The use of by-products for the production of biodiesel presents a number of challenges as opposed to the use of pure plant oils. The content of free fatty acids is significantly higher, which may lead to processing problems if these are not converted into fatty acid methyl esters before the conversion of the fatty substances. This means that the conversion process involves two stages: the first comprises an acid pre-estering using sulphuric acid as catalyst, followed by the re-estering of the fatty acids under alkaline conditions with the addition of an alkaline catalyst, typically potassium or sodium hydroxide.
The use of animal fat also requires an improved final refining process since it contains a number of micro-components that may present a sediment problem unless they are removed. The biodiesel is therefore subjected to a final distillation process, where fatty acid methyl esters are fractionated into light and heavy fractions. This stage also ensures a low sulphur content in the final biodiesel (<10 ppm).
In addition to biodiesel, the production process results in glycerine and potassium sulphate.