Oil bleaching refers to the use of some adsorbents that have strong selective adsorption to certain pigments in oils to remove pigments and other impurities in oils under certain conditions, thereby achieving the purpose of bleaching. Bleaching is a physical refining method, usually carried out after alkali refining or water washing, to remove impurities such as pigments, residual soaps, trace metals, oxidation products, etc. in oils and fats, and improve the color and stability of oils and fats.
The principle of bleaching is to use the interaction between the adsorbent and the impurities in the oils and fats so that the impurities are adsorbed or exchanged by the adsorbent and separated from the oils and fats. During bleaching, the following factors need to be controlled:
(1) Adsorbent: The type, quality, dosage and addition method of the adsorbent have a great influence on the bleaching effect. Commonly used adsorbents include activated clay, activated carbon, silica gel, etc. Activated clay is the most commonly used adsorbent, with strong adsorption capacity and selectivity, and can remove impurities such as pigments, residual soaps, trace metals, etc. in oils and fats. Activated carbon is mainly used to remove impurities such as oxidation products, polycyclic aromatic hydrocarbons, and residual pesticides in oils and fats. Silica gel is mainly used to remove polar substances in oils and fats, such as free fatty acids and colloids. The amount of adsorbent used is generally 1% to 5% of the oil, depending on the type of oil and the impurity content. Too much adsorbent will increase the cost and difficulty of handling, while too little adsorbent will affect the bleaching effect. The adsorbent is generally added by dry method or wet method. The dry method refers to adding the dry adsorbent directly to the oil and stirring it; the wet method refers to mixing the adsorbent with water or other solutions and adding it to the oil and stirring it. The wet method can improve the contact efficiency between the adsorbent and the oil, but it will also increase the moisture and impurities in the solution.
(2) Temperature: Temperature is an important factor affecting the bleaching effect and reaction rate. Too high temperature will cause oil oxidation and deterioration and thermal polymerization, affecting the quality of the oil; too low temperature will cause oil viscosity to increase and crystallization, affecting the transfer and separation of impurities. Generally speaking, the temperature should be controlled at around 90~120℃.
(3) Vacuum: Vacuum is an important condition to ensure the smooth progress of the bleaching process and prevent oil oxidation and deterioration. Vacuum can remove oxygen from the air and reduce oxidation reactions; vacuum can lower the boiling point of water and other volatile substances and promote their escape from oil; vacuum can improve the contact efficiency between oil and adsorbent and accelerate the reaction rate. Generally speaking, vacuum should be controlled at around 0.1~0.5kPa.
(4) Time: Time is an important factor affecting bleaching effect and reaction balance. Too long time will cause oil loss and adsorbent saturation, affecting separation effect; too short time will affect impurity transfer and adsorption, affecting bleaching effect. Generally speaking, time should be controlled at around 10~30 minutes.
(5) Separation: Separation is the process of completely separating oil from adsorbent and discharging adsorbent containing impurities. During separation, attention should be paid to the selection and operation of separation equipment. Commonly used separation equipment includes filter press, centrifuge, plate and frame filter, etc. During separation, attention should be paid to controlling the outlet pressure and flow rate to avoid adsorbent residue and oil loss.