Biodiesel Production and Properties
Biodiesel is an alternative fuel for diesel engines that consists of mono-alkyl esters produced from vegetable oils or animal fats. Its properties depend on the relative amounts of the individual fatty acids that compose the oils and fats. When a high proportion of these fatty acids are saturated, the biodiesel will have a high cetane number and good oxidative stability but it will not be able to go to low temperatures without gelling. Animal fats and tropical oils such as palm and coconut oils are more saturated than common vegetable oils such as soybean and canola. Biodiesel is produced by transesterification of triglycerides with a simple alcohol. When water is present, saponification produces metallic salts, or soaps, that must be removed from the biodiesel before it can be used in engines. Degraded feedstocks such as used cooking oils or crude animal fats contain free fatty acids that are either removed or converted to alkyl esters by acid-catalyzed reactions. The transesterification reaction rate is limited by mass transfer due to the low solubility of alcohol in the oil. Co-solvents and high shear mixers can be used to approach the kinetically limited reaction rate. Strict adherence to industry specifications is important to minimize operational problems. Sediments caused by oxidation reactions and incomplete reaction products and precipitates from sterol compounds are frequent causes of fuel filter plugging.