J Am Oil Chem Soc (2014) 91:525–531 DOI 10.1007/s11746-014-2443-0
Comparison of Fatty Acid Methyl and Ethyl Esters as Biodiesel Base Stock: a Review on Processing and Production Requirements Mohamad Firdaus Mohamad Yusoff Xuebing Xu • Zheng Guo
Received: 15 March 2013 / Revised: 16 October 2013 / Accepted: 3 January 2014 / Published online: 2 March 2014 Ó AOCS 2014
Abstract Fatty acid methyl esters (FAME) were the first fatty acid esters to be introduced for use as biodiesel. However, there is a growing interest in the use of fatty acid ethyl esters (FAEE) in biodiesel. Both FAME and FAEE have their own unique advantages and disadvantages. These differences are ultimately attributable to the structural differences imparted by the alcohols used in their production. Sources of reactants as well as their safety issues, are a focus of this review. Also reviewed are the comparative characteristics and properties of both biodiesel types in terms of physicochemical features and performance. Processing requirements, reaction times and molar ratios of alcohol to oil, together with problems and drawbacks, are discussed. Recent developments on improving the yield of biodiesel, include mixing methanol and ethanol in the same reaction with ethanol acting as a co-solvent, and enzymatic methanolysis and ethanolysis are also highlighted. Keywords Fatty acid methyl esters (FAME) Fatty acid ethyl esters (FAEE) Transesterification Methanolysis Ethanolysis Biodiesel
M. F. M. Yusoff X. Xu Z. Guo (&) Department of Engineering, Aarhus University, Gustav Wieds Vej 10, 8000 Aarhus C, Denmark e-mail: [email protected]
M. F. M. Yusoff School of Chemical Sciences and Food Technology, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Malaysia
Introduction Depletion of petroleum supplies continues to be an important global issue. Additionally, political instability in major oil supplier countries is contributing to increasing prices for this commodity. Fuel derived from petroleum is a major air pollutant. Emissions released as a result of hydrocarbon combustion include carbon dioxide, a contributing factor to global climate change. In order to solve these problems, development of economically and environmentally safer fuel alternatives from renewable resources is critical. Biodiesel has become a reliable and promising substitute for conventional petroleum diesel. Biodiesel is stable and can be used directly as a fuel or mixed in different ratios with petroleum diesel. In addition to its being biodegradable and produced from sustainable resources, biodiesel is less toxic than regular diesel. No major vehicle engine modifications are needed to use biodiesel [1, 2]. Biodiesel is a mixture of fatty acid alkyl esters obtained by transesterification of TAG (triglycerides) of vegetable or animal origin with short chain alcohol. The alcohols that can be used to produce biodiesel include methanol, ethanol, propanol and butanol. However, methanol and ethanol are the most frequently used [2–4]. Most research and industrial production of biodiesel involves transesterification of edible plant oils with methanol, using a homogenous alkali catalyst. However, current research is focused on investigating the production and use of other types of oil feedstocks and different catalysts. Likewise, substitution of ethanol for methanol in biodiesel production has also attracted academia’s interest since it can be obtained from renewable sources. This paper will discuss the factors affecting the choices between methanol and ethanol in the production of
J Am Oil Chem Soc (2014) 91:525–531
Table 1 Physical properties of ethanol and methanol Properties
Molecular mass (g mol-1)
Density at 20 °C (g/cm3)
Vapor pressure (kPa, 20 °C)
Melting point (°C)
Boiling point (°C)
Flash point (°C)
Viscosity (Pas, 20 °C)
1.2 9 10-3
5.9 9 10-4
Kinematic viscosity (m2s-1, 20 °C)
1.82 9 10-6
8.39 9 10-7
biodiesel, primarily on the basis of source and safety requirements during reaction and product handling. General processing requirements, problems faced when producing both biodiesel types, and product quality differences will also be reviewed.
Source and Safety of Methanol and Ethanol Table 1 summarizes the physical properties of methanol and ethanol. Ethanol has a higher boiling point, flash point, viscosity, and kinematic viscosity, while methanol has a higher melting point due to its higher polarity. Methanol also has a slightly higher density and is more volatile than ethanol (Table 1). The differences in their properties can result in differences in the production of their corresponding biodiesel products as reviewed in the following sections. In general, both methanol and ethanol can be obtained from plant materials. Since methanol is cheaper, it is the most commonly employed alcohol. However, since TAG are more soluble in ethanol, it is often considered to be preferable to methanol for transesterification of vegetable oils . Source of Alcohols Methanol, ethanol, propanol, butanol and amyl alcohol have been used in biodiesel production. However, high prices and complex and expensive alcoholysis conditions make all but methanol and ethanol unsuitable for practical use . Methanol Methanol is the most often used alcohol for biodiesel production due to its suitable physicochemical properties, low cost, mild reaction conditions and easy phase separation . The process of methanolysis of vegetable oil is
Fig. 1 Metabolic pathway of methanol (a), and ethanol (b)
easy to conduct, requiring less post treatment to get to a finished product. Today, methanol is mainly obtained from petroleum sources . Previously, it was obtained as a byproduct of the destructive distillation of wood. It can also be synthesized directly from hydrogen and carbon monoxide from any hydrocarbon source. Methanol can also be produced from natural gas or biomass, which is considered to be a renewable source of energy. Ethanol Although methanol is the most widely used alcohol for biodiesel production, the use of ethanol is increasing. Ethanol has several advantages to methanol. As it is generally obtained from agricultural sources such as corn and sugar cane, ethyl esters are considered to be a renewable biofuel and a greener product than methyl esters [2, 7]. However, the process of obtaining ethanol is considerably more expensive. Safety Considerations In biodiesel production, safety is a critical consideration. Methanol has a low boiling point and its vapors are highly flammable. Methanol and methoxide (as a catalyst) are extremely hazardous materials that require careful handling . Methanol is highly toxic to humans, and can cause blindness and severe nausea. Even at low concentrations, methanol vapors can irritate the human respiratory system. Traces of methanol are highly undesirable in foods and other products meant for human consumption [1, 7]. In the body, methanol is metabolized into formaldehyde and then formic acid (Fig. 1a). Although ethanol is more expensive, its advantages include superior solvency properties for vegetable oils and low toxicity relative to methanol. As illustrated in Fig. 1b, ethanol is metabolized in the body to acetaldehyde, which quickly converts to acetic acid. Like methanol, ethanol vapor is flammable but unlike methanol, ethanol vapor
J Am Oil Chem Soc (2014) 91:525–531 Fig. 2 Schematic representation of reversible transesterification towards biodiesel production
CHOCOR2 + HOR'
CHOCOR2 + R'OCOR1
fatty acid alkyl ester
CHOCOR2 + HOR'
CHOCOR2 + R'OCOR3
monoglyceride fatty acid alkyl ester
fatty acid alkyl ester
HOCOR1, HOCOR2 and HOCOR3: Fatty acid; HOR': HOCH3 or HOCH2CH3
does not cause blindness nor is it threatening to the respiratory system.
Biodiesel Production with Methanol/Ethanol as Alcohol Donors Biodiesel production is undergoing rapid technological innovation in both industry and academia. The most common method for producing biodiesel is by transesterification of vegetable oil and animal fats. Catalytic transesterification has a long history of development and biodiesel produced by this method is now available in North America, Europe and Malaysia . The most commonly used catalyst types are alkalis, acids and enzymes. There is also a method whereby the reaction occurs in a supercritical state without the use of a catalyst . Despite the variety of catalysts and production methods that could be employed, alkaline catalysis remains the most effective and widely used method . Processing Requirements Catalytic transesterification can be divided into two main processes: chemical catalysis and biocatalysis. Chemical catalysis may be either heterogenous and homogenous. As for biocatalysis, enzymes are the main catalysts. The transesterification process can also be performed without a catalyst when conducted under supercritical conditions,
whereby the pressure and temperature are above the critical point of the alcohol employed. The transesterification mechanism follows three steps with each step being a reversible reaction (Fig. 2). In each step, one mole of alcohol reacts with one mole of fatty acid in glyceride form to produce one mole each of ester and water. Since a triglyceride or TAG molecule contains three fatty acids, three moles of alcohol per mole of TAG are required to fully convert them into esters. The transesterification reaction is more efficient when conducted with a stoichiometric excess of alcohol, as this will shift the reaction equilibrium toward the formation of esters. Another option is to continuously remove one of the reaction products from the reaction mixture to drive the reaction toward the right. Initial Mole Ratio Effects Generally, in the process of catalytic alcoholysis, the stoichiometric balance of alcohol to oil affects the yield percentage and the quality of the product. Since the reaction is reversible, the increase in the alcohol to oil molar ratio shifts the reaction equilibrium towards biodiesel formation. For ethyl ester production, a low molar ratio of ethanol to oil increases the solubility of the reaction mixture and the homogeneity of the product mixture makes separation difficult . If the initial ratio is increased, the yield will also increase but only up to a certain level. A molar ratio of 9:1 is the optimum ratio for ethyl ester
production, producing the highest yield of product. With this ratio, the reactants were also easier to separate from the glycerol and salt by-products. According to some previous reports [5, 7], a molar ratio greater than 9:1 will result in difficult glycerol separation, as part of the glycerol remains in the ester phase. Stable emulsions formed during vegetable oil ethanolysis are most likely due to the presence of water, excess free fatty acids, or excessive amounts of alkaline catalyst, causing formation of soaps . These emulsions make the separation of ethyl esters from the ethanol-glycerol phase more difficult [12, 13]. In the case of methanolysis, these emulsions are quickly and easily broken down to form a lower glycerolrich layer and an upper methyl ester-rich layer. Baroutian et al.  claimed that methanolysis yields are higher than ethanolysis due to the higher reactivity of methanol.
J Am Oil Chem Soc (2014) 91:525–531
the reaction, phase separation may be observed when water is present at elevated concentrations . According to a study by Azhari et al. , on Jatropha curcas-based methyl and ethyl esters synthesis, a 96 % product yield was obtained when using methanol while a 90 % yield was obtained with ethanol. The differences in yield may be attributable to the differences in the molecular structure of methanol and ethanol. Additionally, his review on previous work done by other groups reported a slower reaction rate when using ethanol in excess as compared to methanol . In the presence of almost any chemical catalyst, transesterification with methanol proceeds much faster than with ethanol. For example, the methanolysis of castor oil achieved a maximum yield after 1 h of reaction time while ethanolysis took about 5 h to obtain the maximum yield of product .
Reaction Temperature Product Quality and Performance The type of feedstock and catalyst as well as other reaction conditions determine the influence of reaction temperature on both rate of reaction and product yield. However, in the case of fatty acid ethyl esters (FAEE) production, the reaction temperature does not greatly influence the yield of the reaction [5, 6]. The reaction rate increases with increasing temperature but after 20–30 min of reaction time, equilibrium is achieved and its effect becomes insignificant. Chemically catalyzed ethanolysis at high temperature is undesirable because the formation of soaps decreases the FAEE yield . Non-catalytic ethanolysis occurs under very high pressure and temperature conditions while at low and mild temperature and pressure, some of the catalyst is needed to initiate the reaction . Common temperatures for the transesterification reactions for methanol and ethanol are 65 and 75 °C, respectively. Above these temperatures, the reactions are no longer effective because of the boiling point constraints of these alcohols . Vegetable oil ethanolysis requires higher energy consumption than methanolysis . Methanolysis is achieved at lower temperatures because methanol is more reactive than ethanol. According to Tapanes et al. , methoxide ion formation is easier to achieve than ethoxide ion. The difference is due to the molecular structure differences between these alcohols. The two carbon hydrocarbon structure of ethanol requires more energy and time to form ethoxide ion than does methanol, which contains only a single carbon . During transesterification, methanol has been shown to be a better acyl acceptor than ethanol. Ethanol-based transesterification is extremely sensitive to minor changes in water content, reaction temperature, oil/ethanol ratio and catalyst concentration. At the end of
The biodiesel quality, definition and standard specifications vary from region to region. The biodiesel standards in Brazil and the US (the ASTM specification D6751) are applicable for both fatty acid methyl esters (FAME) and FAEE, whereas the current European biodiesel standard (EN14214) is only applicable for FAME . Other than that, ethyl esters also show lower smoke opacity, lower exhaust temperature and lower pour point. The evaluation of exhaust gas emissions (including nitrogen oxides, CO2 and smoke density) shows that FAEE has a less negative environmental effect in comparison to FAME . A comparative review of fuel characteristics of some ethyl esters and respective methyl esters produced from different vegetable oils is given in Table 2. As depicted in Table 2, the pour point and cloud point of both substances appear to be slightly higher than standards [14, 15, 18–20]. These properties depend largely on the type of vegetable oil used; the alcohol reactant plays no significant role with regard to the physicochemical properties of the biodiesel. Low cloud point and pour point values are important for biodiesel because it allows vehicle engines to start at a low temperature without the aid of anti-freeze additives . In terms of stability against degradation, FAME show slightly lower stability against oxidation than FAEE. A few reports highlighted that oxidative degradation develops from hydroperoxide formation followed by formation of secondary oxidation products. In this case, the process is influenced by the nature of the oil, specifically the number of double bonds and its quality with particular reference to the presence of hydroperoxide, antioxidant, and pro-oxidants as well as air and high temperature conditions.
J Am Oil Chem Soc (2014) 91:525–531 Table 2 Fuel characteristics of FAEE and FAME produced from different vegetable oils
TAG source/ Reference
Cloud pint (oC)
Pour point (oC)
Acid value (mg KOH/ g)
Sunflower oil 
Soybean oil 
Castor oil 
Jatropha c. oil 
-6 to 5
EN14214 (standard)  Fossil fuel diesel 
Table 3 Comparison of lipase-catalyzed transesterification of triglycerides with methanol and ethanol Oil Soybean Rapeseed Tallow
Ethanol ? 6 mol% H2O
Enzymatic Methanolysis and Ethanolysis An extensive selection of alcohols as well as a few esters have been tested for enzyme catalyzed biodiesel production. The choice of alcohol has some influence on properties of the biodiesel produced such as cold flow and lubricity. Methanol is widely used in the production of biodiesel. However, its enzyme inhibiting properties as compared to other alcohols limits its use in biocatalyzed reactions. For example, methanol was found to irreversibly inhibit the activity of Candida antartica Lipase B (Novozym 435) . To overcome this problem, a stepwise addition of methanol to the reaction mixture was conducted, showing a great improvement in terms of lowering the risk of enzyme inhibition [21–23]. Nelson et al.  conducted research on a series of enzymatic transesterifications using different types of TAG and primary alcohols, using lipases from various sources such as Mucor miehei, C. antartica, Pseudomonas cepacia, Rhizopus delemar and Geotrichum candidum. Focusing on methanol and ethanol specifically, M. miehei lipase produced the best conversion of TAG to alkyl esters, with yields of [70 %. Based on the data presented in Table 3, it
Cold filter plugging point (oC)
Lubricity at 60 °C (lm)
Oxidative stability at 110 °C (h)
is clearly seen that enzymatic transesterification with ethanol produces higher yields than with methanol. This is due to the negative effects from the more polar methanol and by-product glycerol . It has been demonstrated that a concentration of methanol higher than 0.5 M equivalent is insoluble in vegetable oils and that the native lipases are easily inactivated when coming into contact with any insoluble methanol that may be dispersed in the oils. As the glycerol by-product is hydrophilic and insoluble in the oils, it is easily adsorbed onto the surface of the carrier of the immobilized lipase. This could also lead to the negative effect on lipase activity and operational stability. Another interesting observation derived from the investigation is the effect of water, which reduces the conversion of oil to alkyl esters. Both methanol and ethanol are sensitive to the presence of water, which greatly reduces the amount of ester formed. For example when &95 % ethanol was used in the reaction instead of pure spirit, the conversions dropped from 98 to 68 % (Table 3). An acyl acceptor for industrial production must be cheap and available in large quantities . Methanol and ethanol are the most reliable alcohols that fulfill those criteria. Additionally, the influence of the molar ratio of alcohol to oil, water activity and availability influence the final product yield. Determining the exact concentrations of reactants to be employed in the reaction can help manufacturers to reduce energy consumption, equipment size and management of the unreacted alcohol.
Possibilities to Improve Methanolysis and Ethanolysis Alcoholysis of oils and fats with a mixture of methanol and ethanol showed an improvement in the fuel properties of the final product as well as an increase in the reaction rate over ethanol alone. Baroutian et al.  in their research on
the production of palm olein esters, proposed that one should take advantage of the good aspects of both methanol and ethanol in the production of biodiesel. They claimed that ethanol will act as a co-solvent in the mixture, permitting easier mixing of the reactants with palm olein. If a mixture of methanol and ethanol is used for the transesterification reaction, the system can take advantage of both the better solvent properties of ethanol and the desired equilibrium conversion of methanol to produce a mixture of both FAME and FAEE in the same batch of raw material. Based on a reaction system containing a high molar ratio of methyl to ethyl esters, FAME form much faster than FAEE due to the higher reactivity of the methoxide ion intermediate . Kim et al.  explained that combined structures of methoxide–ethanol or ethoxide–methanol greatly improves alcoholysis rate. The reason is that, in this combined system, two reactive sites per alkoxide molecule were formed when the reaction mixture is alkaline enough to form ethoxide. This combined structure can accelerate the reaction rate and can increase the ester yield when using the alcohol mixture and either homogeneous or heterogeneous catalysts. The resultant FAME/FAEE mixtures exhibit enhanced low temperature properties and oxidative stability, as well as superior lubricity in comparison to neat FAME and also satisfies the biodiesel standard with respect to kinematic viscosity and acid value [20, 26]. Another advantage of using a mixture of both alcohols is that, replacing a portion of the methanol with ethanol results in less dependency on synthetic sources of methanol . Several researchers also investigated a way to increase the yield of methyl esters through enzymatic reaction [28, 29]. FAME yield may be increased by using a mixture of two lipases with different substrate specificities such as in the case of soybean oil methanolysis with a mixture of lipase from Rhizopus oryzae and Candida rugosa. Using this mixture, more than 99 % conversion of soybean oil was obtained and the conversion was maintained at over 80 % of its original conversion after 10 cycles of reuse of the mixed lipases . Using a mixture of lipases that has high reactivity towards free fatty acid such as C. antarctica B-lipase and the other on TAG (for example Thermomyces lanuginosus lipase), would allow the use of oils with high FFA content. In order to minimize the processing cost, a continuous process system was developed. The utilization of lipase from Burkholderia cepacia, for ethanolysis of soybean oil in a fixed bed reactor with continuous recirculation of the reaction mixture was investigated . After 46 h of reaction at 50 °C, an ethanol to oil mole ratio of 3:1 and addition of 1 % of water to the reaction medium, a conversion as high as 95 % was observed. A two step process of TAG hydrolysis followed by fatty acid esterification using supercritical alcohol is also an
J Am Oil Chem Soc (2014) 91:525–531
alternative way to prevent the formation of degradation products . This process provided significantly higher FAME yields in the same reaction time as compared to a one-step treatment. Furthermore, applying a reaction with mild conditions of hydrolysis and esterification can reduce energy consumption and prevent the formation of degraded by-product.
Conclusions Biodiesel base stocks vary according to the different types of reactants used in the conversion of oils and fats to fatty acid esters. In terms of safety, ethanol is easier to handle, is less toxic and has a lower safety risk than methanol. However, methyl esters are easier to extract from reaction mixtures than ethyl esters, which tend to form stable emulsions. Combustion characteristics generated by ethyl esters are somewhat better than those of methyl esters but because the price of ethanol is considerably higher than methanol, methyl ester is still the leading biodiesel base stock. Alcohol sources and prices remain the main issue faced by biodiesel producers. If this problem is to be solved, alternative reliable and renewable sources of these reactants must be found. Possibilities include the use of substances from biomass such as manure and biogas. Both chemical and enzymatic transesterification have advantages and disadvantages. By mixing both alcohols in the same reaction, the benefit of ethanol’s solvent properties and the reactivity of methanol can be combined. However, determination of the right reaction conditions, such as alcohol ratios, is required to optimize final product yields. Alternatively, biodiesel producers also have the option of using catalyst-free techniques such as supercritical alcohol reactions, which are rapid, high yielding and environmental friendly.
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