Formaldehyde preparation method
甲醛用途广泛，属生产工艺简单、原料供应充足的大众化工产品。是甲醇下游产品树中的主干，世界30%左 右的甲醇都用来生产甲醛，年产量在2500万吨左右。甲醛溶液是一种浓度较低的水溶液，从经济角度考虑不便于长距离运输，所以一般都在主消费市场附近设厂.进出口贸易也极少。工业上主要采用甲醇氧化法和天然气直接氧化法生产  。
Formaldehyde is widely used as a popular chemical product with simple production process and sufficient supply of raw materials. It is the backbone of methanol downstream product trees. About 30% of the world's methanol is used to produce formaldehyde, with an annual output of about 25 million tons. Formaldehyde solution is a kind of aqueous solution with low concentration, which is not convenient for long-distance transportation from an economic point of view. Therefore, factories are generally set up near the main consumer market, and there is little import and export trade. In industry, methanol oxidation method and natural gas direct oxidation method are mainly used for production .
1. Methanol oxidation method: at 600 ~ 700 ℃, methanol, air and water are directly oxidized to formaldehyde through catalysts such as silver, copper or alum pentoxide, which is absorbed by water to obtain formaldehyde solution
The total reaction is exothermic, but 50-60% formaldehyde is formed by oxidation reaction
The rest is formed by hydrogen reaction. By products are carbon monoxide and carbon dioxide, methyl formate and formic acid. The conversion rate of methanol is 80%, and the yield is 85% ~ 90%. The technology of this method is mature and the yield is high. 
2. Natural gas oxidation method: at 600-680 ℃, the mixture of natural gas and air is directly oxidized to formaldehyde through oxide catalysts such as iron and molybdenum, and the formaldehyde solution is obtained by absorption of water: 
3. Dimethyl ether oxidation method: it is made of dimethyl ether, the by-product of methanol synthesis by high-pressure synthetic gas method, and metal oxide as catalyst.
4. When the methanol vapor is at 300 ℃, the methanol is dehydrogenated and oxidized with the catalyst of copper or silver. Formaldehyde gas absorbs 36% - 40% water, which is formaldehyde solution. Distill the formaldehyde solution on sale to remove impurities, and add methanol as the reagent formaldehyde solution: 
5. Methanol dehydrogenation method: anhydrous formaldehyde can be obtained by direct dehydrogenation of methanol, and hydrogen can be produced at the same time. This process is an attractive method to prepare formaldehyde. The key to its development lies in the improvement of catalyst performance.
6. Mix the gasified methanol with the air and steam after alkali washing in the ratio of 1:1.8-2.0:0.8-1.0 (volume ratio), and heat to 115-120 ℃ for reaction. Under the action of silver catalyst, control the reaction temperature to 600-650 ℃, and pressure to 0.3-0.5mpa
After the reaction, quench the reactant to 80-85 ℃, absorb it with water, distill it, evaporate the unreacted methanol, treat the kettle liquid with anion exchange resin, add a proper amount of polymerization inhibitor into the formaldehyde solution, mix it, and the finished product is obtained.