem me-
diju z namenom razvoja kineti~nega modela. Za primerjavo s predhodno uporabljenim preprostim modelom je bil raz-
vit raz{irjen termo-kineti~ni model. Proizvodnost v heterogenem teko~e-teko~e sistemu smo opisali z izrazom, ki
upo{teva tako kemijsko kinetiko, kakor tudi prenos snovi med obema fazama. Konstanta proizvodnosti je bila pridoblje-
na iz poskusov v homogenem mediju, medtem ko je bil koeficient snovnega prenosa dolo~en posebej. Izmerjeni ter-
mi~ni profili so bili osnova za dolo~itev proizvodnosti reakcije. Prikazane so ~asovne odvisnosti koeficienta snovnega
prenosa in medfazne povr{ine pri razli~nih stopnjah heterogenosti in temperaturah. Iz rezultatov je razvidno, da razlika
v kLa produktu, dobljenim z raz{irjenim in enostavnim modelom obstaja, kar potrjuje pomembnost izbire zanesljivega
kineti~nega modela.
Pe~ar and Gor{ek: Saponification Reaction System: ...

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