catalyst: how much?

Most homebrewers recognize the need to titrate their oil before making biodiesel to determine the amount of catalyst needed.

Free fatty acids (FFAs) in the oil will deactivate a portion of the catalyst, so the amount of catalyst must be adjusted depending on the amount of FFAs present.

If you are using NaOH as your catalyst, each ml of titration solution required corresponds to 0.77% free fatty acids. So, a used frying oil that titrates as 4 will have 3.1% FFAs.

If you are using KOH as your catalyst, then each ml of titration solution is 0.55% free fatty acids. Thus the 3.1% FFA frying oil will titrate to between 5 and 6. Check your KOH because most commercial grades are only about 86% pure. For this KOH purity, each ml of titration solution is 0.47% free fatty acids.

In his original (now dangerously obselete) book From the Fryer to the Fuel Tank, Josh Tickell recommended that, when using NaOH, the homebrewer add a base amount of 3.5 grams to the titration amount and use this total as the number of grams of catalyst to be added to each liter of oil.

This low base number may make it difficult to achieve fuel quality within the ASTM maximum for total glycerin (0.24%).

From my experience, using a little more NaOH than Tickell recommends gives a more complete reaction. I suggest adding a base amount of 9 to the titration amount for both NaOH and KOH.

Basically, in my lab we have always used a baseline amount of 1% NaOH or KOH, calculated as a ratio of mass of catalyst to feedstock, because we know it is more than enough.  Most of the time we are looking at some other effect and we just want to make sure we are not catalyst-limited. 

However, overuse of catalyst can be wasteful.

I've been running a series of experiments that show the effect of using too little catalyst and what happens to the catalyst during the reaction and how it affects the total glycerin number at the end of the reaction.

Stay tuned for updates. S