Drying Biodiesel

By Rick Da Tech


After water washing our biodiesel has an orange juice color to it that indicates it's full of water. We must remove that water before using it in a diesel engine or risk damaging the engine. The oldest method of drying or dessicating biodiesel is settling. In this method the water settles to the bottom of the tank over time. If you have well washed, high conversion biodiesel and live in a relatively dry area, then it works well.

Drying via settling

The pictures above show a couple gallons of well washed, high conversion biodiesel drying on top of it's last wash water. The first photo was just as the bubbler wash was stopped. The last photo is an hour later. Note that with settling, you do not have to remove the water before drying. You can read newsprint through the dried biodiesel on the right. Notice that it turns a darker color as it dries. You can often see a layer of darker biodiesel on top of your batch a few minutes after you stop washing. That's a good indication that you've washed your biodiesel enough. I cheated a little to get it to dry so fast. The final bubble wash was two days continuous. That would be like bubbling a full batch for a week. Not sure why, but extended bubbling on the final wash can really speed up drying by settling. It can also make the fuel acidic.

Low conversion biodiesel contains an elevated level of monoglycerides that are hygroscopic or water absorbing. What that means is that low conversion biodiesel will literally suck the water out of the air. The higher the humidity the faster it will soak up water from the air.

Bubble drying with fan


Raften from Infopop uses this setup. The bubbler pushes the water into the top of the drum and the fan pushes it out. When it's cold or he is in a hurry, he will speed things up by using a water heater element to heat the bio up to about 130°F before adding the bubbler. The fan blows fresh air down into the drum and the wet air goes out through the hole in the plywood.

Biodiesel that is poorly washed will also be hygroscopic due to the soap and trace methanol still in the biodiesel. When using settling to dry your biodiesel it is important to wash your biodiesel until it dries easily. To determine if it will dry easily, take a small sample and sit it on the shelf. If the biodiesel clears up quickly then your done with washing and ready to settle dry. Just remember that the smaller the sample the faster it will dry. A 20ml sample may take 5 minutes to dry, while a 40 gallon batch may take several days.

If you live in muggy wet area, the biodiesel may not dry using settling. The high humidity forces water into the biodiesel faster than it can settle out. In this case we need to look at more active drying methods. With Active drying we set the conditions so that the water evaporates quickly out of the biodiesel. The first trick is to increase the surface are of the biodiesel. Water will evaporate from the biodiesel into the air at a fixed rated per area of surface depending on the biodiesel's desire to give off vapor and the air's desire to take it. Doubling the surface area will double the overall rate of evaporation. The methods most often used to increase surface area are bubbling the biodiesel with air, and pumping the biodiesel through a sprayer or on the side of the drum.


The pictures above, donated by Legal Eagle of B100wh.com, show a spray type dry tank. The FF-316 Stainless Steel 145° Flat Fan nozzle from Bete provides for the increase in surface area. The bypass valve on the nozzle manifold allows adjustment of the spray pattern. The fan keeps fresh low humidity air above the biodiesel for rapid evaporation. The spear heater is used to heat the biodiesel.

We can also alter the biodiesel's desire to give off vapor by heating it. This is often done with either inline or tank mounted immersion heating elements. Heating the wet biodiesel with electric heat is very efficient and the most common way of adding heat. Heating with stirring and a good movement of fresh air over the biodiesel will usually result in drying a full batch in 30 minutes to a couple of hours.

To obtain the fastest drying we can heat the air over the biodiesel. Heating the air increases the air's desire to soak up water. When we heat air, we reduce it's relative humidity making more room for more water. The simplest way I've seen of heating the air, is to stick a hot air gun in the 2" bung of a drum. Heating air is expensive and is not often used except as last addition in areas where high humidity is a problem. As an alternative to heating the air, one ingenious home brewer put his dry tank in a small room with a dehumidifier. The water from the dehumidifier was then filtered and used as wash water on the next batch.

Bob Abby's dry tank


BobAbbey from infopop makes his sprayer out of 1/2" vinyl tubing with 1/8" holes every 4" supplied by an HF pump. A heating element is used in the tank to boost the temperature when necessary.

 {jcomments on}