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Review

Integration of reactive extraction with supercritical uids for processintensication of biodiesel production: Prospects and recent advances

Keat Teong Lee a , *, Steven Lim b , Yean Ling Pang b, Hwai Chyuan Ong b , Wen Tong Chong b

a School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Seberang Perai Selatan, Pulau Pinang, Malaysiab 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

Article history:

Received 16 April 2014

Accepted 6 May 2014

Available online

Keywords:

Supercritical reactive extraction

Biodiesel

Bio-renery

Process intensicationProduct utilization

Biofuels

a b s t r a c t

Current world energy usage is trying to gradually shift away from fossil fuels due to the concerns forthe 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 intoefcient 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, thereare 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 valueproducts. This conceives the idea of bio-renery where different biomass conversion processes are

incorporated and proceed simultaneously at one location. However, the underlying complexity inintegrating different processes with varying process conditions will undoubtly incurs prohibitive cost.

Consequently, process intensication plays an important role in minimizing both the capital andoperating costs associated with process integration in bio-reneries. Recently, process intensicationfor 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 productionsuccessfully combines the extraction and reaction phase together in a single processing unit. In thisreview, the important characteristics and recent progress on both of the intensi cation processes for

biodiesel production will be critically analyzed. The prospects and recent advances of supercriticalreactive 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 setupand more efcient utilizing of all of the products generated.

2014 Elsevier Ltd. All rights reserved.

Contents

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: [email protected](K.T. Lee).

Contents lists available at ScienceDirect

Progress in Energy and Combustion Science

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m/ l o c a t e / p e c s

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
mailto:[email protected]://www.sciencedirect.com/science/journal/03601285http://www.elsevier.com/locate/pecshttp://dx.doi.org/10.1016/j.pecs.2014.07.001http://dx.doi.org/10.1016/j.pecs.2014.07.001http://dx.doi.org/10.1016/j.pecs.2014.07.001http://dx.doi.org/10.1016/j.pecs.2014.07.001http://dx.doi.org/10.1016/j.pecs.2014.07.001http://dx.doi.org/10.1016/j.pecs.2014.07.001http://www.elsevier.com/locate/pecshttp://www.sciencedirect.com/science/journal/03601285http://crossmark.crossref.org/dialog/?doi=10.1016/j.pecs.2014.07.001&domain=pdfmailto:[email protected]


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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

4.1. Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4.2. Recent development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4.2.1. Catalytic processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4.2.2. Co-solvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4.2.3. Enhanced reactive extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105. 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187. 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

1. Introduction

Energy supply and security is one of the most pressing issues

shrouding our civilization development which remained to betackled. For the past century, we have become over-reliant on fossilfuels to generate the energy we required for our technological andsocial development until neglecting the devastating effects theymight 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 twothirds of the world population. The slow transition from fossil fuelsto 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 therenewable energy sources, biofuels from biomass such as biodieselare 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 beblended with mineral diesel up to 20% w/w (B20) and applied toexisting combustion ignition engine without any modications.Apart from that, it is also known to be biodegradable, low toxicity,

lower emissions of harmful pollutants (CO, SOxand 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 conventionallydominated by fossil fuels. Traditionally, biodiesel is produced usinghomogeneous 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. Inaddition, 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 wastefeedstocks to avoid the food versus fuel ethical issue, other

advanced biodiesel production methods have been exploredintensively. In general, they can be categorized into three primaryprocesses; the heterogeneous catalytic process, biological enzy-matic process and supercritical uids non-catalytic process. Eachprocess has its own advantages and disadvantages while ample of

research studies have been performed to further improve theprocesses in terms of the esters yield and cost-competitiveness.

In this context, process intensication has been lauded as havinghuge potential to improve biodiesel production process tremen-

dously through various cost-effective measures. Process intensi-cation can be generally dened as any engineering development ofnovel apparatus or technique which resulted in a substantiallysmaller, 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 assistedprocess [5,6]. Reactive extraction or also known as in situ extraction

combines the extraction and reaction processes together in a singleunit operation. There are usually two routes for this to be done.Conventionally, biodiesel production from edible oils starts withthe extraction of oil from the lipid-bearing solid material eitherthrough 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 toproduce 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


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and time [7]. The second type of reactive extraction deals withsimultaneous 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 mentionhereafter 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 issuessimilar 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 tooccur 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 worksdone on SRE process for biodiesel production[9e15], it is believedthat SRE process can become another sustainable productionmethod for biodiesel especially on non-edible feedstocks such as

Jatropha curcas L. (JCL) and algae. Therefore, in this review, the mostrecent and signicant advancement of technology in biodieselproduction 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 SREprocess. The highlight of this review will be focussing on explainingthe concept of SRE process, its current related research, inuences

of process parameters, advantages and challenges pertaining to thebiodiesel production and development. Last but not least, recom-mendations on future scientic studies are proposed for this novelprocess to move forward and to complement existing biodiesel

production and future bio-renery scheme with a more sustainableapproach.

2. Latest development of biodiesel production

2.1. Current status

Generally, biodiesel can be regarded as a liquid fuel comprises ofalkyl esters derived from transesterication of triglycerides oresterication of fatty acids with short-chain alcohols as acyl ac-ceptors. A typical transesterication and esterication reactions toproduce methyl esters are shown inFig. 1(a) and (b) respectively.

Biodiesel shares a lot of similar physical and chemical propertieswith mineral diesel which makes it an ideal replacement incompression ignition engines. However, its higher viscosity andlower energy density renders pure biodiesel not suitable to be

applied in the engines directly. Instead, it has to be blendedtogether with mineral diesel according to a xed proportion.

Currently, most vehicle and engine manufacturers worldwide haveapproved the usage of B5 biodiesel blend (5% biodiesel and 95%diesel by volume) in their engines with a large part of them haveeven raised the maximum limit up to B20. Combustion of biodiesel

blends in the engines has been proven to contribute to severalencouraging effects such as lubricity enhancement, engine wearreduction and better combustion proles [16,17]. In order toencourage the usage of biodiesel to replace conventional diesel,

many countries have mandated a xed percentage of biodieselvolume (ranging from 1% up to 10% volume) in their diesel supplymix [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 andalso encouraging more investors to develop the industry. Researchand development for biodiesel production is still being performedvigorously 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 biodieselproduction 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 producedfrom 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 sourcessuch as rapeseed, soybean and palm oil to produce biodiesel whichare widely regarded as rst generation biofuels [19e21]. Major

producing countries for rst generation edible feedstocks withtheir respective yields are summarized in Table 1.First generationfeedstocks are readily available since commercial plantations havebegun 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]. Thesecond generation biofuels are then developed primarily from non-edible feedstocks derived from plants such as JCL, Calophyllum

inophyllum,Linseed,Cerbera odallamand from waste materials suchas palm oil mill efuent, waste cooking oil and municipal waste.Non-edible plants for second generation biodiesel production canoften 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 wastematerials are relatively cheap to obtain which can help to reduce

the feedstock cost for biodiesel production. Unfortunately, they stillsuffer from a lot of technical challenges since they are relativelywild and scientic knowledge pertaining to these feedstocks is still

Fig. 1. Schematic diagram for typical (a) transesterication and (b) esterication

process in biodiesel production.

Table 1

Major edible feedstocks for biodiesel production and their major producing

countries.

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

P alm Indonesia, Malaysia,Thailand

20e50 2500e4000

Sunower Ukraine, Russia, EU 30e51 280e700

Cottonseed China, India, Pakistan 18e25 n/aPeanut China, India 36e56 340e440

Coconut Philippines,

Indonesia, India

63e65 600e1500

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insufcient[22]. The taxonomy of these wild plants has not been

explored in details unlike their edible counterparts which results ininconsistent yield and volatile market price. In addition, thesefeedstocks often contain higher amount of impurities in the form ofmoisture and free fatty acids (FFA). Consequently, they are not

suitable to undergo conventional homogeneous basic catalyticprocess and additional purication steps will be required. This will

incur extra expenses to the biodiesel production process and thusthey are generally not favoured among biodiesel developers.

However, most of the recent commercial biodiesel productionswere already being designed to accommodate second-generationfeedstocks 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, biodieselproduction utilizing macroalgae and microalgae is also under

intensive development in recent years. Biodiesel produced fromthese algae is collectively known as third generation biodiesel. Themain difference of using algae for biodiesel productioncompared toprevious generations is their higher photosynthesis capability

which enables them to provide higher product yield per cultivationarea while at the same time sequesters larger amount of CO2fromthe atmosphere [2]. Furthermore, they do not competewith land orfresh water resources if cultivated off-shore in contrast to other

oleaginous oil crops. However, much like the second generationfeedstocks, the cultivation and production technology for algae arenot yet mature. This resulted in less than optimal yield and higherenergy consumption especially during harvesting and drying.

Moreover, the requirement for advanced bio-reactor for efcientmicroalgae production is still very prohibitive and troublesome tomaintain while open pond system is susceptible to be polluted byother microorganisms.

Even though a sustainable and economically competitive thirdgeneration biodiesel production is still in intensive studies, severalresearches have started to develop theories and preparing relevant

technology for the next generation of biodiesel feedstocks. Theo-

retically, fourth generation biodiesel feedstocks will take advantageof the advancement in biotechnology, metabolic engineering andgenome research in order to improve cellular metabolism andcharacteristics of oxygenic photosynthesis plants or microor-

ganism. Through manipulation of the genome and recombinantDNA techniques, it is possible to increase the photosynthesis ef-

ciency by several folds and thus greatly enhancethe output of lipidsfor biodiesel conversion[27]. This enables the realization of the cell

factory concept where the continuous transformation of energyfrom sunlight to biofuels using biomass can be more direct, cleanand cost-effective. In addition, the enhancement of CO2consump-tion by biomass allows the carbon cycle of the relevant biofuels

production to shift from neutral to negative. In other words, thesuperior carbon sequestration ability will allow more CO2 to beabsorbed compared to its total emissions during the complete lifecycle 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 benetswhich can be derived from fourth generation biofuels, several

technical risks still persist due to the lack of fundamental study andknowledge base on the relevant engineering techniques and tech-nology. Furthermore, it will consume additional time in order tolocate 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. Asummary on the progression of biodiesel feedstocks has beendepicted inFig. 2. It is believed that rst generation feedstock will

remain to be the dominant raw materials for biofuels production inthe next decade due to the established supply chains. Secondgeneration feedstock can help to complement the existing biofuelsproduction by increasing its supply security and lower the feed-

stock cost due to volatile market. They are expected to play a biggerrole 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.



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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 thefeedstocks, each biodiesel production process will have its ownadvantages and disadvantages. The most common and commer-

cially established production process for biodiesel from rst gen-eration edible feedstock is homogeneous basic catalytic processusing 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 andpressure. This can be attributed to the low mass transfer resistancesince 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 inthe 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 acidcatalysts such as hydrochloric acid (HCl) and hydrosulphuric acid(H2SO4) 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 catalystcounterparts in transesterication process of triglycerides. Conse-quently, they are usually employed as the esterication reagents ofFFA in a two-step process prior to basic transesterication[31].

Heterogeneous solid catalysts are subsequently popularized tocounter 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 andreduction of corrosion problems since acid sites are chemicallybounded with the solid catalyst [33]. These can render biodieselproduction process to become more economically viable and able

to compete with established petroleum-based diesel fuel. The idealsolid catalyst for transesterication and esterication reactionsshould have characteristics such as an interconnected system oflarge pores, a moderate to high concentration of strong acid or basic

sites and a hydrophobic surface [34]. However, most of the highefciency heterogeneous catalysts involved expensive rare com-pounds such as zirconium dioxide (ZrO2), titanium dioxide (TiO2),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 catalystsderived from low-value feedstock can help to minimize the exor-bitant catalyst cost and without the disposal problem since they are

biodegradable [35]. However, there is still in need of more scienticstudies to enhance their catalytic activity and overcome catalystleaching issue during the reactions.

Bio-catalytic biodiesel production employing enzymes has alsobeen studied intensively in the past decades as an alternative tochemical catalytic production. The most common enzymes used forbiodiesel production are lipases derived from bacteria, yeast and

lamentous fungi such as Burkholderia cepacia [36], Candidaantarctica lipase B [37]and Thermomyces lanuginosus lipase[38].Bio-catalysts are known to exhibit high activity and selectivityunder 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 chemicalcatalyst counterparts. However, the major stumbling blocks for

their large-scale production are the associated expensive cost ofenzyme procurement and rapid inactivation by short-chain alco-hols such as methanol [39]. Nevertheless, the advancement ofbiotechnology engineering and immobilization techniques has the

potential to improve the properties of enzymes and provide morecost-effective strains for biodiesel production in the future.

Apart from conventional catalytic processes, non-catalytic super-criticalprocessand catalytic reactive extractionhave alsobeen lauded

as viable biodiesel production routes. Non-catalytic supercriticalbiodiesel synthesis was rst performed by Saka and Kusdiana[40]using rapeseed oil in a high-pressure batch reactor. Their resultsproved that biodiesel production could be carried out without the

usage of catalyst as opposed to conventional studies. On the otherhand, catalytic reactive extraction process was pioneered by Har-rington and D'Arcy-Evans [41] with sunower oil seeds with theaddition of H2SO4as catalyst. These two processes which are closely

related to the SRE process in this review will be discussed in greaterdetails 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 heatsources. It is believed that these energy sources can improve the

miscibility of the reactants and thus increase the product yields andshorten the processing time by as much as ten folds compared toconventional process [42]. Reactive separation processes whichintegrate reaction and separation of products simultaneously in a

single processing unithave alsogarnered huge interest from biodieselresearchers[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 eachreactant are maintained in large excess majority of the time due toconstant products removal. This can also prevent equilibrium limi-

tation as products areprevented fromaccumulating together withthereactants. Reactive distillation process is one example of the reactiveseparation techniques in which the reaction and separation occur in afractional distillation column. However, reactive distillation is onlyadvantageous when the optimum conditions of the reaction are

compatible with the distillation conditions. Otherwise, the productyield and quality can be greatly affected [45]. The application ofmembrane 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 theirimmiscibility with methanol which is used as the continuous phase.The removal of products from the reaction stream allows the reactionequilibrium 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 inbiodiesel production are the relatively high cost of membrane syn-thesis, prevention of membrane fouling,low mechanical strength and

high energy required to maintain the pressure across the membrane[47]. Available technologies for biodiesel production through trans-esterication and esterication routes are summarized inFig. 3.

It should be noted that while reactive extraction is technically atype of reactive separation process, the extraction route described inthis review functions quite differently from other typical reactiveseparation processes in the context of biodiesel production. Reactive

distillation or membrane separation works primarily on simulta-neous product removal from the reactant mixture during reaction tomaintain the reaction equilibrium on the product side. On the otherhand, reactive extraction in this context aims to transfer the extrac-

tant from the solid biomass into the reactant mixture concurrently



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during reaction. This eliminates the requirement for separateextraction phase pre-reaction. Consequently, reactive extraction

process does not compete directly with other reactive separationprocesses. Comparison between reactive extraction process withother reactive separation processes is listed in Table 2.

2.4. By-product utilization

The main by-products from conventional biodiesel productionare glycerol and de-oiled biomass waste such as seed cake andempty 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 frombiodiesel 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 usedas 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 andenvironmental 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.

Table 2

Comparison between reactive separation processes for biodiesel production.

Comparison Supercritical reactiveextraction Reactive distillation[48,49] Reactive absorption[50] Membrane reactor[48]

Objective Simult aneous extraction and

reaction using solid lipid-

bearing materials directly

Continuous product removal

from reactants through

evaporation using distillation

column

Continuous product removal

from reactants through

absorption in absorption

column

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

extraction process

- High efciency and short re-action time

- Generate higher value of by-

products

- High yield due to improve

equilibrium

- Increase desired productselectivity

- Separation of product mix-

tures with different boiling

points

- High product conversion and

productivity

- More effective usage ofreactor space

- Reboiler or condenser is not

required

- 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

pressure

- High difculty in product

recovery

- Hard to control due to the

existence of three phasessimultaneously

- Reactionand distillation must

have common process

conditions

- Less efcient when product

mixtures have similar boiling

points- Hard to estimate proper resi-

dence time for reaction and

separation

- 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



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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 areusually performed with the aid of appropriate catalyst to producespecialty chemicals such as propylene glycol [52], acrylic acid[53]

and isopropanol[54]. Biological conversion of glycerol can also beachieved through fermentation with yeasts in either aerobic oranaerobic condition. Anaerobic fermentation of glycerol to produce1,3-propanediol is one of the promising processes to convert glyc-

erol to a product with higher demand[55]. Biological conversion isnormally favoured due to their superior yield, selectivity andproduct 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 mechanicalextraction 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 disposedoff. Mismanagement of these biomass wastes can bring adverseeffect 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 areeitherprocessingthem as solid fuel after briquetting or returning them to theirplantations 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 toexplore other methods to upgrade these biomass wastes into

higher value products. As most of these biomass wastes usuallycontain high composition of cellulose and hemi-cellulose, researchworks are available to utilize them for other fuels processing. One ofthe 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 biomasswastes into other bio-based fuels through pyrolysis, hydro-

processing and catalytic cracking processes[61,62]. Information onthe conventional and new product utilization of biodiesel by-products is summarized inFig. 4.

2.5. Challenges in biodiesel production

Global biodiesel production has increased by more than doublefrom 178.83 thousand barrel per day (bpd) in 2007 to 403.74 bpd in2011 with the most dramatic increment coming from SouthAmerica 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 toincrease 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 costrelative to mineral diesel and inconsistency of government policyare several notable problems beleaguered the biodiesel industry

[3]. In order to counter the critics and doubts for biodiesel,continuous research and development in biodiesel processingtechnology 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 hugepotential 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 liquidprocess, catalytic reactive extraction and the combination of both

processes can overcome a lot of the challenges in current biodieselproduction while simultaneously open up the pathway for morepotential benets to be enjoyed by the industry.

3. Supercritical biodiesel production

3.1. Background

Production of biodiesel using non-catalytic supercritical uidswas rst pioneered by Saka and Kusdiana [40]. They wanted toexplore another alternative to conventional catalytic process which

had slow reaction rates due to methanol/oil miscibility and polaritylimitations. Furthermore, usage of homogeneous catalyst in bio-diesel processing will add extra burden to the product purication

Fig. 4. Conventionaland new product utilization for biodiesel by-products.



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after the reaction. Supercritical uid(SCF) is one of the substitutionsproposed 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 (Tc) and pressure (Pc).Under this circumstance, SCF can be treated as an intermediatebetween gas and liquid as their densities are almost identical [77].

One of the important advantages of SCF is that many of its physicalproperties such as density, dielectric constant and solubility can beeasily manipulated through slight variation in temperature andpressure. This allows SCF to become an excellent medium for

extraction and reaction. Moreover, since majority of SCFs have lowboiling points, they can be easily recovered from product mixturesand to be reused again with minimal purication. Supercriticalcarbondioxide (ScCO2) is one of the most popularSCFsand has been

regarded as one of the cleaner extraction agents for lipids or oilsfrom solid seeds[78]. In ScCO2oil extraction, the polarity of CO2isgreatly reduced and thus renders it to be miscible with non-polarlipids inside the solid seeds. Under normal conditions, short-chain

alcohols such as methanol and ethanol have high polarity due tothe existenceof hydrogenbonding in theirmolecules. 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, theirhydrogen bonds will be weakened and dielectric constants will bereduced to less than ve[79]. These transformations allow the al-

cohols to form a single homogeneous mixture with triglyceride andFFA molecules. Ma et al.[80]had proven that the solubility of tri-glycerides in methanol increased by 2e3 wt. % per 10 C incrementof reaction temperature. This will greatly promote the trans-

esterication and estericationprocesses since they depend heavilyon 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 processwill normallyexhibit higher tolerance to impurity contents such as FFA and

moisture[82,83]. This is opposed to conventional catalytic processwhere largeramount of impurities will promote side-reactions suchas hydrolysis and saponication which will in turn reduce the bio-diesel yield[84]. The advantages of non-catalytic SCF technologyrender 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 beenunder intensive research for more than a decade, there is still nocommercial-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 tooperate a continuous supercritical biodiesel plant based on rape-

seed oil and bioethanol as the reagents. Under optimum operatingconditions, the pilot plant could produce up to 144.0 liter/day ofbiodiesel. It is believed that there are several critical issues which

contributed to the sluggish development of SCF technology forbiodiesel production. First and foremost, the relevant technology isstill relatively obscure and unproven compared to the widelyacceptable catalytic process. Consequently, the condence level is

generally low and most conservative biodiesel developers will bereluctant to commit to the unknown risk. Secondly, current bio-diesel production volume per batch is still small due to inconsistentdemand. Therefore, adopting SCF technology in biodiesel produc-

tion may not be able to reap all the bene

ts due to economy of

scale. In addition, most of the relevant infrastructure and humanresources are scarce and exorbitant. The utmost priority is to set up

a successful biodiesel production process utilizing SCF technologyto generate higher interest and quash the unfounded risks. This inturn will be able to create a mushrooming effect to encourage otherdevelopers to adopt similar technology to replicate the success. In

general, development of supercritical liquideliquid trans-esterication process is comprised of three major componentswhich are the choice of solvents, severity of process conditions anddesign of corresponding reactor.

3.2.1. Supercritical solvents

Since alcohols are the basic transesterication and estericationreagents for biodiesel production, most early supercritical process

studies employed either methanol or ethanol as the supercriticalsolvents[40,83,86,87]. Short-chain alcohols are usually preferablein reactions since their hydrogen bonding is more reactive whichcontribute to their increase acidity. Several research works have

also proven that supercritical methanol will invariably producehigher ester yield and reaction rate compared to supercriticalethanol 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 exhibits1.66 debyes at the same temperature. Warabi et al. [88] hadcompared the effect of different alcohols including 1-propanol, 1-

butanol and 1-octanol in supercritical transesterication andesterication with rapeseed oil in a batch process. They found that1-propanol, 1-butanol and 1-octanol required three, four and morethan six times longer reaction duration respectively to achieve

optimum biodiesel yield compared to methanol. The superiorreactivity of supercritical methanol in biodiesel synthesis allowslower amount of reactants and energy consumption per unit ofbiodiesel 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 crudeglycerol supply which far exceeds the demand, the price of glycerolhas been dropping drastically in the international market[51]. Inview of this, many researchers have been focussing on developingglycerol-free process for biodiesel production [89e91]. In general,

there are two main types of solvent investigated for non-glycerolbiodiesel production process which are using dimethyl carbonateand carboxylate esters. Dimethyl carbonate is often regarded as agreen 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 synthesisusing 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 from this process were identied tobe 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 productcompounds. Saka and Isayama [89] proposed the application of

methyl acetate in supercritical biodiesel synthesis using rapeseedoil. At the optimum conditions of 350 C and 45 min, a maximum of97% wt. of methyl ester yield was obtained. It was discovered thatinstead of glycerol, a new by-product triacetin was produced from

the inter-esteri

cation process. Further study showed that the



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presence of triacetin in the biodiesel fuel following the theoretical3:1 methyl oleate to triacetin molar ratio could lead to improve-ment in the pour point and oxidation stability. Consequently, bytaking into account the suitability of triacetin as an addictivemixture 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 volumeof biodiesel fuel can be produced per unit of raw material. Types ofsolvent applied in SCF biodiesel production is listed inTable 3and

the different chemical reactions for glycerol-free solvents areillustrated inFig. 5(a). The ability to utilize different solvents to suitthe market demand of different products further increase theeconomic appeal of biodiesel production using SCF technology.

Future studies should focus on exploring other acyl acceptors andtheir ability to ne-tune the products with added-valueproperties.

3.2.2. Process conditions

The major drawback of applying SCFs in biodiesel production isthe 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 theconversion of esters was low for temperature lower than 270C andpressure 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 to 350 C while the pressure

maintaining at around 20 MPa. These conditions will incur highercost for the fabrication of safety equipment and also consume a lotof energy which might not be sustainable in the long term.

Consequently, current research works have been focussing onmethods in lowering the process severity of supercritical biodieselproduction without affecting the yield signicantly. One of themost popular methods is the addition of a third component to the

supercritical system. Rodriguez-Guerrero et al. [95]had tried theaddition of NaOH to complement the supercritical ethanol forbiodiesel production using castor oil. They discovered that even a0.1% wt. addition of NaOH could almost double the yield compared

to un-catalyzed process while successfully reducing the optimumtemperature from 350 C to 300 C. Meanwhile, Asri et al. [21]experimented on the effect of a heterogeneous base catalyst, CaO/

Kl/g-Al2O3, on the supercritical methanol process using palm oil asfeedstock. They concluded that the catalyst could reduce theeffective reaction time from 90 min to 60 min while increasing thebiodiesel yield from 88% wt. to 95% wt. In both of these studies, therole of catalyst is similar to non-supercritical process in which they

lower the potential energy barrier required for the reaction toproceed effectively. Besides catalysts, several research works hadalso been performed to investigate the effect of different co-solvents in supercritical biodiesel process. Co-solvents such as

CO2, n-hexane and propane were found to be effective in increasingthe mutual solubility between the oil and alcohol mixture and thusenhanced their reaction rates during supercritical process [94,96].

In addition, since most of the co-solvents have lower critical pointscompared 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 ofintensied energy sources such as microwave irradiation and ul-trasonic cavitation can replace the conventional agitation whilepromoting more uniform heat 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 tosupercritical 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,

Table 3

Types of solvent used in SCF biodiesel production.

Solvents Critical temperature,Tc(K) Critical pr essure,

Pc(MPa)Remarks References

Carbon dioxide (CO2) 304.1 7.38 Used as co-solvent in SCF biodiesel process [78]

Methanol (CH3OH) 512.6 8.09 Most common SCF in biodiesel production

due to high reactivity

[93]

Ethanol (C2H5OH) 513.9 6.14 Lower reactivity than methanol but is

renewable and less toxic

[86]

1-propanol (C3H7OH) 536.9 5.20 Low reactivity due to long carbon chain [88]

1-butanol (C4H9OH) 562.9 4.41 Low reactivity due to long carbon chain [88]1-octanol (C5H11OH) 659.0 2.69 Low reactivity due to long carbon chain [88]

Dimethyl carbonate (OC(OCH3)2) 548.0 4.6 Solvent for glycerol-free process [92]

Methyl acetate (CH3COOCH3) 507.3 4.7 Solvent for glycerol-free process [89]

n-Hexane (C6H14) 507.6 3.0 Popular co-solvent with alcohol [94]

Water (H2O) 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.



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Kusdiana and Saka[99]developed a novel two-step supercriticalprocess for the production of biodiesel from rapeseed oil. Hydro-

lysis was rst performed on the oil using subcritical water to breakdown the triglyceride chains into smaller fatty acid molecules. Afterthat, esterication was carried out with supercritical methanol toproduce methyl esters. They managed to obtain a high biodiesel

yield at a lower process severity (270 C and 7 MPa) compared tosingle-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 synthesisprocess depends on its designs either as a batch or continuousprocess. While batch process is preferable in a lot of lab to pilot

plant scale processes due to easy and low risk setup, continuoussupercritical process possesses more benets such as higher pro-duction capacity and lower production cost for commercial-scaleoperation [77]. A typical continuous SCF biodiesel production

usuallyemploys a tubularor plugow reactor in which the reactantinlet streams are pre-heated and pressurized before entering the

reactor. Reactor outlet streams will then need to be depressurizedand cooled downwith heat exchanger before proceed to distillation

column for products separation[100]. Dona et al.[101]had studiedbiodiesel 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 system stabilization

which resulted in low overall experimental error. There are manyother 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 studiesare warranted to take advantage of their unique properties.

4. Catalytic reactive extraction for biodiesel production

4.1. Background

Conventional biodiesel production requires a separate extrac-

tion phase to obtain the oil content inside the solid biomass beforepuried it for conversion to esters. Extraction methods for rstgeneration 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 ofthese methods are very time-consuming and energy extensive. Inaddition, the usage of chemical extraction solvents will generateadditional 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 asreactive extraction[22]. Reactive extraction process combines theextraction 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 contactwith the reaction solvent which acts as both extraction agent and

transesterication reagent. This will eliminate the need for separateextraction process and minimize the losses of yield due to processtransferring. While reactive extraction has a huge potential forbiodiesel synthesis, the main drawback for this process is that

conventional transesterication reagents such as alcohols havepoor miscibility with non-polar lipid compounds. Therefore, theyare not effective in extracting the oil from solid biomass undernormal circumstances. In order to circumvent this problem, reac-

tive extraction is performed under the in

uence of chemical/

biological catalysts, non-polar co-solvents or supercritical condi-tion[10,103e105].

4.2. Recent development

Reactive extraction for biodiesel synthesis was rst initiated by

Harrington and D'Arcy-Evans [41] by using sunower seeds inacidied 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 thesolvent and the reactants. This success has sparked similar researchworks 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. Furtherin-depth studies on their reaction kinetics and mechanism are stillmuch needed to up-scale this process for commercial biodieselproduction. 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 H2SO4and NaOH

were found to be effective in facilitating the reactive extractionprocess for biodiesel production[7,103,106e108]. With the help ofH2SO4, Siler-Marinkovic and Tomasevic[7]found that the biodiesel

produced from in situ transesterication of sunower oil hadsimilar 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 theyield, conversion and reaction rate were substantially differentcompared to conventional two-step process. The solvent to oil

molar ratio had the biggest inuence towards biodiesel yieldamong the other process parameters. Based on the light microscopyand lipid staining technique, they postulated that the catalyst

allowed methanol to diffuse through the cell wall and reacted withtriglyceride molecules when in contact and then forming estersbefore diffused out to the liquid bulk solvent.

Besides conventional catalyst, Hailegiorgis et al.[109]had alsostudied on the benets of adding a phase transfer catalyst (PTC) on

the alkaline reactive extraction of JCL seeds in both methanol andethanol solutions. PTC is usually employed to facilitate the migra-tion of reactants from different phases together so that reaction canoccur without mass transfer limitation. In biodiesel synthesis using

homogeneous basic catalyst, the immiscible oil and alcohol solventinvariably result in slow reaction rate at the initial stage of theprocess. This can be overcome with the help of a PTC which iscommonly derived from quaternary ammonium or phosphonium

salts [110]. Hailegiorgis et al. [109] found that the usage of ben-

zyltrimethylammonium hydroxide resulted in faster formation ofbiodiesel compared to experimental runs without PTC. In fact, theeffect of PTC alone is higher than NaOH in reactive extraction

process as it produced higher biodiesel yield in the same amount ofreaction time. They concluded that this was due to the intrinsic fastreaction of the transesterication reaction and mass transfer be-

tween the phases is the rate-limiting step. PTC can overcome themass transfer limitations more effectively than NaOH and thusexhibits higher improvements. Dong et al. [31] tried to replicate thetwo-steps pre-esterication and transesterication process on

reactive extraction of microalgae, Chlorella sorokiniana. Theyemployed 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



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chemical consumption compared to single-step reactive extractionprocess especially for feedstock with high FFA content. This was

possible since most of the FFA content had been converted to estersand thus reduced the deactivation of basic catalyst from neutrali-zation process.

Apart from chemical 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 theimmobilized 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 theliquid reactants to pass through freely and in contact with theimmobilized enzymes trapped inside as shown inFig 6. After thecompletion of the reaction, the immobilized enzymes could be

recovered easily from the product mixture which helped to reducethe separation cost. However, it was important to choose the op-timum mesh size of the container since the solid seed residuemight 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 couldbe employed to catalyze reactive extraction process after mixingwith n-hexane and methanol under varying temperatures and

water contents. At the optimum condition, high biodiesel yield upto 87.6% wt. was obtained. Jiang et al. [112]carried out similar self-catalyzed reactive extraction using germinated castor seeds withdimethyl carbonate as the acyl acceptor. They found that dimethyl

carbonate could play the role of both extracting agent and reactionreagent effectivelyeven without any co-solvent. Optimum biodieselyield 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, thegermination and purication steps prior to the reaction will stillneed to be optimized in order to reduce the energy consumption

and prevent substantial losses of oil in the seeds duringgermination.

Heterogeneous catalyst is widely perceived as unsuitable forreactive extraction since they are in the same solid phase as thesolid seeds. The huge mass transfer barrier will render both the

extraction and reaction processes too slow to be feasible. Moreover,separation of the catalyst from the solid residue after the process

will almost be impossible without huge energy investment. Thiswill 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 themass 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 themicroalga 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 below average since the solvents were

not in full contactwith the biomass residue throughoutthe 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 inmicroalgae biomass, chlorella pyrenoidosa, to the reactive extraction

of biodiesel synthesis using acidied methanol. They found thathigh water content (90% wt.) originally present in the microalgaecould be detrimental to the biodiesel production as the watermolecules disrupted the contact area between the methanol and

lipids. However, this could be overcome by increasing the reactiontemperature from 90 C to 150 C in which the biodiesel yieldincreased in tandem from 10.3% wt. to 98% wt. At the optimum

conditions, they added 6.0 ml/g ofn-hexane as co-solvent in orderto increase the reactants miscibility. In order to further ascertainthe roles of co-solvent in reactive extraction process, Sanchez et al.[115]studied the inuence of n-hexane using marine macroalgae

(equal mixture ofPelvetia canaliculata and Fucus spiralis) as solidmaterial 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 forreactive extraction process since in its absence the extraction pro-cess was very slow and the biodiesel conversion was almostnegligible. Apart fromn-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).



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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 andthere is no universal co-solvent whicht all types of feedstock inreactive extraction process.

4.2.3. Enhanced reactive extraction

Process improvements for biodiesel production utilizingexternal power sources such as ultrasonic cavitation or microwaveirradiation have been under intensive study. Several research workshad 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 marinemacroalgae, Enteromorpha compressa to produce biodiesel cata-lyzed by H2SO4. Biodiesel yield up to 98.89% wt. was successfully

obtained under the combined inuence of ultrasonic irradiationand THF as co-solvent. Introduction of ultrasonic cavitation couldpotentially reduce the usage of catalyst, solvents and degree ofagitation. Soon et al. [119] concluded that the ultrasonic energy

could cause rapid movement in the reactants mixture whichresulted in cavitation bubbles that were used to break the immis-

cibility of the solvents. Apart from ultrasonic cavitation, microwaveirradiation is also regarded as one of the efcient heat transfer

sources for biodiesel production. Koberg et al. [120]compared theeffect of both ultrasound and microwave on the biodiesel synthesisofNannochloropsis salina using strontium oxide as catalyst. Theyobserved that microwave irradiation was more superior in this case

due to the more efcient heat distribution and its enhanced abilityto destroy the cell walls which trapped the lipids. On the otherhand, Patil et al.[121]studied the application of power dissipationutilizing controlled microwave energy on reactive extraction of the

same microalgae for biodiesel synthesis. They concluded thatincreased power dissipation for microwave irradiation would yieldhigher esters conversion since the orientation of the methanoldipole moment at the elevated condition resulted in lower dielec-

tric constant and polarity. However, high-energy efciency couldonly be achieved when high power dissipation was accompaniedby higher volume of reactants providing equal biodiesel yield. More

studies on the enhanced reactive extraction process for biodieselproduction are needed especially on larger scale to provide better

judgement on their feasibility.

5. Supercritical reactive extraction (SRE)

5.1. Background

As have been discussed previously, conventional reactiveextraction 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 withthe non-polar reactants trapped inside the solid biomass. Conse-quently, the solvents have poor extraction and reaction capabilities

at normal conditions. These result in low extraction and conversionyield even with prolonging reaction time. Addition of catalyst canhelp to overcome the reaction and mass transfer barrier in reactive

extraction. However, it will incur additional cost due to the catalystsynthesis and purication of products in the later stage. In view ofthis, the next logical step seems to combine the reactive extractionprocess 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 processescan 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 involvehigh temperature and pressure. It is therefore interesting to takeinto account the behaviour of the solid oil-bearing material under

the inuence of SCF during SRE process. Generally, SRE process canbe divided into two separate processes which are supercriticalextraction and supercritical reaction. SCF extraction for lipid con-tent in solid biomass has been widely studied through the appli-

cation of supercritical CO2. In fact, studies have shown thatsupercritical CO2 can providehigherextraction efciency comparedto other chemical solvent extractions without affecting the qualityof the lipid extracts[122]. However, supercritical CO2extraction is

usually performed at low temperature range (50e100 C) which isalready sufcient to attain supercritical conditions. Consequently,the solid biomass in the extraction is normally preserved withoutany signicant alteration to its surface morphology and molecular

structure. In contrary, conventional acyl acceptors for biodieselsynthesis such as methanol and ethanol require temperature above250 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 transformationdue 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 ofextraction, ester conversion and biomass solvolysis processes, SREprocess will play a very crucial role in transforming existingbiomass 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 CO2. Theprocess was performed at 30 MPa and 300 C for 5 min. Biodieselpurity 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.% and94.84 wt.%, respectively, under the same operating parameters. Theunsatisfactory yield from SRE process in this study was attributedto 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. Lim et al. [9]also carried out SRE process on JCL oil seeds using supercriticalmethanol and n-hexane as co-solvent. They reported that SRE could

synthesize biodiesel from feedstock with high FFA content sinceesterication of FFA to esters would also occur simultaneously. Thisresulted in higher extraction efciency and FAME yield comparedto 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 ofdifferent 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 comparedwith conventional two-steps process even though subjected to lesspre-treatment steps. Nevertheless, pre-treatment for the solid

seeds was still crucial to obtain higher extraction efciency andsubsequent better conversion to esters. Several SRE research worksby the same authors had also been studied including the effect ofprocess 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 supercriticalhydrolysis and esterication. Conventional algal dewatering and

extraction technique might consume up to 90% of the total



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processing cost. However, high moisture content in algal biomasswould hinder the extraction and subsequent esters yields. Conse-

quently, they made use of the inherent water content in chlorellavulgaris to rst hydrolyze it under subcritical water condition(225e300 C for 15e60 min) to convert the triglycerides intoshorter FFA molecules. The biomass cells were discovered to

conglomerate after hydrolysis into easily lterable solids whichcould still retain most of the lipids. The hydrolyzed wet biomasscells 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 recoveredeven in the absence of pre-drying steps. The main disadvantage ofthis 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 thetwo-steps supercritical hydrolysis and transesterication processinto a single step using the same algal biomass under subcriticalcondition. The algal biomass which contained 80% moisture con-

tent by weight was mixed with methanol in a batch reactor. Theyutilized deionized water under subcritical condition as the catalystto facilitate the extraction, hydrolysis and subsequent conversion to

esters processes. The optimum yield obtained was 89.71% wt. at

175 C, 4 h, 1/4 g/ml biomass to methanol ratio and under contin-uous stirring. However, the effect of temperature and amount ofmoisture 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. Themicrowave irradiation could replace conventional heating andprovide 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 ofmethanol, acetic acid and water in a bid to reduce the process

severity. They found that addition of acetic acid as co-solventenabled the process to operate under subcritical conditions at250 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 andsubsequent 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 whichclaims the failure of SRE to be performed on certain types offeedstock. 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 aftercertain pre-treatment steps. Its versatility might be attributed tothe elevated temperature and pressure conditions during the pro-

cess where lipid-containing cell walls will break down or weakenand 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 besuitable for this process especially when high temperaturethreshold is required. Unsaturated fatty acids chains have a hightendency to undergo cis-trans isomerisation followed by decom-

position to produce dimers and polymers [132]. The decompositionproducts will affect the biodiesel fuel quality in compression igni-tion engines. Apart from that, it is still relatively unclear on theinuence 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 from thesolvents after reaction completion. Consequently, they are usuallyremoved prior to transesterication reaction through dewaxingand 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 arebeing 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 theimpurities 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 extractionand reaction efciency. Lim and Lee[125]had investigated the ef-

fect of de-shelling, sieving, drying and heat treatment on JCL seedstowards 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 insolu

3.- 2014.... lee!!!!, biodiesel production -materias primas.pdf

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