Biodiesel from waste oil

Biodiesel from waste oil

This is more appealing than using new oil, but it's also more complicated.

First, check for water content. Used oil often has some water in it, and it has to be removed before processing. See Removing the water, below.

Refined fats and oils have a Free Fatty Acid (FFA) content of less than 0.1%. FFAs are formed in cooking the oil, the longer and hotter the oil has been cooked the more FFAs it will contain. FFAs interfere with the transesterification process for making biodiesel. With waste oil you have to use more lye catalyst to neutralise the FFAs. The extra lye turns the FFAs into soap which drops out of the reaction along with the glycerine by-product.

It's essential to titrate the oil to find out how much FFA it contains so you can calculate exactly how much extra lye will be required to neutralise it. This means determining the pH -- the acid-alkaline level (pH7 is neutral, lower values are increasingly acidic, higher than 7 is alkaline). An electronic pH meter is best, but you can also use pH test strips (or litmus paper), or, better than test strips, phenolphthalein solution (from a chemicals supplier).

You can also use red cabbage juice, which changes from red in a strong acid, to pink, purple, blue, and finally green in a strong alkali, or one of the other plant-based pH indicators. See Natural test papers -- Cabbage, Brazil, Dahlia, Elderberry, Indigo, Litmus, Rose, Rhubarb, Turmeric.

We didn't have a pH meter when we started making biodiesel in 1999 so we used phenolphthalein solution. Phenolphthalein is colourless up to pH 8.3, then it turns pink (or rather magenta), and red at pH 10.4. When it just starts to turn pink and stays that way for 15 seconds it's reading pH 8.5, which is the measure you want.

Phenolphthalein lasts about a year. It's sensitive to light, store it in a cool, dark place.

Don't be put off or frightened away by titration. It's not difficult, thousands of non-chemist biodiesel makers have learnt how to do it without difficulty and use it every time they make biodiesel. Just follow the directions, step by step. See also More about lye, Better titration, Joe Street's titrator, Accurate measurements.