Altering Distillation Curve of Biodiesel – Olefin Metathesis

June 28, 2017 |


Rudolf Diesel in 1892 received a patent for a compression ignition reciprocating engine. However, his original design, which used coal dust as fuel, did not work as planned. To put things in context – thirty-three years prior, in 1859, crude oil was discovered in Pennsylvania. The first product refined from crude was lamp oil or kerosene. Only a certain fraction of the crude made good lamp oil, refiners were stuck with the question of utilizing the rest of the barrel. Diesel, recognizing that the liquid petroleum byproducts might be better engine fuels than coal dust, began experimenting with one of them. This fuel change, coupled with minor mechanical design changes, resulted in a successful prototype engine in 1895. Today, both the engine and the fuel still bear his name as a result.

Today, diesel engines are used worldwide for transportation, manufacturing, power generation, construction, and farming, Exhibit 1. The types of diesel engines are as varied as their use – from small, high-speed indirect-injection engines to low-speed direct-injection behemoths with cylinders one meter (three feet) in diameter. Their success comes from their efficiency, economy, and reliability.


The urgency to identify a more sustainable way forward for society has become clear with alarming trends in global energy demand, the finite nature of fossil fuel reserves, the need to dramatically curb emissions of greenhouse gases to mitigate the devastating consequences of climate change, the damaging volatility of oil prices (in particular for the transport sector), and the geopolitical instability in supplier regions.

Biodiesel is a sustainable viable fuel substitute that can be employed in current diesel infrastructure without major modification to the engines, offering an interesting alternative to petroleum-based diesel.

In general usage, the term biodiesel covers a variety of materials made from vegetable oils, recycled cooking greases or oils, or animal fats. The definition of the term “biodiesel” is being debated among various pioneers, but for the purposes of the discussion here the following ASTM International definition applies: “a fuel comprised of mono-alkyl esters of long chain fatty acids derived from vegetable oils or animal fats, designated B100”. Vegetable oils and animal fats consist of three fatty acids – hydrocarbon chains of varying lengths, bonded to a glycerol molecule. This structure is commonly known as a triglyceride.


Biodiesel from vegetable oils has proven to be the most viable replacement for petroleum diesel to date. Exhibit 2, shows the global consumption of common vegetable oils. It offers many advantages such as bio-degrability, renewability, reduced exhaust emissions (except nitrous oxides), lack of sulfur, inherent lubricity, higher flash point, and domestic origin. Biodiesel consists of alkyl esters (mainly methyl esters) derived from vegetable oils or animal fats. It may also be from other feedstocks such as used cooking oils. In Europe, the main source for biodiesel production is rapeseed oil followed by sunflower oil with E2IG solutions now promoting a more viable alternative Camelina oil. In tropical areas, palm oil is the major feedstock, while soy bean oil and animal fats predominate in North/South America.

Depending on its source, the fatty acid profile of biodiesel varies. For biodiesel feedstocks of soybean, sunflower, rapeseed (canola), palm and peanut oils, the most common fatty esters are those of palmitic acid, stearic acid, oleic acid, linoleic acid, and linolenic acid. Coconut oil, on the other hand, contains a considerable amount of shorter chain acids, such as lauric acid. Exhibit 3, shows the % unsaturation in different vegetable oils. In contrast, petroleum-derived diesel consists only of carbon and hydrogen atoms (aliphatic straight chain and branched chain as well as some aromatic components). It may also contain unsaturated hydrocarbons although they are not present in large quantities.


The structural features of fatty acid esters such as chain length, chain branching, and degree of unsaturation affect the fuel properties of the biodiesel. Among them are cold-flow properties, oxidation stability, ignition quality, heat of combustion, viscosity, exhaust emissions, and lubricity. Of these, the common problems associated with biodiesel are the oxidative stability and poor low-temperature properties. Polyunsaturated fatty acid esters are more susceptible to oxidation or autoxidation. However, little attention has been given to optimize other fuel properties of biodiesel, such as the – distillation curve.

Biodiesel exhibits a narrow boiling range rather than a distillation curve. An obvious observation, given the fatty acids chains in the raw oils and fats from which biodiesel is produced are mainly comprised of straight chain hydrocarbons with 16-18 carbons that have similar boiling temperatures. The atmospheric boiling point of biodiesel generally ranges from 330 to 357°C, thus the specification value of 360 °C maximum at 90 percent recovered. This specification was incorporated as an added precaution to ensure the fuel has not been adulterated with high boiling contaminants.

For example, rapeseed methyl ester (RME) has an almost constant boiling point (∼330°C) as opposed to the steadily increasing curve of diesel fuel, which contains lower boiling compounds giving rise to a distillation curve starting at ∼160°C and increasing to 400 °C. At present, there is no real pressure or incentive to produce biodiesel with a steadily increasing boiling curve. The standard ASTM 6751 only requires a 90% recovery at 360°C maximum, while no limit is specified in EN 14214. Yet, it is beneficial to have an increasing distillation curve to provide good fuel ignition, and combustion in the cylinder, providing longer engine lifetimes.



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Category: Thought Leadership

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