How To Neutralize And Remove Free Fatty Acids From Biodiesel Oil

Production of high quality Bio-diesel begins with the quality of the oil used. Hence, it is essential that the oil is purified before processing for Bio-diesel. You should recognise that the Fuel is to be fed into engines which requires the best quality possible. As such, careful steps must be taken to remove all undesirable constituents.

In the case of vegetable oil extracted from plant seeds such as the rubber seed or Jatropha, the most recognisable impurities can be categorised into the three below.

1. Solid (solute) content

2. Water and Volatile content

3. Free fatty acid content

Vegetable oil extracted from seeds is regarded as crude and contains a lot of substances that are not required in a high quality Bio-diesel. Suspended particles are examples of these unwanted elements, and are categorised as solid contents or solute.

Although it is required that oil-bearing seeds are dried to remove water before extracting the oil from the seed, experience has shown that the oil usually contains water and other substances that can be remove by heating the oil. Those other substances that are not water are called the volatile contents-

Free fatty acids do not fit directly into the category of impurities. But direct preparation of Bio-diesel without careful consideration of free fatty acid content of the oil can hamper the process.

Solid contents can be removed by simple filtration. The filter used must be of very small pores like a filter paper in other to be able to trap all the solid materials. When the oil has been completely filtered, the next step is to remove the volatile content and water by heating the oil sample. Oil that has been subjected to filtration and heating can finally be put  to the crucial free fatty acid removal process.

There are two ways of removal of free fatty acid from vegetable oil. They are:

1. Washing with soap and water

2. Esterification with acid

Washing under soap is the easier of the two methods. It can be carried out with minimum experience and risk. But the problem with this method is that, it results to wastage of usable free fatty acid. For example, free fatty acid content of Rubber seed oil is said to be up to over 30%. Removal of this amount from the total oil can reduce the quantity of bio-diesel obtained by that amount.

Esterification on the other hand, do not remove the fatty acid. Instead, it helps to shield the fatty acids from the attack of alkalines and preserve it as a constituent of the final oil.

Vegetable oil without free fatty acid exists as C₃H₅(COOR)₃ where R is the alkyl group which could be the same (R) or different groups (R, R', and R''). If R is the same, it implies that the fatty acid component of the oil are of the same type. But if different, it implies that the oil is made up of different type of fatty acids, like in the case of the Rubber seed oil.

The general chemical equation for Vegetable oil is RCOOR', while that of free fatty acid is RCOOH. Washing oil with soap do not mean that soap will be added to the oil. It requires that the oil is treated with lye alkaline which could be Potassium hydroxide (KOH) or Sodium hydroxide (NaOH) to form soap in a process referred to as "Saponification".

If we chose to use KOH as the lye, then KOH can react with the free fatty acid in the oil as RCOOH+KOH→RCOOK+H₂O. Where RCOOK is the soap, and H₂O is the water formed.

The soap can be removed from the oil by washing the oil several times with clean water. At every round of washing, the soap-water is removed (draining the water from the bottom of the oil) by allowing the mixture to settle so that the oil floats on top of the soap-water. Pure oil is eventually obtained when the all the soap water is washed off.

This Technique is the better way of removing the Free fatty acids from Vegetable oil. It involves substituting an alkyl group (R) for the hydrogen of the Carboxyl functional group of the acid. In this way, the fatty acid will transform into an ester which will allow almost all of the oil to be available for bio-fuel.

Details of this technique is available in "Acid catalysed esterification of rubber seed oil" section of the Hub: