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There are a number of ways commonly used for drying biodiesel.
For a home producer the most common way of drying biodiesel is to blow bubbles of air from the bottom of the drying tank. This rising stream of air removes the water from the biodiesel. But if you live in a very humid country you will not be able to remove enough of the water to meet the international standard of 500 ppm (parts per million). Depending on the environment that you live in you may need to heat your biodiesel before it is dried.
Biodiesel is hygroscopic, this means that even if the biodiesel is dried to below 500 ppm, it will soon absorb enough water to exceed this limit from the atmosphere. To maintain a level of water below 500 ppm the biodiesel must be stored in a closed environment.
Another way to dry biodiesel is to pump the biodiesel from the bottom of the drying tank and fan it out over a flat surface at the top of the tank. This surface may be heated to speed up the drying of the biodiesel. Sometimes a fan is used to blow air over or through the biodiesel spray which also speeds up the drying process considerably.
Using air to dry biodiesel has the disadvantage of oxidizing the fuel. This may not be an issue for the small-scale producer as the fuel that is produced on a small scale is used very quickly.
In a commercial environment however this is not acceptable. The shelf life of biodiesel once it enters the distribution chain is very important. Generally anti-oxidants and other expensive additives are added into the biodiesel to extend its shelf life. Drying biodiesel in a manner which reduces the shelf life is unacceptable for commercial producers. In commercial plants drying systems are designed and installed which reduce the chances of oxidation.
There has been some debate in home biodiesel circles, as to the validity of the 500 ppm standard. A standard of 500 ppm would be quite important if the fuel is stored for an extended period of time. It is also an important consideration when the fuel is blended into normal diesel. Seeing as how most homemade biodiesel is used very quickly and is probably used in a B100 ratio the 500 ppm standard is considered by many to be too strict.
Have a look at the "Making Biodiesel" video in our video section which shows one way to dry biodiesel.