Evaluation of fuel properties from free fatty acid compositions of methyl esters obtained from four tropical virgin oils.
Compared to diesel fuel, biodiesel is chemically simple since it contains only six to seven fatty acid esters. However, different esters vary in terms of important fuel properties such as total and free glycerine, cetane number, viscosity, cloud point, pour point, density and degree of unsaturation. This is as a result of the presence of impurities. It is also affected by the choice of feedstock .
Analytical indices related to vegetable oils can be distinguished as structure or quality indices. Structure indices are iodine value; a measure of total unsaturation of an oil, saponification value; an indicator of average molecular weight and hydroxyl value, which is applicable to fatty compounds (or their mixtures) containing hydroxyl groups, such as castor and lesquerell oils. Quality indices are the free fatty acids, peroxide, anisidine, phosphorus and other similar values. Generally, quality indices relate to the quality of vegetable oil obtained after processing and possible further derivatization as well as after extended storage and/or the presence of naturally occurring non-fatty materials. Quality indices were formerly termed processing--related parameters but the term quality indices appears to be more general. .
An adequate cetane number is required for good engine performance. High cetane number ensures good cold start properties and minimizes the formation of white smoke. Standard have been established worldwide for cetane number determination. . Generally, auto-oxidation of fatty compounds is accentuated by factors such as presence of air, heat, traces of metal, peroxides as well as nature (structure) of the fatty acid compound. The reason for auto-oxidation is the presence of double bonds in the chains of fatty compounds. The oxidation stability of triglycerides decreases with increase in its polysaturated fatty acid content. Auto-oxidation of unsaturated fatty compounds proceeds with different rates depending on the number and position of double bonds. The positions allylic to double bonds are especially susceptible to oxidation. The bis-allylic positions in common polyunsaturated fatty acids such as linolenic acid (double bonds at C-9 and C-12) giving one bis-alllylic position at C-11; and linolenic are (double bonds at C-19, C-12 and C-15) giving two bis-allylic position at C-11 and C-14; and more susceptible to ato-oxidation than allylic positions. . One of the major problems associated with the use of biodiesel is poor low temperature flow properties, indicated by relatively high cloud points (temperature at which liquid fatty material becomes cloudy due to the formation of crystals and solidification of saturates) and poor point (the lowest flow temperature of fatty materials). The solids and crystals formed grow and agglomerate, clogging fuel lines and filter and causing operability problems. Saturated fatty compounds have significantly higher melting points than unsaturated fatty compounds and is a mixture, they crystallizes at higher temperature than unsaturated 
High viscosity is a major reason why neat vegetable oils are not as fuel in diesel engines. Viscosity affects the atomization of fuel upon injection into the combustion chamber and thereby ultimately the formation of engine deposits. As the viscosity increases, the greater the tendency of causing engine problem. The viscosity of a variety of fatty compounds measured at 40 [degrees]C and the effects of compound structure on viscosity have received considerable attention in literature [6, 7, and 8]. This piece of research sets out to study the fuel properties of free fatty acid methyl esters obtained from canarium schweinfurthii, hura crepitans, telfaria occidentalis and cucumeropsis manii to evaluate their suitability for use as fuels in diesel engines.
Materials and Methods
Canarium schweinfurthii is a large forest tree which often grows as high as 50 m tall in the Savanna and sub-Savanna belts of Nigeria. They are often cultivated for its fruits which are edible, purplish, ellipsoid but slightly three-angled. The seeds were obtained from Jos, Plateau State during the dry season. The seeds were sun dried for about two weeks to remove moisture after which they were ground into coarse powder and ready for extraction.
Hura crepitans is a large forest tree often found in the tropical rain forest and Savanna regions of Nigeria. The seeds are enclosed in hard protective coat which usually and suddenly splash open and scatters when the seeds are well dried. The tree has broad leaves with thorns all over its trunk. The seeds were collected from Makurdi metropolis during the dry season and sun dried to remove moisture after which it was sun dried, crushed and milled.
Telfaria occidentalis belongs to the family Cucurbitacae spp. It is known as 'Ugu' among the Igede, Idoma peoples of Benue State and the Igbos of Nigeria. There is tremendous genetic diversity within the family, and range of adaptation for Cucurbits species includes tropical and subtropical regions and deserts. The genetic diversity in Cucurbitacae extends to both vegetative and reproductive characteristics. Telfaria occidentalis has become an important medicine source in the last decades. The seeds were obtained from Wurukum market, in Makurdi Local Government of Benue State between December and January. The seeds were extracted from the bulb and dried to a constant weight after which the mesocarps were removed by dehulling. The dried seeds were pounded into coarse powder.
Cucumeropsis manii (white melon) is one of the species of melon commonly found in Nigeria. The seeds are edible and are usually used in preparation of soup among different tribes. The seeds were gotten from Oju Local Government of Benue State during dry season and were dried to a constant weight; the mesocarps were removed while the seeds were milled into a coarse powder and ready for extraction.
Oils were extracted using petroleum ether (60-80[degrees]C) in a soxhlet extractor.
Degumming of Crude Oils
In degumming, the crude oil was mixed with about 3% of warm water and the mixture was agitated mechanically for 30 min at 70[degrees]C. This hydrates the phospholipids and gums thus making them insoluble in the oil. They were thereafter separated by settling.
Production of Biodiesel Fuels
The 100 mL of pretreated oil was poured into a large beaker. The oil was heated at 70 [degrees]C using Bunsen burner to remove the remains of solvent or moisture content. The heated oil was then blended with the prepared methoxy solution in an air tight blender. The mixture was immediately transferred into a separating funnel and closed tightly. The mixture was allowed to settle for 24 h after which a dark color glycerin settled at the bottom while a pale liquid layer which is the methyl ester separated at the top .
Upon completion of reaction, two major products were formed, glycerin and biodiesel. The clear liquid (methyl ester) found at the top layer was decanted into a graduated beaker (NBB).
The methyl ester was mixed with an equal volume of distilled water in a separating funnel. The separating funnel was gently swirled severally and allowed to stand for some minutes and the water was drained off from the bottom of the funnel by turning on the tap of the separating funnel. The tap was turned off when it reaches the methyl ester. This procedure was repeated twice to ensure complete washing. After washing, the methyl ester was dried by heating .
Determination of free fatty acid profile of the neat vegetable oils and the methyl esters obtained using GC
GC and FTIR analysis carried was out with hydrogen flame ionization detector(FID), capillary column, split mode injector, oven temperature programming sufficient to implement a hold - ramp- hold sequence; operating condition; temperature(0 [degrees]C): injector, 285; initial temp, 100 (held 4 min); ramp, 3 [degrees]C/min; final temp 240; hold 15 min: carrier gas, helium; flow rate, 0.75mL/min; linear velocity, 18 cm/s; split ratio, 200:1. Gas chromatography analysis was carried out on a Shimadzu GC-17A chromatograph equipped with a FID detector equipped with a mass spectrometer detector Shimadzu GCMS-QP 5050 using an identical column. 1 [micro]m of each diluted sample with analytical grade dichloromethane was injected.
For example, ASTM D6715 in the United States, the European Norm, EN 14214 and the International Organization for Standardization (ISO), Standard ISO 5165. It can be seen that on the cetane scale, cetane number decreases with decreasing chain length and increasing branching. Some equation correlate cetane with composition of biodiesel the correlation formulated by Clements .
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)
was adopted in this study.
Where CN, is cetane number, [X.sub.ME] is the Wt% of the fatty acid constituent and [CN.sub.ME] is the cetane number of the corresponding fatty acid.
Oxidative Stability => Degree of Un-saturation of the Neat Vegetable Oils
Thus, the oxidative stability of vegetable oil and of methyl esters (biodiesel) obtained therefore may be correlated with the degree of unsaturation of fatty acid component of the oil.
DU = [C.sub.n:1(Wt%)] + [2.sub.Cn:2, 3(Wt%) ...] (2)
Fortunately, there was no oxidation stability requirement in the United States biodiesel standard specification, ASTM D6751, although European standard EN 14212 does contain one .
Cold flow properties
Test methods for cold flow properties evaluation include the ASTM D2500 for cloud point, ASTM D97 for pour point, the ASTM D4539 (the low-temperature flow test) used mainly for conventional diesel fuel and the EN116 (European standard) test for cold filter plugging point (CFPP). However cold flow properties can be predicted form the free fatty acid composition using the following equation.
LCSF ([degrees]C) = (0.1 x [C.sub.16:0] + 0.5 x [C.sub.18:0] + 1 x [C.sub.20:0]) + 1.5 x ([C.sub.22:0] + 2 x [C.sub.24:0]). (3)
CFPP ([degrees]C) = 3.1417 x LCSF - 16.477 (4)
Low temperature properties of fatty compounds are generally considered to depend mainly on the saturated fatty acid content, with the unsaturated fatty content acting essentially as the solvent in which the unsaturated fatty acids are dissolved and from which they precipitate . The correlation between long chain saturated factor (LCSF) and cold filter plugging point (CFPP)
CFPP = 3.1417. LCSF - 16.477 ... (4)
reportedly gave correlation coefficient [R.sup.2] of 0.966. Heat of combustion
The gross calorific value H or heat content of vegetable oils is the quantity of heat evolved when one mole of the oil is burnt to C[O.sub.2] and [H.sub.2]O may be obtained from its structure indices using the relationship .
H = 47.645 - [4.187(IV) + 38.31(SV)] KJ/kg ... (5)
ln = -4.7965 + 2525.92962 (1/T) + 1.6144 [[(SV).sup.2]/[T.sup.2]] - 101.06 x [10.sup.-7][(10).sup.2] (6)
has been used to evaluate the absolute viscosity of vegetable oils.
Determination of Calorific value/ Heating values
Using bomb calorimeter method which involves igniting the oil samples in adiabatic oxygen bomb calorimeter (under a high pressure of [O.sub.2] at 25 atm). This will bring about the oxidation of the organic constituents to [H.sub.2]O, C[O.sub.2] and oxides of some elements as [N.sub.2], S and P with the resultant release of energy as heat. It is this heat that is absorbed by [H.sub.2]O surrounding the bomb and the subsequent increase in temperature of the [H.sub.2]O that is used to estimate the energy value . 2 g of oil sample will be burnt in Parr Adiabatic Oxygen Bomb Calorimeter. The heat of combustion will be calculated as the gross energy.
Gross Energy (G) = w x t - 2.3 L-V g ... (7)
w = energy equivalent of calorimeter or [H.sub.2]O equivalent of calorimeter
T = temperature rise;
2.3 = constant heat of combustion of wire = 2.3 cal/cm or 140cal/g; L = length of wire burnt
V = titre;
g = wt of sample in grammes and
G = KJ/g = KJ/kg.
It is well known that the cetane number of biodiesel depends on the feedstock used for it production. The longer the fatty acid carbon chains and the more saturated the molecules the higher the cetane number [15, 16]. Ramos et al  reported a linear relationship between the degree of unsaturation, and cetane number of biodiesel. The higher the degree of unsaturation, the lower the cetane number.
It was noted that diesel engine vary widely in their cetane requirements, the lower the engine speed, the lower the cetane number (as low as 20) of the fuel it can use . Table 5 shows the values of CN of FAME under study [49 < CN < 65] fall within range acceptable in the broad range of diesel engines as seen in Fig. 2 with TVO-ME and HVO-ME having a cetane number of 51.5 and 51.5 respectively. CVO-ME and CSVO has cetane numbers of 49.6 and 49.3 respectively. Cetane numbers for neat oils were also obtained using the cetane number for individual pure fatty acids and the corresponding fatty acids compositions in Table 1. The results are shown in Table 5, Fig. 2 the profile of cetane number of neat oils and their corresponding FAME which were greatly enhanced in the FAME. Cetane number is one of the prime indicators of the quality of diesel fuel. It relates to the ignition delay time of a fuel upon injection into the combustion chamber. The shorter the ignition delay time, the higher the cetane number and vice versa .
Research has revealed that there is a correlation between high cetane number and low NOx emission. The cetane number of biodiesel is generally higher than conventional diesel. The effect of blending biodiesel on cetane number is almost linear for mixtures of esters with biodiesel fuel or diesel fuel .Research has also shown that the values of cetane number from different feedstock vary widely with soyabean oil derived biodiesel. The value ranges from 48 to as high as 67 with the ASTM D613 standard specification requirement of 47 minimum .
The results show the DU value of HVO, CVO, TVO and CSVO are shown in Table 6 to be 60.88, 90.60, 89.93 and 70.63 respectively, which is well below soyabean oil which has a DU value of 143.60 which has gained commercial status as feedstock for the production of biodiesel.
A triglyceride or molecules under prolonged exposure to oxygen and heat will become diglycerides and one free fatty acid or a monoglyceride and two free fatty acids or potentially three free fatty acids . The main factors affecting biodiesel stability are natural anti-oxidant content, polyunsaturated fatty esters content and the level of mono-and d-glycerides .
Biodiesel with high oxidation stability will take longer to reach an out of specification condition, while biodiesel with low oxidation stability will take less time in storage to reach an out of specification condition [13, 20].
Cold flow properties
A close examination of the results in Table 7 revealed that the values of CFPP bear inverse relationship with DU; the higher the value of DU, the lower the CFPP values as shown in Fig. 3. It should be noted that, CFPP is not a critical biodiesel quality parameter in the tropics where typical values of CFPP should generally be lower than ambient temperatures.
Heat of combustion
The calculated energy densities varied as shown in Table 8 from between 39.2 MJ/kg to 40.0 MJ/kg and are consistent with the range of values obtained previously for other vegetable oils  and compare favourably close with the corresponding value for fossil diesel (Ca 45MJ/kg). The heat content of vegetable oils calculated has been shown to correlate well with the value determined by calorimetric method as shown in Table 9 and indicate that the energy density of vegetable oils is comparable with that of fossil diesel.
A close examination of Eq. (6) shows that the viscosity of vegetable oils should increase with chain length (number of carbon atoms) of constituent fatty acids and with increase in degree of saturation. Factors such as double bond configuration influence viscosity with cis- double bond configuration giving lower viscosity than trans-configuration . The values of absolute viscosity of the vegetable oils under study are given in Table 8. Oils with high SV (and therefore have relatively low average molecular weight) contain short chain fatty acids which would be associated with smaller hydrodynamic volume and hence relatively low viscosity. The result in Table 8 shows a close relationship between the values of absolute viscosity and SV of the oils. It may therefore be possible to deduct relative values of the viscosity of biodiesel from the structure indices of the vegetable oil feedstock.
These empirical data suggests that the mixture of the locally available vegetable oils may be processed safely to methyl ester that would meet the requirement for use in diesel engines. All of the vegetable oils under study are suitable on the basis of oxidative stability index, for biodiesel preparation. CVO-ME shows a lower unsaturated FAME content compared to HVO-ME--hence exhibiting a poor oxidative stability. This can be examined by the one of measure of unsaturation as the dependent variable such as oxidizability. The co-efficient of oleic, linoleic and linolenic fatty esters are proportional to the relative rates of oxidation of these compounds. The heat content of vegetable oils calculated has been shown to correlate well with the value determined by calorimetric method and indicate that the energy density of vegetable oils is comparable with that of fossil diesel. All the oils have high SV and therefore have relatively low average molecular weight and contain short chain fatty acids which would be associated with smaller hydrodynamic volume and hence relatively low viscosity. The results show that all the vegetable oils under study are suitable for biodiesel on the basis of these properties.
The authors will like to appreciate staff of Chemistry Departments in, University on Nigeria Nsukka, Benue State University Makurdi, Uni. of Tech. Awka. Also the staff of NNPC-Warri, and Lighthouse Pet. Eng. Coy. LtdWarri for the use of their laboratories and facilities for analyses. Prof. V.I.E. Ajiwe is specially acknowledged. Scholarships from TETFund Nigeria is also acknowledged.
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Igbum O G (1), * Leke L (1,5) Okoronkwo M U (3,5) Eboka A (2) and Nwadinigwe C A (4)
Department of Chemistry, Benue State University, P M B 102119, Makurdi, Nigeria.
Department of Chemistry, University of Agriculture, P M B 23 73, Makurdi,
Department of Chemistry, Abia State University, Uturu, P.M.B 2000 Uturu, Nigeria
Pure and Industrial Chemistry, university of Nigeria Nsukka, Nigeria
Department of Chemistry, University of Aberdeen, UK.
* Corresponding author: firstname.lastname@example.org
Table 1: Cetane number of pure fatty acid. FFA Composition Lauric acid 61.1 Myristic acid 69.9 Palmitic acid 74.4 Stearic acid 76.3 Oleic acid 57.2 Linoleic acid 36.8 Linolenic acid 21.6 Bangboye and Hansen (2008) Table 2: showing variables and properties of methyl esters obtained and used for the thermal analysis. S/N Feedstock catalyst Cat. Conc. Alcohol/oil Molar ratio 1 TVO-ME NaOH 1 6:1 2 HVO-ME NaOH 1 6:1 3 CSVO-ME KOH 1 6:1 4 CVO-ME KOH 1 9:1 S/N Retention Reaction % yield viscosity Temp([degrees]C) time 1 38 5 97 3.91 2 38 5 97 5.02 3 38 5 94 4.11 4 55 30 96 4.21 S/N Specific gravity 1 0.87 2 0.88 3 0.87 4 0.87 Table 3: Free Fatty Acid Composition of Neat Vegetable Oils Fatty Acid Acronym HVO CVO TVO CSVO Units Stearic C 18:0 0.03 - 2.17 0.54 % Myristic C 14:0 4.33 4.33 5.22 3.66 % Palmitic C 16:0 18.66 22.19 22.93 27.12 % Linolenic C 18:3 9.24 9.11 21.08 19.54 % Lenoleic C 18:2 18.13 20.53 20.24 21.05 % Oleic C 18:1 6.14 11.35 7.29 9.42 % SFA 23.02 . 26.72 30.32 31.32 % UFA 33.51 40.99 48.61 50.01 % U/S 1.46 1.53 1.60 1.60 % L/L 0.51 0.44 1.04 0.93 % Table 4: Free Fatty Acid Methyl Ester (FAME) Composition of Methyl Ester Fatty Acid Acronym HVO-ME CVO-ME TVO-ME CSVO-ME Methyl Stearate C 18:0 0.97 2.17 3.08 0.98 Methyl Myristate C 14:0 6.28 5.97 6.19 5.86 Methyl Palmitate C 16:0 31.28 27.51 29.36 30.34 Methyl Linolenate C 18:3 26.55 8.33 26.17 23.22 Methyl Linoleate C 18:2 25.28 18.06 24.37 24.31 Methyl Oleate C 18:1 8.17 20.69 9.17 10.13 SFA 38.53 35.65 38.63 37.18 UFA 60.00 47.08 59.71 57.66 U/S 1.56 1.32 1.55 1.55 L/L 1.05 0.46 1.07 0.96 Fatty Acid Units Methyl Stearate % Methyl Myristate % Methyl Palmitate % Methyl Linolenate % Methyl Linoleate % Methyl Oleate % SFA % UFA % U/S % L/L % Table 5: Estimated Cetane Number of the Methyl Ester and Neat Vegetable Oils CN = [X.sub.ME] (Wt%). [CN.sub.ME] FAME CN Neat CN Veg. Oil HVO-ME 51.1 HVO 29.10 CVO-ME 49.6 CVO 35.54 TVO-ME 51.5 TVO 38.72 CSVO-ME 49.3 CSVO 40.49 Table 6: Oxidative Stability => Degree of Un-saturation of the Neat Vegetable Oils DU = [C.sub.n:1(Wt%)] + [2C.sub.n:2, 3(Wt%)] Oil DU HVO 60.88 CVO 90.60 TVO 89.93 CSVO 70.63 Table 7: Long Chain Saturated Factor (LCSF) and Cold Filter Plugging Point (CFPP) LCSF ([degrees]C) = (0.1 x [C.sub.16:0] + 0.5 x [C.sub.18:0] + 1 x [C.sub.20:0]) + 1.5 x ([C.sub.22:0] + 2 x [C.sub.24:0]). CFPP ([degrees]C) = 3.1417 x LCSF - 16.477 Oil LCSF ([degrees]C) CFPP ([degrees]C) HVO 1.881 -10.567 CVO 3.173 -9.503 TVO 3.378 -5.864 CSVO 2.219 -9.506 Table 8: The Heat of Combustion (KJ/kg) and the Absolute viscosity Inn Heat of Combustion = 47, 465--[4.187(IV) + 38.31(SV)] KJ/kg Viscosity ln = - 4.7965 + 2525.92962(1/T) + 1.6144 [(SV)2/[T.sup.2]] - 101.06 x [10.sup.-7] [(IV).sup.2] Oil H (KJ/kg) ln HVO 39539.54 96.01 CVO 39197.15 94.76 TVO 39759.41 93.96 CSVO 39964.85 91.99 Where IV = Iodine value SV = Saponification value Table 9: Heating values for neat oils (KJ/kg) and (MJ/kg) Oil HHV (KJ/kg) HHV (MJ/kg) LHV (KJ/kg) LHV (MJ/kg) TVO 39620 39.6 36718 36.7 HVO 40724 40.7 39387 39.4 CSVO 39843 39.8 38147 38.1 CVO 41778 41.8 38672 38.7
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|Author:||Igbum, O.G.; Leke, L.; Okoronkwo, M.U.; Eboka, A.; Nwadinigwe C.A.|
|Publication:||International Journal of Applied Chemistry|
|Date:||Jan 1, 2013|
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