Ensuring Biodiesel Quality Through Lab Analysis

By Deanna Hurum, Brian De Borba, and Jeff Rohrer | August 08, 2008
Economic analysis shows that biofuels have the potential to replace more than 10 billion gallons of the petroleum currently used in the United States by 2030. As the supply of petroleum fuel tightens and its price continues to increase, biofuels will be increasingly used to meet consumer demand. One key example is biodiesel, now widely available in the consumer market both as unmodified biodiesel (B100), and as blends with petroleum diesel that are marketed to consumers for automotive use. Production of these fuels is supported by the U.S. government and by European Union Directive 2003/30/EC.

To ensure reliable quality of biodiesel as it gains widespread acceptance, ASTM International adopted ASTM D 6751. The standard is designed to protect current engine components, thus gaining engine manufacturers' support, and ensure that an engine using the fuel meets environmental regulations. The requirements stated in the ASTM document are predominantly equivalent to the EU standard, EN 14214, and a significant effort is underway to globally harmonize biofuel quality standards.

Currently, D 6751 applies to B100 biodiesel that is used for blending, ensuring that the source material, and therefore blends using B100 and petroleum diesel, will be of high quality. However, in mid-June ASTM International approved a new standard for biodiesel blends between B6 and B20 for on- and off-road diesel. The new standard will take effect once it is officially published.

Among the many parameters controlled by this standard are the concentrations of sodium, potassium, magnesium and calcium in the biodiesel. These impurities are incorporated into the biodiesel during the production process. For most biodiesel currently being produced, a basic catalyst of either sodium or potassium hydroxide or alkoxide is used. Residues from these catalysts persist in the biodiesel and are responsible for elevated sodium or potassium levels in the fuel.

In addition to the residual catalyst, the byproduct glycerol must also be separated from the biodiesel before it can be used. Water is often added during purification steps to separate the glycerol from biodiesel and to remove the residual catalyst. The quality of the water used during these steps has an impact on the cation concentrations in the biodiesel. If the water is hard, magnesium and calcium can transfer into the fuel during the removal of glycerol and the washing steps.

The presence of these alkali and alkaline earth metals in the fuel can form ash and soaps, which may lead to detrimental deposits in engines and damage emissions control systems. To prevent engine damage from the blended fuels, these cations are limited to concentrations less than 5 parts per million (ppm) for sodium and potassium combined and less than 5 ppm for magnesium and calcium combined.

ASTM D 6751 specifies the use of inductively coupled plasma-optical emission spectroscopy by reference to EN 14538 for the determination of alkali and alkaline earth metals in biodiesel. In Europe, methods EN 14108 and EN 14109 are also specified and use atomic absorption for measuring sodium and potassium in biodiesel. These methods suffer several drawbacks, including potential spectral interferences from other elements in the sample, effects of the complex matrix, difficulty in simultaneously determining each of the cations of interest, and complicated sample pretreatment procedures to avoid interferences. Ion chromatography is capable of simultaneously determining, within a reasonable time, the target cations sodium, potassium, magnesium and calcium after a simple aqueous extraction of a biodiesel sample. Additionally, the use of reagent-free ion chromatography systems with eluent generation simplifies the determination of cations in biodiesel by requiring only a source of deionized water to generate the methanesulfonic acid eluent used during the chromatographic method.

Biodiesel Analysis
Biodiesel samples were prepared by dispensing 25 milliliters (mL) into a 125 mL polypropylene separatory funnel, adding 25 mL of deionized water and vigorously shaking the funnel for one minute. Alternatively, for convenience, disposable polypropylene plasticware can be used if it is first rinsed with deionized water to remove potential contamination. The emulsion was allowed to separate for 15 minutes. The aqueous extract was then filtered directly into an analysis vial with a Pall 25 millimeter (mm), 0.2 micron IC Acrodisc syringe filter.

Comparative biodiesel extractions were performed to evaluate the suitability of using deionized water as the extractant. Hydrochloric acid and nitric acid were each tested between concentrations of 1 millimolar (mM) to 10 mM. No improvement in extraction efficiency was observed when using the acidified extractants. Weakly acidic solutions (less than 5 mM) had a disadvantage of significantly stabilizing the water-fuel emulsion leading to poor extractions. Acidic extraction solutions are not recommended due to the lack of improvement in the extraction efficiency and the stabilization of the water-biodiesel emulsion.

Biodiesel analysis was performed on a Dionex ICS-3000 reagent-free ion chromatography system equipped with an EluGen methanesulfonic acid cartridge. An IonPac CS12A-5 micron (3 mm by 150 mm) analytical column and a guard column, CG12A (3 mm by 30 mm), were used for all separations with a flow rate of 0.5 mL per minute. Analytes were detected using suppressed conductivity with a CSRS 300 (2 mm) suppressor operating at 30 milliamps in the recycle mode. Chromeleon Chromatography Management Software was used for system control and automation of data processing.

Analysis Results
The method was used to determine the concentrations of sodium, potassium, magnesium and calcium in commercially available B20 and B99 biodiesel samples from triplicate deionized water extractions. The combined sodium and potassium concentration determined in B20 samples was 0.0463 0.0009 ppm and the combined magnesium and calcium was 0.0306 0.0012 ppm, both well below the ASTM limits. The four cations are well resolved from one another and easily quantified in less than 15 minutes (Figure 1A).

The combined sodium and potassium concentration determined in B99 was 0.991 0.032 ppm with a combined magnesium and calcium concentration of 0.207 0.010 ppm, both of which are also well below the ASTM limits, but significantly higher than the concentrations found in B20 (Figure 1B). In both cases the IonPac CS12A-5 micron provided a fast and high resolution separation allowing the cations to be easily identified and quantified.

The accuracy of the method was verified by determining recoveries of enriched B99 extractions over three days. Recoveries were calculated based on the difference in response between the enriched and unmodified samples, with average recoveries of the four target cations ranging from 98 percent to 108 percent. Sample analysis across three days of triplicate extractions of B99 showed that the error of the determined concentrations ranged from 0.05 percent to 2.4 percent. The recoveries for these samples are excellent, showing that an analysis of B99 is accurate, and that results are highly reproducible with low error across several days of testing. Similar results are expected for analysis of B100 samples.

This method can be used to quickly and easily determine cations in biodiesel samples. The high resolution and column design of the CS12A-5 micron allows for rapid separation of the target cations with a wide response range. This method simplifies determination of the four cations of interest compared to atomic absorption and inductively coupled plasma by requiring a simple sample preparation, simultaneous determination of the target cations, and limited interferences from other elements in the sample.

Additionally, the reagent-free ion chromatography systems with eluent generation ensure high precision and eliminate the need to manually prepare eluents, thereby eliminating eluent preparation errors, as well as providing time and cost savings. Commercially available biodiesel samples were evaluated and samples with low concentrations of cations (B20) and nearly 1 ppm combined sodium and potassium (B99) were successfully analyzed using this method with both samples well below the ASTM specification.

Deanna Hurum, Brian De Borba and Jeff Rohrer are with Sunnyvale, Calif.-based Dionex Corp. Reach Hurum at [email protected] or (408) 481-4295. Reach Rohrer at [email protected].
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