Download Reactive Extraction of Microalgae for Biodiesel Production

Reactive Extraction of Microalgae for Biodiesel Production; an
Optimization Study
S. T. El-Sheltawy, Ph.D.
Department of Chemical Engineering
Cairo University
Giza – Egypt
[email protected]
G. I. El-Diwani, Ph.D.
Department of Chemical Engineering and Pilot Plant
National Research Center
El-Tahrir Street, Dokki, Giza – Egypt
[email protected]
N. K. Attia, Ph.D.
Department of Chemical Engineering and Pilot Plant
National Research Center
El-Tahrir Street, Dokki, Giza – Egypt
[email protected]
H. I. El-Shimi, M.Sc.
Department of Chemical Engineering
Cairo University
Giza – Egypt
[email protected]
Abstract: Concerns about energy security and declining of fossil fuels have led to
growing worldwide interests in renewable energy sources such as biofuels. Algal biodiesel
is a technically alternative and renewable diesel fuel without excessive engine
modifications. This paper provides the biodiesel production from Spirulina-platensis
microalgae via reactive extraction methodology "in-situ transesterification" using alcohol
and acid catalyst, since the engineering factors that strongly affect the percentage yield of
algal biodiesel like alcohol-to-oil molar ratio, acid-catalyst concentration, time and
temperature of reaction were studied. The lipids content of Egyptian Spirulina-platensis
microalgae was estimated to be 11% wt. The weight of the co-product glycerol obtained
was employed to estimate the percentage yield of algal biodiesel. Experiments to look for
algal biodiesel was successful; since 84.7% as a yield was achieved at 1:3714 molar ratio
of oil-to-alcohol, 100% (wt./wt. oil) catalyst concentration, 8h reaction time at temperature
of 65oC with continuous agitation at 650 rpm. Various properties of produced biodiesel
were investigated according to EN 14214 standards. Spirulina-platensis methyl esters
were closest to be applied as biodiesel according to their properties.
Keywords: Renewable energy; Biofuels; Algal biodiesel; Spirulina-platensis; Reactive
extraction; In-situ transesterification.
1. Introduction
The need for a reliable renewable fuel source grows; due to the ever-increasing
demand for energy and depletion of fossil fuels. There are many options in this area, but
unlike solar and nuclear, biofuels such as biodiesel has the capability of providing a fuel
source ideally suited to existing infrastructure within the transportation industry [1-4].
Biodiesel is a green attractive fuel that can be produced from plants' oils, animal
fats, used cooking oil as well as microalgae oils [1-10]. It is a renewable, non toxic and
clean energy alternative to petrol diesel [11]. Biodiesel, when blended with petroleum
diesel, can be used in unmodified diesel engines. It has the added benefit of higher
lubricity and biodegradability, so it helps provide for greater longevity within diesel
engines. To use pure biodiesel (B100), an engine usually needs slightly inexpensive
modifications; to prevent fouling [12-15].
Microalgae are promising candidate bio-resource as potential feedstock for
renewable energy production [4, 16]. Microalgae have high oil productivity (1,000–6,500
gallons of oil per acre per year), high lipid content (usually, 30-65% by wt), low
cultivation cost, and no competition with agricultural land [5]. Biodiesel from microalgae
is renewable and carbon neutral (about 2 kg of CO2) are fixed by 1 kg of algal biomass),
and can use non-arable land [17-20].
The biodiesel production from microalgae oils is well-known as transesterification
reaction that illustrated in Figure (1). This process has previously been demonstrated in
literature using the conventional methods [21], and the process usually uses pre-extracted
algae oil as raw material [22-25]. Extraction of the algal oil by an organic solvent is
required. This step is expensive because it involves cell rupture, solvent extraction,
oil/solvent separation and reuse, etc. So, the combination of the oil extraction and
transesterification steps in one process named "reactive extraction" or "in-situ
transesterification" is considered the available alternative that could minimize the
biodiesel production cost [26]. It includes the simultaneous addition of the acid catalyst
(e.g. conc. H2SO4) and pure alcohol (e.g. methanol) to dried powder microalgal biomass
[27-33].
Figure (1) Transesterification reaction equation
The reactive extraction method was first demonstrated by Harrington and D’ArcyEvans [26] using sunflower seeds as raw material, and an improvement in biodiesel yields
up to 20% compared to the conventional process were done by these authors. This
conversion improvement was considered to be attributable to the improved accessibility of
the oil in the biomass by the acidic medium [1, 33]. The in situ transesterification of
macerated sunflower seeds was also studied by Siler- Marinkovic et al. [29]. Also, this
process was used by E.A. Ehimen et al. [25] on Chlorella microalgae oil using methanol
and 100% (wt/wt oil) sulphuric acid as a catalyst.
The ultimate goal of the present research is to study the engineering variables that
affecting the biodiesel production from Spirulina-platensis microalgae using a novel
process "reactive extraction" that combines the oil extraction and transesterification in a
single step. This ultimate goal is to be realized with the specific objectives of (1)
characterizing the microalgae oil of the selected strain, (2) optimization of the main
reaction factors (catalyst concentration, oil-to-alcohol molar ration, agitation influence,
processing time and temperature) that strongly affect the cost of reactive extraction
process for algal biodiesel production, and (3) measuring the properties of the produced
biodiesel in a comparison with the EN 14214 standards
2. Experimental Background
2.1. Materials
Microalgae biomass
Dried microalgae of Spirulina-Platensis strain was supplied from Microbiology
Department, Soils Water and Environment Res. Inst., Agricultural Research Center, Giza,
Egypt. The cultivation system was similar to that used in Ref. [1]. Conditions of 30oC,
fluorescent lamps of 3.5klx and pH of 8.5±0.5 were used for its cultivation.
Reagents
Sulphuric acid (98% purity) as a catalyst in the trans-esterification process and,
Methanol (99.9% purity) as the reacting alcohol were used in this study. All chemicals
were purchased from El-Nasr Pharmaceutical Chemicals Co.
2.2. Experimental Procedure
First of all, Spirulina-platensis oil was extracted using the Soxtherm extraction
system described by Jie Sheng et al. [21]. An optimization study on the microalgae oil
extraction was performed by El-Shimi, H [1]; to determine the total lipids content on dry
basis. Then, the extracted oil was analyzed for characterization. Different sulphuric acid,
H2SO4 concentrations (30, 50, 100 and 200% wt./wt. oil), were used throughout this study.
Step 1: Preparation of catalyst- alcohol solution
The amount of methanol (CH3OH) is measured in a test-tube and poured into
Erlenmeyer flask. The pre-weighted amount of catalyst is added to the flask. The acidmethanol solution is freshly formed by dissolution of H2SO4 into methanol on a magnetic
stirrer for 5 min. The solution was prepared freshly; in order to maintain the catalyst
activity.
Step 2: Reactive extraction process
Fifteen grams (15gm) of microalgae biomass were previously dried at 110oC; to
remove water traces, and then carefully added to the acid-alcohol mixture in the agitated
vessel that maintained at the desired temperature. At this moment, reactive extraction
process starts. The oil in the biomass reacted with methanol in presence of H2SO4 and
biodiesel with glycerin are produced. The reaction time studied is ranged from 2 - 10 h.
The major reactive extraction processes and products purification steps are shown in
Figure (2). After predetermined reaction time, the warm reaction mixture was allowed to
cool for 20 min, then filtered and the residues are washed three times by re-suspension in
methanol. The filtrate is carefully transferred to separating funnel. Water (60ml) was
added to the filtrate; to facilitate the decantation of the hydrophilic components of the
extract. Further extraction of the product FAME was achieved by extracting three times
using 60 ml of hexane, which resulted in generation of two layers: hydrophobic layer
(hexane, FAME and glycerides), and hydrophilic layer (water, glycerol and excess
methanol). The separation can take about 12 hours to be carried out correctly.
When the products have fully settled, two distinct layers were clearly separated.
The biodiesel layer on top looked clear, lighter in color and thin. The glycerol layer at
bottom looked clear, dark amber color and thick. Most of the settling occurred within the
first hour. Once the glycerol and biodiesel phases have been separated, the bottom layer
which contains glycerol, water, catalyst, and excess methanol was drawn into a preweighted beaker and dissolved in pure water; to purify the glycerol layer, and then
subjected to a flash evaporation process, in which excess alcohol and water are removed.
The recovered alcohol was recycled and reused. Now, the by-product glycerol and the
catalyst are weighted. The biodiesel yield is calculated according to the weight of
produced glycerol.
Step 3: Biodiesel purification
Biodiesel and hexane were separated using a fractional distillation apparatus.
Methanol
Dissolution
Conc. H2SO4
"Acidic
Methanol"
Spirulina-platensis
Powder
Reaction
Processing
Reaction
mixture
Filtration
Algae Residues
Filtrate
Hexane
Decantation
Water
Water layer:
glycerol +
catalyst+ methanol
Hydrophobic layer:
Hexane + Biodiesel
Water
Biodiesel
Washing
Fractional
Distillation
Wastewater
Hexane
Biodiesel
Evaporation
Methanol
Glycerol
Figure (2) Schematic diagram for biodiesel production from Spirulina-platensis
2.3. Engineering factors affecting the reactive extraction process
i. Effect of reaction time and temperature
Fifteen grams of powder microalgae was mixed with 80ml methanol containing
0.82ml H2SO4 at various temperatures (27, 40, 50 and 65oC) with continuous stirring at
650rpm. At each of the interest temperature, the reactive extraction process is given itself
for the required time (2, 4, 8 or 10 h); to investigate the effect in biodiesel yield and it was
repeated in duplicate. The biodiesel yield was calculated.
ii.
Effect of catalyst concentration and alcohol-to-oil molar ratio
Spirulina-platensis powder of 15g was mixed in the reaction vessel with different
alcohol volumes (40, 60, 80 and 100ml) that corresponding to (1857:1, 2786:1, 3714:1 and
4643:1 molar ratio respectively); to study the effect of alcohol-to-oil molar ratio
containing various acid-catalyst concentrations (0, 30, 50, 100 and 200%) on the massbasis of microalgae oil content, this was performed at 65oC for 8h with 650rpm continuous
agitation, assumed as an optimum conditions. A minimum volume of 40 ml methanol was
selected since it was the suitable amount that facilitated a complete submersion of 15 g of
biomass. The respective FAME products and the co-product glycerol at different
investigated variable levels were obtained and their weights determined.
iii. Effect of agitation
The agitation is an important engineering factor that affecting the reaction conversion;
because it facilitate a complete suspension of the particles in the mixture. So, the reactive
extraction process was run with and without agitation for comparison, while the other
variables kept constant; to study the effect of agitation on biodiesel yield.
2.4. Analytical method
Fatty acids composition of the extracted algae oil was determined using gas
chromatographic analysis of the oil ethyl esters. Modification of the oil to its ethyl esters
was made using 2% H2SO4 as catalyst in the presence of dry ethyl alcohol in excess. The
chromatographic analysis was made using Hewlett Packard Model 6890 Chromatograph.
A capillary column 30 m length and 530 μm inner diameter, packed with Apiezon® was
used. Detector temperature, injection temperature and the column temperature were
280°C, 300°C and 100°C to 240°C at 15°C/min, respectively.
3. Results and Discussion
3.1. Characterization of pure microalgae oil
From the optimization study on the Spirulina-platensis oil extraction [1],
microalgae oil was determined to be 0.11g/g of biomass. The oil was characterized as
shown in Table (1).
Table (1) Characterization of algae oil
Characteristics
Spirulina-platensis Oil
3
Density (g/cm )
0.892
o
Viscosity (cp , 40 C)
58
Acidity (mg KOH/g oil)
37.4
The acid value of the oil was 37.4 mg KOH/g oil, which is very higher; therefore
the choice of acidic over alkaline catalysis for the reactive extraction process was justified.
The fatty acids composition of Spirulina-platensis oil is detected via GC-MS and
tabulated in Table (2). It's clear that the predominant fatty acid was the palmitic acid
C16:0 (49.6% by mole), which makes the oil a promising feedstock for biodiesel fuel
synthesis. The percentages of the major fatty acids in the Spirulina used in this study were
in accordance with previous works by other authors [23, 25, 30].
Table (2) Fatty acids profile of Spirulina-platensis oil measured by GC-MS
Fatty acid
Mystic (C14:0)
Palmitic (C16:0)
Palmitoleic (C16:1)
Stearic (C18:0)
Oleic (C18:1)
Linoleic (C18:2)
Linoleuic(C18:3)
Ecosanoic (C20:0)
Eicosic (C20:1)
% in sample (by mole)
22.67
49.58
2.75
5.56
2.24
5.03
7.41
1.06
3.69
The average molecular mass of Spirulina-platensis oil and that of biodiesel were
calculated to be 845.19 and 284 g/gmole respectively with the same procedure published
by H.I.El-Shimi et al. [5]. The reaction yield is calculated from “Equation 1”, as the
biodiesel weight was estimated by knowing the weight of by-product glycerol.
𝐵𝑖𝑜𝑑𝑖𝑒𝑠𝑒𝑙 𝑌𝑖𝑒𝑙𝑑 % =
𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑏𝑖𝑜𝑑𝑖𝑒𝑠𝑒𝑙
𝑥 100
𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑚𝑖𝑐𝑟𝑜𝑎𝑙𝑔𝑎𝑒 𝑜𝑖𝑙
(1)
3.2. Effect of catalyst concentration on biodiesel yield
According to Figure (3), higher yields of microalgae methyl esters were computed
to be 84.716% with ±9% standard error using 0.82 ml H2SO4 (100% by wt. of oil)
dissolved in 80ml methyl-alcohol at 65°C for 8 hr, with further increase in catalyst
concentration the conversion efficiency more or less remains constant. These results agree
with the methanolysis using 100% H2SO4 (wt./wt. of oil) resulted in successful conversion
of Chlorella oil giving the best yields and viscosities of the esters by E.A. Ehimen et al.
[25].
The use of statistical analysis by Excel-program allowed expressing the amount of
methyl esters produced as a polynomial model. As observed in Figure (3), the
concentration of acid catalyst has significant effect on the response (biodiesel yield) and
the data are fitted to a second order with determination coefficient of 0.99 as expressed in
"Equation 2". The optimum catalyst concentration is around 100% wt./wt.oil.
𝑌 = − 0.002 𝑋 2 + 0.736 𝑋 + 39.8 , 𝑅 2 = 0.99
(2)
Where; X is the catalyst concentration, and Y is the biodiesel yield.
100
90
Biodiesel Yield %
80
70
60
50
40
30
20
10
0
0
50
100
150
200
250
Acid Catalyst Conc. %
Figure (3) Investigation of catalyst concentration effect on biodiesel yield
(at 65oC for 8h using 80ml methyl-alcohol)
3.3. Effect of alcohol-to-oil molar ratio on biodiesel yield
The methanol-to-oil molar ratio is an important engineering factor affecting the
conversion of microalgae lipids to biodiesel. This factor was tested in the range of 1857:1
to 4643:1, while the other variables remain constant. The transesterification is an
equilibrium reaction in which an excess of alcohol is required to drive the reaction to the
right [5]. The studied range includes and exceeds that of a similar investigation of the in
situ transesterification of sunflower oil by Siler- Marinkovic and Tomasevic [29]. The
maximum yield achieved was 84.7%, at 65oC during 8h reaction time using 0.82ml acid
catalyst dissolved in 80ml alcohol volume and continuous agitation at 650rpm, concerning
experimental and standard error of ±3.5%. The results are presented in Figure (4) and
indicate an improvement of the microalgae oil conversion with methanol increase.
Dependence of microalgae biodiesel on the reacting alcohol volume was investigated to be
a polynomial model of second order as shown in with 99.5% statistical coefficient of
determination (R2 = 0.995). However, with the use of alcohol volumes over 80 ml (i.e. a
reacting oil-to-alcohol molar ratio of 1:3714) wasn't significant for the FAME yields.
3.4. Effect of reaction time and temperature on biodiesel yield
Reactive extraction of Spirulina-platensis microalgae was carried out at various
temperatures (27-65oC) over range of reaction time (2-10hr), while the other conditions
kept constant. Really, the elevated temperatures improve the initial miscibility of the
reacting species, leading to a significant reduction in the reaction time, as published by all
concerned scientists and researchers [25-32]. The progress of the in-situ transesterification
of microalgae oil to biodiesel of this study is reported in Table (3).
For the samples investigated at room temperature (no process heating), asymptotic
FAME yield % was not reached within the time boundaries of the study. According to
Table (3), when the process was carried out at 65°C, higher equilibrium conversion levels
of biodiesel of 76.22% and 84.7%were attained after reaction time of 4 h and 8 h
respectively. Within the investigated experimental conditions, equilibrium of FAME
conversions was observed to reach similar asymptotic values after a reaction time of 8 and
10 h for temperatures of 50 and 65°C as shown in Figure (5). While at lower temperatures
of 27°C, the process was incomplete and no microalgae biodiesel was observed.
90
Microalgae Biodiesel Yield (%)
85
80
75
y = -2E-06x2 + 0.0202x + 44.391
R² = 0.9957
70
65
60
0
1000
2000
3000
4000
5000
Oil-to-Alcohol Molar Ratio (1:X)
Figure (4) Dependence of Biodiesel Yield % on Oil-to-Alcohol Molar Ratio
Table (3) Influence of reaction time and temperature on biodiesel yield
Reaction Time
(hr)
27oC
2
4
8
10
1.35
10.62
30.22
34.71
Biodiesel Yield (%)
40oC
50oC
25.11
38.2
45.81
70.5
62.3
81.54
62.512
81.63
65oC
43.1
76.22
84.7
84.82
It's observed from Figure (5), temperature has detectable effect on the ultimate
conversion to ester and the data are tend to a polynomial of second degree with high
correlation factor (R2 >97%). However, higher temperatures decrease the time required to
reach maximum conversion of microalgae lipids, and the best reaction conditions seem to
be 65°C for 8 h.
100
y = -1.375x2 + 21.09x + 8.884
R² = 0.978
90
84.7
81.54
y = -1.342x2 + 21x + 3.703
R² = 0.973
80
76.22
70.5
Biodiesel Yield %
70
62.512
62.3
60
27C
y = -0.8537x2 + 14.809x - 0.6505
R² = 0.9979
45.81
50
43.1
38.2
40
84.82
81.63
40C
50C
34.71
30
65C
30.22
25.11
20
10.62
10
y = -0.1992x2 + 6.706x - 11.849
R² = 0.9955
1.35
0
0
2
4
6
8
10
12
Reaction Time (hr)
Figure (5) Effect of reaction time at different temperatures on biodiesel yield %
3.5. Effect of agitation on microalgae biodiesel yield
It's sure that, the agitation is a potential engineering factor affecting the biodiesel
production process; since when the reactive extraction of microalgae was performed
without stirring, no reaction occurred and zero biodiesel yields was obtained. This
indicates that agitating is required to enhance the reaction progress, evidently by aiding
the initial miscibility of the reacting species. However, the reaction was carried out at
65oC using 80ml methanol and 100% H2SO4 (wt/wt oil) as catalyst, and the mixture was
stirred at 650rpm for 8hr intermittently (1h on and 1 h off) to obtain just 74.3% as a
biodiesel yield, and also it subjected to continuously agitation to achieve 84.7% of
biodiesel, which prove the positive influence of agitation during reaction.
3.6. Evaluation of produced algal biodiesel
The main properties of the produced biodiesel are tested and presented in Table
(4). Cetane number of biodiesel is always higher than petrol diesel; because it has
saturated molecules and longer fatty acids carbon chains. The measurement of cetane
number is important; since lower cetane numbers result in poor ignition quality, delay and
excessive engine knock. Cetane number of microalgae biodiesel was measured to be 61,
which is better than 38 for jatropha methyl-esters [33]. The flash point of produced
microalgae biodiesel was estimated to be 172°C which is higher than 160°C for jatropha
biodiesel [28] and much higher than that of 58°C for petrol-diesel, which makes the
biodiesel-based algae, and its blends safer fuels to handle and store near ignition sources.
The cold flow properties "cloud and pour points" of algal biodiesel are measured to be "5
and -1" respectively. These values are not good compared to that given in literatures for
biodiesel from waste vegetable oils (3 and -6 respectively); the reason is that, algal
methyl-esters composed of saturated fatty acids as estimated before in Table (2).
Table (4) Evaluation of algal biodiesel compared to EN 14214 standards
Property
Density (g/ml at 15oC)
Viscosity (cSt at 40oC)
Cetane number
Flash point (oC)
Cloud point (oC)
Pour point (oC)
Value
0.886
4.8
61
172
5
-1
Limits
0.86-0.90
3.5-5.0
Min. 51
Min. 101
-
4. Conclusions
From this research, it may conclude that:
 Oil extraction from microalgae is one of the most important processes that
determine the sustainability of algae-based biodiesel. Lipid content of Spirulinaplatensis was detected to be 11% (by wt.). Fatty acid analysis report that C16:0 is
the dominance in Spirulina-platensis oil that makes it a promising feedstock for
biodiesel production.
 Acid value and viscosity of microalgae oil are very high (37.4 mg KOH/g oil and
58 cp, respectively), so the choice of acid-catalysis reactive extraction process was
justified to get biodiesel from Spirulina-platensis microalgae.
 The results of biodiesel production at the studied conditions using reactive
extraction methodology show that a reaction time of 8hr at 65oC using 100%
concentrated sulphuric acid (wt/wt microalgae oil) as a reaction catalyst dissolved
in 80ml methanol agitated continuously at 650rpm, were enough to get high yield
of biodiesel (84.7%).
 Microalgae biodiesel is close to the practical diesel; as its properties confirm the
EN 14214 standards.
5. Acknowledgements
Financial support from Cairo University-Egypt [FECU] is appreciated.
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