Techniques for Water Washing Biodiesel

Static Washing

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. Contaminates 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 contaminates. Most homebrewers will let the static wash continue over night before draining and starting a different technique. It is particularly effective as a first wash for biodiesel from high FFA oils.

Static washing Tips:

• Use hot water directly from the tap or from any other source.
• Introduce the biodiesel onto the top of the water by pumping the biodiesel into the tank through the standpipe.
• Insulate and cover the drum to keep the heat in.
• Consider using a sealed and vented wash tank to reduce methanol vapors in the workspace.

Mist Washing

Mist washing is the spraying of water over the top of the biodiesel and letting it settle down through the biodiesel collecting contaminates as it goes. It is more aggressive and therefore more effective at removing contaminates that static washing. It is also effective at removing methanol vapors from the air over the biodiesel, making it a safer first wash technique. It uses more water than most of the other techniques unless you re-circulate the water. It can also cause the biodiesel to overflow the wash tank if left running without using overflow prevention equipment. In places where water is abundant and wastewater disposal is not a problem, a continuous mist wash can be implemented allowing very long wash times.

High flow, agressive mist washing is a variation on mist washing that requires high conversion fuel, low soap content, extended settling to remove all the glycerin, hard water, and a heated wash. Not everyone is able to use high flow agressive mist washing because if not all the conditions are met, it will form heavy emulsions. If all the conditions are met, then the wash cycle can be completed in just a few hours.

Mist washing notes:

• The size of the droplet can have an effect on mist washing
• Very small droplets can cause a cooling effect due to evaporation and require the addition of a heat source to compensate for the heat lost due to evaporation. This effect is more pronounced in dry climates. If the tank is sealed the cooling effect goes away as soon as the air over the biodiesel is saturated with water vapor.
• Large droplets can increase agitation and cause emulsions if the source of the spray is too close to the biodiesel. Raising the source to at least a foot above the biodiesel will reduce this effect.
• In summer, a black garden hose laid out in the sun can raise the temperature of the water used for mist washing and improve the wash, although people report mixed results from heating the mist water, not everyone notices an improvement.
• A continuous mist wash is an inexpensive way to prevent tank overflows or run your mist washer for extended periods of time.

Continuous Mist Washing

Mist washing has one problem. If you don’t stop it in time, your wash tank will overflow and spill biodiesel everywhere. This simple drain system will limit how high the biodiesel will go in the tank.

Be sure you connect the "Water Out" to a suitable drain or tank to catch the water. A steel riser pipe would sturdier than a PVC or tubing. It would be a bad idea to use the pipe as a handle to move your wash tank because of the risk of breaking off the pipe. If your drain is kinked or pinched the water will back up and flow spill out the vent pipe.

One note of warning: If you create a big emulsion, it will want to flow out the drain taking your biodiesel with it. It could also stop up the drain and overflow the tank anyway. So you still need to keep an eye on the wash and stop it if you see an emulsion forming.

The water will drag along some biodiesel with it. Best practice is to drain the wash tank into a drum to give the biodiesel time to rise to the surface where it can be removed. If you add epsom salt to the spent wash water, it will react with the soap, making the soap insouble in water and releasing the biodiesel.

A few people have added a limit switch to their tanks. The limit switch would shut off the water using a solenoid valve before the tank overflows.

Bubble Washing

Bubble washing is more aggressive than mist washing. It is done by adding a layer of water beneath the biodiesel and forming air bubbles in the water. The water is dragged up into the biodiesel in a small layer around the air bubble, which falls back down through the biodiesel when the bubble bursts at the top of the tank. The size of the bubbles and the volume of air will determine the aggressiveness of the wash. Small bubbles with low flows are not very aggressive, but large bubbles with high airflows are very aggressive. If the wash tank is not sealed and used as a first wash technique it can release significant amounts of methanol into your workspace area and create a safety issue. This method uses less water than the mist wash technique.

Bubble washing notes:

• Cheap aquarium stones will dissolve in biodiesel
• Limewood wooden air diffusers generate very tiny bubbles for less aggressive bubbles. They hold up well to biodiesel but the plastic hose barb will need to be replaced with a brass one.
• You need a way to adjust airflow to regulate the aggressiveness of the bubbles.
• An aquarium pump with a rheostat adjustment is a great way to allow adjustment of the airflow.
• Airflow can also be adjusted with a needle valve inserted in the line.
• Bubble stones can be made from grinder stones.
• Bubble stones can be made from HDPE pill bottles by using a push pin to poke lots of holes in the bottle.
• Polyethylene tubing can be made into a bubble stone by sealing the end and using a pin to poke lots of holes in the last few inches of the tubing.
• Bubble rings can be made from copper tubing by grinding a thin spot on the tubing then punching a hole using a pushpin.
• Polyethylene tubing will withstand biodiesel but requires the use of compression fittings since it will split when stretched over a hose barb.
• Weights to hold the bubble stone down in the water are best if made from stainless steel. Stainless steel nuts slid over the tubing make good weights.

Stir Washing

Stir washing is a very aggressive technique for washing. Stir washing is accomplished by placing water and biodiesel in the same tank and aggressively mixing to form an emulsion. The emulsion then separates over time. However, it sometimes creates a stable emulsion that does not settle out with time. This is best used on high conversion biodiesel with very little soap using hot, hard water and is primarily used on commercially made biodiesel where new oil is used as a feedstock and very little soap is made.