Rice bran pretreatment is the first crucial step in the oil extraction process. Before processing, raw rice bran is carefully screened to remove any small impurities that may be present. This ensures a cleaner and more efficient subsequent process. Once cleaned, the rice bran is sent into a quenching and tempering system, where a small amount of steam—approximately 2% of the total raw material weight—is introduced. This steam helps soften the rice bran, making it more suitable for pressing and oil extraction. The softening process improves the efficiency of oil recovery and reduces mechanical stress on the equipment.
The next stage is pressing, where the pretreated rice bran is directly fed into a hydraulic press. This cold pressing method is used to extract a significant portion of the oil from the rice bran. The extracted oil, known as rice bran oil, is then sent directly to the physical refining workshop for further purification. Meanwhile, the remaining cake, which still contains some residual oil, is transferred to the leaching workshop for additional oil extraction using solvents. This dual-step approach ensures maximum oil yield and minimizes waste.
In the leaching process, the rice bran cake is placed into an oil leaching device. A solvent, typically n-hexane, is added to dissolve the oils present in the cake. This creates a mixture of oil and solvent, known as mixed oil. The mixture is then filtered through a medium such as a screen to trap solid particles, resulting in a cleaner and more concentrated oil solution. This step is essential for achieving high-quality oil with minimal contamination.
After leaching, the mixed oil undergoes evaporation to separate the solvent from the oil. Since the oil has a high boiling point and is nearly non-volatile, while the solvent (n-hexane) has a low boiling point, steam is used to evaporate most of the solvent. This increases the concentration of the oil in the mixture significantly. The evaporation takes place in a long-tube evaporator, where the mixed oil is preheated and then rapidly boiled inside the heating tubes. As the oil rises, it forms a thin film along the tube walls, allowing for efficient heat transfer and solvent removal.
Once the mixed oil is concentrated, it moves to the stripping stage. Despite the evaporation process, a small amount of solvent may still remain in the oil. To remove this, a direct steam at a certain pressure is introduced into the oil. This steam helps lower the boiling point of the solvent, enabling it to evaporate without condensing. The stripped solvent is then collected and condensed for reuse, ensuring an environmentally friendly and cost-effective process.
The wet mash obtained from the leaching device also contains traces of solvent. To remove this, water vapor is introduced into the wet mash. This process, similar to the stripping of mixed oil, helps desolventize the material by evaporating the remaining solvent. The vapor is then condensed and recycled, maintaining a closed-loop system that minimizes waste and maximizes resource utilization.
Following the stripping and desolvation steps, the oil is ready for further refinement. The next stage involves precipitation filtration, where insoluble impurities such as cake residue, sediment, and plant debris are removed through natural settling. This step helps improve the clarity and purity of the oil before it proceeds to the next stage of processing.
Hydration degumming is another critical step in oil refining. A small amount of water, typically 1% to 3% of the oil weight, is added to the oil. This causes water-soluble impurities, especially phospholipids, to coagulate and precipitate out of the oil. Phospholipids, when dry, can dissolve in the oil, but when they absorb moisture, they become hydrophilic and form larger colloidal particles that settle at the bottom. This process effectively removes these impurities, improving the quality of the final product.
After hydration, the oil is dehydrated using a continuous packing dehydrator under vacuum conditions. The vacuum level is usually around -0.09 MPa, which helps remove any remaining moisture. This dehydration step is important because it prepares the oil for the next phase—adsorption decolorization. By reducing the water content, the oil becomes more receptive to the adsorbent used in the decolorization process.
Adsorption decolorization involves adding a small amount of bleaching clay (about 2% of the oil weight) to the oil. The oil is then stirred in a decolorization tank to allow full contact between the oil and the clay. This process removes color pigments and other impurities. After equilibrium is reached, the oil is filtered to remove the clay, resulting in a clearer and purer oil. This step is vital for achieving a visually appealing and high-quality end product.
Deacidification follows decolorization. The oil is heated to approximately 250°C and passed through a structured packed tower. Saturated steam is injected from the bottom, allowing the oil to flow downward and come into contact with the steam. This process strips away free fatty acids, reducing acidity and improving the stability of the oil. The direct steam used in this step is about 2% of the oil weight, ensuring effective deacidification without excessive energy consumption.
The next stage is deodorization, where the oil is again passed through a structured packed tower. At a temperature of around 230°C, saturated steam is introduced from the bottom, allowing the oil to flow downward and interact with the steam. This step removes volatile compounds responsible for undesirable odors, resulting in a neutral-smelling oil. The direct steam used here is about 1% of the oil weight, ensuring efficient deodorization while minimizing energy use.
During deodorization, any fatty acid vapors produced are collected in a structural packing trap and cooled to around 60–70°C. These cooled fatty acids are then circulated back to the top of the trap, where they come into contact with hot fatty acid gas flowing from the bottom. This heat exchange process allows the gas to condense into liquid form, which is then collected and sent to the fatty acid workshop as a by-product. This step not only enhances the efficiency of the process but also provides additional value from waste streams.
Finally, dewaxing is performed to remove any remaining wax from the deodorized oil. At higher temperatures, wax dissolves in the oil, but when the oil is cooled, the wax crystallizes and separates. The cooled oil is then pressure-filtered through a plate and frame filter press, allowing the refined rice bran oil to pass through while the wax remains trapped on the filter cloth. This step ensures that the final product is clear, stable, and suitable for various applications.
Once all the refining steps are complete, the finished oil is stored in a storage tank. It is then pumped into the filling production line, where it is packaged using a small filling machine and prepared for sale. This final step ensures that the high-quality rice bran oil reaches the market in a safe and convenient format.
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