Mist Washing Biodiesel
By Rick Da Tech
Mist Washing or Spray 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 impurities 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.
An often overlooked upgrade to the continuous mist wash tank is the addition of a timer and overflow shutoffs. The only solenoid valve that will work with low flow misting systems is the direct acting solenoid valve. One place to purchase an inexpensive direct acting solenoid valve is www.omega.com. The 1/8" valve (SV31050) is plenty big for most misting systems and comes with a 110V coil standard. For a timer, a lamp and appliance timer with non removable push pins at 15 minute intervals lets you turn the water on and off every 15 minutes for water conservation. Available online is the Sylvania SA110 15 Amp Heavy Duty Appliance Timer for under $20. Then use float switches on the wash tank and water catch tank with normally closed contacts that open when the float rises to shut off the water if either drum fills for any reason.