Soapstock Acid Oil — Frequently Asked Questions

Soapstock is the wet, alkaline by-product of neutralising crude vegetable oil with caustic soda, consisting mainly of sodium soaps, entrained oil and water. Acid oil is soapstock that has been split with a mineral acid, usually sulphuric acid, so the soaps revert to free fatty acids. The result is drier, more stable and far easier to ship, which is why acid oil rather than soapstock is the form usually traded.

Total fatty matter (TFM) measures the fatty acids and glycerides that can be converted into fuel, so it is a practical measure of value for a material whose FFA varies widely. Contracts commonly set a TFM basis of about 90–95 % or more with price adjustments, plus an MIU maximum. FFA is still reported, because it determines how much esterification a biodiesel plant has to carry out.

Splitting uses sulphuric acid, and if the acid water is not fully separated and the oil is not washed, sulphate and residual acid remain. Sulphur can then run from tens to hundreds of mg/kg. For biodiesel, where the finished ester is limited to 10 mg/kg, set a sulphur cap in the contract and check that your pretreatment or ester distillation can remove the rest. HVO units remove sulphur but consume hydrogen doing so.

Technically yes, after pretreatment to remove phosphorus, sodium, other metals and sulphate. The harder questions are regulatory: acid oils are not named in Annex IX, and some aviation and national schemes restrict or exclude them. Mixed sources also make traceability to each originating refinery more demanding. Confirm eligibility for your compliance route with your certification body before contracting.

Stainless steel or lined ISO tanks are preferred, because high-FFA material with possible residual acid attacks mild steel and some gasket materials. Flexitanks are widely used, but ask the flexitank provider to confirm liner and valve compatibility for acidic cargo. Keep hoses and seals in resistant materials and plan light heating for discharge in cold weather.

Tell us your TFM basis, MIU, sulphur and phosphorus limits, whether you need a single source oil or accept mixed soapstock, the conversion route and the pretreatment available, volume and frequency, packaging, delivery point and Incoterm, and the sustainability documents your market requires. With those, the lot analysis and documentation can be matched to your specification before terms are proposed.