Biofuel, or biodiesel, is fuel derived from living matter, like from crops of corn or soybeans. Triglycerides in the oils in the living material are reacted with methanol to form fatty acid methyl ether (FAME) and glycerin. The glycerin is then removed from the products and the FAME is used for fuel. However, FAME and glycerin aren't really the only products that form from the reaction. Free fatty acids (FFAs), leftover methanol, monoglyceride, diglyceride, and catalyst will also be present in the solution. These products are bad for most engines and so biodiesel in this form is not usable [1]. The biodiesel must be purified so that only FAME remain. There are various processes that one can use to try and purify the biodiesel.
Water Washing vs. Dry Washing
When it cleaning the impurities out of biofuel there are two routes you can take. You can choose to use water to absorb the impurities from the biofuel, or you can use some other substance that will absorb the contaminants. When you use a method that involes water (no matter how it is added) it is called water washing, and a process that doesn't use water is dry washing. There are a number of water and dry washing techniques, and each have their advantages and disadvantages [1]. Water washing tends to be safer than dry washing, and is flexible and simple. Water washing takes longer than dry washing though, some slower methods can take up to a week to clean the biofuel. Water washing also has to be done at a higher temperature, and the bioful must also be dried after it is washed in a water wash process because the water in the fuel can damage engines. Dry washing on the other hand is typically fast, and does not use any water. The fuel does not need to be dried, although it may need to be filtered. Dry washing is usually more dangerous than water washing [2]. Unlike water washing, dry washing sometimes releases harmful chemicals, like methanol, into the air which can harm the environment and/or the people making the biofuel [1]. There are lots of methods that fit into the water washing or dry washing category.
Static Washing
Static washing is a type of water washing where water is added to biodiesel over time impurities move from the biodiesel to the water. The mixture is not mixed making this a very non-aggressive wash technique. It doesn't create emulsion (a mixture of biofuel and impurities like soap that is hard to break down) and is easy but it is very slow [3]. The best way to speed up this process is by mixing the water and biofuel and/or softening the water (making it acidic). Both methods cause the process to be more aggressive though so it is a trade off [1].
Pump Washing
Pump washing is a type of water washing that focuses on breaking down the water and oil into small particles so that the impurities can be cleaned out faster. Pump washing uses pumps and static charges to make the small particles of oil and water, this makes the method fast, and it uses less water than other methods. This method is aggressive but doesn't create emulsion if it is done right. However this method can be complex to set up so doing it right might take some times. Once mastered though, this method is said to have a high pay off [1].
Mist Washing
Mist washing involves spraying a mist of water over the biofuel [1]. As the mist hit the fuel and the water droplet falls through the mixture it picks up impurities. This method is very safe, as the mist in the air will absorb methanol vapors that vaporized from the fuel [4]. This method can also be left to continuously wash the fuel as long as you set up a drainage system so remove the excess water [1]. This method is also very flexible, changing the droplet size of the water mist and how close the mist is sprayed on the biofuel effects how efficient and aggressive the wash is, but it can also cause the biofuel to cool down too much (which would mean a heater would be required), and form emulsion. So these options have to be considered when using this wash method [4].
Bubble Washing
Bubble washing is a wash method similar to mist washing + air washing (which will be mentioned later). Water is added to biofuel and then air is bubbled through the water. The bubbles travel through the fuel, carrying a small amount of water . Then when the bubble pops it sends particles of water back through the biofuel to clean it again. This method is more aggressive than mist washing, and then size of the bubble changes how aggressive the wash is (the bigger, the more aggressive) [1]. This method releases lots of methanol into the air so you should take the proper precautions before trying this method [2].
Magnesol
Magnesol is a synthetic magnesium silicate which can be added to biofuel to clean it. This dry method absorbs soap and FFAs, but doesn't fair well against methanol. Magnesol does not react with water or alcohols which is why it cannot remove methanol very well. Magnesol needs to be mixed very well into the fuel in order to properly clean it, and then must be filtered out. Leftover Magnesol will damage engines if left in the fuel. [5]
ION Exchange Resin Towers and Sawdust
These two methods involve running the biofuel through a tank filled with resin or sawdust. The resin works in different ways, depending on the resin used, but it basically absorbs certain ions and impurities [3]. The sawdust also absorbs impurities, although the type of wood used changes how effective the dust is. One safety hazard with sawdust cleaning is that the sawdust had been known to spontaneously combust days after they are used to clean biofuel [5]. The exact layout and system used to move the biofuel through the resin/sawdust can be altered depending on what you want [3].
Air Washing
Air washing is like bubble washing without water. Air is simply bubbled through the biofuel with no water present. The air bubbles help vaporize methanol, which holds all the impurities in the biofuel. Without the methanol the soap and FFAs can just be scooped out of the bottom of whatever container you are using [1]. This methods whole purpose is to vaporize methanol, but that can be very dangerous. If you are air washing a large amount of fuel a lot of methanol can be released which is toxic to us and is very flammable [5].
Resources
[1] Various, "Make Biodiesel," 2012. [Online]. Available: http://makebiodiesel.org [Accessed 17 April 2013].
[2] I. M. Atadashi, M. K. Aroua and A. A. Aziz, "Biodiesel separation and purification: A
review," Renewable Energy, no. 36, pp. 437-443, 2011.
[3] M. Berrios, M. A. Martin, A. F. Chica and M. A., "Purification of biodiesel from used cooking oils," Applied Energy, no. 88, pp. 3625-3631, 2011.
[4] I. Atadashi, M. Aroua, A. A. Aziz and N. Sulaiman, "Refining technologies for the
purification of crude biodiesel," Applied Energy, no. 88, pp. 4239-4251, 2011.
[5] M. Berrios and R. Skelton, "Comparison of purification methods for biodiesel," Chemical
Engineering Journal, no. 344, pp. 459-465, 2008.
