Critical review on sesame seed oil and its methyl ester on cold flow and oxidation stability

Mujtaba, M. A. et al. Article Critical review on sesame seed oil and its methyl ester on cold flow and oxidation stability Energy Reports Provided in Cooperation with: Elsevier Suggested Citation: Mujtaba, M. A. et al. (2020) : Critical review on sesame seed oil and its methyl ester on cold flow and oxidation stability, Energy Reports, ISSN 2352-4847, Elsevier, Amsterdam, Vol. 6, pp. 40-54, https://doi.org/10.1016/j.egyr.2019.11.160 This Version is available at: https://hdl.handle.net/10419/244014 Standard-Nutzungsbedingungen: Die Dokumente auf EconStor dürfen zu eigenen wissenschaftlichen Zwecken und zum Privatgebrauch gespeichert und kopiert werden. Sie dürfen die Dokumente nicht für öffentliche oder kommerzielle Zwecke vervielfältigen, öffentlich ausstellen, öffentlich zugänglich machen, vertreiben oder anderweitig nutzen. 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If the documents have been made available under an Open Content Licence (especially Creative Commons Licences), you may exercise further usage rights as specified in the indicated licence. https://creativecommons.org/licenses/by-nc-nd/4.0/ Energy Reports 6 (2020) 40–54 Contents lists available at ScienceDirect Energy Reports journal homepage: www.elsevier.com/locate/egyr Review article Critical review on sesame seed oil and its methyl ester on cold flow and oxidation stability M.A. Mujtabaa,b,∗, Haeng Muk Choc,∗∗, H.H. Masjukia,d, M.A. Kalama,∗∗, H.C. Onga, M. Gula,e, M.H. Haritha, M.N.A.M. Yusoffa aCenter for Energy Science, Department of Mechanical Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia bDepartment of Mechanical Engineering, University of Engineering and Technology, City Campus Lahore, Pakistan cDivision of Mechanical Engineering and Automotive Engineering, Kongju National University, 1223-24, Cheonan Daero, Seobook-Gu, Cheonan-City, Choongnam, South Korea dDepartment of Mechanical Engineering, Faculty of Engineering, IIUM, 50728 Kuala Lumpur, Malaysia eDepartment of Mechanical Engineering, University College of Engineering and Technology, Bahauddin Zakariya University, 60000 Multan, Pakistan article info Article history: Received 6 September 2019 Received in revised form 1 November 2019 Accepted 30 November 2019 Available online xxxx Keywords: Sesame oil Biodiesel Transesterification Cold flow properties Oxidation stability Bioenergy abstract The demand for renewable energy is steadily increasing due to rapid population growth and economic development worldwide. An additional reason is that fossil fuel reserves are limited, and this situation results in their non-uniform availability globally. Furthermore, the attitudes of the society, energy policies and technology choices are constantly changing. Thus, renewable energy resources are now considered good alternatives to fossil fuels. In the meantime, liquid energy, such as methyl ester from locally produced vegetable oils, is well accepted by many countries, even though it is currently being blended up to 20% with petroleum fuels. Recently, the industrialisation of biodiesel is a major problem because of its poor cold flow properties and oxidative stability. Vegetable oils are also being blended in an appropriate proportion before transesterification to obtain the desired properties in biodiesel. Similarly, poor cold flow properties and oxidative stability can be improved by choosing suitable vegetable oils for making blends. Amongst all available vegetable oils, sesame seed oil (SSO) has unique cold flow properties and oxidation stability, particularly because of naturally occurring antioxidants and preservatives, which enhance the stability of oil towards rancidity. Therefore, SSO can be used as a potential feedstock for blending with other vegetable oils to enhance the overall cold flow and oxidation stability properties. This overview summarises sesame cultivation, SSO production, the physicochemical properties of SSO and its potential as an alternative renewable fuel source. In this review, the physicochemical properties of sesame biodiesel are compared with those of biodiesel derived from other vegetable oils. Results show that blending SSO with palm oil before transesterification will successfully improve the cold flow properties and oxidation stability of palm methyl ester (biodiesel). ©2019 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Contents 1. Introduction......................................................................................................................................................................................................................... 41 2. Overview of sesame seed plantation ............................................................................................................................................................................... 42 2.1. Background history of the sesame plant ............................................................................................................................................................ 42 2.2. SSO capacity ........................................................................................................................................................................................................... 42 2.3. Medicinal, nutritional and industrial applications of SSO ................................................................................................................................ 43 3. Chemical composition of SSO ........................................................................................................................................................................................... 44 3.1. Physico-chemical characteristics of SSO ............................................................................................................................................................. 44 4. Transesterification of SSO.................................................................................................................................................................................................. 44 4.1. Optimisation of SSO methyl ester ....................................................................................................................................................................... 44 5. Biodiesel standards............................................................................................................................................................................................................. 46 ∗Corresponding author at: Center for Energy Science, Department of Mechanical Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia. ∗∗ Corresponding authors. E-mail addresses: [email protected] (M.A. Mujtaba), [email protected] (H. Muk Cho), [email protected] (M.A. Kalam). https://doi.org/10.1016/j.egyr.2019.11.160 2352-4847/©2019 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). M.A. Mujtaba, H. Muk Cho, H.H. Masjuki et al. / Energy Reports 6 (2020) 40–54 41 6. Physicochemical properties of sesame oil biodiesel ...................................................................................................................................................... 46 6.1. Density .................................................................................................................................................................................................................... 46 6.2. Kinematic viscosity................................................................................................................................................................................................ 47 6.3. Calorific value......................................................................................................................................................................................................... 47 6.4. Cetane number....................................................................................................................................................................................................... 47 6.5. Cold flow properties.............................................................................................................................................................................................. 48 6.6. Oxidation stability ................................................................................................................................................................................................. 48 7. Engine performance and emission characteristics of SSO methyl ester and its blends............................................................................................ 49 8. Cost analysis of SSO........................................................................................................................................................................................................... 49 9. Conclusion ........................................................................................................................................................................................................................... 50 Declaration of competing interest.................................................................................................................................................................................... 51 Acknowledgements ............................................................................................................................................................................................................ 51 References ........................................................................................................................................................................................................................... 51 1. Introduction Rapid population growth has increased energy consumption around the globe. The demand for energy in every sector (e.g. transport, agricultural and domestic) is increasing gradually, and a shortage of fuel will consequently occur due to a gradual decline in fossil fuel reserves (Wakil et al.,2015b). The depletion of fossil fuels poses a serious threat for oil and gas companies to meet the energy demand for all sectors in the future. Oil and gas producers meet energy needs by using available fossil fuel reserves, and this utilisation significantly increases carbon dioxide (CO2) emissions globally (Absi Halabi et al.,2015). The major concern in replacing fossil fuels with alternative renewable fuels is the eradication of combustion emissions, which are directly linked to climate change, global warming and various diseases (Aransiola et al.,2014). Fossil fuel pollutes the air by releasing toxic gases (i.e. nitrogen oxide (NOx), unseen particulate matter (PM), carbon monoxide (CO) and unburned hydrocarbons)). A high concentration of PM in the air can cause cancer and respiratory diseases (e.g. asthma and allergies) (Vardoulakis et al.,2015). According to the Global Status Report (2018), the global proportion of energy generated from different sources is as follows: 10.4% (modern sources of renewable energy), 7.8% (traditional biomass), 2.2% (nuclear energy) and 79.5% (fossil fuels). The transport sector represents the bulk of oil consumption. Nearly 50% of total fossil fuel consumption relates to the energy needs of the transport sector. Approximately one-third of the total global energy consumption is attributed to the transport sector. The global energy demand for the transport sector has increased by 39% over the past decade due to the increase in the movement of freight and overall transport demand in emerging countries globally. The transport sector is estimated to contribute up to 50% of the global greenhouse gas emissions by 2030. Renewable and sustainable fuels can overcome the reliance on fossils fuels to comply with the Kyoto Protocol. The share of renewable energy in the transport sector accounts for 3.1% of global energy consumption. A total of 42 countries worldwide have set national targets to increase the share of renewable energy in transport by 20% and 40% by 2020 and 2030, respectively (2018., 2018). The three major categories of biofuels are first-generation (i.e. bioethanol, a biofuel derived from sugar and starch, such as corn and sugar cane), second-generation (i.e. biodiesel, a biofuel derived from oils of vegetables, such as palm, jatropha, neem, rapeseed and sesame) and third-generation (i.e. biofuel that is derived from algae) biofuels (Mat Yasin et al.,2017;Silitonga et al.,2019). Secondand third-generation biofuels can be used to produce biofuels for the transport sector because the use of first-generation biofuels has a serious impact on food supply. The most suitable replacement for fossil fuels is biodiesel because it is quite similar and has better physicochemical properties (e.g. greater lubricity, higher cetane and low sulphur content) than fossil fuel diesel (Patel and Sankhavara,2017). Biodiesel can be used as an alternative fuel without modifications in an existing diesel engine. The conversion of vegetable oil into biofuel is not a new idea. In 1900, Rudolph Diesel was the first person to test his new invented diesel engine (i.e. compression-ignition engine) with renewable biofuel extracted from peanut oil instead of petroleum diesel, without any modifications to the engine (Dixit et al.,2012). Biodiesel is safe to use, nontoxic and biodegradable, has high cetane number (CN) and combustion efficiency and provides good lubricity with low sulphur content (Wakil et al.,2015b;Imdadul et al.,2016). Previously, many researchers have studied the production of biodiesel from various feedstocks and the behaviour of biodiesel related to engine emissions and performance. Soybean, palm and rapeseed biodiesels are used in the United States, Asia and Europe, respectively (Silitonga et al.,2013). Currently, the major issue with biodiesel commercialisation and industrialisation is its poor oxidation stability and cold flow characteristics. Synthetic antioxidants are usually used to enhance its oxidation stability, and different additives are utilised to improve its cold flow properties. In the past, some researchers have blended biodiesel obtained from different feedstocks to improve these properties. Researchers today are blending the feedstock oil before transesterification to improve its properties in accordance with standards. For instance, palm oil is predominantly used in Asia, especially in Indonesia and Malaysia. The cold flow properties of palm oil are poor due to the high content of saturated fatty acids (FAs). Various researchers have diagnosed the problems associated with engine operation during low-temperature climate as the result of clogging of filters, incomplete combustion, starting problem and fuel starvation (Dwivedi and Sharma,2014). A high degree of unsaturation in biodiesel results in poor oxidation stability but good cold flow properties. Conversely, a high percentage of saturated FAs results in good oxidation stability but poor cold flow properties (Lanjekar and Deshmukh,2016). The selection of feedstock is therefore critical for blending with palm oil to improve its cold flow properties. Vegetable oils with a high degree of unsaturation can be used for blending with palm oil. However, at the same time, the oxidation stability of palm oil will be decreased. According to literature, sesame seed oil (SSO) can be the most suitable option for blending with palm oil compared with all other feedstock oils. SSO has a high degree of unsaturation (up to 85%) and thus has good cold flow properties. Furthermore, it also exhibits high oxidative stability due to naturally occurring antioxidants (i.e. sesamin sesamol and sesamolin) with tocopherols (i.e. vitamin E) (Pullen and Saeed, 2014). This review aims to highlight the potential of SSO biodiesel as a green fuel to replace petroleum diesel. This review explains sesame cultivation and SSO production, applications, composition and properties. Moreover, conversion methods, such as transesterification by conventional methods or ultrasound, of SSO to 42 M.A. Mujtaba, H. Muk Cho, H.H. Masjuki et al. / Energy Reports 6 (2020) 40–54 List of abbreviations ANN Artificial neural network APE Allylic position equivalent ASTM American society for testing and materials BAPE Bis-allylic position equivalent BHA Butylated hydroxyanisole BHT Butylated hydroxytoluene Ca Calcium CCD Central composite design CFPP Cold filter plugging point CH3ONa Sodium methoxide CN Cetane number CO Carbon monoxide CO2Carbon dioxide CP Cloud point CV Calorific value EN European EU European union FAME Fatty acid methyl ester FAO Food and agricultural organisation H2SO4Sulphuric acid HCl Hydraulic acid IPGRI International plant genetic resources institute K Potassium KOH Potassium hydroxide KV Kinematic viscosity Mg Magnesium MPOB Malaysia palm oil board MS Malaysian standard Na Sodium NaOH Sodium hydroxide NOx Nitrogen oxides OS Oxidation stability PDSC Pressurised differential scanning calorimetry PG Propyl gallate PM Particular matter PP Pour point PY Pyrogallol RIP Rancimat induction period RSM Response surface methodology SOME Sesame oil methyl ester SSO Sesame seed oil TBHQ Tert-butyl hydroxyl quinone UHC Unburned hydrocarbons sesame methyl ester are discussed. Techniques used for optimising the biodiesel yield of SSO are also discussed. Lastly, the physicochemical properties of sesame are discussed in detail. In particular, the stability of biodiesel and cold flow properties of biodiesel and sesame methyl ester are compared with those of other vegetable oil methyl esters to demonstrate that sesame biodiesel is a potential feedstock for the future. 2. Overview of sesame seed plantation 2.1. Background history of the sesame plant Sesame (Sesamum indicum L.) belongs to the Pedaliaceae family and is widespread in tropical and subtropical regions of Asia, Africa and South America. The word ‘sesame’ is derived from the Arabic word ‘simsim’ (Moazzami and Kamal-Eldin,2009). Globally, it is known as sesame, as til in Asia and as benniseed or simsim in Africa (Amoo et al.,2017). According to prehistory studies, cultivation of sesame was discovered in South Asian wild populations, and cultivation originated in South Asia before 2000 B.C from the time of the Harappan civilisation (Fuller,2003). The major crop cultivated in Indus valley civilisation was sesame and later on cultivated in west Mesopotamia (TundeAkintundeTY and Akintunde,2012). SSO was used by Assyrians in medicines, food and salves (ointments). Sesame is commonly called the ‘queen of oilseeds’, and SSO was the first oil discovered and consumed by humans; it is also referred to as an ‘orphan crop’ (Moazzami and Kamal-Eldin, 2009). Sesame has been rarely studied and has not been given a crop mandate by any research institute (Bhat et al.,1999). According to Were et al. sesame is listed amongst neglected and underutilised crop species; however, it has high potential according to International Plant Genetic Resources Institute (Were et al., 2006). Sesame is the most valued and oldest oilseed crop due to its high-quality seed oil. Sesame is an erect, annual and herbaceous plant that grows 1–2 m tall (Islam et al.,2016). Sesame seeds are flat, pear-shaped, 2–3.5 mg in weight and 2–3 mm in length. Each capsule contains 50–100 seeds (Moazzami and Kamal-Eldin, 2009). Sesame seeds are oval and small, and they come in various colours, such as yellow, dark brown, white, grey, dark grey, black and reddish brown. The number of seeds per capsule, capsule length and seed size significantly vary depending on the cultivar. A total of 1000 sesame seeds weigh approximately 3 grams (Hegde,2012). Sesame seeds contain nearly 44%–57% oil, 18%–25% protein and 13%–14% carbohydrates (Borchani et al.,2010). In other literature, sesame seed contains 37%–63% oil depending on the variety, growing season and cultivar (Hegde,2012). The most prominent feature of SSO is its resistance towards oxidation rancidity during long exposure to air (Islam et al.,2016). The significant resistance to oxidation is due to the naturally occurring endogenous antioxidants in SSO, such as tocopherols and lignins (i.e. sesamin and sesamolin) (Lee et al.,2008). The remaining part of sesame seeds after the extraction of oil (i.e. cake or meal) is rich in protein (45%–50%) and is typically used for animal feed. The cakes of roasted sesame seeds are rich in antioxidant compounds and can therefore be used as a potential source of antioxidants (Moazzami and Kamal-Eldin,2009). 2.2. SSO capacity According to statistics by the Food and Agricultural Organisation of the United Nations (FAO), the average global yield of sesame seeds in 2017 was 5.53 million metric tons, which was harvested on 9.98 million hectares. In 2017, major producers of sesame countries included the United Republic of Tanzania, Burma (Myanmar), India, Nigeria, Sudan, China (Mainland China), Ethiopia, South Sudan, Burkina Faso and Chad, as shown in Fig. 1. Sesame seed production shares of Africa, Asia, America and Europe in 2017 were 56.9%, 39.7%, 3.4% and 0%, respectively. Africa, Asia and America produced 3.14 million, 2.19 million and 0.189 million tons of sesame seeds, respectively (FAO,2018). Amongst all oilseed crops, sesame is ranked at eighth with respect to oil production globally (Mehmood et al.,2018). M.A. Mujtaba, H. Muk Cho, H.H. Masjuki et al. / Energy Reports 6 (2020) 40–54 43 Fig. 1. Top producers of sesame in the world (FAO,2018). Table 1 Production of sesame seed and oil in different regions of the world (FAO,2018). Region Sesame seed (tons) Sesame oil (tons) Asia 2195089 887199 Africa 3146248 683027 America 189974 25138 Europe 0 36994 Oceania 0 1969 Fig. 2. Production share of sesame oil by region. Source: FAO (2018). According to FAO, average global SSO production in 2014 was 1.63 million tons, and major oil-producing countries were the United Republic of Tanzania, Burma (Myanmar), India, Japan, South Sudan, Sudan, Turkey, Republic of Korea and Uganda. Sesame seed production shares of Africa, Asia, America, Europe and Oceania in 2014 were 41.8%, 54.3%, 1.5%, 2.3% and 0.1%, respectively, as shown in Fig. 2 (FAO,2015) (see Table 1). The average yield of sesame seeds reported in the FAO report (2018) ranged from 256 kg/ha to 1400 kg/ha. The top three countries in the production of the highest yield were China (Mainland China), Nigeria and United Republic of Tanzania with 1400, 1100 and 1074 kg/ha, respectively, as shown in Fig. 1 (FAO,2018). The above-mentioned statistics about sesame seed and oil production should be taken as a rough estimation because most harvested crops are locally consumed and no suitable way to obtain the data for local and domestic production is available. Only several countries properly record data related to sesame seed and oil production and its part in international trade. 2.3. Medicinal, nutritional and industrial applications of SSO Regarding medical use, sesame seeds oil and paste are applied to the skin for treating wounds and burns (Kiran and Asad,2008). Around the globe, sesame is used for treatment of anaemia, amenorrhea, dysentery, respiratory infections, cholera, scorpion poison, dysmenorrhea, tinnitus, diarrhoea, dizziness, memory enhancement and bleeding piles (Hegde,2012;Khan et al.,2014; Kapoor,2017). SSO is used to treat coughs, burns, migraines, snake bites, tuberculosis, hair loss, eye diseases and demulcent in addition to being used as an antitussive (Hegde,2012). Lowquality SSO is also used to produce soap, paints and lubricants (Anilakumar et al.,2010). Sesame seeds are used in food, such as buns, chips, crackers, salads, cakes and breads. SSO has several industrial applications. For instance, sesame is used to prepare perfumes in Africa. Myristic acid made from SSO is used as an ingredient in cosmetics (Anilakumar et al.,2010). Sesamin acts as an antioxidant that can inhibit the absorption of cholesterol and the production of cholesterol in the liver due to its bactericidal and insecticidal effects (Morris,2002). SSO is used as a solvent for skin softeners and drugs and an ingredient for margarines and soaps (Begum et al.,2000). The demand for energy is increasing steadily owing to a growth in the global population. The consumption of fuel has increased to meet the needs of the automotive and energy sectors. The rising global energy demand is barely met due to the depletion of fossil fuel resources. In the future, vegetable oils will replace fossil fuels to fulfil energy requirements. Vegetable oil will be converted to diesel via transesterification reaction to produce a product called ‘biodiesel’. Issues on the commercialisation of biodiesel produced by vegetable oils have been addressed to facilitate its potential to replace fossil fuels. These issues include high viscosity, deterioration of oil and fouling of engine. B20 (80% crude diesel and 20% biodiesel) can be used in existing diesel burned equipment (e.g. compression ignition engine and boilers) without modifications to the engine (Anilakumar et al.,2010). In the past, some researchers have produced biodiesel from SSO by transesterification using a homogeneous and heterogeneous catalyst in with methanol. Sarve et al. obtained optimised biodiesel from SSO using a catalyst (Ba(OH)2) with methanol, which achieved maximum yield at 98.6% at 31.92 ◦C. The sesame biodiesel properties include flash point of 180 ◦C, cloud point (CP) of −5◦C, pour point (PP) of −9◦C, kinematic viscosity (KV) of 40 ◦C 4.47 and CN of 56.32 (Sarve et al.,2015). Various researchers have investigated the use of sesame biodiesel in existing diesel engines and found lower emissions than those from crude diesel. B10 sesame biodiesel was tested in a diesel engine in comparison with other feedstock biodiesels, and the sesame infused biodiesel showed better engine performance than other biodiesels (Naik and Balakrishna,2018). According to Altun et al. and Banapurmath et al. (Altun et al., 2008;Banapurmath et al.,2008) sesame biodiesel reduces the CO and NOxemissions with a slight increment in the brake specific fuel consumption (BSFC). Different researchers have observed that using SSO methyl ester as a fuel reduces exhaust gas emissions, improves brake thermal efficiency (BTE) and increases BSFC. In accordance with the fuel properties of optimised biodiesel, sesame biodiesel has good cold flow properties and CN, which are favourable for commercialising it for diesel engine fuel blends. SSO is a feasible source of vegetable oil for biodiesel production to replace fossil fuels. 44 M.A. Mujtaba, H. Muk Cho, H.H. Masjuki et al. / Energy Reports 6 (2020) 40–54 3. Chemical composition of SSO The chemical composition of SSO contains oil (44%–58%), protein (18%–25%), carbohydrate (∼13.5%) and ash (∼5%) (Elleuch et al.,2007;Sandesh Suresh et al.,2019). The FA composition of SSO varies with geographical region and variety and from feedstock to feedstock. SSO consists of two main FAs: oleic and linoleic acids. The main FAs in SSO are oleic, linoleic, palmitic, stearic and linolenic acids. Table 2 summarises the composition of SSO FAs from different geographical areas (Karmakar et al.,2010; Hassan,2012;Gharby et al.,2017). SSO contains more than 80% of unsaturated FAs, and the major composition belongs to oleic and linoleic acids. The saturated FAs in SSO represent less than 20% of the total chemical composition of SSO. Stearic and palmitic acids are the most important saturated FAs in SSO. SSO is considered to be the most resistant towards oxidation rancidity amongst all vegetable oils. The presence of tocopherols (i.e. vitamin E) in SSO exhibits high resistance to autooxidation. SSO contains 0.5%–1.0% sesamin and 0.3%–0.5% sesamolin with few traces of free sesamol. The high resistance of SSO to oxidative rancidity depends on the presence of natural antioxidants (i.e. sesamin, sesamol and sesamolin) and tocopherols (Hegde, 2012). 3.1. Physico-chemical characteristics of SSO Various researchers have reported the physicochemical characteristics of SSO in the preceding literature. Typical FAs found in SSO are linoleic (C18:2), oleic (C18:1), palmitic (C16:0), stearic (C18:0) and linolenic (C18:3) acids, as shown in Table 2. These properties may vary depending on the extraction method, the geographic location of sesame seeds and analytical methods used for measurement. The physicochemical properties of SSO reported in the literature by various researchers are summarised in Table 3. 4. Transesterification of SSO SSO can be used directly or mixed with petroleum diesel in a diesel engine. The use of pure SSO and its blends as a fuel in diesel engines is problematic due to its high viscosity, as shown in Table 3. The FA content and high viscosity of SSO lead to several problems in diesel engines, such as fuel lines and filter blockage, poor atomisation of fuel, injector coking and piston ring sticking, gum formation (due to oxidation) during storage and combustion, severe carbon deposition in the engine due to incomplete combustion and degradation and thickening of lubricating oil (Cetin and Yüksel,2007;Altun et al.,2008). The solution to these problems caused by the high viscosity of virgin oil can be overcome by converting it to viable biodiesel. Ma and Hanna (1999) reported four possible ways to convert virgin oil to biodiesel as a suitable and viable fuel source for diesel engines, namely, direct use and blends with crude diesel, micro-emulsification, thermal cracking (pyrolysis) and transesterification. The blending of virgin oil with crude diesel and emulsification of virgin oil with solvents (i.e. methanol, ethanol and 1-butanol) reduces the viscosity of virgin oil. Nonetheless, engine performance remains problematic due to carbon deposition and lubricant oil degradation. Thermal cracking or pyrolysis is normally used for producing biogasoline instead of biodiesel. Transesterification is the preferred method for the conversion of virgin oil to biodiesel to improve engine performance. Transesterification is the chemical reaction of a triglyceride (fat or oil) with an alcohol to produce esters and glycerol, as shown in Eq. (1). The transesterification reaction is the conversion of triglyceride into an FA monoalkyl ester (biodiesel) with alcohol (methanol is used normally due to its lower cost and effectiveness Demirbas,2005) using a catalyst, and it produces glycerol as a byproduct. (1) The presence of a catalyst is important during the transesterification reaction because it enhances the reaction rate and yield, and the selection of a catalyst depends on the free FA content, nature of oil and moisture content (Onoji et al.,2016). The alkalicatalysed transesterification reaction includes sodium hydroxide (NaOH), potassium hydroxide (KOH), carbonates and alkoxides of (Na and K), such as sodium methoxide. Acid catalysts include hydrochloric, sulphuric and sulfonic acids. Lipases can also be used as biocatalysts. The alkaline-catalysed transesterification is considerably faster than the acid-catalysed transesterification and is the preferred catalyst in the commercial production of biodiesel (Ma and Hanna,1999). The conversion of triglycerides into FA mono-alkyl ester and glycerol occurs in three reversible steps (Patel and Sankhavara,2017), as shown in Eq. (2)(a–c). The equations of a transesterification reaction are given below: Triglyceride +R′OH ←→ Diglyceride +R′COOR1(2a) Diglyceride +R′OH ←→ Monoglyceride +R′COOR2 (2b) Monoglyceride +R′OH ←→ Glycerol +R′COOR3 (2c) Homogeneous alkali base catalysts (NaOH, CH3ONa and KOH) are the most reactive in the transesterification process for oil with a free FA value less than 1%. Acid catalysts (i.e. H2SO4and HCl) are used for esterification of oil having a free FA value greater than 1% before transesterification reaction. Esterification is necessary to obtain rid of soap formation and a high yield during the transesterification reaction (Onoji et al.,2016). The production methods cited in the literature for the conversion of SSO to sesame methyl ester are conventional and ultrasonic-assisted methods. The transesterification of SSO is conducted with homogeneous base catalysts (KOH, NaOH, CH3ONa) using conventional method (Pullen and Saeed,2014;Dawodu et al.,2014;Betiku and Adepoju,2013;Karim et al.,2014;Ahmad et al.,2009;Wakil et al., 2014) and has biodiesel yields from 87.80% to 98.36%, as shown in Table 4. Sarve et al. (2015) produced biodiesel from SSO with a heterogeneous catalyst (Ba(OH)2) by using ultrasound-assisted transesterification at a frequency of 20 kHz, which produced a maximum optimised biodiesel yield of 98.6%. Therefore, ultrasound techniques can minimise the time and temperature to obtain the maximum yield (see Table 5). 4.1. Optimisation of SSO methyl ester The optimisation of the transesterification process is important to achieve a high yield and ensure the purity of biodiesel. Determining the optimal values for process variables by conventional methods is time consuming because the procedure is costly in terms of labour, money, time consumption and material due to a large number of experiments involved (Ong et al.,2019). During the optimisation, conventional experiments are usually used to M.A. Mujtaba, H. Muk Cho, H.H. Masjuki et al. / Energy Reports 6 (2020) 40–54 45 Table 2 Fatty acid composition (%) of Sesame seed oil. Sudan (El Khier et al., 2008) Congo (Nzikou et al., 2009) Turkey (Ünal and Yalçın,2008) Egypt (Hassan,2012) Morocco (Gharby et al., 2017) Karmakar et al. (2010) Pullen and Saeed (2014) Myristic acid – – 0.02 – 0.1 – 0.1 Palmitic acid 12.9 8.66 8.9 8.47 11.3 7–9 10.1 Stearic acid 3 5.45 5.43 5.53 4.9 4–5 4.0 Oleic acid 47.5 38.86 41.5 41.6 41.9 40–50 37.8 Linoleic acid 36.4 46.2 42.7 42.77 42.1 35–45 45.1 Linolenic acid – – 0.3 0.3 0.2 0.4–1 0.7 Saturated fatty acids 15.9 ±0.2 14.85 14.3 ±0.2 14 16.3 ±0.2 12.5 15.8 Unsaturated fatty acids 83.9 ±0.2 85.15 84.5 ±0.2 84.37 84.3 ±0.2 85.7 84.2 Table 3 Physico-chemical properties of sesame seed oil. Parameters Units Sarve et al. (2015) Kaniz et al. (Ferdous et al.,2012) Dawodu et al. (2014) Betiku and Adepoju (2013) Karim et al. (Karim et al., 2014) (Saydut et al., 2008) Singh and Singh (2010) Borchani et al. (2010) Kinematic viscosity at 40 ◦C, mm2/s 31.51 52.5 22.63 31.39 33.61 25.78 35.5 Density g/cm30.9 0.88 0.8525 0.833 0.936 0.899 0.913 Acid value mg KOH/g oil 0.42 3.15 0.50 0.443 1.64 Iodine value g I2/100 g 75.3 86.15 108 82.45 113 FFA % 6.1 1.58 0.82 Moisture content % 0.03 0.08 0.09 0.00 Saponification value mg KOH/g oil 213 142.2 186 Cetane no. 48.57 50.73 40.2 Flash point ◦C 240 312 245 260 Pour point ◦C−6−3−10 −9.4 Calorific value MJ/kg 38.9 40.20 39.5 39.3 Cloud point ◦C 1 3 1 −3.9 Table 4 Optimum conditions for transesterification of sesame oil. Catalyst type Production technique Operating parameters Biodiesel yield (%) References Methanol/oil ratio Catalyst weight Temp (◦C) Time mins CH3ONa Conventional with CCD 6:1 0.75% 50 30 87.80 Dawodu et al. (2014) NaOH Conventional with RSM 6.24:1 1.04% 63 51.09 98.36 Betiku and Adepoju (2013) KOH Conventional 6:1 1.5% 60 120 96 Karim et al. (2014) NaOH Conventional 6:1 0.5% 60 120 74 Saydut et al. (2008) Ba(OH)2Ultrasonic with 20 kHz RSM +ANN 6.69:1 1.79% 31.92 40.30 98.6 Sarve et al. (2015) CH3ONa Conventional 10:1 – 60 – 92 Ahmad et al. (2009) KOH Conventional 25% (V/V) 1% 60 120 – Wakil et al. (2014) NaOH Conventional 6:1 1% 60 60 96.8 Pullen and Saeed (2014) Table 5 Fuel properties of sesame oil methyl ester. Parameters Units SOME (Betiku and Adepoju,2013) SOME (Karim et al.,2014) SOME (Saydut et al.,2008) SOME (Sarve et al., 2015) SOME (Ahmad et al.,2009) SOME (Wakil et al.,2014) Density g/cm30.833 0.8972 0.8672 0.867 0.871 0.884 Kinematic viscosity at 40 ◦C, mm2/s 4.03 4.58 4.2 4.47 5.77 4.3989 Acid value mg KOH/g oil 0.25 0.32 – 0.12 – – Iodine value g I2/100 g 86.72 – 80.32 – – – FFA % – – – – – – Moisture content % 0.014 0 – 0.017 – – Saponification value mg KOH/g oil – – – – – – Cetane no. 59.80 69.3 50.48 56.32 53 – Flash point ◦C 180 155 170 180 110 208.5 Pour point ◦C 6 −5−14 −9−18 1 Calorific value HHV MJ/kg 41.35 – 40.4 40.1 – 39.996 Cloud point ◦C 18 1 −6−5−6.3 1 CFFP −1 46 M.A. Mujtaba, H. Muk Cho, H.H. Masjuki et al. / Energy Reports 6 (2020) 40–54 determine the optimum values for process variables by trial and error. Response surface methodology (RSM) software is used for process optimisation. It is widely used for optimising and examining the effect of input variables on output variables (operational variables) (Maran et al.,2013b,c). RSM is typically used to optimise the responses or select the best-operating conditions for achieving maximum output result (Maran et al.,2017). The central composite design (CCD), which is also called the Box–Wilson design, is an experimental design to obtain maximum information about a process from a small number of experiments (Prakash Maran et al.,2017). In CCD, the experimental design is used to analyse the behaviour of input variables or parameters on output response. From previous literature or preliminary experiments, process input variables and ranges are normally determined. After the input variables and their ranges are selected, experiments are designed with various factors at three levels, and each input variable is coded between −1, 0 and +1 (Maran et al., 2013a). RSM is used to optimise the response variable (output) depending on various independent variables (input). RSM is the best option with minimal process data; thus, it saves experimental cost and precious time (Shanmugaprakash and Sivakumar, 2013). Artificial neural network (ANN) is a prominent technique for optimising the process in biodiesel research (Gul et al.,2019) because it can model using the mathematical background of the problem and for studying the linear and nonlinear relationships directly from the set of variables (Maran et al.,2013d;Sarve et al.,2015). ANN is considered a superior alternative technique to conventional modelling techniques due to its nonlinearity and complexity (Aghbashlo et al.,2015). A well-trained neural network is a fast, reliable and easy to use tool for solving and optimising engineering problems (Kurtgoz et al.,2017). Sarve et al. (2015) compared both modelling techniques (i.e. ANN and RSM) to predict the yield of SSO methyl ester obtained from ultrasonic-assisted transesterification. ANN is more reliable than RSM in optimising the biodiesel yield. Ultrasound-assisted transesterification is feasible in producing biodiesel whilst using heterogenous catalyst (Tan et al.,2019). Dawodu et al. (2014) used CCD for transesterification of sesame (Sesamum indicum L.) oil to optimise the process variables. The maximum yield of SSO methyl ester was predicted to be 87.80% when the reaction temperature, reaction time, sodium methoxide (CH3ONa) as a catalyst and amount of methanol/oil ratio were 50 ◦C, 30 min, 0.75% and 6:1, respectively. Betiku and Adepoju (2013) used a CCD-based RSM technique to optimise the reaction temperature, the molar ratio (methanol: oil), reaction time and amount of catalyst of the transesterification process to optimise the yield of biodiesel. The biodiesel yield was optimised to 98.36% under the following conditions: (1) molar ratio (methanol to oil): 6.24:1, (2) reaction time: 2 h, (3) sodium hydroxide (catalyst) concentration: 1.04% and (4) reaction temperature: 63 ◦C. Sarve et al. (2015) compared RSM and ANN to optimise the reaction temperature, catalyst concentration, reaction time and molar ratio of methanol to oil to increase the yield of biodiesel. The optimised yield conversion was 98.6% under these process conditions: (1) methanol to oil molar ratio: 6.69:1, (2) barium hydroxide (Ba(OH)2) heterogeneous catalyst concentration: 1.79%, (3) reaction time: 40.30 min and (4) reaction temperature: 31.92 ◦C. The sensitivity analysis was used to analyse the effect of each independent variable on the response to the output variables. Sensitivity analysis showed that the catalyst concentration was the main affecting factor of the FA methyl ester content. The results showed that the lower values of correlation of coefficient, root mean square error, standard error of prediction and relative percent deviation for ANN than RSM verified that ANN was a superior prediction model for FA methyl ester (FAME) content. 5. Biodiesel standards Biodiesel is considered an alternative fuel to crude diesel for diesel engines. Biodiesel is produced from edible and non-edible vegetable oils by transesterification following ASTM D6751 and EN 14214 standards. Most researchers have concluded that biodiesel is fire resistant due to the its higher flash point than that of diesel. Amberyellowish coloured biodiesel has viscosity comparable to that of crude diesel (Al-Dawody and Bhatti,2013;Mat Yasin et al.,2017). Biodiesel producers should follow the fuel standard requirements set by two reputable biodiesel standards that are used for testing fuels. The standards used to ensure the biodiesel quality are EN 14214 for European Union and ASTM D6751 for American biodiesel standard (Atabani et al.,2012). The significant affecting factors of the quality of biodiesel include the technique used to produce biodiesel, the FA composition of vegetable oil, feedstock quality, animal fats and waste oil, the refining process and postproduction parameters (Gautam and Agarwal,2015;Mat Yasin et al.,2017). The low fraction variants of biodiesel, such as B7 or B10, are being utilised in many countries, such as Malaysia. In 2019, Malaysia started utilising B10 biodiesel, which is produced from palm oil, for the automotive industry. Malaysia is the second-largest global producer of palm oil after Indonesia. Malaysia has established its own biodiesel testing standards for palm oil methyl ester. The standard values are mostly taken from ASTM D6751 and EN 14214 standards. According to ASTM D6751 standard, each property of pure biodiesel must be in the range (set by standards) before being utilised neat or blended with diesel in diesel engines. According to EN 14214 European standard, minimum and maximum values of various parameters are defined to ensure the quality of biodiesel. Before the commercialisation of biodiesel as a pure biofuel or blending stock for diesel fuel, it should fulfil the minimum or maximum limits set by the standard. EN 14214 standard specifies the maximum allowable concentration of different parameters within biodiesel to ensure the quality of biodiesel. To facilitate the local implementation of palm oil methyl ester in Malaysia, Malaysia Palm Oil Board played a key role in the publication of Malaysian biodiesel standard (MS 2008:2008) in October 2008 for palm oil methyl ester (Goosen et al.,2007;Lam et al.,2009) (see Table 6). 6. Physicochemical properties of sesame oil biodiesel 6.1. Density The density of biofuel is a key parameter for calculating the precise volume of fuel for adequate combustion (Ramírez Verduzco,2013). In diesel engines, air to fuel mixing mostly depends on the density of the fuel. The fuel–air mixture is conducted at a pressure range of 15–50 MPa and a temperature range of 300 K–350 K in the combustion chamber (Sajjadi et al.,2016). Density directly affects the injection process and efficiency of fuel atomisation because the quantity of fuel injected through injector nozzle in the combustion chamber is assessed by its volume (Kaya et al.,2009). Density is also considered an important property of biodiesel that links with viscosity, CN and heating value (Hoekman et al.,2012). ASTM D6751 standard does not specify the density of a biofuel. EN 14214 standard specifies that a biodiesel’s density should be in the range of 860–900 kg/m3. Compared with petroleum diesel’s density (850 kg/m3), the density of biodiesel (880 kg/m3) is slightly higher; such density can be overcome by increasing the percentage of biodiesel in blends (Silitonga et al., 2013;Wakil et al.,2015b). Sesame, palm, coconut and pequi derived FA methyl esters have the lowest density amongst vegetable oil methyl esters (Sajjadi et al.,2016). Biodiesel (higher unsaturated) with more than two double bonds exhibits a relatively high M.A. Mujtaba, H. Muk Cho, H.H. Masjuki et al. / Energy Reports 6 (2020) 40–54 47 Table 6 ASTM D6751, EN14214 and Malaysian fuel standard MS 2008:2008 for biodiesel (Mat Yasin et al.,2017;Sarve et al.,2015;Betiku and Adepoju,2013;Ahmad et al., 2011;Pullen and Saeed,2012). Properties Unit ASTM D6751 EN14214 Malaysian standard MS 2008:2008 Limit Method Limit Method Limit Method Density at 15 ◦C kg/m3870–890 ASTM D4052–91 860–900 EN ISO 3675 860–900 ASTM D4052 Kinematic viscosity at 40 ◦C mm2/s 1.9–6.0 D445 3.5–5.0 EN ISO 3104 3.5–5.0 MS 1831 Flash point ◦C 130 min D93 >101 EN CD 3679e 120 ASTM D5453 Cetane number – 47 min D613 51.0 EN ISO 5165 51.0 MS 1895 Acid value mg KOH/g <0.50 D664 0.50 EN 14104 0.50 MS 2011 Oxidation stability h >3 EN 14112 6 EN 14112k 6 EN 14112 FAME content % (m/m) – – 95.5 EN14103 – – Water content mg/kg 500 max D2709 500 max EN ISO 12937 500 ASTM E 203 Iodine value % (m/m) – – 120 max EN 14111 110 EN 14111 Sulphur content mg/kg <15 D5453 10 max – 10 – Sulphated ash content % (m/m) 0.02 max D874 0.03 max ISO 3987 0.02 ISO 3987 Methanol content % (m/m) – – 0.2 max EN 141101 0.2 EN 14110 Monoglyceride content % (m/m) – – 0.8 max EN14105m 0.8 max ASTM S 6584 Diglyceride content % (m/m) – – 0.2 max EN14105m 0.2 max ASTM S 6584 Triglyceride content % (m/m) – – 0.2 max EN14105m 0.2 max ASTM S 6584 Free glycerine % (m/m) 0.020 max D6584 0.02 max EN14105m/EN14106 0.02 max ASTM S 6584 Total glycerine % (m/m) 0.240 max D6594 0.25 max EN14105m 0.25 max ASTM S 6584 Total contamination mg/kg – – 24 max EN 12662 24 max ASTM D 5452 Phosphorus content mg/kg 10 max D4951 4 max EN14107p 10 max ASTM D 4951 CFPP ◦C – – max +5 EN 116 15 EN 116 Pour point ◦C−15 to 16 D 97 – – – – Group I metal (Na +K) mg/kg <5 EN14538 <5 max EN14108 5.0 EN 14108 Group II metal (Ca +Mg) mg/kg <5 EN14538 <5 max EN14538 5.0 EN 14109 Carbon residue % (m/m) <0.05 D 4530 <0.3 EN ISO 10370 – – density (Karmakar et al.,2010). According to Wakil et al. (2015b), sesame biodiesel has the trend of increasing density (0.849, 0.853, 0.857 and 0.86 at 50%, 60%, 70% and 80% blend percentage). In Fig. 3, the densities of all the feedstock are greater than those of petroleum diesel (839 kg/m3), and the density (867 kg/m3) of sesame biodiesel is within the range of EN 14214 standard. 6.2. Kinematic viscosity Kinematic viscosity (KV) is the measurement of inherent resistance to liquid flow. The thickness of oil is estimated by the time at 40 ◦C for a volume of liquid to flow through a calibrated liquid in glass in viscometer (Sajjadi et al.,2016;Silitonga et al., 2013). The KB of vegetable oil is usually 10 times higher than that of crude diesel, as shown in Table 3 in the case of SSO. According to Sajjadi et al. (2016), vegetable oils are more viscous at 9 to 17 times and 1.6 times than biodiesel and petroleum diesel, respectively. High viscosity fuels form large droplets during injection and cause problems, such as carbon deposits on engine parts and formation of soot due to poor atomisation during combustion (Silitonga et al.,2013;Wakil et al.,2015b;Sajjadi et al.,2016). During the winter season or in cold weather, high viscosity fuel mixes with air slowly, and this condition leads to weak combustion and increased exhaust emissions. Fuel with low viscosity cannot provide sufficient lubrication during fuel injection and thus results in wear and leakage (Freitas et al.,2010). According to biodiesel standards EN 14214 and ASTM D6571, the limits of biodiesel viscosity are 3.5–5.0 and 1.9–6.0 mm2/s, respectively. Engine operation at low speed causes high injection volumes and pressure due to the high viscosity of fuel; as a result, clogging of fuel lines and poor atomisation and carbon deposits occur (Masjuki et al.,2004). Sarve et al. (2015) observed that the viscosity of SSO was 31.51 mm2/s. After the transesterification, the high viscosity of SSO is significantly decreased to 4.47 mm2/s for SSO methyl ester and thus meets the required limits set by biodiesel standards. The transesterification aims to reduce the viscosity of oil for meeting the limits of biodiesel standards. As shown in Fig. 3, the KV (4.47 mm2/s) of SOME is within the range of EN 14214 standard and is greater than those of crude diesel (2.9 mm2/s), linseed (3.95 mm2/s) and corn (4.363 mm2/s) but is less than those of palm (4.63 mm2/s), jatropha (4.73 mm2/s), soybean (5.429 mm2/s) and sunflower (4.719 mm2/s). 6.3. Calorific value Calorific value (CV) is also considered to be the most important parameter in the selection of fuel. It is the amount of heat released during the combustion of a specified amount of fuel to produce CO2and H2O at its initial temperature. It has no standard value in American (ASTM D6751) and European (EN 14214) standards, but the minimum value of 35 MJ/kg is given in EN 14213 standard (Silitonga et al.,2013). The calorific value for biodiesel is more influenced by high unsaturation than by the length of the carbon chain. The calorific value of biodiesel declines up to 0.21 MJ/kg by an increase in each percentage of unsaturation of FA methyl ester (Ramírez-Verduzco et al.,2012). The heating value of biodiesel (39.57–41.33 MJ/kg) is 12% lower than petroleum diesel (∼46 MJ/kg) due to its higher oxygen content (Sajjadi et al.,2016). As shown in Fig. 3, SOME heating value (40.1 MJ/kg) is the highest of all feedstocks. 6.4. Cetane number Cetane number (CN) is a key parameter of the quality of diesel fuel. CN indicates the ignition delay time within the combustion chamber upon injection. High CN takes short ignition delay time to ignite, and this condition results in low idling noise and good cold startup. By contrast, low CN takes long ignition delay time to ignite, and this condition results in power output reduction, an increase in engine noise, incomplete combustion and inefficiency in fuel conversion (Mat Yasin et al.,2017). Biodiesel easily fulfils the ASTM D6751 minimum specification of 47 for CN, but European standard EN 14214 is more rigorous and is similar to the minimum specification of 51. As shown in Fig. 3, palm oil methyl ester exhibits the highest CN (59.5), followed by SSO methyl ester (56.35) and soybean methyl ester (53.8). The CN of biodiesel produced from linseed oil methyl ester (48) is on borderline. According to previous literature, adding alcohol-based additives, such as methanol, diethyl ether and ethanol, in small proportions can improve the CN significantly for biodiesel and blended fuel of diesel (Ali et al.,2016,2015;Yasin et al.,2014). 54 M.A. Mujtaba, H. Muk Cho, H.H. Masjuki et al. / Energy Reports 6 (2020) 40–54 Wakil, M.A., Kalam, M.A., Masjuki, H.H., Atabani, A.E., Rizwanul Fattah, I.M., 2015b. Influence of biodiesel blending on physicochemical properties and importance of mathematical model for predicting the properties of biodiesel blend. Energy Convers. Manage. 94, 51–67. Wakil, M., Kalam, M., Masjuki, H., Fattah, I.R., Masum, B., 2014. Evaluation of rice bran, sesame and moringa oils as feasible sources of biodiesel and the effect of blending on their physicochemical properties. RSC Adv. 4, 56984–56991. Wakil, M., Masjuki, H.H., Kalam, M., Teoh, Y., How, H., Imtenan, S., 2015a. Influence of engine operating variable on combustion to reduce exhaust emissions using various biodiesels blend. RSC Adv. 5, 100674–100681. Were, B.A., Onkware, A.O., Gudu, S., Welander, M., Carlsson, A.S., 2006. Seed oil content and fatty acid composition in East African sesame (Sesamum indicum L.) accessions evaluated over 3 years. Field Crops Res. 97, 254–260. Yamane, K., Kawasaki, K., Sone, K., Hara, T., Prakoso, T., 2007. Oxidation stability of biodiesel and its effects on diesel combustion and emission characteristics. Int. J. Engine Res. 8, 307–319. Yang, Z., Hollebone, B.P., Wang, Z., Yang, C., Landriault, M., 2013. Factors affecting oxidation stability of commercially available biodiesel products. Fuel Process. Technol. 106, 366–375. Yasin, M.M., Yusaf, T., Mamat, R., Yusop, A.F., 2014. Characterization of a diesel engine operating with a small proportion of methanol as a fuel additive in biodiesel blend. Appl. Energy 114, 865–873.