Biodiesel is one of (and the preferred) products of the reaction between an oil and an alcohol. Vegetable oils and animal fats consist of triglycerides or molecules with a 3-carbon backbone (tri-) that can be chemically written as CH2OCOR1-CHOCOR2-CH2OCOR3. Ideally, the oil (virgin vegetable oil, waste vegetable oil, seaweed oil or animal fat) reacts with 3 molecules of alcohol to form one molecule of glycerin, chemically CH2OH-CHOH-CH2OH and 3 molecules of biodiesel (fatty esters) CH3OCOR1 , CH3OCOR2, and CH3OCOR3, where R1, R2, and R3 are one of several hydrocarbon chains, called fatty acyl groups.
Certain vegetable oils, and in particular waste vegetable oils, consist not only of triglycerides (a 3-carbon backbone with a group R1, R2, and R3), but can also consist of diglycerides (a 3-carbon backbone with an R1 and R2 group), monoglycerides (a 3-carbon backbone with only one R1 group) and / or free fatty acids (chemically RCOOH). Free fatty acids are generally degradation products of vegetable oil, and free fatty acids are separated from triglycerides (hence the name “free”). In other words, a triglyceride molecule exposed to prolonged heat will convert to a diglyceride and a free fatty acid, or a monoglyceride and two free fatty acids, or potentially three free fatty acids.
Vegetable oils with only 1-2 percent free fatty acids have been found to cause difficulties with the production and / or separation of biodiesel. Although some biodiesel reaction systems are designed to convert free fatty acids to biodiesel through an acid catalyzed esterification reaction, the most common biodiesel systems lack this ability. Very often, a home-made biodiesel system or a first-generation commercial system designed to handle virgin vegetable oils uses a base-catalyzed reaction to convert triglycerides to biodiesel and glycerin.
However, with the free fatty acids present, some (perhaps all if there is enough free fatty acid) of the basic catalyst will react with the free fatty acid to create a soap. This depletes the base used to catalyze the transesterification reaction or requires the addition of a larger amount of base to overcome the free fatty acid reaction and can cause problems with soap formation and product separation after the reaction is complete. . In extreme cases, the soaps are mixed with water from the fuel wash stage to create an emulsion that can greatly elongate or even prevent sedimentation of the fuel layer wash water layer.
There are several ways to compensate for the free fatty acids present in vegetable oil:
- With great care and equipment designed for strong acids, add sulfuric acid or another strong acid with alcohol to the oil assuming a certain level of free fatty acids (some users suggest using 1 milliliter of acid per liter of oil. Adding excess acid will increase its chemical costs, both in terms of the acid used and the base required to neutralize the excess acid prior to the base-catalyzed transesterification reaction;
- Test the free fatty acid content, and if it is low enough (less than 1 percent), ignore it and be careful with washing to avoid the formation of an emulsion;
- Test the free fatty acid content and add the appropriate amount of strong acid to convert the free fatty acids into biodiesel. This approach requires the most skill, but will increase your biodiesel fuel efficiency while minimizing your chemical costs and wash / separation problems.