Abstract: The study and optimization of biodiesel production is very important because of its renewable nature and environmentally beneficial attributes. This study highlights methods related to the production and analytical characterization of biodiesel from vegetable and animal oil. Biodiesel is obtained by reacting vegetable and animal oil or fat with an alcohol (transesterification) in the presence of a basic catalyst to give the corresponding mono alkyl ester. Chromatographic and spectroscopic methods besides other miscellaneous methods have been reported in this study in characterizing biodiesel and assessing its quality. It also considers the advantages and disadvantages in an attempt to reduce the cost of biodiesel and other variables affecting the yield and characteristics of biodiesel. The main method that meets the prescribed standard is Gas chromatography. Gas chromatography has to date been the most widely used method for the analysis of biodiesel due to its generally higher accuracy in quantifying minor components. However no method can simultaneously satisfy all criteria of simultaneously determining all trace contaminants with minimal investments of time, cost and labour. Albeit, a fast and easy-to-use method that may be adoptable to production-monitoring such as Near Infra-red spectrometry (or viscometry) can be used for routine analyses.
INTRODUCTION
Energy has always been an important aspect of man’s activities. As demand for energy is excessively increasing, there has been a relentless search for the different forms of energy that will meet up with his activities (Ofoefule and Ibeto, 2010). Consumption of fossil fuels have increased to a great extent and the use of these energy resources is seen as an insurance policy against geopolitical risks and governmental insecurity about fossil fuels cost and safety (Refaat et al., 2008). Many countries in the world are resorting to biofuel technology to solve the problem of the gradual increasing rate of fuel and energy prices resulting from the depletion of the world’s non-renewable fossil fuels. This has led to research for alternative fuels to replace conventional petroleum fuel.
There are more than a dozen alternative and advanced fuels in production and in use today of which biodiesel is one. Biodiesel refers to any diesel-equivalent fuel made from renewable biological materials such as vegetable oils or animal fats (Pinto et al., 2005). It is a non-toxic and biodegradable fuel also made from animals oils and fats and used or recycled oils and fats. It is a clean burning fuel that varies in colour between golden and dark brown depending on the production feedstock.
| Fig. 1: | Biodiesel synthesis pathways (Holbein et al., 2004) |
Biodiesel consist of alkyl esters of long chain fatty acids derived from renewable lipids feedstock like vegetable oil or animal fats through a process called Trans-esterification reaction (Van Gerpen, 2005). This is illustrated with an equation below and the synthesis pathway is also shown in Fig. 1.
Biodiesel is worthy of continued study and production procedures optimization because of its renewable nature and favourable environmental attributes. Infact it has attracted considerable attention during the past decade as a renewable, biodegradable and non-toxic fuel (Ibeto et al., 2009).
By 1983, the process for producing fuel quality engine-tested biodiesel was completed and published internationally (Knothe et al., 2005). An Austrian company, Gaskoks obtained the technology from the South Africa Agricultural Engineers. Pioneering work in Europe and South Africa by researchers such as Martin Mittlebach furthered the development of the biodiesel fuel industry in the early 1990’s with the US industry coming on more slowly due to lower prices of petroleum diesel. Pacific biodiesel became one of the first biodiesel plants in US in 1996. The biodiesel industry became a household name in the US after the terrorist attack of 11th September 2001 resulting in historically high oil prices and an increased awareness of energy security (Knothe et al., 2005). In September 2008, Minnesota became the first US state to mandate that all diesel fuel sold in the state should contain part biodiesel, requiring a content of at least 2% biodiesel. The future of biodiesel lies in the world’s ability to produce renewable feedstocks such as vegetable oils and fats, to keep the cost of biodiesel competitive with petroleum without supplanting land necessary for food production and destroying natural ecosystem in the process. However, these feedstocks has to have specific satisfactory qualities to be suitable for biodiesel production. The vegetable oil specific parameters for good quality of biodiesel specified by different countries are shown in Table 1. The study is aimed at highlighting analytical methods for quality assessment of biodiesel from animal and vegetable oils for effective utilization as biodiesel feedstocks.
Advantages of biodiesel utilization: Apart from reducing the dependency on petroleum-based oil, biodiesel has many other advantages. Its feedstock is renewable i.e its source can be replenished through farming and recycling. It has low idle noise and easy cold starting. Its addition reduces engine wear thereby increasing the life of the fuel injection equipment. It is safe for use in all conventional diesel engines, offers the same performance and engine durability as petroleum diesel fuel. It is non-flammable and nontoxic, reduces tailpipe emissions, visible smoke and noxious fumes and odors (Emil et al., 2009). It has low or no sulphur content and is often used as an additive to Ultra-Low Sulphur Diesel (ULSD) fuel. It has been shown to have high lubricity than any other fuel. It also improves the quality of the environment with a pleasant fruity odour and with less soot generated in the exhaust of the vehicle. It has higher cetane number and actually produces less particulates, carbon monoxide and hydrocarbon emissions (El-Diwani and El-Rafie, 2008).
Disadvantages of biodiesel utilization: Biodiesel fuel though gaining more and more importance because of the depletion and uncontrollable prices of fossil fuel resources, at the same time, has some problems associated with it (Martinot and Eric, 2008). Biodiesel production competes with food industry because the feedstocks used consist of edible oil. It has a shelf life of six months hence its production must be tailored to material demand. It gives out more nitrogen oxide. It cannot be transported in pipelines due to its nature but by truck or rail which increases transportation costs. It does not flow well at low temperature which can cause problem for customers with outdoor storage tank in colder climates. It attacks rubber elements and fuel lines which are exposed to it due to it high acidity. It tends to reduce fuel economy.
Biodiesel blend: Blends of biodiesel with conventional diesel fuel represent a common utilization of biodiesel. In United States, B20 is a blend of 20% biodiesel with 80% conventional diesel fuel and is recognized as an alternative diesel fuel under criteria of the Energy Policy Act (EPACT). In France, it is utilized at a lower level blend with conventional diesel fuel.
| Table 1: | Vegetable oil specific parameters for the quality of biodiesel |
Nevertheless biodiesel can be used in its pure form, but because of the inherent higher temperature cloud point, higher pour point and cold filter plug point of biodiesel, it may require certain engine modification to avoid maintenance and performance problem and as such it is necessary to blend it with the conventional diesel. Pure biodiesel can be mixed with kerosene or diesel fuel. Most of the world uses a system known as the ‘B’ factor to state the amount of biodiesel in any mix (Knothe, 2001a). For Instance, pure biodiesel is referred to as B100 while B80 is 80% biodiesel and 20% diesel fuel. Blending B100 may be accomplished by (1) mixing in tanks at manufacturing point prior to delivery to tanker truck (2) splash -mixing in the tanker truck, i.e., adding specific percentages of biodiesel and diesel or kerosene on-line mixing which involves two components arriving at the tanker truck simultaneously. It is therefore necessary to blend biodiesel because in regions where colder temperature exists, this will keep the fuel from solidifying in tanks and fuel systems.
Properties of biodiesel: Biodiesel has better lubricating properties than today’s lower viscosity diesel (Knothe et al., 2005). The properties depend on the type of feedstock used in the production and the type of analysis carried out on the biodiesel. It has excellent solvent properties hence any deposits in the filter and delivery systems may be dissolved by biodiesel unlike petroleum diesel which forms deposits in vehicular fuel systems. It is biodegradable, immiscible with water and non-toxic. It is a liquid that varies in colour between golden and dark brown depending on the production feedstocks. It has higher cetane rating than today’s lower sulphur diesel fuels which translates to better combustion properties. The calorific value of biodiesel is about 37.27 mg L-1. It has high boiling point and low vapour pressure. Flash point of biodiesel (>30°C, >260F) is significantly higher than petroleum diesel (-45°C, -52F) (Long et al., 2001), making it safer to handle. It has a density of approximately 0.88 g cm-3.
Quality standards of biodiesel: Biodiesel has significant potential for use as an alternative fuel in compression-ignition (Diesel) engines (Prankl et al., 2004). The international standard for biodiesel is ISO 14214, another is American Society for testing Materials which is D6751 as shown in Table 2. These two are the most common standards referenced in the United States as a basis for drawing comparisons between biodiesel and diesel fuels.
| Table 2: | Detailed requirements for S15 biodiesel taken from ASTM D6751-03a |
| Source: (McCormick et al., 2005) | |
In Germany the requirements for biodiesel are fixed on a DIN standard. RME (rapeseed methyl ester from rape products according to DIN E51606), VME (vegetable methyl ester, purely vegetable products according to DIN E51606) and FME (Fat methyl ester, vegetable and animal products according to DIN V51606) are standards for different varieties of biodiesel which are made from different oils. The standards ensure that the important factors such as complete reaction, removal of glycerin, removal of catalyst, absence of free fatty acids removal of alcohol e.t.c. in the fuel production process are satisfied. The specification also includes a higher sulfur grade of biodiesel, S500, which allows 0.05 wt% sulfur.
Need for analytical methods: There is need to carry out quality assessment of biodiesel inorder to determine its chemical characteristics such as acid value, saponification value, iodine value, calorific value, cetane number, flash point, ash content, refractive index, viscosity, specific gravity, fatty acid composition/individual essential oils e.t.c. They all help in the determination of the quality of the biodiesel and specific biodiesel blends (Knothe, 2001b). Also, besides mono-alkyl esters, glycerol (main co-product), alcohol, catalyst, free fatty acids, tri-, di- and monoglycerides compose the final mixture of biodiesel production process. These and other kinds of contaminants can lead to severe operational and environmental problems. Therefore, the quality control of biodiesel is greatly significant to the success of its commercialization and market acceptance. The significance of some of the chemical characteristics of biodiesel are discussed below:
The melt or pour point refers to the temperature at which the oil in solid form starts to melt or pour. In cases where the temperatures fall below the melt point, the entire fuel system including all fuel lines and fuel tank will need to be heated.
The cloud point is the temperature at which waxes first start to crystallize in diesel fuel. It is an indication of the lowest temperature at which diesel fuel can be used before wax crystals will block fuel filters. This is the temperature at which an oil starts to solidify. While operating an engine at temperatures below an oil’s cloud point, heating will be necessary in order to avoid waxing of the fuel. It predicts, therefore, the lowest temperature of the fuel for operability (Dunn, 2003).
The flash point temperature of diesel fuel is the minimum temperature at which the fuel will ignite (flash) on application of an ignition source. It varies inversely with the fuel’s volatility. Minimum flash point temperatures are required for proper safety and handling of diesel fuel. The flash point is used as a safe index for biofuels. It also 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.
The iodine value is a value of the amount of iodine, measured in grams, absorbed by 100 g of a given oil. The iodine value is a measure of the unsaturation of fats and oils. Higher iodine value indicates higher unsaturation of fats and oils (Knothe, 2002). It is also commonly used as a measure of the chemical stability properties of different biodiesel fuels against such oxidation as described above. The Iodine value is determined by measuring the number of double bonds in the mixture of fatty acid chains in the fuel by introducing iodine into 100 g of the sample under test and measuring how many grams of that iodine are absorbed. Iodine absorption occurs at double bond positions - thus a higher IV number indicates a higher quantity of double bonds in the sample, greater potential to polymerise and hence lesser stability.
Viscosity refers to the thickness of the oil and is determined by measuring the amount of time taken for a given measure of oil to pass through an orifice of a specified size. Viscosity affects injector lubrication and fuel atomization. Fuels with low viscosity may not provide sufficient lubrication for the precision fit of fuel injection pumps, resulting in leakage or increased wear. Fuel atomization is also affected by fuel viscosity. Diesel fuels with high viscosity tend to form larger droplets on injection which can cause poor combustion, increased exhaust smoke and emissions. The higher the viscosity, the higher is the tendency of the fuel to form engine deposits (Abayeh et al., 2007).
Aniline Point/Cetane Number (CN) is a relative measure of the interval between the beginning of injection and auto ignition of the fuel. The higher the cetane number of the fuel, the shorter the delay interval and the greater its combustibility. Fuels with low cetane numbers will result in difficult starting, noise and exhaust smoke. In general, diesel engines will operate better on fuels with cetane numbers above 50.
Glycerin is a thick butter-like by-product of the production of biodiesel and must be removed at the manufacturing plant, before delivery. A small amount of glycerin contaminant would cause fuel filter blockage, particularly at the point of delivery. Fuel transfer filters would block quickly if glycerin were present in the biodiesel (Dunn, 2003).
Density is the weight per unit volume. Oils that are denser contain more energy. For example, petrol and diesel fuels give comparable energy by weight, but diesel is denser and hence gives more energy per litre.
Ash is a measure of the amount of metals contained in the fuel. High concentrations of these materials can cause injector tip plugging, combustion deposits and injection system wear. Ash content for bio-fuels is typically lower than for most coals and sulphur content is much lower than for many fossil fuels. Unlike coal ash, which may contain toxic metals and other trace contaminants, biomass ash may be used as a soil amendment to help replenish nutrients removed by harvest.
Sulfur percentage: The percentage by weight, of sulfur in the fuel Sulfur content is limited by law to very small percentages for diesel fuel used in on-road applications (Engine Manufacturers Association, 2010).
The limitation of unsaturated fatty acids is necessary due to the fact that heating higher unsaturated fatty acids results in polymerization of glycerides. This can lead to the formation of deposits or to deterioration of the lubricating fuels. Fuels with this characteristic (e.g. sunflower oil, soybean oil and safflower oil) are also likely to produce thick sludges in the sump of the engine, when fuel seeps down the sides of the cylinder into crankcase. Fatty acids with high unsaturation are prone to air oxidation and polymerization reactions which may lead to blockage of filters and fuel lines during storage (Abayeh et al., 2007). The high FFA content (>1% w/w) will cause soap formation and the separation of products will be exceedingly difficult and as a result, low yield of biodiesel product is obtaiined. The acid-catalyzed esterification of the oil is an alternative (Crabbe et al., 2001).
Analytical methods: Generally, the major categories of analytical methods for analysis of biodiesel are chromatographic and spectroscopic methods. Suitable analytical methods would be able to reliably quantify all contaminants even at trace levels with experimental ease (Mittlebach, 1994).
Chromatographic methods: Chromatography is a technique used to separate a mixture by taking into consideration its different rates of movement in a solvent over an adsorbent material. The first reports on chromatographic analysis of the transesterification of oils used thin layer chromatography with flame ionization detection (TLC/FID) lactroscan instrument (Freedman et al., 1984). In another report TLC/FID was used to correlate bound glycerol content to acyl conversion determine by GC. It was found in this work that the conversion to methyl ester is greater than 96% of the amount of bound glycerol which is less than 25% by weight. Some chromatographic methods are discussed below.
Gas chromatography: The first report on the use of capillary gas chromatography discussed the quantitation of esters as well as mono-, di- and triacylglycerols (Freedman et al., 1986). The samples were treated with N,O-bis(trimethylsilyl)trifluoracetamide (BSTFA) to give the corresponding trimethylsilyl (TMS) derivatives of the hydroxyl groups. This kind of derivatization has been carried out in subsequent quantitation of biodiesel. Derivatization to TMS derivatives is important because it improves the chromatographic properties of the hydroxylated materials and incase of coupling to a mass spectrometer, it facilitates interpretation of their mass spectra. The sample to be analyzed is injected into a micro-syringe from where a steam of an inert gas carries it to the detector of the equipment or machine. The detector gives rise to small electrical signals when components from the column pass through it. These signals are passed through an amplifier before being fed to the recorder which traces the progress of the analysis as series of peaks on a chart paper (Planck and Lorbeer, 1992). However, the accuracy of gas chromatography analyses can be influenced by factors such as base line drift, overlapping signals e.t.c. it is not always clear that such factors are compensated for in biodiesel analysis.
For biodiesel, Gas chromatography forms the basis of the standard ASTM D 6584, which is the prescribed method for measuring free and total glycerol. It determines the amount of glycerol (in derivatized form), mono- and diacylglycerols (both also in derivatized form), triacylglycerols and methyl esters in a biodiesel sample. The derivatized glycerol is the first material to elute, followed sequentially by the methyl esters and the derivatized monoacylglycerols, diacylglycerols and triacylglycerols. A Gas chromatographic method for the simultaneous determination of glycerol, mono-, di- and tri-glycerides in vegetable oil methyl esters has been developed. Trimethylsilylation of glycerol, mono- and di-glycerides, followed by Gas chromatography using a 10 m capillary coated with a 0.1 mm film of DB-5 allows the determination of all analytes in a single Gas chromatography run (Meher et al., 2006). While, many individual compounds can be separated by Gas chromatography, often some overlap in elution time occurs in complex mixtures, especially when they are major components with similar properties. On the other hand, to determine the amount of contaminants in biodiesel you actually do not need to know the complete fatty acid profile in form of the methyl esters. All that is required is the total amount of the specified contaminants relative to the methyl esters (Van Gerpen et al., 2004).
Most reports on the use of Gas chromatography for biodiesel analysis employ Flame-Ionization Detectors (FID), although the use of mass spectrometric detector (MSD) would eliminate any ambiguities about the nature of the eluting materials since mass spectra unique to individual compounds would be obtained (Knothe, 2001b). Determination of the composition C16:0, C16:1, C18:0, C18:1, C18:2 and C18:3 of three waste vegetable oils was done by gas chromatography using fused silica capillary column 60 m x 0.32 mm (ID) at the split ratio 1:5. The oven temperature was planned to remain at 150°C for 1 min, then heated at 30°C min-1. up to 240°C. Helium was used as the carrier gas with a flow rate 1 mL min-1 and also as an auxiliary gas for FID. One ìm of each diluted sample with analytical grade dichloromethane from BDH (England) was injected (Refaat et al., 2008).
A study reports that fatty acid composition of seed oil was determined using agilent 6890 series gas chromatography (GC) equipped with flame ionization detector and capillary column (30 mx0.25 mmx0.25 mm). About 0.1 mL oil was converted to methyl ester using 1 mL NaOMe (1 M) in 1 mL hexane before being injected into the GC. The detector temperature was programmed at 240°C with flow rate of 0.8 mL min-1. The injector temperature was set at 240°C. Hydrogen was used as the carrier gas. The identification of the peaks was achieved by retention times by means of comparing them with authentic standards analyzed under the same conditions (Emil et al., 2009).
Also, fatty acids composition of the used jatropha oil was determined using gas liquid chromatographic analysis of the oil ethyl esters. Modification of the oil to its ethyl esters was made using 2% H2SO4 as catalyst in the presence of dry ethyl alcohol in excess. The chromatographic analysis was made using Hewlett Packard Model 6890 Chromatograph. A capillary column 30 m length and 530 μm inner diameter packed with Apiezon® was used. Detector temperature, injection temperature and the column temperature were 280°C, 300°C and 100 to 240°C at 15°C min-1, respectively (El-Diwani et al., 2009).
High performance liquid chromatography: The technique of HPLC consists of a small column containing adsorbent on which the sample is loaded (Holcapek et al., 1999). The sample is eluted with a solvent under high pressure using pumping system. The components coming out of the column are screened by the detector system and the data is recoded in form of peaks and percentages. Reaction mixtures obtained from lipase-catalyzed transesterfication were analyzed by high performance liquid chromatography using Evaporative Light Scattering Detector (ELSD) (Foglia and Jones, 1997). This method is able to quantify product esters, free fatty acids and the various forms of acylglycerols.
The first report on the use of high performance chromatography by Trathnigg and Mittlebach (1990), described the determination of overall content of mono-, di- and tri-glycerides in fatty acid methyl esters by isocratic liquid chromatography using a density detector. The separation was achieved by coupling a cyano-modified silica column with two Gel Permeation Chromatography (GPC) columns; chloroform with an ethanol content of 0.6% was used as an eluent. This system allowed for the detection of mono-, di- and tri-glycerides as well as methyl esters as classes of compounds. The system was useful for the study of degree of conversion of the transesterification reaction.
A high performance liquid chromatographic method was developed for quantifying blends of biodiesel (simple alkyl esters of fatty acids) in petrodiesel. The method used a silica column with an isocratic mobile phase consisting of hexane and methyl t-butyl ether. Separated components were quantitated using either an Evaporative Light Scattering Detector (ELSD) or UV detector. Precision of injection and linearity of response of the ELSD and UV detectors over a range of biodiesel-petrodiesel blends [1-30 v/v %] were established by use of standards. The method also can be used for quantitating similar levels of oils or fats (triacylglycerols) in petrodiesel (Foglia et al., 2005).
In a study, catalyzed transesterification were analyzed by High Performance Liquid Chromatography (HPLC) using an evaporative light scattering detector (ELSD) (Foglia and Jones, 1997). This method is able to quantitate product esters, free fatty acids and the various forms of acylglycerols. In another study, HPLC with pulsed amperometric detection (the detection limit is usually 10-100 times lower than for amperometric detection and the detection limit is 1 mg/g) was used to determine the amount of free glycerol in vegetable oil esters. The HPLC-PAD method has proved to be simple, rapid and accurate (Meher et al., 2006).
Another study reports that the triacylglycerol (TAG) profile of jatropha oil was determined by using High-Performance Liquid Chromatography (HPLC) equipped with ELSD 800 detector. The TAGs of the oil were separated using commercial column, inertsil ODS 3 (250x4.6 mm). The mobile phase was a mixture of acetonitrile: dichloromethane (60:40) set at a flow rate of 0.8 mL min-1, with pressure 2.3 bar. TAG peaks were identified based on the retention time of available commercial TAGs standard (Emil et al., 2009). Also, purity of glycerol was determined using the HPLC Shimadzu LC 10 with a refractive index detector. The used column was Shim- Pack SCR- 10 N (7.9 mmx30 cm) (Shimadzu column). The mobile phase was water with flow rate of 0.5 mL min-1 at 50°C (El-Diwani et al., 2009).
Gel Permeation chromatography (GPC): Darnoko et al. (2000) in a report, described the use of Gel permeation chromatography as very similar to high performance liquid chromatography in instrumentation except for the nature of the column and the underlying separation principle, namely molecular weight of the analytes for gel permeation chromatography for the analysis of transesterification products. Using a refractive index detector and tetrahydrofuran as mobile phase, mono-, di- and triacylglycerols as well as the methyl esters and glycerol were analyzed (Knothe, 2001b).
Similarly, Gel permeation chromatography was used to evaluate the influence of different variables affecting the transesterification of rapeseed oil with anhydrous ethanol and sodium ethoxide as catalyst (Meher et al., 2006). Gel permeation chromatography has made the quantitation of ethyl esters, mono-, di- and tri-glycerides and glycerol possible.
Liquid chromatography with Gas chromatography (LC-GC): The purpose of combining the two separation techniques is to reduce the complexity of the gas chromatograms and to obtain more reliable peak assignments (Lechner et al., 2002). In this method of analysis also no saponification and off-line pre-separation is required. Despite the sophisticated instrumentation required liquid chromatography with gas chromatography is recommended for analysis because of additional information, short analysis time and reproducibility.
Spectroscopic methods: Spectroscopy is the study of the abruption or emission of light spectroscopic techniques, conveniently categorized by the wavelength of the light and named by the region of electromagnetic spectrum in which they occur. This method analyzes biodiesel when monitoring transesterification reaction. These methods are Nuclear Magnetic Resonance (H-1 Nuclear as well as C-13 magnetic Resonance) spectroscopy and Near-Infra Red (NIR) spectroscopy (Mittlebach, 1994). Recently, stand-alone spectroscopic methods that have been used for biodiesel include Nuclear Magnetic Resonance (NMR) and near-infrared (NIR) spectroscopy. In both cases, certain peaks characteristic for triacylglycerols and methyl esters in the spectra indicate how far the conversion of triacylglycerols to methyl esters (biodiesel) has progressed. NIR is especially easy to use and can give spectra in less than a minute. One of the advantages of these stand-alone methods is that no derivatization is needed (Van Gerpen et al., 2004).
Nuclear magnetic resonance spectroscopy: The first report on spectroscopic determination of the yield of transesterification reaction utilized 1H NMR depicting its progressing spectrum (Gelbard et al., 1995). This is the measurement of radio frequency required for resonance of a nucleus oriented in a string magnetic field. As the orbital electrons shield the nucleus to a certain extent from the applied magnetic field at a given frequency, nucleus in different electronic or chemical environment will resonate at different values of the applied field as compared to a standard compound (Gelbard et al., 1995). This resonance is called chemical shift and it enables the scientists in the deduction of structures of complex molecules (Dimmig et al., 1999).
A study reports that 1H NMR analysis allowed for the calculation of the average degree of fatty acid unsaturation (DU = 1.52) in oil and methyl ester. 1H NMR analysis also provided initial rates of methyl ester formation and activation energy of 27.2 kJ mol-1. The time-dependent concentration data revealed substantial reaction progress toward equilibrium after only 120 sec at a reduced temperature of 10°C. Understanding the resonance shifts in the 1H NMR spectra of starting materials and products allows for quantitation of reaction progress that is in good agreement with results obtained using other analytical methods (Morgenstern et al., 2006).
Turnover and reaction kinetics’ of the transesterification of rapeseed oil with methanol were studied by C-13 NMR (Dimmig et al., 1999) with benzene-d6 as solvent. The signals at approximately 14.5 ppm of the terminal methyl groups unaffected by the transesterification were used as internal quantitation standards. The methyl signals of the product methyl esters registered at around 51 ppm and the glyceridic carbons of the mono-, di- and triacylglycerols registered at 62-71 ppm. Analysis of the latter peak range allowed the determination of transesterification kinetics which showed that the formation of partial acylglycerols from the triglycerols is the slower, rate determining step.
Infra-red spectroscopy: The study of this absorption spectrum gives an important clue to the nature of bonds and the functional groups in molecules (Dimmig et al., 1999). When infra-red light of the same frequency is incident on a molecule, energy is absorbed and amplitude of that vibration increases very small amounts (one milligram or less) of a pure sample, as a solution in a sodium chloride cell or as a film on sodium radiation of increasing wavelength or decreasing frequency and a spectrum is recorded. This spectrum displays absorption maxima at various wave numbers. The finger prints region in IR spectrum of any compound is generally not reproducible by other compounds (Knothe, 1999).
Coronado et al. (2009) reported that regressions based on near-infrared spectroscopy were developed for relatively inexpensive and rapid on-line measurement of the concentration and specific gravity of biodiesel-diesel blends. Methyl esters of five different oils-soybean oil, canola oil, palm oil, waste cooking oil and coconut oil including two different brands of commercial-grade No. 2 on-highway diesel and one brand of off-road No. 2 diesel were used in the calibration and validation processes.
The basis for quantitation of the turnover from triacylglcerol feedstock to methyl ester product are differences in the near-infrared spectroscopy spectra of these classes of compounds. At 6005 cm-1 and at 4425-4430 cm-1, the methyl esters display peaks, while acylglycerols only exhibit shoulders. Ethyl esters could be distinguished in a similar fashion. Using quantitation software, it is possible to develop a method (using partial least squares regression) for quantifying the turnover of triacylglycerols to methyl esters. The absorption at 6005 cm-1 gave better result than the one at 4425 cm-1. Hence ethyl esters and perhaps even higher esters may be distinguished similarly by near-infrared spectroscopy from triacylglycerols (Knothe, 1999).
An analytical method has been developed using Fourier Transform Infrared Spectroscopy (FTIR) to determine biodiesel content in the reaction mixture to monitor the transesterification reaction. It is also shown that it can be used to determine biodiesel content in biodiesel-petrodiesel blends. The method with small modifications can also be used to determine the oil content in the adulteration of biodiesel-petrodiesel blends. Soybean oil is used as the model oil and its methyl ester is used as biodiesel. The software uses the nonlinear Classical Least Square (CLS) method for calibration. It is shown that the method can be used to measure the amount of biodiesel accurately to the extent of 98.11% accuracy for biodiesel-oil mixtures and biodiesel content in the biodiesel-petrodiesel mixture (blend) with an accuracy of 99.99%. The method has also been used to determine the oil content and biodiesel content in the biodiesel-petrodiesel-oil mixture (blend adulteration) with an accuracy of about 95.32% (Naresh and Adewuyi, 2009). Transesterification reaction which yields the methyl esters can be monitored for completion by near-infrared (NIR) spectroscopy using a fiber-optic probe (Knothe, 1999).
A study reports the use of near infrared spectroscopy to determine the content of water and methanol in industrial and laboratory-scale biodiesel samples. A qualitative analysis of the spectra by principle components analysis was carried out and partial least squares regression was used to develop calibration models between spectral and analytical data. The results indicate that the use of NIR spectroscopy in combination with multivariate calibration, is a promising technique to assess the biodiesel quality in both laboratory -scale and industrial scale samples (Felizardo et al., 2007).
Baptista et al. (2008) worked on the use of near infrared (NIR) spectroscopy to determine the ester content in biodiesel as well as the content in inolenic acid methyl esters (C18:3) in industrial and laboratory-scale biodiesel samples. Calibration models for myristic (C14:0), palmitic (C16:0), stearic (C18:0), oleic (C18:1), linoleic (C18:2) acid methyl esters were also obtained. Principal component analysis was used for the qualitative analysis of the spectra, while partial least squares regression was used to develop the calibration models between analytical and spectral data. The obtained results indicated that NIR spectroscopy, in combination with multivariate calibration, is a promising technique to assess the biodiesel quality control in both laboratory-scale and industrial scale samples.
Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES): ICP spectrometry has long been a cost-effective tool for rapid, multi-element analyses of a variety of liquids and materials, including organics. However, elemental analysis of biodiesel presents certain challenges to ICP instrumentation however, which must be met to produce credible results-not least of which is the volatility and plasma loading of diesel and biodiesel. Therefore, the instrument requires:
| • | Good organic capabilities-preferably the ability to analyze undiluted biodiesel at ambient temperatures, reducing evaporation and dilution errors while increasing sensitivity, robust, rapid response radio frequency generator to handle the high plasma loading from organic sample introduction |
| • | High resolution optics for peak separation |
| • | High saturation resistance for the detector to resist blooming due to high carbon and diatomic carbon emissions |
| • | Rapid, multi-element analysis enabling high throughput |
ICP can be used to meet all the above criteria. Since the standards are normally artificially prepared from organo-metallic stock standards and then matrix-matched as closely as possible, an internal standard is commonly used to overcome density, volatility and nebulization differences between the standards and the samples. For example, Yttrium can be added to each sample individually in the same quantity. Alternatively, the internal standards can also be automatically added on line using the Thermo Scientific iCAP 6000’s multi-channel pump and the internal standard mixing kit (Edlund et al., 2002).
ICP-OES was widely used for the analysis of organic samples such as oils or fuels. Typical examples of its applications are the determination of additive elements (such as calcium, magnesium, phosphorus, zinc) or wear metals (such as iron, chromium, nickel, molybdenum) in oils. In other cases the analysis was used to characterize the origin of the oil or to detect destruction of catalysts or metallic surfaces in the refining process caused by metal impurities. Additionally, the determination of chlorine and sulphur has become a frequent task for ICP-OES in the context of environmental monitoring (Edlund et al., 2002).
Viscometry: The viscosity difference forms the basis of viscometry, applied to determine the conversion of vegetable oil to methyl ester. The viscosity difference between the componential triacylglycerols of vegetable oils and their corresponding methyl esters resulting from transesterification is approximately one digit (Knothe, 2001b). Kinematic viscosity has been included in biodiesel standards (1.9-6.0 mm2/sec in ASTM D6751 and 3.5-5.0 mm2/sec in EN 14214) (Knothe, 2005).
Some studies have been carried out on viscosity of biodiesel. Viscosity determined at 20°C and 37.8°C were in good agreement with Gas chromatography analyses conducted for verification purposes. The difference in viscosity between the parent vegetable oil and the corresponding methyl esters can serve to monitor the progress of the transesterification reaction. Otherwise, physical property-based methods do not yield as much detailed analytical information as the other two categories of methods. It also appears that other physical properties may be suitable in a fashion similar to viscosity (Van Gerpen et al., 2004).
In order to obtain the kinematic viscosities of biodiesel fuels at temperatures up to 300°C, a modified Saybolt viscometer was designed. The viscometer was used to measure the efflux times for 60 mL of methyl esters of canola and soy and ethyl esters of fish-oil. The Modified Saybolt Viscometer was calibrated using a standard oil and can be used to measure the kinematic viscosity to within 0.056 mm2/sec with 2% repeatability. Using the measured densities over the same temperature range, the dynamic viscosities were obtained (Tate et al., 2006).
Titration for determining free fatty acids: This entails the determination of the Neutralization Number (NN) of biodiesel (Komers et al., 1997). Two methods were developed in determining strong acids and free fatty acids in one measurement. One method used potentiometery, while the other used two acid-base indicators (neutral red, phenolphthalein). The potentiometric method was more reliable with even the use of two indicators; the neutralization number values derived from the titration method were 10-20% relatively greater than the activity of the sample.
Wet chemical methods: These include those for the determination of iodine and saponification values. The fatty acid profile can only be crudely determined by wet chemical methods. The iodine value is based on the theoretical addition of iodine to the double bonds of fatty compounds. The iodine value is an indicator of the total amount of unsaturated fatty compounds in a sample. Otherwise, it does not give any information on the nature of the unsaturated compounds nor, of course, the saturated compounds. The saponification value is related to the average molecular weight of the sample of fatty compounds. It is probably best to use the iodine and saponification values in conjunction with each other.
There are several wet chemical methods for each one, each having its advantages and limitations. Neither of these values is contained in the biodiesel standard ASTM D6751. There are several wet-chemical AOCS methods for determining glycerol, for example, AOCS Official Method Ca 14-56 entitled Total, Free and Combined Glycerol Iodometric-Periodic Acid Method. Other methods deal with the determination of glycerol in specific products or under specific circumstances and are not applicable. Generally, wet chemical methods are being replaced by the more sophisticated chromatographic, spectroscopic methods or hyphenated methods because of the superior information obtained. The wet chemical methods also are often more time-consuming as they can require complex sample preparations (Van Gerpen et al., 2004).
Comparisn of the analytical methods: Virtually all methods used in the analysis of biodiesel are suitable for all biodiesel feedstock. For gas chromatography, derivatization to TMS (trimethylsilyl) derivative is important because it improves the chromatographic properties of the hydroxylated material and in case of coupling to a mass spectrometer, it facilitates interpretation of their mass spectrometer detector (MSD). Again mass spectral interpretation plays a role in determining the commercial adoption of this detection method, although the benefits of mass spectrometry would likely more than compensate the costs (Vicente et al., 2004). Gas Chromatography (GC) has to date been the most widely used method for the analysis of biodiesel due to its generally higher accuracy in quantifying minor components. However, reports show that high performance liquid chromatography has an advantage over gas chromatography in the sense that it has much faster eluent flow rate due to the high pressure. Time and reagent-consuming derivatization are also not necessary, which reduces analysis time (Meher et al., 2006).
High Performance Liquid Chromatography (HPLC) also has high sensitivity and simultaneous detection of residual alcohol is possible with this technique. The system is useful for quantifying various degrees of conversion of transesterification reactions. Operationally, the high performance liquid chromatographic method is directly applicable to most biodiesel fuels. An important fuel criterion for biodiesel is bound glycerol, which is a function of the residual amount of triglycerides and partial glycerides in the biodiesel. Either high-temperature gas chromatography or high performance liquid chromatography can be used for determining these minor but important components in biodiesel (Foglia et al., 2004).
Near-infrared spectroscopy is among the methods finding increased use. Operational ease, rapidity of measurement and non-destructiveness are among the chief reasons for this development besides accuracy and reliability. NIR spectroscopy is now being used routinely for analyzing the fatty acid composition of oilseeds besides finding other applications in the field of fats and oils (Daun and Williams, 1997; Sato, 1997). More recently, NIR spectroscopy has been used to monitor the transesterification reaction (Knothe, 1999). Although, the Near-infrared spectroscopy (NIR) method is less sensitive than Gas Chromatography (GC) for quantifying minor components, by correlation with existing GC or other analytical data, biodiesel fuel quality can be assessed through the NIR method. The NIR method is easier and faster to use than GC (Knothe, 1999). Nuclear Magnetic Resonance was once used to determine the turnover and reaction kinetics’ of transesterification of vegetable oil and animal oil with methanol while Near Infra-red spectroscopy uses a method called partial least square regression for quantifying turnover of triaclglycerols to methyl esters. The absorption usually gives better result than that of Nuclear Magnetic Resonance. Near infra-red spectroscopy spectra are obtained with the aid of fibre optic probe coupled to a spectrometer which renders the acquisition particularly easy and time-efficient (Pinto et al., 2005).
Gel permeation chromatography is very similar to high performance liquid chromatography in instrumentation. But the reproducibility is better than high performance liquid chromatography with the standard deviation at different rates of conversion (Van Gerpen, 2005). Liquid chromatography with gas chromatography separation methods reduce the complexity of the gas chromatograms and obtain more reliable peak assignments. It gives additional information, short analysis time and reproducibility (Dunn et al., 1997).
With the significant benefits of excellent sensitivity and multi-element capability, ICP is an ideal technique to analyze biodiesel and its precursors for all the required elemental analyses. In addition, simultaneous ICP allows other elements such as copper, silicon and zinc to be analyzed concurrently enabling a comprehensive, cost-effective elemental analysis (Munari et al., 2007).
The wet chemical method is the least costly in terms of equipment since only standard glassware is required. However, this method is both very slow and cumbersome. The gas chromatography method already mentioned requires a gas chromatograph. This method is somewhat faster than the wet chemical method, but in many ways is just as cumbersome. However, near infrared (NIR) also requires appropriate IR equipment. This method is quicker and less cumbersome than the other two methods, however, the equipment must be appropriately calibrated, which requires the necessary expertise (Van Gerpen et al., 2004).
The viscometric method (with results obtained at 20°C) is reported to be suitable for process control purposes due to its rapidity. ICP-OES has been investigated as a method for biodiesel analysis because of the excellent analytical figures of merit, such as multi-element capability, high power of detection, high precision and short analysis time. These figures of merit are required for product and production control of biodiesel (Edlund et al., 2002).
Gas Chromatography (GC) is also often used as the analytical method of choice for free and total glycerin analysis since it is simple, sensitive and reliable, requiring only a small amount of sample preparation. Infrared spectroscopy (FTIR) is often used for rapid on-line assessment of quality control, but is less sensitive to minor components. The two techniques are often used in conjunction with each other for a more complete analysis. UV fluorescence has been used for sulfur determination, but inductively coupled plasma optical emission spectroscopy (ICP-OES) is particularly gaining ground as the analytical method of choice for trace-sulfur determination (Ruppel and Hall, 2010).
CONCLUSION
Several analytical methods have been investigated for fuel quality assessment and production monitoring of biodiesel. Gas chromatography is an appropriate method used for verifying if biodiesel meets prescribed standards due to its ability to detect low-level contaminants, although improvements to this method have been less explored. However no method can simultaneously satisfy all criteria of simultaneously determining all trace contaminants with minimal investments of time, cost and labour. Albeit, a fast and easy-to-use method that may be adoptable to production-monitoring such as Near Infra-red spectrometry (or viscometry) can be used for routine analyses.