The current mainstream method for raw materials is the urea method. The chemical reaction for generating melamine powder through urea pyrolysis is: under heating and certain pressure conditions, 6mol of urea produces 1mol of melamine powder while producing 3mol of carbon dioxide and 6mol of ammonia as by-products.
According to the different pressures of molten urea pyrolysis, it can be divided into high-pressure melamine, normal pressure melamine, and low-pressure melamine.
The reaction equation:
6CO(NH2)2→C3N6H6+6NH3+3C02
The production of melamine powder by high-pressure method belongs to the dry liquid phase reaction, with a temperature range of 370-450 ℃.
High-pressure Melamine Process flow:
The urea solution sent from the urea plant is concentrated to obtain 145 ℃ molten urea. It is pressurized to 8.5MPa and mixed with 8.5MPa and 420 ℃ ammonia before entering the melamine reactor. The reaction pressure is 8.0MPa, and the temperature is 380 ℃.
Urea is directly converted into melamine in the reactor. The liquid phase material containing carbon dioxide, ammonia, melamine, and a small amount of condensed polymer from the reactor is depressurized to 2.5MPa and enters the rapid cooling section.
Most ammonia and carbon dioxide are flash evaporated in the rapid cooling tower and sent out for further treatment in the form of methylamine.
The melamine solution from the bottom of the fast cooling tower is sent to the stripping tower to strip the residual ammonia and carbon dioxide completely, and then condensed polymer decomposition is done to remove solid impurities.
After adsorption decolorization, the melamine product is obtained by crystallization, centrifugal separation, and drying. The yield ranges from 85% to 90%.
The mother liquor produced by centrifugal separation is recycled through ammonia recovery and wastewater treatment, and the ammonia and process water are reused to achieve the purpose of recycling.
Characteristics:
①The process is a liquid-phase reaction, which is not easily blocked by crystallization, But under high temperature and pressure, the reaction medium has strong corrosiveness, the equipment material grade requirements are high, the control system is complex, and the one-time investment is significant.
②The reaction does not require a catalyst, and there is no concern about catalyst poisoning or product contamination.
③The operation is stable, and the product quality is good.
④The device has excellent flexibility in operation, and the time to produce qualified products after one start-up is relatively short: high operating pressure, large scale, low energy consumption, and low operating costs.
⑤Due to its high pressure, the by-product exhaust gas is convenient to use and easy to co-produce urea, reducing product costs.
The production of melamine powder by atmospheric pressure is a gas-phase reaction with a temperature range of around 390 ℃.
Normal-pressure Melamine Powder Process flow:
Solid urea is melted and passed into a storage tank.
Most of it is pumped into a fluidized bed reactor, while the rest is cooled and sent to a washing tower to wash the unreacted urea and unseparated melamine in the reaction tail gas.
After washing, the molten urea is heated to 400 ℃ and enters the reactor. The atomized urea makes the catalyst fluidized. The urea reacts under normal pressure, 380-400 ℃, and the action of catalyst alumina to produce melamine. The molten salt system provides the heat required for the reaction.
The reaction gas is then passed through the waste heat recovery system to produce 2.3MPa steam, which is cooled to 330 ℃ and enters the thermal filter to filter the by-product crystals and catalyst dust. The purified gas enters the crystallizer and is rapidly cooled with 140 ℃ circulating gas from the urea washing tower to maintain the crystallizer at 190-210 ℃, resulting in melamine grains.
After separation and drying of the grains, the product is obtained with a yield of 90%. The tail gas from the cyclone separator enters the urea washing tower for washing, and some of it is used as fluidized gas for the reactor, some as cooling gas for the crystallizer, and the rest is treated with tail gas and sent to the urea plant.
Characteristics:
①Gas phase reaction, dry capture, low corrosion, low equipment material grade, and small one-time investment.
②Simple process, little equipment, short process, simple control, low operating pressure, large equipment, large footprint, cumbersome operation, and certain labor intensity.
③The tail gas pressure of by-products is low, difficult to recover, and has high energy consumption.
④The reaction uses a catalyst, and as the catalyst is used for longer, the production capacity gradually decreases.
⑤The device is prone to blockage and requires continuous shutdown for processing, making long-term operation very difficult.
The production of melamine powder by low-pressure method also belongs to liquid-phase reaction, with a temperature range of 380-440 ℃.
Low-pressure Melamine Powder Process flow:
After the solid urea is melted, the high-speed ammonia (150C) is injected into the fluidized bed reactor from the bottom through a two-phase spray.
The pure ammonia produced by tail gas recovery is compressed and heated to the reaction temperature and then enters the reactor at high speed from the bottom to fluidize the catalyst. The atomized urea produces melamine under the action of the catalyst.
Two cyclone separators are at the top of the reactor to separate the catalyst dust carried in the reaction gas flow and then enter the rapid cooling device.
The fast cooling device is directly quenched with the injected mother liquor and cold slurry, causing melamine to cool and condense into a solid, forming a supersaturated solution that enters the washing tower.
By injecting the mother liquor and circulating slurry, melamine is separated from the gas, liquid, and solid three-phase mixture. The crude slurry containing 10% melamine coming out from the bottom of the tower is transported in two ways: one is pumped back to the rapid cooling device and washing tower as the rapid cooling liquid.
The other route enters the washing tower, and the ammonia and carbon dioxide gas mixture released from the top of the washing tower goes to the tail gas recovery section. The slurry from the washing tower is concentrated to 35% (mass fraction) through a cyclone separator or hydraulic separator and enters the desorption tower, where ammonia and carbon dioxide in the material are stripped out.
The slurry coming out of the desorption tower enters the purification section, and activated carbon is added to the mixer for decolorization. Ammonia is used to adjust the pH value of the slurry to remove condensed matter. The decolorized slurry enters the filter to remove impurities and then goes to the second stage of crystallization.
The crystallizer operates under vacuum and crystallizes melamine through water evaporation and cooling. After centrifugal separation and drying, melamine is obtained with a yield of about 85%.
Characteristics:
①Reaction in the presence of a catalyst, good selectivity, high product quality, few by-products, early technological development, and relatively mature. However, the process is complex and lengthy.
②The process is in the liquid phase. Capture equipment corrosion is significant, material grade is high, and many pieces of equipment occupying a large area require a high initial investment.
③Difficult to operate, prone to blockage, and high energy consumption.
④Low pressure makes it challenging to utilize exhaust gas.
⑤It takes a long time to obtain qualified melamine products after starting production.
As a leading melamine manufactures, if you require high-quality melamine powder products during the production process, we are always ready to meet your requirements.
Whether you need to produce melamine powder through high-pressure, atmospheric pressure, or low-pressure methods, our team can ensure first-class quality and reliable supply. Contact us immediately to discuss your melamine procurement needs.
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