Overview
What Is Soap Stock (Acid Oil)?
Soap stock is generated during the alkali (caustic) refining of crude vegetable oils such as soybean, rapeseed, sunflower and palm oil. When crude oil is treated with caustic soda (NaOH) to remove free fatty acids, the resulting aqueous layer, rich in fatty acid soaps, neutral oil, phospholipids and water, is soap stock.
When acidulated (treated with sulfuric or hydrochloric acid), the soaps are converted back to free fatty acids, producing acid oil (also called crude acid oil or acidulated soapstock). Acid oil typically contains 50-90% FFA, 5-20% neutral oil, and 5-30% water, with the balance being unsaponifiable matter.
Under EU RED III, soap stock from vegetable oil refining is not named explicitly in Annex IX. It often qualifies under Part A (d) as industrial waste/residue not fit for food or feed, but this is stream- and certifier-specific, confirm on the PoS. When accepted as Part A, it sits outside the Part B 1.7% transport energy cap.
Key Advantages
Why Soap Stock for Biofuel Production?
Often Part A (d)*
When accepted as Part A (d), no Part B 1.7% transport energy cap. Classification depends on the stream and certifier, verify on PoS.
Up to 83% GHG Savings
RED III default value for soap stock-based biofuels. Refining by-product carries minimal upstream emissions, GHG benefit allocated almost entirely to the biofuel product.
Multiple Oil Origins
Available from soy, rapeseed, sunflower and palm refining. EU-origin acid oil (rapeseed, sunflower) offers highest traceability and avoids EUDR/deforestation complexity.
Cost-Effective
Refinery by-product trades at steep discount to refined oils. When adjusted for mandate value (double-counting, uncapped), effective cost per unit of compliance is highly competitive.
Technical Data
Quality Specifications
Typical quality parameters for Acid Oil (Acidulated Soap Stock) as supplied by Suat Fuels. High variability between batches and origins, batch-specific CoA essential.
| Parameter | Typical Value | Limit | Test Method |
|---|---|---|---|
| Free Fatty Acids (FFA) | 50-80% | min 50% | AOCS Ca 5a-40 |
| Moisture & Water | 5-20% | max 25% | EN ISO 12937 |
| Neutral Oil | 5-20% | - | AOCS Ca 6b-53 |
| Unsaponifiable Matter | 2-8% | max 10% | AOCS Ca 6b-53 |
| Sulfur | 50-200 ppm | - | EN ISO 20846 |
| Phosphorus | 100-500 ppm | - | EN 14107 |
| Density at 20°C | 0.910-0.940 kg/L | - | EN ISO 3675 |
| Iodine Value | 80-130 (depending on oil origin) | - | AOCS Cd 1d-92 |
Compliance
Regulatory Status & Certifications
EU RED III, Often Annex IX Part A (d)* (Stream-Specific)
- Soap stock from vegetable oil refining, often Part A (d) as industrial waste/residue (confirm on PoS)
- Uncapped, not subject to 1.7% transport energy cap unlike Part B feedstocks
- Double-counted towards EU renewable energy targets
- Eligible for ReFuelEU Aviation SAF mandates as HEFA feedstock (with acid oil pre-treatment)
- ISCC EU chain-of-custody certification required, Suat Fuels holds active ISCC EU licence
End Uses
Applications
FAME Biodiesel
Multi-stage acid esterification + base transesterification. Specialized FAME plants designed for high-FFA feedstocks. EN 14214 compliant output.
HVO (with pre-treatment)
Requires dedicated pre-treatment unit (FFA removal, water removal, phosphorus cleanup) before hydroprocessing. High Annex IX Part A value justifies investment.
HEFA-SAF
ReFuelEU Aviation mandate eligible. Uncapped Part A status makes it highly attractive for SAF producers needing to maximize mandate compliance volume.
Co-blending
Blended with lower-FFA feedstocks (UCO, rapeseed oil) to optimize plant throughput and processing economics at existing FAME or HVO facilities.
Supply Geography
Origins & Availability
Soap stock is produced at every vegetable oil refinery. EU-origin material (rapeseed acid oil from Germany, Poland, France; sunflower acid oil from Ukraine, Romania) is preferred for EU mandate compliance due to traceability advantages. South American soy acid oil (Brazil, Argentina) is price-competitive but requires EUDR traceability. Palm acid oil (Malaysia, Indonesia) is available but carries deforestation risk scrutiny and EUDR compliance requirements.