Biodiesel is made by transesterification: vegetable oil, methanol and a lye catalyst react to give a thin fuel (methyl esters) and glycerol. The carrier’s quantities are 1 litre of oil, 200 ml of methanol and 3.5 g of sodium hydroxide dissolved in the methanol, stirred at 55 °C for 1 hour, settled for 8 hours, then washed and dried. Methanol is toxic and the lye-methanol mixture is caustic, so the work is outdoors with eye and skin protection.
Part of Organic Chemistry
Converting vegetable oil into a diesel-engine fuel by reacting its fatty-acid esters with methanol.
The Reaction Swaps One Alcohol For Another
A vegetable oil is a triglyceride: glycerol carrying three fatty-acid chains as esters (see Key Reactions). Methanol, in the presence of lye, replaces the glycerol on each chain. The three methyl esters are thin enough to burn in a diesel engine; the freed glycerol sinks. The carrier writes it as vegetable oil + methanol → biodiesel + glycerol. The corpus’s chemistry page is Chemistry of Transesterification and the full procedure is Biodiesel Production; the carrier’s quantities are given here.
What You Need
- Oil, new or used, filtered. Heavily degraded oil that is dark, thick or rancid may not work (the carrier); test as in Free Fatty Acid Problem.
- Methanol, from wood distillation or other source.
- Sodium hydroxide, dry solid (Lye Catalyst).
- A vessel that tolerates lye and methanol, a stirrer, a thermometer and a separating vessel.
- Protective gear: Protective Gear.
Method: The Carrier’s Batch, Per Litre Of Oil
Step 1: Dissolve 3.5 g of NaOH in 200 ml of methanol. Do it in a closed-but-vented container outdoors.
Step 2: Heat the oil to 55 °C.
Step 3: Combine and stir vigorously for 1 hour.
Step 4: Settle for 8 hours. Glycerol sinks, biodiesel floats.
Step 5: Drain the glycerol, wash the fuel with water to remove residual lye, then dry and filter. Water left in the fuel causes engine damage and microbial growth (the carrier’s mistakes table).
The glycerol layer carries lye and methanol and stays hazardous: Glycerin Byproduct and Settling and Separation.
Worked Example: Is 200 ml Of Methanol An Excess?
Use rounded atomic masses C = 12, H = 1, O = 16, and typical densities (recalled, not corpus figures): oil 0.92 kg/L and methanol 0.79 kg/L. Treat the oil as tristearin, 890 g per mole (a stand-in for a mixed oil).
- Oil in 1 litre: 920 g ÷ 890 g per mole ≈ 1.03 mol.
- Methanol needed at 3 mol per mol of oil: 3 × 1.03 = 3.1 mol. Methanol is CH4O: 12 + 4 + 16 = 32 g per mole, so 3.1 × 32 ≈ 99 g, which is 99 ÷ 0.79 ≈ 125 ml.
- The carrier uses 200 ml, which is 200 × 0.79 = 158 g, or 158 ÷ 32 ≈ 4.9 mol.
- Ratio: 4.9 ÷ 3.1 ≈ 1.6 times the stoichiometric amount.
The reaction is reversible, so an excess of methanol pushes it toward the ester. The excess does not disappear: it ends up in the glycerol layer and the fuel, which is why washing and hazard handling matter.
Common Mistakes
- Using wood-ash lye. It is a water solution; water stops the reaction (Lye Catalyst).
- Skipping the oil test. Free fatty acids use up the catalyst.
- Leaving water in the fuel.
- Treating methanol like ethanol. It blinds. See the Safety block.
Safety
Methanol is a poison and sodium methoxide is caustic
Methanol passes through intact skin and blinds; it is the substance whose dose figures are set out on Methanol Hazards, a page that treats every lethal figure as a dose of pure methanol, not a volume of liquid. Dissolving lye in methanol makes a caustic, flammable solution. Work outdoors, wear gloves and eye protection, keep flames away, label every container, and never use drink containers.
What’s Next
- Learn next: Bioethanol — a spark-engine fuel
- Combine with: Pressing and Rendering — getting the oil