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Transesterification: Separating Cooking Oil for Biodiesel

written by: ciel s cantoria•edited by: Sarah Malburg•updated: 10/15/2010

Learn about transesterification and find out how to separate cooking oil for biodiesel. More and more families are using biodiesel as a superb substitute for petroleum since the cost of fuel expenses is high. Recycle your used cooking oil instead of simply throwing it away and save money today!

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    What is Transesterification?

    Used vegetable cooking oil 

    Biodiesel is now widely used as a substitute to petroleum-based fuel. It makes use of animal fats and vegetable oils, and it is a good way to recycle used cooking oil. Although Rudolph Diesel, the diesel fuel inventor made use of peanut oil one hundred years ago for his compression-ignition engine, vegetable oils were considered too viscous or too thick for ideal engine performance. In the 1970's, a group of scientists were able to come up with the transesterification method, the most common way to make biodiesel today. It is a simple chemical process to develop vegetable derivatives that can provide almost the same performance levels of hydrocarbon-based fuels.

    Transesterification, is a process of breaking down the materials of vegetable oils, animal fats and used cooking oil by mixing methanol and lye (sodium hydroxide) to produce sodium methoxide. The sodium methoxide substance will then be combined with the vegetable oil, mixed and allowed to stand for several hours. The process of transesterification is completed when glycerin forms and separates by settling at the bottom. The biodiesel fuel or methyl esters will remain afloat at the top of the mixture. The two will then be physically segregated. Below you will find step by step instructions for using old cooking oil and separating it to make biodiesel from home through the most common process of transesterification:

    • It is best to work outdoors or in a well-ventilated place when in the process of producing biodiesel, since the critical issue here is the handling of easily flammable materials.
    • Mix one cup of methanol to 1 ½ teaspoon of lye (sodium hydroxide). Lye can be available in the form of drain opener for household use.
    • Combine the mixture well by stirring until the lye is totally dissolved.
    • Heat 4 cups of vegetable oil or used cooking oil in a saucepan using low fire until it achieves a temperature of 125°-140°.
    • Using a heat resistant glass bottle, pour the heated used cooking/vegetable oil and combine it with the methanol and lye mixture. Put a lid on the bottle and apply vigorous shaking for about 30 seconds.
    • Let the mixture stand for two to three days. Glycerin will slowly appear as a dark layer that will settle at the bottom of the container. The clear liquid material is the biodiesel you will use as fuel, which will clear after three days.
    • Separate the clear liquid biodiesel by transferring the liquid to a glass bottle making sure to leave the dark glycerin substance behind.
    • The biodiesel fuel is now available and ready for use, suitable for any diesel engine vehicle without the need to make any adjustments or enhancements.

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    Some Tips for Biodiesel Fuel Production by Transesterification


    For first time transesterification projects, the use of canola, corn, soy rapeseed, and sunflower oil is recommended because they have a low tendency to harden unlike some vegetable oils or animal fats that harden even at room temperature. On the other hand, the use of peanut oil, tallow, or olive oil may require the need to add a neutralizer to provide better performance since these oils have higher acid contents.

    If you intend to produce 1,000 liters of biodiesel by transesterification, then measure 1,000 liters of used cooking oil or vegetable oil. Combine with sodium methoxide made by mixing 22 kg of lye and 150 liters of methanol. You can make use of a biodiesel processor to mix the methanol and lye first before adding the heated used cooking/vegetable oil. For this amount of biodiesel production, it would be best to do at least one hour of mixing until the lye is dissolved.

    More and more motorists find transesterification, the most common way to make biodiesel, to be quite convenient and simple because the process does not require complicated instructions or hard to find ingredients as additives. Besides, your used cooking oil will finally have a better application instead of being thrown away. The savings on fuel expenses is said to be as much as 50% in the long run!