Improving the Cold Flow Properties of Biodiesel by Fractionation

Abstract

Biodiesel is generally defined as the mono-alkyl esters of fatty acids derived from transesterification of agricultural lipids with a short-chain alcohol. Biodiesel must conform to fuel property specifications such as those outlined in the American Society of Testing and Materials (ASTM) standard D 6751 or the European Committee for Standardization EN 14214 (ASTM, 2008b; CEN, 2003). Although the most common form of biodiesel in the U. S. is made from soybean oil, many other types of lipid feedstocks have been developed. Biodiesel is compatible with compression-ignition (diesel) engines. It may be utilized in neat (100%) form or in blends with conventional diesel fuel (petrodiesel). Biodiesel has been applied as an alternative fuel in transportation trucks, farm and other off-road vehicles, automobiles, locomotives, aircraft, power generators, boilers and heaters. The advantages and disadvantages of biodiesel as an alternative diesel fuel have been efficiently documented (Erhan et al., 2008; Graboski & McCormick, 1998; Knothe & Dunn, 2001, 2005). One major disadvantage is the performance of biodiesel in cold weather which may compromise its year-round commercial viability in moderate temperature climates. Although field studies on cold weather performance are scarce, there is evidence that soybean oil fatty acid methyl esters (SME) develop operability issues as ambient temperatures approach 0-2oC. In contrast, petrodiesel develops similar problems at a significantly lower temperature range, typically between -16 and -20oC. This chapter examines fractionation as an effective process step that may be applied to improve the properties and performance of biodiesel and its blends in petrodiesel during cold weather. This technology is based on separating biodiesel into lowand high-melting point fractions. Unless otherwise noted, the term alkyl esters refers to fatty acid alkyl esters which may specifically include fatty acid methyl esters (FAME), ethyl esters, propyl esters, etc. Similarly, the term petrodiesel refers to petroleum middle distillate fuel, either grade No. 1 or No. 2, as defined in ASTM fuel standard D 975 (ASTM, 2008a).