Separation and Purification Technology 132 (2014) 272–280
Contents lists available at ScienceDirect
Separation and Purification Technology
journal homepage: www.elsevier.com/locate/seppur
Accelerated decantation of biodiesel–glycerol mixtures: Optimization
of a critical stage in biodiesel biorefinery
Afifa Noureddin a,b, Mohmmad Mahdi A. Shirazi c,⇑, Joumana Tofeily a,b,d, Pezhman Kazemi b,1,
Elahe Motaee e,1, Ali Kargari f, Mostafa Mostafaei g, Mandana Akia e, Ali Karout a, Rana Jaber a,
Tayssir Hamieh a, Meisam Tabatabaei b,⇑
a
Laboratory of Materials, Catalysis, Environment and Analytical Methods, Faculty of Sciences, Lebanese University, Campus Rafic Hariri, Beirut, Lebanon
Biofuel Research Team (BRTeam), ABRII, Karaj, Iran
Young Researchers and Elites Club, Omidieh Branch, Islamic Azad University, Omidieh, P.O. Box 164, Iran
d
School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164, USA
e
Chemistry & Chemical Engineering Research Center of Iran, Tehran, Iran
f
Membrane Processes Research Laboratory (MPRL), Department of Petrochemical Engineering, Amirkabir University of Technology (Tehran Polytechnic),
Mahshahr Campus, Mahshahr, Iran
g
Mechanical Engineering of Agricultural Machinery Department, Faculty of Agriculture, Razi University, Kermanshah, Iran
b
c
a r t i c l e
i n f o
Article history:
Received 3 December 2013
Received in revised form 6 May 2014
Accepted 11 May 2014
Available online 17 May 2014
Keywords:
Biodiesel
Glycerol
Decantation
Acceleration
Optimization
a b s t r a c t
When making biodiesel, slow separation of glycerol; the main by-product of the transesterification
reaction, could lead to longer operating times, bigger equipment and larger amount of steel and
consequently increased production cost. Therefore, acceleration of glycerol/biodiesel decantation could
play a significant role in economic viability of the overall biodiesel refinery process. In the present study,
the interactive effects of prominent parameters i.e. temperature (25–65 °C), NaCl addition (0–2 g/
100 ml), and methanol concentrations (10–30 vol.%) on decantation behavior of glycerol/biodiesel
mixture were studied using Box–Behnken design matrix and response surface methodology (RSM). The
findings revealed that in low temperature ranges, the main factor influencing decantation speed was density difference while at high temperature ranges, viscosity variation played the major role. Moreover,
decantation time was significantly decreased by 200% (3 min) at the optimum conditions achieved i.e.
45 °C, 1 g NaCl addition and 20% excess methanol. The methyl ester yield was measured at >90% under
optimum conditions which was increased to ASTM quality (>96.5%) after washing and drying procedures.
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
The soaring price of polluting petroleum-derived fuel, as well as
growing environmental concerns such as global warming and climate change are major factors marking alternative energy sources
indispensible [1]. Various types of biofuels i.e. solid (bio-char),
liquid (bioethanol, vegetable oil and biodiesel) or gaseous (biogas,
biosyngas and biohydrogen) fuels produced from biomass have
attracted a great deal of attention as sustainable alternatives to
meet future energy demands in developing and industrialized
⇑ Corresponding authors. Addresses: Biofuel Research Team (BRTeam), Agricultural Biotechnology Institute of Iran (ABRII), 31535-1897 Karaj, Iran. Tel.: +98
9127496239; fax: +98 26 32701067 (M.M.A. Shirazi). Tel.: +98 26 32703536; fax:
+98 26 32701067 (M. Tabatabaei).
E-mail addresses: mmahdiashirazi@yahoo.com, mmahdiashirazi@srbiau.ac.ir
(M.M.A. Shirazi), meisam_tab@yahoo.com (M. Tabatabaei).
1
These authors contributed equally.
http://dx.doi.org/10.1016/j.seppur.2014.05.011
1383-5866/Ó 2014 Elsevier B.V. All rights reserved.
countries [2–4]. Biofuels sustainably produced from biomass offer
great advantages such as combined energy and food security,
environmental preservation and greenhouse gas reductions,
foreign exchange savings, and socioeconomic development of
esp. the rural areas [5,6].
Among various liquid biofuel, biodiesel is well-known as clean
and biodegradable fuel that could replace petroleum diesel; one
of the major cause of greenhouse gas emissions as well as some
other environmental crises [4]. Biodiesel can be produced from
vegetable oil feedstock (either fresh or waste) and animal fats
using a number of methods. Among those, transesterification is
the most common method which is carried out in the presence
of a catalyst (mostly basic compounds such as potassium hydroxide) and an alcohol e.g. methanol [7]. To complete a transesterification stoichiometrically, a 3:1 M ratio of methanol to triglycerides is
needed. Nevertheless, due to the reversible nature of the reaction,
significantly higher ratios (excess methanol) are usually considered to shift the reaction toward the product side [2,8].
A. Noureddin et al. / Separation and Purification Technology 132 (2014) 272–280
On the other hand, having a good and complete reaction is not
sufficient and the obtained biodiesel should be purified for it
contains impurities such as glycerol (the main by-product of the
transesterification reaction), trace water and soap, excess catalyst,
and un-reacted oil [7,9]. A number of unit operation choices are
available to purify raw biodiesel including extraction and leaching
procedures (water washing and dry washing, respectively) [10,11],
and more recently membrane separation technology [12].
However, prior to any of the above-mentioned downstream unit
operations, implementation of a pre-treatment step in order to
thoroughly separate biodiesel and glycerol is inevitable.
Given the density difference between biodiesel and glycerol
phases (880 kg/m3 and 1050 kg/m3, respectively), the main choice
considered for the pre-treatment step is gravitational settling. In
other words, gravitational settling could be used as the first and
simplest step immediately taken after the completion of the biodiesel production. Fig. 1 presents the general scheme of the batch
settling of glycerol/biodiesel mixture. This involves the lengthy
separation of the polar (the denser phase containing glycerol,
methanol, catalyst, soap, water, etc.) and the non-polar (the lighter
phase containing mono-alkyl esters of long-chain fatty acids)
phases. Long decantation time would increase the cost of the
facilities required considerably e.g. bigger equipment and larger
amount of steel, jeopardizing process profitability [7]. Various factors such as temperature, density difference and presence of excess
methanol in the biodiesel/glycerol mixture could negatively or
positively affect the efficiency and speed of this separation process.
For instance, methanol is a co-solvent which results in lowering
the interfacial tension (IFT) between the two liquids and consequently leads to a more stable emulsion [11,13] and consequently
a considerable amount of glycerol could be retained in the biodiesel phase and vice versa.
In our previous work, a simple but novel idea was developed in
order to accelerate glycerol/biodiesel decantation through NaClassisted gravitational settling [7]. The experiments were carried
out at ambient temperature on raw biodiesel–glycerin mixtures
Fig. 1. Layers formed in the batch settling of glycerol/biodiesel dispersion.
273
and led to 100% faster decantation process at NaCl inclusion rate
of 1 g/100 ml biodiesel–glycerin mixture. Given those findings
and the fact that in a real biodiesel production practice, excess
methanol and heat are generally considered to maximize the yield,
the present study was set to optimize the decantation behavior of
glycerol/biodiesel mixtures by investigating the interactive effects
of methanol and added NaCl concentration as well as temperature.
Statistical experimental design and response surface methodology
(RSM) using Box–Behnken design were used to study the
above-mentioned variables.
2. Materials and methods
2.1. Materials
Raw waste cooking oil biodiesel and glycerol were supplied by
Biofuel Research Team (BRTeam, Iran). Methanol and NaCl with
analytical grade were purchased from Merk (Germany) and used
as received.
2.2. Experiments procedure
One hundred ml of glycerol and biodiesel (1:1) was mixed in a
magnetic stirred vessel for different times (10–80 min at 10 min
intervals) at 400 rpm in order to determine the optimal mixing
time. Fig. 2 shows that increasing the mixing time above 30 min
had no significant impact on the decantation time and hence,
30 min was considered as optimal. Then, different quantities of
NaCl (0, 1 and 2 g/100 ml biodiesel–glycerin mixture) and
methanol (10, 20 and 30 vol.%) were well mixed under the previously-determined optimum mixing conditions (i.e. 400 rpm,
30 min). The obtained mixtures were transferred into A-class
100 ml graduated cylinders (Merienfield, Germany) and their
decantation behaviors were studied in an oven (control system)
at different temperatures i.e. 25, 45 and 65 °C. Experiments were
continued until equilibrium was reached. The equilibrium was
defined as the state at which the position of the interface between
biodiesel (the upper phase) and glycerol (the lower phase) did not
change by time.
Fig. 2. Variation of decantation time of glycerol/biodiesel mixture as a function of
mixing time (400 rpm).
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A. Noureddin et al. / Separation and Purification Technology 132 (2014) 272–280
2.3. Experimental design
Table 2
Box–Behnken design matrix.
RSM method using Box–Behnken design was used for the
optimization of decantation process of glycerol/biodiesel mixture.
Three major parameters affecting the decantation process i.e.
temperature (A), NaCl (B) and methanol concentrations (C), were
selected as independent variables, and decantation time (Y) was
considered as the dependent variable (i.e. the response). In order
to analyze the data and draw the corresponding diagrams, the
Design-Expert software of version (8.0.7.1) was used. In the
process optimization, the responses could be simply related to
the chosen factors by linear or quadratic models. A quadratic
model, which also includes the linear model, was used (Eq. (1)):
Y ¼ b0 þ b1 A þ b2 B þ b3 C þ b12 AB þ b13 AC þ b23 BC þ b11 A2
þ b22 B2 þ b33 C 2 þ e
ð1Þ
where Y is the response, A, B and C represent the effect of the independent variables. Thus, A2, B2, and C2 are the square effects; AB, AC
and BC are the interaction effects; b1, b2, and b3 are the linear coefficients; b12, b13, and b23 are the interaction coefficients. Moreover,
b0 and e represent the constant and random error, respectively.
In this study, a total of 17 experiments were performed in a
randomized order as required in many design procedures. The adequacy of the proposed model was then revealed using the diagnostic checking tests provided by analysis of variance (ANOVA). The
property of the fit polynomial model was represented by the coefficient of determination (R2). The R2 value is a measure of how the
variability in the observed response values could be clarified by
experimental factors and their interactions. These analyses were
performed by the Fisher’s ‘F’ test and P-value (probability). Table 1
shows the independent variables along with their ranges and
levels. The condition for each experimental run given by RSM is
tabulated in Table 2.
2.4. Analytical procedures
2.4.1. Gas chromatography (GC)
At the end of each decantation experiment, biodiesel sample
(2 ml) was taken and stored at 4 °C until GC analysis. About
50 mg of biodiesel was transferred into a micro-tube containing
1 ml internal standard solution (C19; 7 mg/ml). Approximately
0.5 ll of the prepared solution was injected into a Varian CP3800 GC (Varian, Inc., Palo Alto, CA) equipped with a FID detector
and a capillary column (CP9080, Select Biodiesel for FAME,
30 m 0.32 mm ID, Varian, USA). Helium gas was used as the
mobile phase. The concentration of methyl esters in biodiesel
samples was calculated using the GC data by the method presented
in our previous work [7].
2.4.2. Interfacial tension analysis (IFT)
Interfacial tension was measured using a pendant drop
tensiometry with a fully computer-controlled apparatus (OCA 20,
Data Physics; Germany). By using a microsyringe and a syringe
pump, a drop of glycerol was formed at the end of a stainless steel
needle placed in a vessel containing biodiesel. The drop was placed
Table 1
Experimental range and levels of the independent variables.
Variable
Temperature (°C)
NaCl (g)a
Methanol (vol.%)
a
Factor
A
B
C
Range and level
1
0
+1
25
0
10
45
1
20
65
2
30
g NaCl added into 100 ml biodiesel–glycerol mixture.
Exp. no.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
Experimental design
Experimental plan
T (°C)
NaCl (g)
MeOH (wt.%)
A
B
C
+1
0
0
0
0
+1
0
0
+1
1
1
0
+1
0
0
1
1
0
0
+1
1
0
1
0
0
+1
+1
0
+1
0
0
1
0
1
1
0
+1
+1
0
0
0
0
0
0
+1
1
+1
0
1
1
0
65.00
45.00
45.00
45.00
45.00
65.00
45.00
45.00
65.00
25.00
25.00
45.00
65.00
45.00
45.00
25.00
25.00
1.00
1.00
2.00
0.00
1.00
0.00
1.00
1.00
2.00
2.00
1.00
2.00
1.00
1.00
0.00
1.00
0.00
10.00
20.00
30.00
30.00
20.00
20.00
20.00
20.00
20.00
20.00
30.00
10.00
30.00
20.00
10.00
10.00
20.00
in the path of a collimated light beam. The light beam cast a
droplet shape onto the CCD video camera fitted with long-working
distance objective lenses. The camera was connected to a digitizer
device and a computer for the image/data storage. The system was
calibrated using pure water with a built-in calibration function on
the tensiometer.
2.4.3. Residual sodium ion measurement
In order to measure the residual sodium ion in biodiesel, 5 ml
biodiesel sample was washed with 5 ml deionized water, decanted
and the wash wastewater containing sodium ion was collected.
The washing process was repeated for three times and the
wastewater samples collected for each biodiesel sample were well
mixed. Ion chromatography (IC) (Metrohm 861 advanced compact
IC) was then used to measure the amount of Na+ in wastewater
samples.
2.4.4. Density measurement
After the polar (crude glycerol) and non-polar (crude biodiesel)
phases were separated in a decanter for at least 48 h, the density
value of both phases were measured as follows: a dried 25 ml
volumetric flask was weighed using a digital balance with an
accuracy of ±0.0001 g (Sartorius, Germany). Then it was filled with
25 ml of the sample and was weighed again. Density value (kg/
cm3) was calculated based on the following equation (Eq. (2)):
q¼
m2 m1
v
ð2Þ
where q is the density of the measured sample; m1 and m2 are the
mass of empty dried volumetric flask (kg) and the mass of the
sample and the volumetric flask (kg), respectively; and v is the volume of the volumetric flask (cm3).
3. Results and discussion
3.1. Optimization and modeling using RSM
RSM method using Box–Behnken design was employed to
investigate the effect of various factors on the decantation process
of glycerol/biodiesel mixture as well as to determine the interaction between those factors. According to RSM results, the best
model equation obtained (Eq. (3)) was as follows:
Y ¼ 3:20 2:00A 1:75B þ 2:50C þ 3:75AB 3:75AC
0:25BC þ 2:02A2 þ 3:02B2 þ 4:03C 2
ð3Þ
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A. Noureddin et al. / Separation and Purification Technology 132 (2014) 272–280
In the above equation, Y is the decantation time (min), and A, B and
C are the corresponding coded variables of temperature, NaCl, and
methanol, respectively. By using this model, the number of
experiments required to investigate the above-mentioned parameters at three different levels was 17. Moreover, the model was
validated using two additional experiments i.e. (1) 50 °C, 2 g NaCl,
25% methanol and (2) 40 °C, 1.46 g NaCl, 15% methanol. The RSMpredicted and actual decantation time for the two experiments
were 6.5 versus 7 min and 2.34 versus 2.30 min, respectively. The
good agreement achieved between the predicted and actual data
represents the validation of the developed model.
3.2. Analysis of variance (ANOVA)
The ANOVA results for Response Surface Reduced Quadratic
Model is presented in Table 3. The F-value of the model of 72.95
implied that the model was significant for evaluation of the decantation process of glycerol/biodiesel mixture. There was as little as
0.01% chance that the ‘‘Model F-Value’’ could have occured due
to noise. Values of ‘‘Prob > F’’ less than 0.05 indicates that the
model terms were significant. More specifically, all model terms
except BC (the interaction between NaCl and methanol concentration), were found significant. On the other hand, a signal to noise
ratio of 25.306 was obtained. In fact, values greater than 4 reveals
adequate precision in the measurements. Moreover, the obtained
‘‘Lack of Fit F-value’’ of 5.0 revealed that the lack of fit was not significant relative to the pure error and there was a 7.7% chance that
a ‘‘Lack of Fit F-value’’ this large could have occured due to noise.
It has been previously suggested by Joglekar and May that for a
good fit of a model, R2 should be at least 0.8 [14]. Based on the
ANOVA results, the R2 value of the developed model in the present
study was as high as 98.95% indicating a very good fit. Moreover,
the R2 value is expected to be in a good agreement with that of
the adjusted R2 in order for the model to be verified. The adjusted
R2 value obtained was also high at 97.59%. The good agreement
between the R2 and adjusted R2 values and that these values are
close to 1 confirm a high correlation between the observed and
the predicted results as shown in Fig. 3. Furthermore, these results
also indicate that the regression model used could well justify the
response and the independent factors.
3.3. Interactions between independent variables
As mentioned earlier, excess methanol is generally used in a
transesterification reaction in order to drive the reaction toward
higher yield or biodiesel. Also, the reaction is basically endothermic (50–60 °C). It is well understood that both of those factors
i.e. excess heat and methanol could negatively affect phase separation phenomenon. Previously, we reported that NaCl inclusion at
Fig. 3. The actual and predicted decantation times (min).
the rate of 1 g/100 ml glycerol–biodiesel mixture led to 100% acceleration of the biodiesel/glycerol decantation process [7]. The
impact of NaCl on the separation process could be discussed from
two different perspectives i.e. negative and positive. The positive
impact arises from the increased density differences between the
two polar and non-polar phases i.e. glycerol and biodiesel, respectively, through NaCl addition which consequently leads to a faster
decantation process.
On the other hand, electrolyte (i.e. NaCl in this work) inclusion
could also negatively interfere with the biodiesel/methanol/glycerol ternary system through reducing the IFT values between the
polar glycerol and non-polar biodiesel phases. In fact, lower IFT
values accelerate methanol release. Moreover, the monoglyceride
present in the mixture acts as a surfactant [15]. The presence of
surfactant compounds even in very small amounts of around
1 ppm as nuclei for nucleation of glycerol in biodiesel, and the
lowered IFT accumulatively lead to the formation of very small
micrometric droplets or in other words, miniemulsion [16].
Therefore, an important problem encountered during this excess
methanol evaporation (release) stage and consequently upon
formation of a two-phase medium (biodiesel phase with dissolved
glycerol droplets and glycerol phase) is the formation of miniemulsions which not only delays the decantation process but also leads
to increased loss of yield. The latter takes place through retention
of some glycerol microdroplets dispersed and dissolved in the ester
phase.
Furthermore, upon the completion of the reaction, heat is
generally used to recover the excess methanol basically at temperatures close to the boiling point of the alcohol used e.g. methanol
[17]. Traditionally, it is rightly believed that such heat could
accelerate the evaporation rate of the methanol. However, this
Table 3
ANOVA for response surface reduced quadratic model.
Source
Sum of squares
df
Mean square
F value
p-Value (prob > F)
Model
A-temperature
B-salt
C-MeOH
AB
AC
BC
A2
B2
C2
Residual
Lack of fit
Pure error
356.19
32.00
24.50
50.00
56.25
56.25
0.25
17.27
38.53
68.21
3.80
3.00
0.80
9
1
1
1
1
1
1
1
1
1
7
3
4
39.60
32.00
24.50
50.00
56.25
56.25
0.25
17.27
38.53
68.21
0.54
1.00
0.20
72.95
58.95
45.13
92.11
103.62
103.62
0.46
31.81
70.97
125.66
<0.0001
0.0001
0.0003
<0.0001
<0.0001
<0.0001
0.5192
0.0008
<0.0001
<0.0001
5
0.0770
R2 = 0.9895, R2adj = 0.9759, adequate precision = 25.306.
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A. Noureddin et al. / Separation and Purification Technology 132 (2014) 272–280
progressive evaporation of methanol also leads to decreased IFT
values and consequently triggers the miniemulsion phenomenon.
As mentioned above, the formation of miniemulsions could delay
the decantation process. It is worth quoting that the excess methanol present in the system due to the co-solvent nature of the
methanol also results in lowering the IFT value between the two
liquids phases and therefore, causes the formation of a large
amount of miniemulsions and consequently delayed decantation
process.
3.3.1. Interactive effect of NaCl and temperature on the decantation
time
Fig. 4 presents a three dimensional (3D) plot and its corresponding contour plot revealing the interactive effect of temperature and NaCl concentration at a constant methanol
concentration of 20%. As shown in Table 3, the p-value of NaCl/
temperature interaction was significant at <0.0001. Having the
contour and 3D plots analyzed, it could be understood that at the
temperature of 25 °C, the decantation time varied significantly
when NaCl concentration was increased from 0 to 2 g/100 ml
glycerol–biodiesel mixture. More specifically, increasing salt inclusion rate led to increased glycerol density values and consequently
affected the decantation speed positively through increasing the
density value of the polar phase; glycerol (Table 3). On the other
hand, the biodiesel yield decreased due to the formation of
miniemulsions. For instance as revealed by the findings of the
Experiments 10 and 17, an increment of NaCl concentration from
0 to 2 g under similar experimental conditions (25 °C and 20%
methanol), resulted in reduced IFT and yield values and on the
other side, increased glycerol density by approximately 0.1. In this
case, the impact of density increase overcame the IFT effect; and
resulted in 4 times lower decantation time (Table 3 and Fig. 4).
At the maximum temperature tested (65 °C) and in combination with increasing NaCl concentrations, the overall trend of the
decantation process was not favorable. This could be ascribed to
the accumulative impacts of increasing NaCl concentration and
high temperature on reducing the IFT value (Table 3; Experiments
6 and 9). Nevertheless when the decantation behavior at 0 g NaCl
concentration and constant methanol concentration (20%) was
compared at two different temperatures of 25 and 65 °C (i.e. Experiments 6 and 17), increasing temperature seemed to have positively influenced the decantation speed by just over 300% while
the same biodiesel yield of over 90% was achieved (Table 3). This
could be easily explained by the fact that increasing temperature
Fig. 4. Surface and contour plots of decantation time versus temperature (°C) and NaCl concentration (g) at a constant methanol concentration of 20%.
A. Noureddin et al. / Separation and Purification Technology 132 (2014) 272–280
values could lead to break-down of emulsion. This effect was also
observed in a study on water-in-oil (W/O) emulsion by Menon and
Wasan, in which as the temperature increased, viscosity of the
continuous phase decreased which favored an increase in the
flocculation rate between the dispersed droplets [18]. It is worth
quoting that the dispersion of glycerol in biodiesel in the current
study resembles the characteristics of W/O emulsion.
Overall, at low temperature values, glycerol density played the
major role in decantation process. This was in good agreement
with the findings of our previous investigation where at the ambient temperature increasing NaCl concentration significantly
reduced the decantation time through increasing the density
difference [7]. At high temperature values however, IFT variation
played a major role in the phase separation process. In fact, the
IFT-reducing aspect of NaCl inclusion overshadowed its positive
density-increasing aspect and in conjunction with IFT-reducing
impact of temperature accumulatively led to a deteriorated decantation speed.
3.3.2. Interactive effect of methanol and temperature on decantation
time
Fig. 5 presents the three dimensional and its respective contour
plot showing the interactive effects of temperature and methanol
277
(wt.%) at a constant NaCl concentration (1 g/100 glycerol–biodiesel
mixture) on decantation time. According to the ANOVA results, the
p-value of methanol/temperature was 60.0001 implying a significant interaction between these two parameters. Analyzing the 3D
and contour plots show that at the temperature of 25 °C, increasing
the amount of methanol delayed the decantation time. For
instance, the decantation time increased from 5 to 18 min when
methanol concentration was increased from 10% to 30%, respectively (Table 3, Experiments 11 and 16). The resulting delay trigerred by increased methanol concentration could be explained by the
cosolvent role of the methanol. More specifically, methanol is
adsorbed at the interface between the glycerol and biodiesel
phases and consequently reduces the IFT value. In other words,
the adsorbed molecules of methanol would form a mechanicallystrong and elastic interfacial film that would act as a barrier
against aggregation and coalescence [19,20], but leads to the production of miniemulsions. This could be observed in the Experiments 11 and 16 in which increasing methanol concentration by
20% resulted in a decrease in IFT value from 8.64 to 4.87 mN/m.
Generally and as mentioned earlier, presence of methanol and
its progressive evaporation negatively affects the decantation time
through the formation of miniemulsions. However, if the excess
methanol exceeds a certain level, the surplace obviously enters
Fig. 5. Surface and contour plots of decantation time versus temperature (°C) and methanol concentration (wt.%) at a constant NaCl amount of 1 g.
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A. Noureddin et al. / Separation and Purification Technology 132 (2014) 272–280
the glycerol phase more than the biodiesel phase due to the fact
that there are extensive possibilities for hydrogen bonding
between the glycerol molecules and methanol. Therefore, the
contained methanol in the glycerol phase would results in
decreased viscosity of the polar phase (glycerol). It should be noted
that viscosity is a function of temperature as well.
Therefore, given the above-mentioned facts, and the fact that
viscosity reduction is regarded as one of the major players in
facilitating liquid/liquid separation [21], it could be concluded that
increasing temperature in the presence of huge amount of methanol could lead to increased IFT value and consequently shorter
decantation time. These could be clearly seen in the data presented
in Table 3 and Fig. 5. For instance at 30% methanol concentration
and in the presence of 1 g NaCl (Experiments 11 and 13), increasing
the temperature from 25 to 65 °C led to decreased decantation
time from 18 to 6 min, respectively. As explained above, an increment of IFT value from 4.87 to 7.84 mN/m could clearly confirm
this improvement.
3.3.3. Interactive effect of methanol and NaCl on decantation time
Fig. 6 presents the three dimensional and its contour plot
implying the interactive effect of methanol (wt.%) and NaCl concentration (g) on the decantation behavior of glycerol/biodiesel
mixture at a constant temperature of 45 °C. As can be observed
in Table 3, ANOVA results shows a p-value of 0.5192 for NaCl/
methanol interaction indicating a negligible interaction between
the parameters. This also could be comprehended from the circular
shape of the contour plot (Fig. 6). In fact, NaCl inclusion and methanol both influenced the decantation process similarly by IFT
reduction, however, through different mechanisms. The former
decreased IFT for its electrolyte characteristic and the latter
decreased IFT for its co-solvent characteristic. Moreover, the other
conclusion which could be drawn based on the 3D graph provided
in Fig. 6 is that the negative impact of methanol on emulsion stability and consequent lengthier decantation process was more
intense in comparison with that of NaCl.
3.4. Optimization of the decantation process
Basically, the optimum decantation conditions should simultaneously lead to the highest yield of methyl ester and the shortest
decantation time. Such conditions as predicted by the RSM analysis
were as follows: temperature of 39.08 °C, 1.46 g NaCl, and 15.66%
methanol. Stoichiometrically, methanol to oil molar ratio of 3:1
is required to complete the transesterification reaction, however,
in the conventional biodiesel processing activities, higher ratios
Fig. 6. Surface and contour plots of decantation time versus NaCl (g) and methanol concentration (wt.%) at a constant temperature of 45 °C.
279
A. Noureddin et al. / Separation and Purification Technology 132 (2014) 272–280
Table 4
Results obtained by GC, IFT, density analysis.
Exp. no.
Experimental plan
A
B
C
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
65.00
45.00
45.00
45.00
45.00
65.00
45.00
45.00
65.00
25.00
25.00
45.00
65.00
45.00
45.00
25.00
25.00
1.00
1.00
2.00
0.00
1.00
0.00
1.00
1.00
2.00
2.00
1.00
2.00
1.00
1.00
0.00
1.00
0.00
10.00
20.00
30.00
30.00
20.00
20.00
20.00
20.00
20.00
20.00
30.00
10.00
30.00
20.00
10.00
10.00
20.00
Decantation time (min)
Yield (%)
Glycerol density (kg/m3)
IFT (mN/m)
8
3
11
14
4
5
3
3
8
4
18
7
6
3
9
5
16
89.090
90.555
90.694
89.206
90.341
90.080
91.300
90.070
89.796
87.367
90.796
91.885
88.800
90.505
90.124
91.098
90.389
1.147
1.138
1.109
1.082
1.154
1.225
1.180
1.220
1.194
1.221
1.170
1.253
1.157
1.130
1.121
1.204
1.134
6.40
5.88
5.97
1.04
1.39
8.97
1.68
7.15
2.87
6.16
4.87
5.20
7.84
5.77
2.87
8.64
6.62
e.g. 6:1 is usually practiced to compensate for the irreversibility of
the reaction. On such basis, and based on the experimental results
tabulated in Table 4, the optimum conditions for accelerated
decantation process which is also close to a real biodiesel production practice (around 10% excess methanol), would be those of the
Experiments 1 and 12 depending on the operational temperature
used during the transesterification reaction. If the reaction temperature is set at around 45–50 °C, then addition of 2 g NaCl/100 ml
glycerol–biodiesel mixture could lead to the highest yield (as in
the Experiment 12; yield: 91.885%) and the decantation time of
7 min. On the other hand, if the transesterification reaction
temperature is set at around 60–65 °C, then addition of 1 g NaCl/
100 ml glycerol–biodiesel mixture would results in the highest
yield (as in the Experiment 1; 89.09%) and the decantation time
of 8 min.
As also observed in Table 4, increasing excess methanol form 10
to 20 vol.% (at 45 °C, 1 g NaCl/100 ml biodiesel–glycerin mixture)
resulted in a remarkable reduction in decantation time down to
3 min (around 200% speedier decantation process). As mentioned
earlier, application of excess methanol drives the reaction toward
higher biodiesel production due to the reversible nature of the
transesterification reaction. On such basis and according to the
findings presented above, ensuring the presence of as much excess
methanol as 20% could play a significant role in shortening the
decantation process. Thus, if decantation time is of critical concern,
this could be suggested to be considered in well-sealed biodiesel
production practices, provided that the unit operation is wellequipped with an efficient methanol recovery system. Keeping that
in mind, it is also decisive whether such manipulation (considering
20% excess methanol in reaction and its downstream recovery) and
consequent acceleration in decantation time is worth the energy
consumed for methanol recovery.
It is worth quoting that the IC results revealed that Na+ concentration in all the crude biodiesel samples was below 5 ppm. These
results confirmed that the excess Na+ added to the mixture
through NaCl addition entered the polar glycerol phase after
decantation and that glycerol desalting could be conducted as
previously described by Javani et al. [21]. Moreover, washing and
drying procedures increased the methyl ester content in the final
biodiesel product to ASTM quality (>96.5%).
4. Conclusion
The duration of the decantation process could play an economically significant role in the biodiesel production. Therefore,
achieving an accelerated separation of biodiesel and glycerol
phases could enhance profitability. The results obtained revealed
that the volume of excess methanol (unreached alcohol), temperature and NaCl addition could be effectively used to accelerate the
decantation process and consequently further economize the
biodiesel production process. In other words, the higher acceleration rate by 200% achieved herein could be translated into less
capital required for bigger equipment and larger amount of steel
and consequently higher profitability. The fastest decantation time
of 3 min was accomplished at 45 °C, 1 g NaCl addition/100 ml
biodiesel–glycerol mixture and 20% excess methanol. The methyl
ester yield obtained under optimum condition was at over 90%
which was further increased to ASTM quality after washing and
drying procedures.
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