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Page 3

More Chemistry

Biodiesel consists of the monoesters formed when the triglycerides react with an alcohol such as methanol.

What are esters and monoesters?
Esters are a type of chemical compound that contains the following grouping of carbon and oxygen:

               

                    

3333333333333O

                     ||
                  -  C  -  O  -


One example is methyl palmitate:

 

   erereere           O
                       ||
   CH3(CH2)14 - C - O - CH3

        

 

                                  dO

                                    ||

Here the   CH3(CH2)14 - C - O -     part of the molecule represents the palmitic acid and the CH3 designates this as a methyl ester. Other examples of ester molecules are:

 

             
















Note that each of these example contains only one occurrence of the ester functional group  

 

 All of these examples are known as monoesters.  Other organic molecules can contain more than one occurrence of the ester group, such as the triglyceride shown below

 

                      

 

















Note that this molecule contains the ester group three times.  Thus, it is called a triester.  Sometimes people refer to the reaction that converts oil or fat (triglyceride) to methyl esters (biodiesel) as "esterification" as if the reaction were converting something that is not an ester into an ester.  In reality, the reaction converts one type of ester into another type of ester.  This is why the reaction is more properly known as transesterification.

 

Transesterification

 

Transesterification is the process of reacting a triglyceride molecule with an excess of alcohol in the presence of a catalyst (KOH, NaOH, NaOCN3, etc.) to produce glycerin and fatty esters. The chemical reaction with methanol is shown schematically below.

Triglyceride + methanol mixture of fatty esters + glycerol

In this transesterification reaction, one mole of triglyceride reacts with 3 moles of methanol to produce three moles of fatty esters (biodiesel) and one mole of glycerol.  On a weight basis, the mole of triglyceride will weigh about 884 grams, the three moles of methanol about 96 grams, the resulting three moles of biodiesel will weight about 888 grams and the mole of glycerol will weigh 92 grams.  Most biodiesel producers will add from 1.6 to 2.0 times the chemically correct amount of methanol to ensure that the reaction is driven to completion.  This means that a substantial amount of methanol will be left in the products that should be removed and recovered.  Expressed on the basis of 100 grams of oil, the mass balance for the case of 100% excess methanol becomes: 

100 g soybean oil + 21.7 g methanol    100.4 g biodiesel + 10.4 g glycerol  + 10.9 g methanol

 
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