TITLE PAGE TABLE OF CONTENTS Contents TITLE PAGE 1 TABLE OF CONTENTS 3 LIST OF FIGURES 5 LIST OF TABLES 6 LIST OF EQUATIONS 7 Abstract 8 1.0. Introduction 9 2.0. Microalgae harvesting method 10 2.1. Common harvesting technology 10 2.1.1. Centrifugation 10 2.1.2. Sedimentation 11 2.1.3. Flocculation 11 2.1.4. Flotation 13 2.1.5. Filtration 14 2.2. New Emerging Microalgae Biomass Harvesting Techniques 15 2.2.1. Flocculation using magnetic microparticles 16 2.2.2. Flocculation by natural biopolymer 17 2.2.3. Electrical approach 18 3.0. Extraction and Analysis of Lipid from Microalgae Biomass 20 3.1. Lipid extraction 21 3.1.1. Mechanical extraction 21 3.1.2. Chemical/solvent extraction 23 3.1.3. New emerging green solvents systems and process intensification techniques for lipids extraction from microalgae 25 4.0. Heterogeneous transesterification catalysts 29 4.1. Solid Bases Transesterification 33 4.2. Solid Acids Transesterification 35 4.3. Heterogeneous transesterification of algae oil 36 5.0. Reactors 44 5.1. Influence of reactor design and operating conditions 44 6.0. Conclusions 51 References 54 LIST OF FIGURES Figure 1: Flowsheet for biodiesel production from microalgae. Some intensified process techniques highlighted may reduce some downstream steps as it would render the dewatering step unneeded. i.e. MAE – Microwave assisted extraction (MAE), Enzyme assisted extraction (EAE), Ultrasound assisted extraction (UAE), Surfactant assisted extraction 27 Figure 2:Flow sheet of an oscillatory baffled reactor and it mixing features. Also illustrating the solid acid catalyst PrSO3H-SBA-15 undergoing no oscillation but sedimentation and or with about 4.5Hz oscillation traped in the baffles. Figures exuracted from (Eze et al., 2013) 47 Figure 3: Diagram of membrane reactors for producing biodiesel in transesterification reaction through (a) Solid acid catalyst and (b) base catalysts.49 LIST OF TABLES Table 1: Performance comparison of flotation techniques14 Table 2: Performance comparison of filtration methods15 Table 3: Performance of flocculation using biopolymer17 Table 4: performance comparisons for microalgae biomass harvesting by various electrical methods operated in just 1 hour19 Table 5: Reported catalyst used for heterogenous transesterification reaction on various feedstocks30 Table 6: The effect of calcination temperature on the performance of WO3/ZrO2 catalyst (Jothiramalingam & Wang, 2009).39 Table 7: Literature review on biodiesel production via heterogenous catalyst41 LIST OF EQUATIONS Equation 1: Chemical equation showing production of biodiesel from any bio oil 32 Equation 2: Reaction mechanism of transesterification via base catalyst (denoted Y) in the equation. 33 Abstract The dwindling rate of our fossil fuel reserves and general believe of major contribution of CO2 emissions which is linked to the climate change due to the burning of such carbon sources in engines eithe ...

1. TITLE PAGE
TABLE OF CONTENTS
Contents
TITLE PAGE 1
TABLE OF CONTENTS 3
LIST OF FIGURES 5
LIST OF TABLES 6
LIST OF EQUATIONS 7
Abstract 8
1.0. Introduction 9
2.0. Microalgae harvesting method 10
2.1. Common harvesting technology 10
2.1.1. Centrifugation 10
2.1.2. Sedimentation 11
2.1.3. Flocculation 11
2.1.4. Flotation 13
2.1.5. Filtration 14
2.2. New Emerging Microalgae Biomass Harvesting Techniques
15
2.2.1. Flocculation using magnetic microparticles 16
2.2.2. Flocculation by natural biopolymer 17
2.2.3. Electrical approach 18
3.0. Extraction and Analysis of Lipid from Microalgae Biomass
20
3.1. Lipid extraction 21
3.1.1. Mechanical extraction 21
3.1.2. Chemical/solvent extraction 23
3.1.3. New emerging green solvents systems and process
intensification techniques for lipids extraction from microalgae
25
4.0. Heterogeneous transesterification catalysts 29
4.1. Solid Bases Transesterification 33

2. 4.2. Solid Acids Transesterification 35
4.3. Heterogeneous transesterification of algae oil 36
5.0. Reactors 44
5.1. Influence of reactor design and operating conditions 44
6.0. Conclusions 51
References 54
LIST OF FIGURES
Figure 1: Flowsheet for biodiesel production from microalgae.
Some intensified process techniques highlighted may reduce
some downstream steps as it would render the dewatering step
unneeded. i.e. MAE – Microwave assisted extraction (MAE),
Enzyme assisted extraction (EAE), Ultrasound assisted
extraction (UAE), Surfactant assisted extraction 27
Figure 2:Flow sheet of an oscillatory baffled reactor and it
mixing features. Also illustrating the solid acid catalyst
PrSO3H-SBA-15 undergoing no oscillation but sedimentation
and or with about 4.5Hz oscillation traped in the baffles.
Figures exuracted from (Eze et al., 2013) 47
Figure 3: Diagram of membrane reactors for producing biodiesel
in transesterification reaction through (a) Solid acid catalyst

3. and (b) base catalysts.49
LIST OF TABLES
Table 1: Performance comparison of flotation techniques14
Table 2: Performance comparison of filtration methods15
Table 3: Performance of flocculation using biopolymer17
Table 4: performance comparisons for microalgae biomass
harvesting by various electrical methods operated in just 1
hour19
Table 5: Reported catalyst used for heterogenous
transesterification reaction on various feedstocks30
Table 6: The effect of calcination temperature on the
performance of WO3/ZrO2 catalyst (Jothiramalingam & Wang,
2009).39
Table 7: Literature review on biodiesel production via
heterogenous catalyst41

4. LIST OF EQUATIONS
Equation 1: Chemical equation showing production of biodiesel
from any bio oil 32
Equation 2: Reaction mechanism of transesterification via base
catalyst (denoted Y) in the equation. 33
Abstract
The dwindling rate of our fossil fuel reserves and general
believe of major contribution of CO2 emissions which is linked
to the climate change due to the burning of such carbon sources
in engines either for locomotion or power generation have
geared both the academic and industrial research towards the
new routes for renewable and sustainable fuels. However,
microalgae as one of the third-generation biomass feedstock has
recently been proven to be one of the best option considered
employable for biodiesel production. But one of the crucial

5. challenges not yet explicitly attended to is the method of
harvesting, lipids extraction and heterogenous catalyst for
transesterification reaction for oil conversion to biodiesel.
Herein. we reported several techniques for microalgae biomass
harvesting, both the conventional and new emerging ones, as
this helps in building ideas for improvement in the field. We
also present a critical review on the work done on areas of lipid
extraction from microalgae biomass and its conversion to
biodiesel through heterogenous catalysis. it covers the progress
made in this fields from the last decade, available systems for
heterogenous catalysis, mechanism of the reactions and optimal
process conditions. Lastly, we discuss on the reactors employed
in the transesterification, effects of reactors design and way
forward.
1.0. Introduction
In our world, today, the demand for energy is on the increase.
Fossil fuel reserves are running out. The persistent fluctuation
and increase in price of fossil fuels and its adverse effect on the
ecosystem through the emission of greenhouse gases makes it
imperative that we seek alternative, sustainable and more
environmental friendly energy source. The demand for safe
alternative sources of energy such as biofuel is more pressing
than ever before. Top on the list of such sustainable renewable
energy is the feedstock energy source. A good source of
feedstock energy is the Microalgae commonly referred to as the

6. third-generation feedstock (Patrícya et al., 2014; Wawrik &
Harriman, 2010). Various bio-products, such as biofuel and bio-
hydrogen can be manufactured from Microalgae. Microalgae
also have higher biomass yield and lower carbon footprint
requirement compared to other plants (Besson & Guiraud, 2013;
Farooq et al., 2015).
In making microalgae based biofuel production commercially
available and economically viable, the challenge of what
method to use in harvesting microalgae and lipid extraction
must be taken into consideration. Most harvesting methods can
be capital intensive, uneconomical, and produce some level of
environmental pollution. Harvesting microalgae can require as
high as 20-30% of total biomass production budget (Grima et
al., 2003; Mata et al., 2010; Verma et al., 2010) or 50% of the
cost of producing biofuel (Muradov et al., 2015).
Also, it has been proposed that world usage of biodiesel could
increase to two or three times in most part of the globe by year
2020 and numerous influencing factors have not been fully
addressed. Bio-oils or oils as the origin implies contains a key
compound called triglyceride esters
which when react with any monohydric alcohol (i.e. methanol)
would form a group of compounds called mono-alkyl esters i.e.
biodiesel. However, scientist around the world suggested the
use of lower monohydricalcohols (i.e. CH3OH to C3H8O),
without any explicit justification of which gives the best
performance and requirements in terms of its viscosity as
specified by ASTM or related international agencies. Similarly,
finding the best catalyst and optimized operating reaction
conditions is of a great challenge to biofuel industries.
Homogenous catalysis offers faster biodiesel production with
even moderate reaction conditions, but faced with recovery or
separation problem after the transesterification process.
These above highlighted challenges have triggered scientists at
both industries and academics to seek alternative means, while
emphasizing on the feedstock flexibility, and green catalytic
systems. Lately, both microalgae and microalgae emerged as a

7. better option for the biodiesel feedstock. Also, some green
catalyst discussed herein emerged as a better candidate
heterogenous catalysis for the transesterification process. We
therefore present a critical review in these areas and lastly, the
reactors involved.2.0. Microalgae harvesting method
Show et al., (2015) inferred in their work that, when
considering a preferred harvesting procedure two important
issues must first be determined; the attributes of the microalgae
considered and the condition of their growth. The efficiency of
any harvesting method chosen will depend largely on the specie
of microalgae, size of the microalgae, its morphology and
composition of the medium employed. Some harvesting
techniques commonly used are centrifugation, flocculation,
filtration and sedimentation. 2.1. Common harvesting
technology
2.1.1. Centrifugation
Centrifugation is the application of a centrifugal force of higher
intensity than the gravitational force to increase the rate of
separation a suspension. Common centrifugation methods
include Solid-bowl decanter, nozzle-type centrifuge, Hydro-
cyclone, and solid ejecting disc (Milledge & Heaven, 2013).
The challenge with the methods above despite their efficiency at
harvesting majority of the microalgae cell types are high
energy, capital and operational cost required to carry them out
(requirement) (Grima et al., 2003; Milledge & Heaven, 2013;
Yuan et al., 2009).Centrifugation can harvest averagely between
12-25% of microalgae biomass with an energy consumption of
50-75kW (Milledge & Heaven, 2013). The only justification for
this high cost and large amount of energy is that sufficient
biofuel of over 90% of the microalgae biomass must be
harvested.
2.1.2. Sedimentation
Sedimentation is the process by which solids are separated from
liquids by capitalizing on differences in the density of the solids

8. to obtain an effluent of clear liquid (Milledge & Heaven, 2013).
Most wastewater treatment facilities use sedimentation for
sludge treatment. Sedimentation is the most cost effective and
least complicated method for harvesting microalgae biomass,
especially heavy microalgae suspensions. Two difficulty with
using sedimentation for microalgae biomass harvesting is that
for solids with little difference in their densities the process can
be excruciatingly slow and the dry solids concentration of
microalgae biomass that can be harvested is about 0.5-3%
(Grima et al., 2003; Milledge & Heaven, 2013; Yuan et al.,
2009).
Golueke & Oswald, (1965) reported that using alum as a
coagulant in a flocculation-sedimentation process an of 85%
microalgae biomass harvest was achieved.
2.1.3. Flocculation
Flocculation seldom used alone but in conjunction with other
methods (Brennan et al., 2010) such as coagulation-
flocculation and flotation-flocculation. Flocculation through
aggregation improves particle size of microalgae suspension and
speeds up rate of suspension settling (Mata et al., 2010;
Milledge & Heaven, 2013). Auto-flocculation, physical
flocculation, bio-flocculation and physio-chemical flocculation
are four types of flocculation commonly in use.
Auto-flocculation usually cuts the flow of carbon dioxide (CO2)
to the microalgae system when the pH of the culture is above 9,
therefore the microalgae flocculate on its own (Vandamme et
al., 2013). Although auto-flocculation can be a slow, unreliable
process and also requires the presence of calcium and
magnesium ions many researchers have published results of up
to 90% microalgae recovery harvest (Baya et al., 2016; Gerardo
et al., 2015; Milledge & Heaven, 2013; Ras et al., 2011).
Bio-flocculation is a technique in which microorganism are used
in the treatment of wastewater. Such microorganisms include
fungi and bacteria (Gerardo et al., 2015; Vandamme et al.,
2013). Bio-flocculation is a technique yet to be fully

9. comprehended but it is well documented that it improves the
abilities of microalgae to form in suspension (Salim et al.,
2011; Zhou et al., 2013). Zhou et al., (2013) reported almost
100% success harvesting microalgae cells of Chlorella vulgaris
UMN235 by employing palletization-assisted bio-flocculaton.
Their further recommended in their study that adding 20g/L
glucose and of spores in BG-11 medium is desirable for
palletization. Two locally isolated fungi being used as bio-
flocculants are; Aspergillus sp. UMN F01 and Aspergillus sp.
UMN F02. More so, a bio-flocculant from Bacillus
licheniformis CGMCC 2876 was discovered to be an excellent
harvester with 96% efficiency. This level of efficiency results
from reduction in the negative charge of Desmodesmus sp. to
about zero surface charge (Ndikubwimana et al., 2015). From
the findings of the studies above, it can be clearly seen that bio-
flocculation in conjunction with other techniques such as
flotation and electrical approach can be used to solve the
challenge of high quantity of bio-flocculants and time required
to efficiently harvest high amount of microalgae using bio-
flocculation.
2.1.4. Flotation
Microalgae have low densities; this characteristic can be
explored during harvesting using flotation method (Gerardo et
al., 2015; Show et al., 2015). Air bubbles enhance the
movement of microalgae particles upwards. Microalgae cells
become hydrophobic when surfactant or coagulants are added
into the system. Addition of surfactant or coagulants expands
the mass transfer between the air and microalgae particles
improving particles separation (Gerardo et al., 2015; Uduman et
al., 2010).Some readily available surfactants in use are
aluminum sulfate (Al2(SO4)3), iron (III) sulfate (Fe2(SO4)3),
cetyltrimethylammonium bromide (CTAB), chitosan, and iron
(III) chloride (FeCl3). High efficiency harvesting rates between
70 and 99% of microalgae biomass by flotation has been
reported in some studies (Aulenbach et al., 2010; Barrut et al.,

10. 2012; Coward et al., 2014; Show et al., 2015). Flotation
harvesting method requires low initial equipment cost and
shorter period compared to others (Gerardo et al., 2015; Show
et al., 2015).
The choice of surfactants and its effect on reusability of culture
and biofuel manufacturing has only been reported by few
researchers as of today. Recent studies have shown an increase
in the production of biomass and support for the growth of C.
vulgaris – an outstanding discovery using iron (III) chloride
(FeCl3) (Farooq et al., 2015). The challenge with this process is
that ferric acid which is a residue of the iron (III) chloride has a
negative impact on the oxidative stability of biodiesel, upon
completion of the harvesting process it has to be separated from
the biomass.
It is worthy of mention that studies by Farooq et al., (2015);
Kim et al., (2011); Kim et al., (2013) have implied that
cytotoxicity from residual alum from the alum used as
surfactant inhibits the growth of microalgae. Hence the effect of
surfactants need to be properly studied and understood before
harvesting microalgae to be used for production of biofuel and
culture recyclability.
Other flotation techniques have been reported. Some of these
are; dispersed air, micro-flotation, foam flotation, dissolved air,
vacuum gas, electro-flotation, and ozone flotation. A
comparison of these techniques is displayed in Table 1.
Table 1: Performance comparison of flotation techniques
Flotation Techniques
Strains
Harvesting efficiency (%)
References
Foam flotation
Tetraselmis sp.
93
(Garg et al., 2014)
Chlorella sp

11. NR
(Coward et al., 2014)
Microflotation
Dunaliella salina
99
(Hanotu et al., 2012)
Vacuum gas
Mixed culture
23
(Barrut et al., 2012)
Dissolved air
Mixed culture
90
(Phoochinda & White, 2003)
C. zofingiensis
91
(X. Zhang et al., 2014)
Column flotation
Chlorella sp
90
(Liu et al., 2006)
Flocculation flotation
C. vulgaris
93
(Lei et al., 2015)
Dispersed air
Spirulina platensis
80
(Kim et al., 2005)
Electro-flotation
Mixed culture
NR
(Sandbank, 1979)
Ozone flotation
Microcystis

12. 90
(Benoufella et al., 1994)
*NR – Not reported
2.1.5. Filtration
Filtration is the separation of a solid-liquid mixture of
microalgae using a semi-permeable membrane with small pores
that allow the passage of the liquid but retains the solid
microalgae (Gerardo et al., 2015; Show et al., 2015).
Microalgae with low density such as Chlamydomonas sp.,
Chlorella sp. and Scenedesmus sp. can easy have their biomass
harvested using filtration method (Gerardo et al., 2015;
Rickman et al., 2012; Show et al., 2015). One problem with
filtration however is that clogging and fouling brought by
settled cells that can reduce the solid content due to low
volumes of liquid that is able to pass through the filter used
(Huang et al., 2012; Show et al., 2015).
Two filtration setups are common, which are; the dead-end and
the tangential flow. Dead-end setup made up of cartridge
filtration, horizontal filter press, belt filter and vacuum drum
filter are carried out in batch modes. These methods can harvest
5-37% mean solid content. Cross-flow filtration which is an
alternative name for tangential flow filtration was created to
overcome the challenge of fouling and decrease the
accumulation of cake layer such that the filtration time is
accelerated (Gerardo et al., 2015). Shear movement (Morineau-
Thomas et al., 2002; Nurra et al., 2014), back-flushing
(Baerdemaeker et al., 2013), supplementation of coagulant
(Hwang et al., 2013), and alteration of membrane surface
(Baerdemaeker et al., 2013) have been reported by many studies
as ways of minimizing membrane fouling. Table 2 is a summary
of the outcome obtained from using each filtration method in
microalgae biomass harvesting.
Table 2: Performance comparison of filtration methods
Filtration Techniques
Strains

13. Harvesting efficiency (%)
References
Vacuum filter
Coelastrum sp.
NR
Belt filter
Mixed culture
NR
(Grima et al., 2003)
Ultrafiltration
Scenedesmus quadricauda
NR
(Zhang et al., 2010)
Chlorella sp.
94
(Hwang et al., 2013)
Deep-bed filtration
Mixed culture
NR
Vacuum filter
Spirulina sp.
NR
(Goh, 1986)
Ultrafiltration
Dunaliella sp.
99
(Mixson et al., 2014)
*NR – Not reported2.2. New Emerging Microalgae Biomass
Harvesting Techniques
Many scientific studies have been carried out to enhance
microalgae biomass harvesting, which include reducing the
energy requirement and operational cost.
Recent approaches being developed to harvest microalgae

14. biomass include flocculation employing magnetic microparticles
(Seo et al., 2015; Vergini et al., 2015), flocculation using
natural biopolymer (Banerjee et al., 2014; Rahul et al., 2015),
sedimentation involving polymers (Zheng et al., 2015),
magnetic membrane filtration (Bilad et al., 2013) as well as
electrical approaches which includes electro-coagulation-
filtration (ECF) (Gao et al., 2010) and electrochemical
harvesting (ECH) (Misra et al., 2015).
2.2.1. Flocculation using magnetic microparticles
Seo et al., (2015) examined four different magnetic particles,
namely pure copper, iron (III) nitrate nonahydrate,
copper/carbon composite and polyvinylpyrrolidone (PVP).
Vergini et al., (2016) also examined the application of iron
oxide magnetic microparticles (IOMPs). Both studies reported
microalgae biomass harvesting using flocculation with the
various magnetic particles aforementioned (Seo et al., 2015;
Vergini et al., 2015). By using various methods and different
materials these magnetic particles can be coated. Some coating
methods are polyol method, sol-gel processing, thermal
reduction, and spray pyrolysis (Athanassiou et al., 2006;
Grouchko et al., 2009; Yan et al., 2007; Yanhui et al., 2009;
Jacob et al., 2006; Jung et al., 2011; Li & Liu, 2009; Li et al.,
2011; Qian et al., 2012; Wang & Asefa, 2010; Xu et al., 2003).
A 99% harvesting efficiency of Chlorella sp. KR-1 was
achieved by Seo et al., (2015) using polyvinyl-pyrrolidone
(PVP) and iron nitrate with 0.8 () ratio at 10mg/mL. Vergini et
al., (2015) on their part reported a 91% efficiency in harvesting
D. tertiolecta using iron oxide magnetic microparticles (IOMPs)
during 5 min of harvesting. The harvesting procedure was
carried out for a certain period using cylindrical neodymium
magnets (NdFeb) as an external magnetic field. Judging from
the reports by Seo et al., (2015) and Vergini et al., (2015) , a
conclusion can be reached; that flocculation using magnetic
particles brings about cost effectiveness, as far as the magnetic
microparticles can achieve full functionality via one-step

15. synthesis.
It is worthy of note that harvesting efficiency is affected by the
dose of each particular magnetic flocculant. Vergini et al.,
(2015) reported a minimal increase of 16% when the
concentration if IOMPs changed from 6.2mg/L (75%) at the
beginning to 62mg/L (91%). Furthermore, Seo et al., (2015) in
their study reported that increasing the dose of PVP/Fe (0.33)
while reducing the ratio had resulted in reduced harvesting
efficiency (31.4%).
2.2.2. Flocculation by natural biopolymer
Biopolymers are organic flocculants manufactured by
microalgae, bacteria, and plants haven undergone the process of
cationization using (3-chloro-2-hydroxypropyl)
trimethylammonium chloride (HPTAC). Polymeric substances
which are a kind of organic flocculant when linked with
different colloidal particles to produce floc formation have bee n
described as efficient harvesters. An optimized dosage of
60mg/L of cationic inulin resulted in 88.61% harvesting
efficiency within 15 min was reported by Rahul et al., (2015)
while harvesting isolated Botryococcus sp., this study further
showed that cationic inulin is a potent flocculant.
In another study by Banerjee et al., (2014), the potency of
cationic cassia gum to harvest Chlamydomonas sp. CRP7 and
Chlorella sp. CB4 was reported. An harvesting efficiency of
93% was reported for Chlamydomonas sp CRP7 within 15 min
when a 80 mg/L optimized flocculant dosage was used. On the
other hand, the harvesting efficiency for Chlorella sp. CB4 was
92% within 30 min of introducing a 35 mg/L dosage of
optimized flocculant. Finally, these studies showed a similar
trend when the flocculant efficiency increased and the
chlorophyll content decreased. Table 3 contains a summary of
the performance of flocculation using biopolymer.
Table 3: Performance of flocculation using biopolymer
Biopolymer
Strains

16. Optimum concentration (g/L)
Harvesting efficiency (%)
References
bCatinic cassia gum
Chlorella sp. CB4
0.035
92
(Banerjee et al., 2014)
aCatinic cassia gum
Chlamydomonas sp. CRP7
0.08
93
(Banerjee et al., 2014)
aCatinic inulin
Botryococcus sp.
0.06
88.61
(Rahul et al., 2015)
a – run time = 15 min, b – run time = 30min
2.2.3. Electrical approach
Various researchers have worked on the use of electric approach
in biomass harvesting. Methods such as electro-coagulation-
filtration (ECF) (Syafaini et al., 2017), magnetically induced
membrane filtration (MMV) (Bilad et al., 2013) and
electrochemical using non-sacrificial electrodes (Misra et al.,
2015) have been studied. The electrical approach for biomass
harvesting is environmentally friendly, not restrained to certain
microalgae species only, safe, and cost effective.
A comparative study on the treatment of water containing
microalgae using aluminium and iron electrodes by Gao et al.,
(2010) in an electro-coagulation-filtration (ECF) system
reported that aluminum electrodes performed better than the
iron electrode for harvesting. Micro-cystis aeruginosa, as seen
in their harvesting efficiencies which was about 100% for
aluminum and 78.9% for iron. The study further showed that

17. increasing operating time resulted in increased harvesting
efficiency. Gao et al., (2010) also studied the impact of initial
pH on the harvesting efficiency, the best efficiency was
observed for pH values between 4 and 7 where efficiency of
100% was observed. The alkaline broth resulted in the lowest
percentage efficiency. As the initial pH values increased from 8
to 9, and then 10, the harvesting efficiency reduced from 99% to
90% and then 87.2% respectively.
To improve the shear-rates at the liquid-membrane interface
membrane vibrating system was used. From the result of
polyvinyldene fluoride (PVDF) at two different porosities of 9%
and 12% w/w, PVDF-12 presented a greater efficiency for
harvesting Phaeodactylum tricornutum and C. vulgaris, which
was higher than 97% (Bilad et al., 2013). Reduced harvesting
efficiency was observed when using PVDF-9, this was
suspected to be a result of the shortcoming of the membrane
itself.
Misra et al., (2015) reported the use of electrochemical method
in the harvesting of Scenedesmus obliqus FR75119.1 using
carbon plates. This study was interested on the effects of
applied current, initial pH and electrolyte addition. When a
current of 1.5 A was supplied for half an hour, the
electrochemical harvesting (ECH) attained 55.4% efficiency.
This finding was in consonance with that by Gao et al., (2010)
in which the harvesting efficiency relied on the applied current
and conductivity of the broth. Adding electrolytes, for instance
sodium chloride (NaCl) would increase conductivity and reduce
power consumption (S. Gao et al., 2010). The highest harvesting
efficiency reported was 83% when 6 g/L of NaCl was added.
The study further buttressed the report that initial pH within the
acidic range for example pH 5 produced the highest harvesting
efficiency of 73% while initial pH in the alkaline region
between7–9 had produced a lower harvesting efficiency in the
ECH system.
From the aforementioned studies, it can be concluded for the
electrical method, the alkaline broth had the worst performance

18. and that addition of electrolyte was vital in making the process
economically sustainable. Table 4shows a summary for
performance comparisons for microalgae biomass harvesting by
various electrical methods.
Table 4: performance comparisons for microalgae biomass
harvesting by various electrical methods operated in just 1 hour
Electrical approach
Strains
Initial pH
Harvesting efficiency (%)
References
Electrochemical harvesting
Scenedesmus obliqus
9
65
(Misra et al., 2015)
7
66
5
73
Electro-coagulation-floatation
Microcystis aeruginosa
10
87
(S. Gao et al., 2010)
9
90

19. 8
99
4
100
3.0. Extraction and Analysis of Lipid from Microalgae Biomass
Lipids are a major component of microalgae. Microalgae can
contain between 2-60% lipids out of its total dry weight
depending on the condition of growth and the type of specie
being examined. Lipids extracted from microalgae have
attracted major interest for their fatty acids and triglycerides
which can be converted into alcohol esters through
esterification. The oils derived through this process are called
ester fuels, when blended with diesel stocks these oils have
produced efficiency of about 30% without degrading engine
performance (Razzak et al., 2013). A group of researchers have
successfully demonstrated a modified engine that can run on
100% of these fuels (Xu et al., 2008).
Microalgae serve as special chemical production sites using the
instrument of photosynthesis. After over four decades of
studying these microorganisms, they finally displayed their
abilities to produce different varieties of complex compounds
and fossil fuels. The extraction of fatty acids from microalgae
or lipid synthesis which usually results in hydrocarbons with 16
to 22 carbon chain length requires that oxygen be available (Hu
et al., 2008). Lipid content and distribution in the cell are
affected by some elements, some of which are; (a) CO2

20. concentration (b) temperature (c) intensity of light, (d) the
nutrient concentration and (e) the relative proportion of salt -
salinity.
In algae, carbon compounds such as triglycerides-which are
nonpolar lipids act as reservoirs of energy. However, the
function of cell and chloroplast membrane formation is that of
two polar lipids - phospholipids and glycolipids which are
found inside the cell (Razzak et al., 2013). In as much as
biodiesels can be manufactured from conversion of polar lipids,
conventionally the non-polar triglycerides such as feedstocks
are still the favored option.
Therefore, non-polar lipids remain the most important algal
product of interest. The conditions under which an algae grows
during the growth phase will determine greatly the lipid
concentration and productivity (Razzak et al., 2013). The total
lipid content can differ between species beginning at 4.5% for
very low and 80% for very high (Xu et al., 2008).3.1. Lipid
extraction
We can not emphasize enough the fact that lipid extraction from
microalga to be used as biofuel is not only a strenuous process,
but it is both energy intensive and cost demanding. To convert
microalgae into diesel fuel that research into better ways for the
extraction from dry biomass of lipids and further refining is
pivotal. The challenge here is that the specific condition to
achieve this is yet to be fully laid out (Chisti, 2007, 2008).
To extract lipid from biological cell, chemical means, physical
means or a combination of both means are employed. Cell
disruption is mostly needed for releasing lipid and sugar
contents within the cells of microalgae, to be used in the
production of biodiesel and ethanol. Mechanical cell disruption
can lead to extraction of an immense amount of lipid from
microalga when accompanied by chemical solvent extraction.
Furthermore, literatures have reported other mechanical
processes which include: freezing, bead milling, osmotic shock,
homogenization, high pressure, sonication. Also alkali and
organic solvent extraction which is a chemical procedure was

21. also advanced. (Chisti, 2007, 2008; Razzak et al., 2013).
3.1.1. Mechanical extraction
When a dense suspension of microalgae or other organisms is
vigorously stirred together with the purpose of extracting the
lipid in the mills cell disruption occurs. Cell disruption could
happen to the suspended cells when they come into contact with
energetic glass beads with powerful crushing ability during
stirring. Hopkins (Razzak et al., 2013) reported that vigorous
agitation by several small sized glass beads or ceramic beads
was observed while mixing the cells and the beads. A bead mill
is a simple arrangement of an enclosed grinding chamber having
at its center a shaft that rotates. The discs fixed to the shaft
transfer energy of motion to the tiny beads within the enclosure,
bringing about a collision between them. The chamber contains
a sieve or a slot in the direction of the rotating shaft through
which the beads cannot pass. The beads accelerate radially,
creating streams with varying speed and establishing high-
tangential forces. The suspension is channeled into the grinding
column through an external pump.
Recently, the use of ultrasound for disrupting microcell has
increased just as that used during microalgae as well as
microbial processes. Assuming perfect conditions, such an
arrangement can generate strong ultrasonic wave pressures in
the liquid media it transverse leading to the creation of tiny
bubbles. When these bubbles increase in size they generate
cavitation when they collapse and generate huge violent shock
waves of energy breaking cell wall membrane (Samarasinghe et
al., 2012). Converti et al., (2009) demonstrated that ultrasound
can be used (mod. UP100H, Hielscher, Teltow, Germany).
Converti et al., (2009) achieved total extraction of the
microalgae fatty components by a combined use of ultrasound
along with chloroform/methanol. the microalga of
Scenesdesmus almeriensis contains lutein Cerón et al., (2008)
were successful at extracting the lutein from it. The authors also
experimented with three methods of disrupting a cell viz-a-viz

22. (a) bead mill (with ceramic beads of 28 mm diameter, rotation
speed of 120 rpm and, 2L volume), (b) mortar with pestle
(125mL volume), (c) a combination between them and (d)
ultrasound (Pselecta Ultasons unit). They reported the preferred
option as the use of bead mill with alumina as disintegrating
agent, among the treatments tested with regards to industrial use
in the ratio 1:1 w/w for 5 min.
Another study into lipid extraction was carried out by Neto et
al., (2013) the cell disruption step was carried out using
sonication bath pretreatment, after which vortex mixing was
done and solvent extraction was performed using n-hexane.
Neto et al., (2013) verified that increased lipid extraction using
this method was obtained from the alga biomass when compared
with the regular solvent extraction using hexane. These authors
put forward the hypothesis that extraction time and solvent
consumption were reduced when ultrasound was used to assist
the process of lipid extraction because the solvent had greater
penetration of the cell arrangement (Mercer & Armenta, 2011).
Similar to other studied microbial cells, microalga can be
hardened by freezing using liquid nitrogen or the method of
freeze dryer. Frozen microalgae become brittle and easy to
crush simply by the crushing action of the mortar and pestle.
Low temperatures make ice crystals abrasive. Gouveia et al.,
(2007) in their own study, extracted lipids by freezing the
microalga and then crushing mechanically, which was preceded
by supercritical CO2 extraction. Firstly, the microalgae were
manually crushed then moderately mixed with dry ice. The
mixture was then crushed totally with the help of a disk
vibratory mill (NV-TEMA, Labor-Scheibenschwingmuhle, type
T100, 0.75 kW, 1000 V/min).
Zheng et al., (2011) extracted lipids by using C vulgaris sp. The
authors compared different processes through which lipids can
be extracted. Some of the methods examined included (a)
grinding in liquid nitrogen, (b) quartz and grinding under
dehydrated condition, (c) quartz sand grinding under wet
conditions, (d) enzymatic lysis by snailase, (e) bead milling, (f)

23. ultrasonication, (g) enzymatic lysis by cellulose, (h)
microwaves, and (i) enzymatic lysis by lysozyme. Grinding in
liquid nitrogen required a minimum of 2 min.
3.1.2. Chemical/solvent extraction
Using solvent to extract oil usually comes after the algae cells
have been mechanically disrupted and can be carried out
through a two-solvent system. Mechanical disruption takes
place a lot in the Lab setting, such disruption can be done by
the process proposed by Bilgh and Dyer. This process entails
the use of a chloroform/water/methanol method of extraction for
accessing cell lipids. Non-polar (chloroform) and polar
(methanol) two solvent systems are used for fractional lipid
extraction from the cells. A major upside to solvent extractions
is that it allows for high lipid harvest which can be refined
further or used in its raw state.
(Dote et al., 1994) studied how efficiently the algal cells of B.
braunii could be converted into liquid fuels through
liquefaction. Algal cells (~30 g) were charged to the autoclave
while utilizing a catalyst (sodium carbonate) or not, 20 mL of
distilled water was added to enable proper stirring because the
amount of algal cells was inadequate to allow for proper
stirring. The autoclave was later charged with nitrogen at 2 MPa
after purging it using nitrogen, and then heated after sealing it
in an electric furnace.
Supercritical CO2 is also usable as a solvent in the extraction of
lipids from microalgae. Some of the liquid and gas are held
back due to the Supercritical CO2. Supercritical fluid extraction
offers a ‘natural and ecofriendly’ option towards extracting
product. This process is increasingly seen as an alternative to
the usual techniques used for separation. It is an easier, faster
and more reliable method. Also, the process utilizes less organic
solvents, thereby reducing cost and harm. Supercritical CO2
presents some special attributes as a solvent. It is not harmful
(Halim et al., 2012). Hence, Supercritical CO2 can be used as
an appropriate solvent for extracting lipid from microalgae C

24. vulgaris, as clearly shown by (Gouveia et al., 2007; Mendesa et
al., 2003). It was successfully demonstrated that microalgae that
has been mechanically crushed delivered higher extraction
yields using supercritical CO2 compared to others (Razzak et
al., 2013).
3.1.3. New emerging green solvents systems and process
intensification techniques for lipids extraction from microalgae
A major setback for the commercial viability of algae-based
fuels is the cost required for the process. In the process of
producing biodiesel from microalgae a lot of difficulties is
encountered; lipid extraction is one of such challenge, lipid
extraction does not just require a significant quantity of energy
and time but also causes environmental contamination due to
the toxic solvents used. Furthermore, the quality of the lipid
content maybe reduced by unwanted compounds such as
chlorophyll dissolved in it as a result of the conventional
solvents used during lipid extraction. To overcome this
problem, green solvents and process intensification
methods/techniques (green extraction technologies) promise
improvements in the characteristics of energy reduction, eco-
friendliness, non-toxicity and efficient lipid extraction. Hence,
this section examines the prospects of green solvents and
extraction techniques that may enhance the commercial viability
of biodiesel production.
Lipid extraction from microalgae is conventionally carried out
by Folch et al. or Bligh and Dyer (Jeevan et al., 2017) after
which lipid quantification through gravimetric estimation of
lipid content is done. Folch et al. devised an easy method for
extracting total lipids from animal tissues. However, the success
of this method depends on the availability of mineral salts in
the crude extract and use of large amount of solvent. If mineral
salts are not present, majority of the acidic lipids are lost during
the washing step.
On the other hand, Bligh and Dyer devised a fast lipid
extraction technique for estimating the lipid composition of

25. frozen fish tissue. This method works only for tissues wi th 80%
water content, and the efficiency rely on the constancy of
chloroform, methanol and water proportions according to the
water content of the tissue. Chloroform and methanol are not
just toxic and flammable but can also endanger our health and
environment. These solvents reduce the quality of the final
product by dissolving unwanted compounds (chlorophyll)
during the extraction procedure (Archanaa et al., 2010). When
this occurs, the quality of the lipid does not only drop but the
biorefinery goal of a more cost friendly process is greatly
hampered. Moreover, it goes without saying that for biodiesel
production, preference has to be given to saponifiable lipids,
which after transesterification are modified into fatty acid
methyl esters (FAME) and not the whole lipids (Meng et al.,
2009). Hence, those solvent systems for lipid extraction that can
be sustained, non-toxic and improve lipid content yield without
interference of non-lipid compounds should be looked into
(Brennan et al., 2010).
The importance of green solvent in the lipid extraction process
has been discussed above. Furthermore, some new ‘green or
process intensification techniques’ that encourage time and
energy reduction, lower usage of solvent, reduction in the
down-stream processing steps are vital for making the process
viable. These techniques simplify the process and make it cost
effective. For example, during biomass extraction the drying
step could be skipped by using wet algae biomass extraction. In
like vain, transesterification which is used for ester and glycerol
production besides biodiesel process of wet algae biomass may
prevent not just the dewatering and drying steps but also the
lipid extraction process (in situ transesterification). The
possibilities of green solvents and process intensification
technologies and their influence can be compared with a
traditional process for extraction and transesterification to
biodiesel (Figure 1).

26. In-situ transesterification
Other fuels
Chemicals
Conversion
Separation of biodiesel and glycerol / FAME purification
Biodiesel
Glycerol
Transesterification
Cell disruption
Extraction i.e., MAE, SAE, UAE, EAE
Mechanical processing
Grinding
Drying/freeze drying
Wet biomass
Dry biomass
Harvesting
Cultivation of Microalgae
Figure 1: Flowsheet for biodiesel production from microalgae.
Some intensified process techniques highlighted may reduce
some downstream steps as it would render the dewatering step
unneeded. i.e. MAE – Microwave assisted extraction (MAE),
Enzyme assisted extraction (EAE), Ultrasound assisted
extraction (UAE), Surfactant assisted extraction
3.2. Lipid analysis
Fluorophores are plants that exhibit fluorescence when light of
a certain wavelength falls on it when in a non-polar
environment. Nile Red is an example of such a plant and it
causes lipid droplets in algal cells to fluorescents. Keith et al.,
(1987) determined the non-polar lipid content in algal cells
using Nile Red dye. The authors using acetone with a
concentration of 1 μg/mL examined Cell cultures previously

27. stained with Nile Red. The total lipids in Amphora
coffeaeformis, Navicula sp., Tropidoneis sp., and Chlorella sp
were gravimetrically determined by the average peak
fluorescence as measured by a spectrofluorometer.
For detection and characterizing the fatty acids (PAs) levels of
biodiesel, different analytical techniques are available. There
are also diverse techniques for quantifying fatty acid levels
(PAs) in biodiesel. Gas chromatography (GC) and high
performance liquid chromatography (HPLC) are two analytical
methods often used for analyzing fatty acids and triglycerides.
In comparing these methods chromatographic analysis alone
should not be the focus but focus should also be placed on
sample preparation. Although fatty acid analysis using HPLC
has been on the increase in the past 10 years, the most used
method still remains GC (Shantha et al., 1992). When complex
mixtures that cut across a broad molecular range are analyzed
gas chromatography-flame ionization detection {GC-FID} along
with this standard technique has been very effective and fast
(Carelli et al., 1997). Zuo et al., (2013) have exemplified that
separation, quantification and analysis by GC can be very well
carried out using fatty acids without the need for derivatization.
However, while analyzing using GC, to improve the volatility
and separation of the substances; derivatization of fatty acids
can be carried out. This will also reduce tailings. The
technology behind GC has seen recent improvements among
which is the columns having bonded phases that give special
separation opportunities reduced phase bleeding. However,
derivatization can be used for situations requiring improved
sensitivity. In recent studies separation and analysis of
geometric isomers have been done using GC as well as for
positional isomers. For diagnostic fragmentation of saturated
and unsaturated AAS mass spectrometry (MS) has been proven
as a potent technique. It is therefore worthy of note that GC and
MS detection can be used together. MS can also be used for the
analysis of branching positions in Pas (William, 1998).
Otsuka & Morimura, (1966) showed the change in lipid

28. composition over different cell growth stages of synchronously
grown cultures of Chlorella ellipsoidea. Using a silicic acid
column chromatography was preceded by a
methanol/ethanol/ether lipid extraction. Acid methyl ester
fractions, after separation of polar and non-polar individual
fatty acids were then obtained. These authors examined growing
cultured cells at different stages. A change in the relative
distribution of polar and non-polar fatty acids at every next
growth stage was seen by the fatty acid profile.
The kind of lipid, hydrocarbon and other complex oils
produced by microalgae depends on its specie (Banerjee et al.,
2002). Fatty acids in algae were discovered to be abundant in
oleic acid (C18:1) and palmitic acid (C16:1). During cell
division oleic acid is utilized for both cells under light and in
the dark, this suggests that oleic acid in triglycerides contribute
largely to the energy requirement for cell division.4.0.
Heterogeneous transesterification catalysts
Several researches were carried out and reported in other to
explore the reactions of wide range of heterogeneous substance
with the aim of mitigating the diverse challenges experienced
when using homogeneous bases and liquid acids as alcoholysis
catalysts (Choudhury et al., 2014; Dias et al., 2012; Kazemian
et al., 2013). Presented in Table 5 is the list of various solid
acids and bases reported from researchers as biodiesel
producing catalysts. As shown in the table, ZrO2, MoO2,
Al2O3, Si2O3, heteropoly acids and zeolite materials are
catalysts categorized as solid acids (Anderson et al., 2009;
Choudhury et al., 2014; Chouhan & Sarma, 2011; Dias et al.,
2012; Kazemian et al., 2013; Martín & Grossmann, 2012).
Having both Brønsted and Lewis acid sites was the bases for
charactering the above-mentioned materials, which decides their
reactivity in transesterification reactions. Shape selectivities
could also affect the activities of zeolites and heteropoly acids.
Materials with multi-dimensional pore structure stand a better
chance in the formation of alkyl esters without necessarily
cracking. Alternatively, solid bases mainly consist of oxides of

29. common metals, carbonates and basic zeolites. However, in
biodiesel production, the most important active sites in these
materials are the basic sites.
Table 5: Reported catalyst used for heterogenous
transesterification reaction on various feedstocks
Catalyst
Feedstock
Reaction operating conditions
Performance
References
Temp (0F)
Catalyst loading wt%
Time (hr)
Methanol :oil
Yield (%)
Conv. (%)
Solid Acids Heterogenous Catalysis
WO3/ZrO2
Waste cooking oil
167
-
20
19:1
-
85
( Park et al., 2010)
Zeolite Y (Y756)
860
-

30. 0.37
6:1
27
NR
(Brito et al., 2007)
Starch/Carbon-based
176
10
8
30:1
92
(Lou et al., 2008)
H3PW12O40.6H2O
149
4
14
70:1
87
(Cao et al., 2008)
ZrO7H0.2PW12O40-ZrHPW
2
8
20:1
99
(Zhang et al., 2010)

31. Zs/Si
392
3
5
18:1
98
(Jacobson et al., 2008)
SO42-/TiO2-SiO2
4
9:1
90
(Peng et al., 2008)
SO42-/SnO2-SiO2
302
3
15:1
92
(Lam et al., 2009)

32. Solid Bases Heterogenous Catalysis
CaMnO3
animal fat
-
-
-
-
-
NR
(Dias et al., 2012)
Calcined waste coral fragment-CaO
Palm oil, Soybean oil, Rice bran oil, waste cooking oil
-
-
2
-
98
(Roschat et al., 2012)
KF/CaO
Chinese tallow seed oil
149
-
2.5
12:1
97
(Wen et al., 2010)
KNO3/CaO
Rape oil

33. 149
1
3
6:1
98
(Encinara et al., 2010)
CaO/ZnO
Ethyl butyrate
140
1.3
2
12:1
90
(Alba-Rubio et al., 2010)
Li/MgO
Soybean oil
140
9
2
12:1
94
NR
(Wen et al., 2010)
Kl/Mg-Al
158
5
8
20:1
90

34. (Tantirungrotechai et al., 2010)
CaO/SiO3
140
5
8
16:1
95
(Samart et al., 2010)
Na2SiO3
3
1
8:1
100
(Guo et al., 2010)
PzOH/SiO2
167
4
12
60:1
90
(Kim et al., 2011)
Mg-Al-hydrotalcite
446
5

35. 1
13:1
(Cristina et al., 2010)
Sunflower oil
140
2
24
12:1
50
(Campo et al., 2010)
Jatropha curcas oil
113
1
1.5
4:1
95
(Deng et al., 2011)
CaO/Fe3O4
158
2
1.33
15:1
95
(Liu et al., 2010)

36. CaMgO
149
4
6
80
NR
(Taufiq-Yap et al., 2011)
CaZnO
MgO-KOH
Mutton fat
4
0.33
22:1
-
98
(Mutreja et al., 2011)
Dolomite
Canola oil
154
3
3
6:1

37. 92
NR
(Ilgen, 2011)
Palm oil
140
6
3
30:1
98
(Ngamcharussrivichai et al., 2010)
Calcined mollusk & egg shell-CaO
10
2
18:1
90
(Viriya-empikul et al., 2010)
Calcined egg shell-CaO
149
1.5
2
12:1
98
(Cho & Seo, 2010)
KF/Ca-Al-hydrotalcite

38. 5
5
(Gao et al., 2010)
CaO/Al2O3
148
-
5
99
(Zabeti et al., 2010)
Calcined CaCO3-CaO
140
-
1
15:1
94
(Yoosuk et al., 2010)
CaO
7
0.75
96

39. (Yoosuk et al., 2010b)
Sunflower oil
176
1
5.5
6:1
91
(Verziu et al., 2011)
167
-
0.75
4:1
80
(Vujicic et al., 2010)
Waste cooking oil
149
0.9
1
12:1
66
(Kouzu et al., 2017)
K3PO4
140
4

40. 2
6:1
97
(Guan et al., 2009)
Oil palm ash
5.4
0.5
18:1
72
(Chin et al., 2009)
Calcined snail shell-CaO
2
8
6:1
87
99.6
(Birla et al., 2012)
In the heterogeneous catalysis, though regarding if solid basic
or acidic catalyst is employed, a number of factors such as
amount of catalyst, degree of mixing or stirring, the on stream
reaction time, oil/alcohol content, transesterification
temperature and purity index of the feedstock has to be
appropriately considered. Anderson et al., (2009) studied the
distribution of the active material over the catalyst support
which is believed to have influences on the activity of
BaO/Al2O3 in alcoholysis, the author confirmed that higher
distributions have higher activity. To avoid handling

41. difficulties, temperature close to boiling point of monohydric
alcohol should be selected. More so, in other to have a complete
conversion adequate reaction time needs to be provided (for
example 1-3 hours). It is also important to monitor the extent of
catalyst/reactants interaction therefore moderate mixing is very
necessary, because the reaction would be very slow if the
mixing rate is low, meanwhile the reaction may be difficult to
handle as well as side reactions will be experienced when the
mixing rate is high. However, oils with lesser percentage of
fatty acids (such as algae oil with less than 1% fatty acid) are
most desirable because the key impurities in majority of oils are
the free fatty acids.
The equilibrium process through which methanol (monohydric-
alcohol) and triglyceride ester (bio-oil) reacts in ratio 3:1 to
give a mole of glycerol and an equal amount of mono-alkyl
esters (biodiesel) is known as transesterification (Equation 1).
+
Glycerol
3
Methanol
Triglyceride
Catalyst
3
Methyl acetate (Biodiesel)
+
Equation 1: Chemical equation showing production of biodiesel
from any bio oil
This reaction proceeds in three successive phases, each
consisting of the production of biodiesel (mono-alkyl ester) and
initiating alcoholic -OH group into the triglyceride ester chain.
Hence, at the last phase of the reaction, glycerol would be
produced. However, the ratio required of monohydric-alcohol to
oil should be within 4:1 and 12:1 in other to speed up the

42. reaction and generate more biodiesel within the shortest
possible time. Furthermore, negative effect on the total
biodiesel generated may be created as well as reduction in the
general production of biodiesel as such the reaction may shift
backwards all of which are as a result of values below the
above-mentioned ratio. Nevertheless, at higher ratios, greater
challenges may be posed as a result of the removal of excess
alcohol. Other critical factors such as the degree of stirring and
reaction temperature are greatly importance. Thus, to guarantee
adequate interaction between reactants and catalyst particles
sufficient stirring is necessary (Anderson et al., 2009; Chouhan
& Sarma, 2011; Liu et al., 2008; Lotero et al., 2005; Ma et al.,
2008). However, as for the reaction temperature, it must be
close to the boiling point of that of monohydric alcohol,
because slow reactions are favoured by lower temperatures,
while handling difficulties is created by much higher
temperatures.4.1. Solid Bases Transesterification
More often than none, reactions that involve heterogeneous
bases proceed via reactions of either the Brønsted or the Lewis
basic sites of the catalyst generally with ether ethanol or
methanol (monohydric alcohol). Hence, biodiesel is produced
when the alkoxide mixture generated reacts with TAG
(triglyceride - C55H98O6) ester in the oil and also yields
glycerin in the next steps (Equation 2).
+
+
Equation 2: Reaction mechanism of transesterification via base
catalyst (denoted Y) in the equation.

43. The mode of action is Eley-Rideal type, but the extend of its
basicity greatly determines on the speed of the reaction. An
alkoxide group (RO-) is formed and H+ removed when the
alcohol preferentially interacts with the basic sites, been the key
active component which attacks the triglyceride ester at the
intermediate phases. The formation of alkoxide species depends
on strength the basic sites (i.e. the higher the strength of the
basic sites, the more favored the production of the alkoxide
species and vice versa). In the same way, cleavage and glycerol
formation is favoured by stronger basicity and subsequently
enhancing the overall rate of reaction.
Solid basic catalysts i.e. zeolites, ZnO, CuO or other oxides of
first row transition metals, compounds from group 2A elements
and basic polymers (Table 5), particularly the oxides i.e.
Calcium oxide, Magnesium oxide, strontium oxide, Barium
oxide etc., and carbonates which is more prominent for example
Calcium carbonate, Magnesium carbonate, Strontium carbonate
and Barium carbonate, have been focus of research in the area
of heterogenous catalysis for transesterification. Their basi city
is associated with metal-oxygen ion pairs (i.e. M2+ -O2-) and
varies in the order Ba > Sr > Ca > Mg for the oxides. As such
these catalytic materials are easy to prepare, because they are
inexpensive and showed low corrosion properties. The
transesterification reaction of these heterogeneous base
catalysts is determined by the severity of calcinations step
which is a very important factor. The conversion gets reduced at
high calcinations temperatures as a result decrease in active
catalyst surface. For example, only 18% conversion yield was
achieved in 8 hours of reactions for a sample of MgO calcined
at 600OC. Meanwhile, at optimum conditions; lower calcination
temperatures, the methanol/oil content being 12:1 and 5.0 wt %
catalyst 92% high conversion yield was achieved (Chouhan &
Sarma, 2011). This was also noticed over CaO. On the other
hand, after repeated cycles of applications, the latter catalyst
caused reusability problems.
Transition elements such as Fe, Ce, Zr and La and Mixed oxides

44. of Ca have also been studied in transesterification. This has
resulted to above 95% biodiesel yield. Although, to achieve
sufficient shift of the equilibrium position forward, a relatively
high ratio of methanol-oil (i.e. 6:1) could be needed. Sufficient
transesterification time of about 10 hours’ reaction time may as
well be required in addition to these materials. For instance, in
10 hours a CaTiO3 may yields up to 79% of biodiesel, while in
the same reaction time (10hrs) CaCeO3 and CaZrO3 yields
around 70 and 95% of biodiesel at 60OC using oil/methanol
ratio of 1:6. Because of the adequate stability and dispersion
properties of Al2O3 out of other support materials, it shows a
higher activity and also permits improved interaction between
the surface reactants (Anderson et al., 2009; Chouhan & Sarma,
2011; Ma et al., 2008). Basic zeolites and hydrotalcites are
gradually more researched and recently more studied. Materials
like oxides containing faujasites, zeolites and ETS-10 have
basic cations which may be generated through thermal melting
of their supported salts. Ion exchange with highly
electropositive cations is as well essential in other to the
promote transesterification activity. The hydrotalcites (MgeAl)
has good basicity except for the dissolution problems that
necessitate the materials to be carefully prepared. Co-
precipitation preparation methods have thus far shown reliable
stability. 4.2. Solid Acids Transesterification
This category consists of catalysts that are more
environmentally friendly and sustainable compared to
homogeneous catalysts. This catalysts have shown very little or
no recycling and corrosion problems. Nevertheless, to achieve
better efficiency high porosity systems are needed. The porosity
permits sufficient and appropriate adsorption-desorption and
diffusion of the products and reactants. Hence, metal oxides or
carbonates is less effective compared to acidic zeolites (see
Table 5) as such the latter will be more preferable here. In other
to address diffusional impediments, their structural and acidity
properties should constantly be adjusted thereby enhancing and
increasing biodiesel production (Borges et al., 2013; Macario et

45. al., 2008). Nevertheless, selecting the suitable silica alumina
ratio alongside modifying the concentration is a difficult task
with zeolites. High loading can result in the blockage of the
pore systems meanwhile free metals such as Palladium or
Platinum results in hydrogenolysis and then, dehydrogenation,
as a result obstructing biodiesel production.
The performance rating of organosulphonic acids and sulfated
zirconias as materials for transesterification catalysts has been
triggered due to several separation and environmental
challenges related to sulfuric acid. The SO42- in an unsupported
system could effortlessly be lost in the reaction medium and
hence acidity decay which causes catalyst deactivation. Thus to
solve this problem, it becomes necessary to incorporate porous
silica or alumina as support material. ZrO2 generally exists as
tetragonal, cubic and monoclinic phases. It is a well known fact
that the tetragonal phase exerts greater transesterification
activity compared to the other phases, particularly when doped
with appropriate amount of amorphous Tungsten trioxide. In
few cases, sulfated tin oxide or tungstatedzirconia may be used
in place of sulfated zirconia with supported over alumina to
achieve a comparable activity (Jothiramalingam & Wang, 2009).
4.3. Heterogeneous transesterification of algae oil
After harvesting, plants extraction of its oil is usually the first
followed by other significant step in production of biodiesel
(Figure 1). The procedure to be embarked on should be such
that will ensures reduction in the cost of extraction cost and at
the same time ensures high oil production (Martín &
Grossmann, 2012). Chemical and mechanical methods are the
basic and popular techniques obtainable for algae today. The
chemical methods commonly available include hexane solvent,
soxhlet and supercritical fluid extraction while expeller press or
ultrasound-assisted is the main available mechanical method.
The chemical methods are of great health and safety concern
because of the chemical implications meanwhile the mechanical
method is energy intensive because drying of the algae is
required (Martín & Grossmann, 2012). However, adopting

46. supercritical extraction method is also energy intensive because
of the involvement of high pressure equipment used in this
method which is usually expensive. Commercially, Origin Oil
Company widely applies a single step process. This method
entailed chronological steps of harvesting, concentration, and
extraction oil from algae. It split the oil, biomass and water in a
single step (generally in <1hr). This method does not need
initial dewatering of the fresh algae, more so no heavy
equipment or chemicals are required. The Cavitation
Technologies Inc method is another novel technique. In other to
produce cavity bubbles in the solvent the company employed its
Nano-based reactor (Oilgae, 2017). For the bubbles near the cell
wells to collapse, pulses are generated which breakdown the
cell walls to synthesis oils into solvent used for the extraction.
Another method which Nano-technologists would find very
interesting is the Catilin's method (though still under research
and development stage). In other to specially extract and
sequester specific fuel-based compounds present in the algal
lipid feed, specially developed mesoporous nanoparticles will
be used. The T300 catalysts developed by the company will be
used to trans-esterify the free fatty acids and triglycerides rich
balanced algal oil into biodiesel. The major advantage in this
case is that, the technology is potentially very efficient and
involved heterogeneous catalyst as such reduces cost, ensures
sustainability of the environment, recycling of the catalyst as
well as highly purified biodiesel and glycerol (Oilgae, 2017).
Osmotic shock and enzymatic extraction are some other
methods used for the extraction of oil which are still under
investigation, however, the later is considered to be costlier
than the hexane extraction technique. The method uses precise
enzymes molecules to breakdown the cell walls by a common
solvent, thus allowing it to be easier to fractionate the oil.
An estimation of 80,000 liters/acre of algae oil was recently
recorded by Demirbas & Demirbas, (2011). These figures more
than 30 times the quantity that could be obtained from other
feedstocks such as palm oil. They revealed that common species

47. of algae like Schizochytriumsp and Botryococcusbrauni could
produce about 77% oil based on dry matter. They also showed
in one of their model the oil per hectare yield of 100,000 liters
for algae species as compared to just 446 liters for soy plants
and 952 liters per hectare for sunflower plants. Furthermore,
other scientist such as Vazhappilly & Chen, (1998), Volkman et
al., (1989) and Yaguchi et al., (1997)also recorded closer
trends. Algae oil is also discovered to contain unsaturated fatty
acids for example omega-3's, omega-6, docosahexanoic and
ecosapentanoic acids. Interestingly, these compounds can be
isolated and used for other commercial purposes, thereby being
of economic exploitability advantage (Wen & Chen, 2003).
Transesterification of the oil into biodiesel follows after
successfully extracting the oil. Similar conversion techniques
used for other vegetable oils are also adopted for use in the
conversion algae oils (Campbell, 2008; Demirbas & Demirbas,
2011; Miao et al., 2004; Xu et al., 2006). Algae specie,
Chlorella protothecoides was used by Xu et al., (2006) for the
production of biodiesel. The cells were removed via agitation
together with washing with distilled water before using freeze
drying process to dry. Pulverize of powdered cells with mortar
will produce the oil after which extraction is done using n-
hexane, the optimal parameters include equivalent catalyst
concentration (i.e. according to oil weight), methanol/oil ratio
(56:1) at temperature of 30 OC, which reduces product density
from 0.912 to 0.864 during 4 hours on stream. Recently
emphasis is given to the production processes using
homogeneous catalysts with several evidences (Hu et al., 2008;
Martín & Grossmann, 2012; Martın & Grossmann, 2009; Plata
et al., 2010; Santacesaria et al., 2012; Wen et al., 2009).
Meanwhile, shifting to the use of heterogeneous materials
becomes necessary because the materials’ sensitivity towards
fatty acids in the algae oil feed likewise due to low quality
glycerol produced. Nevertheless, the two materials could be
used in line with Figure 1. With the latter catalysts, lesser
energy is needed for the removal of soap and glycerine (i.e.

48. during purification). In the separation process the catalysts are
also removed easily and reused. Hence in the nearest future, the
current used of homologous methods will be substituted by the
heterogeneous process. Acids like Silicotungstic acid
(H4SiW12O40), Phosphotungstic acid - PTA (H3PW12O40),
CsPW (Cs2.5H0.5PW12O40), Phosphotungstic acid/Zirconia
(H3PW12O40/ZrO2) and Phosphotungstic acid/Niobium oxide
(H3PW12O40/Nb2O5) are commonly referred to as heteropoly
acids, have been recorded to show great tolerance towards free
fatty acid concentrations, producing large conversions of
biodiesel at ordinary conditions such as that of vegetable oils
(Alsalme et al., 2008; Katada et al., 2009; Talebian-Kiakalaieh
et al., 2013). This groundbreaking potential can be harness in
algae oils which has greater potential. However, under
controlled conditions acidic zeolites like H-Beta, H-ZSM-5, H-
MOR, H-ETS-10, H-ETS-4 having moderate acidic properties
and adequate porosity permits more fast transesterification with
limited side reactions interference for the vegetable oils (Borges
et al., 2013; Macario et al., 2008) and also for the algae oils.
Materials based on WO3/ZrO2 must be used at the right
loadings and calcinations. The 15% weight of WO3/ZrO2
calcined at 932 OF is recommended, which gives 95% yield
conversion for other oils (see Table 6). Therefore, using
comparable preparations algae oil has more yield potentials
under similarly constant conditions of the reaction.
Table 6: The effect of calcination temperature on the
performance of WO3/ZrO2 catalyst (Jothiramalingam & Wang,
2009).
Conversion (%)
Sample
Calcination temperature (0F)
15 wt% WO3/ZrO2
5 wt% WO3/ZrO2
1

49. 1652
17
5
2
1292
25
5
3
1112
20
10
4
932
81
95
5
752
78
93
Likewise the heterogeneous bases are materials of great
prospect (see Table 7). Mixed oxides with transition metals,
oxides of Calcium, Strontium and Magnesium, and supported
over silica or alumina were widely investigated for oils other
than algae oil and their various properties well evaluated.
Similarly, these materials can intensively be exploited for algae
oils, hoping to mitigate challenges like thermal instability,
dissolution, sintering, and recyclability problems by right
parameters choices.
Selecting suitable conditions of reaction and appropriate
support materials is key to heterogeneous algae
transesterification (Galadima & Muraza, 2014; Krohn et al.,
2011; Santacesaria et al., 2012; Umdu et al., 2009) Duri ng the
conversion of marine microalgae, the properties of aluminum
oxide (Al2O3) doped magnesium oxide (MgO) and calcium
oxide (CaO) were investigated by Umdu et al., (2009), by

50. controlling the concentration of methanol as well as the
quantities of the active materials at 323 K. In their study, they
discovered that unsupported calcium oxide and Magnesium
oxide activity were negligible, while calcium oxide and
aluminum oxide system produced the activity with the most
potential, because of higher basic sites concentration and basic
strengths. Finally, their conclusion was that just like other
vegetable oils, algae could be converted to biodiesel
successfully by the use of the named heterogeneous materials at
low transesterification temperature. Likewise, there are no
unwanted products which needed to be purified. Biodiesel was
also generated by the catalysts at low temperatures for some
eatable vegetable oils containing common impurities such as
free fatty acids. Thus, it means that the materials have strong
potentials for algae oils. A new fixed bed continuous reactor
system was incorporated by McNeff et al., (2008) using porous
titania, alumina and zirconia (including their mixed oxides) as
heterogeneous alcoholysis catalysts at high temperature and
pressure of 300-450 OC and (2500 psi) respectively, in addition
to some feedstocks (such as microalgae, corn and soybean oils).
The catalysts were proven very successful, particularly with
algae oil Table 7.
Table 7: Literature review on biodiesel production via
heterogenous catalyst
S/N
Feedstock
Catalyst
Yield (%)
References
1
1st Generation
Soybean oil
Li/MgO
94
(Borges & Díaz, 2012)

51. 2
ZrO2, TiO2
88
(McNeff et al., 2008)
3
Corn oil
4
Groundnut oil
BaO/Al2O3
80
(Anderson et al., 2009)
5
Cotton seed oil
6
Palm oil
CaO/ Al2O3
99
(Ramachandran et al., 2013)
7
Mutton fat
MgO/KOH
98
(Mutreja et al., 2011)

52. 8
Sunflower oil
ZrO2/La2O3
85
(Ramachandran et al., 2013)
9
Na-X zeolite
84
(Musyoka et al. 2012)
10
2nd Generation
Yellow horn
Cs2.5H0.5PW12O40
96
(Ramachandran et al., 2013)
11
Waste cooking oil
MgO/ TiO2
92
12
Zeolite Y (Y756)
85
(Lam et al., 2010)

53. 13
H3PW12O40.6H2O
87
14
K3PO4
97
15
Chinese tallow seed oil
KF/CaO
(Borges & Díaz, 2012)
16
Jatropha curcas oil
CaO/Fe3O4
95
17
Croton megalocarpus oil
SnO2SO4/ZrO2
18
Algae oil
ZrO2, TiO2
90
(McNeff et al., 2008)

54. 19
NiO, MoO3/ Al2O3
99
(Sani et al., 2013)
20
Pt-SAPO-11
83
21
NiO, MoO3/H-ZSM-5
98
22
Microporous TiO2
95
23
Niobium oxide (HY-340)
94
24
H-beta Zeolite
100

55. 25
Amberlst-15
98.5
(Dong et al., 2013)
3rd Generation
26
Microalgae's lipid
Modified Al2O3
98
(Umdu et al., 2009)
27
Modified TiO2
96
(Chouhan & Sarma, 2011)
28
Porous TiO2 microsphere
Under constant conditions percentage yields of biodiesel gotten
from; algae was 90.2%, corn was 88.3% and soybean oils was
88.1%. More so, economical analysis of the process revealed
that it was less expensive compared with the conventional
homogeneous options. However, they encountered limited

56. interference because of the fatty acids. The reusability of the
catalysts is also made possible because of their thermal
stability. For the sake of increasing the biodiesel yield and
reduce the fatty acids interference experienced with few
heterogeneous base catalysts, some scientists (Dong et al.,
2013), have recently introduced two steps in situ process. This
process consists of a pre-esterification of the algae oil before
the base-catalyzed transesterification. It could permit 98%
recovery of the biodiesel, which is greater than the values
obtained by a single-step catalytic in situ process. The
reusability potential Amberlyst-15 (a heterogeneous material)
was studied by Dong et al., (2013), where it was repeatedly
employed eight times without activity decay. Up to 30% weight
increases in ester yield is caused by catalyst loading, above this,
its concentration does not affect the activities of
transesterification. In the contrast, biodiesel production is
negatively affected as a result of increase in methanol to algae
oil ratio, with an optimal yield of 20%. The technology could
serve as a more cost effective, environmentally sustainable
method because of the optimal biodiesel produced and
recyclable catalyst. Heterogeneous solid acids like NiO-
MoO3/Al2O3, Pt-SAPO-11, NiOeMoO3/H-ZSM-5 and
microporous TiO2 has recently been reported by (Sani et al.,
2013) as being efficiently used for microalgae oil. In general,
the conversions were within the range of 83 and 99% (see Table
7), these materials displaced great prospect for lower
commercial and industrial cost of production compared with the
homogenous system in addition the remarkable stability of
catalysts and lack of reusability problems or corrosion. Many
researchers (Giannakopoulou et al., 2010; Hara, 2010; Lim &
Teong, 2010; MacArio & Giordano, 2013; Peng-lim et al., 2013;
Perego & Millini, 2013; Rathore & Madras, 2007; Serrano et al.,
2013; Sharma et al., 2011; Verma et al., 2011; Zhao et al.,
2013) generally believed that the best future method for the
production of biodiesel from algae or similar non-edible
feedstock would be by heterogeneous catalysis. However,

57. researchers need to integrate economic aspects with science,
technology and policy issues in their studies so as to identify
the technical and economical feasibility of this method.
It is however interesting to note that, diesel range hydrocarbons
and or gasoline (Harman-Ware et al., 2013; Kiss et al., 2006;
Thangalazhy-gopakumare et al., 2012; Tran et al., 2010), animal
feeds and fine chemicals including Hydrogenated Vegetable Oil
(HVO) can also been produced using algae oil. The most
important heterogeneous materials exploited in this regards are
solid acid catalysts. Generally, the conversion processes
involves cracking, hydrotreating and hydrodeoxygenatio n (Kiss
et al., 2006; Savage et al., 2011). Out of the solid catalysts, the
good materials evaluated for cracking reactions includes
aluminum oxide (Al2O3), aluminum chloride (AlCl3), calcium
oxides (CaO) and magnesium oxide (MgO), SAPO-5, SAPO-11
(Kiss et al., 2006), HZSM5, HBEA and USY. Meanwhile,
heterogeneous systems which are mostly used for hydrotreating
of bio-oils even at low temperature include NieMo/g-Al2O3,
CoMo and NiMo-sulphides and their SiO2-Al2O3 supports
though with limited stability challenges. They have interesting
properties such as good thermal stability with catalyst poisons
resistances in the reaction feed. Primary deoxygenation
materials such as Al2O3, SiO2 and zeolites supported Nickel
and/or noble metal catalysts can be used to convert fatty acids
in the algae oil to liquid hydrocarbons of mostly diesel range.
(Hu et al., 2013), recently pyrolyzed oil gotten from microcystis
species (oil-riched blue - green algae) at temperatures ranges of
572-1292OF in a fixed bed reactor. Mostly at optimal
temperature of 932OF, the liquid bio-oils generated were
discovered contain gasoline and diesel range hydrocarbons at
appreciable quantity. C6H6, C6H6O, C7H8O (Cresol), C4H5N,
C8H7N, C5H5N, their families and substituted pyrazines were
also identified at appreciable concentrations. Other researcher
that used different species also had similar outcomes (Choi,
Choi, & Park, 2012; Melligan et al., 2011; Miao et al., 2004; H.
J. Park et al., 2012). 5.0. Reactors

58. Commercial industries and producers have suggested several
techniques for culturing microalgae. Reviews such as (Chen et
al., 2011; Eriksen, 2008; Grobbelaar, 2010; Kumar et al., 2011;
Ugwu et al., 2008; Wang et al., 2012) listed the various
technological solutions. Some of these methods used include
tanks or pools of different type ranging from round ponds with
arms that are movable, large open ponds, cascade systems with
baffles, race way type ponds (tracks). Furthermore, fermenters
(for heterotrophic and mixotrophic cultures), large bags, two
stage systems (cultivation in the reactor in an internal system,
the system of outside pond with paddle wheel, which enforces
growth medium movement and simultaneously aerates the
culture) are also employed (Olaizola, 2003)
The ratio of the culture solution volume to the illuminated
surface is a key factor to the design of reactors used to grow
photosynthetic organisms. The reduction of unwanted
consequences of self-shading or limited access to light from one
cell to another is achievable through the proper selection of the
culture solution irradiated surface (A) and volume (V) ratio.
Nevertheless a raise in the values of the ratio of irradiated
surface to the volume (A/V) is the parameter that is desirable
for photobioreactors (Becker, 2007).5.1. Influence of reactor
design and operating conditions
The design of an innovative chemical reactor in other ease the
continuous processing of viscous bio-oils is most likely to
affect biodiesel production by commercially exploiting
heterogeneous catalysts. Even though a lot of biodiesel
production plants of most industries runs in batch mode and at
an important scale of B7000 tons per year (Aransiola et al.,
2014; Sakai et al., 2009). It becomes very important to employ
continuous flow reactors that is heterogeneously catalyzed, so
as to prevent the separation cases of homogeneous catalysts and
drawbacks of batch mode (particularly raised the capital
investment needed to operate at high volumes as well as raised
the cost for labor of a start or stop process) (Stamenković, &
Veljković, 2014) in addition, also raise the scale of operation

59. ranging from 8000 to 125 000 tons per year (Aransiola et al.,
2014; Sakai et al., 2009). Varieties of process engineering
solutions have been suggested for use in the continuous
esterification of FFAs, as well as the use of fixed bed (Cheng et
al., 2012) or microchannel-flow reactors (Kulkarni et al., 2007)
pervaporation methods (de la Iglesia et al., 2007) and reactive
distillation (Buchaly et al., 2012; Kiss et al., 2008). A depth
review of biodiesel production using process intensification
methods have been conducted elsewhere (Maddikeri et al.,
2012; Qiu et al., 2010).
Chemical conversion and separation steps are combined in a
single stage in reactive distillation. As a result, the process flow
sheets are made easier, cost of production is reduced, as well as
catalyst lifetimes extension by continuously getting rid of water
from the system. Nevertheless, the method is only relevant and
usable when the pressure and temperature needed for reaction is
suitable with that required for the distillation. Kiss et al. used a
range of alcohols catalyzed by sulphated zirconia to
demonstrate this approach for the esterification of dodecanoic
acid (Kiss et al., 2008). However, their reactive distil lation was
100% selective, allowed lesser residence times compared to
similar flow systems, in addition to this it didn’t need excess
alcohol. The fact that it doesn’t need excess alcohol is a key
improvement over the majorities of popular conventional
biodiesel syntheses in which there is a reversible reaction
between the alcohol and triglyceride, hence huge alcohol
excesses are usually needed to accomplish full conversion (to
ensure economic process viability, the excess alcohol should be
separated and re-used after worth).
In other to exploit the full potential of the integrated
heterogeneous catalyst, every continuous flow reactor should be
appropriately designed, however plug flow is an important
characteristic because it allows effective management and
monitoring of the product composition, thus minimizes its
operational costs, downstream separation processes alongside
associated capital investment. The conventional plug flow

60. reactors are poorly designed to lower reactions like TAG
transesterification and FFA esterification, given that,
specification such as very high length, diameter ratios are
required to achieve good mixing, even though they are
problematic because of their pumping duties together with large
footprints, in addition to the difficulties in controlling and
managing it. These difficulties are avoided in Oscillatory
Baffled Reactors (OBRs) by oscillating the reaction fluid via
orifice plate baffles to realize an effective mixing and plug
flow,249 thus decoupling mixing from the net fluid flow in a
scalable fashion, allowing long reaction times on an industrial
scale, as such applied to homogeneously catalysed biodiesel
production (Phan et al., 2012) OBR vortical mixing further
provides an efficient, manageable methods of solid particles
that are uniformly suspended and was utilized recently to
entrain a PrSO3H-SBA-15 mesoporous silica within a glass
OBR under an oscillatory flow for the continuous esterification
of propanoic, hexanoic, lauric and palmitic acid (Figure 2) (Eze
et al., 2013). There is an outstanding semi-quantitative
consensus between the kinetics of hexanoic acid esterification
in the Oscillatory Baffled Reactors and a conventional stirred
batch reactor, in which the important predictor of solid acid
activity was recognized to be the length of the fatty acid chain.
Constant esterification inside the Oscillatory Baffled Reactors
enhanced ester synthesis when compared to the batch operation
because of the water by-product is constantly removed from the
zone of catalyst reaction, proofing the flexibility of the
Oscillatory Baffled Reactors for heterogeneous flow chemistry
and key potential roles as a new clean catalytic technology.
No Oscillation
Oscillation
Net flow out
Baffled tube
Net flow in

61. Oscillation
Sedimentation
Uniform suspension
Upstroke
Down stroke
Figure 2:Flow sheet of an oscillatory baffled reactor and it
mixing features. Also illustrating the solid acid catalyst
PrSO3H-SBA-15 undergoing no oscillation but sedimentation
and or with about 4.5Hz oscillation traped in the baffles.
Figures exuracted from (Eze et al., 2013)
In the production of biodiesel by TAG transesterification with
methanol phase equilibrium considerations are significant,
given that the alcohol and reactant are usually immiscible,
while the FAME product is miscible, obstructing retarding
reaction and mass transport. More complexity and production
cost is added due to the need to separate and purify the product
phase, which comprises of a mixture of solid catalyst, unreacted
oil, biodiesel and glycerol (Lee et al., 2014). These challenges
could be lessened by using membrane reactors (Baroutian et al.,
2011; Falahati & Tremblay, 2012; Xu et al., 2014). where the
semi-permeable material of which the reactor walls are made is
designed to permit the passage of the FAME or glycerol phase,
whereas the oil-rich or MeOH emulsion is retained for more
reaction. However, a MCM-41 supported p-toluenesulfonic acid
catalyst was used by Xu et al., (2014) to pack a ceramic
membrane tube for the transesterification of a recirculating
soybean oil and methanol feed (Figure 3a). It is worthy of note
that membrane reactor produced higher biodiesel compared to a
homogeneous p-toluenesulfonic acid catalyst under similar
conditions in batch mode (84% against 66%). Furthermore,
recycling of catalyst showed just a little reduction of activity,
generally at the end of the third cycle with about 92% of

62. original activity. The production of biodiesel was usually a
strong function of circulation velocity; meanwhile lower
velocities enhanced permeation efficiency, whereas high
velocities improved mixing intensity of the reactant. Even
though membrane reactors provide efficient transesterification
and separation, yet high volumes of catalyst is still required, for
instance, 157g of a microporous TiO2/Al2O3 membrane packed
with potassium hydroxide is employed by a 202cm3 continuous
reactor supported on palm shell activated carbon to increase the
quality of methyl esters produced from palm oil (Figure 3b)
(Baroutian et al., 2011).
(a)
Biodiesel
Glycerol
Circulating pump
Mixing vessel
Membrane reactor
Preheater
Methanol
Oil
Cooler
Membrane tube
back pressure valve
P
Catalyst
(b)
1. KOH/AC Catalyst
2. TiO2/Al2O3 Membrane
3. Permeate Stream
4. Retentate stream
5. FAME
6. Glycerol
7. Methanol

63. 8. Triglyceride
Figure 3: Diagram of membrane reactors for producing biodiesel
in transesterification reaction through (a) Solid acid catalyst
and (b) base catalysts.
Both continuous (Lozano et al., 2011; X. Wang et al., 2011) and
batch modes (Lv et al., 2010) has recorded an enzymatic
catalyzed biodiesel production. Although, a range of lipase
biocatalysts has been developed by nature for the selective
production of FAME at low temperature of the reaction, which
can withstand high FFA levels (Bajaj et al., 2010) Such
biocatalysts are allowed to be employed in continuous mode
with low methanol to oil ratios due to immobilisation on solid
supports (Watanabe et al., 2000). Nevertheless, long residence
times, low biodiesel yields and high enzyme costs are some of
the several shortcomings of biocatalysts. More so, some of the
enzymes could as well be deactivated by short chain alcohols
and the glycerol by-product (Bélafi-Bakó, Kovács, Gubicza, &
Hancsók, 2002); however, these problems could be defeated by
using organic solvents to extract the glycerol and alcohols,
nevertheless it further increase the cost and complexity, as well
as weakens the green credentials of biodiesel production. In
other to prevent the enzyme fro been denatured, the enzyme has
to operate in the presence of water, but for the biodiesel to meet
the biodiesel standards of less than 0.05% of water volume; this
excess water has to be subsequently removed from the resulting
fuel, meanwhile the drying process introduces additional costs.
However, using of near-critical (Lee et al., 2013) or
supercritical CO2(Cao et al., 2007; Cao et al., 2009) as a
reaction medium to decrease enzymatic inhibition by methanol
is an alternative approach, thus boost oil solubility and
diffusion, as well as aid catalyst/biodiesel separation through
simple depressurisation.
Microwaves (Mazubert et al., 2014; Wali et al., 2012) and
ultrasound (Gole & Gogate, 2012; Gude & Grant, 2013) have

64. already been investigated by scientist as a way by which heat
and mass transfer limitations can be eliminated, as well as
reducing the residence time in other to attain a high biodiesel
conversions. Gude et al. used ultrasound rather than thermal
heating for the transesterification of waste cooking oil (Gude &
Grant, 2013) this allows for an efficient heating up to a
temperature of 60–65 OC along with reducing the reaction times
to 1–2 minutes. A similar observation was made by Chand et al.,
in which they noticed progress in heat transfer and reaction time
when using ultrasonication to soybean oil transesterification
(Chand et al., 2010). Although, both groups utilized a
homogeneous NaOH catalyst, which hampered the purification
of the product. Salamatinia et al. used ulterasound and a
heterogeneous catalyst for continuous biodiesel synthesis from
palm oil (Salamatinia et al., 2010). While testing BaO and SrO
catalysts, ultrasound was discovered to again lower the reaction
times and thus the catalyst loadings required to attain less than
95% FAME synthesis.
From the cost analysis of an ultrasonic technique it was
suggested that it would be three times cheaper to operate
conventionally heated continuous biodiesel reactor rather than
an ultrasonic process (Chand et al., 2010). Even at that, the
origin of ultrasonic enhancements regarding reaction mixing
through cavitation or microstreaming, is still an important topic
for debate (Choudhury et al., 2014). However for the
transesterification of waste cooking oil, microwaves have been
coupled with continuous flow reactors which accelerates the
production of biodiesel when compared to conventional thermal
heating, thus increases throughput (Wali et al., 2012). Even
though few innovative combinations of catalysts obtained from
solid waste (eggshell) and microwaves are emerging but up till
date large number of microwave studied still focus on
homogeneously catalysed processes (Khemthong et al., 2012).
However, less catalyst and solvent is require by such microwave
systems. Conversely, the limiting factor is the depth of the
microwave penetration (Mazubert et al., 2014) that could

65. hamper scale-up from laboratory reactor designs, uncontrolled
and irregular heat distribution could lead to ‘hot spots’ and
‘cold spots’ (Gole & Gogate, 2012; Mazubert et al., 2014).6.0.
Conclusions
Many researches revealed rise in price of food and hunger treats
to be seriously associated to the continuous use of edible oils
either from animal or plant as a source of biodiesel feedstock.
Meanwhile, several species of algae can be cultivated having
little or no environmental challenges to produce high grade
biodiesel using methods such as transesterification and oil
extraction which are generally similar to the methods used to
cultivate normal edible plants. Interestingly, even salty
environments do not prevent its cultivation, due to their ability
to utilize CO2 when growing, as such appropriate for
environmental management. These properties stated and more
will definitely increase their future use and consideration as
sustainable feedstocks. The use of heterogeneous catalysts from
oxide, zeolites and their derivatives in transesterification is
always of great industrial interest and attention. At the end of
the transesterification process the solid bases and acids
catalysts could be recovered, recycled and reused moreover they
are potentially less expensive and to the fatty acids present in
the feed they are extremely less sensitive. They usually generate
highly purified biodiesel that has remarkable properties of
international standard especially when effectively and well
constructed. Nevertheless, further studies should be made on the
appropriate mechanisms of reaction and the methods by which
many of these catalysts can be used to optimize triglyceride
esters conversion. In improving biodiesel production and
enhancing the adsorption-desorption of the reactants, good
dispersion of catalysts particles, optimal temperature and the
nature of the active phase or support are prospective
advancement in this method. Zeolite catalysts must be
customized in a way that the frameworks should have the
required basic or acidic properties and porosity. It must be
carefully handled to prevent pore blockage that may result to

66. competitive transesterification versus cracking process when
doped with metals or oxides.
It is important to also identify less expensive methods of oil
extraction and algae cultivation. More so, investigation on more
affordable algae species that is highly rich in oil at the same
time has faster growth rate in specific environments should be
intensified.
Hence, more research should be conducted in other to determine
the most suitable heterogeneous catalysts, their mechanism of
action(s) and most favorable conditions for their reaction.
Oxides and zeolites showed greater prospect in formaldehyde,
diols, allyl alcohols, oleifins, methane and hydrogen fuel (liquid
and gaseous products) production, and in the production of
other liquid hydrocarbons such as diesel and light naphtha.
Glycerol carbonate and derived alcohols such as 1, 3-ditert-
butoxypropan-2-ol, are other important products with
outstanding properties such as combustion improver for diesel
fuel. More research should be encouraged with the aim of
improving their performance.
The distributed biodiesel production will be facilitated by
alternative reactor technologies and process intensification
through for example reactive distillation and oscillatory flow
reactors. In other for biodiesel to remain an important player in
renewable energy sector in this 21st century, it is critical to
facilitate and pursue technical advances in both materials
chemistry and reactor engineering.

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