The core difference lies in the deacidification step: deacidification involves removing free fatty acids (FFA) from crude oil, making it the most critical stage of the refining process.
Chemical refining (alkali refining): Caustic soda (NaOH) is added to neutralize the fatty acids through a chemical reaction, forming soapstock that is then separated out.
Physical refining (distillation deacidification): No alkali is added; instead, high-temperature, high-vacuum conditions combined with steam stripping are used to directly vaporize and remove the fatty acids—a process carried out in the same tower as deodorization.
Chemical Refining Process
Crude oil → Degumming → Alkali refining/deacidification (adding caustic soda, centrifugal separation of soapstock, water washing) → Bleaching (bleaching earth) → Deodorization → Finished oil
Physical Refining Process
Crude oil → Deep degumming → Bleaching → Deacidification + Deodorization (combined in a single tower; high-temperature vacuum steam distillation) → Finished oil
Physical refining places extremely high demands on pretreatment; gums (phospholipids) must be thoroughly removed, as residual gums cause the oil to rapidly deteriorate and darken at high temperatures.
Pros and Cons of Chemical Refining (Alkali Refining)
High adaptability to raw materials: Can handle crude oil with fluctuating acid values and high gum content; offers good stability and is suitable for small-scale plants.
Lower equipment investment and lower operational complexity.
Mature technology; widely used for soybean oil, rapeseed oil, and sunflower oil.
Generates large amounts of soapstock and washing wastewater; creates significant environmental pressure and high wastewater treatment costs.
Alkali removes some neutral oil, resulting in high refining losses; losses are particularly high for crude oil with high acid values.
Significant loss of nutrients—such as gamma-oryzanol and vitamin E—occurs during alkali refining (e.g., in rice bran oil).
Soapstock is a by-product that can be used to produce soap or acidified oil.
Pros and Cons of Physical Refining
No caustic soda used; minimal wastewater, making it more environmentally friendly. Distilled free fatty acids can be condensed and recovered as high-value by-products.
Major advantages for high-acid-value crude oils: For oils like rice bran oil and crude palm oil, which have high acid values, physical refining yields significantly more oil than chemical alkali refining by minimizing neutral oil loss.
Better retention of gamma-oryzanol and tocopherols (provided pretreatment is thorough).
Higher equipment investment; requires high-performance deacidification/deodorization towers and high-vacuum systems. Strict requirements for degumming and bleaching pretreatment; failure to properly remove gums can result in the entire batch being scrapped.
Higher operating temperatures (240–260°C) lead to high energy consumption; complex operation requiring highly skilled personnel.
If the gum content in the crude oil exceeds limits, the oil is prone to oxidation and darkening at high temperatures.
Suitable Oil Types
Chemical Refining Soybean oil, rapeseed oil, and standard sunflower oil; oils where the crude oil has a low acid value but significant fluctuations in gum content.
Small- to medium-sized refineries dealing with inconsistent raw material quality.
Physical RefiningRice bran oil (the most typical example) and palm oil; oils with high crude acid values; the preferred choice for large-scale plants.
Crude rice bran oil often has a Free Fatty Acid (FFA) content of 15–30%; using chemical alkali refining would result in massive oil loss and be economically unviable, so physical refining is the industry standard.
