Physicochemical and Thermal Properties of Biodiesel
Introduction
Fossil fuel usage has increased from 6 Gtoe to 15 Gtoe of energy consumption during the period from 1970 to 2015 (Singh et al. 2020). As the world population increases day by day. the need for petroleum products and the depletion of petroleum reserves also increase rapidly. Demand for energy has made a way for research into alternative fuels (Saravanan et al. 2020). Biodiesel is one alternative fuel similar to petrol diesel w'hich can be derived from biomass. The chemical properties of biodiesel are similar to those of diesel fuel, and biodiesel can help in the substitution of the petroleum fuels. The merits of biodiesel are domestic origin, biodegradability, nontoxicity, and inherent lubricity (Fauzan et al. 2020).
Biodiesel is a combination of methyl esters of fatty acid w'ith specific standards. Biodiesels are mainly produced from vegetable oils, animal fats, algae, and waste oils (Yesilyurt et al. 2020). Biodiesel can be used in diesel engines with blending on diesel without any engine modifications. Research into biodiesel has showm that it produces less toxic emissions compared to diesel (Madheshiya and Vedrtnam 2018). The usage of edible vegetable oils can create an energy imbalance and an availability problem. So to overcome this, non-edible sources have been identified for biodiesel production. All over the world, around 360 species of feedstocks are identified as a potential for biodiesel. Usage of non-edible sources can improve the economy and it will have less impact on the environment (Kumar and Sharma 2016). Normally vegetable oils have greater viscosity, w'hich makes them difficult in direct usage in diesel engines. Biodiesels are prepared using various techniques that are involved in reducing the viscosity of feedstocks. Pyrolysis, dilution, micro-emulsion, and transesterification are the methods. Among these, transesterification is the most commonly used technique for biodiesel production (Atabani, Silitonga, et al. 2013).
Transesterification is a chemical process between vegetable oils and alcohols w'ith the presence of catalyst to form alkyl esters. Alcohol usage does not determine the yield; cost and preparation determines the selection of alcohol. The most frequently consumed alcohols are ethanol and methanol. Transesterification is a three-stage process in which triglycerides of oil are converted to diglycerides following conversion of monoglycerides. During the reaction, methyl ester is formed with glycerol as a by-product, and glycerol has commercial value (Ogunkunle and Ahmed 2019).
The properties of biodiesel depend upon the various pretreatment processes, alcohol used, production techniques, and post-treatment processes. The physicochemical properties of the biodiesel changes from one feedstock to another because the composition of fatty acids varies. To ensure quality biodiesel, international standards EN14214 and ASTM D6751 were established, and these standards give information about the quality requirements and test procedures for determining the properties associated with the biodiesel such as its physicochemical properties and its various blends (Caldeira et al. 2017). During the transesterification reaction, the composition of fatty acids does not change. Strongly dependent on the fatty acid characteristics of feedstocks like the number of bonds and chain length are the physicochemical properties of biodiesel, such as density, viscosity, iodine value (IV), lubricity, calorific value, cetane number, cold flow properties, and oxidation stability (Ya§ar 2020).
Biodiesel has a high flash point, enhanced biodegradability, and less toxicity (Knothe 2006). The oxidation stability of the biodiesel is very low and it also has poor cold flow properties. The decrement in the stability of oxidation of biodiesel is because of the presence of polysaturated fatty acid methyl esters (FAME), as it can further react to form to aldehydes, acids, ketones, and peroxides, which can change properties of biodiesel. Due to oxidation reaction, biodiesel can have saponification at an undesirable rate, which can lead to an increase in kinematic viscosity and acid number (Tang et al. 2008). The oxidation stability is a major factor involved in storage of FAME for longer periods (Adu-Mensah et al. 2019). Atomization and vaporization characteristics of the fuel are highly dependent on the properties of kinematic viscosity and density of the biodiesel (Silitonga et al. 2013). Cetane number improves the performance of the engine and characteristics of the combustion; also, the cetane number of the fuel is a link between density of the fuel and fuel’s calorific value. Biodiesel has a high cetane number because of its rich oxygen content (Miraboutalebi, Kazemi, and Bahrami 2016).
Engine fuel not only provides energy; it should also provide lubricity to the fuel flow system. The tribological properties of engine parts are influenced by the various properties of biodiesel (Fazal, Haseeb, and Masjuki 2013). The lubricity of biodiesel is superior compared to that of ordinary diesel. The lubricity properties of various biodiesels vary depending upon their chemical properties. Before the usage of biodiesel in a conventional diesel engine, its physicochemical properties and tribological behavior of the biodiesel should be found out for better results (Wakil et al. 2015). This chapter is about the determination and influence of the physicochemical and thermal properties of biodiesel for various feedstocks.
Acid Value
The feedstocks used for the biodiesel production should have free fatty acid (FFA) content below 1% by mass (Zhou et al. 2017). If the FFA values are above 2% of its mass, then it requires a reaction of esterification consisting of two step transesterification in order to reduce the acidic value present in biodiesel. Esterification process is the first in which the reaction utilizes methanol and catalyst for producing FFA esters. The transesterification process is the second step, followed by esterification, in which base catalyst is used with the esterification products, and this yields larger amounts of methyl esters of the fatty acid and glycerol. The drawbacks of this two- step transesterification are usage of more alcohol content and increased reaction time and heat energy required for reaction (Sahabdheen and Arivarasu 2019).The high percentage of tri-glycerides content increases the acid value, and the saponification value of the biodiesel because the acid value and saponification value are proportional to the triglycerides content. Similarly, for lower acid value, number of fatty acids will be less in number. It was found that the value of acid of Aegle Marmelos Correa (AMC) oil was 2.6 mg KOH/g of oil. After the two-step transesterification process, the AMC’s acid value was found to be 0.2 mg KOH/g of oil (Thangarasu, Siddharth, and Ramanathan 2020). The acid value of Jatropha oil was 5-10 mg KOH/g and after a two-step transesterification, the acid value of oil was 0.154 with a nano-based catalyst (Deng et al. 2011).
The presence of the amount of free fatty acids in the fat was determined by the acid value. With the standard EN14104 titration method, the acid value of the biodiesel and any oil can be determined. A beaker was taken for titration process with a diethyl ether and ethanol mixture of 50 mL, a 1-g sample to be tested is taken, and it is dissolved. The mixture was added with 2-3 drops of phenolphthalein indicator. The titration was performed with a burette solution of an aqueous solution of KOH also known as potassium hydroxide with a normality of 0.1 N. The following equation was used for calculating the acid value of the tested sample.
where MW. KOH molecular weight, g/mol; V, volume of KOH in mL; N, KOH normality; w, weight of the sample.
Saponification Value
The saponification value is the required quantity of alkali to saponify a definite amount of oil or fat. The titration method of ASTM D1962 standard was used for calculation of the saponification value of biodiesel and oil samples. A unit gram of sample fat was used in the beaker for testing, which was then dissolved with ethanol solvent of 10 mL. In addition, the combination of the oil and the solvent was quantitatively discharged into ethanolic solution of 25 mL and KOH of 0.5 normality, and the result was labeled as the test sample. Another plain sample was prepared with the same procedure without any addition of sample fat. Both the test and plain samples are taken and fitted to the reflux condenser, thereby heating them for 30 min at the temperature of boiling water. After this method, the samples were cooled to reach room temperature. Eventually, a measure of 2-3 decreases of phenolphthalein was mixed to both samples and titration was done against 0.5 N hydrochloric acid (Thangarasu and Anand 2019). The following equation was used to estimate the saponification value.
where MW, KOH molecular weight, g/mol; Vplai,„ HC1 volume for plain sample, mL; Vtesl, HC1 volume for test sample, mL; N, KOH normality, mol/mL; vv, sample weight, g.
The saponification value shows the saponificable unit qualities (acyl groups) per unit weight of the given oil. A higher value of saponification suggests a greater portion of the presence of the higher-molecular-weight fatty acids which are low in the oil or vice versa (Diwakar et al. 2010). The saponification value calculates the oil’s average molecular weight and it is measured in potassium hydroxide milligrams (mg KOH g-1 oil) (Ismail and Ali 2015). It calculates the total conversion potential of the oil through conventional transesterification methods based on biodiesel production. If the saponification value is higher, it can lead to greater corrosion problems in diesel engine parts (Belagur and Chitimi 2013). Saponification values of different feedstocks were reported previously in the ranges of 169-312 mg KOH g-' oil (Azam, Waris, and Nahar 2005), 173-200 mg KOH g'1 oil (Toscano et al. 2012), and 168-202 mg KOH g-1 oil (Belagur and Chitimi 2013). Oil of higher saponification values can be used for soap and shampoo products. The soap-making ability of the oil depends on its saponification value (Odoom and Edusei 2015). The saponification value is 183.9 mg KOH g-' for Jatropha biodiesel and 200.2 mg KOH g-' for Calophyllum inophyllum biodiesel. A higher saponification value of 212-268 mg KOH g-' was seen in coconut oil (Marutani, Soria, and Martinez 2018). The saponification value of Aegle Marmelos Correa biodiesel is 224 mg KOH g-' (Thangarasu, Balaji, and Ramanathan 2019). Karanja biodiesel has a saponification value of 189 mg KOH g-' (Bart, Palmeri, and Cavallaro 2010).
