By Rick Da Tech
If we mix biodiesel and water too violently we create an emulsion. The definition of what is too violent varies. Under some conditions just adding water to biodiesel with a hose is enough to create an emulsion, other times we can mix water and biodiesel with a boat motor and not create an emulsion. There are four major factors, Wash Temperature, Water hardness, Soap content, Conversion.
Commercial biodiesel plants use hot water for washing. It speeds up the process of transferring the impurities from the biodiesel to the water. Heating it up adds energy, causing the individual molecules to move faster. Second, heat prevents emulsions from forming. Heat alone can actually break emulsions.
Water washing biodiesel below about 50°F (10°C) is difficult. It usually results in nasty emulsions. It all goes much smoother when water washing above 80°F (27°C). Commercial biodiesel plants will usually water wash above 120°F (49°C).
Basically water washing in cold weather is a PITA, while water washing in hot weather is easy peasy.
Hard water contains Calcium and Magnesium (lime) ions. These Ions react with soap by replacing the Sodium (Na) or Potassium (K) ions in soap with the more positively charged calcium (Ca) and magnesium (Mg) ions. The new soaps do not dissolve in water. They will still dissolve in biodiesel, at least partially. There are consequences to washing with hard water. The most impressive is that it is nearly impossible to create an emulsion with hard water. Sodium based soaps are what’s called an emulsifier, and help to form and stabilize water-biodiesel emulsions, a.k.a. that white mayonnaise looking stuff between the water and the biodiesel. Calcium and Magnesium based soaps will not dissolve in water and are not emulsifiers. The second consequence is that hard wash water will not turn white from soap like soft water does. The third consequence is that washing with hard water leaves white flakes of calcium soaps in the wash tank. It can be found on the bottom of the wash tank and in a very thin layer between the water and the biodiesel.
So far all the consequences of washing with hard water have been good. There is one bad consequence, it will dissolve in biodiesel. Biodiesel washed in hard water will sometimes have trouble meeting ASTM specification for metals (ASTM D 4951).
The more soap you have in your biodiesel the more quickly it will create an emulsion. In other words, It’s easier to create an emulsion on your first wash than on the last. We can reduce the soap present in unwashed biodiesel by using a prewash. In a prewash we mix in water after the reaction is finished and before settling out the glycerin. It doesn’t take much water; in fact it only takes 5% or one part water to 20 parts biodiesel. If we use more water, it creates a nasty emulsion.
There are other ways to reduce the soap in unwashed biodiesel. Soap is formed when we neutralize FFA during the reaction. Anything that reduces FFA will reduce soap in our unwashed biodiesel. We can use the acid-base two stage recipe to make biodiesel from FFA instead of soap. We can use the glycerin pretreatment to reduce the FFA before we make biodiesel.
Giving the glycerin enough time to completely settle will reduce soaps in the biodiesel. Some people let settling continue for a week before transfering biodiesel to a wash tank. This is very important for extremely agressive washing methods. It helps to settle in a standpipe tank, and flush out the drain pipe before transfering the biodiesel to a washtank.
We can also reduce soap formation by making sure our vegetable oil is bone dry before we try to make biodiesel. Water is a catalyst for turning good oil into soap. In fact if we have much more than about 2% water in the oil, we run the risk of using all of our catalyst to make soap without making any biodiesel.
We measure the quality of our biodiesel by how much oil we convert to biodiesel. In the commercial biodiesel world, they use ASTM D 6584. It uses some expensive equipment to measure how much un-reacted and partially reacted oil is left in the biodiesel. A simpler homebrew test is the 3/27 test. In the 3/27 test, biodiesel dissolves in methanol, but vegetable oil does not.
When oil is converted into biodiesel, it is done in steps. In each step one fatty acid chain is stripped off the glycerin backbone and converted into biodiesel. A glycerin backbone with one fatty acid chain attached is called a monoglyceride. Monoglycerides are emulisfiers. If we have excessive monoglycerides in our biodiesel when we wash with water, we are more likely to create an emulsion, and any emulsion we create is harder to break.