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* For correspondence.

Oxidation Communications 39, No 4-II, 3324–3335 (2016)

Biological fuels – analysis of biodiesel blends

GAS CHROMATOGRAPHY ANALYSIS OF BIODIESEL BLENDS

Z. MUSTAFA, D. YORDANOV*, R. MILINA

University ‘Prof. Dr. Assen Zlatarov’ – Burgas, Organic Campus, Room 323, 1 Prof. Yakimov Street, Burgas, Bulgaria E-mail: dobromirj@abv.bg

ABSTRACT

In the present study a method of simultaneous qualitative and quantitative analysis of fatty acid methyl esters (FAME) in biodiesel blends, suitable for FAMEs concentrations higher than 5% (v/v) is proposed and applied for the determination of FAME in diesel blends and in biodiesel. An SLB-IL100 GC column was employed, characterised by a highly polar stationary phase. The proposed method for the analysis was validated in accordance with the requirements of EA-4/02 ‘Expression of the Uncertainty of Measurement in Calibration’.

Keywords: diesel fuel, biodiesel, biodiesel blends, gas chromatography analysis, fatty acid methyl esters (FAME).

AIMS AND BACKGROUND

Biofuels, produced from biological materials such as vegetable oils, recycled cooking oils, animal fats and plant and waste products, support local agricultural industries and prevent environmental pollution with unnecessary products. Among the biofuels currently in use is receiving much attention as a renewable and sustainable alterna-tive for automobile engine fuels. It contains no hazardous constituents such a sulphur, nitrogen and polycyclic aromatic compounds and reduces pollutant and greenhouse gas emissions. It can be blended with diesel practically at any proportion and can be used without changes in the existing transport vehicles1–3.

Biodiesel is currently produced in commercial quantities from edible oil feedstock. Although biodiesels from these feedstock contributes to a cleaner environment, other issues should not be ignored such as food prices and social impacts associated with using food-based feedstock. This fact has sparked the research on second-generation

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biodiesels (and biofuels in general) from nonedible sources such as castor, microalgae, coffee grounds, etc.4–9

Biodiesel is a fuel comprised of monoalkyl esters of long-chain fatty acids (FAME). The total ester content in biodiesel can be analysed by EN 14103 standard10.

There is no standard method to determine the individual FAMEs in biodiesel. Biodiesel FAMEs can be analysed directly by gas chromatography – flame ionisation detector (GC–FID) (Refs 11–13), gas chromatography – mass spectrometry (GC–MS) (Refs 13–15), two-dimensional gas chromatography16–18, high performance liquid chromatography19.

Usually biodiesel is used mixed with petroleum distillates as blends defined with the abbreviation BX, where X represents the biodiesel percentage (v/v). Biodiesel blends are highly complex and contain not only FAMEs, but many saturated and aromatic hydrocarbons from petroleum. This is why there are few methods reported for the analysis of biodiesel blends, including pre-separation step, limited to a certain percentage of biodiesel as HPLC (Refs 20–22), and comprehensive two-dimensional gas chromatography16,23,24.

The complexity and duration of the procedures, and the high price prompted scientists to work on creating easier and faster methods for determining biodiesel in blends with diesel fuel.

The aim of the present study is to develop a method of simultaneous qualitative and quantitative analysis of FAME in biodiesel blends, suitable for FAMEs concentrations higher than 5% (v/v) and determination of FAME in diesel blends and in biodiesel.

EXPERIMENTAL

Materials. The diesel sample was delivered from the refinery LUKOIL Neftohim Burgas (Burgas, Bulgaria). Biodiesel fuels used were:

– Certified Reference Material (CRM) В100, purchased from SPEX CertiPrep – UK;

– Certified Reference Material FAME Mix Rapeseed oil, from Sigma-Aldrich;– Commercial biodiesel kindly supplied from Astra Bioplant, Bulgaria;– Standard Reference Material (SRM 2772) B100 Biodiesel (Soy-Based), pur-

chased from National Institute of Standards and Technology – USA; – Certified reference materials: Methyl heptadecanoate (C17:0) purchased from

Spex CertiPrep; F.A.M.E. Mix Standard Rapeseed oil (cat No 18917) – from Supelco; Fatty acid methyl esters – myristate (C14:0), palmitate (C16:0), palmitoleate (C16:1), stearate (C18:0), oleate (C18:1), linoleate (C18:2), linolenate (C18:3), arachidate (C20:0), cis-11-eicosenoate (C20:1), behenate (C22:0), cis-13-docosanoate (C22:1), tetracosanoate (C24:0), cis-15-tetracosanoate (C24:1) – from Sigma-Aldrich. Reagents of recognised analytical grade were used.

Biodiesel blends were prepared by adding of biodiesel in diesel fuel at concentra-tions 5, 7, 10, 15 and 20% (v/v) and internal standard (C17:0).

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Gas chromatography. GC analyses were performed on a GC system Agilent Technolo-gies 7890A equipped with FID, split/splitless injector and Agilent 7693A automated liquid sampler. Helium was used as a carrier gas, column flow was 1.5 ml/min. Hydro-gen and air flows were set to 40 and 400 ml/min, respectively, makeup gas (nitrogen) 40 ml/min. The injection volume was 1 µl and split ratio was 1:80. The temperatures of the injector and the detector were 250 and 300 ºC, respectively.

The fussed silica capillary columns used were:1. HP-INNOWAX, 30 m × 0.32 mm ID and 0.25 µm film (Agilent Technologies).

The temperature program was: mode a – 210ºC, 9 min, and then to 230ºC at 20ºC/min and hold 10 min, and mode b – 50ºС, 3ºС/min, 210ºC, 10 min.

2. SLB-IL100, 30 m × 0.25 mm ID and 0.20 µm film (Supelco) at a temperature program 50ºС, 3ºС/min, 210ºC, 10 min.

ChemStation for GC (Agilent Technologies) was used for instrumental control, data acquisition and data analysis.

RESULTS AND DISCUSSION

ANALYSIS OF STANDARD FAME SOLUTIONS AND BIODIESEL

Polyethylene glycol stationary phases have most frequently used for analysis of fatty acids methyl esters in lipid matrices. Such a phase is used in the standard method EN 14103. We modified this method to apply not only for the determination of total FAME content, but of the individual FAMEs, too.

Ionic liquids have been widely used as solvents in organic synthesis and catalysis. In recent years increased efforts to use ionic liquids as GC stationary phases. The reasons for the latter are the low volatility, high thermal stability, excellent selectiv-ity towards specific chemical classes and good wetting abilities on the inner wall of fused silica capillaries.

Both columns SLB-IL100 (ionic liquid phase) and HP-INNOWAX (polyethylene glycol phase) were investigated and compared at several GC conditions (ideal for each column and under the same conditions) to establish the best resolution and correct quantitative analysis of biodiesel and blends. For this purpose certified mixtures of FAME and reference biodiesel fuels from different raw material were used. Some of the chromatograms obtained are shown in Figs 1–3. It is seen that there is a good resolution of all components on both columns in both modes.

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Fig. 1. GC of rapeseed biodiesel on the INOWAX column, mode a

Fig. 2. GC of rapeseed biodiesel on the INOWAX column, mode b

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Fig. 3. GC of rapeseed biodiesel on the SLB-IL 100 column

Validation of the GC method on both columns for analysis of pure biodiesel was performed with respect to the retention time (tR) and the concentration of the individual FAMEs at n = 10. The accuracy and precision, expressed by the standard deviation (SD), relative standard deviation (RSD) and extended uncertainty (U) (Tables 1 and 2) of the method of quantitative analysis of pure biodiesel were calculated and compared with the certified values. It can be seen a good correspondence between the calculated and certified precision characteristics, which confirms suitability of the method for qualitative and quantitative analysis of pure biodiesel.

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Table 1. Precision and accuracy of the FAME retention times and concentrations (GC column HP – IN-NOWAX (210ºC, 9 min, 10ºС/min, 230 ºC, 10 min))FAME Retention time (min) Concentration (%) From certificate of analysis

tR (min)

SD (min)

RSD (%)

C (w.%)

SD (w.%)

RSD (%)

concentra-tion (w.%)

SD (w.%)

RSD (%)

C14:0 2.22 0.011 0.495 0.9 0.007 0.78 1.0 0.01 1.00C16:0 2.71 0.019 0.738 3.9 0.02 0.45 4.0 0.03 0.75C18:0 3.58 0.040 1.117 2.9 0.03 1.03 3.0 0.02 0.67C18:1 3.82 0.068 1.832 60.3 0.57 0.94 60.0 0.45 0.75C18:2 4.20 0.072 1.666 11.9 0.06 0.50 12.0 0.09 0.75C18:3 4.73 0.060 1.286 4.9 0.04 0.82 5.0 0.04 0.80C20:0 5.13 0.072 1.364 2.9 0.02 0.69 3.0 0.03 1.00C20:1 5.53 0.061 1.084 1.0 0.01 1.00 1.0 0.01 1.00C22:0 7.91 0.070 0.884 3.1 0.03 0.96 3.0 0.02 0.67C22:1 8.45 0.081 0.946 4.9 0.03 0.61 5.0 0.03 0.60C24:0 12.83 0.089 0.701 2.9 0.02 0.69 3.0 0.02 0.67

Table 2. Precision and accuracy of the FAME retention times and concentrations (GC column SLB-IL100 (50ºС, 3ºС/min, 210ºC, 10 min))FAME Retention time (min) Concentration (%)

tR (min)

SD (min)

RSD (%)

certified average

concentra-tion (%)

measured average

concentra-tion (%)

calcu-lated

SD (%)

calcu-lated RSD (%)

certifieduncer-tainty

USRM (%)

calculated uncer-taintyU (%)

C14:0 27.239 0.044 0.16 0.075 0.072 0.004 5.5 0.01 0.02C16:0 31.636 0.052 0.16 10.70 10.45 0.166 1.5 0.20 0.13C16:1 32.761 0.050 0.15 0.13 0.12 0.006 5.0 0.02 0.02C18:0 35.614 0.066 0.18 4.30 4.31 0.124 2.8 0.27 0.25C18:1 36.528 0.064 0.18 23.30 22.88 0.26 1.1 0.60 0.52C18:2 38.115 0.070 0.18 1.43 1.39 0.047 3.3 0.15 0.09C18:3 39.917 0,051 0.13 52.30 51.75 0.473 0.9 1.70 0.95C20:0 39.239 0.052 0.13 7.82 7.73 0.12 1.5 0.20 0.24C20:1 40.153 0.052 0.13 0.36 0.39 0.03 7.7 0.05 0.06

ANALYSIS OF BIODIESEL BLENDS

To establish appropriate conditions for the analysis of blends, samples of diesel fuel, pure biodiesel from different raw material and their blends were tested. Some of chromatograms on both GC columns are shown in Figs 4–6. Figure 4 is the chro-matogram of the rapeseed B10 biodiesel blend according to the method EN 14103. It is observed there is no separation between any of the components of diesel and pure biodiesel, so that the standardised method can not be applied to the qualitative

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and quantitative analysis of biodiesel blends. The chromatogram at mode b (Fig. 5) shows better separation, but still some of the main FAME components, irrespective of the raw material, namely, C14: 0, C16: 0, C18: 0 and C18: 2 are coeluted with some diesel alkanes. The possibility of complete separation of the methyl esters of diesel hydrocarbons achieved on the on SLB-IL 100 column (Fig. 6) gives reason IL polar phase to be applied for quantitative analysis of biodiesel blends. Besides the high resolution, the column SLB-IL 100 has another advantage over INNOWAX: analysis time is less (Table 2).

Fig. 4. GC of rapeseed B10 biodiesel blend on the INOWAX column, mode a

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Fig. 5. GC of rapeseed B10 biodiesel blend on the INOWAX column, mode b

Fig. 6. GC of rapeseed B10 biodiesel blend on the SLB-IL100 column

To determine of the total amount (%, v/v) of biodiesel, added to diesel, a four-point calibration curve was constructed using B5, B10, B15 and B20 blends. The ratio between the total corrected area of FAMEs and the corrected area of C17:0 (Internal

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standard), was plotted versus the total content (%, v/v) of FAMEs in the blends. An R2 value of 0.9908 was obtained.

The parameters for validation were calculated using two blends: B2 and B7 (n = 6). The results are shown in Table 3. It is seen that the precision of the method expressed by SD and RSD was very good and ranges from 0.03 to 0.04% (v/v) for SD and 1.54 to 0.58% for RSD. The accuracy expressed by the extended uncertainty was 0.06 to 0.08% (v/v). It is seen also that the maximum difference between the two determinations is 0.10% (v/v), while for method EN 14078 is 0.30% (v/v), i.e. the repeatability of the chromatographic method is better.

Table 3. Precision and accuracy of the GC method of determination of total FAME content in blends (SLB-IL100 GC column)

Added(%)

Measured (%)1 2 3 4 5 6 x SD RSD U.

2.00 1.98 2.04 2.00 1.96 2.03 2.02 2.01 0.031 1.54 0.0627.00 7.05 7.02 6.95 6.98 7.03 6.96 7.00 0.041 0.58 0.082

The main advantage of the proposed method is the possibility to determine the FAME profile of biodiesel in the blend. The method was validated in terms of reten-tion time and concentration of the individual FAME in blends (n = 10). Rapeseed and soya B10 biodiesel blends were used. The results are reported in Tables 4 and 5. The added and measured concentrations coincide within the repeatability of the method.

Table 4. Validation parameters of the analysis of individual FAMEs in blends (SLB-IL100 GC column)FAME

(rapeseed B10 blend)

Retention time (min) Concentration (%)tR SD (min) RSD (%) C SD (%) RSD (%) U

C16:0 32.661 0.083 0.26 0.586 0.003 0.50 0.006C18:0 35.753 0.104 0.29 0.255 0.003 1.18 0.006C18:1 36.729 0.123 0.33 4.854 0.051 1.05 0.102C18:2 38.323 0.118 0.31 3.598 0.033 0.91 0.066C18:3 39.899 0.089 0.22 0.516 0.004 0.82 0.008

Table 5. Average FAMEs concentration (%) in blends of soybean biodiesel in diesel fuel oil (SLB-IL100 GC column)

FAME B5 B7 B10 B15added measured added measured added measured added measured

C16:0 0.53 0.55 0.74 0.70 1.06 1.12 1.60 1.68C18:0 0.21 0.23 0.30 0.30 0.43 0.42 0.64 0.60C18:1 1.16 1.16 1.62 1.61 2.32 2.40 3.48 3.51C18:2 2.61 2.63 3.65 3.57 5.21 5.38 7.80 8.04C18:3 0.69 0.65 0.54 0.57 0.78 0.78 1.16 1.12

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The developed method was applied to analysis of biodiesel from SCG and blends. The chromatograms received are shown in Figs 7 and 8. As can be seen, the resolution between the individual FAMEs in the biodiesel and the blend is good enough to be used for investigations of those fuels.

Fig. 7. GC of SCG biodiesel on the SLB-IL 100 column

Fig. 8. GC of SCG B7 biodiesel blend on the SLB-IL100 column

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CONCLUSIONS

1. In the present study a method of simultaneous qualitative and quantitative analysis of FAME in biodiesel blends, suitable for FAMEs concentrations higher than 5% (v/v) is developed and applied for the determination of FAME in diesel blends and in biodiesel.

2. The main advantage of the proposed method is the possibility to determine the FAME profile of biodiesel in the blend.

3. The proposed method for the analysis was validated in accordance with the requirements of EA-4/02 ‘Expression of the Uncertainty of Measurement in Calibra-tion’25.

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Received 4 February 2016 Revised 15 March 2016