Drying WVO for Biodiesel
By Rick Da Tech
The most common cause of problems such as emulsions and low-quality conversion 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.
There are three types of water and oil mixtures, dissolved, emulsified and free. It is important to know the differences because the various ways of drying oil perform differently on each type of mixture.
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 emulsified in foam or soap bubbles. Often, there is some impurity in the oil that chemically acts to make emulsified water a stable emulsion.
Free water is in an "unstable emulsion." Like emulsified water, it is small bubbles of water in oil, except that it separates out on its 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 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 is where the water is evaporated or boiled rather than settled. These methods usually remove free, emulsified, and dissolved water. The first thing that comes to mind is to heat the oil to above 212F. I have 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 is the most energy-intensive method of drying oil, and it heats the oil to dangerously high temperatures.
Boiling the water out of the oil is also the most dangerous way to dry oil. If you are not actively and vigorously mixing, water settles to the bottom while you are heating. The oil above the water pressurizes 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 is that powerful. Oil can be thrown up to 20 feet away, seriously injuring anyone in the path of the boiling oil. So... boiling the oil is not the way to go unless you are constantly stirring while heating.
Vacuum dehydration is a technique similar to headspace desiccation. If you pull a vacuum on the top of your tank, it draws the water out of the oil. A strong vacuum pump is needed to keep the vacuum. Using a condenser reduces the size of the pump needed to keep the vacuum in place. Circulation or mixing of the oil is necessary since only the water near the surface can be drawn out by the vacuum. Mixing is also needed to prevent the water from settling out to the bottom. However, the cheap pumps we use for pump mixing leak past the seals when under vacuum, so you may need an expensive pump rated for vacuum if you use this technique.
Mechanical Separation removes free water, some emulsified water, but rarely removes dissolved water.
The oldest and easiest method of drying WVO is to use gravity to settle it. Given time the water sinks to the bottom of the container along with sludge and other stuff. If you use cubies to settle your oil, you can see the layering with the good oil on top and the nasty stuff on the bottom. After enough time, the oil forms layers, you can pour the clean oil off the top into another container and leave the nasty stuff behind for more settling. Settling times can be greatly reduced by rough filtering the oil before you start. The particulates in the oil hold onto water and removing them makes 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 settles to the bottom leaving clean, dry oil on top. If the drum is well insulated, it can 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.
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