Energy 44 (2012) 616e622. Contents lists available at SciVerse ScienceDirect. Energy. journal homepage:

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1 Energy 44 (2012) 616e622 Contents lists available at SciVerse ScienceDirect Energy journal homepage: An experimental investigation into biodiesel stability by means of oxidation and property determination M. Shahabuddin, M.A. Kalam *, H.H. Masjuki, M.M.K. Bhuiya, M. Mofijur Centre for Energy Sciences, Faculty of Engineering, University of Malaya, Kuala Lumpur, Malaysia article info abstract Article history: Received 29 January 2012 Received in revised form 25 April 2012 Accepted 18 May 2012 Available online 12 June 2012 Keywords: Biodiesel Oxidation stability Induction period Total acid number Flash point This paper presents the experimental results carried out to evaluate the oxidation and storage stabilities of various biodiesel fuels. The biodiesel fuels are palm methyl ester (), jatropha methyl ester (), coconut oil methyl ester (), 20% blends of with diesel fuel and 20% blends of with diesel fuel. The ordinary diesel fuel was used for comparison purposes. Various ASTM standard methods were used to evaluate all the samples at the interval of 180 h over a 2160 h (three months) duration. Oxidation stability of the samples was measured by induction period (IP) using a Rancimat instrument. Other properties such as density, viscosity, flash point, total acid number (TAN), and total base number (TBN) were measured. The results show that almost all fuel samples met the standard specifications regarding IP. The trends for density, viscosity and TAN increased, while the TBN decreased due to oxidation. For the flash point, the trend also decreased, but the rate was very low. In overall consideration, among the biodiesels, was found to be better in respect to oxidation and storage stabilities. The results of this investigation will be used to sustainable development of biodiesel fuel from various stock resources. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Biodiesel as monoalkyl esters of fatty acids synthesized from renewable vegetable oil, animal fats and waste cooking oil, which emits fewer harmful emissions as compared to mineral diesel fuel [1]. In order to achieve the large-scale production of biodiesel from various stock resources, all the properties of biodiesel should be tested to determine if they meet the standard requirements (as presented in Table 1) or not. Meanwhile, it has already been shown that biodiesel has compatibility with internal-combustion engines as a full or partial replacement of ordinary diesel fuel. Recently, biodiesel has gained potential to partially replacement of petroleum diesel due to its numerous properties such as renewability, biodegradability, lower toxicity, environmental friendly, compatibility, higher flash point, higher cetane number, good lubricity and so on [2e4]. However, biodiesels have some drawbacks, which provide a barrier to their widespread practical applicability. The prominent technical problem of biodiesel is its oxidation, thermal and storage instabilities. Wan Nik et al. [5] have stated that one of the major limitations of biodiesel is its thermal and oxidation instabilities. Auto-oxidation of biodiesel occurs since it is more * Corresponding author. Tel.: þ ; fax: þ addresses: shahabuddin.suzan@yahoo.com (M. Shahabuddin), kalam@ um.edu.my (M.A. Kalam). sensitive to the reaction with fuel-bound oxygen with hot air environment. Oxidation occurs in the form of aldehydes, alcohols, carboxylic acids, insoluble gum and sediment in the biodiesel. The rate of oxidation increases due to the instabilities of thermal and physicochemical properties of biodiesel, and is also affected by its storage instability [6,7]. As the consequences of higher oxygen contents of biodiesel of 10% by weight, result in degradation of oxidation stability [8,9]. Due to the unsaturated free fatty acids in biodiesel, after one-year storage, the acid value, viscosity and density have been shown to be increased by mg KOH/g, cst and kg/m 3, respectively [10]. Furthermore, as the unsaturated fatty acid level decreases, the saturated fatty acid level consequently, increases. Biodiesel oxidation is a very complicated process, and its different unsaturated components can produce several abasement products. Oxidation of this fuel cannot be prevented entirely, but can be retarded [11]. Due to oxidization at higher temperatures, biodiesel leads to the formation of higher amounts of water as well [12]. A variety of mechanisms are associated with the oxidation of unsaturated free fatty acid double bonds, which can be first categories as primary oxidation (presented in Fig. 1) and secondary oxidation. Primary oxidation is involved in initiation, propagation and termination [13]. The oxidation stability of biodiesel is manifested as according to the individual s induction period, which can be measured with /$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi: /j.energy

2 M. Shahabuddin et al. / Energy 44 (2012) 616e Table 1 Specifications related to oxidative stability in biodiesel standards [7,11]. Specification Methods ASTM D6751 EN Oxidative stability (110 C) e 3hmin 6hmin e Content of FAME 4 double e e 1 max e bonds (% m/m) Linolenic acid content (% m/m) EN14103 e 12 max e Iodine value (g iodine/100 g) EN e 120 max e Kinematic viscosity (mm 2 /s) D445; ISO 1.9e e /3105 Acid value D664; 0.50 max 0.50 max e EN Flash point e 130 min > Density at 15 C (kg/m 3 ) e e 860e a Rancimat instrument, usually by heating the biodiesel to 110 C. The induction periods (IP) of different biodiesels such as Jatropha biodiesel, palm biodiesel, sunflower biodiesel, karanja biodiesel and soybean biodiesel have been studied [15]. The results of these studies show that the IP of palm biodiesel is h while that of sunflower biodiesel is only 1.73 h. This difference is due to the various percentages of saturated fatty acid in the biodiesels. Likewise, an experiment was carried out by the Rancimat testing method in the temperature range of 100e140 C using linseed oil methyl ester (LME), soybean oil methyl ester (SME) and rapeseed methyl ester (RME); the results showed that RME possesses a longer induction period while LME has the shortest IP [16]. Again, the difference was due to differences in unsaturated fatty acid compounds. However, the IP of LME shows an improvement at the temperature exceeded 130 C, which is the result of faster polymerization of peroxide radicals before the formation of hydroperoxides. The thermal oxidation is determined by the rate of the oxidation reaction at a high temperature when the fuel is not exposed to air. The oxidation rate increases with the increasing of fuel temperature [17]. A study carried out on soybean oil methyl ester (SME) and used cooking oil methyl ester (U) illustrate that the oxidation rate accelerateswith increasingtemperature caused by a decreased oil stability index (OSI), which is a measure of the relative oxidation stability of fatty acid methyl esters [18]. Therefore, for long-term storage of biodiesel, attention should be concentrated on several conditions, including limiting access to O 2 and exposure to light, metal and moisture [19]. A study was carried out on Jatropha biodiesel, after six months of storage duration, it was found the technical properties changes, with/without antioxidant treatment, and Jatropha biodiesel was not acceptable based on its stability as per standard specification EN However, the stability was found to be increased and met standard specifications after adding 200 ppm antioxidant of pyrogallol (PY) [20]. The induction period of biodiesel can be increased by adding antioxidants to the biodiesel. To test the oxidation stability, different biodiesels fuel has been tested with different antioxidants over a period of 10 weeks. The experimental results show that tertbutylhydroquinone (TBHQ), propyl gallate (PG) and pyrogallol (PA) are the most effective additives in neat methyl ester, whereas butylated hydroxytoluene (BHT) and butylated hydroxyanisol (BHA) are the least effective. Among them, TBHQ is the most effective one to prevent oxidation [21]. Likewise, in another study, a metal deactivator (which reduces the need for costly antioxidants by 30e50%) was doped with an antioxidant and added to Jatropha biodiesel to increase the oxidation stability from 3.95 h to 6 h [22]. In literature, individually some articles on the stability of palm and jatropha biodiesels are found. However, there are very few articles found where the stability of palm methyl ester, jatropha methyl ester and their blends and coconut oil methyl ester are collectively studied by means of their induction period and properties characterization. Thus in this study, we sought to extend our investigations and to specifically test the oxidation and storage stabilities of these biodiesels by measuring IP and the property changes at an interval of 180 h, during the total storage of 2160 h. 2. Experimental There were six different types of fuel samples tested and analysed in this study using different tests and experimental procedures. The tested fuel samples and their initial properties are tabulated in Tables 2 and 3 respectively. The tested biodiesels and diesel were supplied by Xtract Tech Sdn Bhd under forest research institute Malaysia (FRIM) and Hakita Engineering Sdn Bhd of Malaysia respectively. The fuel samples were stored in the glass bottle with the capacity of 300 ml of each at room temperature of w26 C and 65% relative humidity. The glass bottles were tightened with aluminium cover to ensure the fuel sample almost free from air contact during storage. The tested fuel samples were stored in a dark cabinet. In order to measure the oxidation stability of fuel samples (10 ml each), the induction period (IP) was determined using a Rancimat Instrument (model 783; supplied by Metrohm Sdn Bhd, Malaysia) with an air flow rate of 10 l/h at a constant temperature of 110 C. A schematic diagram of the Rancimat instrument is shown in Fig. 2. The properties of the biodiesels related to their oxidation and thermal stabilities, such as density, viscosity, flash point, TAN and TBN, were investigated and compared with the properties of diesel. Total acid number (TAN) was determined using a Metrohm TAN analyser, which is based on standard ASTM D664. This tester is connected to the software, Metrohm TiNet in the personal computer. For TAN analysis, the solvent consist of 500 ml toluene, 495 ml IPA and 5 ml H 2 Owere mixed together in a bottle. This solvent mixing ratio (toluene: IPA: water ¼ 500:495:5) was based on ASTM 664. Then 125 ml of that solvent was taken to mix with 5e10 g of fuel sample in a white beaker of the analyser. Similarly the TBN was analysed by the same instrument based on standard ASTM D2894. For TBN analysis 120 ml solvent consist of 40 ml glacial acetic acid and 80 ml chlorobenzene (C 6 H 5 Cl) were taken to mix with 5e10 g of fuel sample. The standards used for determining other properties like density, kinematic viscosity, and flash point were ASTM D341, ASTM D445, Fig. 1. Typical oxidation reaction [14]. Table 2 Tested fuel samples. Fuel samples 20 Compositions 100% Palm methyl ester 20% and 80% Petroleum diesel 100% Jatropha methyl eater 20% and 80% Coconut oil methyl ester

3 618 M. Shahabuddin et al. / Energy 44 (2012) 616e622 Table 3 Initial properties of the fuel samples. Property Unit 20 Test method Density kg/m ASTM D341 Kinematic Viscosity at 40 C cst ASTM D445 Flash point C ASTM D93 TAN Value mg KOH/g ASTM D664 TBN Value mg KOH/g ASTM D2894 ASTM D93 respectively. During storage, samples were taken out periodically (every 180 h). 3. Error analysis Errors and uncertainties in the experiments can be occurred from instrument selection, condition, calibration, environment, observation, reading and test planning. Uncertainty analysis was needed to prove the accuracy of the experiments. The percentage uncertainties of various parameters like Induction period, density, viscosity, flash point, total acid number (TAN) total base number (TBN) of are presented in Table 4. An uncertainly analysis was performed using the formula given in Ref. [23], which is. Total percentage uncertainty of this experiment ¼ Square root of {(uncertainty of induction period) 2 þ (uncertainty of density) 2 þ (uncertainty of viscosity) 2 þ (uncertainty of flash point) 2 þ (uncertainty of TAN value) 2 þ (uncertainty of TBN value) 2 } ¼ square root of {(0.863) 2 þ (0.168) 2 þ (0.0367) 2 þ (2.415) 2 þ (0.0085) 2 þ (0.591)} ¼2.63%. 4. Results and discussion 4.1. Oxidation stability Oxidation stability is the most important factor to assess biodiesel fuel quality, which is initiated by the chemical reaction between a free-radical and free unsaturated fatty acids. Table 5 shows the fatty acid content of the tested biodiesels parent oil. The primary product of the oxidation reaction, hydroperoxides, are further involved in some complex reactions generating secondary products, including alcohols and carbonyl compounds, which are oxidized to form carboxylic acids [24,25]. As shown in Fig. 3, it is quite clear that the induction period decreases with increasing unsaturated fatty acid percentages in the fuels. Furthermore, it is also depicted from the Fig. 3 that the (10.9% unsaturated fatty acid) had the longest induction period at h, followed by diesel (25% aromatic content) at 22.3 h, 20 (47.4% unsaturated fatty acid) at h, (53.6% unsaturated fatty acid) at h, 20 (68.84% unsaturated fatty acid) at 4.37 h and (78.9% unsaturated fatty acid) at 3.37 h. It is found that, among the fuel samples, all met the standard specification ASTM D6751 (3 h), but the and its biodiesel blend did not meet standard EN (6 h). To meet the standard specification, among several antioxidants, phenolic 2, 6-ditertiarybutyl hydroxytoluene is the most effective at 200 ppm [26]. To improve the IP of jatropha biodiesel above 6 h as required by EN-14112, minimum dosage of 300 ppm from tert-butylated phenol derivative (TBP) and acetylated butyrate biphenyl amine (OBPA) antioxidants is required while the dosages of 200 ppm of tertbutylated hydroxytoluene (BHT) and only 150 ppm of tert-butylhydroxquinone (TBHQ) is required to achieve same IP of 6 h [27]. Whereas, chen et al. [28] stated that the oxidation stability and cold filter plugging point (CFPP) of can be improved by blending of with palm and soybean oil methyl esters, respectively and authors also stated that the use of pyrogallol (PY) as an antioxidant having 100e250 ppm at 25 C temperature ensured the storage stability of 1.9e3.1 years without degradation of the fuel quality. The oxidation stability of biodiesel can also be determined by the following formula: OS ¼ 0:234ðXÞþ22:318ð0 < X 84Þ (1) where X is the unsaturated fatty acid methyl ester (wt%) [29] Density Fig. 2. Schematic diagram of the Rancimat instrument. Density is the measure of the mass per unit volume, which is expressed in kilogram per cubic metre (kg/m 3 ). Fuel density generally increases with increasing molecular weight of the fuel molecules. From Fig. 4, a trend can be seen in the density for all the fuel samples, which increases with storage time. The increasing trend of was most noticeable with a slope equal to This was followed by diesel with a slope of and 20 with a slope of The showed the least increment in density with a slope of The increase in density was caused by the formation of oxidation products, including insoluble sediments. The increase in density of was more which can be explained as the presence of saturated fatty acid percentages in the fuel. Since the fuels containing shorter chain hydrocarbon and more saturated fatty acid have more prone to be crystalized thus, cause of reduction of its volume and consequently increased the density. Simultaneously the mass of the fuel is increased as the consequence of oxidation products as well. Due to oxidation, the residue stored in the bottom of the glass bottles was identified by the weight

4 M. Shahabuddin et al. / Energy 44 (2012) 616e Table 4 Uncertainty calculation for the properties of at time t ¼ 0. parameters Three tests MaxeMin value Average Value with instrument accuracy Test 1 Test 2 Test 3 Max. Min. Induction period (h) Density (kg/m 3 ) Viscosity (cst) Flash point ( C) TAN value (mg KOH/g) TBN value (mg KOH/g) Instrument accuracy for induction period, density, viscosity, flash point TAN and TBN are 3%, 0.02%, 0.1%, 1%, 1% and 5% of the mean measured value respectively. measurement method which was done by subtracting the initial weight to the final weight of the samples. An increases in density of biodiesel from 848 g/l to 885 g/l, increases the viscosity from 2.8 to 5.1 cst which is the most important property as it affect the fuel injection equipment, especially at low temperature the fuel atomization characteristics become very poor due to the higher viscosity [31]. Similarly, the thermal instability of biodiesel is the major contributor to the increase of density, as it increases the rate of oxidation which, in turn, increases the mass of oil and fat due to the formation of insoluble sediments. Hence, as the mass increases, the density also increases. other researchers [34], as it has been noted that the oxidation process leads to the formation of free fatty acids, double bond isomerisation (usually cis to trans), saturation and the production of higher molecular weight molecules, and that viscosity increases with increasing oxidation with a longer storage period. The viscosities of the biodiesels are dominated by the chain length of the methyl or ethyl ester. In this experiment, it is found that the viscosity of was least comparing other biodiesels which might be the cause of shorter chain (C 8 ec 14 ) length of. Whereas the chain length of jatropha and palm methyl ester is belong to C 16 ec 18, which leads to higher viscosity Viscosity Viscosity is the measure of resistance to flow. Particularly, it is important due to the effect on the fuel injection system at low temperatures. Higher viscosity leads to lower atomization characteristics in the fuel injector, which leads to several severe effects on engine performance [32]. Sarin et al. [33] stated that blending of biodiesel over two is a simple but effective method to improve the flow properties at low temperatures. Additionally, a highly viscous fuel would also take longer time to mix with air since the quality of the vaporization and atomization of the fuel is reduced. Kinematic viscosity is increased with the carbon chain length in biodiesel containing free fatty acids and hydrocarbons. However, the viscosity of diesel is lower, and the increasing trend in viscosity over time is lower as diesel is less oxygenated than biodiesel [34]. Fig. 5 shows the changes in viscosity with storage time at 40 C, It can be seen that the viscosity of increases from to 4.92 cst after a storage time of 2160 h (3 months). The viscosity of other samples also increases; the only difference was the rate of the increase. The increasing trend in viscosity was due to the effect of oxidation. The came next with an increase of 0.94 cst (initial: 4.81 cst; final: 5.75 cst). fuel showed good characteristics in terms of viscosity as oxidation did not affect its viscosity very much, with an increase of 0.49 cst (initial: 3.20 cst; final: 3.69 cst). Approximately similar results have been previously observed by 4.4. Flash point The flash point temperature is an important property for a fuel, especially in terms of handling, storage and forming of a combustible mixture. The flash point indicates the difference between a highly flammable, volatile and a relatively non-flammable nonvolatile material [35]. It is expected that a good fuel should have a low auto-ignition temperature, especially in a diesel engine, since it has no extra mechanism to ignite the fuel in the combustion chamber. The auto-ignition temperature of a fuel is the lowest temperature at which the fuel could spontaneously ignite without an external source of ignition. Fuels with a flash point above 66 C can be considered to be safer fuels; therefore, biodiesel is a safer fuel for handling and storage [36]. From the experimental data shown in Fig. 6, it is obvious that after 2160 h (3 months) of storage, the flash points of all biodiesel samples were adequate and above the limiting value for safer fuels. Among these fuel samples, the decreased flash point over the entire storage period for was 33.5 C (max C; min. 208 C), followed by 20 with 25 C (max. 195 C; min. 170 C), with 18 C (max. 206 C; min. 188 C), with 14 C (max. 238 C; min. 224 C), with 12 C (max. 259 C; min. 247 C) and diesel with 11 C (max.75 C; min. 64 C). The showed the worst effect in terms of flash point reduction with a value of 33.5 C over the entire storage period. Table 5 Fatty acid contents (percent) of Jatropha, Palm and Coconut oil [30]. Fatty acid Jatropha Palm Coconut Caprylic acid (C 8:0) e e 8 Capric acid (C 10:0) e e 8 Lauric acid (C 12:0) e e 48 Myristic acid(c 14:0) Palmitic acid (C 16:0) Palmitoleic acid (C 16:1) 1e3.5 Stearic acid (C 18:0) 6e Oleic acid (C 18:1) 42e Linoleic acid (C 18:2) 33e Linolenic acid (C 18:3) 0.8 e e Production kg/ha Induction period (h) 20 Fuel samples Fig. 3. Induction period of different fuel samples.

5 620 M. Shahabuddin et al. / Energy 44 (2012) 616e622 Density (kg/m3) Flash point ( C) 20 Fig. 4. Density vs. storage time. Fig. 6. Flash point vs. storage time. Besides, the other biodiesels were also affected by oxidation. Even though the flash point of biodiesels decreased, the minimum value at the end of the storage period was much higher than the standard limiting value. The flash points of the five biodiesel samples analysed were found to have an influence in the initial boiling point (IBP) such that an increase in flash point leads to an increase in the IBP. fuel recorded high IBP despite having lower flash point than the biodiesel fuels. This could be attributed to the difference in the composition of the fuels, since diesel fuel consists of hundreds of different compounds, while biodiesel usually contains only 4 to 5 major compounds that all boil at about the same temperature [37] Total acid number (TAN) The acid number is the amount of base, expressed in milligrams of potassium hydroxide per gram of sample, required to titrate a sample in the solvent from its initial meter reading to a meter reading corresponding to a freshly prepared non-aqueous basic buffer solution [38]. Fig. 7 shows the changes in TAN over the storage period of 2160 h (3 months). The TAN value increases with increasing storage time for all the biodiesels. The acid number increases as a result of increased hydroperoxides, which may be further oxidised into acids. The esters first oxidise to form peroxides, which then turn into complex reactions, including a split into more reactive aldehydes, which further oxidise into acids. Acids can also be formed when traces of water cause hydrolysis of the esters into alcohols and acids [39]. Fig. 7 indicates that experiences the highest rate of oxidation, while diesel is the lowest (2.54e2.89 mg KOH/g and 0.25e0.59 mg KOH/g, respectively), in terms of the TAN value over the entire storage period. For, the increment in the TAN value was not significant compared to and. But, all biodiesel showed the negative effect of oxidation stability. The shows the lowest TAN value which is because of lowest unsaturated fatty acid content in the fuel, since the fuels, containing unsaturated fatty acid with double bonded long chain hydrocarbon have more susceptibility to oxidation. Aside from the time aspect, temperature also plays an important role in the increasing trend in TAN. If exposed to high running temperatures, the fuel becomes oxidised because of high temperature that causes fuel molecules to react with oxygen in the air, hence causing the TAN value to increase Total base number (TBN) The total base number is a measurement of the capacity of a fuel to neutralise strong acids from the fuel. As the fuel is used, it becomes contaminated with acids and the TBN drops. This is due to the formation of acidic products as a result of oxidation of the fuel. Low TBN reserves provide insufficient neutralisation capacity, leading to the corrosion of engine components, particularly around the piston ring pack, piston ring lands and top end bearing. A study [40] carried out on different fuels, including 15% palm oil biodiesel and 85% ordinary diesel with a corrosion inhibitor, 7.5% palm oil biodiesel and 92.5% ordinary diesel with a corrosion inhibitor and ordinary diesel, in an Isuzu 4FB1 water-cooled four stroke indirect injection (IDI) diesel engine, showed that after 100 h of engine operation, the TBN values were 3.88 mg KOH/g, 3.38 mg KOH/g and 1.76 mg KOH/g respectively, although the initial value was 6.5 mg KOH/g for each sample. The low depletion of TBN in the biodiesels was due to the corrosion inhibitor in the fuel samples. As storage time increases, samples are continually exposed to acidic compounds which form as a result of oxidation, and cause them to turn more acidic. In Fig. 8, among the six samples, diesel shows the kinematic viscosity (cst) 20 TAN value (mg KOH/g) Fig. 5. Viscosity vs. storage time at 40 C. Fig. 7. Total acid number vs. storage time.

6 M. Shahabuddin et al. / Energy 44 (2012) 616e TBN value (mgkoh/g) lowest rate of TBN depletion with a slope of , while has the highest with a value of This variation is due to the formation of acidic compounds during oxidation. The results show that has the poorest ability to neutralise acid. The ranking of the ability to neutralise acid based on Fig. 8 is as follows: > > > > 20 >. It is depicted from the Figs. 7 and 8 that the fuel having higher TAN value have the lower TBN value as well. 5. Conclusions The following conclusions can be drawn from the present study: 1. The unsaturated fatty acid percentages and the longer chain double bonded hydrocarbon in the biodiesel have the great influence on the stability of the biodiesel, as these are higher, the quality of the fuel would be poor as well as the properties would be degraded faster with increasing storage time. 2. The induction period of all biodiesels showed promising results in terms of oxidation stability, and all fuels met the standard specificationastmd6751(3h),exceptforanditsbiodieselblend which did not meet the standard specification EN (6 h). 3. During the storage period of 2160 h (3 months), adverse effects of oxidation were observed in terms of density and kinematic viscosity, but the values did not exceed the limiting value of the standard specification. 4. With respect to property determinations, the flash point of biodiesel showed the best performance among other the properties analysed in this study. However, a decreasing trend in the flash point of was noticeable. 5. The total acid number (TAN) was the most concerning for all the biodiesel samples since none of them met the standard specified value. From these results, it can also be concluded that had the highest potential to prevent oxidation by retaining the properties of the fuel during storage period, while and showed almost similar performance in terms of its properties. It can also be predicted from the trends of the figures that fuel quality would be deteriorated with increasing storage time. Since the oxidation of fuels are largely dependent on the storage conditions. Further study is required to investigate the stabilities of the biodiesel applying various conditions which would help to improve fuel quality by improving the oxidation, thermal and storage stabilities. Acknowledgement Fig. 8. Total base number vs. storage time. 20 The authors would like to thank Department of Mechanical engineering, University of Malaya, Ministry of Higher Education (MOHE) of Malaysia for HIR Grant No. UM.C/HIR/MOHE/ENG/07 and UMR Grant No: RG040-09AET for their financial support which made this study possible. 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