Using Simulated Distillation as Efficient Tool for Modeling Biofuels

Researchers at the University of Graz are proposing the use of simulated distillation (SimDis) as an efficient and effective method to classify biodiesel fuels regarding boiling characteristics and quality. SimDis is a gas chromatographic method widely used in petroleum industries to determine the distillation behavior of different petroleum products and to ensure fuel quality.

In a paper published online 30 December 2009 in the ACS journal Energy & Fuels, Christine Bachler, Sigurd Schober, and Martin Mittelbach show that SimDis can be used to characterize boiling behaviors of different kinds of biodiesel, with a good correlation between data obtained using SimDis and conventional distillation.

Applying the method to two different kinds of biodiesel and biodiesel blends in their study, they show that shorter chain fatty acid methyl esters—e.g., as can be found in coconut oil—can significantly change the distillation characteristic to a more favorable distillation curve, which resembles a fossil diesel fuels boiling behavior.

Biodiesel can differ in chemical composition, unlike fossil hydrocarbon fuels. The fatty acid composition of biodiesel affects critical parameters such as cetane number, cold flow properties, and oxidation stability as well as distillation characteristics.

It is already known that biodiesel basically consists of methyl esters of C₁₆ and C₁₈ fatty acids with similar boiling points. Biodiesel therefore exhibits a narrow boiling range around 350 °C with initial boiling points at 300 °C. In contrast, fossil diesel additionally contains lower boiling compounds, resulting in a steadily increasing boiling behavior starting at 200 °C.

Generally, high-boiling compounds are connected to engine deposits, increase of exhaust gases, and higher cetane numbers, whereas a high amount of volatile compounds can reduce the flash point of the fuel. Further, there is a need to ensure a certain boiling behavior of the diesel fuel due to engine operability, start of the motors, and in the stage of preheating. Recently, the distillation curve is also of main interest in development of diesel fuel surrogates to ensure good engine performance and minimization of pollutants.

—Bachler et al.

Current methods and other procedures based on classic physical distillation suffer from poor reproducibility and are time-consuming and laborious, the authors note. Using simulated distillation simplifies the procedure to obtain distillation characteristics of novel kinds of fuel—e.g., from algae oil or new species of oilseed—where sample amounts are small-sized and fast methods are of great importance.

As a consequence of the general trend to create lower-boiling fuels in order to reduce emissions, simulated distillation is an efficient tool for fuel modeling.

—Bachler et al.

In the study, the researchers used biodiesel produced from rapeseed oil (RME) and from coconut oil (CME)—which differ in their fatty acid composition—along with a petroleum diesel fuel from BP. Among their findings were:

• RME shows a narrow boiling range at temperatures around 350 °C due to the fact of rather high boiling point components, namely fatty acid methyl esters of chain length C₁₆ to C₁₈.

• Coconut oil contains fatty acids of lower boiling point and therefore CME shows a reduced boiling behavior. CME exhibits a distillation characteristic resembling a fossil diesel fuel boiling behavior.

• By mixing RME with CME or fossil diesel fuel, the boiling behavior can be changed, resulting in favorable steadily increasing distillation characteristics.

• The use of pure CME as fuel is not favorable in most European countries due to bad cold properties. However, by mixing CME with RME or diesel fuel the CFPP value can be decreased to adequate values even for moderate climates by maintaining a boiling behavior that still shows ideal characteristics. A certain amount of lower boiling point methyl esters therefore always needs to be selected by consideration of the resulting cold properties of the fuel.

• Fuel blends containing 10% CME seem to represent a good solution.

Further it could be shown that simulated distillation is a powerful tool in analysis, development, and of course in improvement of different kinds of fuels by using widely available laboratory equipment for reduced time and non-negligible potential for automation.

—Bachler et al.

Resources

• Christine Bachler, Sigurd Schober, and Martin Mittelbach (2009) Simulated Distillation for Biofuel Analysis. Energy Fuels doi: 10.1021/ef901295s