Formaldehyde is a popular chemical product with wide uses, simple production process and sufficient raw materials supply. It is the backbone of the downstream product tree of methanol. The annual output of the world is about 25 million tons, and 3% of the left and right methanol is used to produce formaldehyde. However, formaldehyde is a kind of aqueous solution with low concentration, which is not convenient for long-distance transportation from the economic point of view, so factories are generally set up near the main consumer market, and import and export trade is also very few. Methanol oxidation and direct oxidation of natural gas are mainly used in industry:
1. Methanol oxidation: at 6~7℃, methanol, air and water are directly oxidized to generate formaldehyde, and formaldehyde is absorbed by water to obtain formaldehyde solution:
The total reaction is exothermic, but 5~6% of formaldehyde is generated through oxidation, and the rest. By-products are carbon monoxide and carbon dioxide, methyl formate and formic acid. The conversion rate of methanol is 8%, and the yield is 85%~9% based on methanol. This method has mature technology and high yield, and is widely used by domestic and foreign manufacturers.
2. natural gas oxidation method: at 6-68℃, the mixture of natural gas and air is directly oxidized by oxide catalysts such as iron and molybdenum to generate formaldehyde, which is absorbed by water to obtain formaldehyde solution.
3. At 3℃, methanol vapor is introduced into copper or silver catalyst, and methanol is dehydrogenated. Formaldehyde gas absorbs 36% ~ 4% water, which is formaldehyde solution. The commercial formaldehyde solution is distilled to remove impurities, and methanol is added to obtain the reagent formaldehyde solution.
4. dimethyl ether oxidation method: dimethyl ether, a by-product of methanol synthesis by high-pressure synthesis of syngas, is used as raw material and oxidized by metal oxide as catalyst.
5. methanol dehydrogenation method: anhydrous formaldehyde can be obtained by direct dehydrogenation of methanol, and hydrogen is also produced by-product. This process is an attractive method to prepare formaldehyde. The key to its progress lies in the improvement of process catalyst performance.
6. The gasified methanol is mixed with air and water vapor after alkali washing in a volume ratio of 1: 1.8-2.: .8-1., and then heated to 115-12℃ for reaction, and the reaction temperature is controlled to 6-65℃ and the pressure is .3-.5 MPa under the action of silver catalyst.
after the reaction, the reactants are rapidly cooled to 8 ~ 85℃, absorbed by water, then distilled to evaporate unreacted methanol, and the kettle liquid is treated by anion exchange resin, and a proper amount of polymerization inhibitor is added to the obtained formaldehyde solution, and the finished product is obtained by stirring and mixing.