Homogeneous Tubular-Flow Process for Monoolein Preparation

Article abstract of DOI:10.1007/s11746-015-2707-3

Esterification of fatty acids with glycerol is characterized by negligible solubility of the two liquid phases. The reactions to mono-, di- and triglycerides taking place in the fatty acid phase, are limited by chemical equilibrium. The scope of this study is to investigate in a tubular reactor the conversion of a homogeneous mixture of oleic acid and glycerol in tert-butanol. The liquid composition in this study was 1 mol of oleic acid, 6 mol of glycerol and 14 mol of tert-butanol. Experiments were conducted in a tubular reactor at 35 atm over a temperature range of 200-240 °C and residence times of 0.7-17.6 h to determine the kinetics and the chemical equilibrium. The selectivity to monoolein was >95 mol %. A reversible second order reaction fits the data well.

Full text of DOI:10.1007/s11746-015-2707-3

J Am Oil Chem Soc (2015) 92:1525–1529
DOI 10.1007/s11746-015-2707-3
LETTER TO THE EDITOR
Homogeneous Tubular‑Flow Process for Monoolein Preparation
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Moti Herskowitz · Miron V. Landau · Slava Koukouliev · Ruby Zarchin ·
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Roxana Vidruk Nehemya · Lik H. Wee · Johan A. Martens
Received: 11 September 2014 / Revised: 2 August 2015 / Accepted: 6 August 2015 / Published online: 15 August 2015
AOCS 2015
©
Abstract Esterification of fatty acids with glycerol is
characterized by negligible solubility of the two liquid
phases. The reactions to mono-, di- and triglycerides taking
place in the fatty acid phase, are limited by chemical equi-
librium. The scope of this study is to investigate in a tubu-
lar reactor the conversion of a homogeneous mixture of
oleic acid and glycerol in tert-butanol. The liquid composi-
tion in this study was 1 mol of oleic acid, 6 mol of glycerol
and 14 mol of tert-butanol. Experiments were conducted
in a tubular reactor at 35 atm over a temperature range of
MO
TG
TO
ZIF
Monoolein
Triglycerides
Triolein
Zeolitic imidazolate framework
Introduction
The conversion of low-cost and abundant renewable raw
materials into valuable chemical products is a signifi-
cant scientific and technological challenge with a major
environmental and economic impact. Extensive research
has been carried out to develop green technologies for
the utilization of glycerol, a by-product of the biodiesel
industry [1, 2]. An important application is the conver-
sion of glycerol to valuable monoglycerides (MG) that
provide a wide range of applications such as emulsifi-
ers in food, pharmaceutical and cosmetic, detergents and
lubricants [3]. The methods of preparation and produc-
tion of mono- and diglycerides (DG) by esterification,
transesterification and other reactions and their applica-
tions were reviewed recently [4]. Industrially, MG are
produced by glycolysis of fats, oils and fatty acids in
2
00–240 °C and residence times of 0.7–17.6 h to determine
the kinetics and the chemical equilibrium. The selectivity
to monoolein was >95 mol %. A reversible second order
reaction fits the data well.
Keywords Homogeneous tubular-flow process ·
Monoolein production in tert-butanol
Abbreviations
BSTFA N,O-Bis(trimethylsilyl) trifluoroacetamide
DG
DO
GC
HPLC
ID
Diglycerides
Diolein
Gas chromatograph
High-pressure liquid chromatography
Internal diameter
Monoglycerides
presence of inorganic bases (NaOH or Ca(OH) ) at 220–
2
260 °C. The product contains 45–55 % MG, 38–45 %
DG and 8–12 % triglycerides (TG) (Fig. 1) [3]. Many
catalytic, homogeneous and heterogeneous, as well as
enzymatic methods have been studied. Processes with
strong acid catalysts such as H SO and H PO operated
at 240–250 °C [5] yield 40–55 % MG. Operation at such
high temperatures, especially for food applications, may
also degrade the taste, aroma and color. The methods
employed for purification of MG, e.g., distillation, are
very complex, expensive and energy consuming while
reusing the catalyst is not feasible.
MG
*
Moti Herskowitz
herskow@exchange.bgu.ac.il
2
4
3
4
1
Department of Chemical Engineering, Blechner Center
for Industrial Catalysis and Process Development, Ben-
Gurion University of the Negev, 84105 Beer-Sheva, Israel
2
Centre for Surface Chemistry and Catalysis, KU Leuven,
Kasteelpark Arenberg 23, 3001 Heverlee, Belgium
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J Am Oil Chem Soc (2015) 92:1525–1529
Fig. 1 Esterification reaction of glycerol with oleic acid
Stable and active heterogeneous catalysts such as zeo-
lites, sulfonated mesoporous silica, ion-exchange res-
ins, enzymes and metal–organic frameworks [6, 7] were
employed in preparation of MG by esterification of fatty
acids with glycerol at lower temperatures. Corma et al. [8]
reported the transesterification of triolein with glycerol on
solid basic catalysts sepiolite-Cs, MCM-41-Cs, MgO and
hydrotalcites. MgO yielded 97 % triolein conversion and
acid catalyzed reaction. The oleic acid substrate itself
is a catalyst. The esterification reaction of carboxylic
acid with glycerol has also been reported previously
by Jacobs and co-workers [11]. The ionization constant
of the fatty acid chain is very much dependent on the
reaction temperature. Given the fact that the reaction is
autocatalytic with respect to oleic acid, using a solvent
that dissolves both reactants forming a homogeneous
solution is a viable and promising route. Thus the com-
position can be adjusted so as increase the selectivity to
MG and the conversion of the fatty acid. The practical
merits and drawbacks that would determine its commer-
cial application are expected to be done separately. Most
studies published in the literature were performed in
batch reactors. Tubular flow reactors are a preferred con-
figuration that are run at steady-state and could produce
a high selectivity to MG at a relatively high conversion
of oleic acid. The scope of this study is to explore for the
first time the possibility of performing a homogeneous
continuous, tubular-type process.
7
5 % monoolein selectivity after 5 h at 240 °C.
Glycerol and oleic acid are essentially immiscible liq-
uids. A detailed study of this system [9] found that the
glycerol solubility in the phase composed of oleic acid,
monoolein (MO), diolein (DO) and triolein (TO) at 170 °C
was 4–5 wt%, essentially independent of composition. The
oleic acid solubility in glycerol was negligible. Therefore,
the reaction takes place only in the oleic acid phase. Fur-
thermore, it is expected that, given the great excess of oleic
acid, the formed MO reacts further to DO and TO thus
reducing selectivity with increasing oleic acid conversion.
Although the esterification is reversible, the conversion
of oleic acid measured in the batch reactor at 140–160 °C
[
9] reached >90 %, since the water produced in the reac-
tion was transferred to the glycerol phase. The selectivity
to the three products changed little with oleic acid conver-
sion, being very close to its equilibrium values. Enhancing
solubility is expected to improve selectivity. A recent patent
application [10] proposes to use water as co-solvent at high
temperatures (250 °C). The patent claims that a high yield
of MG of up to 90 % was measured. Recently, we reported
Experimental
The esterification was carried out in a bench-scale high-
pressure tubular reactor (Fig. 2), 0.65 cm ID and 132 cm
total length, made of stainless-steel. The reactor was placed
in a high temperature oven to ensure isothermal operation.
The liquid was pumped from the feed tank upflow to the
reactor by a HPLC pump. The residence time in the reactor
was set by adjusting the liquid flow rate. The pressure in the
reactor was kept at 35 atm by a backpressure regulator to
avoid vaporization over the temperature range 200–240 °C.
Most experiments were conducted in the empty reactor. In
some runs, 30 % of the reactor was packed with 70–120 µm
SiC inert particles.
1
00 % selective synthesis of MO at 57 % oleic acid conver-
sion over hierarchical ZIF-8 catalyst at 150 °C after 24 h in
the presence of t-butanol [6, 7].
In spite of the many studies published on this impor-
tant topic, little progress has been made in improving
the performance, especially the selectivity to MG. The
esterification reaction of oleic acid with glycerol is an
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J Am Oil Chem Soc (2015) 92:1525–1529
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Fig. 2 Experimental setup: (1)
balance, (2) feed tank, (3) high
pressure pump, (4) pressure
controller, (5) thermocouple, (6)
thermowell, (7) tubular reactor,
(
(
8) oven, (9) product tank,
10) pressure gauge, (11) back
pressure regulator, (12) nitrogen
cylinder
All experiments were carried with a mixture composed
of oleic acid (Sigma-Aldrich, Belgium), glycerol (Bio-Lab
Ltd, Israel) and tert-butanol (Alfa Aesar, UK) at a molar
ratio of 1:6:14. The concentration of oleic acid was deter-
mined by organic acid–base titration (0.01 M KOH in etha-
nol, Merck, Belgium). Glycerol mono-, di- and triolein
were analyzed by a well-established GC method [12] using
BSTFA (Sigma-Aldrich, Belgium). 0.03 g of product were
mixed with 0.03 g of tetradecane (internal standard). 0.7 g
of BSTFA and 0.7 g of pyridine were added. After 20 min
in oven at 80 °C the solution was injected immediately into
the warm (50 °C) HP6890 GC equipped with injector and
a flame ionization detector and 10 m × 0.32 mm ID fused-
silica capillary column coated with a 0.1 μm film of DB-5.
The GC was programmed to heat from 60 to 190 to 420 °C,
at a ramp rate of 50 °C/min to 190 °C and a second ramp
rate of 6 °C/min to 420 °C.
particular residence time was measured after running the
reactor for at least two times that residence time to ensure
that a steady-state was reached. Indeed, measurements car-
ried out at several times on stream indicated that steady-
state was reached. No apparent effect of packing the reactor
with inert SiC was recorded, as shown in Fig. 3.
The measured selectivity of MO was >0.95. The other
product was DO Neither TO nor butyl ester was detected.
There is no esterification reaction between oleic acid with tert-
butanol as the alcohol part is sterically hindered around the
ester bond. This was expected since the molar ratio of glyc-
erol to oleic acid in the solution was 6. This is by far higher
than the value in the heterogeneous system that contains only
glycerol and oleic acid where the very low glycerol solubility
limits this ratio to ≪1. Therefore, esterification of oleic acid
with glycerol is very selective to MO, in contrast with other
systems published in the literature. Therefore the analysis of
the data will account only for the esterification to MO.
The equilibrium constants of the reaction at 220 and
240 °C, calculated from the data measured at long resi-
dence times, are listed in Table 1. The two values indicate
that the reaction over this temperature range is very mildly
endothermic. The equilibrium constant at 200 °C, calcu-
lated by extrapolation of the two values, is 0.57.
Results and Discussion
Experiments were carried out at 200, 220 and 240 °C. The
residence time in the reactor was varied over a wide range
by changing the liquid flow rate. The performance at a
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J Am Oil Chem Soc (2015) 92:1525–1529
Fig. 3 Second-order reaction
expression displays a very good
fit of kinetic data
0.9
0
0
0
0
0
0
0
0
.8
.7
.6
.5
.4
.3
.2
.1
0
Fiꢁed 200℃
Fiꢁed 220℃
Fiꢁed 240℃
Experimental 200℃
Experimental 220℃
Experimental 240℃
Experimental 200℃ SiC
0
2
4
6
8
10
12
14
16
18
20
Residence ꢀme (h)
Table 1 Chemical equilibrium constants
x is the oleic acid conversion, θ is the residence time, E(t) is
the residence time distribution, E is the activation energy,
Temperature (°C)Residence time Oleic acid con- Equilibrium
a
(h)
version
constant
C10 is the oleic acid feed concentration and K is the chemi-
cal equilibrium constant. The solution [13] of Eq. (3) is:
2
20
40
17.6
6.4
0.8
0.62
0.67
2
0.81
x(t)
ꢀ
ꢁ
√
√
√
√ √
− D K 24 + 25K
−C
1
k
√24+25Kt
0
7
+
K
24 + 25K · tan h 0.5 ·
K
=
−
2 + 2K
(
5)
6)
The kinetic data are plotted in Fig. 3. Calculation of the
Reynolds number from estimated density and viscosity of
the tert-butanol yields values in the order of 10 meaning
that the flow was laminar. Therefore the data were analyzed
according the method developed for a laminar flow reactor
given in expression (1) and assuming a second rate of reac-
tion as shown in Eq. (3):
ꢀ
ꢁ
−
1
√−7
K
2 · tan h
√ √
K 24 + 25K
2
4+25K
(
D =
−
The rate constant was calculated by fitting the calculated
value of x from expression (1) to the measured values at
each temperature. The goodness of fit, defined as ∑ (x
exp–
∞
2
−3
−5
xfitted) , varied from 2.5 × 10 at 220 °C to 6.5 × 10 at
the other two temperatures. This is clearly illustrated by the
curves in Fig. 3. The activation energy calculated from the
x = ∫ x(t)E(t)dt
θ
(
1)
2
7
where E(t) is defined as:
plot in Fig. 4 is 81 kJ/mol, A is 5.3 × 10 L/mol/h and the
2
coefficient of determination is r = 0.98.
θ2
E(t) =
(2)
3
2
· t
Conclusions
ꢀ ꢁ
kC10 (6 − x)(1 − x) − x /K
dx
dt
2
=
The esterification of oleic acid with glycerol (molar ratio
of 1:6) dissolved in tert-butanol was carried out in a tubu-
lar reactor at 35 atm and 200–240 °C. The residence time
in the reactor was varied over a wide range to determine
selectivity to monoolein, diolein and triolein, the reac-
tion kinetics and equilibrium constant. A high selectivity
to monoolein of >95 mol% was measured. It was shown
x = 0 at t = 0
(3)
(4)
The rate constant k is defined as:
ꢀ
ꢁ
E
a
k = A exp −
RT
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J Am Oil Chem Soc (2015) 92:1525–1529
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-
-
-
-
0.50
1.00
1.50
2.00
-
-
-
2.50
3.00
3.50
0
.00190
0.00195
0.00200
0.00205
0.00210
0.00215
5
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1
/T (1/K)
Fig. 4 Estimation of activation energy
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glycerol to oleic acid molar ratio. Fitting the data to a sec-
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energy of 81 kJ/mol.
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government for long-term structural funding (Methusalem).
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