- Production of biodiesel is based on a process involving transesterification. The objective is to separate long chain fatty acid methyl ester (FAME) molecules from short chain glycerol molecules found in vegetable oils and animal fats. The term FAME and biodiesel are often used interchangeably.
A conventional batch process begins with feedstock preparation. The most desirable feedstock is a vegetable oil low in free fatty acids and containing minimal moisture. Moisture and free fatty acids cause incomplete transesterification resulting in excess byproducts at the expense of FAME production.
Based on feedstock analysis an appropriate ratio of alcohol (usually methanol) and a dry catalyst (typically lye/sodium hydroxide) is mixed and pumped into a closed, heated vessel. Feedstock is added and heat is applied to speed up the catalytic reaction. When the reaction is complete you have a mixture of biodiesel and glycerin along with small amounts of unused catalyst and other impurities. Reaction time varies depending on a number of factors, but is typically in the one to five hour range.
Glycerin is much heavier by volume than biodiesel, making it possible to separate the two by allowing the glycerin to settle out, at which point it can be drained off at the bottom of the tank. Alternately, a centrifuge can be used to speed up the process.
The biodiesel then undergoes further purification and filtering. Water is typically used to wash out left over catalyst, soaps and other impurities. Biodiesel doesn't mix with water, making it relatively easy to separate and dry prior to storage. - Rudolph Diesel demonstrated one of his engines powered by peanut oil in 1900. Note that though peanut oil is a bio-fuel, it isn't biodiesel until it is processed to remove byproducts such as glycerin and gums which cause engine problems.
During World War II, vegetable oil was converted to something similar to today's biodiesel. Belgian Patent 422,877, issued in 1937, describes a process similar to transesterification which separates fatty acid ethyl esters (a biodiesel variant) from glycerol.
Following the oil crisis of the late 1970s, a number of similar processes were developed. The past two decades have seen the construction of numerous commercial biodiesel production plants, mostly in Europe. - A flurry of recent research has led to promising alternatives to the conventional batch process described above. Most involve the use of a solid, reusable catalyst and some use a continuous flow process rather than the conventional batch process.
An example is the Mcgyan continuous flow process. It is based on a reusable solid catalyst which reacts with a pressurized, heated mix of feedstock and alcohol in seconds rather than hours. Furthermore, the process is not sensitive to moisture or free fatty acids contained in the feedstock. Lab experiments have been scaled up to a functioning pilot project producing at a rate of 40,000 gallons per year. A three million gallon per year commercial plant is on schedule to begin operation in late 2009.
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