Measuring the oxidative stability of petrodiesel

The oxidation stability of fatty acid methyl esters has been included in various test methods as a standard parameter used to define the minimum quality requirements of biodiesel.

Oxidation stability of diesel and biodiesel blends with the 743 Rancimat according to EN 14112 Test Method.

The oxidation stability of fatty acid methyl esters has already been included in various test methods as a standard parameter used to define the minimum quality requirements of biodiesel: EN 14214 Automotive fuels - Fatty acid methyl esters (FAME) for diesel engines - Requirements and test methods EN 14112 Fat and oil derivatives - Fatty acid methyl esters (FAME) - Determination of oxidation stability (accelerated oxidation test) Metrohm propose that the Rancimat can also be used to define the oxidative stability of petrodiesel and petrodiesel/biodiesel blends.

Sufficient oxidation stability (aging behavior) of any fuel is a basic requirement to guarantee a failure-free operation of the fuel injection equipment within the diesel engine.

EN 590 (European standard for diesel quality requirements) states that aging behavior is characterized as polymer (sludge) content of artificially aged fuel according to EN 12205.

This method was developed for petrodiesels.

The analysis of biodiesel (FAME) blends are not included in this method and EN 12205 does not discriminate well between different diesels and B5 (5% FAME, 95% diesel) blends, the Rancimat method (EN 14112) is a potential alternative.

Sludge determination EN 12205 / ASTM D 2274 350 mL of sample is heated to 95 degC for 16 hours while oxygen is bubbled through the sample at a rate of 3 liters per hour.

The sludge (polymer content) is then determined gravimetrically after filtering.

The limit is 25 g/m (Figure 1).

The disadvantages of this method include a high sample volume, a long measurement time and the determination has many manual, time consuming steps.

Rancimat method EN 14112 - Advantages Unlike EN12205, this method is applicable to FAME, petrodiesel and biodiesel blends and as the oxidative stability of the different types of fuels is determined on the same instrument, the results are directly comparable.

The Rancimat method offers a realistic aging determination, which is not the sludge forming potential of artificially aged fuels which has no field relevance.

The Rancimat also provides automatic evaluation as an induction point is reached which indicates when a sample becomes unstable.

This measured by an increase in conductivity as the sample starts to degrade.

A suggested limit is 20 hours (Figure 1).

I addition, the sample amount (3.0 - 7.5 g) is much smaller than the sludge method and expenditure of human labour is significantly reduced because there is no sample preparation and no time consuming manual procedures.

Figure 1 shows good charactersation of diesel and blends and shows how the Rancimat method gives a direct determination of stability.

This is demonstrated as the age of the fuels and therefore instability of the fuels decreases from left to right on the graph.

The sludge method shows there was no real trend for increased instability as the fuels measured increased in age.

The Rancimat method EN 14112 did pose initially a few problems, mainly sample evapouration due to the low boiling point of petrodiesel, around 170 - 306 degC, while biodiesel is >200 degC.

Therefore the method parameters were modified.

The evapouration problem was solved by an increased sample volume and using longer reaction vessels.

The loss by evapouration was overcome as the sample was reflux condensed in the longer vessels Conclusions When the two methods were compared Metrohm concluded; For Standard test method EN 12205 There was insufficient characterisation of the oxidation stability There was no field relevance of the sludge forming potential For the Rancimat method EN 14112; There was good characterisation of the oxidation stability for biodiesel, petrodiesel and diesel/biodiesel blends and because of the induction point method; there was direct determination of the real aging reserve.

Therefore, the modified Rancimat is suitable for measuring the stability of petrodiesel, biodiesel and blends of both types.

Back to top