Drying WVO for Biodiesel



The most common cause of problems such as emulsions and low quality conversions when making biodiesel fuel is water in the feedstock oil. You cannot tell by looking at the oil if it is dry or wet. Usually Cloudy oil is an indication of wet oil and clear oil is usually an indication of dry oil, but that is not always the case. Dry oil can appear cloudy, and wet oil can appear clear. The only way to be certain is to test your WVO for water to make sure it is dry enough to process. 


Testing oil and biodiesel for water

Testing for Water in WVO

Homebrewers have relied on a number of tests to determine water content. The most common are the hot pan test, and the weigh-heat-weigh test. The Sandy Brae Water Test Kit, and the Carbide Manometer are becoming more popular due to improved accuracy at lower concentrations..



There are three types of water and oil mixtures, dissolved, emulsified and free. It's important to know the differences, because the various ways of drying oil perform differently on each of the mixture types.

Dissolved water is mixed with the oil on a molecular level and is the hardest to dry.

Emulsified water is in a "stable emulsion", very small bubbles of water mixed in oil, just exactly like air is emulsified in foam or soap bubbles. Often, there is some impurity in the oil that acts chemically to make emulsified water a stable emulsion.

Free water is in an "unstable emulsion". Like emulified water, it is small bubbles of water in oil, except that it will seperate out on it's own. This type of mixture is the easiest to separate.

There are three fundamental ways to dry WVO before you process, The first is mechanical separation, the second is chemical separation, and the third is vapor separation. Mechanical separation is where the water stays liquid and is coalesced or collected together and removed as a liquid. This method is better since it can reduce your titration numbers by removing the water soluble acids. Chemical separation is a byproduct of caustic stripping. Vapor separation is where the water is turned to vapor and vented as steam. This method leaves behind water soluble acids in the oil.

Vapor Separation

Vapor Separation is where the water is evaporated or boiled rather than settled. These methods will usually remove free, emulsified, and dissolved water. The first thing that comes to mind is to heat the oil to above 212F. I've heard of people using turkey cookers, natural gas water heaters, giant cast iron caldrons on open wood fires and glycerin fired turk burners to boil the water out of the oil. Boiling the water out uses a lot of energy to heat the oil to dangerously high temperatures and is not a very efficient way to remove the water.

warningBoiling the water out of the oil is also the most dangerous way to dry oil. As you heat, a little water will settle out to the bottom if you are not actively and vigorously mixing. The oil above the water will pressurize it, elevating the boiling point of the water. If the oil is jarred or bumped, it can release the pressure on the water enough for the water to turn instantly to steam. When it does, it expands rapidly throwing boiling oil out of the top of the container. Think of a cannon shooting boiling oil. It's that powerful. Oil can be throw up to 20 feet away. and of course anybody that gets sprayed with this oil will be seriously injured. So... boiling the oil is not really the way to go unless you are constantly stirring while heating.

Testing oil and biodiesel for water

Headspace Desiccation

Headspace Desiccation is the name industry uses for the most common method home brewers use to actively dry biodiesel. It's popular because it not only removes free and emulsified water, also removes dissolved water. It is essentially evaporating water out of biodiesel without boiling it or using a vacuum.


Vacuum dehydration is a technique that has been successfully used. If you pull a vacuum on the top of your tank it lowers the boiling point of the water in the oil. Circulation of the oil is necessary to prevent the water from settling out to the bottom. A strong vacuum pump is needed to keep the vacuum. A condenser will reduce the size of the pump needed to keep the vacuum in place. Vacuum dehydration works best when the oil is circulated, however, most of the cheap pumps we use will develop leaks past the seals when operated under a vacuum.

Mechanical Separation

Mechanical Separation will generally remove free water, some emulsified water, but no dissolved water.

The oldest and easiest method of drying WVO is to use gravity to settle it. Given time the water will fall to the bottom of the container along with sludge and other stuff. If you use cubies to settle your oil, you can actually see the layering with the good oil on top and the nasty stuff on bottom. When the container is half or more filled with good oil on top, then pour the good stuff off the top into another container leaving the nasty stuff for more settling time. Settling times can be greatly reduced by rough filtering the oil before it is settled. The particulates in the oil hold onto water and removing them will make drying WVO easier.


Cold Upflow Settling

Cold Upflow Settling

One method of settling is Cold Upflow Settling. It is more of a continuous flow settling that works best on clean liquid oils. Adding heat is an option if your oil is not already a clean liquid.



You can speed up settling with a little heat. Pour the oil into a 55 gallon drum and heat it to about 120F. Wrap the drum with insulation so it cools down slowly. As the oil cools, the water will settle to the bottom leaving clean dry oil on top. If the drum is well insulated, it will take a few days to cool. I prefer the foil/bubble/foil insulation since it is impervious to oil and biodiesel. I also like using a flexible silicon drum heater since I can move it from drum to drum as needed.


Automated Heat and Settle Tank

Heat and Settle Tank

For a more automated heat and settle tank you can use a hot water heater. It's pretty much automated with safety features built in. Just fill it up, set the timer, push the start button and walk away.




Click on the imageto open a photo gallery with more pictures of this centrifuge.

If you really want to speed things up, you can use a centrifuge. There are a number of inexpensive centrifuges on the market. The DieselCraft OC20 centrifuge will clean and remove all the water from a 55 gallon drum of oil in about 4 hours. Inexpensive centrifuges start at about $300 and will need a pump and motor. Industrial self cleaning centrifuges start at about $10,000. In order for the centrifuge to completely remove all the water, it helps if the oil is heated. Heating the oil to about 190F causes the water to flash off inside the centrifuge making this a combination mechanical and thermal separation when the oil is heated. Heating is not required and some do not heat, some heat, but to lower temperatures. Inline heaters have been found to be the most efficient, but I like the silicon drum heaters with a little insulation. Find our more about using a centrifuge to dry WVO at our SVO Tutorial.  


Chemical Separation


The Glycerin Treatment

The Glycerin Treatment

You can lower the titration number of your WVO by using a technique I'm calling Glycerin Stripping. It's really simple, you add the raw glycerin from a previous batch to your WVO, heat, stir, settle, and decant. The treatment not only reduces the titration number, it also reduces the methanol needed for processing, lowers water content, and cleans the oil before processing.




Related Links

Diffuser for Drying Oil and Biodiesel  -  Biopowered Wiki

Removing Water Contamination from Oil - Machinery Lubrication

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