The majority of oils and fats after extraction (pressing, solvent extraction, combination pressing-extraction, rendering, etc.) are not suitable for the production of biodiesel, especially in large continuous plants using the traditional alkaline transesterification process. Undesirable products are non-triaclyglycerol compounds like free fatty acids (FFAs), phospholipids, oxidation products, metals, protein and carbohydrate residues, waxes, moisture and inorganic matter. Two refining routes are used (chemical and physical refining) which refer to the methodology for FFA removal.

In both refining procedures the first step is a degumming process in which the hydratable phospholipids are removed by water washing and the non-hydratable ones are discarded by treatment with citric or phosphoric acid. Enzymatic degumming becomes more attractive due to the lower losses.

In the chemical process, FFAs are removed as soaps (soapstock) by neutralization with NaOH. In the physical refining, FFAs are discarded by stripping (deodorization). Chemical refining is using a lot of water and the soapstock has to be acidified (production of acid oils) and is environmentally not recommended. In addition, there are considerable losses of lipids during the separation of the soap layer. As 85% of the production costs of biodiesel is based on the feedstock, losses should be kept to a minimum.

A bleaching step (adsorption with activated clay or silica) can be necessary for highly colored oils or high contaminated oils (removal of Ca, Fe, Cu, traces of soaps and phospholipids).

Deodorization, the last step of refining, is performed at 210-260°C, with one to two per cent steam (1 Mbar) for the removal of the FFAs and oxidation products (physical refining). In addition pigments and unwanted contaminants (PAHs are pesticide residues) are degraded. Deodorization can be omitted on condition that FFA content is very low which can be case for freshly extracted soy and rapeseed oils. A detailed overview of refining oils and fats is given by O’Brien et al. (2000).

A feedstock purification technology which may replace the conventional degumming process is Ambersep™ B19 (amberlyst resin) developed by Daw. The product removes proteins and polysaccharides, traces of phospholipids and soaps. By doing a purification step first, the lifetime of Ambersep™ B20 is extended during the esterification of FFAs in crude feedstocks.