| Unwashed biodiesel will not meet ASTM (American Society of Testing and Materials) standards. For more information about ASTM standards, and testing and specifications for biodiesel and other diesel fuels, see Resources. Remember, equipment and engine manufacturers only warranty their equipment and engines for their material and manufacturer defects. Fuel manufacturers (in this case, you) assume responsibility for any damage caused by the fuel. Washing biodiesel is easy to do, and requires only water and time. |
Why wash biodiesel?
The biodiesel produced with the process described above will work in some heating and lighting equipment and may be used to fuel diesel engines. Most impurities settle out into the glycerol layer—including unfiltered particulates, methanol, and glycerin. Some sources encourage using unwashed biodiesel, because washing biodiesel is a time-consuming process. However, some alcohol, sodium hydroxide, and soap remain suspended throughout the biodiesel after the transesterification is complete. Water in biodiesel can lead to biological growth as the fuel degrades. Unreacted methanol in the biodiesel fuel can result in fire or explosion and can corrode engine components. The catalyst, sodium hydroxide, can also attack other engine components. Since the methanol and sodium hydroxide are chemical bases, unwashed biodiesel is caustic and may damage diesel engine components. Soap is not a fuel and will reduce fuel lubricity and cause injector coking and other deposits. At the 5-gallon batch quantity described above, it is not feasible to reclaim the methanol. If you begin making significant quantities of biodiesel, you should reclaim the unreacted methanol, because the reclaimed methanol represents a significant cost savings, and methanol is a pollutant in its own right.
Minimize contamination
To minimize impurities in the biodiesel, filter the oil before you mix it with methoxide. Remove all the water from waste vegetable oil by heating it before it is processed into biodiesel. Do not use any more methanol or sodium hydroxide than is needed to have an efficient reaction.
Most of the unreacted sodium hydroxide ends up in the glycerol layer and can be discarded or used to make soap (see Soap Making Using Biodiesel Waste). Most soap (if there is any water and FFA in the oil you started with) ends up in a white layer between the glycerol and the biodiesel. Drain this layer off with the glycerol before washing the biodiesel.
Remove unreacted methanol
Unreacted alcohol may be distilled from the biodiesel and reclaimed for use in future batches (more than one quarter of the methanol in the recipe will end up unreacted). Although alcohol reclamation is beyond the scope of this publication, note that methanol boils at 148°F at sea level. Methanol can be driven from biodiesel by heating it; do this outside or vent the methanol to the outside. Never breathe methanol fumes. A much better and safer solution is to use a vacuum pump to lower the pressure of a closed tank. The methanol can be collected and re-used. See the Resources section on methanol reclamation.
Washing Techniques
There are three techniques for washing biodiesel: agitation washing, mist washing, and bubble washing. The process of washing biodiesel involves mixing it with water. Water is heavier than biodiesel and absorbs the excess alcohol, sodium hydroxide, and soap suspended in it. After washing and settling, the water and the impurities in the water can be drained from the bottom of the container. Several wash cycles are generally needed. The first water drained off the bottom of the biodiesel will be milky, and the final wash water drained off will be clear. Excess sodium hydroxide in the biodiesel will form soap when mixed with water, and it takes a while for the soap to settle out. Depending on the method you use, it takes roughly as much water as biodiesel for a wash cycle. Initial washings must involve gentle mixing to minimize the formation of soap that will take time to settle out. However, you want the mixing to be thorough and for the water to be dispersed throughout the biodiesel. Agitation washing amounts to stirring water into the biodiesel, letting it settle, and draining it off. Mist washing is spraying a fine mist of water over the surface of the biodiesel. Tiny droplets of water fall through the biodiesel and pick up impurities on the way down. Bubble washing is done by putting a bubbler in a layer of water beneath the biodiesel in a container. As the bubbles rise they are coated with water, which picks up impurities as it travels up and then back down through the biodiesel.
Washing a Small Batch (courtesy of "Squire Tilly")
This is a simple, nearly foolproof method for washing small test batches of biodiesel, and I have often washed a sample of biodiesel in less than an hour using this method.
Be aware that unwashed biodiesel contains soap. If you agitate your first few washes too vigorously, the water, soap, and biodiesel will likely form an emulsion that may take days or weeks to separate.
Prior to washing, let the unwashed biodiesel settle for at least eight hours and possibly as long as a week to get rid of as much excess sodium hydroxide as possible.
In addition to unwashed biodiesel, you will need water, and a container about twice the volume of the biodiesel you are washing (you may use the container in which you reacted the biodiesel, but this ties up that container, so you will probably want to use another container).
The three important things to remember in washing are GENTLY GENTLY GENTLY.
Washing Technique
Pour 1 liter of biodiesel into a 2-liter plastic soft drink bottle. Gently pour about 500 milliliters of lukewarm water into the bottle. Seal with a cap that will not leak. GENTLY rotate bottle end for end for about 30 seconds. After 30 seconds place the bottle upright.
If you have been GENTLE, the water and biodiesel will separate immediately.
You will notice the water is not clear.
Wearing rubber gloves, remove the cap, and using your thumb as a valve, turn the bottle upside down and drain the water. Drain the water into a bucket and allow it to evaporate. Discard any residue.
Repeat the process of adding 500 milliliters of lukewarm water, gently shaking, and draining off the water four or five times. Each time that you repeat the process, you should shake the mixture a little more vigorously and for a little longer, until by the fifth washing you are shaking the mixture very strongly for about a minute or a little more.
Washed biodiesel is VERY CLOUDY, much lighter in color than the original biodiesel, and looks terrible. After a day or two of settling and drying it will clear.
Washing a Larger Batch
Agitation Washing
Gently mix equal parts water and unwashed biodiesel and let settle until clear. Repeat several times, until the water is clear. Pump the biodiesel off of the top of the water (or drain the water off the bottom), dry the biodiesel for a few days in the sun.Bubble Washing
An air pump, hose, and air stone can be bought from an aquarium supply store. Put the weighted air stone in the bottom of your bucket along with the biodiesel. Then gently add about 1/3 as much water as you have biodiesel to wash. Start the air pump and allow the bubbles to gently wash the biodiesel for several hours. Drain off the milky water, and repeat this process, letting the air pump run longer during each washing cycle, until the water remains clear. If the bubbles cause foam to form, use less air. You want to start out GENTLY. After the last water drained is clear, dry the biodiesel (see below) and it is ready to be used as diesel engine fuel.
Mist Washing
Purchase a misting nozzle from a pet store (these are used to keep reptiles cool) or from a drip irrigation company and connect it to your domestic water faucet. Turn the water on to make a fine, gentle mist, and allow the mist to float over the surface of the biodiesel. Keep the misting nozzle above the biodiesel. Mist the biodiesel until you have several gallons of milky water in the bottom of the container, then drain it off. Repeat this process several times, until the water you drain off is clear. Dry the biodiesel until it is clear, and you can use it for diesel fuel.
Drying washed biodiesel
After the biodiesel is washed, it should be dried until it is clear. This can be done by letting the biodiesel sit (covered) in a sunny location for a few days, or it may be heated to about 120°F for a few hours. Reacted, washed, and dried biodiesel may be used in any diesel engine. It should have a pH of close to 7, or chemically neutral, and it should have no methanol left in it.
