Solvents for the Fats and Oils Industry
In the Fats and Oils Industry, solvents are used to extract and refine oils from oil raw materials to maximize the utilization of oil components. Oils can dissolve in various organic solvents, with lower-grade fatty acids or oils with higher unsaturation levels exhibiting greater solubility.
The method of treating oil raw materials with solvents to extract the oil components is called solvent extraction. This is the most effective method in oil processing, particularly for low-oil-content crops such as soybeans, castor beans, and olives, where the extraction efficiency is high. When using the pressing method to extract oil, approximately 4–7% of the oil remains in the oil cake. For example, when soybeans are processed using the pressing method, the soybean cake still contains 20–25% soybean oil, whereas the solvent extraction method can reduce the residual oil content in the soybean cake to below 1%. Although the solvent extraction method has a high oil recovery rate, it has drawbacks such as the residual solvent in the oil having a foul odor, difficulty in separating the solvent from finely ground raw materials, and significant solvent loss. Therefore, the pressing method is generally used first to extract oil, and the pressed oil cake is then subjected to solvent extraction to remove residual oil.
The solvents used in fat and oil extraction should ideally possess the following properties:
Strong penetrating power, fast speed, and selective extraction of the target oil components;
Strong solubility for oil components;
Recyclable (narrow boiling point range, no high-boiling-point components), with high recovery efficiency (solvent loss typically 0.3–0.1%);
Neutral, non-toxic, non-flammable, and leaves no unpleasant odor in oils or oil cakes;
Low vaporization heat, low specific heat capacity, and stable properties;
Non-corrosive to equipment.
It is indeed a challenging task to find a solvent for fat and oil extraction that completely meets all the aforementioned conditions. Currently, petroleum ether is the most widely used solvent, followed by benzene. The table below lists the main solvents used in fat and oil extraction.
Extraction Solvent
| Solvents | Advantages | Disadvantages |
| Petroleum ether | Does not cause rancidity of oils or oil cakes, does not extract pigments or other impurities, can be completely recovered from oil, particularly suitable for the extraction of edible oils, excellent solubility, stability, and penetration, non-corrosive, suitable for oils with a boiling point range of 70–80°C | Flammable, its vapors are slightly anesthetic. |
| Benzene | Strong solubility, constant boiling point | Toxic and flammable. |
| Trichloroethylene | Non-flammable, strong solubility, good stability, no solvent loss | Corrosive to equipment in the presence of moisture, toxic to reducing catalysts when residual in oil., |
| Carbon tetrachloride |
| Pentane | Boiling point: 30–35°C, used for extracting thermally unstable oils. | |
| Hexane | Boiling point: 63–69.4°C. | |
| Heptane | Boiling point: 88–92°C. | |
| Octane | Boiling point: 100–140°C, suitable for extracting castor oil that does not mix well with other oils. | |
| Cyclopentane | Boiling point 68–85°C, primarily cyclohexane | |
| Carbon disulfide | Strong solvent capacity, previously used for olive oil extraction | Flammable, highly toxic. |
| Ethanol | Low solvent capacity for oils at room temperature; solvent capacity increases with temperature. Extraction can be performed at 120°C under pressure, with the alcohol-oil mixture separated upon cooling. However, pigments and other components may also be extracted by the alcohol | Solubility decreases when raw materials contain moisture. |
