A review on assessment of biodiesel production ...

Industrial Crops & Products 114 (2018) 28–44

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Industrial Crops & Products journal homepage: www.elsevier.com/locate/indcrop

A review on assessment of biodiesel production methodologies from Calophyllum inophyllum seed oil

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Mohit Jaina, , Usha Chandrakantb, Valérie Orsata, Vijaya Raghavana a

Department of Bioresource Engineering, McGill University, Sainte-Anne-De-Bellevue, Quebec, Canada, H9X 3V9 Department of Food Science and Agricultural Chemistry, Faculty of Agricultural and Environmental Sciences, McGill University, Sainte-Anne-De-Bellevue, Quebec, Canada, H9X 3V9

b

A R T I C L E I N F O

A B S T R A C T

Keywords: Biodiesel Non-edible oils Calophyllum inophyllum Transesterification Catalyst

In recent times, the world has been confronted with an energy crisis due to the depletion of fossil resources and increased environmental problems. Such a situation has led to an increased research for alternative energy sources such as biofuels from sustainable biomass resources. Tree seed oils are considered as a source of renewable fuels for diesel engine usage, which are presently making its way to becoming a mainstream transportation fuel at the commercial level worldwide. Currently, non-edible tree seed oil resources are gaining worldwide attention because they can be found easily in many parts of the world, especially in wastelands that are not appropriate for cultivating food crops. Important use of non-edible oil resources for biodiesel production alleviates the use of food crops, which leads to reduction of competition in the food market. This paper investigates the potential of Calophyllum inophyllum seed oil, which is found available in plenty in different forest belts of the world, as a promising feedstock for biodiesel production. The main advantages of this crop include its high survival potency in nature which is productive up to 50 years, does not compete with other food crops, it has a high heating value and produces more oil yield than widely used non-edible seed oil, Jatropha curcas. Additionally, C. inophyllum biodiesel so produced, has shown to have very good compatibility with petroleum fuels and has better lubricating capabilities than jatropha. Moreover, the biodiesel fuel produced from the C. inophyllum oil through a multi-step process has the properties in the range of ASTM and EN standards and when blended with diesel fuel gives better engine performance and emission characteristics as compared to diesel fuel. Thus, the biodiesel produced from non-edible Calophyllum inophyllum seed oil could be used as a plausible alternative to diesel fuel in the commercial market.

1. Introduction The escalating petrol and diesel prices and the fast depleting conventional fuel resources along with harmful environmental pollution are resulting in an urge to find alternative fuel sources. Biodiesel, a promising alternate energy source for transport and mechanized agriculture sectors, is a renewable, nontoxic, biodegradable fatty acid methyl ester produced from edible oils, non-edible oils, and animal fat. Fossil fuels have been considered as one of the most important sources of energy. Today, people are widely using fossil fuels as an energy source to carry out their basic activities like transportation, cooking, and agriculture. Furthermore, fossil fuels have uses in many industries like cement manufacturing, steel production, mining, and electricity generation. Today with their major share in electricity generation, fossil fuels play a pivotal role in the development and

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management of the global economy and are considered as an integral part of a country’s economic growth. When considering a developing country like India, its economy depends heavily on imports of fossil fuels from other countries. In 2007, India was importing 4.187 × 1015 British thermal unit (Btu) of fossil fuels and the import has increased to 10.457 × 1015 British thermal unit in 2016 with a compound annual growth rate of 9.6% (Lakshmi et al., 2017). Too much dependency of the population on fossil fuels has raised global issues that affect the existence of life on earth. Recently, a community of scientists has shown concern over the fate of non-renewable energy sources. Some predicted that there has been an exponential increase in the population growth rate, which leads to a spiral increase in the exploitation rate of fossil fuels. In contrast, others supported the idea with a different perspective and stated that the heavy dependency of global life has led fossil fuel reserves to the verge

Corresponding author. E-mail address: [email protected] (M. Jain).

https://doi.org/10.1016/j.indcrop.2018.01.051 Received 5 September 2017; Received in revised form 15 January 2018; Accepted 19 January 2018 0926-6690/ © 2018 Elsevier B.V. All rights reserved.


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quite apparent that the world has been changing and is making headway towards utilization of renewable assets as the main conceivable substitute for fossil fuels. Non-edible biofuel feedstocks are gaining worldwide attention since they are found in many parts of the world. Moreover, they can abridge competition for food, environmental friendly, produce useful by-products (glycerol), and as a feedstock they are more economical compared with edible oils (Atabani et al., 2013). Oil from non-edible plants is viewed as a second-generation biodiesel. Some of the potential nonedible oil producing plants are Jatropha curcas L. (jatropha), Thespesia populnea L. (milo), Pongamia pinnata (karanja or honge), Moringa oleifera (drumstick tree), Calophyllum inophyllum L. (undi), Croton megalocarpus (croton), Ricinus communis L. (castor), Azadirachta indica (neem), Cerbera odllam (sea mango), Triadica sebifera L. (Chinese tallow), Cascabela thevetia L. (yellow oleander), Madhuca indica and Madhuca longifolia (mahua), Ceiba pentandra (silk-cotton), Hevea brasiliensis (rubber), and Eruca Sativa (arugula). Thus, it is possible to speculate that biodiesel production from non-edible plants, like undi (Calophyllum inophyllum), jatropha (Jatropha curcas) (Kamel et al., 2018), neem (Azadirachta indica) (Joshi and Negi, 2017), karanja (Pongamia pinnata) (Patel and Sankhavara, 2017) etc., could be the future fuel sources. Even, biodiesel production from some of the above mentioned non-edible plants have been implemented at industrialized level (Ching et al., 2011), but issues of expense and productivity spur further research development in the area.

of extinction. The Intergovernmental Panel on Climate Change (IPCC) reported that the major reasons for the increase in daily minimum and maximum temperature were climate variability, socioeconomic development, and anthropogenic activities (IPCC, 2007). Not only depletion of fossil fuels, but the harmful changes such as climate change and ozone layer depletion on the environment have raised global concerns. These environmental issues, which were unnoticed for many years, have lately become serious concerns. The plausible reason behind these environmental issues is the increased release of harmful gases such as carbon monoxide, carbon dioxide, nitrogen oxides, sulfur oxides, and other undesirable compounds into the air from the combustion of fossil fuels (Shuba and Kifle, 2018). Only in the last few years due to the rising environmental concerns and serious periodic crises in some of the larger oil exporting countries, biofuels have become a viable and realistic product in the energy market. Although, the concept of biofuel is not new, many individuals were practicing the use of vegetable oils as fuels long back, even before the energy crisis of the late 1970s. However, a drastic change in the production of biofuel was observed only after the energy crisis, i.e. after the early 1980s (Knothe, 2009a). Biofuels are renewable fuel substances produced from biomass, which include biodiesel, bioethanol, and biogas. Although biodiesel and bioethanol are produced from biomass, still perplexity persists among researchers about their superiority. On the one hand, biodiesel has issues of cost and less net energy production on burning as compared with bioethanol, whereas biodiesel works well in any diesel engine without incorporating any specific modulation in the engines upon blending (Jiaqiang et al., 2017). As well, a remarkable decrease (i.e. almost 41%) in the emission of greenhouse gases was observed when using biodiesel fuel as compared with conventional petroleum diesel (Ngee Ann Polytechnic, 2011). Furthermore, biodiesel production is an economical process, unlike several alternative fuel technologies such as solar thermal and photovoltaic collectors which require much larger capital investment (Gude et al., 2013). By keeping the commercial perspective in mind, biodiesel can be considered as a plausible alternative to the conventional diesel fuel. According to current statistical facts, in 2013 biodiesel is considered as the second largest category of biofuels, accounting for 26.12 billion liters worldwide. Over the years, the production of biodiesel fuels has increased substantially from 2.4 to 30.8 billion liters (Fig. 1) (Martinot, 2007; Renewables Global Status Report, 2010, 2013, 2014, 2017). The Global Biofuel Status Report (GBSR), showed that the European Union alone accounts for 10.6 billion liters of biodiesel production in 2013, 40% of the total worldwide production making it the largest producer of biodiesel (Rapier, 2014). Whereas, the United States of America is the largest biodiesel producer with 4.84 billion liters of production followed by Brazil (3.1 billion liters), Germany (2.9 billion liters), and Argentina (2.3 billion liters) (Rapier, 2014). Through all these facts, it’s

1.1. Calophyllum inophyllum oil Calophyllum inophyllum has been chosen for this study over other productive edible and non-edible plants like Jatropha curcas L., Helianthus annuus L. (sunflower), Gossypium arboretum L. (cotton), and Areca catechu L. (palm) because of various advantages, such as their high survival potency in nature i.e. up to 50 years, long productivity and higher oil yield than J. Caracas, however the major disadvantage associated with it, is its availability, which is limited only to tropical and subtropical countries. Calophyllum is extremely sensitive to frost i.e. cannot be used as a feedstock producer in countries with cold winters, however Pennycress is a non-edible plant which can grow well in cold conditions and can be considered a plausible feedstock producer in countries with cold winter (Khalavati and Dincer, 2013). Furthermore, the oil extracted from C. inophyllum has a high heating value, a biodegradable nature, nontoxic, and with a moderately high flash point, which has attracted special attention during recent years as a different option to petroleum-based diesel fuel (Shibasaki-Kitakawa et al., 2007). In addition, biodiesel produced from C. inophyllum oil meets the US ASTM D6751 and European Union EN 14214 biodiesel standards and possesses better lubrication ability than petroleum oil Fig. 1. Substantial worldwide production of Biodiesel from 2004 to 2016.

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Fig. 2. Fatty acid composition of oil extracted from C. inophyllum (Crane et al., 2005).

which are present in very less amount such as myristic acid (< 0.1%), palmitoleic acid (0.2%), linolenic acid (0.3%), arachidic acid (0.6%), Gondoic acid (0.1%), Behenic acid (0.2%), Erucic acid (< 0.1%), Lignoceric acid (0.2%), nervonic aicd (< 0.1%) (Fig. 2) (Crane et al., 2005). It is well known that a biodiesel containing a high degree of unsaturation (such as polyunsaturated and monounsaturated fatty acid methyl esters) is prone to auto-oxidation. Furthermore, oxidative degradation has a negative impact on acid value and kinematic viscosity of the biodiesel. Conversely, higher amount of saturated fatty acids in the oil relates to the high cloud point of the fuel, i.e. the fuel liquifies at a relatively high temperature, which leads to poor cold flow properties (which can be resolved by adding additives such as metal based additives, antioxidants and oxygenated additives) (Jiaqiang et al., 2017) and generally limits the application of the fuel in cold countries. However, a biodiesel containing a high amount of unsaturated fatty acids has good flow properties compared with saturated fatty acids (Ramos et al., 2009; Knothe et al., 2005, Knothe, 2008, 2009b). The principal disadvantage of utilizing crude oils as biodiesel fuel is their high viscosity and low volatility, which causes poor combustion in diesel engines including the formation of deposits and injector cocking. To abridge such effects, further processing of crude oil through various methods involving transesterification, pyrolysis, dilution, and microemulsion (Demirbas, 2002), ought to be implemented. Although with micro-emulsion and dilution techniques, the viscosity of the oil reduces but other engine performance associated problems persist. Similarly, the high cost associated with the pyrolysis method restrains its use at the commercial level, however transesterification has been considered as the ideal strategy in contrast with others. This strategy reduces the viscosity of the oil to a range of 3–5 mm2/s closer to that of diesel and hence improves combustion (Demirbas, 2002). From Table 1 the biodiesel produced from C. inophyllum oil has combustion characteristics like that of diesel fuel (ASTM D6751 and EN 14214). It has a shorter ignition delay, higher ignition temperature and pressure, in addition, the peak heat release is comparable to that of diesel fuel. Moreover, the engine power output and brake power efficiency of the fuel produced from Calophyllum inophyllum is similar to that of diesel fuel (Ong et al., 2011; Demirbas, 2009), however, recent study of Ashok et al. (2017a,b) showed that the efficiency of the combustion engine fuelled with biodiesel can be improved further by adding antioxidant additives such as ethanox. Other properties of biodiesel and blended diesel fuels were tested and reported in prior literature, i.e. it reduces ‘smoke opacity, particulate matters emission, release of un-burnt hydrocarbons, carbon dioxide and carbon monoxide

(Sudradjat, 2011; Ministry of Forestry of the Republic of Indonesia, 2008). Even it has been discussed in a prior literature, that the chemical characteristics of the methyl esters derived from C. inophyllum oil meets the requirement of the diesel engine (Sahoo and Das, 2009a). 1.2. Geographical distribution Calophyllum inophyllum belongs to the class of non-edible oil producing plants widespread in the coastal regions of India, Sri Lanka, East Africa, Australia and Southern Asia. Calophyllum inophyllum is an evergreen tree that grows approximately 8–20 m high (Ong et al., 2011). It grows well in areas with an annual rainfall of 1000–5000 mm (Dweck and Tamanu, 2002) at altitudes from 0 to 200 m. At global level C. inophyllum has been estimated to be the second highest productive feedstock for biodiesel production, i.e. 11.7 kg/tree or 4680 kg/ha of oil (Chauhan et al., 2010) next to palm, i.e. 4000–6000 kg/ha/year (Sahoo and Das, 2009b). Furthermore, the potential to produce oil increases as the seeds of C. inophyllum mature. Indeed, it was reported in prior literature that significant increase in oil content of C. inophyllum occurs upon their maturity, i.e. from 26.3% (6 weeks after anthesis) to 42.7% (after 14 weeks of anthesis) was observed by Hathurusingha et al. (2011a,b). Moreover, the chemical characteristics of the fatty acid methyl esters derived from C. inophyllum oil meet the specifications to use in the diesel engine (Sahoo and Das, 2009a). It’s the fatty acid profile of the oil, which determines the physicochemical properties of the resultant biodiesel (Knothe et al., 2005). The percentage and type of fatty acids composition of Calophyllum inophyllum L. oil vary depending on the quality of the feedstock, growth conditions, the age of the plant and the geographical location in which the plant has grown. According to Slack and Browse (1984), a mature seed contains about 70.8% unsaturated fatty acids and 29.2% saturated fatty acids. The composition of these fatty acids may vary with respect to the maturity of fruits. 1.3. Fatty acid composition of C. inophyllum seed oil The fatty acid composition is an important property for any biodiesel feedstock. It was reported earlier that C. inophyllum oil is mainly dominated by monounsaturated fatty acid (oleic acid C18:1) (39.1 ± 1.4%) followed by polyunsaturated fatty acid (linoleic acid C18:2) (31.1 ± 1.4%) and saturated fatty acids (stearic acid C18:0 (14.3 ± 0.8%) and palmitic acid C16:0 (13.7 ± 0.8%)) (Atabani and César, 2014). Apart from the major contributors, there are few fatty acids 30


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during the transesterification process such as homogeneous catalyst, heterogeneous catalyst or enzymatic catalyst. Finally, the crude biodiesel (FAME) obtained after transesterification is further purified to obtain pure biodiesel fuel suitable for use in the diesel engines.

Table 1 Properties of methyl esters produced from C. inophyllum oil with that of EN and ASTM Standards. Properties

Unit

EN 14214 standard

ASTM D675106 standard

C. inophyllum

Density (15 °C) Cetane number Viscosity (40 °C) Flash point Cloud point Pour point Calorific value Distillation 90% Sulphate ash content Water content Ash content Carbon residue Oxidation Stability FAME content Free Glycerol Methanol content Monoglyceride content Diglyceride content Triglyceride content Acid value

kg/m3 – mm2/s °C °C °C kJ/kg °C %w/w wt.% wt.% wt.% h wt.% wt.% wt.% wt.%

860–900 > 51 3.5–5.0 > 101 – – 35 – 0.02 < 0.05 < 0.02 0.3 max >6 96.5 min 0.25 max 0.20 0.8 max

860–900 > 47 1.9–6.0 > 130 −3 to 12 −15 to 10 – 360 max 0.02 < 0.030 < 0.020 – >3 – – 0.20 –

877.6 57 4 140 2 4.3 41.442 356 0.001 0.005 – – 14.27 98.7 – 0.01 –

wt.% wt.% mg KOH/g

0.2 max 0.2 max 0.5 max

– – 0.8 max

– – 0.34

2.1. Extraction 2.1.1. Soxhlet extraction Soxhlet extraction is the most popular and commonly employed technique for oil extraction. This method works on the principle of extracting lipids from dry or less moisture containing solids. Generally, lipids are soluble in organic solvents (chloroform, hexane, isopropanol, petroleum ether, etc.,) but the solubility shifts with solvent to solvent. Therefore, various properties of solvent should be considered before choosing the appropriate organic solvent for lipid separation as extraction of lipids from a solid material (food, seeds, leaves) might give different yields of fat content when comparing different extraction solvents with varying polarity (Banat et al., 2013). Soxhlet extraction is used when the target molecule (lipids or triglycerides) has solubility in the solvent and the impurities present are insoluble in that solvent. In this process, hexane is used as a solvent in which the target molecule (lipid) has a high solubility, whereas the remaining impurities are insoluble. Moreover, the sample (seeds) used in the soxhlet extraction process should be dried and ground to allow proper percolation of organic solvent through which maximum interaction of lipids and solvent can be achieved. A standard Soxhlet apparatus (Fig. 4) is divided into many parts such as: at the bottom, a heating element with a distillation flask containing organic solvent (hexane) on it, above which is the soxhlet chamber contain porous thimble and the target sample (dried and grounded). On heating, the solvent vapors travel up from the distillation flask and enters the soxhlet chamber via distillation arm. Further on the top, Soxhlet extractor apparatus is equipped with a condenser, which condenses these vapors and channels them into the soxhlet chamber where it passes through the porous thimble and reaches to the dried target sample i.e. seeds. Due to the high solubility of lipids in hexane, it will allow lipids from the target sample to solubilize in the condensed vapors. Later, these condensed vapors are poured back through the siphon exit to the distillation flask. By continuously

References: Ong et al. (2014), Ong et al. (2011), Rutz and Janssen (2006), Alleman et al. (2013).

emissions, however it increases nitrous monoxide emission’ (Jiaqiang et al., 2017; Chaitanya et al., 2015). 2. Processing of Calophyllum inophyllum seed oil Processing of C. inophyllum seed oil is a multistep process as shown in Fig. 3. The initial step of the processing is the extraction of oil from the C. inophyllum seeds, second step involves the formation of fatty acid esters from an extracted oil through the transesterification process. Transesterification is a process involving a chemical reaction between fats̸oils and low molecular weight alcohols (methanol or ethanol) in the presence of a catalyst. It is further divided on the basis of catalyst used

Fig. 3. General scheme of biodiesel production from C. inophyllum seed oil.

Calophyllum inophyllum seeds

EXTRACTION

1. Soxhlet Extraction 2. Super critical fluid Extraction 3. Microwave assisted Extraction 4. Enzymatic Extraction

Calophyllum inophyllum Oil TRANSESTERIFICATION

Homogeneous Catalyst

1585

Solid

Liquid

Heterogeneous Catalyst

Solid

Enzymatic Catalyst

Liquid

Fatty Acid Esters PURIFICATION

1. Wet Washing 2. Dry Washing

Pure Biodiesel

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Fig. 4. Standard Soxhlet Apparatus (Adapted from Wikimedia Commons, 2008.

2.1.2. Supercritical fluid extraction The supercritical fluid extraction (SFE) process has garnered much attention in the last decade, profoundly in food and pharmaceutical industries over other conventional techniques such as organic solvent extraction, steam distillation (Majdi et al., 2012). Moreover, the supercritical fluid extraction process has been applied in an allied area like oil extraction, which has been reported in prior literature such as oil extraction from Apium graveolens L. (celery) seed (Papamichail et al., 2000), Coriandrum sativum L. (coriander) seed (Illes et al., 2000), Prunus avium L. (cherry) seed (Bernardo et al., 2001), Cucumis melo L. (muskmelon) seed (Maran and Priya, 2015), Linum usitatissimum L. (flax) seed (Khattab and Zeitoun, 2013), Helianthus annuus L. (sunflower) seed (Antolin et al., 2002), Corylus avellane L. (hazelnut), Sesamum indicum L. (sesame) seed, Brassica napus L. (rapeseed) (Bernardo et al., 2002), Gossypium arboretum L. (cotton), Olea europaea L. (olive), Glycine max L. (soybean) seed (Hajimahmoodi et al., 2005), and Camellia sinensis L. (tea) seed (Rajaei et al., 2005). Supercritical fluid extraction process works on the principle of extracting oil using solvent at high pressure or at supercritical fluid (SCF) conditions. Generally, supercritical carbon dioxide (when both temperature and pressure increase from standard temperature and pressure conditions) is the most frequently employed supercritical fluid for oil extraction because of its low toxicity, chemical inertness, good safety, and low critical temperature (Pourmortazavi et al., 2005). Compared with conventional techniques SFE has several advantages, such as: (a) the solvent used in SFE process has relatively lower viscosity and higher diffusivity compared with the solvents used in conventional techniques (soxhlet extraction, mechanical disruption followed by solvent separation), (b) the properties of the supercritical fluid can be modulated by changing the pressure and temperature conditions depending on the target sample to gain higher extraction yield, (c) SFE process uses environmental friendly solvents for extraction, (d) due to low temperature of the process and the stability of

repeating this cycle for multiple times the solution gets concentrated in the distillation flask and then the lipids were extracted from it. Recently a study was conducted by Shamsuddin et al. (2015), where they optimized the process of C. inophyllum oil extraction using the Soxhlet method through response surface methodology (based on central composite design). They studied the impact of three parameters on the extraction process, i.e. solvent to seed ratio, extraction time, and drying time. Through their results it was observed that 46.22% yield of C. inophyllum oil can be obtained within 9 h of extraction with drying and with the solvent to seed ratio of 56.82 ml/g. Moreover, an accurate calculation of the fat content of the sample can be done using Eq. (1). According to which the fat percent has been estimated by calculating the difference in the weight of the distillation flask before the beginning of the process i.e. at the empty condition and after the completion of the process i.e. after the evaporation of the solvent from the concentrated solution in the distillation flask (will give total fat extracted from the sample) and divided it with the original weight of the target sample (seeds).

Fat% =

Wf − We × 100 W

(1)

Where, Wf: Weight of flask with extracted fat We: Weight of empty flask W: Original weight of the sample However, soxhlet extraction process has many disadvantages or flaws such as poor extraction of polar lipids, not suitable for high moisture containing seeds, hazards associated with heating of organic solvents, time consuming process (few hours to a couple of days), and the cost input associated with the usage of large amount of solvents, which restricts its use on the commercial scale (de Castro and Garcı́aAyuso, 1998). Thus, an alternative approach is needed which could be economical as well as less time consuming. 32


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Fig. 5. Standard Supercritical Fluid Extraction Apparatus (Sapkale et al., 2010).

2.1.3. Microwave-assisted extraction The concept of microwave-assisted processing has been in the market for about four decades. But its application in the field of extraction has attracted the attention of researcher’s due to its ability to complete the extraction process within very short time, requires less energy input, and less use of chemical solvents (Gude et al., 2013). Even it has been reported earlier by Filly et al. (2014) that microwaves can allow solvent-free extraction of oils from plants. Microwave-assisted extraction process has been successfully implemented for the extraction of natural compounds such as: flavonoids, polyphenols, pigments, antioxidants, isoflavones (Pan et al., 2003; Hong et al., 2001; Kiss et al., 2000; Zigoneanu et al., 2008; Rostagno et al., 2007; Terigar et al., 2010) etc. In addition, there are few other advantages associated with microwave-assisted processing such as rapid thermal modulations, localized superheating, pre-drying of target sample is not required, precise, and controlled processing (Clark and Sutton, 1996; Caddick and Fitzmaurice, 2009; Ku et al., 2002; de la Hoz et al., 2005). With these advantages, microwave assisted extraction could be a best plausible alternative to the conventional solvent extraction technique for oil extraction from biomass. To exploit microwave technique for oil extraction from plant biomass, further knowledge about the microwave characteristics and properties is a prerequisite. Microwaves are electromagnetic waves having a wavelength in the range of 0.01–1 m and frequency in the range of 0.3–300 GHz (Gude et al., 2013). Generally, at commercialscale in industries microwave frequency ranges from 0.9 GHz to 2.45 GHz, whereas, most of the equipment commercially available function at 2.45 GHz frequency. Furthermore, the microwave has energy in the range of 1.24 × 10−6–1.24 × 10−3 eV which is much lower compared with the energy required to ionize any biological compound (13.6 eV) (Chemat-Djenni et al., 2007). Thus, it could be an added advantage with using the microwave for oil extraction as its absorption by the target sample will not denature or affect the molecular structure of lipids (triglycerides) essential for FAME production. The microwave assisted extraction works on a simple principle, i.e. exploiting the dielectric properties of a material. For example, materials having dipoles when irradiated with microwaves will align with the electric field; however, the oscillating nature of the electric field, causes repeated reorientation of the dipoles and the friction between the dipoles that leads to the heat generation thus, resulting into localized superheating. During the microwave assisted extraction, a couple of factors need to be considered, including material properties and their interaction with the microwaves. As all materials respond differently to microwave treatment, i.e. some materials (bulk metals and alloys) reflect microwaves, some (quartz, glass) are transparent to microwaves while others absorb microwaves (aqueous solution and polar solvents) (Li and Yang, 2008; Gude et al., 2013). Further the presence of moisture level in the seeds also affects the extraction efficiency as the microwaves penetrate through the seed, the moisture in the form of water

supercritical carbon dioxide it protects lipids from thermal degradation, (e) SFE takes tens of minutes to complete the process compared with soxhlet extraction which almost uses hours or even days for completion, and (f) separation of the dissolved solutes can be achieved easily by depressurizing supercritical fluid (Lang and Wai, 2001; Khajeh et al., 2004; Bergeron et al., 2005; Yin et al., 2005; Rajaei et al., 2005). For the extraction of oil through this process, a supercritical fluid extraction apparatus is required (Fig. 5). The apparatus is equipped with a pump to push CO2 (solvent) into the pressure cell which maintains the pressure above the critical pressure (7.39 Mpa) in the system. After getting pressurized in pressure cell carbon dioxide enters the thermal cell, where it gets heated above its critical temperature condition (304.25 K) and reaches to a supercritical condition. Later, the supercritical fluid passes through the extraction chamber in which a bed of milled seeds is present. Later, supercritical fluid percolates through the bed of milled seeds and the, lipid molecules get dissolved in it and are swept out from the extraction chamber. Further, it reaches to a separator chamber where the separation of the dissolved molecules takes place by lowering the pressure, the extracted molecules settle down and the carbon dioxide is recycled. A similar extraction process has been employed for C. inophyllum oil by Bingjun (2009) and has obtained 0.48% yield of oil from C. inophyllum when using 35 MPa extraction pressure, 45 °C (318 K) temperature with a CO2 outflow of 20 ml/min for an extraction time of 150 min. Similarly, the yield of oil extracted from Tobacco seed through supercritical fluid extraction was around 11.5% at opimum condition (40.53 MPa, 348 K, 40 min) (Majdi et al., 2012), from flaxseed it was around 36.49% (40 Mpa, 323 K, 2 h) (Khattab and Zeitoun, 2013), from cherry seed it was around 6–9% (313–333 K, 18–22 MPa) (Bernardo et al., 2001). As can be seen, the variation in the yield of the extracted oil through SFE process is governed by many parameters (a) system parameters: pressure, temperature, physicochemical properties of solvent (Abbas et al., 2008), solvent flow rate, (b) sample parameters: type of seed, seed size, seed coat (Papamichail et al., 2000). However, the SFE process has some shortcomings such as high energy input for modulating process temperature and pressure, use of modifiers or co-solvent like methanol or ethanol to increase the solubility of the oils, high pressure and high temperature conditions needed during the process, and the cost associated with the SFE apparatus makes it a cost intensive process. Moreover, the raw material should be freeze dried prior to SFE extraction, to reduce its moisture content to values below 20% as the presence of high amount of water in the sample can interfere with the yield of the oil (Rubio-Rodriguez et al., 2012). Even the extraction efficiency of the process was lower when compared with solvent based extraction (soxhlet extraction) (Bingjun, 2009; Shamsuddin et al., 2015). Therefore, an alternate extraction technology is required, which not only avoids the use of non-environmental friendly solvents, but should be cost effective, and not compromising with the extraction efficiency of the oil. 33


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associated with usage of enzymes, thus, it was not much popular among researchers. However, research has been going on in this area for the last fifteen years. Gai et al. (2013) investigated the enzymatic extraction of oil from Forsythia suspense seeds. Similarly, Zang et al. (2007) reported the enzymatic extraction of oil and protein hydrolysates from rapeseed. A team of researchers from the Philippines (Cruz et al., 2009) has used this approach for the extraction of oil from C. inophyllum seeds where they used 4 crude enzymes, i.e. xylanase, alpha-amylase, cellulose, and protease to extract oil from C. inophyllum seeds. Through their results, it was observed that 1:1 binary mix of xylanase and cellulase or 1:1 binary mix of xylanase and a-amylase can give 95–96% yield of extracted oil. Although, high yield of oil can be obtained through the process, the cost input associated with the usage of enzymes restricts its applicability at the commercial level. Thus, to make this approach to be popular in the future, further in-depth knowledge about the process is required.

molecules in the seed start aligning and realigning with oscillating electric field, with the continuity of this process, leading to the generation of heat because of the friction between the molecules which may cause the water molecules to reach to their boiling point and creates pressure gradient inside the seed which finally results in the breaking of the seed cell wall and causing the mixing of the oil in the extraction solvent (Bhattacharya and Basak, 2006). Moreover, the dielectric properties of the materials (dielectric permissivity) govern their ability to absorb electromagnetic energy and quantifies their conversion to heat (Chaplin, 2015). While, using microwave in the extraction of oil from C. inophyllum seeds (with 51.46% moisture) (Shamsuddin et al., 2015) various parameters that should be focussed on are: the solvent (isopropanol, methanol, acetone, and water) used in microwave assisted extraction should have high dielectric constant for better heat transfer (Spigno and de Faveri, 2009; Pan et al., 2003; Virot et al., 2007; Duvernay et al., 2005), the penetration efficiency of microwave and the moisture level in the seeds. Similar method had been adopted by Kanitkar (2010) for extraction of oil from soybean, rice bran, and Triadica sebifera L. (Chinese tallow tree) seeds. As per his observations, the efficiency of oil extraction through this method was approximately 95% and the time required for the completion was around 20 min. Cooney et al. (2009) tried to explain the microwave assisted extraction kinetics through Arrhenius equation, according to which, microwave assisted extraction process involves the transfer of lipids from seeds to extraction solvent i.e. from the solid phase to immiscible liquid phase. The solubility of the lipid molecules in the solvent is governed by the Gibbs free energy (refer Eqs. (2) and (3)) ΔG = −RT ln K (chemical equilibrium)

2.2. Transesterification of extracted oil Plant oils such as Carthamus tinctorius L. (safflower) oil, rapeseed (canola) oil, vegetable oil, Calophyllum inophyllum oil, jatropha oil, etc., obtained from the plants cannot be used directly as a blend with petroleum-based diesel fuel because of their inherent properties, specifically their viscosity which is incompatible with the internal combustion engine. The high viscosity level of these vegetable oils or waste oils leads to a decrease in the efficiency of the engines when used untreated because of the poor atomization of a fuel in the engine’s combustion chamber (Schuchardt et al., 1998). Moreover, the power and the torque of the engine also decrease with time due to clogging of the injector tip. Therefore, alternative techniques are required to abridge these issues. There are two conceivable approaches in resolving such problems, i.e. either to reduce the kinematic viscosity of the extracted oil or modify the diesel engines such as increasing the injection pressure (Schmidt, 1932; Tatti and Sirtori, 1937). Since modifying engines cannot be considered as a viable solution, researchers have diverted and started focusing on ways such as dilution, thermal cracking (pyrolysis), emulsification and transesterification by which the viscosity of the extracted oil can be altered (Schwab et al., 1987). It had been reported by Ramadhas et al. (2005) that for-biodiesel production from waste or non-edible oil, transesterification technique is the best plausible solution till date. The concept of transesterification existed since mid-1800′s where it was first used by two chemists E. Duffy and J. Patrick for generation of diesel fuel from vegetable oil (Abdalla and Oshaik, 2013). Although, the concept was ahead of that time and moreover using diesel fuel was much cheaper as compared with biomass derived biodiesel at that time. Later in support, a Belgian scientist in 1930s proposed the formation of alkyl esters from vegetable oil using transesterification (Singh and Walia, 2016). In general, the term transesterification means the interchanging of the alkoxy moiety between the alcohol and ester molecules in the presence of a catalyst (used to enhance the rate of reaction) (Schuchardt et al., 1998). Similarly, in terms of biodiesel production from oil, interchanging of alkoxy moiety occurs between the triglyceride and alcohol molecule. Here, the process of transesterification occurs in three sequential and reversible steps in which diglycerides and monoglycerides are the intermediates (Canakci et al., 2006) (Eqs. (4)–(6)).

(2)

K =

(conc. of analyte in solvent phase) × (conc. of solvent in solvent phase) (conc. of analyte in analyte phase) × (conc. of solvent in analyte phase) (3)

As the dissolution of the analyte in the solvent increases with time, the Gibbs free energy becomes the driving force for the reaction to move favorably in this direction and the equilibrium also lies to the right and allows total extraction of lipid molecules into the solvent phase. Although, the solubility of the target molecule in the solvent depends on many parameters such as the chemical interaction of the lipid molecules with the extraction solvent molecules which accounts for the enthalpy and the entropy of the process which depends on the solvent properties and analyte properties (hydrophobicity, hydrophilicity) (Gude et al., 2013). Thus, to optimize the oil yield through microwave assisted extraction one should take extraction kinetics into consideration. Microwave-assisted extraction produces lipids with almost same composition as obtained from conventional techniques, whereas this process is far more economical and less time consuming as it takes only a few minutes for completion, whereas the conventional techniques uses hours or even days for completion. In addition, the efficiency for the oil extraction through this process is far better than the efficiency for oil extraction through the conventional processes (Kanitkar, 2010; Bingjun, 2009; Shamsuddin et al., 2015). Even, selective and rapid extraction of compounds with low solvent and less energy consumption is possible through the microwave-assisted extraction (Pare et al., 1994; Letellier and Budzinski, 1999). Thus, it can be concluded from the above discussion that microwave assisted oil extraction process is a better approach for oil extraction from C. inophyllum seeds when compared with the conventional extraction techniques such as soxhlet extraction (solvent extraction) and supercritical fluid extraction. 2.1.4. Enzymatic extraction Enzymatic extraction is another approach that has been employed for oil extraction from plant biomass. This method has issues of cost

Triglyceride + alcohol ↔ alkyl ester + diglyceride

(4)

Diglyceride + alcohol ↔ alkyl ester + monoglyceride

(5)

Monoglyceride + alcohol ↔ alkyl ester + glycerol

(6)

Together from Eqs. (4)–(6) Eq. (7) was derived: Triglyceride + 3 alcohol ↔ 3 alkyl esters + glycerol (byproduct) 34

(7)


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Table 2 Pros and Cons associated with the catalyst mediated transesterification process. Pros Single-step transesterification: Acid catalyst No soap formation Insensitive to FFA compared with base catalyst

• •

Base catalyst

Solid catalyst

rate is faster than acid catalyst • Reaction Fatty Acid Methyl Ester yield • High reaction temperature compared • Lower with acid catalyst

feedstock value • Low Fatty Acid Methyl Ester (FAME) • High yield with operation stability upto 4–5 cycles

Reaction rate • High separation of the catalyst from the • Easy product Supercritical Higher adaptability to feedstock with • methanol higher water and FFA content downstream processing cost • Less Shorter reaction time • Enzymatic catalyst Operational stability • High rate of byproduct (glycerol) • Higher recovery for feedstock with high FFA • Suitable level. complication of downstream • Reduces process Microwave reaction time • Short of methanol occurs leads to • Evaporation easy downstream processing by product formation • Reduced • Energy efficient process Two-step Transesterification: Chemical catalysts FAME yield • High • Commercially applicable

Cons

Reference

alcohol to oil molar ratio • High action • Corrosive purification cost • High wastewater generation • Huge reaction rate • Slow pretreatment step is needed to lower • Extra down FFA level below 2%. formation • Soap purification cost • High presence reduces catalyst • Water sensitivity energy consumption for refining • Large process cost associated with solid catalyst • High production of salts on continuous usage • Leaching temperature and pressure conditions • High required during process capital investment • High Energy requirement • High • High solvent pumping to use • Expensive reaction time • Longer handling • Culture • Chances of cross contamination

• Not work well with large sample volume. in downstream processing of the • Difficulty final product due to chemical interference operational stability • No intensive process • Cost • Use of extra step for FFA removal

Motasemi and Ani (2012), Atadashi et al. (2013), Nomanbhay and Ong (2017), Freedman et al. (1986), Park et al., 2010

Fukuda et al. (2001), Hideki et al. (2001), Madras and Kolluru (2004), Boz et al. (2009), Leung et al. (2010), Nomanbhay and Ong (2017)

Toda et al. (2005), Zong et al. (2007), Park et al. (2010), Kaur and Ali (2015), Nomanbhay and Ong (2017)

Kusdiana and Saka (2002), Ding et al. (2011), Nomanbhay and Ong (2017)

Fukuda et al. (2001), Leung et al. (2010), Nomanbhay and Ong (2017)

Kanitkar et al. (2011), Gude et al. (2013), Nomanbhay and Ong (2017)

Atadashi et al. (2013), Nomanbhay and Ong (2017)

be speculated that an alkaline catalyst mediated transesterification process is more popular as compared with acid catalyst mediated transesterification (Schuchardt et al., 1998). The most common alkaline catalysts used in the transesterification of oils are potassium hydroxide, potassium methoxide, sodium hydroxide, sodium methoxide, and sodium ethoxide (Lam et al., 2010; Narasimharao et al., 2007; Atadashi et al., 2013; Talha and Sulaiman, 2016). Even though the alkaline mediated transesterification process has few advantages as mentioned in Table 2 and it had been employed for biodiesel production from C. inophyllum oil (Table 3) at laboratory scale, the process has not been accepted at the commercial level due to couple of flaws such as difficulty in final product separation due to catalyst interference, cost associated with the use of chemical catalyst, formation of soap (saponification) which further makes the separation of glycerol and ester phase difficult. In addition, more catalyst loading will be required as part of which will be get utilized in soap formation (Atadashi et al., 2013). Basically, saponification is the process of soap formation through a chemical reaction, i.e. through the alkaline hydrolysis of fatty acid esters (Pinheiro et al., 2017). It was observed in a study (Biodiesel TechNotes, 2006) that soap formation occurs through two sequential steps among which the first step is the hydrolysis of triglycerides occurs in the presence of water, which leads to the formation of free fatty acid, then the next step is the reaction of free fatty acids with an alkaline catalyst which ultimately leads to soap

There are various ways of driving the transesterification process such as (i) conventional heating with acid, base catalysts, and co-solvents, (ii) super-critical methanol conditions with co-solvents and without catalyst (Demirbas, 2005), (iii) enzymatic method using lipases, and (iv) Microwave irradiation with or without catalysts. Further conventional heating was subdivided into two types depending on the phase of the catalyst used such as 1) homogeneous catalysis i.e. all the reactants and the catalyst are in the same phase and 2) heterogeneous catalysis i.e. reactants and catalyst are in different phases. Thus, homogeneous catalysis could be an alternative term for two-step transesterification using a liquid catalyst whereas two-step transesterification involving solid catalyst can be called as heterogeneous catalysis. 2.2.1. Conventional heating with acid or base catalyst In the transesterification process catalysts play a very crucial role. Through prior literature review (Table 3) it was observed that various catalysts such as homogeneous catalysts, heterogeneous catalysts, and enzyme catalysts had been used for biodiesel production from C. inophyllum oil. 2.2.1.1. Single-step transesterification. Initially homogeneous catalysts (which are of two types i.e. alkaline catalysts and acid catalysts) were used for the biodiesel production from oils through transesterification reaction. Based on our understanding through literature review, it can 35

Industrial Crops & Products 114 (2018) 28–44 Dawodu et al. (2014a) Dawodu et al. (2014b) Ayodele and Dawodu (2014) Arumugam and Ponnusami (2014a) Arumugam and Ponnusami (2014b)

formation. Thus, through our above discussion on the fatty acid profile of C. inophyllum oil it was noted that the oil has Linoleic acid in high amount which is directly correlated to the high FFA content (Hathurusingha et al., 2011a,b; Dmytryshyn et al., 2004). Due to the presence of high free fatty acid content, transesterification with alkali based catalyst yields a considerable amount of soap. Thus, the problems associated with alkaline catalysts have urged researchers to focus on acid catalysts as an alternative solution. Acid catalysed transesterification process was found to be a good alternative to this problem. Strong acid catalysts such as sulfuric acid (Freedman et al., 1986; Harrington and D'Arcy-Evans, 1985) and sulfonic acid (Stern and Hillion, 1990) are preferred for transesterification reaction (Schuchardt et al., 1998). Acid catalyses free fatty acid reaction with methanol solvent and form methyl esters, which lead to a considerable reduction in the free fatty acid level in the oil. However, the process has many disadvantages (Table 2), of them the one major disadvantage is its slow reaction rate. A study conducted by Freedman et al. (1986) showed that the transesterification of soybean oil with methanol at 65 °C and in the presence of sulfuric acid, took almost 50 h for completion. Moreover, no such study has ever been carried out with C. inophyllum oil because no probable solutions to the problem of slow reaction rate associated with use of acid catalyst has been discovered till now. Thus, issues associated with single-step transesterification had urged researchers to contemplate different technique. A few researchers proposed an alternative approach to produce biodiesel from waste oils with high FFA value through a two-step process, i.e. acid esterification as a pre-treatment step followed by the alkaline transesterification step (acid–base catalysed transesterification) (Wang et al., 2007; Gerpen and Knothe, 2005; Zhang et al., 2003).

4h 5h 4–5 h 6h 72 h ∼99% 96.6% ∼99% 94% 92% batch 87.5% continuous 7.5 wt.% 7.5 wt.% 5 wt.% 6.25 wt.% 20% cell conc.

2–3 cycles 2–3 cycles 5 cycles ≥10 cycles 6 cycles

Jahirul et al. (2014) Chavan et al. (2013) Ong et al. (2014) Ong et al. (2011) Ramaraju and Kumar (2011) SathyaSelvabala et al. (2011) Deepalakshmi et al. (2015) – – – – – – – 120 min + 90 min 90 min + 90 min 180 min + 120 min 120 min + 120 min 60 min + 30 min - + 60 min 120 + 75 min ∼93% 82–83% 98.92% 89% 92.5% 93% 97% 10% + 1% 0.7% + 1.2% 1% + 1% - + 1.25% 0.75% + 1% 1% + 1% - + 6 wt. %

Silitonga et al. (2014) Habon (2009) – – 60 min 90 min 98.53% 84.22% 1% 1%

Reference Time Duration

30:1 + 7.5:1 6:1 + 8:1 9:1 + 9:1 4:1 + 8:1 ± 0.2 9:1 + 6:1 ± 12:1

55 55–60

75 + 55 50 + 65 60 + 50 60 + 60 60 + 60 60 + 60 60 + 65

180 180 180 35 35

KOH NaOH Two-step processes: H2SO4 + NaOCH3 H2SO4 + KOH H2SO4 + NaOH H2SO4 + KOH H2SO4 + KOH Phosphorylated b zeolite + KOH H2SO4 + calcium based (CaO) catalyst derived from lime sludge

Single-step Processes: Sulphonated incomplete carbonized glucose Sulphonated incomplete carbonized C. inophyllum defatted cake Sulphonated microcrystalline Cellulose catalyst Pure Lipase enzyme immobilize on mesosporous SBA-15 R oryzae cells (immobilized under polyurethane)

2.2.1.2. Two-step transesterification using liquid catalyst (homogeneous catalysis). Till today two-step transesterification is the most popular approach and even more it has been widely accepted on a commercial level. Two-step transesterification comprised of two sequential steps, acid pre-treatment step followed by alkaline treatment step. 2.2.1.2.1. Pre-treatment. The purpose of including a pre-treatment step is to reduce the free fatty acid level, present naturally or produce through hydrolysis of triglycerides, in the oil. In general, the acceptable range of free fatty acids (FFA) in the oils for biodiesel production is around 2% or less (Nomanbhay and Ong, 2017). While considering the fatty acid count of C. inophyllum oil which was found to be around 12% with acid value of 15.5 mg KOH g−1 (Hariram and Kumar, 2012) necessitates a pre-treatment step for biodiesel production. Moreover, it was investigated by Canakci (2007) on soybean oil that with the increasing FFA level from 5% to 33% a significant drop in the yield of methyl esters occurred i.e. from 90.84% to 58.77%. Pre-treatment step involves acid esterification of free fatty acids in the presence of solvent (methanol) and an acid catalyst (Eq. (8)):

30:1 30:1 15:1 6:1 12:1

9:1 –

Temperature (°C) Catalyst

Table 3 Comparative analysis of catalysts mediated transesterification from C. inophyllum oil.

Methanol to oil Ratio

Catalyst conc. (%)

FAME yield (%)

Operational stability

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(8) Where, R is a fatty acid alkyl group Together, it looks to be a straightforward and simple process, but it plays a critical role in optimizing the yield of the end product. However, acid esterification step depends on multiple factors such as temperature, molar ratio of methanol to oil, catalyst concentration, time duration, etc. Thus, to enhance the conversion rate of FFA to methyl esters further optimization is needed. Various studies have been conducted discussing the effect of these parameters on methyl ester formation from oil (Jahirul et al., 2014; Ong et al., 2014; Chavan et al., 2013; Ramaraju and Kumar, 2011; Ong et al., 2011). Thus, it can be concluded that the acid pre-treatment step reduces the excess free fatty acid concentration in the oil and alleviate the chances of soap formation in the alkaline transesterification step. But, it may result into the lower yield of biodiesel and higher cost for 36


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downstream processing as separation of final product will be difficult in the presence of soap (Nomanbhay and Ong, 2017). Although acid pretreatment step reduces free fatty acid concentration, it also leads to water generation which adds up the capital investment required and an extra step in the downstream processing operation. Thus, it opens the scope of further research in the area and to improve the efficiency of the process further. 2.2.1.2.2. Alkaline transesterification. The pre-treatment step is then followed by the alkaline transesterification step. It is the process of production of biodiesel from triglycerides in the presence of a base catalyst. Through prior literature review, it is known that so far various base catalysts, such as KOH (Venkanna and Reddy, 2009; SathyaSelvabala et al., 2011), NaOH (Ong et al., 2014; Habon, 2009), Ba(OH)2 (WeiWei et al., 2008), NaOCH3 (Jahirul et al., 2014) etc., have been used in transesterification process. It was noticed that methoxide catalysts are considered better than other catalysts as it causes less saponification of triglycerides and higher biodiesel yield (though not significant) comparable to the hydroxide catalysts (Biodiesel TechNotes, 2006). Moreover, it was also observed through literature survey that other than the catalyst there are various other parameters such as type of feedstock, moisture content in feedstock, reaction temperature, methanol to oil ratio etc. on which the efficiency of transesterification depends (Melo-Junior et al., 2009). The product obtained from acid esterification contains triglycerides, unconverted free fatty acids, methyl esters, methanol, acid, and water. It is apparent from our above discussion that alkaline transesterification process is sensitive to both free fatty acid content and water content. Though it is evident from the pre-treatment step or the esterification process as mentioned above that it decreases the free fatty acid content to a significant level, but the presence of water content in the oil can lead to the formation of free fatty acid by hydrolysis of triglycerides which finally triggers a saponification reaction (Atadashi et al., 2012). Therefore, for better yield of methyl esters and reduced soap production a close check on the water content in the esterified oil is necessary. Till date two-step transesterification has been the most popular method and commercially acceptable for biodiesel production. Various studies had been conducted for biodiesel production from C. inophyllum oil using this technology (Table 3). Moreover, from Table 3, it can be observed that ˃90% biodiesel yield was obtained through this process within very less treatment time and with low energy input. With advantages on one hand, the process has a few disadvantages on the other such as costs associated with solvent and catalyst, corrosive action associated with acid catalyst and difficulty in separating liquid catalyst from the final product, has forced researchers to contemplate another approach.

(Carmo et al., 2009) for fatty acid esterification (Diamantopoulos et al., 2015). Although the use of solid acid catalyst in two-step transesterification has several advantages, but the major drawback associated with this approach is the high capital investment in the preparation of solid catalyst (Toda et al., 2005; Nomanbhay and Ong, 2017) and low thermal stability of the catalyst. Researchers have proposed numerous ideas to curtail the problem of high capital investment. In 2008, Chung and his team proposed an idea of using zeolites and modified zeolites as a solid catalyst because of their effectiveness in removing free fatty acids from non-edible oil (Chung et al., 2008). Further a team of researchers used Zeolite (or modified zeolite) as a solid catalyst in the esterification of C.inophyllum oil (SathyaSelvabala et al., 2011). In their study, they utilize b-zeolite and phosphoric acid modified-b-zeolite as the catalyst in the pre-treatment step. It was observed through their results that modified-b-zeolite gave better fatty acid methyl ester yield compared with the unmodified form of the catalyst keeping other parameters constant. The difference in the fatty acid methyl ester yield was because of the difference in the activities of the two catalysts which was further correlated with an increase in the hydrophobicity after modification. Initially unmodified catalyst has sites which were preoccupied by water molecules, but on treatment with phosphoric acid the hydrophobicity of the catalyst increases leading to decrease in the preoccupied sites which are available for catalytic conversion (SathyaSelvabala et al., 2010). The approach of using phosphoric acid modified-b-zeolite (Pb) for the esterification in two-step approach dominates compared with other heterogeneous solid acid catalysts which were mentioned in previous literatures. Not only it gave the esterified product with high conversion efficiency, but major advantage of this approach is the high operational stability of catalyst Pb SathyaSelvabala et al., 2010). It is possible to speculate that this study has provided a useful solution to our problem of contamination of the final product because of mineral acids; in addition, its reusability and cost effectiveness compared with other solid acid catalysts made it a plausible technique for commercial scale operations. Although, with numerous advantages it is still a two-step process i.e. in transesterification step mineral base catalyst is used and in purification process base catalyst separation have to be employed. Moreover, contamination of the final product through the leaching of the salts from the modified catalyst on repeated use is the major concern with this approach and with every cycle there is a decrease in the fatty acid methyl ester yield (Nomanbhay and Ong, 2017). To avoid the issues associated with the two-step approach, using solid catalyst, researchers have opted for a different approach, i.e. single-step transesterification using solid catalyst.

2.2.1.3. Two-step transesterification using solid catalyst (heterogeneous catalysis). It’s more than a decade now when people first employed solid catalyst for the alkyl ester production through fatty acids. Twostep transesterification using a solid catalyst is comparatively a newer approach and it works with the similar operating conditions of the process that with the liquid catalyst with few advantages. The switch over from liquid mineral acid catalyst to solid acid catalyst occurs when the disadvantages such as no recyclability of acid catalyst, difficulty in separation from the final product, corrosive action, require high temperature conditions for operation, hydrolysis of esters in presence of water (Park et al., 2010), and an extra water washing step before transesterification step associated with the use of liquid acid catalyst in esterification step came into notice (Kaur and Ali, 2015). The solid acid catalyst approach provides the probable solution to the problems associated with the homogeneous liquid catalyst without compromising on the biodiesel yield. A few literature studies reported the use of solid acid catalysts such as tungstate zirconia (Lopez et al., 2008), sulfated zirconia (Garcia et al., 2008), carbohydrate derived solid acid catalyst (Lou et al., 2008), ion exchange resins (Marchetti et al., 2007), mesoporous aluminosilicate Al-MCM41

2.2.2. Single-step transesterification using solid catalyst It is a one-step process wherein esterification of free fatty acids and transesterification of triglycerides are taking place simultaneously. Moreover, the process eliminates the use of liquid catalyst which avoids the contamination of final product and yield better and cost effective downstream processing (Baig and Ng, 2010). Further advantages associated with it are: (i) saves time as few intermediate steps like pretreatment, water washing, and drying were eliminated, (ii) user friendly approach as it avoids usage of strong acids and bases which leads to corrosive reactions (iii) minimize the cost of the process associated with the wastage of chemicals by utilizing recyclable catalyst. Significant amount of work has been going on in biodiesel production from solid catalyst for the past few years. Various solid catalysts such as metal hydroxides (Dalai et al., 2007), metal oxides (Granados et al., 2007; Macedo et al., 2006; Deepalakshmi et al., 2015; Wang and Yang, 2007; Yacob et al., 2011), and metal complexes (Abreu et al., 2003) have been investigated. Eventually, a team of researchers (Toda et al., 2005) came up with an idea of using low cost carbon-based solid catalyst for biodiesel production to avoid utilization of costly metal catalysts. Toda et al. (2005) in their study used sugars (glucose and 37


M. Jain et al.

be controlled by adapting proper desiccating techniques. Apart from the feedstock the process has its own limitations such as no recovery of solvent, high energy input in the process, usage of liquid chemical catalyst which add difficulties to the final separation step, triggering of saponification reaction which results into lower biodiesel yield comparable to other transesterification methods (Table 3). However, the process has couple of advantages such as averting of separation or water washing step in between extraction and transesterification step which directly benefits the purification step and the required processing cost for it. Secondly, it avoids the use of harmful chemicals unlike solvent extraction approach. The disadvantages and lower biodiesel yield associated with insitu transesterification process has urged researchers to employ alternative approach such as the use of solid base catalyst for biodiesel production from C. inophyllum oil as done by Qian et al. (2015). It can solve the problem of catalyst separation after the reaction and in addition the operational stability of solid catalyst can provide cost advantage to the process. Similarly, Barekati-Goudarzi et al. (2016), employed insitu transesterification for biodiesel from Chinese tallow tree oil in a microwave batch system and they obtained biodiesel yield of 89.14% in very short time as the rate of extraction and conversion to biodiesel increased significantly. Thus, alternative strategies applied by Qian et al. (2015) and Barekati-Goudarzi et al. (2016) can be employed effectively for biodiesel production from C. inophyllum oil to have better biodiesel yield while keeping cost effectiveness of the process in mind.

sucrose) as the carbon source, which were then incompletely carbonized and sulphonated to make active catalyst. Carbon based solid catalysts are more eco-friendly and less expensive as compared with other chemical based solid catalysts (Toda et al., 2005; Zong et al., 2007). Moreover, it was stated in prior literature that sulphonation of carbon based solid catalyst showed higher activity than other solid acid catalysts tested (sulphated zirconia, amberlyst-15, and niobic acid), probably because of higher catalytic activity due to the strong hydrogen bonding of SO3H groups in sulphonated catalyst to water (intermediate product of esterification reaction) which results in strong acidity (Hara, 2010; Zong et al., 2007; Toda et al., 2005). Later, various studies have been conducted with carbon source as a solid catalyst such as D-glucose (Zong et al., 2007), biomass (Dawodu et al., 2014b), and cellulose (Ayodele and Dawodu, 2014) for biodiesel production. A similar strategy has been employed for biodiesel production from C. inophyllum oil by Dawodu et al. (2014a, 2014b), wherein they used C. inophyllum cake (remaining after oil extraction) and glucose as a source of solid catalyst. Through their study, it was observed that C. inophyllum defatted cake (low cost feedstock) derived catalyst produces fatty acid methyl ester yield comparable to that of glucose derived catalyst and a significant difference in the cost of the process was also observed. Although, high fatty acid methyl ester yield was obtained with C. inophyllum cake and glucose catalyst but still the process has one major drawback such as the lower stability of the catalyst. Later, Ayodele and Dawodu (2014) used another catalyst i.e. sulfonated microcrystalline cellulose (MCC) for biodiesel production from C. inophyllum oil. The cellulose derived catalyst gave approximately 99% FAME yield at optimized conditions with high operational stability compared with glucose and C. inophyllum cake catalyst. Moreover, it was also observed that the process involving microcrystalline cellulose catalyst had lesser methanol and catalyst feeding as compared with prior discussed carbon-based solid catalysts (Table 3). Thus, it can be concluded that, sulfonated microcrystalline cellulose catalyst is a plausible option among low cost catalyst which provide higher biodiesel yield and operational stability which can be further used for transesterification of C. inophyllum seed oil. Although, the single-step solid catalyst transesterification has several advantages (Table 2) but one major drawback associated with the process is the leaching of the bound SO3H group from the catalyst with every cycle resulting in the contamination of the final product and reduction in the fatty acid methyl ester yield. This spur further research in the area for devising more robust catalyst which has high stability without compromising on the biodiesel yield.

2.2.4. Supercritical methanol transesterification Recently, the supercritical methanol transesterification method has gained significant importance as an alternative to conventional transesterification methods, as the process works on high temperature (280–400 °C) and pressure (100–300 bar) conditions and avoids the use of catalyst (Marulanda, 2012; Samniang et al., 2014; Manuale et al., 2015; Salar-García et al., 2016; Ortiz-Martínez et al., 2016; AndreoMartínez et al., 2017). Moreover, the setbacks such as high water and free fatty acid content in the oil, and end-product contamination with catalyst and glycerol associated with conventional methods, were not considered as setbacks for supercritical methanol transesterification method (Kusdiana and Saka, 2002; da Silva and Oliveira, 2014). Furthermore, supercritical methanol transesterification causes an increase in the transesterification rate and simultaneously promotes esterification of free fatty acids without any catalyst (Pinnarat and Savage, 2008). A study was done by Salar-García et al. (2016) for biodiesel production from jatropha oil using supercritical methanol transesterification method. Through their study, it was observed that higher yield (99.5%) of biodiesel was obtained within 90 min at 325 °C and 42:1methanol/oil molar ratio, similarly García-Martínez et al. (2017) achieved 92.8% FAME yield from tobacco oil at 300 °C for a process time of 90 min. Moreover, the quality of the biodiesel is well in range of the standards. While considering C. inophyllum oil, which has high content of free fatty acid, transesterification through this approach will be a better alternative as it not only provides simultaneous esterification of free fatty acids, but also reduces the time and cost associated with extra step of pre-treatment present in conventional two-step transesterification process and moreover it gives better biodiesel yield comparable to traditional approaches. Recently, a group of researchers from Thailand had adopted supercritical methanol methodology for biodiesel production from C. inophyllum oil and their result shows 90.4% yield of fatty acid methyl ester and the physical properties of the biodiesel produced were comparable to the biodiesel European 14214 standard (Tipachan et al., 2017). In Supercritical methanol transesterification, temperature and pressure conditions (240 °C and 8.09 MPa supercritical conditions) (Gil et al., 2008) are exploited in such a way that methanol solvent start behaving as a supercritical fluid, i.e. it behaves as a compressible fluid having a density in between that of liquid and gas. Moreover, there are

2.2.3. In situ transesterification (single-step process) A study was conducted by Dalvi et al. (2012), wherein they concocted a novel idea of insitu transesterification, i.e. simultaneous extraction and transesterification for biodiesel production from C. inophyllum oil. Later, a similar approach has been employed for other crops like jatropha (Qian et al., 2015), rapeseed (Qian et al., 2013), rubber (Abdulkadir et al., 2015), Chinese tallow tree (BarekatiGoudarzi et al., 2016). The aim of insitu transesterification process was to avoid an extra separation step between the extraction and transesterification step for biodiesel processing. Moreover, insitu transesterification helps in downsizing three step processing (first extraction, then transesterification followed by purification) to a two-step processing (extraction and transesterification simultaneously followed by purification). Dalvi et al. (2012), studied the effect of various parameters on insitu transesterification approach for biodiesel production from C. inophyllum oil. From their results, it was concluded that the moisture content in seeds and oil plays an important role in altering the yield of fatty acid alkyl esters. They showed that with increasing moisture content the rate of hydrolysis of triglycerides increases, results into formation of free fatty acids (FFA) and soap formation, thus significantly decreases the fatty acid esters yield. However, the moisture content in the seeds can 38


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(2014a), studied the production of biodiesel from C. inophyllum oil using immobilized lipase enzyme where they use mesosporous silica material (Park et al., 2006) obtained from sugarcane leaf ash (Mesosporous SBA-15) for immobilizing lipase enzyme. In another study, Arumugam and Ponnusami (2014b) opted different approach, in place of immobilizing lipase enzyme, they immobilized fungal cells (which have the capability to produce lipase enzymes) using reticulated polyurethane foam; for the production of biodiesel from C. inophyllum oil. If considered both the approaches opted by Arumugam and Ponnusami for biodiesel production from C. inophyllum oil, it can be observed, that immobilizing enzyme has better biodiesel yield, high operational stability with less process duration as compared with that of immobilizing lipase producing organism approach. The use the enzyme lipase as a biocatalyst for the transesterification reaction step in biodiesel production has been extensively investigated. The advantages of using lipases in biodiesel production are: (a) many lipases show considerable activity to catalyze transesterification with long or branched chain alcohols, which can hardly be converted to fatty acid esters in the presence of conventional alkaline catalysts, (b) products and byproduct separation in downstream process are extremely easier, (c) the immobilization of lipases on a carrier has facilitated the repeated use of enzymes after removal from the reaction mixture and when the lipase is in a packed bed reactor, no separation is necessary after transesterification and (d) higher thermostability and short-chain alcohol-tolerant capabilities of lipase make it very convenient for use in biodiesel production (Bacovsky et al., 2007). With various advantages (especially high operational stability, low cost of downstream processing operations, and almost similar FAME yield as compared with chemical methods) associated with this process, it could be suggested that lipase-enzyme method could be used for biodiesel production for C. inophyllum oil.

other properties of methanol that get changed at supercritical condition. The viscosity and surface tension of methanol decreases which leads to effective mass transfer, with increase in temperature the ionic strength of the polar solvent decreases and start behaving as a nonpolar solvent due to which the solubility of oil or lipids further increases in the solvent, and the hydrogen bonding between methanol molecules decreases and they start behaving as an acid catalyst during the transesterification reaction (Modi, 2010). Although this process has several advantages over conventional transesterification approaches, the major setback associated with supercritical transesterification methanol is the high capital investment (for high methanol input (42:1), preheating, and recycling) (Marulanda, 2012) due to which its application at commercial level is difficult. According to the study conducted by West et al. (2008), for biodiesel production from Hysys plant oil, the total capital investment was almost 3.5 times more with supercritical methanol transesterification as compared with conventional acid transesterification. However, several researchers have tried to reduce the cost associated with methanol feed (42:1 methanol-oil ratio) during the transesterification process by increasing the temperature above 350 °C and reducing methanol input, but the onset of decomposition reactions at this temperature had affected the quality of the biodiesel (Imahara et al., 2008; Quesada and Olivares, 2011). Moreover, another disadvantage associated with this approach is the high-energy input required to maintain the supercritical conditions. Thus, keeping the economical perspective of the process in mind it can be concluded that this process of transesterification is good only for laboratory scale. 2.2.5. Enzyme mediated transesterification In the current scenario, two-step (acid-alkaline) transesterification method for biodiesel production is quite popular at industrial level, however, the issues related with the process such as soap formation (saponification), corrosion, difficulty in the separation of the end product have urged researchers to concoct different alternative techniques. Thus, researchers contemplated the use of enzymes (lipases) as a catalyst for biodiesel production. The biodiesel production through enzymatic transesterification is a multi-step process involving hydrolysis of triglycerides followed by the synthesis step involving esterification and transesterification reactions (Vulfson, 1994). In the biodiesel production through enzymatic process, water plays an important role. Generally, the enzymatic methanolysis process takes place on water and oil boundary layer where lipase enzyme catalyses the hydrolysis of triglycerides (Lv et al., 2010). Enzyme catalysed transesterification process have a few advantages (Table 2) over chemical catalysed transesterification process, such as: It avoids the use of any toxic or corrosive chemicals, Very high operational stability i.e. the enzyme can be recycled up to 10 times (Arumugam and Ponnusami, 2014a), Enzymes can esterify both free fatty acids and triglycerides in one step without the need for subsequent steps like degumming, pre-treatment etc., easy recovery of by-product glycerol (Fukuda et al., 2001), Enzymatic transesterification can take place in a wide temperature range due to the thermostability of the lipase enzymes (Shah et al., 2003). However, the major disadvantage associated with the enzymatic transesterification is the high cost input associated with the use of pure enzymes and downstream processing operations. To alleviate this issue, researchers contemplated different ways, such as: efficient production of enzymes from microorganisms i.e., use of microbes in place of using pure enzymes (Vipin et al., 2016; Arumugam and Ponnusami, 2014b; Gog et al., 2012; Openshaw, 2000; Wu et al., 1998; Nelson et al., 1996), use of immobilized enzymes to reduce cost associated with downstream processing operations (Arumugam and Ponnusami, 2014a; Kumari et al., 2009; Abigor et al., 2000; Samukawa et al., 2000; Antolin et al., 2002; Hama et al., 2007). Recently a few researchers have employed this approach for biodiesel production from C. inophyllum oil. Arumugam and Ponnusami

2.2.6. Microwave-assisted transesterification Using microwave for transesterification is a newer approach and it is still in its infancy stage due to lack of research and knowledge in this domain. Patil and Deng (2014) studied microwave enhanced in-situ transesterification of algal biomass for biodiesel production. However, till now no such study had been performed on biodiesel production from C. inophyllum oil. Thus, based on the knowledge gained from the literature review on microwaves and biodiesel production from other plant oils (Bokhari et al., 2015; Zhang et al., 2010; Yaakob et al., 2008; Geuens et al., 2008; Boldor et al., 2010; Refaat et al., 2008), it can be suggested that this technique can provide viable solutions to the current problems of biodiesel production, moreover it can set a new benchmark for biofuels production in near future. Microwave-assisted treatments generally work on the polarization phenomenon and the interaction of microwaves with the material and solvent depends on the latter’s polarity which can be governed by their dielectric properties. Among the alcohols (ethanol or methanol) used as a solvent in transesterification process, methanol has been a suitable alternative for microwave-assisted reaction because of its stronger microwave absorption capacity (Yuan et al., 2009) and the lower cost of solvent. Moreover, the smaller size of the methanol molecule provides free localized rotations which result in localized superheating which assists the reaction to complete at a faster rate, whereas with the increasing size of alcohol molecule the hindrance in the localized rotation of the molecules also increases result in slowing down of the reaction (Tierney and Lidstrom, 2005; Patil and Deng, 2014; Perreux and Loupy, 2001). From the results of earlier studies such as Bokhari et al. (2015) obtained 98.9% methyl ester conversion with kapok (Ceiba pentandra) seed oil using microwave assisted transesterification with a reaction time of 3.29 min, similarly Barekati-Goudarzi et al. (2017) obtained 97% methyl ester conversion with Chinese tallow tree oil within 24 min, it can be observed that higher conversion rate within very short duration can be obtained through microwave-assisted

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2.4. Performance, combustion and emission characteristics of a combustion engine fuelled with C. inophyllum biodiesel

transesterification. Thus, microwave-assisted transesterification for biodiesel production from C. inophyllum oil could be advantageous comparable to other techniques (Table 2). It will enhance the rate of reaction through thermal effect or localized super heating and reduce activation energy of the reaction to favour multistep mediated transesterification reaction to move in a forward direction (Gude et al., 2013). It may also be helpful in evaporation of the solvents (Loupy et al., 1993; Yuan et al., 2009) from the process streams which may further reduce the cost of any purification steps. In addition, the time required to obtain a desired yield of biodiesel can be reduced by almost 30–60 folds comparable to supercritical and conventional approaches (Lertsathapornsuk et al., 2003, 2005). Although the process has many advantages, still there are a few limitations such as the penetration ability of microwave decrease with an increase in the sample volume and the non-uniform temperature distribution in large scale reactors, that urge further research in this arena to bring this technique to commercial levels for industrial biodiesel production. Altogether, the efforts of a few researchers in this field have garnered the interest of other people towards the potential of C. inophyllum as an ideal substitute for biodiesel production comparable to other major oil producing crops like jatropha.

The potential of biodiesel from C. inophyllum seed oil its compatibility with the combustion engine is of utmost importance. In recent years, many research studies have been conducted on the evaluation of engine specific characteristics such as brake thermal efficiency, brake specific energy consumption, unburned hydrocarbon emission, oxides of nitrogen emission, ignition delay, heat release rate etc. of the combustion engine fuelled with the biodiesel obtained from C. inophyllum oil. Dinesh et al. (2016) studied the stability, fuel properties, and emission characteristics of a C. inophyllum biodiesel blends in a CI engine performance and observed that B10 has lower hydrocarbon, carbon monoxide, and nitrogen oxides emission and has better efficiency and higher heat value as compared with other blends and their results were similar to the results obtained by Ong et al. (2014). Ashok et al. (2017a,b) compared the effect of C. inophyllum biodiesel blends with diesel fuel on the engine performance by measuring brake thermal efficiency, brake specific energy consumption, and brake specific fuel combustion. From their results, it was observed that diesel fuel has better efficiency however with increasing biodiesel-diesel blend, brake thermal efficiency decreases, whereas brake specific fuel consumption and brake specific energy consumption increases. Also, they studied the emission and combustion characteristics which shows that biodiesel blends have better emission characteristics as compared with diesel fuel. Thus, from CI engine characteristic studies, it can be concluded that C. inophyllum biodiesel has vastly similar characteristics to diesel fuel and can be used as an alternative to diesel fuel.

2.3. Purification The biodiesel obtained after the transesterification process is not pure enough to comply with the specifications prescribed by the Biodiesel standards such as EN14214 or ASTM D6751 (Stojković et al., 2014) for commercial diesel engines. Due to the presence of a variety of impurities like, free fatty acids, glycerol produced as a by-product of the reaction, methanol or ethanol reaction solvents, unconverted triglycerides, monoacylglycerols, diacylglycerols, water, catalyst, and soap, there is a need for an additional step of purification or downstream processing to obtain pure biodiesel. A stepwise approach has been preferred to obtain crude biodiesel from transesterified product. Since the solubility of the glycerol in the esters is very less and the difference in the densities is very large, the separation of glycerol and esters is very fast. Later, biodiesel is mixed with mineral acids such as hydrochloric acid or sulphuric acid to convert soap in the biodiesel into free fatty acid and inorganic salt. Similar study was conducted by Tan and Raman (2013), where they use Taguchi methodology to design the experiment to investigate the soap removal from biodiesel through acidification. After separation of glycerol, transesterified product should be neutralized either through mineral acid, or base, depending on the pH of the product, as it was mentioned in prior literature, that at neutral pH the separation of the reaction mixture was fast (Stojković et al., 2014). The mixture obtained after neutralization stage has impurities like free fatty acids, unconverted triglycerides, water, salt, etc. To remove FFA (mono-di acylglycerides), an organic solvent such as methanol can be used, as esters get easily dissolved in methanol and the remaining impurities settled down and separated out. Further, solvent (methanol) separation can be carried out through vacuum flash vaporization. Later, in the end, water washing can be employed to remove traces of soap, glycerol, FFA, methanol, and salt which were left over after subsequent processing steps. There are two ways of washing, i.e. (1) Dry washing and (2) Wet washing. Both these methods have advantages and disadvantages. Dry washing is simple in its methodology where, significant reduction in waste water can be observed, is effective for continuous operations, total time of production decreases in dry washing, and it requires less working space. Whereas the wet washing is effective for obtaining the high purity of biodiesel (99%) which met standards of ASTM, moreover it is cheaper than the dry washing method. Hence wet washing approach, if required, would be preferred for purifying biodiesel obtained after transesterification of Calophyllum inophyllum oil.

3. Future perspective There has been a significant increase in the research in biodiesel production from C. inophyllum seed oil in the last few years. Researchers all over the world have started exploring the untapped potential of C. inophyllum seed oil as a valuable feedstock for biodiesel production in the future. Going forward, to improve the efficacy of the fuel usage, one can look into optimizing the efficiency of the combustion engine fuelled with higher blends of biodiesel and diesel fuel, alongside look for cost effective techniques with higher biodiesel production efficiency for C. inophyllum. Once, C. inophyllum biodiesel is made commercially acceptable and is in comparison more viable over carbon based diesel fuel on a large scale, it will most definitely lead to a significant decrease in the level of greenhouse gas emissions, promoting sustainable form of energy balance worldwide. In addition, C. inophyllum oil has various industry based applications such as bio-lubricants, alkoxides etc. (Borugadda et al., 2017; Singh et al., 2017) which further emphasizes the vast potential of this seed oil. 4. Conclusion After detailed literature review of the all the technological processes mentioned that go in the making of C. inophyllum biodiesel in this paper, extraction through Microwave followed by enzyme transesterification reactions would be the optimal process. With a blend of diesel fuel up to 10% (B10) Calophyllum inophyllum biodiesel can be ultimately used in the diesel engines of today without any modification. Calophyllum inophyllum biodiesel is well in scope of ASTM D6751 biodiesel standards (Table 1). Therefore, in conclusion it can be said that non-edible Calophyllum inophyllum seed oil is an attractive feedstock in the future of producing biodiesel in a sustainable process. Conflict of interest We would like to confirm that there are no known conflicts of interest associated with this publication and would also like to confirm that the manuscript has been read and approved by all the authors. 40


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Acknowledgement

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