Gas-phase quenching production process for melamine using urea as raw material

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Granular urea (containing about 1% water) enters the urea melting tank, where it melts into molten urea before entering the urea washing tower. Liquid ammonia is transported via pipelines to the melamine production area, where it is vaporized by a carrier gas compressor to become ammonia gas, which serves as the carrier gas for the melamine polymerization reaction. This reaction carrier gas enters the reactor after being preheated by molten salt in the shell side of a preheater. Urine at 130–150°C also enters the reactor along with the atomized gas; under conditions of 0.4–1.0 MPa and a high temperature of 390°C, urea is converted into melamine through the action of a catalyst. The heat required for the reaction is provided by a molten salt system (the total amount of molten salt is 140 tons; its main components are potassium nitrate, sodium nitrate, and sodium nitrite in a ratio of 53:7:40, and it operates as a closed-loop system). The gaseous products and by-products resulting from the urea reaction enter the tubes of the hot gas cooler, where they exchange heat with the liquid circulating outside the tubes. The total amount of this liquid is 28 tons; its main components are biphenyl and biphenyl ether in a ratio of 73.5:26.5. It operates within a closed-loop system, and as a result, the temperature drops from 390°C to 320–340°C. A very small amount of high-boiling-point by-products such as milleramine and mibeamine crystallize out. The heat released by the reaction gas is carried by the Dowtherm fluid to the Dowtherm condenser, where it exchanges heat with water to produce 2.2 MPa saturated steam; the Dowtherm fluid then returns to the hot gas cooler. The gas-solid mixture exiting the hot gas cooler enters a filter, where solid impurities such as catalyst and high-boiling-point by-products are removed from the gas stream. The dust-free gas coming out of the filter enters the crystallizer, where it mixes with the cold process gas at 140°C coming from the urea scrubber. The temperature drops from 340–320°C to around 210°C, causing melamine to crystallize. This crystallized melamine then enters a cyclone separator along with the mixed gas; the melamine settles at the bottom of the cyclone, and is removed from there by an air stream. After further cooling and packaging, the melamine product is obtained. The gas emerging from the top of the cyclone separator contains trace amounts of triamine dust; one stream is sent to the reactor after being pressurized by a carrier gas compressor, while the other stream enters the urea scrubber, where it is cooled through urea washing, reducing the temperature of the process gas to around 140°C. After cooling, the process gas is divided into two streams: the first stream is pressurized by an air blower and sent back to the crystallizer as quenching gas, while the second stream is waste gas; it is sent via pipelines to the ammonium chloride production line after leaving the melamine plant, where it is used as a raw material for ammonium chloride production.