Vegetable Oil as Biofuel. Chemical Characteristics and Transesterification Procedure


Research Paper (postgraduate), 2012

19 Pages, Grade: 5.00


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Table of Contents

1. Introduction

2. Vegetable Oil
2.1. Composition of oils
2.2 Chemical structures of common fatty acid and their methyl esters

3. Characterization of the oils
3.1. Feedstock for biodiesel production
3.2. Chemical characteristics of oil for biofuel production
3.2.1 Acidity of the oils
3.2.2 Moisture content
3.2.3 Determination of fatty acids composition
3.2.4 Determination of iodine value
3.2.5 Determination of saponification number
3.2.6 Determination of peroxide number
3.2.7 Determination of cetane number
3.2.8 Free fatty acids esterification
3.2.9 Activity tests

4. Conclusion

5. References

Abstract: Reacting oils or fats in an esterification process basically contain monoglycerides, diglycerides, triglycerides, lipids and free fatty acids. Triglyceride (TAGs) nevertheless has a good prospect as an alternative fuel. Triglyceride has a benefit as been renewable and biodegradable with higher cetane number. Biodiesel is the product from a variety of reacting feedstocks. Feedstocks used will vary from vegetable oils (soybean, cottonseed, palm, peanut, rapeseed/canola, sunflower), animal fats (tallow, chicken fat, fish oils) to waste cooking oil and grease. A transesterification reaction involving oil or fat with alcohol will lead to biodiesel which is a mixture of fatty esters. Each ester component contributes to the properties of the fuel. Esters containing higher alcohol content with fatty acids can also be used as biolubricants. This fuel is biodegradable, non-toxic and has low emission profiles than petroleum diesel. Biodiesel can mix with petro-diesel in all distinction and can be used as such with petroleum diesel for direct appliance in diesel engines. Thus, it is very essential and critical to have the data of fatty acid profile of oil and fat used. This should also include their chemical properties.

Keywords : Vegetable oil, Triglycerides, Transesterification, Chemical

1. Introduction

Triglyceides have varying properties. Oils have high Iodine Value usually with high concentration of peroxides. Fats will have low Iodine Value with relatively low concentration of peroxides at the start of rancidity. Peroxide Numbers (PN) is an oil chemical characteristic not specified in current Biodiesel fuel standards. This property can affect cetane number (CN). Cetane number is a parameter that is regulated by the standards which concern biodiesel fuel. PN increases with CN, hence, PN can alter ignition delay period. Saponification number (SN) another property of fuel is an indication of the index number of fatty alkyl chains that can be in a saponification process. Long chain fatty acids have a low Saponifictation number. Their low saponification number arises as a result of relatively fewer numbers of carboxylic functional groups per mass unit of fat compared to short chain fatty acids. In most cases, the experimental SN value is usually lower than it’s calculated theoretically value. This can also be explained in terms of the PN value. The peroxide number value is an indication that a high concentration of oxygen is bound to a fatty alkyl chain.

The number of saturated fatty chains in the fuel determines the fuel’s behaviour at low temperatures. Parameters such as the cloud point, the CFPP (cold filter plugging point) and the freezing point influences fatty acid chain characteristic (Winayanuwattikun et al., 2008). Iodine value another property of fatty acid is similar to its calculated theoretical value. Experimental Iodine value differs from its theoretical one, and most of the time is underestimated. This is explained taking into consideration the peroxide numbers (PN). Hence, states concentration of O2 bound to the fatty alkyl chains as an index of the conservation state of oil.

The reaction where triglycerides are transformed into biodiesel is transesterification. In a transesterification reaction, triacylglycerol (TAG) reacts with alcohol in the presence of a catalyst. The product will take form of alkyl esters of the fatty acids used. In order to achieve high yields of alkyl esters of approximately 99.7 %, typically 50 % - 200 % excess alcohol is needed. The alcohol typically needed will be methanol. It is also possible to get acceptable biodiesel in terms of fuel properties with ethanol or iso-propanol. When methanol is used in biofuel synthesis, the derived biodiesel product is composed of fatty acid methyl esters (FAME). Transesterification is the most common method for biodiesel production. In this reaction type, a fat or oil (triglyceride) reacts with three moles of light alcohol (C1 to C4) to give three moles fatty acid alkyl ester (biodiesel) and one mole of glycerol as the by-product. This process is transesterification of one ester into another. Catalyst may also be required.

Biodiesel quality depends on the quality of the feedstock and materials used for its transesterification process. For an efficient transesterification process, alcohol should be free of moisture and feedstock should have less than 0.5% of FFA. Moisture that can come from alcohol has a potential of reacting with alkyl esters, triglycerides, diglycerides, and monoglycerides to form FFAs. FFA contents of vegetable oil and waste oil are usually high; the raw material should be pretreated. If not, during transesterification will cause separation problems in the biodiesel production process because of lather formation (mainly for homogeneous process). Another factor that determines the quality of biodiesel is the side products that are formed during a transesterification reaction such as intermediate glycerols, mono- and diacylglycerols and unreacted triacylglycerols, FFA, residual alcohol, and catalyst contaminant in the final product. Biodiesel has a variety of benefits ranging from environmental to safety properties comparable to fuel properties of regular diesel fuel.

2. Vegetable Oil

Vegetable oil can be converted to biodiesel by transesterification. Transesterification process is via alcoholysis into its methyl or ethyl ester derivatives. These derivates are an alternative approach to a viable vegetable oil-based diesel fuel. Transesterification significantly reduces vegetable oil viscosity. However, fuel atomization and fuel combustion characteristics may improve. Fatty ester product will possess many similar physical characteristics to those of no. 2 diesel fuel as described by data primarily in Clark’s et al. study (Clark, 1983).

The physical characteristics of vegetable oils pose several technical problems. Indirect injection diesel engines have been reported to operate satisfactorily on crude de-gummed vegetable oil fuels (Plechinger, et al., 1980; Van der Walt and Hugo 1981). Direct injection diesel engines, as are applicable to most agricultural machinery. They can only operate adequately for a brief duration on neat vegetable oil fuels. Longterm use results to injector coking, gum formation and lubricating oil thickening (Van der Walt and Hugo 1981).

2.1. Composition of oils

Fats and oils are primarily water-insoluble hydrophobic substances. These components of plant and animal origin are made up of a mole of glycerol and three moles of fatty acids. Fat and oil are commonly referred to as triglycerides. Fatty acids vary in carbon chain length and in the number of unsaturated bonds. The fatty acids found in vegetable oils are summarized in Natural vegetable oils and animal fats. Some oil or fat are solvent extracted or mechanically pressed to obtain crude oil or fat. These usually contain free fatty acids, phospholipids, sterols, water, odorants and other impurities. Even refined oils and fats contain small amounts of free fatty acids and water. The free fatty acid and water contents have significant effects on the transesterification of glycerides with alcohols using alkaline or acid catalysts. They also interfere with the separation of fatty acid alkyl esters and glycerol because of salt formation in the product.

Chemically synthesized oils or fats are triacylglycerol or triglycerides (TG). Triglyceride is a product of triester of fatty acids with glycerol. The physical and chemical properties of oils and fats depend on their fatty acids composition. The composition of oil is indicated according to its fatty acid profile of the oil or fat. Hence, for example, sunflower oil has fatty acid profile as a representative of palmitic acid (7 wt %), stearic acid (5 wt %), oleic acid (19 wt %), linoleic acid (68 wt %) and linolenic acid (1 wt %). Any transesterification reaction accounts for a reaction of oil or fat with alcohol to produce biodiesel. The biodiesel product of an esterification reaction is a mixture of different fatty esters. Each fatty ester present is a component contributing to the properties of the fuel. Therefore, a vital knowledge of fatty acid profile of oil and fat and their properties is very important and crucial to biofuel production. The physical properties of a fatty acid are classified in terms of its natural oil and fat. Fatty acids are classified according to three schemes in term of the number of double bonds present in the fatty acid .

1. Saturated fatty acids: stearic acid [C18:0], palmitic acid [C16:0], myristic acid [C14:0], lauric acid [C12:0]
2. Mono-unsaturated fatty acids: oleic acid [C18:1], palmitoleic acid [C16:1]
3. Poly-unsaturated fatty acids: linoleic acid [C18:2], linolenic acid [C18:3]

Transesterification involves reacting a triglyceride molecule or a complex fatty acid with alcohol. The process includes neutralizing the free fatty acids, removing the glycerin, and creating an alcohol ester. Transesterification reaction is an equilibrium reaction. In such reaction, the amount of methanol can be used to shift the reaction equilibrium to the right side and hence, produce methyl esters as the proposed product. The more the amount of methanol the more the methyl ester produced. This is the same relation for other alcohols. A catalyst can further be used to improve the reaction rate and yield.

Primary or secondary alcohols are monohydric aliphatic alcohols having 1-8 carbon atoms. Alcohols that can be used in the transesterification reaction are methanol, ethanol, propanol, butanol, hexanol or amyl alcohol. Methanol and ethanol are used most frequently. Ethanol is a preferred alcohol in a transesterification process because it can be derived; it is renewable and biologically less abhorrent to the environment. However, methanol has a low cost and with physical and chemical advantage (polar and the shortest chain alcohol).

Conventional transesterification process involves converting vegetable oils to biodiesel. In this process, free fatty acids and water always produce negative effects. This negative effect arises as a result of the presence of free fatty acids and water which can cause soap formation. This effect also consumes catalyst and reduces catalyst effectiveness, all of which results in a low fuel product conversion (Formo, 1997).

Biodiesels as fuels can be characterized by their viscosity, density, cetane number, cloud and pour points, distillation range, flash point, ash content, sulfur content, carbon residue, copper corrosion, and higher heating value (HHV). The most important parameters affecting the ester yield during a transesterification reaction are molar ratio of alcohol to vegetable oil and reaction temperature. In a typical transesterification process, the viscosity values of vegetable oil methyl esters produce decreases sharply after the process. Vegetable oil methyl esters when compared to a normal D2 fuel, are slightly viscous. The flash point values of vegetable oil methyl esters is also significantly lower than those of vegetable oils. A high regression between the density and viscosity values of vegetable oil methyl esters. The relationships between viscosity and flash point of vegetable oil methyl esters are considerably regular. Reaction parameters are all specified through biodiesel standard, ASTMD 6751. This standard identifies parameters for pure biodiesel (B100) and must meet before being used as a pure fuel or being blended with petroleum-based diesel fuel

Biodiesel is mixture of fatty acid alkyl esters. If methanol is used as a reactant, it will be a mixture of fatty acid methyl esters (FAME). Based on the feed stock, biodiesel has different proportions of fatty acid methyl esters. Table 1.3 shows the chemical composition of common fatty acids and their methyl esters present in the biodiesel.

2.2 Chemical structures of common fatty acid and their methyl esters

Abbildung in dieser Leseprobe nicht enthalten

The choice of vegetable oil will affect the combustion and oxidation properties of the resultant fatty ester. Klopfenstein and Walker (Klopfenstein and Walker 1982) stated that considering methyl esters which had saturated acids, thermal efficiency was inversely related to the chain length of the fatty acid. Increased efficiency was as a result of a double bond. Further increases in unsaturation had negligible effects on thermal efficiencies. They concluded that an ester fuel derived from a vegetable oil with high oleic acid content was better than an ester derived from oil with longer constituent of fatty acids. The methyl oleic ester oxidation rate was because of one double bond. Approximately 10% of a fatty ester contained two double bonds and about 7% that of a three double bond fatty ester. The fatty ester oxidation causes the polymerization resulting in gum formation according to Peterson et al. (1981).

3. Characterization of the oils

Oils can be characterized by its acidity, moisture content, composition, iodine number (IV), saponification number (SN), peroxide number (PN) and cetane number (CN). These parameters can be determined experimentally and theoretically from their acidic compositions.

3.1. Feedstock for biodiesel production

Potentially, the characteristics of any kind of vegetable oil or animal fat are liable for its use as input for biodiesel production. This section includes a description of both the most common sources of triglycerides currently used for biodiesel production and the ones with high potential. The description of triglyceride is important information to determine the alcoholysis of vegetable oil. Alcoholysis of vegetable oils is the process where ester (a triglyceride) reacts with alcohol to form a new ester (biodiesel) and alcohol (glycerol). Alcohols such as methanol, ethanol, propanol and butanol have been used. Though, methanol and ethanol are the most widely used, particularly methanol is preferred owing to its property and availability. This process has been extensively used to minimize high viscosity of triglycerides. If methanol is used in this process, it is called methanolysis.

Oils can be characterized for what concerns acidity (by acid-base titrations) this is reported by Boffito et al., (2012a, 2012b), iodine value (Hannus method (EN 14111:2003)), saponification value (ASTM D5558), peroxide value and composition by GC analyses of the methyl ester yielded by the esterification and transesterification. The next section will describe information on triglyceride which can be synthesized into biofuel.

3.2. Chemical characteristics of oil for biofuel production

Biofuel is a mono alkyl ester of long chain fatty acids (oil). Fuel is mainly derived from transesterification of triacyl glycerol present in renewable feed stocks such as vegetable oils or animal fats. Transesterification is a reversible process and proceeds significantly by the addition of a catalyst. The catalysts can be homogenous, heterogeneous or enzymatic catalysts. This process is affected by molar ratio of oil to alcohol, reaction temperature, reaction time, different catalyst type and amount, free fatty acids and water content of oils or fats. Hitherto, an effort to give insight of catalyst used in the context of biodiesel production is examined.

Biodiesel can be synthesized using several different triacylglycerides, alcohols, and catalysts. In this study, fatty acid methyl ester (FAMEs) is produced from their respective alcohols using either a basic catalyst or an acidic catalyst (H2SO4).

3.2.1 Acidity of the oils

The acidity of oils can be determined by titration analysis of oil with KOH 0.05 M or 0.1 M in ethanol. The neutralization reaction of the FFA is a saponification reaction as shown in the reaction below:

CH3(CH2)7CH=CH(CH2)7COOH + KOH → CH3(CH2)7CH=CH(CH2)7COOK + H2O

Oil and ethanol are not miscible; but if both react together, the effect creates a mixture of diethylether and ethanol in the ratio 9:1 by volume. This mixture can be used as a co-solvent for the acidity titration. 2% phenolphthalein in ethanol can be used as indicator for the titration. The percentage of FFA content per weight can be calculated as follows as usual in similar works (Boffito et al., 2012a; 2012b; Bianchi et al., 2010; Pirola et al., 2011; Russbueldt et al., 2009; Pasias et al., 2006):

Abbildung in dieser Leseprobe nicht enthalten

V is the volume of KOH solution employed for the titration (mL), MW is the average molecular weight of the free fatty acids contained in the oil, obtained from the analysis of the acidic composition by GC. If there is no known acidic composition, an acid with a high concentration should be used. C is the concentration of KOH in mmol mL-1 and W is the weight of the analysed sample (mg).

3.2.2 Moisture content

The Karl Fischer titration method is a method to determine of water in various types of solids or organic liquids. It is possible to detect traces of water in a sample to a few parts per million.

This method developed by Karl Fischer consists of a titration whose end point is usually automatically detected amperometrically. Karl Fischer titration method is based on oxidation / reduction relatively specific to water (Skoog et al., 1998). The used titrant is Karl Fischer reagent. Karl Fischer’s reagent contain the following compounds; pyridine, SO2, I2, and anhydrous methanol.

This classical reaction exists in two forms. I2 and SO2 react in the presence of pyridine and water to form the pyridine sulphite and iodide.

(I) C5H5N·I2 + C5H5N·SO2 + C5H5 + H2O → 2 C5H5N·HI + C5H5N·SO3
(II) C5H5N+·SO[3]- + CH3OH → C5H5N(H)SO4CH3

I2, SO2 and SO3 are complexed with pyridine. Sulphite in pyridine can also of consuming water:

(III) C5H5N+·SO[3]- + H2O → C5H5NH + SO4H-

The last reaction can be avoided using a large excess of methanol. In Reactions I and II the stoichiometry of the Karl Fischer titration, one mole of iodine, one mole of sulphur dioxide and three moles of pyridine for each mole of water. By these two reactions, when all the water has been consumed, an excess of free iodine occurs.

This has an effect of depolarizing across operating electrodes to allow the passage of current. To obtain the amount of water contained in an organic sample, it is necessary to calibrate the reagent, to know how many millilitres are required to titrate a given quantity of water expressed in grams.

The calibration can be performed by adding small amounts of water to 25-30 ml of methanol previously neutralized. It is however preferable not to use distilled water, but solutions or substances of known composition containing water. Sodium tartrate dihydrate (C4H4Na2O6 · 2H2O) has been used to determine the title of the Karl Fischer reagent.

To determine the % of water contained in the samples, the following equation was used.

Abbildung in dieser Leseprobe nicht enthalten

V is the volume in ml of Karl Fischer titrant given at the end point, t is represented in grams of water titrated by 1 ml of reagent. This is obtained from the measurement calibration curve and W is weight in grams of the sample.

3.2.3 Determination of fatty acids composition

Fatty acids composition can be used to determine the content of esters and linoleic acid methyl ester. This analysis can be performed on oils either after the esterification reaction or after the transesterification reaction to determine the overall biodiesel (FAME) content. About 250 mg the sample can be dissolved in 2.50 mL standard solution of 0.1 M of methylheptadecanoate or methylnonadecanoate. Methylheptadecanoate (>99% and methylnonadecanoate (>99%), can be used as an internal standard and heptane as a solvent. The biodiesel yield the FAME content, expressed as mass %, was calculated using the following equation (2.11):

Abbildung in dieser Leseprobe nicht enthalten

where ΣA is the total peak area of the methyl esters, Astd is the peak area corresponding to methylheptadecanoate or methylnonadecanoate; Cstd and Vstd are the concentration (mg ml-3) and the volume (ml), respectively, of methylheptadecanoate or methylnonadecanoate solution being used; W is the mass of the sample in mg.

The content of each methyl ester, expressed as mass %, was determined using the following equation (2.12):

Abbildung in dieser Leseprobe nicht enthalten

where Ai and Astd are the peaks areas represented as ester and as the used standard, respectively, Wi and Wstd is represented as mass (mg) of the sample and the mass standard, used respectively. Average molecular weights can be calculated from the acidic composition using the following equation:

Abbildung in dieser Leseprobe nicht enthalten

MWi is molecular weight of the methyl ester and Ai is its weight percentage of each methyl ester as resulted from the GC analysis

3.2.4 Determination of iodine value

The iodine value (IV) corresponding to the mass of I2 contained in 100 g of sample can be indicated by the number of unsaturations in the oil.

In this reaction, iodine monochloride reacts with the unsaturated bonds to produce a di-halogenated single bond of which one carbon bound an atom of iodine:

CH3(CH2)7CH=CH(CH2)7COOH + ICl → CH3(CH2)7IH-CClH(CH2)7COOK

After this reaction is complete, the amount of iodine reacted can be determined by a solution of potassium iodide. This causes the remaining unreacted ICl to form molecular iodine:

ICl + KI → KCl + I2

The I2 is then titrated with a standard solution of sodium thiosulfate:

I2 + 2Na2S2O3 → 2 NaI + Na2S2O4

Concurrently, a blank test which is a test containing the solvent and reagents identical in amounts with the exception of the sample can carried out. Wijs reagent (ICl in acetic acid) can be used as a source of ICl and as a mixture of glacial acetic acid (>99%) and cyclohexane (>99%) in 1:1 volume ratio was used as a solvent. A KI aqueous 0.1 M solution was used to create an iodine excess. Na2S2O3 0.1 M and starch solution 1% in H2O can be used as a titrating agent and an indicator respectively. Amylose creates a blue black solution on reaction with iodine and is colourless when titrated with iodine.

Two 250 ml- flasks can be used for analyses, one for the sample and one for the blank, respectively. About 0.3 g oil can be used for two analyses, dissolved in 20 ml solvent. 25 ml of Wijs reagent should then be added and made to react for 2 hours in the dark. Afterwards, 20 ml of the KI solution and 150 ml of deionized water can be added. The mixture can then be titrated with 0.1 M Na2S2O3 standard solution. The IV was then calculated with the following equation:

Abbildung in dieser Leseprobe nicht enthalten

12.69 is the equivalent weight of iodine (PM/2), C is solution concentration, V1 and V2 are the volume of the solution required by the titration for the blank and the sample respectively. W is weight oil sample. Precaution should be taken as starch indicator solution should added near the end point (the reaction end point is the point near a fading yellowish iodine colour appears) because at high iodine concentration, starch decomposes the products whose indicator properties are not entirely reversible. The IV can be calculated theoretically from the oil’s acidic composition, using the following equation (Azam et al., 2005):

Abbildung in dieser Leseprobe nicht enthalten

where D is the number of double bonds, Ai the weight % and MWi the molecular weight of each fatty acid contained in the oil.

3.2.5 Determination of saponification number

Saponification value or "saponification number" is the number of milligrams of potassium hydroxide or sodium hydroxide required to carry out a saponification process for 1g of fat under reaction conditions. Saponification number is a measure of the average molecular weight or chain length of all the fatty acids present. Long chain fatty acids present in fats have a low saponification value. This value is because they have a lower number of carboxylic functional groups per unit mass of the fat. Calculated molar mass does not apply to fats or oils containing high amounts of unsaponifiable material, free fatty acids (>0.1%), or mono- and diacylglycerols (>0.1%). Saponification number is measured in terms of a back-titration: the sample is treated with a known amount of solution of potassium hydroxide in ethanol in excess. The resulting solution can then be heated to reflux for at least 1 hour. After 1 hour, cool the excess of potassium hydroxide that remained unreacted. This excess portion remains quantified by titrating with a solution of hydrochloric acid in the presence of phenolphthalein as an indicator. The saponification value was then determined as follows:

Abbildung in dieser Leseprobe nicht enthalten

56.1 is the molecular weight of KOH (mg mmol-1), C is KOH solution concentration of the ethanol used for titration, V1 and V2 are volume of the KOH solution required for the titration of the blank and the oil sample, respectively, and W is the weight of the oil sample.

The SN can be also calculated theoretically from the acidic composition of the oil, using the following equation (Azam et al., 2005):

Abbildung in dieser Leseprobe nicht enthalten

Ai and MWi are the weight % and the molecular weight of each fatty acid contained in the oil, respectively.

3.2.6 Determination of peroxide number

Peroxide number is a measure of the extent to which oil has undergone primary oxidation (Chakrabarty, 2003) and thus, an index of the conservation conditions of oil. The peroxide value is determined by measuring the amount of iodine formed by reacting peroxides (formed from fat or oil) with iodide ion:

2 I- + H2O + ROOH → ROH + 2OH- + I2

The base produced in this reaction is taken up by the excess of acetic acid present. The iodine liberated is titrated with sodium thiosulphate.

2Na2S2O3 + I2 -> Na2S4O6 + 2NaI

For acidic conditions excess acetic acid prevents the formation of hypoiodite (analogous to hypochlorite), this interferes with the reaction condition. Starch solution can be used as an indicator for this reaction; the amylose forms a blue black solution with iodine and is colourless on titration with iodine.

The PN was then calculated as follows :

Abbildung in dieser Leseprobe nicht enthalten

V is volume of the solution required for titration and C is its concentration, W is the weight of the oil sample.

3.2.7 Determination of cetane number

Cetane number (CN) is an expression of diesel fuel quality among a number of other measurements that determine overall diesel fuel quality. Cetane number is a measure of any fuel’s ignition delay. This is the time interval between the start of injection and start of fuel combustion. Fuels with higher CN, which have shorter ignition delays, provide more time for a fuel combustion activity to be completed. Hence, higher speed diesels operate more effectively with higher CN fuels. This is one important parameter which is considered for the selection of oil to be used as feedstock for biodiesel. However it is possible to calculate CN theoretically using the following equation as also carried out in other works (Winayanuwattikun et al., 2008):

Abbildung in dieser Leseprobe nicht enthalten

SN and IV are symbols representing the saponification number and iodine value of the oil, respectively.

3.2.8 Free fatty acids esterification

The suitability of any material such as fuel can be influenced by contaminants from its production process. The components of the fuel ultimately determine the properties of the fuel. Some properties are as a result of the fatty ester’s structure (biodiesel) formed from fatty acids of the parent oil or fat. The term “fatty acids” applies to carboxylic acids obtained from animal and vegetable fats. Fatty acids constitute of all saturated and unsaturated aliphatic carboxylic acids that have carbon chain lengths of C6-C24. Fatty acids extracted from natural fats and oils are quantitatively more important than synthetic fatty acids. Different processes can be used to modify natural fatty acids to achieve special properties for different applications.

There are several hurdles for using vegetable oils as fuel. Vegetable oils viscosities properties ten to twenty times higher than that of fossil fuel. Hence, causes to poor fuel atomization and incomplete combustion. An extremely flash point of vegetable oils leads to thermal and oxidative polymerization, formation of deposits on the injector nozzles, dilution and degradation of lubricating oil, and a sticky piston ring. These issues can be resolved by adapting a fuel engine. The conversion of vegetable oil to fuel has identical properties similar to petroleum-diesel such as:

- Cleavage of triglyceride into its fatty acid or linear hydrophobic components.
- Elimination of polar interactions
- Elimination of the reactive groups (unsaturated groups).

For diesel engines using vegetable oil, different methods can be employed to convert vegetable oil to fuel.

A. Direct use and Blending. Vegetable oils can be used directly after blending with petroleum-diesel. This is however impractical due to the high viscosity, free fatty acid content and gum formation from oxidation and polymerization.
B. Microemulsions. High viscosity of vegetable oils can be solved on forming microemulsions with certain solvents such as methanol, ethanol and 1-butanol. Microemulsion is a colloidal equilibrium dispersion of optically isotropic fluid microstructures. These structures are in dimensions generally ranging from 1 - 150 nm formed spontaneously from two normally immiscible liquids. One liquid is ionic and the other non-ionic amphiphile.
C. Pyrolysis. Heavy molecules such as vegetable oils or animal fats can be converted to smaller molecules by means of heat or heat and catalyst (as in the Scheme below). The fuel obtained in this process is called “diesel-like fuels”. It comprises of a variety of components like olefins and paraffins similar to petroleum-based diesel. Pyrolysis however requires high temperature. Products characterization is difficult because of various reaction products.

Abbildung in dieser Leseprobe nicht enthalten

Pyrolysis of triglycerides

The products from the pyrolysis of triglycerides are chemically similar to petroleum-derived gasoline and diesel fuel.

D. Alcoholysis of Vegetable Oils. In this process, ester (here triglyceride) reacts with alcohol to another new ester (biodiesel) and alcohol (glycerol). Methanol, ethanol, propanol and butanol are Different types of alcohols that can be used for alcoholysis. However, methanol and ethanol are commonly used. Alcoholysis is widely used to reduce high viscosity of triglycerides. When methanol is used in this process, the process is called methanolysis.

3.2.9 Activity tests

Activity tests can be conducted by withdrawing oil samples from the reactors at different pre-established times. Time range usually at 60, 120, 240, 360 minutes. Residual acidity can then be analyzed by acid-base titrations (Boffito et al., 2012, Pirola 2011). The FFA conversion of methyl esters can then be calculated as follows :

Abbildung in dieser Leseprobe nicht enthalten

FFAt=0 is FFA concentration by weight of the starting oil while FFAt is the FFA concentration of the oil sample at time t.

4. Conclusion

Fatty acids composition can be determined by following the normative for the determination of esters and linoleic acid methyl ester content. The analysis can be performed on the oils either after the esterification reaction or after the transesterification reaction to determine the overall biodiesel (FAME) content. The quality of oil is relates to the percentage composition of fatty acids in the oil. In general, 90% is oleic acid (C18:1) and linoleic acid (C18:2) in mutual proportions (Murphy, 1994). Lide (1991) stated that sunflower oil consists of 25.1% oleic acid and 66.2% linoleic acid. Stearic (C18:0) acid and Palmitic acid (C16:0) and make up 7-10% of the oil composition. Ma et al., (1997) found that minor constituents of arachidic (C20:0), behenic (C22:0) and lignoceric acid (C24:0) may be present in oil. The chemical composition of triglycerides can be determined (FAO, 2010). Oil can be extracted from seeds. Oil is mainly used for human consumption. However, vegetable oil can be synthesized into important feedstock for biodiesel production. Oil crops produces higher yields of oil/ha (example ~513 kg/ha) (Nolte, 2007) and oil crops can be grown both in spring and summer (Rashid et al., 2008). Oil crop seeds have a great potential of becoming biodiesel due to their comparable properties with diesel. The properties of seeds include calorific values and cetane number. Oil seeds can be grown for biodiesel production purposes. The seeds provided the highest yields among the varieties tested (Chigeza et al., 2012). Different oils have different compositions of fatty acids, growth characteristics and oil content (Zheljazkov et al., 2008).

5. References

ASTM, Standard specification for biodiesel fuel blend stock (B100) for middle distillate fuels. 2007, Designated D6751-07b.

Azam M.M., Waris A., Nahar N.M. Properties and potential of fatty acid methyl esters of some non-traditional seed oils for use as biodiesel in India Biomass Bioenerg., 29 (2005), pp. 293-302

Bianchi C.L., Boffito D.C., Pirola C., Ragaini V., “Low temperature deacidification process of animal fat as a pre-step to biodiesel production”, Catal. Lett., 2010, 134, 179.

Boffito D.C., Crocellà V., Pirola C., Neppolian B., Cerrato G., Ashokkumar M., Bianchi C.L., “Ultrasonic enhancement of the acidity, surface area and free fatty acids esterification catalytic activity of sulphated ZrO2TiO2 systems”, J. Catal., 2012b, http://dx.doi.org/10.1016/j.jcat.2012.09.013

Boffito D.C., Pirola C., Galli F., Di Michele A., Bianchi C.L., “Free Fatty Acids Esterification of Waste Cooking Oil and its mixtures with Rapeseed Oil and Diesel”, Fuel, 2012, accepted on 19th October 2012, DOI: 10.1016/j.fuel.2012.10.069.

Chakrabarty M.M., “Chemistry and Technology of Oils and Fats”, Allied Publishers Pvt. Ltd, 2003, ISBN : 81-7764-495-5.

Chigeza, G., Mashingaidze, K. and Paul Shanahan, P. 2012. Seed yield and associated trait improvements in sunflower cultivars over four decades of breeding in South Africa. Field Crops Research 130:46-56.

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19 of 19 pages

Details

Title
Vegetable Oil as Biofuel. Chemical Characteristics and Transesterification Procedure
College
Covenant University
Grade
5.00
Author
Year
2012
Pages
19
Catalog Number
V519977
ISBN (Book)
9783346126481
Language
English
Tags
vegetable, biofuel, chemical, characteristics, transesterification, procedure
Quote paper
Joseph Ekpuka (Author), 2012, Vegetable Oil as Biofuel. Chemical Characteristics and Transesterification Procedure, Munich, GRIN Verlag, https://www.grin.com/document/519977

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