By Rick Da Tech
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. Evaporation rates are controlled by three basic properties:
- Relative humidity of the air
- Surface area of the biodiesel
- Temperature of the bio diesel
Headspace is the airspace in your oil tank between the top of the oil and the roof of the tank. As water evaporates, the air in the headspace becomes saturated with water. Headspace desiccation is the process of removing that water from the headspace. If you live in a dry area with low humidity, you can do it with fresh air. If you live in a wet area with high humidity, you must actively dry the air in the headspace. Silica gel is very effective at this and can be regenerated economically to give the silica gel an indefinite life expectancy. It is common and can even be found in most hobby stores as it is used to dry flowers. Just be sure to get the kind with "indicator crystals" that change color as it becomes saturated with water. Another method sometimes used is to dry the air with a room dehumidifier. These appliances can be expensive to purchase and operate but they quickly remove water from air and do not need to be regenerated like silica gel.
Increasing surface area is the easiest way to speed evaporation. Doubling surface area doubles the rate of evaporation as long as everything else remains the same. The methods we use to increase surface area also mix the oil and moves the water near the air/oil interface where it can quickly evaporate. In dry areas, we are able to spray a thin film of wet oil into the top of our drum to satisfy this requirement. Bubbling air through wet oil will mix it and increase its surface area.
Heating the oil increases the energy in the water making it easier for it to evaporate. If we do not add heat somehow, the temperature of the oil will actually drop. It drops because when water evaporates it sucks energy out of its surrounding, causing a chilling effect. The energy absorbed by evaporating water is called the latent heat of evaporation. At minimum we need to supply enough heat to overcome the latent heat of evaporation.
One advantage of headspace desiccation is that you can recycle the air. That's an advantage if you want to use oxygen depleted air to increase your biodiesel's shelf life. Recycling the air requires we actively dry the headspace. When we use Silica Gel to dry the headspace heating the oil will also heat the air in the headspace, reducing the silica gel's ability to remove water vapor.
Examples of Headspace Desiccation
Raften from Infopop uses a bubbler to push 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.
BobAbbey from infopop makes this 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.
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.
imakebiodiesel from Ireland has a humidity problem and uses this setup using 2-300 grams of silica gel in a tray to desiccate the fresh air before blowing it into his150 liter dry tank. He also desiccates the air he uses to bubble it.
update: imakebiodiesel has changed his setup by moving the fan out of his pail. The result is that only the air for bubbling is dessicated with the silica gel. This increases the length of time the silica gel lasts before recharging. His fan is now on a bit of plywood like Raften's above. Link to his description on infopop.
Another related technique is the GL process. With the GL process we use a condenser to desicate the headspace when drying oil before it is processed into biodiesel.