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The well designed biodiesel washtank will remove any methanol, soaps, glycerine and other impurities from the produced biodiesel.
If the fuel has not been washed properly then you will have methanol and free glycerin in it. Despite what some people say I have personally seen a fleet of vehicles ruined with badly washed fuel.
The free glycerin and biodiesel soap is an issue. You would think that soaps would clean an injection system but they do not. Soap does not burn but turns into a black gooey carbon mess in your engine that can bung up your valves and rings leading to catastrophic engine failure.
Free glycerol, because of the high pressures in the injection system polymerizes (turn into plastic) and totally bungs everything up. Aside from polymerizing the glycerol is to viscous to be sprayed properly through the small holes at the tips of the fuel injectors. This leads to poor spray patterns and if the nozzles block up even partially the fuel that is sprayed into the engine can have very bad effects on the engine.
The other more long term problem with inadequate washing is that there are still caustic chemicals in the biodiesel, this will corrode the injectors and pump and over a long period this will lead to failure.
So, with all these problems that can be caused with the contaminants in biodiesel it is a good thing that a washtank is a simple thing to construct.
A wet wash system is just a tank in which biodiesel and water are mixed together.
The best biodiesel washtank will have a conical bottom and will be longer than it is wide. A ratio of 2:1 for height to width is good. The reason that you want a tall tank is that the water can be circulated through a tall column of the biodiesel.
A biodiesel washtank should be double the capacity of the biodiesel. So if you have 100 liters of biodiesel use at least a 200 liter washtank. When you water wash biodiesel the water used for each wash would only typically be 20% of the biodiesel volume. But if something goes wrong with the wash process then the easiest way of fixing it is to add extra water, so you should have the spare space.
The best source of plastic conical tanks is a a farm supply store or a coop. Most conical bottom tanks are produced through a roto molding process. You would probably find them by Goggling "Roto Molder".
When tanks are roto molded the material is typically Poly Ethylene, Cross lInked Poly Ethylene or Poly Propylene. All these materials make good washtanks.
The following arrangement of valves on the bottom of the tank works well. With a conical bottom on the tank you can separate the biodiesel and water layers easily.
The fuel should be filtered on its way to the Holding Tank. On a small system it can be pumped via the filter straight into your biodiesel car.
It is better to get a tank that you can see into as it helps to see what is going on in the tank.
Methods of adding and agitating the water and biodiesel together vary but the two typical methods are :
The water is sprayed from the top of the biodiesel washtank onto the top of the biodiesel. It will then sink to the bottom of the tank through the biodiesel absorbing impurities as it goes. It is important that the water is added gently to the top of the tank as any severe mixing of the biodiesel and water will cause problems.
There is a useful variation on a spray wash system that works as an automatic washer. This is really useful as you can turn the wash system on and come back when the biodiesel is washed. The way it works is a pipe is connected to the bottom of the conical tank. A valve is placed inline so that the pipe can be closed off when required. Water is sprayed into the top of the tank and it then drops through the biodiesel and fills the conical bottom. The rising level of the liquid in the tank causes the level of water in the waste water pipe to rise as well. Eventually the level of the water in the pipe reaches the tee and flows into the drain. It is quite important that the pipe above the Tee is open to the atmosphere as this stops all the liquid in the tank from siphoning out.
Bubble washing biodiesel was invented by the University Of Idaho.
Water is added to the biodiesel washtank gently. Once the water is in the tank, bubbles of air are blown from the bottom. As the bubbles rise they carry a drop of water with them to the top of the biodiesel. There the air bubble breaks and the drop of water is released and falls to the bottom of the tank. This mixes the water and biodiesel together.
Again it is important to do this gently. This is a simple technique and typically a fish tank pump is used to provide the air.