GENERAL INTRODUCTION
In the wake of rising prices and unstable supply besides environmental issues, renewed attention has been paid to shifting away from the use of petroleum based fuels. The world’s energy demand is commencing its dependency on alternative fuels. Such alternative fuels in use today consist of bio-alcohols (such as ethanol), biomass, and natural oil/fat-derived fuels. In search for new energy sources, much attention is focused on biodiesel as a reliable and renewable resource that is to satisfy a significant part of the energy demands (Fan et al., 2009).
Currently, biodiesel is considered a promising alternative due to its renewability, better gas emissions, non toxicity and its biodegradability (Akbar et al., 2009). Biodiesel is defined as mono alkyl esters of long chain fatty acids derived from vegetable oils or animal fats (Knothe et al., 2002). The term ‘mono alkyl esters’ indicates that biodiesel contains only one ester linkage in each molecule. Plant oils and animal fats (triglycerides) contain three ester linkages between fatty acids and glycerol which makes them more viscous.
Generally, it has been observed that transesterification of triglycerides to alkyl esters (biodiesel) generates a mixture that approximates the properties and performance of petroleum diesel fuel, which allows it to be used directly as substitute fuel without modifications or as blending agents for diesel fuel (Bello and Makanju, 2011).
Various vegetable oils are potential feedstock for the production of fatty methyl esters or biodiesel but the quality of the fuel is affected by the oil composition (Akbar et al., 2009). Research results abound in literature on the production of biodiesel through transesterification of edible and non edible oil from different parts of the world (Abayeh et al., 2007; Berchmans and Hirata, 2008) The production of biodiesel from edible vegetable oils has progressively stressed food uses, price, production and availability of oils (Rashid et al., 2008). New oil-seed crops that do not compete with traditional food crops are needed to meet existing energy demands (Xu and Hanna, 2008). In Nigeria, there is an abundance of oil seeds that are relatively unexplored (Abayeh et al., 2007; Eze, 2012), with no competing food uses.
1.1 BACKGROUND OF STUDY
The recent research developments in the exploitation of biodiesel provide a reliable platform for adoption of biodiesel as an alternative energy source. The following could be key reasons to adopt and promote biodiesel production and research;
1.1.1 Availability of feed-stocks
The availability of vast biodiesel resources which include crude oil from avocado pear fruit, Beni seed, soybean, castor seed, cotton seed etc has a reliable potential for production of biodiesel that will immensely help in its utilization an alternative energy source.
1.1.2 Global warming
Another key justifiable reason for embracing and promoting the use of biodiesel is Global warming. This is the increase in the average temperature of the atmosphere, oceans and land mass of the earth (Iduyisi et al, 2012)
Environment Researchers have found out that global warming is humanly induced. Its chief cause includes burning of fossil fuels such as coal, oil and natural gas by automobiles which continually release carbon dioxide, oxides of nitrogen etc into the atmosphere. According to UNDP 2007/2008 Human Development Report, the world temperature has increased around 0.70C since the advent of industrialization and the rate is skyrocketing yearly. It is argued that bio-fuel is environment friendly because carbon dioxide released from burning bio-fuels is balanced by carbon dioxide intake by growing plants from where bio-fuels are made.
1.1.3 Greenhouse effect
Biodiesel has the ability to reduce green house gas emissions when compared to those of fossil fuels. Greenhouse effect is the process that occurs when atmospheric greenhouse gases absorb thermal radiation and re-radiates in all direction, leading to the average increase in the surface temperature. Carbon dioxide emitted by engines is the primary source of greenhouse gas emissions. Burning of biodiesel produces carbon dioxide just as fossil fuels but the former is more advantageous as the carbon dioxide released from burning biodiesel is balanced by carbon dioxide intake by growing plants from where biodiesel are made through the process of photosynthesis.
1.1.4 Pollution
Biodiesel has a higher cetane rating than fossil fuels do. As a result, biodiesel has a higher performance and clean up emissions. When compared to petro diesel, biodiesel contains fewer aromatic hydrocarbons. It has it capacity of reducing direct tailpipe emission of particulates to the environment.
1.1.5 Safety and stability
Biodiesel is safer to handle than petroleum diesel fuel because of its low volatility. Due to the high energy content of all liquid fuels, there is a danger of accidental ignition when the fuel is being stored, transported or transferred. The possibility of having an accidental ignition is related in part to the temperature at which the fuel will create enough vapours to ignite, known as the flash point temperature. The lower the flash point of a fuel, the lower the temperature at which it will form a combustible mixture (Adebayo et al., 2011). For example, petroleum diesel has a flash point of 640C, which means that it can form a combustible mixture at temperature as low as 640C. Biodiesel on the other hand has a flash point of over 1500C, meaning it cannot form a combustible mixture until it is heated well above the boiling point of water (Rodriques-Acosta et al., 2010). It is rare that biodiesel fuel is subjected to these types of condition, making biodiesel quite safe to store, handle and transport. Biodiesel is therefore classified as a non-flammable liquid.
1.2 Disadvantages of biodiesel
Although the advantages make biodiesel seem very appealing, there are also some disadvantages when using biodiesel. Due to the high oxygen content, it releases relatively high NOx levels during combustion. But this can be reduced to below petroleum diesel levels by adjusting engine timing and using a catalytic converter (Rao, 2011). Storage conditions of biodiesel must be monitored strictly as biodiesel has a lower oxidative stability (Afolabi, 2008). Biodiesel has lower temperature flow properties than petroleum diesel which means it will crystallize into a gel at low temperatures when used in its pure form (Abayeh et al., 2007). Biodiesel is also more susceptible to degradation, which is promoted by the presence of oxygen, high temperatures, and the presence of certain metals (Leiner, 1980).
1.3 BIODIESEL LITERATURE
Bio-diesel is the mono alkyl esters of long chain fatty acids obtained when vegetable oil is converted by the process of transesterification which meets the registration for fuels and fuel additives established by the Environmental Protection Agency (EPA) and American Standard of Testing and Materials (ASTM). (Radich, 2003).
This involves the reaction between triglyceride and methanol to give the fatty acid methyl ester (biodiesel) and glycerol.
Biodiesel and petroleum diesel are not chemically similar. Biodiesel is composed of long-chain methyl esters, whereas petroleum diesel is a mixture of aliphatic and aromatic hydrocarbons that contain approximately 10 – 15 carbons. Because biodiesel and petroleum diesel have differing chemical compositions, they have differing fuel properties.
1.3.1 BIODIESEL
The performance of an ester as diesel fuel depends on the chemical composition of the ester, particularly on the length of carbon chain and the degree of saturation (and unsaturation) of fatty acid molecules (Rao, 2011). There are three main types of fatty acids that can be present in a triglyceride which are saturated (Cn: 0), monounsaturated (Cn: 1), and polyunsaturated with two or three double bonds (Cn: 2, 3).
From a chemical point of view, oils from different sources have different fatty acid compositions. The fatty acids are different in relation to the chain length, degree of unsaturation or presence of other chemical functions (Pinto et al., 2005). The relative amounts of the five fatty acids (palmitic, stearic, oleic, linoleic and linolenic) common in most vegetable oils and animal fats determine the physical and chemical properties of the oils and the fuel derived from them (Gerpen, 2004). Transesterification does not alter the fatty acid composition of the feedstock and this composition plays an important role in some critical parameters of biodiesel, such as cetane number and cold flow properties. Good oil for biodiesel production must be rich in long chain and low level unsaturated fatty acid. (Pinto et al., 2005).
1.3.2 BIODIESEL USE
Biodiesel can be used as a blend component in petroleum in any proportion because it is completely miscible with ultra low sulphur diesel fuel (ULSD). Once mixed, the blend will exhibit properties different from neat biodiesel or petroleum fuels. Specifically, the most important fuel properties influenced by blending of biodiesel with petroleum are lubricity, exhaust emissions, CN, flash point, oxidative stability, low-temperature operability, kinematic viscosity, and energy content (Moser, 2009).
Biodiesel can be used in its pure form, also known as neat biodiesel or B100. This is the approach that provides the most reduction in exhaust particles, unburned hydrocarbons and carbon monoxide. This approach is used in countries like Austria and Germany. It is the best way to use biodiesel when its non-toxicity and biodegradability are important.
Biodiesel can also be used as a blend. Typically this can range from 5% to 50% biodiesel in 95% to 50% petroleum diesel and is known as B5, B10 etc depending on the blend. Blends reduce the cost impact of biodiesel while retaining some of the emission reductions. Most of these reductions appear to be proportional to the percentage of biodiesel used (Friedrich, 2003).
Biodiesel can also be used as an additive (1% – 2%) and is known as B1 or B2. Tests for lubricity have shown that biodiesel is a very effective lubricity enhancer. Even as little as 0.25% can have a measurable impact and 1% – 2% is enough to convert a very poor lubricity fuel into an acceptable fuel. Although these levels are too low to have any impact on the CN of the fuel or the emissions from the engine, the lubricity provides a significant advantage at a modest cost (Friedrich, 2003).
Blending petroleum diesel fuel with esters of vegetable oils is presently the most common form of biodiesel. The most common ratio is 80% petroleum diesel and 20% biodiesel also termed “B20â€Â, indicating 20% level of biodiesel. There are numerous reports that significant emission reductions are achieved with these blends (Knothe, 2001).
1.3.3 PETROLEUM DIESEL
Conventional diesel is produced by the distillation of crude oil collecting middle distillate fractions in the range of 175 – 370° C (Scragg, 2003). Diesel fuel typically contains over 400 distinct types of organic compounds which includes approximately 80% (vol.) of saturated hydrocarbons (primarily paraffin’s, the straight chain hydrocarbons) and 20% of aromatic hydrocarbons (naphthalene’s, the cyclic hydrocarbons and alkyl benzenes) (Rao, 2011). The saturated hydrocarbons include approximately 44% of n-paraffin, 29% of i-paraffin and 7% of naphthalene as shown below.
Fig 1: composition of petroleum diesel fuel
Source Rao, 2011
Carbon numbers of these hydrocarbons range from 12 – 18 (Singh and Singh, 2010). The aromatics are a class of hydrocarbons (HCs) that are characterized by stable chemical structures. The aromatics containing multiple benzene rings are known as polyaromatic hydrocarbons (PAH’s). The aromatics include polycyclic aromatic compounds containing 2, 3 4 and 5 fused benzene rings and the benzene will act as nuclei for the growth of undesirable shoot. Aromatics are considered desirable by compression ignition (CI) engine operators because they provide greater energy per litre of diesel fuel, however they may contribute to higher emissions of particulate matter (PM), and NOx, and have lower cetane number.
Some types of feedstock require pretreatment before they can go through the transesterification process. Feedstock with less than 4% free fatty acids such as most plant oils and some food grade animal fats do not require pretreatment. However, feedstock with more than 4% fatty acid requires pretreatment using an acid esterification process. These include inedible animal fats and recycled greases. In this pretreatment step, the feedstock is reacted with an alcohol (like methanol) in the presence of a strong acid catalyst (like sulphuric acid) to convert the free fatty acid into biodiesel. The remaining triglycerides are then converted to biodiesel through the usual transesterification reaction.
The complete process for the production of methyl ester from plant oil and other feedstock therefore involves basically five steps: acid esterification, transesterification, methanol recovery, biodiesel refining, and glycerin refining (Filemon et al, 2010).
Acid esterification: The oil feedstock containing more than 4% free fatty acids is usually pretreated using an acid esterification process in order to increase the yield of biodiesel. The feedstock is first filtered and then pre-processed to remove water and other contaminants such as unwanted solids. The pretreated oil is then fed to the acid esterification process. The catalyst, sulfuric acid, is dissolved in methanol and the mixed with the pretreated oil. The mixture is heated and stirred, and the free fatty acids are converted to biodiesel. Once the reaction is complete, it is dewatered and then fed to the transesterification process.
Transesterification: Transesterification is the general term used to describe the important class of organic reactions where an ester is transformed into another ester by interchange of the alkoxy moiety (Rafaat et al., 2008). In this process, an alcohol reacts with triglycerides in the presence of catalyst.
The main purpose of transesterification is to reduce the viscosity of oil in order to achieve properties that are more suitable for its function as a fuel (Hossain et al., 2010), a catalyst is usually used to improve the reaction rate and yield (Singh and Singh, 2010). Excess alcohol is used for shifting the equilibrium toward the product because of the reversible nature of the reaction (Shereena and Thangaraj, 2009). The alkyl esters produced depend on the alcohol used, where methanol and ethanol are mostly used. Osai (2011), in his comparative studies on the effect of different alcohols on biodiesel yield achieved high conversions of 90%, 85%, and 81% by reacting fluted pumpkin oil with methanol, ethanol and propanol respectively. In another study by Berchmans and Hirata (2008), 90% methyl ester yield was obtained through an alkali catalyzed transesterification process using Jatropha curcas seed oil.
Transesterification has turned out to be an ideal modification process for biodiesel production (Demirbas, 2009). The transesterification of triglycerides into methyl or ethyl esters reduces the molecular weight to one-third that of the triglyceride and also reduces the viscosity by a factor of about eight and increases the volatility marginally (Singh and Singh, 2010). This produces a mixture (biodiesel) with suitable fuel properties. The chemistry of transesterification is mainly centered on triglycerides because oil/fats contain about 98% triglycerides (Ivanoiu, 2011). Therefore, the stoichiometric relationship requires 3mole of alcohol per mol of Triglycerides (3:1) to form one mole of glycerol and three moles of the respective fatty acid alkyl esters. In practice, the ratio needs to be higher to drive the equilibrium to a maximum ester yield (Ma and Hanna, 1999). The transesterification of Triglyceride is a sequence of three reversible reactions, in which the Triglyceride is first converted to monoglyceride and fatty acid. Then, the diglyceride is converted to glycerol liberating an additional ester, and finally the monoglyceride is converted to glycerol liberating the final fatty acid ester. The plant oil, which contains less than 4% free fatty acids, is first filtered and then pre-processed to remove water and other contaminants. The pretreated oil is then fed directly to the transesterification process along with any products of the acid esterification process. The catalyst, potassium hydroxide, is dissolved in methanol and then mixed with the pretreated oil. If an acid esterification process is used, then additional alkaline catalyst must be added to neutralize any excess acid remaining from that step. Once the reaction is complete, the major co-products, biodiesel and glycerin, are separated into two layers.
Methanol recovery: The methanol is usually removed immediately after the biodiesel and glycerin have been separated. This is done to prevent the reaction from reversing itself. The recovered methanol is cleaned and recycled back to the beginning of the process.
Biodiesel refining: Once separated from the glycerin, the biodiesel goes through a series of cleaning-up or purification steps to remove excess alcohol, residual catalyst and soaps. These consist of multistage washings with clean water. The product biodiesel can be further refined through an additional distillation step to produce a colorless, odorless, zero-sulfur, and premium quality biodiesel.
Glycerin refining: The crude glycerin from the transesterification process may be recovered or used in a fuel blend for steam production. The crude glycerin contains unreacted catalyst and soaps that must be neutralized with an acid. The water and alcohol are also removed to produce 50%-80% crude glycerin. The remaining contaminants include unreacted fats and oils. In large biodiesel plants, the glycerin can be further purified through a series of unit operations to produce a product of 99% or higher purity. This purified product is suitable for use in the pharmaceutical and cosmetic industries.
Transesterification is extremely important for biodiesel. Methanol is the preferred alcohol for obtaining biodiesel because it is the cheapest and the most available (Van Gerpen et al, 2004).
For a transesterification process to occur, 6:1 oil to methanol ratio is usually used. This is to enable the equilibrium to shift to the right in order to favor biodiesel production. Transesterification is a catalyzed reaction which can be base or acid catalyzed. Base catalyzed transesterification is most preferred because of fast reaction rate. Bases used are sodium or potassium hydroxide.
It is important to note that soap might be formed instead of biodiesel which is the target product.
It is common for oils and fats to contain small amounts of water and free fatty acids. Free fatty acids consist of the long carbon chains that are disconnected from the glycerol backbone, they are called carboxylic acids.
If an oil or fat containing free fatty acids such as oleic acid is used to produce biodiesel, the alkali catalyst typically used to encourage the reaction will react with this acid to form soap.
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Formation of soap
O
| |
HO-C-(CH2)7 CH=CH (CH2)7CH3         +     KOH
Oleic Acid                                                Potassium Hydroxide
                  O
| |
K+ -O – C – (CH3)7 CH=CH (CH2)7CH3     +    H2O
Potassium oleate (soap)                                        Water
This reaction is undesirable as it binds the catalyst into a form that does not contribute to accelerating the reaction. Excessive soap in the products can inhibit later processing of biodiesel, including glycerol separation and water washing. Water in the oil or fat can also be a problem by the formation of free fatty acid. When an alkali is present, the free fatty acid will react to form soap while water manifests itself through excess soap formation. The transesterification of oils and fats is often accompanied by 2 side reactions when the feedstocks contain free fatty acid and moisture. Influence of FFAs on the feedstock quality used in biodiesel production in large part dictates what type of catalyst or process is needed to produce fatty acid methyl esters that will satisfy relevant biodiesel fuel standards such as
American standard for testing material (ASTM) or European norm (EN). The FFA and moisture contents have significant effects on the transesterification of triglycerides with alcohol using base as catalyst (Berchmans and Hirata, 2008). When the feedstock contains a significant percentage of FFA (>3 wt. %), typical homogenous base catalysts such as sodium or potassium hydroxide or methoxide will not be effective as a result of unwanted side reaction in which the catalyst will react with FFA to form soap and water.
1.4 Variables affecting the process of transesterification
1.4.1 Catalysts
Catalysts used for the transesterification of triglycerides are classified as alkali, acid, or enzyme (Vasudevan and Briggs, 2008; Shereena and Thangaraj, 2009; Singh and Singh, 2010).
1.4.2 Effect of molar ratio
Another important variable affecting the ester (biodiesel) yield is the molar ratio of alcohol to vegetable oil. As indicated earlier, this reaction is reversible and the stoichiometry of the transesterification reaction requires 3moles of alcohol per mole of triglyceride to yield 3moles of fatty acid esters and 1mole of glycerol. Therefore, excess amounts of alcohol are needed to shift the reaction equilibrium to the product side and higher molar ratios result in greater ester conversion in a shorter time (Shereena and Thangaraj, 2009: Xu and Hanna, 2008). However, the high molar ratio of alcohol to vegetable oil makes the recovery of glycerol difficult because there is an increase in solubility (Demirba, 2008). When the glycerol remains in solution, it helps to drive the equilibrium back to the left, lowering the yield of esters.
1.4.3 Effect reaction time and temperature
The rate of reaction is strongly influenced by the reaction temperature. Higher reaction temperatures speed up the reaction and shorten the reaction time. In the transesterification of Triglycerides, the reaction is slow at the beginning for a short time and proceeds quickly and then slows down again (Ma and Hanna, 1999).According to Xu and Hanna (2009), the methyl ester yield increases with increasing reaction temperature. From the research of Xu and Hanna (2009), when the reaction time was 40 minutes, methyl ester yield increased from 74% to 89% and 93% with the reaction temperature increasing from 25 to 45 and then to 650C. This is thought to be the consequence of the favorable effect of the high temperature on diffusion of methanol molecules and reaction with triglyceride molecules.
Hossain et al. (2010) reported 2 hr reaction time gave better ester yield than 6 hour reaction time for the production of biodiesel. It is generally reported that every reaction has a certain time of completion. For the production of biodiesel, it takes about 90 to 120 minutes to complete the conversion (Singh and Pahdi, 2009). The longer the reaction time, the more the hydrolysis of ester would occur. It might produce many free fatty acids at the end and this free fatty acid would participate in soap formation thus reducing the biodiesel yield. Thus excess reaction time does not promote the conversion but favours the reduction in the ester yield.
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1.4.4 Effect of moisture and free fatty acid
The quality of any feedstock has considerable effect on the level of biodiesel production (Shereena and Thangaraj, 2009). For alkali-catalyzed transesterification, the Triglyceride and alcohol must be substantially anhydrous and the free fatty acid level of Triglyceride at minimal because these impurities result to adverse reactions such as saponification and hydrolysis (Drapcho et al., 2008). The soap produced through saponification consumes the catalyst and reduces the catalytic efficiency, as well as causing an increase in viscosity, the formation of gels and difficulty in achieving separation of glycerol (Ma and Hanna, 1999). Fukuda et al. (1999) in their research also noted the influence of feedstock quality (moisture and Free fatty acid) on the transesterification reaction. Excess amount of free fatty acids and water are common features of waste vegetable or animal-based oils’ conversion yield of 65% to 84% esters using crude vegetable oil as compared to 94% to 97%. Yield with refined oil under same reaction conditions has been obtained (Singh and Pahdi, 2009). In many cases, feedstock quality deteriorates gradually due to improper handling and inappropriate storage condition. Improper handling would cause the water content to increase. In addition, exposing the oil to open air and sunlight for a longtime would cause the concentration of FFA to increase significantly (Berchmans and Hirata, 2008).
1.5 REACTIONS INVOLVED IN BIODIESEL PRODUCTION
          Two of the most commonly used catalyst for transesterification is sodium and potassium hydroxide. The catalyst operate by reacting with the alcohol used (methanol).
CH3OH   +   NaOH                     CH3ONa   +   H2O
Similar to H20 consisting of H+ and OH, CH3ONa can be seen consisting of CH3O (alkoxide; alkylate) and Na+. CH3O is the specie that attacks the ester moieties in the glycerol molecule.
The triglyceride anion picks a proton from the methanol and the alkoxide catalyst is recovered. But if the triglyceride anion reacts with water molecule, it will pick a hydroxyl ion and produce fatty acid instead of methyl ester. This fatty acid will react with the base to form soap. This affects the transesterification reaction negatively.
It is more important to use sodium or potassium alkylate directly since effect of too much catalyst leads to formation of mono or diglyceride as well as fatty acid which are undesirable and will definitely lead to soap formation. This will therefore favor transesterification.
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1.6 Why are vegetable oil transesterified to produce biodiesel?
         Vegetable oil methyl esters have lower viscosities (resistance to flow of liquid) than the parent vegetable oils (think of honey or syrup which have high viscosities and flow with difficulty, vs. water or milk, which have low viscosities and flow easily). Compared to the viscosities of the parent vegetable oils, the viscosities of vegetable oil methyl esters are much closer to that of petro diesel. High viscosity causes operational problems in a diesel engine such as poor quality fuel injection and the formation of deposit (Van Gerpen, 2004).
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1.7 Fuel properties and quality standard of biodiesel.
Specifications for biodiesel require particularly close attention due to the large variety of vegetable oils that can be used for biodiesel production and the variability in fuel characteristics that can occur with fuel produced from this feedstock. Today, biodiesel has much stricter definitions in the form of quality standards established to gain wider acceptance from engine manufacturers, distributors, retailers and users (Johnston, 2006). Numerous biodiesel standards are currently in force in a number of countries including in the European countries such as Germany, Italy, France, and the Czech Republic and the ASTM in the USA. These standards provide fuel property values required for a mixture of alkyl esters to be considered as biodiesel. If these limits are met then the biodiesel can be used in modern engines with little or no modification (Abayeh et al., 2007).
1.7.1 Kinematics viscosity
Viscosity is defined as the resistance to shear or flow; it is highly dependent on temperature and it describes the behavior of a liquid in motion near a solid boundary like the walls of a pipe. The presence of strong or weak interactions at the molecular level can greatly affect the way the molecules of an oil or fat slide pass each other, therefore, affecting their resistance to flow (Shannon et al, 2009)
The kinematic viscosity test calls for a glass capillary viscometer with a calibration constant (c) given in mm2/s2. The kinematic viscosity determination requires the measurement of the time (t) the fluid it takes to go from point A to point B inside the viscometer.
Dynamic viscosity is the ratio of applied shear stress and rate of shear of a liquid. Generally viscosity increases with the number of CH2 moieties in the fatty ester chain and decreases with an increasing unsaturation (Knothe, 2008). For oils exposed to oxidizing conditions and high temperatures, degradation of the oil is normally accompanied by an increase in viscosity (Popovich and Hering, 1959). Changes occurring in the oil under these conditions can be followed by viscosity measurement. The kinetic viscosity of biodiesel according to ASTM is within the range of 1.6 – 6.0 as shown in Table 1 Lower viscosity may also indicate the presence of methanol in the biodiesel.
1.7.2 Free fatty acid
The interaction of FFA in the feedstock and sodium methoxide catalyst may form emulsions which make separation of the biodiesel more difficult; possibly leading to yield loss. To minimize the generation of soaps during the reaction, the target reduction for FFA in the feedstock was 0.5wt % maximum.
1.7.3 Flash point temperature
Flash point is the minimum temperature at which the fuel will give off enough vapours to produce an inflammable mixture (fuel vapour and air) above the fuel surface, when the fuel is heated under standard test conditions (Rao, 2011). The fundamental reason for measuring flash point is to assess the safetyhazard of a liquid with regards to its flammability and then classify the liquid into a recognized hazard group. This classification is used to warn of a risk and to enable the correct precautions to be taken when manufacturing, storing, transporting or using the liquid (Belewu et al., 2010). Tests have shown that as little as 1% methanol in biodiesel can lower the flash point from 1700C to less than 400C (Van Gerpen, 2004). Therefore by including a flash point specification of 1300C, the ASTM standard limits the amount of alcohol to a very low level (<0.1%). The flash point is used as a safety index for biofuels because it correlates to the fuel ignitability and varies inversely with the fuel’s volatility. Biodiesel with a flash point of 1500C Falls under the non hazardous category and it is safe for usage. Specifications also quote flash point for quality control purposes. It indicates the level of purification the fuel has undergone; as the presence of a very small amount of alcohol in the biodiesel leads to a significant drop in the flash point (Abayeh et al., 2007). Also, a change in flash point may indicate the presence of potentially dangerous volatile contaminants or the adulteration of one product by another.
1.7.4 Cloud point
Cloud point (CP) is the temperature at which some of the molecules in the fuel first begin to freeze, resulting in the appearance of crystals in the fuel, which gives it an initial cloudy appearance (Abayeh et al., 2007). A major problem of biodiesel is poor temperature flow properties indicated by relatively high CP. This makes CP a critical factor in the cold weather performance of diesel fuel. Therefore, it is an index of the lowest temperature of the fuel’s usability for certain applications. Operating at temperatures below the Cloud point of a biodiesel fuel can result in filter clogging due to wax crystals (Abayeh et al., 2007). Since the saturated methyl esters are the first to precipitate, the amounts of these esters, methyl palmitate and methyl stearate, are the determining factors for the Cloud point. The cloud point of a fuel can be modified in two ways. One is through the use of additives that retard the formation of solid crystals in the biodiesel. The cloud point can also be modified by blending feedstock that are relatively high in saturated fatty acids with feedstock that have lower saturated fatty acid content. The result is a net lower cloud Point for the mixture. Thus, the lower the Cloud point, the higher the quality of the fuel since a high Cloud Point limits the flow properties of biodiesel, which influences its use in a cold environment (Xu and Hanna, 2009). According to Popovich and Hering (1959) the cloud point may also be used in identifying the source of the oil/fat.
1.7.5 Water/moisture content
Biodiesel water content is an important parameter because it affects biodiesel oxidation stability, therefore influencing the storage life of the fuel (Dias et al., 2008). Water can be present in two forms, either as dissolved water or as suspended water droplets. While biodiesel is generally considered to be insoluble in water, it actually takes up (hygroscopic) considerably more water than petroleum diesel. Biodiesel contains as much as 0.15% of dissolved water while petroleum diesel usually takes up about 0.005% (Van Gerpen, 2004). The standards for petroleum diesel fuel (ASTM D975) and biodiesel (ASTM D6751) both limit the amount of water to 0.05% as shown in table 1. Water promotes adverse reaction to transesterification which will convert biodiesel back into free fatty acid. Suspended water is a problem in fuel injection equipment because it contributes to the corrosion of the closely fitting parts in the fuel injection system. The moisture content is particularly important when applied to oils and fuels since it will provide a measure of deterioration and contamination (Popovich and Hering, 1959). Water can also contribute to microbial growth in the fuel. This problem can occur in both biodiesel and petroleum diesel fuel and can result in acidic fuel and sludge’s that plug fuel filters (Van Gerpen, 2004). The water in biodiesel plays an important role in predicting quality performance of the fuel. In commercial practice, the level of moisture and impurities is one of the most important quality characteristics limited by norms and standards (Hoffmann, 1986). The low moisture content often shows that a fuel is good and could not be easily subjected to contamination/rancidity.
1.7.6 Refractive index
The refractive index is the quotient of the sine of the incidence angle of light in the air and the sine of the angle of refraction of light in the substance (Hoffmann, 1986). It employs the principle of critical angle using diffused light. It is used in measuring the concentration of solutions because when the concentration or density of a substance increases, its refractive index increases proportionally (Parthiban et al., 2011). The refractive index is characteristic of each kind of oil/fat. Its value varies with the degree and type of unsaturation of component fatty acids in an oil/fat sample. Refractive index increases with the increase in unsaturation and with the chain length of fatty acid (Nayak and Patel, 2010). Therefore the refractive index of oils is subject to change during processing (hydrogenation) and use (polymerization during heat treatments) hence it can be used successfully in quality control. Refractive index and specific gravity measurements rarely provide sufficient information to quantitatively identify a pure analyte, but are highly useful to check oil contamination/adulteration (Parthiban et al., 2011).
1.7.7 Specific gravity
This is the measure of relative density of the biodiesel compared to the density of water. It is a measure of weight per unit of volume. The relative density of biodiesel is needed to make mass to volume conversions, calculated flow and viscosity properties of biodiesel tanks.
1.7.8 Fire point
This is a measure of the tendency of the test specimen to support combustion. Fire point is a parameter that is not commonly specified, although in some cases, knowledge of this flammability temperature may be desired. It has a direct proportionality to fire point.
1.7.9 Iodine value
The primary products that appear during the autoxidation of fats/oils are hydro peroxides (Liener, 1980). These hydro peroxides contain ‘active or peroxide oxygen’, which if decomposed in a medium (acid), can be measured and the amount of hydro peroxides calculated. Peroxide value is the amount of substances in the sample, expressed in terms of mill equivalent of ‘active or peroxide oxygen’ per kilogram fat which oxidize potassium iodide under the operating conditions (Hoffman, 1986). Metals present in trace amounts are often responsible for the primary initiation, and metals that are oxidized by one electron transfer are the most active. Accordingly, cobalt, copper, iron, nickel, manganese and other such metals have been found to be potent lipid per oxidants (Liener, 1980). The nature and extent of the changes that take place in fats/oils in storage or upon heating and oxidation depend very much on the kind of fat/oil used and the conditions under which it has been heated. The usual method of assessing hydro peroxide is by the determination of peroxide value (Gunston, 1999). Peroxide value is used to monitor the development of rancidity through the evaluation of the quantity of peroxides generated in the initial product of oxidation. Regarding the nature of the fatty acids involved, the results of most studies demonstrate that in nearly all cases, the unsaturated fatty acids were the most susceptible of the fatty acid in question to these effects (Igwenyi et al., 2011).
1.7.1.0 Saponification value
The saponification value is the number of milligrams of potassium hydroxide required to neutralize the Fatty acids liberated on complete hydrolysis or saponification of 1g of the oil (Igwenyi et al, 2011). Saponification value is an index of the average size of fatty acid present, which depends upon the molecular weight and percentage concentration of fatty acid components in the oil (Parthiban et al., 2011). An increase in saponification value in oil increases the volatility of the oil and this enhances the quality of the oil because it shows the presence of lower molecular weight components in 1g of the oil. This is in agreement with the report of Afolabi 2011, that oil fractions with saponification values of 200mgKOH/g and above possess low molecular weight fatty acids. Since 1g of oil/fat containing low molecular weight fatty acids will have more molecules than oil/fat containing higher molecular weight fatty acids (Igwenyi et al., 2011). This principle reveals that the number of milligrams of KOH required to saponify the oil will be greater in the former than in the latter case. Therefore, the higher the saponification value, the lower the molecular weight of the fatty acids and the better the quality of the oil.
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Table 1: International standard of biodiesel
S/N
PARAMETERS
UNIT
USA
(ASTM)
1
Free fatty acid
%
0.5 max
2
Acid value
mgKOH/g
0.8 max
3
Specific gravity
–
0.87-0.9
4
Kinematics viscosity
Cst
1.9-6.0
5
Refractive index
–
–
6
Flash point
0C
130
7
Cloud point
0C
-3-12
8
Fire point
0C
53
9
Water content
%
0.03 max
Source: National standard for biodiesel, 2003
1.7.1.1 Sterculia setigera
Sterculia setigera is a multifunctional forest woody tree species in sub-Sahara Africa, especially known in Senegal for its economic value; its gum is exported since several decades.
The species boiled leaves are used to treat malaria, and the stem bark decoction is used for the treatment of asthma, bronchitis, wound, fever, toothache, etc. sterculia setigera is a deciduous tree with a large, open, spreading crown; it generally grows up to 16 meters tall, but specimens up to 35 meters have been recorded in the Sudan and guineas zones.
1.7.1.2 Research Aims and Objectives.
Worldwide, biodiesel production has been adjusted to the available crops in each region. An oil seed crop amenable to Nigeria’s environmental condition is still in search. The aim of this study was to investigate the properties of oil methyl esters produced from Sterculia setigera seeds by transesterification process. The specific objectives include: