Feedstocks and Production

Feedstocks


Biodiesel is composed of just two ingredients: vegetable oil or animal fat and small alkyl alcohols. [1,2] These ingredients are reacted via transesterification in the present of a catalyst to produce the long-chain alkyl ester structure of biodiesel. This section will briefly describe both of these ingredients and how their chemical structures play a role in the production of biodiesel.

Fat and Oil
The active chemical species in the oils and fats used in biodiesel production are triglycerides. Triglycerides are the product of the esterification of glycerol with three long chain fatty acids. [3]

TriglycerideReaction.GIF
The fatty acids can have varying chain lengths, leading to a multiplicity of triglycerides, and every oil can be made up of a variety of triglycerides leading to further diversity in physical properties in the finished biodiesel. [3] For example, the biodiesel produced from soybean oil is composed of 6 different long-chain alkyl esters each with a differently sized chain. [2] Among the properties that are affected by this variety is lubricity, color, calorific value of combustion, and cloud point. [4] The cloud point is the temperature at which the biodiesel starts to gel, or go solid, and is why many biodiesel vehicles require a separate, traditional fuel tank to initially generate the heat needed to “melt” the biodiesel. [4]



Alcohol
The alcohols used in the production of biodiesel are low-molecular weight mono-functional alcohols. Typically methanol is used due to its low cost, however ethanol, isopropanol, and butanol have been used. The choice of alcohol is something of a trade off in that the higher molecular weight alcohols produce biodiesel with better cloud point properties, but they are more difficult to react with the fat and oil and give less yield. [4]

Production


Biodiesel is the product of a transesterification of fats and oils. This process was first used as early as 1853 and was used to produce a fuel as early as 1893. [3] It was officially patented in 1937. [3] Today, the production levels of useable biodiesel is growing rapidly with 460 million gallons being produced in the United States alone in 2007. {2} The actual capacity for biodiesel production in the United States is in the billion of gallons per year. [2] These numbers are likely higher for the rest of the world, especially in Europe, where the demand for biodesiel is higher. [4].

The biodiesel produced commercially is subject to stringent regulations and specifications to ensure quality and safety. [2] However, there are several internet sites which set out the process, in an almost cookbook recipe format, so that it can be done by anyone at home. (See for example http://journeytoforever.org/biodiesel_make.html). The remainder of this section will describe this process and the chemistry within it.

The Biodiesel Process
In a nutshell, the transesterification reaction that produces biodiesel is a simultaneous undoing of the triglyceride ester bond which links the glycerol and fatty acid and the reesterification of the fatty acid with the alcohol. The reaction is depicted in the following scheme:

Transesterification_of_triglycerides.gif


The reaction can be catalyzed with either an acid or base. [1,2] However, most commercially viable biodiesel production uses a basic catalyst, either sodium or potassium hydroxide, as the reaction requires reasonable temperatures, pressures, and time under these conditions. [2] Typical reactions can be done at approximately 50 °C in 4-8 hours. [1,2] A three-fold excess of alcohol above the stoichiometric amount required is also added to push the reaction forward. [1,2]

The remainder of the process is depicted in the flow chart below.

biodieselprocess.GIF


Following the reaction, the batch is a mixture of 86% biodiesel, 10% glycerol from the triglyceride, and 4% excess alcohol. [2] The glycerol is more dense than the biodiesel and settles to the bottom allowing separation. The biodiesel portion is then washed and purified in warm water to remove residual catalyst and fatty acids and then dried to a finished product. [1] Throughout the process the alcohol is stripped via evaporation or distillation to be reused in the next reaction. [1] The biodiesel produced in this way can be over 98% pure.

The glycerol layer also undergoes the same process resulting in an 80-88% pure glycerol product which can be used in the soap making and various other industries [1,2] In fact, the glycerol produced in the manufacture of biodiesel has largely replaced other processes for manufacturing glycerol. [5]

Refereneces

1.Wikipedia,“Biodiesel Production.” http://en.wikipedia.org/wiki/Biodiesel_production (April 21, 2009)
2.National Biodiesel Board,
http://www.biodiesel.org (April 21, 2009)
3.Wikipedia, “Triglyceride.”
http://en.wikipedia.org/wiki/Triglyceride (April 21, 2009)
4.Wikipedia,"Biodiesel.”
http://en.wikipedia.org/wiki/Biodiesel (April 21, 2009)
5. Wikipedia,“Glycerol.”
http://en.wikipedia.org/wiki/Glycerol (April 21, 2009)