The low-pressure melamine process is used to produce melamine powder from urea.
The melamine process is a catalytic vapor-phase process operated at pressures below 10 bar.
Urea melt is fed into the reactor and is atomized by spray nozzles with the aid of high-pressure ammonia. The reactor is a fluidized bed gas reactor using silica/aluminium oxide as catalyst. The reaction offgas, an ammonia and carbon dioxide mixture, is preheated and is used as fluidizing gas. Conversion of urea to melamine is an endothermic reaction; the necessary heat is supplied via heated molten salt circulated through internal heating coils.
The fluidizing gas leaves the reactor together with gaseous melamine and the byproducts ammonia, carbon dioxide, isocyanic acid and traces of melem. The gas also contains entrained catalyst fines. Melem is separated by desublimation and is removed together with the catalyst fines in a gas filter.
The filtered gas is further cooled in the crystallizer to the desublimation temperature of the melamine product. Cooling is performed using the offgas from the urea scrubber. The melamine forms fine crystals, which are recovered from the process gas in the product-cyclone. Leaving the product-cyclone, the cooled melamine is stored and can be used without further treatment. It has a minimum purity of 99.8%.
The process gas leaving the product-cyclone is fed to the urea scrubber, which is cooled with molten urea. The clean gas leaving the urea scrubber is partially used in the reactor as fluidizing gas and is partially recycled to the crystallizer as quenching gas. The surplus is fed to an offgas treatment unit for further recycling to the urea plant. This outstanding straight-forward low-pressure process without any water quench, features low corrosion tendency, absence of complicated rotating equipment and need for a drying unit. All factors result in very low capital investment and operating costs.
Licensor: Edgein S&T Co. Ltd./Lurgi GmbH, a company of the Air Liquide Group
