Biodiesel Production Techniques – The State of the Art

Introduction

The increasing everyday demand for energy and the threat posed by the crisis linked with the depletion of fossil fuels have made researchers all over the world switch over to alternative sources of energy such as biofuels. Biodiesels, which are alkyl esters of long-chain fatty acids derived from lipids such as animal fats, vegetable oils, alcohols, etc., have properties similar to those of diesel and can be a potential replacement for diesel in the near future. They are known as biodiesels because they are biologically renewable, contrary to conventional diesel, which is non-renewable (Ahmad et al. 2011; Kafuku and Mbarawa 2010; Atabani and Cesar 2014; Canakci and Sanli 2008). Biodiesels are considered as an eco-friendly and sustainable source of energy to reduce dependence on conventional fossil fuels (Kumar and Ali 2013). The demand for fossil fuels, especially high-speed diesel, increases every year at a very steady rate (Mofijur et al. 2013).

Apart from the non-renewability factor of fossil fuels, regular usage of fossil energy has contributed to 25% of the world’s toxic emissions and the greenhouse effect. Since the industrial revolution, many scientists all over the world have presented pieces of evidence about the abnormal increase in the temperature of earth registered due to human actions. This is because of high levels of C02 in the atmosphere due to the burning of fossil fuels. As people are getting more aware of pollution-free fuels and ongoing climate change, there is a rising demand for sustainable production of energy (Sadeghinezhad et al. 2013; Mat Yasin et al. 2017; Tarabet et al. 2014; Varatharajan, Cheralathan, and Velraj 2011; Utlu and Ko^'ak 2008; Kojima, Imazu, and Nishida 2014; Millo et al. 2015; Armas et al. 2005; Maiboom and Tauzia 2011). While biodiesel is not known to suffer from any drawbacks, elements of the process behind producing the feedstock, such as the agricultural input in the form of raw materials and the changes in the land being used for growing the feedstock, have been known to produce ill effects due to the usage of artificial fertilizers and chemicals.

Experimental studies have proved that direct usage of petroleum diesel also causes similar ill effects to the environment. The usage of fertilizers and chemicals can cause harm to waterways in the form of nitrification. Though biodiesel has direct advantages over conventional diesel for a local environment, it does have many drawbacks from the global environment point of view (Devi, Das, and Deka 2017). Biodiesel is a promising fuel for the future due to its extraordinary behavior when compared to other alternatives like non-toxicity, renewability, eco-friendliness, inherent lubricity, high flash and fire point, high cetane number rating, and reduced emissions (Rashid, Anwar, Moser, and Knothe 2008). The entire biodiesel feedstocks at present cannot produce a sufficient amount of biodiesel to meet the global demand. Hence it can be blended with diesel at any proportion and can be used in diesel engines without any modification (Shahabuddin et al. 2012; Jain and Sharma 2011). Biodiesel production is promoted all over the world because of the high availability of a wide range of feedstocks in the world (Janaun and Ellis 2010; Atadashi, Aroua, and Aziz 2010). Around 300 crops have been identified as potential feedstocks for the synthesis of biodiesel (Karmakar, Karmakar, and Mukherjee 2010; Atabani et al. 2012). It has been found that the cost of feedstock constitutes 75% of the entire biodiesel production cost. Hence selection of feedstock is essential and has to be economical to take care of the entire biodiesel cost production. Edible oils have been considered as the primary feedstocks for biodiesel production for a very long time. Nevertheless, this has raised a lot of concerns related to the food crisis due to the usage of these edible feedstocks for biodiesel production. This has also marked a rise in deforestation and destruction of important arable lands with fertile soil. As a result of these, edible oil prices have gone up tremendously (Balat and Balat 2010; Atabani et al. 2013, 2012; Lin et al. 2011; Lim and Teong 2010).

The global production of biodiesels has increased at a steady rate every year. Apart from transportation fuel, biodiesel has several other uses, but it becomes a problem when quantity becomes huge. It can be used as heating oil, lubricant, power-generating oil, solvents in various applications, and a remedy for oil spills (Fernandez-Alvarez et al. 2007; Ng et al. 2015; DeMello et al. 2007; Mudge and Pereira 1999; Mudge 1999; Prince, Haitmanek, and Lee 2008). The feasibility of biodiesel production has always been a critical factor in deciding its superiority over petroleum diesel. New innovative technologies used for synthesizing biodiesels are adopted at a much slower rate all over the globe due to the high investment of capital. On the other hand, the limited availability of fossil fuels has reduced the affordability of these fuels for a normal person. Thus, one cannot completely say that biodiesel is a perfect alternative for petroleum diesel because it is yet to be widely accepted. Thus biodiesel, despite being adopted as a sustainable source of energy, has yet to be put into practical implementation in many places (Knothe and Razon 2017).

Catalyst in Transesterification

Transesterification is the best known and most widely used technology to convert triglycerides into biodiesel. Compared to other technologies, it is a cost-effective and promising method to reduce the viscosity of vegetable oil. Besides, due to its higher conversion efficiency in a shorter time and low cost, this technique is commonly used for industrial biodiesel production (Bhuiya et al. 2016). The transesterification process requires a catalyst to reduce the kinetic energy and hasten the reaction forward. Usually, homogeneous or heterogeneous catalysts are employed to break down the triglycerides into esters. Moreover, many assisted catalytic techniques are available to drive the transesterification reaction, namely (i) conventional heating with mechanical stirring, (ii) ultrasound irradiation, and (iii) microwave irradiation (Jain et al. 2018). The overall transesterification reaction mechanism is presented in Figure 2.1.

The overall transesterification reaction mechanism

FIGURE 2.1 The overall transesterification reaction mechanism.

Homogeneous Catalyst

Acid Catalyst

Esterification of soybean oil with hydrochloric acid (HC1) gives 98.19% of ester yield at an optimum condition of oil to methanol molar ratio of 1:7.9 and a reaction temperature of 77°C for 104 min. The acid-catalyzed transesterification of beef tallow using an HC1 catalyst concentration of 0.5 wt.% and a methanol to beef tallow volume ratio of 6:1 for a 1.5-h reaction time at 60°C yields 96.30% of biodiesel yield (Ehiri, Ikelle, and Ozoaku 2014). Sunflower oil acid-catalyzed transesterification with 1.5 wt.% of HC1 and a reaction temperature of 100°C produced a 95.2% biodiesel yield (Sagiroglu et al. 2011). An optimum biodiesel yield of 92.5% was obtained by acid- catalyzed transesterification of Chlorella pyrenoidosa oil using H2S04 catalyst with a concentration of 0.5 wt.% (Cao et al. 2013). The transesterification of jatropha oil with H2S04 catalyst resulted in a 99% biodiesel yield with a methanol to oil ratio of 0.28 by volume, a reaction temperature of 60°C, and a catalyst concentration of 1.4 v/v for an 88-min reaction time (Sarin et al. 2007). A maximum of 99.7% of oleic methyl ester was obtained with an H2S04 catalyst concentration of 1 wt.% and a methanol to oil molar ratio of 1:3 at 100°C (Lucena, Silva, and Fernandes 2008).

Homogeneous acid catalyst has advantages over homogeneous alkaline catalysts, like the production of biodiesel from low-grade feedstock which has a higher amount of free fatty acid (FFA) than the EN 14104 limits. Moreover, the acid catalyst is not sensitive to water content in the feedstock, hence there is a chance to produce soap in reaction (Thanh et al. 2012). On either side, the homogeneous acid catalyst needs an elevated reaction temperature, a higher methanol to oil ratio, and a longer reaction time for conversion of feedstock into biodiesel than a homogeneous alkaline catalyst (Tariq, Ali, and Khalid 2012). An acid catalyst has a smaller number of active sites and fewer micropores than a base catalyst, which leads to a slow reaction rate in biodiesel production using a homogeneous acid catalyst (Dias, Alvim-Ferraz, and Almeida 2008).

Base Catalyst

Alkali homogeneous catalysts hasten the transesterification reaction by 4000 times as compared to acid catalyst. Among homogeneous catalysts, potassium hydroxide, sodium hydroxide, and sodium methoxide are the most widely used homogeneous catalysts to produce biodiesel, at lower costs and with excellent catalytic activity (Buasri et al. 2012; Fabiano et al. 2010). Figure 2.2 illustrates the reaction mechanism of alkali homogenous catalyzed transesterification. The optimum biodiesel yield of 98.2% was achieved from waste cooking oil using a KOH catalyst of 1 wt.% and an oil to methanol ratio 1:6 for 1 h of reaction time at 70°C (Agarwal et al. 2012). Thanh et al. (2014) achieved 98.1% of fatty acid methyl esters with a KOH catalyst concentration of 1 wt.%, and a 1:6 ratio of oil-methanol for 25 min by an ultrasonic sonicator method (Thanh et al. 2014). Microwave-assisted transesterification of castor oil with a KOH alkaline catalyst concentration of 1.4 wt.%, an 8 w/w ethanol to oil molar ratio, 275 W power, and 7 min of reaction time gives 95% biodiesel yield (Thirugnanasambandham et al. 2016). Rapeseed oil methanolysis with a KOH alkaline catalyst amount of 0.89-1.33 wt.% gives better yield at a temperature range of 45 to 75°C, a methanol to oil molar ratio of 1:6 to 1:7.5, and a reaction time of 3 to 5 h (Hajek et al. 2012). Maximum biodiesel was obtained by the transesterification of sewage sludge with a KOH catalyst amount of 1.5% (Wu et al. 2017). An optimum biodiesel yield of 95% was achieved from transesterification of cottonseed oil with an NaOH amount of 0.75 wt.% and a 7:1 alcohol to oil molar ratio at 110°C (Shankar, Pentapati, and Prasad 2017).

Rapeseed oil transesterification with an NaOH catalyst concentration 1.34 wt.% and an oil to methanol molar ratio of 1:6.9 gives 98% of biodiesel yield (Gu et al.

The reaction mechanism of alkali homogenous catalyzed transesterification

FIGURE 2.2 The reaction mechanism of alkali homogenous catalyzed transesterification.

2015). Alkaline sodium methoxide homogeneous catalyst has a high number of active site and gives a higher yield of biodiesel 98% by methanolysis of Jatropha curcas oil with a 1 wt.% catalyst, an oil to methanol ratio of 1:9, a stirring speed of 1000 rpm, and 30 min of reaction time at 50°C (Silitonga et al. 2013). The transesterification of Sesamum indicum L. seed oil gives 87.8% of biodiesel yield with 0.75 wt.% of sodium methoxide catalyst concentration and an oil to methanol molar ratio of 1:6 (Dawodu, Ayodele, and Bolanle-Ojo 2014). Using potassium methoxide homogeneous alkaline catalyst obtained 98.46% yield of methyl esters from transesterification of sunflower oil with 1 wt.% catalyst concentration, 65°C reaction temperature, and a 1:6 oil to methanol molar ratio (Rashid, Anwar, Moser, and Ashraf 2008). The highest biodiesel yield, 98.9%, was obtained from soybean oil by using a potassium methoxide alkaline catalyst concentration of 1 wt.% and a 1:6 molar ratio of oil-methanol at 60°C (Saydut et al. 2016).

Saponification of triglycerides or esters is less with a metal methoxide catalyst, such as potassium methoxide (CH,OK), and sodium methoxide (CH,ONa), than it is with a metal hydroxide (KOH, NaOH) catalyst. During the reaction, the metal alk- oxide catalyst is converted into СИЗО- and K+ or Na+, thereby giving no chance for water formation (Atadashi et al. 2013). Vicente et al. studied a comparative analysis of four different homogeneous alkaline catalysts, namely KOH, NaOH, CH,OK, and CH,ONa. The highest biodiesel yields, of 99.33% and 98.46%, were obtained for CH,OK and CH,ONa catalyst (Vicente, Martinez, and Aracil 2004). Homogeneous alkaline catalyst has many advantages like a high reaction rate, shorter period of conversion, and lower reaction temperature required for transesterification (Lam, Lee, and Mohamed 2010).

However, homogenous catalysts have many drawbacks over heterogeneous catalysts. Homogeneous catalysts do not work with oils with higher FFA content. Use of a homogeneous alkali catalyst for transesterification of high-FFA oil leads to soap formation. The soap formation is illustrated in Figure 2.3. Separation of homogenous catalysts is difficult, and the process generates a large amount of wastewater. Hence, the production process is costly and ultimately difficult (Mansir et al. 2018). In order to overcome the issues related to the homogeneous catalyst, there is a need for heterogeneous catalysts. Heterogeneous catalysts have some advantages over homogeneous catalysts like reusability, less toxicity, a decrease in the soaping problem, ease

Formation of soap during transesterification process

FIGURE 2.3 Formation of soap during transesterification process.

of separation, and a decrease in water usage (Yan et al. 2010; Bournay et al. 2005; DaSilveira Neto et al. 2007; Lopez Granados et al. 2010; Boffito et al. 2013).

 
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