The reaction of oil and water makes oil crack into fatty acid and glycerol (sweet water). Hydrolysis of oil is the main method of producing fatty acid nowadays, and also one of the most important methods of producing natural glycerol. In soap-making industry, more and more fatty acid neutralization processes have been used. In recent years, the annual production of fatty acids in China has increased dramatically (about 50,000 tons in 1980s, about 300,000 tons in 2000 and more than 1 million tons in 2006). Natural glycerol in China mainly comes from the fatty acid industry.
The reaction of oil and water to produce fatty acid and glycerol. Because of the different control methods of pyrolysis operation, the concentration of glycerol and the quality of sweet water vary greatly, which also has a great influence on the treatment of sweet water. Oil hydrolysis methods are generally divided into low temperature and atmospheric pressure hydrolysis, medium temperature and medium temperature hydrolysis, high temperature and high pressure hydrolysis. The speed of oil hydrolysis is affected by catalyst, water content, temperature and oil varieties. The higher the temperature is, the faster the cracking rate is. In addition, affected by catalysts, the cracking speed of oils with short carbon chain fatty acids is faster than that without catalysts, and that of oils with long carbon chain fatty acids is faster than that of oils with short carbon chain fatty acids.
Low temperature and atmospheric pressure hydrolysis is an open reactor with 50% grease, 1% catalyst (alkyl benzene sulfonic acid, etc.) and 49% sulfuric acid aqueous solution. It is heated and stirred by direct steam for 20-24 hours continuously under atmospheric pressure until the hydrolysis rate is 60%-80%. Then it is still and releases sweet water. Catalyst and sulfuric acid solution were added again, and the above operations were repeated until the hydrolysis rate reached 92%~95%. The hydrolysis rate of oil at low temperature and atmospheric pressure is low, the period is long, the glycerol content of sweet water is low (10%), and it contains impurities such as inorganic acids. Therefore, the application of low temperature and atmospheric pressure hydrolysis method is less and less.
Medium temperature and medium pressure hydrolysis is the oldest method of oil hydrolysis published in 1854 as a patent. Up to now, batch hydrolysis with catalyst and batch hydrolysis without catalyst and continuous hydrolysis with multi-tower in series have been formed. Commonly used catalysts are ZnO, MgO, CaO and so on. The hydrolysis temperature was 190~230℃, and the pressure of the autoclave was 1.0 – 3.0 MPa.
At present, many small and medium-sized factories in China use batch hydrolysis of 1.0-1.5 MPa (oil-water ratio is 1:0.5, secondary hydrolysis, hydrolysis rate is 95%, hydrolysis period is 15-20 h, sweet water contains about 10% glycerol). In order to avoid the pollution of sulfate wastewater from sulfuric acid treatment and improve the yield and quality of glycerol, the method of prolonging the reaction time without catalyst under the condition of original steam pressure is often adopted in the process of Glycerol Recovery to treat sweet water.
High temperature and high pressure continuous hydrolysis of oils and fats was first published by Mils in 1935. After many improvements, it has become the main method to produce fatty acids on a large scale (at present, many single tower 50,000-100,000 tons of hydrolysis units have been built in China). The high pressure steam is directly flowed into the reaction tower, keeping the working pressure between 5.0 MPa and 6.0 MPa and the temperature between 250℃ and 260 ℃. The operation process is: oil is transported from the high pressure pump to the lower part of the hydrolysis tower, oil rises through the glycerol-water phase, process water is transported from the high pressure pump to the upper part of the tower and flows downstream through the heat exchanger distributor, oil and fatty acid float upward because of the lightness of the water, and the glycerol produced flows downstream together with water, and is washed by counter-current hydrolysis. Fatty acids and glycerol are separately discharged from the top and bottom of the tower through a pressure reducer. The residence time of oil in the tower is about 2.5 hours, and the hydrolysis rate is over 98%. The content of glycerol in sweet water can reach 25%~40%.
