Caustic Soda in Biodiesel Production

Biodiesel is made by reacting vegetable oils or animal fats with methanol, using a catalyst. Caustic soda is one of the most common catalysts for small and medium producers. This guide explains its role, how much to use, and why water and free fatty acids matter so much.

The reaction

In transesterification, triglycerides in the oil react with methanol to form methyl esters (biodiesel) and glycerine. Caustic soda, dissolved in the methanol to form sodium methoxide, speeds up the reaction so it completes in about an hour at moderate temperature.

How much caustic soda

For clean, refined oil with very low free fatty acids, a commonly used starting dose is around 1% caustic soda by weight of oil. Used cooking oil and crude oils contain free fatty acids (FFA), which consume caustic and form soap. For these oils:

1. Titrate a sample to measure FFA.
2. Add extra caustic to neutralise the measured FFA, on top of the base catalyst dose.
3. If FFA is high, pre-treat the oil with acid esterification first, because too much soap makes separation difficult.

Keep water out

Water in the oil, methanol or caustic causes soap formation and lowers yield. Use dry oil, anhydrous methanol and fresh, well-sealed caustic soda flakes. Flakes left open absorb moisture and carbon dioxide and lose strength.

After the reaction

• Let the glycerine settle and separate it.
• Wash or dry-polish the biodiesel to remove remaining catalyst, soap and methanol.
• Test the fuel for quality against the applicable standard before use or sale.

Safety

Sodium methoxide is both corrosive and made with methanol, which is toxic and highly flammable. Mix in a closed container, in a ventilated area, with no ignition sources. Wear goggles, chemical gloves and protective clothing, and keep eyewash and fire extinguishers nearby.

Sourcing caustic soda in Oman

Producers and laboratories in Oman can source caustic soda flakes 99% in sealed 25 kg bags from Muscat Chemical, with a published daily price and delivery within Oman.