the importance of washing fuel

Yes, DIP, washing is essential. You wouldn't dream of lathering up in the shower and stepping out without rinsing all that soap off. So why would you expect your fuel to enter the world without a good rinse?

Next to a good reaction, washing is the most important part of quality control (QC). And with all quality control issues good data is at the heart of your success. So, it is critical to quantitatively measure the amount of soap in your fuel. I draw off a sample after each wash and run a soap test (www.biodieselcommunity.org/testingforsoap). If there are more than 20 parts per million (PPM) of soap I keep on washing. Even though the American Society for Testing Materials (ASTM) does not regulate soap content it is one of the more important QC factors in making good biodiesel.

Our dirty biodiesel esters are contaminated with soap, residual catalyst, and free glycerol (among others). All of which have a greater affinity for water than they do for biodiesel. Hence, rinsing dirty fuel with water washes contaminants out. It's worth mentioning that water washing is just one of many strategies for washing. I favor good old fashioned water washing because I don't like hauling piles of Magnesol, Amberlite, or their equivalents to the landfill.

For most of us, it makes sense to perform an initial mist wash without agitating the fuel. If you have a lot of soap in your fuel, add water and agitate and you will end up with a massive lump that resembles mayonnaise. This condition is called an emulsification. Mist washing is an emulsification avoidance technique. If your biodiesel is made from high FFA feedstocks you will want to perform multiple mist washes before moving on to more vigorous washing.

Bubble washing is by far the most common water wash technique in commercial and backyard brewing. Bubble washing is accomplished by placing a diffuser (air stone or sparge device) attached to an air source (air pump, compressor, or blower) at the bottom of the wash tank. I like to think of the air bubbles as elevators for little water drops. The water drops stick to the air bubbles and travel up through the fuel absorbing contaminants. When the air bubble reaches the surface of the fuel it pops and the water drops that were along for the ride fall back down through the fuel sucking up more contaminants on their way back to the bottom of the tank. When the water is saturated with contaminants it must be exchanged for fresh water. I consider washing to be complete when soap levels are below 20 PPM.

Some fuel makers use pumps, mixers, or various types of agitators to mix the water and the fuel. This type of mix washing is very effective and is best suited for relatively clean fuel unless you needed mayo with that.

I'll mention a few tricks I've learned over the years:

-Never wash in tanks that can't be completely drained. I stick to vertical conical tanks.

-Initial washes don't need to be more than an hour. Final washes can be longer.

-Hot water does a better job washing than cold water.

-Soft water is better than hard water. Another advantage of soft water is that hard water can sometimes impart trace metals to the fuel. The latest spec (ASTM D6751-06a) includes limits on calcium/magnesium and sodium/potassium.

-A small amount of acid added to the fuel will break soaps into FFA and metallic salt. Acidulating soap can compliment water washing but be aware that it increases acid value too.

The last word is about safety. Unless you recover methanol from the biodiesel before washing it will gas off during washing, especially if bubbling and attach to the water. Closed wash systems with safe ventilation systems is important. Wash water needs to be handled accordingly. Washing safely is fun and easy!