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Jurnal Bagus
Production of Renewable Liquid Fuels for Diesel Engine Applications – A Review
Biodiesel Com Etanol
brun_energyenvironmentalscience_2011
e 0362030037
New Energy Hub in Batac
Produksi Biodiesel Skala Kecil
CWSF 5-Page Report
Integration of reactive extraction with supercritical uids for process
intensication of biodiesel production: Prospects and recent advances
Keat Teong Lee
a, *
, Steven Lim
, Yean Ling Pang
, Hwai Chyuan Ong
, Wen Tong Chong
School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Seberang Perai Selatan, Pulau Pinang, Malaysia
Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
a r t i c l e i n f o
Received 16 April 2014
Accepted 6 May 2014
Supercritical reactive extraction
Bio-renery
Process intensication
Product utilization
Current world energy usage is trying to gradually shift away from fossil fuels due to the concerns for
the climate change and environmental pollutions. Liquid energy from renewable biomass is widely
regarded as one of the greener alternatives to partially full the ever-growing energy demand.
Contemporary research and technology has been focussing on transforming these bio-resources into
efcient liquid and gaseous fuels which are compatible with existing petrochemical energy infra-
structure. Due to the wide range of properties and compositions from different types of biomass, there
are ample of processing routes available to cater for different demands and requirements. In addition,
they can produce multi-component products which can be further upgraded into higher value
products. This conceives the idea of bio-renery where different biomass conversion processes are
incorporated and proceed simultaneously at one location. However, the underlying complexity in
integrating different processes with varying process conditions will undoubtly incurs prohibitive cost.
Consequently, process intensication plays an important role in minimizing both the capital and
operating costs associated with process integration in bio-reneries. Recently, process intensication
for biodiesel production has been developing rigorously due to increasing demand for cost-cutting
measures. Supercritical uid process allows biodiesel production to be performed without any addi-
tion of catalyst. Meanwhile, catalytic in situ or reactive extraction process for biodiesel production
successfully combines the extraction and reaction phase together in a single processing unit. In this
review, the important characteristics and recent progress on both of the intensication processes for
biodiesel production will be critically analyzed. The prospects and recent advances of supercritical
reactive extraction (SRE) process which integrates both of the processes will also be discussed. This
review will also scrutinize on the methods for these processes to compliment future bio-renery setup
and more efcient utilizing of all of the products generated.
2014 Elsevier Ltd. All rights reserved.
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2. Latest development of biodiesel production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.1. Current status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.2. Feedstocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.3. Production processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.4. By-product utilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.5. Challenges in biodiesel production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Supercritical biodiesel production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Recent development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
* Corresponding author. Tel.: 60 4 5996467; fax: 60 4 5941013.
E-mail address: ktlee@usm.my (K.T. Lee).
j ournal homepage: www. el sevi er. com/ l ocat e/ pecs
http://dx.doi.org/10.1016/j.pecs.2014.07.001
0360-1285/ 2014 Elsevier Ltd. All rights reserved.
Progress in Energy and Combustion Science 45 (2014) 54e78
3.2.1. Supercritical solvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2.2. Process conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2.3. Reactor configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Catalytic reactive extraction for biodiesel production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.2.1. Catalytic processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.2.2. Co-solvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.2.3. Enhanced reactive extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5. Supercritical reactive extraction (SRE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1. Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.2. Recent advances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.3. Process characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.3.1. Feedstocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.3.2. Pre-treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.3.3. Supercritical solvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.3.4. Process variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.3.5. Agitation effect and heat sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.3.6. Product separation and characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.3.7. Thermal stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.3.8. Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6. Prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6.1. Integration in bio-refinery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6.2. Product utilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
7. Critical issues and recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
7.1. Impact to biodiesel production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
7.2. Energy and cost assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.3. Future research needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Uncited references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Energy supply and security is one of the most pressing issues
shrouding our civilization development which remained to be
tackled. For the past century, we have become over-reliant on fossil
fuels to generate the energy we required for our technological and
social development until neglecting the devastating effects they
might bring to our ecosystem. However, the quest to replace fossil
fuels to more sustainable energy sources remains sluggish espe-
cially in developing countries which account for more than two
thirds of the world population. The slow transition from fossil fuels
to alternative energy sources can be attributed to various factors
such as low accessibility, high cost, insufcient infrastructure,
inadequate technology and sub-par efciency [1]. Among the
renewable energy sources, biofuels from biomass such as biodiesel
are currently recognized as one of the best alternatives to partially
displace the usage of fossil fuels in the energy sector [2]. Biodiesel,
which is usually derived from plant oils or animal fats, can be
blended with mineral diesel up to 20% w/w (B20) and applied to
existing combustion ignition engine without any modications.
Apart from that, it is also known to be biodegradable, low toxicity,
lower emissions of harmful pollutants (CO, SO
and unburned hy-
drocarbons), easy handling and distribution [3].
Despite these advantages, biodiesel advocates and developers
still nd it difcult to break into the energy market conventionally
dominated by fossil fuels. Traditionally, biodiesel is produced using
homogeneous basic catalysts such as sodium hydroxide (NaOH)
and potassium hydroxide (KOH) [2]. This production route de-
mands a high purity oil feedstock which will otherwise reduce the
process yield due to side-reactions such as saponication. In
addition, homogeneous catalysts are usually difcult to be removed
from the product stream and this will incur extra purication cost.
In lieu with the shift from edible feedstocks to non-edible or waste
feedstocks to avoid the food versus fuel ethical issue, other
advanced biodiesel production methods have been explored
intensively. In general, they can be categorized into three primary
processes; the heterogeneous catalytic process, biological enzy-
matic process and supercritical uids non-catalytic process. Each
process has its own advantages and disadvantages while ample of
research studies have been performed to further improve the
processes in terms of the esters yield and cost-competitiveness.
In this context, process intensication has been lauded as having
huge potential to improve biodiesel production process tremen-
dously through various cost-effective measures. Process intensi-
cation can be generally dened as any engineering development of
novel apparatus or technique which resulted in a substantially
smaller, cleaner and more energy-efcient production technology
[4]. Several process intensication measures proposed for biodiesel
production include the novel oscillatory bafed reactor, hetero-
genization of the catalysis, supercritical non-catalytic reactions,
reactive extraction process and ultrasound/microwave assisted
process [5,6]. Reactive extraction or also known as in situ extraction
combines the extraction and reaction processes together in a single
unit operation. There are usually two routes for this to be done.
Conventionally, biodiesel production from edible oils starts with
the extraction of oil from the lipid-bearing solid material either
through mechanical pressing or chemical extraction. The extracted
liquid oil will then undergo several purication stages before sub-
jected to transesterication process with short-chain alcohol to
produce esters which are equivalent to biodiesel. The imple-
mentation of reactive extraction allows the elimination of pre-
extraction step which can potentially reduce the operating cost
K.T. Lee et al. / Progress in Energy and Combustion Science 45 (2014) 54e78 55
and time [7]. The second type of reactive extraction deals with
simultaneous removal of the glycerol from the ester phase during
the reaction in an extraction column [8]. This review focuses pri-
marily on the former method and reactive extraction mention
hereafter is referred to the former method unless specied other-
wise. However, the usage of homogeneous base/acid catalysts in
reactive extraction still resulted in several challenging issues
similar to the conventional two-step homogeneous trans-
esterication process.
In order to avoid falling into the same quandary, supercritical
reactive extraction (SRE) process is proposed for biodiesel pro-
duction which enables the extraction and reaction processes to
occur at a fast rate even without addition of any catalyst. Currently,
no review has been done on the potential and challenges of SRE
application on biodiesel production. From the past research works
done on SRE process for biodiesel production [9e15], it is believed
that SRE process can become another sustainable production
method for biodiesel especially on non-edible feedstocks such as
Jatropha curcas L. (JCL) and algae. Therefore, in this review, the most
recent and signicant advancement of technology in biodiesel
production will be discussed. More in-depth discussions will be
placed on supercritical uids technology and catalytic reactive
extraction process which act as the fundamental study for SRE
process. The highlight of this reviewwill be focussing on explaining
the concept of SRE process, its current related research, inuences
of process parameters, advantages and challenges pertaining to the
biodiesel production and development. Last but not least, recom-
mendations on future scientic studies are proposed for this novel
process to move forward and to complement existing biodiesel
production and future bio-renery scheme with a more sustainable
2. Latest development of biodiesel production
2.1. Current status
Generally, biodiesel can be regarded as a liquid fuel comprises of
alkyl esters derived from transesterication of triglycerides or
esterication of fatty acids with short-chain alcohols as acyl ac-
ceptors. A typical transesterication and esterication reactions to
produce methyl esters are shown in Fig. 1(a) and (b) respectively.
Biodiesel shares a lot of similar physical and chemical properties
with mineral diesel which makes it an ideal replacement in
compression ignition engines. However, its higher viscosity and
lower energy density renders pure biodiesel not suitable to be
applied in the engines directly. Instead, it has to be blended
together with mineral diesel according to a xed proportion.
Currently, most vehicle and engine manufacturers worldwide have
approved the usage of B5 biodiesel blend (5% biodiesel and 95%
diesel by volume) in their engines with a large part of them have
even raised the maximum limit up to B20. Combustion of biodiesel
blends in the engines has been proven to contribute to several
encouraging effects such as lubricity enhancement, engine wear
reduction and better combustion proles [16,17]. In order to
encourage the usage of biodiesel to replace conventional diesel,
many countries have mandated a xed percentage of biodiesel
volume (ranging from 1% up to 10% volume) in their diesel supply
mix [18]. Several countries have also introduced nancial in-
centives such as carbon credit or tax exemption to lower the price
of biodiesel blends to become more economically competitive and
also encouraging more investors to develop the industry. Research
and development for biodiesel production is still being performed
vigorously by researchers from all over the world to overcome the
underlying challenges and to fully realizing its potentials as a sus-
tainable energy source. Generally, the research works for biodiesel
production are focused on three primary areas which are the
feedstocks, the process and the by-products.
2.2. Feedstocks
One of the advantages of biodiesel is that it can be produced
from a variety of biomass sources and thus not limited to any
geographical region unlike fossil fuels. Established biodiesel pro-
duction usually employs feedstock derived from edible sources
such as rapeseed, soybean and palm oil to produce biodiesel which
are widely regarded as rst generation biofuels [19e21]. Major
producing countries for rst generation edible feedstocks with
their respective yields are summarized in Table 1. First generation
feedstocks are readily available since commercial plantations have
begun a long time ago and their supply chains are rmly estab-
lished. However, ethical issues such as food shortage and forest
encroachment result in a call to shift to more sustainable alterna-
tive feedstocks which are not t for human consumption [3]. The
second generation biofuels are then developed primarily fromnon-
edible feedstocks derived from plants such as JCL, Calophyllum
inophyllum, Linseed, Cerbera odallam and fromwaste materials such
as palm oil mill efuent, waste cooking oil and municipal waste.
Non-edible plants for second generation biodiesel production can
often be planted in semi or non-arable lands and thus avoiding the
land shortage issue while generating higher revenues for under-
utilized lands. These feedstocks together with those from waste
materials are relatively cheap to obtain which can help to reduce
the feedstock cost for biodiesel production. Unfortunately, they still
suffer from a lot of technical challenges since they are relatively
wild and scientic knowledge pertaining to these feedstocks is still
Fig. 1. Schematic diagram for typical (a) transesterication and (b) esterication
process in biodiesel production.
Major edible feedstocks for biodiesel production and their major producing
Feedstocks Major producing
countries [25]
Oil content (%) [26] Oil yield
(kg/ha/yr) [26]
Rapeseed EU, China, Canada 35e50 600e1000
Soybean China, US, Brazil 15e21 300e450
Palm Indonesia, Malaysia,
20e50 2500e4000
Sunower Ukraine, Russia, EU 30e51 280e700
Cottonseed China, India, Pakistan 18e25 n/a
Peanut China, India 36e56 340e440
Coconut Philippines,
Indonesia, India
63e65 600e1500
insufcient [22]. The taxonomy of these wild plants has not been
explored in details unlike their edible counterparts which results in
inconsistent yield and volatile market price. In addition, these
feedstocks often contain higher amount of impurities in the formof
moisture and free fatty acids (FFA). Consequently, they are not
suitable to undergo conventional homogeneous basic catalytic
process and additional purication steps will be required. This will
incur extra expenses to the biodiesel production process and thus
they are generally not favoured among biodiesel developers.
However, most of the recent commercial biodiesel productions
were already being designed to accommodate second-generation
feedstocks to complement existing rst generation biodiesel pro-
duction [23,24]. Once a cost-effective production method has being
established, it will undoubtedly encourage more biodiesel de-
velopers to follow suit and set the trend for the future.
Besides second generation non-edible feedstocks, biodiesel
production utilizing macroalgae and microalgae is also under
intensive development in recent years. Biodiesel produced from
these algae is collectively known as third generation biodiesel. The
main difference of using algae for biodiesel production compared to
previous generations is their higher photosynthesis capability
which enables them to provide higher product yield per cultivation
area while at the same time sequesters larger amount of CO
the atmosphere [2]. Furthermore, they do not compete with land or
fresh water resources if cultivated off-shore in contrast to other
oleaginous oil crops. However, much like the second generation
feedstocks, the cultivation and production technology for algae are
not yet mature. This resulted in less than optimal yield and higher
energy consumption especially during harvesting and drying.
Moreover, the requirement for advanced bio-reactor for efcient
microalgae production is still very prohibitive and troublesome to
maintain while open pond system is susceptible to be polluted by
other microorganisms.
Even though a sustainable and economically competitive third
generation biodiesel production is still in intensive studies, several
researches have started to develop theories and preparing relevant
technology for the next generation of biodiesel feedstocks. Theo-
retically, fourth generation biodiesel feedstocks will take advantage
of the advancement in biotechnology, metabolic engineering and
genome research in order to improve cellular metabolism and
characteristics of oxygenic photosynthesis plants or microor-
ganism. Through manipulation of the genome and recombinant
DNA techniques, it is possible to increase the photosynthesis ef-
ciency by several folds and thus greatly enhance the output of lipids
for biodiesel conversion [27]. This enables the realization of the cell
factory concept where the continuous transformation of energy
from sunlight to biofuels using biomass can be more direct, clean
and cost-effective. In addition, the enhancement of CO
consump-
tion by biomass allows the carbon cycle of the relevant biofuels
production to shift from neutral to negative. In other words, the
superior carbon sequestration ability will allow more CO
absorbed compared to its total emissions during the complete life
cycle of biofuels production. Preliminary laboratory research works
have already managed to produce volatile biofuels such as short-
chain alcohols or aldehydes from metabolic engineering of cyn-
obacteria [28]. While there are a lot of potentials and benets
which can be derived from fourth generation biofuels, several
technical risks still persist due to the lack of fundamental study and
knowledge base on the relevant engineering techniques and tech-
nology. Furthermore, it will consume additional time in order to
locate the most optimum production method and cost-effective
equipment to ensure fourth generation biofuels to be economi-
cally competitive with other energy sources in the market. A
summary on the progression of biodiesel feedstocks has been
depicted in Fig. 2. It is believed that rst generation feedstock will
remain to be the dominant raw materials for biofuels production in
the next decade due to the established supply chains. Second
generation feedstock can help to complement the existing biofuels
production by increasing its supply security and lower the feed-
stock cost due to volatile market. They are expected to play a bigger
role especially after improvement in their productivity and culti-
vation techniques. The ultimate objective is undoubtedly to move
Fig. 2. Progression of biodiesel feedstocks and their important characteristics.
progressively towards utilizing carbon negative feedstock for sus-
tainable biofuels production.
2.3. Production processes
Due to the wide variation of feedstocks for biodiesel production,
it will be impossible to have only one-size-ts-all production pro-
cess. Depending on the physical and chemical properties of the
feedstocks, each biodiesel production process will have its own
advantages and disadvantages. The most common and commer-
cially established production process for biodiesel from rst gen-
eration edible feedstock is homogeneous basic catalytic process
using sodium hydroxide (NaOH) or potassium hydroxide (KOH)
[29]. The process is relatively simple and easy to maintain since
high yield can be achieved close to normal room temperature and
pressure. This can be attributed to the low mass transfer resistance
since the catalysts exist in the same liquid phase as the reactants.
However, they are not suitable for feedstocks which contain high
amount of impurities (water and FFA) commonly found in non-
edible feedstocks and algae. These impurities are capable of deac-
tivating the basic catalysts through side-reactions such as saponi-
cation which form soaps and reduce the desirable esters yield
[30]. Since homogeneous catalysts will usually being retained in
the same phase as the products post-reaction, additional purica-
tion and separation steps need to be introduced in order to full
minimum fuel standards. On the other hand, homogeneous acid
catalysts such as hydrochloric acid (HCl) and hydrosulphuric acid
(H
) can withstand higher content of impurities in their feed-
stocks. However, their basic reaction rates are slower by approxi-
mately 10 order of magnitudes compared to basic catalyst
counterparts in transesterication process of triglycerides. Conse-
quently, they are usually employed as the esterication reagents of
FFA in a two-step process prior to basic transesterication [31].
Heterogeneous solid catalysts are subsequently popularized to
counter the inherent weaknesses in homogeneous catalytic system.
The advantages of using solid catalysts are elimination of washing
step [32], easier separation of catalyst from the product stream,
lower product contamination levels, easy catalyst recycling and
reduction of corrosion problems since acid sites are chemically
bounded with the solid catalyst [33]. These can render biodiesel
production process to become more economically viable and able
to compete with established petroleum-based diesel fuel. The ideal
solid catalyst for transesterication and esterication reactions
should have characteristics such as an interconnected system of
large pores, a moderate to high concentration of strong acid or basic
sites and a hydrophobic surface [34]. However, most of the high
efciency heterogeneous catalysts involved expensive rare com-
pounds such as zirconium dioxide (ZrO
), titanium dioxide (TiO
zeolites and other alkaline earth metal oxides. Their preparation
and synthesizing steps are also tedious, time-consuming and pro-
hibitive [6]. Recently, researches on carbon-based solid catalysts
derived from low-value feedstock can help to minimize the exor-
bitant catalyst cost and without the disposal problemsince they are
biodegradable [35]. However, there is still in need of more scientic
studies to enhance their catalytic activity and overcome catalyst
leaching issue during the reactions.
Bio-catalytic biodiesel production employing enzymes has also
been studied intensively in the past decades as an alternative to
chemical catalytic production. The most common enzymes used for
biodiesel production are lipases derived from bacteria, yeast and
lamentous fungi such as Burkholderia cepacia [36], Candida
antarctica lipase B [37] and Thermomyces lanuginosus lipase [38].
Bio-catalysts are known to exhibit high activity and selectivity
under room conditions which are suitable for feedstocks with high
FFA and moisture content. They also produce fewer amounts of
wastewater and less energy demanding compared to their chemical
catalyst counterparts. However, the major stumbling blocks for
their large-scale production are the associated expensive cost of
enzyme procurement and rapid inactivation by short-chain alco-
hols such as methanol [39]. Nevertheless, the advancement of
biotechnology engineering and immobilization techniques has the
potential to improve the properties of enzymes and provide more
cost-effective strains for biodiesel production in the future.
Apart from conventional catalytic processes, non-catalytic super-
critical process andcatalytic reactive extractionhave alsobeenlauded
as viable biodiesel production routes. Non-catalytic supercritical
biodiesel synthesis was rst performed by Saka and Kusdiana [40]
using rapeseed oil in a high-pressure batch reactor. Their results
proved that biodiesel production could be carried out without the
usage of catalyst as opposed to conventional studies. On the other
hand, catalytic reactive extraction process was pioneered by Har-
rington and D'Arcy-Evans [41] with sunower oil seeds with the
addition of H
as catalyst. These two processes which are closely
related to the SRE process in this review will be discussed in greater
details in the subsequent sections. Over the years, other process in-
tensications have also been introduced to biodiesel processing in a
bid to minimize the chemical equilibrium limitations and economic
penalties. These include the introduction of ultrasound and micro-
wave irradiation to the reaction as different agitation effects and heat
sources. It is believed that these energy sources can improve the
miscibility of the reactants and thus increase the product yields and
shorten the processing time by as much as ten folds compared to
conventional process [42]. Reactive separation processes which
integrate reaction and separation of products simultaneously in a
singleprocessing unit have alsogarneredhuge interest frombiodiesel
researchers [43]. Continuous removal of products from the liquid-
phase reaction can theoretically improve the productivity and selec-
tivity while minimizing energy usage for subsequent product sepa-
ration [44]. This can be achieved since the concentrations of each
reactant are maintained in large excess majority of the time due to
constant products removal. This can also prevent equilibrium limi-
tationas products are preventedfromaccumulatingtogether withthe
reactants. Reactive distillation process is one example of the reactive
separation techniques inwhich the reaction and separation occur ina
fractional distillation column. However, reactive distillation is only
advantageous when the optimum conditions of the reaction are
compatible with the distillation conditions. Otherwise, the product
yield and quality can be greatly affected [45]. The application of
membrane technology in biodiesel production also enables the sep-
aration of esters and glycerol from the un-reacted triglyceride mole-
cules. They cannot pass through the pores of membrane due to their
immiscibility with methanol which is used as the continuous phase.
The removal of products fromthe reaction streamallows the reaction
equilibrium to maintain at the product side while at the same time
alleviate major product contamination at the downstream purica-
tion processes [46]. The main challenges of membrane separation in
biodiesel production are the relatively high cost of membrane syn-
thesis, preventionof membrane fouling, lowmechanical strengthand
high energy required to maintain the pressure across the membrane
[47]. Available technologies for biodiesel production through trans-
esterication and esterication routes are summarized in Fig. 3.
It should be noted that while reactive extraction is technically a
type of reactive separation process, the extraction route described in
this review functions quite differently from other typical reactive
separation processes in the context of biodiesel production. Reactive
distillation or membrane separation works primarily on simulta-
neous product removal fromthe reactant mixture during reaction to
maintain the reaction equilibrium on the product side. On the other
hand, reactive extraction in this context aims to transfer the extrac-
tant from the solid biomass into the reactant mixture concurrently
during reaction. This eliminates the requirement for separate
extraction phase pre-reaction. Consequently, reactive extraction
process does not compete directly with other reactive separation
processes. Comparison between reactive extraction process with
other reactive separation processes is listed in Table 2.
2.4. By-product utilization
The main by-products from conventional biodiesel production
are glycerol and de-oiled biomass waste such as seed cake and
empty fruit bunch. It is estimated that every 1 kg of biodiesel
production will accompany by 0.1 kg of crude glycerol (10 wt.%)
while the exact amount of leftover biomass depends on its oil
content and can vary from 1 to 1.5 kg [51]. Crude glycerol from
biodiesel production is normally perceived as low value (approxi-
mately $0.1/kg) due to high impurity content such as FFA, soap,
solvent and catalyst residues. Conventionally, they are being used
as fuel for boilers or as animal feed supplements. The rapid
development of biodiesel has created a large excess of glycerol
supply in the market. This has resulted it to become nancial and
environmental liabilities which need to be addressed. Conse-
quently, it is paramount for biodiesel developers to explore
Fig. 3. Schematic diagram on the different biodiesel processing routes based on transesterication and/or esterication.
Comparison between reactive separation processes for biodiesel production.
Comparison Supercritical reactive extraction Reactive distillation [48,49] Reactive absorption [50] Membrane reactor [48]
Objective Simultaneous extraction and
reaction using solid lipid-
bearing materials directly
Continuous product removal
from reactants through
evaporation using distillation
absorption in absorption
Continuous product removal from
reactants during reaction through
membrane reactor
Energy consumption High Moderate Low Low
Process conditions Reactants have to be in
supercritical state
Reaction and distillation
conditions must be equal
Low reux ratio Less severe process conditions
Advantages - Eliminate separate lipid
- High efciency and short re-
action time
- Generate higher value of by-
- High yield due to improve
- Increase desired product
- Separation of product mix-
tures with different boiling
- High product conversion and
- More effective usage of
reactor space
- Reboiler or condenser is not
- High resistance to chemical & ther-
mal stress
- High surface area per unit volume
- Enhancement of mass transfer be-
tween solvent and reactant
Drawbacks - High temperature and
- High difculty in product
- Hard to control due to the
existence of three phases
- Reaction and distillation must
have common process
- Less efcient when product
mixtures have similar boiling
- Hard to estimate proper resi-
dence time for reaction and
- Effective only in the region of
low gas phase concentrations
- Easily affected by the heat
from the reaction
- Difcult solvent regeneration
- Fabrication cost is still high
- Fouling can occur which reduce the
effectiveness over time
- Highly dependable on the interaction
with efcient catalyst to generate
high product yield
Cost High Medium Low High
Development stage Laboratory process study Laboratory process study with
simulation results
Only simulation studies Laboratory process study
alternative usage of glycerol as a higher value-added product.
Currently, there are two common routes under scientic studies to
upgrade crude glycerol to other important chemical commodities.
Chemical oxidation and/or reduction processes of glycerol are
usually performed with the aid of appropriate catalyst to produce
specialty chemicals such as propylene glycol [52], acrylic acid [53]
and isopropanol [54]. Biological conversion of glycerol can also be
achieved through fermentation with yeasts in either aerobic or
anaerobic condition. Anaerobic fermentation of glycerol to produce
1,3-propanediol is one of the promising processes to convert glyc-
erol to a product with higher demand [55]. Biological conversion is
normally favoured due to their superior yield, selectivity and
product recovery [51].
Conventional biodiesel production requires pre-extraction of oil
or lipids from the lipid-bearing material before conversion to es-
ters. Pre-extraction process can be done through mechanical
extraction using screw press or chemical extraction using non-
polar solvents such as n-hexane [3]. After extraction, a huge
amount of leftover de-oiled biomass cake will need to be disposed
off. Mismanagement of these biomass wastes can bring adverse
effect to the environment especially for non-edible seed cakes such
as JCL which still contain toxic compounds in considerable con-
centration [56]. Conventional disposal options are either processing
them as solid fuel after briquetting or returning them to their
plantations as organic fertilizers. Both of these ways do not provide
much value to the market or carbon credit in the biodiesel pro-
duction life-cycle assessment (LCA) which is crucial for sustain-
ability benchmarking [57,58]. Consequently, it is imperative to
explore other methods to upgrade these biomass wastes into
higher value products. As most of these biomass wastes usually
contain high composition of cellulose and hemi-cellulose, research
works are available to utilize themfor other fuels processing. One of
the examples is the usage of empty fruit bunch from palm oil to
produce bioethanol using acid pre-treatment followed by fermen-
tation process [59,60]. It is also possible to upgrade the biomass
wastes into other bio-based fuels through pyrolysis, hydro-
processing and catalytic cracking processes [61,62]. Information on
the conventional and new product utilization of biodiesel by-
products is summarized in Fig. 4.
2.5. Challenges in biodiesel production
Global biodiesel production has increased by more than double
from178.83 thousand barrel per day (bpd) in 2007 to 403.74 bpd in
2011 with the most dramatic increment coming from South
America and Asia regions [74,75]. As more countries starting to
adopt a mandatory minimum blending of biodiesel in their con-
ventional diesel fuels, global production of biodiesel is expected to
increase by more than 10% in the next few years [26,76]. However,
its sustainability prospects, cost-effectiveness and stability of raw
material supply still remain questionable for biodiesel production.
The food versus fuel debate, encroachment of forests, higher cost
relative to mineral diesel and inconsistency of government policy
are several notable problems beleaguered the biodiesel industry
[3]. In order to counter the critics and doubts for biodiesel,
continuous research and development in biodiesel processing
technology and its related downstream processes will be required.
Advanced processing techniques which are able to synergize all the
three main components in biodiesel production will have huge
potential to revolutionize the industry to the next level. The pro-
cessing techniques should be highly versatile to accommodate
feedstocks with different properties, easily integrate with major
thermo-chemical processes and adding high value to all its prod-
ucts. In this context, the implementation of supercritical liquid
process, catalytic reactive extraction and the combination of both
processes can overcome a lot of the challenges in current biodiesel
production while simultaneously open up the pathway for more
potential benets to be enjoyed by the industry.
3. Supercritical biodiesel production
3.1. Background
Production of biodiesel using non-catalytic supercritical uids
was rst pioneered by Saka and Kusdiana [40]. They wanted to
explore another alternative to conventional catalytic process which
had slowreaction rates due to methanol/oil miscibility and polarity
limitations. Furthermore, usage of homogeneous catalyst in bio-
diesel processing will add extra burden to the product purication
Fig. 4. Conventional and new product utilization for biodiesel by-products.
after the reaction. Supercritical uid (SCF) is one of the substitutions
proposed to overcome the inherent problems associated with cat-
alytic biodiesel production. SCF is dened as any substance at con-
ditions higher than its critical temperature (T
) and pressure (P
Under this circumstance, SCF can be treated as an intermediate
between gas and liquid as their densities are almost identical [77].
One of the important advantages of SCF is that many of its physical
properties such as density, dielectric constant and solubility can be
easily manipulated through slight variation in temperature and
pressure. This allows SCF to become an excellent medium for
extraction and reaction. Moreover, since majority of SCFs have low
boiling points, they can be easily recovered from product mixtures
and to be reused again with minimal purication. Supercritical
carbon dioxide (ScCO
) is one of the most popular SCFs and has been
regarded as one of the cleaner extraction agents for lipids or oils
from solid seeds [78]. In ScCO
oil extraction, the polarity of CO
greatly reduced and thus renders it to be miscible with non-polar
lipids inside the solid seeds. Under normal conditions, short-chain
alcohols such as methanol and ethanol have high polarity due to
the existence of hydrogenbondingintheir molecules. Consequently,
they are often immiscible with other non-polar compounds
including triglyceride and FFA which are the major reactants for
biodiesel synthesis. However, under supercritical conditions, their
hydrogen bonds will be weakened and dielectric constants will be
reduced to less than ve [79]. These transformations allow the al-
cohols to forma single homogeneous mixture with triglyceride and
FFA molecules. Ma et al. [80] had proven that the solubility of tri-
glycerides in methanol increased by 2e3 wt. % per 10
C increment
of reaction temperature. This will greatly promote the trans-
esterication and estericationprocesses since they depend heavily
on the homogeneity and effective contact area between the re-
actants [81]. Furthermore, reactions perform at elevated tempera-
ture and pressure will usually have higher reaction rate and thus
minimize the time required for optimum conversion [40]. In addi-
tion, numerous studies have reported that SCF process will normally
exhibit higher tolerance to impurity contents such as FFA and
moisture [82,83]. This is opposed to conventional catalytic process
where larger amount of impurities will promote side-reactions such
as hydrolysis and saponication which will in turn reduce the bio-
diesel yield [84]. The advantages of non-catalytic SCF technology
render it a promising alternative to create a more robust and cost-
effective biodiesel production in the future.
3.2. Recent development
Although SCF technology for biodiesel production has been
under intensive research for more than a decade, there is still no
commercial-scale production available based solely on this tech-
nology. BioFuelBox, a private company from U.S., had designed and
built 1 million-gallon-per-year (MMgy) supercritical biodiesel
plant in 2010 but was shut down soon after due to lack of funding
[85]. Bensaid et al. [86] had reported that their team managed to
operate a continuous supercritical biodiesel plant based on rape-
seed oil and bioethanol as the reagents. Under optimum operating
conditions, the pilot plant could produce up to 144.0 liter/day of
biodiesel. It is believed that there are several critical issues which
contributed to the sluggish development of SCF technology for
biodiesel production. First and foremost, the relevant technology is
still relatively obscure and unproven compared to the widely
acceptable catalytic process. Consequently, the condence level is
generally low and most conservative biodiesel developers will be
reluctant to commit to the unknown risk. Secondly, current bio-
diesel production volume per batch is still small due to inconsistent
demand. Therefore, adopting SCF technology in biodiesel produc-
tion may not be able to reap all the benets due to economy of
scale. In addition, most of the relevant infrastructure and human
resources are scarce and exorbitant. The utmost priority is to set up
a successful biodiesel production process utilizing SCF technology
to generate higher interest and quash the unfounded risks. This in
turn will be able to create a mushrooming effect to encourage other
developers to adopt similar technology to replicate the success. In
general, development of supercritical liquideliquid trans-
esterication process is comprised of three major components
which are the choice of solvents, severity of process conditions and
design of corresponding reactor.
3.2.1. Supercritical solvents
Since alcohols are the basic transesterication and esterication
reagents for biodiesel production, most early supercritical process
studies employed either methanol or ethanol as the supercritical
solvents [40,83,86,87]. Short-chain alcohols are usually preferable
in reactions since their hydrogen bonding is more reactive which
contribute to their increase acidity. Several research works have
also proven that supercritical methanol will invariably produce
higher ester yield and reaction rate compared to supercritical
ethanol under the same conditions [20,83,87]. This is supported by
the investigation of their dipole moment where methyl alcohol
exhibits 2.87 debyes at 20
C while ethyl alcohol only exhibits
1.66 debyes at the same temperature. Warabi et al. [88] had
compared the effect of different alcohols including 1-propanol, 1-
butanol and 1-octanol in supercritical transesterication and
esterication with rapeseed oil in a batch process. They found that
1-propanol, 1-butanol and 1-octanol required three, four and more
than six times longer reaction duration respectively to achieve
optimum biodiesel yield compared to methanol. The superior
reactivity of supercritical methanol in biodiesel synthesis allows
lower amount of reactants and energy consumption per unit of
biodiesel produced and thus exhibits more favourable global
warming potential in LCA [87].
As has been mentioned earlier, glycerol is the main by-product
from transesterication due to the usage of alcohol-type solvent
as transesterication reagent. Due to the sudden glut of crude
glycerol supply which far exceeds the demand, the price of glycerol
has been dropping drastically in the international market [51]. In
view of this, many researchers have been focussing on developing
glycerol-free process for biodiesel production [89e91]. In general,
there are two main types of solvent investigated for non-glycerol
biodiesel production process which are using dimethyl carbonate
and carboxylate esters. Dimethyl carbonate is often regarded as a
green solvent due to its low-toxicity, low ammability and biode-
gradability [92]. Ilham and Saka [90] rst conducted the investi-
gation of supercritical dimethyl carbonate in biodiesel synthesis
using rapeseed oil. The optimum ester yield of 94% wt. was suc-
cessfully obtained under 350
C, 20 MPa and 12 min reaction time
which were comparable with supercritical methanol process. In
addition, the main by-products fromthis process were identied to
be glycerol carbonate, citramalic acid and glyoxal. All these prod-
ucts were of higher value than glycerol and could be used exten-
sively in printing, painting, dyeing and pharmaceutical industries.
On the other hand, methyl acetate has also been investigated as
a replacement solvent for alcohol in SCF biodiesel synthesis. In this
process, inter-esterication process will take place instead of con-
ventional transesterication since the two different ester com-
pounds will exchange their acyl groups to produce new product
compounds. Saka and Isayama [89] proposed the application of
methyl acetate in supercritical biodiesel synthesis using rapeseed
oil. At the optimumconditions of 350
C and 45 min, a maximumof
97% wt. of methyl ester yield was obtained. It was discovered that
instead of glycerol, a new by-product triacetin was produced from
the inter-esterication process. Further study showed that the
presence of triacetin in the biodiesel fuel following the theoretical
3:1 methyl oleate to triacetin molar ratio could lead to improve-
ment in the pour point and oxidation stability. Consequently, by
taking into account the suitability of triacetin as an addictive
mixture in the biodiesel fuel, the theoretical yield for supercritical
methyl acetate process can increase up to 105% wt. compared to the
yield of supercritical alcohol process. In other words, higher volume
of biodiesel fuel can be produced per unit of raw material. Types of
solvent applied in SCF biodiesel production is listed in Table 3 and
the different chemical reactions for glycerol-free solvents are
illustrated in Fig. 5(a). The ability to utilize different solvents to suit
the market demand of different products further increase the
economic appeal of biodiesel production using SCF technology.
Future studies should focus on exploring other acyl acceptors and
their ability to ne-tune the products with added-value
3.2.2. Process conditions
The major drawback of applying SCFs in biodiesel production is
the huge cost related to the high pressure and temperature
required to achieve reasonable yield. Kusdiana and Saka [93] had
investigated the methyl ester conversion on rapeseed oil using
supercritical methanol at 200
C to 500
C. They found that the
conversion of esters was lowfor temperature lower than 270
C and
pressure at 14 MPa. To ensure that complete supercritical condi-
tions had been achieved for the reaction, they suggested the reac-
tion temperature to be kept at 300
C while the pressure
maintaining at around 20 MPa. These conditions will incur higher
cost for the fabrication of safety equipment and also consume a lot
of energy which might not be sustainable in the long term.
Consequently, current research works have been focussing on
methods in lowering the process severity of supercritical biodiesel
production without affecting the yield signicantly. One of the
most popular methods is the addition of a third component to the
supercritical system. Rodriguez-Guerrero et al. [95] had tried the
addition of NaOH to complement the supercritical ethanol for
biodiesel production using castor oil. They discovered that even a
0.1% wt. addition of NaOH could almost double the yield compared
to un-catalyzed process while successfully reducing the optimum
temperature from 350
C. Meanwhile, Asri et al. [21]
experimented on the effect of a heterogeneous base catalyst, CaO/
Kl/g-Al
, on the supercritical methanol process using palm oil as
feedstock. They concluded that the catalyst could reduce the
effective reaction time from 90 min to 60 min while increasing the
biodiesel yield from 88% wt. to 95% wt. In both of these studies, the
role of catalyst is similar to non-supercritical process in which they
lower the potential energy barrier required for the reaction to
proceed effectively. Besides catalysts, several research works had
also been performed to investigate the effect of different co-
solvents in supercritical biodiesel process. Co-solvents such as
, n-hexane and propane were found to be effective in increasing
the mutual solubility between the oil and alcohol mixture and thus
enhanced their reaction rates during supercritical process [94,96].
In addition, since most of the co-solvents have lower critical points
compared to alcohols, they can help to decrease the critical tem-
perature and pressure of the binary mixture. This allows the su-
percritical conditions to be attained at a milder condition.
Apart from that, it has been known that implementation of
intensied energy sources such as microwave irradiation and ul-
trasonic cavitation can replace the conventional agitation while
promoting more uniformheat transfer [97]. Gobikrishnan et al. [98]
studied the application of ultrasonic as pre-treatment before per-
forming supercritical methanol process using soybean oil. By son-
icating the oil and methanol mixture for 1 h at 65
C prior to
supercritical process, they managed to obtain optimum biodiesel
yield of 84.2% wt. at 265.7
C, 1:44.7 oil to methanol molar ratio,
8.8 min and 10 MPa. They postulated that the sonication pre-
treatment could help to overcome the initial mass transfer resis-
tance between the oil and methanol mixture and thus allowed the
supercritical reaction to proceed at a faster rate. On the other hand,
Types of solvent used in SCF biodiesel production.
Solvents Critical temperature, T
(K) Critical pressure,
Remarks References
Carbon dioxide (CO
) 304.1 7.38 Used as co-solvent in SCF biodiesel process [78]
Methanol (CH
OH) 512.6 8.09 Most common SCF in biodiesel production
due to high reactivity
Ethanol (C
OH) 513.9 6.14 Lower reactivity than methanol but is
renewable and less toxic
1-propanol (C
OH) 536.9 5.20 Low reactivity due to long carbon chain [88]
1-butanol (C
1-octanol (C
Dimethyl carbonate (OC(OCH
) 548.0 4.6 Solvent for glycerol-free process [92]
Methyl acetate (CH
COOCH
n-Hexane (C
) 507.6 3.0 Popular co-solvent with alcohol [94]
Water (H
O) 647.3 22.1 Hydrolysis reagent in SCF biodiesel process [83]
Fig. 5. Glycerol-free chemical reaction for biodiesel synthesis using (a) dimethyl car-
bonate and (b) methyl acetate.
Kusdiana and Saka [99] developed a novel two-step supercritical
process for the production of biodiesel from rapeseed oil. Hydro-
lysis was rst performed on the oil using subcritical water to break
down the triglyceride chains into smaller fatty acid molecules. After
that, esterication was carried out with supercritical methanol to
produce methyl esters. They managed to obtain a high biodiesel
yield at a lower process severity (270
C and 7 MPa) compared to
single-step supercritical methanol process.
3.2.3. Reactor congurations
The design of reactor for supercritical process can be vital to
improve its efciency while consuming fewer resources. The gen-
eral reactor conguration for supercritical biodiesel synthesis
process depends on its designs either as a batch or continuous
process. While batch process is preferable in a lot of lab to pilot
plant scale processes due to easy and low risk setup, continuous
supercritical process possesses more benets such as higher pro-
duction capacity and lower production cost for commercial-scale
operation [77]. A typical continuous SCF biodiesel production
usually employs a tubular or plug owreactor inwhich the reactant
inlet streams are pre-heated and pressurized before entering the
reactor. Reactor outlet streams will then need to be depressurized
and cooled downwith heat exchanger before proceed to distillation
column for products separation [100]. Dona et al. [101] had studied
biodiesel production using methyl acetate with soybean and mac-
auba oil in a continuous tubular packed bed reactor. They discov-
ered that the reactor design provided excellent systemstabilization
which resulted in low overall experimental error. There are many
other reactor designs for biodiesel synthesis which include cav-
itational reactor, microwave reactor, oscillatory ow reactor,
microchannel reactor, rotating tube reactor, reactive distillation and
membrane reactor [97,102]. Unfortunately, they are not yet opti-
mized for supercritical biodiesel process and thus further studies
are warranted to take advantage of their unique properties.
4. Catalytic reactive extraction for biodiesel production
Conventional biodiesel production requires a separate extrac-
tion phase to obtain the oil content inside the solid biomass before
puried it for conversion to esters. Extraction methods for rst
generation biofuel feedstock such as rapeseed, soybean and sun-
ower usually involve mechanical pressing for small to moderate
scale or chemical extraction for large commercial scale [3]. Both of
these methods are very time-consuming and energy extensive. In
addition, the usage of chemical extraction solvents will generate
additional waste stream which will incur extra cost. The develop-
ment of process intensication in biodiesel processing has resulted
in the discovery of in situ biodiesel production or also known as
reactive extraction [22]. Reactive extraction process combines the
extraction and reaction together in a single unit operation where
they proceed simultaneously. In the context of biodiesel synthesis,
the solid oil-bearing material (oil seeds) will be in direct contact
with the reaction solvent which acts as both extraction agent and
transesterication reagent. This will eliminate the need for separate
extraction process and minimize the losses of yield due to process
transferring. While reactive extraction has a huge potential for
biodiesel synthesis, the main drawback for this process is that
conventional transesterication reagents such as alcohols have
poor miscibility with non-polar lipid compounds. Therefore, they
are not effective in extracting the oil from solid biomass under
normal circumstances. In order to circumvent this problem, reac-
tive extraction is performed under the inuence of chemical/
biological catalysts, non-polar co-solvents or supercritical condi-
tion [10,103e105].
Reactive extraction for biodiesel synthesis was rst initiated by
Harrington and D'Arcy-Evans [41] by using sunower seeds in
acidied methanol solution. They discovered an increment of 20%
in product yield compared to conventional two-step process (pre-
extraction followed by transesterication). They attributed this to
the minimal loss of reactants and better miscibility between the
solvent and the reactants. This success has sparked similar research
works over the years to further investigate the process. However,
most of the research works on reactive extraction for biodiesel
production still remain on laboratory scale up to this date. Further
in-depth studies on their reaction kinetics and mechanism are still
much needed to up-scale this process for commercial biodiesel
production. Details about the development og catalytic reactive
extraction process are separated into several sections as follows.
4.2.1. Catalytic processes
Homogeneous acid and basic catalysts such as H
and NaOH
were found to be effective in facilitating the reactive extraction
process for biodiesel production [7,103,106e108]. With the help of
, Siler-Marinkovic and Tomasevic [7] found that the biodiesel
produced from in situ transesterication of sunower oil had
similar characteristics with conventional two-step process. Mean-
while, Zakaria and Harvey [19] studied the effect of NaOH, a ho-
mogeneous basic catalyst in methanol for the reactive extraction of
rapeseed to produce biodiesel. From their experimental results,
they discovered that the effects of process parameters towards the
yield, conversion and reaction rate were substantially different
compared to conventional two-step process. The solvent to oil
molar ratio had the biggest inuence towards biodiesel yield
among the other process parameters. Based on the light microscopy
and lipid staining technique, they postulated that the catalyst
allowed methanol to diffuse through the cell wall and reacted with
triglyceride molecules when in contact and then forming esters
before diffused out to the liquid bulk solvent.
Besides conventional catalyst, Hailegiorgis et al. [109] had also
studied on the benets of adding a phase transfer catalyst (PTC) on
the alkaline reactive extraction of JCL seeds in both methanol and
ethanol solutions. PTC is usually employed to facilitate the migra-
tion of reactants fromdifferent phases together so that reaction can
occur without mass transfer limitation. In biodiesel synthesis using
homogeneous basic catalyst, the immiscible oil and alcohol solvent
invariably result in slow reaction rate at the initial stage of the
process. This can be overcome with the help of a PTC which is
commonly derived from quaternary ammonium or phosphonium
salts [110]. Hailegiorgis et al. [109] found that the usage of ben-
zyltrimethylammonium hydroxide resulted in faster formation of
biodiesel compared to experimental runs without PTC. In fact, the
effect of PTC alone is higher than NaOH in reactive extraction
process as it produced higher biodiesel yield in the same amount of
reaction time. They concluded that this was due to the intrinsic fast
reaction of the transesterication reaction and mass transfer be-
tween the phases is the rate-limiting step. PTC can overcome the
mass transfer limitations more effectively than NaOH and thus
exhibits higher improvements. Dong et al. [31] tried to replicate the
two-steps pre-esterication and transesterication process on
reactive extraction of microalgae, Chlorella sorokiniana. They
employed a heterogeneous acidic ion-exchange resin, Amberlyst-
15 as the esterication reagent followed by KOH for base-
catalyzed transesterication. They proved that the two-steps pro-
cess could provide higher biodiesel yield with faster rate and lower
chemical consumption compared to single-step reactive extraction
process especially for feedstock with high FFA content. This was
possible since most of the FFA content had been converted to esters
and thus reduced the deactivation of basic catalyst from neutrali-
zation process.
Apart fromchemical catalysis process, reactive extraction is also
applicable on biological catalysis process using enzymes. Jiang et al.
[105] fabricated a novel facile separation device to recover the
immobilized enzymes, Lipozyme TL IM, after reactive extraction of
JCL seeds with methanol. The device consisted of several bucket-
type containers with different wire mesh sizes that allowed the
liquid reactants to pass through freely and in contact with the
immobilized enzymes trapped inside as shown in Fig 6. After the
completion of the reaction, the immobilized enzymes could be
recovered easily from the product mixture which helped to reduce
the separation cost. However, it was important to choose the op-
timum mesh size of the container since the solid seed residue
might block the mesh apertures and thus prevented contact with
the immobilized enzymes. Besides addition of external enzymes,
Gu et al. [111] suggested that it was also possible to perform cata-
lytic reactive extraction for biodiesel production using inherent
lipase of oil seeds during germination as bio-catalyst. Germinated
JCL oil seeds could produce high lipase activity which in turn could
be employed to catalyze reactive extraction process after mixing
with n-hexane and methanol under varying temperatures and
water contents. At the optimum condition, high biodiesel yield up
to 87.6% wt. was obtained. Jiang et al. [112] carried out similar self-
catalyzed reactive extraction using germinated castor seeds with
dimethyl carbonate as the acyl acceptor. They found that dimethyl
carbonate could play the role of both extracting agent and reaction
reagent effectively evenwithout any co-solvent. Optimumbiodiesel
yield was successfully obtained at 87.41% wt. at 35
C for 8 h re-
action time. Self-catalyzed reactive extraction can eliminate the
expensive usage of free or immobilized enzymes. However, the
germination and purication steps prior to the reaction will still
need to be optimized in order to reduce the energy consumption
and prevent substantial losses of oil in the seeds during
germination.
Heterogeneous catalyst is widely perceived as unsuitable for
reactive extraction since they are in the same solid phase as the
solid seeds. The huge mass transfer barrier will render both the
extraction and reaction processes too slowto be feasible. Moreover,
separation of the catalyst from the solid residue after the process
will almost be impossible without huge energy investment. This
will prevent catalyst reusability and greatly undermines the pro-
cess economic competitiveness. Nevertheless, Li et al. [113]
experimented on reactive extraction of a green microalga, Nanno-
chloropsis sp., with a solid base catalyst, MgeZr. They overcame the
mass transfer resistance during the extraction by adding a non-
polar co-solvent, methylene dichloride with methanol. In addi-
tion, the solid catalyst was easily recoverable since the experiment
was performed in an amended Soxhlet extractor where the
microalga residue was not in direct contact with the solid catalyst.
However, the process required 4 h to obtain approximately 60% wt.
of biodiesel yield which was belowaverage since the solvents were
not in full contact with the biomass residue throughout the process.
4.2.2. Co-solvents
Since reactive extraction process employs solid biomass directly
as reactants, its inherent compositions and characteristics can in-
uence the corresponding product yield and process conversion.
Cao et al. [114] experimented on the effect of moisture content in
microalgae biomass, chlorella pyrenoidosa, to the reactive extraction
of biodiesel synthesis using acidied methanol. They found that
high water content (90% wt.) originally present in the microalgae
could be detrimental to the biodiesel production as the water
molecules disrupted the contact area between the methanol and
lipids. However, this could be overcome by increasing the reaction
temperature from 90
C in which the biodiesel yield
increased in tandem from 10.3% wt. to 98% wt. At the optimum
conditions, they added 6.0 ml/g of n-hexane as co-solvent in order
to increase the reactants miscibility. In order to further ascertain
the roles of co-solvent in reactive extraction process, Sanchez et al.
[115] studied the inuence of n-hexane using marine macroalgae
(equal mixture of Pelvetia canaliculata and Fucus spiralis) as solid
material and compared with using sunower oil. They noticed that
n-hexane had detrimental effect towards the FAME yield when
using liquid oil directly due to its high solvency power for non-
polar reactants. However, n-hexane was absolutely required for
reactive extraction process since in its absence the extraction pro-
cess was very slow and the biodiesel conversion was almost
negligible. Apart from n-hexane, different types of co-solvents had
Fig. 6. Schematic diagram of the experimental setup with novel facile separation device for immobilized lipase (reprinted with permission from Ref. [105]. Copyright 2012
American Chemical Society).
also been studied which included isopropanol, tetrahydrofuran
(THF), benzene and chloroform [104,116,117]. It is postulated that
different co-solvents function uniquely under different system and
there is no universal co-solvent which t all types of feedstock in
reactive extraction process.
4.2.3. Enhanced reactive extraction
Process improvements for biodiesel production utilizing
external power sources such as ultrasonic cavitation or microwave
irradiation have been under intensive study. Several research works
had experimented on these technologies in reactive extraction in a
bid to replicate their benets on process severity and product yield.
Suganya et al. [118] performed reactive extraction on marine
macroalgae, Enteromorpha compressa to produce biodiesel cata-
lyzed by H
. Biodiesel yield up to 98.89% wt. was successfully
obtained under the combined inuence of ultrasonic irradiation
and THF as co-solvent. Introduction of ultrasonic cavitation could
potentially reduce the usage of catalyst, solvents and degree of
agitation. Soon et al. [119] concluded that the ultrasonic energy
could cause rapid movement in the reactants mixture which
resulted in cavitation bubbles that were used to break the immis-
cibility of the solvents. Apart from ultrasonic cavitation, microwave
irradiation is also regarded as one of the efcient heat transfer
sources for biodiesel production. Koberg et al. [120] compared the
effect of both ultrasound and microwave on the biodiesel synthesis
of Nannochloropsis salina using strontium oxide as catalyst. They
observed that microwave irradiation was more superior in this case
due to the more efcient heat distribution and its enhanced ability
to destroy the cell walls which trapped the lipids. On the other
hand, Patil et al. [121] studied the application of power dissipation
utilizing controlled microwave energy on reactive extraction of the
same microalgae for biodiesel synthesis. They concluded that
increased power dissipation for microwave irradiation would yield
higher esters conversion since the orientation of the methanol
dipole moment at the elevated condition resulted in lower dielec-
tric constant and polarity. However, high-energy efciency could
only be achieved when high power dissipation was accompanied
by higher volume of reactants providing equal biodiesel yield. More
studies on the enhanced reactive extraction process for biodiesel
production are needed especially on larger scale to provide better
judgement on their feasibility.
5. Supercritical reactive extraction (SRE)
As have been discussed previously, conventional reactive
extraction process requires the aid of appropriate catalyst to syn-
thesize biodiesel under acceptable reaction rate. Catalyst is usually
required due to the immiscibility of the acyl acceptor solvents with
the non-polar reactants trapped inside the solid biomass. Conse-
quently, the solvents have poor extraction and reaction capabilities
at normal conditions. These result in lowextraction and conversion
yield even with prolonging reaction time. Addition of catalyst can
help to overcome the reaction and mass transfer barrier in reactive
extraction. However, it will incur additional cost due to the catalyst
synthesis and purication of products in the later stage. In view of
this, the next logical step seems to combine the reactive extraction
process with SCF technology which can produce biodiesel without
any usage of catalyst. Since both SCF and catalytic reactive extrac-
tion processes have been proven to be viable for biodiesel pro-
duction, it was expected that the combination of these processes
can bring forward tremendous opportunities to existing chemical
and physical limitations.
The main difference between conventional liquid-liquid super-
critical transesterication process and SRE is the introduction of a
solid carbon compound which possesses high lipid content. Like-
wise, catalytic reactive extraction process often does not involve
high temperature and pressure. It is therefore interesting to take
into account the behaviour of the solid oil-bearing material under
the inuence of SCF during SRE process. Generally, SRE process can
be divided into two separate processes which are supercritical
extraction and supercritical reaction. SCF extraction for lipid con-
tent in solid biomass has been widely studied through the appli-
cation of supercritical CO
. In fact, studies have shown that
supercritical CO
can provide higher extraction efciency compared
to other chemical solvent extractions without affecting the quality
of the lipid extracts [122]. However, supercritical CO
extraction is
usually performed at low temperature range (50e100
C) which is
already sufcient to attain supercritical conditions. Consequently,
the solid biomass in the extraction is normally preserved without
any signicant alteration to its surface morphology and molecular
structure. In contrary, conventional acyl acceptors for biodiesel
synthesis such as methanol and ethanol require temperature above
C to reach their critical points [77]. At this high temperature
range, solid biomass which contains a lot of volatile reactive com-
pounds can undergo rapid physical and chemical transformation
due to various side-reactions depending on the choice of solvents.
This is similar to the typical solvolysis process from lignocellulosic
biomass to produce bio-oil [123]. Through this intensication of
extraction, ester conversion and biomass solvolysis processes, SRE
process will play a very crucial role in transforming existing
biomass processing technology.
5.2. Recent advances
SRE for biodiesel production was rst reported by Kasim et al.
[124] using rice bran with supercritical methanol and CO
. The
process was performed at 30 MPa and 300
C for 5 min. Biodiesel
purity and yield were recorded at 52.52% wt. and 51.28% wt.,
respectively. In comparison, supercritical biodiesel production us-
ing puried pre-extracted rice bran oil recorded 89.25 wt.% and
94.84 wt.%, respectively, under the same operating parameters. The
unsatisfactory yield from SRE process in this study was attributed
to the low extraction efciency since moderate amount of lipids
was still trapped in the matrix of the rice bran. However, no addi-
tional effort was made to further optimize the process. Limet al. [9]
also carried out SRE process on JCL oil seeds using supercritical
methanol and n-hexane as co-solvent. They reported that SRE could
synthesize biodiesel from feedstock with high FFA content since
esterication of FFA to esters would also occur simultaneously. This
resulted in higher extraction efciency and FAME yield compared
to conventional process using soxhlet extraction. However, the
process required a longer reaction time (45e80 min) due to the
slow reactor heating rate. They proceeded to study the effect of
different pre-treatments on the solid JCL seeds through a combi-
nation of sieving, heat treatment and de-shelling [125]. They
discovered that SRE could provide better product yield compared
with conventional two-steps process even though subjected to less
pre-treatment steps. Nevertheless, pre-treatment for the solid
seeds was still crucial to obtain higher extraction efciency and
subsequent better conversion to esters. Several SRE research works
by the same authors had also been studied including the effect of
process parameters, different co-solvents and optimization process
using response surface methodology [10,11,126].
Apart from JCL, Levine et al. [127] tried to process wet algal
biomass directly into biodiesel through two-steps supercritical
hydrolysis and esterication. Conventional algal dewatering and
extraction technique might consume up to 90% of the total
processing cost. However, high moisture content in algal biomass
would hinder the extraction and subsequent esters yields. Conse-
quently, they made use of the inherent water content in chlorella
vulgaris to rst hydrolyze it under subcritical water condition
(225e300
C for 15e60 min) to convert the triglycerides into
shorter FFA molecules. The biomass cells were discovered to
conglomerate after hydrolysis into easily lterable solids which
could still retain most of the lipids. The hydrolyzed wet biomass
cells were then subjected to supercritical ethanol reactive extrac-
tion to produce biodiesel. Optimization of the reaction time and
temperature allowed ester yield as high as 79.2% to be recovered
even in the absence of pre-drying steps. The main disadvantage of
this process was the possible losses of lipid content during hydro-
lysis and also the huge amount of non-ester content in the nal
product due to incomplete conversion. Tsigie et al. [128] rened the
two-steps supercritical hydrolysis and transesterication process
into a single step using the same algal biomass under subcritical
condition. The algal biomass which contained 80% moisture con-
tent by weight was mixed with methanol in a batch reactor. They
utilized deionized water under subcritical condition as the catalyst
to facilitate the extraction, hydrolysis and subsequent conversion to
esters processes. The optimum yield obtained was 89.71% wt. at
C, 4 h, 1/4 g/ml biomass to methanol ratio and under contin-
uous stirring. However, the effect of temperature and amount of
moisture for this single-step process were not being investigated.
On the other hand, Patil et al. [14] studied the introduction of mi-
crowave irradiation to SRE of wet algal biomass using ethanol. The
microwave irradiation could replace conventional heating and
provide a more efcient heat transfer to the reaction system. They
claimed that microwave-mediated SRE could increase the extrac-
tion efciency, reduce reaction time and improve biodiesel yield.
Go et al. [129] also carried out SRE of JCL seeds with a mixture of
methanol, acetic acid and water in a bid to reduce the process
severity. They found that addition of acetic acid as co-solvent
enabled the process to operate under subcritical conditions at
C while maintaining 94e98% wt. biodiesel yield.
5.3. Process characteristics
5.3.1. Feedstocks
For SRE process, the feedstocks will have to be solid biomass
which contains high extractable lipids content for extraction and
subsequent esters conversion. Feedstocks which have been sub-
jected to SRE process in the literature include JCL seeds, rice bran,
spent coffee ground, activated sludge, oleaginous yeast and
microalgae (Chlorella vulgaris and Nannochloropsis salina)
[9,12,14,124,127,130,131]. So far there is no published report which
claims the failure of SRE to be performed on certain types of
feedstock. This suggested that the process is highly versatile and
most other solid oil-bearing material from edible or non-edible
sources will have no trouble to be processed into biodiesel after
certain pre-treatment steps. Its versatility might be attributed to
the elevated temperature and pressure conditions during the pro-
cess where lipid-containing cell walls will break down or weaken
and thus allow solvents to diffuse easily. It should be noted how-
ever that feedstocks which contain high amount of poly-
unsaturated fatty acids (with multiple double bonds) might not be
suitable for this process especially when high temperature
threshold is required. Unsaturated fatty acids chains have a high
tendency to undergo cis-trans isomerisation followed by decom-
position to produce dimers and polymers [132]. The decomposition
products will affect the biodiesel fuel quality in compression igni-
tion engines. Apart from that, it is still relatively unclear on the
inuence of impurities such as phospholipids and glycolipids on
SRE. For conventional biodiesel production, these impurities will
normally hinder the esters conversion reaction due to the forma-
tion of amphiphilic emulsion with the solvents [133]. The emulsion
will also increase the difculty of ester-glycerol separation fromthe
solvents after reaction completion. Consequently, they are usually
removed prior to transesterication reaction through dewaxing
and degumming processes. However, similar renement cannot be
performed onto SRE process since pre-extraction stage is skipped.
All the non-fatty acid lipids inside the solid biomass which are
being extracted will not be separated prior to the reaction. There-
fore, it is still dubious on the effectiveness of SRE process towards
feedstock with high inherent phospholipids and glycolipids con-
tent. More experimental studies will be needed to investigate the
impurities effect in SRE process.
5.3.2. Pre-treatments
Since SRE requires the input of solid biomass as reactant directly,
pre-treatment steps may be crucial in determining the extraction
and reaction efciency. Lim and Lee [125] had investigated the ef-
fect of de-shelling, sieving, drying and heat treatment on JCL seeds
towards biodiesel production using SRE. They compared the pre-
treatment processes with conventional two-steps process and
found that SRE required less pre-treatment stages and intensity due
to its higher reactivity. The existence of insoluble seed shell was
discovered to hinder the ester conversion efciency and could in-
crease the ester yield by 18.7% w/w after being removed. Go et al.
[134] also experimented on the JCL seeds using hydrolysis pre-
treatment prior to SRE. They discovered that hydrolysis pre-
treatment could allow the reaction to proceed at lower process
severity (250
C and 13.0 MPa) without drying or aking albeit
requiring higher methanol to solid ratio and longer reaction time.
On the other hand, unicellular biomass such as microalgae and
yeast usually require less pre-treatment intensity compared to
higher plant seeds oil. This can be attributed to the absence of hard
lignin or impervious cell wall normally found in oleaginous oil
plants such as Jatropha and rapeseed. Moreover, the simplicity of its
cell structure allows solvents to get into contact with the intra-
cellular lipids easily. However, Takisawa et al. [135] claimed that
hydrolysis of wet microalgae prior to the SRE could still play an
important role to reduce the effect of water inhibition on ester
conversion. In addition, the hydrolysis pre-treatment using
subcritical water could also enhance the overall reaction rate since
it promoted esterication process which had higher reaction ki-
netics compared to transesterication process. Its shortcoming was
that the nal product yield might be lower due to lipid losses
during hydrolysis at severe conditions [127]. Generally, the main
objective of pre-treatment steps is to increase the effective contact
area between the solvents and convertible lipids inside the dis-
rupted cell wall. While pre-treatment steps are not a necessity for
supercritical process, optimum pre-treatments will allow high
biodiesel yield to be obtained at lower process intensity and thus
reduce the operating cost. Moreover, the yield-enhancing effect of
pre-treatments on solid biomass is more pronounced in SRE and
thus can be more cost-effective compared to conventional two-step
process [125].
Currently, only methanol and ethanol have been applied as
primary reagents in SRE. This is understandable since short-chain
alcohols are known to possess high reactivity as acyl acceptors
during ester conversion process under supercritical conditions. Few
direct comparison studies have been done on the effect of different
alcohol types on SRE process and thus the results are still not
conclusive. Levine et al. [136] showed that SRE process on lipid-rich
microalgae, Chlorella protothecoides using ethanol provided slightly
higher ester yield compared to methanol. They attributed this
difference to the higher dissolving power of ethanol for lipids in the
microalgae. However, it has beenwidely acknowledged that shorter
chain alcohols will exhibit higher reactivity than longer chain al-
cohols even during supercritical state [20]. SRE experimental
studies on Nannochloropsis microalgae were conducted both by
Reddy et al. [15] using ethanol and Patil et al. [13] using methanol. It
appeared that Patil et al. obtained higher ester yield at 84.15%
compared to Reddy et al. study which recorded optimum yield at
67.0% although the optimum conditions for both studies were quite
different. Apart from the amount of the yield, different alcohol
structures can also potentially affect the ester fuel properties.
Reddy et al. claimed that ethyl esters exhibited a higher energy
content (43.0 vs 39.8 MJ/kg) compared to methyl esters. The usage
of other types of acyl acceptor such as methyl acetate and dimethyl
carbonate in SRE should be feasible as well since their supercritical
conditions and properties closely resembled methanol and ethanol.
In view that these solvents do not produce glycerol as by-products,
their potential in supercritical reactive extraction should also be
explored especially in regards to solvolysis of the solid biomass.
Apart from primary solvents, the usage of co-solvents is also an
important element in SRE. In conventional biodiesel production
process, co-solvents are applied to increase the mutual miscibility
between polar acyl acceptors and non-polar fatty acids [118]. The
reduction in mass transfer resistance will allow reaction to proceed
at a much faster rate at lower temperature and pressure level.
Meanwhile in supercritical biodiesel production, co-solvents also
play a role in reducing the process severity to attain supercritical
condition [94]. A binary or tertiary mixture will often contain
different critical points depending on its composition. As a general
rule of thumb, the addition of another solvent with lower critical
point will allow the previous mixture to attain supercritical state at
a milder condition. The combination of these two benets from the
addition of co-solvents enables SRE to perform at a lower process
severity without sacricing its product yield. However, selection of
appropriate co-solvents will need to be done carefully since their
effects may differ from a system to another [126]. Co-solvents for
SRE should possess several important characteristics which include
highly inert towards other side-reactions, low critical point, easily
recoverable from products, high reusability, non-toxic and inex-
pensive. Examples of co-solvents studied for SRE are n-hexane [9],
[12,124], water [128,130,136], acetic acid [134] and tetrahy-
drofuran [126]. Co-solvents which are slightly acidic during su-
percritical or subcritical condition are more favoured since they can
also act as Lewis acid to further promote the reaction process. Tsigie
et al. [131] claimed that esterication of feedstock with high FFA
content could be accelerated when acid concentration in the re-
actants was more than 0.25% wt. for SRE process. In addition, Patil
et al. [14,137] also pointed out that addition of water as co-solvent
in SRE process could reduce thermal degradation of fatty acids due
to inhibitory effect towards hydroproxides mechanism in addition
to help speeding up the reaction.
5.3.4. Process variables
5.3.4.1. Temperature and pressure. For SRE process, temperature
and pressure will play a dominant role since they are directly
proportional to the reactivity of the reactants. The effect of
increasing temperature and pressure are two-folds as they not only
help to overcome the activation energy barrier but also increase the
mutual solubility of the reactants. Consequently, there is normally a
huge jump in process yield especially when supercritical state is
attained although the temperature and pressure are still increasing
linearly. The exact supercritical point for a reactive system such as
SRE is often difcult to be known precisely especially when
involving more than one phase. Therefore, the optimum tempera-
ture and pressure for SRE process utilizing different feedstocks may
deviate from each other and are often times higher than critical
point for the corresponding pure solvent. However, temperature
and pressure exceeding a certain threshold will backre as thermal
degradation of fatty acids will occur [132]. High temperature and
pressure also often require higher equipment and maintenance
cost. Consequently, it is necessary to nd a lower optimum condi-
tion for SRE through introduction of appropriate co-solvents or
solid pre-treatment. The optimum temperature for SRE in the
literature ranges from 175
C while the pressure ranges
from 2.2 MPa to 30 MPa as shown in Table 4. The normal optimum
temperature for the process without any co-solvent is in the range
of 250e300
C although it can be further reduced to 175
C under
the inuence of subcritical water [128,130]. The effect of pressure in
SRE process is harder to quantify since it is usually co-related to the
amount of reactants and types of solvent employed. Application of
back-pressure regulator [12] is one of the options although it faces
the risk of disrupting the amount of initial reactants during
elevated temperature and pressure. Another viable method will be
limited to controlling the initial pressure of the starting reactants
by pumping gases such as nitrogen (N
) or CO
into the reactor
before heating [126]. Lim and Lee [10] performed a statistical
optimization on SRE of JCL seeds using response surface method-
ology. They found that the effect of temperature was the most
signicant variables compared to solvent to solid ratio (SSR), re-
action time and pressure. This nding was also agreed by study
done by Patil et al. [13] where reaction temperature was the most
signicant variables compared to reaction time and SSR ratio in SRE
process of wet Nannochloropsis salina. Meanwhile, Calixto et al. [12]
discovered that only temperatures above critical point of methanol
C) at constant pressure would provide high ester yield from
spent coffee seeds. Effect of pressure on the other hand was
negligible at subcritical temperature range since the density of
methanol was still high. They also concluded that temperature and
pressure had a synergistic effect where both had to be sufciently
high to obtain higher ester yield and lower unconverted fatty acids
5.3.4.2. Reaction time. Reaction time is one of the important traits
of SRE process. Conventional catalytic biodiesel process requires
about 1 h for basic catalysts and even longer for acidic catalysts up
to 5e6 h. Supercritical liquid process is known to have shorter re-
action time (less than 1 h) due to the elevated temperature and
pressure which greatly promotes the reaction kinetics. Sufcient
reaction time is necessary to allow fatty acids to react with acyl
acceptor to form ester compounds. This will also ensure that the
nal product contains high ester yield and low un-reacted fatty
acids concentration which will be detrimental to the diesel engine.
However, longer reaction time is deemed as unproductive and not
favourable for large-scale production. For SRE process, the
involvement of solid biomass to interact with the liquid solvent is
anticipated to require longer reaction time compared to super-
critical liquideliquid process. This has been shown in Table 4 where
majority of the SRE processes have optimum reaction time around
20e50 min excluding heating time. Nevertheless, they are still
much faster than conventional two-steps pre-extraction and cata-
lytic reaction of biodiesel production processes. Several SRE process
optimization studies have pointed out that optimum reaction time
should be inversely proportional to temperature for optimum ester
yield [10,11,131]. Consequently, the intention to produce high ester
yield under less severe subcritical condition is plausible for SRE
process by increasing the reaction time. Kasim et al. [124] tried to
perform SRE process on rice bran only for 5 min which resulted in
sub-optimal ester purity and yield at 52.52% and 51.28%, respec-
tively. They concluded that there were still leftover lipids trapped
inside the solid matrix and thus unable to participate in the
reaction. Therefore, higher reaction time was needed to allow
diffusion of the lipids outwards to the bulk liquid solvent. Tsigie
et al. [128] also agreed with the postulation as they claimed that
longer time for un-catalyzed SRE process was needed to break the
cell wall and subsequently reacted with methanol. It should also be
noted that reaction time for SRE is not advisable to be too long
especially at higher reaction temperature and pressure since un-
saturated fatty acids have a higher tendency to undergo thermal
decomposition. Levine et al. [127] concluded that at 325
C tem-
perature, increasing reaction time from 60 min to 120 min resulted
in higher decrement of biodiesel yield which was less pronounced
C. Reddy et al. [15] agreed with the above results when they
performed SRE process on microalgae Nannochloropsis salina.
Extension of the reaction time did not result in any noticeable drop
of ester yield at 260
5.3.4.3. Solvent to solid ratio (SSR). Similar to conventional liquid
biodiesel production process, the amount of solvent are one of the
important variables affecting the process yield. Based on the reac-
tion stoichiometry, every mole of fatty acid in the form of triglyc-
eride will require three mole of acyl acceptor to form three mole of
ester compound. In reality, the molar ratio of solvent to oil often
requires more than the minimum three to ensure complete con-
version. Since transesterication process is known to be reversible,
higher concentration of acyl acceptor will be needed to push the
equilibrium to the product side. However, larger amount of solvent
will also incur higher energycost for product separation. Inaddition,
it will more likely forming emulsion with the ester and glycerol
layers and thus reduces their yields during recovery phase. In SRE
process, the amount of solvent is correlated to the mass of solid in
the starting reactant rather than the amount of liquid oil. SRE pro-
cess is perceived to require higher molar ratio of solvent to oil (more
than 100) in the solid seeds compared to conventional process since
the solvent will also simultaneously act as the extraction agent.
According to Ficks Law, higher concentration gradient establishes
across the solvent and solute boundary will promote faster solute
diffusion into the bulk solution. However, this may not apply to the
function of co-solvent in SRE process. Since co-solvents are only
used to mediate the mass transfer between polar primary solvent
and non-polar fatty acids, they do not have to be presence in large
amount. In fact, large amount of co-solvents may be detrimental to
the reaction as they will lower the volumetric productivity and
retain more fatty acids from diffuse into bulk solvent solution.
Tsigie et al. [131] found that even small addition of water as co-
solvent from 0 to 0.50 ml/g for SRE process on Yarrowia lipolytica
Po1g, an oleaginous yeast could increase more than double the
ester content in the product from37.23% wt. to 92.18% wt. However,
water content above 0.50 ml/g decreased the yield slightly due to
hydrolysis of ester back into FFA. The optimum SSR ratio was ob-
tained at 30 ml/g which was the most effective volume to extract
the lipids from the yeast. Meanwhile, Levine et al. [127] discovered
that addition of water as co-solvent in SRE of Chlorella vulgaris
exceeding 10% wt. would generally result in system dilution and
ester hydrolysis. Similarly, lower ethanol loading in the process
would encourage higher ester yield. However, they also found that
higher ethanol loading could reduce isomerisation of unsaturated
linoleic acid if the reaction time was short. Lim and Lee [11] varied
the solvent to solid ratio from 2.5 ml/g to 15.0 ml/g for methanol
and 0e6.0 ml/g for n-hexane in SRE of JCL seeds. They discovered an
optimum point at SSR 5.0 ml/g for methanol while n-hexane did
Review on different SRE processes with their process parameters and yields.
Feedstock Pre-treatment Solvent/co-solvent Temperature/pressure Time Solid loading Yield Additional info References
Rice bran e Methanol (30 ml)
C/30 MPa 5 min e 51.28 e [124]
Jatropha curcas L. seeds Dried and
sieved
Methanol (200 ml)
n-hexane (50 ml)
C/24 MPa 80 min
(including
heating)
20 g 103.5 30 min pre-stirring
at 400 rpm
Microalgae
Chlorella vulgaris
Subcritical
(6.6 ethanol/solids
mass ratio)
O (10.1% wt.)
C 120 min
60 mg 100.0 Wet biomass with
80% wt. moisture
Spent coffee ground Dried and
(0.11 CO
/MeOH
molar ratio)
C/10 MPa 50 min 2 g 93.4 e [12]
Microalgae Chlorella
e Methanol (1/4 g/ml)
O (80% wt.)
C/2.2 MPa 4 h (including
5 g 89.71 Wet biomass with
Continuous stirring
Activated sludge e Methanol (30 ml/g)
C/3.5 MPa 24 h (including
1 g 90 Wet activated sludge [130]
Nannochloropsis
(CCMP1776)
e Methanol (9.0 ml/g)
C/8.27 MPa 25 min 4 g 84.15 Stirring [13]
Yarrowia lipolytica
Po1g
Freeze-dried Methanol (30 ml)
O (0.5 ml)
1 g 38.42 e [131]
Nannochloropsis Salina e Ethanol (9 ml/g)
C/8.27e9.30 MPa 20 min 4 g 67.0 e [15]
Jatropha curcas L. seeds Subcritical
Methanol (7.5 ml/g)
Acetic acid (2.5 ml/g)
C/3.0 MPa 105 min
7.5 g 101.7 Kernels with 59% wt.
C/8.0 MPa 25 min 2 g 30.9 Microwave irradiation
(800e1400 W)
Chlorella protothecoids Subcritical
Ethanol (20:1
ethanol/fatty
acid molar ratio)
C/20.0 MPa 180 min e 89.0 e [136]
(4 MPa)
C/21.0 MPa 105 min
20 g 94.43 Continuous stirring [129]
not provide any benet to the process yield at any amount. This was
attributed to the dilution of the oil/methanol inter-phase since
supercritical methanol alone was already sufcient to extract the
oil from the solid seeds. Tsigie et al. [128] also discovered that the
optimum SSR at 4.0 ml/g for wet Chlorella vulgaris with 4.0 ml/g
water content was needed to facilitate the cleavage of glycerine-
fatty acid linkages. For gaseous co-solvent, Calixto et al. [12]
investigated the effect of CO
as co-solvent for SRE process on
spent coffee seeds. They concluded that a higher ester yield
(93.4% wt.) could be obtained at 0.11 molar ratio of CO
to methanol
compared to without addition of CO
(84.9% wt.). In addition, the
temperature and pressure were lowered by 30
C and 20 MPa,
respectively. This proved that the addition of CO
could improve the
mutual solubility of methanol and fatty acids. However, much like
liquid co-solvents, higher pressure of CO
resulted in higher
amount of fatty acids solubilised in CO
phase instead of methanol
and thus prevented reaction from taking place smoothly. For
feedstock with high impurity content such as activated sludge,
Huynh et al. [130] stated that higher amount of SSR would be
needed up to 30 ml/g to obtain the optimum ester conversion at
44.56% wt. after 8 h. Increasing SSR beyond 30 ml/g hindered the
ester conversion as the solid residue post-reactionwas still found to
contain high FFA content. They speculated that other impurities
which were more polar such as protein and carbohydrates were
being selectively extracted rather than fatty acids. SRE process on
the activated sludge from food processing factory was also
discovered to benet from the effect of subcritical water as co-
solvent up to 84% wt. Selection of appropriate co-solvent is thus
crucial to prevent undermining the prospects of SRE process.
5.3.4.4. Solid loading. Solid loading is a new parameter dened for
SRE process since conventional biodiesel production does not
involve thepresence of solidbiomass. Since solidbiomass usually has
a high density, it is important to ensure that they do not conglom-
erate together into a larger entity which will reduce the effective
contact area with the solvent. Moreover, this solid biomass is nor-
mally sticky in nature due to their inherent high oil content after size
reduction which can easily result in the formation of large paste
material [125]. From the production standpoint, it is crucial to pro-
cess as much feedstock as possible due to the cost efciency factor
especially for batchoperation. However, it is alsoimportant toensure
adequate space is available inthereactor for liquidandgas expansion
during supercritical process. Consequently, optimum solid loading
will need to be investigated to ensure maximum production output
without sacricing the product yield. The optimum solid loading
depends not only on the process conditions but also varies with the
design of the reactor itself. The elevated temperature and pressure
during SRE process will enable the reactor to accommodate higher
amount of solid compared to conventional process. Lim and Lee [11]
investigated the effect of solid loading on SRE of JCL seeds. They
discovered that solid loading was inter-related to the volume of the
reactor and 54.0 ml/g reactor loading provided the optimum
extraction and ester conversion efciency. High solid loading would
result in the solid compounded at the bottom which increased the
diffusion resistance. Go et al. [129] also varied the solid loading from
80 to 12 ml/g for SRE process using the same feedstock. They found
that higher solid loading will generally result in lower ester yield
although higher CO
pressure could help to negate the reduction of
yield. Most other SRE processes did not investigate the effect of solid
loading towards ester yield but they are usually ranged under 10 g as
shown in Table 4. Therefore, the amount might be negligible for the
effect of solid loading to be signicant. Nevertheless, up-scaling of
the SRE process is still paramount for future commercialization
production and thus investigation of optimum solid loading should
be continuously emphasized.
5.3.5. Agitation effect and heat sources
For a chemical process, adequate mixing of the reactants to form
a homogeneous phase is important to improve the mass transfer
and reaction rate. Conventional catalytic biodiesel production
emphasizes on high agitation effect to overcome the low mutual
miscibility between the polar solvent and non-polar fatty acid.
Sufcient agitation will also enable more uniform distribution of
reactants and heat transfer across the reactor volume to prevent
creation of cold-spots. For gas phase reaction, agitation effect is
usually not an important criterion since their molecules will
disperse randomly in a conned reactor space. In view of this, su-
percritical state during biodiesel production has already introduced
an inherent agitation effect to the reactants. In contrary, SRE pro-
cess consisted of both solid and liquid phases during initial of the
reaction. Tsigie et al. [128] claimed that stirring would be important
in this type of process as it could prevent clumping and ensured
that the solid biomass was exposed to the solvent uniformly. It was
evident from their results which showed that continuous stirring
could shorten the reaction time by half from 4 h to 8 h under the
same conditions and product yield. On the other hand, Lim and Lee
[11] investigated the mechanical stirring effect on SRE process of
JCL by varying the speed from 0 to 500 rpm. They discovered that
once supercritical state had been attained by the reactants at
C, mechanical stirring had no signicant inuence to either
the extraction or ester yields. On the contrary, Go et al. [129] found
that adequate stirring could still reduce one-third of the reaction
time for SRE process of pre-hydrolyzed JCL seeds at 250
Henceforth, it can be concluded that agitation effect is still impor-
tant for SRE process especially when it is needed to perform below
its supercritical state conditions. Apart from stirring, agitation ef-
fect can also be introduced to a process through the usage of ul-
trasonic cavitation or microwave irradiation as mentioned earlier.
To the best of the authors' knowledge, there is still no SRE process
assisted by ultrasonic cavitation being published in the literature.
However, several successful studies on implementation of ultra-
sonic cavitation to catalytic reactive extraction process for biodiesel
production seem to be promising [118,138]. Microwave irradiation
has been applied for lipid extraction from biomass due to its su-
perior efciency and rapid heat transfer compared to conventional
extraction methods [139]. Due to the same reasons, this technique
has also been experimented on biodiesel production using catalytic
reactive extraction [140e143]. Chemical process involving non-
supercritical liquideliquid or liquidesolid phases assisted by mi-
crowave irradiation could potentially transfer heat more efciently
through radiation rather than convection or conduction. This can
result in localized superheating of the reactants and thus enable the
reaction to proceed to completion at a faster rate [121]. Unfortu-
nately, these unique benets will diminish during supercritical
process since the solvents will become more resistant to heat
transfer when reaching supercritical state. Patil et al. [14] experi-
mented on using microwave irradiation to replace conventional
heating in SRE process using wet Nannochloropsis salina as the
feedstock. To overcome this disadvantage during SRE process, a
passive heating element made of silicon carbide was added to
augment the heat transfer. With this addition, they discovered that
microwave irradiation in SRE process could increase the initial
extraction efciency and thus improves the ester yield at a shorter
time frame. These were possible since microwave interaction with
the reactants allowed higher interfacial polarization mechanism
and thus reduced the activation energy required [144].
5.3.6. Product separation and characterization
Conventional catalytic biodiesel production requires extensive
energy for catalyst separation from the product particularly if ho-
mogeneous catalysts are employed. Non-catalytic SRE process
allows the elimination of catalyst separation phase which greatly
improves its economic viability. However, ltration is still needed
to remove the solid residue from the biomass after reaction from
the liquid product stream. This should be relatively easier and less
energy consuming since the solid is heterogeneous to the liquid
product. Product separation for supercritical liquid biodiesel pro-
duction can be divided into two main phases which are non-polar
phase and polar phase. Non-polar phase usually consists of the
converted esters with any un-reacted fatty acids while major
components in the polar phase will be the solvent and glycerol.
Their high immiscibility at normal roomconditions enables themto
be separated easily through gravity settling into two distinct pha-
ses. The main issue of product separation in SRE process lies on its
multi-component liquid miscibility. Since the lipid content in the
solid biomass do not undergo any pre-treatment prior to the re-
action, majority of the non-fatty acid impurities will also be mixed
together in the nal product stream. Furthermore, the high
extraction efciency at elevated process condition in SRE process
may result in liquefaction of the biomass which produces more
polar and non-polar compounds. The presence of these additional
compounds may create a third intermediate layer between the
ester rich phase and solvent rich phase and thus increases their
mutual solubility. Consequently, simple gravity settling separation
is no longer possible to produce biodiesel with high purity. Since
biodiesel production is the major focus of SRE process, product
analyses are mostly emphasized on the ester rich phase. The
composition and properties of the solvent rich phase are relatively
unexplored which add to the difculty in searching for their opti-
mum separation.
Most SRE process studies overcome this problem by adding
alternating hexane and water to the product liquid mixture to force
a better separation between polar and non-polar phase [11,15,124].
The hexane rich phase will extract most of the non-polar ester
content while the aqueous rich phase will retain the solvent and
glycerol from mixing in the hexane rich phase. This liquideliquid
extraction is repeated several times before vaporizing the solvent to
obtain the nal biodiesel product. Leftover ne solid particles in-
side the liquid product could be further removed through centri-
fugation. It is still unclear if there is a better separation method for
the SRE liquid product mixture as this separation method will
undoubtedly incur additional cost. Nevertheless, the volume of the
solvent required for this liquideliquid extraction technique is only
approximately half of that required for lipid extraction using
chemical extraction. Characterizations on the ester content after
SRE process at optimum conditions show that it is not much
different than other conventional biodiesel production methods
and mostly still meet the requirement of ASTM 6571 and EN 14214
international biodiesel standards [10,130].
Kasim et al. [124] observed that the rice bran solid residue after
SRE process was black and semi-solid at room temperature. They
concluded that complete carbonization occurred for the solid
biomass at temperature higher than 200
C. Further analysis of the
non-polar liquid mixture using thin-layer chromatography found
that it contained slightly polar compounds believed to be origi-
nated from protein and carbohydrate degradation of the biomass.
Lim and Lee [10] performed a thorough characterization on the
products from SRE process of JCL seeds. They found that the
products could be divided into four major categories which are
solid residue, non-polar, polar and gaseous compounds with per-
centages as shown in Fig. 7. The properties of solid residue shared
several similarities with lignite which suggested possible usage as
solid fuel. The analysis of the polar compounds through gas chro-
matography mass spectrometry (GCeMS) showed a complex
composition of oxygenated cyclic compounds, phenols and car-
boxylic acids with energy content up to 35.8 MJ/kg. Levine et al.
[127] obtained a black charcoal-like solid of Chlorella vulgaris after
performing hydrolysis pre-treatment above 225
C. They postu-
lated that the conglomerate was due to dehydration, polymeriza-
tion and aromatization reactions of the polysaccharides in the cell
wall and intracellular matrix. In another study conducted by Levine
et al. [136], they analyzed the lipid-extracted hydrochar of Chlorella
protothecoids and found that it only possessed minor heavy metal
content with heating value up to 27 MJ/kg. On the other hand, Patil
et al. [14] observed that the nutrient contents for algal biomass
before and after SRE process were not much different and were still
high enough to process into animal feed supplement. They also
found that the algal biodiesel contained olens, fatty alcohols, al-
dehydes, sterols and vitamin in minor quantities through GCeMS
analysis which could potentially served as additives for better fuel
quality. Moreover, they claimed that SRE process could produce
product with higher purity compared to conventional catalytic
process which greatly simplied the subsequent separation
5.3.7. Thermal stability
For supercritical biodiesel production process, thermal stability
has always been a great concern particularly if the feedstocks
contain high amount of unsaturated fatty acids [145,146]. Imahara
et al. [146] had performed supercritical liquid biodiesel production
on various vegetable oils to investigate their thermal stability
during the reaction. They concluded that polyunsaturated fatty
acids were more prone to thermal degradation than saturated or
Fig. 7. A typical product mass balance for SRE process using JCL seeds (reprinted with permission from Ref. [10]. Copyright 2013 Elsevier).
monounsaturated fatty acids. Thermal degradation was noticeable
starting at temperature above 270
C where isomerisation would
transform cis-type C]C bonding into trans-type C]C bonding on
fatty acids even while maintaining similar overall ester yield.
Higher temperature above 350
C, however, would start to result in
huge reduction of ester yield due to the denaturation of esters into
smaller molecular weight compounds and gaseous products. While
denaturation of esters should be avoided, there is still no conclusive
evidence that isomerisation of fatty acids will signicantly impact
the biodiesel fuel quality. Levine et al. [127] found that higher
temperature would create more transisomerization reactions spe-
cically on polyunsaturated C18:2 and C18:3 ethyl esters which
could help to meet biodiesel EN14103 specications. Since poly-
unsaturated fatty acids usually leads to low oxidative stability in
biodiesel, a carefully controlled mild thermal degradation of these
fatty acids may increase its resistance to oxidative decomposition
while shorter chain fatty acids may also help to improve its cold
ow properties [147]. Another SRE process study conducted by
Levine et al. [136] using Chlorella protothecoids suggested that
higher solvent volume could potentially inhibit thermal degrada-
tion and polymerization of fatty acids either through dilution or
free radical stabilization. Lim et al. [145] conducted a thermal sta-
bility testing on SRE process of JCL seeds using methanol. They
discovered that only methyl linoleate was unstable when subjected
to SRE process above 300
C for 20e60 min. Based on their
experimental studies, they postulated that the existence of solid
biomass during the process might contribute to the thermal
retardation effect as opposed to supercritical liquid biodiesel pro-
duction. This allowed the ester content to undergo higher tem-
perature and pressure at longer residence time before signicantly
impact its fuel quality.
5.3.8. Mechanism
Understanding the fundamental behaviour of SRE process is
crucial in order to establish optimum conditions and also for future
process up-scaling to commercial production. Fundamental
behaviour of SRE process can be established through its mecha-
nism, kinetics, thermodynamics and phase equilibrium study. Due
to its relatively new and unexplored status, there are only few
scientic SRE process studies available on investigating its funda-
mental behaviour. Levine et al. [136] tried to model the kinetics of
SRE process on Chlorella protothecoids by assuming pseudo rst
order kinetics and that ethanol was present in excess. The rate
constants calculated were 0.012, 0.014 and 0.016 min
at tem-
peratures 275
C, 285
C and 295
C, respectively. In addition,
activation energy was determined to be 44 kJ/mol. Preliminary
study on SRE process on JCL seeds conducted by Lim et al. [9]
showed that extraction efciencies were a lot higher than ester
content at temperature 260
C. This proved that majority of the
lipids inside the solid biomass had already being extracted out by
the solvents to the liquid bulk solution. Majority of the fatty acids in
the liquid bulk solution were not transesteried yet to esters since
supercritical conditions had not being attained. Consequently, the
reaction kinetics remained sluggish and the overall SRE process
was assumed to be kinetically controlled. This explains the ndings
by Lim and Lee [11] where addition of n-hexane as co-solvent did
not bring signicant improvement to the process conditions as it
only increased the miscibility between the lipids and the solvent.
On the contrary, addition of CO
as co-solvent was able to bring
down the overall system critical point to reach supercritical state at
a lower process condition [126]. Consequently, fatty acids in the
bulk liquid solution could be transesteried to ester at a faster rate.
This is also in agreement with SRE experimental study conducted
by Calixto et al. [12] on spent coffee grounds where he discovered a
large percentage of extracted but unconverted triglycerides at
lower temperature and pressure. They also claimed that at higher
than optimum pressure, addition of CO
could create a two-phase
system inside the reaction where triglycerides concentration in
the methanol phase decreased which affected the ester conversion.
These differ fromthe postulations made by Zakaria and Harvey [19]
for catalytic reactive extraction process where the reaction front
was assumed to take place inside the solid biomass matrix before
diffused out to the bulk liquid solution.
Besides transesterication reaction, supercritical process usu-
ally also enables esterication of FFA to proceed simultaneously.
This allows the process to withstand feedstocks with high FFA
content which is otherwise not possible for basic catalytic biodiesel
production process. This was proven by study performed by Lim
and Lee [145] where SRE of JCL oil seeds could withstand up to
30% wt. FFA without signicant impact on its process yield. Ester-
ication of FFA generally can be carried out under milder process
conditions compared to transesterication. Since FFA is more
reactive than higher molecular weight of triglyceride molecules,
esterication process is assumed to be initiated rst before trans-
esterication of triglycerides [88]. Consequently, the starting ester
content in minor concentration detected during SRE process before
supercritical state has been achieved is believed to be mostly
originated from esterication of FFA. This is also in agreement with
the process improvement of SRE process pre-treated with
subcritical hydrolysis [134,136]. Hydrolysis of the lipids enables
triglyceride molecules to be broken down into smaller FFA mole-
cules. Subsequently, they will be esteried into esters during SRE
process. This can result in high ester yield within a shorter time
frame compared to un-hydrolyzed SRE process. The solid biomass
in SRE process is believed to be undergoing similar process mech-
anism as that of lignin solvolysis [127]. Solvolysis or also known as
solvent liquefaction is a type of thermo-chemical process to convert
biomass into smaller molecular weight fragments at moderate
temperature and pressure in the presence of suitable solvents.
Fig. 8. Proposed chemical reaction pathway for cellulose decomposition under su-
percritical methanol (reprinted with permission from Ref. [149]. Copyright 2001
Springer).
Solvolysis can be performed with or without catalyst and is one of
the popular methods to produce bio-oil with high energy content
from solid biomass [123,148]. Coincidentally, conventional SRE
process solvents such as alcohol and water are also commonly used
in solvolysis of biomass. The major difference between SRE and
solvolysis process is that SRE process emphasizes on feedstocks
with high lipid content for biodiesel production while solvolysis
stresses on synthesis of bio-oil from biomass with low lipid value.
Ishikawa and Saka [149] had performed supercritical methanolysis
on cellulose on a batch process to investigate the reaction decom-
position rates and products. They deduced that the cellulose
compound would rst methylated into cellotriose and cellobiose
before further decomposed into methyl glucosides as shown in
Fig. 8. Anomerization and degradation would then yield other
liquid products such as 5-HMF. Lim and Lee [10] agreed with the
ndings as their characterizations on polar liquid fraction after SRE
process using cellulose-rich JCL solid residue showed compounds
with similar properties described by Ishikawa and Saka [10].
Nevertheless, the exact degradation product in the liquid mixture
depends on the composition of the solid biomass subjected for SRE
process. It remains unclear if the solvolysis mechanism of the solid
biomass affects the conversion of fatty acids to esters. Theoretically,
solvolysis will rupture the cell wall and inner cellular matrix which
will help to free up trapped lipids within and promote the SRE
process extraction efciency. A schematic diagram depicting the
processes involved during SRE reaction is shown in Fig. 9.
6. Prospects
6.1. Integration in bio-renery
Experimental studies on transforming biomass to bioenergy to
replace fossil fuels have been investigated intensively in the past
and will most likely continue to develop fervently in the future. The
existence of a huge variety of biomass with different properties and
compositions allow production of various fuels and specialty
chemicals. There are also a lot of thermo-chemical reaction path-
ways available to cater for different feedstocks. This prompted the
concept of bio-renery, an analogous to petroleum renery, where
integration of multiple processes were carried out in one place to
produce a spectrum of desired bio-based products. This integration
will allow maximizing the production value for each feedstock
since the outputs can be converted fully into useful high value
products. There are four major operations in a bio-renery which
include pre-treatment, processing, separation and purication
processes. These processes invariably involve several major com-
ponents which are heat, pressure, catalysts, solvents or any com-
bination of the above. While the usage of catalysts is unavoidable in
certain processes, their usage should be minimized especially at the
starting point of the bio-renery reaction chain since they have
higher possibility to contaminate downstream processing. In view
of this, supercritical solvent is one of the best non-catalytic re-
placements for thermo-chemical processes. By reducing the usage
of additional component, supercritical process could produce
products with higher purity which is essential in multi-processing
bio-renery. These advantages are clearly reected in the SRE
process. In addition, process intensication for bio-renery oper-
ations is extremely critical as cost-cutting measure since it not only
reduces the number of hardware in terms of unit operation but also
minimizes the generation of waste streams. Apart from biodiesel
production, SRE process can also potentially be integrated into
bioethanol synthesis from lignocellulosic biomass in the bio-
renery. Lignocellulosic biomass often has to be pre-treated prior
to cellulose hydrolysis and fermentation due to the hard lignin and
complex cellulose crystalline structure [150]. Popular pre-
treatment methods for the biomass include acid hydrolysis and
Fig. 9. Schematic diagram showing all the processes and corresponding products involve in SRE process.
organosolvation at moderate temperature and pressure. These pre-
treatment techniques coincide with most of the SRE process con-
ditions. Thus, it remains a high possibility for the lipid-depleted
solid residue after SRE process to be subjected to fermentation
for bioethanol production. Co-production of both biodiesel and
bioethanol can bring tremendous benets to the entire biomass to
bioenergy processing scheme.
6.2. Product utilization
Besides lowchemical and energy demand, a sustainable process
also requires minimum generation of waste or un-wanted by-
products. Product utilization is thus an important criterion to
determine the process sustainability as well as economic competi-
tiveness. Successful application of by-products from a process can
improve its LCA through displacement methodology and results in
reduction of disposal cost [57]. By-products can also be upgraded
through various thermo-chemical processes into higher value
products which further increase their appeal and marketability.
However, since the process enhancements usually requires certain
process severity, it is important to ensure that the input re-
quirements do not exceed the product outputs after the process.
Consequently, it is imperative for the product to be subjected to
process enhancement which is intensied into the main reaction
itself. This can potentially minimize the losses of yield during
product transferring phase while at the same time also reduces the
requirements for more severe by-product enhancement processes.
In this context, SRE process enables the solid biomass to be sub-
jected to supercritical solvolysis simultaneously together with the
ester conversion reaction. Conventionally, lipid-extracted biomass
residue is oftenbeingdisposedoff as either animal feedif the protein
content is high or as soil fertilizer back to the plantations. These
options do not provide much value to the solid residue while puri-
cation process such as detoxication still needs to be performed
especially on solid residue from non-edible feedstocks such as
Jatropha which contains toxic substances. SRE process takes
advantage of the solvolysis of solid residue at supercritical condi-
tions to produce other added-value products such as solid fuel and
bio-oil. Furthermore, toxic substances in non-edible feedstock such
as phorbol ester can be denaturated or extracted out during the
reaction at elevated temperature and pressure with alcohol solvents
[151]. This eliminates the requirement for separate intensive
detoxication stage after the reaction. Solvolysis of the solid
biomass during SRE process will also produce polar liquid bio-oil
mixed together with glycerol. Separation can be done if there is a
highdemandfor the specialtychemicals containinthe bio-oil which
includes a wide spectrum of phenols, aldehydes, carboxylic acids
and esters functional compounds [10]. Alternatively, it can also be
employed as liquid fuel directly with minimum further upgrading
process. Reyhanitash et al. [152] claimed that the existence of
glycerol in bio-oil could act as a stabilizer during catalytic hydro-
deoxygenationprocess. Levine et al. [136] suggestedthat microalgae
biochar after hydrothermal and SRE process could potentially be re-
applied as combustion fuel to provide energy back to the process. In
another study, Levine et al. [127] concluded that glycerol and ester
decomposition products during SRE process might still contribute
positively to the overall fuel quality such as viscosity and cloud/pour
point. Consequently, they suggested that the total yield from the
process tobe extendedtoinclude non-ester compounds whichwere
still valuable. The solid residue fromSRE process of Chlorella vulgaris
was also characterized and found to contain high carbon content
(68e72% wt.). They suggested that it might be suitable to be re-used
as soil amendment similar to biochars. Go et al. [134] performed a
preliminary assessment on utilizing the hydrolysate from hydro-
thermal treatment of JCL seeds prior to SRE process to cultivate
yeast. They characterized the hydrolysate and found that 30e40% of
the organics dissolved in it contained high reducing sugar content.
Concentrations of inhibitors such as furfural and 5-HMF were also
discovered to be negligible. They successfully applied the hydroly-
sate to promote the growth of Yarrowia lipolytica without any
further pre-treatment. In short, SRE process can open up a lot of
opportunities for higher product utilization compared to conven-
tional biodiesel production process.
7. Critical issues and recommendations
7.1. Impact to biodiesel production
Currently, most biodiesel production process employed either
homogeneous or heterogeneous catalysts as they are relatively
simpler to set up and the optimization technique has been well
established. Eventhough non-catalytic supercritical liquid biodiesel
synthesis process has been introduced since a decade ago, only few
commercial-scale biodiesel productions have adopted this tech-
nology [40]. However, as supercritical technology becomes pro-
gressively mature over time, it remains a favourite to replace
conventional catalytic process in certain applications. One of the
major challenges for biodiesel production lies in its cost relative to
mineral diesel [3]. Without nancial incentive, biodiesel is yet to be
able to compete competitively with diesel using the current estab-
lished technology. The fact that SRE process can provide signicant
advantages to biodiesel production may bring a totally different
perspective to the industry. Firstly, supercritical process is known to
be able to accommodate a wider range of feedstocks due to its high
impurities tolerance [83,145]. This greatly reduces the price vola-
tility of biodiesel due to over-dependency on specic feedstocks
muchlike the fossil fuels today. This is also inline withthe transition
of biodiesel feedstocks from rst generation to second and third
generations. Secondly, it eliminates the requirement for separate
extraction and renement processes for the solid biomass which
will greatly help in its process intensication. Chemical extraction
process has been known to be very energy-intensive especially
during the solvent evaporation phase [153]. By combining the pro-
cesses in a single step, less generation of waste streams is also
anticipated. Thirdly, SRE process opens up better opportunities for
product utilization which will be favourable towards biodiesel
production life-cycle assessment. Solid biomass subjected to sol-
volysis at elevated temperature and pressure allows simultaneous
production of reusable solid residue and liquid bio-oil. Depending
on separation methods and market demand, theycan be channelled
into a spectrum of higher value products and increase the biodiesel
production economic appeal. Due to the wide range of possible
products produced from SRE process, it may even be feasible to
integrate SRE process with other types of reactive separation pro-
cesses such as reactive distillation and membrane separation which
are being actively explored recently [48]. Multi-stage reactive
distillation column canpotentially be modied to separate products
according to their different boiling points as analogous to the frac-
tional distillationcolumninpetroleumrenery [154]. Subsequently,
membrane reactor can be applied to purify the product streams and
remove other contaminants utilizing their different permeabilities
[155]. All these benets and higher efciency of SRE process is very
promising and can totally revamp the existing biodiesel industry.
The comparison between the SRE process with other existing bio-
diesel productions is listed in Table 5.
7.2. Energy and cost assessment
One of the major concerns for supercritical process lies in its
huge energy for elevated temperature and exorbitant cost for high-
pressure equipment. Undoubtedly, supercritical process such as
SRE requires higher initial capital cost compared to catalytic pro-
cess for procurement of suitable equipment which can withstand
the extreme process severity. Reaction start up also requires a huge
energy input in order for the reactor to heat up to the desired
temperature and pressure. However, the process immense benets
as mentioned earlier coupled with higher process efciency is
believed able to compensate for the huge initial cost in the long
term. In a continuous production process, advancement of heat
recovery technology from the product streams is also able to
minimize the nancial burden of the operating cost. To the best of
the authors knowledge, there is no energy and cost assessment
being done yet on SRE process. However, economic assessment on
biodiesel production using liquid supercritical technology has been
performed in several studies as shown in Table 6. Lim et al. [156]
discovered that supercritical process required a higher total capi-
tal cost due to the high cost of pumps, heaters and heat exchangers
compared to base-catalyzed production process. However, this was
compensated by the much lower manufacturing cost from higher
glycerol credit and absence of catalysts. Lee et al. [157] compared
the economic analysis from base-catalyzed and supercritical
methanol process using both fresh and waste vegetable oil. They
concluded that although supercritical process had a higher total
capital investment, it was the most economically feasible process.
This was due to the higher net present value, lower manufacturing
cost and outstanding discounted cash owrate of return. In another
study, Yusuf and Kamarudin [158] performed cost estimation on
supercritical methanol biodiesel production from JCL and
compared with base-catalyzed biodiesel production from canola
oil. They found that in overall supercritical biodiesel production
provided lower production cost due to higher product selectivity
and less competing side-reactions. Even though most of the eco-
nomic studies for supercritical process are based upon technical
data from lab-scale experiments, these have proven that
supercritical process is still economically competitive compared to
conventional homogeneous catalytic process. Economic assess-
ment for SRE process which promises more benets and exibility
to the biodiesel production chain should be very promising as well.
In addition, the higher tolerance to accommodate low-value multi-
properties feedstocks is much sought after since the cost of feed-
stock usually constitutes more than half of the biodiesel production
cost in the economic assessment [158].
Although no energy assessment has been done on SRE process,
its feasibility can be hypothetically drawn from LCA analysis using
liquid supercritical biodiesel production technology. Rosmeika et al.
[162] compared the LCA analysis between supercritical methanol
and homogeneous basic catalytic processes for biodiesel produc-
tion. They concluded that initial energy consumption of supercrit-
ical methanol was higher than catalytic process (250 vs 170 MJ/kg
biodiesel) due to the higher temperature and amount of solvent
required. However, implementation of by-product utilization such
as recycling biomass residue as heat sources could lower the energy
consumption of supercritical technology down to 160 MJ/kg bio-
diesel. This observation was also supported by LCA study per-
formed by Sawangkeawet al. [163] where environmental impact of
supercritical production technology could be improved tremen-
dously by changing fossil fuels to biomass-based fuels. In view that
SRE process is able to exhibit more efcient by-product utilization,
its energy consumption should be favourable compared to existing
production process.
7.3. Future research needs
Undeniably, SRE process holds great potential to address most of
the challenges in the current biodiesel production. However, this
relatively new technology still requires a lot of fundamental
research and development before it becomes viable for large-scale
commercial production. In particular, there are still several
Comparison of the different process characteristics and requirements for biodiesel synthesis.
Catalytic process Supercritical process
Homogeneous Heterogeneous Enzymatic Reactive extraction Supercritical liquid SRE
Feedstocks 1st Generation 1st and 2nd Generation 1st and 2nd Generation 1ste3rd Generation 1ste3rd Generation 1ste3rd Generation
Temperature and pressure Low Moderate Low Low High High
Reaction time Fast Slow Moderate Moderate Fast Moderate
Pre-treatment intensity High Medium High Medium Low Low
Energy demand Low Moderate Low Low High High
Waste generation High Moderate High High Low Low
Product separation High Low High High Low Moderate
Product utilization Low Low Moderate Moderate Moderate High
Compatibility Low Low Low Moderate Moderate High
Cost Low Medium High Low Medium Medium
Comparison of economic assessment between supercritical and catalytic process for biodiesel synthesis.
Feedstocks Process Capacity
(tonnes/yr)
Total production
cost (USD million/yr)
Total operating cost
(USD million/yr)
Unit production cost
(USD/kg)
JCL oil Supercritical methanol 40,000 46.43 31.20 0.78 [159]
Waste canola oil Supercritical methanol 40,000 55.61 32.49 0.81 [158]
Canola oil Homogeneous base-catalyzed 40,000 90.79 50.9 1.27 [158]
Waste cooking oil Supercritical methanol 40,000 51.40 35.51 0.98 [160]
Castor oil Supercritical ethanol 40,000
63.14 38.99 1.05 [161]
Rapeseed oil Homogeneous base-catalyzed 40,000
Rapeseed oil Supercritical methanol 40,000
Waste cooking oil Homogeneous base-catalyzed 40,000
28.90 26.00 e [162]
Homogeneous acid-catalyzed 26.85 23.80 e
Heterogeneous acid-catalyzed 22.25 19.40 e
Supercritical methanol 25.95 22.95 e
Up-scale.
scientic uncertainties pertaining to the true efciency of SRE
process when dealing with multiple phases during the reaction.
Furthermore, laboratory bench scale results do not necessary
follow the economy of scale for process up-scaling. More specic
knowledge gaps shrouding SRE process are listed as follows:
1) SRE process involves three distinct phases (solid biomass, liquid
solvent and gaseous mixture) which exist simultaneously dur-
ing the reaction. Consequently, there is a need to pin point the
occurrence of phase changes, mixture critical points and equi-
librium conditions. Unfortunately, for a reactive process such as
SRE, depicting its complete phase diagram to ascertain their
relationships at different stages of the process will be quite
complex and tedious. Nevertheless, this fundamental informa-
tion will be very crucial in understanding the behaviour of each
reactant during the process study.
2) Since SRE process involves the solid biomass directly during the
reaction phase, solid pre-treatment seems to be an important
consideration especially in maximizing the extraction efciency.
It is important to optimize the solid pre-treatment for the
biomass so that it can improve the process efciency while at
the same time does not incur exorbitant cost. One of the po-
tential optimization methods is to look at the possibility of
integrating solid pre-treatment steps into the reaction itself. For
example, hydrolysis pre-treatment of algae biomass could be
incorporated into SRE process through the addition of small
amount of water together as co-solvent.
3) There is still a lack of fundamental study pertaining to the ki-
netics and mechanism of SRE process. Most supercritical pro-
cesses are assumed to be pseudo rst order due to the excess of
methanol. However, the extraction efciency is also one of the
reaction limiting steps for SRE process. In addition, competing
reactions such as hydrolysis and esterication may also affect
the kinetics of the process. It is still doubtful on the role of the
solid biomass and its degradation product towards the extrac-
tion efciency and ester conversion. Consequently, more thor-
ough studies are needed to ascertain their relationships before,
during and after SRE process.
4) As have being mentioned earlier, solvolysis of the solid biomass
enables generation of solid residue and bio-oil for better product
utilization. So far, alcohols such as methanol and ethanol are the
most common acyl acceptors being employed for SRE process.
Since other acyl acceptors such as dimethyl carbonate and
methyl acetate have also proven their potential in liquid su-
percritical production process, it may be worthwhile to consider
their implementation in glycerol-free SRE process. Most
importantly, it is important to investigate the correlation of the
properties of the primary solvent and co-solvent not only to-
wards the extraction efciency and ester conversion but also
emphasizes on by-products generation and upgrading.
5) While SRE process does exhibit superior product utilization, its
nal product usually contains an assortment of solid and liquid
compounds. It still remains a huge challenge to nd a cost-
effective method to separate the desired compounds from the
product slurry especially if the initial feedstocks possess high
amount of impurities. These impurities can affect the miscibility
between the esters and the polar compounds. Consequently,
additional separation or purication phase such as liquid-liquid
extraction may need to be employed which will incur additional
energy and cost. In view of this, experimental studies on SRE
process should also be designed to minimize the impact of
product separation post-reaction.
Apart from research studies, technological suppliers also play a
major role in bridging the gap between laboratory results and
commercial production for SRE process. Advancement in super-
critical reactor design and heat recovery technology can improve
tremendously the viability of large-scale biodiesel production uti-
lizing SRE process. Current biodiesel stakeholders and industrial
developers should also continue to show their support in these
innovations and provide more nancial incentives for ground-
breaking researches in this eld. The full benets of intensica-
tion processes such as SRE can only be realized with the full
cooperation and commitment fromall parties in biodiesel industry.
As the pressure to shift from fossil fuels energy to more sus-
tainable renewable energy increase day by day, conversion of bio-
energy from biomass remains one of the most suitable alternatives
to ll in the gap. The ability to transform most if not all parts of the
biomass into usable energy and added-value products in various
applications through thermo-chemical and biological methods
proves that development of bio-reneries will be very promising.
As such, process intensication such as SRE process can play a huge
role in integrating various processes into a single unit operation
and minimizes the loss of product yield. More importantly, its
involvement of different compounds in different phases can allowa
deeper understanding on their interactions at molecular level
especially during supercritical conditions. Besides being more
efcient, SRE process can also provide higher exibility in con-
trolling the properties of the nal ester content and by-products.
This can promote more innovative ways in product utilizing while
at the same time reduces the generation of waste. These benets
can totally rejuvenate the biodiesel industry which is often deemed
as lacklustre in terms of economic and productivity compared to
fossil fuels production. Unfortunately, there are still several tech-
nical barriers which need to be overcome for SRE process before
most of its potential can be realized. Development for supercritical
liquid biodiesel production and catalytic reactive extraction will
need to be proven rst especially for large-scale commercial pro-
duction. There are already ample of scientic studies reported in
this review which propose a lot of constructive suggestions and
recommendations to improve the processes. By then, it is hopeful
that the SRE process technology will be more mature and estab-
lished for commercial production to attract more biodiesel stake-
holders to participate in its development.
The authors are grateful for the nancial support from The
University of Malaya, Kuala Lumpur, Malaysia (UM.C/625/1/HIR/
MOHE/ENG/15) (D000015-16001) and FRGS (6071233) which fully
supported this research.
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