Quality Assurance Of Biofuels

Metrohm UK Ltd offer instrumentation for the analysis of biodiesel and bioethanol

Interest in biofuels has greatly increased in recent years. The reasons are the diminishing reserves of crude oil and the simultaneous increasing demand for mineral oil as well as the environmental problems linked with burning fossil fuels.

In addition to a neutral CO2 balance, biogenic fuels are generally characterized by their very good microbiological degradability. However, with biofuels based on fatty acids there is an increased risk of polymerization at higher temperatures in the absence of antioxidants or in the presence of nonferrous metals. A too high water content and the presence of alkali metal or alkaline earth metal ions as well as sulfate and chloride ions also have a negative effect on the fuel quality and can damage the fuel system of the vehicle. With the aim of avoiding these problems, the manufacturers of biofuels and motor vehicles have developed quality standards in which testing methods and quality parameters for biofuels are defined.

Several of the stipulated tests can be carried out with Metrohm instruments and Metrohm application methods.

The most important advantages at a glance are; • Biofuel analysis from a single supplier • Many years of application know-how in the ion analysis sector • «Swiss made» technology for the determination of the biofuel quality parameters • Three-year instrument warranty

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Oxidation stability of fatty acid methyl esters (FAME) and vegetable oils as per EN 14112

873 Biodiesel Rancimat
Fatty acid methyl esters are usually obtained from oil seeds. During manufacture the vegetable oil is transesterified with methanol. This produces the methyl esters of the fatty acids present in the oil with glycerol as a byproduct. Vegetable oils and fatty acid methyl esters have a relatively short storage life as they are slowly oxidized by atmospheric oxygen. The resulting oxidation products can damage vehicle motors. For this reason the oxidation stability is an important quality criterion for biodiesel and vegetable oils and must therefore be checked regularly during manufacture and storage. With the 873 Biodiesel Rancimat this determination can be carried out simply and reliably. The addition of suitable antioxidants slows down the oxidation process. The effectiveness of antioxidants can also be determined with the 873 Biodiesel Rancimat.

The 873 Biodiesel Rancimat is a modern, PC-controlled instrument for the convenient determination of the oxidation stability of oils and fats. The temperature can be set very accurately and reproducibly by using the GLP Set with certified temperature sensor; this further improves the precision of the results.

Water determination by Karl Fischer titration

Water content of biodiesel
The water content of biodiesel determines the calorific value and, above all, the storage life of the fuel. Biodiesel with a high water content has a clearly lower oxidation stability. The lower the oxidation stability, the greater the probability that oxidation products will be formed during a longer storage period. These could damage the motor, in particular the fuel injection system, by forming deposits. In order to avoid this, DIN EN 14 214 limits the water content of biodiesel to 500 ppm. The ISO standard 12 937 «Petroleum products – Determination of water – Coulometric Karl Fischer titration method» describes the coulometric Karl Fischer titration for determining the water content. The 756 and 831 KF Coulometers from Metrohm easily meet all the specifications stipulated in the standard and are outstandingly suitable for this application. In most cases the sample can be injected directly into the reaction solution. In order to improve the solubility of the samples, xylene (dimethylbenzene) is added to the KF reagent. Some biodiesel fuels contain additives that could participate in side-reactions during the direct coulometric Karl Fischer titration. In such cases Metrohm recommends that the biodiesel sample is not injected directly into the reaction solution. Instead the water contained in the biodiesel should be driven off in a KF oven. The 832 KF Thermoprep oven is suitable for this. The water is driven off at 120 °C and transported to the titration cell of the KF Coulometer in a stream of carrier gas (dry air or inert gas).

This process can be completely automated with the 774 Oven Sample Processor.

Water content of vegetable oils
Natural or simply filtered vegetable oils can also be used as a fuel for diesel motors. In contrast to biodiesel, which is obtained by chemically altering the vegetable oil and which can be used in standardized quality with existing motor technology, natural vegetable oils require adaptation of the diesel motor. As the presence of water favors the cleavage of fatty acids in the vegetable oil, standard DIN V 51 605 limits the permitted water content to 750 ppm.

The ISO standard 12 937 «Petroleum products – Determination of water – Coulometric Karl Fischer titration method» is also stipulated as a test method for vegetable oils. Because of the poor solubility of oils in methanol the use of a solubility promoter is also recommended here.

Water content in bioethanol
Bioethanol is mixed with gasoline in various ratios in order to reduce both the demand for gasoline and environmental pollution. The gasoline-alcohol blend is known in the USA as gasohol and in Brazil as Gasolina Tipo C. In the USA the mixtures E10 and E85, which contain 10% and 85% bioethanol respectively, are widespread. In Brazil the bioethanol content of most blends lies between 21% and 23%. The ASTM standard E 1064 «Standard Test Method for Water in Organic Liquids by Coulometric Karl Fischer Titration» describes the coulometric Karl Fischer titration for determining the water content. For a water content >2% the recommended test method is volumetric titration as per ASTM E 203. The volumetric KF titrators from Metrohm meet all the specifications required by the standard and are therefore extremely suitable for this application.

pH measurement and titrimetric analysis

pH measurement
The pH value is an important quality criterion for all biofuels. A combined pH glass electrode with ground-joint diaphragm is recommended for measuring the pH in organic solvents. Because of the expected low conductivity of the sample, the electrode should be particularly well shielded in order to suppress electrostatic influences. The Solvotrode with separable ground-joint diaphragm is particularly suitable for measuring the pH of biofuels. The pH meter used for the measurement should possess the following GLP functions, among others: • Three-point calibration with automatic buffer recognition • Temperature compensation • Calibration data monitoring • Automatic measured value recording • Result memory with identification The pH meter 827 pH lab for laboratory use and the handy, battery-operated 826 pH mobile offer all these functions with extremely comfortable operation.

Titrimetric determinations

• Acid number
The acid number is a sum parameter for all acidic components; at the same time it is a measure for the long-term stability and corrosiveness of the biofuel. The smaller the value, the higher the quality. Standard EN 14 104 stipulates a non-aqueous potentiometric acid-base titration for determining the acid number. This involves dissolving the sample in a bioethanol/diethyl ether mixture and titrating it with potassium hydroxide dissolved in 2-propanol (1 mol/L). As organic solvents are used, the manual working steps should be reduced to a minimum. With its space-saving Dosino technology the Titrando allows fully automatic and safe solvent addition. The Solvotrode, an excellently shielded combined pH glass electrode, has been specially developed for non-aqueous titrations. The separable ground-joint diaphragm guarantees stable potentials and is easy to clean. • Iodine number in fatty acids and fatty acid methyl esters The iodine number is a measure for the number of double bonds in a sample. It is the amount of iodine (in g/100 g  sample) that can be added to the sample under the given conditions. The determination of the iodine number is covered by European standard EN 14 111. An aliquot of the biofuel sample is dissolved in a mixture of cylcohexane/glacial acetic acid and treated with Wijs reagent. After a defined time has elapsed, potassium iodide and water are added. The released elemental iodine is then titrated with sodium thiosulfate solution. EN 14

111 allows both optical (with starch solution as the indicator) and potentiometric endpoint determination.

Ion chromatographic analysis

Ion chromatography for quality assurance

In addition to the determination of the oxidation stability, the water content and the acid and iodine numbers, the quality requirements for biofuels also cover limits for ionic species. Alkali metals and alkaline earth metals, antioxidants and anions such as chloride and sulfate can be determined by ion chromatography in order to check fulfillment of the requirements of DIN EN 14 214, DIN V 51 605 and ASTM D 4806 and therefore prevent motor damage.

Determining alkali metals and alkaline earth metals After esterification and subsequent treatment, alkali metals and alkaline earth metals may be present in biodiesel as unwanted residues. They form salts with the anions contained in biodiesel and these block the fuel filter. In addition to this primary damage, secondary damage is caused to the injection pump. Standard DIN EN 14 214 permits a cumulative concentration of 5 mg/kg for both the alkali metals sodium and potassium and also for the alkaline earth metals magnesium and calcium. Both groups of cations can be determined rapidly and accurately in a single ion chromatographic run. This saves time and money.

The samples are extracted with dilute nitric acid, dialyzed and then injected directly into the IC system. The complete sample preparation procedure and analysis takes place fully automatically. The combination of instruments used consists of the 861 Advanced Compact IC with Metrohm inline extraction and dialysis. Both inline extraction and inline dialysis represent a form of the Metrohm Inline Sample Preparation «MISP». Metrohm IC systems are characterized by their compact and robust design. The costs for consumables and reagents are reduced to a minimum. The complete system is PC-controlled. The IC Net software used can be understood intuitively and is therefore very user-friendly.

Determination of antioxidants
The oxidation stability of fatty acids and of the fatty acid methyl esters synthesized from them can be improved by the addition of antioxidants.
Biodiesel based on rapeseed oil already contains an outstanding natural protection against oxidation – vitamin E (d-á-tocopherol). In addition Baynox®, whose structure was inspired by that of vitamin E, is added to the biodiesel as a stabilizer. This inhibits both oxidation to corrosive acids and the formation of insoluble polymers and thus prevents the injection nozzles from being blocked. Both these substances are therefore determined within the context of quality monitoring and for determining the amounts of additives to be added. Because of their structural similarities, vitamin E and Baynox® can be determined together in a single analysis. To improve solubility, dichloromethane is added to the eluent and analyte solutions.

The biodiesel samples should be diluted 1:1000. The analytes are separated at 35 °C and then determined quantitatively using UV detection. For the determination of antioxidants for quality assurance purposes, Metrohm recommends the 844 UV/VIS Compact IC with built-in column heating. This unique instrument in the ion chromatography sector combines the advantages of a compact IC system with a diode array detector (DAD) in a single housing. The method can be completely automated by combination with the 838 Advanced Sample Processor. This saves both processing time and costs.

Chloride and sulfate
Bioethanol is either used in pure form as a fuel or mixed with fossil fuels.
Contamination in the form of inorganic salts affects both the functioning and the working life of the motor. Against this background the ethanol specification ASTM D 4806 limits the sulfate content to 4 ppm and chloride to 40 ppm. Chloride and sulfate are separated on an anion exchange column and then determined quantitatively by conductivity detection. The recommended setup is an 861 Advanced IC with Metrohm CO2 Suppressor «MCS».
The analysis can be completely automated by using an 838 Advanced IC Filtration Sample Processor, which allows inline ultrafiltration of the samples.

Metrohm UK Ltd have a large network of service and support personnel throughout the UK including dedicated training staff for each product and an application/support laboratory. They have a lot of expertise in the measurement of biofuels and offer Application Bulletins and periodic seminars. For further information or to arrange a demonstration please contact Metrohm UK Ltd Tel 01280824824, Fax 01280824800, Email enquiry@metrohm.co.uk, Website www.metrohm.co.uk