After the reaction, let the mixture stand undisturbed in a tall narrow container for 8-24 hours: the dark, thick glycerin sinks and the thinner, golden biodiesel floats, and the glycerin is drained off the bottom. If no clean layers form, the reaction failed, usually from too much water or too little lye; the drained glycerin still contains lye and methanol and must be treated as caustic and poisonous.
Part of Biodiesel Production
Settling separates the denser glycerin layer from the lighter biodiesel layer by gravity after the reaction.
Gravity Does The Separation
Biodiesel is less dense than glycerin (about 0.88 against 1.26 kg/L; both recalled), and the two are not miscible once the reaction has run. Standing them still lets the glycerin drop to the bottom. A tall narrow container gives a deep layer that is easy to see and drain; the parent suggests a clear water-cooler jug with a valve at the bottom.
How much comes out: from the parent’s rule of 1-1.5 litres of glycerin per 10 litres of biodiesel, a 20-litre batch gives about 2-3 litres of glycerin under about 18-20 litres of biodiesel (the mass balance gives up to 1.05 litres of biodiesel per litre of oil before losses; see Triglyceride + Methanol → FAME + Glycerin).
What You Need
- A tall, narrow, clear container with a drain tap at the bottom, or a way to siphon
- A receiving container for glycerin, marked and made for alkalis (Container Selection)
- Goggles, gloves, apron
- A place outdoors, away from flame
Method
Step 1: Pour the finished reaction into the settling container while still warm, and leave it undisturbed.
Step 2: Wait 8-24 hours. In the first hour a dark layer begins to drop. Expect biodiesel golden or amber on top, glycerin dark brown and thick below.
Step 3: Drain the glycerin. Open the bottom valve slowly and stop as soon as clear amber biodiesel starts to follow. With NaOH the glycerin may set solid on cooling (parent); drain it while it is still liquid or warm the container gently.
Step 4: Treat the biodiesel as unfinished. It still holds methanol, lye, soap and traces of glycerin and must be washed (Washing & Drying).
Step 5: Store the glycerin sealed and labelled (Glycerin Byproduct).
If It Does Not Separate
The parent’s diagnosis: too much water or insufficient lye. Other pointers from the chemistry: a soapy batch from high free fatty acid makes a milky emulsion that settles slowly or sets as a gel (Free Fatty Acid Problem). Give it more time, keep it warm, and keep notes for the next batch. A failed batch is still hazardous: do not pour it away.
Common Mistakes
- Draining too fast and pulling biodiesel into the glycerin.
- Stirring up the layers by moving the container.
- Treating the glycerin as harmless. It carries lye and a large share of the excess methanol.
- Pouring in a drain, stream or soil. Methanol and lye reach water.
The bottom layer is caustic and toxic
The glycerin layer holds unused lye and much of the leftover methanol. Handle it with gloves and goggles, outdoors, away from flame, keep it in a closed labelled container and do not breathe the fumes. Wash any splash off the skin at once with plenty of water (Spill Response).
What’s Next
- Learn next: Washing & Drying, to clean the fuel
- Combine with: Glycerin Byproduct, for the layer you drained