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144 Anca Elena Eliza Sterpu and Anca Iuliana Dumitriu 2. Methods 2.1. Blend Preparation The biodiesel used in this work was produced by basic methanolysis of rapeseed oil using a methanol/oil molar ratio of 12:1, with 0.5% NaOH by weight as the catalyst. The reaction temperature and time were 65 o C and 1 h, respectively. The biodiesel thus produced by this process is totally miscible with mineral diesel in any proportion. The experimental samples were prepared by mixing a commercial diesel fuel with rapeseed biodiesel in different proportions (B0, B5, B10, B15, B20, and B100). Blends were prepared on a volume basis at 25 o C. 2.2. Experimental Both density and kinematic viscosity were simultaneously measured accor<strong>din</strong>g to ASTM D445 using a Anton Paar device, SVM 3000 type. The Closed-cup Flash Point Analyzer, Pensky-Martens type, was used to measure the flash point of biodiesel samples with different compositions. This flash point analyzer is operated accor<strong>din</strong>g to the standard test method, ASTM D93. The analyzer incorporates control devices to adjust the heating rate of a sample. The test interval is 1 o C and the heat rate is 2 o C/min. The low heating value was determined following ASTM D4868, as a function of the fuel density, sulfur, water, and ash content. All fuels tested were distilled following the procedure established by ASTM D86. In addition to the initial and final boiling points, temperatures for distilled percentage multiples of 10 were also registered. The calculated cetane index for the fuels tested was determined as a function of their densities and distillation curve data using the empirical correlations recommended by ASTM D976 and D4737. 3. Results and Discussion 3.1. Basic Properties of Pure Fuels Experimental data on the properties of rapeseed oil biodiesel and diesel fuel are presented in Table 1. Rapeseed oil biodiesel (ROB) is slightly denser and more viscous, and has a narrower boiling interval than diesel fuel, indicating that the fatty acid methyl esters in biodiesel have similar boiling points. Diesel fuel, meanwhile, is composed of a wide variety of hydrocarbons with different volatilities. ROB has a low heating value lower than diesel fuel due to the presence of oxygen in the methyl ester molecules. This difference in heating values, expressed as energy per unit mass, is slightly reduced when it is reported as energy per unit volume, given the greater density of ROB. As a result of these differences, the calculated cetane index for ROB is higher than for diesel fuel; this agrees with the tendency reported by several researchers for both cetane numbers and CCI.
Bul. Inst. Polit. Iaşi, t. LVIII (LXII), f. 4, 2012 145 Table 1 The main properties of rapeseed oil biodiesel and diesel fuel Properties Units ASTM Diesel Rapeseed method fuel oil biodiesel Density at 25 o C g/cm 3 D445 0.8374 0.8695 Kinematic viscosity at 40 o C cSt D445 4.6597 6.1415 Flash Point o C D93 67 107 Initial boiling point o C D86 170 296 Temperature at 50% recovered o C D86 260 334.5 Final boiling point o C D86 328 343 Calculated cetane index - D976 46.99 52.23 D4737 46.65 54.25 Low heating value kJ/kg D4868 42650 40301 Refractive index - D1218 1.467 1.45 Sulphur content % D5453 0.07 - Water content % D6304 0.012 0.017 Ash content % D482 0.001 0.003 3.2. Effect of Biodiesel Content Mixing rules are used for estimating the properties of blends as a function of pure fuel properties and biodiesel content. The suitability of these rules is evaluated by means of the absolute average deviation (AAD), calculated as (Benjumea et al., 2008) 100 AAD NP i φ − φ NP EXP PR = ∑ , (1) = 1 φEXP where NP is the number of experimental points, φ is the property to be predicted and the subscripts are EXP for experimental and PR for predicted. For predicting viscosity of liquid mixture, a geometric volume average form equation (Benjumea et al., 2008) (Arrhenius-type equation) can be used ln η = V ln η + V ln η . B ROB ROB B0 B0 In the above equation, V is the volume fraction and the subscripts are B for blend, ROB for biodiesel and B0 for diesel fuel. For the remaining properties, Kay’s mixing rule (Benjumea et al., 2008) is used n B i i i= 1 (2) φ = ∑ x φ , (3) where: φB is the property of the blend and φi is the respective property of the i th component. Using volume fraction instead of molar fraction, Eq. (3) for a binary mixture takes the form of an arithmetic volume average:
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