Vegetable Oil Biorefining via Olefin Metathesis

Fig. 1: First-generation Grubb's catalyst.

As the world's supply of fossil fuels reaches imminent depletion over the next handful of decades, significant attention and funding have been given to the development of renewable alternative fuel sources. One of the more attractive energy sources currently in use is biodiesel. Made from vegetable oils, biodiesel has the advantage of use in the present diesel infrastructure with minimal adjustments. [1] Vegetable oils and their methyl esters are composed of long chain hydrocarbons terminating in an oxygen functionality. [1] The long chain hydrocarbons vary in length and degree of unsaturation as shown in Table 1. [1]

Conventional No. 2 diesel contains primarily saturated hydrocarbons ten to twenty carbons in length. The existence of carbon-carbon double bonds and ester functionality give rise to the primary performance differences observed between biodiesels and conventional diesel. Biodiesels boast a higher flash point and centane number than No. 2 diesel; of the presently available alternative fuels, biodiesel offers the highest percentage of the total energy content of conventional No. 2 diesel, between 75.6 and 89.9%. [1] When injection volume is taken into account, the fuel energy delivered by biodiesels can reach as high as 96.2% of the fuel energy delivered by fossil diesel per unit volume. [1] However, biodiesels are not currently used widely in place of conventional petrodiesels.

Unsurprisingly, one of the major deterrents to adaptation of biodiesels is profit-driven. Relative to crude oil, feedstock oils are very expensive. The cost drops significantly if non-edible oils are used as competition with the food supply is removed. [1] Even after cost is removed as an obstacle, inferior performance characteristics of biodiesel limit its use. The ester functionality on vegetable oils make it more dense and viscous than hydrocarbons. Depending on the vegetable oil identity, the viscosity of biodiesel is 30 to 160% higher than fossil diesel. [1] This leads to reduced cold flow and eventual inoperability at cold temperatures for biodiesels. In an attempt to correct for this high viscosity, companies often turn to fuel additives. The additive identity and amount depends on the carbon chain length and degree of unsaturation in the oil. Reexamining Table 1, it is apparent that a high degree of batch variation exists and developing a universal additive poses a significant challenge. Companies also design these additives to improve the poor oxidative stability, due to double bonds, of biodiesels. More recently, advanced chemical techniques have been used to take advantage of the increased reactivity of double bonds in the biorefinery of vegetable oils.

Olefin metathesis is an efficient and broadly used process in petroleum refining and other industries. [2] This catalyst driven chemical reaction swaps the molecular groups on either side of two carbon-carbon double bonds as shown in Fig. 2.

Fig. 2: Basic schematic of olefin metathesis.

This "swapping" process gives reasonably precise control over the chemical compounds generated. The development of an effective catalyst, a Grubb's first generation catalyst is shown in Fig. 1, allowed for the manufacturing of chemicals previously thought unattainable. Olefin metathesis of biodiesels can be carried out with relatively simple procedures on readily available industrial equipment. [2] The feedstock oils preferred for this treatment possess multiple degrees of unsaturation, as it is at these sites that the chemical reactions occur. [2] These feedstock oils are then rendered desirable as biodiesels where they were previously disregarded because of poor oxidative stability. Using olefin metathesis, "enhanced" biodiesels with shorter carbon chain lengths can be generated. [3] Because of the shorter chain, these "enhanced" fuels should have better cold flow properties than their unrefined predecessors. The attractive qualities of olefin metathesis as a refining technique have led to serious interest in the construction of biorefineries operating under this chemical process.

At the close of 2009, Elevance Renewable Science, Inc. received a $2.5 million grant from the US Department of Energy to fund a demonstration of biorefinery based on olefin metathesis. [2] This biorefinery would produce not only biofuels, but other specialty chemicals traditionally obtained through refining of petroleum. Elevance plans to scale up this technology with $40 million from private equity funding to produce chemicals ranging from advanced biohydrocarbon renewable jet fuel to Victoria's Secret body lotions. [2,3] Elevance projects its new biorefinery to be profitable at $45/barrel crude oil.2 In addition, the olefin metathesis based plant is projected to be $300-900 per metric ton more profitable than a traditional biodiesel plant. [2] Based on these estimates, Elevance has entered into a joint venture with Wilmar International Limited to build an olefin metathesis based biorefinery within Wilmar's new integrated manufacturing complex in Surabaya, Indonesia. [4] The plant is slated to come online in 2011. [4] As the demonstration-scale biorefinery is completed, the performance of "enhanced" biodiesels will be examined to determine their ability to compete with conventional biodiesels and No. 2 diesel itself.

© Katie Sokolowsky. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

[1] J.-H. Ng, H. K. Ng and S. Gan, "Advances in Biodiesel Fuel for Application in Compression Ignition Engines," Clean Technologies and Environmental Policy 12, 459 (2009).

[2] " Elevance Renewable Sciences Planning Biorefinery Based on Olefin Metathesis Process; Range of Products Includes Renewable Jet and Diesel, and Biodiesel," Green Car Congress, 24 Dec 09.

[3] Kotrba, Ron. " Newton Plant to Become Biorefinery Showcase," Biodiesel Magazine, Apr. 2010.