Notes for GL's Biodiesel Quiz #2


Methanol or methoxide are less dense than oil, so will tend to float on the oil surface unless they are vigorously mixed in.

If you have an undersized pump and you introduce your methoxide too rapidly to the inlet side of the pump, you may find the methoxide sits on top of your oil in the reactor and doesn't get propely mixed in.

If you repeatedly get poor conversion, for no apparent reason, try reducing your methoxide feed rate, which can help to properly mix it in with the oil, so that it doesn't sit on top.



Methanol cannot be effectively filtered from air, so the safest approach it breath fresh air from an outside source. Don't use a standard industrial air compressor to feed air to a face mask, because the compressor can introduce a fine oil mist into the air stream , which can be very damaging to lungs.

Either invest in a purpose-made breathing apparatus, or if you can't afford that, you can make a simple and effective breathing aid here.



The glycerides in under-reacted biodiesel (as mono-glyceride, di-glyceride or tri-glyceride) are more viscous or thicker than biodiesel, and can over-stress your engine's injection pump, especially in cold weather.



You can reduce the heat power from an electric heating element by running it on a reduced voltage. If you run it on half its rated voltage it will give 1/4 its rated heating power.

This is because power = voltage x current, and if you reduce voltage by a factor of two, you will also reduce current by a factor of two. 1/2 x 1/2 = 1/4



It is imposiible to tell how 'good' used oil is, by looking at it or smelling it, because there are so many different oil types which look different to each other. The food which has been cooked in the oil can give it a darker colour and a different smell too, and this may have little effect on the oil's quality.

The most useful measure of an oil's quality is its acid number, as this will tell you how much extra lye you will need to add, in order to get good conversion.



Lye will firstly be used up in neutraling FFA and creating soap. This is a wasteful part of the process. The remaining lye will then act as a catalyst to help strip the long chain fatty acids from the glycerol 'backbone' of the oil. The reason we titrate is to know how much extra lye needs to be added for the first wasteful 'sacrifice' when the FFA is neutralised.



The full procedure for carrying out a titration is given here.



Pure methanol boils at 65C. Methanol in the biodiesel reaction boils at a somewhat higher temperature, but if you use 65C as your upper limit you should avoid excessive loss of methanol.

You may find when you add methoxide to your reactor that a puff of vapour exits the vent. This is not due to boiling. It is because the methanol has a much higher vapour pressure than air at processing temperatures, so the air in the reactore will be quickly displaced by methanol vapour, after which no more puffs should be seen.

If you vent via a condenser, such as the plumber's delight condenser, you will be able to capture any displaced methanol, for re-use.



The 3 main ingredients are glycerides, alcohol and catalyst.

It is possible to make biodiesel from FFA, containing no glycerides, but this is less common.

Mineral oil is never used to make true biodiesel, nor is kerosene or gasoline.



It appears from numerous posts on the forums that the biggest contributor to good biodiesel reaction success is good mixing, all other things being equal.

From what I have read, my 'rule of thumb' succestion for pump sizing is to use a pump which can re-circulate the whole contents of your reactor tank within a minute or less.

So , if you have a 400 gallon tank, a 400 gallon per minute pump will almost certainly allow you to achieve full conversion at 60C within an hour, if using 5g NaOH or 7g KOH base (assuming at least 90% purity), with dry oil.