The Influence of Antioxidant Additives on Oxidation Stability in Spark-Ignition Fuels

Oxidation stability is an integral part of the standard specification for spark-ignition fuels (ASTM D4814 / EN 228). Typical gasoline is primarily composed of naphtha (cycloalkanes), paraffins (alkanes) and olefins (alkenes). As refiners seek to maximize the production of high-value products from low-value stocks, there is increased pressure to include catalytically cracked naphtha component streams with high olefin content in the gasoline blending pool.

In particular, the unsaturated components are inherently unstable and subject to deterioration due to oxidation, which can cause undesirable fuel characteristics such as darkening, formation of polymers and reduced induction periods. Aging of gasoline can occur both during storage and use in an engine with hydroperoxides as initial reaction products, which – followed by a series of subsequent reactions – give rise to aldehydes, acids and polymers. Ultimately, these oxidation products are responsible for an acidity increase leading to rusting and corrosion, oil thickening leading to loss of viscosity control and the appearance of insoluble materials leading to filter and nozzle blocking. The primary purpose of antioxidant additives is to extend the induction period and period for which gasoline can be stored before unwanted oxidation products are formed. However, the response of a gasoline to an antioxidant in terms of increased induction period and thus oxidation stability depends on both the antioxidant and the fuel composition. In order to achieve the best possible results for a certain fuel blend a thorough screening of antioxidants and their concentration is essential to obtain the most cost-effective solution.

Rapid Small Scale Oxidation Test

A solution for this would be to use a RSSOT (Rapid Small Scale Oxidation Test) according to the ASTM standard D7525. The RSSOT was developed as an alternative to the standard test method for oxidation stability of gasoline (ASTM D525), which is the traditional bomb method. An automatic RSSOT instrument uses this new test method to measure the induction period of spark-ignition fuels, including those containing alcohols or other oxygenates under accelerated oxidation conditions. A typical test is conducted with a small sample volume of 5 mL. This is introduced into a test cell and charged with oxygen to 500 kPa at a temperature of between 15 °C to 25 °C. The measurement is started by heating the test cell to 140 °C until the break point is reached. The break point is defined as the pressure in the test cell which is 10 % below the maximum pressure of the test run. The induction period, which can be used as a measure of the oxidation stability, is specified as the time elapsed between starting the heating procedure of the test cell and the break point (pmax -10%), which is commonly measured in minutes.

Oxidation-Stability-Tester-RapidOxy

Oxidation Stability Tester RapidOxy

The fundamental parameter influencing the induction period of the RSSOT is the applied temperature. Investigating the temperature dependence from 110 °C up to 160 °C in 10 °C increments at 700 kPa has revealed that the RSSOT induction time is highly dependent on the applied test temperature and that the rate constant k increases with temperature. The Arrhenius plot shows a linear correlation of the rate constant against the applied test temperature with the general conclusion that the RSSOT induction period of a fuel sample is approximately cut by half for each temperature increase of 10 °C.

Applied-Temperature

In comparison to the RSSOT, the standard test method for oxidation stability of gasoline, ASTM D525, is typically conducted with a large sample size of 50 mL in an oxidation pressure vessel at lower temperatures of 100 °C and an oxygen pressure of 700 kPa. The break point for the ASTM D525 is defined as the point in the time-pressure plot that is preceded by two consecutive pressure drops of 14 kPa each within 15 minutes. The corresponding induction period is defined as the time elapsed between the application of temperature and the break point. Although ASTM D7525 and ASTM D525 are similar in some aspects, the test temperature, sample size and break point definition, in particular, make the RSSOT method a faster, safer and more user-friendly alternative (Table 1).

Method ASTM D7525 ASTM D525
Sample size 5 mL 50 mL
Test temperature 140 °C 100 °C
Oxygen pressure 500 kPa 700 kPa
Break point pmax -10% 2 consecutive Δp of -14 kPa each within 15 min
Induction period Time between application of temperature and break point Time between application of temperature and break point

Table 1: Comparison of the RSSOT / ASTM D7525 with ASTM D525

Thanks to the higher test temperature and smaller sample size the ASTM D7525 test method reaches the break point approx. 15 to 25 times faster than with ASTM D525. Moreover, the break point definition of the ASTM D525 with two consecutive timed pressure drops of 14 kPa can lead to tests in which the break point criterion is never reached and thus no induction period can be measured. The clear definition of the ASTM D7525 break point as pmax -10% eliminates this problem and an induction period is obtained in every test. Additionally, in contrast to ASTM D525 no additional safety measures like a burst disk is needed because of the apparatus design of the RSSOT. The eruption of a burst disk during a test can be a major hazard to the surrounding laboratory staff and laboratory environment.

These advantages make the RSSOT ideal for the fast optimization and release of gasoline.

Measuring the influence of antioxidant additives on gasoline with high olefin content

A “raw” gasoline with an olefin content of approx. 40 % and a commercially available antioxidant were used for this investigation. The influence of the antioxidant additive on the induction period was screened for concentration from 0 ppm up to 50 ppm and tests were done in parallel using both test methods ASTM D7525 and ASTM D525 (Table 2). The results show that with ASTM D7525 a short induction time of 23.55 minutes is obtained with 0 ppm of antioxidant, which can be gradually prolonged up to 17.60 min with an antioxidant concentration of 50 ppm. As expected ASTM D525 has a much longer induction time of 199.34 min with 0 ppm antioxidant, which likewise for ASTM D7525 is prolonged with increasing concentration of antioxidant additive. Overall, the measured induction periods were approx. 20 times faster with the RSSOT in comparison to ASTM D525.

513.93243.501014.63271.002515.45348.005017.60453.00

Antioxidant (ppm) ASTM D7525 (min) ASTM D525 (min)
0 13.55 199.34

Table 2: Influence of antioxidant additives on the induction period of gasoline with high olefin content

Plotting the induction periods of ASTM D7525 and ASTM D525 against each other shows a linear correlation with an excellent data fitting of R2 = 0.98.

In ASTM D4814 the limit for automotive spark-ignition engine fuels is set to a minimum induction period of 240 minutes using ASTM D525 as the test method or a minimum induction period of 360 minutes for EN 228. Correlating these minimum ASTM D525 induction periods to ASTM D7525 the minimum induction period with the RSSOT has to be approx. 14 min for ASTM D4814, respectively 16 min for the EN 228.

To assess the influence of the antioxidant, the additive concentration was plotted against the ASTM D7525 induction period. The linear slope with a fitting of R2 = 0.99 indicates an excellent correlation between RSSOT induction period and antioxidant additive concentration. Using this plot the minimum antioxidant concentration needed to meet the limits defined in the specifications of ASTM D4814 and EN 228 can be determined. The ASTM D4814 oxidation stability criterion of 240 minutes induction period with ASTM D525 correlated to 14 minutes induction period with ASTM D7525 and can be met with 6 ppm of antioxidant concentration. The EN 228 oxidation stability criterion of 360 minutes induction period with ASTM D525 correlated to 16 minutes induction period with ASTM D7525 and can be met with 30 ppm of antioxidant concentration.

D525-vs-PetroOxy

Oxidation-Stability-ASTM-D4814

Ultimately, these results show that ASTM D7525 allows fast product optimization and helps to prevent excessive use of antioxidant additives to get the most cost-effective solution.

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First published in Petro Industry News volume 15 Issue 6

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