The Modified Rancimat Method for the Determination of Oxidation Stability of Biodiesel Blends According to EN15751

Oxidation stability is an essential test for quality of BXX blends of petrodiesel and biodiesel. EN 15751 outlines the modified Metrohm 873 Rancimat method to analyse the stability of these blends.
As of the 15th April 2008, all petrol and diesel which is sold at UK pumps now has to include at least 2.5% biofuels. The idea behind the Renewable Transport Fuels Obligation (RTFO) is to reduce climate change emissions from transport - which produced more than a quarter of overall greenhouse gases in the UK - by using renewable fuels instead of fossil fuels. These renewable fuels, made from crops such as oilseed rape, soya, oil palm, sugar cane or maize, have been added to fuel sold around the country. The move is aimed at making transport fuels increasingly environmentally friendly without changing how the vehicle engine works.

Biodiesel production has been in the headlines in the UK recently due to political and environmental concerns over vegetable oil supply and production. This article however will concentrate on technical aspects and concerns about biodiesel.

Biodiesel can be produced from straight vegetable oil, animal oil/fats, tallow and waste oils. Almost all biodiesel is produced using base catalyzed transesterification as it is the most economical process requiring only low temperatures and pressures and producing a 98% conversion yield. Rape, oil palm and soya oil are the most common crops used for biodeisel production.

The Transesterification process is the reaction of a triglyceride (fat/oil) with an alcohol to form esters and glycerol. A triglyceride has a glycerine molecule as its base with three long chain fatty acids attached. The characteristics of the fat are determined by the nature of the fatty acids attached to the glycerine. The nature of the fatty acids can in turn affect the characteristics of the biodiesel. During the esterification process, the triglyceride is reacted with alcohol in the presence of a catalyst, usually a strong alkaline like sodium hydroxide. The alcohol reacts with the fatty acids to form the mono-alkyl ester, or biodiesel and crude glycerol. In most production methanol or ethanol is the alcohol used (methanol produces methyl esters, ethanol produces ethyl esters) and is base catalysed by either potassium or sodium hydroxide. Potassium hydroxide has been found to be more suitable for the ethyl ester biodiesel production; either base can be used for the methyl ester. Fatty Acid Methyl Esters (FAME) are the result and commonly known as biodiesel.

Oxidation stability of biodiesel is an essential test according to EN 14214. Blends of biodiesel and petrochemical diesel also now need testing.

Biodiesel contains no petroleum, but it can be blended at any level with petroleum diesel to create a biodiesel blend. It can be used in compression-ignition (diesel) engines with little or no modifications. Biodiesel is simple to use, biodegradable, nontoxic, and essentially free of sulphur and aromatics.

Most commercially available biodiesel fuels are actually biodiesel blends that are properly referenced with the letter B followed by a one- or two-digit number that represents the percentage of biodiesel used in the blend with petroleum diesel fuel. Pure biodiesel is sometimes called “neat” biodiesel and is also referred to as B100. The most common biodiesel blends are B2, B5, B10, B20 and B50. The remaining fraction is petroleum-based diesel fuel, which is often referred to as petrodiesel.

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 (EN 14112). 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.

The latest revision of the oxidation stability standard is EN 15751 and will also include specifications of determining the stability in not only biodiesel but also blends of biodiesel and petrochemical diesel.

The modified EN 15751 method Metrohm have perfected for blends accounts for the volatility of petrodiesel and the longer stability of blends by increasing the reaction tube length to 250 mm and increasing the minimum analysis time to 20 hours. The modified Rancimat EN 15751 method will be an alternative to the 16 hour long EN 12205 sludge test specified in EN590.

As of now, mainly biodiesel blends are sold, therefore assessing the stability of these blends as well as neat biodiesel provides more meaningful and representative assessment of fuel quality.

In summary, measuring oxidation stability of biodiesel is essential as stated by EN 14214 and EN 14112 as degraded fuels will cause numerous engine problems. Metrohm have perfected the means to test blends of biodiesel to EN 15751 which provides a more realistic assessment of fuels for the current market.

The 873 Rancimat is supplied with as standard the modified glassware essential for blend analysis.