Water Washing Biodiesel
By Rick Da Tech
Washing Biodiesel with water is the oldest and most common method of cleaning biodiesel. About 3% of raw, unwashed biodiesel is methanol. Methanol is a solvent, it captures soap and other impurities and holds them dissolved in the biodiesel. Water soaks up that methanol, releasing impurities to be washed away with water.
Keeping the methanol liquid and diluted in water makes water washing the safest way to clean biodiesel. Most dry wash methods require we evaporate or distill the methanol into a flammable and toxic gas as part of the purification process.
Water washing is the most flexible way to purify biodiesel. Under the right conditions, you can power wash in just a few hours with extremely aggressive wash methods. Alternatively, you can take up to a week using less aggressive methods. Some methods conserve water, letting you wash with 1 part water for every 5 parts biodiesel, while most use a 1:1 water to biodiesel ratio. Water washing can be automated, and you can mix and match different wash methods to fit your personal needs.
Water Wash Methods
It takes several washes to clean your biodiesel. Most people start with a less aggressive wash method, moving on to more aggressive methods with later wash cycles. I like to start with an overnight Static Wash, then move on to bubble washing. With each wash, I put one part water for 5 parts biodiesel. If I have a 40-gallon batch, I replace the water with 8 gallons of fresh water for each wash. The first two bubble washes usually last about 8 hours each, with the last water wash usually lasting 48 hours. I use a lamp and appliance timer to control the bubble wash, so I only need to attend to it for a few minutes a day.
Static or gravity washing is the least aggressive and least likely to generate an emulsion. It is simply placing water and biodiesel in the same tank without any mixing. Impurities migrate from the biodiesel to the water through the boundary layer over time. This process takes anywhere from 4 hours to 24 hours to saturate the water with contaminants. Most homebrewers let the static wash continue overnight before draining and starting a different technique. It is particularly effective as the first wash on very soapy biodiesel made from high FFA oils.
Sooner or later everyone makes an emulsion that doesn't break down on its own. Emulsions are a mixture of water and biodiesel and other stuff like soap, and sometimes glycerin. It usually looks like mayonnaise. It can have other appearances as well. It is a problem for the beginner. Emulsions can be avoided and broken with a little care.
Tests for When to Stop Washing
It usually takes three to four washes to clean biodiesel completely. We can use one of several tests to help us decide when we have washed enough. There are three ways to tell when to quit washing.
The Shakem'up Test
Many people recommend the Shakem’up Test as a test for soap. In the Shakem'up test we fill a bottle about a third full of biodiesel, then add an equal amount of water, and Shake like crazy. If it separates into water and biodiesel quickly with the water being not too white (soapy), then it is a pass.
The test needs a lot of experience to read properly. If you wash with soft water, the water on bottom will always be soapy. If you have hard water, it may be clear when there is still soap in the biodiesel. Different oils can also give different readings. Even though it is subjective, it can still be useful as a quick test, once you have sufficient experience with it.
Appearance of the Wash Water
It takes some experience to read this test properly. If you wash with soft water, the water on the bottom is always soapy. If you have hard water, it may be clear when there is still soap in the biodiesel. Different oils can also give different readings. Even though it is subjective, it can still be a useful quick test, once you have sufficient experience with it.
It ain't over till the paperwork is done.
Biodiesel Wash Tank Plans
University of Idaho Tech Note 25 - The advantages of water washing
Water Washing Biodiesel 101 - Utah Biodiesel Supply
Biodiesel Emulsions 101 – Preventing And Curing Them - Utah Biodiesel Supply