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The most common cause of problems such as emulsions and low quality conversions is water in the feedstock oil. You can not 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. The video below shows the hot pan test for water.
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.
Mechanical Separation
The oldest and easiest method of drying WVO is to let it settle. 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.
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.
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 BurnVeg.com.
Chemical Separation
Chemical Separation is a byproduct of caustic stripping. Caustic stripping is the neutralization of FFA in WVO using a caustic. The popular form of caustic stripping in the homebrew world is the Glycerin Treatment. In this treatment we add our raw glycerin to the WVO before processing. After mixing and settling, the oil will be clean and dry and have a reduced titration value. Note that this process does not work the same for everyone. That is because of the variations in oils and raw glycerin. I suspect that if we refined the raw glycerin, then added caustics and water in a controlled manner the results would be more predictable. However that would end up being just as much hassle as doing an Acid Base process without the improved yield of the acid/base process. The advantage of the Glycerin Treatment is that it is cheap and easy to do. It's cheap because you don't have to buy anything, it just uses byproduct and easy because it's just pour it in the WVO, Stir, Settle, and Decant.
Vapor Separation
Vapor Separation is where the water is evaporated or boiled rather than settled. 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.
 Boiling 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. 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.
If we add enough heat to overcome the latent heat of evaporation, we can evaporate the water out of the oil in short order. You will need to heat the oil to 110F to 120F for this method. That calls for passing dry or hot dry air over WVO that is being agitated. One method is to pump the oil off the bottom of a drum of oil back to the top. Aim the output of the pump on the side of the drum so it flows around the side before falling back into the drum. This will increase the surface area and speed evaporation. Adding a fan that blows over the drum will also help remove the water vapor and speed things up. Finally if you have high humidity, you will need to heat the air. Heating the air lowers it's relative humidity and improves the rate at which the water evaporates. I've seen people using hair dryers, heat guns and solar dryers to provide hot dry air.
Another related technique is the GL process. We heat the oil and use a venturi in the circulation line to draw air into the tank. The oil heats up the air so it can absorb more oil and pushes it out the vent. You can use a condenser and draw the air out of the top of the tank, through the condenser, and venturi back into the tank. Doing it that way reduces the oxygen levels the oil sees while hot and thereby reduces the chances of oxidation (spoiling) of the oil.
Vacuum dehydration is also 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.
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Drying Oil for Biodiesel
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