The water washing process of oil is a common step in oil refining, which is usually used to remove residual impurities, soap stock and some soluble substances (such as phospholipids, free fatty acid neutralization products) in oil. The water washing process can not only improve the purity of oil, but also improve the color, flavor and storage stability of oil. It is generally used in the further purification process after deacidification, degumming and desoaping of oil.
Introduction to oil washing process
Principle:
The principle of the water washing process is to use the affinity of water and impurities in oil (such as soap stock, phospholipids, etc.) to dissolve or suspend impurities in water through the mixing of water and oil, and then remove these impurities from the oil through separation equipment.
Water washing is often used in refined oil after alkali refining and deacidification to remove soap stock and other water-soluble impurities generated by neutralization reaction.
Process flow:
Step 1: Add water to mix
After deacidification or other treatment, a certain proportion of warm water is added to mix. The proportion of water is usually 10-20% of the weight of the oil, which is adjusted according to the nature of the oil and the amount of impurities to be removed.
Step 2: Heating and stirring
During the oil-water mixing process, the oil is usually heated to 60-80°C, and the water and oil are fully mixed by stirring to increase the contact area between water and impurities and promote the dissolution of impurities into the water.
Step 3: Centrifugal separation
The mixed oil and water enter the centrifuge, and the water phase and oil phase are separated by centrifugal force. The water phase contains most impurities, such as soap stock, phospholipids and soluble impurities, while the oil phase is the refined oil.
Step 4: Repeated water washing
Depending on the oil purity requirements, water washing may need to be performed multiple times. After each water washing, the oil will be heated and stirred again, and then separated.
Function:
Remove residual impurities: Water washing can effectively remove chemical reaction products such as soap stock and soluble phospholipids that remain in the refining steps such as deacidification and degumming.
Reducing the acid value of oil: Through water washing, a small amount of free fatty acids and water-soluble acid substances in the oil can also be removed, further reducing the acid value of the oil.
Improve the purity and stability of oil: After removing impurities, the color, smell and shelf life of oil are improved, which is suitable for consumption or further processing.
Factors affecting the water washing process
Water temperature: Higher water temperature can improve the solubility and diffusion rate of impurities, but too high temperature may cause oxidation or loss of oil. Therefore, it is usually controlled at 60-80°C.
Water quality: Water quality has a great influence on the water washing effect. Using soft water or deionized water can reduce the interference of calcium and magnesium ions and avoid the combination of soap stock with ions in water to form new precipitates.
Stirring speed and time: Full stirring helps the water to fully contact with impurities in oil, but excessive stirring may cause emulsification, making it difficult to separate oil and water. The general stirring time is controlled at about 30-60 minutes.
Detailed introduction of water washing equipment
Mixing reactor:
Function: Used for the mixing reaction of oil and water. The reactor is equipped with a stirring device and a heating device to ensure that the oil and water are fully mixed and maintained at a suitable temperature.
Features: It has a good mixing effect. The reactor is usually designed as a closed structure to prevent oxidation and water loss.
Centrifuge:
Function: Through high-speed rotation, centrifugal force is used to separate the water phase and oil phase in the oil-water mixture. Impurities in the water phase are separated and the oil phase is purified.
Features: The separation efficiency of modern centrifuges is very high, and most impurities in the water phase can be effectively removed. Centrifuges are usually automated equipment, capable of continuous operation, and suitable for large-scale production.
Type:
Disc centrifuge: Suitable for processing large-flow oil-water mixtures, and can quickly separate the oil phase and water phase.
Centrifuge: Mainly used to process oil and fat mixtures with high viscosity or high impurity content, with good separation effect.
Sedimentation tank:
Function: Sedimentation tank is a low-speed separation of oil and water equipment, mainly used for small-scale production or as a pretreatment equipment before centrifugal separation. The oil-water mixture is left in the tank for a period of time, and the water phase will gradually sink due to its high density, while the oil phase will float.
Features: Sedimentation tank equipment is simple and low-cost, but the separation efficiency is low. It is usually used for preliminary separation of water and oil after water washing.
Water washing tower:
Function: In large-scale continuous refining production, the water washing tower continuously washes the oil through multi-stage spraying. The water in the tower contacts the oil in countercurrent, the impurities are taken away with the water, and the oil is purified layer by layer.
Features: Multi-stage separation can be set in the tower to ensure the purification effect of the oil, which is suitable for large-scale continuous production.
Filtering equipment:
Function: The filtering equipment is used to remove solid impurities and moisture in the oil, and further improve the purity of the oil. The filtering equipment is usually installed after the water washing process as the final purification step.
Features: Different types of filtering equipment such as plate and frame filters and bag filters can be selected according to the different properties of the oil.
Advantages and challenges of water washing process
Advantages:
Simple and efficient: The water washing process is simple and easy to operate, especially suitable for removing soluble impurities.
Low cost: Compared with other refining processes, the water washing cost is relatively low and the equipment investment is small.
Environmental protection: No chemical agents are used in the water washing process, and the generated wastewater is easier to handle.
Challenges:
Difficulty in oil-water separation: The water washing process easily leads to emulsification of oil and water, especially when the oil contains more phospholipids, the difficulty of separating oil and water increases.
Oil loss: During the water washing process, some neutral oil will be lost with the water phase, resulting in oil loss, especially when dealing with low acid value oil.
In general, the water washing process is an indispensable step in oil refining. Through effective water washing and advanced separation equipment, the purity and quality of oil can be greatly improved, and the final quality of refined oil can be guaranteed.