Most biodiesel in the USA is made by reacting vegetable (Soy) oils with methanol in the presence of a catalyst by a chemical process known as transesterification. The reaction process forms two products: methyl ester (which is the bio-diesel) and glycerol. To force the reaction to completion, an excess amount (20-100% more methanol than actually required for reaction) of methanol is used. The two products are separated by decanting or centrifuge, and the excess (un-reacted) methanol is divided between the two products: approximately 40% with the biodiesel, and 60% with the glycerol.
Methanol Recovery from both the biodiesel and glycerol products is imperative to the economics of any biodiesel production facility. The preferred result is to recover the methanol in a form for re-use in production to reduce the overall methanol costs. There are a couple steps involved: methanol extraction from biodiesel (or glycerol), and recovery of the methanol.
There are several different methods of extracting methanol from biodiesel and glycerin: vacuum flash evaporation, distillation, and water washing. However, at low concentrations, methanol/biodiesel and methanol/glycerin mixtures are non-ideal, meaning due to molecular level interactions, they do not behave in the same way as typical ideal solutions do. If a simple flash evaporation operation is performed based solely on the vapor pressure of methanol, only a portion of the methanol will be removed which means more methanol may remain in the biodiesel than desired. To remove the final amounts of methanol to the low levels of ASTM requirements is more complex.
Biodiesel water washing to remove the total amount of methanol from biodiesel generally is a lower equipment cost method of extraction, however the separation of methanol from the resulting methanol-water mix becomes expensive.
Distillation of methanol from biodiesel can be performed in a single column, however the column is relatively complex in design, control and operation, and more expensive.