A Method for Determining Oxidation Stability of Petrodiesel, Biodiesel, and Blended Fuels

The diesel engine was developed in 1892 to allow the use of cost-saving diesel fuel. At the time, the production of diesel was a relatively simple single cut within the atmospheric distillation process. Fuel quality was only a concern during winter months, and quality concerns were limited to the fuel’s cold properties, which were easily modified using additives or spark ignition fuel (gasoline). With the success of the diesel engine, diesel fuel started being used in passenger vehicles, commercial vehicles, construction equipment, ships, locomotives, and all manner of stationary engines and generators.

The high levels of demand for this important fuel supply, as well as environmental concerns, have driven the development and production of biodiesel produced from renewable, bio-based raw materials. Today biodiesel, also known as fatty acid methyl ester (FAME), has reached an important position in the global fuel market. Biodiesel is produced from rapeseed (in Europe), palm oil, soybean oil, used fat and animal fat, as well as many other feedstocks. To take advantage of this new, environmentally friendly source of diesel, numerous car manufacturers have adapted their engines to use biodiesel. With the growing production and use of biodiesel, there has been an increasing emphasis on the quality of biodiesel fuels as well as blends of bio- and petrodiesel.

Unique quality issues

The quality of petrodiesel is governed by the standards EN 590 and ASTM D 975. Unfortunately, many test methods developed for petrodiesel have proven unsatisfactory for biodiesel and blends containing biodiesel. Unlike petrodiesel, the physical and chemical properties of biodiesel fuel vary with both the refining process employed and the feedstock from which it is produced. Consequently, many established ASTM test methods are either unproven for use with biodiesel or suspected of giving erroneous results; therefore, biodiesel (FAME) has its own quality standards: EN 14 214 and ASTM D 6751.

Oxidation stability—an important FAME quality parameter

Biodiesel is more susceptible to degradation during storage than petrodiesel. Unsaturated FAME components significantly decrease the oxidative stability of the fuel. Oxidative degradation results in the plugging of filters and the formation of sludge polymers throughout the entire fuel system. With the blending of biodiesel into petrodiesel and the complexity of modern sophisticated fuel injection systems (e.g., pump-jet, common rail), oxidation stability has become a very important quality parameter for the strategic storage of large quantities of fuel.

In practice, oxidation stability covers two important physical characteristics: storage stability and thermal stability. Storage stability is influenced by humidity, sunlight, microorganisms, temperature, oxygen in the air, etc. Thermal stability is the tendency to generate gum and solid deposits at elevated temperatures. For pure biodiesel (B100) as well as blends containing biodiesel, both of these characteristics need to be measured by the oxidation stability testing method employed.

Current oxidation stability test methods

To ensure good-quality fuel, specification standards (e.g., EN 590—ASTM D 975) have been developed in which specific test methods for measuring each quality parameter are listed. Presently, the available methods for determining oxidation stability are very time consuming. Additionally, their relevance to practical use is rather limited. This paper examines the two methods presently used to determine oxidation stability and compares them with the PetroOXY method (Petrotest® Instruments GmbH & Co. KG, Dahlewitz, Germany), which has recently completed numerous round-robin tests at various petroleum testing laboratories.

TOST—Methods ISO 12 205/ASTM D 2274

Presently, the most important test methods for the oxidation stability testing of petrodiesel are ISO 12 205 and ASTM D 2274. These methods require a 16-hr-long aging process at 95 °C under oxygen flow. The fuel is subsequently filtered and the vessel cleaned in order to collect the entire amount of residue created during the test for weighing. The amount of residue collected determines the quality of the fuel.

The amount of residue to be measured is so small that these methods are close to the resolution limits of analytical balances. Consequently, it is only possible to discriminate very good and relatively good fuels from very bad fuels. A more precise differentiation is not possible. Making these test methods even less reliable is the fact that the correlation of the tests with actual storage stability is still unknown. Of additional concern is that stability may also depend upon field conditions and fuel composition. The combination of limited analytical resolution and a lack of correlation to storage stability has resulted in a lack of confidence in the methods’ results.

Method EN 14 112

Currently, the most important test method for biodiesel and blends is EN 14 112. This method determines oxidation stability through a combination of distillation and conductivity. As a consequence of the method, only the highly volatile oxidation products are detected. The nonvolatile oxidation products, such as gum, remain undetected in the 110 °C heated vessel. Therefore, the results obtained are an incomplete analysis of the sample’s oxidation stability. This may be the reason that EN 14 112 results are difficult to interpret, requiring at least partial analysis by highly skilled personnel. Additionally, utmost cleanliness is required in order to properly perform this test method. Thus, the cleaning process is often considered problematic by those performing the tests.

PetroOXY method

Responding to a demand from the U.S. gasoline industry, a smaller instrument was developed based on the oxidation stability tests according to ISO 7536—ASTM D 525—IP 40. Unlike EN 14 112, results from the PetroOXY method include all volatile and nonvolatile oxidation products, thereby providing a complete analysis of the sample’s oxidation stability.

Within a small, hermetically sealed test chamber, a 5-mL sample is combined with oxygen at a pressure of 700 kPa (approx. 7 bar) and heated to a temperature of 140 °C. These test conditions initiate a very fast artificial aging process, which is measured by a pressure drop within the chamber. It has been determined that the time needed to achieve a fixed pressure drop is directly related to the oxidation stability of the fuel.

Figure 1 - Graphs of pressure curves of various biodiesels (NYSERDA [New York State Energy Research and Development Authority]-USA).

In the PetroOXY method, the oxidation stability is characterized by the induction period. The induction period is the elapsed time between starting the test and the breaking point, which is defined as a pressure drop of 10% below the maximum pressure as detected in the pressure versus time curve (Figure 1).

PetroOXY apparatus

Figure 2 - PetroOXY automatic oxidation stability tester.

The PetroOXY apparatus (Figure 2) complies with user requirements for improvements to the widely used Oxidation Stability Pressure Vessel Test. Features include:

  • Small sample volume of 5 mL
  • Short test time of less than 1 hr for product release
  • Clear, understandable test results
  • Improved user safety
  • Automated oxygen charging and relief
  • Easy, 5-min cleaning procedure
  • Simple sample handling; no special training needed.

Comparison of methods: repeatability, reproducibility, and correlation

1. TOST method. The TOST method, according to ISO 12 205 and ASTM D 2274, specifies that fuel samples are tested in a glass container at a temperature of 95 °C for 16 hr, while oxygen is bubbled through the sample at a controlled flow. The aged sample must then go through a filtration process to collect filterable insolubles, and the vessel must be put through a cleaning procedure to collect adherent insolubles. The total insolubles collected must then be weighed. The small amounts of insolubles collected often push the limits of resolution of analytical balances. 

In total, it takes approximately 24 hr to complete all of the procedures in the TOST method. Due to the difficulty of collecting the small amounts of insolubles created, the method’s repeatability is merely 25%, and its reproducibility only 45%. These are not satisfactory performance specifications for a parameter as important to fuel quality as oxidation stability. Consequently, correlation to other standardized methods is very difficult, if not impossible.

2. Method EN 14 112. A pilot study was conducted with a large German research laboratory to develop correlation data between the EN 14 112 method for biodiesel and the newly proposed PetroOXY method. The data were developed by testing samples of petrodiesel, biodiesel (B100), blends B5 and B20, as well as native plant oils.

Per EN 14214, the induction period (time consumption to the specified oxidation event) for biodiesel must exceed 6 hr in order to pass the test. If biodiesel already shows induction periods up to 10 hr, then even longer periods can be expected for petrodiesel and blends.

During performance of the pilot study, the following induction periods were found using the EN 14 112 method (results in hh:mm):

Diesel22:00up to>33:00hr
B58:30up to10:30hr
B2017:30up to20:30hr

For petrodiesel, EN 14 112’s repeatability is expected to be 15%, which, although better than the TOST method, is still not very satisfactory for such an important quality test. Reproducibility is expected to be 25%, which is also not very satisfactory. The repeatability and reproducibility are similar when testing biodiesel.

On a positive note, for biodiesel testing there is a good correlation between the EN 14 112 method and the bomb method (ISO 7536—ASTM D 525) according to the literature (SAE-Document 2004-01-3031).

PetroOXY method

The PetroOXY method was developed from the bomb method (ISO 7536—ASTM D 525—IP 40). As with the bomb method, the test results are directly measured as the pressure drop inside the test chamber as the oxidation process is accelerated by heat and oxygen at pressure.

In the previously mentioned pilot study with nationally and internationally sourced biodiesel samples, induction periods of less than 1 hr, and often less than ½ hr, were found when using the PetroOXY method. These induction periods were measured on the same biodiesel samples that were tested with the EN 14 112 method.

The PetroOXY method provided the following corresponding results (hh:mm):

Diesel1:01up to1:45hr
B50:36up to0:39hr
B200:47up to0:53hr

Comparing these results with the previously mentioned results for EN 14 112 illustrates the tremendous productivity improvement achievable by using the PetroOXY method.

The repeatability and reproducibility of the PetroOXY method demonstrated tremendous improvement over the other methods. With biodiesel, repeatability was better than 5% and reproducibility was better than 8%. Furthermore, a good correlation between EN 14 112 and the PetroOXY method was found in almost all tests. The PetroOXY’s performance improvements over EN 14 112 as well as the differences in results between the methods may be due to the fact that the PetroOXY accounts for both volatile and nonvolatile oxidation components, while EN 14 112 cannot.

Figure 3 - Results of round-robin testing.

Figure 4 - PetroOXY demonstrates an excellent correlation between methods and a significant savings in time.

Compared to the TOST method, the PetroOXY method allows for a more precise discrimination between very good, good, and bad samples. Although the test results using the PetroOXY method demonstrated a similarity to TOST method results, the differences were too great to be considered correlation. This inability to correlate may be due to the differences in the test methodology, the lack of repeatability and reproducibility using the TOST method, or a combination of all these factors (see Figures 3 and 4).

PetroOXY standardization efforts

ASTM is undertaking a standardization project for “Oxidation Stability of Spark Ignition Fuel” and “Middle Distillates.” An official “Work Item Number” was issued for this project and further evaluation and study of the PetroOXY method are underway. It is expected that an official ASTM method number will be assigned to the PetroOXY method, and it will become the preferred standard for determining the oxidation stability of both petro- and bio-based fuels.

The EI-IP (Energy Institute-Institute of Petroleum) in London, U.K., also has a standardization project for the “Oxidation Stability of FAME” and a first proposed method already has appeared in the 2007 IP-Book of Standards. The PetroOXY method also was recently presented to FAM of DIN in Germany. (DIN is the Deutches Institut für Normung e.V., i.e., German Institute for Standardization; FAM is the Board of Standardization committee within DIN for petroleum, fuels, lubricants, and related products.)

Economic aspects and advantages

It has been demonstrated in round-robin testing that the PetroOXY method results in a dramatic reduction in testing time. In addition to time savings, the handling and cleaning time required per test is limited to approximately 5 min.

Comparing the PetroOXY method with the TOST method, the reduction in time required is approximately 95%. Additionally, PetroOXY provides a highly improved ability to precisely discriminate between the oxidation stability of different fuel samples along with a tremendous improvement in repeatability and reproducibility, which are critical when two separate laboratories are comparing results. Similarly, the PetroOXY method provided an almost 95% reduction in time during one comparison against the EN 14 112 method.

In addition to the time savings, use of the PetroOXY can enable fuel producers and blenders to optimize the amount of additives they mix into their fuels. With the system’s improved repeatability, oxidation stability additives can be blended much closer to the actual required level. The greatly improved reproducibility also improves the consistency of test results between laboratories.

Conclusion

As of the writing of this paper, the PetroOXY method for determining the oxidation stability of fuels, although analytically proven, has not yet been issued an official standard. Draft standards for testing gasoline, petrodiesel, biodiesel, and blends are presently under discussion and review in the working groups of several standards organizations. Consequently, the benefits of this method can only be used when standardization is not mandated by law or required by contract. Although limiting, this does not prevent those involved with the production, storage, and use of fuels from enjoying the tremendous benefits offered by the method.

The PetroOXY can be used in the research and development of engines and fuel systems as well as in the development and testing of new fuel additives. Fuel producers can evaluate the performance of new blends much faster than before and with greater sensitivity and precision. Refinery laboratories and production control personnel can effectively reduce their reaction times to process and feedstock changes. Test times can be reduced even further with the method simply by elevating the test temperature. It is also now possible to perform quick spot checks whenever the stability of a fuel is under suspicion as well as for strategic and routine monitoring of stored fuels. Fuel producers and terminal facilities have a powerful new tool for the monitoring and control of their fuel’s oxidation stability. The PetroOXY method is set to transform the manner in which all fuels are tested for oxidation stability.

The authors are with Petrotest® Instruments GmbH & Co. KG, Ludwig-Erhard-Ring 13, 15827 Dahlewitz, Germany; tel.: +49 (0) 33 708 56-300; fax: +49 (0) 33 708 56-556; e-mail: sales@petrotest.com.Petrotest instruments are distributed in North America by AMETEK Petrolab, Albany, NY.

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